IFNγ/IL-10 co-producing cells dominate the CD4 response to malaria in highly exposed children

Advances and challenges in investigating B-cells via single-cell transcriptomics

Single-cell RNA sequencing (scRNAseq) and Variable, Diversity, Joining (VDJ) profiling have improved our understanding of B-cells. Recent scRNAseq-based approaches have led to the discovery of intermediate B-cell states, including preplasma cells and pregerminal centre B-cells, as well as unveiling protective roles for B-cells within tertiary lymphoid structures in respiratory infections and cancers. These studies have improved our understanding of transcriptional and epigenetic control of B-cell development and of atypical and memory B-cell differentiation. Advancements in temporal profiling in parallel with transcriptomic and VDJ sequencing have consolidated our understanding of the trajectory of B-cell clones over the course of infection and vaccination. Challenges remain in studying B-cell states across tissues in humans, in relating spatial location with B-cell phenotype and function, in examining antibody isotype switching events, and in unequivocal determination of clonal relationships. Nevertheless, ongoing multiomic assessments and studies of cellular interactions within tissues promise new avenues for improving humoral immunity and combatting autoimmune conditions.

CD4+ T cells display a spectrum of recall dynamics during re-infection with malaria parasites

Children in malaria-endemic regions can experience repeated Plasmodium infections over short periods of time. Effects of re-infection on multiple co-existing CD4+ T cell subsets remain unresolved. Here, we examine antigen-experienced CD4+ T cells during re-infection in mice, using scRNA-seq/TCR-seq and spatial transcriptomics. TCR transgenic TEM cells initiate rapid Th1/Tr1 recall responses prior to proliferating, while GC Tfh counterparts are refractory, with TCM/Tfh-like cells exhibiting modest non-proliferative responses. Th1-recall is a partial facsimile of primary Th1-responses, with no upregulated effector-associated genes being unique to recall. Polyclonal, TCR-diverse, CD4+ T cells exhibit similar recall dynamics, with individual clones giving rise to multiple effectors including highly proliferative Th1/Tr1 cells, as well as GC Tfh and Tfh-like cells lacking proliferative capacity. Thus, we show substantial diversity in recall responses mounted by multiple co-existing CD4+ T cell subsets in the spleen, and present graphical user interfaces for studying gene expression dynamics and clonal relationships during re-infection.

The impact of Plasmodium-driven immunoregulatory networks on immunity to malaria

Malaria, caused by infection with Plasmodium parasites, drives multiple regulatory responses across the immune landscape. These regulatory responses help to protect against inflammatory disease but may in some situations hamper the acquisition of adaptive immune responses that clear parasites. In addition, the regulatory responses that occur during Plasmodium infection may negatively affect malaria vaccine efficacy in the most at-risk populations. Here, we discuss the specific cellular mechanisms of immunoregulatory networks that develop during malaria, with a focus on knowledge gained from human studies and studies that involve the main malaria parasite to affect humans, Plasmodium falciparum. Leveraging this knowledge may lead to the development of new therapeutic approaches to increase protective immunity to malaria during infection or after vaccination.

Disease-specific plasma protein profiles in patients with fever after traveling to tropical areas

Fever is common among individuals seeking healthcare after traveling to tropical regions. Despite the association with potentially severe disease, the etiology is often not determined. Plasma protein patterns can be informative to understand the host response to infection and can potentially indicate the pathogen causing the disease. In this study, we measured 49 proteins in the plasma of 124 patients with fever after travel to tropical or subtropical regions. The patients had confirmed diagnoses of either malaria, dengue fever, influenza, bacterial respiratory tract infection, or bacterial gastroenteritis, representing the most common etiologies. We used multivariate and machine learning methods to identify combinations of proteins that contributed to distinguishing infected patients from healthy controls, and each other. Malaria displayed the most unique protein signature, indicating a strong immunoregulatory response with high levels of IL10, sTNFRI and II, and sCD25 but low levels of sCD40L. In contrast, bacterial gastroenteritis had high levels of sCD40L, APRIL, and IFN-γ, while dengue was the only infection with elevated IFN-α2. These results suggest that characterization of the inflammatory profile of individuals with fever can help to identify disease-specific host responses, which in turn can be used to guide future research on diagnostic strategies and therapeutic interventions.

Distinct transcriptomic signatures define febrile malaria depending on initial infective states, asymptomatic or uninfected

BACKGROUND

Cumulative malaria parasite exposure in endemic regions often results in the acquisition of partial immunity and asymptomatic infections. There is limited information on how host-parasite interactions mediate the maintenance of chronic symptomless infections that sustain malaria transmission.

METHODS

Here, we determined the gene expression profiles of the parasite population and the corresponding host peripheral blood mononuclear cells (PBMCs) from 21 children (< 15 years). We compared children who were defined as uninfected, asymptomatic and those with febrile malaria.

RESULTS

Children with asymptomatic infections had a parasite transcriptional profile characterized by a bias toward trophozoite stage (~ 12 h-post invasion) parasites and low parasite levels, while early ring stage parasites were characteristic of febrile malaria. The host response of asymptomatic children was characterized by downregulated transcription of genes associated with inflammatory responses, compared with children with febrile malaria,. Interestingly, the host responses during febrile infections that followed an asymptomatic infection featured stronger inflammatory responses, whereas the febrile host responses from previously uninfected children featured increased humoral immune responses.

CONCLUSIONS

The priming effect of prior asymptomatic infection may explain the blunted acquisition of antibody responses seen to malaria antigens following natural exposure or vaccination in malaria endemic areas.

A molecular signature for IL-10-producing Th1 cells in protozoan parasitic diseases

Control of visceral leishmaniasis (VL) depends on proinflammatory Th1 cells that activate infected tissue macrophages to kill resident intracellular parasites. However, proinflammatory cytokines produced by Th1 cells can damage tissues and require tight regulation. Th1 cell IL-10 production is an important cell-autologous mechanism to prevent such damage. However, IL-10-producing Th1 (type 1 regulatory; Tr1) cells can also delay control of parasites and the generation of immunity following drug treatment or vaccination. To identify molecules to target in order to alter the balance between Th1 and Tr1 cells for improved antiparasitic immunity, we compared the molecular and phenotypic profiles of Th1 and Tr1 cells in experimental VL caused by Leishmania donovani infection of C57BL/6J mice. We also identified a shared Tr1 cell protozoan signature by comparing the transcriptional profiles of Tr1 cells from mice with experimental VL and malaria. We identified LAG3 as an important coinhibitory receptor in patients with VL and experimental VL, and we reveal tissue-specific heterogeneity of coinhibitory receptor expression by Tr1 cells. We also discovered a role for the transcription factor Pbx1 in suppressing CD4+ T cell cytokine production. This work provides insights into the development and function of CD4+ T cells during protozoan parasitic infections and identifies key immunoregulatory molecules.

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.

