Acquisition of antibodies against Plasmodium falciparum merozoites and malaria immunity in young children and the influence of age, force of infection, and magnitude of response

Proactive home-based malaria management in rural communities of Bassar Health District in northern Togo from 2014 to 2017: PECADOM + , a pilot experiment

BACKGROUND

Togo's National Malaria Control Programme has initiated an active home-based malaria management model for all age groups in rural areas of Bassar Health District. This report describes the model, reports its main results, and determines the factors associated with positive rapid diagnostic test results.

METHODS

From 2014 to 2017, in three peripheral care units of Bassar Health District (Binaparba, Nangbani, and Baghan), community health workers visited residents' homes weekly to identify patients with malaria symptoms, perform rapid diagnostic tests in symptomatic patients, and give medication to positive cases. Univariate and multivariate logistic regression models were used to determine the factors associated with positive tests.

RESULTS

The study covered 11,337 people (817 in 2014, 1804 in 2015, 2638 in 2016, and 6078 in 2017). The overall mean age was 18 years (95% CI 5-29; min-max: 0-112 years). The median age was 10 years (SD: 16.9). The proportions of people tested positive were 75.3% in Binaparba, 77.4% in Nangbani, and 56.6% in Baghan. The 5-10 age group was the most affected category (24.2% positive tests). Positive tests were more frequent during the rainy than during the dry season (62 vs. 38%) and the probability of positive test was 1.76 times higher during the rainy than during the dry season (adjusted OR = 1.74; 95% CI 1.60-1.90). A fever (37.5 °C or higher) increased significantly the probability of positive test (adjusted OR = 2.19; 95% CI 1.89-2.54). The risk of positive test was 1.89 times higher in passive than in active malaria detection (adjusted OR = 1.89; 95% CI 1.73-2.0).

CONCLUSIONS

This novel experimental community and home-based malaria management in Togo suggested that active detection of malaria cases is feasible within 24 h, which allows rapid treatments before progression to often-fatal complications. This PECADOM + program will help Togo's National Malaria Control Programme reduce malaria morbidity and mortality in remote and hard-to-reach communities.

Role of cytokines in immunomodulation during malaria clearance

Malaria remains a significant global health challenge, demanding a deeper understanding of host immune responses for effective clearance of the parasitic infection. Cytokines, as crucial mediators of the immune system, orchestrate a complex interplay during the various stages of malaria infection. Throughout the course of the disease, an intricate balance of pro-inflammatory and anti-inflammatory cytokines dictate the immune response's outcome, influencing parasitic clearance and disease severity. During the initial stages, interleukins such as interleukin-12 (IL-12), interferon-gamma (IFN-γ), and tumour necrosis factor-alpha (TNF-α) play pivotal roles in activating innate immune cells, initiating the anti-parasitic response. Simultaneously, regulatory cytokines like interleukin-10 (IL-10) and transforming growth factor-beta (TGF-β) modulate this immune activation, preventing excessive inflammation and tissue damage. As the infection progresses, a delicate shift occurs, characterized by a transition to adaptive immunity, guided by cytokines like interleukin-4 (IL-4), interleukin-5 (IL-5), and interleukin-13 (IL-13), promoting antibody production and T-cell responses. Notably, the resolution of malaria infection crucially relies on a fine-tuned balance of cytokine networks. Dysregulation or imbalances in these mediators often result in immune hyperactivation, contributing to severe manifestations and prolonged infection. Understanding the multi-faceted roles of cytokines in malaria clearance offers promising avenues for therapeutic interventions. Targeting cytokine pathways to restore immune equilibrium or bolster protective responses could potentially enhance treatment strategies and vaccine development. In conclusion, the pivotal role of cytokines in immunomodulation during malaria clearance underscores their significance as potential targets for therapeutic interventions, offering promising prospects in the global fight against this infectious disease.

Cross-reactivity of rPvs48/45, a recombinant Plasmodium vivax protein, with sera from Plasmodium falciparum endemic areas of Africa

Background

Ps48/45, a Plasmodium gametocyte surface protein, is a promising candidate for malaria transmission-blocking (TB) vaccine. Due to its relevance for a multispecies vaccine, we explored the cross-reactivity and TB activity of a recombinant P. vivax Ps48/45 protein (rPvs48/45) with sera from P. falciparum-exposed African donors.

Methods

rPvs48/45 was produced in Chinese hamster ovary cell lines and tested by ELISA for its cross-reactivity with sera from Burkina Faso, Tanzania, Mali, and Nigeria - In addition, BALB/c mice were immunized with the rPvs48/45 protein formulated in Montanide ISA-51 and inoculated with a crude extract of P. falciparum NF-54 gametocytes to evaluate the parasite-boosting effect on rPvs48/45 antibody titers. Specific anti-rPvs48/45 IgG purified from African sera was used to evaluate the ex vivo TB activity on P. falciparum, using standard mosquito membrane feeding assays (SMFA).

Results

rPvs48/45 protein showed cross-reactivity with sera of individuals from all four African countries, in proportions ranging from 94% (Tanzania) to 40% (Nigeria). Also, the level of cross-reactive antibodies varied significantly between countries (p<0.0001), with a higher antibody level in Mali and the lowest in Nigeria. In addition, antibody levels were higher in adults (≥ 17 years) than young children (≤ 5 years) in both Mali and Tanzania, with a higher proportion of responders in adults (90%) than in children (61%) (p<0.0001) in Mali, where male (75%) and female (80%) displayed similar antibody responses. Furthermore, immunization of mice with P. falciparum gametocytes boosted anti-Pvs48/45 antibody responses, recognizing P. falciparum gametocytes in indirect immunofluorescence antibody test. Notably, rPvs48/45 affinity-purified African IgG exhibited a TB activity of 61% against P. falciparum in SMFA.

Conclusion

African sera (exposed only to P. falciparum) cross-recognized the rPvs48/45 protein. This, together with the functional activity of IgG, warrants further studies for the potential development of a P. vivax and P. falciparum cross-protective TB vaccine.

Antibody response to malaria vaccine candidates in pregnant women with Plasmodium falciparum and Schistosoma haematobium infections

Malaria in pregnancy has severe consequences for the mother and foetus. Antibody response to specific malaria vaccine candidates (MVC) has been associated with a decreased risk of clinical malaria and its outcomes. We studied Plasmodium falciparum (Pf) and Schistosoma haematobium (Sh) infections and factors that could influence antibody responses to MVC in pregnant women. A total of 337 pregnant women receiving antenatal care (ANC) and 139 for delivery participated in this study. Pf infection was detected by qPCR and Sh infection using urine filtration method. Antibody levels against CSP, AMA-1, GLURP-R0, VAR2CSA and Pfs48/45 MVC were quantified by ELISA. Multivariable linear regression models identified factors associated with the modulation of antibody responses. The prevalence of Pf and Sh infections was 27% and 4% at ANC and 7% and 4% at delivery. Pf infection, residing in Adidome and multigravidae were positively associated with specific IgG response to CSP, AMA-1, GLURP-R0 and VAR2CSA. ITN use and IPTp were negatively associated with specific IgG response to GLURP-R0 and Pfs48/45. There was no association between Sh infection and antibody response to MVC at ANC or delivery. Pf infections in pregnant women were positively associated with antibody response to CSP, GLURP-R0 and AMA-1. Antibody response to GLURP-R0 and Pfs48/45 was low for IPTp and ITN users. This could indicate a lower exposure to Pf infection and low malaria prevalence observed at delivery.

Can incorporating genotyping data into efficacy estimators improve efficiency of early phase malaria vaccine trials?

BACKGROUND

Early phase malaria vaccine field trials typically measure malaria infection by PCR or thick blood smear microscopy performed on serially sampled blood. Vaccine efficacy (VE) is the proportion reduction in an endpoint due to vaccination and is often calculated as VEHR = 1-hazard ratio or VERR = 1-risk ratio. Genotyping information can distinguish different clones and distinguish multiple infections over time, potentially increasing statistical power. This paper investigates two alternative VE endpoints incorporating genotyping information: VEmolFOI, the vaccine-induced proportion reduction in incidence of new clones acquired over time, and VEC, the vaccine-induced proportion reduction in mean number of infecting clones per exposure.

METHODS

Power of VEmolFOI and VEC was compared to that of VEHR and VERR by simulations and analytic derivations, and the four VE methods were applied to three data sets: a Phase 3 trial of RTS,S malaria vaccine in 6912 African infants, a Phase 2 trial of PfSPZ Vaccine in 80 Burkina Faso adults, and a trial comparing Plasmodium vivax incidence in 466 Papua New Guinean children after receiving chloroquine + artemether lumefantrine with or without primaquine (as these VE methods can also quantify effects of other prevention measures). By destroying hibernating liver-stage P. vivax, primaquine reduces subsequent reactivations after treatment completion.

RESULTS

In the trial of RTS,S vaccine, a significantly reduced number of clones at first infection was observed, but this was not the case in trials of PfSPZ Vaccine or primaquine, although the PfSPZ trial lacked power to show a reduction. Resampling smaller data sets from the large RTS,S trial to simulate phase 2 trials showed modest power gains from VEC compared to VEHR for data like those from RTS,S, but VEC is less powerful than VEHR for trials in which the number of clones at first infection is not reduced. VEmolFOI was most powerful in model-based simulations, but only the primaquine trial collected enough serial samples to precisely estimate VEmolFOI. The primaquine VEmolFOI estimate decreased after most control arm liver-stage infections reactivated (which mathematically resembles a waning vaccine), preventing VEmolFOI from improving power.

CONCLUSIONS

The power gain from the genotyping methods depends on the context. Because input parameters for early phase power calculations are often uncertain, these estimators are not recommended as primary endpoints for small trials unless supported by targeted data analysis.

TRIAL REGISTRATIONS

NCT00866619, NCT02663700, NCT02143934.

Impact of population based indoor residual spraying in combination with mass drug administration on malaria incidence and test positivity in a high transmission setting in north eastern Uganda

BACKGROUND

Mass drug administration (MDA) and indoor residual spraying (IRS) are potent malaria burden reduction tools. The impact of combining MDA and IRS is not well documented. We evaluated the impact of MDA + IRS compared to IRS alone at a high transmission site in Eastern Uganda.

METHODS

A quasi-experimental study was implemented in Toroma and Kapujan subcounties in north eastern Uganda. Both subcounties received four rounds of IRS using primiphos-methyl (Acttellic SC300) 6-8 months apart from December 2016 to December 2018. Eligible residents of Kapujan simultaneously received MDA using dihydroartemesinin-piperaquine (DHA-PQ). Health facility data was used to monitor malaria case incidence rate and test positivity rates.

