CELL SPECIFICITY OF CHALONE‐TYPE INHIBITORS OF DNA SYNTHESIS RELEASED BY BLOOD LEUCOCYTES AND ERYTHROCYTES

Inhibitors of DNA synthesis released into balanced salt solution by rat erythrocytes and by rat leucocytes have been found to possess target‐cell‐specific properties which would be expected of chalones. When assayed in short‐term in vitro cultures the erythrocyte product reduced DNA synthesis (as measured autoradiographically) in erythroblasts present in populations of bone‐marrow cells but did not affect the DNA synthesis in myeloid or lymphoid cells. The leucocyte product, under the same culture conditions, reduced DNA synthesis in leucocyte precursor cells. The grain counts over nuclei of different cell types were recorded as well as the DNA labelling index. Results so far obtained cannot ascribe the erythrocyte‐chalone‐produced reduction in labelling index to a blockage of entry into S phase. This cell‐specific inhibitor may reduce continuing DNA synthesis in S phase cells to undetectable levels, compared with synthesis in control media. The leucocyte product, however, most probably prevents entry of leucocyte precursor cells into S phase. Possible relevance of these inhibitors as components of physiological control mechanisms or as therapeutic agents is discussed.