In acute Plasmodium falciparum (P. falciparum) malaria, the pro- and anti-inflammatory immune pathways must be delicately balanced so that the parasitemia is controlled without inducing immunopathology. An important mechanism to fine-tune T cell responses in the periphery is the induction of coinhibitory receptors such as CTLA4 and PD1. However, their role in acute infections such as P. falciparum malaria remains poorly understood. To test whether coinhibitory receptors modulate CD4⁺ T cell functions in malaria, blood samples were obtained from patients with acute P. falciparum malaria treated in Germany. Flow cytometric analysis showed a more frequent expression of CTLA4 and PD1 on CD4⁺ T cells of malaria patients than of healthy control subjects. In vitro stimulation with P. falciparum-infected red blood cells revealed a distinct population of PD1⁺CTLA4⁺CD4⁺ T cells that simultaneously produced IFNγ and IL10. This antigen-specific cytokine production was enhanced by blocking PD1/PDL1 and CTLA4. PD1⁺CTLA4⁺CD4⁺ T cells were further isolated based on surface expression of PD1 and their inhibitory function investigated in-vitro. Isolated PD1⁺CTLA4⁺CD4⁺ T cells suppressed the proliferation of the total CD4⁺ population in response to anti-CD3/28 and plasmodial antigens in a cell-extrinsic manner. The response to other specific antigens was not suppressed. Thus, acute P. falciparum malaria induces P. falciparum-specific PD1⁺CTLA4⁺CD4⁺ T_effector cells that coproduce IFNγ and IL10, and inhibit other CD4⁺ T cells. Transient induction of regulatory T_effector cells may be an important mechanism that controls T cell responses and might prevent severe inflammation in patients with malaria and potentially other acute infections. In acute infections like malaria, our immune systems must achieve a careful balance between inflammatory and anti-inflammatory responses to successfully fight the infection without causing harm to the host. In this study, we examined the CD4⁺ T cell response and CD4⁺ T cell regulation in patients with acute malaria. Important mechanisms to control CD4⁺ T cell activity include specific regulatory T cell populations which suppress other T cells or the expression of so-called coinhibitory receptors which inhibit the inflammatory response of the expressing cells. We showed that in malaria patients, high numbers of T cells expressed the coinhibitory receptors CTLA4 and PD1. Surprisingly, despite the high expression of coinhibitory receptors, malaria-specific effector function was predominantly found in the PD1⁺CTLA4⁺CD4⁺ T cell population. A blockade of the receptors enhanced the effector response. Even more surprising, we found that although PD1⁺ CTLA4⁺CD4⁺ T cells contained the majority of malaria-specific T cells, they showed, at the same time, cell-extrinsic suppressor activity and actively downregulated T cell proliferation. Thus, our observations describe a new population of “regulatory” T effector cells which are induced during acute malaria. This will contribute to our understanding of the complex immune pathways activated during acute malaria.

The immune response of malaria patients is a main factor influencing the clinical severity of malaria. A tight regulation of the CD4⁺ T cell response or the induction of tolerance have been proposed to contribute to protection from severe or clinical disease. We therefore compared the CD4⁺ T cell phenotypes of Ghanaian children with complicated malaria, uncomplicated malaria, asymptomatic Plasmodium falciparum (Pf) infection or no infection. Using flow cytometric analysis and automated multivariate clustering, we characterized the expression of the co-inhibitory molecules CTLA-4, PD-1, Tim-3, and LAG-3 and other molecules implicated in regulatory function on CD4⁺ T cells. Children with complicated malaria had higher frequencies of CTLA-4⁺ or PD-1⁺ CD4⁺ T cells than children with uncomplicated malaria. Conversely, children with uncomplicated malaria showed a higher proportion of CD4⁺ T cells expressing CD39 and Granzyme B, compared to children with complicated malaria. In contrast, asymptomatically infected children expressed only low levels of co-inhibitory molecules. Thus, different CD4⁺ T cell phenotypes are associated with complicated versus uncomplicated malaria, suggesting a two-sided role of CD4⁺ T cells in malaria pathogenesis and protection. Deciphering the signals that shape the CD4⁺ T cell phenotype in malaria will be important for new treatment and immunization strategies.

In Plasmodium falciparum malaria, CD8⁺ T cells play a double-edged role. Liver-stage specific CD8⁺ T cells can confer protection, as has been shown in several vaccine studies. Blood-stage specific CD8⁺ T cells, on the other hand, contribute to the development of cerebral malaria in murine models of malaria. The role of CD8⁺ T cells in humans during the blood-stage of P. falciparum remains unclear. As part of a cross-sectional malaria study in Ghana, granzyme B levels and CD8⁺ T cells phenotypes were compared in the peripheral blood of children with complicated malaria, uncomplicated malaria, afebrile but asymptomatically infected children and non-infected children. Granzyme B levels in the plasma were significantly higher in children with febrile malaria than in afebrile children. CD8⁺ T cells were the main T cell subset expressing granzyme B. The proportion of granzyme B⁺ CD8⁺ T cells was significantly higher in children with complicated malaria than in uncomplicated malaria, whereas the activation marker CD38 on CD8⁺ T cells showed similar expression levels. This suggests a pathogenic role of cytotoxic CD8⁺ T cells in the development of malaria complications in humans.

Plasmodium knowlesi has been identified in the last decade as a fifth species causing malaria in areas of South East Asia. Due to its short erythrocytic cycle, rapid development of high parasitemia and severe manifestations are frequently observed. Therefore, prompt diagnosis of infection is essential to prevent complications, but the low sensitivity of rapid diagnostic tests for P knowlesi pose a diagnostic challenge in acute settings. In this study, we report the case of a German traveler to Thailand, who was treated for P knowlesi malaria after returning to Germany. Rapid antigen test for malaria was negative on presentation. Diagnosis of a nonfalciparum malaria was made based on microscopy, and species definition was determined using polymerase chain reaction technique.

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