RESULTS

In the MDA + IRS arm, malaria incidence dropped by 83% (IRR: 0·17 (0.16-0.18); p < 0.001) in children under 5 year and by 78% (IRR: 0·22 (0.22-0.23); p < 0.001) in persons aged ≥ 5 years from the pre-intervention to the intervention period. In the IRS arm malaria incidence dropped by 47% (IRR: 0.53 (0.51, 0.56); p < 0.001) in children under 5 years and by 71% 0.29 (0.28, 0.30); p < 0.001) in persons aged ≥ 5 years. A drastic drop occurred immediately after the intervention after which cases slowly increased in both arms. Malaria test positivity rate (TPR) dropped at a rate of 21 (p = 0.003) percentage points per 1000 persons in the MDA + IRS arm compared to the IRS arm. There was a mean decrease of 60 (p-value, 0.040) malaria cases among children under five years and a mean decrease in TPR of 16·16 (p-value, 0.001) in the MDA + IRS arm compared to IRS arm.

INTERPRETATION

MDA significantly reduced malaria burden among children < 5 years however the duration of this impact needs to be further investigated.

Acquisition of complement fixing antibodies targeting Plasmodium falciparum merozoites in infants and their mothers in Uganda

Background

Antibody-mediated complement fixation has previously been associated with protection against malaria in naturally acquired immunity. However, the process of early-life development of complement-fixing antibodies in infants, both in comparison to their respective mothers and to other immune parameters, remains less clear.

Results

We measured complement-fixing antibodies in newborns and their mothers in a malaria endemic area over 5 years follow-up and found that infants' complement-fixing antibody levels were highest at birth, decreased until six months, then increased progressively until they were similar to birth at five years. Infants with high levels at birth experienced a faster decay of complement-fixing antibodies but showed similar levels to the low response group of newborns thereafter. No difference was observed in antibody levels between infant cord blood and mothers at delivery. The same result was found when categorized into high and low response groups, indicating placental transfer of antibodies. Complement-fixing antibodies were positively correlated with total schizont-specific IgG and IgM levels in mothers and infants at several time points. At nine months, complement-fixing antibodies were negatively correlated with total B cell frequency and osteopontin concentrations in the infants, while positively correlated with atypical memory B cells and P. falciparum-positive atypical memory B cells.

Conclusion

This study indicates that complement-fixing antibodies against P. falciparum merozoites are produced in the mothers and placentally-transferred, and they are acquired in infants over time during the first years of life. Understanding early life immune responses is crucial for developing a functional, long lasting malaria vaccine.

IgM antibody responses against Plasmodium antigens in neotropical primates in the Brazilian Atlantic Forest

Introduction

Zoonotic transmission is a challenge for the control and elimination of malaria. It has been recorded in the Atlantic Forest, outside the Amazon which is the endemic region in Brazil. However, only very few studies have assessed the antibody response, especially of IgM antibodies, in Neotropical primates (NP). Therefore, in order to contribute to a better understanding of the immune response in different hosts and facilitate the identification of potential reservoirs, in this study, naturally acquired IgM antibody responses against Plasmodium antigens were evaluated, for the first time, in NP from the Atlantic Forest.

Methods

The study was carried out using 154 NP samples from three different areas of the Atlantic Forest. IgM antibodies against peptides of the circumsporozoite protein (CSP) from different Plasmodium species and different erythrocytic stage antigens were detected by ELISA.

Results

Fifty-nine percent of NP had IgM antibodies against at least one CSP peptide and 87% against at least one Plasmodium vivax erythrocytic stage antigen. Levels of antibodies against PvAMA-1 were the highest compared to the other antigens. All families of NP showed IgM antibodies against CSP peptides, and, most strikingly, against erythrocytic stage antigens. Generalized linear models demonstrated that IgM positivity against PvCSP and PvAMA-1 was associated with PCR-detectable blood-stage malaria infection and the host being free-living. Interestingly, animals with IgM against both PvCSP and PvAMA-1 were 4.7 times more likely to be PCR positive than animals that did not have IgM for these two antigens simultaneously.

Discussion

IgM antibodies against different Plasmodium spp. antigens are present in NP from the Atlantic Forest. High seroprevalence and antibody levels against blood-stage antigens were observed, which had a significant association with molecular evidence of infection. IgM antibodies against CSP and AMA-1 may be used as a potential marker for the identification of NP infected with Plasmodium, which are reservoirs of malaria in the Brazilian Atlantic Forest.

Can incorporating genotyping data into efficacy estimators improve efficiency of early phase malaria vaccine trials?

Background

Early phase malaria vaccine field trials typically measure malaria infection by PCR or thick blood smear microscopy performed on serially sampled blood. Vaccine efficacy (VE) is the proportion reduction in an endpoint due to vaccination and is often calculated as V E H R = 1 - hazard ratio or V E R R = 1 - risk ratio. Genotyping information can distinguish different clones and distinguish multiple infections over time, potentially increasing statistical power. This paper investigates two alternative VE endpoints incorporating genotyping information: V E m o l F O I , the vaccine-induced proportion reduction in incidence of new clones acquired over time, and V E C , the vaccine-induced proportion reduction in mean number of infecting clones per exposure.

Methods

We used simulations and analytic derivations to compare power of these methods to V E H R and V E R R and applied them to three data sets: a Phase 3 trial of RTS,S malaria vaccine in 6912 African infants, a Phase 2 trial of PfSPZ Vaccine in 80 Burkina Faso adults, and a trial comparing Plasmodium vivax incidence in 466 Papua New Guinean children after receiving chloroquine + artemether lumefantrine with or without primaquine (as these VE methods can also quantify effects of other prevention measures). By destroying hibernating liver-stage P. vivax, primaquine reduces subsequent reactivations after treatment completion.

Results

The RTS,S vaccine significantly reduced the number of clones at first infection, but PfSPZ vaccine and primaquine did not. Resampling smaller data sets from the large RTS,S trial to simulate phase 2 trials showed modest power gains from V E C compared to V E H R for data like RTS,S, but V E C is less powerful than V E H R for vaccines which do not reduce the number of clones at first infection. V E m o l F O I was most powerful in model-based simulations, but only the primaquine trial collected enough serial samples to precisely estimate V E m o l F O I . The primaquine V E m o l F O I estimate decreased after most control arm liver-stage infections reactivated (which mathematically resembles a waning vaccine), preventing V E m o l F O I from improving power.

Conclusions

The power gain from the genotyping methods depends on the context. Because input parameters for early phase power calculations are often uncertain, we recommend against these estimators as primary endpoints for small trials unless supported by targeted data analysis.

Trial registrations

NCT00866619, NCT02663700, NCT02143934.

The temporal dynamics of Plasmodium species infection after artemisinin-based combination therapy (ACT) among asymptomatic children in the Hohoe municipality, Ghana

BACKGROUND

The routine surveillance of asymptomatic malaria using nucleic acid-based amplification tests is essential in obtaining reliable data that would inform malaria policy formulation and the implementation of appropriate control measures.

METHODS

In this study, the prevalence rate and the dynamics of Plasmodium species among asymptomatic children (n = 1697) under 5 years from 30 communities within the Hohoe municipality in Ghana were determined.

RESULTS AND DISCUSSION

The observed prevalence of Plasmodium parasite infection by polymerase chain reaction (PCR) was 33.6% (571/1697), which was significantly higher compared to that obtained by microscopy [26.6% (451/1697)] (P < 0.0001). Based on species-specific analysis by nested PCR, Plasmodium falciparum infection [33.6% (570/1697)] was dominant, with Plasmodium malariae, Plasmodium ovale and Plasmodium vivax infections accounting for 0.1% (1/1697), 0.0% (0/1697), and 0.0% (0/1697), respectively. The prevalence of P. falciparum infection among the 30 communities ranged from 0.0 to 82.5%. Following artesunate-amodiaquine (AS + AQ, 25 mg/kg) treatment of a sub-population of the participants (n = 184), there was a substantial reduction in Plasmodium parasite prevalence by 100% and 79.2% on day 7 based on microscopy and nested PCR analysis, respectively. However, there was an increase in parasite prevalence from day 14 to day 42, with a subsequent decline on day 70 by both microscopy and nested PCR. For parasite clearance rate analysis, we found a significant proportion of the participants harbouring residual Plasmodium parasites or parasite genomic DNA on day 1 [65.0% (13/20)], day 2 [65.0% (13/20)] and day 3 [60.0% (12/20)] after initiating treatment. Of note, gametocyte carriage among participants was low before and after treatment.

CONCLUSION

Taken together, the results indicate that a significant number of individuals could harbour residual Plasmodium parasites or parasite genomic DNA after treatment. The study demonstrates the importance of routine surveillance of asymptomatic malaria using sensitive nucleic acid-based amplification techniques.

Analytical approaches for antimalarial antibody responses to confirm historical and recent malaria transmission: an example from the Philippines

Background

Assessing the status of malaria transmission in endemic areas becomes increasingly challenging as countries approach elimination. Serology can provide robust estimates of malaria transmission intensities, and multiplex serological assays allow for simultaneous assessment of markers of recent and historical malaria exposure.

Methods

Here, we evaluated different statistical and machine learning methods for analyzing multiplex malaria-specific antibody response data to classify recent and historical exposure to Plasmodium falciparum and Plasmodium vivax. To assess these methods, we utilized samples from a health-facility based survey (n = 9132) in the Philippines, where we quantified antibody responses against 8 P. falciparum and 6 P. vivax-specific antigens from 3 sites with varying transmission intensity.

Findings

Measurements of antibody responses and seroprevalence were consistent with the 3 sites' known endemicity status. Among the models tested, a machine learning (ML) approach (Random Forest model) using 4 serological markers (PfGLURP R2, Etramp5.Ag1, GEXP18, and PfMSP119) gave better predictions for P. falciparum recent infection in Palawan (AUC: 0.9591, CI 0.9497-0.9684) than individual antigen seropositivity. Although the ML approach did not improve P. vivax infection predictions, ML classifications confirmed the absence of recent exposure to P. falciparum and P. vivax in both Occidental Mindoro and Bataan. For predicting historical P. falciparum and P. vivax transmission, seroprevalence and seroconversion rates based on cumulative exposure markers AMA1 and MSP119 showed reliable trends in the 3 sites.

Interpretation

Our study emphasizes the utility of serological markers in predicting recent and historical exposure in a sub-national elimination setting, and also highlights the potential use of machine learning models using multiplex antibody responses to improve assessment of the malaria transmission status of countries aiming for elimination. This work also provides baseline antibody data for monitoring risk in malaria-endemic areas in the Philippines.

Funding

Newton Fund, Philippine Council for Health Research and Development, UK Medical Research Council.

