CD40 and infection: To determine whether differences in costimulatory expression may be contributing to the impaired proliferation of adult OT-I cells in aged recipient mice (Figs. 1 and 2), the expression levels of MHC-II, CD40, and CD86 were measured on splenic DC subsets prior to and on days 1, 3, and 5 following infection with Lm-OVA.