HLA-DRB1 and rheumatoid arthritis: When considering a possible association between clinical variables, HLA and Pg positivity, UPIA and RA patients who were smokers and/or positive for ACPA autoantibodies and/or positive for HLA DRB1*04 allele were more frequently positive for Pg DNA in the synovial tissue, compared to patients who were negative for all these clinical/genetic variables (42.9% vs. 10.5%, P = 0.05) (Additional file 3, Table S2).