TACI deficiency does not impair B-cell activation and germinal center formation. (A) The proportion of NP-specific B cells (idiotype-positive) in the spleen of QM-TACI-wt or QM-TACI-ko mice was analyzed by flow cytometry in mice immunized 10 days earlier. Splenocytes were stained with an anti–idiotypic antibody¹⁰  and with an anti–IgM^a antibody that detects the allotype produced by the heavy chain-targeted allele. (B) Average proportions (above) or average number (below) of idiotype-positive cells obtained from 3 QM-TACI-wt or 3 QM-TACI-ko mice. Averages were compared by t test. Significant differences: *P < .05; **P < .01. (C) TACI deficiency does not decrease B-cell activation after immunization. Splenocytes were obtained from QM-TACI-wt or QM-TACI-ko mice immunized with 100 μg NP-OVA 10 days earlier. Splenocytes were stained with anti-CD19 and CD69 antibodies and analyzed by FACS. Activated B cells (CD19⁺, CD69⁺) were as frequent in TACI-proficient as in TACI-deficient mice. Average proportions of CD19⁺, CD69⁺ cells obtained from 3 QM-TACI-wt, or 3 QM-TACI-ko mice. Averages were compared by t test. Differences between groups were not significant. (D) TACI deficiency caused increases in size and number of germinal centers 10 days after immunization. Frozen sections of Peyer patches were stained with anti–GL7 antibody (brown), which specifically labels germinal center B cells. The figure is representative of 3 independent experiments. Digital images were obtained by Leica DM6000 B microscope (Leica) and with QCapture Pro Version 6.0.0 software (QImaging).