Activation of Macrophages by Extracellular Vesicles Derived from Babesia-infected Red Blood Cells

Babesia microti is the primary cause of human babesiosis in North America. Despite an emergence of the disease in recent years, the pathogenesis and immune response to B. microti infection remain poorly understood. Studies in laboratory mice have shown a critical role for macrophages in the elimination of parasites and infected red blood cells (iRBCs). Importantly, the underlying mechanisms that activate macrophages are still unknown. Recent evidence identified the release of extracellular vesicles (EVs) from Babesia iRBCs. EVs are spherical particles released from cell membranes under natural or pathological conditions that have been suggested to play roles in host-pathogen interactions among diseases caused by protozoan parasites. The present study examined whether EVs released from cultured Babesia iRBCs activated macrophages resulting in changes in the secretion of cytokines. An analysis of vesicle size in EV fractions from Babesia iRBCs showed diverse populations in the <100 nm size range compared to EVs from uninfected RBCs. Uptake of EVs released from B. microti-iRBCs was observed in macrophages in vitro. In addition, incubation of macrophages with EVs isolated from Babesia iRBC culture supernatants resulted in the activation of NF-kappa B and modulation of pro-inflammatory cytokines. These results support a role for EVs in the activation of macrophage functions and provide new insights into the mechanisms involved in the induction of the innate immune response during babesiosis.