SUMMARY Despite several studies describing the secretion of exosomes by Leishmania in vitro, observation of their formation and release in vivo has remained a major challenge. Herein, we show that Leishmania constitutively secretes exosomes within the lumen of the sand fly midgut through a mechanism homologous to the mammalian pathway. Through egestion experiments, we demonstrate that Leishmania exosomes are part of the sand fly inoculum and are co-egested with the parasite during the insect’s bite possibly influencing the host infectious process. Indeed, co-inoculation of mice footpads with L. major plus midgut-isolated or in vitro-isolated L. major exosomes resulted in a significant increase in footpad swelling. Notably, co-injections produced exacerbated lesions through overinduction of inflammatory cytokines, in particular IL-17a. Our data indicate that Leishmania exosomes are an integral part of the parasite’s infectious life cycle and propose to add these vesicles to the repertoire of virulence factors associated to vector-transmitted infections.

Leishmania donovani parasites are the cause of visceral leishmaniasis and are transmitted by bites from phlebotomine sand flies. A prominent feature of vector-transmitted Leishmania is the persistence of neutrophils at bite sites, where they protect captured parasites, leading to enhanced disease. Here, we demonstrate that gut microbes from the sand fly are egested into host skin alongside Leishmania parasites. The egested microbes trigger the inflammasome, leading to a rapid production of interleukin-1β (IL-1β), which sustains neutrophil infiltration. Reducing midgut microbiota by pretreatment of Leishmania-infected sand flies with antibiotics or neutralizing the effect of IL-1β in bitten mice abrogates neutrophil recruitment. These early events are associated with impairment of parasite visceralization, indicating that both gut microbiota and IL-1β are important for the establishment of Leishmania infections. Considering that arthropods harbor a rich microbiota, its potential egestion after bites may be a shared mechanism that contributes to severity of vector-borne disease.

Currently, there are no commercially available human vaccines against leishmaniasis. In rodents, cellular immunity to salivary proteins of sand fly vectors is associated to protection against leishmaniasis, making them worthy targets for further exploration as vaccines. We demonstrate that nonhuman primates (NHP) exposed to Phlebotomus duboscqi uninfected sand fly bites or immunized with salivary protein PdSP15 are protected against cutaneous leishmaniasis initiated by infected bites. Uninfected sand fly-exposed and 7 of 10 PdSP15-immunized rhesus macaques displayed a significant reduction in disease and parasite burden compared to controls. Protection correlated to the early appearance of Leishmania-specific CD4(+)IFN-γ(+) lymphocytes, suggesting that immunity to saliva or PdSP15 augments the host immune response to the parasites while maintaining minimal pathology. Notably, the 30% unprotected PdSP15-immunized NHP developed neither immunity to PdSP15 nor an accelerated Leishmania-specific immunity. Sera and peripheral blood mononuclear cells from individuals naturally exposed to P. duboscqi bites recognized PdSP15, demonstrating its immunogenicity in humans. PdSP15 sequence and structure show no homology to mammalian proteins, further demonstrating its potential as a component of a vaccine for human leishmaniasis.