miRNAs and T cell-mediated Immune Response in Disease

Abstract

We reviewed the role of miRNAs in the regulation of T cell differentiation and function in cardiometabolic inflammatory diseases, such as obesity, type 2 diabetes, atherosclerosis, and autoimmune diseases, such as type 1 diabetes, rheumatoid arthritis, asthma, and cancer. Cardiometabolic diseases, type 1 diabetes, and rheumatoid arthritis are characterized by miRNA expression profiles that favor the differentiation of T helper 1 and 17 cells and cytotoxic cells and a decrease in T helper 2 cells, regulatory T cells, and myeloid-derived suppressor cells. Asthma is characterized by changes in miRNAs that favor the differentiation of T helper 2 cells. Finally, cancer is characterized by miRNA profiles that cause a decrease in T helper 1 and 17 cells and cytotoxic cells and an increase in T helper 2 cells, regulatory T cells, and myeloid-derived suppressor cells. In particular, differences in the expression of miR-155 and a cluster containing Let-7, miR-10a, miR-17-92, miR-34a, miR-142, and miR-150 may determine whether the balance flips towards cytotoxicity or immunosuppression. High levels of miR-21 and miR-29 and low levels of miR-150 are associated with T cell profiles that protect against inflammatory and autoimmune diseases associated with tissue damage but also induce tumor growth. All these miRNAs were found to be associated with disease progression and/or response to therapy in one or more of the diseases under study. Therefore, studies on the value of the identified miRNA clusters in predicting disease progression and selecting therapies that may yield gains in treatment costs are warranted.

Introduction

Appropriate interactions between innate and adaptive immune responses are required for protection against pathogens, metabolic diseases, autoimmunity, allergies, and cancer. The innate immune response is a rapid defense mechanism. Herein M1 macrophages, dendritic cells (DCs), and natural killer (NK) cells contribute to inflammation and tissue damage in cardiometabolic diseases, such as obesity, type 2 diabetes, atherosclerosis, cardiomyopathy, and autoimmune diseases, such as type 1 diabetes and rheumatoid arthritis. In asthma, eosinophils, mast cells, basophils, and neutrophils cause chronic inflammation, bronchoconstriction, and airway hyper-responsiveness. Tumor-associated M1 macrophages, activated neutrophils, NK Cells, and DCs kill cancer cells, whereas tumor-associated M2 macrophages, myeloid-derived suppressor cells (MDSCs), and mast cells suppress the immune response [1-3].

Adaptive immune responses depend on several combinations of T cell types [4-8]. T cell-mediated immunity in inflammatory cardiometabolic diseases is characterized by the increased differentiation of CD4+ cells to T helper 1 (Th1) cells, which produce interferon (IFN)-γ, tumor necrosis factor (TNF)-α, and interleukin (IL)-22. Th1 cells activate CD8+ cytotoxic T cells, which produce IFN-γ and TNF-α and cause apoptosis. The action of Th1 cells may be reinforced by retinoid-related orphan receptor γt (RORγt) protein in Th17 cells [9,10]. In contrast, there is a decrease in regulatory T (Treg) cells that produce IL-10 and transforming growth factor (TGF)-β to suppress excessive immune response. TGF-β is an important regulator of T cell differentiation. It inhibits the differentiation and function of Th1 and Th2 cells and promotes the differentiation of Th17 cells. TGF-β is required for the induction of FOXP3 in naive T cells and the development of Treg cells. In addition, TGF-β is crucial for the differentiation of tissue-resident memory CD8+ T cells and their retention in the tissue, whereas it suppresses effector T cell function. TGF-β regulates the generation or function of natural killer T cells and innate lymphoid cells (ICLs) [11].

There is also a decrease in MDSCs, which links the innate and adaptive immune responses, resulting in decreased Treg cell differentiation, increased activation of M1 macrophages, and loss of anti-NK cell activity. Th1 and Th17 cells also play an important role in the pathogenesis of type 1 diabetes [12], whereas Th17 cells in rheumatoid arthritis inhibit the differentiation of Tregs and Th2 cells [13-15].

Airway inflammation in asthma is mainly characterized by an increase in Th2 and mast cells in conjunction with an increase in NK and CD8+ T cells. In the initial phases, Th2 immune responses may dominate Th1 responses, enabling escape from inflammation, whereas, in the chronic phase, Th1 and Th17 cells may restrict Th2 cell activation and induce neutrophilic inflammation in severe asthma [16,17].

Cancer is characterized by a reduction in Th1 and Th17 cells, an increase in Th2 and Treg cells, and a decreased recruitment of cytotoxic T lymphocytes to the tumor. An increase in the number of MDSCs was also observed. In particular, monocytic MDSCs, resembling monocytes, suppress Th1, Th17, and cytotoxic T cells via nitric oxide (NO) and immunosuppressive IL-10 and TGF-β. Furthermore, they can differentiate into tumor-associated macrophages (TAMs), which increases their immunosuppressive activity in the tumor microenvironment. All these changes allow tumors to grow and metastasize [18,19].

