Clinical and immunological aspects of anti-peptidylarginine deiminase type 4 (anti-PAD4) autoantibodies in rheumatoid arthritis

Rheumatoid arthritis (RA) is the most common rheumatic autoimmune disease worldwide, which causes progressive joint damage and can lead to functional disability. Despite prominent advances in RA diagnosis and treatment during the last 20 years, there is still a need for novel biomarkers that aid in diagnosis and prognosis of this heterogeneous disease. Citrullination is a key post-translational modification implicated on anti-citrullinated protein/peptide antibodies (ACPA) production in RA, catalyzed by human peptidylarginine deiminases (PADs). Among these enzymes, PAD4 has been recognized as an important player in RA pathogenesis and the enzyme itself is a target of autoantibodies (anti-PAD4) in a subgroup of RA patients. Accumulating evidence suggests that anti-PAD4 autoantibodies may be useful as a severity biomarker in RA and recent studies have also shed light on the functional significance of these autoantibodies. This review summarizes the evidence on anti-PAD4 autoantibodies in RA, and addresses its usefulness for disease diagnosis and prognosis. Novel immunological aspects of anti-PAD4 antibodies and their relevance to RA pathogenesis are also discussed.