Role of IL-17 and TH-17 in Atherosclerosis
1. Introduction
Cardiovascular diseases (CVD) are the most common causes of mortality worldwide and coronary artery diseases (CAD) are the most prevalent type of them. Atherosclerosis, as a chronic, multifactorial, and complex inflammatory process, is the main underlying mechanism of CAD and inflammation is the key mechanism in every atherosclerosis stage. Several biochemical agents, such as cytokines, chemokines, lipid mediators, and leukocytes, contribute to atherosclerosis and thus bring this possibility to mind that intervening with them might modify the process of atherosclerosis. Therefore, applying these inflammatory agents as immunotherapeutic targets leads to a decline in CAD risk. Various studies have been done to target inflammation and thus reduction of CVD risk, For example, applying anti-inflammatory targets to decrease cardiovascular events in rheumatoid arthritis patients. However, there is still a long road ahead to discover the atherosclerosis mechanism completely. T-helper 17 (Th-17), a new distinct subset of naïve CD4+ T cells, and its secreted cytokine, interleukin 17 (IL-17) have been identified in inflammatory diseases recently; many studies have been done to clarify their roles in atherosclerosis too. But, their role in atherosclerosis is still controversial. Since incongruent results to determine them as protective or pathogenic agents, this review was performed to summarize their roles in coronary atherosclerosis plaque.
2. Origin of TH-17 and IL-17
T-helper 17 is a new distinct subset of naïve CD4+ T cells that seems to act separately from TH-1 and TH-2. TH-17 was identified by secretion of IL-17A, IL-17F, IL-22, and granulocyte-macrophage colony-stimulating factor(GM-CSF) and expression of transcription factor retinoic acid-receptor-related orphan receptor gamma-T (RORγt), IL-23R, and the chemokine receptor CCR6.
TGF-β is the most important cytokine to develop naïve CD4+ T cells into TH-17 cells, in the absence of IL-6, TGF-β predisposes naïve CD4 T cells to Treg cells, both of TGF-β and IL-6 are required for the initial step of TH-17 differentiation from CD4+ T cells. IL-23 is required for TH-17 maintenance and differentiation but it is not involved in initial differentiation. In the absence of IL-23, inducible TH-17 differentiates to non-pathogenic TH-17, and by the presence of IL-23, pathogenic TH-17 with inflammatory functions will be acquired. IL-21, IL-12, IL-18, IL-1β, and TNF-alpha are other cytokines with suggested roles in the differentiation and maintenance of TH-17. On the contrary, IL-2, IFN-γ, and IL-27 prevent naïve CD4+ T cells from TH-17 polarization.
IL-17 cytokine family includes 6 members: IL-17A (also called IL-17A), IL-17B, IL-17C, IL-17D, IL-17E (also called IL-25), and IL-17F. IL-17F shares the highest (55%) and IL-25 shares the least homology with IL-17A. Due to this similarity, IL-17A homodimers, IL-17F homodimers, and IL-17A/F heterodimers attach to the same receptor complex, including IL-17RA and IL-17RC subunits. Thus IL-17RA alone is not enough to activate IL-17A signals. IL-17RA must bind IL-17RC to create a complete IL-17A receptor heterodimer. There are 3 other receptor subunits (B, D, and E), these 5 subunits all together are called the IL-17R family. IL-17RA is expressed in many tissues throughout the body, especially hematopoietic tissues. The main responses to IL-17A occur in epithelial cells, endothelial cells, and fibroblasts, while macrophages, and dendritic cells are also receptive.
Besides TH-17 cells, many other immune cells produce IL-17, this list is still increasing and consists of CD8+ T cells, γδ T cells, invariant natural killer T cells (iNKT), natural killer (NK) cells, natural Th17 cells, lymphoid tissue inducer (LTi) cells, group 3 innate lymphoid (ILC3) cells, macrophages, neutrophils, and mast cells. Although mast cells and neutrophils' capability is still debatable, Noordenbos T et al. showed that primary human tissue mast cells do not produce IL-17A themselves but actively uptake exogenous IL-17A by receptor-mediated endocytosis. The exogenous IL-17A is contained in intracellular granules and may be released in a structurally and functionally intact form.