Ninety-eight percent of the genome is transcribed into non-coding RNAs [20-22]. Among them, microRNAs (miRNAs or miRs), which contain approximately 22 nucleotides, bind to the target sequence of their target mRNA and regulate mRNA stability or protein translation [23,24]. The activity of miRNAs is regulated by long non-coding RNAs (lncRNAs) [25]. Some lncRNAs modify chromatin segments, whereas others regulate the stability of mRNA or act as miRNA sponges [26]. The activity of miRNAs is also regulated by circular RNAs, which code for amino acids and are not strictly non-coding RNAs [27]. Upon exposure to stress conditions, the expression of miRNAs changes more rapidly than that of proteins, and the change in their expression is reversible, in contrast to genome mutations [28,29]. A single miRNA may affect several targets within one or more pathways in one or more cell types. Additionally, the same target may be affected by several miRNAs, which may affect each other’s expression or activity. Furthermore, they may be secreted into extracellular vesicles or exosomes, contributing to the communication between several cell types within one tissue or between several tissues [30].

We reviewed the role of miRNAs in the regulation of innate immunity in cardiometabolic diseases and cancer [31]. Here, we review the role of miRNAs in the regulation of innate immunity in cardiometabolic diseases, autoimmune diseases, and asthma. Different imbalances between T cell types are associated with these diseases, which have the potential to identify clusters of miRNAs that regulate the activation or inhibition of several T cell types. Understanding the molecular mechanisms by which these miRNA clusters influence T cell behavior could reveal novel diagnostic or therapeutic targets for these diseases.

T Cells in Cardiometabolic Diseases

First, we reviewed the miRNAs involved in the regulation of adaptive immune responses in chronic cardiometabolic diseases, including obesity, type 2 diabetes, atherosclerosis, and cardiomyopathy. They are mainly driven by the activation of Th1, Th17, and cytotoxic cells that induce cell death. Their activation is also a consequence of impaired activity of Tregs, Th2 cells, and MDSCs.

T Cells in Obesity and Type 2 Diabetes

More CD4+ T cells accumulate in obese adipose tissue (AT). Leptin causes a shift from Th2 to Th1 cells, decreased IL-4, IL-5, IL-10, and IL-13 levels, and increased IL-2 and IFN-α levels further increased by DCs, causing Treg cell death. This loss of Tregs hampers the reversal of obesity and type 2 diabetes. In addition, Th17 cells impair adipocyte differentiation and cause glucose intolerance and insulin resistance. Importantly, AT-derived stem cells from obese persons induced Th17 polarization and inflammation depending on physical contact between these stem cells and mononuclear cells via increased expression of intracellular adhesion molecule-1 and activation of the inflammasome and phosphatidylinositol 3-kinase pathways also involved in insulin signaling [32]. In addition, obesity is associated with a loss of Tregs in AT due to the downregulation of their defining transcription factor, peroxisome proliferator-activated receptor γ (PPARγ), coupled with strikingly enhanced responses to proinflammatory cytokines [33].

High levels of glucose and reactive oxygen species in type 2 diabetes increase the number of Th1 and Th17 cells and reduce the number of Treg and Th2 cells, which are associated with higher levels of IL-1β, TNF-α, IFN-γ, and IL-6 and lower levels of IL-4 and IL-13. In particular, the loss of IL-13 is associated with more M1 macrophages, further enhancing IL-6, IL-1β, and TNF-α levels, and causing Th1 cell proliferation and β-cell death. In addition, the shift from ICL2s to ICL1s is associated with a shift from M2 to M1 macrophages and the loss of Treg cell function [34,35].

Mechanistically, high levels of TNF-α in obese subjects cause a decrease in miR-10a, which targets the TGF-β1/Smad3 signaling pathway. The decrease in miR-10a, especially in mesenchymal stem cell-derived exosomes, may reduce Th17 and Treg responses while increasing the Th1 response by decreasing RORγt and FOXP3 and enhancing T-bet [36]. Impaired transfer of miR-10a from adipose-derived mesenchymal stem cells to CD4+ cells increases the frequency of Th1 cells and decreases that of Treg cells. A mimic of miR-10a reduced the expression of TGF-β1, the transcription factor Krüppel-like factor 4, and IL-17F and induced the expression of FOXP3 [37,38]. Inflammatory IL-6 and IFNs and the metabolic factors insulin and leptin are associated with an increase in miR-17-92 [39]. This upregulation of the miR-17-92 family in obese AT reduces IL-10 secretion by Tregs and increases IFN-γ secretion by Th1 cells [40]. In contrast, ox-LDL-induced miR-23b-3p stimulates Treg cell differentiation and decreases Th17 cell differentiation via the miR-23b-3p/NEU1 axis, thereby reducing body weight, fat percentage, inflammatory cytokine levels, and insulin resistance [41]. Inflammatory cytokines in the AT of high-fat diet-induced obese mice decreased miR-30e-5p, which was associated with increased Th1 cell polarization involving DLL4-Notch signaling. Th1 cell numbers increased after weight loss [42,43].

