Phenotype and proliferative responses of CD21^low and CD21^high V_H1-69⁺ B cells. (A-B) FACS plots from a representative (n = 8) MC patient with predominance of CD21^low V_H1-69⁺ B cells. (A) Expression of surface markers by electronically gated V_H1-69⁺ B cells stained with the G6 antibody.⁴  (B) Gating strategy for the analysis of proliferative responses of V_H1-69⁺ and V_H1-69⁻ B cells to CpG (2.5 μg/mL, 5-day culture). Cells were permeabilized before staining. The regions of analysis encompass CD20⁺ B cells and CD20^low/neg plasmablasts, which were also characterized as CD38⁺ cytoplasmic-IgM^high cells (not shown). Plasmablasts are present only in the V_H1-69⁻ gating region. Percent values in the histograms denote the number of cells entering division (precursor cohort⁵ ). (C-D) FACS plots from a representative (n = 3) MC patient with highly enriched CD21^high V_H1-69⁺ B cells. Analyses of surface marker expression (C) and of proliferative responses to CpG (D) were done as in panels A and B, respectively.