CD4+ CD25+ Foxp3+ regulatory T cells, dendritic cells, and circulating cytokines in uncomplicated malaria: do different parasite species elicit similar host responses?

Effects of co-infection with Clonorchis sinensis on T cell exhaustion levels in patients with chronic hepatitis B

To investigate the effects of co-infection with Clonorchis sinensis (C. sinensis) on T cell exhaustion levels in patients with chronic hepatitis B, we enrolled clinical cases in this study, including the patients with concomitant C. sinensis and HBV infection. In this study, we detected inhibitory receptors and cytokine expression in circulating CD4+ and CD8+ T cells by flow cytometry. PD-1 and TIM-3 expression levels were significantly higher on CD4+ T and CD8+ T cells from co-infected patients than on those from the HBV patients. In addition, CD4+ T cells and CD8+ T cells function were significantly inhibited by C. sinensis and HBV co-infection compared with HBV single infection, secreting lower levels of Interferon gamma (IFN-γ), Interleukin-2 (IL-2), and TNF-α. Our current results suggested that C. sinensis co-infection could exacerbate T cell exhaustion in patients with chronic hepatitis B. PD-1 and TIM-3 could be novel biomarkers for T cell exhaustion in patients with Clonorchis sinensis and chronic hepatitis B co-infection. Furthermore, it may be one possible reason for the weaker response to antiviral therapies and the chronicity of HBV infection in co-infected patients. We must realize the importance of C. sinensis treatment for HBV-infected patients. It might provide useful information for clinical doctors to choose the right treatment plans.

The effect of HLA-DP gene polymorphisms in Plasmodium Vivax-induced malaria susceptibility

Plasmodium vivax is the second most common Plasmodium parasite causing clinically serious symptoms and death from malaria. It is an important cause of morbidity and mortality, especially in Asia, the Middle East, and South America. Human leukocyte antigen molecules are responsible for presenting foreign antigens to T cells. Polymorphisms in HLA genes affect antigen presentation. HLA alleles involved in the presentation of P. vivax antigens affect the antibody response. The present study aimed to reveal the relationship of rs3077 and rs9277535 polymorphisms in HLA-DP genes with malaria caused by P. vivax for the first time in the worldwide. In the present research, rs3077 and rs9277535 polymorphisms were investigated in a case-control study of 124 patients with P. vivax-induced malaria and 211 healthy persons by using a real-time polymerase chain reaction (RT-PCR). The results showed that the G alleles of rs3077 and rs9277535 polymorphisms were detected as protective alleles, while the A alleles of both polymorphisms increase the risk of susceptibility to malaria disease. The results of the present study showed that both polymorphisms have a major effect on the susceptibility to malaria caused by P. vivax. We recommend that this study should be conducted in a different population with a larger sample size to confirm our results.

Plasmodium vivax infection alters the peripheral immunoregulatory network of CD4 T follicular cells and B cells

Regulatory and effector cell responses to Plasmodium vivax, the most common human malaria parasite outside Africa, remain understudied in naturally infected populations. Here, we describe peripheral CD4+ T- and B-cell populations during and shortly after an uncomplicated P. vivax infection in 38 continuously exposed adult Amazonians. Consistent with previous observations, we found an increased frequency in CD4+ CD45RA- CD25+ FoxP3+ T regulatory cells that express the inhibitory molecule CTLA-4 during the acute infection, with a sustained expansion of CD21- CD27- atypical memory cells within the CD19+ B-cell compartment. Both Th1- and Th2-type subsets of CXCR5+ ICOShi PD-1+ circulating T follicular helper (cTfh) cells, which are thought to contribute to antibody production, were induced during P. vivax infection, with a positive correlation between overall cTfh cell frequency and IgG antibody titers to the P. vivax blood-stage antigen MSP119 . We identified significant changes in cell populations that had not been described in human malaria, such as an increased frequency of CTLA-4+ T follicular regulatory cells that antagonize Tfh cells, and a decreased frequency of circulating CD24hi CD27+ B regulatory cells in response to acute infection. In conclusion, we disclose a complex immunoregulatory network that is critical to understand how naturally acquired immunity develops in P. vivax malaria.

Identification and Characterization of P0 Protein as a Vaccine Candidate Against Babesia divergens, Blood Parasite of Veterinary and Zoonotic Importance

The molecular identification and antigenic characterization of P0 protein in Babesia divergens, a blood parasite of veterinary and zoonotic importance, were carried out in this study for use in developing subunit vaccines against B. divergens infection. Recombinant protein encoding P0 (BdP0) was developed in Escherichia coli, and its antiserum was generated in mice for further molecular characterization. Anti-rBdP0 serum had a specific interaction with the corresponding legitimate B. divergens protein, as confirmed by Western blotting and indirect fluorescent antibody tests. ELISA was used to assess the immunogenicity of BdP0 in a group of 68 bovine field samples, and significant immunological reactivity was found in 19 and 20 positive samples of rBdp0 and B. divergens lysate, respectively. The in vitro growth of B. divergens cultures treated with anti-rBdP0 serum was significantly inhibited (p < 0.05). Furthermore, after 6 h of incubation with 2 mg/ml anti-rBdP0 serum, the ability of pre-incubated free merozoites to invade bovine erythrocytes was reduced by 59.88%. The obtained data suggest the possible use of rBdP0 as diagnostic antigen and may serve as a vaccine candidate against babesiosis caused by B. divergens either in animal or human.

Immunosuppression in Malaria: Do Plasmodium falciparum Parasites Hijack the Host?

Malaria reflects not only a state of immune activation, but also a state of general immune defect or immunosuppression, of complex etiology that can last longer than the actual episode. Inhabitants of malaria-endemic regions with lifelong exposure to the parasite show an exhausted or immune regulatory profile compared to non- or minimally exposed subjects. Several studies and experiments to identify and characterize the cause of this malaria-related immunosuppression have shown that malaria suppresses humoral and cellular responses to both homologous (Plasmodium) and heterologous antigens (e.g., vaccines). However, neither the underlying mechanisms nor the relative involvement of different types of immune cells in immunosuppression during malaria is well understood. Moreover, the implication of the parasite during the different stages of the modulation of immunity has not been addressed in detail. There is growing evidence of a role of immune regulators and cellular components in malaria that may lead to immunosuppression that needs further research. In this review, we summarize the current evidence on how malaria parasites may directly and indirectly induce immunosuppression and investigate the potential role of specific cell types, effector molecules and other immunoregulatory factors.

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.

Dendritic cell responses to Plasmodium falciparum in a malaria-endemic setting

Background

Plasmodium falciparum causes the majority of malaria cases worldwide and children in sub-Saharan Africa are the most vulnerable group affected. Non-sterile clinical immunity that protects from symptoms develops slowly and is relatively short-lived. Moreover, current malaria vaccine candidates fail to induce durable high-level protection in endemic settings, possibly due to the immunomodulatory effects of the malaria parasite itself. Because dendritic cells play a crucial role in initiating immune responses, the aim of this study was to better understand the impact of cumulative malaria exposure as well as concurrent P. falciparum infection on dendritic cell phenotype and function.

Methods

In this cross-sectional study, the phenotype and function of dendritic cells freshly isolated from peripheral blood samples of Malian adults with a lifelong history of malaria exposure who were either uninfected (n = 27) or asymptomatically infected with P. falciparum (n = 8) was assessed. Additionally, plasma cytokine and chemokine levels were measured in these adults and in Malian children (n = 19) with acute symptomatic malaria.

Results

With the exception of lower plasmacytoid dendritic cell frequencies in asymptomatically infected Malian adults, peripheral blood dendritic cell subset frequencies and HLA-DR surface expression did not differ by infection status. Peripheral blood myeloid dendritic cells of uninfected Malian adults responded to in vitro stimulation with P. falciparum blood-stage parasites by up-regulating the costimulatory molecules HLA-DR, CD80, CD86 and CD40 and secreting IL-10, CXCL9 and CXCL10. In contrast, myeloid dendritic cells of asymptomatically infected Malian adults exhibited no significant responses above the uninfected red blood cell control. IL-10 and CXCL9 plasma levels were elevated in both asymptomatic adults and children with acute malaria.

Conclusions

The findings of this study indicate that myeloid dendritic cells of uninfected adults with a lifelong history of malaria exposure are able to up-regulate co-stimulatory molecules and produce cytokines. Whether mDCs of malaria-exposed individuals are efficient antigen-presenting cells capable of mounting an appropriate immune response remains to be determined. The data also highlights IL-10 and CXCL9 as important factors in both asymptomatic and acute malaria and add to the understanding of asymptomatic P. falciparum infections in malaria-endemic areas.

Reduction in total leukocytes in malaria patients compared to febrile controls: A systematic review and meta-analysis

BACKGROUND

Leukocyte alterations are a common hematological alteration among malaria patients.

OBJECTIVES

This systematic review and meta-analysis aimed to provide data and evidence comparing alterations in total leukocyte counts in malaria patients compared to febrile/healthy subjects at baseline before treatment. A systematic review was conducted by following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement for reporting systematic reviews and meta-analyses.

DATA SOURCES

Web of Science (ISI), Scopus, and Medline.

