Malaria drives T cells to exhaustion

Characteristics of splenic PD-1+ γδT cells in Plasmodium yoelii nigeriensis infection

Although the functions of programmed death-1 (PD-1) on αβ T cells have been extensively reported, a role for PD-1 in regulating γδT cell function is only beginning to emerge. Here, we investigated the phenotypic and functional characteristics of PD-1-expressing γδT cells, and the molecular mechanism was also explored in the Plasmodium yoelii nigeriensis (P. yoelii NSM)-infected mice. Flow cytometry and single-cell RNA sequencing (scRNA-seq) were performed. An inverse agonist of RORα, SR3335, was used to investigate the role of RORα in regulating PD-1+ γδT cells. The results indicated that γδT cells continuously upregulated PD-1 expression during the infection period. Higher levels of CD94, IL-10, CX3CR1, and CD107a; and lower levels of CD25, CD69, and CD127 were found in PD-1+ γδT cells from infected mice than in PD-1- γδT cells. Furthermore, GO enrichment analysis revealed that the marker genes in PD-1+ γδT cells were involved in autophagy and processes utilizing autophagic mechanisms. ScRNA-seq results showed that RORα was increased significantly in PD-1+ γδT cells. GSEA identified that RORα was mainly involved in the regulation of I-kappaB kinase/NF-κB signaling and the positive regulation of cytokine production. Consistent with this, PD-1-expressing γδT cells upregulated RORα following Plasmodium yoelii infection. Additionally, in vitro studies revealed that higher levels of p-p65 were found in PD-1+ γδT cells after treatment with a RORα selective synthetic inhibitor. Collectively, these data suggest that RORα-mediated attenuation of NF-κB signaling may be fundamental for PD-1-expressing γδT cells to modulate host immune responses in the spleen of Plasmodium yoelii nigeriensis-infected C57BL/6 mice, and it requires further investigation.

Chronic malaria exposure is associated with inhibitory markers on T cells that correlate with atypical memory and marginal zone-like B cells

Chronic immune activation from persistent malaria infections can induce immunophenotypic changes associated with T-cell exhaustion. However, associations between T and B cells during chronic exposure remain undefined. We analyzed peripheral blood mononuclear cells from malaria-exposed pregnant women from Papua New Guinea and Spanish malaria-naïve individuals using flow cytometry to profile T-cell exhaustion markers phenotypically. T-cell lineage (CD3, CD4, and CD8), inhibitory (PD1, TIM3, LAG3, CTLA4, and 2B4), and senescence (CD28-) markers were assessed. Dimensionality reduction methods revealed increased PD1, TIM3, and LAG3 expression in malaria-exposed individuals. Manual gating confirmed significantly higher frequencies of PD1+CD4+ and CD4+, CD8+, and double-negative (DN) T cells expressing TIM3 in malaria-exposed individuals. Increased frequencies of T cells co-expressing multiple markers were also found in malaria-exposed individuals. T-cell data were analyzed with B-cell populations from a previous study where we reported an alteration of B-cell subsets, including increased frequencies of atypical memory B cells (aMBC) and reduction in marginal zone (MZ-like) B cells during malaria exposure. Frequencies of aMBC subsets and MZ-like B cells expressing CD95+ had significant positive correlations with CD28+PD1+TIM3+CD4+ and DN T cells and CD28+TIM3+2B4+CD8+ T cells. Frequencies of aMBC, known to associate with malaria anemia, were inversely correlated with hemoglobin levels in malaria-exposed women. Similarly, inverse correlations with hemoglobin levels were found for TIM3+CD8+ and CD28+PD1+TIM3+CD4+ T cells. Our findings provide further insights into the effects of chronic malaria exposure on circulating B- and T-cell populations, which could impact immunity and responses to vaccination.

Cytolytic circumsporozoite-specific memory CD4+ T cell clones are expanded during Plasmodium falciparum infection

Clinical immunity against Plasmodium falciparum infection develops in residents of malaria endemic regions, manifesting in reduced clinical symptoms during infection and in protection against severe disease but the mechanisms are not fully understood. Here, we compare the cellular and humoral immune response of clinically immune (0-1 episode over 18 months) and susceptible (at least 3 episodes) during a mild episode of Pf malaria infection in a malaria endemic region of Malawi, by analysing peripheral blood samples using high dimensional mass cytometry (CyTOF), spectral flow cytometry and single-cell transcriptomic analyses. In the clinically immune, we find increased proportions of circulating follicular helper T cells and classical monocytes, while the humoral immune response shows characteristic age-related differences in the protected. Presence of memory CD4+ T cell clones with a strong cytolytic ZEB2+ T helper 1 effector signature, sharing identical T cell receptor clonotypes and recognizing the Pf-derived circumsporozoite protein (CSP) antigen are found in the blood of the Pf-infected participants gaining protection. Moreover, in clinically protected participants, ZEB2+ memory CD4+ T cells express lower level of inhibitory and chemotactic receptors. We thus propose that clonally expanded ZEB2+ CSP-specific cytolytic memory CD4+ Th1 cells may contribute to clinical immunity against the sporozoite and liver-stage Pf malaria.

Background malaria incidence and parasitemia during the three-dose RTS,S/AS01 vaccination series do not reduce magnitude of antibody response nor efficacy against the first case of malaria

BACKGROUND

RTS,S/AS01 has been recommended by WHO for widespread implementation in medium to high malaria transmission settings. Previous analyses have noted lower vaccine efficacies in higher transmission settings, possibly due to the more rapid development of naturally acquired immunity in the control group.

METHODS

To investigate a reduced immune response to vaccination as a potential mechanism behind lower efficacy in high transmission areas, we examine initial vaccine antibody (anti-CSP IgG) response and vaccine efficacy against the first case of malaria (to exclude the effect of naturally acquired immunity) using data from three study areas (Kintampo, Ghana; Lilongwe, Malawi; Lambaréné, Gabon) from the 2009-2014 phase III trial (NCT00866619). Our key exposures are parasitemia during the vaccination series and background malaria incidence. We calculate vaccine efficacy (one minus hazard ratio) using a cox-proportional hazards model and allowing for the time-varying effect of RTS,S/AS01.

RESULTS

We find that antibody responses to the primary three-dose vaccination series were higher in Ghana than in Malawi and Gabon, but that neither antibody levels nor vaccine efficacy against the first case of malaria varied by background incidence or parasitemia during the primary vaccination series.

CONCLUSIONS

We find that vaccine efficacy is unrelated to infections during vaccination. Contributing to a conflicting literature, our results suggest that vaccine efficacy is also unrelated to infections before vaccination, meaning that control-group immunity is likely a major reason for lower efficacy in high transmission settings, not reduced immune responses to RTS,S/AS01. This may be reassuring for implementation in high transmission settings, though further studies are needed.

Genetic variations of Plasmodium falciparum circumsporozoite protein and the impact on interactions with human immunoproteins and malaria vaccine efficacy

In October 2021, the world's first malaria vaccine RTS,S was endorsed by WHO for broad use in children, despite its low efficacy. This study examined polyclonal infections and the associations of parasite genetic variations with binding affinity to human leukocyte antigen (HLA). Multiplicity of infection was determined by amplicon deep sequencing of PfMSP1. Genetic variations in PfCSP were examined across 88 samples from Ghana and analyzed together with 1655 PfCSP sequences from other African and non-African isolates. Binding interactions of PfCSP peptide variants and HLA were predicted using NetChop and HADDOCK. High polyclonality was detected among infections, with each infection harboring multiple non-3D7 PfCSP variants. Twenty-seven PfCSP haplotypes were detected in the Ghanaian samples, and they broadly represented PfCSP diversity across Africa. The number of genetic differences between 3D7 and non-3D7 PfCSP variants does not influence binding to HLA. However, CSP peptide length after proteolytic degradation significantly affects its molecular weight and binding affinity to HLA. Despite the high diversity of HLA, the majority of the HLAI and II alleles interacted/bound with all Ghana CSP peptides. Multiple non-3D7 strains among P. falciparum infections could impact the effectiveness of RTS,S. Longer peptides of the Th2R/Th3R CSP regions should be considered in future versions of RTS,S.

