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Vγ9Vδ2 T cells rapidly respond to phosphoantigens produced by Plasmodium falciparum in an innate-like manner, without prior antigen exposure or processing. Vδ2 T cells have been shown to inhibit parasite replication in vitro and are associated with protection from P. falciparum parasitemia in vivo. Although a marked expansion of Vδ2 T cells is seen after acute malaria infection in naïve individuals, repeated malaria causes Vδ2 T cells to decline both in frequency and in malaria-responsiveness, and to exhibit numerous transcriptional and phenotypic changes, including upregulation of the Fc receptor CD16. Here we investigate the functional role of CD16 on Vδ2 T cells in the immune response to malaria. We show that CD16+ Vδ2 T cells possess more cytolytic potential than their CD16- counterparts, and bear many of the hallmarks of mature NK cells, including KIR expression. Furthermore, we demonstrate that Vδ2 T cells from heavily malaria-exposed individuals are able to respond to opsonized P.falciparum-infected red blood cells through CD16, representing a second, distinct pathway by which Vδ2 T cells may contribute to anti-parasite effector functions. This response was independent of TCR engagement, as demonstrated by blockade of the phosphoantigen presenting molecule Butyrophilin 3A1. Together these results indicate that Vδ2 T cells in heavily malaria-exposed individuals retain the capacity for antimalarial effector function, and demonstrate their activation by opsonized parasite antigen. This represents a new role both for Vδ2 T cells and for opsonizing antibodies in parasite clearance, emphasizing cooperation between the cellular and humoral arms of the immune system., Author summary T cells that express the Vδ2 and Vγ9 TCR chains (Vδ2 T cells) have been shown to play an important role in controlling parasitemia during P.falciparum infection. A better understanding of how these cells interact with malaria parasites to control infection is necessary. We have previously shown that after multiple P. falciparum infections, Vδ2 T cells decrease in frequency, become less responsive to TCR stimulation, and upregulate the Fc receptor CD16. Here we investigate whether Vδ2 T cells from chronically malaria-exposed individuals can be activated directly through CD16 to release proinflammatory cytokines and degranulate. We show that in these individuals, TCR is downregulated on CD16+ Vδ2 T cells, and that these cells are more likely to express a variety of cytotoxic effector molecules. Importantly, we show that these CD16+ Vδ2 T cells can be activated directly through CD16, independent of TCR, by antibody bound to parasite antigen. These results are notable because they indicate many Vδ2 T cells from chronically-exposed individuals may not be exhausted but instead favor an alternative activation pathway, one that cooperates with a mature anti-malarial antibody response.

Background. Experimental inoculation of viable Plasmodium falciparum sporozoites administered with chemoprevention targeting blood-stage parasites results in protective immunity. It is unclear whether chemoprevention similarly enhances immunity following natural exposure to malaria. Methods. We assessed P. falciparum–specific T-cell responses among Ugandan children who were randomly assigned to receive monthly dihydroartemisinin-piperaquine (DP; n = 87) or no chemoprevention (n = 90) from 6 to 24 months of age, with pharmacologic assessments for adherence, and then clinically followed for an additional year. Results. During the intervention, monthly DP reduced malaria episodes by 55% overall (P < .001) and by 97% among children who were highly adherent to DP (P < .001). In the year after the cessation of chemoprevention, children who were highly adherent to DP had a 55% reduction in malaria incidence as compared to children given no chemoprevention (P = .004). Children randomly assigned to receive DP had higher frequencies of blood-stage specific CD4+ T cells coproducing interleukin-2 and tumor necrosis factor α (P = .003), which were associated with protection from subsequent clinical malaria and parasitemia, and fewer blood-stage specific CD4+ T cells coproducing interleukin-10 and interferon γ (P = .001), which were associated with increased risk of malaria. Conclusions. In this setting, effective antimalarial chemoprevention fostered the development of CD4+ T cells that coproduced interleukin 2 and tumor necrosis factor α and were associated with prospective protection, while limiting CD4+ T-cell production of the immunoregulatory cytokine IL-10.

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.

