The role of leucyl-leucine methyl ester-sensitive cytotoxic cells in skin allograft rejection

Treatment of murine spleen cells (SpC) with L-leucyl-L-leucine methyl ester (Leu-Leu-OMe) depletes L3T4(+) and Lyt2(+) cytotoxic T lymphocyte precursors and the capacity to generate lethal graft-versus-host disease in semiallogeneic class I + II MHC and multiple non-MHC-disparate recipient mice, whereas T helper cell function is preserved. In the present studies the role of Leu-Leu-OMe-sensitive CTL in skin graft rejection was examined. C57BL/6J (B6) mice were serially thymectomized, lethally irradiated, reconstituted with T cell-depleted bone marrow, and treated with intraperitoneal injections of anti-L3T4 and anti-Lyt2 monoclonal antibodies. These adult thymectomized, bone marrow-reconstituted, T cell-depleted (ATXBM, TCD) mice were unable to reject B6xDBA/2F1 (B6D2F1) skin grafts. When such ATXBM, TCD mice were reconstituted with 7 x 10(7) control B6 SpC, acute rejection of B6D2F1 skin was observed. When B6 donor SpC were Leu-Leu-OMe-treated prior to transfer to ATXBM, TCD mice, uniform rejection of B6D2F1 skin grafts was still observed, although a significant delay in the time to rejection was observed. More rigorous T cell depletion of ATXBM, TCD host mice by infusion of antithymocyte globulin did not prevent delayed rejection of B6D2F1 skin initiated by transfer of Leu-Leu-OMe-treated B6 SpC. Despite the lack of complete prevention of skin allograft rejection, Leu-Leu-OMe treatment of B6 donor cells prevented lethal GVHD even in thymectomized B6D2F1 recipients. Precursors of anti-B6D2F1-specific CTL were greatly reduced or undetectable in unreconstituted ATXBM, TCD mice or in irradiated B6D2F1 recipients of Leu-Leu-OMe-treated B6 SpC. By contrast, ATXBM, TCD recipients of Leu-Leu-OMe-treated B6 SpC were found to contain a population of anti-class I MHC-specific CTL precursors of host origin within 28 days of reconstitution. These findings have indicated a number of features of the cells involved in skin graft rejection. First, Leu-Leu-OMe-sensitive CTL play a major role in acute rejection of class I + II MHC and multiple non-MHC antigen-disparate skin grafts. Moreover, the thymus-independent expansion of host-derived CTL precursors in ATXBM, TCD mice reconstituted with syngeneic Leu-Leu-OMe-resistant T helper cells also appears to play a role in mediating rejection of allogeneic skin grafts.