Dose-dependence of the augmentation of hemopoiesis by thymocytes

We have used the spleen colony assay and the 24-h uptake of 59Fe by hemopoietic tissues to study certain quantitative aspects of the interaction of parent-strain thymocytes and marrow cells transplanted into lethally irradiated F1 hybrid mice. The data show that thymocytes augment bone marrow growth at least partly by increasing the proportion of stem cells from parental marrow that from colonies in the hybrid animal. This increase in spleen colony number is a linear function of the number of thymocytes injected within certain dose ranges. At higher thymocytes doses, a plateau is reached where further increase in the number of thymocytes injected does not result in an increase in the number of spleen colonies; this plateau occurs at just that level of response given by the same marrow dose when transplanted into isogeneic (parent-strain) recipients. Experiments in which 59Fe uptake up red blood cells and spleen was used to measure marrow growth yielded similar results, except that the plateau in uptake occurred at a much lower level than that measured for the same marrow dose in isogeneic recipients. This apparent discrepancy in results obtained from the two different assays may best be explained by the recent observation that thymocytes shift the differentiation pattern of marrow stem cells away from erythropoiesis and toward granulopoiesis. The absolute thymocyte dose seems to be more important than the thymocyte: marrow-cell ratio in determining the degree of augmentation observed.