Anti–Peptidylarginine Deiminase 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 patients with rheumatoid arthritis (RA) 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 patients with RA 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. In addition, PAD4 gene and protein expression was assessed using human fibroblast-like synoviocyte in vitro cultures and a single-cell RNA sequencing data set obtained from patients with RA. 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 interleukin-17A (IL-17A), tumor necrosis factor α (TNFα), and interferon-γ (IFNγ)–producing T cells. Anti-PAD4 antibodies preferentially bound monocytes in both humans and mice, eliciting proinflammatory chemokine production by human monocytes in vitro. T cell cytokines enhanced by anti-PAD4 antibodies in the CIA model (ie, IL-17A, TNFα, and IFNγ) synergized to induce a proinflammatory 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. (Figure presented.).