Figure 4. Comparison of the phenotype and fraction of Tfh cells in spleens of immunized WT or TACI-deficient mice.

(A) TACI expression by GC cells and Tfh cells. Tfh cells were identified as CD4⁺CXCR5⁺ and PD1⁺ by FACS (upper panel) and tested for expression of Bcl6 or Taci mRNA by qPCR (middle and lower panels, respectively). Tfh cells from TACI-deficient mice expressed as much Bcl6 as those from WT mice (middle panel). Taci expression by T cells was equivalent to that in GC B cells (lower panel). (B) Relative number of Tfh cells in spleens of WT and TACI-deficient mice before (Pre-I) and 5, 8, and 28 days after immunization with SRBCs. Frequencies of Tfh cells in C57BL/6 and Taci-KO mice were compared by 1-way ANOVA and Holm-Sidak multiple comparison tests. Pre-I, P = 0.0015; day 5, P = 0.0004; day 8, P < 0.0001; day 28, P > 0.9999 was nonsignificant. (C) Impact of TACI expression by B and T cells on the size of the Th cell population. Number of Th cells of donor (CD45.2⁺) or recipient origin (CD45.1⁺) was determined in chimeras reconstituted as shown in Figure 3 relative to chimeras reconstituted with WT-T and WT-QM B cells, 8 days after immunization with 100 μg NP conjugated with OVA. Increase in the number of CD4⁺ Th cells was secondary to B cell–intrinsic TACI deficiency. Comparisons by 1-way ANOVA and Holm-Sidak multiple comparison tests. ***P = 0.0001; **P = 0.0013.