EVALUATING THE ROLE OF PLASMODIUM FALCIPARUM EXTRACELLULAR VESICLES IN NEGATIVE IMMUNE REGULATION

Abstract

Malaria is still a major global challenge, particularly in sub-Saharan Africa where nearly two-thirds of the global malaria cases are reported annually. The majority of the cases are attributed to Plasmodium falciparum infection which is also accountable for most of the severe and fatal cases. To survive and grow within the host, the parasite is thought to manipulate the host immunity in a way that leads to inadequate immune responses. Even though parasite effectors used to manipulate the host immune response are not well recognized, parasite secreted factors are thought to play a part. One of the ways cells release secreted effectors is through extracellular vesicles (EVs). EVs are bio-sphere membranes released from both eukaryotic and prokaryotic cells. They contain signaling competent molecules (e.g., proteins, miRNA) and are released by all cells to regulate their microenvironment and to exchange information with distant cells and tissues. P. falciparum-infected erythrocytes release EVs that can mediate an interaction between infected red blood cells and the host immune cells. However, the impact of P. falciparum EVs on host immune cells has not been well investigated with the few studies done using EVs from long-term laboratory-adapted parasites. This introduces the possibility that the EVs have not been interacting with the host immune cells for a long period. Here, we assessed the induction of programed death-ligand 1 (PDL-1) on monocytes and B cells and programmed death protein-1 (PD-1) expression on CD4T cells and CD8T cells by PfEVs and further investigated their effect on T cell proliferation and differentiation. Frequency of PDL-1+ monocytes, PDL-1+ B cells and PD-1+ CD4T cells as well as PD-1+CD8T cells was significantly increased in PfEVs stimulated peripheral blood mononuclear cells PBMCs. Furthermore, PfEVs inhibited CD8T cell proliferation. We observed a significant rise in the proportion of CD3T cells double positive for IL-10 and IFNγ. No significance difference was observed in the proportions of IL-10+CD4T, IFN- x g+CD4T cells, IL-10+CD8T cells, IFN-g+CD8T cells and IL-10+/IFNγ+ CD8T cells. We also show significant decrease in the number of CD3T cells post stimulation. On further analysis we show a significant reduction in the absolute number of CD8T cells post stimulation which implies that CD8T cells are more susceptible to the PDL-1/PD-1 interaction consequently negatively affecting the host immune responses. Overall, we show that PfEVs play a role in host immune modulation by inducing expression of inhibitory cell markers and secretion of inhibitory cytokines. However, the exact mechanisms through which PfEVs induce the expression of these markers remain to be elucidated. It will be interesting to look at the specific T cell subsets affected and the mechanisms and pathways involved in the induction of inhibitory markers by PfEVs during malaria infection as well explore the specific cell subsets induced by PfEVs in negative immune downregulation.