MicroRNAs: Emerging Regulators of Immune-Mediated Diseases

Authors

  • T. Tomankova,

    1. Laboratory of Immunogenomics and Immunoproteomics, Faculty of Medicine and Dentistry, Institute of Molecular and Translational Medicine, Palacky University, Olomouc, Czech Republic
    Search for more papers by this author
  • M. Petrek,

    1. Laboratory of Immunogenomics and Immunoproteomics, Faculty of Medicine and Dentistry, Institute of Molecular and Translational Medicine, Palacky University, Olomouc, Czech Republic
    Search for more papers by this author
  • J. Gallo,

    1. Department of Orthopaedics, Faculty of Medicine and Dentistry, Palacky University, Olomouc, Czech Republic
    Search for more papers by this author
  • E. Kriegova

    1. Laboratory of Immunogenomics and Immunoproteomics, Faculty of Medicine and Dentistry, Institute of Molecular and Translational Medicine, Palacky University, Olomouc, Czech Republic
    Search for more papers by this author

Dr E. Kriegova, Laboratory of Immunogenomics and Immunoproteomics, Palacky University, I.P. Pavlova 6, CZ-77520 Olomouc, Czech Republic. E-mail: kriegova@yahoo.com

Abstract

MicroRNAs (miRNAs) represent the most abundant class of regulators of gene expression in humans: they regulate one-third of human protein-coding genes. These small noncoding ∼22-nucleotides (nt)-long RNAs originate by multistep process from miRNA genes localized in the genomic DNA. To date, more than 1420 miRNAs have been identified in humans (miRBase v17). The main mechanism of miRNA action is the posttranscriptional regulation via RNA interference with their target mRNAs. The majority of target mRNAs (more than 80%) undergo degradation after recognition by complementary miRNA; the translational inhibition with little or no influence on mRNA levels has been also reported. Each miRNA may suppress multiple mRNA targets (average ∼200), and at the same time, one mRNA can be targeted by many miRNAs enabling to control a spectrum wide range of cellular processes. Recently, the role of miRNAs in the development of immune cells and the maintenance of immune system homeostasis gained attention, and the involvement of miRNAs in the pathogenesis of several immune system diseases has emerged. This review focuses on the role of miRNAs in autoimmune disorders (systemic lupus erythematosus, rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease and psoriasis), inflammatory pathologies of distinct organ (atherosclerosis, osteoarthritis and atopic eczema) and/or systemic locations such as allergy. The role of miRNAs, their predicted and known mRNA targets and description of their actions in physiological immune reactions and in the pathological processes ongoing in immune-mediated human disorders will be discussed. Finally, miRNA-based diagnostics and therapeutic potentials will be highlighted.

Introduction

MicroRNAs (miRNAs) are highly conserved small ∼22 nucleotides (nt) long RNAs involved in the negative posttranscriptional regulation of target mRNAs [1]. miRNAs belong to the one of the most abundant classes of human genome regulators: more than 30% of human genes are regulated by miRNAs, whereas the main action is the degradation of the target mRNAs [2]. Because of high similarity and binding on different sequences on mRNA, each miRNA may suppress multiple mRNA targets (average ∼200) and one mRNA can be targeted by many miRNAs [3]. Their multistep genesis from miRNA genes localized in the genomic DNA is shown in Fig. 1 [4, 5]. Since their first discovery in humans in 2001 [6], more than 1420 human miRNAs have been identified to date (miRBase v17).

Figure 1.

 Biogenesis and mechanism of action of miRNAs. miRNAs are transcribed by RNA polymerase II from the genomic DNA to 100- to 1000-nt-long primary miRNA transcripts (pri-miRNAs). In the nucleus, a pri-miRNA is then cleaved by the ribonuclease Drosha/Pasha to a 70-nt-long precursor miRNA (pre-miRNA). After transport to the cytoplasm, the pre-miRNAs are cleaved by RNase III Dicer into ∼22-nt miRNA duplexes: the ‘passenger’ (miR*) strand undergoes degradation, and the ‘guide’ (miR) strand is incorporated into the RNA-induced silencing complex (RISC) and serves as a functional, mature miRNA. This miRNA–RISC complex acts by two different mechanisms: (A) deadenylation and subsequent degradation of the target mRNA occurs when miRNA is near-perfectly complementary with 3′ untranslated region of target mRNA (major mechanism); (B) translational inhibition occurs when miRNA is only partially complementary to its target mRNA. Adopted from Brodersen et al. [4], Kim [5].

miRNAs participate in the regulation of almost every aspect of cell physiology [7, 8]. Besides their involvement in developmental timing, cell differentiation, apoptosis or anti-viral defence, recent studies showed that miRNAs play a crucial role also in the development of immune cells and function of immune system, including the differentiation and survival of immune cells, antibody production and the inflammatory mediator release (Table 1, Fig. 2). The recent findings about the role of miRNAs in the innate and adaptive immune responses have been summarized in Table 1.

