TLR9 engagement on CD4 T lymphocytes in vivo show greater radioresistance than non-TLR–stimulated CD4 T lymphoyctes. (A) A schematic representation of the protocol used for the experiments described here. CD4 T cells were purified from WT CD45.1 and MyD88^–/– CD45.2 BL/6 mice. Cells were activated with anti-CD3/CD28 Ab for 48 hours, pooled at a 1:1 ratio, labeled with CFSE, and intravenously injected into MyD88^–/– mice (n = 5) followed by 3 intraperitoneal injections of TLR9 ligand (50 μg/injection) on days 1, 3, and 5. Mice were irradiated on day 6 (300 cGy). (B) The number of WT and MyD88^–/– CD4 T lymphocytes recovered from the spleen and lymph nodes of IR and non-IR animals was determined using flow cytomety. The expression of the congenic markers CD45.1 and CD45.2 were used to identify transferred cells. The endogenous population of MyD88^–/– T cells was distinguished from transferred MyD88^–/– T cells based on their labeling with CFSE. The Student t test was used to determine P values between the indicated groups; *P < .05; ns indicates not significant.