Human β-selection and Notch signaling requirements. (A) Flow cytometric analysis of human CD4⁺CD3⁻CD8α⁻CD28⁻ (CD4⁺CD28⁻) and CD4⁺CD3⁻CD8α⁻CD28⁺ (CD4⁺CD28⁺) postnatal thymocytes after 6 days of coculture on OP9-GFP or OP9-DL1 stromal cells. Numbers in quadrants indicate the percentage of the corresponding population. Results shown are representative for 5 independent experiments. (B) Flow cytometric analysis of CFSE labeled CD4⁺CD28⁻ and CD4⁺CD28⁺ thymocytes after OP9-DL1 (solid line) and OP9-GFP (dotted line) stromal coculture shows Notch-DL1 dependent optimal proliferation of both populations. (C) Corresponding total, CD3⁺TCR-αβ⁺, and CD3⁺TCR-γδ⁺ cellularity for cultures depicted in panel A. Data represent mean (± SEM; n = 5). * indicates significant differences (P < .05).