Participant perceptions and experiences of a novel community-based respiratory longitudinal sampling method in Liverpool, UK: A mixed methods feasibility study

Longitudinal, community-based sampling is important for understanding prevalence and transmission of respiratory pathogens. Using a minimally invasive sampling method, the FAMILY Micro study monitored the oral, nasal and hand microbiota of families for 6 months. Here, we explore participant experiences and opinions. A mixed methods approach was utilised. A quantitative questionnaire was completed after every sampling timepoint to report levels of discomfort and pain, as well as time taken to collect samples. Participants were also invited to discuss their experiences in a qualitative structured exit interview. We received questionnaires from 36 families. Most adults and children >5y experienced no pain (94% and 70%) and little discomfort (73% and 47% no discomfort) regardless of sample type, whereas children ≤5y experienced variable levels of pain and discomfort (48% no pain but 14% hurts even more, whole lot or worst; 38% no discomfort but 33% moderate, severe, or extreme discomfort). The time taken for saliva and hand sampling decreased over the study. We conducted interviews with 24 families. Families found the sampling method straightforward, and adults and children >5y preferred nasal sampling using a synthetic absorptive matrix over nasopharyngeal swabs. It remained challenging for families to fit sampling into their busy schedules. Adequate fridge/freezer space and regular sample pick-ups were found to be important factors for feasibility. Messaging apps proved extremely effective for engaging with participants. Our findings provide key information to inform the design of future studies, specifically that self-sampling at home using minimally invasive procedures is feasible in a family context.

Combinatorial multimer staining and spectral flow cytometry facilitate quantification and characterization of polysaccharide-specific B cell immunity

Bacterial capsular polysaccharides are important vaccine immunogens. However, the study of polysaccharide-specific immune responses has been hindered by technical restrictions. Here, we developed and validated a high-throughput method to analyse antigen-specific B cells using combinatorial staining with fluorescently-labelled capsular polysaccharide multimers. Concurrent staining of 25 cellular markers further enables the in-depth characterization of polysaccharide-specific cells. We used this assay to simultaneously analyse 14 Streptococcus pneumoniae or 5 Streptococcus agalactiae serotype-specific B cell populations. The phenotype of polysaccharide-specific B cells was associated with serotype specificity, vaccination history and donor population. For example, we observed a link between non-class switched (IgM+) memory B cells and vaccine-inefficient S. pneumoniae serotypes 1 and 3. Moreover, B cells had increased activation in donors from South Africa, which has high-incidence of S. agalactiae invasive disease, compared to Dutch donors. This assay allows for the characterization of heterogeneity in B cell immunity that may underlie immunization efficacy.

Respiratory mucosal immune memory to SARS-CoV-2 after infection and vaccination

Respiratory mucosal immunity induced by vaccination is vital for protection from coronavirus infection in animal models. In humans, the capacity of peripheral vaccination to generate sustained immunity in the lung mucosa, and how this is influenced by prior SARS-CoV-2 infection, is unknown. Here we show using bronchoalveolar lavage samples that donors with history of both infection and vaccination have more airway mucosal SARS-CoV-2 antibodies and memory B cells than those only vaccinated. Infection also induces populations of airway spike-specific memory CD4+ and CD8+ T cells that are not expanded by vaccination alone. Airway mucosal T cells induced by infection have a distinct hierarchy of antigen specificity compared to the periphery. Spike-specific T cells persist in the lung mucosa for 7 months after the last immunising event. Thus, peripheral vaccination alone does not appear to induce durable lung mucosal immunity against SARS-CoV-2, supporting an argument for the need for vaccines targeting the airways.

Assessing the use of minimally invasive self-sampling at home for long-term monitoring of the microbiota within UK families

Monitoring the presence of commensal and pathogenic respiratory microorganisms is of critical global importance. However, community-based surveillance is difficult because nasopharyngeal swabs are uncomfortable and painful for a wide age range of participants. We designed a methodology for minimally invasive self-sampling at home and assessed its use for longitudinal monitoring of the oral, nasal and hand microbiota of adults and children within families. Healthy families with two adults and up to three children, living in and near Liverpool, United Kingdom, self-collected saliva, nasal lining fluid using synthetic absorptive matrices and hand swabs at home every two weeks for six months. Questionnaires were used to collect demographic and epidemiological data and assess feasibility and acceptability. Participants were invited to take part in an exit interview. Thirty-three families completed the study. Sampling using our approach was acceptable to 25/33 (76%) families, as sampling was fast (76%), easy (76%) and painless (60%). Saliva and hand sampling was acceptable to all participants of any age, whereas nasal sampling was accepted mostly by adults and children older than 5 years. Multi-niche self-sampling at home can be used by adults and children for longitudinal surveillance of respiratory microorganisms, providing key data for design of future studies.

The global transcriptome of Plasmodium falciparum mid-stage gametocytes (stages II-IV) appears largely conserved and gametocyte-specific gene expression patterns vary in clinical isolates

Our overall understanding of the developmental biology of malaria parasites has been greatly enhanced by recent advances in transcriptomic analysis. However, most of these investigations rely on laboratory strains (LS) that were adapted into in vitro culture many years ago, and the transcriptomes of clinical isolates (CI) circulating in human populations have not been assessed. In this study, RNA-seq was used to compare the global transcriptome of mid-stage gametocytes derived from three short-term cultured CI, with gametocytes derived from the NF54 reference laboratory strain. The core transcriptome appeared to be consistent between CI- and LS-derived gametocyte preparations, but some important differences were also observed. A majority of gametocyte-specific genes (43/53) appear to have relatively higher expression in CI-derived gametocytes than in LS-derived gametocytes, but a K-means clustering analysis showed that genes involved in flagellum- and microtubule-based processes (movement/motility) were more abundant in both groups, albeit with some differences between them. In addition, gametocytes from one CI described as CI group II gametocytes (CI:GGII) showed gene expression variation in the form of reduced gametocyte-specific gene expression compared to the other two CI-derived gametocytes (CI gametocyte group I, CI:GGI), although the mixed developmental stages used in our study is a potential confounder, only partially mitigated by the inclusion of multiple replicates for each CI. Overall, our study suggests that there may be subtle differences in the gene expression profiles of mid-stage gametocytes from CI relative to the NF54 reference strain of Plasmodium falciparum. Thus, it is necessary to deploy gametocyte-producing clinical parasite isolates to fully understand the diversity of gene expression strategies that may occur during the sequestered development of parasite sexual stages. IMPORTANCE Maturing gametocytes of Plasmodium falciparum are known to sequester away from peripheral circulation into the bone marrow until they are mature. Blocking gametocyte sequestration can prevent malaria transmission from humans to mosquitoes, but most studies aim to understand gametocyte development utilizing long-term adapted laboratory lines instead of clinical isolates. This is a particular issue for our understanding of the sexual stages, which are known to decrease rapidly during adaptation to long-term culture, meaning that many LS are unable to produce transmissible gametocytes. Using RNA-seq, we investigated the global transcriptome of mid-stage gametocytes derived from three clinical isolates and a reference strain (NF54). This identified important differences in gene expression profiles between immature gametocytes of CI and the NF54 reference strain of P. falciparum, suggesting increased investment in gametocytogenesis in clinical isolates. Our transcriptomic data highlight the use of clinical isolates in studying the morphological, cellular features and molecular biology of gametocytes.

Prognostic Value of C-Reactive Protein in SARS-CoV-2 Infection: A Simplified Biomarker of COVID-19 Severity in Northern Ethiopia

Purpose

To evaluate the role of C-reactive protein (CRP) in predicting severe COVID-19 patients.

Methods

A prospective observational cohort study was conducted from July 15 to October 28, 2020, at Kuyha COVID-19 isolation and treatment center hospital, Mekelle City, Northern Ethiopia. A total of 670 blood samples were collected serially. SARS-CoV-2 infection was confirmed by RT-PCR from nasopharyngeal swabs and CRP concentration was determined using Cobas Integra 400 Plus (Roche). Data were analyzed using STATA version 14. P-value <0.05 was considered statistically significant.

