Fas-mediated suicide of tumor-reactive T cells following activation by specific tumor: selective rescue by caspase inhibition

CD8+ T lymphocytes that specifically recognize tumor cells can be isolated and expanded ex vivo. While the lytic properties of these cells have been well described, their fate upon encounter with cognate tumor is not known. We performed reverse 51Cr release assays in which the lymphocyte effectors rather than the tumor cell targets were radioactively labeled. We found that melanoma tumor cells caused the apoptotic death of tumor-specific T cells only upon specific MHC class I-restricted recognition. This death was entirely blockable by the addition of an Ab directed against the Fas death receptor (APO-1, CD95). Contrary to the prevailing view that tumor cells cause the death of anti-tumor T cells by expressing Fas ligand (FasL), our data suggested that FasL was instead expressed by T lymphocytes upon activation. While the tumor cells did not express FasL by any measure (including RT-PCR), functional FasL (as well as FasL mRNA) was consistently found on activated anti-tumor T cells. We could successfully block the activation-induced cell death with z-VAD-fmk, a tripeptide inhibitor of IL-1 beta-converting enzyme homologues, or with anti-Fas mAbs. Most importantly, these interventions did not inhibit T cell recognition as measured by IFN-gamma release, nor did they adversely affect the specific lysis of tumor cell targets. These results imply that Fas-mediated activation-induced cell death could be a limiting factor in the in vivo efficacy of adoptive transfer of class I-restricted CD8+ T cells and provide a means of potentially enhancing their growth in vitro as well as their function in vivo.