Naturally acquired antibody response to Plasmodium falciparum and Plasmodium vivax among indigenous Orang Asli communities in Peninsular Malaysia

Malaria remains a public health problem in many parts of the world. In Malaysia, the significant progress towards the national elimination programme and effective disease notification on malaria has resulted in zero indigenous human malaria cases since 2018. However, the country still needs to determine the extent of malaria exposure and transmission patterns, particularly in high-risk populations. In this study, a serological method was used to measure transmission levels of Plasmodium falciparum and Plasmodium vivax among indigenous Orang Asli communities in Kelantan, Peninsular Malaysia. A community-based cross-sectional survey was conducted in three Orang Asli communities (i.e., Pos Bihai, Pos Gob, and Pos Kuala Betis) in Kelantan from June to July 2019. Antibody responses to malaria were assessed by enzyme-linked immunosorbent assay (ELISA) using two P. falciparum (PfAMA-1 and PfMSP-1₁₉) and two P. vivax (PvAMA-1 and PvMSP-1₁₉) antigens. Age-adjusted antibody responses were analysed using a reversible catalytic model to calculate seroconversion rates (SCRs). Multiple logistic regression was used to investigate factors associated with malaria exposure. The overall malaria seroprevalence was 38.8% for PfAMA-1, 36.4% for PfMSP-1₁₉, 2.2% for PvAMA-1, and 9.3% for PvMSP-1₁₉. Between study areas, the proportion of seropositivity for any P. falciparum and P. vivax antigens was significantly highest in Pos Kuala Betis with 34.7% (p < 0.001) and 13.6% (p < 0.001), respectively. For all parasite antigens except for PvAMA-1, the proportion of seropositive individuals significantly increased with age (all p < 0.001). Based on the SCR, there was a higher level of P. falciparum transmission than P. vivax in the study area. Multivariate regression analyses showed that living in Pos Kuala Betis was associated with both P. falciparum (adjusted odds ratio [aOR] 5.6, p < 0.001) and P. vivax (aOR 2.1, p < 0.001) seropositivities. Significant associations were also found between age and seropositivity to P. falciparum and P. vivax antigens. Analysis of community-based serological data helps describe the level of transmission, heterogeneity, and factors associated with malaria exposure among indigenous communities in Peninsular Malaysia. This approach could be an important adjunct tool for malaria monitoring and surveillance in low malaria transmission settings in the country.

Humoral Immune Responses to P. falciparum Circumsporozoite Protein (Pfcsp) Induced by the RTS, S Vaccine - Current Update

Malaria vaccines targeting the circumsporozoite protein (CSP) of the P. falciparum parasite have been overall relatively promising. RTS, S is a pre-erythrocytic recombinant protein-based malaria vaccine that targets CSP. RTS, S effectiveness shows some limited success regardless of its 58% efficacy for severe disease. P. falciparum circumsporozoite protein (Pfcsp) has stood to be the main candidate protein for most pre-erythrocytic stage vaccines. Studies on the structural and biophysical characteristics of antibodies specific to CSP (anti-CSP) are underway to achieve fine specificity with the CSP polymorphic regions. More recent studies have proposed the use of different kinds of monoclonal antibodies, the use of appropriate adjuvants, ideal vaccination dose and frequency, and improved targeting of particular epitopes for the robust production of functional antibodies and high complement-fixing activity as other potential methods for achieving long-lasting RTS, S. This review highlights recent findings regarding humoral immune responses to CSP elicited by RTS, S vaccine.

Naturally acquired antibodies from Beninese infants promote Plasmodium falciparum merozoite-phagocytosis by human blood leukocytes: implications for control of asymptomatic malaria infections

Background

Immunoglobulin G (IgG) antibodies are thought to play important roles in the protection against Plasmodium falciparum (P. falciparum) malaria. A longitudinal cohort study performed in the Southern part of Benin, identified a group of infants who were able to control asymptomatic malaria infections (CAIG).

Methods

IgG antibodies against distinct merozoite antigens were quantified in plasma from Beninese infants. Functionality of these antibodies was assessed by the merozoite-phagocytosis assay using THP-1 cells and primary neutrophils as effector cells. Gm allotypes were determined by a serological method of haemagglutination inhibition.

Results

Purified IgG from infants in CAIG promoted higher levels of merozoite-phagocytosis than did IgG from children who were unable to control asymptomatic infections (Ologit multivariate regression model, Coef. = 0.06, 95% CI 0.02;0.10, P = 0.002). High level of merozoite-phagocytosis activity was significantly associated with high levels of IgG against AMA1 (Coef. = 1.76, 95% CI 0.39;3.14, P = 0.012) and GLURP-R2 (Coef. = 12.24, 95% CI 1.35;23.12, P = 0.028). Moreover, infants of the G3m5,6,10,11,13,14,24 phenotype showed higher merozoite-phagocytosis activity (Generalized linear model multivariate regression, Coef. = 7.46, 95% CI 0.31;14.61, P = 0.041) than those presenting other G3m phenotypes.

Conclusion

The results of the present study confirm the importance of antibodies to merozoite surface antigens in the control of asymptomatic malaria infection in Beninese infants. The study also demonstrated that G3m phenotypes impact the functional activity of IgG. This last point could have a considerable impact in the research of candidate vaccines against malaria parasites or other pathogens.

The ecology and epidemiology of malaria parasitism in wild chimpanzee reservoirs

Chimpanzees (Pan troglodytes) harbor rich assemblages of malaria parasites, including three species closely related to P. falciparum (sub-genus Laverania), the most malignant human malaria parasite. Here, we characterize the ecology and epidemiology of malaria infection in wild chimpanzee reservoirs. We used molecular assays to screen chimpanzee fecal samples, collected longitudinally and cross-sectionally from wild populations, for malaria parasite mitochondrial DNA. We found that chimpanzee malaria parasitism has an early age of onset and varies seasonally in prevalence. A subset of samples revealed Hepatocystis mitochondrial DNA, with phylogenetic analyses suggesting that Hepatocystis appears to cross species barriers more easily than Laverania. Longitudinal and cross-sectional sampling independently support the hypothesis that mean ambient temperature drives spatiotemporal variation in chimpanzee Laverania infection. Infection probability peaked at ~24.5 °C, consistent with the empirical transmission optimum of P. falciparum in humans. Forest cover was also positively correlated with spatial variation in Laverania prevalence, consistent with the observation that forest-dwelling Anophelines are the primary vectors. Extrapolating these relationships across equatorial Africa, we map spatiotemporal variation in the suitability of chimpanzee habitat for Laverania transmission, offering a hypothetical baseline indicator of human exposure risk.

Microscopic and submicroscopic infection by Plasmodium falciparum: Immunoglobulin M and A profiles as markers of intensity and exposure

Assessment of serological Plasmodium falciparum–specific antibodies in highly endemic areas provides valuable information about malaria status and parasite exposure in the population. Although serological evidence of Plasmodium exposure is commonly determined by Plasmodium-specific immunoglobulin G (IgG) levels; IgM and IgA are likely markers of malaria status that remain relatively unexplored. Previous studies on IgM and IgA responses have been based on their affinity for single antigens with shortage of immune responses analysis against the whole Plasmodium proteome. Here, we provide evidence of how P. falciparum infection triggers the production of specific IgM and IgA in plasma and its relationship with parasite density and changes in hematological parameters. A total of 201 individuals attending a hospital in Breman Asikuma, Ghana, were recruited into this study. Total and P. falciparum–specific IgM, IgA, and IgG were assessed by ELISA and examined in relation to age (0–5, 14–49, and ≥50 age ranges); infection (submicroscopic vs. microscopic malaria); pregnancy and hematological parameters. Well-known IgG response was used as baseline control. P. falciparum–specific IgM and IgA levels increased in the population with the age, similarly to IgG. These data confirm that acquired humoral immunity develops by repeated infections through the years endorsing IgM and IgA as exposure markers in endemic malaria regions. High levels of specific IgA and IgM in children were associated with microscopic malaria and worse prognosis, because most of them showed severe anemia. This new finding shows that IgM and IgA may be used as diagnostic markers in this age group. We also found an extremely high prevalence of submicroscopic malaria (46.27% on average) accompanied by IgM and IgA levels indistinguishable from those of uninfected individuals. These data, together with the observed lack of sensitivity of rapid diagnostic tests (RDTs) compared to PCR, invoke the urgent need to implement diagnostic markers for submicroscopic malaria. Overall, this study opens the potential use of P. falciparum–specific IgM and IgA as new serological markers to predict malaria status in children and parasite exposure in endemic populations. The difficulties in finding markers of submicroscopic malaria are highlighted, emphasizing the need to explore this field in depth.

Identification of factors associated with residual malaria transmission using school-based serological surveys in settings pursuing elimination

Background

Targeted research on residual malaria transmission is important to improve strategies in settings pursuing elimination, where transmission reductions prove challenging. This study aimed to detect and characterize spatial heterogeneity and factors associated with Plasmodium falciparum infections and exposure, P. falciparum apical membrane antigen 1 (PfAMA1) antibody (Ab) response, in the Central Highlands of Madagascar (CHL).

Methods

From May to July 2014, a cross-sectional school-based survey was carried out in 182 fokontany (villages) within 7 health districts of the CHL. Rapid diagnostic tests (RDTs) and a bead-based immunoassay including PfAMA1 antigen biomarker were used to estimate malaria prevalence and seroprevalence, respectively. Local Moran’s I index was used to detect spatial “hotspots”. Remotely sensed environmental data—temperature, vegetation indices, land covers, and elevation—were used in multivariable mixed-effects logistic regression models to characterize factors associated with malaria infection and cumulative exposure.

Results

Among 6,293 school-children ages 2–14 years surveyed, RDT prevalence was low at 0.8% (95% CI 0.6–1.1%), while PfAMA1 Ab seroprevalence was 7.0% (95% CI 6.4–7.7%). Hotspots of PfAMA1 Ab seroprevalence were observed in two districts (Ankazobe and Mandoto). Seroprevalence increased for children living > 5 km from a health centre (adjusted odds ratio (OR) = 1.6, 95% CI 1.2–2.2), and for those experiencing a fever episode in the previous 2 weeks (OR 1.7, 95% CI 1.2–2.4), but decreased at higher elevation (for each 100-m increase, OR = 0.7, 95% CI 0.6–0.8). A clear age pattern was observed whereby children 9–10 years old had an OR of 1.8 (95% CI 1.2–2.4), children 11–12 years an OR of 3.7 (95% CI 2.8–5.0), and children 13–14 years an OR of 5.7 (95% CI 4.0–8.0) for seropositivity, compared with younger children (2–8 years).

Conclusion

The use of serology in this study provided a better understanding of malaria hotspots and associated factors, revealing a pattern of higher transmission linked to geographical barriers in health care access. The integration of antibody-assays into existing surveillance activities could improve exposure assessment, and may help to monitor the effectiveness of malaria control efforts and adapt elimination interventions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04260-0.

Age-dependent changes in circulating Tfh cells influence development of functional malaria antibodies in children

T-follicular helper (Tfh) cells are key drivers of antibodies that protect from malaria. However, little is known regarding the host and parasite factors that influence Tfh and functional antibody development. Here, we use samples from a large cross-sectional study of children residing in an area of high malaria transmission in Uganda to characterize Tfh cells and functional antibodies to multiple parasites stages. We identify a dramatic re-distribution of the Tfh cell compartment with age that is independent of malaria exposure, with Th2-Tfh cells predominating in early childhood, while Th1-Tfh cell gradually increase to adult levels over the first decade of life. Functional antibody acquisition is age-dependent and hierarchical acquired based on parasite stage, with merozoite responses followed by sporozoite and gametocyte antibodies. Antibodies are boosted in children with current infection, and are higher in females. The children with the very highest antibody levels have increased Tfh cell activation and proliferation, consistent with a key role of Tfh cells in antibody development. Together, these data reveal a complex relationship between the circulating Tfh compartment, antibody development and protection from malaria.

