Veto activity of CD34⁺ cells.

(A) A dose-response curve testing the veto activity of CD34⁺ cells at different ratios of veto to responder cells. Equal numbers (1 × 10⁶/mL) of responder cells and irradiated allogeneic stimulator cells from the donor of the CD34⁺ cells (▨) and a third-party donor (▪) were cocultured for 5 days. Different numbers of purified CD34⁺cells were added to obtain the indicated ratios of CD34⁺ to responder cells. The responder cells were then cultured again for 7 days under limiting dilution, and the CTL activity was determined by⁵¹Cr-release assay. Two experiments were carried out. Data are from 1 representative experiment and represent the percentage of specific lysis at a cell concentration of 4 × 10⁴effector cells/well. The asterisk indicates a significant difference (P < .001 on t test compared with control cultures without CD34⁺ cells). (B) The average CTL response (±SD) in the presence (▨) and absence of CD34⁺ cells (▪) at a veto-to-responder cell ratio of 0.5. The veto effect was tested as described above, and data from 11 independent experiments using different donors were pooled. Data represent the percentage of responding cultures at a cell concentration of 1 × 10⁴cells/well. The asterisk indicates a significant difference (P < .001 on t test compared with control cultures without CD34⁺ cells).