Opposing impact of B cell-intrinsic TLR7 and TLR9 signals on autoantibody repertoire and systemic inflammation

B Cell αv Integrins Regulate TLR-Driven Autoimmunity

Systemic lupus erythematosus (SLE) is defined by loss of B cell tolerance, resulting in production of autoantibodies against nucleic acids and other cellular Ags. Aberrant activation of TLRs by self-derived RNA and DNA is strongly associated with SLE in patients and in mouse models, but the mechanism by which TLR signaling to self-ligands is regulated remains poorly understood. In this study, we show that αv integrin plays a critical role in regulating B cell TLR signaling to self-antigens in mice. We show that deletion of αv from B cells accelerates autoantibody production and autoimmune kidney disease in the Tlr7.1 transgenic mouse model of SLE. Increased autoimmunity was associated with specific expansion of transitional B cells, extrafollicular IgG2c-producing plasma cells, and activation of CD4 and CD8 T cells. Our data show that αv-mediated regulation of TLR signaling in B cells is critical for preventing autoimmunity and indicate that loss of αv promotes escape from tolerance. Thus, we identify a new regulatory pathway in autoimmunity and elucidate upstream signals that adjust B cell activation to prevent development of autoimmunity in a mouse model.

New insights into the role of antinuclear antibodies in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by antinuclear antibodies (ANAs) that form immune complexes that mediate pathogenesis by tissue deposition or cytokine induction. Some ANAs bind DNA or associated nucleosome proteins, whereas other ANAs bind protein components of complexes of RNA and RNA-binding proteins (RBPs). Levels of anti-DNA antibodies can fluctuate widely, unlike those of anti-RBP antibodies, which tend to be stable. Because anti-DNA antibody levels can reflect disease activity, repeat testing is common; by contrast, a single anti-RBP antibody determination is thought to suffice for clinical purposes. Experience from clinical trials of novel therapies has provided a new perspective on ANA expression during disease, as many patients with SLE are ANA negative at screening despite previously testing positive. Because trial results suggest that patients who are ANA negative might not respond to certain agents, screening strategies now involve ANA and anti-DNA antibody testing to identify patients with so-called 'active, autoantibody-positive SLE'. Evidence suggests that ANA responses can decrease over time because of the natural history of disease or the effects of therapy. Together, these findings suggest that, during established disease, more regular serological testing could illuminate changes relevant to pathogenesis and disease status.

TLR7 Negatively Regulates B10 Cells Predominantly in an IFNγ Signaling Dependent Manner

IL-10 producing B cells (B10 cells) play an important immunoregulatory role in various autoimmune and infection conditions. However, the factors that regulate their development and maintenance are incompletely understood. Recently, we and others have established a requirement for TLR7 in promoting autoimmune antibody forming cell (AFC) and germinal center (GC) responses. Here we report an important additional role of TLR7 in the negative regulation of B10 cell development. TLR7 overexpression or overstimulation promoted the reduction of B10 cells whereas TLR7 deficiency rescued these cells in both non-autoimmune and autoimmune-prone mice. TLR7 expression was further inversely correlated with B cell-dependent IL-10 production and its inhibition of CD4 T cell proliferation and IFNγ production in an in vitro B cell and T cell co-culture system. Further, B10 cells displayed elevated TLR7, IFNγR, and STAT1 expression compared to non-B10 cells. Interestingly, deficiency of IFNγR in TLR7 overexpressing lupus-prone mice rescued B10 cells from TLR7-mediated reduction. Finally, B cell intrinsic deletion of IFNγR was sufficient to restore B10 cells in the spleens of TLR7-promoted autoimmune mouse model. In conclusion, our findings demonstrate a novel role for the IFNγR-STAT1 pathway in TLR7-mediated negative regulation of B10 cell development.

Plasmacytoid Dendritic Cells and Type I Interferon Promote Extrafollicular B Cell Responses to Extracellular Self-DNA

Class-switched antibodies to double-stranded DNA (dsDNA) are prevalent and pathogenic in systemic lupus erythematosus (SLE), yet mechanisms of their development remain poorly understood. Humans and mice lacking secreted DNase DNASE1L3 develop rapid anti-dsDNA antibody responses and SLE-like disease. We report that anti-DNA responses in Dnase1l3-/- mice require CD40L-mediated T cell help, but proceed independently of germinal center formation via short-lived antibody-forming cells (AFCs) localized to extrafollicular regions. Type I interferon (IFN-I) signaling and IFN-I-producing plasmacytoid dendritic cells (pDCs) facilitate the differentiation of DNA-reactive AFCs in vivo and in vitro and are required for downstream manifestations of autoimmunity. Moreover, the endosomal DNA sensor TLR9 promotes anti-dsDNA responses and SLE-like disease in Dnase1l3-/- mice redundantly with another nucleic acid-sensing receptor, TLR7. These results establish extrafollicular B cell differentiation into short-lived AFCs as a key mechanism of anti-DNA autoreactivity and reveal a major contribution of pDCs, endosomal Toll-like receptors (TLRs), and IFN-I to this pathway.

TJ-M2010-5, a novel MyD88 inhibitor, corrects R848-induced lupus-like immune disorders of B cells in vitro

B cell hyperactivities are involved in the development of systemic lupus erythematosus (SLE). Toll-like receptor 7 (TLR7) in the B cells plays a pivotal role in the pathogenesis of SLE. Previous studies have focused on the intrinsic role of B cells in TLR7/MyD88 signaling and consequently on immune activation, autoantibody production, and systemic inflammation. However, a feasible treatment for this immune disorder remains to be discovered. The in vitro cellular response that have been studied likely plays a central role in the production of some important autoantibodies in SLE. We successfully used R848 to build a lupus-like B cell model in vitro; these B cells were overactivated, differentiated into plasma cells, escaped apoptosis, massively proliferated, and produced large amounts of autoantibodies and cytokines. In the present study, we found that TJ-M2010-5, a novel MyD88 inhibitor previously synthesized in our lab, seemed to inhibit the lupus-like condition of B cells, including overactivation, massive proliferation, differentiation into plasma cells, and overproduction of autoantibodies and cytokines. TJ-M2010-5 also induce B cells apoptosis. Furthermore, TJ-M2010-5 was found to remarkably inhibit NF-κB and MAPK signaling. In summary, TJ-M2010-5 might correct R848-induced lupus-like immune disorders of B cells by blocking the TLR7/MyD88/NF-κB and TLR7/MyD88/MAPK signaling pathways.

B cell-intrinsic TLR9 expression is protective in murine lupus

Toll-like receptor 9 (TLR9) is a regulator of disease pathogenesis in systemic lupus erythematosus (SLE). Why TLR9 represses disease while TLR7 and MyD88 have the opposite effect remains undefined. To begin to address this question, we created 2 alleles to manipulate TLR9 expression, allowing for either selective deletion or overexpression. We used these to test cell type-specific effects of Tlr9 expression on the regulation of SLE pathogenesis. Notably, Tlr9 deficiency in B cells was sufficient to exacerbate nephritis while extinguishing anti-nucleosome antibodies, whereas Tlr9 deficiency in dendritic cells (DCs), plasmacytoid DCs, and neutrophils had no discernable effect on disease. Thus, B cell-specific Tlr9 deficiency unlinked disease from autoantibody production. Critically, B cell-specific Tlr9 overexpression resulted in ameliorated nephritis, opposite of the effect of deleting Tlr9. Our findings highlight the nonredundant role of B cell-expressed TLR9 in regulating lupus and suggest therapeutic potential in modulating and perhaps even enhancing TLR9 signals in B cells.