STUDY ELIGIBILITY CRITERIA, PARTICIPANTS, AND INTERVENTIONS

All published articles reporting a total leukocyte count of patients infected with malaria, non-malaria (febrile or healthy group) at baseline before treatment before August 27, 2019, were retrieved, and data were extracted by two main reviewers independently.

STUDY APPRAISAL AND SYNTHESIS METHODS

We used a forest plot, heterogeneity test (Cochran's Q), and the degree of heterogeneity (I2) to test whether the included studies were heterogeneous. The quality of the included studies was determined by a quality assessment guide based on the quality assessment tool developed by the Newcastle-Ottawa Scale (NOS). Cochran's Q (Chi-square) and Moran's I2 were used to evaluate heterogeneity. Meta-regression using STATA software was conducted to find the source of heterogeneity. A funnel plot with Egger's test was used to examine the significance of publication bias among the included studies. The mean differences were estimated using a random-effects model.

RESULTS

Out of the 2,261 articles screened, 29 articles were included in this systematic review and meta-analysis. The heterogeneity test indicated that there was heterogeneity among the included studies with no publication bias. The meta-analysis demonstrated that the total leukocyte count was significantly lower in patients with malaria (n = 4,619) than in those without malaria (n = 10,056) (Z = 4.0, P-value < 0.00001, mean difference = -1.38, 95% CI = -2.06-(-0.71)). Leukocyte differential alterations, low lymphocyte counts (P-value <0.0001, mean difference = -1.03, 95% CI = -1.53-(-0.53)) and a high NL ratio were found in the malaria group (n = 1,579) compared to the non-malaria group (n = 4,991) (P-value <0.0001, mean difference = 0.6, 95% CI = 0.32-0.88). The subgroup analysis indicated that there was a significantly lower total leukocyte count in the malaria group (n = 3,545) than in the febrile group (n = 8,947) (Z = 1.33, P-value < 0.0001, mean difference = -1.76, 95% CI = -2.56-(-0.96)), but no significant difference was found between the malaria group (n = 1,232) and the healthy group (n = 1,679) (P-value > 0.05).

LIMITATIONS

As the specific diagnoses in the febrile groups were not reported in the included studies so that the results of the present study need to be carefully interpreted.

CONCLUSIONS AND IMPLICATIONS OF KEY FINDINGS

This systematic review demonstrated that the total leukocyte count was affected by malarial infection at baseline despite the heterogeneity of the included studies. Future work must aim to understand the treatment-related total leukocyte reduction during follow-up or post-treatment outcomes in malaria-endemic settings.

Kynurenine elevation correlates with T regulatory cells increase in acute Plasmodium vivax infection: A pilot study

BACKGROUND

Disease-tolerance mechanisms limit infection severity by preventing tissue damage; however, the underlying mechanisms in human malaria are still unclear. Tryptophan (TRP), an essential amino acid, is catabolized into tolerogenic metabolites, kynurenines (KYN), by indoleamine 2,3-dioxygenase 1 (IDO1), which can induce Foxp3+ T regulatory cells (Tregs). In this study, we evaluated the relationship of these metabolites with Treg-mediated tolerance induction in acute malaria infections.

METHODS

We performed a cross-sectional study that evaluated asymptomatic, symptomatic malaria patients and endemic control patient groups. We assessed plasmatic concentration of cytokines by ELISA. Plasmatic TRP and KYN levels were measured by HPLC. Peripheral T regulatory cells were measured and phenotyped by flow cytometry.

RESULTS

The KYN/TRP ratio was significantly elevated in asymptomatic and symptomatic Plasmodium infection, compared to healthy controls. Also, Th1 and Th2 cytokines were elevated in the acute phase of malaria disease. IFN-γ increase in acute phase was positively correlated with the KYN/TRP ratio and KYN elevation was positively correlated with the increase of peripheral FoxP3+ T regulatory cells.

CONCLUSIONS

Additional studies are needed not only to identify innate mechanisms that increase tryptophan catabolism but also the role of Tregs in controlling malaria-induced pathology and malaria tolerance by the host.

The immunology of Plasmodium vivax malaria

Plasmodium vivax infection, the predominant cause of malaria in Asia and Latin America, affects ~14 million individuals annually, with considerable adverse effects on wellbeing and socioeconomic development. A clinical hallmark of Plasmodium infection, the paroxysm, is driven by pyrogenic cytokines produced during the immune response. Here, we review studies on the role of specific immune cell types, cognate innate immune receptors, and inflammatory cytokines on parasite control and disease symptoms. This review also summarizes studies on recurrent infections in individuals living in endemic regions as well as asymptomatic infections, a serious barrier to eliminating this disease. We propose potential mechanisms behind these repeated and subclinical infections, such as poor induction of immunological memory cells and inefficient T effector cells. We address the role of antibody-mediated resistance to P. vivax infection and discuss current progress in vaccine development. Finally, we review immunoregulatory mechanisms, such as inhibitory receptors, T regulatory cells, and the anti-inflammatory cytokine, IL-10, that antagonizes both innate and acquired immune responses, interfering with the development of protective immunity and parasite clearance. These studies provide new insights for the clinical management of symptomatic as well as asymptomatic individuals and the development of an efficacious vaccine for vivax malaria.

Monocyte-derived dendritic cells in malaria

The pathogenesis of malaria is a multifactorial syndrome associated with a deleterious inflammatory response that is responsible for many of the clinical manifestations. While dendritic cells (DCs) play a critical role in initiating acquired immunity and host resistance to infection, they also play a pathogenic role in inflammatory diseases. In our recent studies, we found in different rodent malaria models that the monocyte-derived DCs (MO-DCs) become, transiently, a main DC population in spleens and inflamed non-lymphoid organs. These studies suggest that acute infection with Plasmodium berghei promotes the differentiation of splenic monocytes into inflammatory monocytes (iMOs) and thereafter into MO-DCs that play a pathogenic role by promoting inflammation and tissue damage. The recruitment of MO-DCs to the lungs and brain are dependent on expression of CCR4 and CCR5, respectively, and expression of respective chemokine ligands in each organ. Once they reach the target organ the MO-DCs produce the CXCR3 ligands (CXCL9 and CXCL10), recruit CD8⁺ T cells, and produce toxic metabolites that play an important role in the development of experimental cerebral malaria (ECM) and acute respiratory distress syndrome (ARDS).

Immunogenic Evaluation of Ribosomal P-Protein Antigen P0, P1, and P2 and Pentameric Protein Complex P0-(P1-P2)2 of Plasmodium falciparum in a Mouse Model

Malaria remains one the most infectious and destructive protozoan diseases worldwide. Plasmodium falciparum, a protozoan parasite with a complex life cycle and high genetic variability responsible for the difficulties in vaccine development, is implicated in most malaria-related deaths. In the course of study, we prepared a set of antigens based on P-proteins from P. falciparum and determined their immunogenicity in an in vivo assay on a mouse model. The pentameric complex P0-(P1-P2)₂ was prepared along with individual P1, P2, and P0 antigens. We determined the level of cellular- and humoral-type immunological response followed by development of specific immunological memory. We have shown that the number of Tc cells increased significantly after the first immunization with P2 and after the second immunization with P1 and P0-(P1-P2)₂, which highly correlated with the number of Th1 cells. P0 appeared as a poor inducer of cellular response. After the third boost with P1, P2, or P0-(P1-P2)₂, the initially high cellular response dropped to the control level accompanied by elevation of the number of activated Treg cells and a high level of suppressive TGF-β. Subsequently, the humoral response against the examined antigens was activated. Although the titers of specific IgG were increasing during the course of immunization for all antigens used, P2 and P0-(P1-P2)₂ were found to be significantly stronger than P1 and P0. A positive correlation between the Th2 cell abundance and the level of IL-10 was observed exclusively after immunization with P0-(P1-P2)₂. An in vitro exposure of spleen lymphocytes from the immunized mice especially to the P1, P2, and P0-(P1-P2)₂ protein caused 2-3-fold higher cell proliferation than that in the case of lymphocytes from the nonimmunized animals, suggesting development of immune memory. Our results demonstrate for the first time that the native-like P-protein pentameric complex represents much stronger immune potential than individual P-antigens.

Dendritic Cell Responses and Function in Malaria

Malaria remains a serious threat to global health. Sustained malaria control and, eventually, eradication will only be achieved with a broadly effective malaria vaccine. Yet a fundamental lack of knowledge about how antimalarial immunity is acquired has hindered vaccine development efforts to date. Understanding how malaria-causing parasites modulate the host immune system, specifically dendritic cells (DCs), key initiators of adaptive and vaccine antigen-based immune responses, is vital for effective vaccine design. This review comprehensively summarizes how exposure to Plasmodium spp. impacts human DC function in vivo and in vitro. We have highlighted the heterogeneity of the data observed in these studies, compared and critiqued the models used to generate our current understanding of DC function in malaria, and examined the mechanisms by which Plasmodium spp. mediate these effects. This review highlights potential research directions which could lead to improved efficacy of existing vaccines, and outlines novel targets for next-generation vaccine strategies to target malaria.

