Rejection of an IA+ variant line of FBL-3 leukemia by cytotoxic T lymphocytes with CD4+ and CD4-CD8- T cell receptor-alpha beta phenotypes generated in CD8-depleted C57BL/6 mice.

FBL-3N is an MHC class II Ag+ variant line that was obtained spontaneously during maintenance of Friend virus-induced leukemia FBL-3 in an athymic C57BL/6 (B6) mouse. Inocula of FBL-3N, but not the parental FBL-3 tumor, regressed after initial growth in CD8-depleted, syngeneic B6 mice. The cellular mechanisms by which FBL-3N was rejected in these mice were investigated in this study. We demonstrated that CTL with both CD4+ and CD4-CD8-TCR-alpha beta phenotypes were generated in mixed lymphocyte tumor cell culture spleen cells obtained from CD8-depleted B6 mice that had rejected FBL-3N by in vitro stimulation with mitomycin C-treated FBL-3N. After adoptive transfer of these CTL that were generated in vitro into athymic B6 mice, challenge with the FBL-3N tumor resulted in tumor regression after its initial growth. Thus, CD4+ and CD4-CD8-TCR-alpha beta CTL mediated rejection of the FBL-3N tumor in CD8-depleted B6 mice. Furthermore, the findings that depletion of B6 mice of CD4+ cells in addition to CD8+ cells abrogated the rejection of FBL-3N and generation of CTL in mixed lymphocyte tumor cell culture spleen cells suggest that CD4+ cells were required not only as a source of CD4+ CTL, but also as helper cells for generation of CD4-CD8-TCR-alpha beta CTL. Tumor Ag recognition of CD4-CD8-TCR-alpha beta CTL was restricted to Db, like that of classical CD8+ CTL, but the restriction appeared to be less obligatory than that of CD8+ CTL.