For LPS-induced plasmablast differentiation assays, splenic B220+ B cells were enriched using MACS technology (Miltenyi Biotec) according to the manufacturer’s instructions. Purified B cells were plated at a density of 150,000 cells/ml in IMDM (Life Technologies) supplemented with 10% fetal calf serum (Sigma), 50 µM 2-mercaptoethanol, 2 mM glutamine (Gibco), 100 U/ml penicillin (Sigma), and 100 µg/ml streptomycin sulfate (Sigma) and were subsequently treated for up to 4 d with 25 µg/ml LPS (Sigma). For the iGB cell differentiation system (Nojima et al., 2011 (link); Wöhner et al., 2016 (link)), 40LB stromal cells (cultured in DMEM supplemented with 10% fetal calf serum) were inactivated by irradiation. 100,000 splenic B cells, which were enriched using CD43 MACS beads (Miltenyi Biotec), were plated onto the 40LB cells in 6-well plates in RPMI medium (supplemented with 10% fetal calf serum [ThermoFisher Scientific; lot42A0368K], 1 mM glutamine, 50 µM 2-mercaptoethanol, 10 mM Hepes, and 1 mM sodium pyruvate) and stimulated with recombinant IL-4 (20 ng/ml) for 4 d. At day 4, 100,000 stimulated B cells were transferred onto fresh 40LB cells in a 6-well plate and stimulated with recombinant IL-21 (10 ng/ml) for another 4 d until cells were analyzed by flow cytometry.