The role of indoleamine 2,3 dioxygenase 1 in the osteoarthritis

Abstract

Osteoarthritis (OA) is a chronic degenerative joint disease and a leading cause of disability. It involves articular cartilage destruction and a whole joint inflammation. In spite of OA pathogenesis is still unclear, new studies on the OA pathophysiological aetiology and immunomodulation therapy continuously achieve significant advances with new concepts. Here, we focus on the indoleamine-2,3-dioxygenase1 (IDO1) activity in the osteoarthritis (OA), which is one of the noticeable enzymes in the synovial fluid of arthritis patients. It was recognized as an essential mediator of autoreactive B and T cell responses in rheumatoid arthritis (RA) and an interesting therapeutic target against RA. However, the role IDO1 plays in the OA pathogenesis hasn’t been discussed. The new OA experimental analysis evidenced IDO1 overexpression in the synovial fluid of OA patients, and recent studies reported that IDO1 metabolites were found higher in the OA synovial fluid than RA and spondyloarthropathies (SpA) patients. Moreover, the positive relation of IDO1 metabolites with OA pain and joint stiffness has been confirmed. Thus, the IDO1 plays a pivotal role in the pathogenesis of OA. In this review, the role IDO1 plays in the OA pathogenesis has been deeply discussed. It could be a promising target in the immunotherapy of OA disease.

Introduction

Osteoarthritis (OA) pathophysiology remains a serious challenge that hinders therapeutic researches. Clinically it is referred to severe joint cartilage degeneration [1-4]. Different reports described OA as an inflammatory disease according to the high levels of proinflammatory factors such as IL-1β, TNF-α, IFN-γ, IL-6 and IL-13 [5-7] as seen in Table 1. However, anti-inflammatory mediators such as TGF-β1 and regulatory T cells have been studied in OA pathophysiology as well [8-10]. Recently, Merlo and colleagues reported that indoleamine 2,3 dioxygenase1/2 (IDO1/2) are candidate therapeutic targets to treat inflammations in the autoimmune diseases [11,12]. IDO2 works as an essential mediator of autoreactive B and T cell responses in the rheumatoid arthritis (RA) [12]. He developed a novel monoclonal antibody (mAb)-based approach to target IDO2 in the preclinical arthritis models. Results showed an efficient treatment against RA in mice. Moreover, using of 1-Methyl-d-tryptophan (IDO inhibitor) blocked autoreactive B cell activation and recurrence of arthritis in mice [11]. In our hospital, 52 synovial fluid samples were obtained from OA patients in comparison to 5 synovial fluid samples were obtained from non- OA patients (during surgeries of joint replacement). By ELISA kits all the OA SF samples showed overexpression of IDO1 in comparison to non-OA samples as seen in the Table 2. Suggesting a possible role of IDO1 in the OA pathogenesis. Furthermore, the role IDO1 plays in the autoimmune arthritis is unclear yet. Some studies suggesting a regulatory function of IDO1 in the autoimmune RA [13] while others reported a proinflammatory role [14,15], or no role [16].

Table 1.

List of cytokines and chemokines involving osteoarthritis pathophysiology

Cytokine/chemokine	Function	Situ in The Joint	Reference
IL-10, IL13, IL-4, IL-19, IL-32a, IL-32b, IL-32g and IL-32d, TGF-β	It has a chondroprotective role. Other literature suggesting an inflammatory enhancing role	Synovial fluid/subchondral bone/articular cartilage	[19,36,49,51,56,58,59,60,82]
TNF-α, IL-1β, IL-1α, IFN-γ, IL-6	It increases inflammation and cartilage degradation by inducing synovitis and mmp13 in the chondrocytes	Synovial fluid/subchondral bone/articular cartilage	[49,50,51,53,58,101]
IL-8	It induces inflammatory responses in the synovium	Synovial fluid	[19,51]
IL-15, IL-17	It has inflammatory influence and some anti-inflammatory functions	Synovial fluid	[50,52]

Table 2.

The obtained synovial fluid from OA patients and non-OA patients

Patients No	Gender	Age	OA grade	Samples	IDO1 average level
33	Female*	> 67±13	3-4	SF	25.23±6.5 IU/mL
19	Male	> 65±11	3/4	SF	22.75±8.2 IU/mL
5	Male	> 42±9	Negative	SF	2.26±1.4 IU/ml

OA: Osteoarthritis, SF: synovial fluid;

^* P<0.05.

