R123 expression highlights three distinct populations of immature transitional, maturing transitional, and mature B cells.

(A) Baseline staining of unstimulated FACS-sorted transitional B cells with R123 (left panel), CD21 and CD23 (middle panel), and sIgD and sIgM (right panel). (B) Characteristics of transitional B cells cultured with IL-4: percentages of R123neg, R123lo, and R123hi cells (panel 1); graphical distribution of R123neg (orange), R123lo (blue), and R123hi (red) subpopulations (panel 2); CD21 and CD23 expression of R123 subpopulations (panel 3); and sIgD and sIgM expression of R123 subpopulations (panel 4). (C) Characteristics of transitional B cells cultured with IL-4+R848: percentages of R123neg, R123lo, and R123hi cells (panel 1); graphical distribution of R123neg, R123lo, and R123hi cells (panel 2); CD21 and CD23 expression of R123 subpopulations (panel 3); and sIgD and sIgM expression of R123 subpopulations (panel 4).(D) Model of human transitional B cell maturation stimulated by IL-4 alone or IL-4 + PRR ligand. The immature, transitional B cell possesses a characteristic phenotype as shown. Stimulation with IL-4 alone or IL-4 + PRR ligand results in the development of both CD23+ and CD23- mature B cells that express ABCB1. The CD23+ population upregulates expression of CD23, sIgM, and sIgD prior to full induction of ABCB1. Expression of CD23 and ABCB1 by these B cells is markedly enhanced when they are stimulated with both IL-4 and certain PRR ligands, including R848 and CpG. The CD23- population upregulates sIgM and sIgD expression and ABCB1 without inducing CD23. This population is more prominent in cells stimulated with IL-4 alone than in those stimulated with the IL-4+PRR ligand combination.