The Impact of Plasmodium falciparum Small-Sized Extracellular Vesicles (PfsEVs) and Plasmodium falciparum-Infected Red Blood Cells (PfiRBCs) on Host Immune Cells in vitro

Abstract

Overly increased immune activation is thought to play a key role in malaria pathogenesis. During blood stage infection, Plasmodium falciparum can directly interact with host immune cells through the infected red blood cells (PfiRBCs) but can also do so indirectly via the release of extracellular vesicles (EVs). This study compared the impact of PfiRBCs and P. falciparum small-sized EVs (PfsEVs, also known as exosomes) from a clinical parasite isolate (PfKE12) adapted to short-term laboratory culture conditions on host peripheral blood mononuclear cells (PBMC). PfsEVs were isolated from cell-free culture-conditioned media by ultracentrifugation while mature trophozoite PfiRBCs were purified by magnetic column separation. The PfsEVs and the PfiRBCs were then co-cultured for 18 hours with the study PBMC. Cellular responses acquired were quantified by quantifying the cell surface expression of activation markers (CD25 & CD69) and cytokine/chemokine levels in the supernatant. PfsEVs induced CD25 expression on CD4+, CD19+ and CD14+ cells while PfiRBCs induced the same on CD19+ and CD14+ cells relative to the background. Both PfsEVs and PfiRBCs induced CD69 on CD4+, CD8+ and CD19+ cells. Additionally, PfiRBCs induced higher expression of CD69 on CD14+ cells. CD69 induced by PfiRBCs on CD4+ and CD19+ cells were significantly higher than that induced by PfsEVs. The secretion of MIP1𝛂, MIP1β, GM-CSF, IL-6, IL-8, and TNF𝛂 were significantly induced by both PfsEVs and PfiRBCs whereas MCP-1, IL-10, IL-17𝛂 were preferentially induced by PfsEVs and IP-10 and IFN-𝛄 by PfiRBCs. Prior exposure to malaria (judged by antibodies to schizont extract) was associated with lower monocyte responses to PfsEVs. PfsEVs and PfiRBCs showed differential abilities to induce secretion of IL-17𝛂 and IFN-𝛄 suggesting that the former is better at inducing Th17, whilst the latter induce Th1 immune responses, respectively. Prior exposure to malaria significantly reduces the ability of PfsEVs to activate monocytes, suggesting immune tolerance to PfsEVs may play a role in naturally acquired anti-disease immunity.