Exogenous and endogenous TLR ligands activate anti-chromatin and polyreactive B cells

Toll-Like Receptors in the Pathogenesis of Autoimmune Diseases

Human Toll-like receptors (TLRs) are a family of transmembrane receptors, which play a key role in both innate and adaptive immune responses. Beside of recognizing specific molecular patterns that associated with different types of pathogens, TLRs may also detect a number of self-proteins and endogenous nucleic acids. Activating TLRs lead to the heightened expression of various inflammatory genes, which have a protective role against infection. Data rising predominantly from human patients and animal models of autoimmune disease indicate that, inappropriate triggering of TLR pathways by exogenous or endogenous ligands may cause the initiation and/or perpetuation of autoimmune reactions and tissue damage. Given their important role in infectious and non-infectious disease process, TLRs and its signaling pathways emerge as appealing targets for therapeutics. In this review, we demonstrate how TLRs pathways could be involved in autoimmune disorders and their therapeutic application.

Microbial TLR Agonists and Humoral Immunopathogenesis in HIV Disease

Although T cells are the primary and most-studied targets of the Human Immunodeficiency Virus (HIV), B cells, especially memory B lymphocytes, are also chronically depleted in the course of HIV disease. Although the lack of CD4+ T cell help may explain these deficiencies, intrinsic defects in B lymphocytes appear to contribute to B cell depletion and reduced antibody (Ab) production in the setting of HIV, especially of some antigens eliciting T cell-independent responses. The gut mucosal barrier is disrupted in HIV disease, resulting in increased systemic exposure to microbial products such as Toll-Like Receptor (TLR) agonists. The association of enhanced systemic levels of TLR agonists and B cell dysfunction in HIV disease is not understood. This review discusses the potential role of microbial TLR agonists in the B cell depletion, enhanced autoantibody production and impaired responses to vaccination observed in HIV-infected hosts. Increased microbial translocation in HIV infection may drive B cells to produce autoantibodies and increase susceptibilities of B cells to apoptosis through activation-induced cell death. Determining the mechanisms of B cell perturbations in HIV disease will inform the design of novel strategies of improve immune responses to vaccines, reduce opportunistic infections and slow disease progression.

TLR9 promotes tolerance by restricting survival of anergic anti-DNA B cells, yet is also required for their activation

Nucleic acid-reactive B cells frequently arise in the bone marrow but are tolerized by mechanisms including receptor editing, functional anergy, and/or deletion. TLR9, a sensor of endosomal dsDNA, both promotes and regulates systemic autoimmunity in vivo, but the precise nature of its apparently contradictory roles in autoimmunity remained unclear. In this study, using the 3H9 anti-DNA BCR transgene in the autoimmune-prone MRL.Fas(lpr) mouse model of systemic lupus erythematosus, we identify the stages at which TLR9 contributes to establishing and breaking B cell tolerance. Although TLR9 is dispensable for L chain editing during B cell development in the bone marrow, TLR9 limits anti-DNA B cell life span in the periphery and is thus tolerogenic. In the absence of TLR9, anti-DNA B cells have much longer life spans and accumulate in the follicle, neither activated nor deleted. These cells retain some characteristics of anergic cells, in that they have elevated basal BCR signaling but impaired induced responses and downregulate their cell-surface BCR expression. In contrast, whereas TLR9-intact anergic B cells accumulate near the T/B border, TLR9-deficient anti-DNA B cells are somewhat more dispersed throughout the follicle. Nonetheless, in older autoimmune-prone animals, TLR9 expression specifically within the B cell compartment is required for spontaneous peripheral activation of anti-DNA B cells and their differentiation into Ab-forming cells via an extrafollicular pathway. Thus, TLR9 has paradoxical roles in regulating anti-DNA B cells: it helps purge the peripheral repertoire of autoreactive cells, yet is also required for their activation.

