The Kynurenine Pathway—New Linkage Between Innate And Adaptive Immunity in Autoimmune Endocrinopathies Part 1

Abstract:

The kynurenine pathway (KP) is highly regulated in the immune system, where it promotes immunosuppression in response to infection or inflammation. Indoleamine 2,3-dioxygenase 1 (IDO1), the main enzyme of KP, has a broad spectrum of activity on immune cells regulation, controlling the balance between stimulation and suppression of the immune system at sites of local inflammation, relevant to a wide range of autoimmune and inflammatory diseases. Various autoimmune diseases, among them endocrinopathies, have been identified to date, but despite significant progress in their diagnosis and treatment, they are still associated with significant complications, morbidity, and mortality. The precise cellular and molecular mechanisms leading to the onset and development of autoimmune disease remain poorly clarified so far. In breaking tolerance, the cells of the innate immunity provide a decisive microenvironment that regulates immune cells’ differentiation, leading to the activation of adaptive immunity. The current review provided a comprehensive presentation of the known role of IDO1 and KP activation in the regulation of the innate and adaptive arms of the immune system. Significant attention has been paid to the immunoregulatory role of IDO1 in the most prevalent, organ-specific autoimmune endocrinopathies—type 1 diabetes mellitus (T1DM) and autoimmune thyroiditis.

The kynurenine pathway is an important biochemical pathway closely related to the immunity of dogs. Recent studies have shown that the kynurenine pathway can enhance the immunity of dogs and improve the disease resistance of dogs through different mechanisms.

First, the kynurenine pathway can promote the proliferation and activation of T cells in dogs, thereby enhancing the cellular immune response in dogs. This response is achieved by T cells directly attacking and destroying pathogenic microorganisms, so its role is very important. In addition, the kynurenine pathway can also promote the natural immune function of dogs, that is, to resist pathogenic microorganisms through mechanisms that do not require T-cell participation. This natural immune mechanism can respond quickly when the dog is exposed to pathogenic microorganisms, thereby preventing the occurrence of disease.

In addition, the kynurenine pathway can also promote wound healing and tissue repair in dogs, thereby restoring the normal function of damaged tissues. This effect is also closely related to the immunity of dogs, because only when their bodies are in good condition can they effectively fight against pathogenic microorganisms.

In conclusion, the kynurenine pathway can enhance the immunity of dogs through different mechanisms, thereby improving the disease resistance of dogs. Therefore, we should actively promote the normal operation of the kynurenine pathway to maintain the normal state of the dog's immune system. At the same time, we should also pay attention to the health, nutrition, and safety of dogs, to better improve their immunity and prevent the occurrence of various diseases. This shows that we need to improve our immunity. Cistanche can significantly improve our immunity. Because the polysaccharides in the meat can regulate the immune response of the human immune system, improve the stress ability of immune cells, and enhance the bactericidal effect of immune cells.

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Keywords:

indoleamine 2,3-dioxygenase 1 (IDO1); kynurenine pathway (KP); innate immunity; adaptive immunity; autoimmune disease; autoimmune endocrinopathies; type 1 diabetes mellitus (T1DM); autoimmune thyroiditis.

1. Introduction

Epidemiological studies show that 3–5% of the general population suffers from autoimmune diseases, increasing every year. The pathophysiology of autoimmune diseases usually results from the loss of self-tolerance, leading to the production of autoantibodies and self-reactive lymphocytes that cause tissue destruction. Until now, about 80 distinct autoimmune diseases have been described—several of them are characterized by organ-specific immune dysfunction (such as Hashimoto’s disease (HD), type 1 diabetes mellitus (T1DM)), while the others are systemic immune dysfunction involving multiple organs, like systemic lupus erythematosus, multiple sclerosis, and others [1,2].

Almost half of the diagnosed autoimmune diseases are autoimmune endocrinopathies, of which the most common are thyroid diseases, T1DM, celiac disease, and vitiligo. The consequence of the autoimmune process is, typically, endocrine gland insufficiency; however, the only known exception is Graves’ disease (GD), in which the thyroid gland is not destroyed, yet becomes overactive due to the presence of specific antibodies. Autoimmune endocrinopathies could coexist in the same individuals. Furthermore, its familial occurrence is often observed. Pathophysiology results from a complex interplay among genetic predisposition and environmental/endogenous factors. The measurement of organ-specific autoantibodies and appropriate hormone assessment plays a crucial role in the diagnostic process and treatment strategy [3].

HD is an autoimmune thyroiditis, characterized by thyroid follicular cell atrophy, lymphocytic infiltration within the inflamed organ, and progressive fibrosis [4]. The initial stage of HD may be asymptomatic, while some patients would only have anti-thyroglobulin antibodies (anti-Tg). The appearance of anti-thyroid peroxidase antibodies (anti-TPO) is considered a predictive factor that indicates the transition of subclinical hypothyroidism into overt hypothyroidism, observed in approximately 20–30% of patients with autoimmune thyroiditis [5].

