Polyamine oxidation down-regulates IL-2 production by human peripheral blood mononuclear cells

Polyamine biosynthesis by mononuclear cells (MNC) appears to regulate T cell activity since 1) inhibition of polyamine biosynthesis increases IL-2 production and 2) H2O2 (a product of polyamine oxidase, PAO), suppresses lymphocyte proliferation. We investigated this immunoregulatory mechanism and find that catalase, an H2O2 inhibitor, also enhances IL-2 production by MNC. The addition of PAO, but only in the presence of either endogenous or exogenous spermidine (a polyamine), decreases IL-2 production; this effect is catalase inhibitable. Pre-incubation with spermidine, but not any of three diamines tested, suppresses PHA-stimulated IL-2 production and this effect requires monocytes. Three PAO inhibitors have an enhancing effect on IL-2 production which is again monocyte dependent. These results suggest that: 1) H2O2 produced by PHA-activated PBMC down-regulates IL-2 production; 2) products of the interaction between PAO and spermidine inhibit IL-2 production and H2O2 is essential but apparently not sufficient to mediate this effect; 3) human PBMC contain PAO activity. This polyamine-PAO-dependent inhibitory mechanism might constitute a feedback loop that limits human T cell proliferation and, consequently, also the immune responses mediated by these cells.