Meta-Analysis of Human Antibodies Against Plasmodium falciparum Variable Surface and Merozoite Stage Antigens

Concerted efforts to fight malaria have caused significant reductions in global malaria cases and mortality. Sustaining this will be critical to avoid rebound and outbreaks of seasonal malaria. Identifying predictive attributes that define clinical malaria will be key to guide development of second-generation tools to fight malaria. Broadly reactive antibodies against variable surface antigens that are expressed on the surface of infected erythrocytes and merozoites stage antigens are targets of naturally acquired immunity and prime candidates for anti-malaria therapeutics and vaccines. However, predicting the relationship between the antigen-specific antibodies and protection from clinical malaria remains unresolved. Here, we used new datasets and multiple approaches combined with re-analysis of our previous data to assess the multi-dimensional and complex relationship between antibody responses and clinical malaria outcomes. We observed 22 antigens (17 PfEMP1 domains, 3 RIFIN family members, merozoite surface protein 3 (PF3D7_1035400), and merozoites-associated armadillo repeats protein (PF3D7_1035900) that were selected across three different clinical malaria definitions (1,000/2,500/5,000 parasites/µl plus fever). In addition, Principal Components Analysis (PCA) indicated that the first three components (Dim1, Dim2 and Dim3 with eigenvalues of 306, 48, and 29, respectively) accounted for 66.1% of the total variations seen. Specifically, the Dim1, Dim2 and Dim3 explained 52.8%, 8.2% and 5% of variability, respectively. We further observed a significant relationship between the first component scores and age with antibodies to PfEMP1 domains being the key contributing variables. This is consistent with a recent proposal suggesting that there is an ordered acquisition of antibodies targeting PfEMP1 proteins. Thus, although limited, and further work on the significance of the selected antigens will be required, these approaches may provide insights for identification of drivers of naturally acquired protective immunity as well as guide development of additional tools for malaria elimination and eradication.

A Molecular Analysis of Memory B Cell and Antibody Responses Against Plasmodium falciparum Merozoite Surface Protein 1 in Children and Adults From Uganda

Memory B cells (MBCs) and plasma antibodies against Plasmodium falciparum (Pf) merozoite antigens are important components of the protective immune response against malaria. To gain understanding of how responses against Pf develop in these two arms of the humoral immune system, we evaluated MBC and antibody responses against the most abundant merozoite antigen, full-length Pf merozoite surface protein 1 (PfMSP1FL), in individuals from a region in Uganda with high Pf transmission. Our results showed that PfMSP1FL-specific B cells in adults with immunological protection against malaria were predominantly IgG+ classical MBCs, while children with incomplete protection mainly harbored IgM+ PfMSP1FL-specific classical MBCs. In contrast, anti-PfMSP1FL plasma IgM reactivity was minimal in both children and adults. Instead, both groups showed high plasma IgG reactivity against PfMSP1FL, with broadening of the response against non-3D7 strains in adults. The B cell receptors encoded by PfMSP1FL-specific IgG+ MBCs carried high levels of amino acid substitutions and recognized relatively conserved epitopes on the highly variable PfMSP1 protein. Proteomics analysis of PfMSP119-specific IgG in plasma of an adult revealed a limited repertoire of anti-MSP1 antibodies, most of which were IgG1 or IgG3. Similar to B cell receptors of PfMSP1FL-specific MBCs, anti-PfMSP119 IgGs had high levels of amino acid substitutions and their sequences were predominantly found in classical MBCs, not atypical MBCs. Collectively, these results showed evolution of the PfMSP1-specific humoral immune response with cumulative Pf exposure, with a shift from IgM+ to IgG+ B cell memory, diversification of B cells from germline, and stronger recognition of PfMSP1 variants by the plasma IgG repertoire.

Quantification of the dynamics of antibody response to malaria to inform sero-surveillance in pregnant women

Background

Malaria remains a major public health threat and tools sensitive to detect infections in low malaria transmission areas are needed to progress elimination efforts. Pregnant women are particularly vulnerable to malaria infections. Throughout pregnancy they access routine antenatal care, presenting a unique sentinel population to apply novel sero-surveillance tools to measure malaria transmission. The aim of this study was to quantify the dynamic antibody responses to multiple antigens during pregnancy so as to identify a single or multiple antibody response of exposure to malaria in pregnancy.

Methods

This study involved a secondary analysis of antibody responses to six parasite antigens [five commonly studied merozoite antigens and the variant surface antigen 2-chondroitin sulphate A (VAR2CSA), a pregnancy-specific erythrocytic antigen] measured by enzyme-linked immunosorbent assay (ELISA) over the gestation period until delivery (median of 7 measurements/woman) in 250 pregnant women who attended antenatal clinics located at the Thai-Myanmar border. A multivariate mixture linear mixed model was used to cluster the pregnant women into groups that have similar longitudinal antibody responses to all six antigens over the gestational period using a Bayesian approach. The variable-specific entropy was calculated to identify the antibody responses that have the highest influence on the classification of the women into clusters, and subsequent agreement with grouping of women based on exposure to malaria during pregnancy.

Results

Of the 250 pregnant women, 135 had a Plasmodium infection detected by light microscopy during pregnancy (39% Plasmodium falciparum only, 33% Plasmodium vivax only and 28% mixed/other species), defined as cases. The antibody responses to all six antigens accurately identified the women who did not have a malaria infection detected during pregnancy (93%, 107/115 controls). Antibody responses to P. falciparum merozoite surface protein 3 (PfMSP3) and P. vivax apical membrane antigen 1 (PvAMA1) were the least dynamic. Antibody responses to the antigens P. falciparum apical membrane antigen 1 (PfAMA1) and PfVAR2CSA were able to identify the majority of the cases more accurately (63%, 85/135).

Conclusion

These findings suggest that the combination of antibodies, PfAMA1 and PfVAR2CSA, may be useful for sero-surveillance of malaria infections in pregnant women, particularly in low malaria transmission settings. Further investigation of other antibody markers is warranted considering these antibodies combined only detected 63% of the malaria infections during pregnancy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04111-y.

Intermittent preventive treatment with Sulphadoxine-Pyrimethamine (IPTp-SP) is associated with protection against sub-microscopic P. falciparum infection in pregnant women during the low transmission dry season in southwestern Cameroon: A Semi - longitudinal study

The current guidelines for malaria prevention and control during pregnancy in Africa is predicated on the prevention of infection and/or disease through intermittent preventive treatment in pregnancy (IPTp), insecticide-treated nets (ITNs) and effective malaria case diagnosis and management. Concerns that increasing SP resistance in some areas of SSA may have compromised IPTp-SP efficacy prompted this contemporaneous study, designed to assess the prevalence and risk factors of sub-microscopic infection in parturient women during the low transmission season in Mutengene, a rapidly growing semi-urban area in Southwest Region, Cameroon. Pregnant women originally reporting for the establishment of antenatal clinic care during the dry season were followed-up to term and their pregnancy outcomes recorded. About 2 ml of venous blood was collected for malaria diagnosis using PfHRP2/pLDH malaria rapid diagnostic kit and light microscopy. DNA was extracted from dried blood spots by the Chelex-100 method and the Plasmodium falciparum status detected by nested PCR amplification of the 18SrRNA gene using specific predesigned primers. Of the 300 women enrolled, the proportion of malaria parasite infected as determined by microscopy, RDT and PCR was 12.9%, 16.4% and 29.4% respectively, with 39.9% overall infected with P. falciparum by microscopy and/or RDT and/or PCR and a very low-density infection, averaging 271 parasites per microliter of blood. About 25.0% (68/272) of women who were negative by microscopy were positive by PCR (submicroscopic P. falciparum infection), with primigravidae and IPTp-SP non usage identified as independent risk factors for submicroscopic P. falciparum parasitaemia while fever history (aOR = 4.83, 95% CI = 1.28-18.22, p = 0.020) was associated with risk of malaria parasite infection overall. IPTp-SP use (p = 0.007) and dosage (p = 0.005) significantly influenced whether or not the participant will be malaria parasite negative or carry submicroscopic or microscopic infection. Although Infant birthweight and APGAR score were independent of the mother's P. falciparum infection and submicroscopic status, infant's birthweight varied with the gravidity status (p = 0.001) of the mother, with significantly lower birthweight neonates born to primigravidae compared to secundigravidae (p = 0.001) and multigravidae (p = 0.003). Even in holo-endemic dry season, there exists a large proportion of pregnant women with very low density parasitaemia. IPTp-SP seems to be relevant in controlling submicroscopic P. falciparum infections, which remains common in pregnant women, and are hard to diagnose, with potentially deleterious consequences for maternal and fetal health. Future studies should be carried out in hyperendemic malaria foci where the parasitemia levels are substantially higher in order to confirm the efficacy of IPTp-SP.

Adults with Plasmodium falciparum malaria have higher magnitude and quality of circulating T-follicular helper cells compared to children

BACKGROUND

Protective malarial antibodies are acquired more rapidly in adults than children, independently of cumulative exposure, however the cellular responses mediating these differences are unknown. CD4 T-follicular helper (Tfh) cells have key roles in inducing antibodies, with Th2-Tfh cell activation associated with antibody development in malaria. Whether Tfh cell activation in malaria is age dependent is unknown and no studies have compared Tfh cell activation in children and adults with malaria.

METHODS

We undertook a comprehensive study of Tfh cells, along with B cells and antibody induction in children and adults with malaria. Activation and proliferation of circulating Tfh (cTfh) cell subsets was measured ex vivo and parasite-specific Tfh cell frequencies and functions studied with Activation Induced Marker (AIM) assays and intracellular cytokine staining.

FINDINGS

During acute malaria, the magnitude of cTfh cell activation was higher in adults than in children and occurred across all cTfh cell subsets in adults but was restricted only to the Th1-cTfh subset in children. Further, adults had higher levels of parasite-specific cTfh cells, and cTfh cells which produced more Th2-Tfh associated cytokine IL-4. Consistent with a role of higher Tfh cell activation in rapid immune development in adults, adults had higher activation of B cells during infection and higher induction of antibodies 7 and 28 days after malaria compared to children.

INTERPRETATION

Our data provide evidence that age impacts Tfh cell activation during malaria, and that these differences may influence antibody induction after treatment. Findings have important implications for vaccine development in children.

FUNDING

This word was supported by the National Health and Medical Research Council of Australia, Wellcome Trust, Charles Darwin University Menzies School of Health Research, Channel 7 Children's Research Foundation, and National Health Institute.

