HLA-B27 (B27) designates a group of major histocompatibility complex class I proteins directly involved in pathogenesis of spondyloarthropathies (SpAs), inflammatory arthritic diseases affecting both axial and peripheral joints. While the role of B27 is unknown, the B pocket, a region of the peptide binding groove common to all B27 subtypes yet unique to this allele, is particularly important. This region dramatically influences peptide selection, leading to the idea that B27 may present arthritogenic peptides, although this remains unproven. However, our studies indicate the B pocket also causes B27 to misfold. Thus, we hypothesize that B27 may differ from other alleles in ways that are unrelated to peptide binding specificity per se, and that understanding these differences may have important consequences for elucidating the cause of SpAs. This proposal is designed to determine the mechanism and consequences of B27 misfolding. In Specific Aim 1 we will examine the influence of B pocket amino acids, peptide, and beta2 microglobulin, on the folding kinetics and misfolding of B27 to determine the mechanism. In Specific Aim 2 we will identify molecular chaperones involved in the earliest stages of B27 heavy chain (HC) folding in the endoplasmic reticulum (ER), and characterize high molecular weight HC complexes that may represent misfolding intermediates. We will determine the influence of chaperone overexpression on folding kinetics and misfolding. Specific Aim 3 will assess consequences of HLA-B27 misfolding. We will test the hypothesis that some misfolded high molecular weight HC escape the ER and are expressed on the cell surface. We will also determine whether B27 misfolding signals an ER stress response resulting in NF-kappaB activation and/or induction of chaperone synthesis. These experiments will characterize fundamental differences between B27 and other MHC class I alleles. They will provide insights into the mechanisms responsible for these differences, how they may relate to the pathogenesis of SpAs, and how they may be corrected.