Recognition of CD34⁺ cells by HLA-DP–specific T-cell clones. Clones 501 (A,C) and 538 (B,D) were incubated at different effector-target cell ratios with purified CD34⁺ cells obtained from 2 healthy adult donors expressing HLA-DPB1*0901,*1001 (•), or *0401 (○). Lysis of CD34⁺ target cells was measured in a standard 4-hour cytotoxicity assay (A-B). Inhibition of the colony-forming capacity of CD34⁺ cells was analyzed in a clonogenic assay (C-D).