HLA-B*2703, a subtype of HLA-B27 that has not been associated with susceptibility to spondyloarthropathies, differs from all other major histocompatibility complex (MHC) class I molecules including other B27 subtypes at position 59 in the A pocket. Alloreactive cytotoxic T lymphocytes (CTL) directed against the most common B27 subtype, B*2705, show partial recognition of B*2703, while the vast majority of anti-B*2703 CTL clones recognize B*2705. Furthermore, B*2703 is deficient in the binding and presentation of a B*2705-restricted influenza A nucleoprotein peptide (SRYWAIRTR). Binding and presentation are restored by substituting Arg for the naturally-occurring Ser at position 1 (P1) of the peptide, indicating the importance of this amino acid in maintaining high affinity peptide binding to B*2703. These and other preliminary results suggest that B*2703 may bind and present only a subset of those peptides presented by B*2705. The consequences of this unique polymorphism at position 59, on both the physiologic function of B27 in protective immunity, as well aas the pathogenic role this molecule may play in susceptibility to spondyloarthropathies, are unknown. We are engaged in several lines of investigation to determine the extent of peptide binding and presentation differences between these two closely related HLA-B27 subtypes. In this Developmental and Feasibility project application we propose to use B*2703/humanbeta2-microglobulin (hbeta2m) transgenic mice that we are in the process of producing, along with currently existing B*2705/hbeta2m animals, to determine the functional significance of these differences. First, we will address the hypothesis that B*2703 presents only a subset of B*2705-restricted self peptides by performing reciprocal skin grafts and assessing tissue rejection. If our hypothesis in correct, B*2703/hbeta2m mice will reject B*2705/hbeta2m tissue, while B*2705 animals will accept B*2703 tissue. Alloreactive cytotoxic T lymphocyte responses accompanying graft rejection will be evaluated. Second, we will examine B*2705 and B*2703-restricted responses to known viral or self peptide epitopes introduced by immunization with DNA vectors encoding these peptides. We will vary the P1 amino acid to determine its role in influencing in vivo peptide presentation by these HLA-B27 subtypes, and how this affects immune responsiveness. Third, we will produce B*2703 transgenic rats to determine whether this subtype can induce the spontaneous inflammatory disease seen in rats transgenic for B*2705. These studies in transgenic animals will help to define the role of a unique A pocket polymorphism in peptide presentation by MHC class I molecules, and may provide the basis for understanding differential susceptibility to spondyloarthropathies conferred by two closely related HLA-B27 subtypes.