Effect of radiation and 4-hydroperoxycyclophosphamide on thymic regulators of erythropoietic growth

In vitro growth of erythroid progenitors (BFU-e and CFU-e) from mouse bone marrow (BM) can be influenced by the addition of graded doses of thymocytes (THY). At low THY:BM ratios (1:50), inhibition of erythroid growth occurred, whereas at high THY:BM ratios (20:1), enhancement of erythroid growth was observed. Experiments were designed to examine the sensitivity of the thymocytes to gamma radiation or treatment in vitro with an alkylating agent, 4-hydroperoxycyclophosphamide (4HC), to determine if two subpopulations with different physical characteristics were responsible for the observations. We reasoned that if there were selective effects of either radiation or drug treatment on enhancement or inhibition, this would provide evidence for separate populations which regulated erythroid cell growth by cell-cell interaction. At the small doses of drug or X-ray studies (i.e., 10 μM 4 HC or 3.0 Gys), elimination of the inhibitory effect of low THY:BM ratios resulted. Enhancement of erythroid growth was unaffected by drug or radiation until 80 μM 4HC or 30.0 Gys were employed. Fresh putative inhibitory thymocytes (low numbers) added to their 4HC or radiation-treated inhibitory cells restored the suppressor effect on erythroid growth. The helper effect, however, was not restored by the addition of fresh thymocytes. We conclude that there are at least two populations which provide an inhibitory influence than is relatively sensitive to radiation and cyclophosphamide in vitro, and an enhancing effect which is relatively insensitive to these treatments.