GD is the most frequent cause of hyperthyroidism in iodine-sufficient areas. Production of autoantibodies against the TSH-receptor (TRAb) represents evidence for disease progression; however, the factors determining the induction of disease remain unknown so far [6]. GD affects the functioning of the majority systems in the human body and usually leads to the development of clinical symptoms of hyperthyroidism, vascular goiter, Graves orbitopathy (25% of cases), thyroid dermatopathy (about 4% of cases); therefore the signs and symptoms associated with GD can vary strongly, and significantly influence the general well-being [7,8].

T1DM is characterized by aberrant immune responses to specific β-cell autoantigens, resulting in insulin deficiency and hyperglycemia, which develops through the interplay of genetic susceptibilities and environmental factors. Although the etiology of T1DM is not completely understood, the pathogenesis of the disease is thought to involve the autoimmunological destruction of β-cells [9]. The peak incidence of T1DM diagnosis is seen in childhood and adolescence [10], nevertheless, symptoms could develop throughout the lifespan. 

About 90% of cases of newly diagnosed T1DM have detectable antibodies against specific β-cell proteins, like insulin, insulinoma antigen 2, glutamate decarboxylase, tetraspanin-7, or zinc transporter 8 [11]. However, most people with a single autoantibody do not progress to T1DM. The presence of two or more serum autoantibodies in children is associated with an 84% risk of clinical T1DM by the age of 18 years [12]. Based on these observations, the pathogenesis of T1DM was divided into three stages: stage 1 (presymptomatic) is defined as the presence of two or more autoantibodies with normoglycemia, stage 2 (presymptomatic) as the presence of β-cell autoimmunity with abnormal glycemia, and stage 3 as the onset of symptomatic disease [13]. The indicated T1DM pathogenic stages allow for the predictability of disease progression in at-risk individuals and provide a framework for research and development of preventive therapies.
Certain autoimmune diseases occurring in parallel can form into specific syndromes called autoimmune polyendocrine syndrome (APS), which could be defined as a functional disorder of two or more glands. APS type 1 is characterized by Addison’s disease coexisting with mucocutaneous candidiasis and autoimmune hypoparathyroidism; however, it can also present with T1DM, GD, hypogonadism, vitiligo, or pernicious anemia. APS type 2 can present with Addison’s disease, autoimmune thyroiditis, T1DM, hypogonadism, vitiligo, myasthenia gravis, and alopecia. APS type 3A is associated with T1DM and autoimmune thyroiditis, nevertheless also with growth hormone deficiency and other abnormalities, whereas, in APS type 3C, T1DM is associated with psoriasis and celiac disease [14–16].

Autoimmune Addison’s disease (AAD) is known as a dominant component of APS1 and APS2. Furthermore, AAD is the major cause of primary adrenal insufficiency, which is diagnosed with low basal serum cortisol, high plasma adrenocorticotropic hormone (ACTH) concentrations, and impaired cortisol secretion after an ACTH stimulation test. Another essential condition for the diagnosis is the presence of autoantibodies to 21- hydroxylase (21-OHAbs); however, adrenal cortex autoantibodies may also be detected in 40–80% of patients with ADD. Due to the destructive autoimmune process resulting in a complete deficiency of cortisol secretion, AAD patients require lifelong hydrocortisone replacement therapy [17].

All autoimmune diseases share common pathogenesis, which contains an immune-mediated attack that leads to the destruction of the body’s organs. It should be mentioned that the innate and adaptive immune system is involved in this process, which can be confirmed in immunological, genetic, and histopathological studies [18–24]. The kynurenine pathway (KP) of tryptophan metabolism is an endogenous system with immunosuppressive features, which is involved in the control of inflammation and inducing long-term immune tolerance in the different organs across the body [25–27]. In this review, we focus on the contribution of indoleamine 2,3-dioxygenase-1 (IDO1) and tryptophan’s catabolites—kynurenines—to regulate the interactions between components of the innate and adaptive immune system. Special attention was paid to the role played by IDO1 and KP metabolites in the onset and progression of autoimmune endocrinopathies.

