Optimally “rested” T15 CTL cells undergo significant in vitro autonomous expansion in response to B7 costimulation by leukemia, but do not have an apparent equivalent response to costimulation in vivo. CTL line T15 was stimulated 2 weeks previously with irradiated C1498B7-2 and expanded the first week in IL-2 100 U/mL. In the second week, “resting” T15 CTL were maintained in 5 U/mL IL-2 while “stimulated” T15 CTL continued to expand in 100 U/mL of IL-2. These 2 populations were studied in vitro and in vivo on the same day. (A) “resting” versus “stimulated” T15 CTL were stimulated with paraformaldehyde fixed C1498 or C1498B7-2 in vitro in the absence of exogenous IL-2 for 48 hours, and thymidine incorporation measured the last 16 hours. As a positive control incorporation of thymidine without leukemia stimulation in the presence of IL-2 100 U/mL was measured and found to be 153,967 for “resting” T15 CTL and 185,437 for “stimulated” T15 CTL. (B) Cytotoxicity of chromium labeled C1498 by “resting” versus “stimulated” T15 CTL measured in a 4-hour chromium release assay. (C) Female B6 mice were irradiated to 500 cGy on day −1, and received C1498 or C1498B7-2 105 IV on day 0. On day +1 mice received either 5 × 106 “rested” or “stimulated” T15 CTL or no treatment. (D) Survivors from the above experiment were rechallenged on day 120 with C1498 105 IV with aged matched female B6 mice serving as controls.