The role of B cell antigen presentation in the initiation of CD4+ T cell response

B cells have been known for their ability to present antigens to T cells for almost 40 years. However, the precise roles of B cell antigen presentation in various immune responses are not completely understood. The term "professional" antigen-presenting cells (APCs) was proposed to distinguish APCs that are required for initiating the immune responses from those use antigen presentation to enhance their own effector functions. Unlike dendritic cells, which are defined as professional APCs for their well-established functions in activating naive T cells, B cells have been shown in the past to mostly present antigens to activated CD4+ T cells mainly to seek help from T helper cells. However, recent evidence suggested that B cells can act as professional APCs under infectious conditions or conditions mimicking viral infections. B cell antigen receptors (BCRs) and the innate receptor Toll-like receptors are activated synergistically in response to pathogens or virus-like particles, under which conditions B cells are not only potent but also the predominant APCs to turn naive CD4+ T cells into T follicular helper cells. The discovery of B cells as professional APCs to initiate CD4+ T cell response provides a new insight for both autoimmune diseases and vaccine development.

Gut microbiota differently contributes to intestinal immune phenotype and systemic autoimmune progression in female and male lupus-prone mice

The risk of developing systemic lupus erythematosus (SLE) is about 9 times higher in women as compared to men. Our recent report, which used (SWRxNZB) F1 (SNF1) mouse model of spontaneous lupus, showed a potential link between immune response initiated in the gut mucosa at juvenile age (sex hormone independent) and SLE susceptibility. Here, using this mouse model, we show that gut microbiota contributes differently to pro-inflammatory immune response in the intestine and autoimmune progression in lupus-prone males and females. We found that gut microbiota composition in male and female littermates are significantly different only at adult ages. However, depletion of gut microbes causes suppression of autoimmune progression only in females. In agreement, microbiota depletion suppressed the pro-inflammatory cytokine response of gut mucosa in juvenile and adult females. Nevertheless, microbiota from females and males showed, upon cross-transfer, contrasting abilities to modulate disease progression. Furthermore, orchidectomy (castration) not only caused changes in the composition of gut microbiota, but also a modest acceleration of autoimmune progression. Overall, our work shows that microbiota-dependent pro-inflammatory immune response in the gut mucosa of females initiated at juvenile ages and androgen-dependent protection of males contribute to gender differences in the intestinal immune phenotype and systemic autoimmune progression.

Type II but Not Type I IFN Signaling Is Indispensable for TLR7-Promoted Development of Autoreactive B Cells and Systemic Autoimmunity

TLR7 is associated with development of systemic lupus erythematosus (SLE), but the underlying mechanisms are incompletely understood. Although TLRs are known to activate type I IFN (T1IFN) signaling, the role of T1IFN and IFN-γ signaling in differential regulation of TLR7-mediated Ab-forming cell (AFC) and germinal center (GC) responses, and SLE development has never been directly investigated. Using TLR7-induced and TLR7 overexpression models of SLE, we report in this study a previously unrecognized indispensable role of TLR7-induced IFN-γ signaling in promoting AFC and GC responses, leading to autoreactive B cell and SLE development. T1IFN signaling in contrast, only modestly contributed to autoimmune responses and the disease process in these mice. TLR7 ligand imiquimod treated IFN-γ reporter mice show that CD4⁺ effector T cells including follicular helper T (Tfh) cells are the major producers of TLR7-induced IFN-γ. Transcriptomic analysis of splenic tissues from imiquimod-treated autoimmune-prone B6.Sle1b mice sufficient and deficient for IFN-γR indicates that TLR7-induced IFN-γ activates multiple signaling pathways to regulate TLR7-promoted SLE. Conditional deletion of Ifngr1 gene in peripheral B cells further demonstrates that TLR7-driven autoimmune AFC, GC and Tfh responses and SLE development are dependent on IFN-γ signaling in B cells. Finally, we show crucial B cell-intrinsic roles of STAT1 and T-bet in TLR7-driven GC, Tfh and plasma cell differentiation. Altogether, we uncover a nonredundant role for IFN-γ and its downstream signaling molecules STAT1 and T-bet in B cells in promoting TLR7-driven AFC, GC, and SLE development whereas T1IFN signaling moderately contributes to these processes.

B Cell Abnormalities in Systemic Lupus Erythematosus and Lupus Nephritis-Role in Pathogenesis and Effect of Immunosuppressive Treatments

Abnormalities in B cells play pivotal roles in the pathogenesis of systemic lupus erythematosus (SLE) and lupus nephritis (LN). Breach in central and peripheral tolerance mechanisms generates autoreactive B cells which contribute to the pathogenesis of SLE and LN. Dysregulation of B cell transcription factors, cytokines and B cell-T cell interaction can result in aberrant B cell maturation and autoantibody production. These immunological abnormalities also lead to perturbations in circulating and infiltrating B cells in SLE and LN patients. Conventional and novel immunosuppressive medications confer differential effects on B cells which have important clinical implications. While cyclophosphamide and mycophenolate mofetil (MMF) showed comparable clinical efficacy in active LN, MMF induction was associated with earlier reduction in circulating plasmablasts and plasma cells. Accumulating evidence suggests that MMF maintenance is associated with lower risk of disease relapse than azathioprine, which may be explained by its more potent and selective suppression of B cell proliferation. Novel therapeutic approaches targeting the B cell repertoire include B cell depletion with monoclonal antibodies binding to cell surface markers, inhibition of B cell cytokines, and modulation of costimulatory signals in B cell-T cell interaction. These biologics, despite showing improvements in serological parameters and proteinuria, did not achieve primary endpoints when used as add-on therapy to standard treatments in active LN patients. Other emerging treatments such as calcineurin inhibitors, mammalian target of rapamycin inhibitors and proteasome inhibitors also show distinct inhibitory effects on the B cell repertoire. Advancement in the knowledge on B cell biology has fueled the development of new therapeutic strategies in SLE and LN. Modification in background treatments, study endpoints and selective recruitment of subjects showing aberrant B cells or its signaling pathways when designing future clinical trials may better elucidate the roles of these novel therapies for SLE and LN patients.

Identification of U11snRNA as an endogenous agonist of TLR7-mediated immune pathogenesis

The activation of innate immune receptors by pathogen-associated molecular patterns (PAMPs) is central to host defense against infections. On the other hand, these receptors are also activated by immunogenic damage-associated molecular patterns (DAMPs), typically released from dying cells, and the activation can evoke chronic inflammatory or autoimmune disorders. One of the best known receptors involved in the immune pathogenesis is Toll-like receptor 7 (TLR7), which recognizes RNA with single-stranded structure. However, the causative DAMP RNA(s) in the pathogenesis has yet to be identified. Here, we first developed a chemical compound, termed KN69, that suppresses autoimmunity in several established mouse models. A subsequent search for KN69-binding partners led to the identification of U11 small nuclear RNA (U11snRNA) as a candidate DAMP RNA involved in TLR7-induced autoimmunity. We then showed that U11snRNA robustly activated the TLR7 pathway in vitro and induced arthritis disease in vivo. We also found a correlation between high serum level of U11snRNA and autoimmune diseases in human subjects and established mouse models. Finally, by revealing the structural basis for U11snRNA's ability to activate TLR7, we developed more potent TLR7 agonists and TLR7 antagonists, which may offer new therapeutic approaches for autoimmunity or other immune-driven diseases. Thus, our study has revealed a hitherto unknown immune function of U11snRNA, providing insight into TLR7-mediated autoimmunity and its potential for further therapeutic applications.

Functional Characterization of CD11c+ Age-Associated B Cells as Memory B Cells

Age-associated B cells (ABCs) are a unique subset of B cells defined by surface CD11b and CD11c expression. Although ABC expansion has been observed in both human and animal studies in the setting of advanced age, during humoral autoimmunity and following viral infection, the functional properties of this cellular subset remain incompletely defined. In the current study, we demonstrate that ABCs fulfill the criteria for memory B cells (MBCs), based on evidence of Ag-dependent expansion and persistence in a state poised for rapid differentiation into Ab-secreting plasma cells during secondary responses. First, we show that a majority of ABCs are not actively cycling but exhibit an extensive replication history consistent with prior Ag engagement. Second, despite unswitched surface IgM expression, ABCs show evidence of activation-induced cytidine deaminase (AID)-dependent somatic hypermutation. Third, BCRs cloned from sorted ABCs exhibit broad autoreactivity and polyreactivity. Although the overall level of ABC self-reactivity was not increased relative to naive B cells, ABCs lacked features of functional anergy characteristic of autoreactive B cells. Fourth, ABCs express MBC surface markers consistent with being poised for rapid plasma cell differentiation during recall responses. Finally, in a murine model of viral infection, adoptively transferred CD11c⁺ B cells rapidly differentiated into class-switched Ab-secreting cells upon Ag rechallenge. In summary, we phenotypically and functionally characterize ABCs as IgM-expressing MBCs, findings that together implicate ABCs in the pathogenesis of systemic autoimmunity.