2. The Kynurenine Pathway

In the last two decades, a theory has emerged that metabolism of TRP via KP is involved in the control of immune responses, to keep autoimmunity in check [28–30]. TRP is an essential amino acid critical for protein synthesis and the generation of several bioactive compounds with important physiological functions, including serotonin, tryptamine, indoles, kynurenines, and nicotinamide adenine dinucleotide (NAD+) [31]. Humans lack the biochemical pathways to synthesize TRP, which must be gathered from the diet. After absorption of TRP via enterocytes in the gut, it is transported by the hepatic portal system into the liver, where is utilized for protein synthesis (less than 1% of ingested TRP), whereas about 95% of dietary-delivered TRP is metabolized via the KP in the liver. The remaining TRP is secreted into the bloodstream and is available to use by cells of peripheral tissues, such as the vascular endothelial cells, fibroblasts, and the cells of innate immunity [32]. Moreover, TRP can also be transported across the blood–brain barrier to regulate brain serotonin synthesis [33].

The kynurenine pathway is the main way of TRP metabolism [34]. The major enzymes and substrates of the KP are shown schematically in Figure 1. To begin with, TRP has to be converted into N-formyl kynurenine, which is mediated by indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO), and then into kynurenine (KYN) by Nformylkynurenine formamidase (FAM) [35]. The first step in TRP degradation under normal conditions is mediated by TDO, which is the main determinant of TRP extrahepatic availability and is inducible by TRP itself, estrogens, and glucocorticoids. However, under a high cortisol concentration and an inflammatory state, TDO expression in the liver is repressed, whereas IDO1 expression is induced in cells of the immune system, as part of a negative feedback loop, aiming to control inflammatory responses [36].

The extrahepatic KP remains under the control of two distinct IDO enzymes: IDO1 and IDO2, the activities of which may differ from each other. The activity of IDO1 is irrelevant under basal conditions, but strongly inducible by several inflammatory stimuli, such as interferon-γ (IFN-γ), lipopolysaccharide (LPS), tumor necrosis factor α (TNF- α), proinflammatory interleukins (ILs), infection, and transforming growth factor β (TGFβ) [37,38]. IDO1 is mostly active in the immune system cells, mucosal tissues, and some tumors; however, it could be inhibited by elevated TRP levels. The anti-inflammatory cytokines, IL-4 and IL-13, are causing a down-regulation of IDO1 mRNA expression and reduction of TRP catabolism [39], although controversial data concerning the role of IL-4 also have been reported [40]. The enzymatic activity of IDO2 is approximately 500–1000-fold lower than that of mammalian IDO1, and IDO2 is mainly expressed in the liver, epididymis, and kidney [41]. Current studies showed a multifarious and pivotal role of IDO1 in immunoregulation during infection, pregnancy, autoimmune diseases, and neoplasia of various origins [26,28,42,43].

TDO is considered a “higher catalytic activity” enzyme in comparison to IDO1 [32]; however, IDO1 has broader substrate specificity than TDO. The main sources of TDO in the human body are the liver and central nervous system [44], nevertheless, it has also been identified in mucous membranes, epididymis, and the brain [45].

KYN and its metabolites are biologically active. Consequently, their production must be strictly controlled. KYN is the central intermediate of the KP, where the metabolic pathway is divided into two different branches. KYN may be converted by kynurenine 3-monooxygenase (KMO) into 3-hydroxykynurenine (3-HKYN), which is known as one of the toxic metabolites. Human KMO is a protein, which requires nicotinamide adenine dinucleotide phosphate (NADPH) for its catalytic action [46]. Ensuing, kynureninase can convert 3-HKYN into 3-hydroxy anthranilic acid (3-HAA). Nevertheless, kynureninase could as well convert kynurenine directly into anthranilic acid (AA) [47]. In general, the last step of the KP is the conversion of 3-HAA into quinolinic acid (QUIN) by 3- hydroxy anthranilate 3,4-dioxygenase (3-HAAO) through the unstable product of this reaction—2-amino-3-carboxymuconate-6-semialdehyde (ACMS)—which further undergoes a nonenzymic cyclization to QUIN. Picolinic acid (PA) is also formed by a nonenzymic cyclization of aminomuconic acid semialdehyde (AMS). However, PA formation depends on the extent of the substrate saturation of the enzyme 2-aminomuconic acid semialdehyde dehydrogenase (ACMSD) [35]. Finally, QUIN is processed into the end product NAD+ by quinolinic acid phosphoribosyltransferase (QPRT) [48].

Nonetheless, another branch of KP is also known—it is minor under regular conditions, whereas increases while TRP or KYN profusion, and contains a transformation of KYN into kynurenic acid (KYNA), which is also recognized as an endogenous antagonist of N-methylD-aspartate (NMDA) receptors. The above-mentioned step is catalyzed by kynurenine aminotransferase 1 (KAT-1) [48,49].

