Background:  Although vaccination with live attenuated SIV is currently the most effective means of inducing protection against lentivirus infections, the immunologic mechanisms responsible remain unknown. Previous studies have yielded conflicting data regarding the role of cellular and humoral immune responses in mediating protection, suggesting that innate immune responses, including natural killer (NK) cells, could contribute to protection.

Methods:  Rhesus macaques were vaccinated with SIV∆nef and then challenged with SIV_mac239 or SIV_mac251. Cellular dynamics were measured by polychromatic flow cytometry and absolute counts of lymphocytes in blood and rectal biopsies were determined by a bead-based, flow cytometry assay.

Results:  Following vaccination with SIV∆nef up to 8-fold increases of circulating NK cells were observed, peaking at 2 weeks post-vaccination and preceding SIV-specific T cell responses. Furthermore, the gut-homing marker a4b7 was up-regulated on circulating NK cells, coinciding with a SIV∆nef-induced 2.5-fold increase in total NK cells in colorectal tissue. Ki67 expression was also up-regulated 2- to 5-fold in circulating, lymphoid, and mucosal NK cells. Upon further examination, expansion of NK cells was stratified by MHC genotype — Mamu A*01⁺ macaques showed significant and sustained expansion of NK cells, but not Mamu A*02⁺ or Mamu B*17⁺ macaques. SIV∆nef also induced a significant expansion of KIR⁺ and cytotoxic NK cells in the colorectal mucosa. Interestingly, in vaccinated macaques subsequently challenged with wild-type SIV, a4b7 and Ki67 were both up-regulated in circulating and mucosal NK cells, even in protected animals and in the absence of an obvious anamnestic response.

Conclusions:  Although current lentiviral vaccines stress the importance of induction of SIV-specific T and B cell responses, NK cells could contribute to protection by inhibiting initial rounds of wild-type virus replication at the virus-mucosa interface. Our data indicate the live attenuated virus, SIV∆nef, induces a robust and sustained expansion of NK cells and could suggest the protective SIV∆nef NK responses are modulated in part by interaction of Mamu A*01 with an as of yet unidentified KIR. Perhaps most importantly, our data show that SIV∆nef induces trafficking and accumulation of NK cells in the gut mucosa, the primary site of virus replication, a feature that will likely be a critical component of an effective HIV/SIV vaccine.