TLR2 and TLR4 in autoimmune diseases: a comprehensive review

Innate immunity in diabetes and diabetic nephropathy

The innate immune system includes several classes of pattern recognition receptors (PRRs), including membrane-bound Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs). These receptors detect pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) in the extracellular and intracellular space. Intracellular NLRs constitute inflammasomes, which activate and release caspase-1, IL-1β, and IL-18 thereby initiating an inflammatory response. Systemic and local low-grade inflammation and release of proinflammatory cytokines are implicated in the development and progression of diabetes mellitus and diabetic nephropathy. TLR2, TLR4, and the NLRP3 inflammasome can induce the production of various proinflammatory cytokines and are critically involved in inflammatory responses in pancreatic islets, and in adipose, liver and kidney tissues. This Review describes how innate immune system-driven inflammatory processes can lead to apoptosis, tissue fibrosis, and organ dysfunction resulting in insulin resistance, impaired insulin secretion, and renal failure. We propose that careful targeting of TLR2, TLR4, and NLRP3 signalling pathways could be beneficial for the treatment of diabetes mellitus and diabetic nephropathy.

Annexin A2 binds to endosomes and negatively regulates TLR4-triggered inflammatory responses via the TRAM-TRIF pathway

Lipopolysaccharide (LPS) derived from Gram-negative bacteria activates plasma membrane signaling via Toll-like receptor 4 (TLR4) on host cells and triggers innate inflammatory responses, but the underlying mechanisms remain to be fully elucidated. Here we reveal a role for annexin A2 (AnxA2) in host defense against infection as anxa2^−/− mice were highly susceptible to Gram-negative bacteria-induced sepsis with enhanced inflammatory responses. Computing analysis and biochemical experiments identified that constitutive AnxA2 expression facilitated TLR4 internalization and its subsequent translocation into early endosomal membranes. It activated the TRAM-dependent endosomal signaling, leading to the release of anti-inflammatory cytokines. Importantly, AnxA2 deficiency prolonged TLR4-mediated signaling from the plasma membrane, which was attributable to pro-inflammatory cytokine production (IL-6, TNFα and IL-1β). Thus, AnxA2 directly exerted negative regulation of inflammatory responses through TLR4-initiated TRAM-TRIF pathway occurring on endosomes. This study reveals AnxA2 as a critical regulator in infection-initiated inflammation, which protects the host from excessive inflammatory damage.

Polymorphisms in Toll-like receptor genes are associated with vitiligo

Background: The members of Toll-like receptor (TLR) family are responsible for recognizing various molecular patterns associated with pathogens. Their expression is not confined to immune cells and have been detected in skin cells such as keratinocytes and melanocytes. As part of a generated response to pathogens, TLRs are involved in inducing inflammatory mediators to combat these threats. It is therefore not surprising that TLRs have been implicated in inflammatory skin diseases, including atopic dermatitis and psoriasis. Likewise, as key players in autoimmunity, they have been associated with a number of autoimmune diseases. Based on this, the role of TLRs in vitiligo could be suspected, but is yet to be clearly established.

Methods: In order to conduct a genetic association analysis, 30 SNPs were selected from TLR1-TLR8 and TLR10 regions to be genotyped in Estonian case-control cohort consisting of 139 vitiligo patients and 307 healthy control individuals. The patients were further analyzed in subgroups based on sex, age of onset, occurrence of vitiligo among relatives, extent of depigmented areas, vitiligo progression activity, appearance of Köbner's phenomenon, existence of halo naevi, and incidence of spontaneous repigmentation.

Results: The most notable finding came with SNP rs179020 situated in TLR7 gene, that was associated in entire vitiligo (Padj = 0.0065) and also several subgroup analyses. Other single marker and haplotype analyses pointed to TLR3, TLR4, and TLR10 genes.

Conclusions: This study investigated the genetic regions of nine TLR genes in relation to vitiligo susceptibility. The main results were the associations of TLR7 SNPs with vitiligo, while several other associations were obtained from the remaining TLR gene regions. This suggests that in addition to other inflammatory skin diseases, TLRs affect the development of vitiligo, thus making them interesting targets for future research.

