Protective immunity and expansion of transferred CTLs is lost 7 days after infusion. Loss of protection is prevented by anti-CD40 antibody (FGK45) treatment. (a) Recipients received 6 × 10⁶ LLO_91-99–specific CTLs and were infected with wild-type L. monocytogenes 7 days later. Seventy-two hours after infection bacteria were counted in liver and spleen. Left panel: no FGK45; right panel 100 μg FGK45 intraperitoneally 2 days and 1 day before CTL transfer and 12 hours after infection. Control animals received PBS. (b) Left panel: Forward scatter (FSC) for LLO_91-99–specific CTLs 7 days after transfer in the presence (gray histogram) or absence (black line) of FGK45. Right panel: mean FSC intensity for two to three mice per group. (c) Seventy-two hours after infection, splenocytes were stained for CD8α, Thy1.1, and with LLO_91-99 H2-K^d tetramers. Right upper quadrants show percentages of total CD8 lymphocytes. (d) Two weeks after transfer of CTLs and administration of FGK45, as described in a but without infection of the recipients, splenocytes were restimulated in vitro with LLO_91-99. Percentage of specific lysis in the presence of different concentrations of LLO_91-99 peptide was determined by standard ⁵¹Cr-release assay, using P815 (H2^d) target cells. Diamonds, plus in vivo FGK45; squares, without in vivo FGK45. Data in a are the mean and SD of two to three animals per group and are representative of two independent experiments. Dot plots in c are representative of five to six animals. Data in d are the mean and SD of two animals.