Nucleic Acid-sensing Toll-like Receptors Research Articles

Nucleic Acid-Sensing Toll-like Receptors Are Essential for the Control of Endogenous Retrovirus Viremia and ERV-Induced Tumors

The genome of vertebrates contains endogenous retroviruses (ERVs) that are largely nonfunctional relicts of ancestral germline infection by exogenous retroviruses. However, in some mouse strains ERVs are actively involved in disease. Here we report that nucleic acid-recognizing Toll-like receptors 3, 7, and 9 (TLR 3, TLR7, and TLR9) are essential for the control of ERVs. Loss of TLR7 function caused spontaneous retroviral viremia that coincided with the absence of ERV-specific antibodies. Importantly, additional TLR3 and TLR9 deficiency led to acute T cell lymphoblastic leukemia, underscoring a prominent role for TLR3 and TLR9 in surveillance of ERV-induced tumors. Experimental ERV infection induced a TLR3-, TLR7-, and TLR9-dependent group of "acute-phase" genes previously described in HIV and SIV infections. Our study suggests that in addition to their role in innate immunity against exogenous pathogens, nucleic acid-recognizing TLRs contribute to the immune control of activated ERVs and ERV-induced tumors.

UNC93B1 mediates innate inflammation and antiviral defense in the liver during acute murine cytomegalovirus infection.

Antiviral defense in the liver during acute infection with the hepatotropic virus murine cytomegalovirus (MCMV) involves complex cytokine and cellular interactions. However, the mechanism of viral sensing in the liver that promotes these cytokine and cellular responses has remained unclear. Studies here were undertaken to investigate the role of nucleic acid-sensing Toll-like receptors (TLRs) in initiating antiviral immunity in the liver during infection with MCMV. We examined the host response of UNC93B1 mutant mice, which do not signal properly through TLR3, TLR7 and TLR9, to acute MCMV infection to determine whether liver antiviral defense depends on signaling through these molecules. Infection of UNC93B1 mutant mice revealed reduced production of systemic and liver proinflammatory cytokines including IFN-α, IFN-γ, IL-12 and TNF-α when compared to wild-type. UNC93B1 deficiency also contributed to a transient hepatitis later in acute infection, evidenced by augmented liver pathology and elevated systemic alanine aminotransferase levels. Moreover, viral clearance was impaired in UNC93B1 mutant mice, despite intact virus-specific CD8+ T cell responses in the liver. Altogether, these results suggest a combined role for nucleic acid-sensing TLRs in promoting early liver antiviral defense during MCMV infection.

Heat shock protein gp96 regulates Toll-like receptor 9 proteolytic processing and conformational stability

Nucleic acid-sensing Toll-like receptors (TLRs) initiate innate immune responses to foreign RNA and DNA, yet can detect and respond to host DNA. To avoid autoimmune pathologies, nucleic acid sensing TLRs are tightly regulated. TLR9 primarily resides in the endoplasmic reticulum, traffics to endosomes, is proteolytically processed and responds to DNA. The heat shock protein gp96 is one of several accessory proteins that regulate intracellular trafficking of TLR9. In the absence of gp96, TLR9 fails to exit the endoplasmic reticulum, and therefore gp96-deficient macrophages fail to respond to CpG DNA. However, absence of gp96 precludes studies on potential chaperoning functions of gp96 for TLR9. Here we demonstrate that pharmacologic interference with gp96 function inhibits TLR9 signaling. TLR9 remains associated with gp96 during intracellular trafficking, and gp96-specific inhibitors increase TLR9 sensitivity to proteolytic degradation. We propose that gp96 is critical for both TLR9 egress from the ER, and for protein conformational stability in the endosomal compartment. These studies highlight the importance of examining gp96-specific inhibitors for modulating TLR9 activation, and the treatment autoimmune diseases.

