PT-100 stimulates the survival and growth of primitive hematopoietic progenitors in vitro and induces the production of cytokines by BM stromal cells. (A) CFU levels after 4-week bulk culture of human CD34⁺ cells in the presence of varying concentrations of PT-100, as indicated on abscissa. CD34⁺ cells were propagated in vitro on supportive layers of X-irradiated stromal cells, and CFUs were assayed as described in “Materials and methods.” Data are expressed as mean ± SD from triplicate cultures and are representative of 6 independent experiments. (B) Clonal frequency of LTC-ICs in human CD34⁺ cell population cultured in the presence (•) or absence (○) of 10 nM PT-100. Limiting-dilution assay of LTC-ICs on supportive layers of stromal cells was performed as described in “Materials and methods.” Data are representative of 2 assays. (C-D) Changes in G-CSF (○,•), IL-11 (▵,▴), and IL-6 (□,▪) levels in supernatants of BM stromal cells cultured either with (filled symbols) or without (open symbols) 1 μM PT-100. Data are expressed as means from triplicate cultures and are representative of 5 replicate experiments. (E) Comparison of gene expression in BM stromal cells incubated for 6 hours in the presence (▪) or absence (□) of 1 μM PT-100. Cytokine gene expression was determined by microarray hybridization, as described in “Materials and methods,” and data were replicated in 2 independent experiments.