CD8(+) T cells are essential for long-term, vaccine-induced resistance against intracellular pathogens. Here we show that natural antibodies, acting in concert with complement, are endogenous adjuvants for the generation of protective CD8(+) T cells after vaccination against visceral leishmaniasis. IL-4 was crucial for the priming of vaccine-specific CD8(+) T cells, and we defined the primary source of IL-4 as a CD11b(+)CD11c(lo) phagocyte. IL-4 secretion was not observed in antibody-deficient mice and could be reconstituted with serum from normal, but not Btk immune-deficient, mice. Similarly, no IL-4 response or CD8(+) T-cell priming was seen in C1qa(-/-) mice. These results identify a new pathway by which immune complex-mediated complement activation can regulate T-cell-mediated immunity. We propose that this function of natural antibodies could be exploited when developing new vaccines for infectious diseases.