Secretion of Extracellular Vesicles from Babesia microti-infected Erythrocytes: Biological Roles in Host Macrophage Activation

Babesia microti is the primary cause of human babesiosis in North America. Despite recent increases in the disease, the pathogenesis and immune response to B. microti infection remain poorly understood. Studies in laboratory mice have shown a critical role for macrophages in eliminating parasites and parasitized red blood cells (RBCs). Notably, the effector parasite molecules that activate macrophages are still unknown. Recent evidence identified a novel protein export mechanism in B. microti that features a network of tubes of vesicles that extend from the parasite plasma membrane to the RBC cytoplasm. Vesicles harboring Babesia proteins are released from the infected RBC (iRBC) to the extracellular environment. We postulate that these parasite-derived extracellular vesicles (EVs) participate in intercellular communication between iRBCs and neighboring cells. When macrophages function as recipients, changes in the production of cytokines with key roles in the host innate immune response occur. To test this hypothesis, we examined cytokine responses in macrophages exposed to B. microti-iRBCs using an in vitro co-culture model. Pro-inflammatory cytokines such as IP-10, G-CSF, IL-6, TNF-α, MIP-1α, MIP-2, IL-1rα, and RANTES were markedly increased in the supernatants of macrophages co-incubated with iRBCs, as compared to uninfected RBCs, using antibody arrays. These effects were dependent on parasite growth, as treatment with the antiparasitic drug clindamycin resulted in significant decreases in cytokine secretion by macrophages exposed to iRBCs. These results support the hypothesis that soluble factors released by B. microti-iRBCs cause phenotypic changes in macrophages. Identification of secreted parasite antigens found in EVs isolated from iRBCs will provide insights into the mechanisms of intercellular communication between B. microti and macrophages that contribute to the induction of the innate immune response in the mammalian host.