Plasmodium falciparum Activates CD16+ Dendritic Cells to Produce Tumor Necrosis Factor and Interleukin-10 in Subpatent Malaria

Background

The malaria causing parasite Plasmodium subverts host immune responses by several strategies including the modulation of dendritic cells (DCs).

Methods

In this study, we show that Plasmodium falciparum skewed CD16+ DC cytokine responses towards interleukin (IL)-10 production in vitro, distinct to the cytokine profile induced by Toll-like receptor ligation. To determine CD16+ DC responsiveness in vivo, we assessed their function after induced P falciparum infection in malaria-naive volunteers.

Results

CD16+ DCs underwent distinctive activation, with increased expression of maturation markers human leukocyte antigen (HLA)-DR and CD86, enhanced tumor necrosis factor (TNF) production, and coproduction of TNF/IL-10. In vitro restimulation with P falciparum further increased IL-10 production. In contrast, during naturally acquired malaria episode, CD16+ DCs showed diminished maturation, suggesting increased parasite burden and previous exposure influence DC subset function.

Conclusions

These findings identify CD16+ DCs as the only DC subset activated during primary blood-stage human Plasmodium infection. As dual cytokine producers, CD16+ DCs contribute to inflammatory as well as regulatory innate immune processes.

Deciphering genetic regulation of CD14 by SP1 through characterization of peripheral blood mononuclear transcriptome of P. faiciparum and P. vivax infected malaria patients

BACKGROUND

Plasmodium falciparum and Plasmodium vivax are two major parasites responsible for malaria which remains a threat to almost 50% of world's population despite decade-long eradication program. One possible reason behind this conundrum is that the bases of clinical variability in malaria caused by either species are complex and poorly understood.

METHODS

Whole-genome transcriptome was analyzed to identify the active and predominant pathways in the PBMC of P. falciparum and P. vivax infected malaria patients. Deregulated genes were identified and annotated using R Bioconductor and DAVID/KEGG respectively. Genetic and functional regulation of CD14, a prioritized candidate, were established by quantitative RT-PCR, genotyping using RFLP and resequencing, mapping of transcription factor binding using CONSITE and TFBIND, dual luciferase assay, western blot analysis, RNAi- mediated gene knockdown and chromatin-immunoprecipation.

FINDINGS

The study highlighted that deregulation of host immune and inflammatory genes particularly CD14 as a key event in P. falciparum malaria. An abundance of allele-C of rs5744454, located in CD14 promoter, in severe malaria motivated us to establish an allele-specific regulation of CD14 by SP1. An enhancement of SP1 and CD14 expression was observed in artemisinin treated human monocyte cell line.

INTERPRETATION

Our data not only reinstates that CD14 of TLR pathway plays a predominant role in P. falciparum malaria, it establishes a functional basis for genetic association of rs5744454 with P. falciparum severe malaria by demonstrating a cis-regulatory role of this promoter polymorphism. Moreover, the study points towards a novel pharmacogenetic aspect of artemisinin-based anti-malarial therapy. FUND: DST-SERB, Govt. of India, SR/SO/HS-0056/2013.

Analysis of the lymphocyte cell population during malaria caused by Plasmodium vivax and its correlation with parasitaemia and thrombocytopaenia

BACKGROUND

The mechanisms of activation and regulation of T lymphocytes and their cytokines in malaria caused by Plasmodium vivax are complex and poorly understood. Previous data suggest that T cells balance protective immune responses with immune mediated pathology in malaria. This study investigates the lymphocytic profile of patients infected with P. vivax by identifying and quantifying the specific sub-populations of Th1, Th2, Th17 and Treg cells and observing the correlation between parasitaemia and the number of platelets.

METHODS

A cross-sectional study was carried out in an endemic area of the state of Acre, Brazil. In order to obtain identification and quantification of lymphocyte sub-populations through flow cytometry, blood samples were collected from 50 individuals infected with P. vivax and 20 non-infected controls. To differentiate Th1 from Th2, the presence of cytokines IL-4 and TNF was examined by enzyme-linked immunosorbent assay. Utilizing the Mann-Whitney and Spearman coefficient tests, comparison and correlation analysis were rendered to test the parasitaemia and the number of platelets relationship.

RESULTS

The data indicate that individuals infected with P. vivax present a significant reduction in Th1, Th2 and Th17 cell sub-populations when compared to the non-infected control group. A negative correlation exists between parasitaemia and platelet counts in individuals infected with P. vivax. There is no correlation of parasitaemia or thrombocytopaenia with any sub-population of T lymphocytes analysed. Interestingly, patients with serum Th1 cytokine profile present inversely proportional parasitaemia to the increase in the number of Th1, Th2, Th17 and Treg cells while patients with serum Th2 cytokine profile present directly proportional parasitaemia to the increase in number of Th1 and Th2 cells. Regarding the number of platelets, patients with serum Th1 cytokine profile show a correlation directly proportional to the Th17 sub-population. In contrast, platelet counts are directly proportional only to Treg and activated Treg cells in patients with serum Th2 cytokine profile.

CONCLUSIONS

During the P. vivax infection patients with serum Th1 versus Th2 cytokine profile present different biological mechanisms for activating the immune system against parasite load.

Maternal exposure to farming environment protects offspring against allergic diseases by modulating the neonatal TLR-Tregs-Th axis

Background

As the development of urbanization in China, the morbidity of allergic disease rise up prominently even in children, which may be partially associated with the excessively clean environment. It has been reported that common microorganism in rural environment shows protective effects on allergic disease by modulating TLRs-Tregs/Th cell axis. But the mechanism of this protection still needs to be elucidated in detail. We investigated the effects of maternal exposure to farming environment on the neonatal innate immune system, especially on the TLR-Treg-Th (Th1, Th2, Th9, and Th17) axis, in the Jilin province of China.

Methods

Eighty-four non-farming and 42 farming pregnant women were recruited. Endotoxins and glucans in dust from the living rooms of the pregnant mothers were measured. Cord blood mononuclear cells were challenged with phytohemagglutinin, lipopolysaccharide, or peptidoglycan. Proliferative response of lymphocyte was measured by 3H-TdR incorporation methods, CD4 + CD25 + FOXP3 + T cells percentage was assessed with flow cytometry, Tregs specific genes (FOXP3, LAG3, GITR, CTLA-4 and TGF-β) and TLR2, TLR4 genes expression were detected by RT-PCR, specific cytokines of Th1, Th2, Th9, Th17 and Tregs were measured with flow cytometer, suppressive capacity of Tregs was tested by culturing with effector cells in vitro, and TLR2/4 gene polymorphism was detected.

Results

Higher endotoxin content was observed in the living rooms of the farming mothers. Compared with that in the non-farming group, in farming neonatal CBMCs, lymphocyte proliferation declined; the IFN-γ/IL-13 ratio increased; and the quantity of Tregs and gene expression of FOXP3, GITR, CTLA4 and TLR2 increased significantly (P < 0.05). Isolated Tregs suppressed the proliferation of effector T cells and IL-13 production more strongly in vitro (P = 0.04, 0.03, respectively), and the TLR2 polymorphism affected FOXP3 expression and IFN-γ and IL-13 production.

Conclusions

Maternal exposure to farming affected the quantity and function of neonatal Tregs upon stimulation with PPG and LPS, which partly contributed to reducing the risk for allergic diseases in the offspring. The results of our study will lay the theoretical foundation for allergic disease prevention in early life.

Electronic supplementary material

The online version of this article (10.1186/s13601-018-0220-0) contains supplementary material, which is available to authorized users.

Characterization of T cell activation and regulation in children with asymptomatic Plasmodium falciparum infection

Background

Asymptomatic Plasmodium infections are characterized by the absence of clinical disease and the ability to restrict parasite replication. Increasing levels of regulatory T cells (Tregs) in Plasmodium falciparum infections have been associated with the risk of developing clinical disease, suggesting that individuals with asymptomatic infections may have reduced Treg frequency. However, the relationship between Tregs, cellular activation and parasite control in asymptomatic malaria remains unclear.

Methods

In a cross-sectional study, the levels of Tregs and other T cell activation phenotypes were compared using flow cytometry in symptomatic, asymptomatic and uninfected children before and after stimulation with infected red blood cell lysates (iRBCs). In addition, the association between these T cell phenotypes and parasitaemia were investigated.

Results

In children with asymptomatic infections, levels of Tregs and activated T cells were comparable to those in healthy controls but significantly lower than those in symptomatic children. After iRBC stimulation, levels of Tregs remained lower for asymptomatic versus symptomatic children. In contrast, levels of activated T cells were higher for asymptomatic children. Strikingly, the pre-stimulation levels of two T cell activation phenotypes (CD8+CD69+ and CD8+CD25+CD69+) and the post-stimulation levels of two regulatory phenotypes (CD4+CD25+Foxp3+ and CD8+CD25+Foxp3+) were significantly positively correlated with and explained 68% of the individual variation in parasitaemia. A machine-learning model based on levels of these four phenotypes accurately distinguished between asymptomatic and symptomatic children (sensitivity = 86%, specificity = 94%), suggesting that these phenotypes govern the observed variation in disease status.

