Thymidine-dependent effect of granulocyte-derived inhibitor on granulocyte-macrophage progenitor cells (GM-CFC)

We have compared the effect of granulocyte extract (GRE) on proliferation of hemopoietic cells from various sources, using two different cultures media, CMRL 1066 and McCoy 5A. GRE caused a strong (80-90%) inhibition of granulocyte-macrophage colony forming cells (GM-CFC) in cultures with medium CMRL 1066. GM-CFC in human blood and human and mouse bone marrow were equally sensitive to the inhibitor. The inhibitor had a maximal effect in the concentration range corresponding to GRE from 2 x 10(5) to 2 x 10(6) cells per 1 ml culture dish. At higher GRE concentration the inhibition was reduced. GM-CFC from human blood and mouse marrow were suppressed in cultures with McCoy's medium as well, but to a lesser extent than in CMRL 1066 cultures. On the other hand, in cultures with human bone marrow cells (BMC) and McCoy's medium, GRE had no inhibitory effect. CMRL 1066 medium contains a number of components not present in McCoy's medium. In a systematic study where these substances were added one by one to McCoy's medium we found that inhibition by GRE depended upon the presence of thymidine. At a thymidine concentration of 3 x 10(-5) mol/l GRE strongly suppressed GM-CFC in human blood and bone marrow. This thymidine concentration itself had no effect. Other nucleosides or components of the CMRL 1066 did not potentiate the suppressive effect of GRE.