Antigen-driven induction of polyreactive IgM during intracellular bacterial infection

Polyreactivity is well known as a property of natural IgM produced by B-1 cells. We demonstrate that polyreactive IgM is also generated during infection of mice with Ehrlichia muris, a tick-borne intracellular bacterial pathogen. The polyreactive IgM bound self and foreign Ags, including single-stranded and double-stranded DNA, insulin, thyroglobulin, LPS, influenza virus, and Borrelia burgdorferi. Production of polyreactive IgM during infection was Ag driven, not due to polyclonal B cell activation, as the majority of polyreactive IgM recognized ehrlichial Ag(s), including an immunodominant outer membrane protein. Monoclonal polyreactive IgM derived from T cell-independent spleen plasmablasts, which was germline-encoded, also bound cytoplasmic and nuclear Ags in HEp-2 cells. Polyreactive IgM protected immunocompromised mice against lethal bacterial challenge infection. Serum from human ehrlichiosis patients also contained polyreactive and self-reactive IgM. We propose that polyreactivity increases IgM efficacy during infection but may also exacerbate or mollify the response to foreign and self Ags.

WASp-deficient B cells play a critical, cell-intrinsic role in triggering autoimmunity

In a manner dependent on CD4 T cell help and Toll-like receptor signals, B cells lacking WASp induce autoantibody production and autoimmune disease in mice.

Patients with the immunodeficiency Wiskott-Aldrich syndrome (WAS) frequently develop systemic autoimmunity. Here, we demonstrate that mutation of the WAS gene results in B cells that are hyperresponsive to B cell receptor and Toll-like receptor (TLR) signals in vitro, thereby promoting a B cell–intrinsic break in tolerance. Whereas this defect leads to autoantibody production in WAS protein–deficient (WASp^−/−) mice without overt disease, chimeric mice in which only the B cell lineage lacks WASp exhibit severe autoimmunity characterized by spontaneous germinal center formation, class-switched autoantibodies, renal histopathology, and early mortality. Both T cell help and B cell–intrinsic TLR engagement play important roles in promoting disease in this model, as depletion with anti-CD4 antibodies or generation of chimeric mice with B cells deficient in both WASp and MyD88 prevented development of autoimmune disease. These data highlight the potentially harmful role for cell-intrinsic loss of B cell tolerance in the setting of normal T cell function, and may explain why WAS patients with mixed chimerism after stem cell transplantation often develop severe humoral autoimmunity.

Anergic responses characterize a large fraction of human autoreactive naive B cells expressing low levels of surface IgM

B cell anergy represents an important mechanism of peripheral immunological tolerance for mature autoreactive B cells that escape central tolerance enforced by receptor editing and clonal deletion. Although well documented in mice, the extent of its participation in human B cell tolerance remains to be fully established. In this study, we characterize the functional behavior of strictly defined human naive B cells separated on the basis of their surface IgM (sIgM) expression levels. We demonstrate that cells with lower sIgM levels (IgM(lo)) are impaired in their ability to flux calcium in response to either anti-IgM or anti-IgD cross-linking and contain a significantly increased frequency of autoreactive cells compared with naive B cells with higher levels of sIgM. Phenotypically, in healthy subjects, IgM(lo) cells are characterized by the absence of activation markers, reduction of costimulatory molecules (CD19 and CD21), and increased levels of inhibitory CD22. Functionally, IgM(lo) cells display significantly weaker proliferation, impaired differentiation, and poor Ab production. In aggregate, the data indicate that hyporesponsiveness to BCR cross-linking associated with sIgM downregulation is present in a much larger fraction of all human naive B cells than previously reported and is likely to reflect a state of anergy induced by chronic autoantigen stimulation. Finally, our results indicate that in systemic lupus erythematosus patients, naive IgM(lo) cells display increased levels of CD95 and decreased levels of CD22, a phenotype consistent with enhanced activation of autoreactive naive B cells in this autoimmune disease.