Plasmodium falciparum malaria continues to be a leading cause of morbidity and mortality in Sub-Saharan Africa [1, 2]. Although children living in endemic settings eventually develop clinical immunity to malaria, most individuals experience numerous infections before clinical protection is achieved. A vaccine to prevent malaria is urgently needed, but progress has been limited by our lack of understanding of both the critical P. falciparum antigenic targets and the immune effector mechanisms needed to confer protective immunity. Growing evidence suggests that malaria-specific T cells induced by natural infection or by vaccination may protect against clinical disease [3–8]. T-cell responses to blood-stage antigens, including merozoite surface antigen 1 (MSP1), are frequently observed among children living in endemic settings, and a few studies have found them to be associated with protection from future malaria [4, 8]. However, thus far blood-stage vaccines have not proven efficacious [9]. Several lines of evidence have prompted a growing interest in pre-erythrocytic stage malaria antigens as potential vaccine targets. T-cell responses to the pre-erythrocytic circumsporozoite (CSP) antigen have been shown to correlate with protection from future parasitemia [3, 6], and a subunit vaccine (RTS,S) incorporating CSP has modestly reduced clinical P. falciparum malaria in African infants in phase 2 and 3 trials [10–12]. T-cell responses to other pre-erythrocytic proteins including TRAP and LSA-1 have also been associated with protection in humans [5, 7, 13, 14]. Moreover, it has long been known that vaccination with irradiated sporozoites, which arrest development during the liver stage, confers sterile protective immunity in humans [15–18], suggesting an important role for the T-cell response to pre-erythrocytic antigens in mediating vaccine-induced immune protection. The use of chemoprevention, either seasonal or year-round, has recently been explored as a public health strategy to prevent mortality and morbidity due to childhood malaria in endemic settings [19, 20]. Although it has been shown to be effective in reducing malaria, concerns have been raised that a “rebound” increase in malaria incidence may be observed once chemoprevention is stopped, due to delayed development of protective immune responses [21, 22]. However, recent studies suggest that provision of antimalarial drugs that target blood-stage malaria may actually enhance the development of cellular immune responses directed at pre-erythrocytic antigens and, somewhat paradoxically, foster the development of protective immunity, a strategy termed “infection-treatment vaccination” [23–27]. In these studies, individuals experimentally infected by sporozoites while receiving chloroquine, which prevents blood-stage malaria but allows the clinically silent liver stage infection to develop, consistently exhibited sterile protection upon rechallenge [25–27]. These data suggest that limiting exposure to blood-stage infection may actually enhance the development of immune responses to pre-erythrocytic stages, perhaps due to enhanced exposure to liver stage antigens [28] or avoidance of immunoregulatory mechanisms induced by parasitemia [29]. By analogy, provision of chemoprevention to heavily exposed children might actually encourage pre-erythrocytic responses and foster the development of protective immunity. In this study, we performed a longitudinal evaluation of malaria-specific T-cell responses generated in response to natural infection and compared the responses of children receiving monthly chemoprevention with dihydroartemisinin-piperaquine (DP) to those receiving no chemoprevention as part of a randomized clinical trial. We hypothesized that interferon γ (IFNγ) responses to pre-erythrocytic antigens would be associated with protection from malaria, and that selective suppression of blood-stage malaria by chemoprevention given to children living in a high endemicity setting may limit the development of T-cell responses to blood-stage antigens and enhance the development of responses to pre-erythrocytic antigens.

Cytokine-producing CD4 T cells have important roles in immunity against Plasmodium falciparum (Pf) malaria. However, the factors influencing functional differentiation of Pf-specific CD4 T cells in naturally exposed children are not well understood. Moreover, it is not known which CD4 T-cell cytokine-producing subsets are most critical for protection. We measured Pf-specific IFNγ-, IL10-, and TNFα-producing CD4 T-cell responses by multi-parametric flow cytometry in 265 children aged 6 months to 10 years enrolled in a longitudinal observational cohort in a high malaria transmission site in Uganda. We found that both age and parasite burden were independently associated with cytokine production by CD4 T cells. IL10 production by IFNγ+ CD4 T cells was higher in younger children and in those with high-parasite burden during recent infection. To investigate the role of CD4 T cells in immunity to malaria, we measured associations of Pf-specific CD4 cytokine-producing cells with the prospective risk of Pf infection and clinical malaria, adjusting for household exposure to Pf-infected mosquitos. Overall, the prospective risk of infection was not associated with the total frequency of Pf-specific CD4 T cells, nor of any cytokine-producing CD4 subset. However, the frequency of CD4 cells producing IL10 but not inflammatory cytokines (IFNγ and TNFα) was associated with a decreased risk of clinical malaria once infected. These data suggest that functional polarization of the CD4 T-cell response may modulate the clinical manifestations of malaria and play a role in naturally acquired immunity.

Vδ2+ γδ T cells are semi-innate T cells that expand markedly following P. falciparum (Pf) infection in naïve adults, but are lost and become dysfunctional among children repeatedly exposed to malaria. The role of these cells in mediating clinical immunity (i.e. protection against symptoms) to malaria remains unclear. We measured Vδ2+ T cell absolute counts at acute and convalescent malaria timepoints (n = 43), and Vδ2+ counts, cellular phenotype, and cytokine production following in vitro stimulation at asymptomatic visits (n = 377), among children aged 6 months to 10 years living in Uganda. Increasing age was associated with diminished in vivo expansion following malaria, and lower Vδ2 absolute counts overall, among children living in a high transmission setting. Microscopic parasitemia and expression of the immunoregulatory markers Tim-3 and CD57 were associated with diminished Vδ2+ T cell pro-inflammatory cytokine production. Higher Vδ2 pro-inflammatory cytokine production was associated with protection from subsequent Pf infection, but also with an increased odds of symptoms once infected. Vδ2+ T cells may play a role in preventing malaria infection in children living in endemic settings; progressive loss and dysfunction of these cells may represent a disease tolerance mechanism that contributes to the development of clinical immunity to malaria.