Ox-LDL and TNF-α induce miR-155 expression and impair Treg differentiation by impairing TGF-β [44].

T Cells in Atherosclerosis and Cardiomyopathy

A shift from Th2 to Th1 cells occurs in atherosclerotic plaques, which increases the secretion of IL-12 and IL-18 by M1 macrophages. A decrease in Th2-specific IL4 increases Th1-specific IFN-γ levels, whereas a decrease in Treg cells reduces the levels of IL-5, IL-10, and IL-13. The increase in Th17 cells, which secrete IL-17 and IL-22, activates M1 macrophages, further enhancing TNF-α, IL-1β, and IFN-γ levels. Ox-LDL-induced DC maturation is associated with Th1 and Th17 cell differentiation [45-47].

An increase in Th1 and Th17 cells, and a decrease in Th2 cells and Tregs, are associated with cardiac injury. A decrease in Tregs is particularly hazardous because they mediate myocardial repair and promote stable scar formation. In addition, Tregs inhibit the recruitment of inflammatory cells and suppress the local expression of proinflammatory cytokines. Reperfusion is associated with a decrease in CD8+ T cells, probably because of their recruitment to ischemic cardiac tissue, where they amplify inflammation by direct cytotoxicity to healthy cardiomyocytes and activate macrophage-mediated clearance of dead cell debris [48].

Mechanistically, ox-LDL-induced TREM-1-mediated activation of DCs in atherosclerotic lesions is associated with the activation of Th1 and Th17 cells through Let-7c and hampered Treg differentiation. Atorvastatin blocks this effect [49,50]. TNF-α and IL-1β are positively correlated with long non-coding urothelial cancer-associated 1 RNA, which sponges miR-26a [51]. A decrease in miR-26a expression in cardiac tissues 24 h after ischemia-reperfusion injury causes an increase in cytotoxic T, Th1, and Th17 cells by impairing the targeting of high-mobility group box protein 1 (HMGB1) [52]. Ox-LDL-induced miR-33 increases M1 macrophages and reduces the number of Treg cells in plaques, which are associated with inflammation and atherosclerosis progression [53]. In addition, IL-1β is associated with an increased expression of miR-146a in macrophage-derived exosomes [54]. This increase in patients with acute coronary syndrome is associated with increased Th1 cell differentiation and inflammation [55]. Moreover, increased expression of miR-146a is associated with increased levels of TNF-α, MCP-1, and NF-κB p65, which are key proinflammatory cytokines and critical transcription factors in acute coronary syndromes. Furthermore, PVT1 expression was lower, and miR-146a expression was higher in Tregs following cardiac transplant rejection. Restoring PVT1 expression, thereby sponging miR-146a, improved the survival of Treg cells, and alleviated cardiac rejection [56,57].

High levels of miR-155 are correlated with Th17 differentiation in patients with acute coronary syndrome [58]. High glucose and ox-LDL levels increase miR-155 secretion by DCs and endothelial cells, promoting Th17 cells and decreasing Treg cell differentiation by interfering with the TGF-β1-Smad 2 signaling pathway [59]. Inhibition of miR-155 increases Th2 and Treg cells [60]. High miR-155 levels in CD4+ T cells of patients with unstable angina cause a shift from Th2 to Th1 cells [61]. IL-6 and hypoxia-inducible factor-1α induce miR-210 to suppress Treg cell function by targeting FOXP3 and promote fibrosis [62].

Figure 1 shows the possible interactions between metabolic risk factors, changes in differentiation, and interactions between T cells in the pathogenesis of cardiometabolic diseases. This shows how miRNAs play an important role in differentiation patterns and interactions. In summary, the most important miRNA is miR-155 which induces Th1 cells, stimulates the activation of cytotoxic CD8+ cells by Th1 cells, and inhibits the differentiation of Th2 cells and Tregs. Let-7c, miR-17-92, and miR146a induce Th1 cell differentiation and inhibit Treg cell differentiation. A decrease in miR-26a expression induced Th1 and cytotoxic T cells, whereas a decrease in miR-10a increased the Th1/Treg ratio.

Figure 1.

T cells in cardiometabolic diseases. High levels of glucose, ox-LDL, and an imbalance between inflammatory and anti-inflammatory cytokines are associated with increased differentiation of CD4+ cells into Th1 and CD8+ cytotoxic T cells, and a reduction in Th2 cells and MDSCs, resulting in impaired Treg and increased cytotoxic T cell differentiation. This imbalance in T cells is caused by changes in the miRNA levels. Arrowheads reflect activation, and hammerheads reflect inhibition. Inhibited cytokines and miRNAs are shown in blue and those induced are brown.

T Cells in Autoimmune Diseases

Here, we review the miRNAs involved in the regulation of adaptive immune responses in type 1 diabetes and rheumatoid arthritis. Because they are also mainly driven by the activation of Th1, Th17, and cytotoxic cells, and impaired activity of Tregs, Th2 cells, and MDSCs, including in these diseases, allowed us to compare the expression of miRNAs or to find more specific miRNAs.

