Diminished plasmablast formation and Ig secretion in response to CD40L/IL-21 in SCID patients with poor donor-derived B-cell reconstitution. PBMCs of the indicated patient-groups were either unstimulated, or cultured with CD40L/IL-21 (A,C) or CpG (B,D). After 4 days, the frequency of B cells that differentiated into plasmablasts, as determined by acquisition of a CD38^hiCD27^hi phenotype (A-B), and secretion of IgM and IgG (C-D) were determined by flow cytometry and ELISA, respectively. Representative FACS plots derived from healthy donors are presented in panels A and B. Statistical analysis was performed using 1-way ANOVA and Bonferroni posttest analysis (*P < .05; **P < .01; ***P < .001). NS indicates nonsignificant changes.