TLR7 influences germinal center selection in murine SLE

Toll-like receptors 7 and 9 regulate the proliferation and differentiation of B cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune illness marked by the loss of immune tolerance and the production of autoantibodies against nucleic acids and other nuclear antigens (Ags). B lymphocytes are important in the immunopathogenesis of SLE. Multiple receptors control abnormal B-cell activation in SLE patients, including intrinsic Toll-like receptors (TLRs), B-cell receptors (BCRs), and cytokine receptors. The role of TLRs, notably TLR7 and TLR9, in the pathophysiology of SLE has been extensively explored in recent years. When endogenous or exogenous nucleic acid ligands are recognized by BCRs and internalized into B cells, they bind TLR7 or TLR9 to activate related signalling pathways and thus govern the proliferation and differentiation of B cells. Surprisingly, TLR7 and TLR9 appear to play opposing roles in SLE B cells, and the interaction between them is still poorly understood. In addition, other cells can enhance TLR signalling in B cells of SLE patients by releasing cytokines that accelerate the differentiation of B cells into plasma cells. Therefore, the delineation of how TLR7 and TLR9 regulate the abnormal activation of B cells in SLE may aid the understanding of the mechanisms of SLE and provide directions for TLR-targeted therapies for SLE.

Rubicon promotes rather than restricts murine lupus and is not required for LC3-associated phagocytosis

NADPH oxidase deficiency exacerbates lupus in murine models and patients, but the mechanisms remain unknown. It is hypothesized that NADPH oxidase suppresses autoimmunity by facilitating dead cell clearance via LC3-associated phagocytosis (LAP). The absence of LAP reportedly causes an autoinflammatory syndrome in aged, nonautoimmune mice. Prior work implicated cytochrome b-245, β polypeptide (CYBB), a component of the NADPH oxidase complex, and the RUN and cysteine-rich domain-containing Beclin 1-interacting protein (RUBICON) as requisite for LAP. To test the hypothesis that NADPH oxidase deficiency exacerbates lupus via a defect in LAP, we deleted Rubicon in the B6.Sle1.Yaa and MRL.Faslpr lupus mouse models. Under this hypothesis, RUBICON deficiency should phenocopy NADPH oxidase deficiency, as both work in the same pathway. However, we observed the opposite - RUBICON deficiency resulted in reduced mortality, renal disease, and autoantibody titers to RNA-associated autoantigens. Given that our data contradict the published role for LAP in autoimmunity, we assessed whether CYBB and RUBICON are requisite for LAP. We found that LAP is not dependent on either of these 2 pathways. To our knowledge, our data reveal RUBICON as a novel regulator of SLE, possibly by a B cell-intrinsic mechanism, but do not support a role for LAP in lupus.

Context dependent induction of autoimmunity by TNF signaling deficiency

TNF inhibitors are widely used to treat inflammatory diseases; however, 30%–50% of treated patients develop new autoantibodies, and 0.5%–1% develop secondary autoimmune diseases, including lupus. TNF is required for formation of germinal centers (GCs), the site where high-affinity autoantibodies are often made. We found that TNF deficiency in Sle1 mice induced TH17 T cells and enhanced the production of germline encoded, T-dependent IgG anti-cardiolipin antibodies but did not induce GC formation or precipitate clinical disease. We then asked whether a second hit could restore GC formation or induce pathogenic autoimmunity in TNF-deficient mice. By using a range of immune stimuli, we found that somatically mutated autoantibodies and clinical disease can arise in the setting of TNF deficiency via extrafollicular pathways or via atypical GC-like pathways. This breach of tolerance may be due to defects in regulatory signals that modulate the negative selection of pathogenic autoreactive B cells.

