Benefits of immunosuppressive therapy currently adopted for organ transplant are achieved with the trade-off of severe side effects. Although treatment with multiple drugs have led to considerable improvement in survival rates for both patients and grafts but have also exposed patients to opportunistic infections. Novel immunotherapeutic strategies that take advantage of normal mechanisms of tolerance to self antigen such as inhibiting T-cell costimulatory pathways may be an effective way to permit the development of antigen specific tolerance of transplants. The impact of costimulatory signals for complete T cell activation has allowed investigators to develop new strategies for immune intervention, both for suppression of the immune system (automimmune diseases and transplantation) and for stimulation of the immune system (vaccines). Attempts to block T cell costimulation as a therapeutic strategy in autoimmunity and transplantation has gained tremendous momentum. Several receptor ligand pairs have been identified and characterized with CD 28 being one of the immune system (vaccines). Attempts to block T cell costimulation as a therapeutic strategy in autoimmunity and transplantation has gained tremendous momentum. Several receptor ligand pairs have been identified and characterized with CD 28 being one of the principal and most effective T cell costimulatory receptor that binds APC costimulatory ligands B7.1 (CD80) and B7.2 (CD86). Although the role of CD28 as a costimulatory molecule is of paramount importance, blackade of this pathway cannot uniformly block T-cell activation. CD40L/CD40 engagement delivers a powerful costimulatory signal for T cell activation also. We hypothesized that a peptide dervied from the ligand binding CD$03 like region of CD28 will selectively block B7:CD28 interactions without affecting the high affinity B7:CTLA-4 interactions. We designed a 20 residue retro-inverso D-peptide mimic from the ligand binding epitope of the costimulatory molecule CD28 that comprised the highly conserved hexapeptide motif 'MYPPY' that exhibit a Polyproline type II structure. This peptide has been demonstrated to inhibit encephalitogenic T cell in vitro and prevented disease induction and ameliorated established disease in EAE. The inital goal of this proposal is to extend our studies with the CD28 peptide mimic to transplantation. Thus, we propose to initially evaluate whether the retro-inverso engineered D-peptide mimic of CD28 will allow allograft acceptance, and block rejection skin grafts. Another immediate goal is to further extend our design principles to the development of peptide mimics for the blockade of the CD40L:CD40 costimulatory pathway. We have identified and designed ligand- binding epitopes of CD40L and CD40 that we will structurally and functionally characterize both invitro and in vivo. We propose the evaluate the impact of co-administration of retro-inverso CD28 and CD40L peptide mimics on allograft acceptance. The long term objective of this project is not only to develop a widely applicable blockade stretegy targeting constimulatory molecules but also to elucidate the underlining mechanisms in downregulating immune responses as well as to gain understanding the biological active conformation of these peptide mimics and how they interact with the costimulatory molecules.