In the ido deficient mice, no skeletal defects were observed [17], but it showed increased Th1/Th17 cells in arthritis joints which suggest a protective role of IDO1 in the joints [18]. A cross sectional studies reported that IDO1 increases peripheral inflammatory cells in the RA patients [19,20]. Altogether suggesting an imbalance of IDO1 in the synovium may seriously lead to arthritis development. However, the role of IDO in the OA hasn’t been searched. As known, mesenchymal stem cells (MSCs) produce IDO1 in response to inflammatory factors in the joint [21]. High levels of proinflammatory mediators such IL-1β, TNF-α, and IFN-γ in the OA [22] are supposed to induce DCs, Monocytes, and even MSCs release IDO1 in the synovial fluid in response to inflammation [23] as presented in the Figure 1. High inflammatory levels make inflammatory stress on the synovial MSCs that induce high regulatory mediators to rebalance immune response but the overexpression of regulatory mediators could also affect the cartilage and chondrogenesis by inducing hypertrophy and mmp-13 leading to induce cartilage degradation, see Figure 2 [23-25]. As reported, a metabolite of IDO controls the TNF-stimulated gene 6 (TSG-6)-mediated anti-inflammatory effects of human MSCs [26]. Wang and colleagues [26] concluded that kynurenine activates aryl hydrocarbon receptor (AhR) in MSCs that directly binds to the TSG-6 promoter that leads to induce TSG-6 expression. TSG-6 has anti-inflammatory role and chondroprotective effects in various models of inflammation and arthritis [27]. However, in recent few years the association between TSG-6 activities and osteoarthritis progression was determined at 3-year of follow-up [28]. Thus, the activation of TSG-6 in the synovial fluid of OA patients is strongly supposed to be stimulated by IDO1 metabolite (Kynurenic acid) which stimulates MSCs to release TSG-6 via activation of AhR receptor as presented in Figure 3.

Figure 1.

The potential role IDO could play in the pathogenesis of OA disease. As seen, in response As seen in response to cartilage injury proinflammatory cytokines such as IL-1β, TNF-α, and IFN-γ and inflammatory MSCs stimulates DCs to produce IDO1 to rebalance immune status. However, high IDO1 levels produced by DCs and MSCs inhibits chondrogenesis and increase the risk of cartilage erosion by enhancing cartilage inflammations.

Figure 2.

The mechanism by which proinflammatory cytokines in the OA knee joint stimulates MSCs to produce IDO1. IL-1β directly activates ISRE, while TNF-α, and IFN-γ activate ISRE via IRF1 molecule. ISRE signaling stimulates STAT1 which block IDO1 suppressor gene Bin1 and stimulate IDO1 production via NF-κβ pathway. High levels of IDO1 affects chondrocytes biology and inhibits chondrogenesis.

Figure 3.

The mechanism by which IDO1 metabolites (kynurenine) impacts MSCs function in the OA pathogenesis. Kynurenine activates AhR receptor that induces TSG-6 production.

Osteoarthritis and indoleamine 2,3 dioxygenase activity

As known, during OA development the joints functional units comprising cartilage and bone undergo decontrolled catabolic and anabolic remodeling processes to adapt to local biochemical and biological signals [29]. Changes in cartilage, synovial fluid, and subchondral bone directly contribute to the OA virulence [30,31]. Increased vascularization and formation of micro-cracks in joints during OA have suggested the facilitation of molecules such as cytokines or enzymes from cartilage to bone and vice versa. The further investigation reported TGF-β as a key cytokine involved osteoarthritis fibrosis [32,33]. Zhen and colleagues [34] reported that inhibition of TGF-β activity attenuated degeneration of osteoarthritic articular cartilage. In contrast, TGF-β1 initiates pathological changes of osteoarthritis and may increase disease severity [35,36]. However, the role of TGF-β1 mechanism in the OA pathogenesis remain controversial, and its upstream molecules are unknown. There is a specific promoter initiates upstream mechanisms hasn’t been discussed. IDO1 is supposed to be the responsible of TGF-β1 upregulation through smad2/4 and subsequently IL-6 [37,38]. Moreover, IDO1 has the potential to activate pDCs and other immune cells to sustain TGF-β1 functions and other regulatory mediators in the synovial [39].

Although, the effect of IDO1 on chondrocytes is unknown, it has been reported that IDO1 could support chondroprotective potential in the RA by reducing functional autoreactive Th1/Th17 in the joints and draining lymph nodes [40]. Inhibition of IDO activity, or knockout of the gene encoding IDO in the RA animal model caused an increase in the severity of collagen-induced arthritis [40]. However, the overexpression of IDO1 in the RA synovial patients was a serious challenge suggesting a pathogenic role of IDO1 in the RA. In the OA patients IDO1 overexpression has been found in all collected samples as presented in Table 1. Thus, maybe it has a role in the inhibition of chondrogenic differentiation by stimulating TGS-6 overexpression that induces chondrocytes to produce mmp13 leading to break down collagen type II and degrade extracellular matrix. Furthermore, the therapeutic effect of IDO1 down regulation in the RA animal model [12] importantly suggest IDO1 involves inflammation mechanisms which suppose the same role in the synovial of OA patients. So, the study of IDO molecular mechanism in the OA pathogenesis will open new intellectual perspectives for OA novel treatment strategy. Moreover, the determination of IDO1 release and molecular mechanism in early stage of OA disease could also represent immunomodulatory effects of IDO1 involving the synovitis of OA disease.