Persistent Plasmodium falciparum infections enhance transmission-reducing immunity development

Subclinical infections that serve as reservoir populations to drive transmission remain a hurdle to malaria control. Data on infection dynamics in a geographical area is required to strategically design and implement malaria interventions. In a longitudinal cohort, we monitored Plasmodium falciparum infection prevalence and persistence, and anti-parasite immunity to gametocyte and asexual antigens for 10 weeks. Of the 100 participants, only 11 were never infected, whilst 16 had persistent infections detected by reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR), and one participant had microscopic parasites at all visits. Over 70% of the participants were infected three or more times, and submicroscopic gametocyte prevalence was high, ≥ 48% of the parasite carriers. Naturally induced responses against recombinant Pfs48/45.6C, Pfs230proC, and EBA175RIII-V antigens were not associated with either infection status or gametocyte carriage, but the antigen-specific IgG titers inversely correlated with parasite and gametocyte densities consistent with partial immunity. Longitudinal analysis of gametocyte diversity indicated at least four distinct clones circulated throughout the study period. The high prevalence of children infected with distinct gametocyte clones coupled with marked variation in infection status at the individual level suggests ongoing transmission and should be targeted in malaria control programs.

Predictive markers of transmission in areas with different malaria endemicity in north-eastern Tanzania based on seroprevalence of antibodies against Plasmodium falciparum

OBJECTIVE

A community-based cross-sectional study was done to assess Plasmodium falciparum exposure in areas with different malaria endemicity in north-eastern Tanzania using serological markers; PfAMA-1 and PfMSP-1₁₉.

RESULTS

Bondo had a higher seroprevalence 36.6% (188) for PfAMA-1 as compared to Hai 13.8% (33), χ² = 34.66, p < 0.01. Likewise, Bondo had a higher seroprevalence 201(36.6%) for PfMSP-1 as compared to Hai 41 (17.2%), χ² = 29.62, p < 0.01. Anti-PfAMA-1 titters were higher in malaria positive individuals (n = 47) than in malaria negative individuals (n = 741) (p = 0.07). Anti-PfMSP-1 antibody concentrations were significantly higher in malaria-positive individuals (n = 47) than in malaria-negative individuals (n = 741) (p = 0.003). Antibody response against PfAMA-1 was significantly different between the three age groups; < 5 years, 5 to 15 years and > 15 years in both sites of Bondo and Hai. Likewise, antibody response against PfMSP-1₁₉ was significantly different between the three age groups in the two sites (p < 0.001). We also found significant differences in the anti-PfAMA-1and anti-PfMSP-1₁₉ antibody concentrations among the three age groups in the two sites (p = 0.004 and 0.005) respectively. Immunological indicators of P. falciparum exposure have proven to be useful in explaining long-term changes in the transmission dynamics, especially in low transmission settings.

Influence of Host-Related Factors and Exposure to Mosquito Bites on the Dynamics of Antibody Response to Plasmodium falciparum Antigens

Humoral immunity to Plasmodium falciparum is acquired after repeated infections, and can lead to clinical protection. This study aimed to evaluate how human-, parasite-, and environment-related determinants can modulate the dynamics of IgG responses to Plasmodium falciparum after an infection. Individuals (n = 68, average age = 8.2 years) with uncomplicated malaria were treated with ACT and followed up for 42 days. IgG responses to P. falciparum merozoite antigens (PfMSP1, PfMSP3, PfAMA1, PfGLURP-R0), to whole schizont extract (PfSchz), and to Anopheles gSG6-P1 and Aedes Nterm–34 kDa salivary peptides were measured. Regression analyses were used to identify factors that influence the dynamics of IgG response to P. falciparum antigen between D0 and D42, including demographic and biological factors and the level of exposure to mosquito bites. The dynamics of IgG response to P. falciparum differed according to the antigen. According to multivariate analysis, IgG responses to PfSchz and to PfGLURP-R0 appear to be affected by exposure to Aedes saliva and are associated with age, parasite density, and anti-Plasmodium pre-existing immune response at study inclusion. The present work shows that human exposure to Aedes saliva may contribute, in addition to other factors, to the regulation of anti-Plasmodium immune responses during a natural infection.

The role of different components of the immune system against Plasmodium falciparum malaria: Possible contribution towards malaria vaccine development

Plasmodium falciparum malaria still remains a major global public health challenge with over 220 million new cases and well over 400,000 deaths annually. Most of the deaths occur in sub-Saharan Africa which bears 90 % of the malaria cases. Such high P. falciparum malaria-related morbidity and mortality rates pose a huge burden on the health and economic wellbeing of the countries affected. Lately, substantial gains have been made in reducing malaria morbidity and mortality through intense malaria control initiatives such as use of effective antimalarials, intensive distribution and use of insecticide-treated nets (ITNs), and implementation of massive indoor residual spraying (IRS) campaigns. However, these gains are being threatened by widespread resistance of the parasite to antimalarials, and the vector to insecticides. Over the years the use of vaccines has proven to be the most reliable, cost-effective and efficient method for controlling the burden and spread of many infectious diseases, especially in resource poor settings with limited public health infrastructure. Nonetheless, this had not been the case with malaria until the most promising malaria vaccine candidate, RTS,S/AS01, was approved for pilot implementation programme in three African countries in 2015. This was regarded as the most important breakthrough in the fight against malaria. However, RTS,S/AS01 has been found to have some limitations, the main ones being low efficacy in certain age groups, poor immunogenicity and need for almost three boosters to attain a reasonable efficacy. Thus, the search for a more robust and effective malaria vaccine still continues and a better understanding of naturally acquired immune responses to the various stages, including the transmissible stages of the parasite, could be crucial in rational vaccine design. This review therefore compiles what is currently known about the basic biology of P. falciparum and the natural malaria immune response against malaria and progress made towards vaccine development.

Plasmodium falciparum Malaria Vaccines and Vaccine Adjuvants

Malaria-a parasite vector-borne disease-is a global health problem, and Plasmodium falciparum has proven to be the deadliest among Plasmodium spp., which causes malaria in humans. Symptoms of the disease range from mild fever and shivering to hemolytic anemia and neurological dysfunctions. The spread of drug resistance and the absence of effective vaccines has made malaria disease an ever-emerging problem. Although progress has been made in understanding the host response to the parasite, various aspects of its biology in its mammalian host are still unclear. In this context, there is a pressing demand for the development of effective preventive and therapeutic strategies, including new drugs and novel adjuvanted vaccines that elicit protective immunity. The present article provides an overview of the current knowledge of anti-malarial immunity against P. falciparum and different options of vaccine candidates in development. A special emphasis has been made on the mechanism of action of clinically used vaccine adjuvants.

High-dimensional mass cytometry identifies T cell and B cell signatures predicting reduced risk of Plasmodium vivax malaria

IFN-γ-driven responses to malaria have been shown to modulate the development and function of T follicular helper (TFH) cells and memory B cells (MBCs), with conflicting evidence of their involvement in the induction of antibody responses required to achieve clinical immunity and their association with disease outcomes. Using high-dimensional single-cell mass cytometry, we identified distinct populations of TH1-polarized CD4+ T cells and MBCs expressing the TH1-defining transcription factor T-bet, associated with either increased or reduced risk of Plasmodium vivax (P. vivax) malaria, demonstrating that inflammatory responses to malaria are not universally detrimental for infection. Furthermore, we found that, whereas class-switched but not IgM+ MBCs were associated with a reduced risk of symptomatic malaria, populations of TH1 cells with a stem central memory phenotype, TH17 cells, and T regulatory cells were associated with protection from asymptomatic infection, suggesting that activation of cell-mediated immunity might also be required to control persistent P. vivax infection with low parasite burden.

Molecular characterization of Plasmodium falciparum PHISTb proteins as potential targets of naturally-acquired immunity against malaria

Background: Plasmodium falciparum causes the deadliest form of malaria in humans. Upon infection, the host’s infected red blood cells (iRBCs) are remodelled by exported parasite proteins to provide a niche for parasite development and maturation. Methods: Here we analysed the role of three PHISTb proteins Pf3D7_0532400, Pf3D7_1401600, and Pf3D7_1102500 by expressing recombinant proteins and evaluated antibody responses against these proteins using immune sera from malaria-exposed individuals from Kenya and The Gambia in Africa. Results: Children and adults from malaria-endemic regions recognized the three PHISTb proteins. Responses against PHISTb proteins varied with malaria transmission intensity in three different geographical sites in Kenya (Siaya and Takaungu) and The Gambia (Sukuta). Antibody responses against PHISTb antigens Pf3D7_1102500 and Pf3D7_1401600 were higher in Sukuta, a low transmission region in Gambia, compared to Siaya, a high transmission region in western Kenya, unlike Pf3D7_0532400. Anti-PHIST responses indicate negative correlation between antibody levels and malaria transmission intensity for Pf3D7_1102500 and Pf3D7_1401600. We report a correlation in antibody responses between schizont and gametocyte extract, but this is not statistically significant (cor=0.102, p=0.2851, CI=95%) and, Pf3D7_0532400 (cor=0.11, p=0.249, CI=95%) and Pf3D7_1401600 (cor=0.02, p=0.7968, CI=95%). We report a negative correlation in antibody responses between schizont and Pf3D7_1102500 (cor=-0.008, p=0.9348, CI=95%). There is a correlation between gametocyte extract and Pf3D7_1401600 (cor=-0.0402, p=0.6735, CI=95%), Pf3D7_1102500 (cor=0.0758, p=0.4271, CI=95%) and Pf3D7_0532400 (cor=0.155, p=0.1028, CI=95%). Acquisition of anti-PHIST antibodies correlates with exposure to malaria for Pf3D7_0532400 (p=0.009) but not Pf3D7_1102500 and Pf3D7_1401600 (p=0.507 and p=0.15, respectively, CI=95%). Children aged below 2 years had the lowest antibody levels which do not correlate with age differences. Conclusions: Collectively, these findings provide evidence of natural immunity against PHISTb antigens that varies with level of malaria exposure and underscore their potential as possible serological markers to P. falciparum infection aimed at contributing to malaria control through vaccine development.

