Autonomous proliferation and expansion of CTL line T15 with B7 mediated costimulation by leukemia cells. (A) The anti-C1498 CTL line T15 was placed into microwells at 2.5 × 10⁴ cells/well and the indicated paraformaldehyde fixed stimulators were added at the indicated concentration (in triplicate). Thymidine incorporation was assessed the last 16 hours of a 48-hour incubation and expressed as CPM. Values shown are after subtraction of CPM in wells without T15 CTL added. (B) T15 CTL cells were stimulated with irradiated (10,000 rads) leukemia cells as indicated at a 20:1 responder stimulator ratio with either 5 U or 50 U of IL-2 added on day 2 of stimulation. Viable T cells were counted by trypan exclusion.