Dissociation of CCDC25 dimer into NETs-DNA-bound monomer licenses inflammatory activation of fibroblast-like synoviocytes in rheumatoid arthritis.

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by synovium infiltration of immune cells, leading to inflammatory activation of fibroblast-like synoviocyte (FLS). Neutrophils infiltrate the synovium early in RA and exacerbate disease by releasing neutrophil extracellular traps (NETs). CCDC25 has been identified as a sensor for NETs-DNA, but its role in FLS activation remains to be investigated. Here, CCDC25 was found upregulated in RA-FLS, with reduced dimeric and increased monomeric forms relative to controls, and only monomeric CCDC25 bound to NETs-DNA. NETs-DNA binding to monomeric CCDC25 enhanced Rac1-Sar1 interaction, disrupted COPII-dependent ER-to-Golgi trafficking, induced endoplasmic reticulum stress (ERS), and promoted inflammatory activation of FLS. In vivo, CCDC25 deficiency alleviated arthritis severity in adjuvant-induced, collagen-induced and serum-transfer models, reduced synovitis, and weakened the neutrophil-FLS feedback loop. These findings support monomeric CCDC25 as a NETs-DNA sensor that induces inflammatory activation of FLS, with potential as a therapeutic target in RA.