CD4⁺CD25⁺ T cells preferentially kill antigen-presenting B cells. (A) CD45.1 and CD45.2 B cell blasts were prepulsed with OVA (100 μM) for 2 hours or left untreated, respectively, then were equally mixed and cocultured with activated OT-II CD4⁺CD25⁺ T cells at an E/T ratio of 5:1 for 8 hours. The culture was then stained with FITC-conjugated anti–mouse CD45.1, PE-conjugated anti–mouse B220, and 7-AAD. The gate was set on B220⁺ cells. Results are expressed as means ± SD (n = 4). P indicates cells that are pulsed with ova peptide; N, cells that are not pulsed with peptide; *P < .01 between pulsed and nonpulsed CD45.1 B cells; **P < .01 between nonpulsed and pulsed CD45.2 B cells. (B) B cells from C57BL/6 or BALB/c mice were equally mixed and cocultured with OVA (10 μM) or anti-CD3 (2μg/mL) in the presence or absence of activated OT-II CD4⁺CD25⁺ T cells. The cells were then stained with FITC-anti–mouse H-2D^d, PE–anti–mouse B220, and 7-AAD and gated on B220⁺ cells. Results are expressed as means ± SD (n = 4).