Impaired transitional B-cell differentiation in mice deficient in PU.1 and/or Spi-B. (A) Increased splenic transitional B-cell frequencies in mice deficient in PU.1 or Spi-B. Flow cytometric analysis was performed to determine the frequency of transitional B cells expressing both B220 (x-axis) and CD93 (y-axis) in the spleen of 6- to 10-week-old mice of the indicated genotypes (see 4 panels). (B) Quantitation of results shown in Panel A for groups of 5 mice of each indicated genotype. The frequency of CD93⁺ cells is shown gated on B220⁺ cells. (C) Altered differentiation of splenic transitional B cells in mice deficient in PU.1 or Spi-B. Flow cytometric analysis was performed to determine the frequency of transitional-1 (IgM⁺CD23⁻), transitional-2 (IgM⁺CD23⁺), and transitional-3 (IgM^lowCD23⁺) B cells (indicated by boxes and legend in right panel). Results shown were gated on B cells expressing both B220 and CD93. (D-F) Quantitation of results shown in panel C for transitional-1, transitional-2, or transitional-3 B cells for groups of 5 mice of the indicated genotypes.