The veto activity of CD34⁺ cells: inhibition of antidonor response requires addition of CD34⁺ cells in the initial 24 hours of culture.

Responder cells and irradiated allogeneic stimulator cells were cocultured for 5 days with (▪) or without (▨) addition of CD34⁺ cells. CD34⁺ cells were added at different time intervals. At the end of the culture period, the responders were cultured again in limiting-dilution cultures for 7 days and the CTL activity was determined.