Toll-Like Receptor 9 Deficiency Breaks Tolerance to RNA-Associated Antigens and Up-Regulates Toll-Like Receptor 7 Protein in Sle1 Mice

Objective

Toll‐like receptors (TLRs) 7 and 9 are important innate signaling molecules with opposing roles in the development and progression of systemic lupus erythematosus (SLE). While multiple studies support the notion of a dependency on TLR‐7 for disease development, genetic ablation of TLR‐9 results in severe disease with glomerulonephritis (GN) by a largely unknown mechanism. This study was undertaken to examine the suppressive role of TLR‐9 in the development of severe lupus in a mouse model.

Methods

We crossed Sle1 lupus‐prone mice with TLR‐9–deficient mice to generate Sle1TLR‐9^−/− mice. Mice ages 4.5–6.5 months were evaluated for severe autoimmunity by assessing splenomegaly, GN, immune cell populations, autoantibody and total Ig profiles, kidney dendritic cell (DC) function, and TLR‐7 protein expression. Mice ages 8–10 weeks were used for functional B cell studies, Ig profiling, and determination of TLR‐7 expression.

Results

Sle1TLR‐9^−/− mice developed severe disease similar to TLR‐9–deficient MRL and Nba2 models. Sle1TLR‐9^−/− mouse B cells produced more class‐switched antibodies, and the autoantibody repertoire was skewed toward RNA‐containing antigens. GN in these mice was associated with DC infiltration, and purified Sle1TLR‐9^−/− mouse renal DCs were more efficient at TLR‐7–dependent antigen presentation and expressed higher levels of TLR‐7 protein. Importantly, this increase in TLR‐7 expression occurred prior to disease development, indicating a role in the initiation stages of tissue destruction.

Conclusion

The increase in TLR‐7–reactive immune complexes, and the concomitant enhanced expression of their receptor, promotes inflammation and disease in Sle1TLR9^−/− mice.

Anti-double Stranded DNA Antibodies: Origin, Pathogenicity, and Targeted Therapies

Systemic lupus erythematosus (SLE) is characterized by high-titer serological autoantibodies, including antibodies that bind to double-stranded DNA (dsDNA). The origin, specificity, and pathogenicity of anti-dsDNA antibodies have been studied from a wider perspective. These autoantibodies have been suggested to contribute to multiple end-organ injuries, especially to lupus nephritis, in patients with SLE. Moreover, serum levels of anti-DNA antibodies fluctuate with disease activity in patients with SLE. By directly binding to self-antigens or indirectly forming immune complexes, anti-dsDNA antibodies can accumulate in the glomerular and tubular basement membrane. These autoantibodies can also trigger the complement cascade, penetrate into living cells, modulate gene expression, and even induce profibrotic phenotypes of renal cells. In addition, the expression of suppressor of cytokine signaling 1 is reduced by anti-DNA antibodies simultaneously with upregulation of profibrotic genes. Anti-dsDNA antibodies may even participate in the pathogenesis of SLE by catalyzing hydrolysis of certain DNA molecules or peptides in cells. Recently, anti-dsDNA antibodies have been explored in greater depth as a therapeutic target in the management of SLE. A substantial amount of data indicates that blockade of pathogenic anti-dsDNA antibodies can prevent or even reverse organ damage in murine models of SLE. This review focuses on the recent research advances regarding the origin, specificity, classification, and pathogenicity of anti-dsDNA antibodies and highlights the emerging therapies associated with them.

Self-DNA at the Epicenter of SLE: Immunogenic Forms, Regulation, and Effects

Self-reactive B cells generated through V(D)J recombination in the bone marrow or through accrual of random mutations in secondary lymphoid tissues are mostly purged or edited to prevent autoimmunity. Yet, 10–20% of all mature naïve B cells in healthy individuals have self-reactive B cell receptors (BCRs). In patients with serologically active systemic lupus erythematosus (SLE) the percentage increases up to 50%, with significant self-DNA reactivity that correlates with disease severity. Endogenous or self-DNA has emerged as a potent antigen in several autoimmune disorders, particularly in SLE. However, the mechanism(s) regulating or preventing anti-DNA antibody production remain elusive. It is likely that in healthy subjects, DNA-reactive B cells avoid activation due to the unavailability of endogenous DNA, which is efficiently degraded through efferocytosis and various DNA-processing proteins. Genetic defects, physiological, and/or pathological conditions can override these protective checkpoints, leading to autoimmunity. Plausibly, increased availability of immunogenic self-DNA may be the key initiating event in the loss of tolerance of otherwise quiescent DNA-reactive B cells. Indeed, mutations impairing apoptotic cell clearance pathways and nucleic acid metabolism-associated genes like DNases, RNases, and their sensors are known to cause autoimmune disorders including SLE. Here we review the literature supporting the idea that increased availability of DNA as an immunogen or adjuvant, or both, may cause the production of pathogenic anti-DNA antibodies and subsequent manifestations of clinical disease such as SLE. We discuss the main cellular players involved in anti-DNA responses; the physical forms and sources of immunogenic DNA in autoimmunity; the DNA-protein complexes that render DNA immunogenic; the regulation of DNA availability by intracellular and extracellular DNases and the autoimmune pathologies associated with their dysfunction; the cytosolic and endosomal sensors of immunogenic DNA; and the cytokines such as interferons that drive auto-inflammatory and autoimmune pathways leading to clinical disease. We propose that prevention of DNA availability by aiding extracellular DNase activity could be a viable therapeutic modality in controlling SLE.

TLR7 Protein Expression in Mild and Severe Lupus-Prone Models Is Regulated in a Leukocyte, Genetic, and IRAK4 Dependent Manner

The global increase in autoimmunity, together with the emerging autoimmune-related side effects of cancer immunotherapy, have furthered a need for understanding of immune tolerance and activation. Systemic lupus erythematosus (SLE) is the archetypical autoimmune disease, affecting multiple organs, and tissues. Studying SLE creates knowledge relevant not just for autoimmunity, but the immune system in general. Murine models and patient studies have provided increasing evidence for the innate immune toll like receptor-7 (TLR7) in disease initiation and progression. Here, we demonstrated that the kinase activity of the TLR7-downstream signaling molecule, interleukin-1 receptor associated kinase 4 (IRAK4), is essential for mild and severe autoimmune traits of the Sle1 and Sle1-TLR7 transgenic (Sle1Tg7) murine models, respectively. Elimination of IRAK4 signaling prevented all pathological traits associated with murine lupus, including splenomegaly with leukocyte expansion, detectable circulating antinuclear antibodies and glomerulonephritis, in both Sle1 and Sle1Tg7 mice. The expansion of germinal center B cells and increased effector memory T cell phenotypes that are typical of lupus-prone strains, were also prevented with IRAK4 kinase elimination. Analysis of renal leukocyte infiltrates confirmed our earlier findings of an expanded conventional dendritic cell (cDC) within the kidneys of nephritic mice, and this was prevented with IRAK4 kinase elimination. Analysis of TLR7 at the protein level revealed that the expression in immune cells is dependent on the TLR7-transgene itself and/or autoimmune disease factors in a cell-specific manner. Increased TLR7 protein expression in renal macrophages and cDCs correlated with disease parameters such as blood urea nitrogen (BUN) levels and the frequency of leukocytes infiltrating the kidney. These findings suggest that controlling the level of TLR7 or downstream signaling within myeloid populations may prevent chronic inflammation and severe nephritis.

