In vivo CD40 stimulation promotes differentiation of memory T cells from infused effector T cells. (a) CTLs were infused into recipient mice that received anti-CD40 antibody 1 day and 2 days earlier. Recipients were infected 7 days later and also received another dose of anti-CD40 antibody 12 hours after the infection. Three weeks later, mice were rechallenged with 100,000 wild-type L. monocytogenes, and 72 hours later, splenocytes were stained for CD8α, Thy1.1, and CD62L, and with LLO_91-99 H2-K^d tetramers. Dot plots are gated on live CD8 T cells. Thy1.1 is shown on the y axis, and tetramer staining is shown on the x axis. Numbers in the upper quadrants represent percentage of total CD8 T cells. (b) Recipient mice were infected 30 minutes after CTL infusion without anti-CD40 antibody administration. These recipients were challenged 3 weeks later and analyzed as described in a. These plots are representative of three animals per group.