Integrative Single-Cell RNA Sequencing and Spatial Transcriptomics Uncover Distinct Fibroblast-Macrophage Crosstalk...

Femoroacetabular impingement (FAI) and synovitis have been recognized as essential factors for developing osteoarthritis (OA) in the hip joints. However, little is known about altered synovial cellular compositions, their associated transcriptomic profiles, and cell-cell interactions in FAI and hip OA.

Synovial samples from a sex-matched cohort of patients with FAI and hip OA (n = 6 per condition) were analyzed using integrative single-cell RNA sequencing and spatial transcriptomics.

Compared to FAI, epiregulin (EREG)-enriched lining synovial fibroblast-like synoviocytes (FLS) were significantly increased in hip OA. EREG⁺ FLS are proinflammatory due to elevated expression of CXCL1, IL8, and MMPs. Pseudotime analysis predicts that EREG⁺ FLS are derived from DPP4⁺PI16⁺ sublining FLS, likely regulated by NFIX and REL, as well as ELK3 and ETV6 transcription factors under FAI or OA conditions, respectively. Importantly, the only putative cell-cell interaction is fibroblast growth factor 2 (FGF2)-syndecan 4 (SDC4) communication between COL1A1⁺IGFBP5⁺ fibrotic macrophages and EREG⁺ FLS. This interaction may induce expression of interleukin (IL)-6, IL-8, and matrix metalloproteinase 1 (MMP1) in hip OA synovium. The Gene Ontology analysis of activated genes downstream of FGF2-SDC4 signaling revealed that inflammation and angiogenesis were up-regulated in hip OA, whereas positive gene transcription and skeletal muscle differentiation were dominant in FAI. Moreover, we also found that EREG⁺CCL20⁺MMP3^hi lining FLS along with most macrophage and monocyte populations are unique to patients with hip OA compared to patients with knee OA and rheumatoid arthritis.

The findings of this study offer a groundwork in tailoring novel targets and therapies for patients with FAI and hip OA.