Stimulation of DNA Synthesis In Vitro in Mouse Thymic Cells by Bone Marrow Preparation

Some characteristics of a bone marrow factor were ascertained, on the basis of its ability to increase ³H-thymidine incorporation into thymic cells in vitro. It was found that the factor was not detectable in kidney or spleen preparations derived from the same mice from which active bone marrow preparations were obtained. The activity of the factor was reduced shortly after total-body irradiation of the mice, and increased again during the regeneration of the bone marrow. It is further shown here that the bone marrow factor has no effect on ³H-thymidine incorporation into mouse bone marrow cells. Leukemic mouse thymic cells show a significantly reduced response towards the bone marrow factor in comparison to normal thymic cells. The bone marrow factor has less effect on protein synthesis than on DNA synthesis in thymic cells, and its stimulatory effect on DNA synthesis is not dependent on protein synthesis. The isolated bone marrow factor is resistant towards heating at 100°C, even at a very low or very high pH. However, it is more labile to heat when in an impure state. Its activity is not affected by RNase, DNase, pronase and carboxypeptidase. It is insoluble in ether, but soluble in absolute ethanol.