TLR9-stimulated OT-2 CD4 T lymphocytes respond to antigenic stimulation after total body irradiation. (A) A schematic representation of the protocol used for the experiments described here. Purified OT-2 CD4 T cells (10⁶) were labeled with CFSE and intravenously injected into MyD88^–/– mice or WT mice (n = 3). Twenty-four hours after adoptive T-cell transfer mice were vaccinated via intraperitoneal injection with ovalbumin protein (500 μg), α-CD40 antibody (50 μg) plus or minus TLR9 ligand (50 μg). One day after vaccination, mice were irradiated (TBI; 300 cGy) or not. OT-2 T-cell proliferation was evaluated in WT (B) and MyD88^–/– (C) mice using flow cytometry 10 days after vaccination. Each CFSE peak, after the first peak, represents one cycle of cell division. The average number of OT-2 T cells recovered from the spleens of MyD88 or WT mice is shown on the dot plot. The Student t test was used to determine P values between the numbers of OT-2 cells in irradiated, TLR9 ligand–treated and irradiated, non-TLR9 ligand–treated mice; *P < .05, **P < .005.