Reduced BCR affinity in Noxa^−/− mice after antigen stimulation results from a B cell intrinsic defect. WT (CD45.2) recipients (n = 10) were lethally irradiated and transferred with mixed WT (CD45.2) and Noxa^−/− (CD45.1) BM in a 4:1 ratio. Two months after transfer, 8 mice were immunized with PE, and after 12 days B-cell responses were assessed. (A) Ratio of Noxa^−/− cells in indicated lymphocyte populations before immunization in the blood of bone marrow chimeric mice 8 weeks after transfer. Presence of Noxa^−/− CD45.1⁺ cells within B cells (B220⁺CD11b⁻), CD4 (CD4⁺CD3⁺) and CD8 (CD8⁺CD3⁺) T cells was quantified. (B) Histograms of FB cells and GC B cells in BM chimeric mice 12 days after PE/Alum immunization. Top panels: fraction of Noxa^−/− cells (CD45.1⁺) within FB cells and GC B cells. Bottom panels: overlay of CD45.1⁺ and CD45.1⁻ cells within the FB and GC B cell pools, stained with PE. (C) Quantification of the ratio of Noxa^−/−/WT cells in the FB versus GC B cell fractions as shown in panel B. Noxa^−/− cells are enriched within the antigen-specific pool. (D) Quantification of the average PE binding of GC B cells 12 days after immunization. MFI data shows significantly reduced PE-binding of antigen-specific Noxa^−/− GC B cells. Mean and individual data points are shown (*P < .05, **P < .005; Student t test)