Bhlhe40 limits early IL-10 production from CD4+ T cells during Plasmodium yoelii 17X infection

The cytokine IL-10 suppresses T-cell-mediated immunity, which is required to control infection with Plasmodium yoelii. Consequently, IL-10 can delay the time needed to resolve this infection, leading to a higher parasite burden. While the pathways that lead to IL-10 production by CD4+ T cells are well defined, much less is known about the mediators that suppress the expression of this potent anti-inflammatory cytokine. Here, we show that the transcription factor basic helix-loop-helix family member e40 (Bhlhe40) contributes to controlling parasite burden in response to P. yoelii infection in mice. Loss of Bhlhe40 expression in mice results in higher Il10 expression, higher peak parasitemia, and a delay in parasite clearance. The observed phenotype was not due to defects in T-cell activation and proliferation or the humoral response. Nor was it due to changes in regulatory T-cell numbers. However, blocking IL-10 signaling reversed the outcome in Bhlhe40-/ - mice, suggesting that excess IL-10 production limits their ability to control the infection properly. In addition to suppressing Il10 expression in CD4+ T cells, Bhlhe40 can promote Ifng expression. Indeed, IFN-γ production by CD4+ T cells isolated from the liver was significantly affected by the loss of Bhlhe40. Lastly, Bhlhe40 deletion in T cells resulted in a phenotype similar to that observed in the Bhlhe40-/ - mice, indicating that Bhlhe40 expression in T cells contributes to the ability of mice to control infection with P. yoelii.

Single cell transcriptomics shows that malaria promotes unique regulatory responses across multiple immune cell subsets

Plasmodium falciparum malaria drives immunoregulatory responses across multiple cell subsets, which protects from immunopathogenesis, but also hampers the development of effective anti-parasitic immunity. Understanding malaria induced tolerogenic responses in specific cell subsets may inform development of strategies to boost protective immunity during drug treatment and vaccination. Here, we analyse the immune landscape with single cell RNA sequencing during P. falciparum malaria. We identify cell type specific responses in sub-clustered major immune cell types. Malaria is associated with an increase in immunosuppressive monocytes, alongside NK and γδ T cells which up-regulate tolerogenic markers. IL-10-producing Tr1 CD4 T cells and IL-10-producing regulatory B cells are also induced. Type I interferon responses are identified across all cell types, suggesting Type I interferon signalling may be linked to induction of immunoregulatory networks during malaria. These findings provide insights into cell-specific and shared immunoregulatory changes during malaria and provide a data resource for further analysis.

STING activation promotes autologous type I interferon-dependent development of type 1 regulatory T cells during malaria

The development of highly effective malaria vaccines and improvement of drug-treatment protocols to boost antiparasitic immunity are critical for malaria elimination. However, the rapid establishment of parasite-specific immune regulatory networks following exposure to malaria parasites hampers these efforts. Here, we identified stimulator of interferon genes (STING) as a critical mediator of type I interferon production by CD4+ T cells during blood-stage Plasmodium falciparum infection. The activation of STING in CD4+ T cells by cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) stimulated IFNB gene transcription, which promoted development of IL-10- and IFN-γ-coproducing CD4+ T (type I regulatory [Tr1]) cells. The critical role for type I IFN signaling for Tr1 cell development was confirmed in vivo using a preclinical malaria model. CD4+ T cell sensitivity to STING phosphorylation was increased in healthy volunteers following P. falciparum infection, particularly in Tr1 cells. These findings identified STING expressed by CD4+ T cells as an important mediator of type I IFN production and Tr1 cell development and activation during malaria.

Malaria-specific Type 1 regulatory T cells are more abundant in first pregnancies and associated with placental malaria

BACKGROUND

Malaria in pregnancy (MIP) causes higher morbidity in primigravid compared to multigravid women; however, the correlates and mechanisms underlying this gravidity-dependent protection remain incompletely understood. We aimed to compare the cellular immune response between primigravid and multigravid women living in a malaria-endemic region and assess for correlates of protection against MIP.

METHODS

We characterised the second trimester cellular immune response among 203 primigravid and multigravid pregnant women enrolled in two clinical trials of chemoprevention in eastern Uganda, utilizing RNA sequencing, flow cytometry, and functional assays. We compared responses across gravidity and determined associations with parasitaemia during pregnancy and placental malaria.

FINDINGS

Using whole blood RNA sequencing, no significant differentially expressed genes were identified between primigravid (n = 12) and multigravid (n = 11) women overall (log 2(FC) > 2, FDR < 0.1). However, primigravid (n = 49) women had higher percentages of malaria-specific, non-naïve CD4+ T cells that co-expressed IL-10 and IFNγ compared with multigravid (n = 85) women (p = 0.000023), and higher percentages of these CD4+ T cells were associated with greater risks of parasitaemia in pregnancy (Rs = 0.49, p = 0.001) and placental malaria (p = 0.0073). These IL-10 and IFNγ co-producing CD4+ T cells had a genomic signature of Tr1 cells, including expression of transcription factors cMAF and BATF and cell surface makers CTLA4 and LAG-3.

INTERPRETATION

Malaria-specific Tr1 cells were highly prevalent in primigravid Ugandan women, and their presence correlated with a higher risk of malaria in pregnancy. Understanding whether suppression of Tr1 cells plays a role in naturally acquired gravidity-dependent immunity may aid the development of new vaccines or treatments for MIP.

FUNDING

This work was funded by NIH (PO1 HD059454, U01 AI141308, U19 AI089674, U01 AI155325, U01 AI150741), the March of Dimes (Basil O'Connor award), and the Bill and Melinda Gates Foundation (OPP 1113682).

Cytotoxic CD4 T cells in the mucosa and in cancer

CD4 T cells were initially described as helper cells that promote either the cellular immune response (Th1 cells) or the humoral immune response (Th2 cells). Since then, a plethora of functionally distinct helper and regulatory CD4 T cell subsets have been described. CD4 T cells with cytotoxic function were first described in the setting of viral infections and autoimmunity, and more recently in cancer and gut dysbiosis. Regulatory CD4 T cell subsets such as Tregs and T-regulatory type 1 (Tr1) cells have also been shown to have cytotoxic potential. Indeed, Tr1 cells have been shown to be important for maintenance of stem cell niches in the bone marrow and the gut. This review will provide an overview of cytotoxic CD4 T cell development, and discuss the role of inflammatory and Tr1-like cytotoxic CD4 T cells in maintenance of intestinal stem cells and in anti-cancer immune responses.

Characterization of Duffy Binding Protein II-specific CD4+T cell responses in Plasmodium vivax patients

Plasmodium vivax Duffy Binding Protein region II (PvDBPII) is a leading vaccine candidate against blood-stage vivax malaria. Anti-PvDBPII antibodies potentially block parasite invasion by inhibition of erythrocyte binding. However, knowledge of PvDBPII-specific T cell responses is limited. Here, to assess the responses of PvDBPII-specific CD4⁺T cells in natural P. vivax infection, three cross-sectional studies were conducted in recovered subjects. In silico analysis was used for potential T cell epitope prediction and selection. PBMCs from P. vivax subjects were stimulated with selected peptides and examined for cytokine production by ELISPOT or intracellular cytokine staining. Six dominant T cell epitopes were identified. Peptide-driven T cell responses showed effector memory CD4⁺T cell phenotype, secreting both IFN-γ and TNF-α cytokines. Single amino acid substitutions in three T cell epitopes altered levels of IFN-γ memory T cell responses. Seropositivity of anti-PvDBPII antibodies were detected during acute malaria (62%) and persisted up to 12 months (11%) following P. vivax infection. Further correlation analysis showed four out of eighteen subjects had positive antibody and CD4⁺T cell responses to PvDBPII. Altogether, PvDBPII-specific CD4⁺T cells were developed in natural P. vivax infections. Data on their antigenicity could facilitate development of an efficacious vivax malaria vaccine.