Table 1.   miRNAs involved in the development and function of the immune system.
miRNAFunctionTarget(s)ReferencesDeregulated in immune disease
  1. Adopted from Tilli et al. [100], O′Connell et al. [101].

  2. AC9, adenylate cyclase 9; BIM, BCL-2-interacting mediator of cell death; DUSP, dual-specificity protein phosphatase; ETS1, v-ets erythroblastosis virus E26 oncogene homolog 1; FADD, Fas-associated death domain; FOXP, Forkhead box protein; HCV, hepatitis C virus; HOX, homeobox; HSC, haematopoietic stem cell; IBD, irritable bowel disease; IKKε, IκB kinase ε; IL12A, interleukin-12-A; IRAK1, interleukin-1 receptor-associated kinase 1; JAG1, protein jagged-1 precursor; KIT, tyrosine protein kinase; MAF, v-maf musculoaponeurotic fibrosarcoma oncogene homolog; Mef2c, myeloid ELF1-like factor 2C; MS, multiple sclerosis; Myb, v-myb myeloblastosis viral oncogene homolog; NFIA, nuclear factor I/A; NF-κB1, nuclear factor-κB subunit 1; PDCD4, programmed cell death 4; PLK2, polo-like kinase 2; PTEN, phosphatase and tensin homologue; PTPN22, protein tyrosine phosphatase, nonreceptor type 22; RA, rheumatoid arthritis; RIP, receptor-interacting protein; RUNX1, runt-related transcription factor 1; SHP2, SH2-domain-containing protein tyrosine phosphatase 2; SLE, systemic lupus erythematosus; SPI1, haematopoietic transcription factor PU.1; TLR, toll-like receptor; TNF, tumour necrosis factor; TRAF6, tumour necrosis factor receptor-associated factor 6; WNT1, wingless-type MMTV integration site family, member 1.

miR-155Regulation of bacterial and viral infection response in macrophages/monocytesIKKε, FADD, RIP[100]SLE, RA, MS, IBD, atherosclerosis, allergy, atopic eczema
B and T cell differentiationEnhances TNF-α production[100]
Regulator of germinal centre B cell responseMAF[102, 103]
Regulator of immunoglobulin productionSPI1[104]
Regulator of acute inflammatory response (IL-8 and CCL5/RANTES release)IKKε[8, 105]
miR-150B and T cell developmentMyb[106]RA, atherosclerosis
miR-181B and T cell development
T cell sensitivity to antigens
DUSP5,-6, SHP2, PTPN22[107]
miR-146Th1-effector cell specificity
Negative regulator of LPS signalling
Regulator of acute inflammatory response (IL-8 and CCL5/RANTES release)
IRAK1, TRAF6[8, 20]SLE, RA, psoriasis, atherosclerosis, osteoarthritis
miR-125bRegulator of LPS signallingTNF-α[43]Psoriasis, atopic eczema
let-7iHighly expressed in human cholangiocytes, regulates TLR4 expressionTLR-4[108]
miR-223Maturation of promyelocytic precursors into granulocytes
Negative regulator of the proliferation/activation of neutrophils
Mef2c[109]RA
miR-221/miR-222Regulators of cell proliferation and engraftmentKIT[110]Psoriasis, atherosclerosis
miR-10Regulator of HOX gene familyHOX family[111]Atherosclerosis
miR-196bModulator of haematopoietic stem cell homeostasis and lineage commitmentHOX family[112]SLE
miR-126Enhancer of colony formation in vitro may promote the production of downstream progenitors by HSCsHOXA9, PLK2[113, 114]IBD, atherosclerosis, allergy
miR-17-92 clusterEnhancer of T cell survival during developmentBIM, PTEN[115]
miR-326Promote Th17 cell developmentETS1[32]MS
miR-142-3pRepressed by FOXP3, leading to increase in cyclic AMP and suppressor function of T regulatory cellsAC9[116]SLE, psoriasis
miR-424Promote monocytic differentiationNFIA, SPI1[117]
miR-21Suppressor of inflammatory pathway activation in myeloid cellsPTEN, PDCD4, IL12A[118]SLE, IBD, psoriasis, atherosclerosis, allergy, atopic eczema
miR-17-5p/-20a/-106aPromote monocyte differentiationRUNX1[119]SLE, MS, psoriasis, atherosclerosis
let-7eRegulator of TLR signallingTLR-4[120]
miR-9Regulator of NF-κB expression during TLR4-mediated activation of monocytes and neutrophilsNF-κB1[121]Osteoarthritis
miR-34Perturbs B cell development by causing a cell increase at pro-B to pre-B cell transitionJAG1, WNT1, FOXP1[122]MS
miR-122Required for hepatitis C virus replicationHCV genomic RNA[123]
miR-196, miR-296, miR-351, miR-431, miR-448Antiviral defence; overexpression of these miRNAs in infected liver cells attenuates the viral replicationHCV genomic RNA[124]
miR-16Restriction of inflammatory mediators production, regulator of immunity through cooperation with other miRNAsTNF-α[125]RA, IBD, atherosclerosis, osteoarthritis
Figure 2.