Furthermore, as known human MSCs release IDO to reduce high inflammatory responses especially in the bone [21,41]. In OA disease, MSCs positively express nestin to modulate subchondral bone structure [42]. Increases in nestin is partially regulated by vascular endothelial growth factor (VEGF) in the OA [43]. Since VEGF increases the expression and activity of IDO1 in the DCs [44], we wonder whether the nestin+MSCs could depend on IDO pathway to develop changes leading to initiate OA disease. OA-nestin+MSCs could use IDO pathway long-term immune tolerance to modulate synovial immune status because plasmacytoid dendritic cells (pDCs) that were confirmed in the OA synovium efficiently enhance immune long-term tolerance in the presence of IDO1 [45,46]. Moreover, genomic and experimental studies evidenced that VEGF is associated with OA development, and its severity [47,48], while VEGF high expression increases IDO1 overexpression and activity, which importantly suggests a role played by IDO1 in the OA development.

The association of osteoarthritic cytokines and chemokines with IDO1 pathway

The most reported cytokines in the osteoarthritis are proinflammatory such as IL-1β, IFN-γ, IL-6, TNF-α, IL-8, and IL-17 [49-55]. However, regulatory chemokines also have been documented such as TGF-β, IL-10, IL-4, IL-13 and IL-19 [36,51,56-62] see Table 1. These inconsistent data presented incomprehensible OA mechanism [63]. The most interesting point here is that these proinflammatory cytokines have a pivotal role in the pathogenesis of RA (inflammatory disease) with significant expression but its expression in the OA was shown less than RA [63,64]. Moreover, recent reports revealed that ani-inflammatory cytokines and Treg cells showed significant increasing in the OA [61,64], suggesting a secondary role of proinflammatory cytokines in the OA. Also, its worthy mention that IFN-γ, TNF-α in combination with IL-1β could actively induce IDO enzyme secretion by MSCs, APCs or stromal cells to enhance underlying immunosuppressive responses [21,65]. However, high inflammatory responses induce IDO1 activity which contribute in the production of other mediators affecting chondrogenesis and induce chondrocyte apoptosis as seen in Figure 4. As well as IL-32 has a potential to induce IDO1 secretion [66]. Thus, the OA inflammatory agents are suggested to promote IDO pathway and subsequently TGF-β, but whether IL-1β is the essential factor to initiate IDO1 pathway or IFN-γ and TNF-α is still uncertain. Therefore, we suggest a deep investigation for these proinflammatory agents regarding IDO1 upregulation and consequently chondrocytes responses in the OA. Therefore, we provide overview on the major pathophysiological cytokines involving OA and a possible relation with IDO1 pathway as the following.

Figure 4.

Schematic diagram shows the potential role IDO1 coud play in the cartilage. Proinflammatory cytokines in the cartilage in response to injury can elicit MSCs to producing IDO1. High levels of IDO1 me-tabolites modulate MSCs to produce TSG-6 which has a chondrogenic inhibition potential. It stimulates cleavage of casepase3 in the chondrocytes that induces chondrocyte apoptosis and extracellular matrix (ECM) degradation by mmp13.

Interleukin-1 beta (IL-1β)

IL-1β is one of the main proinflammatory and catabolic cytokines in the pathophysiology of OA disease [55,67]. Long monitoring of OA patients synovial fluids presented rapid increasing of IL-1β with OA early development [68]. Later, it was classified as an important biochemical marker of OA prognosis. IL-1β works to suppress type II collagen and aggrecan synthesis in the articular chondrocytes [69]. It could also contribute in the production of other inflammatory interleukins such as IL-6, IL-8 which also showed overexpression in the OA [70,71]. The blockage of IL-1β formation showed significant elimination in the OA severity but the results of treatment with these drugs were not entirely satisfactory this is suggesting a secondary role of IL-1β in the OA pathophysiology. However, recently kynurenine (IDO1 metabolite) exhibited capability to stimulate bioactive IL-1β secretion from myeloid cells through caspase 1 induction, while IDO-/- mice field to induce IL-1β cleavage and production [72]. So, IDO1 could has a role to initiate synovitis through promoting proinflammatory IL-1β.

Tumor necrosis factor alpha (TNF-α)

TNF-α is a kind of protein classified as proinflammatory factor that produced by antigen presenting cells (APCs) to stimulate inflammatory responses [73]. The increasing levels of TNF-α in the OA patients serum specimens showed significant association with joint stiffness and pain [62]. Many literatures reported that TNF-α works to recruit monocytes to increase IDO1 activity in the brain and some other organs [74-76]. Importantly, Pantsulaia and his colleagues revealed that there is no association between hand OA and plasma TNF-α levels [77]. Moreover, TNF-α in canine models did not show an association with mild osteoarthritic developments when increased in articular cartilage [78], these evidence suggest a secondary role of TNF-α in the OA pathogenesis that strongly expect its role to stimulate IDO pathway to stimulate synovitis.

