Consequences of extremely high doses of irradiation on bone marrow stromal cells and the release of hematopoietic growth factors.

Although hematopoietic growth factors have previously been difficult to demonstrate in long-term murine bone marrow cultures, it is possible to demonstrate release of growth factors from adherent cells of these long-term cultures following modest doses of irradiation. The present studies were undertaken to determine the maximally tolerated dose of irradiation for growth factor-producing stromal cells and to characterize the growth factor activities. It was discovered that stromal cells could survive extremely high doses of irradiation (500 Gy) and continue to elaborate hematopoietic growth factors. Using escalating doses of irradiation, a dose-dependent increment in detectable hematopoietic growth factors was detectable in unconcentrated conditioned medium. Conditioned medium from long-term cultures exposed to 500 Gy stimulated both fresh murine bone marrow cells (15 +/- 2 to 81 +/- 5 CFU-C/5 X 10(4) target cells) and the interleukin-3/GM-CSF-responsive cell line FDC-P1. In the CFU-C assay, this activity appeared to be predominantly monocyte/macrophage differentiating activity (M-CSF), based upon colony morphology. However, following stimulation of these irradiated stromal cells with endotoxin, there was a significant increase in FDC-P1 growth-promoting activity, and in the CFU-C assay there was increased production of granulocyte, megakaryocyte, and blast-cell type colonies indicating the increased release of a multilineage growth factor. The stromal cells surviving these extremely high doses of irradiation represent a heterogeneous population as demonstrated by morphologic, histochemical, and functional characterization. The two predominant cell populations included a macrophage-like cell and a large flat cell previously referred to as a "blanket" cell.