 Contribution of miRNAs to physiological and pathological processes in humans. miRNAs have been implicated in numerous biological processes such as developmental timing, cell proliferation and differentiation, apoptosis, anti-viral defence, development and function of immune system, control of inflammation and other unknown functions. The imbalance in expression of miRNAs crucial for physiological processes may lead to the development of pathological processes such as developmental abnormalities, musculoskeletal and cardiovascular disorders, schizophrenia, cancer and various inflammatory and immune system diseases.

Based on the crucial role of miRNAs in human physiology, their abnormal expression (upregulation or downregulation) may lead to the development of such a diverse diseases such as cancer, cardiovascular disorders, schizophrenia, musculoskeletal disorders, lung diseases and developmental abnormalities [9] (Fig. 2). Recently, an intensive research has focused on delineating the role of miRNAs in the development of inflammatory and immune-mediated diseases (Table 2). In this review, therefore, we describe the current state of the art on the miRNA expression profiles in immune-mediated diseases in humans. It is obvious that we simplified the classification of immune-mediated diseases to show the potential of miRNAs in a broad range of pathologies of immune system. We are also aware that the list of references is far from complete owing to the extreme pace of research developments in this area.

Table 2.   Selection of miRNAs and their targets associated with immune-mediated diseases.
DiseaseExpression ↑/TargetaReferenceExpression ↓/TargetaReference
  1. aOnly those targets cited in the corresponding reference are listed.

  2. *The less predominant form arising from precursor miRNA (the ‘passenger’ strand).

  3. AT1R, angiotensin II type 1 receptor; CDK-2, cell division protein kinase 2; CTLA-4, cytotoxic T lymphocyte protein 4; ERK1/2, mitogen-activated protein kinase 3; ETS1, v-ets erythroblastosis virus E26 oncogene homolog 1; E2F1, E2F transcription factor 1; FADD, Fas-associated death domain; FGFR2, fibroblast growth factor receptor 2; HOX, homeobox; IGFBP-5, insulin-like growth factor-binding protein 5; IKKε, IκB kinase ε; IRAK1, interleukin-1 receptor-associated kinase 1; IGF-1R, insulin-like growth factor 1 receptor; KLF13, Krueppel-like factor 13; MCP-1, monocyte chemoattractant protein 1; (MIP)-2α, lens fibre major intrinsic protein; MMP, matrix metalloproteinase; ORP9, oxysterol-binding protein-like 9; PDCD4, programmed cell death 4; PI3KR1, phosphatidylinositol 3-kinase regulatory alpha subunit; POU2AF1, POU domain class 2 associating factor 1; PTEN, phosphatase and tensin homologue; RhoA, ras homolog gene family, member A; Ripk1, receptor-interacting serine/threonine protein kinase 1; SMAD, mothers against decapentaplegic homolog; SOCS-3, Suppressor of cytokine signalling 3; SPRY, protein sprouty homolog; STAT5A, signal transducer and activator of transcription 5A; TIMP3, metalloproteinase inhibitor 3; TNF, tumour necrosis factor; TRAF6, tumour necrosis factor receptor-associated factor 6; VCAM-1, vascular cell adhesion protein; VEGF, vascular endothelial growth factor.

Autoimmune systemic/organ disorders
 Systemic lupus erythematosusmiR-142-3p,-342,-299-3p,-198,-298,-21,-189, -61,-78
miR-21,-148a
[12, 126]

[15]
miR-17-5p/E2F1
miR-196a/Hox-C8
miR-383, -409-3p, -184, -141, -112
miR-125a/
KLF13
miR-146a, miR-155
[12, 126]



[14]
[127, 128]
 Rheumatoid arthritismiR-146a/TRAF6, IRAK1
miR-155/FADD, IKKε, Ripk1, MMP-1/MMP-3
miR-132, miR-16
miR-203
/MMP-1, IL-6
miR-346/Bruton’s tyrosine kinase, control of TNF-α
miR-124a/CDK-2, MCP-1
let-7a, miR-26,-146a/b, -150, -155
miR-223
[17–19, 21, 23]
[17, 19]

[19, 88]
[24]
[25, 26]