Results

Overall, COVID-19 patients had significantly elevated CRP at baseline when compared to PCR-negative controls [median 11.1 (IQR: 2.0-127.8) mg/L vs 0.9 (IQR: 0.5-1.9) mg/L; p=0.0004)]. Those with severe COVID-19 clinical presentation had significantly higher median CRP levels compared to those with non-severe cases [166.1 (IQR: 48.6-332.5) mg/L vs 2.4 (IQR: 1.2-7.6) mg/L; p<0.00001)]. Moreover, COVID-19 patients exhibited higher median CRP levels at baseline [58 (IQR: 2.0-127.8) mg/L] that decreased significantly to 2.4 (IQR: 1.4-3.9) mg/L after 40 days after symptom onset (p<0.0001). Performance of CRP levels determined using ROC analysis distinguished severe from non-severe COVID-19 patients, with an AUC value of 0.83 (95% CI: 0.73-0.91; p=0.001; 77.4% sensitivity and 89.4% specificity). In multivariable analysis, CRP levels above 30 mg/L were significantly associated with an increased risk of developing severe COVID-19 for those who have higher ages and comorbidities (ARR 3.99, 95% CI: 1.35-11.82; p=0.013).

Conclusion

CRP was found to be an independent determinant factor for severe COVID-19 patients. Therefore, CRP levels in COVID-19 patients in African settings may provide a simple, prompt, and inexpensive assessment of the severity status at baseline and monitoring of treatment outcomes.

Parasite clearance dynamics in children hospitalised with severe malaria in the Ho Teaching Hospital, Volta Region, Ghana

Background

Over 90% of severe malaria (SM) cases occur in African children. Parenteral artesunate is currently the recommended treatment for SM. Studies of parasite clearance in paediatric SM cases are needed for assessment of therapeutic outcomes but are lacking in Africa.

Methods

Severe malaria patients were recruited in the children's emergency ward at Ho Teaching Hospital, Ghana, in 2018. Blood samples were taken upon admission, every 24 h for 3 days and 1 week after treatment, and DNA extracted. Parasitaemia and parasite densities were performed by microscopy at enrolment and the follow-up days wherever possible. Relative parasite density was measured at each timepoint by duplex qPCR and parameters of parasite clearance estimated.

Results

Of 25 evaluable SM patients, clearance of qPCR-detectable parasites occurred within 48 h for 17 patients, but three out of the remaining eight were still qPCR-positive on day 3. Increased time to parasite clearance was seen in children ≥5 years old, those with lower haemoglobin levels and those with a high number of previous malaria diagnoses, but these associations were not statistically significant.

Conclusion

We examined parasite clearance dynamics among paediatric cases of SM. Our observations suggest that daily sampling for qPCR estimation of P. falciparum peripheral density is a useful method for assessing treatment response in hospitalised SM cases. The study demonstrated varied parasite clearance response, thus illuminating the complex nature of the mechanism in this important patient group, and further investigations utilizing larger sample sizes are needed to confirm our findings.

Streptococcus pneumoniae colonization associates with impaired adaptive immune responses against SARS-CoV-2

Background

Although recent epidemiological data suggest that pneumococci may contribute to the risk of SARS-CoV-2 disease, cases of coinfection with Streptococcus pneumoniae in patients with coronavirus disease 2019 (COVID-19) during hospitalization have been reported infrequently. This apparent contradiction may be explained by interactions of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and pneumococci in the upper airway, resulting in the escape of SARS-CoV-2 from protective host immune responses.

Methods

Here, we investigated the relationship of these 2 respiratory pathogens in 2 distinct cohorts of health care workers with asymptomatic or mildly symptomatic SARS-CoV-2 infection identified by systematic screening and patients with moderate to severe disease who presented to the hospital. We assessed the effect of coinfection on host antibody, cellular, and inflammatory responses to the virus.

Results

In both cohorts, pneumococcal colonization was associated with diminished antiviral immune responses, which primarily affected mucosal IgA levels among individuals with mild or asymptomatic infection and cellular memory responses in infected patients.

Conclusion

Our findings suggest that S. pneumoniae impair host immunity to SARS-CoV-2 and raise the question of whether pneumococcal carriage also enables immune escape of other respiratory viruses and facilitates reinfection.

Trial registration

ISRCTN89159899 (FASTER study) and ClinicalTrials.gov NCT03502291 (LAIV study).

Longitudinal profile of antibody response to SARS-CoV-2 in patients with COVID-19 in a setting from Sub-Saharan Africa: A prospective longitudinal study

Background

Serological testing for SARS-CoV-2 plays an important role for epidemiological studies, in aiding the diagnosis of COVID-19, and assess vaccine responses. Little is known on dynamics of SARS-CoV-2 serology in African settings. Here, we aimed to characterize the longitudinal antibody response profile to SARS-CoV-2 in Ethiopia.

Methods

In this prospective study, a total of 102 PCR-confirmed COVID-19 patients were enrolled. We obtained 802 plasma samples collected serially. SARS-CoV-2 antibodies were determined using four lateral flow immune-assays (LFIAs), and an electrochemiluminescent immunoassay. We determined longitudinal antibody response to SARS-CoV-2 as well as seroconversion dynamics.

Results

Serological positivity rate ranged between 12%-91%, depending on timing after symptom onset. There was no difference in positivity rate between severe and non-severe COVID-19 cases. The specificity ranged between 90%-97%. Agreement between different assays ranged between 84%-92%. The estimated positive predictive value (PPV) for IgM or IgG in a scenario with seroprevalence at 5% varies from 33% to 58%. Nonetheless, when the population seroprevalence increases to 25% and 50%, there is a corresponding increases in the estimated PPVs. The estimated negative-predictive value (NPV) in a low seroprevalence scenario (5%) is high (>99%). However, the estimated NPV in a high seroprevalence scenario (50%) for IgM or IgG is reduced significantly to 80% to 85%. Overall, 28/102 (27.5%) seroconverted by one or more assays tested, within a median time of 11 (IQR: 9–15) days post symptom onset. The median seroconversion time among symptomatic cases tended to be shorter when compared to asymptomatic patients [9 (IQR: 6–11) vs. 15 (IQR: 13–21) days; p = 0.002]. Overall, seroconversion reached 100% 5.5 weeks after the onset of symptoms. Notably, of the remaining 74 COVID-19 patients included in the cohort, 64 (62.8%) were positive for antibody at the time of enrollment, and 10 (9.8%) patients failed to mount a detectable antibody response by any of the assays tested during follow-up.

Conclusions

Longitudinal assessment of antibody response in African COVID-19 patients revealed heterogeneous responses. This underscores the need for a comprehensive evaluation of seroassays before implementation. Factors associated with failure to seroconvert needs further research.

Effect of co-infection with intestinal parasites on COVID-19 severity: A prospective observational cohort study

Background: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection results in a spectrum of clinical presentations. Evidence from Africa indicates that significantly less COVID-19 patients suffer from serious symptoms than in the industrialized world. We and others previously postulated a partial explanation for this phenomenon, being a different, more activated immune system due to parasite infections. Here, we aimed to test this hypothesis by investigating a potential correlation of co-infection with parasites with COVID-19 severity in an endemic area in Africa. Methods: Ethiopian COVID-19 patients were enrolled and screened for intestinal parasites, between July 2020 and March 2021. The primary outcome was the proportion of patients with severe COVID-19. Ordinal logistic regression models were used to estimate the association between parasite infection, and COVID-19 severity. Models were adjusted for sex, age, residence, education level, occupation, body mass index, and comorbidities. Findings: 751 SARS-CoV-2 infected patients were enrolled, of whom 284 (37.8%) had intestinal parasitic infection. Only 27/255 (10.6%) severe COVID-19 patients were co-infected with intestinal parasites, while 257/496 (51.8%) non-severe COVID-19 patients were parasite positive (p<0.0001). Patients co-infected with parasites had lower odds of developing severe COVID-19, with an adjusted odds ratio (aOR) of 0.23 (95% CI 0.17-0.30; p<0.0001) for all parasites, aOR 0.37 ([95% CI 0.26-0.51]; p<0.0001) for protozoa, and aOR 0.26 ([95% CI 0.19-0.35]; p<0.0001) for helminths. When stratified by species, co-infection with Entamoeba spp., Hymenolepis nana, Schistosoma mansoni, and Trichuris trichiura implied lower probability of developing severe COVID-19. There were 11 deaths (1.5%), and all were among patients without parasites (p = 0.009). Interpretation: Parasite co-infection is associated with a reduced risk of severe COVID-19 in African patients. Parasite-driven immunomodulatory responses may mute hyper-inflammation associated with severe COVID-19. Funding: European and Developing Countries Clinical Trials Partnership (EDCTP) - European Union, and Joep Lange Institute (JLI), The Netherlands. Trial registration: Clinicaltrials.gov: NCT04473365.

