Neutrophil-dendritic cell interaction plays an important role in live attenuated Leishmania vaccine induced immunity

BACKGROUND: Neutrophils are involved in the initial host responses to pathogens. Neutrophils can activate T cell responses either independently or through indirect involvement of Dendritic cells (DCs). Recently we have demonstrated direct neutrophil-T cell interactions that initiate adaptive immune responses following immunization with live attenuated Leishmania donovani centrin deleted parasite vaccine (LdCen(-/-)). However, neutrophil-DC interactions in T cell priming in vaccine immunity in general are not known. In this study we evaluated the interaction between neutrophils and DCs during LdCen(-/-) infection and compared with wild type parasite (LdWT) both in vitro and in vivo. METHODOLOGY/FINDINGS: LdCen(-/-) parasite induced increased expression of CCL3 in neutrophils caused higher recruitment of DCs capable of inducing a strong proinflammatory response and elevated co-stimulatory molecule expression compared to LdWT infection. To further illustrate neutrophil-DCs interactions in vivo, we infected LYS-eGFP mice with red fluorescent LdWT/LdCen(-/-) parasites and sort selected DCs that engulfed the neutrophil containing parasites or DCs that acquired the parasites directly in the ear draining lymph nodes (dLN) 5d post infection. The DCs predominantly acquired the parasites by phagocytosing infected neutrophils. Specifically, DCs containing LdCen(-/-) parasitized neutrophils exhibited a proinflammatory phenotype, increased expression of costimulatory molecules and initiated higher CD4(+)T cell priming ex-vivo. Notably, potent DC activation occurred when LdCen(-/-) parasites were acquired indirectly via engulfment of parasitized neutrophils compared to direct engulfment of LdCen(-/-) parasites by DCs. Neutrophil depletion in LdCen(-/-) infected mice significantly abrogated expression of CCL3 resulting in decreased DC recruitment in ear dLN. This event led to poor CD4(+)Th1 cell priming ex vivo that correlated with attenuated Tbet expression in ear dLN derived CD4(+) T cells in vivo. CONCLUSIONS: Collectively, LdCen(-/-) containing neutrophils phagocytized by DC markedly influence the phenotype and antigen presenting capacity of DCs early on and thus play an immune-regulatory role in shaping vaccine induced host protective response.