Deconstructing the sex bias in allergy and autoimmunity: From sex hormones and beyond

Men and women differ in their susceptibility to develop autoimmunity and allergy but also in their capacity to cope with infections. Mechanisms responsible for this sexual dimorphism are still poorly documented and probably multifactorial. This review discusses the recent development in our understanding of the cell-intrinsic actions of biological factors linked to sex, sex hormones and sex chromosome complement, on immune cells, which may account for the sex differences in the enhanced susceptibility of women to develop immunological disorders, such as allergic asthma or systemic lupus erythematosus (SLE). We choose to more specifically discuss the impact of sex hormones on the development and function of immune cell populations directly involved in type-2 immunity, and the role of the X-linked Toll like receptor 7 (TLR7) in anti-viral immunity and in SLE. We will also elaborate on the recent evidence demonstrating that TLR7 escapes from X chromosome inactivation in the immune cells of women, and how this may contribute to endow woman immune system with enhanced responsiveness to RNA-virus and susceptibility to SLE.

Evidence of an anti-inflammatory toll-like receptor 9 (TLR 9) pathway in adipocytes

Adipocytes express various pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs) and actively participate in anti-bacterial and anti-viral host defence. Obesity is associated with adipose tissue PRR expression. The potential role of Toll-like receptor 9 (TLR9) in adipocytes has not yet been investigated. Here, we evaluated TLR9 expression during adipocyte differentiation (AD) of 3T3-L1 adipocytes, in primary murine adipocytes and in different murine and human adipose tissue depots by real-time PCR, immunocytochemistry and immunohistochemistry. TLR9 expression was inhibited using specific siRNA-mediated knockdown, and TLR9 signaling was induced using specific class A, B and C agonistic CpG-oligodeoxynucleotide (ODN) treatment vs ODN controls in 3T3-L1 adipocytes and in primary murine adipocytes from Tlr9wt/wt vs Tlr9-/- mice. We found that TLR9 gene expression is induced during AD and that TLR9 protein is expressed in murine gonadal and human visceral adipocytes. AD depends on intact TLR9 expression. Tlr9-/- mice demonstrate significantly reduced adiponectin serum levels, while siRNA-mediated TLR9 knockdown led to reduced adiponectin mRNA expression in adipocytes. TLR9 ligands (CpG-ODNs) inhibit pro-inflammatory resistin secretion in mature 3T3-L1 adipocytes. Tlr9-/- as compared to Tlr9wt/wt adipocytes exhibit increased resistin and MCP1 secretion and reduced adiponectin secretion into cell culture supernatants, while TLR9 ligands (ODNs) show differential effects in Tlr9-/- vs Tlr9wt/wt primary murine adipocytes. TLR9 expression is significantly increased in visceral compared to subcutaneous adipose tissue depots in non-diabetic obese patients and correlates with systemic resistin levels in a compartment-specific manner. Thus, adipocytic TLR9 is a putative, new protective factor during (obesity-associated) adipose tissue inflammation.

The TYK2-P1104A Autoimmune Protective Variant Limits Coordinate Signals Required to Generate Specialized T Cell Subsets

TYK2 is a JAK family member that functions downstream of multiple cytokine receptors. Genome wide association studies have linked a SNP (rs34536443) within TYK2 encoding a Proline to Alanine substitution at amino acid 1104, to protection from multiple autoimmune diseases including systemic lupus erythematosus (SLE) and multiple sclerosis (MS). The protective role of this SNP in autoimmune pathogenesis, however, remains incompletely understood. Here we found that T follicular helper (Tfh) cells, switched memory B cells, and IFNAR signaling were decreased in healthy individuals that expressed the protective variant TYK2 ^A1104 (TYK2 ^P ). To study this variant in vivo, we developed a knock-in murine model of this allele. Murine Tyk2 ^P expressing T cells homozygous for the protective allele, but not cells heterozygous for this change, manifest decreased IL-12 receptor signaling, important for Tfh lineage commitment. Further, homozygous Tyk2 ^P T cells exhibited diminished in vitro Th1 skewing. Surprisingly, despite these signaling changes, in vivo formation of Tfh and GC B cells was unaffected in two models of T cell dependent immune responses and in two alternative SLE models. TYK2 is also activated downstream of IL-23 receptor engagement. Here, we found that Tyk2 ^P expressing T cells had reduced IL-23 dependent signaling as well as a diminished ability to skew toward Th17 in vitro. Consistent with these findings, homozygous, but not heterozygous, Tyk2 ^P mice were fully protected in a murine model of MS. Homozygous Tyk2 ^P mice had fewer infiltrating CD4⁺ T cells within the CNS. Most strikingly, homozygous mice had a decreased proportion of IL-17⁺/IFNγ⁺, double positive, pathogenic CD4⁺ T cells in both the draining lymph nodes (LN) and CNS. Thus, in an autoimmune model, such as EAE, impacted by both altered Th1 and Th17 signaling, the Tyk2 ^P allele can effectively shield animals from disease. Taken together, our findings suggest that TYK2^P diminishes IL-12, IL-23, and IFN I signaling and that its protective effect is most likely manifest in the setting of autoimmune triggers that concurrently dysregulate at least two of these important signaling cascades.

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.

Generation of functional murine CD11c+ age-associated B cells in the absence of B cell T-bet expression

Age-associated B cells (ABC), a novel subset of activated B cells defined by CD11b and CD11c expression, have been linked with both protective anti-viral responses and the pathogenesis of systemic autoimmunity. Expression of the T_H 1 lineage transcription factor T-bet has been identified as a defining feature of ABC biology, with B cell-intrinsic expression of this transcription factor proposed to be required for ABC formation. In contrast to this model, we report that Tbx21 (encoding T-bet)-deficient B cells upregulate CD11b and CD11c surface expression in vitro in response to integrated TLR and cytokine signals. Moreover, B cell-intrinsic T-bet deletion in a murine lupus model exerted no impact of ABC generation in vivo, with Tbx21^-/- ABCs exhibiting an identical surface phenotype to wild-type (WT) ABCs. Importantly, WT and Tbx21^-/- ABCs sorted from autoimmune mice produced equivalent amounts of IgM and IgG ex vivo following TLR stimulation, indicating that T-bet-deficient ABCs are likely functional in vivo. In summary, our data contradict the established literature by demonstrating that T-bet expression is not uniformly required for ABC generation.

Integrated B Cell, Toll-like, and BAFF Receptor Signals Promote Autoantibody Production by Transitional B Cells

The B cell survival cytokine BAFF has been linked with the pathogenesis of systemic lupus erythematosus (SLE). BAFF binds distinct BAFF-family surface receptors, including the BAFF-R and transmembrane activator and CAML interactor (TACI). Although originally characterized as a negative regulator of B cell activation, TACI signals are critical for class-switched autoantibody (autoAb) production in BAFF transgenic mice. Consistent with this finding, a subset of transitional splenic B cells upregulate surface TACI expression and contribute to BAFF-driven autoAb. In the current study, we interrogated the B cell signals required for transitional B cell TACI expression and Ab production. Surprisingly, despite established roles for dual BCR and TLR signals in autoAb production in SLE, signals downstream of these receptors exerted distinct impacts on transitional B cell TACI expression and autoAb titers. Whereas loss of BCR signals prevented transitional B cell TACI expression and resulted in loss of serum autoAb across all Ig isotypes, lack of TLR signals exerted a more limited impact restricted to autoAb class-switch recombination without altering transitional B cell TACI expression. Finally, in parallel with the protective effect of TACI deletion, loss of BAFF-R signaling also protected against BAFF-driven autoimmunity. Together, these findings highlight how multiple signaling pathways integrate to promote class-switched autoAb production by transitional B cells, events that likely impact the pathogenesis of SLE and other BAFF-dependent autoimmune diseases.

