Protective immunity conferred by transferred CTLs correlates with their expansion 72 hours after infection. (a) We infused 6 × 106 T cells into recipients, followed by infection with wild-type L. monocytogenes or LLOSer92L. monocytogenes 30 minutes or 48 hours after transfer. Seventy-two hours after infection, splenocytes were stained for CD8α, Thy1.1, and with LLO91-99 H2-Kd tetramers. Dot plots are gated on live CD8 lymphocytes. Staining for Thy1.1 is shown on the y axis, tetramer staining on the x axis. Dot plots represent 4–6 animals per group. (b) CD8 CTLs were labeled with CFSE and transferred into recipient mice that were either infected or left uninfected. 72 hours after transfer CFSE fluorescence of Thy1.1+, LLO91-99–specific T cells was determined. (c) Absolute number of transferred T cells, determined by Thy1.1 and H2-Kd tetramers, is plotted for various conditions. Lane 1, LmSer92 infection 30 minutes after CTL transfer; lane 2, Lm infection 30 minutes after CTL transfer; lane 3, Lm infection 48 hours after CTL transfer; lane 4, Lm infection 7 days after CTL transfer; lane 5, no Lm infection, day 3 after CTL transfer; lane 6, no Lm infection, day 7 after CTL transfer; lane 7, Lm infection 7 days after CTL transfer. 5, 6, and 7 received 100 μg anti-CD40 antibody intraperitoneally 2 days and 1 day before CTL infusion and 12 hours after infection. Each point represents an individual mouse. *,†Difference in absolute numbers of transferred T cells between conditions * and † achieves statistical significance (P < 0.05 by Student’s t test).