The human organism is constantly exposed to numerous immunological stimuli, to which it must respond with appropriate feedback. For a successful defence a balance must be achieved between activity and overreaction of the immune system.

An underactivity of the immune system allows pathogens that have entered or malignant cells that have formed, to stay. Overreaction of the immune system leads to autoimmune reactions.

The products of kynurenine metabolism are important regulators of the immune balance. The main enzyme in kynurenine metabolism, indolamin 2,3-dioxigenase (IDO), and its main agent kynurenine, are discussed in this review article.

* Refer to Portable Document Format for figures and tables


Biochemical Basis

Tryptophan metabolism along with the kynurenine pathway play an essential role in the regulation of the immune system. Approximately 95% of absorbed tryptophan is degraded via this pathway (Beadle, et al. 1947).

The rate-limiting enzymes of the kynurenine pathway are indolamin-2,3-dioxygenases, including isoforms 1 and 2, and tryptophan 2,3-dioxigenase. The enzymes catalyze the degradation of tryptophan to unstable n-formyl kynurenine, finally resulting in kynurenine.

IDO 1, important for the prevention of immune overreaction, is present in all tissues and immune cells which come into contact with the outside world and plays an important role in the control of immunity, mainly in immune cells such as macrophages as well as dendritic cells. Formation of IDO 1 is induced by several factors, such as lipopolysaccharides (Fujigaki, et al. 2001), inflammatory cytokines INF-alpha (Hassanain, et al. 1993), INF-gamma (Taylor, et al. 1991) or tumor necrosis factors (TNF alpha) (Babcock, et al. 2000) with the participation of the corresponding receptors. However, it can also be induced via the so called aryl hydrocarbon receptor (AHR) through toxic substances (Busbee, et al. 2013) or in a positive feedback via products of the kynurenine pathway (Orabona & Pallotta, 2012).

The influence of kynurenine on cytotoxic T cells or killer cells is noteworthy.

(I) The tryptophan depletion resulting from IDO activation in immune cells via the stress response kinase general control nonderepressible 2 kinase (GCN2) induces a G1 T cell arrest with decreased proliferation or apoptosis of killer cells (Munn, et al. 2005; Fallarino, et al. 2006; Manlapat, et al. 2007).

(II) The vitality of toxic T cells, regulatory T cells (Tregs), and antigen presenting cells (APCs) of the adaptive immune system are influenced. A high activity of IDO and an accompanying high kynurenine level shift the immune balance toward more Tregs and less cytotoxic T cells and, therefore, in the direction of immune tolerance (Puccetti, et al. 2007; Jasperson, et al. 2008). It should be noted that the main product of the IDO pathway, kynurenine, (i) is a ligand of the aryl hydrocarbon receptor, which modulates the immune response (Opitz, et al. 2011) and (ii) stimulates the production of IDO (Vogel, et al. 2008).

This can then lead to a downright vicious cycle following the induction of IDO 1:

(i) Various inductors activate IDO 1 in immune cells (dendritic cells and macrophages).

(ii) Immune cells steadily form kynurenine which supports its own production through positive feedback.

(iii) The effect of kynurenine on AHR results in an imbalance between Tregs and cytotoxic T cells.

This results in an imbalance of the immune system characterized by a high level of kynurenine at low levels of tryptophan and is self-perpetuating with a negative effect on health. In the following, we would like to underscore the clinical meaning of this imbalance using two clinical pictures.


Immune Tolerance and Infections

Viruses, bacteria, and fungi can have the unpleasant characteristic of being able to be dormant in the target organism over a longer period (months to years) without being significantly attacked by the immune system. This is called latent infection. The disease is not cured completely.

Kynurenine plays a deciding role here. In the normal run of an infection the pro-inflammatory path of the immune system is initially activated. To prevent an overshooting immune response, an immune brake is activated by simultaneously activating the kynurenine pathway. This then leads to a fine-tuning of the pro- and anti-inflammatory factors as with kynurenine and a balancing of the immune system.

Following the infection, pro- and anti-inflammatory factors (immune block) return to original levels. If the immune block is maintained this is not the case. The activity of IDO and the level of kynurenine remain high. (Figure 5) The immune block remains active. However, if the activity of IDO remains high, infections cannot be combated effectively.

Already in 1991 the significance of the IDO pathway in HIV was discussed. IDO activity and the level of kynurenine increase as a consequence of HIV infection (Fuchs, et al. 1991; Huengsberg, et al. 1998; Werner, et al. 1998). At the same time it was proven that the number of cytotoxic T cells is reduced in patients with high levels of kynurenine. This worsens the chance of survival for patients (Bipath, et al. 2015).