Erratum: Type 1 regulatory T cell-mediated tolerance in health and disease

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Type I regulatory T cells in malaria: of mice and men

Type I regulatory T (Tr1) cells are a population of regulatory CD4+ T cells implicated in the suppression of pathological immune responses across multiple diseases, but a unifying transcriptional signature of Tr1 identity across disease contexts has not been characterized. In this issue of the JCI, Edward, Ng, and colleagues identified a conserved transcriptional signature that distinguished Tr1 (IL-10+IFN-γ+) from Th1 (IL-10-IFN-γ+) cells in human and mouse malaria. This signature implicated genes encoding inhibitory receptors - including CTLA-4 and LAG-3 - and transcription factors - including cMAF. The authors identified coinhibitory receptor expression that distinguished Tr1 cells from other CD4+ T cell subsets. Furthermore, cMAF - and, to a lesser extent, BLIMP-1 - promoted IL-10 production in human CD4+ T cells. BLIMP-1 also played a role in supporting the expression of inhibitory receptors. These findings describe a few key features that seem to be conserved by Tr1 cells across multiple species, disease contexts, and marker definitions.

IL-10-producing Th1 cells possess a distinct molecular signature in malaria

Control of intracellular parasites responsible for malaria requires host IFN-γ+T-bet+CD4+ T cells (Th1 cells) with IL-10 produced by Th1 cells to mitigate the pathology induced by this inflammatory response. However, these IL-10-producing Th1 (induced type I regulatory [Tr1]) cells can also promote parasite persistence or impair immunity to reinfection or vaccination. Here, we identified molecular and phenotypic signatures that distinguished IL-10-Th1 cells from IL-10+Tr1 cells in Plasmodium falciparum-infected people who participated in controlled human malaria infection studies, as well as C57BL/6 mice with experimental malaria caused by P. berghei ANKA. We also identified a conserved Tr1 cell molecular signature shared between patients with malaria, dengue, and graft-versus-host disease. Genetic manipulation of primary human CD4+ T cells showed that the transcription factor cMAF played an important role in the induction of IL-10, while BLIMP-1 promoted the development of human CD4+ T cells expressing multiple coinhibitory receptors. We also describe heterogeneity of Tr1 cell coinhibitory receptor expression that has implications for targeting these molecules for clinical advantage during infection. Overall, this work provides insights into CD4+ T cell development during malaria that offer opportunities for creation of strategies to modulate CD4+ T cell functions and improve antiparasitic immunity.

Type 1 regulatory T cell-mediated tolerance in health and disease

Type 1 regulatory T (Tr1) cells, in addition to other regulatory cells, contribute to immunological tolerance to prevent autoimmunity and excessive inflammation. Tr1 cells arise in the periphery upon antigen stimulation in the presence of tolerogenic antigen presenting cells and secrete large amounts of the immunosuppressive cytokine IL-10. The protective role of Tr1 cells in autoimmune diseases and inflammatory bowel disease has been well established, and this led to the exploration of this population as a potential cell therapy. On the other hand, the role of Tr1 cells in infectious disease is not well characterized, thus raising concern that these tolerogenic cells may cause general immune suppression which would prevent pathogen clearance. In this review, we summarize current literature surrounding Tr1-mediated tolerance and its role in health and disease settings including autoimmunity, inflammatory bowel disease, and infectious diseases.

Mouse Models for Unravelling Immunology of Blood Stage Malaria

Malaria comprises a spectrum of disease syndromes and the immune system is a major participant in malarial disease. This is particularly true in relation to the immune responses elicited against blood stages of Plasmodium-parasites that are responsible for the pathogenesis of infection. Mouse models of malaria are commonly used to dissect the immune mechanisms underlying disease. While no single mouse model of Plasmodium infection completely recapitulates all the features of malaria in humans, collectively the existing models are invaluable for defining the events that lead to the immunopathogenesis of malaria. Here we review the different mouse models of Plasmodium infection that are available, and highlight some of the main contributions these models have made with regards to identifying immune mechanisms of parasite control and the immunopathogenesis of malaria.

Immunological profiles associated with distinct parasitemic states in volunteers undergoing malaria challenge in Gabon

Controlled human malaria infection (CHMI) using cryopreserved non-attenuated Plasmodium falciparum sporozoites (PfSPZ) offers a unique opportunity to investigate naturally acquired immunity (NAI). By analyzing blood samples from 5 malaria-naïve European and 20 African adults with lifelong exposure to malaria, before, 5, and 11 days after direct venous inoculation (DVI) with Sanaria^R PfSPZ Challenge, we assessed the immunological patterns associated with control of microscopic and submicroscopic parasitemia. All (5/5) European individuals developed parasitemia as defined by thick blood smear (TBS), but 40% (8/20) of the African individuals controlled their parasitemia, and therefore remained thick blood smear-negative (TBS⁻ Africans). In the TBS⁻ Africans, we observed higher baseline frequencies of CD4⁺ T cells producing interferon-gamma (IFNγ) that significantly decreased 5 days after PfSPZ DVI. The TBS⁻ Africans, which represent individuals with either very strong and rapid blood-stage immunity or with immunity to liver stages, were stratified into subjects with sub-microscopic parasitemia (TBS^-PCR⁺) or those with possibly sterilizing immunity (TBS⁻PCR⁻). Higher frequencies of IFNγ⁺TNF⁺CD8⁺ γδ T cells at baseline, which later decreased within five days after PfSPZ DVI, were associated with those who remained TBS⁻PCR⁻. These findings suggest that naturally acquired immunity is characterized by different cell types that show varying strengths of malaria parasite control. While the high frequencies of antigen responsive IFNγ⁺CD4⁺ T cells in peripheral blood keep the blood-stage parasites to a sub-microscopic level, it is the IFNγ⁺TNF⁺CD8⁺ γδ T cells that are associated with either immunity to the liver-stage, or rapid elimination of blood-stage parasites.

TNF-α+ CD4+ T cells dominate the SARS-CoV-2 specific T cell response in COVID-19 outpatients and are associated with durable antibodies

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific CD4⁺ T cells are likely important in immunity against coronavirus 2019 (COVID-19), but our understanding of CD4⁺ longitudinal dynamics following infection and of specific features that correlate with the maintenance of neutralizing antibodies remains limited. Here, we characterize SARS-CoV-2-specific CD4⁺ T cells in a longitudinal cohort of 109 COVID-19 outpatients enrolled during acute infection. The quality of the SARS-CoV-2-specific CD4⁺ response shifts from cells producing interferon gamma (IFNγ) to tumor necrosis factor alpha (TNF-α) from 5 days to 4 months post-enrollment, with IFNγ^-IL-21^-TNF-α⁺ CD4⁺ T cells the predominant population detected at later time points. Greater percentages of IFNγ^-IL-21^-TNF-α⁺ CD4⁺ T cells on day 28 correlate with SARS-CoV-2-neutralizing antibodies measured 7 months post-infection (⍴ = 0.4, p = 0.01). mRNA vaccination following SARS-CoV-2 infection boosts both IFNγ- and TNF-α-producing, spike-protein-specific CD4⁺ T cells. These data suggest that SARS-CoV-2-specific, TNF-α-producing CD4⁺ T cells may play an important role in antibody maintenance following COVID-19.