An intrinsic B-cell defect supports autoimmunity in New Zealand black chromosome 13 congenic mice

Introgression of a New Zealand Black (NZB) chromosome 13 interval onto a C57BL/6 (B6) background (B6.NZBc13) is sufficient to produce many hallmarks of lupus, including high-titre anti-chromatin antibody production, abnormal B- and T-cell activation, and renal disease. In this study we sought to characterize the immune defects leading to these abnormalities. By generating hematopoietic chimeras and BCR transgenic mice, we show that the congenic autoimmune phenotype can be transferred by BM cells and requires the presence of autoreactive B cells. Using the hen egg white lysozyme immunoglobulin transgenic mouse model, we demonstrate that B-cell anergy, deletion, and receptor editing are intact. Nevertheless, congenic B cells exhibit altered peripheral B-cell selection, as demonstrated by enhanced survival and activation of endogenous B cells with autoreactivity to chromatin and Sm/ribonucleoprotein. Given the autoantibody specificities to nuclear antigens, TLR signalling was assessed. B6.NZBc13 B cells were hyper-responsive to poly(I:C), a TLR3 ligand, demonstrating enhanced proliferation and survival as compared to B6 B cells. Our findings indicate the presence of an intrinsic B-cell defect on NZB chromosome 13 that results in hyper-responsiveness to a dsRNA analogue and implicates its potential supporting role in the generation of autoimmunity in B6.NZBc13 mice.

B cells and autoantibodies: complex roles in CNS injury

Emerging data indicate that traumatic injury to the brain or spinal cord activates B lymphocytes, culminating in the production of antibodies specific for antigens found within and outside the central nervous system (CNS). Here, we summarize what is known about the effects of CNS injury on B cells. We outline the potential mechanisms for CNS trauma-induced B cell activation and discuss the potential consequences of these injury-induced B cell responses. On the basis of recent data, we hypothesize that a subset of autoimmune B cell responses initiated by CNS injury are pathogenic and that targeted inhibition of B cells could improve recovery in cases of brain and spinal cord injury.

Antigen-specific responses and ANA production in B6.Sle1b mice: a role for SAP

B6.Sle1b mice, which contain the Sle1b gene interval derived from lupus prone NZM2410 mice on a C57BL/6 background, present with gender-biased, highly penetrant anti-nuclear antibody (ANA) production. To obtain some insight into the possible induction mechanism of autoantibodies in these mice we compared antigen-specific T dependent (TD) and T independent (TI-II) responses between B6.Sle1b and B6 mice before the development of high ANA titers. Our results show that B6.Sle1b mice mount enhanced responses to a TI-II antigen. Additionally, the memory T cell response generated by a TD antigen also increased. This enhancement correlates with the greater ability of B cells from B6.Sle1b mice to present antigen to T cells. The SLAM Associated Protein (SAP) is critical for signaling of many of the molecules encoded by the SLAM/CD2 gene cluster, candidates for mediating the Sle1b phenotype; therefore, we also investigated the effect of sap deletion in these strains on the TD and TI-II responses as well as on ANA production. The results of these studies of responses to non-self-antigens provide further insight into the mechanism by which responses to self-antigens might be initiated in the context of specific genetic alterations.

Role of B cells in systemic lupus erythematosus and rheumatoid arthritis

B cell tolerance to many self-proteins is actively maintained by either purging self-reactive B receptors through clonal deletion and receptor editing, or by functional silencing known as anergy. However, these processes are clearly incomplete as B cell driven autoimmune diseases still occur. The significance of B cells in two such diseases, rheumatoid arthritis and systemic lupus erythematosus, is highlighted by the ameliorative effects of B cell depletion. It remains to be determined, however, whether the key role of the B cell in autoimmune disease is autoantibody production or another antibody-independent function.

