MZB1 and rheumatoid arthritis: As overexpression of MZB1 has been detected in RA synovium and periodontitis gingival tissues and citrullination very likely promotes the function of MZB1, one may wonder whether aberrant expression along with hyper-citrullination of MZB1 contribute to the altered IgM and IgA immune response and the development of RF, ACPA and RA.