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SARS-CoV-2-specific CD4⁺ response shifts from cells producing IFNγ to TNF-α

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SARS-CoV-2-specific IFNγ⁻TNF-α⁺ CD4⁺ T cells predominate at later timepoints

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IFNγ^-TNF-α⁺ CD4⁺ T cells correlate with durable SARS-CoV-2-neutralizing antibodies

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Post-infection mRNA vaccination boosts both IFNγ⁺ and TNF-α⁺ S-specific CD4⁺ T cells

IL-10 Producing Regulatory B Cells Mediated Protection against Murine Malaria Pathogenesis

Simple Summary

The immunomodulatory role of B cell subset called regulatory B cells was evaluated during Plasmodium infection to study their role in susceptibility or resistance during infection. The expansion of regulatory B cells during Plasmodium infection indicated their important role in regulating the immune response. Adoptive transfer of regulatory B cells following infection with a lethal parasite resulted in enhanced survival of mice and inhibited growth of the Plasmodium parasite. Moreover, by inhibiting the production of the pro-inflammatory cytokine, IFN-γ, and stimulating anti-inflammatory IL-10 production, regulatory B cells may serve as an important contributor to protective immune response.

Abstract

Various immune cells are known to participate in combating infection. Regulatory B cells represent a subset of B cells that take part in immunomodulation and control inflammation. The immunoregulatory function of regulatory B cells has been shown in various murine models of several disorders. In this study, a comparable IL-10 competent B-10 cell subset (regulatory B cells) was characterized during lethal and non-lethal infection with malaria parasites using the mouse model. We observed that infection of Balb/c mice with P. yoelii I 7XL was lethal, and a rapid increase in dynamics of IL-10 producing B220⁺CD5⁺CD1d⁺ regulatory B cells over the course of infection was observed. However, animals infected with a less virulent strain of the parasite P. yoelii I7XNL attained complete resistance. It was observed that there is an increase in the population of regulatory B cells with an increase of parasitemia; however, a sudden drop in the frequency of these cells was observed with parasite clearance. Adoptive transfer of regulatory B cells to naïve mice followed by infection results in slow parasite growth and enhancement of survival in P. yoelii 17XL (lethal) infected animals. Adoptively transferred regulatory B cells also resulted in decreased production of pro-inflammatory cytokine (IFN-γ) and enhanced production of anti-inflammatory cytokine (IL-10). It infers that these regulatory B cells may contribute in immune protection by preventing the inflammation associated with disease and inhibiting the parasite growth.

Inflammatory cytokine responses in children with asymptomatic malaria infection living in rural, semi-urban and urban areas in south-eastern Gabon

Cytokines are soluble mediators of the immune response, and their evolution influences the disease outcome. Gaining knowledge on cytokines has become important, as they can constitute biomarkers allowing the diagnosis of malaria and preventing severe forms of the disease. Here, we investigated 10 cytokines and their circulating levels in asymptomatic Gabonese children with Plasmodium falciparum infection living in urban, semi-urban and rural areas. Blood samples were collected from 273 schoolchildren (153 uninfected and 120 infected) aged 6 to 192 months. Hematological parameters were determined and P. falciparum diagnosis was performed using a rapid diagnosis test, microscopy and nested polymerase chain reaction (PCR). Plasma pro- [interferon (IFN)-γ, tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-12p70, IL-17A and IL-22] and anti-inflammatory [IL-10, IL-4, IL-13 and transforming growth factor (TGF)-β] cytokine levels were measured by enzyme-linked immunosorbent assay (ELISA) and compared between asymptomatic-infected and uninfected children. Results revealed that without distinction of area, IL-10 and IL-6 levels were higher in infected compared to uninfected children; however, the pro- and anti-inflammatory ratios (IL-6/IL-10 and TNF-α/IL-10) were similar. Furthermore, with area distinction significantly elevated levels of IL-10 in these asymptomatic children were always accompanied by either significantly low or high levels of a proinflammatory cytokine. Also, comparison between asymptomatic-infected children from the three areas showed significantly lower IL-17A, IL-22 and TGF-β levels in urban area compared to semi-urban and rural areas. These results suggest that asymptomatic malaria infections induce significantly high inflammatory cytokine levels without modifying the balanced between pro- and anti-inflammatory cytokines and underline the higher exposure to infections of children in rural areas.

Safety, Tolerability, Pharmacokinetics and Pharmacodynamics of Co-administered Ruxolitinib and Artemether-Lumefantrine in Healthy Adults

Despite repeated malaria infection, individuals living in malaria endemic areas remain vulnerable to re-infection. The Janus kinase (JAK1/2) inhibitor ruxolitinib could potentially disrupt the parasite-induced dysfunctional immune response when administered with anti-malarial therapy. This randomized, single-blind, placebo-controlled, single center phase 1 trial investigated the safety, tolerability, pharmacokinetic and pharmacodynamic profile of ruxolitinib and the approved antimalarial artemether-lumefantrine in combination. Ruxolitinib pharmacodynamics were assessed by inhibition of phosphorylation of signal transducer and activator of transcription 3 (pSTAT3). Eight healthy male and female participants aged 18-55 years were randomized to either ruxolitinib (20 mg) (n = 6) or placebo (n = 2) administered 2 h after artemether-lumefantrine (80/480 mg) twice daily for three days. Mild adverse events occurred in six participants (four ruxolitinib; two placebo). The combination of artemether-lumefantrine and ruxolitinib was well tolerated, with adverse events and pharmacokinetics consistent with the known profiles of both drugs. The incidence of adverse events and artemether, dihydroartemisinin (the major active metabolite of artemether) and lumefantrine exposure were not affected by ruxolitinib co-administration. Ruxolitinib co-administration resulted in a 3-fold greater pSTAT3 inhibition compared to placebo (geometric mean ratio: 3.01 [90%CI 2.14, 4.24]), with a direct and predictable relationship between ruxolitinib plasma concentrations and %pSTAT3 inhibition. This study supports the investigation of the combination of artemether-lumefantrine and ruxolitinib in healthy volunteers infected with Plasmodium falciparum malaria. (This study has been registered at ClinicalTrials.gov under registration no. NCT04456634).

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.

Altered Ex-Vivo Cytokine Responses in Children With Asymptomatic Plasmodium falciparum Infection in Burkina Faso: An Additional Argument to Treat Asymptomatic Malaria?

Introduction

Patients with clinical malaria have an increased risk for bacterial bloodstream infections. We hypothesized that asymptomatic malaria parasitemia increases susceptibility for bacterial infections through an effect on the innate immune system. We measured circulating cytokine levels and ex-vivo cytokine production capacity in asymptomatic malaria and compared with controls.

Methods

Data were collected from asymptomatic participants <5 years old with and without positive malaria microscopy, as well as from hospitalized patients <5 years old with clinical malaria, bacteremia, or malaria/bacteremia co-infections in a malaria endemic region of Burkina Faso. Circulating cytokines (TNF-α, IFN-γ, IL-6, IL-10) were measured using multiplex assays. Whole blood from asymptomatic participants with and without positive malaria microscopy were ex-vivo stimulated with S. aureus, E. coli LPS and Salmonella Typhimurium; cytokine concentrations (TNF-α, IFN-γ, IL-1β, IL-6, IL-10) were measured on supernatants using ELISA.

