PRTN3 and rheumatoid arthritis: The role of RA autoimmunity in the pathogenesis of systemic bone loss has been extensively investigated over the years, specifically the action of anti-citrullinated protein (ACPA) antibodies, the Immunoglobulin G (IgG) subclass of antibodies, which target citrullinated proteins such as vimentin, fibrinogen, and collagen.