Extracellular spermidine attenuates tryptophan breakdown in mitogen-stimulated peripheral human mononuclear blood cells

Polyamines, particularly spermidine, play a vital role in regulating cellular functions and influencing processes such as proliferation, ageing, and immune modulation. This study explores the effects of spermidine on the immunoregulatory pathway of tryptophan breakdown, which is mediated by indoleamine 2,3-dioxygenase 1 (IDO-1), as well as on neopterin synthesis, in peripheral blood mononuclear cells. Spermidine treatment was found to suppress IDO-1 activity in mitogen-stimulated cells in a dose-dependent manner without affecting neopterin synthesis. This suppressive effect on tryptophan breakdown contributes to the debate surrounding the health-promoting antioxidant properties of spermidine. However, the divergent regulation of the two immunobiochemical pathways investigated, which share common upstream signalling events, requires further exploration.