Characterization of the Early Inflammatory Infiltrate at the Feeding Site of Infected Sand Flies in Mice Protected from Vector-Transmitted Leishmania major by Exposure to Uninfected Bites

BACKGROUND Mice exposed to sand fly saliva are protected against vector-transmitted Leishmania major. Although protection has been related to IFN-γ producing T cells, the early inflammatory response orchestrating this outcome has not been defined. METHODOLOGY/PRINCIPAL FINDINGS Mice exposed to uninfected P. duboscqi bites and naïve mice were challenged with L. major-infected flies to characterize their early immune response at the bite site. Mostly, chemokine and cytokine transcript expression post-infected bites was amplified in exposed compared to naïve mice. In exposed mice, induced chemokines were mostly involved in leukocyte recruitment and T cell and NK cell activation; IL-4 was expressed at 6 h followed by IFN-γ and iNOS2 as well as IL-5 and IL-10 expression. In naïve animals, the transcript expression following Leishmania-infected sand fly bites was suppressed. Expression profiles translated to an earlier and significantly larger recruitment of leukocytes including neutrophils, macrophages, Gr+ monocytes, NK cells and CD4+ T cells to the bite site of exposed compared to naïve mice post-infected bites. Additionally, up to 48 hours post-infected bites the number of IFN-γ-producing CD4+ T cells and NK cells arriving at the bite site was significantly higher in exposed compared to naïve mice. Thereafter, NK cells become cytolytic and persist at the bite site up to a week post-bite. CONCLUSION/SIGNIFICANCE The quiet environment induced by a Leishmania-infected sand fly bite in naïve mice was significantly altered in animals previously exposed to saliva of uninfected flies. We propose that the enhanced recruitment of Gr+ monocytes, NK cells and CD4 Th1 cells observed at the bite site of exposed mice creates an inhospitable environment that counters the establishment of L. major infection.