Activation of macrophages by extracellular vesicles derived from <i>Babesia</i> -infected red blood cells

<i>Babesia microti</i> is the primary cause of human babesiosis in North America. Despite the emergence of the disease in recent years, the pathogenesis and immune response to <i>B. microti</i> 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 <i>Babesia</i> 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 <i>Babesia</i> iRBCs could activate macrophages and alter cytokine secretion. An analysis of vesicle size in EV fractions from <i>Babesia</i> iRBCs showed diverse populations in the <100 nm size range compared to EVs from uninfected RBCs. In co-culture experiments, EVs released by <i>B. microti</i> iRBCs appeared to be associated with macrophage membranes and cytoplasm, indicating uptake of these vesicles <i>in vitro</i>. Interestingly, the incubation of macrophages with EVs isolated from <i>Babesia</i> iRBC culture supernatants resulted in the activation of NF-κB and modulation of pro-inflammatory cytokines. These results support a role for <i>Babesia</i>-derived EVs in macrophage activation and provide new insights into the mechanisms involved in the induction of the innate immune response during babesiosis.