Profiling Humoral Immune Response Against Pre-Erythrocytic and Erythrocytic Antigens of Malaria Parasites Among Neotropical Primates in the Brazilian Atlantic Forest

Human malaria due to zoonotic transmission has been recorded in the Atlantic Forest, an extra-Amazonian area in Brazil, which are a challenge for malaria control. Naturally acquired humoral immune response against pre-erythrocytic and erythrocytic antigens of Neotropical primates (NP) was evaluated here to improve the knowledge about the exposure of those animals to the malaria transmission and support the identification of the potential reservoirs of the disease in the Atlantic Forest. Blood samples of 154 monkeys from three areas of the Atlantic Forest were used to identify IgG antibodies against peptides of the repeat region of the major pre-erythrocytic antigen, the circumsporozoite protein (CSP), of Plasmodium vivax (PvCSP), Plasmodium brasilianum/Plasmodium malariae (Pb/PmCSP), and Plasmodium falciparum (PfCSP) by ELISA. Antibodies against erythrocytic recombinant antigens of P. vivax, Apical membrane antigen 1 (PvAMA-1), Erythrocyte binding protein 2 (PvEBP-2) and domain II of Duffy binding protein (PvDBPII) were also evaluated. Parameters, such as age, sex, PCR positivity, and captivity, potentially associated with humoral immune response were analyzed. Eighty-five percent of NP had antibodies against at least one CSP peptide, and 76% against at least one P. vivax erythrocytic antigen. A high percentage of adults compared to non-adults were seropositive and showed increased antibody levels. Neotropical primates with PCR positive for P. simium had a significantly higher frequency of positivity rate for immune response against PvEBP-2, PvDBPII and also higher antibody levels against PvDBPII, compared to PCR negative NPs for this species. Monkeys with PCR positive for P. brasilianum/P. malariae showed higher frequency of seropositivity and antibody levels against Pb/PmCSP. Levels of antibodies against Pb/PmCSP, PvEBP-2 and PvDBPII were higher in free-living than in captive monkeys from the same area. All Platyrrhine families showed antibodies against CSP peptides, however not all showed IgG against erythrocytic antigens. These findings showed a high prevalence of naturally acquired antibodies against CSP repeats in all studied areas, suggesting an intense exposure to infected-mosquitoes bites of NP from all families. However, mainly monkeys of Atelidae family showed antibodies against P. vivax erythrocytic antigens, suggesting blood infection, which might serve as potential reservoirs of malaria in the Atlantic Forest.

Suitability of IgG responses to multiple Plasmodium falciparum antigens as markers of transmission intensity and pattern

Detection of antibody reactivity to appropriate, specific parasite antigens may constitute a sensitive and cost-effective alternative to current tools to monitor malaria transmission across different endemicity settings. This study aimed to determine the suitability of IgG responses to a number of P. falciparum antigens as markers of transmission intensity and pattern. Antibody responses to multiple malaria antigens were determined in 905 participants aged 1–12 years from three districts with low (Keta), medium (Hohoe) and high (Krachi) transmission intensity in the Volta region of Ghana. Blood film microscopy slides and dry blood spots (DBS) were obtained for parasitaemia detection and antibody measurement, respectively. Sera were eluted from DBS and levels of IgG specific for 10 malaria antigens determined by a multiplex assay. Results were compared within and among the districts. Total IgG responses to MSPDBL1, MSPDBL_Leucine, MSP2-_FC27, RAMA, and PfRh2a and PfRh2b were higher in Krachi than in Hohoe and Keta. Seroprevalence of IgG specific for MSPDBL_Leucine, RON4, and PfRh2b were also highest in Krachi. Responses to RALP-1, PfRh2a and PfRh2b were associated with patent but asymptomatic parasitaemia in Keta, while responses to MSPDBL1, MSPDBL_Leucine, MSP2-_FC27, RAMA, Rh2-₂₀₃₀, and PfRh2b were associated with parasite carriage in Hohoe, but not in Krachi. Using ROC analysis, only PfRh2b was found to predict patent, but asymptomatic, parasitaemia in Keta and Hohoe. Antibody breadth correlated positively with age (r = 0.29, p<0.0001) and parasitaemia (β = 3.91; CI = 1.53 to 6.29), and medium to high transmission (p<0.0001). Our findings suggest differences in malaria-specific antibody responses across the three transmission zones and that PfRh2b has potential as a marker of malaria transmission intensity and pattern. This could have implications for malaria control programs and vaccine trials.

Correlates of malaria vaccine efficacy

Introduction: An effective vaccine against malaria forms a global health priority. Both naturally acquired immunity and sterile protection induced by irradiated sporozoite immunization were described decades ago. Still no vaccine exists that sufficiently protects children in endemic areas. Identifying immunological correlates of vaccine efficacy can inform rational vaccine design and potentially accelerate clinical development.Areas covered: We discuss recent research on immunological correlates of malaria vaccine efficacy, including: insights from state-of-the-art omics platforms and systems vaccinology analyses; functional anti-parasitic assays; pre-immunization predictors of vaccine efficacy; and comparison of correlates of vaccine efficacy against controlled human malaria infections (CHMI) and against naturally acquired infections.Expert Opinion: Effective vaccination may be achievable without necessarily understanding immunological correlates, but the relatively disappointing efficacy of malaria vaccine candidates in target populations is concerning. Hypothesis-generating omics and systems vaccinology analyses, alongside assessment of pre-immunization correlates, have the potential to bring about paradigm-shifts in malaria vaccinology. Functional assays may represent in vivo effector mechanisms, but have scarcely been formally assessed as correlates. Crucially, evidence is still meager that correlates of vaccine efficacy against CHMI correspond with those against naturally acquired infections in target populations. Finally, the diversity of immunological assays and efficacy endpoints across malaria vaccine trials remains a major confounder.

Genomic investigation of atypical malaria cases in Kanel, northern Senegal

BACKGROUND

The diagnosis of malaria cases in regions where the malaria burden has decreased significantly and prevalence is very low is more challenging, in part because of reduced clinical presumption of malaria. The appearance of a cluster of malaria cases with atypical symptoms in Mbounguiel, a village in northern Senegal where malaria transmission is low, in September 2018 exemplifies this scenario. The collaboration between the National Malaria Control Programme (NMCP) at the Senegal Ministry of Health and the Laboratory of Parasitology and Mycology at Cheikh Anta Diop University worked together to evaluate this cluster of malaria cases using molecular and serological tools.

METHODS

Malaria cases were diagnosed primarily by rapid diagnostic test (RDT), and confirmed by photo-induced electron transfer-polymerase chain reaction (PET-PCR). 24 single nucleotide polymorphisms (SNPs) barcoding was used for Plasmodium falciparum genotyping. Unbiased metagenomic sequencing and Luminex-based multi-pathogen antibody and antigen profiling were used to assess exposure to other pathogens.

RESULTS

Nine patients, of 15 suspected cases, were evaluated, and all nine samples were found to be positive for P. falciparum only. The 24 SNPs molecular barcode showed the predominance of polygenomic infections, with identifiable strains being different from one another. All patients tested positive for the P. falciparum antigens. No other pathogenic infection was detected by either the serological panel or metagenomic sequencing.

CONCLUSIONS

This work, undertaken locally within Senegal as a collaboration between the NMCP and a research laboratory at University of Cheikh Anta Diop (UCAD) revealed that a cluster of malaria cases were caused by different strains of P. falciparum. The public health response in real time demonstrates the value of local molecular and genomics capacity in affected countries for disease control and elimination.

Blockade of LAG-3 in PD-L1-Deficient Mice Enhances Clearance of Blood Stage Malaria Independent of Humoral Responses

T cells expressing high levels of inhibitory receptors such as PD-1 and LAG-3 are a hallmark of chronic infections and cancer. Checkpoint blockade therapies targeting these receptors have been largely validated as promising strategies to restore exhausted T cell functions and clearance of chronic infections and tumors. The inability to develop long-term natural immunity in malaria-infected patients has been proposed to be at least partially accounted for by sustained expression of high levels of inhibitory receptors on T and B lymphocytes. While blockade or lack of PD-1/PD-L1 and/or LAG-3 was reported to promote better clearance of Plasmodium parasites in various mouse models, how exactly blockade of these pathways contributes to enhanced protection is not known. Herein, using the mouse model of non-lethal P. yoelii (Py) infection, we reveal that the kinetics of blood parasitemia as well as CD4⁺ T follicular helper (T_FH) and germinal center (GC) B cell responses are indistinguishable between PD-1^-/-, PD-L1^-/- and WT mice. Yet, we also report that monoclonal antibody (mAb) blockade of LAG-3 in PD-L1^-/- mice promotes accelerated control of blood parasite growth and clearance, consistent with prior therapeutic blockade experiments. However, neither CD4⁺ T_FH and GC B cell responses, nor parasite-specific Ab serum titers and capacity to transfer protection differed. We also found that i) the majority of LAG-3⁺ cells are T cells, ii) selective depletion of CD4⁺ but not CD8⁺ T cells prevents anti-LAG-3-mediated protection, and iii) production of effector cytokines by CD4⁺ T cells is increased in anti-LAG-3-treated versus control mice. Thus, taken together, these results are consistent with a model in which blockade and/or deficiency of PD-L1 and LAG-3 on parasite-specific CD4⁺ T cells unleashes their ability to effectively clear blood parasites, independently from humoral responses.

Acquisition and decay of IgM and IgG responses to merozoite antigens after Plasmodium falciparum malaria in Ghanaian children

Developing a vaccine against Plasmodium falciparum malaria has been challenging, primarily due to high levels of antigen polymorphism and a complex parasite lifecycle. Immunization with the P. falciparum merozoite antigens PfMSRP5, PfSERA9, PfRAMA, PfCyRPA and PfRH5 has been shown to give rise to growth inhibitory and synergistic antisera. Therefore, these five merozoite proteins are considered to be promising candidates for a second-generation multivalent malaria vaccine. Nevertheless, little is known about IgG and IgM responses to these antigens in populations that are naturally exposed to P. falciparum. In this study, serum samples from clinically immune adults and malaria exposed children from Ghana were studied to compare levels of IgG and IgM specific for PfMSRP5, PfSERA9, PfRAMA, PfCyRPA and PfRH5. All five antigens were found to be specifically recognized by both IgM and IgG in serum from clinically immune adults and from children with malaria. Longitudinal analysis of the latter group showed an early, transient IgM response that was followed by IgG, which peaked 14 days after the initial diagnosis. IgG levels and parasitemia did not correlate, whereas parasitemia was weakly positively correlated with IgM levels. These findings show that IgG and IgM specific for merozoite antigens PfMSRP5, PfSERA9, PfRAMA, PfCyRPA and PfRH5 are high in children during P. falciparum malaria, but that the IgM induction and decline occurs earlier in infection than that of IgG.

IL-4 Treatment Mitigates Experimental Cerebral Malaria by Reducing Parasitemia, Dampening Inflammation, and Lessening the Cytotoxicity of T Cells

Cytokine responses to malaria play important roles in both protective immunity development and pathogenesis. Although the roles of cytokines such as TNF-α, IL-12, IFN-γ, and IL-10 in immunity and pathogenesis to the blood stage malaria are largely known, the role of IL-4 remains less understood. IL-4 targets many cell types and induces multiple effects, including cell proliferation, gene expression, protection from apoptosis, and immune regulation. Accordingly, IL-4 has been exploited as a therapeutic for several inflammatory diseases. Malaria caused by Plasmodium falciparum manifests in many organ-specific fatal pathologies, including cerebral malaria (CM), driven by a high parasite load, leading to parasite sequestration in organs and consequent excessive inflammatory responses and endothelial damage. We investigated the therapeutic potential of IL-4 against fatal malaria in Plasmodium berghei ANKA-infected C57BL/6J mice, an experimental CM model. IL-4 treatment significantly reduced parasitemia, CM pathology, and mortality. The therapeutic effect of IL-4 is mediated through multiple mechanisms, including enhanced parasite clearance mediated by upregulation of phagocytic receptors and increased IgM production, and decreased brain inflammatory responses, including reduced chemokine (CXCL10) production, reduced chemokine receptor (CXCR3) and adhesion molecule (LFA-1) expression by T cells, and downregulation of cytotoxic T cell lytic potential. IL-4 treatment markedly reduced the infiltration of CD8⁺ T cells and brain pathology. STAT6, PI3K-Akt-NF-κB, and Src signaling mediated the cellular and molecular events that contributed to the IL-4-dependent decrease in parasitemia. Overall, our results provide mechanistic insights into how IL-4 treatment mitigates experimental CM and have implications in developing treatment strategies for organ-specific fatal malaria.

