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

Introduction  

Systemic 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.     

Differential roles for RIG-I-like receptors and nucleic acid-sensing TLR pathways in controlling a chronic viral infection

The necessity for pathogen recognition of viral infection by the innate immune system in initiating early innate and adaptive host defenses is well documented. However, little is known about the role these receptors play in the maintenance of adaptive immune responses and their contribution to resolution of persistent viral infections. In this study, we demonstrate a nonredundant functional requirement for both nucleic acid-sensing TLRs and RIG-I-like receptors in the control of a mouse model of chronic viral infection. Whereas the RIG-I-like receptor pathway was important for production of type I IFNs and optimal CD8(+) T cell responses, nucleic acid-sensing TLRs were largely dispensable. In contrast, optimal anti-viral Ab responses required intact signaling through nucleic acid-sensing TLRs, and the absence of this pathway correlated with less virus-specific Ab and deficient long-term virus control of a chronic infection. Surprisingly, absence of the TLR pathway had only modest effects on Ab production in an acute infection with a closely related virus strain, suggesting that persistent TLR stimulation may be necessary for optimal Ab responses in a chronic infection. These results indicate that innate virus recognition pathways may play critical roles in the outcome of chronic viral infections through distinct mechanisms.     

Toll-like receptors

Toll-like receptors are a family of transmembrane receptors responsible for recognition and initiation of a response to invading microbes by the immune system. As part of the innate immune system, Toll-like receptors recognise pathogen-associated molecular patterns, highly conserved components that are essential to microbial function. Some of ten toll-like receptors identified in humans are able to recognise several pathogen-associated molecular patterns.     

Toll-like receptors

Toll-like receptors (TLRs) are a growing family of molecules involved in innate immunity. Accumulating evidence suggests that TLR molecules are involved in signalling receptor complexes which recognise components of Gram-positive and Gram-negative bacteria and mycobacteria. Differential expression and regulation as well as distinct though overlapping ligand recognition patterns may underlie the existence of a vast TLR family. Apparent structural and functional redundancy may render certain outputs of the TLR family robust.     

Toll-like receptors as adjuvant receptors

Poly(ethylene glycol)-lipid (PEG-lipid) conjugates are widely used in the field of liposomal drug delivery to provide a polymer coat that can confer favorable pharmacokinetic characteristics on particles in the circulation. More recently these lipids have been employed as an essential component in the self-assembly of cationic and neutral lipids with polynucleic acids to form small, stable lipid/DNA complexes that exhibit long circulation times in vivo and accumulate at sites of disease. However, the presence of a steric barrier lipid might be expected to inhibit the transfection activity of lipid/DNA complexes by reducing particle-membrane contact. In this study we examine what effect varying the size of the hydrophobic anchor and hydrophilic head group of PEG-lipids has on both gene and antisense delivery into cells in culture. Lipid/DNA complexes were made using unilamellar vesicles composed of 5 mole% PEG-lipids in combination with equimolar dioleoylphosphatidylethanolamine and the cationic lipid dioleyldimethylammonium chloride. Using HeLa and HepG2 cells we show that under the conditions employed PEG-lipids had a minimal effect on the binding and subsequent endocytosis of lipid/DNA complexes but they severely inhibited active gene transfer and the endosomal release of antisense oligodeoxynucleotides into the cytoplasm. Decreasing the size of the hydrophobic anchor or the size of the grafted hydrophilic PEG moiety enhanced DNA transfer by the complexes.     

IgM and its receptors: Structural and functional aspects

This review combines the data obtained before the beginning of the 1990s with results published during the last two decades. The predominant form of the IgM molecule is a closed ring composed of five 7S subunits and a J chain. The new model of spatial structure of the pentamer postulates nonplanar mushroom-shaped form of the molecule with the plane formed by a radially-directed Fab regions and central protruding portion consisting of Cμ4 domains. Up to the year 2000 the only known Fc-receptor for IgM was pIgR. Interaction of IgM with pIgR results in secretory IgM formation, whose functions are poorly studied. The receptor designated as Fcα/μR is able to bind IgM and IgA. It is expressed on lymphocytes, follicular dendritic cells, and macrophages. A receptor binding IgM only named FcμR has also been described. It is expressed on T- and B-lymphocytes. The discovery of new Fc-receptors for IgM requires revision of notions that interactions between humoral reactions involving IgM and the cells of the immune system are mediated exclusively by complement receptors. In the whole organism, apart from IgM induced by immunization, natural antibodies (NA) are present and comprise in adults a considerable part of the circulating IgM. NA are polyreactive, germ-line-encoded, and emerge during embryogenesis without apparent antigenic stimuli. They demonstrate a broad spectrum of antibacterial activity and serve as first line of defense against microbial and viral infections. NA may be regarded as a transitional molecular form from invariable receptors of innate immunity to highly diverse receptors of adaptive immunity. By means of interaction with autoantigens, NA participate in maintenance of immunological tolerance and in clearance of dying cells. At the same time, NA may act as a pathogenic factor in atherosclerotic lesion formation and in development of tissue damage due to ischemia/reperfusion.     

Viruses and Toll-like receptors

Recently a number of viruses, including a poxvirus, herpesvirus, retrovirus and two paramyxoviruses, have been shown to activate cells via Toll-like receptor family members. Here we postulate that although activation via Toll-like receptor molecules can lead to anti-viral innate immune responses, in some cases viruses may use these responses to ameliorate infection.     

Viruses and Toll-like receptors

Production of inflammatory cytokines and type I interferons by mammalian cells is mediated through virus-specific activation of Toll-like receptors (TLRs). Known roles for different TLRs and speculation as to their roles in viral pathogenesis and immunity are discussed in this review.     

