I&ONS and the development of endotoxin tolerance via IDO upregulation
Chronic I&ONS can also provoke the development of endotoxin tolerance by inducing the transcriptional activation of IDO (Kim et al.
2015c; Wichers and Maes
2004) leading to upregulation of the kynurenine pathway, aryl hydrocarbon receptor (AhR) activity and increased levels of TGF-β1 (Bessede et al.
2014; Wirthgen and Hoeflich
2015) and IL-10 (Alexeev et al.
2016; Lanis et al.
2017) via well documented mechanisms (reviewed (Wirthgen and Hoeflich
2015)).
The upregulation of AhR activity is of interest given data presented in the previous section as increased activity of this cytosolic transcription factor leads to upregulation of RelB and non-canonical NF-κB signalling (Salazar et al.
2017; Vogel et al.
2013). Mechanistically, these effects appear to be mediated by transcriptional upregulation of RelB (de Souza et al.
2014;
Thatcher et al.
2007) and subsequent physical engagement between RelB and AhR to produce dimers capable of modulating the expression of NF-κB-sensitive genes (Vogel et al.
2008). AhR-upregulated RelB also stimulates and maintains the transcription of miR-146a (Zago et al.
2014,
2017).
This is of importance as miR-146a is a dominant player in the development and maintenance of the hypo-inflammatory environment characteristic of endotoxin tolerance (Banerjee et al.
2013; Nahid et al.
2009).
Mechanistically, this inhibitory effect is enabled by suppressing TLR signalling pathways by reducing the translation of
TNF receptor associated factor 6 (
TRAF6),
interleukin-1 receptor-associated kinase 1 (
IRAK1),
IRAK2 and
interferon regulatory factor 3 (
IRF3), which are positive adaptor kinases of MyD88-mediated signalling and hence their inactivation results in reduced activity of both NF-κB and IRF3 (Nahid et al.
2011) (reviewed (Testa et al.
2017)).
The upregulation of TGF-β1 also results in upregulated non-canonical NF-κB signalling (Pallotta et al.
2011; Shi and Massague
2003). Increased activation of this cytokine also upregulates pseudokinase IRAK-M (Pan et al.
2010; Srivastav et al.
2015; Standiford et al.
2011). This is significant because IRAK-M would appear to be the ‘master regulator’ of the TLR pathway suppression characteristic of the state of endotoxin tolerance in PMBCs (del Fresno et al.
2007; Escoll et al.
2003; Stiehm et al.
2013; van’t Veer et al.
2007; Wiersinga et al.
2009).
Indeed, the weight of evidence suggests that increased activity of this enzyme alone is sufficient to maintain an LPS-induced hypo-inflammatory state in human macrophages and monocytes (van’t Veer et al.
2007). This is unsurprising given that this molecule can inhibit TLR signalling at multiple levels. TGF-β1 has been established as an indispensable element in the development of endotoxin tolerance-associated SHIP upregulation (Sly et al.
2004; Yang et al.
2015).
This may be of particular relevance from the perspective of a putative explanatory model of CFS aetiology as elevated levels of this cytokine in PMBCs and whole blood are a common finding in patients diagnosed according to narrow international consensus criteria and correlate with the severity of a range of symptoms (Blundell et al.
2015; Wyller et al.
2017). Once again, it is noteworthy that this phenomenon is not observed in patients diagnosed according to broader schema which are not internationally recognised such as the ‘alternative CDC criteria’ (Clark et al.
2017).
Upregulated IL-10 also exerts negative effects on TLR signalling by increasing the ubiquination and proteasome-mediated degradation of a range of MyD88-dependent signalling effector molecules such as IRAK-4 and TRAF6 ultimately resulting in reduced phosphorylation and activity of inhibitor of kappa B kinase (IKK), p38 and JNK (Chang et al.
2009).
IL-10 is produced by monocytes, macrophages, Tregs and Th2-polarised T cells in a state of endotoxin tolerance, and suppresses the CD8 T and CD4 Th1 type cell response making an indispensable contribution to the development of an anti-inflammatory environment (Jiang and Chess
2006; Littman and Rudensky
2010).
The indispensable contribution of IL-10 to the development of endotoxin tolerance (Liu et al.
2011b; Quinn et al.
2012) is of importance from the perspective of this paper as the upregulation of this cytokine is a common observation in CFS patients (Roerink et al.
2017; Wong et al.
2015).
It should be noted that once activated, IDO activity can be maintained by two positive feedback mechanisms. First, TGF-β can target its cellular receptor leading to the upregulation of NF-κB-RelB signalling leading to further transcription of IDO (Pallotta et al. 2011; Shi and Massague 2003). Second, IDO-activated AhRs can in turn upregulate the transcription of IDO1 (the gene that encodes IDO) via genomic and non-genomic routes (Li et al. 2016b; Litzenburger et al. 2014). Hence once activated, IDO upregulation could be protracted or even chronic.
In addition, there is evidence obtained from human studies that chronic or intermittent translocation of LPS into the systemic circulation can induce a state of tolerance and alternative activation in macrophages and monocytes characteristic of endotoxin tolerance via the activation of IDO, kynurenine and the AhR (Banerjee et al.
2013; del Campo et al.
2011; del Fresno et al.
2008; Pena et al.
2011; Wisnik et al.
2017).
Given the existence of LPS translocation in CFS, this mechanism could also contribute to the development of a chronic state resembling endotoxin tolerance.
The importance of IDO activation in the development of endotoxin tolerance is further emphasised by data confirming that interactions between the AhR, kynurenine and TGF-β1 are responsible for the polarisation of activated naïve T cells into the Treg phenotype by the presentation of antigen by tolerogenic antigen-presenting cells (Gandhi et al.
2010; Mezrich et al.
2010). Such phenotypic presentations are considered below.
IDO2 is a homologue of IDO (also known as IDO1), being an immunomodulatory enzyme which catalyses L-trytophan; like
IDO1,
IDO2 is also located on chromosome 8 in humans but
IDO2 is not as widely expressed as
IDO1 and IDO2 has a distinct signalling role (Metz et al.
2007; Cha et al.
2018). B cell
IDO2 expression has recently been identified as being an essential mediator of autoreactive B and T cells in autoimmune responses (Merlo and Mandik-Nayak
2016; Merlo et al.
2016,
2017). It seems likely, therefore, that IDO2 may be found to play an important role in ME/CFS.