Toll-like receptor signalling in B cells during systemic lupus erythematosus

B lymphocytes have a central role in autoimmune diseases, which are often defined by specific autoantibody patterns and feature a loss of B cell tolerance. A prototypic disease associated with B cell hyperactivity is systemic lupus erythematosus (SLE). In patients with SLE, the loss of B cell tolerance to autoantigens is controlled in a cell-intrinsic manner by Toll-like receptors (TLRs), which sense nucleic acids in endosomes. TLR7 drives the extrafollicular B cell response and the germinal centre reaction that are involved in autoantibody production and disease pathogenesis. Surprisingly, TLR9 seems to protect against SLE, even though it is required for the production of autoantibodies recognizing double-stranded DNA-associated antigens, which are abundant in SLE and are a hallmark of this disease. The protective function of TLR9 is at least partly mediated by its capacity to limit the stimulatory activity of TLR7. The roles of TLR7 and TLR9 in the effector function of B cells in lupus-like disease and in patients with SLE, and the unique features of TLR signalling in B cells, suggest that targeting TLR signalling in SLE might be therapeutically beneficial.

Loss of B cell tolerance to autoantigens in systemic lupus erythematosus (SLE) is driven by TLR7, whereas TLR9 appears to protect against SLE by limiting the stimulatory activity of TLR7. The unique features of Toll-like receptor signalling in B cells implicate it as a therapeutic target in SLE.

Intrinsic TLR7 and TLR9 signalling in B cells plays an important role in the development and pathogenesis of systemic lupus erythematosus (SLE).

In patients with SLE, effector plasma cells are generated via the extrafollicular response and via the formation of spontaneous germinal centres.

TLR7 plays key roles in the extrafollicular response and the response mediated by germinal centres.

Some plasma cells produce IL-10 and can have protective roles in lupus-like disease.

TLR7 escapes X chromosome inactivation in immune cells

Toll-like receptor 7 (TLR7) is critical to the induction of antiviral immunity, but TLR7 dosage is also a key pathogenic factor in systemic lupus erythematosus (SLE), an autoimmune disease with strong female bias. SLE prevalence is also elevated in individuals with Klinefelter syndrome, who carry one or more supernumerary X chromosomes, suggesting that the X chromosome complement contributes to SLE susceptibility. TLR7 is encoded by an X chromosome locus, and we examined here whether the TLR7 gene evades silencing by X chromosome inactivation in immune cells from women and Klinefelter syndrome males. Single-cell analyses of TLR7 allelic expression demonstrated that substantial fractions of primary B lymphocytes, monocytes, and plasmacytoid dendritic cells not only in women but also in Klinefelter syndrome males express TLR7 on both X chromosomes. Biallelic B lymphocytes from women displayed greater TLR7 transcriptional expression than the monoallelic cells, correlated with higher TLR7 protein expression in female than in male leukocyte populations. Biallelic B cells were preferentially enriched during the TLR7-driven proliferation of CD27⁺ plasma cells. In addition, biallelic cells showed a greater than twofold increase over monoallelic cells in the propensity to immunoglobulin G class switch during the TLR7-driven, T cell-dependent differentiation of naive B lymphocytes into immunoglobulin-secreting cells. TLR7 escape from X inactivation endows the B cell compartment with added responsiveness to TLR7 ligands. This finding supports the hypothesis that enhanced TLR7 expression owing to biallelism contributes to the higher risk of developing SLE and other autoimmune disorders in women and in men with Klinefelter syndrome.

UC-1V150, a potent TLR7 agonist capable of activating macrophages and potentiating mAb-mediated target cell deletion

Toll-like receptors (TLR) are critical mediators of the immune system with their activation linked to infection, inflammation and the pathogenesis of immune diseases including autoimmunity and cancer. For this reason, over the last 2 decades, TLR and their associated signalling pathways have been targeted therapeutically to enhance innate and adaptive immunity. Several TLR ligands, both endogenous and synthetic are at various phases of clinical testing, and new ligands are continually emerging. Agonists of TLR7 are known immune response modifiers, simultaneously stimulating several cell types, resulting in immune cell activation and cytokine and chemokine release. The immune stimulating properties of the TLR7 agonist Imiquimod has also been exploited for use in the treatment of malignant superficial tumours of the skin. Here, we investigated a novel TLR7 agonist UC-1V150 and demonstrate it activates both human and mouse myeloid cells in vitro and in vivo, to deliver potent FcγR-mediated engulfment of opsonized target cells.