Effect of co-infection with intestinal parasites on COVID-19 severity: A prospective observational cohort study

Background: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection results in a spectrum of clinical presentations. Evidence from Africa indicates that significantly less COVID-19 patients suffer from serious symptoms than in the industrialized world. We and others previously postulated a partial explanation for this phenomenon, being a different, more activated immune system due to parasite infections. Here, we aimed to test this hypothesis by investigating a potential correlation of co-infection with parasites with COVID-19 severity in an endemic area in Africa. Methods: Ethiopian COVID-19 patients were enrolled and screened for intestinal parasites, between July 2020 and March 2021. The primary outcome was the proportion of patients with severe COVID-19. Ordinal logistic regression models were used to estimate the association between parasite infection, and COVID-19 severity. Models were adjusted for sex, age, residence, education level, occupation, body mass index, and comorbidities. Findings: 751 SARS-CoV-2 infected patients were enrolled, of whom 284 (37.8%) had intestinal parasitic infection. Only 27/255 (10.6%) severe COVID-19 patients were co-infected with intestinal parasites, while 257/496 (51.8%) non-severe COVID-19 patients were parasite positive (p<0.0001). Patients co-infected with parasites had lower odds of developing severe COVID-19, with an adjusted odds ratio (aOR) of 0.23 (95% CI 0.17-0.30; p<0.0001) for all parasites, aOR 0.37 ([95% CI 0.26-0.51]; p<0.0001) for protozoa, and aOR 0.26 ([95% CI 0.19-0.35]; p<0.0001) for helminths. When stratified by species, co-infection with Entamoeba spp., Hymenolepis nana, Schistosoma mansoni, and Trichuris trichiura implied lower probability of developing severe COVID-19. There were 11 deaths (1.5%), and all were among patients without parasites (p = 0.009). Interpretation: Parasite co-infection is associated with a reduced risk of severe COVID-19 in African patients. Parasite-driven immunomodulatory responses may mute hyper-inflammation associated with severe COVID-19. Funding: European and Developing Countries Clinical Trials Partnership (EDCTP) - European Union, and Joep Lange Institute (JLI), The Netherlands. Trial registration: Clinicaltrials.gov: NCT04473365.

Innate and adaptive nasal mucosal immune responses following experimental human pneumococcal colonization

Streptococcus pneumoniae (Spn) is a common cause of respiratory infection, but also frequently colonizes the nasopharynx in the absence of disease. We used mass cytometry to study immune cells from nasal biopsy samples collected following experimental human pneumococcal challenge in order to identify immunological mechanisms of control of Spn colonization. Using 37 markers, we characterized 293 nasal immune cell clusters, of which 7 were associated with Spn colonization. B cell and CD161⁺CD8⁺ T cell clusters were significantly lower in colonized than in noncolonized subjects. By following a second cohort before and after pneumococcal challenge we observed that B cells were depleted from the nasal mucosa upon Spn colonization. This associated with an expansion of Spn polysaccharide–specific and total plasmablasts in blood. Moreover, increased responses of blood mucosa-associated invariant T (MAIT) cells against in vitro stimulation with pneumococcus prior to challenge associated with protection against establishment of Spn colonization and with increased mucosal MAIT cell populations. These results implicate MAIT cells in the protection against pneumococcal colonization and demonstrate that colonization affects mucosal and circulating B cell populations.

Antigenic cartography of immune responses to Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1)

Naturally acquired clinical immunity to Plasmodium falciparum is partly mediated by antibodies directed at parasite-derived antigens expressed on the surface of red blood cells which mediate disease and are extremely diverse. Unlike children, adults recognize a broad range of variant surface antigens (VSAs) and are protected from severe disease. Though crucial to the design and feasibility of an effective malaria vaccine, it is not yet known whether immunity arises through cumulative exposure to each of many antigenic types, cross-reactivity between antigenic types, or some other mechanism. In this study, we measured plasma antibody responses of 36 children with symptomatic malaria to a diverse panel of 36 recombinant proteins comprising part of the DBLα domain (the 'DBLα-tag') of PfEMP1, a major class of VSAs. We found that although plasma antibody responses were highly specific to individual antigens, serological profiles of responses across antigens fell into one of just two distinct types. One type was found almost exclusively in children that succumbed to severe disease (19 out of 20) while the other occurred in all children with mild disease (16 out of 16). Moreover, children with severe malaria had serological profiles that were narrower in antigen specificity and shorter-lived than those in children with mild malaria. Borrowing a novel technique used in influenza-antigenic cartography-we mapped these dichotomous serological profiles to amino acid sequence variation within a small sub-region of the PfEMP1 DBLα domain. By applying our methodology on a larger scale, it should be possible to identify epitopes responsible for eliciting the protective version of serological profiles to PfEMP1 thereby accelerating development of a broadly effective anti-disease malaria vaccine.

Immune Responses to the Sexual Stages of Plasmodium falciparum Parasites

Malaria infections remain a serious global health problem in the world, particularly among children and pregnant women in Sub-Saharan Africa. Moreover, malaria control and elimination is hampered by rapid development of resistance by the parasite and the vector to commonly used antimalarial drugs and insecticides, respectively. Therefore, vaccine-based strategies are sorely needed, including those designed to interrupt disease transmission. However, a prerequisite for such a vaccine strategy is the understanding of both the human and vector immune responses to parasite developmental stages involved in parasite transmission in both man and mosquito. Here, we review the naturally acquired humoral and cellular responses to sexual stages of the parasite while in the human host and the Anopheles vector. In addition, updates on current anti-gametocyte, anti-gamete, and anti-mosquito transmission blocking vaccines are given. We conclude with our views on some important future directions of research into P. falciparum sexual stage immunity relevant to the search for the most appropriate transmission-blocking vaccine.

Gametocyte Development and Carriage in Ghanaian Individuals with Uncomplicated Plasmodium falciparum Malaria

Plasmodium falciparum gametocytes develop over 9-12 days while sequestered in deep tissues. On emergence into the bloodstream, they circulate for varied amounts of time during which certain host factors might influence their further development. We aimed to evaluate the potential association of patient clinical parameters with gametocyte development and carriage via in vivo methods. Seventy-two patients were enrolled from three hospitals in the Volta region of Ghana in 2016. Clinical parameters were documented for all patients, and gametocyte prevalence by microscopy was estimated at 12.5%. By measuring RNA transcripts representing two distinct gametocyte developmental stages using reverse transcriptase quantitative polymerase chain reaction (RT-qPCR), we obtained a more precise estimate of gametocyte carriage while also inferring gametocyte maturation. Fifty-three percent of the study participants harbored parasites expressing transcripts of the immature gametocyte-specific gene (PF3D7_1477700), whereas 36% harbored PF3D7_1438800 RNA-positive parasites, which is enriched in mid and mature gametocytes, suggesting the presence of more immature stages. Linear logistic regression showed that patients older than 5 years but less than 16 years were more likely to carry gametocytes expressing both PF3D7_1477700 and PF3D7_1438800 compared with younger participants, and gametocytemia was more likely in mildly anemic individuals compared with those with severe/moderate anemia. These data provide further evidence that a greater number of malaria patients harbor gametocytes than typically estimated by microscopy and suggest a possible association between age, fever, anemia, and gametocytemia.

