(A) Expression of B7-1 and B7-2 molecules by CHO-transfectants, professional APCs, and pre-B ALL cells (unstimulated and CD40-stimulated). The histograms shown for DCs and macrophages are from a single donor and are representative of five donors. Open areas represent fluorescence distribution of the B7-1 or B7-2 molecules and solid areas represent that of isotype-matched control antibodies. The cell number is shown on the Y-axis.

(B) Proliferative responses of purified allogeneic CD3⁺ T cells to irradiated pre-B ALL cells in the presence or absence of direct or bystander costimulation. Proliferation was assessed by measuring [³H] thymidine incorporation for the last 18 hours of the 6-day assay. The SI were calculated for each individual experiment as follows: SI = cpm_{(T cells + ALL cells)}/cpm_{(T cells)} . Bars represent mean ± SEM of the SI from the MLRs using 4 different patients and 5 different allogeneic T-cell donors. Allogeneic DCs and monocytes were syngeneic with the T cells. Control-Ig (C-Ig) and CTLA4-Ig fusion proteins were used at 10 μg/mL. Anti-CD28 MoAb (CD28) was used at 2.5 μg/mL. (C) Secondary proliferative responses of T cells primed by pre-B ALL cells in the presence of bystander costimulation. Proliferation was assessed by measuring [³H] thymidine incorporation for the last 18 hours of the 6-day assay. Bars represent mean ± SEM of the SI.