EXAMINATION OF THE SENSITIVITY OF T CELLS TO FAS LIGATION

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Abstract
Background. Alloantigen-reactive T cells represent the major barrier to successful organ transplantation. However, it has been shown that cotransplantation of Fas ligand (FasL)-expressing cells can induce functional allograft tolerance in some model systems. In this study, the basis for this tolerance was investigated using a sensitive in vitro assay system. Methods. T lymphocytes were activated by coculture with an allogeneic Epstein Barr virus-transformed B-cell line. Samples of the lymphocytes were taken daily and treated with agonistic anti-Fas antibodies or FasL-expressing cells. The time in culture required for development of optimal sensitivity to Fas-mediated apoptosis was assessed by Tdt-mediated nick end labeling (TUNEL) staining and the JAM assay of DNA fragmentation. After the induction of optimal apoptosis, a series of experiments was performed to assess the response of the T-cell population to antigen-specific rechallenge. Results. Treatment of the allospecific lymphocyte population with anti-Fas antibodies or Fas-L-expressing cells did not induce apoptosis efficiently until between 6 and 7 days after initiation of the mixed lymphocyte culture; this time corresponded with decreases in the ambient interleukin 2 concentration and in Bcl-2 expression. In addition, induction of apoptosis by treatment with the agonistic anti-Fas antibody reduced the lymphoproliferative response of the T-cell population after antigen-specific rechallenge. Conclusions. These results give an important indication of the mechanism by which FasL-expressing third-party cells can reduce an allospecific T-cell response by an apoptotic mechanism. Furthermore, they demonstrate that apoptotic tolerance in vivo may only occur after the prolonged period of potentially graft-damaging T-cell activation required for acquisition of sensitivity to Fas-mediated apoptosis.