SFRs on T cells and B cells are required for proper GC reaction and T cell–dependent antibody production. (A–D) Purified CD4⁺ T cells from WT or SFR KO mice were injected into mice expressing SFRs but lacking all T cells (TCR KO mice). Mice were then immunized with NP-OVA and analyzed as detailed in A. Total CD4⁺ cells, Tfh cells, and GC Tfh cells are enumerated in B. GC B cells, NIP⁺IgM⁺ GC B cells, and NIP⁺IgG1⁺ GC B cells are enumerated in C. NP-specific IgM and IgG1 are quantified in D. TCR KO mice without injection of any CD4⁺ cells are represented as “none.” For B and C (day 7), mice receiving WT CD4⁺ T cells: n = 7; SFR KO CD4⁺ T cells: n = 6. For C (day 9), mice receiving WT CD4⁺ T cells: n = 8; and SFR KO CD4⁺ T cells: n = 7. Data are pooled from three (day 7) or two (day 9) independent experiments. For D, mice receiving no cells (“none”): n = 6; WT CD4⁺ T cells: n = 9; and SFR KO CD4⁺ T cells: n = 9. Data are pooled from two independent experiments. (E–H) Purified B cells from WT or SFR KO mice were injected into mice expressing SFRs but lacking all B cells (BCR KO mice). Mice were then immunized with NP-OVA and analyzed as detailed in E. Total B cells (F), GC B cells (G), and NP-specific IgM and IgG1 (H) were quantified. For F and G, mice receiving WT CD19⁺ B cells: n = 4; and SFR KO CD19⁺ B cells: n = 5. Data are from one experiment. For H, mice receiving no cells (“none”): n = 8; WT CD19⁺ B cells: n = 13; and SFR KO CD19⁺ B cells: n = 12. Data are pooled from three independent experiments. Each symbol represents one mouse. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001 (two-tailed Student’s t test). Data are shown as mean ± SEM. D, day; ns, not significant; SPC, spleen cells; SS, side scatter.