[27]
[129]
[130]
miR-363, miR-498
miR-132
[23]
[88]
 Multiple sclerosismiR-326,-34,-155/CD47
miR-18b,-599,-96
miR-326
/ETS1
miR-93,-106b,-19a/b
miR-17-5p/
PI3KR1, PTEN
miR-145
[28]
[30]
[32]
[34]
[35]
[84]
miR-17, -20a[33]
 Inflammatory bowel diseasemiR-16,-21,-23a,-24,-29a,-126,-195,-let-7f
miR-21, miR-155
[38]

[131]
miR-192/(MIP)-2α
miR-375,-422b
miR-149*, miRplus-F1065, miR-505*
[38]

[85]
 PsoriasismiR-21,-205/TIMP3
miR-203/SOCS-3
miR-146a/TRAF6, IRAK1
miR-17,-20a,-31,-141, -142-3p,-146b-5p,-146a,-200a,-203
miR-99a
/IGF-1R
[40]
[41]
[42]
[42]

[45]
miR-221,-222/TIMP3
miR-125b/FGFR2, regulation of TNF-α pathway
[40]
[42, 44]
Inflammatory disorders
 Atherosclerosis (vascular inflammation)miR-155/AT1R, ERK1/2
miR-155, -221/ETS1
miR-21/PDCD4, PTEN, SPRY1/2
miR-125a-5p/ORP9
miR-222/STAT5A
miR-1,-133a,-133b,-499-5p
miR-146a
miR-150
miR-210
[49]
[50]
[51–55]

[56]
[57]
[87]
[99]
[132]
[133]
miR-126/VCAM-1
miR-26a/SMAD1/4
miR-122, -375
miR-15b,-16,-20a/b
/VEGF
miR-10a/Hox-A1
[48]
[58]
[87]
[134]
[135]
 OsteoarthritismiR-146a
miR-9/
MMP-13
miR-98
[63]
[64]
[64]
miR-140,-27a/MMP-13, IGFBP-5
miR-146
miR-27b/
MMP-13
miR-132, -16
[61, 62]

[64]
[65]
[88]
 Allergy and immune-mediated lung disordersmiR-21/IL-12p35
miR-126/POU2AF1
let-7, miR-29a, -155
miR-133a
/RhoA
[69]
[70]
[71]
[72]
  
 Atopic eczemamiR-21
miR-155/
CTLA-4
[41]
[74]
miR-125b/TNF-α pathway[41]

miRNAs in immune system diseases

Given the role of miRNAs in the physiology of immune system including the fine-tuning of T cell reactivity to antigens and of antibody response [10] (Table 1), it is conceivable that deregulation of involved miRNAs may lead to impaired tolerance against self-antigens and to the development of autoimmune diseases. Hereby, we provide an overview of the current knowledge about the miRNA profiles in autoimmune diseases, both organ-specific [rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease (IBD) and psoriasis] and systemic [systemic lupus erythematosus (SLE)]. Moreover, deregulation of miRNAs may lead to sustained inflammation, which is a hallmark of chronic inflammatory diseases [8]. We, therefore, summarize the findings about the role of miRNAs in immune system diseases associated with inflammation of distinct, representative body systems such as cardiovascular (atherosclerosis), musculoskeletal (osteoarthritis, OA), dermal (atopic eczema) and/or systemic states such as allergy. Even though intensive research in this area, the biological function of many deregulated miRNAs in immune-related diseases remains to be uncovered. For this reason, we concentrate only to those miRNAs, where the target genes in studied diseases have been identified. Nevertheless, to obtain a complete spectrum of miRNAs associated with immune-related diseases, we included also those miRNAs where the target mRNA has not been identified yet (Table 2). Many studies of miRNA profiling have been published in animal models of immune diseases; we, however, restricted our review mainly on reports from studies in humans.

Autoimmune disorders

A/Systemic disorders

Systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is characterized by the production of pathogenic autoantibodies to nuclear antigens and development of lupus nephritis; the disease affects the skin, joints, kidneys and other organs [11]. The miRNA profiling analysis in peripheral blood mononuclear cells (PBMCs) from patients with SLE revealed downregulation of miR-17-5p, a miRNA involved in the regulation of transcription factor E2F1 [12]. E2F1 factor has been shown to regulate the expression of interferon-inducible Ifi200 family genes (p200-family proteins), which are able to sense cytoplasmic DNA and thus contribute to the chronic inflammation [13]. In the same study, downregulation of miR-196a in SLE was associated with the upregulation of other transcription factor Hox-C8, a member of HOX transcription factors involved in the regulation of numerous downstream genes [12]. A recent study demonstrated that miR-125a has been also downregulated in patients with SLE; this miRNA has been shown to regulate CCL5/RANTES expression by targeting the transcription factor Kruppel-like factor (KLF) 13 in activated T cells [14]. Investigations of CD4+ T cells identified upregulation of miR-21 and miR-148a in patients with SLE; both miRNAs were linked to the DNA hypomethylation, an essential characteristics of apoptotic DNA against the antibodies in lupus are induced through inhibition of DNA methyltransferase 1 (DNMT1) [15].