Autoimmunity in 2013

The peer-reviewed publications in the field of autoimmunity published in 2013 represented a significant proportion of immunology articles and grew since the previous year to indicate that more immune-mediated phenomena may recognize an autoimmune mechanism and illustrated by osteoarthritis and atherosclerosis. As a result, our understanding of the mechanisms of autoimmunity is becoming the paradigm for translational research in which the progress in disease pathogenesis for both tolerance breakdown and inflammation perpetuation is rapidly followed by new treatment approaches and clinical management changes. The similarities across the autoimmune disease spectrum outnumber differences, particularly when treatments are compared. Indeed, the therapeutics of autoimmune diseases are based on a growing armamentarium that currently includes monoclonal antibodies and small molecules which act by targeting molecular markers or intracellular mediators with high specificity. Among the over 100 conditions considered as autoimmune, the common grounds are well illustrated by the data reported for systemic lupus erythematosus and rheumatoid arthritis or by the plethora of studies on Th17 cells and biomarkers, particularly serum autoantibodies. Further, we are particularly intrigued by studies on the genomics, epigenetics, and microRNA at different stages of disease development or on the safe and effective use of abatacept acting on the costimulation of T and B cells in rheumatoid arthritis. We are convinced that the data published in 2013 represent a promising background for future developments that will exponentially impact the work of laboratory and clinical scientists over the next years.

Structural basis for inhibition of TLR2 by staphylococcal superantigen-like protein 3 (SSL3)

Toll-like receptors (TLRs) are crucial in innate recognition of invading micro-organisms and their subsequent clearance. Bacteria are not passive bystanders and have evolved complex evasion mechanisms. Staphylococcus aureus secretes a potent TLR2 antagonist, staphylococcal superantigen-like protein 3 (SSL3), which prevents receptor stimulation by pathogen-associated lipopeptides. Here, we present crystal structures of SSL3 and its complex with TLR2. The structure reveals that formation of the specific inhibitory complex is predominantly mediated by hydrophobic contacts between SSL3 and TLR2 and does not involve interaction of TLR2-glycans with the conserved Lewis(X) binding site of SSL3. In the complex, SSL3 partially covers the entrance to the lipopeptide binding pocket in TLR2, reducing its size by ∼50%. We show that this is sufficient to inhibit binding of agonist Pam2CSK4 effectively, yet allows SSL3 to bind to an already formed TLR2-Pam2CSK4 complex. The binding site of SSL3 overlaps those of TLR2 dimerization partners TLR1 and TLR6 extensively. Combined, our data reveal a robust dual mechanism in which SSL3 interferes with TLR2 activation at two stages: by binding to TLR2, it blocks ligand binding and thus inhibits activation. Second, by interacting with an already formed TLR2-lipopeptide complex, it prevents TLR heterodimerization and downstream signaling.

Increased expression of TLR2 in CD4(+) T cells from SLE patients enhances immune reactivity and promotes IL-17 expression through histone modifications

The innate immune system has been shown to play an important pathologic role in systemic lupus erythematosus (SLE). TLR2, a PRR, recognizes exogenous PAMPs, and endogenous damage-associated molecular patterns and has been implicated in the initiation and maintenance of the perpetuated inflammatory reactions in autoimmune diseases. Here, we report increased expression of TLR2 in CD4(+) and CD8(+) T cells, CD19(+) B cells, and CD14(+) monocytes from SLE patients. Conventional treatment, such as hydroxychloroquine and corticosteroids, showed no effect on TLR2 expression in CD4(+) T cells from SLE patients. In vitro stimulation of TLR2 in CD4(+) T cells from SLE patients increased CD40L and CD70 expression, as well as secretion of IL-6, IL-17A, IL-17F, and TNF-α, while Foxp3 transcription decreased. This effect was reversed by TLR2 siRNA. Moreover, TLR2 activation upregulated H3K4 tri-methylation and H4 acetylation levels while downregulated H3K9 tri-methylation level in the IL-17A promoter region. In addition, it also increased H4 acetylation levels and decreased H3K9 tri-methylation levels in the IL-17F promoter region. In summary, our findings demonstrate that increased expression of TLR2 contributes to immune reactivity and promotes IL-17A and IL-17F expression through histone modifications in SLE.