Distinct Effector B Cells Induced by Unregulated Toll-like Receptor 7 Contribute to Pathogenic Responses in Systemic Lupus Erythematosus

Systemic Lupus Erythematosus (SLE) is characterized by B cells lacking IgD and CD27 (double negative; DN). We show that DN cell expansions reflected a subset of CXCR5- CD11c+ cells (DN2) representing pre-plasma cells (PC). DN2 cells predominated in African-American patients with active disease and nephritis, anti-Smith and anti-RNA autoantibodies. They expressed a T-bet transcriptional network; increased Toll-like receptor-7 (TLR7); lacked the negative TLR regulator TRAF5; and were hyper-responsive to TLR7. DN2 cells shared with activated naive cells (aNAV), phenotypic and functional features, and similar transcriptomes. Their PC differentiation and autoantibody production was driven by TLR7 in an interleukin-21 (IL-21)-mediated fashion. An in vivo developmental link between aNAV, DN2 cells, and PC was demonstrated by clonal sharing. This study defines a distinct differentiation fate of autoreactive naive B cells into PC precursors with hyper-responsiveness to innate stimuli, as well as establishes prominence of extra-follicular B cell activation in SLE, and identifies therapeutic targets.

Female predisposition to TLR7-driven autoimmunity: gene dosage and the escape from X chromosome inactivation

Women develop stronger immune responses than men, with positive effects on the resistance to viral or bacterial infections but magnifying also the susceptibility to autoimmune diseases like systemic lupus erythematosus (SLE). In SLE, the dosage of the endosomal Toll-like receptor 7 (TLR7) is crucial. Murine models have shown that TLR7 overexpression suffices to induce spontaneous lupus-like disease. Conversely, suppressing TLR7 in lupus-prone mice abolishes SLE development. TLR7 is encoded by a gene on the X chromosome gene, denoted TLR7 in humans and Tlr7 in the mouse, and expressed in plasmacytoid dendritic cells (pDC), monocytes/macrophages, and B cells. The receptor recognizes single-stranded RNA, and its engagement promotes B cell maturation and the production of pro-inflammatory cytokines and antibodies. In female mammals, each cell randomly inactivates one of its two X chromosomes to equalize gene dosage with XY males. However, 15 to 23% of X-linked human genes escape X chromosome inactivation so that both alleles can be expressed simultaneously. It has been hypothesized that biallelic expression of X-linked genes could occur in female immune cells, hence fostering harmful autoreactive and inflammatory responses. We review here the current knowledge of the role of TLR7 in SLE, and recent evidence demonstrating that TLR7 escapes from X chromosome inactivation in pDCs, monocytes, and B lymphocytes from women and Klinefelter syndrome men. Female B cells where TLR7 is thus biallelically expressed display higher TLR7-driven functional responses, connecting the presence of two X chromosomes with the enhanced immunity of women and their increased susceptibility to TLR7-dependent autoimmune syndromes.

αv Integrins regulate germinal center B cell responses through noncanonical autophagy

Germinal centers (GCs) are major sites of clonal B cell expansion and generation of long-lived, high-affinity antibody responses to pathogens. Signaling through TLRs on B cells promotes many aspects of GC B cell responses, including affinity maturation, class switching, and differentiation into long-lived memory and plasma cells. A major challenge for effective vaccination is identifying strategies to specifically promote GC B cell responses. Here, we have identified a mechanism of regulation of GC B cell TLR signaling, mediated by αv integrins and noncanonical autophagy. Using B cell-specific αv-KO mice, we show that loss of αv-mediated TLR regulation increased GC B cell expansion, somatic hypermutation, class switching, and generation of long-lived plasma cells after immunization with virus-like particles (VLPs) or antigens associated with TLR ligand adjuvants. Furthermore, targeting αv-mediated regulation increased the magnitude and breadth of antibody responses to influenza virus vaccination. These data therefore identify a mechanism of regulation of GC B cells that can be targeted to enhance antibody responses to vaccination.

Potential Chronotherapeutic Optimization of Antimalarials in Systemic Lupus Erythematosus: Is Toll-Like Receptor 9 Expression Dependent on the Circadian Cycle in Humans?

Toll-like receptor 9 (TLR9) belongs to the group of endosomal receptors of the innate immune system with the ability to recognize hypomethylated CpG sequences from DNA. There is scarce information about TLR9 expression and its association with the circadian cycle (CC). Different patterns of TLR9 expression are regulated by the CC in mice, with an elevated expression at Zeitgeber time 19 (1:00 a.m.); nevertheless, we still need to corroborate this in humans. In systemic lupus erythematosus (SLE), the inhibitory effect of chloroquine (CQ) on TLR9 is limited. TLR9 activation has been associated with the presence of some autoantibodies: anti-Sm/RNP, anti-histone, anti-Ro, anti-La, and anti-double-stranded DNA. Treatment with CQ for SLE has been proven to be useful, in part by interfering with HLA-antigen coupling and with TLR9 ligand recognition. Studies have shown that TLR9 inhibitors such as antimalarial drugs are able to mask TLR9-binding sites on nucleic acids. The data presented here provide the basic information that could be useful for other clinical researchers to design studies that will have an impact in achieving a chronotherapeutic effect by defining the ideal time for CQ administration in SLE patients, consequently reducing the pathological effects that follow the activation of TLR9.

TACI deletion protects against progressive murine lupus nephritis induced by BAFF overexpression

B cells are known to promote the pathogenesis of systemic lupus erythematosus (SLE) via the production of pathogenic anti-nuclear antibodies. However, the signals required for autoreactive B cell activation and the immune mechanisms whereby B cells impact lupus nephritis pathology remain poorly understood. The B cell survival cytokine B cell activating factor of the TNF Family (BAFF) has been implicated in the pathogenesis of SLE and lupus nephritis in both animal models and human clinical studies. Although the BAFF receptor has been predicted to be the primary BAFF family receptor responsible for BAFF-driven humoral autoimmunity, in the current study we identify a critical role for signals downstream of Transmembrane Activator and CAML Interactor (TACI) in BAFF-dependent lupus nephritis. Whereas transgenic mice overexpressing BAFF develop progressive membranoproliferative glomerulonephritis, albuminuria and renal dysfunction, TACI deletion in BAFF-transgenic mice provided long-term (about 1 year) protection from renal disease. Surprisingly, disease protection in this context was not explained by complete loss of glomerular immune complex deposits. Rather, TACI deletion specifically reduced endocapillary, but not mesangial, immune deposits. Notably, although excess BAFF promoted widespread breaks in B cell tolerance, BAFF-transgenic antibodies were enriched for RNA- relative to DNA-associated autoantigen reactivity. These RNA-associated autoantibody specificities were specifically reduced by TACI or Toll-like receptor 7 deletion. Thus, our study provides important insights into the autoantibody specificities driving proliferative lupus nephritis, and suggests that TACI inhibition may be novel and effective treatment strategy in lupus nephritis.

Accelerated model of lupus autoimmunity and vasculopathy driven by toll-like receptor 7/9 imbalance

Objectives

Activation of endosomal toll-like receptor (TLR)7 or TLR9 has been proposed as a critical step for the initiation and development of SLE. Traditional spontaneous lupus models normally introduce multiple risk alleles, thereby adding additional confounding factors. In the induced lupus models, the role of TLR9 remains unclear. In the present study, we explored the role of an imbalance between TLR7 and TLR9 pathways in the pathogenesis of lupus and its associated vasculopathy using the imiquimod model in TLR9 KO/B6 background.

Methods

Wild type (WT) and Tlr9^-/- mice were epicutaneously treated with imiquimod cream 5% on both ears three times per week for indicated times. At euthanasia, mice were analysed for organ involvement, endothelium-dependent vasorelaxation, serum autoantibodies, and innate and adaptive immune responses.

Results

Compared with the lupus-like phenotype that develops in imiquimod-treated WT mice, Tlr9^-/- mice exposed to imiquimod have increased severity of autoimmunity features and inflammatory phenotype that develops at earlier stages. These abnormalities are characterised by enhanced TLR7 expression and immune activation, increased immune complex deposition, Th1 T cells and dendritic cell kidney infiltration and significant impairments in endothelial function. Modulation of TLR7 expression was observed in the Tlr9^-/- mice.

