Date Published: November 1, 2016
Publisher: Public Library of Science
Author(s): Maria Sophia Mackroth, Annemieke Abel, Christiane Steeg, Julian Schulze zur Wiesch, Thomas Jacobs, Christian R. Engwerda.
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+ Teffector cells that coproduce IFNγ and IL10, and inhibit other CD4+ T cells. Transient induction of regulatory Teffector 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.
Malaria remains one of the leading health burdens worldwide with about 600 000 deaths per year. Most of these deaths are attributable to the species Plasmodium falciparum (P. falciparum) . Primary infection with P. falciparum initially induces a strong Th1-type CD4+ T cell response. While a strong proinflammatory Th1 response can contribute to control of parasitemia and protection to subsequent infections [2, 3], it can also be pathological as it activates the endothelium and thereby promotes sequestration of parasitized red blood cells in the microvessels of vital organs such as the brain [4, 5]. This impedes parasite clearance by the spleen and enhances severe manifestations of malaria such as cerebral malaria [6–8]. Therefore, a tight coordination of the immune response is needed to ensure the optimal outcome for the patient. Strong proinflammatory responses activate counteracting pathways such as the induction of regulatory T cell (Treg) populations and the production of anti-inflammatory cytokines, which both are crucial for preventing immunopathology in malaria and other parasitic diseases [9–12]. Another key mechanism that regulates potentially immunopathological T cell responses in the periphery is the induction of coinhibitory receptors such as cytotoxic T-Lymphocyte attenuator 4 (CTLA4) and programmed death 1 (PD1) on T cells.
The present study explored the role of coinhibitory receptors as an immune regulatory pathway in acute P. falciparum malaria. We initially hypothesized that the induction of coinhibitory receptors, primarily CTLA4 and PD1, is an important and adaptable mechanism by which CD4+ T cell responses in malaria are downregulated. Indeed, we found that the majority of P. falciparum-specific CD4+ T cells expressed both CTLA4 and PD1 and that blocking both receptors enhanced their P. falciparum-specific effector responses. Surprisingly, subsequent experiments showed that the PD1+CTLA4+CD4+ Teff cells in malaria patients had an acquired extrinsic regulatory function by which they suppressed the P. falciparum-specific and polyclonal proliferation of other T cells. These PD1+CTLA4+CD4+ Teff cells therefore constitute a distinct regulatory T cell population which is induced during acute malaria.