A Lower Dose of Infection Generates a Better Long-Term Immune Response against Toxoplasma gondii

Toxoplasma gondii, an obligate intracellular pathogen, induces a strong immune response in the infected host. In the encephalitis model of infection, long-term protective immunity is mediated by CD8 T cells, with the CD4 T cell population providing important help. Most of the immune studies have used a 10- to 20-cyst dose of T. gondii, which leads to T cell dysfunctionality during the late phase of chronic infection and increases the chances of reactivation. In the current study, we compared the immune response of mice orally infected with either 2 or 10 cysts of T. gondii. During the acute phase, we demonstrate that the lower dose of infection generates a reduced number of CD4 and CD8 T cells, but the frequency of functional CD4 or CD8 T cells is similar in animals infected with two different doses. However, Ag-experienced T cells (both CD4 and CD8) are better maintained in lower dose-infected mice at 8 wk postinfection, with an increase number functional cells that exhibit lower multiple inhibitory receptor expression. In addition to better long-term T cell immunity, animals infected with a lower dose display reduced inflammation manifested by lesser Ag-specific T cell and cytokine responses during the very early stage of the acute infection. Our studies suggest a previously unappreciated role of dose-dependent early programming/imprinting of the long-term CD4/CD8 T cell response during T. gondii infection. These observations point to the need for an in-depth analysis of how early events shape long-term immunity against this pathogen.

Host-parasite interactions during Plasmodium infection: Implications for immunotherapies

Malaria is a global infectious disease that remains a leading cause of morbidity and mortality in the developing world. Multiple environmental and host and parasite factors govern the clinical outcomes of malaria. The host immune response against the Plasmodium parasite is heterogenous and stage-specific both in the human host and mosquito vector. The Plasmodium parasite virulence is predominantly associated with its ability to evade the host's immune response. Despite the availability of drug-based therapies, Plasmodium parasites can acquire drug resistance due to high antigenic variations and allelic polymorphisms. The lack of licensed vaccines against Plasmodium infection necessitates the development of effective, safe and successful therapeutics. To design an effective vaccine, it is important to study the immune evasion strategies and stage-specific Plasmodium proteins, which are targets of the host immune response. This review provides an overview of the host immune defense mechanisms and parasite immune evasion strategies during Plasmodium infection. Furthermore, we also summarize and discuss the current progress in various anti-malarial vaccine approaches, along with antibody-based therapy involving monoclonal antibodies, and research advancements in host-directed therapy, which can together open new avenues for developing novel immunotherapies against malaria infection and transmission.

Impact of exposure to malaria and nutritional status on responses to the experimental malaria vaccine ChAd63 MVA ME-TRAP in 5-17 month-old children in Burkina Faso

The experimental malaria vaccine ChAd63 MVA ME-TRAP previously showed protective efficacy against Plasmodium falciparum infection in Phase IIa sporozoite challenge studies in adults in the United Kingdom and in a Phase IIb field efficacy trial in Kenyan adults. However, it failed to demonstrate efficacy in a phase IIb trial in 5-17 month-old children in an area of high malaria transmission in Burkina Faso. This secondary analysis investigated whether exposure to malaria or nutritional status might be associated with reduced responses to vaccination in this cohort. Parasite blood smears and anti-AMA-1 IgG titres were used to assess history of exposure to malaria and weight-for-length Z scores were calculated to assess nutritional status. Differences in vaccine-specific anti-TRAP IgG titre and ex vivo IFNγ ELISpot response were measured between groups. In total, n = 336 volunteers randomised to receive the experimental vaccine regimen were included in this analysis. A positive smear microscopy result was associated with reduced anti-TRAP IgG titre (geometric mean titre: 2775 (uninfected) vs 1968 (infected), p = 0.025), whilst anti-AMA-1 IgG titres were weakly negatively correlated with reduced ex vivo IFNγ ELISpot response (r = -0.18, p = 0.008). Nutritional status was not associated with either humoral or cellular immunogenicity. Vaccine efficacy was also measured separately for vaccinees with positive and negative blood smears. Although not significant in either group compared to controls, vaccine efficacy measured by Cox hazard ratio was higher in uninfected compared to infected individuals (19.8% [p = 0.50] vs 3.3% [p = 0.69]). Overall, this data suggests exposure to malaria may be associated with impaired vaccine immunogenicity. This may have consequences for the testing and eventual deployment of various vaccines, in areas with high endemicity for malaria.

Trial Registration

Pactr.org, identifier PACTR201208000404131; ClinicalTrials.gov, identifier NCT01635647.

Role of the PD-1/PD-L1 Pathway in Experimental Trypanosoma cruzi Infection and Potential Therapeutic Options

Chagas disease (CD) is a neglected chronic infection caused by the protozoan parasite Trypanosoma cruzi (T. cruzi). A significant portion of infected people develops cardiac or digestive alterations over a lifetime. Since several chronic infections associated with antigen persistence and inflammation have been shown to lead to T cell exhaustion, new therapies targeting co-inhibitory receptors to regain T cell activity are under consideration. This study explored immune therapeutic approaches targeting the inhibitory PD-1/PD-L pathway in an experimental model for CD. Infected PD-L1 knockout mice (PD-L1 KO) showed increased systemic parasitemia in blood although no significant differences in parasite load were observed in different organs. Furthermore, we found no significant differences in the frequency of activated T cells or proinflammatory cytokine production when compared to WT counterparts. PD-L1 deficiency led to the production of IL-10 by CD8+ T cells and an upregulation of Tim-3 and CD244 (2B4). Unexpectedly, the lack of PD-L1 did not contribute to a significantly improved T cell response to infection. Single blockade and combined blockade of PD-1 and Tim-3 using monoclonal antibodies confirmed the results observed in infected. PD-L1 KO mice. Our results describe for the first time that the interruption of the PD-1/PD-L1 axis during acute T. cruzi infection does not necessarily enhance the immune response against this parasite. Its interruption favors increased levels of parasitemia and sustained upregulation of other co-inhibitory receptors as well as the production of regulatory cytokines. These results suggest that the clinical application of immune therapeutic approaches targeting the PD-1/PD-L1 axis in CD might be risky and associated with adverse events. It highlights that more research is urgently needed to better understand the immune regulation of T cells in CD before designing immune therapeutic approaches for a clinical context.

Eryptosis as a New Insight in Malaria Pathogenesis

Eryptosis is a programmed cell death-like process that occurs in red blood cells. Although the red blood cells are anucleated, there are similarities between eryptosis and apoptosis, such as increased calcium efflux, calpain activation, phosphatidylserine exposure, cell blebbing and cell shrinkage. Eryptosis occurs physiologically in red blood cells, as a consequence of the natural senescence process of these cells, but it can also be stimulated in pathological situations such as metabolic syndromes, uremic syndromes, polycythemia vera, anemias such as sickle cell anemia and thalassemia, and infectious processes including Plasmodium infection. Infection-induced eryptosis is believed to contribute to damage caused by Plasmodium, but it’s still a topic of debate in the literature. In this review, we provided an overview of eryptosis mechanisms and its possible pathogenic role in malaria.

Influenza virus-like particle vaccine containing both apical membrane antigen 1 and microneme-associated antigen proteins of Plasmodium berghei confers protection in mice

Background

Apical membrane antigen 1 (AMA1) and microneme-associated antigen (MIC) of Plasmodium parasites are important factors involved in host cell invasion.

Methods

In this study, influenza VLP vaccines containing both codon-optimized AMA1 and MIC were generated and the vaccine efficacy was evaluated in mice.

