FIGURE 1.

The regulatory activity of CD34 cells: evidence for target specificity. The average CTL response (SD) in the presence (black bars) or absence (white bars) of CD34⁺ cells at a veto-to-responder cell ratio of 0.5. The veto effect was tested by a limiting dilution assay as follows: 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 co-cultured for 5 days. The responder cells were then cultured again for 7 days under limiting dilution, and the CTL activity was determined by ⁵¹Cr-release assay. Data represent the average ± SD of 11 independent experiments using different donor and third-party pairs. A significant difference (p < 0.001 on t-test compared with control cultures without CD34 cells) between control cultures and those including CD34 cells was found upon stimulation against donor cells (Rachamim et al., 1998).