Toll-like receptors control autophagy

Autophagy is a newly recognized innate defense mechanism, acting as a cell-autonomous system for elimination of intracellular pathogens. The signals and signalling pathways inducing autophagy in response to pathogen invasion are presently not known. Here we show that autophagy is controlled by recognizing conserved pathogen-associated molecular patterns (PAMPs). We screened a PAMP library for effects on autophagy in RAW 264.7 macrophages and found that several prototype Toll-like receptor (TLR) ligands induced autophagy. Single-stranded RNA and TLR7 generated the most potent effects. Induction of autophagy via TLR7 depended on MyD88 expression. Stimulation of autophagy with TLR7 ligands was functional in eliminating intracellular microbes, even when the target pathogen was normally not associated with TLR7 signalling. These findings link two innate immunity defense systems, TLR signalling and autophagy, provide a potential molecular mechanism for induction of autophagy in response to pathogen invasion, and show that the newly recognized ability of TLR ligands to stimulate autophagy can be used to treat intracellular pathogens.     

Pleiotropic function of Toll-like receptors

A group of type I transmembrane proteins, Toll-like receptors (TLRs) discriminate various microorganism-associated molecular structures that can function as immune adjuvants. Each TLR signaling has an overlapping but distinct function, which largely depends on intracellular adaptor molecules. Clarifying the functions and signaling of TLRs should provide us with critical information for manipulating the host defense mechanism.     

The biology of Toll-like receptors

In 1997, a human homologue of the Drosophila Toll protein was described, a protein later to be designated Toll-like receptor 4 (TLR4). Since that time, additional human and murine TLR proteins have been identified. Mammalian TLR proteins appear to represent a conserved family of innate immune recognition receptors. These receptors are coupled to a signaling pathway that is conserved in mammals, insects, and plants, resulting in the activation of genes that mediate innate immune defenses. Numerous studies have now identified a wide variety of chemically-diverse bacterial products that serve as putative ligands for TLR proteins. More recent studies have identified the first endogenous protein ligands for TLR proteins. TLR signaling represents a key feature of innate immune response to pathogen invasion.     

Heterogeneity of neuronal nicotinic acetylcholine receptors: Structural and functional aspects

Decay kinetics of the postsynaptic excitatory currents (EPSC), distribution of the antibodies specific to different α-subunits of neuronal nicotinic acetylcholine receptors (nAChR), and the effects of these antibodies on ACh-induced membrane currents were studied in neurons of different autonomic ganglia of rats. It was shown that α3-, α5- and α7-subunits were present in all studied cultured neurons of the rat superior cervical ganglion (SCG), while the α4-subunit was present only in about half of the neurons; this α-subunit distribution differed from that in cultured intracardial neurons of rats. Two nAChR populations were found in rat SCG neurons, and a series of nAChR populations were found in murine superior mesenteric ganglion neurons; they differed in kinetics of their ion channel activity, voltage dependence and the rate of their open channel blockade. The possible functional role of neuronal nAChR heterogeneity is discussed.     

Structural and functional aspects of the receptors for platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a 30 kDa dimer of disulfide-bonded A and B chains. Three isoforms of PDGF have been isolated (PDGF-AA, PDGF-AB and PDGF-BB). These bind with different affinities and specificities to two structurally related cell surface receptors, viz. the α-receptor and the β-receptor. The receptors are transmembrane proteins with an intracellular, ligand-stimulatable protein tyrosine kinase domain. Activation of the receptors is intimately associated with receptor dimerization, and available data suggest that PDGF is a divalent ligand such that one molecule of PDGF binds and dimerizes two receptor molecules. Stimulation of PDGF receptors leads to a cascade of cellular events, which have been shown to require an intact receptor tyrosine kinase activity. However, ligand-induced internalization and degradation of the β-receptor occur essentially independent of the receptor kinase activity. Receptor activation leads to the phosphorylation on tyrosine residues of three enzymes, probably by direct phosphorylation: phospholipase C-γ, phosphatidylinositol 3′ kinase and Raf-1. In certain cells, PDGF β-receptor expression is inducible such that cells in normal tissue in vivo do not express receptors; only in inflammatory lesions or when cells are explanted in vitro, are receptors being expressed. Transformation by the v-sis oncogene is mediated by an autocrine PDGF-like growth factor. Although both the α- and β-receptors are structurally related to the v-fms and v-kit oncogenes, it is not known if the PDGF receptors have a transforming potential. In conclusion, the finding of three isoforms of PDGF that interact with two structurally related receptors implies a finely tuned regulatory network, the role of which in cell growth and transformation remains to be clarified.     

Lung cancer and Toll-like receptors

Lung carcinoma is one of the leading causes of death worldwide. It is a non-immunogenic cancer, resistant to immune surveillance. Toll-like receptors (TLRs) connect the innate to the adaptive immune system. Given that cancerous cells evade the immune system, the activation of TLRs could represent a potential target for cancer therapy. The induction of Th1-like and cytotoxic immunity by TLR signalling could lead to tumour cell death, resulting in tumour regression or arrest. However, basic research and clinical trials revealed that the activation of specific TLRs, such as TLR2, TLR4 and TLR9, do not have any anti-tumour activity in lung carcinoma. Increasing evidence suggests that TLRs are important regulators of tumour biology; however, little is known about their function in lung cancer. Thus, in order to develop new therapeutic approaches, further studies are needed to understand the connection between TLRs and lung cancer progression. This review focuses on the potential mechanisms by which TLR ligands can facilitate or not lung cancer and lung metastases establishment/progression.     

Recognition highlights: Toll-like receptors

As part of our first line of defense against invading organisms, Toll-like receptors recognize bacterial and viral molecules, and launch an inflammatory response.