PRTN3 and rheumatoid arthritis: Later, Bongartz et al. have shown that citrullinated proteins are also present in the lung tissue of patients with interstitial pneumonia associated with RA (RA-IP) as well as in patients with idiopathic interstitial pneumonia (IIP), but high antibodies to citrullinated protein antigen (ACPA) titers have been observed only in RA-IP and not in IIP.