Requirement of Toll-like receptor 7 for pristane-induced production of autoantibodies and development of murine lupus nephritis

OBJECTIVE

The detection of high titers of antibodies against small nuclear ribonucleoproteins (snRNP) is a diagnostic finding in patients in whom systemic lupus erythematosus (SLE) is suspected. Endogenous RNA molecules within snRNP trigger Toll-like receptor 7 (TLR-7) activation in B cells and dendritic cells, leading to anti-snRNP antibody production, which is associated with the development of immune complex nephritis in SLE. The purpose of this study was to investigate the role of TLR-7 in anti-snRNP antibody production and renal disease in SLE induced by an exogenous factor in the absence of genetic predisposition, using the pristane-induced murine lupus model.

METHODS

Serum autoantibodies, IgG isotypes, and cytokine levels in pristane-treated wild-type and TLR-7-deficient mice were analyzed by enzyme-linked immunosorbent assay. Histopathologic changes in mouse kidneys were determined by light immunofluorescence microscopy. Cell subsets in splenocytes and peritoneal lavage cells from the mice were examined by flow cytometry.

RESULTS

We found that anti-snRNP antibody production induced by pristane treatment was entirely dependent on the expression of TLR-7, whereas anti-double-stranded DNA antibody production was not affected by a lack of TLR-7. Impaired anti-snRNP antibody production in TLR-7-deficient mice was paralleled by lower levels of glomerular IgG and complement deposits, as well as less severe glomerulonephritis.

CONCLUSION

TLR-7 is specifically required for the production of RNA-reactive autoantibodies and the development of glomerulonephritis in pristane-induced murine lupus, a model of environmentally triggered SLE in the absence of genetic susceptibility to autoimmunity. Specific interference with TLR-7 activation by endogenous TLR-7 ligands may therefore be a promising novel strategy for the treatment of SLE.

The grateful dead: damage-associated molecular pattern molecules and reduction/oxidation regulate immunity

The response to pathogens and damage in plants and animals involves a series of carefully orchestrated, highly evolved, molecular mechanisms resulting in pathogen resistance and wound healing. In metazoans, damage- or pathogen-associated molecular pattern molecules (DAMPs, PAMPs) execute precise intracellular tasks and are also able to exert disparate functions when released into the extracellular space. The emergent consequence for both inflammation and wound healing of the abnormal extracellular persistence of these factors may underlie many clinical disorders. DAMPs/PAMPs are recognized by hereditable receptors including the Toll-like receptors, the NOD1-like receptors and retinoic-acid-inducible gene I-like receptors, as well as the receptor for advanced glycation end products. These host molecules 'sense' not only pathogens but also misfolded/glycated proteins or exposed hydrophobic portions of molecules, activating intracellular cascades that lead to an inflammatory response. Equally important are means to not only respond to these molecules but also to eradicate them. We have speculated that their destruction through oxidative mechanisms normally exerted by myeloid cells, such as neutrophils and eosinophils, or their persistence in the setting of pathologic extracellular reducing environments, maintained by exuberant necrotic cell death and/or oxidoreductases, represent important molecular means enabling chronic inflammatory states.

Structural requirements and applications of inhibitory oligodeoxyribonucleotides

Synthetic oligodeoxyribonucleotides (ODN) bearing certain sequence characteristics mimic bacterial DNA by activating B cells and dendritic cells through Toll-like receptor (TLR) 9, an event that potentiates both humoral and cell-mediated immunity. ODN sharing some of the sequence characteristics of strong stimulatory (ST-) ODN, but substituting GGG for CGTT, competitively inhibit ST-ODN-driven events. An ODN with the same length and base composition as a strong ST-ODN, but lacking both ST- and IN-sequence requirements, has neither ST- nor IN-activity. Whereas, certain sequence changes strongly influence ST-ODN activity in human cells relative to mouse cells and B cells relative to non B cells, the strongest IN-ODN appear to work well in both species and multiple cell types. Converting from the natural phosphodiester backbone to a nuclease-resistant phosphorothioate backbone increases the sensitivity to ST-ODN about 2 logs and to IN-ODN 3 logs, while increasing the impact of critical base changes in ST-ODN and diminishing it in IN-ODN. Examples where IN-ODN have been used in vivo to interrupt autoimmune and other TLR-9-induced inflammatory states are described.

