Mechanical Strain‐Programmed SDC1 ⁺ Sheath Fibroblasts Trigger CXCR4 ^hi Neutrophil‐Mediated Enthesitis in Ankylosing Spondylitis

ABSTRACT Ankylosing spondylitis (AS) is an osteoimmune disease characterized by pathological enthesitis related to mechanical strain. However, the cell interactions and molecular mechanisms of AS enthesitis are still unclear. Herein, we constructed a hind paw loading/unloading model using experimental spondyloarthritis SKG mice, and generated a single‐cell RNA sequencing atlas of mechanical strain‐related AS enthesitis. In this context, a disease‐specific subpopulation of SDC1 ⁺ sheath fibroblasts was identified to arise under mechanical strain, and these cells secreted higher levels of CXCL5 to recruit and promote the activation of CXCR4 ^hi neutrophils, which exacerbated CXCR4 ^hi neutrophil‐mediated enthesitis by increasing their neutrophil extracellular trap formation. Administering CXCL5 neutralizing antibody relieved disease progression in SKG mice. Additionally, computational trajectory analysis revealed a distinct fate branch of the mechanical strain‐responding SDC1 ⁺ sheath fibroblasts under the control of SOX5‐mediated enhancers and super‐enhancers. Specifically inhibiting SOX5 in enthesis fibroblasts via rAAV9.HAP‐1 carrying a shRNA targeting Sox5 blocked the generation of SDC1 ⁺ sheath fibroblasts in response to mechanical strain and markedly reversed the development of CXCR4 ^hi neutrophil‐mediated enthesitis. This study identifies the specific cell interactions and molecular mechanisms involved in mechanical strain‐related AS enthesitis, therefore contributing to the understanding of AS pathogenesis and providing insight into potential clinical treatments for AS.