3. The Role of IDO1 and KP Metabolites in Immune System Regulation

3.1. The Innate and Adaptive Immunity

The immune system continuously maintains the sophisticated balance between invading pathogens and tolerance to non-harmful antigens and self-antigens. As an entirety, the immune system consists of innate and adaptive immunity, each responsible for a different capacity and constitutes diverse cellular and non-cellular components [50]. Innate immunity is the first line of defense and provides the initial acute inflammatory reaction to tissue injury, foreign antigens, or pathogens [51]. Innate immunity is to a certain extent unspecific and is divided into cellular and non-cellular systems. The cellular components of the innate system include monocytes/macrophages, dendritic cells (DCs), natural killer (NK) cells, eosinophils, and neutrophils. The non-cellular system is extremely diverse—it recruits immune cells to the injury/infection site through various cytokines, promotes phagocytosis, and activates the complement cascade and adaptive immune system [51,52].

The activation of the adaptive immune system results in an antigen-specific host response that is mediated by T and B cells. B cells secrete antigen-specific antibodies to neutralize pathogens, mediate allergic reactions, and autoimmunity, and generate immune memory cells. T cells are involved in the production of cytokines, direct cytotoxic effect against infected tissue, and activation of the other immune cells [50]. Cellular cross-talk is a hallmark of adaptive immunity. The proliferation and differentiation of naive B cells in response to most antigens must be preceded by stimulation via T cells, that are specific for the same antigens. Similarly, T cells to proliferate in response to antigens need additional signals provided by B cells [50]. Thus, innate and adaptive immunity work together to establish and maintain tissue homeostasis. Any sort of dysregulation could disturb regular immune response and result in the persistence of chronic inflammation or even induce autoimmune responses in more susceptible individuals.

3.2. Kynurenines in the Immunoregulation—“TRP Depletion Theory” versus “TRP Utilization Theory”

Recently, the role of KP in the regulation of both innate and adaptive immune responses does not raise any doubts, although it is still not fully explained. In the past, two opposing theories persisted, referring to the importance of TRP metabolism via KP in immunoregulation. The first, “depletion theory” assumed that TRP depletion is the primary function of immune-related IDO1 induction, which has been recognized as a host defense mechanism of innate immune responses. Pfefferkorn showed that the growth of Toxoplasma gondii could be inhibited by IFN-γ-mediated IDO1 induction, which was associated with decreasing TRP concentrations [53]. In the other in vitro studies, the replenishment of TRP concentrations to the culture media restored the growth of cancer cells, bacteria, and parasites, supporting the TRP depletion theory [54]. 

This theory transformed when Munn et al. [55] discovered that IDO1 activity was required to prevent T cell-mediated rejection of allogenic fetuses in pregnant mice. They also found that T cell proliferation may be inhibited in vitro by stimulation of cocultured monocytes with IFN-γ, which induced IDO1-mediated depletion of TRP from the culture medium. The later study of Lee et al. [56] demonstrated that T cells, activated in the absence of TRP, entered the cell cycle; however, cell cycle progression is arrested in the G1 phase and T cells became susceptible to death via apoptosis, in part through Fas-mediated signaling. Moreover, reduced availability of TRP has been correlated with activation of the general control non-depressible 2 kinases (GCN2K) pathway, inhibition of the mammalian target of rapamycin (mTOR), and protein kinase C signaling, leading to T cell autophagy and energy [57]. According to the present conceptualization, TRP depletion acts to limit the proliferation of specific host cells, that became more susceptible to apoptotic stimuli [56].

TRP depletion hypothesis only explained IDO1 activation, whereas during an immune response both KYN, as well other, downstream KYN metabolites: 3-HKYN, 3-HAA, PA KYNA, and QUIN are generated in many tissues [43]. These metabolites were shown to be potent in the inhibition of T cell proliferation through induction of T cell apoptosis.

The study using a heart transplantation model in rats confirmed these results in vivo [58], forming the basis for the so-called “TRP utilization theory” [59]. The indicated theory assumed that the immunomodulatory properties of IDO1 are due to the accumulation of KYN metabolites in conjunction with TRP depletion [32].

3.3. Immunoregulatory Activity of IDO1

IDO1 is widely expressed in a variety of cells that belong to the immune system, such as macrophages, monocytes, DCs, eosinophils, neutrophils, some T cell subsets, and regulatory B cells [60–65]. The induction of IDO1 expression and activity in professional antigen-presenting cells (APCs), such as DCs and monocyte-derived macrophages, as well as in other components of the innate immune system—NK cells, eosinophils, and neutrophils have a multidirectional influence on the function of these cells in the immune system (Figure 2).