Proportions of circulating follicular helper T cells are reduced and correlate with memory B cells in HIV-infected children

INTRODUCTION

HIV causes defects in memory B cells in children, but the mechanisms of those defects have not been fully elucidated. One possible mechanism is the lack of T-cell help to B cells during immune reactions. However, few studies have assessed the effect of HIV on follicular helper T cells (TFH cells) in children.

METHODS

In this study, follicular-homing CD4 T cells and memory B cells were assessed in HIV-infected children and compared with children from the community. CXCR5 and CD45RO were used as markers of follicular-homing T cells and memory T cells, respectively. Memory TFH cells were identified as CD3+CD8-CD4+CXCR5+CD45RO+PD1+. Central memory T cells were identified based on CCR7 expression. Relationship between the proportions of follicular-homing CD4 T cells and memory B cells were determined in multivariable regression models.

RESULTS

Highly viremic HIV-infected children had lower proportions of memory TFH cells when compared with community control children. In multivariable analyses, high proportions of memory TFH cells were associated with increased percentages of resting memory B cells after adjusting for other covariates.

CONCLUSION

The impact of HIV on follicular helper T cells could influence the accumulation of memory B cells in HIV-infected children.

A recombinant two-module form of human properdin is an inhibitor of the complement alternative pathway

Properdin upregulates the alternative complement pathway by binding and stabilising the C3 convertase complex (C3bBb). Properdin is a soluble glycoprotein and its flexible rod-like 53kDa monomers form cyclic polymers (dimers, trimers, tetramers and pentamers). The properdin monomer consists of seven thrombospondin type I repeats (TSR 0-6), which are similar and homologous to domains found in circumsporozoite and thrombospondin-related anonymous proteins of Plasmodium species, ETP100 of Eimeria tenella, various complement components C6-C9, and thrombospondin I and II. Using deletion constructs, TSR4 and TSR5 of human properdin were implicated in C3b binding and stabilising C3 convertase. However, individually expressed TSR4 or TSR5 failed to bind properdin ligands. Here, we have expressed and characterized biologically active TSR4 and TSR5 together (TSR4+5) in tandem in Escherichia coli, fused to maltose-binding protein. MBP-TSR4+5 bind solid-phase C3b, sulfatides and glycosaminoglycans. In addition, functionally active recombinant TSR4+5 modules inhibit the alternative pathway of complement.

Increased adhesion of Plasmodium falciparum infected erythrocytes to ICAM-1 in children with acute intestinal injury

BACKGROUND

Children with severe malaria are at increased risk of invasive bacterial disease particularly infection with enteric gram-negative organisms. These organisms are likely to originate from the gut, however, how and why they breach the intestinal interface in the context of malaria infection remains unclear. One explanation is that accumulation of infected red blood cells (iRBCs) in the intestinal microvasculature contributes to tissue damage and subsequent microbial translocation which can be addressed through investigation of the impact of cytoadhesion in patients with malaria and intestinal damage.

METHODS

Using a static adhesion assay, cytoadhesion of iRBCs was quantified in 48 children with malaria to recombinant proteins constitutively expressed on endothelial cell surfaces. Cytoadhesive phenotypes between children with and without biochemical evidence of intestinal damage [defined as endotoxemia or elevated plasma intestinal fatty acid binding protein (I-FABP)] was compared.

RESULTS

The majority of parasites demonstrated binding to the endothelial receptors CD36 and to a lesser extent to ICAM-1. Reduced adhesion to CD36 but not adhesion to ICAM-1 or rosetting was associated with malarial anaemia (p = 0.004). Increased adhesion of iRBCs to ICAM-1 in children who had evidence of elevated I-FABP (p = 0.022), a marker of intestinal ischaemia was observed. There was no correlation between the presence of endotoxemia and increased adhesion to any of the recombinant proteins.

CONCLUSION

Increased parasite adhesion to ICAM-1 in children with evidence of intestinal ischaemia lends further evidence to a link between the cytoadherence of iRBCs in gut microvasculature and intestinal damage.

A Glucuronoxylomannan-Associated Immune Signature, Characterized by Monocyte Deactivation and an Increased Interleukin 10 Level, Is a Predictor of Death in Cryptococcal Meningitis

Background. Cryptococcal meningitis remains a significant cause of death among human immunodeficiency virus type 1 (HIV)–infected persons in Africa. We aimed to better understand the pathogenesis and identify immune correlates of mortality, particularly the role of monocyte activation.

Methods. A prospective cohort study was conducted in Cape Town, South Africa. Patients with a first episode of cryptococcal meningitis were enrolled, and their immune responses were assessed in unstimulated and stimulated blood specimens, using flow cytometry and cytokine analysis.

Results. Sixty participants were enrolled (median CD4⁺ T-cell count, 34 cells/µL). Mortality was 23% (14 of 60 participants) at 14 days and 39% (22 of 57) at 12 weeks. Nonsurvivors were more likely to have an altered consciousness and higher cerebrospinal fluid fungal burden at presentation. Principal component analysis identified an immune signature associated with early mortality, characterized by monocyte deactivation (reduced HLA-DR expression and tumor necrosis factor α response to lipopolysaccharide); increased serum interleukin 6, CXCL10, and interleukin 10 levels; increased neutrophil counts; and decreased T-helper cell type 1 responses. This immune signature remained an independent predictor of early mortality after adjustment for consciousness level and fungal burden and was associated with higher serum titers of cryptococcal glucuronoxylomannan.

Conclusions. Cryptococcal-related mortality is associated with monocyte deactivation and an antiinflammatory blood immune signature, possibly due to Cryptococcus modulation of the host immune response. Validation in other cohorts is required.

Control of Viremia Enables Acquisition of Resting Memory B Cells with Age and Normalization of Activated B Cell Phenotypes in HIV-Infected Children

HIV affects the function of all lymphocyte populations, including B cells. Phenotypic and functional defects of B cells in HIV-infected adults have been well characterized, but defects in children have not been studied to the same extent. We determined the proportion of B cell subsets and frequencies of Ag-specific memory B cells in peripheral blood from HIV-infected children and healthy controls, using flow cytometry and B cell ELISPOT, respectively. In addition, we measured the quantities and avidities of plasma Abs against various Ags by ELISA. We also determined plasma levels of BAFF and expression of BAFF receptors on B cells. Children with high HIV viremia had increased proportions of activated mature B cells, tissue-like memory B cells and plasmablasts, and low proportions of naive B cells when compared with community controls and children with low HIV viremia, similar to adults infected with HIV. HIV-infected groups had lower proportions of resting memory B cells than did community controls. Notably, high HIV viremia prevented the age-dependent accumulation of class-switched resting memory B cells. HIV-infected children, regardless of the level of viremia, showed lower quantities and avidities of IgG and lower frequencies of memory B cells against Expanded Program on Immunization vaccines. The HIV-infected children had an altered BAFF profile that could have affected their B cell compartment. Therefore, B cell defects in HIV-infected children are similar to those seen in HIV-infected adults. However, control of HIV viremia is associated with normalization of activated B cell subsets and allows age-dependent accumulation of resting memory B cells.

10-valent pneumococcal non-typeable Haemophilus influenzae protein-D conjugate vaccine (PHiD-CV) induces memory B cell responses in healthy Kenyan toddlers

Memory B cells are long‐lived and could contribute to persistence of humoral immunity by maintaining the plasma‐cell pool or making recall responses upon re‐exposure to an antigen. We determined the ability of a pneumococcal conjugate vaccine to induce anti‐pneumococcal memory B cells. Frequencies of memory B cells against pneumococcal capsular polysaccharides from serotypes 1, 6B, 14, 19F and 23F were determined by cultured B cell enzyme‐linked immunospot (ELISPOT) in 35 children aged 12–23 months who received pneumococcal non‐typeable Haemophilus influenzae protein‐D conjugate vaccine (PHiD‐CV). The relationships between plasma antibodies and memory B cell frequencies were also assessed. After two doses of PHiD‐CV, the proportion of subjects with detectable memory B cells against pneumococcal capsular polysaccharides increased significantly for serotypes 1 (3–45%; P < 0·01), 19F (21–66%; P < 0·01) and 23F (13–36%; P = 0·02), but not serotypes 6B (24–42%; P = 0·24) and 14 (21–40%; P = 0·06). Correlations between antibodies and memory B cells were weak. Carriage of serotype 19F at enrolment was associated with poor memory B cell responses against this serotype at subsequent time‐points (day 30: non‐carriers, 82% versus carriers, 0%, P < 0·01; day 210: non‐carriers, 72% versus carriers, 33%, P = 0·07). PHiD‐CV is capable of inducing memory B cells against some of the component pneumococcal capsular polysaccharides.