Effects of Gui Zhi Ma Huang Ge Ban Tang on the TLR7 Pathway in Influenza Virus Infected Mouse Lungs in a Cold Environment

OBJECTIVE

We wished to investigate the effects of the traditional Chinese medicine Gui Zhi Ma Huang Ge Ban Tang on controlling influenza A virus (IAV) infection and improving inflammation in mouse lungs.

METHOD

Mice were maintained in normal and cold environments and infected with IAV by intranasal application, respectively. Real-time quantitative polymerase chain reaction was used to measure mRNA expression of TLR7, myeloid differentiation primary response 88 (MyD88), and nuclear factor-kappa B (NF-κB)p65 in the TLR7 signaling pathway and virus replication in lungs. Western blotting was used to measure expression levels of TLR7, MyD88, and NF-κB p65 proteins. Flow cytometry was used to detect the proportion of T-helper (Th)1/Th2 and Th17/T-regulatory (Treg) cells.

RESULTS

Application of Gui Zhi Ma Huang Ge Ban Tang in influenza-infected mice in a cold environment showed (i) downregulation of TLR7, MyD88, and NF-κBp65; (ii) inhibition of transcriptional activities of promoters coding for TLR7, MyD88, and NF-κBp65; (iii) reduction in the proportion of Th1/Th2 and Th17/Treg cells.

CONCLUSIONS

Gui Zhi Ma Huang Ge Ban Tang had a good therapeutic effect on mice infected with IAV, especially in the cold environment. It could reduce lung inflammation in mice significantly and elicit an anti-influenza effect by downregulating expression of the key factors in TLR7 signaling pathway.

Mouse models of lupus: what they tell us and what they don't

Lupus is a complex heterogeneous disease characterised by autoantibody production and immune complex deposition followed by damage to target tissues. Animal models of human diseases are an invaluable tool for defining pathogenic mechanisms and testing of novel therapeutic agents. There are perhaps more applicable murine models of lupus than any other human disease. There are spontaneous models of lupus, inducible models of lupus, transgenic-induced lupus, gene knockout induced lupus and humanised mouse models of lupus. These mouse models of lupus have contributed significantly to our knowledge of the pathogenesis of lupus and served as valuable preclinical models for proof of concept for new therapies. Despite their utility, mouse models of lupus have their distinct limitations. Although similar, mouse and human immune systems are different and thus one cannot assume a mechanism for disease in one is translatable to the other. Efficacy and toxicity of compounds can vary significantly between humans and mice, also limiting direct translation. Finally, the heterogeneous aspects of human lupus, both in clinical presentation, underlying pathogenesis and genetics, are not completely represented in current mouse models. Thus, proving a therapy or mechanism of disease in one mouse model is similar to proving a mechanism/therapy in a limited subset of human lupus. These limitations, however, do not marginalise the importance of animal models nor the significant contributions they have made to our understanding of lupus.

Use of Toll-Like Receptor Agonists to Induce Ectopic Lymphoid Structures in Myasthenia Gravis Mouse Models

