PRTN3 and rheumatoid arthritis: To determine whether HLA-DR4Vim-64cit59-71 and HLA-DR4α-eno-15cit10-22-reactive CD4+ T cells could be identified in HLA-DR4+ humans, PBMCs from an ACPA+ RA donor 2 were magnetically enriched using either the HLA-DR4Vim-64cit59-71 or HLA-DR4α-eno-15cit10-22, or HLA-DR4 influenza hemagglutinin epitope (HLA-DR4HA) tetramers, using an established protocol38,39.