3. Function of TH-17 and IL-17
As well as other IL-17-producing cells, the main function of TH-17 cells in humans is the inflammatory response to several bacterial and fungi, especially in the epithelium and mucosal surfaces. As can be observed in hyper-IgE syndrome, a mutant form of the STAT3 gene (one of the indispensable transcription factors in the TH-17 cells), results in recurrent C. Albicans and Staphylococcus aureus infections in these patients. On the other hand, excessive activity of TH-17 and oversecretion of IL-17 lead to chronic inflammation and autoimmune diseases such as rheumatoid arthritis.
Sarah L. Gaffen suggested that TH-17 cells can be considered to be a bridge between innate and adaptive immunity, IL-17A and IL-17F bind to their receptors and activate ACT1 (also known as CIKS), TNFR-associated factor 6 (TRAF6), and then nuclear factor-κB pathway, resembling innate immunity pathways. On the other side, IL-22 is also produced by TH-17 cells, and affects cells through the JAK1– and TYK2–STAT3 pathways, similar to adaptive immune signaling.
Regarding proinflammatory activities of IL-17, it induces granulopoiesis and CXC chemokines which lead to neutrophilia and neutrophil recruitment. It also affects macrophages to augment and attract them and induces proinflammatory cytokines secretion.
4. Regulators of TH-17 and IL-17 production
Apart from the necessity of IL-6 for TH-17 differentiation, some studies reported that a raised production of IL-17 coincides with a higher level of IL-16, suggesting a positive feedback loop between IL-16 and IL-17. In addition to the positive effect of IL-1β on TH-17 differentiation, reduction of IL-1β secretion is accompanied by decreased aortic IL-17A production and reduced size of atherosclerotic plaques. Transforming growth factor-β (TGF-β) and IL-1β together also considerably enhances IL-17A production in invariant natural killer T (iNKT) cells. Both IL-21 and IL-23 are capable of activating the transcription signal transducer and transcription 3 activators (STAT3) and receptor-related orphan receptor retinoic acid (RORÿ), which eventually lead to increased production of IL-17A in Th17 cells. The transcription factor IRF4 can directly bind to the IL-17 promoter and activates mucosal RORγt signaling in TH-17 cells and consequently increasing IL-17 expression. It has been reported that miR-326, a TH-17 cell-associated microRNA, affects Ets-1, a negative regulator of TH-17 differentiation. Hence, Suppression and overexpression of miR-326 result in lower and higher rates of TH-17 differentiation, respectively.
Another microRNA, miR-155, through suppressing SOCS1, the significant negative regulator of the JAK/STAT signaling pathway, promotes Th17 cell differentiation, as well as IL-17 production. Intracellular signaling mechanisms to upregulate IL-17A production have also been identified, It has been suggested that β-arrestin2, an intracellular protein, induced IL-17 production by targeting the ERK 1/2 pathway of CD4+ T lymphocytes in a murine asthma model. Furthermore, bacterial toxins can affect TH-17 activity, for instance, cholera toxins via a cAMP-dependent pathway, directly induce Th17 cells to express IL-17. Kotla S. et al. reported that 15(S)-hydroxyeicosatetraenoic acid, the main 15-LO-dependent metabolite of arachidonic acid, induced the production of reactive oxygen species (ROS), which results in enhancing CREB-dependent IL-17A production and thus atherosclerosis. On the contrary, Studies show that IFN-α administration is able to decrease IL-17 production in CD4 + T cells. All-trans retinoic acid (ATRA), an active derivative of vitamin A, has been suggested to have a role in TH-17/Treg balance. According to studies, it reduces the rate of TH-17 while enhancing the number of Treg cells. Furthermore, ATRA decreases the expression of IL-17A and increases the expression of IL-10 and TGF- β1.
