In retrospective studies, we have previously seen elevated antibody levels against arginine gingipain (Rgp), an important Pg virulence factor, in RA versus controls, even before RA symptom onset,6 9 10 and we have observed an interaction between anti-Rgp IgG and HLA-DRB1 shared epitope (SE) in ACPA-positive RA.6 This evidence concerns the gene HLA-DRB1 and rheumatoid arthritis.