The changing faces of IgG4-related disease: Clinical manifestations and pathogenesis

Since the earliest reports in 2001, immunoglobulin G4 (IgG4)-related disease has been defined as an autoimmune systemic disease characterized by the lymphoplasmacytic infiltration of affected tissues leading to fibrosis and obliterative phlebitis along with elevated serum IgG4 levels. Prior to this unifying hypothesis, a plethora of clinical manifestations were considered as separate entities despite the similar laboratory profile. The pathology can be observed in virtually all organs and may thus be a challenging diagnosis, especially when the adequate clinical suspicion is not present or when obtaining a tissue biopsy is not feasible. Nonetheless, the most frequently involved organs are the pancreas and exocrine glands but these may be spared. Immunosuppressants lead to a prompt clinical response in virtually all cases and prevent histological sequelae and, as a consequence, an early differential diagnosis from other conditions, particularly infections and cancer, as well as an early treatment should be pursued. We describe herein two cases in which atypical disease manifestations were observed, i.e., one with recurrent neck lymph node enlargement and proptosis, and one with jaundice. Our understanding of the pathogenesis of IgG4-related disease is largely incomplete but data support a significant role for Th2 cytokines with the contribution of innate immunity factors such as Toll-like receptors, macrophages and basophils. Further, macrophages activated by IL4 overexpress B cell activating factors and contribute to chronic inflammation and the development of fibrosis. We cannot rule out the possibility that the largely variable disease phenotypes reflect different pathogenetic mechanisms and the tissue microenvironment may then contribute to the organ involvement.

Innate immunity and the failing heart: the cytokine hypothesis revisited

Elevated levels of inflammatory mediators have been identified in patients with heart failure, including heart failure with reduced and preserved ejection fraction, as well as acute decompensated heart failure. Moreover, experimental studies have shown repeatedly that activation of inflammation in the heart provokes left ventricular remodeling and left ventricular dysfunction. Nonetheless, phase III clinical trials that have attempted to antagonize inflammatory mediators have been negative with respect to the primary end points of the trials, and in some patients, resulted in worsening heart failure or death. The following review will discuss how recent developments in the field of innate immunity have advanced our understanding of the role of inflammation in the pathogenesis of heart failure and will discuss the negative outcomes of the existing clinical trials in light of this new information.

Vascular smooth muscle cell in atherosclerosis

Vascular smooth muscle cells (VSMCs) exhibit phenotypic and functional plasticity in order to respond to vascular injury. In case of the vessel damage, VSMCs are able to switch from the quiescent 'contractile' phenotype to the 'proinflammatory' phenotype. This change is accompanied by decrease in expression of smooth muscle (SM)-specific markers responsible for SM contraction and production of proinflammatory mediators that modulate induction of proliferation and chemotaxis. Indeed, activated VSMCs could efficiently proliferate and migrate contributing to the vascular wall repair. However, in chronic inflammation that occurs in atherosclerosis, arterial VSMCs become aberrantly regulated and this leads to increased VSMC dedifferentiation and extracellular matrix formation in plaque areas. Proatherosclerotic switch in VSMC phenotype is a complex and multistep mechanism that may be induced by a variety of proinflammatory stimuli and hemodynamic alterations. Disturbances in hemodynamic forces could initiate the proinflammatory switch in VSMC phenotype even in pre-clinical stages of atherosclerosis. Proinflammatory signals play a crucial role in further dedifferentiation of VSMCs in affected vessels and propagation of pathological vascular remodelling.

Effect of blueberry on hepatic expression of TLR4 in a rat model of alcoholic liver injury

AIM: To investigate the protective effect of blueberry in rats with alcoholic hepatic injury and explore the possible mechanism.

METHODS: Forty-eight healthy SD rats were randomly divided into a control group, a blueberry group, a magnesium isoglycyrrhizinate group and a model group. The rats in the control group were given physiological saline, and other groups were all given 56 mL/L ethanol solution intragastrically. The blueberry group and magnesium isoglycyrrhizinate group were, respectively, given blueberry juice and magnesium isoglycyrrhizinate solution 1 h after intragastric alcohol infusion. All SD rats were sacrificed after 4 wk. Pathological changes in the hepatic tissue were assessed by hematoxylin-eosin (HE) staining, and alanine aminotransferase (ALT), aspartate aminotransferase (AST), triglyceride (TG) and total cholesterol (TC) in serum were detected with automatic biochemical analyzer. The activity an of superoxide dismutase (SOD), the contents of tumor necrosis factor alpha (TNF-α) and interleukin 8 (IL-8) in liver homogenates were determined by enzyme linked immunosorbent assay (ELISA). The expression of Toll-like receptor 4 (TLR4) in liver tissue was detected by reverse transcription-PCR (RT-PCR) and Western blot.