Interleukin-6 (IL-6)

IL-6 is an amino acid residue that mostly works as proinflammatory factor [79,80]. Usually, osteoblasts secrete IL-6 to stimulate osteoclast formation [81]. In addition, normal chondrocytes secretes IL-6 with low expression to enhance chondrocytes proliferation [62]. However, high levels of IL-6 have been reported in the RA and OA synovial which being induced by TNF-α or IL-1β [82,83]. The overexpression of IL-6 inhibits collagen II expression in the chondrocytes [84,85]. Also, the dysregulated overproduction of IL-6 is responsible for the systemic inflammatory manifestations in RA patients [86]. Furthermore, IL-6 upregulates mmP13 expression in human and bovine cartilage explant cultures. In addition, IL-6 knock out mice displayed a low number of in the inflammatory cells in knee joints and a limited response to collagen induced arthritis [87]. Rübenhagen and his colleagues revealed that synovial IL-6 levels haven’t any correlation with OA severity in a study of 82 knee OA patients [88], since Van and his colleagues conducted that IL-6 joint cavity injection in IL6 deficient mice reduces cartilage destruction [89]. Worthy mention, IL-6 induces IDO1 expression through the JAK/STAT pathway [90]. This is suggesting that IL-6 could enhance OA synovitis through stimulation of IDO1 production that actively contributes to reduce collagen II production, as well as induces capspase3 cleavage.

Interferon-γ (IFN-γ)

IFN-γ is a soluble proinflammatory cytokine that rapidly induces inflammation responses [91]. It is secreted by T cells, natural killer (NK) cells and macrophages [92]. IFN-γ is a homodimer protein that binds to the interferon γ receptor that triggers a cellular response to viral, microbial or acute inflammation stimulation. It was early mentioned that IFN-γ actively participates in the RA synovitis [93,94]. Later, IFN-γ was reported with increasing levels in the OA patients synovial fluids [95]. It was suggested that IFN-γ stimulates inflammatory cells CD4 in the synovium that actively inhibits chondrocytes proliferation [64,96]. Others reported that IFN-γ induces the production of matrix metalloproteinases-13 (mmp-13) that inhibits collagen II production in the chondrocytes [97,98]. Besides, IFN-γ and TNF-α induce IL-6 production in the synovial fluid [70]. Importantly, IFN-γ induces IDO1 upregulation in the endothelial cells and tumor tissues [74,99,100]. Moreover, the induction of IDO1 in the RA and OA is associating with IFN-γ overexpression and other inflammatory factors [101]. Also, IFN-γ KO mice displayed increased levels of IL-17 producing T cells and the exacerbation of arthritis. Indeed, splenocytes of the IFN-γ KO mice increased IL-17 production when cultured with type II collagen [102]. Furthermore, the addition of IFN-γ to the culture of APCs from IFN-γ KO mice significantly reduced IL-17, while the inhibition of IDO1 by 1-methyl-DL-tryptophan abolished the inhibitory effects of IFN-γ. These results illustrate that IFN-γ regulates IL-17 production through IDO1 in the arthritis, since IFN-γ presented strong relation with IL-1β in the OA pathophysiology, which represent a strong relation of IDO1 with OA initiation.

Interleukin-10 (IL-10)

IL-10 is a pleiotropic anti-inflammatory cytokine that inhibits proinflammatory cytokines production [103]. It elicits diverse host defense mechanisms and maintaining the integrity and homeostasis of tissue epithelial layers [104]. Although, IL-10 classified as inhibitory cytokine it could block the activity of catabolic cytokines leading to joint stiffness and destruction [105]. High levels of IL-10 has been detected in the serum of OA patients [106] but it was suggested to provide a chondroprotective function [107,108]. Moreover, LPS stimulation induces low ex vivo production of IL-10 that associates increased risks of familial OA [109]. Meanwhile, recombinant human IL-10 is being tested in clinical trials to treat RA and other inflammatory diseases [110]. Van and colleagues [111] suggested that in the osteoarthritic synovium and cartilage explant culture MSCs could produce inhibitory factors such as IL-10, TGF-β, and IDO to reduce inflammation in short term. Recently, IL-10 mediates the inhibitory effect of umbilical cord-derived mesenchymal stem cells (UCMSC) that regulate Cadherin-11 (CDH11) expression by fibroblast-like synoviocytes (FLS) in RA by inducing IDO1 production [112]. In the OA there is no reports confirmed the role of IDO1 yet, since IL-10 high levels have been reported. This review reports the high expression of IDO1 in the synovial fluid of OA patients. So, the expected role of IDO1 in the OA could has a relation with IL-10 chondroprotective function which suggesting deep investigation of this point in the future.

