The present invention provides peptidylarginine deiminase 4 (PAD-4) polypeptides comprising one or more citrullinated Arginine sites. In one embodiment, a PAD-4 polypeptide comprises a citrulline residue at the following sites: Arg-205, Arg-212, Arg-218, Arg-372, Arg-374, Arg-383, Arg-394, Arg-495, Arg-536, and Arg-544. The present invention also provides methods for detecting the presence of autoantibodies to citrullinated PAD-4. The methods of the present invention may also be used to qualify Rheumatoid Arthritis status in a subject. In other embodiments, the present invention provides methods for inducing tolerance to citrullinated PAD-4.