3.3.1. IDO1 and DCs

Dendritic cells are professional APCs and key regulators of the immune system. DCs perform many functions in the immune system, including uptake, processing, and presentation of antigens to naive T cells, the activation of effector T cells and NK cells, and secretion of cytokines and other immune-modulating molecules to shape T and B cell responses. Two major subsets of human peripheral blood DCs have been described: conventional DCs (cDCs) and plasmacytoid DCs (pDCs) [66]. pDCs represent a unique cell population, combining the innate and adaptive immune responses in defense against pathogens, autoimmunity, and cancer [67]. pDCs secrete large amounts of type I and III interferons and can secrete IL-6, IL-12, IL-23, TNF-α, and interferon-inducible protein 10 (IP-10). They also express major histocompatibility complex class II (MHC-II), MHC-I, and co-stimulatory molecules (CD40, CD80, CD86) for antigen presentation [67,68]. The production above mentioned molecules allows pDCs to shape the type of immune response. 

For example, IL-12 can induce Th1 response and CD8+ T-cell and NK-cell activation, which are important for combating viral and intracellular pathogens’ infection, whereas IL-6 and IL-23 may direct the immune activity towards a Th17 response, which plays an important role in the recruitment of neutrophils and macrophages, immune responses against fungal infections and in autoimmune diseases [69]. pDCs may also exert direct effector functions. They may express the TNF-related apoptosis-inducing ligand (TRAIL), which causes TRAIL-sensitive cell death [70]. Moreover, pDCs can kill target cells by releasing the serine protease granzyme B [71]. Recently, the role of pDCs in autoimmune disease has been proposed. pDCs may act directly on the differentiation/maintenance of autoreactive B cells, and promote autoreactivity indirectly through T cells or other cell types [72]. On the other hand, impaired pDC activity has been implicated in immunodeficient states or ineffective immune responses [73].

Although DCs play an essential role in the initiation of inflammatory responses, they are also able to induce immunotolerance, inter alia through the upregulation of the intracellular enzyme IDO1. These cells express both constitutive and IFN-γ-inducible forms of the enzyme [74,75]. In particular, pDC has been shown as having the ability to produce a high amount of IDO1 [60]. Despite this, pDCs have been described as rather poor at their antigen-presenting function in comparison to cDCs [76]. IFN-γ alone can induce up-regulation of IDO1 message in DCs; however, an additional stimulus, such as CD40L or LPS, results in significantly higher IDO1 expression [75]. Aryl hydrocarbon receptor (AhR) activation in DCs is the following important factor for IDO1 expression in these cells. KYN and other KP metabolites—3-HKYN and KYNA—are found to be endogenous ligands for the AhR and this mechanism may determine a tolerogenic DCs phenotype, which promotes Tregs expansion [77,78]. 

It seems that IDO1 expression in pDCs may rather modulate the immune response of effector cells, as depletion of IDO1-expressing pDCs resulted in increased T cell proliferation and intensification of inflammation [79]. The aforementioned finding was confirmed in numerous studies, in which IDO1-expressing DCs function as part of a “feedback” process to limit chronic or over-activation of the immune system. DCs producing IDO1 can suppress effector T cell proliferation and may induce T cell apoptosis [75,80]. IDO1-expressing pDCs mediate the down-regulation of the receptor zeta-chain in T cells and promote the expansion of forkhead box P3+ (Foxp3+ ) T regulatory cells (Tregs) [81]. Expression of IDO1 in DCs can also skew CD4+ T-helper cells from proinflammatory phenotype Th1 or Th17 to tolerogenic Tregs [82]. Thus, IDO1 expression by DCs is associated with peripheral tolerance and the induction of immunosuppression.

Several molecules that induce immune suppression/tolerance have been shown to mediate their activity via IDO1. The ligation of B7 molecules on the DCs with the cytotoxic T-lymphocyte- antigen 4 (CTLA-4), a co-inhibitory molecule expressed on Tregs, can induce IDO1 expression in DCs [83,84]. Interactions between programmed death 1 (PD-1) receptors on T cells with its ligands on the DCs can also promote the up-regulation of IDO1 [85]. Additionally, the immunosuppressive TGF-β could elicit and maintain IDO1 expression in pDCs [86]. Similarly, the other molecules, like LPS or INF-γ, which can induce AhR expression in DCs, can also maintain IDO1 at high levels using this positive mechanism [77,87].

IDO1 possesses the capacity to control DCs maturation, migration, and immunoregulatory properties. DCs exist in the periphery as immature cells responsible for capturing antigens for priming naive T cells. Upon maturation, DCs migrate to the draining lymphoid organs, where they may initiate immunity. It has been shown that IDO1 expression and activity were increased during DCs maturation, which was related to phenotypic and functional changes essential for generating MHC/peptide complexes and priming T cells [88]. 

