COMPARATIVE STRATEGIES TO INDUCE LONG-TERM GRAFT ACCEPTANCE IN FULLY ALLOGENEIC RENAL VERSUS CARDIAC ALLOGRAFT MODELS BY CD28-B7 T-CELL COSTIMULATORY BLOCKADE - ROLE OF THYMUS AND SPLEEN

Blocking CD28-B7 T cell costimulatory activation by the fusion protein CTLA4Ig prevents rejection and induces long-term graft acceptance in various experimental transplant models. There are reported differences in the efficacy of CTLA4Ig in renal and cardiac rodent allograft mode ls, but it is not clear whether these are due to the strain or species differences investigated in the different studies reported. This stud y investigates the effect of blocking CD28-B7 T cell costimulation wit h murine CTLA4Ig in rat models of acute renal and cardiac allograft re jection models, using the same complete major histocompatibility compl ex-incompatible strain combination. A single injection of murine CTLA4 Ig 2 d after engraftment was able to induce long-term graft acceptance (>100 d) in 54% of Lewis rat recipients of Wistar-Furth kidneys. Tran sferring this protocol into the acute Wistar-Furth to Lewis heart allo graft model resulted in a mean graft survival time of 24.7 +/- 16.9 d, and all grafts were ultimately rejected. Only concomitant injection o f donor cells (4 x 10(7) splenocytes) plus a single injection of CTLA4 Ig on the day of transplant could induce long-term graft acceptance in 50% of animals. In both the cardiac and renal transplant models, the thymus and spleen were required for induction of tolerance. The mainte nance phase of tolerance, however, did not require an intact thymus bu t did require the presence of a spleen. These data have important clin ical applicability because human studies with T cell costimulatory blo ckade are being planned.