PRTN3 and rheumatoid arthritis: To further evaluate the differences detected in RA naïve and unmutated B cells by the phenotype and repertoire studies, we performed a serological screening for total IgM levels in a subset of the cohort with 193 ACPA+ RA patients, 50 ACPA− RA and 154 population-based controls (Fig. 4, Supplemental Table 10).