Regulatory substances produced by lymphocytes—VII

IDS was prepared from supernatants of concanavalin A-stimulated rat LNC cultures in serumfree medium and partially purified as in earlier studies. Its action in inhibiting³H-thymidine incorporation by LNC stimulated with PHA was markedly enhanced in the presence of isobutyl-methylxanthine, a cAMP phosphodiesterase inhibitor, and diminished by imidazole. a phosphodiesterase activator, both used in physiologically active concentrations.

IDS acted on homogenates or partially purified membranes of PHA-stimulated LNC to stimulate synthesis of cAMP in the presence of ATP and a suitable regenerating system. This effect was observed with preparations from cells harvested 20 hr but not 3 hr after PHA stimulation. Added exogenous cAMP was not destroyed al an increased rate by 20-hr homogenates.

This evidence, together with the earlier demonstration that IDS acts only in G₁, that it produces a rise in cAMP only at this phase of the cell cycle in stimulated lymphocytes or other types of cells, and that IDS action is mimicked by elevation of cAMP, led to the conclusion that IDS action is mediated by activation of cell membrane adenylate cyclase and elevation of intracellular cAMP, in the absence of an effect on phosphodiesterase.