T Cells in Type 1 Diabetes

DCs, M1 macrophages, and NK cells initiate type 1 diabetes. DCs secrete IL-12 and IL-15 and activate autoreactive T cells. IFN-γ, TNF-α, and IL-2 secreted by Th1 cells activate cytotoxic CD8+ T cells, leading to the loss of β-cells. Although CD4+ and CD8+ T cells are simultaneously recruited to the islets, CD8+ T cells rapidly outnumber CD4+ T cells. Additionally, activated macrophages amplify inflammation and β-cell death. The loss of Th2 cytokines also induces antigen-presenting cell activation, inflammation, and the loss of Treg cell activity [63]. The latter is associated with a decrease in IL-35 levels leading to an increase in inflammatory cells [64,65].

Mechanistically, TGF-β blocks the expression of miR-10a in Tregs of mice with autoimmune diabetes by inhibiting FOXP3 expression [66]. Higher miR-142 levels in adults with type 1 diabetes are associated with lower numbers of Tregs, most likely by targeting TGF-β [67,68]. CD46 initiates Th1 responses by upregulating miR-155, which is especially needed for extensive T cell proliferation and requires glycolysis to meet the high glucose demand, by upregulating glucose transporter (GLUT)-1. T lymphocytes release exosomes containing miR-142-3p, miR-142-5p, and miR-155, which induce β-cell apoptosis in non-obese diabetic mice [69].

T Cells in Rheumatoid Arthritis

IFN-γ produced by Th1 cells in the human rheumatoid synovium induces toll-like receptor signaling, which activates Th1 cell and M1 macrophage polarization, and increases the levels of matrix metalloproteinases, which induce erosion and progressive joint destruction. Th1 cells and M1 macrophages secrete IL-6 that stimulates Th17 cell differentiation, whereas IFN-γ secreted by Th1 and Th17 cells induces the levels of granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-22, IL-17, TNF-α, IL-1β, and IL-6 in synovial cells and macrophages. IL-6 and IL-1β prevent Treg differentiation [11,70-72].

Low Let-7g-5p levels in patients with rheumatoid arthritis and a collagen-induced arthritis model are associated with increased Th17 cell differentiation by increasing RORγt expression [73]. High levels of miR-10b, upregulated by pro-inflammatory cytokines in PBMCs and CD4+ T cells of rheumatoid patients, correlate positively with Th1 cells and negatively with Th2 and Treg cells. GATA3 and PTEN have been confirmed to be targets of miR-10b. GATA3 siRNA increased Th1 and reduced Th2 cells, whereas PTEN siRNA increased Th17 and decreased Treg cells [74,75]. Low levels of miR-26b-5p in rheumatoid arthritis patients and a model of collagen-induced arthritis are associated with increased differentiation of Th17 cells [76]. IL-1β induces circular RNA (Circ_SEC24A) expression in osteoarthritic cartilage tissues associated with miR-26b-5p downregulation [77]. IL-23, IL-6, and IL-1β in CD1c+ inflammatory DCs induce miR-34a-stimulated Th17 cell propagation and TNF-α expression in inflammatory arthritis [78]. The transcription factor Yin Yang 1 (YY1) regulates the pathogenicity of Th17 cells by binding to the promoter region of the transcription factor T-bet and interacting with the T-bet protein. MiR-124, positively related to the inflammation index [79], correlates negatively with YY1 and positively with Th17 cells but not with Th1 cells [80]. High levels of inflammatory miR-146a in rheumatoid arthritis inhibit FOXP3 expression and TGF-β secretion and correlate positively with Th17 cells and negatively with Treg cells [81,82]. MiR-155-deficient mice with fewer Th17 and antigen-specific cytotoxic T cells do not develop collagen-induced arthritis [83]. In addition, PEG liposomes delivering antagomiR-155-5p to monocytes and macrophages reduced joint inflammation in murine models of rheumatoid arthritis [84]. MiR-206 targets SOCS3 and FOXP3, which are associated with a Th17/Treg imbalance in patients with osteoarthritis, possibly by downregulating Let-7a, Let-7d, and Let-7g [85].

In summary, type 1 diabetes and rheumatoid arthritis are also characterized by an increase in Th1 and cytotoxic T cells and a reduction in Th2 and Treg cells. In particular, arthritis is associated with an increased differentiation of Th17 cells. Table 1 shows similar functions of miR-10a, miR-146a, and miR-155 as in cardiometabolic diseases. In addition, miR-10b has been identified as a regulator of Th1, Th2, and Treg differentiation and function. Furthermore, miR-142 and miR-206 were identified as regulators of Tregs, and mR-34a, miR-124, miR-146a, and miR-206 were identified as regulators of Th17 cell differentiation and function.

Table 1. Overview of Effects of Changes in miRNAs on T Cells in Cardiometabolic and Autoimmune Diseases.

