Synthesis and release of thymidine by macrophages.

An inhibitor of DNA synthesis and cell proliferation found in macrophage supernatants was subjected to further analysis. The following conclusions were drawn: a. Biochemically, the inhibitor appeared to be thymidine, judging from the migration in three different thin-layer systems and on gel filtration. b. The inhibitor was shown to be synthesized de novo by macrophage monolayers pulsed with a radiolabeled precursor of thymidine such as 14C-formate. c. The amount of inhibitor present in tissue culture fluids proved to be sufficient to block the growth of the EL-4 leukemia cell line in a manner that could be prevented with the addition of 2'-deoxycytidine. The EL-4 line was considerably more sensitive to thymidine blockade than a number of different cells tested.