Toll-Like Receptor 7 Ligands Inhibit Influenza A Infection in Chickens

Avian influenza virus is endemic in many regions around the world and remains a pandemic threat, a scenario tied closely to outbreaks of the virus in poultry. The innate immune system, in particular the nucleic acid-sensing toll-like receptors (TLRs) -3, -7, -8, and -9, play a major role in coordinating antiviral immune responses. In this study we have investigated the use of TLR ligands as antivirals against influenza A in chickens. The TLR7 ligand poly-C inhibited low-path influenza A growth in the chicken macrophage cell line HD-11 more effectively than poly(I:C), which acts via TLR3. The TLR7 ligand 7-allyl-8-oxoguanosine (loxoribine) inhibited influenza A replication in vitro and in ovo in a dose-dependent manner. Treatment of primary chicken splenocytes with loxoribine resulted in the induction of interferons-α, -β, and -λ, and interferon-stimulated genes PKR and Mx. These results demonstrate that nucleic acid-sensing TLR ligands show considerable potential as antivirals in chickens and could be incorporated into antiviral strategies.

Nucleic acid-binding polymers as anti-inflammatory agents

Dead and dying cells release nucleic acids. These extracellular RNAs and DNAs can be taken up by inflammatory cells and activate multiple nucleic acid-sensing toll-like receptors (TLR3, 7, 8, and 9). The inappropriate activation of these TLRs can engender a variety of inflammatory and autoimmune diseases. The redundancy of the TLR family encouraged us to seek materials that can neutralize the proinflammatory effects of any nucleic acid regardless of its sequence, structure or chemistry. Herein we demonstrate that certain nucleic acid-binding polymers can inhibit activation of all nucleic acid-sensing TLRs irrespective of whether they recognize ssRNA, dsRNA or hypomethylated DNA. Furthermore, systemic administration of such polymers can prevent fatal liver injury engendered by proinflammatory nucleic acids in an acute toxic shock model in mice. Therefore these polymers represent a novel class of anti-inflammatory agent that can act as molecular scavengers to neutralize the proinflammatory effects of various nucleic acids.

Negative regulation of signaling by a soluble form of toll‐like receptor 9

Nucleic acid structures are highly conserved through evolution and when self nucleic acids are aberrantly detected by toll-like receptors (TLRs) they contribute to autoimmune disease. For this reason, multiple regulatory mechanisms exist to prevent immune responses to self nucleic acids. TLR9 is a nucleic acid-sensing TLR that is regulated at multiple levels including association with accessory proteins, intracellular localization and proteolytic processing. In the endolysosomal compartment TLR9 is proteolytically processed to an 80 kDa form (p80) and this processing is a prerequisite for activation. Here, we identified a soluble form of TLR9 (sTLR9) generated by a novel proteolytic event that cleaved TLR9 between amino acids 724-735. Similar to p80, sTLR9 was generated in endosomes. However, generation of sTLR9 was independent of the cysteine protease cathepsin B, active at acidic pH, but partially dependent on cathepsin S, a protease active at neutral pH. Most importantly, sTLR9 inhibited TLR9-dependent signaling. Altogether, these data support a model where an intrinsic proteolytic processing mechanism negatively regulates TLR9 signaling. A proper balance between the independent proteolytic events probabably contributes to regulation of TLR9-mediated innate immunity and prevention of autoimmune disease.

Differential role of nucleic acid-sensing toll-like receptors in autoantibody production. (44.29)

Abstract SLE is associated with a diversity of autoAbs, of which some are relatively specific for SLE, whereas others are also found in other autoimmune diseases. Nucleic acid sensing-TLRs (TLR3, 7-9; NA-TLRs) through the engagement of NA-containing material alone or in immune-complexes have been shown to play a critical role in lupus pathogenesis. However it is unclear to what extent NA-TLR signaling also promotes autoAbs to self-antigens (Ags) unrelated to NA. Here, we studied the role of NA-TLRs in MRL-lpr mice using the 3d mutation that abolishes NA-TLR signaling. In 3d mice, IgG and IgM antibodies to nuclear and RNA-associated Ags were essentially completely abolished, whereas other autoAbs with potential to cross-react to NAs (cardiolipin, and β2-GP1) were reduced to a lesser extent, and Abs unrelated to NAs (desmoglein3) were unaffected. Using a novel model that addresses loss of tolerance to B cells that acquire self-reactivity after class-switch recombination, it was found that NA-TLRs did not play an essential role in this process. Absence of NA-TLR signaling reduced lymphoproliferation, splenomegaly, and glomerulonephritis, but did not affect the incidence or severity of skin disease consistent with no reduction in cryoglobulins. Thus, in the MRL-lpr model, NA-TLRs are necessary for nucleic acid-associated autoantibody specificities and glomerulonephritis, but to varying degrees for other autoAb specificities and cutaneous disease.

