Indoleamine 2,3 dioxygenase

2.6.1. Tryptophan degradation and Indoleamine 2,3-dioxygenase

Tryptophan (trp) is one of the seven essential amino acids necessary for the human body. It is found in relatively low amounts in the body. Unlike other amino acids, the majority of tryptophan circulates in blood mainly bound to albumin (Pardridge 1979). Tryptophan is also the precursor of N- formulkynurenine, which is converted into kynurenine (kyn) by kynurenine formamidase. N-formylkynurenine and kynurenine are the first metabolites of a complex metabolic pathway ending in quinolic acid, niacin, kynurenic and xanturenic acid (Grohmann, Fallarino & Puccetti 2003). Two enzymes, in humans, are able to catalyse conversion of tryptophan into N- formylkynurenine: tryptophan 2,3 dioxygenase (TDO) and Indoleamine 2, 3 dioxygenase (IDO) (Le Floc'h, Otten & Merlot 2011, Luukkainen, Toppila- Salmi 2013). IDO initiates the first and rate-limiting step of tryptophan breakdown along the kynurenine pathway (Grohmann, Fallarino & Puccetti 2003) IDO is expressed in both non-myeloid cells (e.g. epithelial cells, vascular endothelium, tumours) and myeloid cells (e.g. macrophages, DCs, B-cells) (Munn, Mellor 2013). IDO expression is found primarily in mucosal tissues.

The IDO pathway is induced in many tissues during inflammation majorly because IDO gene expression is induced by interferons (IFN) (Munn, Mellor 2013, Popov, Schultze 2008, Scheler et al. 2007).

2.6.2.The mechanisms of action of IDO

IDO is able to modify immune responses in several ways. Firstly, IDO produces kynurenine, which acts as a natural ligand for the aryl hydrocarbon receptor (Munn, Mellor 2013). Kynurenine-pathway metabolites act as both immunologically active ligands for the aryl hydrocarbon receptor and as excitatory neurotoxins in central nervous system inflammation (Munn, Mellor 2013). The aryl hydrocarbon receptor is a ligand-activated transcription factor that appears to be

immunosuppressive (Munn, Mellor 2013). It promotes differentiation of Tregs, suppresses anti-tumour responses and decreases the immunogenicity of DCs. The second effect of IDO is the rapid consumption of tryptophan from the local microenvironment (Munn, Mellor 2013). Tryptophan depletion can act as a potent regulatory signal via molecular stress-response pathways e.g. general control nonderepressible 2 (GCN2) kinase, which is a mammalian target of rapamycin (Munn, Mellor 2013). IDO-induced GCN2 pathway seems to enhance Treg activity, whilst inhibiting T effector cells. The shift in favour of Tregs over T cells may control excessive inflammation (Munn, Mellor 2013). Thirdly, IDO has been reported to act as a direct intracellular signalling molecule in DCs expressing it, independently of its enzymatic activity (Munn, Mellor 2013). The metabolic effects of IDO begin as local signals, as it is a non-secreted intracellular enzyme.

Neighbouring cells may sense and respond to both secreted kynurenine metabolites and reduced access to tryptophan (Munn, Mellor 2013). In addition, the signalling activity of IDO in DCs turns them into regulatory DCs (Fallarino, Grohmann 2012). DCs can present antigens in an immunogenic or tolerogenic fashion, depending on environmental factors (Grohmann et al. 2003). In certain pathways, antigen-presenting cells and Tregs are involved in an interplay which results in further up-regulation of IDO and also four other amino acid-consuming enzymes, capable of restraining T cell proliferation and promoting Treg expansion (Fallarino, Grohmann 2012, Luukkainen, Toppila-Salmi 2013).

2.6.3. IDO and diseases

2.6.3.1.The role of IDO in airway inflammation

IDO induction in eosinophils might mediate a Th2 polarization, in vitro and in vivo eosinophils displayed intracellular IDO immunoreactivity (Odemuyiwa et al. 2004).

