CD4 and neoplasm: Previously, we showed that ACT of naive CD4+ T cells into lymphopenic hosts could treat large, established tumors.11 The naive CD4+ T cells differentiated in vivo to Th1 cytotoxic T cells and rejected established melanoma in both irradiated wild-type (WT)10 and RAG−/− mice.11 To determine the mechanism of tumor rejection, we sought to deplete specific cell subsets by using antibodies and genetic knockouts.