Conclusion

Compared to symptomatic P. falciparum infections, in children asymptomatic infections are characterized by lower levels of Tregs and activated T cells, which are associated with lower parasitaemia. The results indicate that T cell regulatory and activation phenotypes govern both parasitaemia and disease status in paediatric malaria in the studied sub-Saharan African population.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2410-6) contains supplementary material, which is available to authorized users.

The significance role of regulatory T cells in the persistence of infections by intracellular bacteria

Regulatory T cells (Treg cells), are considered as effective immune cells playing a key role in immune response during cancers, autoimmune and infectious diseases. Regulatory T lymphocytes are divided into two main subgroups: natural Treg cells that generated during maturation in the thymus and have the suppressive activity that is critical for the establishment and maintenance of homeostasis in the body and induced Treg cells (iTreg) that are originated from naive T cells following the self-antigen recognition. In recent years, the roles of Treg in immune responses to microbial infections have received increased attention in researches. Several reports suggested the pivotal role of Treg cells in controlling responses to bacterial infections and demonstrated the impact of regulatory cells on one or more stages in the pathogenesis of bacterial infections. In this review, we describe the significance of regulatory T cells in the immunopathology of bacterial infections by focusing on specific bacterial infections including Mycobacteria, Listeria monocytogenes, and Bordetella pertussis. Moreover, suppressive mechanisms of regulatory T cells during bacterial infection including cell-cell contact, local secretion of inhibitory cytokines and local competition for growth factors will be discussed.

CD39 and immune regulation in a chronic helminth infection: The puzzling case of Mansonella ozzardi

Background

Chronic helminth infections typically induce an immunoregulatory environment, with markedly reduced immune responses to both parasite-specific and unrelated bystander antigens. Here we tested whether these changes are also observed in human infections with Mansonella ozzardi, a neglected filarial nematode widely distributed across Latin America.

Methods

CD4⁺ T cell populations from microfilaremic (Fil+) and uninfected (Fil-) inhabitants in M. ozzardi-endemic riverine communities in Brazil were characterized by flow cytometry analysis. Plasma concentrations of a wide range of cytokines and chemokines were measured. We examined whether M. ozzardi infection is associated with suppressed in vitro lymphoproliferative and inflammatory cytokine responses upon stimulation with filarial antigen, unrelated antigens or mitogens.

Principal findings/Conclusions

Fil+ subjects had lower plasma levels of selected inflammatory cytokines, such as TNF-α, IL-8, and IL-6, than their Fil- counterparts. However, we found no evidence for attenuated T-cell responses to filarial antigens or co-endemic pathogens, such as malaria parasites and Toxoplasma gondii. CD4⁺ T cells expressing CD39, an ectonucleosidase involved in the generation of the anti-inflammatory molecule adenosine, were increased in frequency in Fil+ subjects, compared to uninfected controls. Significantly, such an expansion was directly proportional to microfilarial loads. Surprisingly, CD39 blocking with a neutralizing antibody suppressed antigen-driven lymphoproliferation in vitro, while decreasing inflammatory cytokine responses, in Fil+ and Fil- individuals. These findings suggest that circulating CD4⁺ CD39⁺ T cells comprise subsets with both regulatory and stimulatory roles that contribute to the immune homeostasis in chronic M. ozzardi infection.

Helminth infections downregulate immunity and reduce host’s inflammatory responses, but the filarial nematode Mansonella ozzardi, which is widely distributed across Latin America, appears to represent an exception to this rule. We found similar lymphoproliferative responses to filarial and unrelated antigens and comparable regulatory cytokine responses in subjects harboring M. ozzardi microfilariae, compared to local uninfected controls. The proportion of CD4⁺ T cell subtypes expressing CD39 was significantly increased in infected subjects and correlated positively with their microfilarial density. However, antibody blocking of CD39, an ectonucleosidase involved in the synthesis of the immunosuppresive molecule adenosine, paradoxically reduced, rather than promoted, antigen-driven lymphoproliferation in vitro. We suggest that CD39⁺ CD4⁺ T cells circulating in microfilaremics comprise both regulatory and stimulatory cell subsets that are concomitantly expanded. The balance between these cell subsets with opposing regulatory functions may be crucial to maintain immune homeostasis during chronic M. ozzardi infections.

Severe thrombocytopaenia in patients with vivax malaria compared to falciparum malaria: a systematic review and meta-analysis

Background

Plasmodium vivax is the most geographically widespread species among human malaria parasites. Immunopathological studies have shown that platelets are an important component of the host innate immune response against malaria infections. The objectives of this study were to quantify thrombocytopaenia in P. vivax malaria patients and to determine the associated risks of severe thrombocytopaenia in patients with vivax malaria compared to patients with P. falciparum malaria.

Main body

A systematic review and meta-analysis of the available literature on thrombocytopaenia in P. vivax malaria patients was undertaken. Relevant studies in health-related electronic databases were identified and reviewed. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed. Fifty-eight observational studies (n = 29 664) were included in the current review. Severe thrombocytopaenia (< 50 000/mm³) to very severe thrombocytopaenia (< 20 000/mm³) was observed in 10.1% of patients with P. vivax infection. A meta-analysis of 11 observational studies showed an equal risk of developing severe/very severe thrombocytopaenia between the patients with P. vivax malaria and those with P. falciparum malaria (OR: 1.98, 95% CI: 0.92–4.25). This indicates that thrombocytopaenia is as equally a common manifestation in P. vivax and P. falciparum malaria patients. One study showed a higher risk of developing very severe thrombocytopaenia in children with severe P. vivax malaria than with severe P. falciparum malaria (OR: 2.80, 95% CI: 1.48–5.29). However, a pooled analysis of two studies showed an equal risk among adult severe cases (OR: 1.19, 95% CI: 0.51–2.77). This indicates that the risk of developing thrombocytopaenia in P. vivax malaria can vary with immune status in both children and adults. One study reported higher levels of urea and serum bilirubin in patients with P. vivax malaria and severe thrombocytopaenia compared with patients mild thrombocytopaenia or no thrombocytopaenia, (P < 0.001 in all comparisons). A pooled analysis of two other studies showed a similar proportion of bleeding episodes with thrombocytopaenia in severe P. vivax patients and severe P. falciparum patients (P = 0.09). This implied that both P. vivax and P. falciparum infections could present with bleeding episodes, if there had been a change in platelet counts in the infected patients. A pooled analysis of another two studies showed an equal risk of mortality with severe thrombocytopaenia in both P. vivax and P. falciparum malaria patients (OR: 1.16, 95% CI: 0.30–4.60). However, due to the low number of studies with small sample sizes within the subset of studies that provided clinically relevant information, our confidence in the estimates is limited.

Conclusion

The current review has provided some evidence of the clinical relevance of severe thrombocytopaenia in P. vivax malaria. To substantiate these findings, there is a need for well designed, large-scale, prospective studies among patients infected with P. vivax. These should include patients from different countries and epidemiological settings with various age and gender groups represented.

Electronic supplementary material

The online version of this article (10.1186/s40249-018-0392-9) contains supplementary material, which is available to authorized users.

Plasmacytoid dendritic cells appear inactive during sub-microscopic Plasmodium falciparum blood-stage infection, yet retain their ability to respond to TLR stimulation

Plasmacytoid dendritic cells (pDC) are activators of innate and adaptive immune responses that express HLA-DR, toll-like receptor (TLR) 7, TLR9 and produce type I interferons. The role of human pDC in malaria remains poorly characterised. pDC activation and cytokine production were assessed in 59 malaria-naive volunteers during experimental infection with 150 or 1,800 P. falciparum-parasitized red blood cells. Using RNA sequencing, longitudinal changes in pDC gene expression were examined in five adults before and at peak-infection. pDC responsiveness to TLR7 and TLR9 stimulation was assessed in-vitro. Circulating pDC remained transcriptionally stable with gene expression altered for 8 genes (FDR < 0.07). There was no upregulation of co-stimulatory molecules CD86, CD80, CD40, and reduced surface expression of HLA-DR and CD123 (IL-3R-α). pDC loss from the circulation was associated with active caspase-3, suggesting pDC apoptosis during primary infection. pDC remained responsive to TLR stimulation, producing IFN-α and upregulating HLA-DR, CD86, CD123 at peak-infection. In clinical malaria, pDC retained HLA-DR but reduced CD123 expression compared to convalescence. These data demonstrate pDC retain function during a first blood-stage P. falciparum exposure despite sub-microscopic parasitaemia downregulating HLA-DR. The lack of evident pDC activation in both early infection and malaria suggests little response of circulating pDC to infection.

