Distinct T Cell Repertoires in Spinal Entheses and Synovial Fluids in Ankylosing Spondylitis.

UNLABELLED: Ankylosing spondylitis (AS) is a chronic inflammatory condition primarily impacting the spine and sacroiliac joints, driven by immune dysregulation. While T cells are known contributors to AS, the specific cellular and molecular changes within affected tissues, especially in the spine, remain to be fully elucidated. We conducted single-cell RNA (scRNA-seq) and T cell receptor (TCR) sequencing on synovial fluids and spinal ligament samples from AS patients, supplemented by public AS and healthy control data. The analysis identified nine T cell clusters, with CD8 + memory T cells enriched in spine ligaments (SL), while mucosal-associated invariant T lymphocytes (MAIT) cells and regulatory T lymphocytes (Tregs) were enriched in synovial fluids. Differential expression and pathway analyses revealed that senescence-related genes, such as calmodulin 1 (CALM1) and cyclin-dependent kinase inhibitor 1A (CDKN1A), were enriched in SL, suggesting the presence of senescent T cells, and cytotoxic genes, including natural killer cell granule protein 7 (NKG7), were enriched in synovial fluids. Furthermore, TCR repertoire analysis uncovered biased usage of T cell receptor gamma variable (TRGV) genes in SL and T cell receptor beta variable 9 (TRBV9) in synovial fluid, indicating tissue-specific clonal expansion. Lastly, peptide prediction identified potential human leukocyte antigen-B27 (HLA-B27)-presented peptides, highlighting their contribution to inflammation. Our study demonstrates a unique immune landscape in AS, characterized by differential clonal expansion in peripheral and axial joints. These findings underscore T cell-mediated immune responses in AS pathology, contributing to the understanding of the immunological mechanisms of AS and potential immune-focused therapies. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s43657-025-00292-z.