Results

VLPs vaccine immunization elicited higher levels of parasite-specific IgG and IgG2a antibody responses in sera. CD4⁺ and CD8⁺ T cells and germinal center B cells in blood, inguinal lymph nodes (ILN) and spleen were found to be significantly increased. Importantly, VLPs vaccination significantly reduced the levels of pro-inflammatory cytokines IFN-γ and TNF-α, decreased parasitemia in blood, resulting in lower body weight loss and longer survival time compared to control.

Conclusion

These results indicated that VLPs containing P. berghei AMA1 and MIC could be a candidate for malaria blood-stage vaccine design.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12865-022-00494-4.

Dysfunctional State of T Cells or Exhaustion During Chronic Viral Infections and COVID-19: A Review

Coronavirus disease 2019 (COVID-19) continuously affecting the lives of millions of people. The virus is spread through the respiratory route to an uninfected person, causing mild-to-moderate respiratory disease-like symptoms that sometimes progress to severe form and can be fatal. When the host is infected with the virus, both innate and adaptive immunity comes into play. The effector T cells act as the master player of adaptive immune response in eradicating the virus from the system. But during cancer and chronic viral infections, the fate of an effector T cell is altered, and the T cell may enters a state of exhaustion, which is marked by loss of effector function, depleted proliferative capacity and cytotoxic effect accomplished by an increased expression of numerous inhibitory receptors such as programmed cell death protein 1 (PD-1), lymphocyte-activation protein 3 (LAG-3), and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) on their surface. Various other transcriptional and epigenetic changes take place inside the T cell when it enters into an exhausted state. Latest studies point toward the induction of an abnormal immune response such as lymphopenia, cytokine storm, and T cell exhaustion during SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) infection. This review sheds light on the dysfunctional state of T cells during chronic viral infection and COVID-19. Understanding the cause and the effect of T cell exhaustion observed during COVID-19 may help resolve new therapeutic potentials for treating chronic infections and other diseases.

Deciphering the Plasmodium falciparum malaria-specific CD4+ T-cell response: ex vivo detection of high frequencies of PD-1+TIGIT+ EXP1-specific CD4+ T cells using a novel HLA-DR11-restricted MHC class II tetramer

Relatively little is known about the ex vivo frequency and phenotype of the Plasmodium falciparum-specific CD4+ T-cell response in humans. The exported protein 1 (EXP1) is expressed by plasmodia at both, the liver stage and blood stage, of infection making it a potential target for CD4+ and CD8+ effector T cells. Here, a fluorochrome-labelled HLA-DRB1∗11:01-restriced MHC class II tetramer derived from the P. falciparum EXP1 (aa62-74) was established for ex vivo tetramer analysis and magnetic bead enrichment in 10 patients with acute malaria. EXP1-specific CD4+ T cells were detectable in 9 out of 10 (90%) malaria patients expressing the HLA-DRB1∗11 molecule with an average ex vivo frequency of 0.11% (0-0.22%) of total CD4+ T cells. The phenotype of EXP1-specific CD4+ T cells was further assessed using co-staining with activation (CD38, HLA-DR, CD26), differentiation (CD45RO, CCR7, KLRG1, CD127), senescence (CD57), and co-inhibitory (PD-1, TIGIT, LAG-3, TIM-3) markers as well as the ectonucleotidases CD39 and CD73. EXP1-specific tetramer+ CD4+ T cells had a distinct phenotype compared to bulk CD4+ T cells and displayed a highly activated effector memory phenotype with elevated levels of co-inhibitory receptors and activation markers: EXP1-specific CD4+ T cells universally expressed the co-inhibitory receptors PD-1 and TIGIT as well as the activation marker CD38 and showed elevated frequencies of CD39. These results demonstrate that MHC class II tetramer enrichment is a sensitive approach to investigate ex vivo antigen-specific CD4+ T cells in malaria patients that will aid further analysis of the role of CD4+ T cells during malaria.

Xylopic acid-amodiaquine and xylopic acid-artesunate combinations are effective in managing malaria in Plasmodium berghei-infected mice

Background

Evidence of Plasmodium resistance to some of the current anti-malarial agents makes it imperative to search for newer and effective drugs to combat malaria. Therefore, this study evaluated whether the co-administrations of xylopic acid-amodiaquine and xylopic acid-artesunate combinations will produce a synergistic anti-malarial effect.

Methods

Antiplasmodial effect of xylopic acid (XA: 3, 10, 30, 100, 150 mg kg⁻¹), artesunate (ART: 1, 2, 4, 8, 16 mg kg⁻¹), and amodiaquine (AQ: 1.25, 2.5, 5, 10, 20 mg kg⁻¹) were evaluated in Plasmodium berghei (strain ANKA)-infected mice to determine respective ED₅₀s. Artemether/lumefantrine was used as the positive control. XA/ART and XA/AQ were subsequently administered in a fixed-dose combination of their ED₅₀s (1:1) and the combination fractions of their ED₅₀s (1/2, 1/4, 1/8, 1/16, and 1/32) to determine the experimental ED₅₀s (Z_exp). An isobologram was constructed to determine the nature of the interaction between XA/ART, and XA/AQ combinations by comparing Z_exp with the theoretical ED₅₀ (Z_add). Bodyweight and 30-day survival post-treatment were additionally recorded.

Results

ED₅₀s for XA, ART, and AQ were 9.0 ± 3.2, 1.61 ± 0.6, and 3.1 ± 0.8 mg kg⁻¹, respectively. The Z_add, Z_exp, and interaction index for XA/ART co-administration was 5.3 ± 2.61, 1.98 ± 0.25, and 0.37, respectively while that of XA/AQ were 6.05 ± 2.0, 1.69 ± 0.42, and 0.28, respectively. The Z_exp for both combination therapies lay significantly (p < 0.001) below the additive isoboles showing XA acts synergistically with both ART and AQ in clearing the parasites. High doses of XA/ART combination significantly (p < 0.05) increased the survival days of infected mice with a mean hazard ratio of 0.40 while all the XA/AQ combination doses showed a significant (p < 0.05) increase in the survival days of infected mice with a mean hazard ratio of 0.27 similar to AL. Both XA/ART and XA/AQ combined treatments significantly (p < 0.05) reduced weight loss.

Conclusion

Xylopic acid co-administration with either artesunate or amodiaquine produces a synergistic anti-plasmodial effect in mice infected with P. berghei.

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

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

Adaptive immune responses mediated age-related Plasmodium yoelii 17XL and 17XNL infections in 4 and 8-week-old BALB/c mice

BACKGROUD

It is important to expound the opposite clinical outcomes between children and adulthood for eradicate malaria. There remains unknown about the correlation between adaptive immune response and age-related in malaria.

METHODS

4 and 8-week-old mice were used to mimic children and adulthood, respectively. Parasitemia and the survival rate were monitored. The proportion and function of Th1 and Th2 cells were detected by FACS. The levels of IFN-γ, IL-4, total IgG, IgG1, IgG2a and Plasmodium yoelii MSP-1-specific IgG were measured by ELISA.

RESULTS

The adult group showed greater resistance to P. yoelii 17XL infection, with lower parasitemia. Compared with 4-week-old mice, the percentage of CD4⁺T-bet⁺IFN-γ⁺ Th1 cells as well as IFN-γ production were significantly increased on day 5 p.i. in the 8-week-old mice after P. yoelii 17XNL infection. The percentage of CD4⁺GATA3⁺IL-4⁺ Th2 cells and CD4⁺CXCR5⁺ Tfh cells, and IL-4 production in the 8-week-old mice significantly increased on day 5 and day 10 after P. yoelii 17XNL infection. Notably, the levels of total IgG, IgG1, IgG2a and P. yoelii MSP-1-specific IgG were also significantly increased in the 8-week-old mice. PD-1, a marker of exhaustion, was up-regulated on CD4⁺ or activated CD4⁺ T cells in the 8-week-old mice as compared to the 4-week-old group.

CONCLUSIONS

Thus, we consider that enhanced cellular and humoral adaptive immunity might contribute to rapid clearance of malaria among adults, likely in a PD-1-dependent manner due to induction of CD4⁺ T cells exhaustion in P. yoelii 17XNL infected 8-week-old mice.

