Lymphocyte isolation and cell assays
SF mononuclear cells (SFMC) and paired peripheral blood samples (PBMC) from two SLE, three SpA and two seronegative RA patients (matched for age and sex, Supplementary Table S2), were analyzed by flow cytometry panels for T cells and when possible for B cells, monocytes and dendritic cells (Supplementary Table S4). Cryopreserved PBMC and SFMC were thawed and stained with the corresponding antibody mixtures and viability dye (Supplementary Table S4 and S5). For functional studies, duplicates of 0.5×106 SFMC were stimulated with anti‐CD3/CD28 beads (1bead/cell, Dynabeads, Thermo-Fisher, Waltham, USA) for 16h in 96‐well flat‐bottom plate. The last 4h brefeldin-A (5ug/ml, Sigma Aldrich, St. Louis, USA) was added. Analyses were run on a Fortessa LSR cell analyzer (BD Biosciences, San Jose, USA) and data were processed by FlowJo software (BD Biosciences, San Jose, USA). Gating strategies are displayed in Supplementary Fig. S1.