AID^−/− GC B cells have reduced apoptotic activity in vivo. (A) Mice were immunized with NP-CGG and 11 days later were harvested to assess caspase levels in the GC via flow cytometry. Cells were surface stained to mark GC populations and then incubated with fluorescently labeled substrates specific for caspases 8, 9, or 3. Plots were gated either on the follicular B-cell population (B220⁺, Fas⁻, PNA^low) or the GC B-cell population (B220⁺, Fas⁺, PNA^high) and assessed for caspase activity (shown is a representative WT sample). (B) Data were accumulated for immunized mice for caspases 8, 9, and 3. Statistics represent comparisons between WT GC versus AID^−/− GC. Data are mean ± SD of n = 6 to 9 mice per genotype. (C) Caspase 3 activity for AID^−/− and WT GC B cells is shown for immunized mice reconstituted with a 1:1 mixture of WT and AID^−/− BM. Data are mean ± SD of n = 6 chimeric mice. (D) Mice were immunized with NP-CGG and 11 days later were harvested and stained to mark GC populations and fixed for use in a flow cytometry–based TdT-mediated dUTP nick-end labeling assay. Gated cells were assessed for apoptotic activity (shown is a representative WT sample). (E) Accumulated data were expressed as a percentage of positive cells. Statistics represent comparisons between WT GC versus AID^−/− GC. Data are mean ± SD of n = 6 to 9 mice per genotype. **P ≤ .01. ***P ≤ .001.