Early Immune Regulatory Changes in a Primary Controlled Human Plasmodium vivax Infection: CD1c+ Myeloid Dendritic Cell Maturation Arrest, Induction of the Kynurenine Pathway, and Regulatory T Cell Activation

Plasmodium vivax malaria remains a major public health problem. The requirements for acquisition of protective immunity to the species are not clear. Dendritic cells (DC) are essential for immune cell priming but also perform immune regulatory functions, along with regulatory T cells (Treg). An important function of DC involves activation of the kynurenine pathway via indoleamine 2,3-dioxygenase (IDO). Using a controlled human experimental infection study with blood-stage P. vivax, we characterized plasmacytoid DC (pDC) and myeloid DC (mDC) subset maturation, CD4⁺ CD25⁺ CD127^lo Treg activation, and IDO activity. Blood samples were collected from six healthy adults preinoculation, at peak parasitemia (day 14; ∼31,400 parasites/ml), and 24 and 48 h after antimalarial treatment. CD1c⁺ and CD141⁺ mDC and pDC numbers markedly declined at peak parasitemia, while CD16⁺ mDC numbers appeared less affected. HLA-DR expression was selectively reduced on CD1c⁺ mDC, increased on CD16⁺ mDC, and was unaltered on pDC. Plasma IFN-γ increased significantly and was correlated with an increased kynurenine/tryptophan (KT) ratio, a measure of IDO activity. At peak parasitemia, Treg presented an activated CD4⁺ CD25⁺ CD127^lo CD45RA^- phenotype and upregulated TNFR2 expression. In a mixed-effects model, the KT ratio was positively associated with an increase in activated Treg. Our data demonstrate that a primary P. vivax infection exerts immune modulatory effects by impairing HLA-DR expression on CD1c⁺ mDC while activating CD16⁺ mDC. Induction of the kynurenine pathway and increased Treg activation, together with skewed mDC maturation, suggest P. vivax promotes an immunosuppressive environment, likely impairing the development of a protective host immune response.

Artesunate and erythropoietin synergistically improve the outcome of experimental cerebral malaria

Cerebral malaria (CM) is a severe neurological syndrome in humans and the main fatal cause of malaria. In malaria epidemic regions, despite appropriate anti-malarial treatment, 10-20% of deaths still occur during the acute phase. This is largely attributable to poor treatment access, therapeutic complexity and drug resistance; thus, developing additional clinical adjunctive therapies is an urgent necessity. In this study, we investigated the effect of artesunate (AST) and recombinant human erythropoietin (rhEPO) using an experimental cerebral malaria (ECM) model-C57BL/6 mice infected with Plasmodium berghei ANKA (PbA). Treatment with the combination of AST and rhEPO reduced endothelial activation and improved the integrity of blood brain barrier, which led to increased survival rate and reduced pathology in the ECM. In addition, this combination treatment down-regulated the Th1 response during PbA infection, which was correlated with the reduction of CCL2, TNF-α, IFN-γ, IL-12, IL-18, CXCL9 and CXCL10 levels, leading to reduced accumulation of pathogenic T cells in the brain. Meanwhile, AST and rhEPO combination led to decreased maturation and activation of splenic dendritic cells, expansion of regulatory T cells, and increased IL-10 and TGF-β production. In conclusion, these data provide a theoretical basis for clinical adjunct therapy with rhEPO and AST in human cerebral malaria patients.

On the cytokine/chemokine network during Plasmodium vivax malaria: new insights to understand the disease

Background

The clinical outcome of malaria depends on the delicate balance between pro-inflammatory and immunomodulatory cytokine responses triggered during infection. Despite the numerous reports on characterization of plasma levels of cytokines/chemokines, there is no consensus on the profile of these mediators during blood stage malaria. The identification of acute phase biomarkers might contribute to a better understanding of the disease, allowing the use of more effective therapeutic approaches to prevent the progression towards severe disease. In the present study, the plasma levels of cytokines and chemokines and their association with parasitaemia and number of previous malaria episodes were evaluated in Plasmodium vivax-infected patients during acute and convalescence phase, as well as in healthy donors.

Methods

Samples of plasma were obtained from peripheral blood samples from four different groups: P. vivax-infected, P. vivax-treated, endemic control and malaria-naïve control. The cytokine (IL-6, IL-10, IL-17, IL-27, TGF-β, IFN-γ and TNF) and chemokine (MCP-1/CCL2, IP-10/CXCL10 and RANTES/CCL5) plasma levels were measured by CBA or ELISA. The network analysis was performed using Spearman correlation coefficient.

Results

Plasmodium vivax infection induced a pro-inflammatory response driven by IL-6 and IL-17 associated with an immunomodulatory profile mediated by IL-10 and TGF-β. In addition, a reduction was observed of IFN-γ plasma levels in P. vivax group. A lower level of IL-27 was observed in endemic control group in comparison to malaria-naïve control group. No significant results were found for IL-12p40 and TNF. It was also observed that P. vivax infection promoted higher levels of MCP-1/CCL2 and IP-10/CXCL10 and lower levels of RANTES/CCL5. The plasma level of IL-10 was elevated in patients with high parasitaemia and with more than five previous malaria episodes. Furthermore, association profile between cytokine and chemokine levels were observed by correlation network analysis indicating signature patterns associated with different parasitaemia levels.

Conclusions

The P. vivax infection triggers a balanced immune response mediated by IL-6 and MCP-1/CCL2, which is modulated by IL-10. In addition, the results indicated that IL-10 plasma levels are influenced by parasitaemia and number of previous malaria episodes.

Plasmodium vivax but Not Plasmodium falciparum Blood-Stage Infection in Humans Is Associated with the Expansion of a CD8+ T Cell Population with Cytotoxic Potential

P. vivax and P. falciparum parasites display different tropism for host cells and induce very different clinical symptoms and pathology, suggesting that the immune responses required for protection may differ between these two species. However, no study has qualitatively compared the immune responses to P. falciparum or P. vivax in humans following primary exposure and infection. Here, we show that the two species differ in terms of the cellular immune responses elicited following primary infection. Specifically, P. vivax induced the expansion of a subset of CD8⁺ T cells expressing the activation marker CD38, whereas P. falciparum induced the expansion of CD38⁺ CD4⁺ T cells. The CD38⁺ CD8⁺ T cell population that expanded following P. vivax infection displayed greater cytotoxic potential compared to CD38^- CD8⁺ T cells, and compared to CD38⁺ CD8⁺ T cells circulating during P. falciparum infection. We hypothesize that P. vivax infection leads to a stronger CD38⁺ CD8⁺ T cell activation because of its preferred tropism for MHC-I-expressing reticulocytes that, unlike mature red blood cells, can present antigen directly to CD8⁺ T cells. This study provides the first line of evidence to suggest an effector role for CD8⁺ T cells in P. vivax blood-stage immunity. It is also the first report of species-specific differences in the subset of T cells that are expanded following primary Plasmodium infection, suggesting that malaria vaccine development may require optimization according to the target parasite.

The specific immune responses that contribute to protective immunity in humans following Plasmodium infection are yet to be fully characterized. The species P. vivax and P. falciparum account for most human infections, yet little is known about P. vivax specific immune responses and whether they are similar to or distinct from P. falciparum. Here, we establish that P. vivax and P. falciparum elicit distinct cellular immune responses following primary infection, with the expansion of a subset of CD38⁺ CD8⁺ T cells with a cytotoxic potential in P. vivax but not in P. falciparum infection. This study provides the first evidence for the activation of CD8⁺ T cells in P. vivax blood-stage infection and demonstrates the existence of species-dependent host immune responses to malaria. These findings have important implications for P. vivax vaccine development, and suggest that future malaria vaccine studies should be adapted according to the target Plasmodium spp.

Cytokine gene polymorphisms are not associated with anti-PvDBP, anti-PvAMA-1 or anti-PvMSP-119 IgG antibody levels in a malaria-endemic area of the Brazilian Amazon

Background

The immune response against Plasmodium vivax immunogenic epitopes is regulated by pro- and anti-inflammatory cytokines that determine antibody levels and class switching. Cytokine gene polymorphisms may be responsible for changes in the humoral immune response against malaria. The aim of this study was to evaluate whether polymorphisms in the TNFA, IFNG and IL10 genes would alter the levels of anti-PvAMA1, PvDBP and -PvMSP-1₁₉ IgG antibodies in patients with vivax malaria.

Methods

Samples from 90 vivax malaria-infected and 51 uninfected subjects from an endemic area of the Brazilian Amazon were genotyped using polymerase chain reaction-restriction fragment length polymorphism (PCR–RFLP) to identify polymorphisms of the genes TNFA (−1031T > C, −308G > A, −238G > A), IFNG (+874T > A) and IL10 (−819C > T, −592C > A). The levels of total IgG against PvAMA1, PvDBP and PvMSP-1₁₉ were determined using an enzyme-linked immunosorbent assay (ELISA). Associations between the polymorphisms and the antibody response were assessed by means of logistic regression models.

Results

No significant differences were found in the levels of IgG antibodies against the PvAMA-1, PvDBP or PvMSP-1₁₉ proteins in relation to the studied polymorphisms.

Conclusions

Although no associations were found among the evaluated genotypes and alleles and anti-merozoite IgG class P. vivax antibody levels, this study helps elucidate the immunogenic profile involved in the humoral immune response in malaria.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1414-3) contains supplementary material, which is available to authorized users.

