Numerous experimental
vaccines have been developed to protect against the cutaneous and visceral forms of
leishmaniasis caused by
infection with the obligate intracellular protozoan Leishmania, but a human
vaccine still does not exist. Remarkably, the efficacy of anti-
Leishmania vaccines has never been fully evaluated under experimental conditions following natural vector transmission by infected sand fly
bite. The only immunization strategy known to protect humans against natural exposure is "leishmanization," in which viable L. major parasites are intentionally inoculated into a selected site in the skin. We employed mice with healed L. major
infections to mimic leishmanization, and found tissue-seeking,
cytokine-producing CD4+ T cells specific for Leishmania at the site of challenge by infected sand fly
bite within 24 hours, and these mice were highly resistant to sand fly transmitted
infection. In contrast, mice vaccinated with a
killed vaccine comprised of autoclaved L. major
antigen (ALM)+CpG
oligodeoxynucleotides that protected against needle inoculation of parasites, showed delayed expression of protective immunity and failed to protect against infected sand fly challenge. Two-photon intra-vital microscopy and flow cytometric analysis revealed that sand fly, but not needle challenge, resulted in the maintenance of a localized neutrophilic response at the inoculation site, and removal of neutrophils following vector transmission led to increased parasite-specific immune responses and promoted the efficacy of the
killed vaccine. These observations identify the critical
immunological factors influencing
vaccine efficacy following natural transmission of Leishmania.