Th1 cells	Th17 cells	Cytotoxic T cells	Th2 cells	Treg cells	MDSCs
Positive effect on T cells
Low levels of miRNAs
miR-10a	Let-7g-5p	miR-26a
miR-26a	miR-26a
miR-30e-5p	miR-26b-5p
High levels of miRNAs
Let-7c	Let-7c	miR-155a		mir-23b-3p
miR-10b	miR-34a
miR-17-92	miR-124
miR-146a	miR-146a
miR-155a	miR-155a
	miR-206
Negative effects on T cells
Low levels of miRNAs
	miR-10a			miR-10aa
High levels of miRNAs
	miR-23b-3p		miR-10b	Let-7c
			miR-142	miR-10b
			miR-155	miR-17-92
				miR-33
				miR-142
				miR-146aa
				miR-155
				miR-206
				miR-210

^aIndicates miRNAs with the same functions in cardiometabolic and auto-immune diseases. miRNAs that are associated only with auto-immune diseases or with other functions in auto-immune diseases are in italics.

T Cells in Asthma

Compared to the previously discussed diseases, airway inflammation in asthma is mainly characterized by an increase in Th2 and mast cells in conjunction with an increase in NK and CD8+ T cells. IL-4 and IL-13 and epithelial cell-derived IL-33 and IL-25 activate Th2 cells. In addition, thymic stromal lymphopoietin activates Th2 cells and recruits eosinophils, NK, and CD8+ T cells. Th2 cells in the airways of asthmatic patients produce IL-4, IL-5, and IL-13. IL-4 induces IgE production, IL-5 eosinophil survival, and IL-13 tissue remodeling. In addition, Th1 and Th17 levels increase in steroid-resistant asthma with infiltration of neutrophils. The reduction in Treg cells activates antigen-presenting cells by enhancing MHC class II, CD80, and CD86 expression [86-88].

The increase in maternally expressed lncRNA MEG3 in asthmatic patients could sponge miR-17 and reduce Th17 cell activity by targeting RORγt [89]. Another member of the miR-17-92 family, miR-18a, decreases Th17 cell differentiation by targeting RORγt [90]. High levels of miR-19, a third member of the miR-17-92 family, in airway-infiltrating T cells of patients with asthma, promote Th2 cytokine production by directly targeting inositol phosphatase PTEN, the signaling inhibitor SOCS1, and deubiquitinase A20 [91,92]. Elevated miR-21 levels may be associated with Th2 cell activation by impairing the IL-12/IFN-γ pathway via direct binding to IL-12p35 [93,94]. MiR-34a overexpression reduces the frequency of FOXP3-expressing Treg cells in OVA-induced allergic asthma in mice [95].

Low miR-130b-3p levels are associated with increased Th2 cell numbers in an experimental asthma model. MiR-130b-3p targets the HMGB1/TLR4/mitochondrial fission protein signaling pathway [96]. Low levels of miR-143 in asthma are associated with high numbers of Th2 cells and low numbers of Th1 cells. MiR-143-3p negatively regulated the expression of nuclear factor of activated T cells 1 (NFATc1), reducing the number of lung Th2 cells and increasing the number of Th1 cells by decreasing the expression of the basic leucine zipper ATF-like transcription factor gene [97,98]. Elevated levels of miR-145 increase Th2 hyperactivity and cause Th1 deficiency by targeting the RUNX family transcription factor 3 [99]. The decrease in miR-146a-3p levels in severe asthma patients and animal models has been associated with increased Th2 cell differentiation by decreased targeting of serpin family B member 2 [100]. Downregulation of miRNA-451a promotes Th2 cell differentiation in pediatric asthma by targeting ETS proto-oncogene 1 [101]. Low lncRNA STAT4-AS1 levels in T cells from patients with asthma may be responsible for increased Th17 cell differentiation via increased binding of the retinoid-related orphan receptor γt protein to the IL-17A promoter [102].

Asthma in this review included most miRs that affected Th2 cell differentiation. Low levels of miR-130-3p, miR-143-3p, miR-146a, and miR-451a and high levels of miR-19, miR-21, and miR-145 were associated with increased Th2 cell differentiation (Table 2). MiR-143 and miR-145 decreased the Th1/Th2 ratio. Relatively fewer miRNAs regulating Th17 (miR-17 and miR-18) and Treg differentiation (miR-34a) have been linked to asthma. However, the abundance of the identified miRNAs related to Th2 cells may be due to selection bias. First, it is possible that the original studies aimed to selectively identify miRNAs related to Th2 response. It is also possible that only patients with type 2 (T2)-“high” asthma characterized by upregulation of T2 immune pathways (ie, IL-4 and IL-13 gene sets) and eosinophilic airway inflammation were included. Therefore, a comparison with patients with T2-“low” asthma is warranted [103]. We further discuss this in the section on therapy.

Table 2. Overview of Changes of miRNAs on T Cells in Asthma.