A population of CD4hiCD38hi T cells correlates with disease severity in patients with acute malaria

OBJECTIVE

CD4⁺ T cells are critical mediators of immunity to Plasmodium spp. infection, but their characteristics during malarial episodes and immunopathology in naturally infected adults are poorly defined. Flow cytometric analysis of PBMCs from patients with either P. falciparum or P. knowlesi malaria revealed a pronounced population of CD4⁺ T cells co-expressing very high levels of CD4 and CD38 we have termed CD4^hiCD38^hi T cells. We set out to gain insight into the function of these novel cells.

METHODS

CD4⁺ T cells from 18 patients with P. falciparum or P. knowlesi malaria were assessed by flow cytometry and sorted into populations of CD4^hiCD38^hi or CD4^norm T cells. Gene expression in the sorted populations was assessed by qPCR and NanoString.

RESULTS

CD4^hiCD38^hi T cells expressed high levels of CD4 mRNA and canonical type 1 regulatory T-cell (TR1) genes including IL10, IFNG, LAG3 and HAVCR2 (TIM3), and other genes with relevance to cell migration and immunomodulation. These cells increased in proportion to malaria disease severity and were absent after parasite clearance with antimalarials.

CONCLUSION

In naturally infected adults with acute malaria, a prominent population of type 1 regulatory T cells arises that can be defined by high co-expression of CD4 and CD38 (CD4^hiCD38^hi) and that correlates with disease severity in patients with falciparum malaria. This study provides fundamental insights into T-cell biology, including the first evidence that CD4 expression is modulated at the mRNA level. These findings have important implications for understanding the balance between immunity and immunopathology during malaria.

Toll-like receptor 4, Toll-like receptor 7 and Toll-like receptor 9 agonists enhance immune responses against blood-stage Plasmodium chabaudi infection in BALB/c mice

BACKGROUND

Toll-like receptor (TLR) signals play vital roles during the blood-stage of malaria infections. However, the roles of TLR agonists in the regulation of immune responses and the development of protective immunity to malaria remain poorly understood.

METHOD

BALB/c mice were pre-treated with TLR4, TLR7 and TLR9 agonists, followed by infection with Plasmodium chabaudi. After infection, splenic dendritic cells (DCs), Th1 cells and programmed death-1 (PD-1) expressed on Th1 cells, as well as regulatory T cells (Tregs) were analyzed by flow cytometry. The levels of IFN-γ, TNF-α, TGF-β and IL-10 in splenocytes and IgG1 and IgG2a in serum were measured by ELISA.

RESULT

Administration of TLR4, TLR7 and TLR9 agonists prior to infection improved disease outcomes. All TLR agonists promoted DC activation, and the proportions of Th1 cells increased. In TLR4, TLR7 and TLR9 agonist treated groups the levels of pro-inflammatory cytokines IFN-γ and TNF-α were elevated, and IgG1 and IgG2a serum levels were also significantly increased. TLR4, TLR7 and TLR9 agonists diminished the activation of Tregs and down-regulated the anti-inflammatory cytokines TGF-β and IL-10. Finally, PD-1 expressed on Th1 cells were decreased in TLR4, TLR7 and TLR9 agonist treated groups compared with control groups.

CONCLUSION

TLR4, TLR7 and TLR9 agonists activated DC-mediated innate immune responses and adaptive immune response, which against the blood-stage of Plasmodium and might be applied to malaria protection and treatment.

Immune responses to malaria pre-erythrocytic stages: Implications for vaccine development

Radiation-attenuated sporozoites induce sterilizing immunity and remain the 'gold standard' for malaria vaccine development. Despite practical challenges in translating these whole sporozoite vaccines to large-scale intervention programmes, they have provided an excellent platform to dissect the immune responses to malaria pre-erythrocytic (PE) stages, comprising both sporozoites and exoerythrocytic forms. Investigations in rodent models have provided insights that led to the clinical translation of various vaccine candidates-including RTS,S/AS01, the most advanced candidate currently in a trial implementation programme in three African countries. With advances in immunology, transcriptomics and proteomics, and application of lessons from past failures, an effective, long-lasting and wide-scale malaria PE vaccine remains feasible. This review underscores the progress in PE vaccine development, focusing on our understanding of host-parasite immunological crosstalk in the tissue environments of the skin and the liver. We highlight possible gaps in the current knowledge of PE immunity that can impact future malaria vaccine development efforts.

Cellular and Molecular Mechanisms of CD8+ T Cell Differentiation, Dysfunction and Exhaustion

T cells follow a triphasic distinct pathway of activation, proliferation and differentiation before becoming functionally and phenotypically “exhausted” in settings of chronic infection, autoimmunity and in cancer. Exhausted T cells progressively lose canonical effector functions, exhibit altered transcriptional networks and epigenetic signatures and gain constitutive expression of a broad coinhibitory receptor suite. This review outlines recent advances in our understanding of exhausted T cell biology and examines cellular and molecular mechanisms by which a state of dysfunction or exhaustion is established, and mechanisms by which exhausted T cells may still contribute to pathogen or tumour control. Further, this review describes our understanding of exhausted T cell heterogeneity and outlines the mechanisms by which checkpoint blockade differentially engages exhausted T cell subsets to overcome exhaustion and recover T cell function.

Production of glycosylphosphatidylinositol-anchored proteins for vaccines and directed binding of immunoliposomes to specific cell types

BACKGROUND

Liposomes are highly useful carriers for delivering drugs or antigens. The association of glycosylphosphatidylinositol (GPI)-anchored proteins to liposomes potentially enhances the immunogenic effect of vaccine antigens by increasing their surface concentration. Furthermore, the introduction of a universal immunoglobulin-binding domain can make liposomes targetable to virtually any desired receptor for which antibodies exist.

METHODS

We developed a system for the production of recombinant proteins with GPI anchors and histidine tags and Strep-tags for simplified purification from cells. This system was applied to i) the green fluorescent protein (GFP) as a reporter, ii) the promising Plasmodium falciparum vaccine antigen PfRH5 and iii) a doubled immunoglobulin Fc-binding domain termed ZZ from protein A of Staphylococcus aureus. As the GPI-attachment domain, the C-terminus of murine CD14 was used. After the recovery of these three recombinant proteins from Chinese hamster ovary (CHO) cells and association with liposomes, their vaccine potential and ability to target the CD4 receptor on lymphocytes in ex vivo conditions were tested.

RESULTS

Upon immunization in mice, the PfRH5-GPI-loaded liposomes generated antibody titers of 103 to 104, and showed a 45% inhibitory effect on in vitro growth at an IgG concentration of 600 µg/mL in P. falciparum cultures. Using GPI-anchored ZZ to couple anti-CD4 antibodies to liposomes, we created immunoliposomes with a binding efficiency of 75% to CD4+ cells in splenocytes and minimal off-target binding.

CONCLUSIONS

Proteins are very effectively associated with liposomes via a GPI-anchor to form proteoliposome particles and these are useful for a variety of applications including vaccines and antibody-mediated targeting of liposomes. Importantly, the CHO-cell and GPI-tagged produced PfRH5 elicited invasion-blocking antibodies qualitatively comparable to other approaches.

Toll/IL-1 receptor-containing proteins STIR-1, STIR-2 and STIR-3 synergistically assist Yersinia ruckeri SC09 immune escape

Immune escape is a common feature of bacteria, viruses, parasites and even cancer cells. Our earlier work on an integrative and conjugative element (ICEr2) of Yersinia ruckeri SC09 demonstrated contributory roles of stir-1, stir-2 and stir-3 in bacterial toxicity and ability to code for immune evasion. Here, we further examined the ability of stir-4 in ICE (r2) and its encoded STIR-4 protein to mediate immune evasion using comparative genomic analysis. Additionally, the mechanisms underlying the synergistic activities of STIR-1, STIR-2, STIR-3 and STIR-4 in immune evasion were examined. Our results showed that STIR-4 did not contribute to bacterial toxicity, either in vivo nor in vitro, or show the ability to assist in bacterial immune escape. STIR-1, STIR-2, and STIR-3 formed heterotrimers in bacteria while facilitating immune evasion, which we speculate may be essential to maintain their stability. This discovery also partially explains the previous finding that a single gene can mediate immune evasion. Our data provide further knowledge on the distribution of ICE (r2)-like elements in bacteria, validating the prevalence of large-scale gene transfer in pathogens and its potential for enhancing virulence levels. Further studies are necessary to establish the biological significance of the ICE (r2) component.