Prime-boost vaccination with chimpanzee adenovirus and modified vaccinia Ankara encoding TRAP provides partial protection against Plasmodium falciparum infection in Kenyan adults

Protective immunity to the liver stage of the malaria parasite can be conferred by vaccine-induced T cells, but no subunit vaccination approach based on cellular immunity has shown efficacy in field studies. We randomly allocated 121 healthy adult male volunteers in Kilifi, Kenya, to vaccination with the recombinant viral vectors chimpanzee adenovirus 63 (ChAd63) and modified vaccinia Ankara (MVA), both encoding the malaria peptide sequence ME-TRAP (the multiple epitope string and thrombospondin-related adhesion protein), or to vaccination with rabies vaccine as a control. We gave antimalarials to clear parasitemia and conducted PCR (polymerase chain reaction) analysis on blood samples three times a week to identify infection with the malaria parasite Plasmodium falciparum. On Cox regression, vaccination reduced the risk of infection by 67% [95% confidence interval (CI), 33 to 83%; P = 0.002] during 8 weeks of monitoring. T cell responses to TRAP peptides 21 to 30 were significantly associated with protection (hazard ratio, 0.24; 95% CI, 0.08 to 0.75; P = 0.016).

HIV-exposed uninfected children: a growing population with a vulnerable immune system?

Through the successful implementation of policies to prevent mother-to-child-transmission (PMTCT) of HIV-1 infection, children born to HIV-1-infected mothers are now much less likely to acquire HIV-1 infection than previously. Nevertheless, HIV-1-exposed uninfected (HEU) children have substantially increased morbidity and mortality compared with children born to uninfected mothers (unexposed uninfected, UU), predominantly from infectious causes. Moreover, a range of phenotypical and functional immunological differences between HEU and UU children has been reported. As the number of HEU children continues to increase worldwide, two questions with clear public health importance need to be addressed: first, does exposure to HIV-1 and/or ART in utero or during infancy have direct immunological consequences, or are these poor outcomes simply attributable to the obvious disadvantages of being born into an HIV-affected household? Secondly, can we expect improved maternal care and ART regimens during and after pregnancy, together with optimized infant immunization schedules, to reduce the excess morbidity and mortality of HEU children?

CD4+ T cell responses to the Plasmodium falciparum erythrocyte membrane protein 1 in children with mild malaria

The immune response against the variant surface Ag Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a key component of clinical immunity against malaria. We have investigated the development and maintenance of CD4⁺ T cell responses to a small semiconserved area of the Duffy binding–like domain (DBL)α–domain of PfEMP1, the DBLα-tag. Young children were followed up longitudinally, and parasites and PBMCs were isolated from 35 patients presenting with an acute case of uncomplicated malaria. The DBLα-tag from the PfEMP1 dominantly expressed by the homologous parasite isolate was cloned and expressed as recombinant protein. The recombinant DBLα-tag was used to activate PBMCs collected from each acute episode and from an annual cross-sectional survey performed after the acute malaria episode. In this article, we report that CD4⁺ T cell responses to the homologous DBLα-tag were induced in 75% of the children at the time of the acute episode and in 62% of the children at the following cross-sectional survey on average 235 d later. Furthermore, children who had induced DBLα-tag–specific CD4⁺IL-4⁺ T cells at the acute episode remained episode free for longer than children who induced other types of CD4⁺ T cell responses. These results suggest that a wide range of DBLα-tag–specific CD4⁺ T cell responses were induced in children with mild malaria and, in the case of CD4⁺IL-4⁺ T cell responses, were associated with protection from clinical episodes.

Value of Plasmodium falciparum histidine-rich protein 2 level and malaria retinopathy in distinguishing cerebral malaria from other acute encephalopathies in Kenyan children

Background. The diagnosis of cerebral malaria is problematic in malaria-endemic areas because encephalopathy in patients with parasitemia may have another cause. Abnormal retinal findings are thought to increase the specificity of the diagnosis, and the level of histidine-rich protein 2 (HRP2) may reflect the parasite biomass.

Methods. We examined the retina and measured plasma HRP2 levels in children with acute nontraumatic encephalopathy in Kenya. Logistic regression, with HRP2 level as an independent variable and World Health Organization–defined cerebral malaria and/or retinopathy as the outcome, was used to calculate malaria-attributable fractions (MAFs) and retinopathy-attributable fractions (RAFs).

Results. Of 270 children, 140 (52%) had peripheral parasitemia, 80 (30%) had malaria retinopathy, and 164 (61%) had an HRP2 level of >0 U/mL. During 2006–2011, the incidence of HRP2 positivity among admitted children declined by 49 cases per 100 000 per year (a 78% reduction). An HRP2 level of >0 U/mL had a MAF of 93% for cerebral malaria, with a MAF of 97% observed for HRP2 levels of ≥10 U/mL (the level of the best combined sensitivity and specificity). HRP2 levels of >0 U/mL had a RAF of 77% for features of retinopathy combined, with the highest RAFs for macular whitening (99%), peripheral whitening (98%), and hemorrhages (90%).

Conclusion. HRP2 has a high attributable fraction for features of malarial retinopathy, supporting its use in the diagnosis of cerebral malaria. HRP2 thresholds improve the specificity of the definition.

Inflammatory Flt3l is essential to mobilize dendritic cells and for T cell responses during Plasmodium infection

Innate sensing mechanisms trigger a variety of humoral and cellular events that are essential to adaptive immune responses. Here we describe an innate sensing pathway triggered by Plasmodium infection that regulates dendritic cell homeostasis and adaptive immunity through Flt3 ligand (Flt3l) release. Plasmodium-induced Flt3l release in mice requires Toll-like receptor (TLR) activation and type I interferon (IFN) production. We found that type I IFN supports the upregulation of xanthine dehydrogenase, which metabolizes the xanthine accumulating in infected erythrocytes to uric acid. Uric acid crystals trigger mast cells to release soluble Flt3l from a pre-synthesized membrane-associated precursor. During infection, Flt3l preferentially stimulates expansion of the CD8-α(+) dendritic cell subset or its BDCA3(+) human dendritic cell equivalent and has a substantial impact on the magnitude of T cell activation, mostly in the CD8(+) compartment. Our findings highlight a new mechanism that regulates dendritic cell homeostasis and T cell responses to infection.

Safety and immunogenicity of heterologous prime-boost immunisation with Plasmodium falciparum malaria candidate vaccines, ChAd63 ME-TRAP and MVA ME-TRAP, in healthy Gambian and Kenyan adults

BACKGROUND

Heterologous prime boost immunization with chimpanzee adenovirus 63 (ChAd63) and Modified vaccinia Virus Ankara (MVA) vectored vaccines is a strategy recently shown to be capable of inducing strong cell mediated responses against several antigens from the malaria parasite. ChAd63-MVA expressing the Plasmodium falciparum pre-erythrocytic antigen ME-TRAP (multiple epitope string with thrombospondin-related adhesion protein) is a leading malaria vaccine candidate, capable of inducing sterile protection in malaria naïve adults following controlled human malaria infection (CHMI).

METHODOLOGY

We conducted two Phase Ib dose escalation clinical trials assessing the safety and immunogenicity of ChAd63-MVA ME-TRAP in 46 healthy malaria exposed adults in two African countries with similar malaria transmission patterns.

RESULTS

ChAd63-MVA ME-TRAP was shown to be safe and immunogenic, inducing high-level T cell responses (median >1300 SFU/million PBMC).