Analogous to this, it was recently observed that in other viral infections, such as cytomegaly, Epstein-Barr, HBV and HBC or influenza, a suppression of the immune system is mediated via IDO activity. Figure 6 shows an example of the inhibition of CMV- specific cytotoxic T-cells (so called CD8+ T-cells) via IDO activation (Hong, et al. 2016).

In the following clinical study the effect of a possible immune block was proven due to high IDO activity or high kynurenine level in a patient with pneumonia (Suzuki, Y. et al. 2012). The above Kaplan-Mayer diagram shows a clear worse prognosis when the kynurenine level is high.


Immune Tolerance and Cancer

All types of cancer cells can express IDO and create an immunosuppressive environment by synthesizing kynurenine, leading to apoptosis of killer cells and acquisition of T regulatory cells. Figure 3 shows a model of immunosuppression via IDO activation and the formation of kynurenine on the cellular level.

This hypothesis is supported by numerous clinical studies and their significant results are summarized in the following:

• High IDO activity supports the development of cancer with a hazard-ratio of 20%. This was shown in a study including more than 7015 patients (Zuo, et al. 2016).

• High IDO activity complicates the recovery in the course of cancer and increases the risk of relapse in most types of cancer. Examples include colorectal cancer (Cavia-Saiz, et al. 2014), lung cancer (Creelan, et al. 2013; Chuang, et al. 2014) Leukemia (Folgiero, et al. 2014), Hodgkin‘s Lymphoma (Choe, et al. 2014) as well as cervical cancer (Ferns, et al. 2015), etc.

A study by Creelan et al. impressively shows how IDO activity in cancer patients can be changed during treatment. Due to radiation and chemotherapy, the level of kynurenine increases in some patients. These patients must expect a worse prognosis (refer to Figure 8). Low levels of IDO activity or kynurenine are therefore of great significance for patients during and following cancer treatment.


Modulation of Immune Tolerance Through Active Substances

For the past 10 years, cancer researchers have been searching for adequate possibilities to influence IDO activity, thereby, controlling its disastrous effects. Physicians and patients urgently await active substances for clinical Phases I to III. One of the early substances, epacadostat, now is successfully evaluated for clinical phase III.

However, natural medicine also offers substances for the modulation of IDO. The aryl hydrocarbon receptor, shown in Figure 2 is important in the regulation of IDO activity as well as in the formation of kynurenine, and it can be inhibited with substances such as flavonoids, for example, curcumin, quercetin, or resveratrol. While environmental toxins such as dioxin activate IDO, the above-mentioned substances lead to a modulation of the AHR so that its regular signalling to IDO formation or activation is interrupted. Busbee et al. offer a review of effects of the molecular mechanisms of the natural substances on IDO activity (Busbee et al. 2013).

At this point we would like to give an example. The effect of curcumin on the activity of IDO in macrophages was clearly demonstrated by Jeong et al. (2008). IDO, induced by INF-gamma is blocked by curcumin in a dose-dependent manner (Jeong, et al. 2009).


Modulation of Immune Tolerance Through Sports

It has been known for a long time that moderate sports activity positively influences the immune system and the healing as well as self-healing energy, both before, and after illnesses. In 2015, the Karolinska Institute proved the decrease of stress-induced kynurenine levels through sports activity in a path breaking study. (Agudelo, et al. 2014) This realization can be used for the neurological side-effects such as fatigue induced by cancer treatment. It is known from the studies that (i) fatigue and elevated kynurenine levels are linked (Kurz, et al. 2012) but (ii) sports fatigue decreases its levels (Schuler, et al. 2016). The correlation of the level of kynurenine and sports activity closes the circle on the management of activity for patients with the goal to modulate IDO activity in favour of an optimal immune balance.


Summary and A Look Ahead

Our health and ability to overcome disease are decisively dependent on immune balance. This balance is driven primarily by the activity of indolamin 2,3-dioxygenase. High IDO activity and high kynurenine levels pull a virtual immune blockade along with it.

Clinical studies show that a dysregulation of IDO activity is followed with worse prognoses in infections or malignant diseases. On the other hand, studies show that in addition to classical treatment, sports activity or natural substances may be of advantage to decrease IDO activity in these diseases.

IDO activity as well as the level of kynurenine can easily be determined using measurement of kynurenine and tryptophan in serum or plasma, offering a meaningful diagnostic approach for prevention and treatment follow-up.



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