Conclusions

These findings further underscore the protective role of TLR9 in TLR7-driven autoimmunity and also in the development of vasculopathy, further strengthening the importance of tightly manipulating TLRs in putative therapeutic strategies. This study provides a new model of accelerated lupus phenotype driven by danger-associated molecular patterns.

Toll-like receptors in lupus nephritis

The pathogenesis of systemic autoimmune diseases such as systemic lupus erythematosus (SLE) is based on the loss of self-tolerance against ubiquitous autoantigens involving all mechanisms of adaptive immunity. However, data accumulating over the last decade imply an important role also for numerous elements of innate immunity, namely the Toll-like receptors in the pathogenesis of SLE. Here we discuss their role in the most common organ complication of SLE, i.e. lupus nephritis. We summarize experimental and clinical data on the expression and functional contribution of the Toll-like receptors in immune complex glomerulonephritis, and intrarenal inflammation. Based on these discoveries Toll-like receptors are evolving as therapeutic targets for the treatment of SLE and lupus nephritis.

Plasma Cell Differentiation Pathways in Systemic Lupus Erythematosus

Plasma cells (PCs) are responsible for the production of protective antibodies against infectious agents but they also produce pathogenic antibodies in autoimmune diseases, such as systemic lupus erythematosus (SLE). Traditionally, high affinity IgG autoantibodies are thought to arise through germinal center (GC) responses. However, class switching and somatic hypermutation can occur in extrafollicular (EF) locations, and this pathway has also been implicated in SLE. The pathway from which PCs originate may determine several characteristics, such as PC lifespan and sensitivity to therapeutics. Although both GC and EF responses have been implicated in SLE, we hypothesize that one of these pathways dominates in each individual patient and genetic risk factors may drive this predominance. While it will be important to distinguish polymorphisms that contribute to a GC-driven or EF B cell response to develop targeted treatments, the challenge will be not only to identify the differentiation pathway but the molecular mechanisms involved. In B cells, this task is complicated by the cross-talk between the B cell receptor, toll-like receptors (TLR), and cytokine signaling molecules, which contribute to both GC and EF responses. While risk variants that affect the function of dendritic cells and T follicular helper cells are likely to primarily influence GC responses, it will be important to discover whether some risk variants in the interferon and TLR pathways preferentially influence EF responses. Identifying the pathways of autoreactive PC differentiation in SLE may help us to understand patient heterogeneity and thereby guide precision therapy.

Spontaneous germinal centers and autoimmunity

Germinal centers (GCs) are dynamic microenvironments that form in the secondary lymphoid organs and generate somatically mutated high-affinity antibodies necessary to establish an effective humoral immune response. Tight regulation of GC responses is critical for maintaining self-tolerance. GCs can arise in the absence of purposeful immunization or overt infection (called spontaneous GCs, Spt-GCs). In autoimmune-prone mice and patients with autoimmune disease, aberrant regulation of Spt-GCs is thought to promote the development of somatically mutated pathogenic autoantibodies and the subsequent development of autoimmunity. The mechanisms that control the formation of Spt-GCs and promote systemic autoimmune diseases remain an open question and the focus of ongoing studies. Here, we discuss the most current studies on the role of Spt-GCs in autoimmunity.

Co-Stimulation of BCR and Toll-Like Receptor 7 Increases Somatic Hypermutation, Memory B Cell Formation, and Secondary Antibody Response to Protein Antigen

The goal of immunization is to produce both a flood of antibodies to neutralize antigen and memory cells to accelerate the secondary response. To enhance the generation of memory B cells, we examined the effect of co-engaging BCR and toll-like receptor (TLR) 7 receptors by immunizing mice with a hapten-protein antigen, NP-CGG, and a ligand, R837 (imiquimod). During the early and late primary responses, there was no augmentation with R837 on the number of germinal center B cells or serum antibody. However, in the niche of germinal centers, R837 increased somatic hypermutation in the canonical V_H1-72 gene that encodes NP-specific antibody. Increased mutation was not due to enhanced expression of activation-induced deaminase, but was likely a result of selection for high-affinity B cells with altered codons in the gene. This correlated with the appearance of antigen-specific B cells with a memory phenotype, which was intrinsic to TLR7 on B cells. To determine if these memory cells produced a recall response after a secondary challenge, spleen cells from mice that were immunized with NP-CGG and R837 were adoptively transferred into muMT recipients, and boosted with NP-CGG. Cells from mice that initially received both antigen and R837 generated a robust increase in germinal center B cells, plasmablasts, plasma cells, and serum antibody, compared with their cohorts who received antigen alone. These results support the use of co-immunization with TLR7 ligands to promote vigorous memory B cell responses to protein antigens.

Spoiling for a Fight: B Lymphocytes As Initiator and Effector Populations within Tertiary Lymphoid Organs in Autoimmunity and Transplantation

Tertiary lymphoid organs (TLOs) develop at ectopic sites within chronically inflamed tissues, such as in autoimmunity and rejecting organ allografts. TLOs differ structurally from canonical secondary lymphoid organs (SLOs), in that they lack a mantle zone and are not encapsulated, suggesting that they may provide unique immune function. A notable feature of TLOs is the frequent presence of structures typical of germinal centers (GCs). However, little is known about the role of such GCs, and in particular, it is not clear if the B cell response within is autonomous, or whether it synergizes with concurrent responses in SLOs. This review will discuss ectopic lymphoneogenesis and the role of the B cell in TLO formation and subsequent effector output in the context of autoimmunity and transplantation, with particular focus on the contribution of ectopic GCs to affinity maturation in humoral immune responses and to the potential breakdown of self-tolerance and development of humoral autoimmunity.

Intracellular B Lymphocyte Signalling and the Regulation of Humoral Immunity and Autoimmunity

B lymphocytes are critical for effective immunity; they produce antibodies and cytokines, present antigens to T lymphocytes and regulate immune responses. However, because of the inherent randomness in the process of generating their vast repertoire of antigen-specific receptors, B cells can also cause diseases through recognizing and reacting to self. Therefore, B lymphocyte selection and responses require tight regulation at multiple levels and at all stages of their development and activation to avoid diseases. Indeed, newly generated B lymphocytes undergo rigorous tolerance mechanisms in the bone marrow and, subsequently, in the periphery after their migration. Furthermore, activation of mature B cells is regulated through controlled expression of co-stimulatory receptors and intracellular signalling thresholds. All these regulatory events determine whether and how B lymphocytes respond to antigens, by undergoing apoptosis or proliferation. However, defects that alter regulated co-stimulatory receptor expression or intracellular signalling thresholds can lead to diseases. For example, autoimmune diseases can result from altered regulation of B cell responses leading to the emergence of high-affinity autoreactive B cells, autoantibody production and tissue damage. The exact cause(s) of defective B cell responses in autoimmune diseases remains unknown. However, there is evidence that defects or mutations in genes that encode individual intracellular signalling proteins lead to autoimmune diseases, thus confirming that defects in intracellular pathways mediate autoimmune diseases. This review provides a synopsis of current knowledge of signalling proteins and pathways that regulate B lymphocyte responses and how defects in these could promote autoimmune diseases. Most of the evidence comes from studies of mouse models of disease and from genetically engineered mice. Some, however, also come from studying B lymphocytes from patients and from genome-wide association studies. Defining proteins and signalling pathways that underpin atypical B cell response in diseases will help in understanding disease mechanisms and provide new therapeutic avenues for precision therapy.

B cell-derived IL-6 initiates spontaneous germinal center formation during systemic autoimmunity

Arkatkar et al. report that B cell–derived IL-6 is critical for T follicular helper cell differentiation, spontaneous germinal center formation, and class-switched autoantibody production during humoral autoimmunity.

