T CELL COSTIMULATORY BLOCKADE SUPPRESSES INDIRECT BETTER THAN DIRECT ALLOIMMUNE RESPONSES IN VIVO

Abstract

73 There are two pathways for stimulating CD4+ T cells during graft rejection, called the direct and indirect pathways of allorecognition. The relative contributions of these pathways to acute rejection and their susceptibility to immunosuppression have been hard to assess. We investigated these issues using skin grafts from donors and to recipients that have been genetically modified to eliminate one or other of the pathways for CD4+ stimulation. To examine the indirect pathway we placed skin grafts from B6 class II knockout donors (B6 II-) onto normal BALB/c recipients. To examine the direct pathway we placed normal BALB/c skin grafts onto B6 class II deficient recipients that have a reconstituted CD4+ population (B6 II-4+). B6 II-4+ mice were generated by crossing B6 II- mice with expressing a class II transgene only on thymic epithelium, and selecting offspring that express class II antigens only on thymic epithelium, thus achieving positive selection of CD4+ T cells without class II antigen expression in the periphery. We compared the effectiveness of combined CsA and anti-CD8 treatment or of combined CTLA4Ig and anti-CD40L treatment on skin allograft survival for the two types of transplants. CsA was given at 50 mg/kg/day sc from day 1, anti-CD8 was administered ip x3 prior to and weekly after transplantation. Both CTLA4Ig and anti-CD40L were given at 500µg/day ip on days 0,2,4,6 relative to the transplant. TableTableIn the absence of immunosuppression rejection depending on the indirect pathway was at least as fast as rejection depending on direct recognition (gp 4 vs 3). Both strategies for immunosuppression were more effective for BALB/c recipients than for B6 recipients (gps 6 vs 5 and 10 vs 9). Compared to their controls, neither the direct nor indirect pathway was especially sensitive to CsA and anti-CD8 immunosuppression (gps 7 vs 5 and 8 vs 6). Costimulatory blockade was more effective in the absence of a direct response (gp 12 vs 10) than in the absence of an indirect response; and only gp 12 had longterm surviving grafts, although they eventually rejected at >100 days. These results emphasize the importance of strain combinations when testing immunosuppressive strategies. They indicate that T cell costimulatory blockade suppresses the indirect pathway better then the direct pathway. They also suggest that T cell costimulatory blockade will be most effective when combined with strategies to prevent direct allo-responses.