Surface expression of inhibitory (CTLA-4) and stimulatory (OX40) receptors by CD4+ regulatory T cell subsets circulating in human malaria

Several CD4⁺ T cell subtypes contribute to immune homeostasis in malaria, but the markers that define the main suppressive T cell subsets induced by this infection remain largely unknown. Here we provide a detailed phenotypic characterization of immunoregulatory CD4⁺ T cell populations in uncomplicated human malaria. We found an increased proportion of CD4⁺ T cells expressing CTLA-4, OX40, GITR, TNFRII, and CD69 in acute-phase single-species infections with Plasmodium vivax, Plasmodium falciparum, or both. Such an increase was not proportional to parasite density in P. vivax infections, and did not persist after parasite clearance. Significantly, less than 10% of CD4⁺ T cells expressing these regulatory molecules had the classical T regulatory (Treg) phenotype (CD4⁺CD25⁺CD127^-FoxP3⁺). Two major Treg cell subpopulations, which together accounted for 19-23% of all Treg cells circulating in malaria patients, expressed surface receptors with opposing regulatory functions, either CTLA-4 or OX40. OX40⁺ Treg cells outnumbered their CTLA-4⁺ counterparts (1.8:1) during acute P. vivax infection, while a more balanced ratio (1.3:1) was observed following parasite clearance These data reveal new players in the complex CD4⁺ Treg cell network that maintains immune homeostasis in malaria and suggest potential targets for therapeutic interventions to improve parasite-specific effector immune responses.

Dendritic Cells and Their Multiple Roles during Malaria Infection

Dendritic cells (DCs) play a central role in the initiation of adaptive immune responses, efficiently presenting antigens to T cells. This ability relies on the presence of numerous surface and intracellular receptors capable of sensing microbial components as well as inflammation and on a very efficient machinery for antigen presentation. In this way, DCs sense the presence of a myriad of pathogens, including Plasmodium spp., the causative agent of malaria. Despite many efforts to control this infection, malaria is still responsible for high rates of morbidity and mortality. Different groups have shown that DCs act during Plasmodium infection, and data suggest that the phenotypically distinct DCs subsets are key factors in the regulation of immunity during infection. In this review, we will discuss the importance of DCs for the induction of immunity against the different stages of Plasmodium, the outcomes of DCs activation, and also what is currently known about Plasmodium components that trigger such activation.

Polymorphisms in Plasmodium vivax Circumsporozoite Protein (CSP) Influence Parasite Burden and Cytokine Balance in a Pre-Amazon Endemic Area from Brazil

Mechanisms involved in severe P. vivax malaria remain unclear. Parasite polymorphisms, parasite load and host cytokine profile may influence the course of infection. In this study, we investigated the influence of circumsporozoite protein (CSP) polymorphisms on parasite load and cytokine profile in patients with vivax malaria. A cross-sectional study was carried out in three cities: São Luís, Cedral and Buriticupu, Maranhão state, Brazil, areas of high prevalence of P. vivax. Interleukin (IL)-2, IL-4, IL-10, IL-6, IL-17, tumor necrosis factor alpha (TNF-α, interferon gamma (IFN-γ and transforming growth factor beta (TGF-β were quantified in blood plasma of patients and in supernatants from peripheral blood mononuclear cell (PBMC) cultures. Furthermore, the levels of cytokines and parasite load were correlated with VK210, VK247 and P. vivax-like CSP variants. Patients infected with P. vivax showed increased IL-10 and IL-6 levels, which correlated with the parasite load, however, in multiple comparisons, only IL-10 kept this association. A regulatory cytokine profile prevailed in plasma, while an inflammatory profile prevailed in PBMC culture supernatants and these patterns were related to CSP polymorphisms. VK247 infected patients showed higher parasitaemia and IL-6 concentrations, which were not associated to IL-10 anti-inflammatory effect. By contrast, in VK210 patients, these two cytokines showed a strong positive correlation and the parasite load was lower. Patients with the VK210 variant showed a regulatory cytokine profile in plasma, while those infected with the VK247 variant have a predominantly inflammatory cytokine profile and higher parasite loads, which altogether may result in more complications in infection. In conclusion, we propose that CSP polymorphisms is associated to the increase of non-regulated inflammatory immune responses, which in turn may be associated with the outcome of infection.

Recent evidences have associated P. vivax infections with clinical complications, previously only attributed to P. falciparum malaria. The interaction between host and parasite may contribute to severity of the disease, however, the specific contribution of each factor remains unclear. Previous studies have shown that polymorphisms in Plasmodium vivax CSP may interfere in systemic reactions, response to drug treatments, leading to different symptoms as well as humoral responses. In this study, we investigate whether these polymorphisms could influence the parasite load and cytokine profile, which altogether may influence the malaria outcome. In this sense, studies have shown that subjects with high parasitic loads, responding with production of pro-inflammatory cytokines develop a more severe disease. The present data indicate that VK247 variant are associated with significant higher parasite loads and pro-inflammatory cytokine profile compared to VK210 variant. In this regard, this study demonstrates that P. vivax CSP polymorphisms have systemic effects in the host immune response, and the investigation of immunogenicity of parasite proteins may provide evidences for a better understanding of this infection.

Examining cellular immune responses to inform development of a blood-stage malaria vaccine

Naturally acquired immunity to the blood-stage of the malaria parasite develops slowly in areas of high endemicity, but is not sterilizing. It manifests as a reduction in parasite density and clinical symptoms. Immunity as a result of blood-stage vaccination has not yet been achieved in humans, although there are many animal models where vaccination has been successful. The development of a blood-stage vaccine has been complicated by a number of factors including limited knowledge of human-parasite interactions and which antigens and immune responses are critical for protection. Opinion is divided as to whether this vaccine should aim to accelerate the acquisition of responses acquired following natural exposure, or whether it should induce a different response. Animal and experimental human models suggest that cell-mediated immune responses can control parasite growth, but these responses can also contribute to significant immunopathology if unregulated. They are largely ignored in most blood-stage malaria vaccine development strategies. Here, we discuss key observations relating to cell-mediated immune responses in the context of experimental human systems and field studies involving naturally exposed individuals and how this may inform the development of a blood-stage malaria vaccine.

Insights into the naturally acquired immune response toPlasmodium vivaxmalaria

Plasmodium vivaxis the most geographically widespread of the malaria parasites causing human disease, yet it is comparatively understudied compared withPlasmodium falciparum.In this article we review what is known about naturally acquired immunity toP. vivax, and importantly, how this differs to that acquired againstP. falciparum.Immunity to clinicalP. vivaxinfection is acquired more quickly than toP. falciparum, and evidence suggests humans in endemic areas also have a greater capacity to mount a successful immunological memory response to this pathogen. Both of these factors give promise to the idea of a successfulP. vivaxvaccine. We review what is known about both the cellular and humoral immune response, including the role of cytokines, antibodies, immunoregulation, immune memory and immune dysfunction. Furthermore, we discuss where the future lies in terms of advancing our understanding of naturally acquired immunity toP. vivax, through the use of well-designed longitudinal epidemiological studies and modern tools available to immunologists.

Immunoregulation in human malaria: the challenge of understanding asymptomatic infection

Asymptomatic Plasmodium infection carriers represent a major threat to malaria control worldwide as they are silent natural reservoirs and do not seek medical care. There are no standard criteria for asymptomaticPlasmodium infection; therefore, its diagnosis relies on the presence of the parasite during a specific period of symptomless infection. The antiparasitic immune response can result in reducedPlasmodium sp. load with control of disease manifestations, which leads to asymptomatic infection. Both the innate and adaptive immune responses seem to play major roles in asymptomatic Plasmodiuminfection; T regulatory cell activity (through the production of interleukin-10 and transforming growth factor-β) and B-cells (with a broad antibody response) both play prominent roles. Furthermore, molecules involved in the haem detoxification pathway (such as haptoglobin and haeme oxygenase-1) and iron metabolism (ferritin and activated c-Jun N-terminal kinase) have emerged in recent years as potential biomarkers and thus are helping to unravel the immune response underlying asymptomatic Plasmodium infection. The acquisition of large data sets and the use of robust statistical tools, including network analysis, associated with well-designed malaria studies will likely help elucidate the immune mechanisms responsible for asymptomatic infection.

The immunological balance between host and parasite in malaria

Coevolution of humans and malaria parasites has generated an intricate balance between the immune system of the host and virulence factors of the parasite, equilibrating maximal parasite transmission with limited host damage. Focusing on the blood stage of the disease, we discuss how the balance between anti-parasite immunity versus immunomodulatory and evasion mechanisms of the parasite may result in parasite clearance or chronic infection without major symptoms, whereas imbalances characterized by excessive parasite growth, exaggerated immune reactions or a combination of both cause severe pathology and death, which is detrimental for both parasite and host. A thorough understanding of the immunological balance of malaria and its relation to other physiological balances in the body is of crucial importance for developing effective interventions to reduce malaria-related morbidity and to diminish fatal outcomes due to severe complications. Therefore, we discuss in this review the detailed mechanisms of anti-malarial immunity, parasite virulence factors including immune evasion mechanisms and pathogenesis. Furthermore, we propose a comprehensive classification of malaria complications according to the different types of imbalances.

Immune Escape Mechanisms are Plasmodium's Secret Weapons Foiling the Success of Potent and Persistently Efficacious Malaria Vaccines

Despite decades of active research, an efficacious vaccine mediating long-term protection is still not available. This review highlights various mechanisms and the different facets by which the parasites outsmart the immune system. An understanding of how the parasites escape immune recognition and interfere with the induction of a protective immune response that provides sterilizing immunity will be crucial to vaccine design.

