Control of mitogen-induced lymphocyte activation

Exposure of human peripheral blood lymphocytes to increased doses of PHA (0–2000 μg/ml PHA) leads to a decrease in thymidine incorporation. The addition of exogenous cyclic AMP (10 −3 –10 −4 M ) to lymphocyte cultures exposed to such high levels of PHA results in a recovery of the ability to incorporate both thymidine and uridine. Using this system as a model for the study of cyclic nucleotide metabolism, experiments have demonstrated a differential sensitivity to changes in cyclic nucleotide levels among lymphocyte subpopulations exposed to PHA, concanavalin A, or pokeweed mitogen. Further evidence for differential sensitivity is also obtained using lymphocyte populations separated on the basis of rosette formation. Measurement of intracellular cAMP and cGMP levels in lymphocyte cultures exposed to PHA, concanavalin A, or pokeweed mitogen suggests that while cGMP is the mitogenic signal, cAMP may control deactivation, thereby exerting an overide effect on cGMP mediated events. In addition, metabolic studies suggest that exogenous cAMP may influence PHA-induced transformation up to 67 hr after culture initiation and that the effects of Ca 2+ ions and cyclic nucleotides are interrelated in the activation of lymphocytes by PHA.