Myasthenia gravis (MG) is an autoimmune disease mediated by autoantibodies against the acetylcholine receptor (AChR) at the neuromuscular junction. MG symptoms are characterized by muscle weaknesses. The thymus of MG patients is very often abnormal and possesses all the characteristics of tertiary lymphoid organs such as neoangiogenesis processes, overexpression of inflammatory cytokines and chemokines, and infiltration of B lymphocytes leading to ectopic germinal center (GC) development. We previously demonstrated that injections of mice with polyinosinic–polycytidylic acid [Poly(I:C)], a synthetic double-stranded RNA mimicking viral infection, induce thymic changes and trigger MG symptoms. Upon Poly(I:C) injections, we observed increased thymic expressions of α-AChR, interferon-β and chemokines such as CXCL13 and CCL21 leading to B-cell recruitment. However, these changes were only transient. In order to develop an experimental MG model associated with thymic GCs, we used Poly(I:C) in the classical experimental autoimmune MG model induced by immunizations with purified AChR emulsified in complete Freund’s adjuvant. We observed that Poly(I:C) strongly favored the development of MG as almost all mice displayed MG symptoms. Nevertheless, we did not observe any ectopic GC development. We next challenged mice with Poly(I:C) together with other toll-like receptor (TLR) agonists known to be involved in GC development and that are overexpressed in MG thymuses. Imiquimod and CpG oligodeoxynucleotides that activate TLR7 and TLR9, respectively, did not induce thymic changes. In contrast, lipopolysaccharide that activates TLR4 potentiated Poly(I:C) effects and induced a significant expression of CXCL13 mRNA in the thymus associated with a higher recruitment of B cells that induced over time thymic B-lymphoid structures. Altogether, these data suggest that tertiary lymphoid genesis in MG thymus could result from a combined activation of TLR signaling pathways.

Contrasting expression pattern of RNA-sensing receptors TLR7, RIG-I and MDA5 in interferon-positive and interferon-negative patients with primary Sjögren's syndrome

OBJECTIVE

The interferon (IFN) type I signature is present in over half of patients with primary Sjögren's syndrome (pSS) and associated with higher disease-activity and autoantibody presence. Plasmacytoid dendritic cells (pDCs) are considered as the main source of enhanced IFN type I expression. The objective of this study was to unravel the molecular pathways underlying IFN type I bioactivity in pDCs of patients with pSS.

METHODS

Blood samples from 42 healthy controls (HC) and 115 patients with pSS were stratified according to their IFN type I signature. CD123+BDCA4+ pDCs and CD14+ monocytes were isolated from peripheral blood mononuclear cells (PBMCs). Genome-wide microarray analysis was conducted on sorted pDCs in a small sample set, followed by validation of differentially expressed genes of interest in pDCs and monocytes.

RESULTS

We found an upregulation of endosomal toll-like receptor (TLR) 7, but not TLR9, in IFN-positive (IFNpos) pDCs (p<0.05) and monocytes (p=0.024). Additionally, the downstream signalling molecules MyD88, RSAD2 and IRF7 were upregulated, as were the cytoplasmic RNA-sensing receptors DDX58/retinoic acid inducible gene-I (RIG-I) and IFIH1/melanoma differentiation associated gene-5 (MDA5). In vitro triggering of the TLR7-pathway in HC PBMCs induced upregulation of DDX58/RIG-I and IFIH1/MDA5, and downregulated TLR9. The upregulation of TLR7, its downstream signalling pathway, DDX58/RIG-I and IFIH1/MDA5 were confined to patients with IFN-positive pSS. IFN-negative patients had a contrasting expression pattern-TLR7 normal, and decreased TLR9, RIG-I and MDA5.

CONCLUSIONS

Here we conclude a contrasting expression pattern of the RNA-sensing receptors TLR7, RIG-I and MDA5 in pDCs and monocytes of patients with IFNpos pSS. This profile could explain the pathogenic IFN production and might reveal novel therapeutic targets in these patients.

The potential role of retroviruses in autoimmunity

In the last 20 years research in Immunology underwent fundamental changes. Most importantly, the identification of the key role of innate immune pattern recognition receptors (PRRs) that recognize evolutionarily conserved molecular patterns on infectious pathogens. This results in priming of innate immune cells, which in turn activate and direct the adaptive immune response. Progress in innate immune recognition instigated the current working hypothesis, that recognition of endogenous ligands by PRRs results in innate immune cell activation (autoinflammation) or activation of adaptive cells, with self-reactive antigen receptors (autoimmunity). In particular, nucleic acid-sensing innate immune receptors seem to be prime candidates for a mechanistic understanding of autoreactive activation of the immune system. However, it remains uncertain what the actual source of nucleic acid ligands is and what other signals are needed to drive activation of autoreactive innate immune cells and break self-tolerance of the adaptive immune system. Here, I will review our present understanding about whether the infection with exogenous retroviruses or the reactivation of endogenous retroviruses might play an etiological role in certain autoimmune conditions of humans and murine experimental models.