Sex differences in the immune response and in infectious disease susceptibility have been well described, although the mechanisms underlying these differences remain incompletely understood. We evaluated the frequency of cord blood CD4 T cell subsets in a highly malaria-exposed birth cohort of mother-infant pairs in Uganda by sex. We found that frequencies of cord blood regulatory T cell ([Treg] CD4+CD25+FoxP3+CD127lo/−) differed by infant sex, with significantly lower frequencies of Tregs in female than in male neonates (P = .006). When stratified by in utero malaria exposure status, this difference was observed in the exposed, but not in the unexposed infants.

BackgroundIn malaria-endemic areas, the first exposure to malaria antigens often occurs in utero when the fetal immune system is poised towards the development of tolerance. Children exposed to placental malaria have an increased risk of clinical malaria in the first few years of life compared to unexposed children. Recent work has suggested the potential of pregnancy-associated malaria to induce immune tolerance in children living in malaria-endemic areas. A study was completed to evaluate the effect of malaria exposure during pregnancy on fetal immune tolerance and effector responses.MethodsUsing cord blood samples from a cohort of mother-infant pairs followed from early in pregnancy until delivery, flow cytometry analysis was completed to assess the relationship between pregnancy-associated malaria and fetal cord blood CD4 and dendritic cell phenotypes.ResultsCord blood FoxP3+ Treg counts were higher in infants born to mothers with Plasmodium parasitaemia early in pregnancy (12-20weeks of gestation; p=0.048), but there was no association between Treg counts and the presence of parasites in the placenta at the time of delivery (by loop-mediated isothermal amplification (LAMP); p=0.810). In contrast, higher frequencies of activated CD4 T cells (CD25+FoxP3-CD127+) were observed in the cord blood of neonates with active placental Plasmodium infection at the time of delivery (p=0.035). This population exhibited evidence of effector memory differentiation, suggesting priming of effector T cells in utero. Lastly, myeloid dendritic cells were higher in the cord blood of infants with histopathologic evidence of placental malaria (p&nbsp

γδ T cells expressing Vδ2 may be instrumental in the control of malaria, because they inhibit the replication of blood-stage parasites in vitro and expand during acute malaria infection. However, Vδ2 T-cell frequencies and function are lower among children with heavy prior malaria exposure. It remains unclear whether malaria itself is driving this loss. Here we measure Vδ2 T-cell frequency, cytokine production, and degranulation longitudinally in Ugandan children enrolled in a malaria chemoprevention trial from 6 to 36 months of age. We observed a progressive attenuation of the Vδ2 response only among children incurring high rates of malaria. Unresponsive Vδ2 T cells were marked by expression of CD16, which was elevated in the setting of high malaria transmission. Moreover, chemoprevention during early childhood prevented the development of dysfunctional Vδ2 T cells. These observations provide insight into the role of Vδ2 T cells in the immune response to chronic malaria.

FoxP3+ regulatory CD4 T cells (Tregs) help to maintain the delicate balance between pathogen-specific immunity and immune-mediated pathology. Prior studies suggest that Tregs are induced by P. falciparum both in vivo and in vitro; however, the factors influencing Treg homeostasis during acute and chronic infections, and their role in malaria immunopathogenesis, remain unclear. We assessed the frequency and phenotype of Tregs in well-characterized cohorts of children residing in a region of high malaria endemicity in Uganda. We found that both the frequency and absolute numbers of FoxP3+ Tregs in peripheral blood declined markedly with increasing prior malaria incidence. Longitudinal measurements confirmed that this decline occurred only among highly malaria-exposed children. The decline of Tregs from peripheral blood was accompanied by reduced in vitro induction of Tregs by parasite antigen and decreased expression of TNFR2 on Tregs among children who had intense prior exposure to malaria. While Treg frequencies were not associated with protection from malaria, there was a trend toward reduced risk of symptomatic malaria once infected with P. falciparum among children with lower Treg frequencies. These data demonstrate that chronic malaria exposure results in altered Treg homeostasis, which may impact the development of antimalarial immunity in naturally exposed populations., Author Summary In malaria endemic regions, immunity is slow to develop and does not provide substantial protection against reinfection. Rather, following repeated exposure, older children and adults eventually develop protection from most symptomatic manifestations of the infection. This may be due in part to the induction of immunoregulatory mechanisms by the P. falciparum parasite, such as FoxP3+ regulatory T cells (Tregs). Prior human studies have shown that Tregs are induced by malaria parasites both in vivo and in vitro, but the role of these cells in immunity in children who are chronically exposed to malaria remains unclear. In this study, we assessed the frequency and features of Tregs among children from areas of high malaria transmission in Uganda. We found that this regulatory T cell population declined markedly with increasing malaria episodes. This loss was associated with decreased expression of TNFR2, which is a protein implicated in stability of Tregs. Additionally, T cells from highly malaria exposed children demonstrated a reduced propensity to differentiate into Tregs following parasite stimulation. Together our data suggest that repeated episodes of malaria alter Treg homeostasis, which may influence the development of immunity to malaria in children.