Acute Malaria Induces PD1 ⁺CTLA4 ⁺ Effector T Cells with Cell-Extrinsic Suppressor Function

Abstract

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.

Author Summary

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.