text: Isolation and growth of B cells.
  Splenocytes were prepared by homogenizing spleen tissue with two frosted slides and debris was filtered through a 100-mum cell strainer. Erythrocytes were lysed using 0.8% ammonium chloride solution (StemCell Technologies) for 10 min on ice and washed twice with PBS. B cells were isolated using CD19-MACS beads according to the manufacturer's instructions (Miltenyi Biotec) and grown in Iscove's medium supplemented with heat-inactivated 10% fetal bovine serum and antibiotic/antimycotic (GIBCO). Splenocytes isolated from SCID-passaged lymphomas consisted of 80%-90% B cells as determined by flow cytometry, and were hence not further purified with CD19-MACS beads. Splenocytes were seeded at 1.25 x 106 cells/ml and where applicable, recombinant mouse IL4 was added at 100 ng/ml, recombinant mouse IL10 at 10 ng/ml, and rat IgG1 anti-mouse IL10 and rat IgG1 isotype control at 10 mug/ml (R&D Systems). For BrdU incorporation assays, splenocytes were pulsed for 24 h with 10 muM BrdU 1 d post-harvest. 
  