CTLA4-Ig prevents CD34⁺ cell proliferation upon stimulation with allogeneic T cells. (A) Proliferation of purified CD34⁺ cells was assessed after 6 days in culture with medium alone or with irradiated allogeneic T cells (irrad T-cells) and CTLA4-IgG4m (CTLA4-Ig), with or without the addition of exogenous cytokines (GM-CSF/TNFα). CTLA4-Ig significantly inhibited CD34⁺ cell proliferation (P = .05). Results are represented as the mean cpm ± SD of 3 separate experiments. (B) The number of CFU-Cs and CFU-GMs generated from CD34⁺ cells in clonogenic assays was not affected by addition of CTLA4-Ig. The results are represented as mean colonies ± SD in 3 separate experiments. ^*P = .05.