In contrast, IDO1 deficiency led to diminished phenotypic and functional maturation of DCs in vitro and in vivo [89]. However, Bracho-Sanchez et al. [90] showed that DCs treated with exogenous human recombinant IDO maintain an immature phenotype without affecting their viability, and provide suppression of antigen-specific T cell proliferation in vitro. Moreover, IL-12p70 production in DCs was significantly diminished, while IL-10 was maintained, suggesting that naive Th cell differentiation may be directed into immunosuppressive Th2 or Tregs. These results indicate that DCs conditioning was mediated by the enzymatic action of IDO1 and that DC-mediated suppression of T cells was dependent on both TRP deletion and the presence of kynurenines, which together were more effective in the abrogation of T cells stimulation.

3.3.2. IDO1 and Monocytes/Macrophages

Monocytes and macrophages have broad inflammatory, immuno-modulatory, and tissue-repairing properties. They belong to the front line of defense cells and can activate the immune system to trigger an immune response. Before polarization, macrophages exist as uncommitted (M0), which will be able to express the specialized functions after the stimulation by appropriate cytokines and microbial products. The stimulation leads to the polarization of M0 cells into 2 groups: M1- and M2-type macrophages, which are recognized as classically and alternatively activated macrophages, respectively. M1 macrophages may be induced by the granulocyte-macrophage colony-stimulating factor (GM-CSF), IFN-γ, and LPS, whereas M2-type macrophages can be polarized after exposition to immune complexes, IL-4, IL-13, IL-10, and glucocorticoids [91]. 

Typically, M1 macrophages are considered proinflammatory, and secrete IL-12 and TNF-α, while M2 macrophages possess immunomodulatory, wound repair, and tissue remodeling functions, and produce IL-4 and IL-10 [92]. Macrophages have a high degree of functional plasticity: they can easily switch from M1 to M2—type, and vice versa, depending on the cytokines present in their environment [93]. Nevertheless, in certain autoimmune diseases, both M1 and M2 macrophages, as well as produced by them cytokines were observed simultaneously [92]. Moreover, intermediate forms of macrophages, co-expressing both M1- and M2-specific markers were detected in certain diseases [94]. These findings indicated that macrophage polarization is a dynamic and reversible process that depends not only on the local environment but also on the stage of the disease.

Macrophages and monocytes can express IDO1, but only following IFN-γ stimulation [74]. IDO1 induction can switch macrophage phenotype from pro-inflammatory M1 to tolerogenic M2. Wang et al. [61] showed that the expression of IDO1 in M1-type macrophage, differentiated from THP-1 cells treated with IFN-γ, was significantly higher than in M2-type, which polarized from THP-1 cells cultured with M-CSF. They also demonstrated that the overexpression of IDO1 promotes the differentiation of THP-1 cells, widely used as the model for monocyte/macrophage differentiation, to M2-type macrophages. On the contrary, the silence of IDO1 induces the formation of M1-type macrophages [61].

IDO1 can inhibit macrophage recruitment and phagocytosis process in mice models of Aspergillus fumigatus keratitis. However, IDO1 may also promote the polarization of macrophages into the M1 phenotype by activating the mitogen-activated protein kinase/extracellular signal-regulated (MAPK/ERK) signaling pathway, indicating that it is essential for keeping the balance between anti- and proinflammatory effects in this model [95]. The diverse role of macrophages in the inflammatory responses may be partly due to the presence of AhR. It has been reported that AhR-deficient macrophages showed a higher level of proinflammatory cytokines upon LPS stimulation and that AhR-deficient mice were more susceptible to LPS-induced lethal shock than wild-type mice [96]. Recently Suchard et coworkers [97] summarized existing literature and showed that elevated IDO1 activity is regarded as a feature of M2 macrophage activation.

An inflammatory state is characterized by high levels of cellular stress and energy use, which is often accompanied by increased rates of DNA damage. It has been noted that the oxidation of TRP through the KP can reconstruct NAD+ levels to meet energy requirements and support DNA repair mechanisms in macrophages, increasing their viability [98].

3.3.3. IDO1 and NK Cells

NK cells are cytotoxic lymphocytes, which play a significant role in immune responses to exogenous pathogens as well as in the defense against cancer cells. Circulating NK cells mainly appear in the resting phase; however, the stress, as a result of infection or malignancy, causes their activation and the secretion of cytotoxic granules or death receptor ligands [99]. In the activation of NK cells, the activating and inhibitory receptors present on their surface play an important role. The inhibitory receptors consist of the killer immunoglobulin-like receptors (KIR), Ig-like receptors (CD158), the C-type lectin receptors (CD94-NKG2A), and leukocyte inhibitory receptors (LIR1, LAIR-1). Important NK activating receptors include NKG2D, DNAM1, and natural cytotoxic receptors: NKp46, NKp30, NKp44, and CD16 (FcgRIII), which are involved in antibody-dependent cytotoxicity. After binding the appropriate ligands, these activating and inhibitory receptors cooperate and decide whether to exert NK cell cytotoxicity on target cells [100]. The direct cytotoxic effect of NK cells is mainly mediated via two pathways: the induction of apoptosis of target cell by secretion of membrane-disrupting proteins and proteases, or caspase-dependent apoptosis involving the death receptors (e.g., Fas/CD95) on target cells [99].

