Essential Role of Neutrophils in the Protective Immune Response Induced by a Live Attenuated Leishmania Vaccine

No licensed vaccine exists against visceral leishmaniasis (VL), a disease caused by the Leishmania donovani parasite. We have previously reported both macrophages and dendritic cells play important role in the protection induced by a live attenuated centrin gene-deleted L. donovani (LdCen(-/-)) parasite vaccine. The role of neutrophils in orchestrating the initial innate response to pathogens is widely recognized. To investigate the early interaction of LdCen(-/- )with neutrophils, we immunized mice intradermally in the ear pinna with LdCen(-/-). Compared with LdWT infection, LdCen(-/-) parasites induced higher recruitment of neutrophils to the ear dermis and ear draining lymph nodes (dLN) as early as 6-18 h after immunization, which were predominantly proinflammatory in nature. Neutrophils from ear dLN of LdCen(-/-)-immunized mice exhibited heightened expression of costimulatory molecules and attenuated expression of coinhibitory molecules necessary for higher T cell activation. Further phenotypic characterization revealed heterogeneous neutrophil populations containing N alpha and N beta subtypes in the ear dLN. Of the two, the parasitized N alpha subset from LdCen(-/-)-immunized mice exhibited much stronger Ag-specific CD4(+) T cell proliferation ex vivo. Adoptive transfer of neutrophils bearing LdCen(-/-) parasites induced an increased Th1 response in naive mice. Importantly, neutrophil depletion significantly abrogated Ag-specific CD4(+) T cell proliferation in LdCen(-/-)-immunized mice and impaired protection against virulent challenge. Conversely, replenishing of neutrophils significantly restored the LdCen(-/-)-induced host-protective response. These results suggest that neutrophils are indispensable for protective immunity induced by LdCen(-/-) parasite vaccine.