CD4(+) T cells eliminate MHC class II-negative cancer cells in vivo by indirect effects of IFN-gamma

CD4(+) T cells can eliminate tumor cells in vivo in the absence of CD8(+) T cells. We have CD4(+) T cells specific for a MHC class II-restricted, tumo r-specific peptide derived from a mutant ribosomal protein expressed by the UV light-induced tumor 6132A-PRO. By using neutralizing mAb specific for m urine IFN-gamma and adoptive transfer of CD4(+) T cells into severe combine d immunodeficient mice, we show that anti-IFN-gamma treatment abolishes the CD4(+) T cell-mediated rejection of the tumor cells in vivo. The tumor cel ls were MHC class II negative, and IFN-gamma did not induce MHC class II ex pression in vitro. Therefore, the tumor-specific antigenic peptide must be presented by host cells and not the tumor cells. Tumor cells transduced to secrete IFN-gamma had a markedly reduced growth rate in severe combined imm unodeficient mice, but IFN-gamma did not inhibit the growth of the tumor ce lls in vitro. Furthermore, tumor cells stably expressing a dominant-negativ e truncated form of the murine IFN-gamma receptor a chain, and therefore in sensitive to IFN-gamma, nevertheless were rejected by the adoptively transf erred CD4(+) T cells. Thus, host cells, and not tumor cells, seem to be the target of IFN-gamma. Together, these results show that CD4(+) T cells can eliminate IFN-gamma-insensitive, MHC class II-negative cancer cells by an i ndirect mechanism that depends on IFN-gamma.