Human Indoleamine 2,3-Dioxygenase 1 Is an Efficient Mammalian Nitrite Reductase.

The heme enzyme indoleamine 2,3-dioxygenase-1 (IDO1) catalyzes the first reaction of l-tryptophan oxidation along the kynurenine pathway. IDO1 is a central immunoregulatory enzyme with important implications for inflammation, infectious disease, autoimmune disorders, and cancer. Here we demonstrate that IDO1 is a mammalian nitrite reductase capable of chemically reducing nitrite to nitric oxide (NO) under hypoxia. Ultraviolet-visible absorption and resonance Raman spectroscopy showed that incubation of dithionite-reduced, ferrous-IDO1 protein (FeII-IDO1) with nitrite under anaerobic conditions resulted in the time-dependent formation of an FeII-nitrosyl IDO1 species, which was inhibited by substrate l-tryptophan, dependent on the concentration of nitrite or IDO1, and independent of the concentration of the reductant, dithionite. The bimolecular rate constant for IDO1 nitrite reductase activity was determined as 5.4 M-1 s-1 (pH 7.4, 23 °C), which was comparable to that measured for myoglobin (3.6 M-1 s-1; pH 7.4, 23 °C), an efficient and biologically important mammalian heme-based nitrite reductase. IDO1 nitrite reductase activity was pH-dependent but differed with myoglobin in that it showed a reduced proton dependency at pH >7. Electron paramagnetic resonance studies measuring NO production showed that the conventional IDO1 dioxygenase reducing cofactors, ascorbate and methylene blue, enhanced IDO1's nitrite reductase activity and the time- and IDO1 concentration-dependent release of NO in a manner inhibited by l-tryptophan or the IDO inhibitor 1-methyl-l-tryptophan. These data identify IDO1 as an efficient mammalian nitrite reductase that is capable of generating NO under anaerobic conditions. IDO1's nitrite reductase activity may have important implications for the enzyme's biological actions when expressed within hypoxic tissues.

Identification of Native and Posttranslationally Modified HLA-B*57:01-Restricted HIV Envelope Derived Epitopes Using Immunoproteomics.

The recognition of pathogen-derived peptides by T lymphocytes is the cornerstone of adaptive immunity, whereby intracellular antigens are degraded in the cytosol and short peptides assemble with class I human leukocyte antigen (HLA) molecules in the ER. These peptide-HLA complexes egress to the cell surface and are scrutinized by cytotoxic CD8+ T-cells leading to the eradication of the infected cell. Here, naturally presented HLA-B*57:01 bound peptides derived from the envelope protein of the human immunodeficiency virus (HIVenv) are identified. HIVenv peptides are present at a very small percentage of the overall HLA-B*57:01 peptidome (<0.1%) and both native and posttranslationally modified forms of two distinct HIV peptides are identified. Notably, a peptide bearing a natively encoded C-terminal tryptophan residue is also present in a modified form containing a kynurenine residue. Kynurenine is a major product of tryptophan catabolism and is abundant during inflammation and infection. Binding of these peptides at a molecular level and their immunogenicity in preliminary functional studies are examined. Modest immune responses are observed to the modified HIVenv peptide, highlighting a potential role for kynurenine-modified peptides in the immune response to HIV and other viral infections.

Role of indoleamine 2,3-dioxygenase in health and disease.

IDO1 (indoleamine 2,3-dioxygenase 1) is a member of a unique class of mammalian haem dioxygenases that catalyse the oxidative catabolism of the least-abundant essential amino acid, L-Trp (L-tryptophan), along the kynurenine pathway. Significant increases in knowledge have been recently gained with respect to understanding the fundamental biochemistry of IDO1 including its catalytic reaction mechanism, the scope of enzyme reactions it catalyses, the biochemical mechanisms controlling IDO1 expression and enzyme activity, and the discovery of enzyme inhibitors. Major advances in understanding the roles of IDO1 in physiology and disease have also been realised. IDO1 is recognised as a prominent immune regulatory enzyme capable of modulating immune cell activation status and phenotype via several molecular mechanisms including enzyme-dependent deprivation of L-Trp and its conversion into the aryl hydrocarbon receptor ligand kynurenine and other bioactive kynurenine pathway metabolites, or non-enzymatic cell signalling actions involving tyrosine phosphorylation of IDO1. Through these different modes of biochemical signalling, IDO1 regulates certain physiological functions (e.g. pregnancy) and modulates the pathogenesis and severity of diverse conditions including chronic inflammation, infectious disease, allergic and autoimmune disorders, transplantation, neuropathology and cancer. In the present review, we detail the current understanding of IDO1's catalytic actions and the biochemical mechanisms regulating IDO1 expression and activity. We also discuss the biological functions of IDO1 with a focus on the enzyme's immune-modulatory function, its medical implications in diverse pathological settings and its utility as a therapeutic target.

Flavivirus infection induces indoleamine 2,3-dioxygenase in human monocyte-derived macrophages via tumor necrosis factor and NF-κB.

Infection with West Nile virus (WNV) via a mosquito bite results in local viral replication in the skin, followed by viremia. Thus, tissue macrophages are ideally located to prevent the dissemination of WNV throughout the host. The current study shows that WNV infection of human monocyte-derived macrophages (MDM) results in increased WNV mRNA, protein, and infectious virions at 24 h p.i. with a decline in titer after 48 h. Concomitant with viral control was the robust induction of indoleamine 2,3-dioxygenase (IDO) and resultant metabolism of L-tryptophan (L-Trp) to kynurenine. In WNV-exposed cultures, IDO protein was induced primarily in noninfected versus viral-infected MDM. Whereas WNV infection increased the production of IFN-α, IFN-ß, and TNF, only antibody neutralization of TNF attenuated IDO expression and activity. WNV infection also activated NF-κB, and inhibition of this pathway with BMS-345541 abrogated IDO induction. Similar results were also obtained with MDM infected with the related flavivirus, Japanese encephalitis virus. Whereas IDO-mediated L-Trp metabolism can exhibit antiviral properties, inhibition of IDO activity in MDM with L-1-MT or the addition of excess L-Trp did not affect viral control. However, culturing MDM in L-Trp-deficient medium or overexpression of IDO in cells prior to infection significantly attenuated WNV replication, which was reversed by adding excess L-Trp. Together, these data support that although IDO is not required by MDM for the clearance of established viral infection, the spread of flavivirus infection is limited by IDO expressed in uninfected, neighboring cells.