B/Organ-specific disorders

Rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic inflammation within the joint tissue infiltrated by activated immune cells and by synovial hyperplasia, leading to cartilage and bone destruction after several years [16]. In synovial fibroblasts and tissues derived from patients with RA, an increased expression of miR-155, miR-146 and miR-146a has been reported [17–19]. It has been demonstrated that enforced expression of miR-155 in RA synovial fibroblasts represses the MMP-3 production and counteracts the induction of MMP-1 and MMP-3 by proinflammatory cytokines and toll-like receptor (TLR) ligands [17]. IRAK1 and TRAF6 genes were identified as miR-146a targets [20, 21]. Both IRAK1 and TRAF6 proteins are crucial early mediators for the regulation of the IL-1-induced MMP-13, a prominent matrix metalloproteinase involved in the degradation of cartilage in arthritis [22]. Moreover, the level of miR-146a expression positively correlated with TNF-α levels and negatively with Fas-associated factor 1, thus regulating T cell apoptosis [23]. miR-203, miR-346 and miR-124a are further miRNAs upregulated in RA synovial cells when compared with OA or healthy synovium [24–27]. The elevated miR-203 resulted in the increased secretion of MMP-1 and IL-6 via the NF-κB pathway [24]. miR-346 has been shown to suppress the IL-18 response of fibroblast-like synoviocytes by inhibiting the Bruton’s tyrosine kinase gene transcription [25]. Recent study showed that miR-346 controls also release of TNF-α, a major cytokine implicated in RA, by stabilization of tristetraprolin [26]. Finally, the decrease in miR-124a was associated with an increased proliferation of RA synovial cells and regulation of inflammation, whereas CDK-2 and CCL2/MCP-1 have been identified as miR-124a targets [27].

Multiple sclerosis

Multiple sclerosis (MS) is an autoimmune chronic disease of the central nervous system, characterized by inflammation and demyelination [28], which is driven by a T cell-mediated autoimmune reaction against the proteins of the myelin sheath [29]. Although numerous of miRNAs have been found deregulated in MS (Table 2), the function is described only for few of them. Of these with known functions, upregulated miR-96 in patients with MS has been associated with the regulation of interleukin and Wnt signalling pathways and, therefore, supposed to be an important player in the development/activation of the effector and regulatory T cells [30, 31]. Another miRNA involved in MS is a Th-17 cell–associated miR-326, expression of which was highly correlated with disease severity in patients with MS [32]. It has been shown that miR-326 promotes Th-17 differentiation by targeting Ets-1, a negative regulator of Th-17 differentiation [32]. Importantly, upregulation of miR-326, together with miR-34a and miR-155, was detected in the active MS lesions [28]. These miRNAs target CD47, a ‘don’t eat me’ signal involved in the inhibition of macrophage activity via interaction with signal regulatory protein α (SIRP)-α [28]. The downregulation of self-recognition signal CD47 caused by these miRNAs in MS lesions results in the reduced interaction of CD47:(SIRP)-α, thus promoting phagocytosis of myelin [28]. In another study in peripheral blood of patients with MS, miR-17 and miR-20a were downregulated, thus modulating T cell activation genes in a knock-in and knock-down T cell model [33]. Another miRNA found upregulated in PBMCs of patients with MS is miR-106b-25 cluster: it has been postulated that upregulation of miR-106b-25 cluster may lead to the disruption of the TGF-β signalling, thus promoting the MS development owing to the loss of a suppressive phenotype of the naive T cells [34]. Finally, miR-17-5p has been found upregulated in CD4+ cells from patients with MS: its expression correlated with downregulation of phosphatase and tensin homology (PTEN) and PI3KR1 (phosphatidyl-inositol-3-kinase regulatory subunit 1) [35], genes involved in the regulation of oligodendrocyte differentiation and myelination [36].

