Abstract The spondyloarthropathies (SpAs) are human diseases, characterized by sacroiliitis and overlapping clinical features, of unknown pathogenesis. The prototypical SpA, ankylosing spondylitis (AS), is one of the best known and strongest examples of a disease associated with an HLA allele, specifically HLA-B27, an association known for 50 years. Since HLA-B27 is a major histocompatibility complex class I (MHC-I) molecule, it seems likely that it is involved in disease pathogenesis by presenting peptide antigens on antigen-presenting cells (APCs) to CD8+ T cells, the canonical function of MHC-I molecules. However, due to lack of direct support of this hypothesis, and conflicting data from previous HLA-B27 rodent models, other hypotheses became more favored, even though genome-wide association studies and lack of association of AS with certain HLA-B27 alleles that affect peptide display support a role for HLA-B27 in antigen presentation in disease. Recent studies from the applicant’s laboratory in collaboration with an international group showed clonal expansion of certain T cell receptors (TCRs) on CD8+ T cells in the joints of patients with AS and eye of patients with anterior uveitis, a frequent complication of the SpAs, that can occur in isolation and is also associated with HLA-B27. These TCRs were used to identify peptides from bacterial and human proteins that are presented by HLA-B27 to trigger TCR reporter cells. Importantly, several of these TCRs recognized both bacterial and self-peptides presented by HLA-B27. Crystallographic studies showed the basis for this TCR cross-reactivity. These studies strongly suggest the overall hypothesis to explain the pathogenesis of AS, the basis for this proposal: Antigen- specific CD8+ T cells in HLA-B27+ individuals clonally expand in response to bacterial challenge then attack normal tissues expressing the cross-reactive self-protein(s). To test this model, the applicant proposes here to create a mouse expressing a relevant TCR and HLA-B27 for such mechanistic immune response studies that are otherwise impossible to pursue in humans. To avoid other technical and knowledge limitations of prior HLA-B27 rodent models, they will produce other modifications. Therefore, the Specific Aims of this proposal are to: 1) Generate HLA-B27 mouse models; and 2) Perform functional analyses of T cells and APCs in HLA- B27 mice. Thus, this proposal will lead to the development of a mouse model for detailed mechanistic studies to further explore the pathogenesis of HLA-B27 in AS and related diseases, and indeed lay the foundation for mechanistic studies of other HLA-associated autoimmune disorders.