Preferential localization of B lymphocytes in the splenic MZ of irf4^−/− mice and inducible deletion of irf4 in B cells in vivo. (A) Spleen sections from irf4^+/+, irf4^+/−, and irf4^−/− mice were analyzed for CD3, MOMA1, and B220 expression by IF. One representative mouse out of four per group is shown. Bar, 500 µm. (B) Schematic representation of the Cre-ER transgene under the control of the human (h) CD20 promoter (middle) and of the irf4 gene before (top) and after (bottom) Cre-mediated recombination in the irf4^fl/flCD20-TAM-Cre model. (C–E) Mice were challenged with three doses of TAM on three consecutive days and analyzed 11–14 d after the final TAM injection. (C) Spleen cells from irf4^fl/flCD20-TAM-Cre mice after administration of corn oil or TAM (n = 4 per group, two independent experiments) were analyzed for CD19 and eGFP expression by flow cytometry. The number above the gate indicates percentage of eGFP⁺ B cells among lymphocytes. (D) eGFP expression level on splenic B cells from irf4^fl/flCD20-TAM-Cre, irf4^fl/+CD20-TAM-Cre, and CD20-TAM-Cre mice was assessed by flow cytometry (one representative mouse out of four to eight per group is shown; four independent experiments). (E) B cells were sorted from the spleen of irf4^fl/flCD20-TAM-Cre, irf4^fl/+CD20-TAM-Cre, and CD20-TAM-Cre mice, and IRF4 expression was determined by Western blot (one representative of two independent experiments).