Background: Indoleamine 2,3-dioxygenase (IDO1), a tryptophan catabolizing enzyme, is recognized as an authentic regulator of immunity in several physiopathologic conditions. In cancer, IDO1 can either be expressed directly by the tumor cells themselves, or induced indirectly in host antigen presenting cells by the tumor and its expression has been associated with a worse clinical outcome in a variety of cancers, such as melanoma, ovarian cancer, and colorectal cancer. This consideration has driven to the definition of IDO1 as an investigational immune checkpoint target and to the development of several IDO1 inhibitors, some of which have entered clinical evaluation. Its mechanisms of action as an immune regulator, is composite and involves tryptophan deprivation and production of immunosuppressive metabolites (kynurenines). We recently demonstrated that IDO1 also acts as a signal-transducing molecule, independently of its enzymic function. In particular, in a microenvironment dominated by TGF-β, we found that IDO1 is involved in intracellular signaling events responsible for the self-amplification and maintenance of a stably regulatory phenotype in plasmacytoid dendritic cells (pDCs), a DC subset. In the literature, IDO1 has been described as a protein with a cytoplasmic localization. However, no thorough analysis of modifications of this localization in different conditions has been performed so far. Methods: We investigated the intra-cellular localization of IDO1 by means of confocal microscopy and western blot analysis of sucrose isopycnic gradient. Results: Besides confirming the main cytoplasmic localization of the protein IDO1, we detected the presence of a significant amount of IDO1 in subcellular structures, corresponding to early-endosomes, especially when it acts as a signal-transducing molecule. Conclusions: Thanks to these data, it is possible to assume the enzyme IDO1 has not exclusively a cytoplasmic localization, but there could be a balance between its localization in the cytosol and early endosomes, possibly related to two distinct IDO1-mediated (catalysis vs. signaling) tolerogenic mechanisms.
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