Transforming growth factor beta (TGF-β)

TGF-β is a multifunctional regulatory cytokine that mostly enhances immunosuppressive process [113]. It appears in three different mammalian isoforms (TGF-β1, TGF-β2, and TGF-β3) and other signaling proteins [114,115]. TGF-β plays a critical role in the development and maintenance of the skeletal tissues [116]. Osteoblasts has the potential to secret the three isoforms of TGF-β which acts as anabolic factor that promotes osteoclasts differentiation and proliferation [114]. Moreover, the effect of TGF-β on the osteoblast is controversial, it can enhance osteoblasts differentiation, induce bone formation, and extracellular bone matrix production [117]. In contrast, osteoblasts showed decreased expression of TGF-β receptor I and receptor II in human, murine, and rat during differentiation, which indicates less sensitivity of osteoblasts to TGF-β1 in the differentiation late phase. Furthermore, TGF-β1 induces osteoclasts maturation and osteoclastogenesis of hematopoietic precursor [116,118]. However, TGF-β1 high levels could attenuates osteoclastogenesis through osteoproteogerin as well as down regulating RANKL pathway [116]. In addition, inflammatory factors such TNF-α and IL-1β increase the expression of RANKL in the chondrocytes leading to reduce collagen II expression. Altogether, TGF-β1 plays a role to reduce cartilage and bone degeneration by reducing RNAKL pathway [119]. In contrast, recently noticed that TGF-β1 modulate SMAD receptor signaling that affects chondrocyte differentiation and potentially induces osteoarthritis development [60]. Besides, alterations in TGF-β1 molecular activity actively contributes in the OA progression [120]. Importantly, TGF-β1 increasing levels associated the overexpression of IDO in many diseases that contribute in the immune tolerance enhancement [121,122]. In the antigen induced arthritis IDO/TGF-β1 mediates a protective effect of IFN-α [37]. However, the role of TGF-β1 in the OA is still controversial but the detection of IDO high levels in the OA patients synovial could largely help to understand TGF-β1 functions. Therefore, IDO mechanism in the OA needs an urgent study to provide clear interpretation for many inflammatory and regulatory factors involving OA pathogenesis.

Indoleamine 2,3 dioxygenase and possible activity in the joint

IDO is an intracellular immunomodulatory enzyme [123,124]. It engages immunotolerance mechanisms such as kynurenine pathway that catabolizing L-tryptophan through O₂ dependent oxidation to enhance immunosuppressive processes [38,125]. It is secreted by many cell types such as antigen presenting cells (APCs), stromal cells, mesenchymal stem cells (MSCs), tumour cells, endothelial cells and some of the alternatively activated macrophages [126-128]. By 2018 the function of IDO in different diseases was a focus of research and drug discovery efforts. As well as, the efforts to use IDO as a biomarker for tumor prognosis [129]. Also, it associates with some neurodegenerative pathogenesis [130,131]. It has the potential to alter the immune status from inflammatory to regulatory and orchestrate intracellular mechanisms to elicit suppressive cytokines and signals [38,132]. Pathogenesis of OA and RA present high association with NF-κβ, RANKL, TGF-β, COX and IL-6 pathways that essentially has a strong relation with IDO1 [38,133]. In addition, many literatures evidenced the involvement of IDO in the pathogenesis of RA and some of autoimmune diseases [20,134,135]. The activity of IDO1 in the OA pathogenesis is the current hot topic in the new projects because promising results in the RA importantly suggest possible role of IDO1 in the OA pathogenesis. As presented in Figure 4 IDO1 could efficiently affects the chondrogenic functions of MSCs by different mechanisms, one of the proposed mechanisms by stimulating TSG-6 as described above.

The possible theories about the effect of IDO1 on chondrogenic functions of synovial MSCs as seen in Figure 5 could be summarized as the following 1) may be IDO1 induce Wnt signaling as reported in the cancer cell [136] which has been proved to regulate Sox9 activity [137]. 2) IDO1 could attenuates CCR2, Hifα, or PKA [38], which directly inhibits Sox9 activity [138,139]. 3) the activation of ERK1/2 could inhibited by IDO1 that leads to inactivate Sox9 in the MSCs [140]. All these theories suggest an indirect effect of IDO1 on the chondrogenic signaling Sox9 which directly mediates collagen type II and 9 production as well as aggrecan.

Figure 5.

Schematic diagram displays the possible theories of IDO1 mechanisms by which IDO effects Sox9 signaling in the MSCs leading to inhibit chondrogenic genes. As known IDO1 induces Wnt signaling that directly inhibits Sox9 and consequently Col2a, Col9a, and aggrecan. Also, IDO has the capability to inhibit PKA, CCR2, or Hif-α which work as Sox9 upstream molecules. Indeed, IDO1 could directly elicits ERK1/2 signaling that directly inhibits Sox9 and subsequently inhibits chondrogenic genes.

