Synovial Telocytes Are Characterized by Efhd1, Dpp4 , and Pi16 Expression, Show High Responsiveness to Osmotic Stress, and Exhibit Fibroblast Plasticity Potential

Objective Telocytes are interstitial mesenchymal cells that facilitate intercellular communication and mediate tissue homeostasis and remodeling. Telocyte loss is associated with autoimmune and fibrotic diseases, including rheumatoid arthritis (RA). Thus, we aimed to characterize the genomics, functions, and fibroblast phenotypic transition potential of synovial telocytes. Methods We performed single‐cell RNA sequencing (scRNA‐seq) to phenotype mesenchymal populations in mouse knees and analyzed human synovial scRNA‐seq data from publicly available databases. Immunohistochemistry (IHC) on normal and arthritic joints from Efhd1 ‐CreER T2 × Ai9 tdTtomato (tdT) reporter and tumor necrosis factor transgenic (TNF‐tg) mice assessed synovial telocytes versus fibroblasts. Cocultures of tdT + telocytes and tdT fibroblasts were assessed for responses to osmotic shock via fluorescent microscopy of Fluo4 and MitoSOX Green. Invasive phenotypes were assessed in Matrigel assays. Telocyte‐to‐fibroblast plasticity in response to matrix stiffness was evaluated using bulk RNA‐seq. Results Uniform manifold approximation and projections demonstrated that Efhd1, Dpp4, and Pi16 are specific marker genes that distinguish telocytes from Comp ‐, Dkk3 ‐, and Cdh11 ‐expressing fibroblasts in normal murine and human synovium. IHC confirmed tdT + synovial telocytes are CD34 + /Comp − , primarily reside in the sublining, and are lost during TNF‐induced arthritis. In vitro, telocytes are more sensitive to osmotic pressure than fibroblasts, have a unique mitoflash response to this stimulus, and lose marker gene expression while acquiring myofibroblast markers in response to a stiff matrix. Conclusion Synovial telocytes are Efhd1 + / Dpp4 + /Pi16 + , function as lymphatic sensors to monitor osmotic pressure, and differentiate into fibroblasts in response to matrix stiffness. Their relative absence in RA synovium is associated with decreased lymphatic function and disease progression.