Sero-epidemiological evaluation of malaria transmission in The Gambia before and after mass drug administration

BACKGROUND

As The Gambia aims to achieve malaria elimination by 2030, serological assays are a useful surveillance tool to monitor trends in malaria incidence and evaluate community-based interventions.

METHODS

Within a mass drug administration (MDA) study in The Gambia, where reduced malaria infection and clinical disease were observed after the intervention, a serological sub-study was conducted in four study villages. Spatio-temporal variation in transmission was measured with a panel of recombinant Pf antigens on a multiplexed bead-based assay. Village-level antibody levels were quantified as under-15 sero-prevalence, sero-conversion rates, and age-adjusted antibody acquisition rates. Antibody levels prior to MDA were assessed for association with persistent malaria infection after community chemoprophylaxis.

RESULTS

Seasonal changes in antibodies to Etramp5.Ag1 were observed in children under 15 years in two transmission settings-the West Coast and Upper River Regions (4.32% and 31.30% Pf prevalence, respectively). At the end of the malaria season, short-lived antibody responses to Etramp5.Ag1, GEXP18, HSP40.Ag1, EBA175 RIII-V, and Rh2.2030 were lower amongst 1-15 year olds in the West Coast compared to the Upper River, reflecting known differences in transmission. Prior to MDA, individuals in the top 50th percentile of antibody levels had two-fold higher odds of clinical malaria during the transmission season, consistent with previous findings from the Malaria Transmission Dynamics Study, where individuals infected before the implementation of MDA had two-fold higher odds of re-infection post-MDA.

CONCLUSIONS

Serological markers can serve dual functions as indicators of malaria exposure and incidence. By monitoring age-specific sero-prevalence, the magnitude of age-stratified antibody levels, or identifying groups of individuals with above-average antibody responses, these antigens have the potential to complement conventional malaria surveillance tools. Further studies, particularly cluster randomised trials, can help establish standardised serological protocols to reliably measure transmission across endemic settings.

Naturally Acquired Humoral Immunity Against Plasmodium falciparum Malaria

Malaria remains a significant contributor to the global burden of disease, with around 40% of the world's population at risk of Plasmodium infections. The development of an effective vaccine against the malaria parasite would mark a breakthrough in the fight to eradicate the disease. Over time, natural infection elicits a robust immune response against the blood stage of the parasite, providing protection against malaria. In recent years, we have gained valuable insight into the mechanisms by which IgG acts to prevent pathology and inhibit parasite replication, as well as the potential role of immunoglobulin M (IgM) in these processes. Here, we discuss recent advances in our understanding of the mechanisms, acquisition, and maintenance of naturally acquired immunity, and the relevance of these discoveries for the development of a potential vaccine against the blood stage of Plasmodium falciparum.

Antibody responses to a suite of novel serological markers for malaria surveillance demonstrate strong correlation with clinical and parasitological infection across seasons and transmission settings in The Gambia

BACKGROUND

As malaria transmission declines, sensitive diagnostics are needed to evaluate interventions and monitor transmission. Serological assays measuring malaria antibody responses offer a cost-effective detection method to supplement existing surveillance tools.

METHODS

A prospective cohort study was conducted from 2013 to 2015 in 12 villages across five administrative regions in The Gambia. Serological analysis included samples from the West Coast Region at the start and end of the season (July and December 2013) and from the Upper River Region in July and December 2013 and April and December 2014. Antigen-specific antibody responses to eight Plasmodium falciparum (P. falciparum) antigens-Etramp5.Ag1, GEXP18, HSP40.Ag1, Rh2.2030, EBA175 RIII-V, PfMSP1₁₉, PfAMA1, and PfGLURP.R2-were quantified using a multiplexed bead-based assay. The association between antibody responses and clinical and parasitological endpoints was estimated at the individual, household, and population level.

RESULTS

Strong associations were observed between clinical malaria and concurrent sero-positivity to Etramp5.Ag1 (aOR 4.60 95% CI 2.98-7.12), PfMSP1₁₉ (aOR 4.09 95% CI 2.60-6.44), PfAMA1 (aOR 2.32 95% CI 1.40-3.85), and PfGLURP.R2 (aOR 3.12, 95% CI 2.92-4.95), while asymptomatic infection was associated with sero-positivity to all antigens. Village-level sero-prevalence amongst children 2-10 years against Etramp5.Ag1, HSP40.Ag1, and PfMSP1₁₉ showed the highest correlations with clinical and P. falciparum infection incidence rates. For all antigens, there were increased odds of asymptomatic P. falciparum infection in subjects residing in a compound with greater than 50% sero-prevalence, with a 2- to 3-fold increase in odds of infection associated with Etramp5.Ag1, GEXP18, Rh2.2030, PfMSP1₁₉, and PfAMA1. For individuals residing in sero-positive compounds, the odds of clinical malaria were reduced, suggesting a protective effect.

CONCLUSIONS

At low transmission, long-lived antibody responses could indicate foci of malaria transmission that have been ongoing for several seasons or years. In settings where sub-patent infections are prevalent and fluctuate below the detection limit of polymerase chain reaction (PCR), the presence of short-lived antibodies may indicate recent infectivity, particularly in the dry season when clinical cases are rare. Serological responses may reflect a persistent reservoir of infection, warranting community-targeted interventions if individuals are not clinically apparent but have the potential to transmit. Therefore, serological surveillance at the individual and household level may be used to target interventions where there are foci of asymptomatically infected individuals, such as by measuring the magnitude of age-stratified antibody levels or identifying areas with clustering of above-average antibody responses across a diverse range of serological markers.

Evaluating the predictive performance of malaria antibodies and FCGR3B gene polymorphisms on Plasmodium falciparum infection outcome: a prospective cohort study

Background

Malaria antigen-specific antibodies and polymorphisms in host receptors involved in antibody functionality have been associated with different outcomes of Plasmodium falciparum infections. Thus, to identify key prospective malaria antigens for vaccine development, there is the need to evaluate the associations between malaria antibodies and antibody dependent host factors with more rigorous statistical methods. In this study, different statistical models were used to evaluate the predictive performance of malaria-specific antibodies and host gene polymorphisms on P. falciparum infection in a longitudinal cohort study involving Ghanaian children.

Methods

Models with different functional forms were built using known predictors (age, sickle cell status, blood group status, parasite density, and mosquito bed net use) and malaria antigen-specific immunoglobulin (Ig) G and IgG subclasses and FCGR3B polymorphisms shown to mediate antibody-dependent cellular functions. Malaria antigens studied were Merozoite surface proteins (MSP-1 and MSP-3), Glutamate Rich Protein (GLURP)-R0, R2, and the Apical Membrane Antigen (AMA-1). The models were evaluated through visualization and assessment of differences between the Area Under the Receiver Operating Characteristic Curve and Brier Score estimated by suitable internal cross-validation designs.

Results

This study found that the FCGR3B-c.233C>A genotype and IgG against AMA1 were relatively better compared to the other antibodies and FCGR3B genotypes studied in classifying or predicting malaria risk among children.

Conclusions

The data supports the P. falciparum, AMA1 as an important malaria vaccine antigen, while FCGR3B-c.233C>A under the additive and dominant models of inheritance could be an important modifier of the effect of malaria protective antibodies.

Variations in killer-cell immunoglobulin-like receptor and human leukocyte antigen genes and immunity to malaria

Malaria is one of the deadliest infectious diseases in the world. Immune responses to Plasmodium falciparum malaria vary among individuals and between populations. Human genetic variation in immune system genes is likely to play a role in this heterogeneity. Natural killer (NK) cells produce inflammatory cytokines in response to malaria infection, kill intraerythrocytic Plasmodium falciparum parasites by cytolysis, and participate in the initiation and development of adaptive immune responses to plasmodial infection. These functions are modulated by interactions between killer-cell immunoglobulin-like receptors (KIRs) and human leukocyte antigens (HLAs). Therefore, variations in KIR and HLA genes can have a direct impact on NK cell functions. Understanding the role of KIRs and HLAs in immunity to malaria can help to better characterize antimalarial immune responses. In this review, we summarize the different KIRs and HLAs associated with immunity to malaria thus far.

Molecular characterization of Plasmodium falciparum PHISTb proteins as potential targets of naturally-acquired immunity against malaria

Background: Plasmodium falciparum causes the deadliest form of malaria in humans. Upon infection, the host’s infected red blood cells (iRBCs) are remodelled by exported parasite proteins in order to provide a niche for parasite development and maturation. Methods: Here we analysed the role of three PHISTb proteins Pf3D7_0532400, Pf3D7_1401600, and Pf3D7_1102500 by expressing recombinant proteins and evaluated antibody responses against these proteins using immune sera from malaria-exposed individuals from Kenya and The Gambia in Africa. Results: Our findings show that children and adults from malaria-endemic regions recognized the three PHISTb proteins. Responses against the PHISTb proteins varied with malaria transmission intensity in three different geographical sites in Kenya (Siaya and Takaungu) and The Gambia (Sukuta). Antibody responses against PHISTb antigens Pf3D7_1102500 and Pf3D7_1401600 were higher in Sukuta, a low transmission region in the Gambia, as compared to Siaya, a high transmission region in western Kenya, unlike Pf3D7_0532400. Anti-PHIST responses show a negative correlation between antibody levels and malaria transmission intensity for two PHIST antigens, Pf3D7_1102500 and Pf3D7_1401600. However, we report a correlation in antibody responses between schizont extract and Pf3D7_0532400 (p=0.00582). Acquisition of anti-PHIST antibodies was correlated with exposure to malaria for PHISTb protein Pf3D7_0532400 (p=0.009) but not the other PHIST antigens Pf3D7_1102500 and Pf3D7_1401600 (p=0.507 and p=0.15, respectively, CI=95%). Children aged below 2 years had the lowest antibody levels, but the responses do not correlate with age differences. Conclusions: Collectively, these findings provide evidence of natural immunity against PHISTb antigens that varies with level of malaria exposure and underscore potential for these parasite antigens as possible serological markers to P. falciparum infection aimed at contributing to malaria control through vaccine development.