Th1	Th17	Th2	Treg
Positive effects on T cells
Low levels of miRNAs
		miR-130-3p
		miR-143
		miR-146a
		miR-451a
High levels of miRNAs
		miR-19
		miR-21
		miR-145
Negative effects on T cells
Low levels of miRNAs
miR-143	miR-17
High levels
miR-145	miR-18		miR-34a

T Cells in Cancer

Having outlined the critical role of miRNAs in modulating T cell responses in cardiometabolic and autoimmune diseases characterized by inflammation, tissue injury, and cell death, we now turn our attention to their impact on cancer, where the interplay between T cells and miRNAs follows a distinct pattern directed at tumor growth and metastasis.

Hypoxia reduces the number of Th1 cells, CD8+ cytotoxic T cells, and NK cells, and increases the number of Th2 and Treg cells. Cancer-associated fibroblasts (CAFs) promote Th2 responses at the expense of Th1 responses. Thymic stromal lymphopoietin (TSLP)-stimulated DCs recruit Th2 cells and polarize naive CD4+ T cells into Th2 cells. Th2 cytokines, such as IL-4, IL-10, and IL-13, secreted by Th2 cells decrease the cytolytic ability of CD8+ T and NK cells, downregulating IL-12 from antigen-presenting cells and inhibiting DC maturation, which is crucial for CD8+ T cell activation and differentiation, as well as promoting Treg expansion, which also suppresses CD8+ effector function. In addition, Th2 cytokines promote M2-like polarization of MDSCs, enhancing arginase-1 and ROS production in MDSCs and boosting their suppressive activity against NK, cytotoxic, and Th1 cells. Finally, Th2 cytokines induce FOXP3 expression and Treg cell differentiation. Treg cells then inhibit the Th1 and Th17 responses via IL-10 and TGF-β. Moreover, CAFs secrete IL-33, which decreases Th1 differentiation. In addition, MDSCs promote the metastatic potential of tumor cells by decreasing the levels of inflammatory and immunosuppressive cytokines. Wnt ligands drive tumor macrophages into the M2 phenotype and promote anti-apoptotic signaling in tumor cells [104-107] (Figure 2).

Figure 2.

T cells in tumors. Overall, miRNAs promote tumor growth by inhibiting the differentiation of Th1, Th17, and cytotoxic cells, and inducing the differentiation of Th2 cells, Tregs, and MDSCs. Arrowheads reflect activation and hammerheads reflect inhibition. Inhibited miRNAs are shown in blue and those induced are brown.

MiR-150 targets cMyb and increases cytotoxic CD8+ T cells [108]. IFN-α decreases miR-378 levels [109], leading to the induction of granzyme B (GZMB) secretion by NK cells and increased NK cell cytotoxicity [110,111].

CIRC_0089761 binds to miR-27b-3p to upregulate PD-L1 and prevent CD8+ T cell apoptosis [112]. MiR-140-3p facilitated tumor growth by suppressing NK cell function in an ovarian cancer model by targeting mitogen-activated protein kinase 1 [113]. MiR-142-5p deficiency is associated with a decrease in both CD4+ and CD8+ T lymphocytes owing to impaired regulation of PD-L1 [114]. MiR-155 deficiency in CD8+ T cells inhibited anti-tumor activity by impairing the activation of Akt and Stat5 signaling by inducing FoxO3a and Treg cells [115]. Secretomes from human metastatic melanoma cells also silence miR-155, resulting ifn the upregulation of FOXP3 expression in Treg cells [116]. MiR-155 deficiency may be associated with decreased CD8+ T cell anti-tumor function by elevating T cell senescence and functional induction by indirectly inhibiting polycomb repressor complex 2 activity by reducing the expression of PRC2-associated PHD finger protein 19 through upregulation of the Akt inhibitor, inositol polyphosphate-5-phosphatase D, or Ship1 [117].

MYC sponges miR-34a [118], leading to enhanced production of the chemokine CCL22, which recruits Treg cells to facilitate immune escape [119].

MiR-9 correlates positively with the number of MDSCs in mice by targeting SOCS2 and SOCS3 [120,121]. Elevated levels of miR-9 and miR-181a promote tumor growth and immune escape by enhancing MDSCs infiltration in situ by separately inhibiting SOCS3 and protein inhibitor of activated STAT3, two crucial regulators in the negative feedback loop of the JAK/STAT signaling pathway [122]. Hypoxia-inducible expression of miR-10a and miR-21 in glioma-derived exosomes induces MDSC expansion and activation by targeting RAR-related orphan receptor alpha and PTEN [123]. The irradiation-induced upregulation of miR-26b-5p in exosomes of dying esophageal squamous cell carcinoma cells enhances MDSC expansion and activation by targeting PTEN [124]. The increase in miR-30a levels in a B-cell lymphoma mouse model stimulated differentiation and immunosuppression of MDSCs by inhibiting SOCS3 suppression [125]. GM-CSF induces miR-200c expression in the tumor environment, promoting MDSC expansion by targeting PTEN [126]. HIF1α-induced miR-210 retains the immunosuppressive action of MDSCs against cytotoxic T cells by increasing arginase activity and nitric oxide production without affecting reactive oxygen species, IL6, or IL10 production, or PD-L1 expression [127].