A Review of Plasmodium coatneyi-Macaque Models of Severe Malaria

Malaria remains one of the most significant public health concerns in the world today. Approximately half the human population is at risk for infection, with children and pregnant women being most vulnerable. More than 90% of the total human malaria burden, which numbers in excess of 200 million annually, is due to Plasmodium falciparum. Lack of an effective vaccine and a dwindling stockpile of antimalarial drugs due to increased plasmodial resistance underscore the critical need for valid animal models. Plasmodium coatneyi was described in Southeast Asia 50 years ago. This plasmodium of nonhuman primates has been used sporadically as a model for severe malaria, as it mimics many of the pathophysiologic features of human disease. This review covers the reported macroscopic, microscopic, ultrastructural, and molecular pathology of P. coatneyi infection in macaques, specifically focusing on the rhesus macaque, as well as describing the critical needs still outstanding in the validation of this crucial model of human disease.

Proinflammatory responses and higher IL-10 production by T cells correlate with protection against malaria during pregnancy and delivery outcomes

Pregnancy triggers immunological changes aimed to tolerate the fetus. However, it has not been properly addressed whether similar changes occur in tropical areas with high infection pressure and whether these changes render women more susceptible to infectious diseases. We compared the frequencies of T cell subsets, including regulatory T cells, in pregnant and nonpregnant women from Papua New Guinea, a high malaria transmission area, and from Spain, a malaria-free country. We also assessed the relationship among these cellular subsets, malaria infection, and delivery outcomes. CD4(+)FOXP3(+)CD127(low) T cells (Tregs) were decreased in pregnant women in both countries but were not associated with malaria infection or poor delivery outcomes. An expansion of IFN-γ-producing cells and intracytoplasmic IFN-γ levels was found in pregnant compared with nonpregnant women only in Papua New Guinea. Increased CD4(+)IL-10(+)IFN-γ(+) frequencies and Treg-IFN-γ production were found in women with current Plasmodium falciparum infection. Higher CD4(+)IL-10(-)IFN-γ(+) T cells frequencies and production of proinflammatory cytokines (including TNF and IL-2) at recruitment (first antenatal visit) had a protective association with birth weight and future (delivery) P. falciparum infection, respectively. Higher intracellular IL-10 levels in T cells had a protective association with future P. falciparum infection and hemoglobin levels at delivery. The protective associations were found also with nonmalaria-specific T cell responses. Treg frequencies positively correlated with plasma eotaxin concentrations, but this subset did not express eotaxin receptor CCR3. Thus, an activated immune system during pregnancy might contribute to protection against malaria during pregnancy and poor delivery outcomes.

CD8+ T lymphocyte expansion, proliferation and activation in dengue fever

Dengue fever induces a robust immune response, including massive T cell activation. The level of T cell activation may, however, be associated with more severe disease. In this study, we explored the level of CD8+ T lymphocyte activation in the first six days after onset of symptoms during a DENV2 outbreak in early 2010 on the coast of São Paulo State, Brazil. Using flow cytometry we detected a progressive increase in the percentage of CD8+ T cells in 74 dengue fever cases. Peripheral blood mononuclear cells from 30 cases were thawed and evaluated using expanded phenotyping. The expansion of the CD8+ T cells was coupled with increased Ki67 expression. Cell activation was observed later in the course of disease, as determined by the expression of the activation markers CD38 and HLA-DR. This increased CD8+ T lymphocyte activation was observed in all memory subsets, but was more pronounced in the effector memory subset, as defined by higher CD38 expression. Our results show that most CD8+ T cell subsets are expanded during DENV2 infection and that the effector memory subset is the predominantly affected sub population.

Dengue is a disease affecting approximately 400 million people annually, especially in tropical and subtropical areas of the globe. The immune response against the dengue virus is still under investigation and it is important to understand why the disease can be fatal in a small proportion of cases. In this work, we explored how an important cell type of the immune system, namely the CD8+ T cell, reacts during dengue infection. Using a method known as flow cytometry, we demonstrated that these cells expand and become highly activated, during the days following the onset of dengue fever symptoms. This expansion is associated with a decreased dengue virus load in the patients’ blood, suggesting that CD8+ T cells play an important role in viral control. Interestingly, we found that a subset of CD8+ T cells, called effector memory, is greatly expanded during dengue infection. Our results are important because they might contribute to the understanding of disease mechanisms during dengue infection and may help in the development of a novel vaccine against dengue.

Phenotypic profiling of CD8(+) T cells during Plasmodium vivax blood-stage infection

Background

For a long time, the role of CD8⁺ T cells in blood-stage malaria was not considered important because erythrocytes do not express major histocompatibility complex (MHC) class I proteins. While recent evidences suggest that CD8⁺ T cells may play an important role during the erythrocytic phase of infection by eliminating parasites, CD8⁺ T cells might also contribute to modulate the host response through production of regulatory cytokines. Thus, the role of CD8⁺ T cells during blood-stage malaria is unclear. Here, we report the phenotypic profiling of CD8⁺ T cells subsets from patients with uncomplicated symptomatic P. vivax malaria.

Methods

Blood samples were collected from 20 Plasmodium vivax-infected individuals and 12 healthy individuals. Immunophenotyping was conducted by flow cytometry. Plasma levels of IFN-γ, TNF-α and IL-10 were determined by ELISA/CBA. Unpaired t-test or Mann–Whitney test was used depending on the data distribution.

Results

P. vivax-infected subjects had lower percentages and absolute numbers of CD8⁺CD45RA⁺ and CD8⁺CD45RO⁺ T cells when compared to uninfected individuals (p ≤ 0.0002). A significantly lower absolute number of circulating CD8⁺CD45⁺CCR7⁺ cells (p = 0.002) was observed in P. vivax-infected individuals indicating that infection reduces the number of central memory T cells. Cytokine expression was significantly reduced in the naïve T cells from infected individuals compared with negative controls, as shown by lower numbers of IFN-γ⁺ (p = 0.001), TNF-α⁺ (p < 0.0001) and IL-10⁺ (p < 0.0001) CD8⁺ T cells. Despite the reduction in the number of CD8⁺ memory T cells producing IFN-γ (p < 0.0001), P. vivax-infected individuals demonstrated a significant increase in memory CD8⁺TNF-α⁺ (p = 0.016) and CD8⁺IL-10⁺ (p = 0.004) cells. Positive correlations were observed between absolute numbers of CD8⁺IL-10⁺ and numbers of CD8⁺IFN-γ⁺ (p < 0.001) and CD8⁺TNF-α⁺ T cells (p ≤ 0.0001). Finally, an increase in the plasma levels of TNF-α (p = 0.017) and IL-10 (p = 0.006) and a decrease in the IFN-γ plasma level (p <0.0001) were observed in the P. vivax-infected individuals.

Conclusions

P. vivax infection reduces the numbers of different subsets of CD8⁺ T cells, particularly the memory cells, during blood-stage of infection and enhances the number of CD8⁺ memory T cells expressing IL-10, which positively correlates with the number of cells expressing TNF-α and IFN-γ.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-015-0762-x) contains supplementary material, which is available to authorized users.

CD4+ T cells apoptosis in Plasmodium vivax infection is mediated by activation of both intrinsic and extrinsic pathways

BACKGROUND

Reduction in the number of circulating blood lymphocytes (lymphocytopaenia) has been reported during clinical episodes of malaria and is normalized after treatment with anti-malaria drugs. While this phenomenon is well established in malaria infection, the underlying mechanisms are still not fully elucidated. In the present study, the occurrence of apoptosis and its pathways in CD4+ T cells was investigated in naturally Plasmodium vivax-infected individuals from a Brazilian endemic area (Porto Velho - RO).

METHODS

Blood samples were collected from P. vivax-infected individuals and healthy donors. The apoptosis was characterized by cell staining with Annexin V/FITC and propidium iodide and the apoptosis-associated gene expression profile was carried out using RT2 Profiler PCR Array-Human Apoptosis. The plasma TNF level was determined by ELISA. The unpaired t-test or Mann-Whitney test was applied according to the data distribution.

RESULTS

Plasmodium vivax-infected individuals present low number of leukocytes and lymphocytes with a higher percentage of CD4+ T cells in early and/or late apoptosis. Increased gene expression was observed for TNFRSF1B and Bid, associated with a reduction of Bcl-2, in individuals with P. vivax malaria. Furthermore, these individuals showed increased plasma levels of TNF compared to malaria-naive donors.

CONCLUSIONS

The results of the present study suggest that P. vivax infection induces apoptosis of CD4+ T cells mediated by two types of signaling: by activation of the TNFR1 death receptor (extrinsic pathway), which is amplified by Bid, and by decreased expression of the anti-apoptotic protein Bcl-2 (intrinsic pathway). The T lymphocytes apoptosis could reflect a strategy of immune evasion triggered by the parasite, enabling their persistence but also limiting the occurrence of immunopathology.

