Biologic effects of in vitro X-irradiation of murine long-term bone marrow cultures on the production of granulocyte-macrophage colony-stimulating factors

The production of colony-stimulating factor (CSF) by murine long-term bone marrow culture (LTBMC) was studied by a technique involving measurement of colony formation in agar overlay by fresh marrow target cells. Colonies were removed and microscopically examined for morphology and histochemistry. LTBMCs were exposed to x-irradiation at 200 rad/min prior to the overlay. Non-irradiated control LTBMCs induced 51.5 ± 11 granulocyte-macrophage colonies per 2 x 10⁵ target cells. Irradiation of LTBMCs to 6000 rad revealed a six-fold plateau-maximum increase in the number of colonies. There was occasional appearance of macroscopic mixed colonies containing granulocytes, macrophages, and megakaryocytes over irradiated but not control LTBMCs. Irradiated cells in the adherent stromal layer of LTBMCs continuously produced CSF that was detectable in the cell-free supernatants for up to seven weeks after irradiation and after doses as high as 10,000 rad. Shielding of the x-ray beam over half of the culture surface by a 10-half-value-layer lead block produced increased colony formation by target cells near the exposed surface area. The data indicate that CSF production by adherent cells within LTBMC persists after supralethal doses of x-irradiation. The mechanism of the increased colony formation by target cells overlaid on irradiated stromal cells involves factors relative to the local microenvironment.