Recent studies have identified critical roles for B cells in triggering autoimmune germinal centers (GCs) in systemic lupus erythematosus (SLE) and other disorders. The mechanisms whereby B cells facilitate loss of T cell tolerance, however, remain incompletely defined. Activated B cells produce interleukin 6 (IL-6), a proinflammatory cytokine that promotes T follicular helper (T_FH) cell differentiation. Although B cell IL-6 production correlates with disease severity in humoral autoimmunity, whether B cell–derived IL-6 is required to trigger autoimmune GCs has not, to our knowledge, been addressed. Here, we report the unexpected finding that a lack of B cell–derived IL-6 abrogates spontaneous GC formation in mouse SLE, resulting in loss of class-switched autoantibodies and protection from systemic autoimmunity. Mechanistically, B cell IL-6 production was enhanced by IFN-γ, consistent with the critical roles for B cell–intrinsic IFN-γ receptor signals in driving autoimmune GC formation. Together, these findings identify a key mechanism whereby B cells drive autoimmunity via local IL-6 production required for T_FH differentiation and autoimmune GC formation.

Inhibition of Toll-Like Receptor Signaling as a Promising Therapy for Inflammatory Diseases: A Journey from Molecular to Nano Therapeutics

The recognition of invading pathogens and endogenous molecules from damaged tissues by toll-like receptors (TLRs) triggers protective self-defense mechanisms. However, excessive TLR activation disrupts the immune homeostasis by sustained pro-inflammatory cytokines and chemokines production and consequently contributes to the development of many inflammatory and autoimmune diseases, such as systemic lupus erythematosus (SLE), infection-associated sepsis, atherosclerosis, and asthma. Therefore, inhibitors/antagonists targeting TLR signals may be beneficial to treat these disorders. In this article, we first briefly summarize the pathophysiological role of TLRs in the inflammatory diseases. We then focus on reviewing the current knowledge in both preclinical and clinical studies of various TLR antagonists/inhibitors for the prevention and treatment of inflammatory diseases. These compounds range from conventional small molecules to therapeutic biologics and nanodevices. In particular, nanodevices are emerging as a new class of potent TLR inhibitors for their unique properties in desired bio-distribution, sustained circulation, and preferred pharmacodynamic and pharmacokinetic profiles. More interestingly, the inhibitory activity of these nanodevices can be regulated through precise nano-functionalization, making them the next generation therapeutics or “nano-drugs.” Although, significant efforts have been made in developing different kinds of new TLR inhibitors/antagonists, only limited numbers of them have undergone clinical trials, and none have been approved for clinical uses to date. Nevertheless, these findings and continuous studies of TLR inhibition highlight the pharmacological regulation of TLR signaling, especially on multiple TLR pathways, as future promising therapeutic strategy for various inflammatory and autoimmune diseases.

TLR7/TLR9- and B Cell Receptor-Signaling Crosstalk: Promotion of Potentially Dangerous B Cells

B cells are capable of receptor-mediated responses to foreign antigens. Recognition of microbial-derived nucleic acid (NA) by toll-like receptors (TLRs) 7 and 9 in B cells has been substantiated. Endogenous NA released from damaged or dying cells can also be immunogenic in certain contexts and can incite aberrant activation of B cells. When TLR-driven B cell receptor (BCR)-activated B cells are not properly constrained, pathologic autoantibodies are produced. It is also clear that endosomal TLR7/TLR9 can operate in conjunction with BCR. In addition to BCR signaling, a balance between TLR7 and TLR9 is pivotal in the development of B cell autoreactivity. While TLR9 is important in normal memory B cell responses through BCR, TLR9 activation has been implicated in autoantibody production. Paradoxically, TLR9 also plays known protective roles against autoimmunity by directly and indirectly inhibiting TLR7-mediated autoantibody production. Herein, we summarize literature supporting mechanisms underpinning the promotion of pathological BCR-activated B cells by TLR7 and TLR9. We focus on the literature regarding known points of TLR7/TLR9 and BCR crosstalk. Data also suggest that the degree of TLR responsiveness relies on alterations of certain intrinsic B-cell signaling molecules and is also context specific. Because allogeneic hematopoietic stem cell transplantation is a high NA and B cell-activating factor environment, we conclude that B cell studies of synergistic TLR-BCR signaling in human diseases like chronic graft-versus-host disease are warranted. Further understanding of the distinct molecular pathways mediating TLR-BCR synergy will lead to the development of therapeutic strategies in autoimmune disease states.

The A946T variant of the RNA sensor IFIH1 mediates an interferon program that limits viral infection but increases the risk for autoimmunity

The single-nucleotide polymorphism rs1990760 in the gene encoding the cytosolic viral sensor IFIH1 results in an amino-acid change (A946T; IFIH1^T946) that is associated with multiple autoimmune diseases. The effect of this polymorphism on both viral sensing and autoimmune pathogenesis remains poorly understood. Here we found that human peripheral blood mononuclear cells (PBMCs) and cell lines expressing the risk variant IFIH1^T946 exhibited heightened basal and ligand-triggered production of type I interferons. Consistent with those findings, mice with a knock-in mutation encoding IFIH1^T946 displayed enhanced basal expression of type I interferons, survived a lethal viral challenge and exhibited increased penetrance in autoimmune models, including a combinatorial effect with other risk variants. Furthermore, IFIH1^T946 mice manifested an embryonic survival defect consistent with enhanced responsiveness to RNA self ligands. Together our data support a model wherein the production of type I interferons driven by an autoimmune risk variant and triggered by ligand functions to protect against viral challenge, which probably accounts for its selection within human populations but provides this advantage at the cost of modestly promoting the risk of autoimmunity.

Altered B cell signalling in autoimmunity

Recent work has provided new insights into how altered B cell-intrinsic signals - through the B cell receptor (BCR) and key co-receptors - function together to promote the pathogenesis of autoimmunity. These combined signals affect B cells at two distinct stages: first, in the selection of the naive repertoire; and second, during extrafollicular or germinal centre activation responses. Thus, dysregulated signalling can lead to both an altered naive BCR repertoire and the generation of autoantibody-producing B cells. Strikingly, high-affinity autoantibodies predate and predict disease in several autoimmune disorders, including type 1 diabetes and systemic lupus erythematosus. This Review summarizes how, rather than being a downstream consequence of autoreactive T cell activation, dysregulated B cell signalling can function as a primary driver of many human autoimmune diseases.

Btk-specific inhibition blocks pathogenic plasma cell signatures and myeloid cell-associated damage in IFN-driven lupus nephritis

Systemic lupus erythematosus (SLE) is often associated with exaggerated B cell activation promoting plasma cell generation, immune-complex deposition in the kidney, renal infiltration of myeloid cells, and glomerular nephritis. Type-I IFNs amplify these autoimmune processes and promote severe disease. Bruton's tyrosine kinase (Btk) inhibitors are considered novel therapies for SLE. We describe the characterization of a highly selective reversible Btk inhibitor, G-744. G-744 is efficacious, and superior to blocking BAFF and Syk, in ameliorating severe lupus nephritis in both spontaneous and IFNα-accelerated lupus in NZB/W_F1 mice in therapeutic regimens. Selective Btk inhibition ablated plasmablast generation, reduced autoantibodies, and - similar to cyclophosphamide - improved renal pathology in IFNα-accelerated lupus. Employing global transcriptional profiling of spleen and kidney coupled with cross-species human modular repertoire analyses, we identify similarities in the inflammatory process between mice and humans, and we demonstrate that G-744 reduced gene expression signatures essential for splenic B cell terminal differentiation, particularly the secretory pathway, as well as renal transcriptional profiles coupled with myeloid cell-mediated pathology and glomerular plus tubulointerstitial disease in human glomerulonephritis patients. These findings reveal the mechanism through which a selective Btk inhibitor blocks murine autoimmune kidney disease, highlighting pathway activity that may translate to human SLE.

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.

Excessive activation of the TLR9/TGF-β1/PDGF-B pathway in the peripheral blood of patients with systemic lupus erythematosus

Background

Our aim is to study the existence of the TLR9/TGF-β1/PDGF-B pathway in healthy humans and patients with systemic lupus erythematosus (SLE), and to explore its possible involvement in the pathogenesis of lupus nephritis (LN).