CONCLUSIONS

ChAd63-MVA ME-TRAP is a safe and highly immunogenic vaccine regimen in adults with prior exposure to malaria. Further clinical trials to assess safety and immunogenicity in children and infants and protective efficacy in the field are now warranted.

TRIAL REGISTRATION

Pactr.org PACTR2010020001771828 Pactr.org PACTR201008000221638 ClinicalTrials.gov NCT01373879 NCT01373879 ClinicalTrials.gov NCT01379430 NCT01379430.

Endotoxaemia is common in children with Plasmodium falciparum malaria

Background

Children presenting to hospital with recent or current Plasmodium falciparum malaria are at increased the risk of invasive bacterial disease, largely enteric gram-negative organisms (ENGO), which is associated with increased mortality and recurrent morbidity. Although incompletely understood, the most likely source of EGNO is the bowel. We hypothesised that as a result of impaired gut-barrier function endotoxin (lipopolysaccharide), present in the cell-wall of EGNO and in substantial quantities in the gut, is translocated into the bloodstream, and contributes to the pathophysiology of children with severe malaria.

Methods

We conducted a prospective study in 257 children presenting with malaria to two hospitals in Kenya and Uganda. We analysed the clinical presentation, endotoxin and cytokine concentration.

Results

Endotoxaemia (endotoxin activity ≥0.4 EAA Units) was observed in 71 (27.6%) children but its presence was independent of both disease severity and outcome. Endotoxaemia was more frequent in children with severe anaemia but not specifically associated with other complications of malaria. Endotoxaemia was associated with a depressed inflammatory and anti-inflammatory cytokine response. Plasma endotoxin levels in severe malaria negatively correlated with IL6, IL10 and TGFβ (Spearman rho: TNFα: r=−0.122, p=0.121; IL6: r=−0.330, p<0.0001; IL10: r=−0.461, p<0.0001; TGFβ: r=−0.173, p<0.027).

Conclusions

Endotoxaemia is common in malaria and results in temporary immune paralysis, similar to that observed in patients with sepsis and experimentally-induced endotoxaemia. Intense sequestration of P. falciparum-infected erythrocytes within the endothelial bed of the gut has been observed in pathological studies and may lead to gut-barrier dysfuction. The association of endotoxaemia with the anaemia phenotype implies that it may contribute to the dyserythropoesis accompanying malaria through inflammation. Both of these factors feasibly underpin the susceptibility to EGNO co-infection. Further research is required to investigate this initial finding, with a view to future treatment trials targeting mechanism and appropriate antimicrobial treatment.

The retinoblastoma protein (Rb) as an anti-apoptotic factor: expression of Rb is required for the anti-apoptotic function of BAG-1 protein in colorectal tumour cells

Although the retinoblastoma-susceptibility gene RB1 is inactivated in a wide range of human tumours, in colorectal cancer, the retinoblastoma protein (Rb) function is often preserved and the RB locus even amplified. Importantly, we have previously shown that Rb interacts with the anti-apoptotic Bcl-2 associated athanogene 1 (BAG-1) protein, which is highly expressed in colorectal carcinogenesis. Here we show for the first time that Rb expression is critical for BAG-1 anti-apoptotic activity in colorectal tumour cells. We demonstrate that Rb expression not only increases the nuclear localisation of the anti-apoptotic BAG-1 protein, but that expression of Rb is required for inhibition of apoptosis by BAG-1 both in a γ-irradiated Saos-2 osteosarcoma cell line and colorectal adenoma and carcinoma cell lines. Further, consistent with the fact that nuclear BAG-1 has previously been shown to promote cell survival through increasing nuclear factor (NF)-κB activity, we demonstrate that the ability of BAG-1 to promote NF-κB activity is significantly inhibited by repression of Rb expression. Taken together, data presented suggest a novel function for Rb, promoting cell survival through regulating the function of BAG-1. As BAG-1 is highly expressed in the majority of colorectal tumours, targeting the Rb–BAG-1 complex to promote apoptosis has exciting potential for future therapeutic development.

T-cell responses to the DBLα-tag, a short semi-conserved region of the Plasmodium falciparum membrane erythrocyte protein 1

The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a variant surface antigen expressed on mature forms of infected erythrocytes. It is considered an important target of naturally acquired immunity. Despite its extreme sequence heterogeneity, variants of PfEMP1 can be stratified into distinct groups. Group A PfEMP1 have been independently associated with low host immunity and severe disease in several studies and are now of potential interest as vaccine candidates. Although antigen-specific antibodies are considered the main effector mechanism in immunity to malaria, the induction of efficient and long-lasting antibody responses requires CD4+ T-cell help. To date, very little is known about CD4+ T-cell responses to PfEMP1 expressed on clinical isolates. The DBLα-tag is a small region from the DBLα-domain of PfEMP1 that can be amplified with universal primers and is accessible in clinical parasite isolates. We identified the dominant expressed PfEMP1 in 41 individual clinical parasite isolates and expressed the corresponding DBLα-tag as recombinant antigen. Individual DBLα-tags were then used to activate CD4+ T-cells from acute and convalescent blood samples in children who were infected with the respective clinical parasite isolate. Here we show that CD4+ T-cell responses to the homologous DBLα-tag were induced in almost all children during acute malaria and maintained in some for 4 months. Children infected with parasites that dominantly expressed group A-like PfEMP1 were more likely to maintain antigen-specific IFNγ-producing CD4+ T-cells than children infected with parasites dominantly expressing other PfEMP1. These results suggest that group A-like PfEMP1 may induce long-lasting effector memory T-cells that might be able to provide rapid help to variant-specific B cells. Furthermore, a number of children induced CD4+ T-cell responses to heterologous DBLα-tags, suggesting that CD4+ T-cells may recognise shared epitopes between several DBLα-tags.

Human complement Factor H modulates C1q-mediated phagocytosis of apoptotic cells

Complement is implicated in the clearance of apoptotic cells by phagocytes. Deficiencies in early complement components, particularly C1q, are associated with an increased risk of the development of systemic lupus erythematosus. C1q is considered to be important in this process through interaction with apoptotic cells and phagocytes. In the present study, we confirm that apoptotic cells are recognized not only by C1q but also by the complement regulatory protein Factor H. Both C1q and Factor H bind to apoptotic cells in a dose-dependent and saturable manner. We further examined the role of C1q and Factor H in the clearance of apoptotic cells by monocytes. C1q enhanced uptake/adhesion of apoptotic cells by monocytes whereas Factor H alone had no effect on this process. However, when both C1q and Factor H were present on the apoptotic cell surface, C1q-mediated enhancement of uptake/adhesion of the apoptotic cells by monocytes was reduced. This effect of Factor H also occurred if monocytes were pre-treated with Factor H, and then exposed to C1q-coated apoptotic cells. The results were consistent with Factor H interacting with monocytes through the integrin CD11b/CD18. We conclude that under physiological conditions, Factor H may be important in controlling the inflammation which might arise from C1q deposition on apoptotic cells.

The plasma concentration of the B cell activating factor is increased in children with acute malaria

Malaria-specific antibody responses in children often appear to be short-lived but the mechanisms underlying this phenomenon are not well understood. In this study, we investigated the relationship between the B-cell activating factor (BAFF) and its receptors expressed on B cells with antibody responses during and after acute malaria in children. Our results demonstrate that BAFF plasma levels increased during acute malarial disease and reflected disease severity. The expression profiles for BAFF receptors on B cells agreed with rapid activation and differentiation of a proportion of B cells to plasma cells. However, BAFF receptor (BAFF-R) expression was reduced on all peripheral blood B cells during acute infection, but those children with the highest level of BAFF-R expression on B cells maintained schizont-specific immunoglobin G (IgG) over a period of 4 months, indicating that dysregulation of BAFF-R expression on B cells may contribute to short-lived antibody responses to malarial antigens in children. In summary, this study suggests a potential role for BAFF during malaria disease, both as a marker for disease severity and in shaping the differentiation pattern of antigen-specific B cells.

