While IgA responses did not significantly differ between different disease types from the SCREEN-RA/SCQM or the Rheuma-VOR cohort (Supplementary Figures 2A, B), we also identified the highest IgA signal in both cohorts, independently of disease status, against the P. copri strain RPC01 (Supplementary Figures 2C, D). Here, CD79A is linked to rheumatoid arthritis.