Adjuvanted Leishmania infantum extracellular vesicles induce protective immunity in experimental visceral leishmaniasis

The lack of an effective vaccine remains a critical barrier to controlling visceral leishmaniasis (VL), a lethal parasitic disease with expanding global incidence. Here, we evaluate extracellular vesicles (EVs) released by stationary-phase Leishmania infantum promastigotes as a biologically derived, cell-free vaccine platform when formulated with the clinically relevant oil-in-water adjuvant Addavax. Parasite-derived EVs contained a broad repertoire of immunologically relevant proteins, including GP63, and induced functional maturation of bone marrow–derived dendritic cells in vitro . In vivo , immunization of BALB/c mice with adjuvanted EVs elicited antigen-specific antibody responses and robust Th1-biased cellular immunity, characterized by enhanced T-cell proliferation and increased frequencies of IFN-γ–producing CD4⁺ T cells. Following experimental challenge, vaccinated mice exhibited significantly reduced hepatic and splenic parasite burdens during both acute and chronic infection, accompanied by preserved tissue architecture. Protection correlated with a favorable IFN-γ/IL-10 balance, supporting adjuvanted parasite-derived EVs as a modular, cell-free vaccine strategy for VL.