Protective Immunity Induced by Virus-Like Particle Containing Merozoite Surface Protein 9 of Plasmodium berghei

Merozoite surface protein 9 (MSP-9) from Plasmodium has shown promise as a vaccine candidate due to its location and possible role in erythrocyte invasion. In this study, we generated virus-like particles (VLPs) targeting P. berghei MSP-9, and investigated the protection against lethal doses of P. berghei in a mouse model. We found that VLP vaccination induced a P. berghei-specific IgG antibody response in the sera and CD4⁺ and CD8⁺ T cell populations in blood compared to a naïve control group. Upon challenge infection with P. berghei, vaccinated mice showed a significant increase in CD4⁺ and CD8⁺ effector memory T cell and memory B cell populations. Importantly, MSP-9 VLP immunization inhibited levels of the pro-inflammatory cytokines IFN-γ and IL-6 in the spleen and parasite replication in blood, resulting in significantly prolonged survival time. These results suggest that the MSP-9 VLP vaccine may constitute an effective malaria vaccine.

Novel Peptide-Based PD1 Immunomodulators Demonstrate Efficacy in Infectious Disease Vaccines and Therapeutics

Many pathogens use the same immune evasion mechanisms as cancer cells. Patients with chronic infections have elevated levels of checkpoint receptors (e.g., programed cell death 1, PD1) on T cells. Monoclonal antibody (mAb)-based inhibitors to checkpoint receptors have also been shown to enhance T-cell responses in models of chronic infection. Therefore, inhibitors have the potential to act as a vaccine “adjuvant” by facilitating the expansion of vaccine antigen-specific T-cell repertoires. Here, we report the discovery and characterization of a peptide-based class of PD1 checkpoint inhibitors, which have a potent adaptive immunity adjuvant capability for vaccines against infectious diseases. Briefly, after identifying peptides that bind to the recombinant human PD1, we screened for in vitro efficacy in reporter assays and human peripheral blood mononuclear cells (PBMC) readouts. We first found the baseline in vivo performance of the peptides in a standard mouse oncology model that demonstrated equivalent efficacy compared to mAbs against the PD1 checkpoint. Subsequently, two strategies were used to demonstrate the utility of our peptides in infectious disease indications: (1) as a therapeutic in a bacteria-induced lethal sepsis model in which our peptides were found to increase survival with enhanced bacterial clearance and increased macrophage function; and (2) as an adjuvant in combination with a prophylactic malaria vaccine in which our peptides increased T-cell immunogenicity and the protective efficacy of the vaccine. Therefore, our peptides are promising as both a therapeutic agent and a vaccine adjuvant for infectious disease with a potentially safer and more cost-effective target product profile compared to mAbs. These findings are essential for deploying a new immunomodulatory regimen in infectious disease primary and clinical care settings.

A key control point in the T cell response to chronic infection and neoplasia: FOXO1

T cells able to control neoplasia or chronic infections display a signature gene expression profile similar or identical to that of central memory T cells. These cells have qualities of self-renewal and a plasticity that allow them to repeatedly undergo activation (growth, proliferation, and differentiation), followed by quiescence. It is these qualities that define the ability of T cells to establish an equilibrium with chronic infectious agents, and also preserve the ability of T cells to be re-activated (by checkpoint therapy) in response to malignant cancers. Here we describe distinctions between the forms of inhibition mediated by tumors and persistent viruses, we review the properties of T cells associated with long-term immunity, and we identify the transcription factor, FOXO1, as the control point for a program of gene expression that allows CD8⁺ T cells to undergo serial reactivation and self-renewal.

PD-1 blockade promotes immune memory following Plasmodium berghei ANKA reinfection

The establishment of malaria immune memory is slow, incomplete, and short-lived. The mechanisms underpinning the generation and maintenance of anti-malarial immune memory remain unclear. This study evaluated the possible role of programmed cell death-1 (PD-1) in the establishment of malaria immune memory. Following infection by Plasmodium berghei ANKA (Pb ANKA) we compared natural immunity, acquired immunity, and immune memory between WT and mice lacking PD-1 via monoclonal antibody treatment. We found that parasitemia levels were significantly lower in the PD-1 knockdown group. After PD-1 elimination, dendritic cells, Th1, and T-follicular helper cells increased significantly. In addition, memory T, long-lived plasma cells, and serum antibody production also increased significantly. Therefore, PD-1 elimination induced stronger natural and acquired immune responses and enhanced immune memory against the parasite.

Reassessing therapeutic antibodies for neglected and tropical diseases

In the past two decades there has been a significant expansion in the number of new therapeutic monoclonal antibodies (mAbs) that are approved by regulators. The discovery of these new medicines has been driven primarily by new approaches in inflammatory diseases and oncology, especially in immuno-oncology. Other recent successes have included new antibodies for use in viral diseases, including HIV. The perception of very high costs associated with mAbs has led to the assumption that they play no role in prophylaxis for diseases of poverty. However, improvements in antibody-expression yields and manufacturing processes indicate this is a cost-effective option for providing protection from many types of infection that should be revisited. Recent technology developments also indicate that several months of protection could be achieved with a single dose. Moreover, new methods in B cell sorting now enable the systematic identification of high-quality antibodies from humanized mice, or patients. This Review discusses the potential for passive immunization against schistosomiasis, fungal infections, dengue, and other neglected diseases.

Protection induced by malaria virus-like particles containing codon-optimized AMA-1 of Plasmodium berghei

Background

Despite the extensive endeavours, developing an effective malaria vaccine remains as a great challenge. Apical membrane antigen 1 (AMA-1) located on the merozoite surface of parasites belonging to the genus Plasmodium is involved in red blood cell invasion.

Methods

Influenza virus-like particle (VLP) vaccines containing codon-optimized or native (non-codon optimized) AMA-1 from Plasmodium berghei were generated. VLP-induced protective immunity was evaluated in a mouse model.

Results

Mice immunized with VLP vaccine containing the codon-optimized AMA-1 elicited higher levels of P. berghei-specific IgG and IgG2a antibody responses compared to VLPs containing non-codon optimized AMA-1 before and after challenge infection. Codon-optimized AMA-1 VLP vaccination induced higher levels of CD4⁺ T cells, CD8⁺ T cells, B cells, and germinal centre cell responses compared to non-codon optimized AMA-1 VLPs. Importantly, the codon-optimized AMA-1 VLP vaccination showed lower body weight loss, longer survival and a significant decrease in parasitaemia compared to non-codon optimized VLP vaccination.

Conclusion

Overall, VLP vaccine expressing codon-optimized AMA-1 induced better protective efficacy than VLPs expressing the non-codon optimized AMA-1. Current findings highlight the importance of codon-optimization for vaccine use and its potential involvement in future malaria vaccine design strategies.

