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

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, tumor-specific peptide derived from a mutant ribosomal protein expressed by the UV light-induced tumor 6132A-PRO. By using neutralizing mAb specific for murine IFN-γ and adoptive transfer of CD4 + T cells into severe combined immunodeficient mice, we show that anti-IFN-γ treatment abolishes the CD4 + T cell-mediated rejection of the tumor cells in vivo . The tumor cells were MHC class II negative, and IFN-γ did not induce MHC class II expression 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-γ had a markedly reduced growth rate in severe combined immunodeficient mice, but IFN-γ did not inhibit the growth of the tumor cells in vitro . Furthermore, tumor cells stably expressing a dominant-negative truncated form of the murine IFN-γ receptor α chain, and therefore insensitive to IFN-γ, nevertheless were rejected by the adoptively transferred CD4 + T cells. Thus, host cells, and not tumor cells, seem to be the target of IFN-γ. Together, these results show that CD4 + T cells can eliminate IFN-γ-insensitive, MHC class II-negative cancer cells by an indirect mechanism that depends on IFN-γ.