PADI4 and rheumatoid arthritis: PAD-mediated citrullination of nuclear antigens such as histones has previously been reported to play an essential role in the unique form of cell death known as neutrophil extracellular trap formation (NETosis) (15, 16), and it has been postulated that enhanced NET production could provide an important source of autoantigens within the inflamed joints of RA patients (7).