Emerging biochemical, microbial and immunological evidence in the search for why HLA-B∗27 confers risk for spondyloarthritis

The strong association of the human leukocyte antigen B∗27 alleles (HLA-B∗27) with spondyloarthritis and related rheumatic conditions has long fascinated researchers, yet the precise mechanisms underlying its pathogenicity remain elusive. Here, we review how interplay between the microbiome, the immune system, and the enigmatic HLA-B∗27 could trigger spondyloarthritis, with a focus on whether HLA-B∗27 presents an arthritogenic peptide. We propose mechanisms by which the unique biochemical characteristics of the HLA-B∗27 protein structure, particularly its peptide binding groove, could dictate its propensity to induce pathological T cell responses. We further provide new insights into how TRBV9