Cortisol, one of the most strong immunomodulatory hormones, greatly reduces lipopolysaccharide-stimulated IL-17A production in peripheral mononuclear blood cells (PBMCs) in patients with rheumatoid arthritis by blocking Toll-like receptor 4 (TLR4). Dexamethasone therapy significantly down-regulates IL-17 production in asthmatic mice's bronchoalveolar lavage fluid (BALF). Adding adrenaline (Ad) or noradrenaline (NAd) to peritoneal macrophages stimulated by lipopolysaccharides dose-dependently inhibits the production and secretion of IL-17A through inactivation of c-Jun N-terminal kinase (JNK). Eicosapentaenoic acid (EPA), one of the n-3 polyunsaturated fatty acids, is a neuroprotective lipid that increases the activity of the peroxisome proliferator-receptor π (PPARÿ) and thereby decreases the production ofIL-17A in the central nervous system-CD4 + T cells of the EPA-treated autoimmune encephalomyelitis mice. Early growth response gene-2 (Egr-2), a zinc-finger transcription factor, down-regulates IL-17A production, and Th17 cell polarization by blocking the Batf. T cells lacking Egr-2 have a higher tendency to Th17 but not to Th1 or Th2. therefore, Egr-2 has a significant role in Th17 regulation. Trivalent inorganic arsenic (As III), an important anticancer compound used to treat patients with acute promyelocytic leukemia, significantly decreases the expression of IL-17A mRNA in human-activated Th17 cells, possibly via inhibiting the JNK / c-Jun signaling pathway. Infection with murine cytomegalovirus reduces IL-17 expression by activating suppressors of cytokine signaling (SOCS)-3 and interleukin-10.
It has been reported that cigarette smoking enhanced TH-17 production and consequently IL-17 production. Conversely, vitamin D blocks the polarization of T cells towards TH17.
Intracellular cholesterol induces the RORγt signaling pathway and thus, increases the IL-17 production in CD4+ T cells and γδT cells. Extracellular lipids like oxidized (ox)LDL, via dendritic cells mediated mechanism, enhance the polarization of naïve CD4+ T cells towards TH17 and IL-17 production.
Some studies have reported that hypertension is associated with a higher level of IL-17, and also angiotensin II can increase IL-17 production and TH-17 polarization in an inflammatory condition. Similarly, chronic kidney disease, especially in the advanced stage, induce IL-17 production and induce TH-17 differentiation.
As mentioned above, there is a growing list of clinical and molecular factors that affects IL-17 and TH-17 and thus influence atherosclerosis through them.
5. Role of IL-17 and TH-17 in atherosclerosis
5.1. Proatherogenic
To investigate the role of IL-17 and TH-17 in atherosclerosis, several studies have been done yet. One of the first stages of atherosclerosis is endothelial dysfunction which is defined by the enhanced production of adhesion molecules. These molecules are expressed on the surface of endothelial cells of vessels and thus the interactions between leukocytes and endothelial cells increase. Shiatsu et al. reported that IL-17 has a positive effect on the expression of these adhesion molecules. On the other hand, they observed that IL-17 stimulates macrophages to become foam cells by scavenging Ox-LDL. These two findings are hypothesized to be underline the mechanisms of IL-17 atherogenic role. T. van Es et al. studied LDLr deficient recipient mice which were transplanted with IL-17R─/─ bone marrow. They detected a considerable reduction in atherosclerotic lesions size, the mast cell number in lesions, and also IL-6 expression in IL-17R─/─ mice.
Conversely, IL-10 expression decreased in them whereas the plasma level of IL-17 was the same in both mice types. In this study, only bone marrow-derived cells were affected and collagen is produced by smooth muscle cells too. Thus, the collagen densities of the two groups and subsequently plaque stability were similar. F. Usui et al. investigated two groups of mice which were double deficient for both IL-17 and Apo-E. The diets of the two groups were different (Western diet and Paigen diet) to compare the effect of IL-17 on atherosclerosis between groups. They observed that IL-17 deficiency decreases atherosclerotic lesions sizes in mice that were fed the Western diet (diet with moderate hypercholesterolemia) not the Paigen diet (diet with severe hypercholesterolemia). They showed that IL-17 deficiency decreased macrophage infiltration, production of inflammatory cytokines as well as atherosclerosis development in mice fed the lower cholesterol amount. The mice that were fed the Paigen diet showed no change. This result emphasizes the diet importance in the atherogenic role of IL-17. Interestingly, Butcher et al. generated double deficiency for both IL-17 and Apo-E too, one group was fed a Western diet (21% fat) and another group fed a chow diet (3% fat). In comparison to Apoe−/− mice, the first group showed a considerable lesion size reduction while the second group demonstrated no difference. In addition to studies that reported the role of IL-17 and TH-17 in the early phases of atherosclerosis and were mentioned before, Erbel c et al. investigated their role in the advanced phase of atherosclerotic lesions. According to their study, treatment with IL-17A mAb inhibits the development of atherosclerotic lesions in the advanced phase and provokes plaque stabilization in mice, primarily by reducing the inflammatory interactions. They also suggested that IL-17 induces a distinct form of macrophage with particular functions and features which has a key role in the atherosclerosis mechanism. Additionally, Benagiano M et al. reported that Chlamydophila pneumoniae infection starts an atherosclerosis cascade which TH-17 is one of the main regulators of it. TH-17 has a crucial role in atherosclerosis and also the enhanced level of TH-17 correlates with the severity of atherosclerotic plaque-forming.