On the other hand, IDO enzyme works on the tryptophan to produce kynurenic acid [141], which mainly stimulates calcium mobilization and inositol phosphate production in a GPR35-dependent manner in the presence of Gqi/o chimeric G proteins [142,143]. In the autoimmune diseases, the accumulation of kynurenic acid showed significant relation with mechanistic target of rapamycin (mTOR) activity [144]. Dai and his colleagues [145] reported that mTOR signaling is regulating runx2 in the skeletogenesis in the mice. Also, runx2 activates p13k/akt signaling in the skeletogenesis [146]. These lines of evidence strongly support the expected role of IDO in the chondrogenesis inhibition because the IDO deficient mice didn’t display any skeletal defects that supports the idea of IDO involves OA pathogenesis. Consequently, runx2 cooperates with mmp13 to degrade cartilage [147]. Further analysis showed that DNA methylation of runx2 P1 mediates mmp13 transaction in the chondrocytes [148]. So, the molecular activity of IDO to engage OA pathogenesis is extremely expected which encourage the deep investigation of IDO molecular activity in the chondrocytes. Upon the previous evidence it is supposed that activation of runx2 and mmp13 in the chondrocytes could be stimulated by IDO high levels in the synovial fluid which lead to initiate chondrocytes apoptosis and inhibition of chondrogenic proliferation but these suggestions need deep research.

Cellular populations in the OA synovitis and possible relation with IDO

Although, there are many evidence have consistently presented immunological changes in the subchondral bone and synovium before and during OA initiation [149-151] yet the role of infiltrating immune cells and molecular signals remain largely uncertain [152-155]. Furthermore, OA synovial fluid and bone marrow MSCs, osteoblasts, osteoclasts, and chondrocytes were investigated deeply, it presents changes in the function in comparison to the same normal cells [156,157]. However, the reason for these changes and its immune cellular motives remains a big challenge. Many literatures have discussed the immune cell involvement in the OA pathophysiology [154,158-161] as seen in Table 3. CD4⁺ T cells were significantly described in the synovium and subchondral bone of OA [162,163], but T cell subsets population haven’t being discussed enough, which makes T cells role in OA is very complex. Moreover, there are various immune cells have been reported in the OA such as NK, Mast cells, Macrophage, mDCs, and pDCs etc164 [164-166]. However, there are no clear roles for these cells in the pathogenesis process. Also, reported cytokines or secreted chemokines didn’t figure out a clear working mechanism, which also suggests a role for other molecules haven’t been searched yet. As the population of MSCs and pDCs has been massively described in the OA patients synovial. Thus, it’s supposed that IDO1 has a potential role to orchestrate immune cell functions in the synovium cavity. Moreover, IDO1 overexpression has been detected in the RA synovial [12,14], as well as in OA synovial as shown in Table 2. Furthermore, recent reports described the active role of regulatory cells in the OA pathogenesis such as Treg cells (CD4⁺CD25⁺FOXP3⁺) [30,155,167], which strongly supports that regulatory pathways could has a key secret in the OA pathophysiology. Nevertheless, interesting evidence of IDO1 in the OA specimens is encouraging deep searching of IDO1 accumulation rate at OA different grades.

Table 3.

List of the Immune Cells in the Osteoarthritis which have a relation with IDO1 immune activity

OA-Immune cell	Activity in the OA-Synovial fluid	Reference
CD19+CD20+	Low activity	[166]
CD3+CD8+	Low activity	[160,166]
T cells/B Cells CD4+/CD8+	High activity, it could stimulate synovitis by production of proinflammatory cytokines	[152,161,162]
CD4+ T Cells/Active Macrophage	Med activity by secretion of TNF-α	[153,154]
Treg CD4+CD25+	High activity, it could contribute to increase TGF-β1 and IL-10 that increase pain and joint stiffness. It increases activated effector memory cells (CD62L-CD69+)	[8,167]
CD4+CD25+ Tim-3+	Significant levels in the OA more than RA that enhance regulatory responses	[155]
Macrophage CD68+/B Cell CD20+	Low expression in the OA synoviits	[161]
Macrophage CD68+	Significantly high in RA but limited increasing in the OA	[164,165]
Mast Cells	It was noticed in the OA synovial but with low populations	[64,168]
CD56+CD16-/Low cytotoxicity NK Cells/Neutrophils	Recently it was reported but with low numbers	[169,170]
plasmacytoid DCs/Myeloid DCs	These cells were significantly noticed in the synovial and bone marrow of OA patients but its function isn’t clear	[45,171]