Pathogenic Th1 responses in CHIKV-induced inflammation and their modulation upon Plasmodium parasites co-infection

The induction of polyarthritis and polyarthralgia is a hallmark of arthritogenic alphavirus infections, with an exceptionally higher morbidity observed with chikungunya virus (CHIKV). While the mechanisms underlying these incapacitating acute symptoms remain partially understood, the progression to chronic conditions in some cases remains unanswered. The highly pro‐inflammatory nature of alphavirus disease has suggested the involvement of virus‐specific, joint‐infiltrating Th1 cells as one of the main pathogenic mediators of CHIKV‐induced joint pathologies. This review summarizes the role of cell‐mediated immune responses in CHIKV pathogenesis, with a specific focus on pro‐inflammatory Th1 responses in the development of CHIKV joint inflammation. Furthermore, due to the explosive nature of arthritogenic alphavirus outbreaks and their recent expansion across the world, co‐infections with other highly prevalent pathogens such as malaria are likely to occur but the pathological outcomes of such interactions in humans are unknown. This review will also discuss the potential impact of malaria co‐infections on CHIKV pathogenesis and their relevance in alphavirus control programs in endemic areas.

Liver-Mediated Adaptive Immune Tolerance

The liver is an immunologically tolerant organ that is uniquely equipped to limit hypersensitivity to food-derived antigens and bacterial products through the portal vein and can feasibly accept liver allografts. The adaptive immune response is a major branch of the immune system that induces organ/tissue-localized and systematic responses against pathogens and tumors while promoting self-tolerance. Persistent infection of the liver with a virus or other pathogen typically results in tolerance, which is a key feature of the liver. The liver's immunosuppressive microenvironment means that hepatic adaptive immune cells become readily tolerogenic, promoting the death of effector cells and the “education” of regulatory cells. The above mechanisms may result in the clonal deletion, exhaustion, or inhibition of peripheral T cells, which are key players in the adaptive immune response. These tolerance mechanisms are believed to be responsible for almost all liver diseases. However, optimal protective adaptive immune responses may be achieved through checkpoint immunotherapy and the modulation of hepatic innate immune cells in the host. In this review, we focus on the mechanisms involved in hepatic adaptive immune tolerance, the liver diseases caused thereby, and the therapeutic strategies needed to overcome this tolerance.

Immune Exhaustion: Past Lessons and New Insights from Lymphocytic Choriomeningitis Virus

Lymphocytic choriomeningitis virus (LCMV) is a paradigm-forming experimental system with a remarkable track record of contributing to the discovery of many of the fundamental concepts of modern immunology. The ability of LCMV to establish a chronic infection in immunocompetent adult mice was instrumental for identifying T cell exhaustion and this system has been invaluable for uncovering the complexity, regulators, and consequences of this state. These findings have been directly relevant for understanding why ineffective T cell responses commonly arise during many chronic infections including HIV and HCV, as well as during tumor outgrowth. The principal feature of exhausted T cells is the inability to elaborate the array of effector functions necessary to contain the underlying infection or tumor. Using LCMV to determine how to prevent and reverse T cell exhaustion has highlighted the potential of checkpoint blockade therapies, most notably PD-1 inhibition strategies, for improving cellular immunity under conditions of antigen persistence. Here, we discuss the discovery, properties, and regulators of exhausted T cells and highlight how LCMV has been at the forefront of advancing our understanding of these ineffective responses.

Controlled Infection Immunization Using Delayed Death Drug Treatment Elicits Protective Immune Responses to Blood-Stage Malaria Parasites

Naturally acquired immunity to malaria is robust and protective against all strains of the same species of Plasmodium. This develops as a result of repeated natural infection, taking several years to develop.

Naturally acquired immunity to malaria is robust and protective against all strains of the same species of Plasmodium. This develops as a result of repeated natural infection, taking several years to develop. Evidence suggests that apoptosis of immune lymphocytes due to uncontrolled parasite growth contributes to the slow acquisition of immunity. To hasten and augment the development of natural immunity, we studied controlled infection immunization (CII) using low-dose exposure to different parasite species (Plasmodium chabaudi, P. yoelii, or P. falciparum) in two rodent systems (BALB/c and C57BL/6 mice) and in human volunteers, with drug therapy commencing at the time of initiation of infection. CIIs with infected erythrocytes and in conjunction with doxycycline or azithromycin, which are delayed death drugs targeting the parasite’s apicoplast, allowed extended exposure to parasites at low levels. In turn, this induced strong protection against homologous challenge in all immunized mice. We show that P. chabaudi/P. yoelii infection initiated at the commencement of doxycycline therapy leads to cellular or antibody-mediated protective immune responses in mice, with a broad Th1 cytokine response providing the best correlate of protection against homologous and heterologous species of Plasmodium. P. falciparum CII with doxycycline was additionally tested in a pilot clinical study (n = 4) and was found to be well tolerated and immunogenic, with immunological studies primarily detecting increased cell-associated immune responses. Furthermore, we report that a single dose of the longer-acting drug, azithromycin, given to mice (n = 5) as a single subcutaneous treatment at the initiation of infection controlled P. yoelii infection and protected all mice against subsequent challenge.

Soluble markers of neutrophil, T-cell and monocyte activation are associated with disease severity and parasitemia in falciparum malaria

Background

The immune response during P. falciparum infection is a two-edged sword, involving dysregulation of the inflammatory responses with several types of immune cells participating. Here we examined T-cell, monocyte/macrophage and neutrophil activation during P. falciparum infection by using soluble activation markers for these leukocyte subsets.

Methods

In a prospective cross-sectional study clinical data and blood samples were collected from adults in Mozambique with P. falciparum infection, with (n = 70) and without (n = 61) co-infection with HIV-1, as well as HIV-infected patients with similar symptoms but without malaria (n = 58) and healthy controls (n = 52). Soluble (s)CD25, sCD14, sCD163 and myeloperoxidase (MPO) as markers for T-cell, monocyte/macrophage and neutrophil activation, respectively as well as CX3CL1, granzyme B and TIM-3 as markers of T-cell subsets and T-cell exhaustion, were analyzed.

Results

All patient groups had raised levels of activation markers compared with healthy controls. Levels of sCD25 and MPO increased gradually from patient with HIV only to patient with malaria only, with the highest levels in the HIV/malaria group. In the malaria group as a whole, MPO, sCD14 and in particular sCD25 were correlated with disease severity. sCD163, sCD25 and in particular MPO correlated with the degree of parasitemia as assessed by qPCR. Patients with falciparum malaria also had signs of T-cell subset activation (i.e. increased granzyme B and CX3CL1) and T-cell exhaustion as assessed by high levels of TIM-3 particularly in patients co-infected with HIV.

Conclusion

Our data support a marked immune activation in falciparum malaria involving all major leukocyte subsets with particular enhanced activation of neutrophils and T-cells in patients co-infected with HIV. Our findings also support a link between immune activation and immune exhaustion during falciparum malaria, particularly in relation to T-cell responses in patients co-infected with HIV.

Electronic supplementary material

The online version of this article (10.1186/s12879-018-3593-8) contains supplementary material, which is available to authorized users.

Plasmodium genomics: an approach for learning about and ending human malaria

Malaria causes high levels of morbidity and mortality in human beings worldwide. According to the World Health Organization (WHO), about half a million people die of this disease each year. Malaria is caused by six species of parasites belonging to the Plasmodium genus: P. falciparum, P. knowlesi, P. vivax, P. malariae, P. ovale curtisi, and P. ovale wallikeri. Currently, malaria is being kept under control with varying levels of elimination success in different countries. The development of new molecular tools as well as the use of next-generation sequencing (NGS) technologies and novel bioinformatic approaches has improved our knowledge of malarial epidemiology, diagnosis, treatment, vaccine development, and surveillance strategies. In this work, the genetics and genomics of human malarias have been analyzed. Since the first P. falciparum genome was sequenced in 2002, various population-level genetic and genomic surveys, together with transcriptomic and proteomic studies, have shown the importance of molecular approaches in supporting malaria elimination.

