In the present study, we investigated the possibility to use irradiated, non-replicating class II transcriptional activator (CIITA)-transfected tumor TS/A cells as a cell-based vaccine. Eighty-three percent of TS/A-CIITA-vaccinated mice were completely protected from tumor growth and the remaining 17% displayed significant reduction of tumor growth. In contrast, only 30% of mice injected with irradiated TS/A parental cells were protected from tumor growth, whereas the remaining 70% of animals remained unprotected. Immunity generated in the TS/A-CIITA-vaccinated mice correlated with an efficient priming of CD4+ T cells and consequent triggering and maintenance of CD8+ CTL effectors, as assessed by adoptive transfer assays. Important qualitative differences were observed between the two cell-based vaccines, as TS/A-CIITA-vaccinated mice developed a CTL response containing a large proportion of anti-gp70 AH1 epitope-specific cells, completely absent in TS/A-vaccinated mice, and a mixed Th1/Th2 type of response as opposed to a Th2 type of response in TS/A-vaccinated mice. Finally, in TS/A-CIITA-vaccinated mice, a statistically significant reduction in the percentage and absolute number of CD4+ CD25+ T regulatory cells as compared with those of untreated mice with growing tumors (P < 0.001) or mice vaccinated with TS/A parental cells were observed. These results let to envisage the use of CIITA-transfected non-replicating tumor cells as a vaccination strategy for prevention and, possibly, adjuvant immunotherapy in human settings.