PRTN3 and rheumatoid arthritis: We tested a panel of RA patient-derived monoclonal ACPA generated at the Karolinska Institutet and found that one of them, L201_10D0721, was particularly suitable for our purpose: it did not stain neutrophils from healthy donors, but stained them in a granular pattern after a brief treatment with 2 μM ionomycin, which induces a strong Ca2+-dependent intracellular citrullination of numerous proteins16,17 (Supplemental Fig. S2).