The timing of anti-CD40 antibody administration determines outcome of CTL response to in vivo infection. CTLs were infused into recipient mice and challenged with L. monocytogenes 7 days later. Bacterial counts in liver and spleen were determined 72 hours after infection. Animals received anti-CD40 antibody or an isotype control at the indicated time points. Experimental animals received 6 × 106 CTLs 7 days before infection (lanes 1–4). Control animals received PBS intraperitoneally (lanes 5 and 6). Data are representative of three animals per group. Lane 1, CD40 mAb 1 day and 2 days before CTL transfer and 12 hours after infection; lane 2, CD40 mAb 1 day and 2 days before CTL transfer; lane 3, CD40 mAb 12 hours after infection; lane 4, isotype control 1 day and 2 days before CTL transfer and 12 hours after infection; lane 5, isotype control 1 day and 2 days before CTL transfer and 12 hours after infection; lane 6, CD40 mAb 1 day and 2 days before CTL transfer and 12 hours after infection. Lower limit of detection, 50.