Plasma cell differentiation requires Cdc42. (A–E) CTV-labeled splenic WT and Cdc42 KO B cells were stimulated for 4 d with LPS or CD40L in the presence of IL-4 and IL-5. Flow cytometry was used to measure class switch recombination (IgG1⁺ cells; A), proliferation (CTV dilution; B), and plasma cell differentiation (CD138^hi; C). Secreted IgM in culture supernatants was measured by sandwich ELISA (D). Cultured cells were sorted on the basis of IgG1 and CD138 expression as well as CTV dilution. Cells were then prepared for transmission electron microscopy (TEM). (E) Representative TEM images of IgG1⁺ cells; plasmablasts (IgG1⁻CTV^loCD138^int), and plasma cells (IgG1⁻CTV^loCD138^hi) are shown. Quantitation of percentage of cells with expanded or nonexpanded ER is shown in graphs at the bottom. Data are representative at least 4 experiments with one mouse in each. Mean and SEM. Student’s t test, *, P < 0.05; **, P < 0.01.