Antiviral responses induced by the TLR3 pathway

Antiviral responses are successively induced in virus-infected animals, and include primary innate immune responses such as type I interferon (IFN) and cytokine production, secondary natural killer (NK) cell responses, and final cytotoxic T lymphocyte (CTL) responses and antibody production. The endosomal Toll-like receptors (TLRs) and cytoplasmic RIG-I-like receptors (RLRs), which recognize viral nucleic acids, are responsible for virus-induced type I IFN production. RLRs are expressed in most tissues and cells and are primarily implicated in innate immune responses against various viruses through type I IFN production, whereas nucleic acid-sensing TLRs, TLRs 3, 7, 8 and 9, are expressed on the endosomal membrane of dendritic cells (DCs) and play distinct roles in antiviral immunity. TLR3 recognizes viral double-stranded RNA taken up into the endosome and serves to protect the host against viral infection by the induction of a range of responses including type I IFN production and DC-mediated activation of NK cells and CTLs, although the deteriorative role of TLR3 has also been reported in some virus infections. Here, we review the current knowledge on the role of TLR3 during viral infection, and the current understanding of the TLR3-signalling cascade that operates via the adaptor protein TICAM-1 (also called TRIF).

Targeting Toll-Like Receptors for Treatment of SLE

Toll-like receptors (TLRs) are important innate immune receptors for the identification and clearance of invading pathogens. Twelve TLRs that recognize various conserved components of microorganisms are currently known. Among these, the endosomal TLRs 3, 7/8, and 9 recognize dsRNA, ssRNA, and CpG DNA, respectively. Nucleic acid-sensing TLRs, TLR 7 in particular, have been implicated in systemic lupus erythematosus (SLE) and are thought to exacerbate disease pathology. Activation of these TLRs results in the production of inflammatory cytokines and type I interferon. Genome-wide association studies, single nucleotide polymorphism analyses as well as experimental mouse models have provided evidence of TLR signaling involvement in SLE and other autoimmune diseases. Since activation of these receptor pathways promotes autoimmune phenotypes, inhibitory drugs that target these pathways constitute important new therapeutic strategies for the treatment of systemic autoimmunity.

Endosomal TLR signaling is required for anti-nucleic acid and rheumatoid factor autoantibodies in lupus

Using the Unc93b1 3d mutation that selectively abolishes nucleic acid-binding Toll-like receptor (TLR) (TLR3, -7, -9) signaling, we show these endosomal TLRs are required for optimal production of IgG autoAbs, IgM rheumatoid factor, and other clinical parameters of disease in 2 lupus strains, B6-Fas(lpr) and BXSB. Strikingly, treatment with lipid A, an autoAb-inducing TLR4 agonist, could not overcome this requirement. The 3d mutation slightly reduced complete Freund's adjuvant (CFA)-mediated antigen presentation, but did not affect T-independent type 1 or alum-mediated T-dependent humoral responses or TLR-independent IFN production induced by cytoplasmic nucleic acids. These findings suggest that nucleic acid-sensing TLRs might act as an Achilles' heel in susceptible individuals by providing a critical pathway by which relative tolerance for nucleic acid-containing antigens is breached and systemic autoimmunity ensues. Importantly, this helps provide an explanation for the high frequency of anti-nucleic acid Abs in lupus-like systemic autoimmunity.

Type I interferon receptor controls B-cell expression of nucleic acid-sensing Toll-like receptors and autoantibody production in a murine model of lupus

IntroductionSystemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by the production of high-titer IgG autoantibodies directed against nuclear autoantigens. Type I interferon (IFN-I) has been shown to play a pathogenic role in this disease. In the current study, we characterized the role of the IFNAR2 chain of the type I IFN (IFN-I) receptor in the targeting of nucleic acid-associated autoantigens and in B-cell expression of the nucleic acid-sensing Toll-like receptors (TLRs), TLR7 and TLR9, in the pristane model of lupus.MethodsWild-type (WT) and IFNAR2-/- mice were treated with pristane and monitored for proteinuria on a monthly basis. Autoantibody production was determined by autoantigen microarrays and confirmed using enzyme-linked immunosorbent assay (ELISA) and immunoprecipitation. Serum immunoglobulin isotype levels, as well as B-cell cytokine production in vitro, were quantified by ELISA. B-cell proliferation was measured by thymidine incorporation assay.ResultsAutoantigen microarray profiling revealed that pristane-treated IFNAR2-/- mice lacked autoantibodies directed against components of the RNA-associated autoantigen complexes Smith antigen/ribonucleoprotein (Sm/RNP) and ribosomal phosphoprotein P0 (RiboP). The level of IgG anti-single-stranded DNA and anti-histone autoantibodies in pristane-treated IFNAR2-/- mice was decreased compared to pristane-treated WT mice. TLR7 expression and activation by a TLR7 agonist were dramatically reduced in B cells from IFNAR2-/- mice. IFNAR2-/- B cells failed to upregulate TLR7 as well as TLR9 expression in response to IFN-I, and effector responses to TLR7 and TLR9 agonists were significantly decreased as compared to B cells from WT mice following treatment with IFN-α.ConclusionsOur studies provide a critical link between the IFN-I pathway and the regulation of TLR-specific B-cell responses in a murine model of SLE.

PDC expressing CD36, CD61 and IL-10 may contribute to propagation of immune tolerance

Human plasmacytoid dendritic cells (PDC) are blood dendritic cell antigen 2 (BDCA2) and blood dendritic cell antigen 4 (BDCA4) positive leukocytes that do not express common lineage markers. They have been described as proinflammatory innate immune cells and are the major source of αIFN in the human body. PDC-derived secretion of type I IFNs upon triggering of nucleic acid-sensing toll-like receptors (TLR) primes immune cells to rapidly respond to microbial stimuli and promotes a Th1 response. Here, we report that human PDC express CD36 and CD61 (β3 integrin), both involved in uptake of apoptotic cells and in induction of tolerance. Freshly isolated PDC and PDC within human blood leukocytes constitutively express IL-10. Thus, PDC may possess a so far neglected role in propagation of immune tolerance.

A critical link between Toll-like receptor 3 and type II interferon signaling pathways in antiviral innate immunity

A conundrum of innate antiviral immunity is how nucleic acid-sensing Toll-like receptors (TLRs) and RIG-I/MDA5 receptors cooperate during virus infection. The conventional wisdom has been that the activation of these receptor pathways evokes type I IFN (IFN) responses. Here, we provide evidence for a critical role of a Toll-like receptor 3 (TLR3)-dependent type II IFN signaling pathway in antiviral innate immune response against Coxsackievirus group B serotype 3 (CVB3), a member of the positive-stranded RNA virus family picornaviridae and most prevalent virus associated with chronic dilated cardiomyopathy. TLR3-deficient mice show a vulnerability to CVB3, accompanied by acute myocarditis, whereas transgenic expression of TLR3 endows even type I IFN signal-deficient mice resistance to CVB3 and other types of viruses, provided that type II IFN signaling remains intact. Taken together, our results indicate a critical cooperation of the RIG-I/MDA5-type I IFN and the TLR3-type II IFN signaling axes for efficient innate antiviral immune responses.

TLR7/9 Antagonists as Therapeutics for Immune-Mediated Inflammatory Disorders

There is an increasing interest in ligands of nucleic acid-sensing Toll-like receptors (TLR), especially TLR7 and TLR9, for pharmacological intervention in various diseases. The TLR7 agonist imiquimod is currently used as a topical treatment for genital warts caused by human papillomavirus (HPV), actinic keratosis (AK) and superficial basal cell carcinoma. Oligodeoxynucleotides (ODN) TLR9 agonists are currently in clinical trials for use in lung cancer, as anti-viral therapy, as adjuvants and as immune modulators in asthma and allergies. TLR7/9 antagonists, such as the anti-malaria drugs chloroquine, hydroxychloroquine and quinacrine, have been used since the 1950s to treat immune-mediated inflammatory disorders (IMID) such as rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and Sjögren's syndrome. However, the use of these anti-malarials in IMID is limited due to the side effects or suboptimal efficacy. Pre-clinical animal models as well as genetic linkage studies have indicated that TLR7/9 play a pivotal role in the aforementioned as well as other IMID such as multiple sclerosis (MS), inflammatory bowl disease (IBD)/colitis and psoriasis. Recent evidence has suggested that selective, specific antagonists for TLR7 and/or 9 might be more beneficial in certain diseases, such as SLE. Thus, the use of suppressive ODN or novel small molecule TLR7/9 inhibitors with a larger safety window and differentiated selectivity may potentially have significant clinical utility in these IMID. Herein, we review efforts to develop novel TLR7/9 antagonists and the rationale for the use of such therapeutics in a variety of IMID.