Methods

Protein levels of the cytokines were detected by ELISA. mRNA levels of the cytokines were analyzed by real-time PCR. MTT assay was used to test the proliferation of mesangial cells under different treatments.

Results

Compared to healthy controls (N_Control = 56), levels of Toll-like receptor (TLR)9, transforming growth factor (TGF)-β1, and platelet-derived growth factor B (PDGF-B) were increased significantly in the peripheral blood of SLE patients (N_SLE = 112). Significant correlations between the levels of TLR9, TGF-β1, and PDGF-B were observed in both healthy controls and SLE patients. The levels of TGF-β1 and PDGF-B were greatly enhanced by TLR9 activation in primary cell cultures. The proliferation of mesangial cells induced by the plasma of SLE patients was significantly higher than that induced by healthy controls; PDGF-B was involved in this process. The protein levels of PDGF-B homodimer correlated with the levels of urine protein in SLE patients with LN (N_LN =38).

Conclusions

The TLR9/TGF-β1/PDGF-B pathway exists in humans and can be excessively activated in SLE patients. High levels of PDGF-B may result in overproliferation of mesangial cells in the kidney that are involved in the development of glomerulonephritis and LN. Further studies are necessary to identify TLR9, TGF-β1, and PDGF-B as new therapeutic targets to prevent the development of glomerulonephritis and LN.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-017-1238-8) contains supplementary material, which is available to authorized users.

A TLR9-dependent checkpoint governs B cell responses to DNA-containing antigens

Mature B cell pools retain a substantial proportion of polyreactive and self-reactive clonotypes, suggesting that activation checkpoints exist to reduce the initiation of autoreactive B cell responses. Here, we have described a relationship among the B cell receptor (BCR), TLR9, and cytokine signals that regulate B cell responses to DNA-containing antigens. In both mouse and human B cells, BCR ligands that deliver a TLR9 agonist induce an initial proliferative burst that is followed by apoptotic death. The latter mechanism involves p38-dependent G1 cell-cycle arrest and subsequent intrinsic mitochondrial apoptosis and is shared by all preimmune murine B cell subsets and CD27- human B cells. Survival or costimulatory signals rescue B cells from this fate, but the outcome varies depending on the signals involved. B lymphocyte stimulator (BLyS) engenders survival and antibody secretion, whereas CD40 costimulation with IL-21 or IFN-γ promotes a T-bet+ B cell phenotype. Finally, in vivo immunization studies revealed that when protein antigens are conjugated with DNA, the humoral immune response is blunted and acquires features associated with T-bet+ B cell differentiation. We propose that this mechanism integrating BCR, TLR9, and cytokine signals provides a peripheral checkpoint for DNA-containing antigens that, if circumvented by survival and differentiative cues, yields B cells with the autoimmune-associated T-bet+ phenotype.

Toll-like receptor 9 suppresses lupus disease in Fas-sufficient MRL Mice

Genetic deficiency in TLR9 accelerates pathogenesis in the spontaneous polygenic MRL.Fas^lpr murine model of systemic lupus erythematosus, despite the absence of anti-nucleosome autoantibodies. However, it could be argued that this result was dependent on Fas-deficiency rather than lupus-promoting genes in the MRL genetic background. Here we report the effects of TLR9 deficiency on autoimmune disease independent of the lpr mutation in Fas by characterizing Tlr9^-/- and Tlr9^+/+ mice on the Fas-intact MRL/+ genetic background. By 30 weeks of age, Tlr9-deficient MRL/+ had more severe renal disease, increased T cell activation, and higher titers of anti-Sm and anti-RNA autoantibodies than Tlr9-intact animals, as had been the case in the MRL.Fas^lpr model. In addition, Tlr9-deficient MRL/+ mice had increased numbers of germinal center phenotype B cells and an increase in splenic neutrophils and conventional dendritic cell populations. Thus, the disease accelerating effects of Tlr9 deficiency are separable from those mediated by the Fas mutation in the lupus-prone MRL genetic background. Nonetheless, disease acceleration in Tlr9-deficient MRL/+ mice was phenotypically distinct from that in Fas-deficient counterparts, which has important implications.

Autoreactive helper T cells alleviate the need for intrinsic TLR signaling in autoreactive B cell activation

T cells play a significant role in the pathogenesis of systemic autoimmune diseases, including systemic lupus erythematosus; however, there is relatively little information on the nature and specificity of autoreactive T cells. Identifying such cells has been technically difficult because they are likely to be rare and low affinity. Here, we report a method for identifying autoreactive T cell clones that recognize proteins contained in autoantibody immune complexes, providing direct evidence that functional autoreactive helper T cells exist in the periphery of normal mice. These T cells significantly enhanced autoreactive B cell proliferation and altered B cell differentiation in vivo. Most importantly, these autoreactive T cells were able to rescue many aspects of the TLR-deficient AM14 (anti-IgG2a rheumatoid factor) B cell response, suggesting that TLR requirements can be bypassed. This result has implications for the efficacy of TLR-targeted therapy in the treatment of ongoing disease.

Safe and Effective Gene Therapy for Murine Wiskott-Aldrich Syndrome Using an Insulated Lentiviral Vector

Wiskott-Aldrich syndrome (WAS) is a life-threatening immunodeficiency caused by mutations within the WAS gene. Viral gene therapy to restore WAS protein (WASp) expression in hematopoietic cells of patients with WAS has the potential to improve outcomes relative to the current standard of care, allogeneic bone marrow transplantation. However, the development of viral vectors that are both safe and effective has been problematic. While use of viral transcriptional promoters may increase the risk of insertional mutagenesis, cellular promoters may not achieve WASp expression levels necessary for optimal therapeutic effect. Here we evaluate a self-inactivating (SIN) lentiviral vector combining a chromatin insulator upstream of a viral MND (MPSV LTR, NCR deleted, dl587 PBS) promoter driving WASp expression. Used as a gene therapeutic in Was^−/− mice, this vector resulted in stable WASp⁺ cells in all hematopoietic lineages and rescue of T and B cell defects with a low number of viral integrations per cell, without evidence of insertional mutagenesis in serial bone marrow transplants. In a gene transfer experiment in non-human primates, the insulated MND promoter (driving GFP expression) demonstrated long-term polyclonal engraftment of GFP⁺ cells. These observations demonstrate that the insulated MND promoter safely and efficiently reconstitutes clinically effective WASp expression and should be considered for future WAS therapy.

BCR and co-receptor crosstalk facilitate the positive selection of self-reactive transitional B cells

The establishment of a diverse B cell repertoire requires fine-tuning of antigen receptor selection during development in order to permit sufficient diversity while reducing the potential for autoimmunity. In this review, we highlight recent studies demonstrating the central role of the B cell antigen receptor (BCR), in coordination with other key pro-survival signals mediated by CD40, BAFF-R, TACI and/or TLRs, in regulating both negative and positive selection of autoreactive B cells. In particular, we show how altered antigen or co-stimulatory signaling can facilitate positive selection of transitional B cells with self-reactive BCRs, ultimately leading to their entry into the mature, naive B cell compartment. We propose a model wherein altered receptor signals (due to inherited genetic changes) leads: first, to enhanced positive selection of autoreactive cells into the naïve B cell repertoire; subsequently, to an increased probability of pathogenic germinal center responses in individuals with a broad range of autoimmune disorders.

Neutrophils, Dendritic Cells, Toll-Like Receptors, and Interferon-α in Lupus Nephritis

Finding better treatments for lupus nephritis requires an understanding of the pathogenesis of the causative systemic disease, how this leads to kidney disease, and how lupus nephritis progresses to end-stage kidney disease. Here, we provide a brief conceptual overview on the related pathomechanisms. As a main focus we discuss in detail the roles of neutrophils, dendritic cells, Toll-like receptors, and interferon-α in the pathogenesis of lupus nephritis by separately reviewing their roles in extrarenal systemic autoimmunity and in intrarenal inflammation and immunopathology.