Regulation of IDO Activity by Oxygen Supply: Inhibitory Effects on Antimicrobial and Immunoregulatory Functions

Abstract

Tryptophan is an essential amino acid for human beings as well as for some microorganisms. In human cells the interferon-γ (IFN-γ) inducible enzyme indoleamine 2,3-dioxygenase (IDO) reduces local tryptophan levels and is therefore able to mediate broad-spectrum effector functions: IDO activity restricts the growth of various clinically relevant pathogens such as bacteria, parasites and viruses. On the other hand, it has been observed that IDO has immunoregulatory functions as it efficiently controls the activation and survival of T-cells. Although these important effects have been analysed in much detail, they have been observed in vitro using cells cultured in the presence of 20% O2 (normoxia). Such high oxygen concentrations are not present in vivo especially within infected and inflamed tissues. We therefore analysed IDO-mediated effects under lower oxygen concentrations in vitro and observed that the function of IDO is substantially impaired in tumour cells as well as in native cells. Hypoxia led to reduced IDO expression and as a result to reduced production of kynurenine, the downstream product of tryptophan degradation. Consequently, effector functions of IDO were abrogated under hypoxic conditions: in different human cell lines such as tumour cells (glioblastoma, HeLa) but also in native cells (human foreskin fibroblasts; HFF) IDO lost the capacity to inhibit the growth of bacteria (Staphylococcus aureus), parasites (Toxoplasma gondii) or viruses (herpes simplex virus type 1). Additionally, IDO could no longer efficiently control the proliferation of T-cells that have been co-cultured with IDO expressing HFF cells in vitro. In conclusion, the potent antimicrobial as well as immunoregulatory functions of IDO were substantially impaired under hypoxic conditions that pathophysiologically occurs in vivo.

Hypoxia impairs mesenchymal stromal cell-induced macrophage M1 to M2 transition

“…As result, we evaluated the expressions of IDO, IL-10, and TNF-α in LPS-stimulated M1 macrophages cultured under normoxia (21% O 2 ) or hypoxia (1% O 2 ). The enzyme IDO is a key regulator of the innate immune system; IDO catalyzes tryptophan degradation, thus limiting the growth of bacteria, parasites and viruses 17, 27, 28 . While macrophages do not constitutively express IDO 14, 27 , IDO expression can be induced by a variety of inflammatory stimuli such as IFN-γ 14, 17, 27, 29 .…”

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

“…This inhibition was due to the depletion of L-Trp, since replication was restored upon supplementation of L-Trp to the culture medium. Likewise, L-Trp depletion due to IDO1 activity is also responsible for the IFNγ -mediated, but not IFNα-or IFNβ-mediated, suppression of HSV-1 and -2 in vitro [78,546], although the IDO1-mediated inhibition of HSV-1 infection is abrogated during hypoxia [124], which inhibits IDO1 expression and activity [124]. In vitro replication of vaccinia (used in smallpox vaccination) virus, dengue virus, measles virus and HBV (hepatitis B virus) are all limited by IFNγ , in a manner that is either wholly or partially dependent on L-Trp deprivation [535,[547][548][549].…”

“…Many of these negative regulators affect IDO1 gene transcription indirectly, e.g., via modulation of IFN production or the down-regulation of IFNγ receptors. IDO1 expression and activity is also suppressed under hypoxic conditions, with the defect in expression linked to impaired JAK-STAT signalling [122][123][124]. This has important implications for the effectiveness of IDO1's immune control and host defence roles in in vivo conditions where local tissue O 2 tension is reduced (e.g.…”

“…As a dioxygenase, IDO1 naturally requires O 2 as a substrate. Accordingly, several studies report decreased IDO1 enzyme activity in cells cultured under reduced O 2 tension or hypoxia [123,124].…”

“…For example, IFNγ and TNFα have been reported to inhibit Neospora caninum (a parasite causing abortion in livestock) in human cell lines, primary cells and bovine fibroblast-like cells [492,493], Encephalitozoon intestinalis (an opportunistic diarrhoeal parasite associated with AIDS) in mouse enterocyte cells and human epithelial colorectal adenocarcinoma cells [494], Leishmania donovani (an intracellular protozoan that causes leishmaniasis) in human macrophages [441], as well as the growth of T. gondii (which can cause severe toxoplasmosis, particularly in the immunocompromised) in primary human cells and human cell lines, primarily via IDO1-mediated L-Trp starvation [441,471,[495][496][497][498][499][500]. The effectiveness of IFNγ -induced IDO1 at inhibiting T. gondii infection in vitro is, however, impaired in cells cultured under low O 2 tension [124], i.e. physiological O 2 levels similar to that apparent in tissues that normally vary between 3 and 5 %, but can fall to <1 % in infected tissues.…”