Inflammatory bowel disease

Inflammatory bowel disease (IBD), comprising Crohn’s disease (CD) and ulcerative colitis (UC), is an autoimmune chronic, relapsing, inflammatory disorder caused by the enteric microflora in genetically predisposed patients with an immune dysregulation in the gastrointestinal tract [37]. miR-192, predominantly expressed in colonic epithelial cells, was found significantly decreased in the mucosa of patients with active UC compared to healthy tissues [38]. The macrophage inflammatory peptide (MIP)-2α, a chemokine expressed by epithelial cells, has been identified as the target of miR-192 [38]. In colon epithelial cells, induction of (MIP)-2α expression by TNF-α was accompanied by a reduction in miR-192 expression, and miR-192 was involved in the regulation of (MIP)-2α expression [38]. Although a number of differentially expressed miRNAs have been found in inactive colonic mucosa of patients with IBD, their targets remain unknown. However, it has been suggested that dysregulation of miRNA expression pre-exists in the quiescent colonic mucosa of patients with UC and CD and may play a key role in the sensitization of the quiescent mucosa to environmental factors and/or to IBD inducers such as commensal flora, thus leading to the onset and/or relapse of inflammation [39].

Psoriasis

Psoriasis is an autoimmune chronic inflammatory skin disease characterized by intense proliferation and abnormal differentiation of keratinocytes [40]. The psoriasis-affected skin showed upregulation of miR-203 when compared with healthy skin [41]. miR-203 in psoriatic plaques correlated with the downregulation of suppressor of cytokine signalling 3 (SOCS-3), which is involved in inflammatory response and keratinocyte functions [41]. Also, miR-146a has been shown to be overexpressed in psoriasis [42]. Because both of its targets (TRAF6 and IRAK1) are involved in the TNF-α signalling pathway regulation [20], it has been postulated that miR-146a may control TNF-α signalling in the skin [42]. Similarly, downregulation of miR-125b in psoriasis has been associated with elevated TNF-α production [42, 43]. Additionally, miR-125b has also been shown to modulate keratinocyte proliferation and differentiation, partially through the regulation of fibroblast growth factor receptor 2 (FGFR2) [44]. Another psoriasis-associated miRNA is miR-99a involved in the differentiation of keratinocytes through regulation of IGF-1R [45]. The elevation of IGF-1R in the psoriatic lesion drives the cells towards proliferation, and one of the possible physiological responses to such stimuli is the upregulation of miR-99a expression, which may slow keratinocyte proliferation and induce their differentiation [45]. Also, several other miRNAs, including miR-21, miR-205, miR-221 and miR-222, have been found deregulated in psoriatic skin [40]. Target prediction identified numerous potential mRNA targets of these miRNAs, including TIMP3, regulating cellular growth, proliferation, apoptosis and degradation of the extracellular matrix [40].

Inflammatory disorders

Atherosclerosis (Vascular inflammation)

Atherosclerosis is a multifactorial disease driven, in part, by chronic inflammation and leukocyte influx in response to cholesterol accumulation in the arterial wall causing the loss of endothelial integrity [46, 47]. miR-126 has been shown to suppress the expression of a vascular cell adhesion molecule 1 (VCAM-1), thereby decreasing leukocyte adherence to endothelial cells, a critical event leading to vascular inflammation [48]. Another miRNA associated with cardiovascular disease, miR-155, has been shown to repress angiotensin II type 1 receptor (AT1R), thus promoting inflammation, oxidative stress and cardiovascular remodelling when activated by angiotensin II [49]. Recently, high expression of miR-155 and miR-221 was detected in human umbilical vein endothelial cells and vascular smooth muscle cells [50]. Ets-1, a key endothelial transcription factor for inflammation, angiogenesis and vascular remodelling, has been identified as a candidate target for both miR-155 and miR-221 [50]. Further, miR-21 has been shown to have a cardioprotective effect resulting in lesser apoptosis [51–53] and to regulate the neointima lesion formation after angioplasty [54]. Programmed cell death 4 (PDCD4), phosphatase and tensin homology deleted from chromosome 10 (PTEN), sprouty1 (SPRY1) and sprouty2 (SPRY2) are the target genes of miR-21 [55]. Targets relevant for vascular inflammation have been identified also for miR-125a-5p, miR-222 and miR-26a. miR-125a-5p has been found to mediate lipid uptake and to decrease the secretion of some inflammatory cytokines (IL-2, IL-6, TNF-α and TGF-β) in oxLDL-stimulated monocyte-derived macrophages; oxysterol-binding protein-like 9 (ORP9) has been identified as a direct miR-125a-5p target [56]. Moreover, miR-222 has been involved in inflammation-mediated vascular remodelling through the regulation of a signal transducer and activator of transcription 5A (STAT5A) [57]. miR-26a alters TGF-β pathway signalling and promotes vascular smooth muscle cell proliferation while inhibiting cellular differentiation and apoptosis via targeting the SMAD1/4, members of the TGF-β superfamily signalling cascade [58]. Of note, several other miRNAs have been implicated in atherosclerosis [59] affecting the cell survival, migration and differentiation, but their direct targets remain uncovered.

