Anti‐Peptidylarginine‐4 Autoantibodies Derived from Patients with Rheumatoid Arthritis Exert Pathogenic Effects by Activating Monocytes and Exacerbating Inflammatory Arthritis

Objective Autoantibodies targeting peptidylarginine deiminase 4 (PAD4), an enzyme involved in protein citrullination, are found in a subset of rheumatoid arthritis (RA) patients with severe joint disease. However, the mechanisms by which anti‐PAD4 antibodies participate in disease pathogenesis are incompletely defined. Methods We investigated the role of anti‐PAD4 monoclonal antibodies derived from RA patients using a collagen‐induced arthritis (CIA) mouse model and human monocyte in vitro cultures. The cellular targets of anti‐PAD4 antibodies were identified using mouse knee joint cells and human peripheral blood mononuclear cells (PBMC). In addition, PAD4 gene and protein expression were assessed using human fibroblast‐like synoviocyte in vitro cultures and a single‐cell RNA sequencing dataset obtained from RA patients. Results We show that anti‐PAD4 antibody treatment augmented disease severity in the CIA mouse model, with increased joint damage, myeloid cell infiltration, and synovial fibroblast activation. Arthritic mice administered with anti‐PAD4 antibodies had an increased proportion of IL‐17A, TNF‐α, and IFN‐γ‐producing T cells . Anti‐PAD4 antibodies preferentially bound monocytes in both humans and mice, eliciting pro‐inflammatory chemokine production by human monocytes in vitro . T cell cytokines enhanced by anti‐PAD4 antibodies in the CIA model, i.e. IL‐17A, TNF‐α, and IFN‐γ, synergized to induce a pro‐inflammatory phenotype in human fibroblast‐like synoviocytes. Conclusion Our findings suggest a model in which anti‐PAD4 antibody binding to monocytes triggers an inflammatory cascade that promotes immune cell recruitment to the joint and T cell activation, culminating in synovial fibroblast activation and the development of more severe arthritis. image