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

Abstract

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 Nα and Nβ subtypes in the ear dLN. Of the two, the parasitized Nα 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.