The murine mammary adenocarcinoma cell line TS/A is a highly malignant MHC class II-negative tumor. We show that transfection of TS/A cells with the MHC class II transactivator CIITA renders them MHC class II-positive and highly immunogenic in vivo. These cells were fully rejected by 51% of syngeneic recipients and had a significantly lower growth rate in the remaining 49% of animals. This directly correlated to the amount of MHC class II molecules expressed in the transfected tumor. Tumor rejecting animals were protected against rechallenge with the parental TS/A tumor. The rejection required CD4(+) and CD8(+) T cells. CD4(+) T cells were fundamental in the priming phase of the antitumor response VSports手机版. CTL-specific for a peptide of the envelope gp70 of an endogenous ecotropic retrovirus were identified and explained the specificity of the effector mechanism of rejection against the TS/A and the antigenically related C26 carcinoma cells but not against the unrelated gp70-negative syngeneic fibrosarcoma F1F cells. This is the first example of successful tumor vaccination by genetic transfer of CIITA. These results open the way to a possible use of CIITA for increasing both the inducing and the effector phase of the antitumor immune response. .