The function of IDO in eosinophils is either stimulatory or inhibitory on Th1 and Th2 cells, depending on the inflammatory model and previous sensitization (Swanson et al. 2004, Grohmann et al. 2007, Luukkainen, Toppila-Salmi 2013).

Tryptophan and kynurenine serum concentrations have been found to be higher in seasonal AR patients, in comparison to controls and to be higher out of pollen season than during pollen season (Ciprandi et al. 2010). Moreover, aeroallergen exposure in vivo increased serum IDO activity in asymptomatic atopics compared with either symptomatic atopic or non-atopic individuals (Paveglio et al. 2011).

High baseline serum IDO activity associates with better response to immunotherapy (Kositz et al. 2008). Also, signs of decreased IDO activity have been detected after immunotherapy (Kofler et al. 2012).

In patients with CRS, Plasmacytoid- (pDCS) and myeloid DCs were found to be elevated in nasal polyp tissue, in comparison to non-inflamed nasal mucosa. In addition, pDCS were down regulated in more severe cases (e.g. CRSwNP with asthma)(Pezato et al. 2014). IDO levels were also found to be increased in NP, in

comparison to control nasal mucosa, and correlated negatively with the number of pDCS (Pezato et al. 2014).

In patients with asthma, baseline IDO activity in sputum has been shown to be significantly lower than control levels (Maneechotesuwan et al. 2008). Normal baseline activity in asthmatics could be induced by using inhaled corticosteroids (Maneechotesuwan et al. 2008). Similarly, lower IDO activity has been observed in atopics and in allergic asthmatics, in comparison to non-atopics (Maneechotesuwan et al. 2008). Pharmacologic inhibition of IDO worsened symptoms in experimental allergic asthma (van der Sluijs et al. 2013). When exposing in vitro monocyte- derived DCs with house dust mite Dermatophagoides pteronyssinus 1, functionally active IDO decreased in cells from patients with house dust mite-sensitive asthma compared to non-atopic asthmatics(Xu et al. 2008b). In murine asthma models, IDO inhibits eosinophilic inflammation, immature DCS expressing IDO improved lung inflammation, eosinophil numbers and lowered cytokine levels (Loughman, Hunstad 2012). Increased CD4+ T cell apoptosis was observed in mice receiving IDO-expressing immature DCs, in comparison to the control groups (Maneechotesuwan et al. 2009).

2.6.3.2. The role of IDO in other diseases

IDO1, IDO2 and TDO have all been shown to associate with cancer phenotype or with inferior prognosis, in several malignancies, such as gastric, colon, renal, breast, and ovarian cancer, acute myeloid leukaemia, malignant gliomas, and melanoma) (Lob et al. 2009, Adams et al. 2012, Ott et al. 2014, Gerlini et al. 2010, Munn et al. 2004, Ryan et al. 2014, Mansfield et al. 200, Theate et al. 2014, Goyne, Cannon 2013, Fukuno et al. 2014, Chen et al. 2014).

Sioud et al gave an IDO-silenced DC vaccine to four patients with gynaecological cancers. IDO silencing enhanced the immunogenic function of DCS, both in vitro and in vivo, and this was related to objective clinical response (Sioud et al. 2013).

IDO can have opposing roles in host defence against infection. IDO can play a dominant role in directly suppressing pathogen replication (e.g. toxoplasmosis, chlamydial infections, Mycobacteria tuberculosis) (Pfefferkorn 1984, Carlin, Borden & Byrne 1989) or by limiting the spread of virus infection (e.g. human herpes simplex virus type 2, human cytomegalovirus) (Adams et al. 2004, Bodaghi et al. 1999, Obojes et al. 2005, Terajima, Leporati 2005, Schmidt, Schultze 2014).

However, disadvantageous effects of IDO have also been reported in viral infections (hepatitis B and C virus, human immunodeficiency virus 1) (Hoshi et al.

2012, Planes, Bahraoui 2013, Boasso et al. 2009, Schmidt, Schultze 2014).

Escherichia coli has been found to induce IDO expression in host cells, as a pathogen strategy to promote colonization and establishment of infection (Loughman, Hunstad 2012).