Development of HEL-specific EFPBs cells is greatly impaired in the absence of T cell–expressed Bcl-6. (A–D) Representative flow cytometric contour plots (A and C) and quantification (B and D) of SW_HEL GC B cells identified as B220⁺ GL-7⁺ Fas⁺ HEL-binding CD45.1 cells (A and B) and EFPBs identified as CD45.1⁺ B220^lo intracellular HEL-binding cells (C and D) in CD45.2 chimeric mice sufficient (Tc Bcl6^+/+) or deficient (Tc Bcl6^−/−) in Bcl-6–expressing T cells 4.5 d after adoptive transfer of SW_HEL B cells and HEL-SRBC immunization. Control (HEL⁻) mice received SW_HEL B cells and were immunized with SRBCs that had not been conjugated to HEL. (E) HEL-specific antibody titers in the same HEL-SRBC–immunized chimeric mice. These data are representative of two independent experiments with five mice per group immunized with HEL-SRBC and two mice per group in the HEL-only control group for each independent experiment.