IL-10 blocks FL + IL-7–stimulated growth of CD19⁺ proB cells. An average of 22,000 sorted CD19⁺ cells (mean of four experiments) generated from Lin⁻Sca-1⁺c-kit⁺ cells in response to FL + IL-7 (as in Fig 8), were replated in FL + IL-7 or FL + IL-7 + IL-10 as indicated. After 10 days of incubation, viable cells were counted and CD19 expression determined by flow cytometry. (▪), CD19⁺ cells; (□), CD19⁻ cells. Data represent absolute cell numbers (mean ± SEM) of four separate experiments.