Osteoarthritis

Osteoarthritis (OA) is a chronic, progressive, degenerative disease of the entire joint accompanied by a variable degree of inflammation of the synovial membrane driven by proinflammatory cytokines, namely IL-1β [60]. Study in articular cartilage showed that miR-140 expression is reduced in OA versus normal tissues [61]. In vitro treatment of chondrocytes with IL-1β, a cytokine involved in OA pathogenesis, suppressed miR-140 expression [61]. By contrast, transfection of chondrocytes with miR-140 downregulated IL-1β caused induction of ADAMTS5 [61]. Decrease in miR-140 and miR-27a was reported also in OA chondrocytes [62]. It has been postulated that miR-140 and miR-27a may target MMP-13 and IGFBP-5 and miR-140 may regulate also TGF-β [62]. Another miRNA induced by IL-1β is miR-146a, found intensely expressed in low-grade OA cartilage [63]. In the late-stage OA cartilage and bone, a number of differentially expressed miRNAs have been reported [64]. Of these, miR-9 and miR-98 were upregulated in OA bone and cartilage, while miR-146 was downregulated in OA cartilage [64]. Functional analysis of these three miRNAs revealed that they mediated the IL-1β-induced production of TNF-α in primary chondrocytes [64]. Moreover, miR-9 inhibited MMP-13 secretion in vitro [64]. Another miRNA associated with OA chondrocytes is miR-27b [65]. It has been shown that IL-1β-induced activation of MMP-13-associated signal transduction pathways resulted in miR-27b downregulation [65]. Although number of other OA-associated miRNAs has been reported, their mRNA targets need to be discovered in the future studies.

Allergy and immune-mediated lung disorders

miRNAs play a pivotal role also in the allergy and immune-mediated lung disorders. The knowledge about the miRNAs and their targets in the lung conditions is summarized in the recent reviews [66–68]. Briefly, miR-21 [69] and miR-126 [70] have been found associated with allergic lung inflammation. IL-12p35, a cytokine contributing to polarization of Th cells towards Th2 cells, has been identified as the direct target of miR-21 [69]. In relevance to the Th2 paradigm of allergic reaction, blockade of miR-126 resulted in augmented expression of POU domain class 2 associating factor 1 (POU2AF1), which activates the transcription factor PU.1 (SPI1), altering Th2 cell function via negative regulation of GATA3 expression [70]. Importantly, in vitro stimulation of bronchial epithelial cells with IL-4 and TNF-α showed that miRNAs such as let-7, miR-29a and miR-155 may help cells reset their protein profile in response to external stimuli in allergic inflammation; the exact mechanism is under investigation [71]. Also, miR-133a has been involved in the regulation of allergic inflammation: it negatively regulated the expression of RhoA, a key protein of bronchial smooth muscle contraction, thus contributing to the airway hyperresponsiveness in individuals with asthma [72]. Moreover, disruption of the binding sites for miR-148a, miR-148b and miR-152 in the HLA-G gene has been shown to contribute to the asthma susceptibility [73].

Atopic eczema

Atopic eczema (AE) is a common chronic skin disorder characterized by the presence of activated T cells within the skin accompanied by the activation of inflammatory cytokine network [74]. Downregulation of miR-125b in atopic eczema has been shown to regulate the TNF-α pathway [42, 43]. Recently, miR-155 has been found overexpressed in CD4+ T cells of patients with atopic eczema [74]. It has been postulated that miR-155 might contribute to chronic skin inflammation by increasing the proliferative response of Th cells through the downregulation of CTLA-4 – a critical costimulatory receptor of the T cell receptor required to counterbalance the proliferative nature of immune response [74].

miRNAs: promising diagnostic biomarkers

Recently, it has been shown that extracellular miRNAs are detectable in almost all body fluids and excretions, including urine, faeces, saliva, tear, ascetic, pleural and amniotic fluid [75–77]. Moreover, miRNAs have been shown to be relatively resistant against external impacts such as enzymatic degradation, freezing and thawing, or intense pH conditions [78, 79], probably due to their coexistence in lipid or lipoprotein complexes [80]. Based on these facts, miRNAs may represent promising diagnostic biomarkers. miR-21 was the first serum miRNA biomarker discovered: patients with diffuse large B cell lymphoma had high serum levels of miR-21, which was associated with increased relapse-free survival [81]. Subsequently, usefulness of the serum miRNAs for diagnosis and prognosis has been reported for solid cancers and leukaemias [80, 82, 83]. Recently, several studies have reported the potential of circulating miRNAs also for the diagnosis of various autoimmune and inflammatory disorders, such as multiple sclerosis [84], active UC and Crohn’s disease [85, 86], acute myocardial infarction [87], rheumatoid arthritis and OA [88] or SLE [89]. Taken together, there is growing evidence that the knowledge of disease-specific miRNAs in immune-mediated and inflammatory diseases may open new avenues for future miRNA-based diagnostics. Detailed investigations, directed at diagnostic performance (sensitivity, specificity, etc.), into these promising novel biomarkers are however required before miRNAs can enter the clinic.