Results

Included were children with clinical malaria (n=118), bacteremia (n=22), malaria and bacteremia co-infection (n=9), asymptomatic malaria (n=125), and asymptomatic controls (n=237). Children with either clinical or asymptomatic malaria had higher plasma cytokine concentrations than controls. Cytokine concentrations correlated positively with malaria parasite density with the strongest correlation for IL-10 in both asymptomatic (r=0.63) and clinical malaria (r=0.53). Patients with bacteremia had lower circulating IL-10, TNF-α and IFN-γ and higher IL-6 concentrations, compared to clinical malaria. Ex-vivo whole blood cytokine production to LPS and S. aureus was significantly lower in asymptomatic malaria compared to controls. Whole blood IFN-γ and IL-10 production in response to Salmonella was also lower in asymptomatic malaria.

Interpretation

In children with asymptomatic malaria, cytokine responses upon ex-vivo bacterial stimulation are downregulated. Further studies are needed to explore if the suggested impaired innate immune response to bacterial pathogens also translates into impaired control of pathogens such as Salmonella spp.

T-follicular helper cells in malaria infection and roles in antibody induction

Immunity to malaria is mediated by antibodies that block parasite replication to limit parasite burden and prevent disease. Cytophilic antibodies have been consistently shown to be associated with protection, and recent work has improved our understanding of the direct and Fc-mediated mechanisms of protective antibodies. Antibodies also have important roles in vaccine-mediated immunity. Antibody induction is driven by the specialized CD4⁺ T cells, T-follicular helper (Tfh) cells, which function within the germinal centre to drive B-cell activation and antibody induction. In humans, circulating Tfh cells can be identified in peripheral blood and are differentiated into subsets that appear to have pathogen/vaccination-specific roles in antibody induction. Tfh cell responses are essential for protective immunity from Plasmodium infection in murine models of malaria. Our understanding of the activation of Tfh cells during human malaria infection and the importance of different Tfh cell subsets in antibody development is still emerging. This review will discuss our current knowledge of Tfh cell activation and development in malaria, and the potential avenues and pitfalls of targeting Tfh cells to improve malaria vaccines.

Immunological Considerations for Schistosoma Vaccine Development: Transitioning to Endemic Settings

Despite mass drug administration programmes with praziquantel, the prevalence of schistosomiasis remains high. A vaccine is urgently needed to control transmission of this debilitating disease. As some promising schistosomiasis vaccine candidates are moving through pre-clinical and clinical testing, we review the immunological challenges that these vaccine candidates may encounter in transitioning through the clinical trial phases in endemic settings. Prior exposure of the target population to schistosomes and other infections may impact vaccine response and efficacy and therefore requires considerable attention. Schistosomes are known for their potential to induce T-reg/IL-10 mediated immune suppression in populations which are chronically infected. Moreover, endemicity of schistosomiasis is focal whereby target and trial populations may exhibit several degrees of prior exposure as well as in utero exposure which may increase heterogeneity of vaccine responses. The age dependent distribution of exposure and development of acquired immunity, and general differences in the baseline immunological profile, adds to the complexity of selecting suitable trial populations. Similarly, prior or concurrent infections with other parasitic helminths, viral and bacterial infections, may alter immunological responses. Consequently, treatment of co-infections may benefit the immunogenicity of vaccines and may be considered despite logistical challenges. On the other hand, viral infections leave a life-long immunological imprint on the human host. Screening for serostatus may be needed to facilitate interpretation of vaccine responses. Co-delivery of schistosome vaccines with PZQ is attractive from a perspective of implementation but may complicate the immunogenicity of schistosomiasis vaccines. Several studies have reported PZQ treatment to induce both transient and long-term immuno-modulatory effects as a result of tegument destruction, worm killing and subsequent exposure of worm antigens to the host immune system. These in turn may augment or antagonize vaccine immunogenicity. Understanding the complex immunological interactions between vaccine, co-infections or prior exposure is essential in early stages of clinical development to facilitate phase 3 clinical trial design and implementation policies. Besides well-designed studies in different target populations using schistosome candidate vaccines or other vaccines as models, controlled human infections could also help identify markers of immune protection in populations with different disease and immunological backgrounds.

Genetic variation in the immune system and malaria susceptibility in infants: a nested case-control study in Nanoro, Burkina Faso

BACKGROUND

Genetic polymorphisms in the human immune system modulate susceptibility to malaria. However, there is a paucity of data on the contribution of immunogenetic variants to malaria susceptibility in infants, who present differential biological features related to the immaturity of their adaptive immune system, the protective effect of maternal antibodies and fetal haemoglobin. This study investigated the association between genetic variation in innate immune response genes and malaria susceptibility during the first year of life in 656 infants from a birth cohort survey performed in Nanoro, Burkina Faso.

METHODS

Seventeen single nucleotide polymorphisms (SNPs) in 11 genes of the immune system previously associated with different malaria phenotypes were genotyped using TaqMan allelic hybridization assays in a Fluidigm platform. Plasmodium falciparum infection and clinical disease were documented by active and passive case detection. Case-control association analyses for both alleles and genotypes were carried out using univariate and multivariate logistic regression. For cytokines showing significant SNP associations in multivariate analyses, cord blood supernatant concentrations were measured by quantitative suspension array technology (Luminex).

RESULTS

Genetic variants in IL-1β (rs1143634) and FcγRIIA/CD32 (rs1801274)-both in allelic, dominant and co-dominant models-were significantly associated with protection from both P. falciparum infection and clinical malaria. Furthermore, heterozygote individuals with rs1801274 SNP in FcγRIIA/CD32 showed higher IL-1RA levels compared to wild-type homozygotes (P = 0.024), a cytokine whose production is promoted by the binding of IgG immune complexes to Fcγ receptors on effector immune cells.

CONCLUSIONS

These findings indicate that genetic polymorphisms in genes driving innate immune responses are associated to malaria susceptibility during the first year of life, possibly by modulating production of inflammatory mediators.

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.

Red blood cell homeostasis in children and adults with and without asymptomatic malaria infection in Burkina Faso

Asymptomatic malaria infections may affect red blood cell (RBC) homeostasis. Reports indicate a role for chronic hemolysis and splenomegaly, however, the underlying processes are incompletely understood. New hematology analysers provide parameters for a more comprehensive analysis of RBC hemostasis. Complete blood counts were analysed in subjects from all age groups (n = 1118) living in a malaria hyperendemic area and cytokines and iron biomarkers were also measured. Subjects were divided into age groups (<2 years, 2–4, 5–14 and ≥15 years old) and clinical categories (smear-negative healthy subjects, asymptomatic malaria and clinical malaria). We found that hemoglobin levels were similar in smear-negative healthy children and asymptomatic malaria children but significantly lower in clinical malaria with a maximum difference of 2.2 g/dl in children <2 years decreasing to 0.1 g/dl in those aged ≥15 years. Delta-He, presenting different hemoglobinization of reticulocytes and RBC, levels were lower in asymptomatic and clinial malaria, indicating a recent effect of malaria on erythropoiesis. Reticulocyte counts and reticulocyte production index (RPI), indicating the erythropoietic capacity of the bone marrow, were higher in young children with malaria compared to smear-negative subjects. A negative correlation between reticulocyte counts and Hb levels was found in asymptomatic malaria (ρ = -0.32, p<0.001) unlike in clinical malaria (ρ = -0.008, p = 0.92). Free-Hb levels, indicating hemolysis, were only higher in clinical malaria. Phagocytozing monocytes, indicating erythophagocytosis, were highest in clinical malaria, followed by asymptomatic malaria and smear-negative subjects. Circulating cytokines and iron biomarkers (hepcidin, ferritin) showed similar patterns. Pro/anti-inflammatory (IL-6/IL-10) ratio was higher in clinical than asymptomatic malaria. Cytokine production capacity of ex-vivo whole blood stimulation with LPS was lower in children with asymptomatic malaria compared to smear-negative healthy children. Bone marrow response can compensate the increased red blood cell loss in asymptomatic malaria, unlike in clinical malaria, possibly because of limited level and length of inflammation.

