SDC1 and enthesitis: In this context, a disease-specific subpopulation of SDC1<sup>+</sup> 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<sup>hi</sup> neutrophils, which exacerbated CXCR4<sup>hi</sup> neutrophil-mediated enthesitis by increasing their neutrophil extracellular trap formation.