Indoleamine 2,3-dioxygenase (IDO) catabolizes tryptophan to kynurenine. In the immune system, the reduction in tryptophan and increase in kynurenine act to suppress T-cell function. In the nervous system, kynurenine can be further metabolized to quinolinic acid, which can be neurotoxic. IDO is known to be expressed by microglia and its levels are upregulated by interferon-γ (IFNγ). We report here that IDO immunoreactivity is also localized in neurons, and that IDO is upregulated by IFNγ in neurons of the hippocampus. Thus, neuronal IDO could contribute to the vulnerability of neurons to inflammatory conditions.
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