Exacerbation of autoimmune neuro-inflammation in mice cured from blood-stage Plasmodium berghei infection

The thymus plays an important role shaping the T cell repertoire in the periphery, partly, through the elimination of inflammatory auto-reactive cells. It has been shown that, during Plasmodium berghei infection, the thymus is rendered atrophic by the premature egress of CD4+CD8+ double-positive (DP) T cells to the periphery. To investigate whether autoimmune diseases are affected after Plasmodium berghei NK65 infection, we immunized C57BL/6 mice, which was previously infected with P. berghei NK65 and treated with chloroquine (CQ), with MOG35-55 peptide and the clinical course of Experimental Autoimmune Encephalomyelitis (EAE) was evaluated. Our results showed that NK65+CQ+EAE mice developed a more severe disease than control EAE mice. The same pattern of disease severity was observed in MOG35-55-immunized mice after adoptive transfer of P. berghei-elicited splenic DP-T cells. The higher frequency of IL-17+- and IFN-γ+-producing DP lymphocytes in the Central Nervous System of these mice suggests that immature lymphocytes contribute to disease worsening. To our knowledge, this is the first study to integrate the possible relationship between malaria and multiple sclerosis through the contribution of the thymus. Notwithstanding, further studies must be conducted to assert the relevance of malaria-induced thymic atrophy in the susceptibility and clinical course of other inflammatory autoimmune diseases.

Intestinal parasites coinfection does not alter plasma cytokines profile elicited in acute malaria in subjects from endemic area of Brazil

In Brazil, malaria is prevalent in the Amazon region and these regions coincide with high prevalence of intestinal parasites but few studies explore the interaction between malaria and other parasites. Therefore, the present study evaluates changes in cytokine, chemokine, C-reactive protein, and nitric oxide (NO) concentrations in 264 individuals, comparing plasma from infected individuals with concurrent malaria and intestinal parasites to individuals with either malaria infection alone and uninfected. In the studied population 24% of the individuals were infected with Plasmodium and 18% coinfected with intestinal parasites. Protozoan parasites comprised the bulk of the intestinal parasites infections and subjects infected with intestinal parasites were more likely to have malaria. The use of principal component analysis and cluster analysis associated increased levels of IL-6, TNF-α, IL-10, and CRP and low levels of IL-17A predominantly with individuals with malaria alone and coinfected individuals. In contrast, low levels of almost all inflammatory mediators were associated predominantly with individuals uninfected while increased levels of IL-17A were associated predominantly with individuals with intestinal parasites only. In conclusion, our data suggest that, in our population, the infection with intestinal parasites (mainly protozoan) does not modify the pattern of cytokine production in individuals infected with P. falciparum and P. vivax.

Maladjusted host immune responses induce experimental cerebral malaria-like pathology in a murine Borrelia and Plasmodium co-infection model

In the Plasmodium infected host, a balance between pro- and anti-inflammatory responses is required to clear the parasites without inducing major host pathology. Clinical reports suggest that bacterial infection in conjunction with malaria aggravates disease and raises both mortality and morbidity in these patients. In this study, we investigated the immune responses in BALB/c mice, co-infected with Plasmodium berghei NK65 parasites and the relapsing fever bacterium Borrelia duttonii. In contrast to single infections, we identified in the co-infected mice a reduction of L-Arginine levels in the serum. It indicated diminished bioavailability of NO, which argued for a dysfunctional endothelium. Consistent with this, we observed increased sequestration of CD8+ cells in the brain as well over expression of ICAM-1 and VCAM by brain endothelial cells. Co-infected mice further showed an increased inflammatory response through IL-1β and TNF-α, as well as inability to down regulate the same through IL-10. In addition we found loss of synchronicity of pro- and anti-inflammatory signals seen in dendritic cells and macrophages, as well as increased numbers of regulatory T-cells. Our study shows that a situation mimicking experimental cerebral malaria (ECM) is induced in co-infected mice due to loss of timing and control over regulatory mechanisms in antigen presenting cells.

Vitamin D inhibits the occurrence of experimental cerebral malaria in mice by suppressing the host inflammatory response

In animal models of experimental cerebral malaria (ECM), neuropathology is associated with an overwhelming inflammatory response and sequestration of leukocytes and parasite-infected RBCs in the brain. In this study, we explored the effect of vitamin D (VD; cholecalciferol) treatment on host immunity and outcome of ECM in C57BL/6 mice during Plasmodium berghei ANKA (PbA) infection. We observed that oral administration of VD both before and after PbA infection completely protected mice from ECM. VD administration significantly dampened the inducible systemic inflammatory responses with reduced circulating cytokines IFN-γ and TNF and decreased expression of these cytokines by the spleen cells. Meanwhile, VD also resulted in decreased expression of the chemokines CXCL9 and CXCL10 and cytoadhesion molecules (ICAM-1, VCAM-1, and CD36) in the brain, leading to reduced accumulation of pathogenic T cells in the brain and ultimately substantial improvement of the blood-brain barriers of PbA-infected mice. In addition, VD inhibited the differentiation, activation, and maturation of splenic dendritic cells. Meanwhile, regulatory T cells and IL-10 expression levels were upregulated upon VD treatment. These data collectively demonstrated the suppressive function of VD on host inflammatory responses, which provides significant survival benefits in the murine ECM model.

Relationship of regulatory T cells to Plasmodium falciparum malaria symptomatology in a hypoendemic region

BACKGROUND

Previous data have suggested that regulatory T cells (Tregs) balance protective immune responses with immune mediated pathology in malaria. This study aimed to determine to test the hypothesis that Treg proportions or absolute levels are associated with parasitaemia and malaria symptoms.

METHODS

Treg cells were quantified by flow cytometry as CD4+ CD25+, Foxp3+, CD127(low) T cells. Three patient groups were assessed: patients with symptomatic Plasmodium falciparum malaria (S), subjects with asymptomatic P. falciparum parasitaemia (AS) and uninfected control individuals (C).

RESULTS

S, AS and C groups had similar absolute numbers and percentage of Tregs (3.9%, 3.5% and 3.5% respectively). Levels of parasitaemia were not associated with Treg percentage (p = 0.47).

CONCLUSION

Neither relative nor absolute regulatory T cell numbers were found to be associated with malaria-related symptomatology in this study. Immune mechanisms other than Tregs are likely to be responsible for the state of asymptomatic P. falciparum parasitaemia in the Peruvian Amazon; but further study to explore these mechanisms is needed.

The robust and modulated biomarker network elicited by the Plasmodium vivax infection is mainly mediated by the IL-6/IL-10 axis and is associated with the parasite load

Background. Recent studies have shown that the inflammatory process, including the biomarker production, and the intense activation of innate immune responses are greater in the malaria caused by Plasmodium vivax than other species. Here, we examined the levels of serum biomarkers and their interaction during acute malaria. Material and Methods. Blood samples were collected from P. vivax-infected patients at admission and from healthy donors. Levels of serum biomarkers were measured by Cytometric Bead Assay or ELISA. Results. P. vivax infection triggered the production of both inflammatory and regulatory biomarkers. Levels of IL-6, CXCL-8, IFN-γ, IL-5, and IL-10 were higher in P. vivax-infected patients than in healthy donors. On the other hand, malaria patients produced lower levels of TNF-α, IL-12p70, and IL-2 than healthy individuals. While the levels of IL-10 and IL-6 were found independent on the number of malaria episodes, higher levels of these cytokines were seen in patients with higher parasite load. Conclusion. A mixed pattern of proinflammatory and regulatory biomarkers is produced in P. vivax malaria. Analysis of biomarker network suggests that IL-10 and IL-6 are a robust axis in malaria patients and that this interaction seems to be associated with the parasite load.

Liver accumulation of Plasmodium chabaudi-infected red blood cells and modulation of regulatory T cell and dendritic cell responses

It is postulated that accumulation of malaria-infected Red Blood Cells (iRBCs) in the liver could be a parasitic escape mechanism against full destruction by the host immune system. Therefore, we evaluated the in vivo mechanism of this accumulation and its potential immunological consequences. A massive liver accumulation of P. c. chabaudi AS-iRBCs (Pc-iRBCs) was observed by intravital microscopy along with an over expression of ICAM-1 on day 7 of the infection, as measured by qRT-PCR. Phenotypic changes were also observed in regulatory T cells (Tregs) and dendritic cells (DCs) that were isolated from infected livers, which indicate a functional role for Tregs in the regulation of the liver inflammatory immune response. In fact, the suppressive function of liver-Tregs was in vitro tested, which demonstrated the capacity of these cells to suppress naive T cell activation to the same extent as that observed for spleen-Tregs. On the other hand, it is already known that CD4+ T cells isolated from spleens of protozoan parasite-infected mice are refractory to proliferate in vivo. In our experiments, we observed a similar lack of in vitro proliferative capacity in liver CD4+ T cells that were isolated on day 7 of infection. It is also known that nitric oxide and IL-10 are partially involved in acute phase immunosuppression; we found high expression levels of IL-10 and iNOS mRNA in day 7-infected livers, which indicates a possible role for these molecules in the observed immune suppression. Taken together, these results indicate that malaria parasite accumulation within the liver could be an escape mechanism to avoid sterile immunity sponsored by a tolerogenic environment.