The mechanisms by which dendritic cell (DC) vaccines prime host T cells in vivo was analyzed. Mice were immunized with syngeneic bone marrow-derived DC and as surrogate antigen beta-galactosidase (beta-gal) was used. DC either pulsed with peptide, loaded with beta-gal antigen or gene-modified induced beta-gal-specific cytotoxic T lymphocytes (CTL) and moderate rejection of an in vivo challenge with beta-gal expressing tumors. In addition, beta-gal-specific CTL lysed the syngeneic DC that were used as vaccines. Using SCID mice reconstituted with F1 lymphocytes, direct priming by gene-modified DC vaccines was demonstrated by the presence of beta-gal-specific CTL of the haplotype exclusively expressed by DC while indirect priming by host antigen-presenting cells (APC) was shown by the detection of CTL of the haplotype exclusively present on host APC and absent on DC vaccines. Since DC immunization in syngeneic mice was associated with an increase in NK1.1+/Ly49C- cells and detectable lysis of DC in vitro by lymphokine-activated killer cells, DC vaccines appear to interact with host natural killer cells as well as with antigen-specific T cells. These effector cells in turn may lyse DC vaccines thereby leading to the release of antigens that can be taken up by host APC.