Plasmodium falciparum-infected erythrocytes and beta-hematin induce partial maturation of human dendritic cells and increase their migratory ability in response to lymphoid chemokines

Acute and chronic Plasmodium falciparum infections alter the immune competence of the host possibly through changes in dendritic cell (DC) functionality. DCs are the most potent activators of T cells, and migration is integral to their function. Mature DCs express lymphoid chemokine receptors (CCRs), expression of which enables them to migrate to the lymph nodes, where they encounter naïve T cells. The present study aimed to investigate the impact of the synthetic analog to malaria parasite pigment hemozoin, i.e., β-hematin, or infected erythrocytes (iRBCs) on the activation status of human monocyte-derived DCs and on their expression of CCRs. Human monocyte-derived DCs partially matured upon incubation with β-hematin as indicated by an increased expression of CD80 and CD83. Both β-hematin and iRBCs provoked the release of proinflammatory and anti-inflammatory cytokines, such as interleukin-6 (IL-6), IL-10, and tumor necrosis factor alpha, but not IL-12, and induced upregulation of the lymphoid chemokine receptor CXCR4, which was coupled to an increased migration to lymphoid ligands. Taken together, these results suggest that the partial and transient maturation of human myeloid DCs upon stimulation with malaria parasite-derived products and the increased IL-10 but lack of IL-12 secretion may lead to suboptimal activation of T cells. This may in turn lead to impaired adaptive immune responses and therefore insufficient clearance of the parasites.

Specific receptor usage in Plasmodium falciparum cytoadherence is associated with disease outcome

Our understanding of the basis of severe disease in malaria is incomplete. It is clear that pathology is in part related to the pro-inflammatory nature of the host response but a number of other factors are also thought to be involved, including the interaction between infected erythrocytes and endothelium. This is a complex system involving several host receptors and a major parasite-derived variant antigen (PfEMP1) expressed on the surface of the infected erythrocyte membrane. Previous studies have suggested a role for ICAM-1 in the pathology of cerebral malaria, although these have been inconclusive. In this study we have examined the cytoadherence patterns of 101 patient isolates from varying clinical syndromes to CD36 and ICAM-1, and have used variant ICAM-1 proteins to further characterise this adhesive phenotype. Our results show that increased binding to CD36 is associated with uncomplicated malaria while ICAM-1 adhesion is raised in parasites from cerebral malaria cases.

Distinct kinetics of memory B-cell and plasma-cell responses in peripheral blood following a blood-stage Plasmodium chabaudi infection in mice

B cell and plasma cell responses take place in lymphoid organs, but because of the inaccessibility of these organs, analyses of human responses are largely performed using peripheral blood mononuclear cells (PBMC). To determine whether PBMC are a useful source of memory B cells and plasma cells in malaria, and whether they reflect Plasmodium-specific B cell responses in spleen or bone marrow, we have investigated these components of the humoral response in PBMC using a model of Plasmodium chabaudi blood-stage infections in C57BL/6 mice. We detected memory B cells, defined as isotype-switched IgD⁻ IgM⁻ CD19⁺ B cells, and low numbers of Plasmodium chabaudi Merozoite Surface Protein-1 (MSP1)-specific memory B cells, in PBMC at all time points sampled for up to 90 days following primary or secondary infection. By contrast, we only detected CD138⁺ plasma cells and MSP1-specific antibody-secreting cells within a narrow time frame following primary (days 10 to 25) or secondary (day 10) infection. CD138⁺ plasma cells in PBMC at these times expressed CD19, B220 and MHC class II, suggesting that they were not dislodged bone-marrow long-lived plasma cells, but newly differentiated migratory plasmablasts migrating to the bone marrow; thus reflective of an ongoing or developing immune response. Our data indicates that PBMC can be a useful source for malaria-specific memory B cells and plasma cells, but extrapolation of the results to human malaria infections suggests that timing of sampling, particularly for plasma cells, may be critical. Studies should therefore include multiple sampling points, and at times of infection/immunisation when the B-cell phenotypes of interest are likely to be found in peripheral blood.

Copy number, linkage disequilibrium and disease association in the FCGR locus

The response of a leukocyte to immune complexes (ICs) is modulated by receptors for the Fc region of IgG (FcγRs), and alterations in their affinity or function have been associated with risk of autoimmune diseases, including systemic lupus erythematosus (SLE). The low-affinity FcγR genomic locus is complex, containing regions of copy number variation (CNV) which can alter receptor expression and leukocyte responses to IgG. Combined paralogue ratio tests (PRTs) were used to distinguish three intervals within the FCGR locus which undergo CNV, and to determine FCGR gene copy number (CN). There were significant differences in FCGR3B and FCGR3A CNV profiles between Caucasian, East Asian and Kenyan populations. A previously noted association of low FCGR3B CN with SLE in Caucasians was supported [OR = 1.57 (1.08–2.27), P = 0.018], and replicated in Chinese [OR = 1.65 (1.25–2.18), P = 4 × 10⁻⁴]. There was no association of FCGR3B CNV with vasculitis, nor with malarial or bacterial infection. Linkage disequilibrium (LD) between multi-allelic FCGR3B CNV and SLE-associated SNPs in the FCGR locus was defined for the first time. Despite LD between FCGR3B CNV and a variant in FcγRIIB (I232T) which abolishes inhibitory function, both reduced CN of FCGR3B and homozygosity of the FcγRIIB-232T allele were individually strongly associated with SLE risk. Thus CN of FCGR3B, which controls IC responses and uptake by neutrophils, and variations in FCGR2B, which controls factors such as antibody production and macrophage activation, are important in SLE pathogenesis. Further interpretations of contributions to pathogenesis by FcγRs must be made in the context of LD involving CNV regions.

Multiple functions of human T cells generated by experimental malaria challenge

Protective immunity generated following malaria infection may be comprised of Ab or T cells against malaria Ag of different stages; however, the short-lived immunity that is observed suggests deficiency in immune memory or regulatory activity. In this study, cellular immune responses were investigated in individuals receiving Plasmodium falciparum sporozoite challenge by the natural (mosquito bite) route as part of a malaria vaccine efficacy trial. Parasitemia, monitored by blood film microscopy and PCR, was subsequently cleared with drugs. All individuals demonstrated stable IFN-gamma, IL-2 and IL-4 ex vivo ELISPOT effector responses against P. falciparum-infected RBC (iRBC) Ag, 28 and 90 days after challenge. However, infected RBC-specific central memory responses, as measured by IFN-gamma cultured ELISPOT, were low and unstable over time, despite CD4(+) T cells being highly proliferative by CFSE dilution, and showed an inverse relationship to parasite density. In support of the observation of poor memory, co-culture experiments showed reduced responses to common recall Ag, indicating malaria-specific regulatory activity. This activity could not be accounted for by the expression of IL-10, TGF-beta, FOXP3 or CTLA-4, but proliferating T cells expressed high levels of CD95, indicating a pro-apoptotic phenotype. Lastly, there was an inverse relationship between FOXP3 expression, when measured 10 days after challenge, and ex vivo IFN-gamma measured more than 100 days later. This study shows that malaria infection elicits specific Th1 and Th2 effector cells, but concomitant weak central memory and regulatory activity, which may help to explain the short-lived immunity observed.

Dendritic cells in Plasmodium infection

Infection with Plasmodium parasites (malaria) contributes greatly to morbidity and mortality in affected areas. Interaction of the protozoan with the immune system has a critical role in the pathogenesis of the disease, but may also hold a key to containing parasite numbers through specific immune responses, which vaccine development aims to harness. A central player in the generation of such immune responses is the dendritic cell. However, Plasmodium parasites appear to have profound activating and suppressing effects on dendritic cell function, which may enhance immunopathology or facilitate the parasite's survival by depressing beneficial immunity. Furthermore, immune responses to other infections and vaccines may be impaired. A greater understanding of the effects of the parasite on dendritic cells will contribute to insight and potential defeat of this infectious disease.

