Radioprotective and antileukemic effects of growth factors in regenerating hematopoietic tissue.

We have examined the role of growth factors present in regenerating hematopoietic tissue in the prevention of some radiation-induced effects. These factors, extracted from calf spleen undergoing reparative regeneration, increased the 30-day survival of irradiated mice and partially decreased the incidence of leukemia in surviving mice. The growth factors modified the properties of leukemia cells in vitro, and could suppress residual leukemia cells in vivo. The antileukemic activity of regenerating hematopoietic tissue can be purified to a homogeneous state. The radioprotective activity is associated with the production of regulatory molecules that have been partially characterized. These findings provide evidence that the natural resistance of regenerating hematopoietic tissue (which has an increased number of cell targets for radiation and other damaging agents) results from concurrent local production of a battery of defensive regulatory molecules.