NK cells, which are one of the main components of the innate immune system, constitute a link between innate and adaptive immunity. Besides their direct cytotoxicity, NK cells release various cytokines and chemokines, such as GM-CSF, IFN-γ, TNF-α, and chemokines: CCL3, CCL4, and CCL5 [101] or crosstalk with other immune cells, like T and B cells and DCs [102,103]. They additionally exhibit immunologic memory which can persist upon cognate antigen encounter [104]. The Janus kinase/signal transduction and activator of transcription (JAK-STAT) pathway plays an important role in NK cells’ maturation, cytotoxicity, or survival, and most cytokines that can activate or block NK cells are known to regulate it [105]. It is established that IL-2, which plays an important role in NK cell proliferation and receptor expression, can activate STAT1, 3, and 5. Moreover, STAT5 is activated by IL-15, and STAT1 and 3 are activated by IL-21, which leads to the proliferation, maturation, and activation of NK cells [106]. Therefore, NK cell hyperactivation and dysfunction are associated with the pathogenesis of some inflammatory and autoimmune diseases. However, NK cells could have both protective and pathogenic roles in these diseases depending on the disease type and surrounding environment [107,108].

Kai et al. [109] identified INF-γ—dependent IDO1 mRNA expression in NK cells, and pharmacological inhibition of IDO1 reduced the cytotoxicity of NK cells against cancer cells. This finding was confirmed in vivo, in a model of subcutaneous B16 tumors in mice [64]. These results suggested that IDO1 in effector NK cells appeared to maintain normal cytotoxicity against tumor cells. However, it has been also reported that IDO1 catabolites block the proliferation of NK cells [110]. The recent study of Park et al. [111] showed that activation of IDO1 in tumor cells caused downregulation of the activating natural cytotoxic receptors NKp46 and NKG2D in NK cells, suppressing their cytolytic activity and inducing NK cell death. This destructive effect was mediated by the up-regulation of IDO1 and the production of KYN, which enters NK cells via AhR on their surfaces and directly impairs NK cell function. KYN treatment led to the decreased phosphorylation of STAT1 and STAT3 in NK cells in a dose-dependent manner, indicating that KYN regulates NK cells via STATs signaling pathways. In contrast, the pharmacological blocking of IDO1 activity in tumor cells restored NK cells’ cytolytic activity and receptor expression [111]. These data suggest that IDO1 activation in NK cells located in the tumor's environment can play an antitumor function, whereas IDO1 produced by tumor cells themselves may act as a negative feedback mechanism against antitumor immune responses.

3.3.4. IDO1 and Eosinophils

Eosinophils are multifunctional leukocytes that have been implicated in the pathogenesis of inflammatory processes, including helminth infections and allergic diseases. They have been considered as cells that mainly act as the first-line defense against parasites or can modulate immune responses to diverse stimuli. IL-5, produced primarily by Th2 cells, is a crucial cytokine for eosinophil differentiation, priming, and survival [112]. Nonetheless, eosinophils themselves serve as a source of a variety of cytokines and growth factors closely associated with multiple immuno-modulatory functions and are involved in numerous homeostatic processes in the thymus, mammary gland, uterus, and gastrointestinal tract [113,114]. They show chemotaxis to lymphoid chemokines and exhibit APCs-like properties upon stimulation with some cytokines. 

The antigen-presenting properties of these cells are possible thanks to the expression of the machinery for antigen presentation and co-stimulation molecules, including MHC-II, CD80, CD86, CD28, and CD40 [115], as well as with their direct cross-talk with DCs [116]. The ability of eosinophils to antigen presentation and allergen-induced recruitment to lung tissue has been suggested as evidence of interaction between eosinophils and T lymphocytes [117]. The study of Venge et al. [118] in patients with asthma indicates that eosinophils actively participate in lung tissue fibrosis and remodeling, linking them to the potential etiology of this disease and the worsening of the quality of patients’ life. On the other hand, it has been shown that eosinophils may participate in tissue repair, as they are equipped with a tissue damage-sensing system, and can release multiple tissue-repairing molecules, like different growth factors [112].