Trial registration: Prospective diagnostic study: ClinicalTrials.gov identifier: NCT02669823.

Explorative cross-sectional field study: ClinicalTrials.gov identifier: NCT03176719.

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.

Asymptomatic Malaria Infection Is Maintained by a Balanced Pro- and Anti-inflammatory Response

Background

Pro- and anti-inflammatory cytokines are important mediators of immunity and are associated with malaria disease outcomes. However, their role in the establishment of asymptomatic infections, which may precede the development of clinical symptoms, is not as well-understood.

Methods

We determined the association of pro and anti-inflammatory cytokines and other immune effector molecules with the development of asymptomatic malaria. We measured and compared the plasma levels of pro-inflammatory mediators including tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ), interleukin (IL)-6, IL-12p70, IL-17A, and granzyme B, the anti-inflammatory cytokine IL-4 and the regulatory cytokine IL-10 from children with asymptomatic malaria infections (either microscopic or submicroscopic) and uninfected controls using Luminex.

Results

We show that individuals with microscopic asymptomatic malaria had significantly increased levels of TNF-α and IL-6 compared to uninfected controls. Children with either microscopic or submicroscopic asymptomatic malaria exhibited higher levels of IFN-γ, IL-17A, and IL-4 compared to uninfected controls. The levels of most of the pro and anti-inflammatory cytokines were comparable between children with microscopic and submicroscopic infections. The ratio of IFN-γ/IL-10, TNF-α/IL-10, IL-6/IL-10 as well as IFN-γ/IL-4 and IL-6/IL-4 did not differ significantly between the groups. Additionally, using a principal component analysis, the cytokines measured could not distinguish amongst the three study populations. This may imply that neither microscopic nor submicroscopic asymptomatic infections were polarized toward a pro-inflammatory or anti-inflammatory response.

Conclusion

The data show that asymptomatic malaria infections result in increased plasma levels of both pro and anti-inflammatory cytokines relative to uninfected persons. The balance between pro- and anti-inflammatory cytokines are, however, largely maintained and this may in part, explain the lack of clinical symptoms. This is consistent with the generally accepted observation that clinical symptoms develop as a result of immunopathology involving dysregulation of inflammatory mediator balance in favor of pro-inflammatory mediators.

A new era of rational malaria vaccine development

Holz et al. report a glycolipid-conjugate vaccine that provides sterile immunity in mice against Plasmodium berghei ANKA sporozoite challenge by inducing long-lasting tissue-resident memory (TRM) CD8+ T cells in the liver.

Effect of Seasonal Malaria Chemoprevention on Immune Markers of Exhaustion and Regulation

BACKGROUND

Seasonal malaria chemoprevention (SMC) is a novel strategy to reduce malaria infections in children. Infection with Plasmodium falciparum results in immune dysfunction characterized by elevated expression of markers associated with exhaustion, such as PD1 and LAG3, and regulatory CD4+FOXP3+ T cells.

METHODS

In the current study, the impact of seasonal malaria chemoprevention on malaria-induced immune dysfunction, as measured by markers associated with exhaustion and regulatory T cells, was explored by flow cytometry.

RESULTS

Children that received seasonal malaria chemoprevention had fewer malaria episodes and showed significantly lower fold changes in CD4+PD1+ and CD4+PD1+LAG3+ compared to those that did not receive SMC. Seasonal malaria chemoprevention had no observable effect on fold changes in CD8 T cells expressing PD1 or CD160. However, children receiving SMC showed greater increases in CD4+FOXP3+ T regulatory cells compared to children not receiving SMC.

CONCLUSIONS

These results provide important insights into the dynamics of malaria-induced changes in the CD4 T-cell compartment of the immune system and suggest that the reduction of infections due to seasonal malaria chemoprevention may also prevent immune dysfunction.

CLINICAL TRIALS REGISTRATION

NCT02504918.

The Impact of Malaria Parasites on Dendritic Cell-T Cell Interaction

Malaria is caused by apicomplexan parasites of the genus Plasmodium. While infection continues to pose a risk for the majority of the global population, the burden of disease mainly resides in Sub-Saharan Africa. Although immunity develops against disease, this requires years of persistent exposure and is not associated with protection against infection. Repeat infections occur due to the parasite's ability to disrupt or evade the host immune responses. However, despite many years of study, the mechanisms of this disruption remain unclear. Previous studies have demonstrated a parasite-induced failure in dendritic cell (DCs) function affecting the generation of helper T cell responses. These T cells fail to help B cell responses, reducing the production of antibodies that are necessary to control malaria infection. This review focuses on our current understanding of the effect of Plasmodium parasite on DC function, DC-T cell interaction, and T cell activation. A better understanding of how parasites disrupt DC-T cell interactions will lead to new targets and approaches to reinstate adaptive immune responses and enhance parasite immunity.

Host-pathogen interaction in the tissue environment during Plasmodium blood-stage infection

Human malarial infection occurs after an infectious Anopheles mosquito bites. Following the initial liver-stage infection, parasites transform into merozoites, infecting red blood cells (RBCs). Repeated RBC infection then occurs during the blood-stage infection, while patients experience various malarial symptoms. Protective immune responses are elicited by this systemic infection, but excessive responses are sometimes harmful for hosts. As parasites infect only RBCs and their immediate precursors during this stage, direct parasite-host interactions occur primarily in the environment surrounded by endothelial lining of blood vessels. The spleen is the major organ where the immune system encounters infected RBCs, causing immunological responses. Its tissue structure is markedly altered during malarial infection in mice and humans. Plasmodium falciparum parasites inside RBCs express proteins, such as PfEMP-1 and RIFIN, transported to the RBC surfaces in order to evade immunological attack by sequestering themselves in the peripheral vasculature avoiding spleen or by direct immune cell inhibition through inhibitory receptors. Host cell production of regulatory cytokines IL-10 and IL-27 limits excessive immune responses, avoiding tissue damage. The regulation of the protective and inhibitory immune responses through host-parasite interactions allows chronic Plasmodium infection. In this review, we discuss underlying interaction mechanisms relevant for developing effective strategies against malaria.

