Evaluation of an oral CXCR3 antagonist in a rat model of acute cardiac allograft rejection

and TCR-tg mice (n 6) do not acutely reject bm12 hearts (graft survival 33-100 days). However, TCR-tg mice primed with bm12 skin grafts acutely reject bm12 heart grafts (MST 22 days, n 6). Interestingly, TCR-tg recipients of bm12 heart grafts fail to develop chronic rejection compared with B6 recipients. We developed a model to visualize dynamics of CD4 T cell mediated direct allograft rejection and to investigate mechanisms underlying development of transplant tolerance. B6 nude mice were adoptively transferred with 15x10 TCR-tg spleen cells and transplanted with bm12 skin grafts. Expansion of allospecific TCR-tg CD4 T cells was observed primarily in the draining lymph nodes at early time points. Alloantigen-driven TCR-tg CD4 T cell expansion increased 7 fold over homeostatic expansion, peaked 14 days after transplantation, and declined thereafter. TCR-tg CD4 T cells exhibited upregulation of early activation markers, expression of effector/memory markers, and IFNproduction mirroring the kinetics of clonal expansion. We investigated the role of B7-CD28 and B7h-ICOS costimulatory blockade in MHC class II alloreactive CD4 T cell responses using CTLA4Ig and anti-B7h mAb. CTLA4Ig treated recipients exhibited a drastic reduction in TCR-tg CD4 T cell expansion. TCR-tg CD4 T cells also retained a naive phenotype. In contrast, anti-B7h mAb had no effect on expansion or activation of TCR-tg CD4 T cells. CTLA4Ig reduced IFNproduction by 80%, anti-B7h mAb by 20%. Our results represent the first model of MHC class II alloreactive CD4 T cell activation in vivo. It provides a powerful tool for tracking CD4 T cell mediated alloimmune responses and investigating mechanisms of tolerance induction in vivo.