KLF3 expression overcomes the MZ B-cell developmental arrest caused by CD19 deficiency. (A) Splenocytes from mice of the indicated genotypes were stained for CD19, CD93, CD21/35, and CD23, and analyzed by FACS. (B) Spleen cryosections were stained for Moma-1 and IgM and analyzed by immunofluorescent microscopy. The white arrow indicates an area rich in MZ B cells. Such areas were lacking entirely in CD19-deficient mice. (C) Cell numbers of follicular and MZ B cells in mice of the indicated genotype. Dashed lines indicate sex-matched littermates analyzed in parallel. Three FACS experiments were performed. For histology, 3 CD19:KLF3 and 3 nontransgenic CD19cre/cre mice were analyzed. Micrographs were obtained with a LSM610 meta confocal microscope, using a 16×/0.5 NA objective.