Which comes first: the antigen or the adjuvant?

Recent attempts to develop an HIV-1 vaccine indicate that viral replication can be limited by the induction of viral-specific T cell responses; however, recent trials of vaccine candidates designed to target CD8+ T cell responses were unsuccessful. In this issue, Sui and colleagues used a nonhuman primate model to investigate the effect of various vaccine adjuvants on the efficacy of SIV immunization. Unexpectedly, Sui et al. discovered that animals given adjuvant alone in the absence of SIV antigen exhibited a pronounced decrease in viral load following viral challenge. Vaccination with viral antigens combined with adjuvant correlated with the expansion of a population of cells with similarity to myeloid-derived suppressor cells (MDSCs) that may have suppressed vaccine-elicited T cell responses. Together, these results suggest that both innate and adaptive vaccine-elicited immune responses will need to be considered in future HIV-1 vaccine development.