The time course of CTLAIg effect on cardiac allograft rejection.

T cell response to alloantigen is dependent not only on T cell receptor activation, but also upon co-stimulation through the CD28 receptor, as T cell receptor activation alone is insufficient for an optimal immune response. The CD28 receptor on helper T cells interacts with its ligand B7 on activated B cells/macrophages as the co-stimulus to support T cell activity. The natural ligand for CD28, B7 is expressed in both constituitive and inducible forms. Expression of the inducible form, B7.1 is the most co-stimulatory and may peak at 48 hr following antigen presentation. CTLA4Ig (a soluble CD28 receptor analog) binds both B7's and inhibits CD28 activation. This experiment was undertaken to examine the optimal time course of CTLA4Ig effect at or following antigen presentation. In vivo studies used a rat MHC mismatch heterotopic cardiac allograft transplant model (Brown-Norway to Lewis). Controls received no immunotherapy. Experimental recipients received CTLA4Ig (0.05 mg i.p.) the day of transplant only or had CTLA4Ig x 7-21 days begun at Day 0, 3, or 5 following transplant. Rejection was defined as a lack of allograft contraction. Allograft survival was best when CTLA4Ig was present on Day 2. These findings demonstrate that CTLA4Ig was most effective 48 hr following antigen presentation, perhaps reflecting induction of tolerance at a time of maximal CTLA4Ig/B7.1 blockade. CTLA4Ig given later, at a time when the recognition/rejection process has already begun was not as effective indicating the lack of immunosuppressive function of CTLA4Ig itself and confirms CTLA4Ig's mechanism of co-stimulation blockade.