M2 tumor macrophages secrete exosomes enriched in miR-29a-3p and miR-21-5p, inducing an increase in Tregs and a decrease in Th17 cells by targeting STAT3 [128]. A decrease in HOXA transcript antisense RNA myeloid-specific 1 (HOTAIRM1) in patients with lung cancer is associated with more MDSCs and fewer Th1 and cytotoxic T cells [129].

Cancer in this review includes a cluster of miRNAs that induce the differentiation of MDSCs, including miR-9, miR-10a, miR-26b, miR-30a, miR-181a, miR-200, and miR-210. MiR-21 also regulates tumor growth by inhibiting Th17 cells and increasing the number of Th2 and Treg cells. Downregulation of miR-155 reduces the activity of cytotoxic cells and increases that of Treg cells.

miRNAs and Disease Risk and Response to Treatment

In the present study, we searched for miRNAs that are mechanistically related to T cell differentiation in a specific disease and that correlate with disease progression or response to treatment. Although we identified miRNAs related to disease and therapy, the reported studies did not examine changes in T cell levels.

Obesity and Type 2 Diabetes

Serum miR-21-5p, miR-22-3p, miR-150-5p, and miR-155-5p levels were higher in obese adolescents with insulin resistance (IR; ie, HOMA-IR > 4) and non-alcoholic fatty liver disease (NAFLD), and their levels correlated with hepatic fat and serum triglyceride. In patients with NAFLD, miR-155-5p is correlated with ALT and AST, and miR-21-5p and -22-3p levels are negatively correlated with plasma adiponectin levels [130]. A comparison of 112 patients with type 2 diabetes, 72 individuals with impaired fasting glucose (IFG), and 94 healthy controls revealed that increased expression of let-7b, miR-144, and miR-29a and decreased expression of miR-142 were significant independent predictors of type 2 diabetes, IFG, and IR [131].

Liraglutide upregulated the expression of browning-related genes in epididymal WAT. The expression levels of miR-196a and miR-378a were positively associated with WAT browning, whereas the expression levels of miR-155, miR-199a, and miR-382 were negatively correlated with WAT browning in rats [132].

Atherosclerosis

Let-7c-5p, let-7d-5p, let-7f-5p, miR-376a-3p, and miR-376c-3p are overexpressed in the plasma of high-risk cardiovascular patients with statin intolerance [133].

Rheumatoid Arthritis

Lower levels of miR-146a-5p, miR-155-5p, and mi-132 at baseline levels predicted a better response to methotrexate after 4 months of treatment of 92 patients with rheumatoid arthritis according to the American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) criteria with active disease (disease-modifying anti-rheumatic drug (DMARD)-naïve and Disease Activity Score 28 [134].

The decrease in miR-26b levels predicted the response to olokizumab in 103 patients with rheumatoid arthritis at weeks 12 and 24 of treatment [135]. Twenty-one patients treated with the drug candidate ABX464 (Obefazimod, 50 mg/day), which upregulates miR-124, were compared to 20 placebo-treated patients. It improved multiple early efficacy endpoints in patients who failed to respond to methotrexate and/or anti-TNF-α therapy or were intolerant of anti-TNF-α therapy [136]. Patients with rheumatoid arthritis were treated with conventional synthetic disease-modifying anti-rheumatic drugs (csDMARs) alone (n = 20), monoclonal anti-TNF antibodies (ADA or infliximab (IFX)) (n = 9), TNFR2–Ig soluble receptor, etanercept (ETA) (n = 15), and non-TNFi biologics (n = 14). Patients treated with ADA, but not with ETA or other biologics, had a correction of the defect in monocyte differentiation into anti-inflammatory M2 macrophages, paralleled by a decrease in miR-155 [137].

Asthma

MiR-21 was significantly upregulated in 27 asthmatic children compared to 21 controls. Inhaled corticosteroids increase the forced expiratory volume in the first second (FEV1) and negatively correlate with miR-21 [138]. MiR-21 expression levels were negatively correlated with FEV1 in 175 children (95 asthmatic patients and 80 controls); asthmatic children were subdivided into three groups: 40 asthmatic children without inhaled corticosteroids, 40 steroid-sensitive asthma children, and 15 steroid-resistant asthma children. MiR-21 was positively correlated with both sputum and blood eosinophil counts. Importantly, serum miR-21 tends to differentiate between therapy-sensitive and therapy-resistant children [139]. MiR-146a expression was elevated in asthma and associated with measures related to worse asthma outcomes, including elevated blood eosinophil counts, higher asthma control questionnaire scores, and the need for higher doses of inhaled glucocorticoids in 35 asthmatic patients [140]. Circulating miR-155-5p and miR-532-5p were prognostic biomarkers of the response to inhaled glucocorticoids in 86 treated subjects, including 43 highly responsive and 43 poorly responsive subjects, with miR-155-5p and miR-532-5p decreasing and increasing transrepression of NF-κB, respectively [141].