T-cells producing multiple combinations of IFNγ, TNF and IL10 are associated with mild forms of dengue infection

Multifunctional interleukin 10 (IL10)⁺ Th1 cells have been implicated in favorable evolution of many infectious diseases, promoting an efficacious immune response while limiting immunopathology. Here, we investigated the presence of multifunctional CD4⁺ and CD8⁺ T-cells that expressed interferon gamma (IFNγ), IL10 and tumor necrosis factor (TNF), or its combinations during dengue infection. Peripheral blood mononuclear cells (PBMCs) from outpatients with dengue (mild dengue forms) and hospitalized patients (or patients with dengue with warning signs and severe dengue) were cultured in the presence of envelope (ENV) or NS3 peptide libraries of DENV during critical (hospitalization period) and convalescence phases. The production of IFNγ, IL10 and TNF by CD4⁺ and CD8⁺ T-cells was assessed by flow cytometry. Our data show that patients with mild dengue, when compared with patients with dengue with warning signs and severe dengue, presented higher frequencies of multifunctional T-cells like NS3-specific IFNγ/IL10-producing CD4⁺ T-cells in critical phase and NS3- and ENV-specific CD8⁺ T-cells producing IFNγ/IL10. In addition, NS3-specific CD8⁺ T-cells producing high levels of IFNγ/TNF and IFNγ/TNF/IL10 were also observed in the mild dengue group. We observed that multifunctional T-cells produced higher levels of cytokines as measured by intracellular content when compared with single producer T-cells. Importantly, multifunctional CD4⁺ and CD8⁺ T-cells producing IFNγ, TNF and IL10 simultaneously displayed positive correlation with platelet levels, suggesting a protective role of this population. The presence of IL10⁺ Th1 and IL10⁺ Tc1 multifunctional cells was associated with mild dengue presentation, suggesting that these cells play a role in clinical evolution of dengue infection.

T cell-mediated immunity to malaria

Immunity to malaria has been linked to the availability and function of helper CD4⁺ T cells, cytotoxic CD8⁺ T cells and γδ T cells that can respond to both the asymptomatic liver stage and the symptomatic blood stage of Plasmodium sp. infection. These T cell responses are also thought to be modulated by regulatory T cells. However, the precise mechanisms governing the development and function of Plasmodium-specific T cells and their capacity to form tissue-resident and long-lived memory populations are less well understood. The field has arrived at a point where the push for vaccines that exploit T cell-mediated immunity to malaria has made it imperative to define and reconcile the mechanisms that regulate the development and functions of Plasmodium-specific T cells. Here, we review our current understanding of the mechanisms by which T cell subsets orchestrate host resistance to Plasmodium infection on the basis of observational and mechanistic studies in humans, non-human primates and rodent models. We also examine the potential of new experimental strategies and human infection systems to inform a new generation of approaches to harness T cell responses against malaria.

Using two phases of the CD4 T cell response to blood-stage murine malaria to understand regulation of systemic immunity and placental pathology in Plasmodium falciparum infection

Plasmodium falciparum infection and malaria remain a risk for millions of children and pregnant women. Here, we seek to integrate knowledge of mouse and human T helper cell (Th) responses to blood-stage Plasmodium infection to understand their contribution to protection and pathology. Although there is no complete Th subset differentiation, the adaptive response occurs in two phases in non-lethal rodent Plasmodium infection, coordinated by Th cells. In short, cellular immune responses limit the peak of parasitemia during the first phase; in the second phase, humoral immunity from T cell-dependent germinal centers is critical for complete clearance of rapidly changing parasite. A strong IFN-γ response kills parasite, but an excess of TNF compared with regulatory cytokines (IL-10, TGF-β) can cause immunopathology. This common pathway for pathology is associated with anemia, cerebral malaria, and placental malaria. These two phases can be used to both understand how the host responds to rapidly growing parasite and how it attempts to control immunopathology and variation. This dual nature of T cell immunity to Plasmodium is discussed, with particular reference to the protective nature of the continuous generation of effector T cells, and the unique contribution of effector memory T cells.

Molecular switches for regulating the differentiation of inflammatory and IL-10-producing anti-inflammatory T-helper cells

CD4 T-helper (Th) cells secret a variety of inflammatory cytokines and play critical roles in host defense against invading foreign pathogens. On the other hand, uncontrolled inflammatory responses mediated by Th cells may result in tissue damage and inflammatory disorders including autoimmune and allergic diseases. Thus, the induction of anti-inflammatory cytokine expression becomes an important "brake" to repress and/or terminate aberrant and/or unnecessary immune responses. Interleukin-10 (IL-10) is one of the most important anti-inflammatory cytokines to limit inflammatory Th cells and immunopathology and to maintain tissue homeostasis. Many studies have indicated that Th cells can be a major source of IL-10 under specific conditions both in mouse and human and that extracellular signals and cell intrinsic molecular switches are required to turn on and off Il10 expression in different Th cells. In this review, we will highlight the recent findings that have enhanced our understanding on the mechanisms of IL-10 induction in distinct Th-cell subsets, including Th1, Th2, and Th17 cells, as well as the importance of these IL-10-producing anti-inflammatory Th cells in immunity and inflammation.

Molecular Signatures of Dengue Virus-Specific IL-10/IFN-γ Co-producing CD4 T Cells and Their Association with Dengue Disease

Dengue virus (DENV) can cause diseases ranging from dengue fever (DF) to more severe dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS). Whether antiviral T cells contribute to the protection against or pathogenesis of severe disease is not well defined. Here, we identified antigen-specific IL-10+IFN-γ+ double-positive (DP) CD4 T cells during acute DENV infection. While the transcriptomic signatures of DP cells partially overlapped with those of cytotoxic and type 1 regulatory CD4 T cells, the majority of them were non-cytotoxic/Tr1 and included IL21, IL22, CD109, and CCR1. Although we observed a higher frequency of DP cells in DHF, the transcriptomic profile of DP cells was similar in DF and DHF, suggesting that DHF is not associated with the altered phenotypic or functional attributes of DP cells. Overall, this study revealed a DENV-specific DP cell subset in patients with acute dengue disease and argues against altered DP cells as a determinant of DHF.

Understanding P. falciparum Asymptomatic Infections: A Proposition for a Transcriptomic Approach

Malaria is still a significant public health burden in the tropics. Infection with malaria causing parasites results in a wide range of clinical disease presentations, from severe to uncomplicated or mild, and in the poorly understood asymptomatic infections. The complexity of asymptomatic infections is due to the intricate interplay between factors derived from the human host, parasite, and environment. Asymptomatic infections often go undetected and provide a silent natural reservoir that sustains malaria transmission. This creates a major obstacle for malaria control and elimination efforts. Numerous studies have tried to characterize asymptomatic infections, unanimously revealing that host immunity is the underlying factor in the maintenance of these infections and in the risk of developing febrile malaria infections. An in-depth understanding of how host immunity and parasite factors interact to cause malaria disease tolerance is thus required. This review primarily focuses on understanding anti-inflammatory and pro-inflammatory responses to asymptomatic infections in malaria endemic areas, to present the view that it is potentially the shift in host immunity toward an anti-inflammatory profile that maintains asymptomatic infections after multiple exposures to malaria. Conversely, symptomatic infections are skewed toward a pro-inflammatory immune profile. Moreover, we propose that these infections can be better interrogated using next generation sequencing technologies, in particular RNA sequencing (RNA-seq), to investigate the immune system using the transcriptome sampled during a clearly defined asymptomatic infection.

