Splenic suppressing factor: purification and characterization of a factor suppressing thymidine incorporation into activated lymphocytes.

Suppressing activity upon the mitogen-activated lymphocytes was found in the supernatant (SUP) from the culture of mouse spleen, high-density subpopulation of thymocytes, and peritoneal exudate cells. Suppressing factor was obtained from the non-stimulated lymphocytes cultured for 24 to 36 hr with or without serum. Suppressing activity in the SUP was observed in the incorporation of 3H-thymidine, 3H-uridine, and 3H-leucine into Con A-activated lymphocytes or in the proliferation of L cells. Suppressing factor partially purified by Sephadex G-25 column chromatography was a heat-stable and dialyzable substance(s). Further purification and isolation of this factor by two-dimensional thin layer chromatography revealed that this was thymidine and thymidine monophosphate. The suppression in 3H-thymidine incorporation was attributed to the dilution effect of cold thymidine released from cultured lymphocytes.