Impaired B-cell proliferation and IgE production by the CD40L^G219R mutant. (A) In vitro proliferation of B cells from PBMCs of healthy donors was assessed by measuring [³H]thymidine incorporation after a 5-day stimulation in the presence of interleukin-4 and irradiated 3T3 cells (3T3), 3T3-expressing CD40L, or 3T3-expressing CD40L^G219R. *P < .02. (B) Production of IgE by B cells. PBMCs were stimulated to activate CSR to IgE production for 12 days in the presence of interleukin-4 and irradiated 3T3 cells (3T3), 3T3-expressing CD40L, or 3T3-expressing CD40L^G219R. At day 12, production of IgE was measured by enzyme-linked immunosorbent assay in the supernatants of activated B cells. ***P < .001.