In utero priming of highly functional effector T cell responses to human malaria

Malaria remains a significant cause of morbidity and mortality worldwide, particularly in infants and children. Some studies have reported that exposure to malaria antigens in utero results in the development of tolerance, which could contribute to poor immunity to malaria in early life. However, the effector T cell response to pathogen-derived antigens encountered in utero, including malaria, has not been well characterized. Here, we assessed the frequency, phenotype, and function of cord blood T cells from Ugandan infants born to mothers with and without placental malaria. We found that infants born to mothers with active placental malaria had elevated frequencies of proliferating effector memory fetal CD4⁺ T cells and higher frequencies of CD4⁺ and CD8⁺ T cells that produced inflammatory cytokines. Fetal CD4⁺ and CD8⁺ T cells from placental malaria-exposed infants exhibited greater in vitro proliferation to malaria antigens. Malaria-specific CD4⁺ T cell proliferation correlated with prospective protection from malaria during childhood. These data demonstrate that placental malaria is associated with the generation of proinflammatory malaria-responsive fetal T cells. These findings add to our current understanding of fetal immunity and indicate that a functional and protective pathogen-specific T cell response can be generated in utero.

The immune response to malaria in utero

Malaria causes tremendous early childhood morbidity and mortality, providing an urgent impetus for the development of a vaccine that is effective in neonates. However, the infant immune response to malaria may be influenced by events that occur well before birth. Placental malaria infection complicates one quarter of all pregnancies in Africa and frequently results in exposure of the fetus to malaria antigens in utero, while the immune system is still developing. Some data suggest that in utero exposure to malaria may induce immunologic tolerance that interferes with the development of protective immunity during childhood. More recently, however, a growing body of evidence suggests that fetal malaria exposure can prime highly functional malaria-specific T- and B-cells, which may contribute to postnatal protection from malaria. In utero exposure to malaria also impacts the activation and maturation of fetal antigen presenting cells and innate lymphocytes, which could have implications for global immunity in the infant. Here, we review recent advances in our understanding of how various components of the fetal immune system are altered by in utero exposure to malaria, discuss factors that may tilt the critical balance between tolerance and adaptive immunity, and consider the implications of these findings for malaria prevention strategies.

Early Changes in CD4+ T-Cell Activation During Blood-Stage Plasmodium falciparum Infection

We examined transcriptional changes in CD4+ T cells during blood-stage Plasmodium falciparum infection in individuals without a history of previous parasite exposure. Transcription of CXCL8 (encoding interleukin 8) in CD4+ T cells was identified as an early biomarker of submicroscopic P. falciparum infection, with predictive power for parasite growth. Following antiparasitic drug treatment, a CD4+ T-cell regulatory phenotype developed. PD1 expression on CD49b+CD4+ T (putative type I regulatory T) cells after drug treatment negatively correlated with earlier parasite growth. Blockade of PD1 but no other immune checkpoint molecules tested increased interferon γ and interleukin 10 production in an ex vivo antigen-specific cellular assay at the peak of infection. These results demonstrate the early development of an immunoregulatory CD4+ T-cell phenotype in blood-stage P. falciparum infection and show that a selective immune checkpoint blockade may be used to modulate early developing antiparasitic immunoregulatory pathways as part of malaria vaccine and/or drug treatment protocols.

TH1-Polarized TFH Cells Delay Naturally-Acquired Immunity to Malaria

Humoral immunity is a critical effector arm for protection against malaria but develops only slowly after repeated infections. T cell-mediated regulatory dynamics affect the development of antibody responses to Plasmodium parasites. Here, we hypothesize that T follicular helper cell (T_FH) polarization generated by repeated Plasmodium asexual blood-stage infections delays the onset of protective humoral responses. IFN-γ production promotes polarization toward T_FH1 and increased generation of regulatory follicular helper cells (T_FR). Delineating the mechanisms that drive T_H1 polarization will provide clues for appropriate induction of lasting, protective immunity against malaria.

Immune effector mechanisms in malaria: An update focusing on human immunity

The past decade has witnessed dramatic decreases in malaria-associated mortality and morbidity around the world. This progress has largely been due to intensified malaria control measures, implementation of rapid diagnostics and establishing a network to anticipate and mitigate antimalarial drug resistance. However, the ultimate tool for malaria prevention is the development and implementation of an effective vaccine. To date, malaria vaccine efforts have focused on determining which of the thousands of antigens expressed by Plasmodium falciparum are instrumental targets of protective immunity. The antigenic variation and antigenic polymorphisms arising in parasite genes under immune selection present a daunting challenge for target antigen selection and prioritization, and is a given caveat when interpreting immune recall responses or results from monovalent vaccine trials. Other immune evasion strategies executed by the parasite highlight the myriad of ways in which it can become a recurrent infection. This review provides an update on immune effector mechanisms in malaria and focuses on our improved ability to interrogate the complexity of human immune system, accelerated by recent methodological advances. Appreciating how the human immune landscape influences the effectiveness and longevity of antimalarial immunity will help explain which conditions are necessary for immune effector mechanisms to prevail.

Repeated clinical malaria episodes are associated with modification of the immune system in children

BACKGROUND

There are over 200 million reported cases of malaria each year, and most children living in endemic areas will experience multiple episodes of clinical disease before puberty. We set out to understand how frequent clinical malaria, which elicits a strong inflammatory response, affects the immune system and whether these modifications are observable in the absence of detectable parasitaemia.

METHODS

We used a multi-dimensional approach comprising whole blood transcriptomic, cellular and plasma cytokine analyses on a cohort of children living with endemic malaria, but uninfected at sampling, who had been under active surveillance for malaria for 8 years. Children were categorised into two groups depending on the cumulative number of episodes experienced: high (≥ 8) or low (< 5).

RESULTS

We observe that multiple episodes of malaria are associated with modification of the immune system. Children who had experienced a large number of episodes demonstrated upregulation of interferon-inducible genes, a clear increase in circulating levels of the immunoregulatory cytokine IL-10 and enhanced activation of neutrophils, B cells and CD8⁺ T cells.

CONCLUSION

Transcriptomic analysis together with cytokine and immune cell profiling of peripheral blood can robustly detect immune differences between children with different numbers of prior malaria episodes. Multiple episodes of malaria are associated with modification of the immune system in children. Such immune modifications may have implications for the initiation of subsequent immune responses and the induction of vaccine-mediated protection.

The Role of IL-10 in Malaria: A Double Edged Sword

IL-10 produced by CD4⁺ T cells suppresses inflammation by inhibiting T cell functions and the upstream activities of antigen presenting cells (APCs). IL-10 was first identified in Th2 cells, but has since been described in IFNγ-producing Tbet⁺ Th1, FoxP3⁺ CD4⁺ regulatory T (Treg) and IL-17-producing CD4⁺ T (Th17) cells, as well as many innate and innate-like immune cell populations. IL-10 production by Th1 cells has emerged as an important mechanism to dampen inflammation in the face of intractable infection, including in African children with malaria. However, although these type I regulatory T (Tr1) cells protect tissue from inflammation, they may also promote disease by suppressing Th1 cell-mediated immunity, thereby allowing infection to persist. IL-10 produced by other immune cells during malaria can also influence disease outcome, but the full impact of this IL-10 production is still unclear. Together, the actions of this potent anti-inflammatory cytokine along with other immunoregulatory mechanisms that emerge following Plasmodium infection represent a potential hurdle for the development of immunity against malaria, whether naturally acquired or vaccine-induced. Recent advances in understanding how IL-10 production is initiated and regulated have revealed new opportunities for manipulating IL-10 for therapeutic advantage. In this review, we will summarize our current knowledge about IL-10 production during malaria and discuss its impact on disease outcome. We will highlight recent advances in our understanding about how IL-10 production by specific immune cell subsets is regulated and consider how this knowledge may be used in drug delivery and vaccination strategies to help eliminate malaria.