PRTN3 and Autoimmunity: Studies of seropositive rheumatoid arthritis (RA) show that markers of autoimmunity, including antibodies to citrullinated protein/peptide antigens (ACPA), rheumatoid factor (RF) and others can be present in blood many years prior to the clinically-apparent onset of inflammatory arthritis (IA) and classified RA [(reviewed in ref. 1)].