Novel miRNA-based therapy

The first evidence about the usefulness of miRNA-based therapy was demonstrated in mouse models using miR-122 antisense oligonucleotide, which resulted in the decrease in hepatic fatty acid and cholesterol synthesis [90]. Recent reports showed that correction of the miRNA deficiencies by either antagonizing (antagomirs) or restoring (mimics) miRNA function may provide a therapeutic benefit also in human cells [91]. First reports in humans have been focused on the cancer therapy. It has been shown that the delivery of miRNAs that are highly expressed and, therefore, tolerated in normal tissues but lost in diseased cells may provide a general strategy for miRNA replacement therapies [92, 93]. When the miR-143/miR-145 were restored in pancreatic cancer cells in which their levels were repressed, the cells were no longer tumourigenic [92]. Similarly, the replacement of miR-26a, a miRNA expressed at lower levels in hepatocellular carcinoma cells, induced cell cycle arrest by targeting cyclins D2 and E2 [93].

Blockade of viral infections represents another highly promising field for miRNA-based therapy. There is evidence that miRNAs are crucial endogenous ‘host factors’ for the replication of viruses. The blockade of human miR-122 has been shown to block hepatitis C virus infection [94, 95]. Very promising is the blockade of miR-199a-3p, which possesses a broad antiviral capacity against single-stranded RNA viruses [96].

Although the miRNA-based therapy has not been applied in human immune disease yet, there has been growing evidence about their therapeutic potential in animal models of immune diseases. Antagonism of miR-126 function by anti-miR-126 suppressed the asthmatic phenotype in mice model of allergic asthma, whereas the decreasing Th2 response, airway hyperresponsiveness, infiltrating eosinophils and neutrophils, and mucus hypersecretion were reported when compared with control mice [70]. Also, recent studies in humans suggest that miRNAs may represent the basis for future therapeutic targets not only in cancer [97] but also in IBD [85, 98] and atherosclerosis [99]. Despite growing amount of promising data, further investigations and preclinical trials are prerequisite prior any therapeutic applications.

Comparison of miRNA profiles within immunopathologies

miRNAs have emerged as regulators of processes ongoing in a range of immune system diseases, including autoimmunity and chronic inflammatory diseases. Because most of the immune-mediated diseases involve similar pathways such as those comprising action of proinflammatory cytokines, molecules associated with extracellular matrix and/or impaired apoptosis, one may also anticipate similarities in miRNA profiles within the immune-pathologies. As expected, several identical miRNAs have been found deregulated across various immune-related diseases (e.g. miR-146, miR-155, miR-150 and miR-21). Of these, the majority has been already identified as key regulators of physiological immune reactions. The known targets of these miRNAs comprise proinflammatory cytokines, matrix metalloproteinases and regulatory molecules involved in TLR and NF-κB signalling pathways, as well as in apoptosis. However, the spectrum of mRNA targets for these ‘immune system-related’ miRNAs in particular immune diseases is still far from complete, and future studies are needed to identify them.

Besides the deregulation of miRNA expression targeting central components of the innate and adaptive immune system, recent studies showed that some miRNAs associated with immune-mediated disorders may regulate also disease-specific processes such as demyelination in multiple sclerosis, hypomethylation of DNA in lupus, expression of adhesive molecules in atherosclerosis etc. In addition, miRNAs can also play pivotal role in the regulation of the survival, migration and differentiation of characteristic disease-associated cell populations. In this context, it has been shown that miRNAs can control proliferation and differentiation of keratinocytes in psoriasis, proliferation of Th cells in atopic eczema, proliferation of synovial cells in rheumatoid arthritis, etc. Nevertheless, the actions/effects of many deregulated miRNAs in pathological processes of the immune system have been yet unexplored, and the identification of their respective mRNA targets has been still under investigation.

Conclusion

Taken together, miRNAs play a crucial role in physiology and pathology of immune system. However, the knowledge of miRNA targets and their mechanisms of function in immune-mediated diseases remains limited and needs to be explored. The understanding of the roles of miRNAs in the pathogenesis of immune-mediated diseases may provide a point from which we may embark into the promising new arena of miRNA therapeutics and diagnostics.

Acknowledgment

Funding was obtained from the Czech Ministry of Health (IGA MZ CR NT/11117, IGA MZ CR NT/11049) and in part by the Internal Grant Agency of Palacky University (IGA PU LF_2010_008) and by the Operational Programme Research and Development for Innovations (CZ.1.05/2.1.00/01.0030). The authors declare no conflicting financial interests.

Ancillary