Recent Advances in the Immunopathology of Axial Spondyloarthritis: with and without HLA-B27.

PURPOSE OF REVIEW: Axial spondyloarthritis (axSpA) is a chronic multi-organ rheumatic disease affecting various tissues, and recent key advancements have provided significant insight into understanding its immunopathology.

RECENT FINDINGS: Expanded CD8⁺ T cell subsets bearing unique T cell receptor (TCR) motifs (TRAV21/TRBV9) in HLA-B27-positive axSpA patients or those with acute anterior uveitis were recently identified. These T cells can bind to self- and bacterial-derived peptides, supporting the "arthritogenic peptide" hypothesis. Monoclonal antibody-mediated elimination of these expanded T cells has shown potential as a novel treatment for axSpA in a phase II clinical trial, offering a groundbreaking therapeutic approach. In addition to T cells, neutrophils have emerged as important mediators of both inflammation and new bone formation in axSpA, potentially contributing to HLA-B27-independent immune pathomechanisms. Key neutrophil-derived molecules-such as macrophage migration inhibitory factor, hypoxia-inducible factor 1-alpha, caspase recruitment domain-containing protein 9, and neutrophil extracellular traps (NETs)-have been shown in preclinical models to promote Th17-mediated inflammation and reduce regulatory T cell numbers. These mechanisms may be relevant to axSpA pathogenesis and represent potential new therapeutic targets. In recent years, significant progress has been made in understanding the immunopathology of axSpA through both HLA-B27-dependent and -independent mechanisms. However, key questions remain. The pathogenic HLA-B27:05-bound peptides driving CD8⁺ T cell expansion remain unidentified. Moreover, due to the diversity of TCR motifs that recognize self- and microbial peptides, therapies targeting a single TCR (e.g., TRBV9) may have limited efficacy. Further research is needed to clarify axSpA pathogenesis across both pathways.