Nucleic acid recognition receptors in autoimmunity

Recent studies in mouse models of systemic autoimmune diseases have drawn attention to the involvement of Toll-like receptors (TLRs) in the generation of autoreactive immune responses. The endosomally localized TLRs7 and 9 are activated by autoimmune complexes containing self DNA and RNA in B lymphocytes and dendritic cells. These endogenous TLR ligands act as autoadjuvants providing a stimulatory signal together with the autoantigen and thus contribute to break peripheral tolerance against self antigens in systemic lupus erythematosus (SLE), for example. In vivo studies in SLE mouse models demonstrate an essential role for TLR7 in the generation of RNA-containing antinuclear antibodies and deposition of pathogenic immune complexes in the kidney. TLR9, however, appears to have immunostimulatory as well as regulatory functions in SLE mouse models. Type I Interferon, which is produced by plasmacytoid dendritic cells in response to autoimmune complexes containing RNA and DNA recognized by TLR7 and 9 acts as a potent amplifier of the autoimmune response. TLR-independent recognition of self nucleic acids by cytosolic RNA and DNA sensors may also play a role in the generation of autoimmune responses. Defects in protective mechanisms, which normally prevent immunostimulation by self nucleic acids in healthy individuals, promote the development of autoimmune diseases. For example, defects in nucleases that clear nucleic acids derived from apoptotic material, changes in the level and localization of TLR expression, defects in negative regulators of TLR signaling, or changes in the posttranscriptional modification of mammalian DNA and RNA may contribute to autoreactive responses. A better understanding of the exact function of different nucleic acid recognition receptors in the development of systemic autoimmunity will allow targeting of these innate immune receptors for the therapy of patients with systemic autoimmune diseases.

Control of toll-like receptor 7 expression is essential to restrict autoimmunity and dendritic cell proliferation

Nucleic acid-binding innate immune receptors such as Toll-like receptor 7 (TLR7) and TLR9 have been implicated in the development of some autoimmune pathologies. The Y chromosome-linked genomic modifier Yaa, which correlates with a duplication of Tlr7 and 16 other genes, exacerbates lupus-like syndromes in several mouse strains. Here we demonstrated that duplication of the Tlr7 gene was the sole requirement for this accelerated autoimmunity, because reduction of Tlr7 gene dosage abolished the Yaa phenotype. Further, we described new transgenic lines that overexpressed TLR7 alone and found that spontaneous autoimmunity developed beyond a 2-fold increase in TLR7 expression. Whereas a modest increase in Tlr7 gene dosage promoted autoreactive lymphocytes with RNA specificities and myeloid cell proliferation, a substantial increase in TLR7 expression caused fatal acute inflammatory pathology and profound dendritic cell dysregulation. These results underscore the importance of tightly regulating expression of TLR7 to prevent spontaneous triggering of harmful autoreactive and inflammatory responses.

Immunologically active autoantigens: the role of toll-like receptors in the development of chronic inflammatory disease

Pattern recognition receptors (PRRs), expressed on cells of both the innate and adaptive immune systems, serve as sentinels, waiting to alert the host to the first signs of microbial infection and to activate the initial line of immune defense. Research has increasingly demonstrated that many of the same PRRs also recognize self-epitopes that either are released from dying or damaged cells or are present at the surface of apoptotic cells or apoptotic bodies. In this context, PRRs play a critical role in tissue repair and the clearance of cellular debris. However, failure to appropriately regulate self-responses triggered by certain PRRs can have serious pathological consequences. The Toll-like receptor (TLR) gene family represents a case in point. TLR7, 8, and 9 were originally identified as receptors specific for bacterial and viral RNA and DNA, but more recent in vitro and in vivo studies have now linked these receptors to the detection of host RNA, DNA, and RNA- or DNA-associated proteins. In this context, they likely play a key role in the development of systemic autoimmune diseases.

