In one or more embodiments, the present invention provides a novel biomarker which provides a link between a distinct clinical phenotype and a biochemical effect of an autoantibody on an enzyme implicated in disease pathogenesis. In particular, the present invention provides an isolated or purified human autoantibody to PAD3 protein. Methods of diagnosis of subjects for rheumatoid arthritis (RA) using these antibodies as well as diagnosis of the severity of RA in the subject, and methods for monitoring treatment of a subject with RA are also provided. The biomarkers provided herein are also useful in the diagnosis of connective tissue-interstitial lung disease (CT-ILD) in patients having or suspected of having RA.