Inhibition of polyamine synthesis suppresses human lymphocyte proliferation without decreasing cytokine production or interleukin 2 receptor expression

Difluoromethylornithine (DFMO) irreversibly inhibits ornithine decarboxylase (ODC), a crucial enzyme in polyamine synthesis, and impairs mitogen-induced lymphocyte proliferation. To examine the mechanism of action of DFMO, we studied the effect of this ODC inhibitor on lymphokine production and interleukin 2 (IL 2) receptor expression. DFMO decreased thymidine uptake of peripheral blood mononuclear cells stimulated by the mitogens phytohemagglutinin, concanavalin A, phorbol myristate acetate and ionomycin 60–70% compared with untreated cells, and the inhibition could be completely reversed by 10 mM spermidine. DFMO had no effect on IL 1 production by monocytes exposed to silica particles. Concentrations of IL 2 increased 7-fold in DFMO-treated, PHA-stimulated PBMC cultures, compared with untreated cells; whereas IL 2 receptor expression as measured by the anti-Tac monoclonal antibody was not affected by the inhibition of ODC. Mixing experiments using cells cultured with or without DFMO indicated that the inhibition by DFMO was not mediated by suppressor cells. Our results strongly support the concept that polyamines are required for a relatively late event in lymphocyte activation occuring after the interaction of IL 2 and its receptor.