(A) Scheme of the coculture experiments. (B and C) CFU-C activity (B) and endogenous GM-CSF expression (C) of BM-derived naive LSK cells after coculture with CD45^– stromal cells from the indicated tissues. ***P < 0.001, by 2-way ANOVA followed by Dunnett’s test. (D) Cartoon depicting the adoptive transfer of CD45.1^– BM-derived naive LSK cells into the spleens of CD45.1⁺ tumor-free or tumor-bearing recipient mice. (E) Suppressive activity of donor-derived CD11b⁺Gr-1⁺ myeloid cells retrieved from the spleens of the recipient mice described in D. ***P < 0.001, by 2-way ANOVA with Bonferroni’s correction. (F and G) CFU-C activity (F) and endogenous GM-CSF expression (G) of BM-derived naive LSK cells after coculture in Transwells with splenic stromal cells from Hepa mice with the indicated Abs (1 μg/ml) in the cultures. ***P < 0.001, by 2-way ANOVA with Bonferroni’s correction. (H) After 4 days of coculture in Transwells as described in F and G, Lin^– HSPCs were isolated and transferred into serum-free medium supplemented with SCF only. After another 3 days of culture, the Gr-1⁺ myeloid descendants were isolated and tested for their suppressive activity. CFSE⁺ splenocyte proliferation when cocultured with myeloid descendant cells at the indicated ratios (solid lines) or cultured alone (shaded) is shown (E and H). Numbers in the flow cytometric plots indicate the proportions of gated cells (C, E, G, and H). Data are shown as the mean ± SEM of 3 experiments, with cells pooled from 6 to 8 mice (B, E, and F) or are representative of 3 experiments, with cells pooled from 6 to 8 mice (C, G, and H).