5.2. Atheroprotective
Danzaki K et al. developed apolipoprotein E (ApoE)/IL-17A double-deficient (ApoE −/− IL-17A −/−) mice which have lacking IL-17 signaling in both BM cells and VSMC, after 6 to 8 weeks of high-fat diet, atherosclerosis was increased significantly in ApoE −/− IL-17A −/− mice comparison with ApoE −/− mice. The composition of vulnerable plaques was accelerated also. They concluded that IL-17A plays an important role in the early phase of atherosclerosis development and plaque stability. Another report showed that CD20 antibody-mediated B-cell depletion in ApoE−/− and Ldlr−/− mice result in polarization of T-cell differentiation, particularly toward Th17 cell differentiation. Therefore, the IL-17A level increases and IFN-γ production decreases. Consequently, the atherosclerosis burden drops off. Similarly, Taleb et al. reported that loss of suppressor of cytokine signaling (SOCS) 3 in T cells enhances IL-17 level which results in the reduction of the lesion size and plaque vulnerability in Ldlr−/− mice. They noted a marked decrease in IFN-γ in mice lacking SOCS3 expression. Furthermore, they observed that atheroprotection was associated with strong suppression of vascular inflammation according to a major reduction of vascular T cell infiltration and a 60% decrease in endothelial expression of VCAM-1.
IFN-γ, as an important pathogenic agent in inflammatory disorders, is mostly expressed by the TH-1 cells and plays a significant role in atherosclerosis. Thus, It has been hypothesized that IL-17 may have an atheroprotective role if its signaling is adequate to minimize the expression of IFN-γ and to enhance the production of IL-10. In contrast, proatherogenic effects can be exerted by a concurrent rise in IL-17 and IFN-γ.
5.3. Limited effects
Despite several studies on the proatherogenic and atheroprotective roles of IL-17 and TH-17, some studies report no significant role for them in atherosclerosis. Madhur et al. demonstrated that IL-17 elimination reduces vascular inflammation while it has no impact on lesion size and the burden of atherosclerosis. Although, IL-17 makes plaque more vulnerable and IL-17 elimination stabilizes them.
6. Role of IL-17 in coronary atherosclerotic plaque
Plaque destabilization leads to rupture and acute coronary syndrome. Thus, discovering its mechanism can reduce mortality considerably. The effect of IL-17 on the stabilization of atherosclerotic plaque has been investigated in many studies. While some studies suggested that IL-17 promotes plaque stability, others reported a destabilizing role for it. Additionally, the imbalance of Th-17/Treg can be a marker of the acute coronary syndrome (ACS). Some studies reported that in patients with ACS, levels of TH-17 and IL-17 enhances notably, and Treg levels decrease conversely. On the contrary, Su et al. suggested that IL-17 is not involved in the acute events of ACS, but participates in the subsequent chronic process of inflammation after acute events. Interestingly, Zhang et al. reported that platelet aggregation and IL-17 are associated in patients with ACS.
7. Conclusion
As mentioned above, several studies about the role of IL-17 and TH-17 in atherosclerosis has been done yet, but the exact mechanism of their functions is still unclear, these controversial results may be due to different method for investigation, for example, different generations of mice, different compositions of diets and various durations of observing. Further studies should be done to clarify their role and create the opportunity of using them as therapeutic agents.