Cancer

MiR-9 expression in lung cancer tissues of 116 patients with non-small cell lung cancer (NSCLC) was significantly higher than that in adjacent normal tissues, and its upregulation was significantly correlated with advanced tumor node metastasis, tumor size, and lymph node metastasis [142]. Increased levels of miR-9 and miR-221 in breast tissue were associated with an elevated risk of malignancy, such as larger tumor size, poor differentiation, late-stage evolution, and lymph node metastasis in 206 patients with breast cancer [143]. High levels of miR-10a and miR-126 have been associated with a longer relapse-free time in 81 postmenopausal ER-positive breast cancer patients treated with tamoxifen [144]. High plasma levels of miR-18a, miR-20a, and miR-92a are associated with poor outcomes in 196 patients with NSCLC [145]. Meta-analyses revealed that high miR-21 expression was associated with poor prognosis of prostate cancer [146]. MiR-21 may be combined with let-7, miR-34a, miR-146a, miR-155, and members of the miR-17/92 cluster to predict the radiosensitivity of prostate cancer patients undergoing radiation treatment [147]. MiR-21 also predicted the prognosis of 100 patients with gastric cancer [148], and together with miR-126-3p, predicted metastasis in 60 patients with NSCLC [149]. Elevated miR-21 expression is significantly associated with radioresistance in 140 patients with cervical cancer [150] and 86 patients with breast cancer [151]. Low miR-26a and miR-29a levels were identified as independent risk factors for predicting poor disease-free survival in 120 patients with hepatocellular carcinoma who underwent hepatic resection (n = 63) or radiofrequency ablation (n = 57) [152]. Reduced miR-26b expression was correlated with tumor development and poor prognosis in 88 patients with cervical cancer [153]. An miRNA classifier containing miR-29a, miR-29c, miR-133a, miR-143, miR-145, miR-192, and miR-505 could detect hepatocellular, with 257 participants in the training cohort and 352 and 139 participants in the two independent validation cohorts [154]. High miR-34a expression in 1172 tumors from patients with breast cancer was associated with a non-favorable tumor phenotype with a positive nodal status, high tumor grade, ER negativity, and high proliferation rate [155]. High miR-34a expression was associated with unfavorable prognostic outcomes in 113 primary colorectal adenocarcinoma specimens compared with 61 paired non-cancerous colorectal tissue samples [156]. Low miR-140-3p levels are associated with poor survival in patients with lung adenocarcinoma [157]. miR-155 and miR-24 predicted relapse in 133 patients with early breast cancer 4 years after diagnosis [158]. miRNA profiling of exosomes isolated from the plasma of 245 patients with advanced cell lung cancer who received nivolumab as second-line therapy was performed, and it was found that miR-181a-5p and miR-574-5p could discriminate between patients, unlike those that are likely to benefit from immunotherapy [159]. MiR-181a-5p and miR-630 expression levels have the potential to predict the outcome of disease and treatment response in patients with NSCLC [160]. Serum miR-210 levels were higher in 60 patients with NSCLC compared with 30 healthy control subjects and correlated with the clinical stage and the presence of regional lymph node metastasis [161].

Conclusion

This review identifies clusters of miRNAs that regulate the differentiation of Th1, Th2, Th17, and Treg cells in cardiometabolic and autoimmune diseases. A review of miRNAs in asthma identified additional candidates that regulate Th2 cell differentiation. Information on the role of miRNAs in the differentiation of cytotoxic cells and MDSCs in former diseases is limited, at least in comparison with cancer. In addition, miRNAs selected based on mechanistic insights were found to be associated with disease progression, poor outcomes, and/or resistance to therapy. Large-scale studies on the value of these miRNA clusters in predicting disease progression and in selecting therapies that may yield gains in treatment costs are warranted.

Glossary

AT

adipose tissue

DC

dendritic cell

FOXP3

forkhead box P3

GM-CSF

granulocyte-macrophage colony-stimulating factor

HIF-1

hypoxia-inducible factor-1

HMGB1

high-mobility group box protein 1

IFN

interferon

IL

interleukin

MDSC

myeloid-derived suppressor cell

MEG3

long microRNA maternally expressed 3

MHC

major histocompatibility complex

miR

microRNA or miRNA

MYC

MYC proto-oncogene

NK

natural killer

NFATc

nuclear factor of activated T cells 1

NSCLC

non-small cell lung cancer

ox-LDL

oxidized LDL

PPAR

peroxisome proliferator-activated receptor

PTEN

phosphatase and tensin homolog

RORγt

retinoid-related orphan receptor γt protein

SOCS

suppressor of cytokine signaling

STAT

signal transducer and activator of transcription

TGF

transforming growth factor

Th

T helper cell

TNF

tumor necrosis factor

Treg

regulatory T cells

TUG1

long miRNA of taurine upregulated gene 1

Author Contributions

PH performed the literature search, evaluated the quality of published data, and wrote the review.

Conflicts of Interest

none.

Funding Statement

No external funds were available for this study.