RESULTS: In the control group, hepatocytes were intact, and no vacuoles were observed. In the model group, vacuoles were noted in the cytoplasm of hepatocytes. The degree of cytoplasmic vacuolation was significantly alleviated in the blueberry group and magnesium isoglycyrrhizinate group. The levles of ALT, AST, TG, and TC in the model group were significantly higher than those in the blueberry group and magnesium isoglycyrrhizinate group (F = 47.984, 14.974, 149.338, 94.896, P < 0.01 for all). Compared with the model group, the activities of SOD in liver homogenates were significantly higher (F = 33.102, P < 0.01) and the contents of TNF-α and IL-8 in liver homogenates were significantly lower (F = 76.250, 37.971, P < 0.01 for both) in the blueberry group and magnesium isoglycyrrhizinate group. The expression of hepatic TLR4 in the model group was significantly higher than that in the blueberry group and magnesium isoglycyrrhizinate group (F = 141.932, 62.750, P < 0.01 for both).

CONCLUSION: Blueberry exerts a protective effect on alcoholic hepatic injury in rats, and the mechanism might be associated with down-regulating the expression of TLR4, reducing the contents of hepatic TNF-α and IL-8, and up-regulating the activity of hepatic SOD.

Signaling mechanisms regulating B-lymphocyte activation and tolerance

It is becoming more and more accepted that, in addition to producing autoantibodies, B lymphocytes have other important functions that influence the development of autoimmunity. For example, autoreactive B cells are able to produce inflammatory cytokines and activate pathogenic T cells. B lymphocytes can react to extracellular signals with a range of responses from anergy to autoreactivity. The final outcome is determined by the relative contribution of signaling events mediated by activating and inhibitory pathways. Besides the B cell antigen receptor (BCR), several costimulatory receptors expressed on B cells can also induce B cell proliferation and survival, or regulate antibody production. These include CD19, CD40, the B cell activating factor receptor, and Toll-like receptors. Hyperactivity of these receptors clearly contributes to breaking B-cell tolerance in several autoimmune diseases. Inhibitors of these activating signals (including protein tyrosine phosphatases, deubiquitinating enzymes and several adaptor proteins) are crucial to control B-cell activation and maintain B-cell tolerance. In this review, we summarize the inhibitory signaling mechanisms that counteract B-cell activation triggered by the BCR and the coreceptors.

Innate immunity alterations in idiopathic interstitial pneumonias and rheumatoid arthritis-associated interstitial lung diseases

BACKGROUND

This is a prospective cohort study elucidating innate immunity in idiopathic pulmonary fibrosis (IPF), cryptogenic organizing pneumonia (COP), rheumatoid arthritis-associated usual interstitial pneumonia (RA-UIP) and RA-associated non specific interstitial pneumonia (RA-NSIP).

METHODS

23 IPF subjects, 9 COP subjects, 5 RA-UIP subjects, 8 RA-NSIP subjects were enrolled. 10 subjects were excluded. 19 healthy subjects served as controls. Blood and bronchoalveolar lavage (BAL) were obtained. Natural killer (NK) and NKT cells, NK cells apoptosis and the expression of triggering receptor expressed on myeloid cells type 1 (TREM-1) were assessed. Tumor necrosis factor-α (TNF-α) production was measured in cell cultures after stimulation with lipopolysaccharide endotoxin (LPS) and Pam3CysSK3, and in BAL. Surface expression of Toll-like receptors (TLR) 2 and 4 on peripheral blood monocytes (PBMC's) and circulating NK cells was also assessed.

RESULTS

RA-NSIP had low blood NKs, marginally insignificant (p=0.07). These NKs poorly produced TNF-α after LPS stimulation. TLR's expression on NK cells was similar throughout disease groups and controls. PBMC's mainly from IPF patients exhibited low TNF-α production after LPS stimulation but not after Pam3CysSK3 stimulation, while TLR4 expression on PBMC's was found normal in all study groups. TLR2 expression on PBMC's was increased in IPF, but mainly in COP, RA-UIP and RA-NSIP (p=0.015). TREM-1 expression was significant on COP monocytes and on COP neutrophils versus controls. RA-NSIP monocytes also exhibited TREM-1 expression (p=0.07). Decreased TNF-α concentration in BAL was finally observed in IPF and RA-UIP.

CONCLUSIONS

Innate immunity in the lungs and the peripheral circulation in IPF and RA-UIP are similar and more fibrotic than in RA-NSIP which is characterized by NK cell depletion and dysfunction. TREM-1 and TLR's likely affect patterns of inflammation in various interstitial lung diseases.

TLR2, TLR4 and CD86 gene polymorphisms in recurrent aphthous stomatitis

BACKGROUND

Recurrent aphthous stomatitis (RAS) is an inflammatory disease induced by genetic and environmental factors. Toll-like receptor (TLR) and CD86 are essential components for innate immunity and cellular immune response. We aimed to determine whether inheritance of specific TLR2, TLR4and CD86 gene polymorphisms are associated with RAS.

