Reduced absolute cell counts after culture of thymic precursors on OP9-DL1 in the presence of IFN-α is due to reduced IL-7–mediated proliferation but not to increased apoptosis. CD34⁺CD1a^– thymic precursors were cultured on OP9-DL1 with IL-7 and different concentrations of exogenously added IFN-α. (A) Absolute cell numbers were determined after 1 week of coculture by counting live cells using trypan blue exclusion. Shown are the fold expansions in cell number compared with the number of cells at the start of the culture of 8 independent experiments. Average values are indicated by a black bar; statistical analysis was performed using a paired 2-tailed Student t test; *P < .05, **P < .001. (B) Three days after culture without or with IFN-α (1000 U/mL), flow cytometric analysis was performed after intracellular staining with anti–Bcl-2 antibody (open curve). Shaded curves represent isotype controls. Numbers indicate MFI of the specific stainings. (C) Directly after sorting, CD34⁺CD1a^– thymic precursors were labeled with CFSE, followed by coculture on OP9-DL1 cells with IL-7 in the presence or absence of different concentrations of IFN-α. On day 6 of coculture, CFSE levels were determined by flow cytometric analysis. (D) Thymic progenitor cells were labeled with CFSE and cultured in cell suspension with recombinant IL-7 (1 ng/mL) in the presence or absence of 100 U/mL IFN-α. After 96 (top panel) and 120 hours (bottom panel), CFSE levels were determined by flow cytometry. Results are representative of 5 independent experiments.