Moreover, as presented in Table 3 recent references reported an interesting population of mast cells, NKs, and neutrophils in the synovium of OA patients [64,168-170]. Interestingly, others reported the role of plasmacytoid DCs in the OA [45,171], which is supposed to produce IDO enzyme under the effect of proinflammatory mediators. So, this is confirming a possible role of IDO pathway in the OA pathophysiology because IDO1 expression showed direct effect on chondrocytes proliferation and collagen II in the matrix that suggesting a possible effect on the metalloproteinases. In addition, all previous literature searched OA have considered the same cell populations of rheumatoid arthritis (RA), so they concluded an inflammatory status but less than RA, while OA is distinctly different from RA. Therefore, there are some critical questions need to be answered; why the same immune populations were determined in the RA also have been reported in the OA? We speculate that OA infiltrating cells yet haven’t clear interpretation specify their mechanisms in the OA pathogenesis. Also, high expressed cytokines in OA has been ascribed to the same reasons of RA, which is a real confusing point. Immune cells mentioned in the OA synovial showed proinflammatory and sometimes regulatory functions as seen in Table 3. This contradiction presents serious obstacles hinder OA pathology understanding. Moreover, several reports concluded that T cell’s specific functions aren’t clear in the pathophysiology of OA [64,172]. A thorough screening for active immune cells in the subchondral bone or synovium of OA still need deep understanding. Most active cells, signals, and overexpressed molecules leading to subchondral bone thickness, sclerotic and inflammations haven’t been investigated well because the amount of literature reporting OA and disease mediating immune cells showed massive conflicts in their functions [82]. Hence, our experimental findings open a new route to study OA pathophysiology regarding IDO1 activity that importantly could contribute to solve OA pathogenesis secrete.

Signaling pathways in the OA and possible relation with IDO1

There are many signaling pathways have been reported in the OA presented in Table 4, but the main pathway orchestrating pathological mechanism is undetermined. Thus, deep understanding of signaling is needed to provide superior strategy to treat OA. Virous pathways play important roles involving chondrocyte metabolism, differentiation, proliferation, apoptosis, synthesis, and degradation of extracellular matrix (ECM) in the OA pathological stages, see in Table 4 such as SRY-related protein 9 (Sox9) [173], insulin-like growth factor (IGF), nuclear factor-κappa beta (NF-κβ), bone morphogenetic protein (BMP), transforming growth factor β (TGF-β) that make osteoarthritis pathogenesis understanding is too difficult [174,175], because of paradox functions, some of these pathways are proinflammatory and others are anti-inflammatory [176]. This ambiguity hinders therapeutic strategies advancement. Therefore, the discussion of OA reported pathways regarding their association with IDO could help to declare many important points because IDO pathway works as a bridge between inflammatory and anti-inflammatory mechanisms. In the following some of the most important signaling pathways in the AO pathogenesis with brief description of possible relation with IDO.

Table 4.

List of OA signaling pathways that has a relation with IDO activity

Pathway	Function	Situ in The Joint	IDO Relation	Reference
RNAKL/NF-KBP65/IGFBP5/GDF5/BMP-7/PPRA/ACVR2B/SMAD2/IRAK1/iNOS2/TRFA6/CHRD1/TIR/CCN2	Signaling	Synovial fluid/subchondral bone	These pathways induce IDO secretion and function	[177,178,179]
RALA	Sox9 regulation	Synovial	Inhibits IDO	[173]
MMP13 /MMP-1/2/3/9/ADAMTS4	Matrix degradation	Synovial fluid/subchondral bone	These pathways could be stimulated by IDO1	[7,59,176]
COX2/TNFα/JAK2/STAT3	Induce inflammations	Synovial fluid	These factors have a potential to induce IDO production	[59,68,102,201,207]
COLA1/COL2A1/ACAN	Intracellular matrix	Articular cartilage	IDO1 regulates COAL2A1 production while enhances COLA1	[87,97,188]
SOX9/NF-κB	Transcription	Synovial fluid/articular cartilage	IDO1 regulates soxy9 expression	[198,200]
TGF-β	Regulatory	Synovial fluid/subchondral bone/articular cartilage	Induces IDO activity and massively expressed in the presence of IDO	[32,60,120,140]

Receptor activator of nuclear factor-κβ ligand (RANKL)

