Multi-omic and functional analyses identified an endoplasmic reticulum stress-driven TIMD4⁺ IL1β⁺ macrophage...

Osteoarthritis (OA) is a chronic degenerative disease lacking effective non-operative managements so far. Although the joint synovium and infrapatellar fat pad (IPFP) have been demonstrated to contribute to joint formation and homeostasis, their roles in OA progression remain incompletely investigated. Thus, in the current study, the cellular components and spatial distribution in synovium and IPFP were explored via performing an integrated single-cell RNA sequencing (scRNA-seq) and spatial multi-omics sequencing analysis on human synovium and IPFP as well as in vivo interference on OA mice. The bioinformatic and pathological data showed that a TIMD4⁺ IL1β⁺ macrophage subpopulation co-localizes with PDPN⁺ lining fibroblasts (L-FLS) in both synovium and IPFP and the macrophage subpopulation expanded during OA progression. Further analysis revealed that endoplasmic reticulum stress (ERS) induced TIMD4⁺ IL1β⁺ macrophage activation and promoted the secretion of pro-inflammatory cytokines and matrix-degrading enzymes that fuel tissue inflammation and fibrosis. In addition, the ERS-driven macrophages was conserved in murine OA. Furthermore, pharmacological inhibition of ERS in OA mouse significantly alleviated inflammatory infiltration and fibrosis, therefore effectively attenuating joint damage. In summary, our study identified an ERS-driven TIMD4⁺ IL1β⁺ macrophage subpopulation and demonstrated that these macrophages contribute to OA progression. These findings suggest that targeting the ERS-macrophage axis represents a promising therapeutic strategy for OA.