Identification of soluble CD14 as an endogenous agonist for Toll-like receptor 2 on human astrocytes by genome-scale functional screening of glial cell derived proteins

Human astrocytes express a limited repertoire of Toll-like receptor (TLR) family members including TLR1-4, which are expressed on the cell surface. Also, TLR3 but not TLR4 activation on astrocytes induces expression of several factors involved in neuroprotection and down-regulation of inflammation rather than in the onset of traditional pro-inflammatory reactions. The notion that astrocyte TLR may thus play a role not only in host defense but also in tissue repair responses prompted us to examine the possibility that endogenous TLR agonists could be expressed in the human central nervous system to regulate the apparently dual astrocyte functions during trauma or inflammation. As a potential source of endogenous agonists, a cDNA library derived from several human brain tumor cell lines was used. Gene pools of this library were transfected into COS-7 cells and the expression products were screened for their ability to induce TLR activation in human primary astrocytes. The screening resulted in the identification of soluble CD14. By using a panel of TLR-transfected HEK293 cells, we found that signaling by soluble CD14 was TLR2 dependent. Moreover, the CD14-triggered TLR2-mediated response in astrocytes lead to the production of CXCL8, IL-6, and IL12p40, whereas typical TLR-induced pro-inflammatory cytokines, like TNF-alpha and IL-1beta, were not produced at detectable levels. In conclusion, our data indicate that apart from its well-known ability to act as a co-receptor for TLR-dependent signaling by peptidoglycans or LPS, soluble CD14 can also act as a direct agonist for TLR2.

Nucleic acid-sensing TLRs as modifiers of autoimmunity

The immune system requires precise regulation of activating and inhibitory signals so that it can mount effective responses against pathogens while ensuring tolerance to self-components. Some of the most potent activation signals are triggered by innate immune molecules, particularly those in the TLR family. Recent studies have shown that engagement of TLRs plays a significant role in both innate and adaptive immunity. This review focuses on the ways that TLR function might contribute to the etiology of lupus-like syndromes in the context of an autoimmune-prone environment. By considering the sources, localization, and expression of both nucleic acids and the molecules that bind them, we discuss several ways that innate immunity can play a role in the development of systemic autoimmunity.

Modulation of autoimmunity by TLR9 in the chronic graft-vs-host model of systemic lupus erythematosus

Chronic graft-vs-host (cGVH) disease is induced in nonautoimmune mice by the transfer of alloreactive T cells that recognize foreign MHC class II. It closely resembles systemic lupus erythematosus, with antinuclear Abs and immune-mediated nephritis. Recent work has implicated TLRs, particularly TLR9, in the recognition of certain autoantigens in vitro and in vivo. To explore further the role of TLR9 in systemic autoimmunity, we induced cGVH disease in C57BL/6 (B6) mice lacking TLR9, including B6 mice expressing the anti-DNA-encoding IgH transgenes 3H9 or 56R (B6.3H9.TLR9(-/-), B6.56R.TLR9(-/-)). We found that cGVH disease caused breakdown of B cell tolerance to chromatin and DNA in TLR9(-/-) recipients of alloreactive cells, yet that nephritis was less severe and that some autoantibody titers were lower compared with B6-cGVH controls. Spleen lymphocyte analysis showed that cGVH disease strikingly depleted marginal zone B cells in B6 mice, but did not influence T cell subsets in either B6 or B6-TLR9(-/-) hosts. B6.56R.TLR9(-/-) mice had less spontaneous production of autoantibodies than B6.56R mice, but there were no significant differences between B6.56R and B6.56R.TLR9(-/-) postinduction of cGVH disease. Taken together, these results suggested that TLR9 may worsen some aspects of systemic autoimmunity while alleviating others.