Peripheral blood mononuclear cells of 15 RA patients positive for anti‐citrullinated protein antibodies (ACPA) were analyzed directly ex vivo using spectral flow cytometry and B‐cell differentiation markers, citrullinated antigen‐biotin‐streptavidin tetramers, and intracellular (phosphoflow) markers. Here, PRTN3 is linked to rheumatoid arthritis.