Rheumatoid arthritis is associated with the development of autoantibodies to citrullinated self-proteins. Citrullinated synovial proteins, which are generated via the actions of the protein arginine deiminases (PADs), are known to develop in the murine collagen-induced arthritis (CIA) model of inflammatory arthritis. Given these findings, we evaluated whether N-α-benzoyl-N5-(2-chloro-1-iminoethyl)-l-ornithine amide (Cl-amidine), a recently described pan-PAD inhibitor, could affect the development of arthritis and autoimmunity by treating mice in the CIA model with Cl-amidine on days 0–35. Cl-amidine treatment reduced total synovial and serum citrullination, decreased clinical disease activity by∼ 50%, and significantly decreased IgG2a anti-mouse type II collagen Abs. Additionally, histopathology scores and total complement C3 deposition were significantly lower in Cl-amidine–treated mice compared with vehicle controls. Synovial microarray analyses demonstrated decreased IgG reactivity to several native and citrullinated epitopes compared with vehicle controls. Cl-amidine treatment had no ameliorative effect on collagen Ab-induced arthritis, suggesting its primary protective mechanism was not mediated through effector pathways. Reduced levels of citrullinated synovial proteins observed in mice treated with Cl-amidine are consistent with the notion that Cl-amidine derives its efficacy from its ability to inhibit the deiminating activity of PADs. In total, these results suggested that PADs are necessary participants in the autoimmune and subsequent inflammatory processes in CIA. Cl-amidine may represent a novel class of disease-modifying agents that modulate aberrant citrullination, and perhaps other immune processes, necessary for the development of inflammatory arthritis.

Rheumatoid arthritis (RA) is an autoimmune disorder that is estimated to affect nearly 1% of the population. RA is characterized by chronic inflammation of the synovium, resulting in pannus formation and joint destruction. The presence of autoantibodies designated rheumatoid factors, which recognize Ig Fc domains, have been well-characterized in RA. However, a new class of autoantibodies, designated Abs to citrullinated protein Ags (ACPA), have proven to be more specific than rheumatoid factors for the presence of RA (1–3). Indeed, several studies showed that ACPA are present in the sera of individuals who ultimately develop seropositive RA for an average of 4–5 y prior to the onset of clinically apparent disease (4–6), suggesting that the development of these autoantibodies may be an early event in the onset and progression of RA.