Activation of Nucleic Acid-Sensing Toll-Like Receptors Induces Proliferation, Cytokine Production, Immunogenic Phenotype, and Plasma Cell Differentiation of CLL Cells and Immunoglobulin Production.

Weak immune function of chronic lymphocytic leukemia (CLL) cells may contribute to disease progression and inhibit effective immunotherapy. Accordingly, agents that enhance the function of CLL cells may be useful in immunotherapeutic approaches to this disease. Since Toll-like receptors (TLRs) are major regulators of B-cell function in innate and adaptive immunity, we asked to what extent stimulating two nucleic acid-sensing TLRs affected the activation and differentiation of CLL B cells. Isolated CLL cells were treated with either the TLR7-activating ligand, gardiquimod, or the TLR9-activating ligand ODN 2006-G5. CLL cells were also treated with the contents of apoptotic leukemic B cells, produced by repeated freezing and thawing. Ability to induce proliferation, as defined by CFSE dilution studies, cytokine production, differentiation, and altered surface molecule expression was measured. Both TLR 7 and TLR 9 ligands individually induced high levels of proliferation and plasma cell differentiation of CLL B cells. A dramatic increase in percentage of dividing cells was seen with simultaneous TLR7 and TLR9 activation, while blocking the TLR 7/9 pathway by pre-treating CLL B cells with chloroquine or bafilomycin reduced the proliferative capacity by 90%. Furthermore, stimulating CLL B cells with TLR7 and 9 ligands significantly up-regulated activation (CD38, CD69 and CD83), adhesion (CD54), and other immunophenotypic (CD40, CD80 and CD86) markers. Enzyme immunoassays of culture supernatants after TLR 7 and TLR 9 stimulation indicated that the levels of soluble IgM increased considerably, although levels of cytoplasmic IgM were unchanged. Production of IL6 and IL10 was also induced. We propose that ligands for TLRs 9 and 7 stimulate CLL B cells to proliferate and differentiate into plasma cells in the absence of T-cell help. In addition these stimuli alter CLL cell surface phenotype to a more immunogenetic profile, providing a rationale for CpG oligonucleotides and gardiquimod as humoral vaccine adjuvants for both normal immune reactions and potentially for anti-leukemic therapy.

The role of toll-like receptors in systemic lupus erythematosus

Systemic lupus erythematosus is an autoimmune disease characterized by the production of autoantibodies against a relatively limited range of nuclear antigens. These autoantibodies result in the formation of immune complexes that deposit in tissues and induce inflammation, thereby contributing to disease pathology. Growing evidence suggests that recognition of nucleic acid motifs by Toll-like receptors may play a role in both the activation of antinuclear B cells and in the subsequent disease progression after immune complex formation. The endosomal localization of the nucleic acid-sensing Toll-like receptors (TLRs), TLR3, 7, and 9, is believed to contribute to the distinction between endogenous nucleic acids and those of foreign origin. In this article we review recent work that suggests a role for the B-cell receptor and Fcgamma receptors in delivering nuclear antigens to intracellular compartments allowing TLR activation by endogenous nucleic acids. A number of in vitro studies have presented evidence supporting a role for TLRs in SLE pathology. However, recent studies that have examined the contributions of individual TLRs to SLE by using TLR-deficient mice suggest that the situation is far more complicated in vivo. These studies show that under different circumstances TLR signaling may either exacerbate or protect against SLE-associated pathology. Further understanding of the role of TLRs in pathological autoreactivity of the adaptive immune system will likely lead to important insights into the etiopathogenesis of SLE and potential targets for novel therapies.