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

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.