The immune effects of TRYCATs (tryptophan catabolites along the IDO pathway): Relevance for depression and other conditions characterized by tryptophan depletion induced by inflammation

Immune activation is accompanied by induction of indoleamine (2,3)-dioxygenase (IDO), an enzyme which degrades tryptophan, a phenomenon which plays a role in the pathophysiology of major depression and post-natal depression and anxiety states. TRYCATs - tryptophan catabolites along the IDO pathway - such as kynurenine, kynurenic acid, xanthurenic acid, and quinolinic acid, have multiple effects, e.g. apoptotic, anti- versus pro-oxidant, neurotoxic versus neuroprotective, and anxiolytic versus anxiogenic effects. The aim of the present study was to study the immune effects of the above TRYCATS. Toward this end we examined the effects of the above TRYCATs on the LPS + PHA-induced production of interferon-gamma (IFN gamma), interleukin-10 (IL-10), and tumor necrosis factor-alpha (TNF alpha) in 18 normal volunteers. We found that the production of IFN gamma was significantly decreased by all 4 catabolites. Xanthurenic acid and quinolinic acid decreased the production of IL-10. Kynurenine, kynurenic acid, and xanthurenic acid, decreased the IFN gamma/IL-10 production ratio, whereas quinolinic acid increased this ratio. Kynurenic acid significantly reduced the stimulated production of TNF alpha. It is concluded that kynurenine, kynurenic acid, and xanthurenic acid have anti-inflammatory effects trough a reduction of IFN gamma, whereas quinolinic acid has pro-inflammatory effects in particular via significant decreases in IL-10. Following inflammation-induced IDO activation, some TRYCATs, i.e. kynurenine, kynurenic acid, and xanthurenic acid, exert a negative feedback control over IFN gamma production thus downregulating the initial inflammation, whereas an excess of quinolinic acid further aggravates the initial inflammation.