Plasmodium falciparum immunodominant IgG epitopes in subclinical malaria

Incomplete non-sterile immunity to malaria is attained in endemic regions after recurrent infections by a large percentage of the adult population, who carry the malaria parasite asymptomatically. Although blood-stage Plasmodium falciparum rapidly elicits IgG responses, the target antigens of partially protective and non-protective IgG antibodies as well as the basis for the acquisition of these antibodies remain largely unknown. We performed IgG-immunomics to screen for P. falciparum antigens and to identify epitopes associated with exposure and clinical disease. Sera from malaria cases identified five prevalent antigens recognized by all analyzed patients' IgGs. Epitope mapping of them, using adult and children sera samples from an endemic malaria region in Ghana segregated into patients with positive or negative subclinical detection of P. falciparum, revealed binding specificity for two 20-mer immunodominant antigenic regions within the START-related lipid transfer protein and the protein disulfide isomerase PDI8. These 20-mer epitopes challenged with sera samples from children under 5 years old displayed specific IgG binding in those with detectable parasitemia, even at subclinical level. These results suggest that humoral response against START and PDI8 antigens may be triggered at submicroscopic parasitemia levels in children and may eventually be used to differentially diagnose subclinical malaria in children.

Geographic location determines beta-cell autoimmunity among adult Ghanaians: Findings from the RODAM study

Introduction

Beta‐cell autoantibodies are established markers of autoimmunity, which we compared between Ghanaian adults with or without diabetes, living in rural and urban Ghana and in three European cities.

Methods

In the multicenter cross‐sectional Research on Obesity and Diabetes among African Migrants (RODAM) study (N = 5898), we quantified autoantibodies against glutamic acid decarboxylase (GAD65Ab) by radioligand binding assay (RBA) and established cut‐offs for positivity by displacement analysis. In a subsample, we performed RBA for zinc transporter‐8 autoantibodies (ZnT8Ab). Associations of environmental, sociodemographic, and clinical factors with GAD65Ab were calculated.

Results

In this study population (age: 46.1 ± 11.9 years; female: 62%; Ghana‐rural: 1111; Ghana‐urban: 1455; Europe: 3332), 9.2% had diabetes with adult‐onset. GAD65Ab concentrations were the highest in Ghana‐rural (32.4; 10.8‐71.3 U/mL), followed by Ghana‐urban (26.0; 12.3‐49.1 U/mL) and Europe (11.9; 3.0‐22.8 U/mL) with no differences between European cities. These distributions were similar for ZnT8Ab. Current fever, history of fever, and higher concentrations of liver enzymes marginally explained site‐specific GAD65Ab concentrations. GAD65Ab positivity was as frequent in diabetes as in nondiabetes (5.4% vs 6.1%; P = .25). This was also true for ZnT8Ab positivity.

Conclusion

Geographic location determines the occurrence of GAD65Ab and ZnT8Ab more than the diabetes status. Beta‐cell autoimmunity may not be feasible to differentiate diabetes subgroups in this population.

Dynamics of IgM and IgG responses to the next generation of engineered Duffy binding protein II immunogen: Strain-specific and strain-transcending immune responses over a nine-year period

BACKGROUND

A low proportion of P. vivax-exposed individuals acquire protective strain-transcending neutralizing IgG antibodies that are able to block the interaction between the Duffy binding protein II (DBPII) and its erythrocyte-specific invasion receptor. In a recent study, a novel surface-engineered DBPII-based vaccine termed DEKnull-2, whose antibody response target conserved DBPII epitopes, was able to induce broadly binding-inhibitory IgG antibodies (BIAbs) that inhibit P. vivax reticulocyte invasion. Toward the development of DEKnull-2 as an effective P. vivax blood-stage vaccine, we investigate the relationship between naturally acquired DBPII-specific IgM response and the profile of IgG antibodies/BIAbs activity over time.

METHODOLOGY/PRINCIPAL FINDINGS

A nine-year follow-up study was carried-out among long-term P. vivax-exposed Amazonian individuals and included six cross-sectional surveys at periods of high and low malaria transmission. DBPII immune responses associated with either strain-specific (Sal1, natural DBPII variant circulating in the study area) or conserved epitopes (DEKnull-2) were monitored by conventional serology (ELISA-detected IgM and IgG antibodies), with IgG BIAbs activity evaluated by functional assays (in vitro inhibition of DBPII-erythrocyte binding). The results showed a tendency of IgM antibodies toward Sal1-specific response; the profile of Sal1 over DEKnull-2 was not associated with acute malaria and sustained throughout the observation period. The low malaria incidence in two consecutive years allowed us to demonstrate that variant-specific IgG (but not IgM) antibodies waned over time, which resulted in IgG skewed to the DEKnull-2 response. A persistent DBPII-specific IgM response was not associated with the presence (or absence) of broadly neutralizing IgG antibody response.

CONCLUSIONS/SIGNIFICANCE

The current study demonstrates that long-term exposure to low and unstable levels of P. vivax transmission led to a sustained DBPII-specific IgM response against variant-specific epitopes, while sustained IgG responses are skewed to conserved epitopes. Further studies should investigate on the role of a stable and persistent IgM antibody response in the immune response mediated by DBPII.

Screening for malaria antigen and anti-malarial IgG antibody in forcibly-displaced Myanmar nationals: Cox's Bazar district, Bangladesh, 2018

Background

Several refugee settlements in Bangladesh have provided housing and medical care for the forcibly-displaced Myanmar nationals (FDMN, also known as Rohingya) population. The identification of malaria infection status in the refugee settlements is useful in treating infected persons and in developing malaria prevention recommendations. Assays for Plasmodium antigens and human IgG against Plasmodium parasites can be used as indicators to determine malaria infection status and exposure.

Methods

Dried blood spot (DBS) samples (N = 1239) from a household survey performed April–May 2018 in three settlements in Cox’s Bazar district, Bangladesh were utilized for a sample population of children from ages 1–14 years of age. The samples were tested using a bead-based multiplex antigen assay for presence of the pan-Plasmodium antigen aldolase as well as Plasmodium falciparum histidine rich protein 2 (HRP2). A bead-based multiplex assay was also used to measure human IgG antibody response to P. falciparum, Plasmodium malariae, and Plasmodium vivax merozoite surface protein 1 antigen (MSP1) isoforms, and P. falciparum antigens LSA1, CSP, and GLURP-R0.

Results

There were no detectable Plasmodium antigens in any samples, suggesting no active malaria parasite infections in the tested children. IgG seroprevalence was highest to P. vivax (3.1%), but this was not significantly different from the percentages of children antibody responses to P. falciparum (2.1%) and P. malariae (1.8%). The likelihood of an anti-Plasmodium IgG response increased with age for all three malaria species. Evidence of exposure to any malaria species was highest for children residing 8–10 months in the settlements, and was lower for children arriving before and after this period of time.

Conclusions

Absence of Plasmodium antigen in this population provides evidence that children in these three Bangladeshi refugee settlements did not have malaria at time of sampling. Higher rates of anti-malarial IgG carriage from children who were leaving Myanmar during the malaria high-transmission season indicate these migrant populations were likely at increased risk of malaria exposure during their transit.

Molecular Characterization and Immuno-Reactivity Patterns of a Novel Plasmodium falciparum Armadillo-Type Repeat Protein, PfATRP

Nearly half of the genes in the Plasmodium falciparum genome have not yet been functionally investigated. We used homology-based structural modeling to identify multiple copies of Armadillo repeats within one uncharacterized gene expressed during the intraerythrocytic stages, PF3D7_0410600, subsequently referred to as P. falciparum Armadillo-Type Repeat Protein (PfATRP). Soluble recombinant PfATRP was expressed in a bacterial expression system, purified to apparent homogeneity and the identity of the recombinant PfATRP was confirmed by mass spectrometry. Affinity-purified α-PfATRP rabbit antibodies specifically recognized the recombinant protein. Immunofluorescence assays revealed that α-PfATRP rabbit antibodies reacted with P. falciparum schizonts. Anti-PfATRP antibody exhibited peripheral staining patterns around the merozoites. Given the localization of PfATRP in merozoites, we tested for an egress phenotype during schizont arrest assays and demonstrated that native PfATRP is inaccessible on the surface of merozoites in intact schizonts. Dual immunofluorescence assays with markers for the inner membrane complex (IMC) and microtubules suggest partial colocalization in both asexual and sexual stage parasites. Using the soluble recombinant PfATRP in a screen of plasma samples revealed that malaria-infected children have naturally acquired PfATRP-specific antibodies.

Protective Immunity against Severe Malaria in Children Is Associated with a Limited Repertoire of Antibodies to Conserved PfEMP1 Variants

Extreme diversity of the major Plasmodium falciparum antigen, PfEMP1, poses a barrier to identifying targets of immunity to malaria. Here, we used protein microarrays containing hundreds of variants of the DBLα domain of PfEMP1 to cover the diversity of Papua New Guinean (PNG) parasites. Probing the plasma of a longitudinal cohort of malaria-exposed PNG children showed that group 2 DBLα antibodies were moderately associated with a lower risk of uncomplicated malaria, whereas individual variants were only weakly associated with clinical immunity. In contrast, antibodies to 85 individual group 1 and 2 DBLα variants were associated with a 70%-100% reduction in severe malaria. Of these, 17 variants were strong predictors of severe malaria. Analysis of full-length PfEMP1 sequences from PNG samples shows that these 17 variants are linked to pathogenic CIDR domains. This suggests that whereas immunity to uncomplicated malaria requires a broad repertoire of antibodies, immunity to severe malaria targets a subset of conserved variants. These findings provide insights into antimalarial immunity and potential antibody biomarkers for disease risk.

IgG antibody response against Plasmodium falciparum aminopeptidase 1 antigen in Gabonese children living in Makokou and Franceville

The search for novel chemical classes of anti-malarial compounds to cope with the current state of chemoresistance of malaria parasites has led to the identification of Plasmodium falciparum aminopeptidase 1 (PfA-M1) as a new therapeutic target. PfA-M1, known to be involved in the hemoglobin digestion cascade which helps to provide most of the amino acids necessary to the parasite's metabolism, is currently considered as a promising target for anti-malarial chemotherapy. However, its immunogenic properties have not yet been tested in the Gabonese population. In Gabon, the prevalence of malaria remains three times higher in semi-urban areas (60·12%) than in urban areas (17·06%). We show that malaria-specific PfA-M1 antibodies are present in children and increase with the level of infection. Children living in semi-urban areas have higher anti-PfA-M1 antibody titers (0·14 ± 0·02 AU) than those living in urban areas (0·08 ± 0·02 AU, P = 0·03), and their antibody titers increase with age (P < 0·0001). Moreover, anti-PfA-M1 antibody titers decrease in children with hyperparasitemia (0·027 ± 0·055 AU) but they remain high in children with low parasite density (0·21 ± 0·034 AU, P = 0·034). In conclusion, our results suggest that malaria-specific PfA-M1 antibodies may play an important role in the immune response of the host against P. falciparum in Gabonese children. Further studies on the role of PfA-M1 during anemia are needed.