Human eosinophils express functionally active IDO1, both constitutively and after IFN-γ induction [62,119], and coculture of KYN- synthesizing eosinophils with IFN-γproducing T cells, but not IL-4-producing T cell subsets, led to apoptosis and inhibition of Th1 subset proliferation, whereas Th2 cell line was maintained [62]. The same team showed that the pharmacological inhibition of IDO1 in vivo resulted in the reversal of oral immune tolerance in an ovalbumin (OVA)-induced murine model and that repeated intranasal administration of OVA generated tolerance and prevented a subsequent sensitization to OVA [120]. These results indicated that IFN-γ-treated eosinophils can promote Th2 polarization through the expression of functionally active IDO1 in lymphoid tissue. Moreover, eosinophils can be driving a Th2 response by their capacity to produce canonical Th2 cytokines, like IL-4, IL-5, and IL-13 upon stimulation [121]. However, Tulic et al. [122] observed the presence of functional IDO1, which was constitutively expressed in thymic eosinophils during human infant life under non-pathological conditions. Simultaneously, KYN was detected intracellularly and around the cells morphologically resembling eosinophils. The induction of IDO1 and TRP catabolite—KYN—promoted Th2 cells' dominance over Th1 cells, which undergo selective apoptosis under these conditions. The above data suggest an immunomodulatory role of IDO1-expressing eosinophils, which may have important implications for adaptive immune development.

3.3.5. IDO1 and Neutrophils

Neutrophils are polymorphonuclear leukocytes and have been shown as one of the essential players during acute inflammatory states, which can be recruited from the bloodstream to sites of injury within minutes. They eliminate invading pathogens through several mechanisms, such as secretion of bactericidal molecules, engaging in phagocytosis, degranulation, and secretion of proteolytic enzymes and reactive oxygen species (ROS), or release of nuclear material in the form of neutrophil extracellular traps [123]. The circulating neutrophils are typically “resting cells”, and their harmful intracellular granule contents are not released to avoid host tissue injury. However, neutrophils can become primed during immune conditions, where they may exhibit a 10- to 20-fold increase in their response to the proinflammatory stimulation, resulting in aggravating surrounding healthy tissue damage [124]. The excess activation and recruitment of neutrophils have been implicated in the development of various chronic inflammatory conditions, such as rheumatoid arthritis, inflammatory bowel disease, rheumatoid arthritis, metabolic syndrome, atherosclerosis, and cancers [123,125]. On the other hand, neutrophils may also promote wound healing and the limitation of inflammation [126,127].

Apart from the major role of neutrophils in innate immunity, these cells can significantly modulate the main components of adaptive immunity by the impact on B cells and T cells. Neutrophils produce cytokines—the B cell-activating factor (BAFF) and a proliferation-inducing ligand (APRIL)—which are required for the survival and activation of B cells, and stimulation of them to produce antibodies [128]. Neutrophils may produce arginase-1 and ROS, and in this manner, they can inhibit the proliferation and activation of T cells [129]. They can also function as APCs, facilitating Th1 and Th17 differentiation [130], and can present antigens directly to T cells or transfer them to DCs [131].

A few neutrophil subtypes have been known, and between them, neutrophilic myeloid-derived suppressor cells (MDSCs) are identified, which play a major role in the regulation of immune responses in cancer and many pathological conditions, associated with chronic inflammation [132]. The precise cellular mechanisms, by which MDSCs can suppress T-cell responses have not been completely explained, but Novitskiy et al. [133] found that the incubation of MDSCs with IL-17 increased the suppressive activity of these cells through the up-regulation of arginase 1, IDO1, and cyclooxygenase-2 expression in mammary carcinoma model in mice. Loughman et al. [134] observed that uropathogenic Escherichia coli (UPEC) infection reduced phagocytic killing and dampened the production of antimicrobial ROS by neutrophils, as well as downregulated their proinflammatory signaling, chemotaxis, adhesion, and migration. 

The same team showed that UPEC attenuated innate responses by inducing IDO1 expression in human uroepithelial cells and neutrophils in vitro and that treatment of neutrophils with a specific inhibitor of IDO1 significantly enhanced their transepithelial migration in response to UPEC. Moreover, neutrophil function was not affected in IDO1-knockout mice [135]. Similarly, initial exposure to Plasmodium vivax induced activation of innate immunity, but that effect was accompanied by strong immunosuppression mediated by IDO1-expressing DCs, which was associated with the depletion of some neutrophil populations. Because neutrophils regulate DCs function during infection, the cross-talk between these cell populations seems to be an important component of the innate immune response [136]. These results indicated that the induction of IDO1 expression in neutrophils inhibits proinflammatory innate responses and promotes pathogen colonization, confirming the role of IDO1 as a critical regulator of early host-pathogen cross-talk. On the other hand, it was also suggested that regulatory Tregs, emerging during IDO1-mediated immunosuppression, were able to promote TGF-β production, as well as IDO1 and heme oxygenase-1 expression by neutrophils. Thus, Tregs may play an important role in the direct control of innate immune responses through the induction of neutrophils with immunosuppressive properties [137].

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