HLA-B27 and spondyloarthritis: at the crossroads of innate and adaptive immunity.

HLA-B*27 confers a strong risk of developing spondyloarthritis (SpA), which includes axial SpA with or without peripheral arthritis, enthesitis, acute anterior uveitis and gastrointestinal inflammation. Although no definitive mechanism has been established to explain the role of this HLA class I protein in the pathogenesis of SpA, three main hypotheses have emerged. First is the idea that self-peptides displayed by HLA-B27 resemble microorganism-derived peptides, leading to the expansion of autoreactive CD8+ T cells that trigger disease. The second and third hypotheses focus on aberrant properties of HLA-B27, including its tendency to form cell-surface dimers that can activate innate killer immunoglobulin-like receptors on CD4+ T helper 17 cells, triggering the production of pathogenic cytokines. HLA-B27 also misfolds in the endoplasmic reticulum, which can activate the unfolded protein response, increasing IL-23 expression and thereby promoting the production of type 17 cytokines. HLA-B27 misfolding in mesenchymal stem cells has also been linked to enhanced bone formation by mesenchymal stem cell-derived osteoblasts, which could contribute to structural damage in axial SpA. In this Review we summarize prevailing ideas about the role of HLA-B27 in SpA, discuss the latest developments as well as the gaps in current knowledge, and provide recommendations for future research to address these unmet needs.