The Tfh-skewed epitope-specific CD4⁺ T cell response is B cell dependent. (A) Representative flow plots demonstrating CXCR5 expression on antigen-specific CD4⁺ T cells at acute time points after Py-GP66 infection in WT and μMT mice. Plots are gated on dump⁻ CD3⁺ CD8⁻ CD4⁺ CD44⁺ GP66⁺ cells. (B) Summary data of the percentage of CXCR5⁺ GP66⁺ cells and number of GP66⁺ cells from A. Data are pooled from three to six mice per cohort from at least two independent experiments and were analyzed by unpaired t test. (C) WT mice were treated with anti-CD20 antibody or isotype control immediately before infection and on day 3 after infection. Representative flow plots of GP66⁺ cells from the secondary lymphoid organs of mice infected 4 d prior with Py-GP66. (D) Summary data of the percentage of CXCR5⁺ GP66⁺ cells and number of GP66⁺ cells from C. Data are pooled from six mice per cohort from two independent experiments and were analyzed by unpaired t test. For B and D, data are shown as means ± SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001. Ab, antibody; dpi, days post-infection; ns, not significant.