Characterization of a Plasmodium falciparum macrophage-migration inhibitory factor homologue

BACKGROUND

Macrophage-migration inhibitory factor (MIF), one of the first cytokines described, has a broad range of proinflammatory properties. The genome sequencing project of Plasmodium falciparum identified a parasite homologue of MIF. The protein is expressed during the asexual blood stages of the parasite life cycle that cause malarial disease. The identification of a parasite homologue of MIF raised the question of whether it affects monocyte function in a manner similar to its human counterpart.

METHODS

Recombinant P. falciparum MIF (PfMIF) was generated and used in vitro to assess its influence on monocyte function. Antibodies generated against PfMIF were used to determine the expression profile and localization of the protein in blood-stage parasites. Antibody responses to PfMIF were determined in Kenyan children with acute malaria and in control subjects.

RESULTS

PfMIF protein was expressed in asexual blood-stage parasites, localized to the Maurer's cleft. In vitro treatment of monocytes with PfMIF inhibited random migration and reduced the surface expression of Toll-like receptor (TLR) 2, TLR4, and CD86.

CONCLUSIONS

These results indicate that PfMIF is released during blood-stage malaria and potentially modulates the function of monocytes during acute P. falciparum infection.

The effect of Plasmodium falciparum infection on expression of monocyte surface molecules

Plasmodium falciparum infection may result in severe malaria in susceptible individuals. The pathogenesis of severe disease is probably a combination of the sequestration of infected erythrocytes and overstimulation of the immune response. Monocytes are a key source of many of the pro-inflammatory agents implicated but also are found sequestered in blood vessels. However, little is known about the monocyte phenotype in malaria disease. Flow cytometry was performed on fresh whole blood to determine surface expression of four receptors during acute severe and non-severe malaria and again during convalescence when uninfected. Three hundred and fifty-six children with P. falciparum infection were studied and were found to show increased expression of intercellular adhesion molecule-1 (ICAM-1), urokinase plasminogen activator receptor (uPAR), CD23 and chemokine receptor 5 (CCR5) (P<0.001) during acute disease compared with convalescent levels. Using multivariate analysis, it was found that large increases in expression of ICAM-1 (odds ratio (OR) 2.44, 95% CI 1.80-3.32) and uPAR (OR 3.14, 95% CI 1.93-5.09) but small increases in expression of CD23 (OR 0.82, 95% CI 0.68-0.96) were independently associated with severe malaria. These results give an insight into the cellular processes occurring in severe malaria and suggest that pathology is based on a complex repertoire of pro- and anti-inflammatory processes.

The frequency of BDCA3-positive dendritic cells is increased in the peripheral circulation of Kenyan children with severe malaria

The ability of Plasmodium falciparum-infected erythrocytes to adhere to host endothelial cells via receptor molecules such as ICAM-1 and CD36 is considered a hallmark for the development of severe malaria syndromes. These molecules are also expressed on leukocytes such as dendritic cells. Dendritic cells are antigen-presenting cells that are crucial for the initiation of adaptive immune responses. In many human diseases, their frequency and function is perturbed. We analyzed the frequency of peripheral blood dendritic cell subsets and the plasma concentrations of interleukin-10 (IL-10) and IL-12 in Kenyan children with severe malaria and during convalescence and related these parameters to the adhesion phenotype of the acute parasite isolates. The frequency of CD1c(+) dendritic cells in children with acute malaria was comparable to that in healthy controls, but the frequency of BDCA3(+) dendritic cells was significantly increased. Analysis of the adhesion phenotypes of parasite isolates revealed that adhesion to ICAM-1 was associated with the frequency of peripheral blood CD1c(+) dendritic cells, whereas the adhesion of infected erythrocytes to CD36 correlated with high concentrations of IL-10 and low concentrations of IL-12 in plasma.

Malaria pigment paralyzes dendritic cells

The capacity of malarial infection to suppress the patient's immune responses both to the parasite and to other antigens has long puzzled researchers. A prime suspect, the parasite-produced pigment hemozoin, has now been clearly shown to mediate immunosuppression by inhibiting dendritic cell activity.

Frequencies of peripheral blood myeloid cells in healthy Kenyan children with alpha+ thalassemia and the sickle cell trait

The high frequencies of both alpha+ thalassemia and the sickle cell trait (hemoglobin AS [HbAS]) found in many tropical populations are thought to reflect selection pressure from Plasmodium falciparum malaria. For HbAS, but not for alpha+ thalassemia, protection appears to be mediated by the enhanced phagocytic clearance of ring-infected erythrocytes. We have investigated the genotype-specific distributions of peripheral blood leukocyte populations in two groups of children living on the coast of Kenya: a group of healthy P. falciparum parasite-negative children sampled at cross-sectional survey during a period of low malaria transmission, and a group of children attending the hospital with acute malaria. We report distinctive distributions of peripheral blood myeloid dendritic cells and monocytes in children with alpha+ thalassemia and HbAS during healthy periods and disease, and suggest ways in which these might relate to the mechanisms of protection afforded by these conditions.

Antigen presentation and dendritic cell biology in malaria

Dendritic cells (DCs) are important both in amplifying the innate immune response and in initiating adaptive immunity and shaping the type of T helper (Th) response. Although the role of DCs in immune responses to many intracellular pathogens has been delineated and research is underway to identify the mechanisms involved, relatively little is known concerning the role of DCs in immunity to malaria. In this review, we provide an overview and summary of previous and current studies aimed to investigate the role of DCs as antigen presenting cells (APCs). In addition, the role of DCs in inducing innate and adaptive immunity to blood-stage malaria is discussed and, where information is available, the mechanisms involved are presented. Data from studies in humans infected with Plasmodium falciparum, the major human parasite responsible for the high morbidity and mortality associated with malaria throughout many regions of the developing world, as well as data from experimental mouse models are presented. Overall, the data from these studies are conflicting. The possible reasons for these differences, including the use of different parasite species and parasite strains in the mouse studies, are discussed. Nevertheless, together the data have important implications for development of an effective malaria vaccine since the selection of appropriate Plasmodium antigens and/or adjuvants, targeting innate immune responses involving DCs, may provide optimal protection against malaria. It is hoped that this review promotes more investigation among malariologists and immunologists alike on DCs and malaria.

Early interactions between blood-stage plasmodium parasites and the immune system

Accumulating evidence provides strong support for the importance of innate immunity in shaping the subsequent adaptive immune response to blood-stage Plasmodium parasites, the causative agents of malaria. Early interactions between blood-stage parasites and cells of the innate immune system, including dendritic cells, monocytes/macrophages, natural killer (NK) cells, NKT cells, and gamma6 T cells, are important in the timely control of parasite replication and in the subsequent elimination and resolution of the infection. The major role of innate immunity appears to be the production of immunoregulatory cytokines, such as interleukin (IL)-12 and interferon (IFN)-gamma, which are critical for the development of type 1 immune responses involving CD4+ Thl cells, B cells, and effector cells which mediate cell-mediated and antibody-dependent adaptive immune responses. In addition, it is likely that cells of the innate immune system, especially dendritic cells, serve as antigen-presenting cells. Here, we review recent data from rodent models of blood-stage malaria and from human studies, and outline the early interactions of infected red blood cells with the innate immune system. We compare and contrast the results derived from studies in infected laboratory mice and humans. These host species are sufficiently different with respect to the identity of the infecting Plasmodium species, the resulting pathologies, and immune responses, particularly where the innate immune response is concerned. The implications of these findings for the development of an effective and safe malaria vaccine are also discussed.

Regulation of immune response by Plasmodium-infected red blood cells

During the asexual blood stage infection of the human malaria parasite, Plasmodium falciparum, parasite-derived proteins are inserted onto the surface of the host red blood cell membrane. These proteins are highly variable and were originally thought only to mediate antigenic variation, and sequestration of parasites from peripheral circulation, thus enabling immune evasion. Recent studies have revealed that PfEMP-1 and other molecules on the P. falciparum-infected red blood cell (PfRBC) activate and modulate the immune response. In this review, we discuss how PfRBCs interact with antigen-presenting cells (APCs) and other cells of the immune system, and how such interactions could modulate the host response to Plasmodium infections.