RANKL is one of TNF family that located on the osteoblasts. It triggers osteoclastogenesis by interaction with RANK receptor on the osteoclasts [177]. Its early expression after fracture suggests that RANKL plays a pivotal role to regulate immune response [177,178]. So, it has been widely studied in the RA and OA pathophysiology [179]. Scientists suggest that RANKL is induced by vascular endothelial growth factor (VEGF) in the RA synovitis [180,181]. Besides, RANKL levels in the synovial of RA patients was correlated with VEGF concentration [182]. They play unique role to induce osteoclastogenesis and inhibition of chondrogenesis. In the murine and chicken, chondrocytes regulate osteoclastogenesis by producing RANKL [183]. Moreover, RANKL has the potential to regulate osteoclasts differentiation [184]. In the OA RANKL was significantly associated with osteoproteogerin (OPG) that produced by osteoblasts [185]. Recent evidence suggested that OPG/RANKL may be implicated in the subchondral bone changes that lead to develop articular cartilage degeneration [185]. In addition, the functional consequence of RANKL expression by the articular chondrocytes remains unknown. It’s thought that RANKL reduced function participates in the induction of chondrocytes apoptosis [186]. Few years ago, by the study of New Zealand (NZ) rabbits, articular cartilage in response to synovitis diffuses RANKL to subchondral bone which extremely induces subchondral bone changes and mineralization that reduce cartilage nourishment leading to degeneration [187]. Recent reports revealed that RANKL/OPG affects chondrocytes to produce high levels of matrix metalloproteinase-13 leading to inhibit collagen II production and induce cartilage degradation [188]. On the other hand, regulatory function of RANKL has been noticed [189]. In addition, two studies reported immunosuppressive role of RANKL in the regulatory DCs and Treg cells by inducing IL-10 expression [190,191]. As known, regulatory DCs and IL-10 strongly increase IDO1 production, as well as regulatory DCs produces high levels of IDO [192,193]. This is suggesting that RANKL could has a role to induce IDO1 production in the synovial fluid and subchondral bone area that could enhance IDO related factors to promote OA development. This point needs more investigation because no direct relation has been reported between RANKL and IDO in the RA or any other arthritis.

Nuclear factor kappa-BP65 (NF-κβ P65)

NF-κβ is a transcriptional pathway that play critical role to regulate the expression of many genes. It contains five family inducible transcription factors (p65/RelA, RelB, cRel, p50, and p52), which form homodimers or heterodimers that bind DNA differentially [194,195]. Moreover, NF-κβ has a potential to enhance inflammatory responses and regulate immunosuppressive mechanisms through phosphorylation and release NF-κβ from its inhibitor to bind DNA [196]. Moreover, NF-κβ p65 phosphorylation has not yet been fully characterized, it mainly depends on the stimulus and cell type [197]. Recently, it was reported that NF-κβ p65 is induced in chondrocytes by IL-1β [198]. Importantly, in 2008 Chen and colleagues [199] suppressed the experimental OA by targeting NF-κβ p65 expression. They used adenoviral vector-mediated nuclear factor-κBp65 in the rat model to reduce OA. Results showed that downregulation of NF-κβ p65 significantly reduced early OA development in the rat model. Additional studies revealed that NF-κβ p65 mediates cellular proliferation, inflammatory cytokines, and apoptosis mediators [200]. Many researchers suggest that NF-κβ p65 works to induce many cytokines such as TNF-α, IL-6, IL-1β and IL-2, as well as induces enzymes such as cyclooxygenase (COX)-2, and nitric oxide synthase (iNOS) [201-203] that actively contribute in the OA pathophysiology. This is strongly suggesting that NF-κβ p65 could stimulate IDO secretion in the synovial fluid, because most of the factors being induced by NF-κβ p65 have the potential to stimulate IDO1 expression. Furthermore, targeting NF-κβ p65 in the OA was reported as potential therapeutic strategy [204,205] but because the association of this pathway with many biological activities makes this choice unsatisfied. Moreover, it was reported that NF-κβ p65 facilitates chondrogenic differentiation by early transient activation as well as it determines Sox9 early expression and other chondrogenic pathways [206]. It means targeting NF-κβ p65 to treat OA isn’t a good choice. There are another interesting targets have been reported recently such as stat3 [207]. However, the investigation of IDO pathway in OA pathogenesis could explore a novel therapeutic option.

Conclusion

This review is the first article discusses the role of IDO1 in the OA pathogenesis according to the previous literature and new experimental evidence from synovial fluid of OA patients. IDO1 could play an important role in the early stage of OA pathogenesis and participates in the immune modulations that inhibits chondrogenesis and enhance cartilage degeneration. Therefore, we recommend a deep investigation of IDO1 molecular mechanism in the OA disease.

Disclosure of conflict of interest

None.

Abbreviations

OA

Osteoarthritis

RA

Rheumatoid Arthritis

IDO1

Indoleamine 2,3 dioxygenase 1

SF

Synovial fluid

SCB

Subchondral bone

KO

knock out

APC

Antigen presenting cell

Ihh gene

Indian hedgehog gene

MSCs

Mesenchymal Stem Cells

ECM

Extracellular matrix

OPG

Osteoproteogerin

VEGF

Vascular endothelial growth factor

mDCs

myeloid dendritic cells

pDCs

plasmacytoid dendritic cells

LPS

Lipopolysaccharides

CDH11

Cadherin-11

FLS

Fibroblast-like synoviocytes

TDO

Tryptophan-2,3-dioxygenase