Detection of host pathways universally inhibited after Plasmodium yoelii infection for immune intervention

Malaria is a disease with diverse symptoms depending on host immune status and pathogenicity of Plasmodium parasites. The continuous parasite growth within a host suggests mechanisms of immune evasion by the parasite and/or immune inhibition in response to infection. To identify pathways commonly inhibited after malaria infection, we infected C57BL/6 mice with four Plasmodium yoelii strains causing different disease phenotypes and 24 progeny of a genetic cross. mRNAs from mouse spleens day 1 and/or day 4 post infection (p.i.) were hybridized to a mouse microarray to identify activated or inhibited pathways, upstream regulators, and host genes playing an important role in malaria infection. Strong interferon responses were observed after infection with the N67 strain, whereas initial inhibition and later activation of hematopoietic pathways were found after infection with 17XNL parasite, showing unique responses to individual parasite strains. Inhibitions of pathways such as Th1 activation, dendritic cell (DC) maturation, and NFAT immune regulation were observed in mice infected with all the parasite strains day 4 p.i., suggesting universally inhibited immune pathways. As a proof of principle, treatment of N67-infected mice with antibodies against T cell receptors OX40 or CD28 to activate the inhibited pathways enhanced host survival. Controlled activation of these pathways may provide important strategies for better disease management and for developing an effective vaccine.

Neutralization of the Plasmodium-encoded MIF ortholog confers protective immunity against malaria infection

Plasmodium species produce an ortholog of the cytokine macrophage migration inhibitory factor, PMIF, which modulates the host inflammatory response to malaria. Using a novel RNA replicon-based vaccine, we show the impact of PMIF immunoneutralization on the host response and observed improved control of liver and blood-stage Plasmodium infection, and complete protection from re-infection. Vaccination against PMIF delayed blood-stage patency after sporozoite infection, reduced the expression of the Th1-associated inflammatory markers TNF-α, IL-12, and IFN-γ during blood-stage infection, augmented Tfh cell and germinal center responses, increased anti-Plasmodium antibody titers, and enhanced the differentiation of antigen-experienced memory CD4 T cells and liver-resident CD8 T cells. Protection from re-infection was recapitulated by the adoptive transfer of CD8 or CD4 T cells from PMIF RNA immunized hosts. Parasite MIF inhibition may be a useful approach to promote immunity to Plasmodium and potentially other parasite genera that produce MIF orthologous proteins.

Plasmodium species produce an ortholog of the cytokine macrophage migration inhibitory factor, PMIF, which modulates the host inflammatory response to malaria. Here, the authors show that inhibition of PMIF may have translational benefits for managing malaria infections.

Both inflammatory and regulatory cytokine responses to malaria are blunted with increasing age in highly exposed children

Background

Young children are at greatest risk for malaria-associated morbidity and mortality. The immune response of young children differs in fundamental ways from that of adults, and these differences likely contribute to the increased susceptibility of children to severe malaria and to their delayed development of immunity. Elevated levels of pro-inflammatory cytokines and chemokines in the peripheral blood during acute infection contribute to the control of parasitaemia, but are also responsible for much of the immunopathology seen during symptomatic disease. Clinical immunity to malaria may depend upon the ability to regulate these pro-inflammatory responses, possibly through mechanisms of immunologic tolerance. In order to explore the effect of age on the immune response to malaria and the development of clinical immunity, cytokines and chemokines were measured in the plasma of children at day 0 of an acute malaria episode and during convalescence.

Results

Younger children presenting with acute malaria exhibited much higher levels of TNF, IL2, and IL6, as well as increased Th1 associated chemokines IP10, MIG, and MCP1, compared to older children with acute malaria. Additionally, the regulatory cytokines IL10 and TNFRI were dramatically elevated in younger children compared to older children during acute infection, indicating that regulatory as well as pro-inflammatory cytokine responses are dampened in later childhood.

Conclusions

Together these data suggest that there is a profound blunting of the cytokine and chemokine response to malaria among older children residing in endemic settings, which may be due to repeated malaria exposure, intrinsic age-based differences in the immune response, or both.

Evaluating antidisease immunity to malaria and implications for vaccine design

Immunity to malaria could be categorized broadly as antiparasite or antidisease immunity. While most vaccine research efforts have focused on antiparasite immunity, the evidence from endemic populations suggest that antidisease immunity is an important component of natural immunity to malaria. The processes that mediate antidisease immunity have, however, attracted little to no attention, and most interests have been directed towards the antibody responses. This review evaluates the evidence for antidisease immunity in endemic areas and discusses the possible mechanisms responsible for it. Given the key role that inflammation plays in the pathogenesis of malaria, regulation of the inflammatory response appears to be a major mechanism for antidisease immunity in naturally exposed individuals.

Impact of Chronic Viral Infection on T-Cell Dependent Humoral Immune Response

During the last decades, considerable efforts have been done to decipher mechanisms supported by microorganisms or viruses involved in the development, differentiation, and function of immune cells. Pathogens and their associated secretome as well as the continuous inflammation observed in chronic infection are shaping both innate and adaptive immunity. Secondary lymphoid organs are functional structures ensuring the mounting of adaptive immune response against microorganisms and viruses. Inside these organs, germinal centers (GCs) are the specialized sites where mature B-cell differentiation occurs leading to the release of high-affinity immunoglobulin (Ig)-secreting cells. Different steps are critical to complete B-cell differentiation process, including proliferation, somatic hypermutations in Ig variable genes, affinity-based selection, and class switch recombination. All these steps require intense interactions with cognate CD4⁺ helper T cells belonging to follicular helper lineage. Interestingly, pathogens can disturb this subtle machinery affecting the classical adaptive immune response. In this review, we describe how viruses could act directly on GC B cells, either through B-cell infection or by their contribution to B-cell cancer development and maintenance. In addition, we depict the indirect impact of viruses on B-cell response through infection of GC T cells and stromal cells, leading to immune response modulation.

Mechanism of splenic cell death and host mortality in a Plasmodium yoelii malaria model

Malaria is a fatal disease that displays a spectrum of symptoms and severity, which are determined by complex host-parasite interactions. It has been difficult to study the effects of parasite strains on disease severity in human infections, but the mechanisms leading to specific disease phenotypes can be investigated using strains of rodent malaria parasites that cause different disease symptoms in inbred mice. Using a unique mouse malaria model, here we investigated the mechanisms of splenic cell death and their relationship to control of parasitemia and host mortality. C57BL/6 mice infected with Plasmodium yoelii nigeriensis N67C display high levels of pro-inflammatory cytokines and chemokines (IL-6, IFN-γ, TNF-α, CXCL1, and CCL2) and extensive splenic damage with dramatic reduction of splenic cell populations. These disease phenotypes were rescued in RAG2^−/−, IFN-γ^−/−, or T cell depleted mice, suggesting IFN-γ and T cell mediated disease mechanisms. Additionally, apoptosis was one of the major pathways involved in splenic cell death, which coincides with the peaks of pro-inflammatory cytokines. Our results demonstrate the critical roles of T cells and IFN-γ in mediating splenic cell apoptosis, parasitemia control, and host lethality and thus may provide important insights for preventing/reducing morbidity associated with severe malaria in humans.

Protective immunity differs between routes of administration of attenuated malaria parasites independent of parasite liver load

In humans and murine models of malaria, intradermal immunization (ID-I) with genetically attenuated sporozoites that arrest in liver induces lower protective immunity than intravenous immunization (IV-I). It is unclear whether this difference is caused by fewer sporozoites migrating into the liver or by suboptimal hepatic and injection site-dependent immune responses. We therefore developed a Plasmodium yoelii immunization/boost/challenge model to examine parasite liver loads as well as hepatic and lymph node immune responses in protected and unprotected ID-I and IV-I animals. Despite introducing the same numbers of genetically attenuated parasites in the liver, ID-I resulted in lower sterile protection (53-68%) than IV-I (93-95%). Unprotected mice developed less sporozoite-specific CD8⁺ and CD4⁺ effector T-cell responses than protected mice. After immunization, ID-I mice showed more interleukin-10-producing B and T cells in livers and skin-draining lymph nodes, but fewer hepatic CD8 memory T cells and CD8⁺ dendritic cells compared to IV-I mice. Our results indicate that the lower protection efficacy obtained by intradermal sporozoite administration is not linked to low hepatic parasite numbers as presumed before, but correlates with a shift towards regulatory immune responses. Overcoming these immune suppressive responses is important not only for live-attenuated malaria vaccines but also for other live vaccines administered in the skin.

