The involvement of cyclic nucleotide metabolism in the initiation of lymphocyte proliferation induced by mitogens.

Murine lymphocytes incubated with specific mitogens were examined for alterations in levels of intracellular cyclic AMP and cyclic GMP. Mitogenic concentrations of LPS elevate cyclic GMP levels; however, DxSO4, Poly I:C, PPD, Con A, and PHA show little immediate effect on cyclic GMP levels. Higher concentrations of all of these mitogens consistently elevate cyclic AMP levels. Exogenous cyclic GMP stimulates DNA synthesis in homozygous nude (nu/nu) spleen cultures but not in murine thymocyte cultures. Both LPS and cyclic GMP enhance the mitogenic response of thymocytes in the presence of suboptimal concentrations of Con A. Exogenous cyclic AMP inhibits mitogenic responses to both LPS and Con A; however, these inhibitory effects are partially reversed when cyclic GMP is also added to cultures. The mitogenic response of nu/nu spleen cells to LPS can be inhibited by high concentrations of Con A. This inhibition by Con A is also partially reversed by cyclic GMP, indicating that Con A may exert its effect by elevating intracellular levels of cyclic AMP. Thus, while lymphocytee mitogens may affect intracellular levels of both cyclic AMP and cyclic GMP, three effects show a consistent pattern. 1) Exogenous cyclic GMP is either mitogenic for B cells or agents that elevate cyclic GMP levels enhance the mitogenic response of T cells to Con A. 2) Increasing cyclic AMP levels inhibit cell proliferation induced by both B and T cell mitogens. 3) The inhibitory effects of cyclic AMP or cyclic AMP-elevating agents on mitogen-induced proliferation can be partially reversed by the presence of cyclic GMP. The opposing effects of these two cyclic nucleotides may reflect either their involvement in the initiation of cell proliferation or thei modulating effects on cell cycle events after the initiation process has been completed.