The Effect of BAFF Inhibition on Autoreactive B-Cell Selection in Murine Systemic Lupus Erythematosus

The goal of this study was to determine how B-cell-activating factor of the TNF family (BAFF) availability influences selection of the autoreactive B-cell repertoire in NZB/W and NZW/BXSB lupus-prone mice bearing the site-directed heavy-chain transgene 3H9 that encodes for anti-dsDNA and anti-cardiolipin (CL) autoantibodies. We used a bone marrow chimera system in which autoreactive 3H9 transgenic B cells were allowed to mature in competition with wild-type cells and could be identified by green fluorescent protein. The light-chain repertoire associated with the 3H9 heavy chain in naive and antigen-activated B-cell subsets was assessed using single-cell polymerase chain reaction. We found that deletion of autoreactive transgenic B cells occurred in the bone marrow of both strains regardless of BAFF availability, and there were only modest and physiologically non-relevant effects on the naive B-cell repertoire. BAFF inhibition had different effects on selection of the germinal center repertoire in the two strains. In the NZW/BXSB strain, BAFF inhibition phenocopied the loss of one TLR7 allele in that it influenced the selection of 3H9-encoded autoreactive B cells in the germinal center but did not prevent somatic mutation. In the NZB/W strain, BAFF inhibition did not alter the selection of 3H9-encoded B cells in the germinal center, but it influenced selection of a subset of germinal center cells into the plasma cell compartment. Our data underscore the complexity of regulation of the autoreactive B-cell repertoire by BAFF and may help to explain the heterogeneity of responses observed after BAFF inhibition in humans.

Defects in Germinal Center Selection in SLE

Germinal centers (GCs) are the primary site at which clonal expansion and affinity maturation of B cells occur. B cells encounter antigen and receive T cell help in the GC light zone (LZ) and then migrate to the dark zone where they proliferate and undergo somatic mutation before cycling back to the LZ for further rounds of selection. Tolerance to autoantigens is frequently lost de novo as GC B cells undergo class switching and somatic mutation. This loss of tolerance is regulated by a variety of mechanisms including cell death, failure to compete for T cell help, and failure to differentiate into effector cells. Systemic lupus erythematosus (SLE) is characterized by loss of tolerance to nucleic acid antigens. While defects in tolerance occur in the naïve repertoire of SLE patients, pathogenic autoantibodies also arise in the GC by somatic mutation from non-autoreactive precursors. Several B cell defects contribute to the loss of GC tolerance in SLE, including polymorphisms of genes encoded by the Sle1 locus, excess TLR7 signaling, defects in FcRIIB expression, or defects of B cell apoptosis. Extrinsic soluble factors, such as Type-1 IFN and B cell-activating factor, or an increased number of T follicular helper cells in the GC also alter B cell-negative selection. Finally, defects in clearance of apoptotic debris within the GC result in BCR-mediated internalization of nucleic acid containing material and stimulation of autoantibody production by endosomal TLR-driven mechanisms.

Roles of B Cell-Intrinsic TLR Signals in Systemic Lupus Erythematosus

Toll-like receptors (TLRs) are a large family of pattern recognition receptors. TLR signals are involved in the pathogenesis of systemic lupus erythematosus. Mouse and human B cells constitutively express most TLRs. Many B cell subpopulations are highly responsive to certain TLR ligation, including B-1 B cells, transitional B cells, marginal zone B cells, germinal center B cell and memory B cells. The B cell-intrinsic TLR signals play critical roles during lupus process. In this review, roles of B cell-intrinsic TLR2, 4, 7, 8 and 9 signals are discussed during lupus pathogenesis in both mouse model and patients. Moreover, mechanisms underlying TLR ligation-triggered B cell activation and signaling pathways are highlighted.