Immunological memory to blood-stage malaria infection is controlled by the histamine releasing factor (HRF) of the parasite

While most subunit malaria vaccines provide only limited efficacy, pre-erythrocytic and erythrocytic genetically attenuated parasites (GAP) have been shown to confer complete sterilizing immunity. We recently generated a Plasmodium berghei (PbNK65) parasite that lacks a secreted factor, the histamine releasing factor (HRF) (PbNK65 hrfΔ), and induces in infected mice a self-resolving blood stage infection accompanied by a long lasting immunity. Here, we explore the immunological mechanisms underlying the anti-parasite protective properties of the mutant PbNK65 hrfΔ and demonstrate that in addition to an up-regulation of IL-6 production, CD4⁺ but not CD8⁺ T effector lymphocytes are indispensable for the clearance of malaria infection. Maintenance of T cell-associated protection is associated with the reduction in CD4⁺PD-1⁺ and CD8⁺PD-1⁺ T cell numbers. A higher number of central and effector memory B cells in mutant-infected mice also plays a pivotal role in protection. Importantly, we also demonstrate that prior infection with WT parasites followed by a drug cure does not prevent the induction of PbNK65 hrfΔ-induced protection, suggesting that such protection in humans may be efficient even in individuals that have been infected and who repeatedly received antimalarial drugs.

Malaria

Malaria is caused in humans by five species of single-celled eukaryotic Plasmodium parasites (mainly Plasmodium falciparum and Plasmodium vivax) that are transmitted by the bite of Anopheles spp. mosquitoes. Malaria remains one of the most serious infectious diseases; it threatens nearly half of the world's population and led to hundreds of thousands of deaths in 2015, predominantly among children in Africa. Malaria is managed through a combination of vector control approaches (such as insecticide spraying and the use of insecticide-treated bed nets) and drugs for both treatment and prevention. The widespread use of artemisinin-based combination therapies has contributed to substantial declines in the number of malaria-related deaths; however, the emergence of drug resistance threatens to reverse this progress. Advances in our understanding of the underlying molecular basis of pathogenesis have fuelled the development of new diagnostics, drugs and insecticides. Several new combination therapies are in clinical development that have efficacy against drug-resistant parasites and the potential to be used in single-dose regimens to improve compliance. This ambitious programme to eliminate malaria also includes new approaches that could yield malaria vaccines or novel vector control strategies. However, despite these achievements, a well-coordinated global effort on multiple fronts is needed if malaria elimination is to be achieved.

Virus-Like Particle (VLP) Plus Microcrystalline Tyrosine (MCT) Adjuvants Enhance Vaccine Efficacy Improving T and B Cell Immunogenicity and Protection against Plasmodium berghei/vivax

Vaccination is the most effective prophylactic tool against infectious diseases. Despite continued efforts to control malaria, the disease still generally represents a significant unmet medical need. Microcrystalline tyrosine (MCT) is a well described depot used in licensed allergy immunotherapy products and in clinical development. However, its proof of concept in prophylactic vaccines has only recently been explored. MCT has never been used in combination with virus-like particles (VLPs), which are considered to be one of the most potent inducers of cellular and humoral immune responses in mice and humans. In the current study we assessed the potential of MCT to serve as an adjuvant in the development of a vaccine against malaria either alone or combined with VLP using Plasmodium vivax thrombospondin-related adhesive protein (TRAP) as a target antigen. We chemically coupled PvTRAP to VLPs derived from the cucumber mosaic virus fused to a universal T-cell epitope of the tetanus toxin (CMVtt), formulated with MCT and compared the induced immune responses to PvTRAP formulated in PBS or Alum. The protective capacity of the various formulations was assessed using Plasmodium berghei expressing PvTRAP. All vaccine formulations using adjuvants and/or VLP increased humoral immunogenicity for PvTRAP compared to the antigen alone. The most proficient responder was the group of mice immunized with the vaccine formulated with PvTRAP-VLP + MCT. The VLP-based vaccine formulated in MCT also induced the strongest T cell response and conferred best protection against challenge with recombinant Plasmodium berghei. Thus, the combination of VLP with MCT may take advantage of the properties of each component and appears to be an alternative biodegradable depot adjuvant for development of novel prophylactic vaccines.

Spleen-Dependent Immune Protection Elicited by CpG Adjuvanted Reticulocyte-Derived Exosomes from Malaria Infection Is Associated with Changes in T cell Subsets' Distribution

Reticulocyte-derived exosomes (rex) are 30–100 nm membrane vesicles of endocytic origin released during the maturation of reticulocytes to erythrocytes upon fusion of multivesicular bodies with the plasma membrane. Combination of CpG-ODN with rex obtained from BALB/c mice infected with the reticulocyte-prone non-lethal P. yoelii 17X malaria strain (rexPy), had been shown to induce survival and long lasting protection. Here, we show that splenectomized mice are not protected upon rexPy+CpG inmunizations and that protection is restored upon passive transfer of splenocytes obtained from animals immunized with rexPy+CpG. Notably, rexPy immunization of mice induced changes in PD1⁻ memory T cells with effector phenotype. Proteomics analysis of rexPy confirmed their reticulocyte origin and demonstrated the presence of parasite antigens. Our studies thus prove, for what we believe is the first time, that rex from reticulocyte-prone malarial infections are associated with splenic long-lasting memory responses. To try extrapolating these data to human infections, in vitro experiments with spleen cells of human transplantation donors were performed. Plasma-derived exosomes from vivax malaria patients (exPv) were actively uptaken by human splenocytes and stimulated spleen cells leading to changes in T cell subsets.

Immune Escape Strategies of Malaria Parasites

Malaria is one of the most life-threatening infectious diseases worldwide. Immunity to malaria is slow and short-lived despite the repeated parasite exposure in endemic areas. Malaria parasites have evolved refined machinery to evade the immune system based on a range of genetic changes that include allelic variation, biomolecular exposure of proteins, and intracellular replication. All of these features increase the probability of survival in both mosquitoes and the vertebrate host. Plasmodium species escape from the first immunological trap in its invertebrate vector host, the Anopheles mosquitoes. The parasites have to pass through various immunological barriers within the mosquito such as anti-microbial molecules and the mosquito microbiota in order to achieve successful transmission to the vertebrate host. Within these hosts, Plasmodium species employ various immune evasion strategies during different life cycle stages. Parasite persistence against the vertebrate immune response depends on the balance among virulence factors, pathology, metabolic cost of the host immune response, and the parasites ability to evade the immune response. In this review we discuss the strategies that Plasmodium parasites use to avoid the vertebrate host immune system and how they promote successful infection and transmission.

T-Cell Immunoglobulin- and Mucin-Domain-Containing Molecule 3 Signaling Blockade Improves Cell-Mediated Immunity Against Malaria

Cell-mediated immune responses play important roles in immune protection against Plasmodium infection. However, impaired immunity, such as lymphocyte exhaustion, is a common phenomenon in malaria. T-cell immunoglobulin- and mucin-domain-containing molecule 3 (Tim-3) is an important regulatory molecule in cell-mediated immunity and has been implicated in malaria. In this study, it was found that Tim-3 expression on key populations of lymphocytes was significantly increased in both Plasmodium falciparum-infected patients and Plasmodium berghei ANKA (PbANKA)-infected C57BL/6 mice. Upregulation of Tim-3 led to lymphocyte exhaustion, while blocking Tim-3 signaling with an anti-Tim-3 antibody restored lymphocyte activity in Plasmodium infections. Further, anti-Tim-3 treatment accelerated the parasite clearance and relieved the symptoms of cerebral malaria in PbANKA-infected mice. In conclusion, Tim-3 on immune cells negatively regulates cell-mediated immunity against Plasmodium infection, and blocking Tim-3 signaling enhances sterile immunity and may play a protective role during malarial parasite infections.