METHODS

Ninety-six patients with RAS and 153 controls were studied. Eight SNPs were genotyped using PCR-RFLP technique; four in TLR2 gene: rs4696480, rs3804100, rs121917864, rs5743708; three in TLR4 gene: rs10759931, rs4986790 rs1927911; and one in CD86 gene rs17281995. Association was assessed by logistic regression analysis. Linkage disequilibrium (LD) was assessed using the Haploview program.

RESULTS

Significant increase in inheritance of A allele (OR = 1.6, P = 0.01) and AA genotype (OR = 3.89, P = 0.01) of TLR4 rs10759931 was observed in cases. TLR4rs1927911 C allele and CC genotype were also increased (OR = 1.60 and 2.78 respectively); however, this was not statistically significant (P = 0.02 and 0.03 respectively). TLR2 and CD86 did not show association with RAS.

CONCLUSIONS

This is the first study to investigate the association of TLR and CD86 with RAS. We found a significant association between TLR4 rs10759931 polymorphism and RAS. Confirmatory studies in other populations and functional investigations are needed to determine the role of TLR4 in RAS.

Tenascin-C aggravates autoimmune myocarditis via dendritic cell activation and Th17 cell differentiation

Background

Tenascin‐C (TN‐C), an extracellular matrix glycoprotein, appears at several important steps of cardiac development in the embryo, but is sparse in the normal adult heart. TN‐C re‐expresses under pathological conditions including myocarditis, and is closely associated with tissue injury and inflammation in both experimental and clinical settings. However, the pathophysiological role of TN‐C in the development of myocarditis is not clear. We examined how TN‐C affects the initiation of experimental autoimmune myocarditis, immunologically.

Methods and Results

A model of experimental autoimmune myocarditis was established in BALB/c mice by immunization with murine α‐myosin heavy chains. We found that TN‐C knockout mice were protected from severe myocarditis compared to wild‐type mice. TN‐C induced synthesis of proinflammatory cytokines, including interleukin (IL)‐6, in dendritic cells via activation of a Toll‐like receptor 4, which led to T‐helper (Th)17 cell differentiation and exacerbated the myocardial inflammation. In the transfer experiment, dendritic cells loaded with cardiac myosin peptide acquired the functional capacity to induce myocarditis when stimulated with TN‐C; however, TN‐C‐stimulated dendritic cells generated from Toll‐like receptor 4 knockout mice did not induce myocarditis in recipients.

Conclusions

Our results demonstrated that TN‐C aggravates autoimmune myocarditis by driving the dendritic cell activation and Th17 differentiation via Toll‐like receptor 4. The blockade of Toll‐like receptor 4‐mediated signaling to inhibit the proinflammatory effects of TN‐C could be a promising therapeutic strategy against autoimmune myocarditis.

Interaction of Pattern Recognition Receptors with Mycobacterium Tuberculosis

Tuberculosis (TB) is considered a major worldwide health problem with 10 million new cases diagnosed each year. Our understanding of TB immunology has become greater and more refined since the identification of Mycobacterium tuberculosis (MTB) as an etiologic agent and the recognition of new signaling pathways modulating infection. Understanding the mechanisms through which the cells of the immune system recognize MTB can be an important step in designing novel therapeutic approaches, as well as improving the limited success of current vaccination strategies. A great challenge in chronic disease is to understand the complexities, mechanisms, and consequences of host interactions with pathogens. Innate immune responses along with the involvement of distinct inflammatory mediators and cells play an important role in the host defense against the MTB. Several classes of pattern recognition receptors (PRRs) are involved in the recognition of MTB including Toll-Like Receptors (TLRs), C-type lectin receptors (CLRs) and Nod-like receptors (NLRs) linked to inflammasome activation. Among the TLR family, TLR1, TLR2, TLR4, and TLR9 and their down-stream signaling proteins play critical roles in the initiation of the immune response in the pathogenesis of TB. The inflammasome pathway is associated with the coordinated release of cytokines such as IL-1β and IL-18 which also play a role in the pathogenesis of TB. Understanding the cross-talk between these signaling pathways will impact on the design of novel therapeutic strategies and in the development of vaccines and immunotherapy regimes. Abnormalities in PRR signaling pathways regulated by TB will affect disease pathogenesis and need to be elucidated. In this review we provide an update on PRR signaling during M. tuberculosis infection and indicate how greater knowledge of these pathways may lead to new therapeutic opportunities.