Optimal therapy to prevent graft rejection should be based on the induction of specific tolerance to the donor tissue. Current concepts on immunological tolerance hold that anergy is the result of intercellular signalling after TCR/MHC-peptide interaction, in the absence of a so- called Costimulatory signal. This co-stimulatory signal is provided by cell surface of antigen presenting cells (APC). At present, the best candidate co-stimulatory signal that determines whether TGR-stimulation leads to full T-cell activation or to T-cell tolerance, is generated by interaction of CD28 on the T cells with B7 on APC. B7-24 is a novel monoclonal antibody (MAB) that binds to the B7 molecule. Recently obtained in vitro data suggests that blocking the B7/CD28 interaction with MAB B7- 24 indeed results in antigen-specific tolerance. Because any in vivo use of this MAB will be limited by the human anti-mouse antibody (HAMA) response the overall goal is to use PCR technology to humanize B7-24. In phase I we will clone and sequence heavy and light chain variable regions of B7-24, then utilize the CDRs to reshape B7-24 into a human IgGI monoclonal antibody (hB7-24). We will express hB7-24 using an amplifiable high level antibody expression system developed in this laboratory and test activity of B7-24 for in vitro binding activity. In phase II this humanized antibody will be produced and tested for functional activity. At the completion of phase II we plan to initiate preclinical development of hB7-24. Use of hB7-24 may be a useful approach for refractory solid organ rejection episodes or recalcitrant autoimmune diseases such as rheumatoid arthritis.