Toll-like receptor 9-dependent and -independent dendritic cell activation by chromatin-immunoglobulin G complexes

Dosage of X-linked Toll-like receptor 8 determines gender differences in the development of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease with a high incidence in females and a complex phenotype. Using 564Igi mice, a model of SLE with knock-in genes encoding an autoreactive anti-RNA Ab, we investigated how expression of Toll-like receptors (TLRs) in B cells and neutrophils affects pathogenesis. We established that TLR signaling through MyD88 is necessary for disease. Autoantibody was produced in mice with single deletions of Tlr7, Tlr8, or Tlr9 or combined deletions of Tlr7 and Tlr9. Autoantibody was not produced in the combined absence of Tlr7 and Tlr8, indicating that TLR8 contributes to the break in tolerance. Furthermore, TLR8 was sufficient for the loss of B-cell tolerance, the production of class-switched autoantibody, heightened granulopoiesis, and increased production of type I IFN by neutrophils as well as glomerulonephritis and death. We show that dosage of X-linked Tlr8 plays a major role in the high incidence of disease in females. In addition, we show that the negative regulation of disease by TLR9 is exerted primarily on granulopoiesis and type I IFN production by neutrophils. Collectively, we suggest that individual TLRs play unique roles in the pathogenesis of systemic lupus erythematosus, suggesting new targets for treatment.

TLR Agonists as Modulators of the Innate Immune Response and Their Potential as Agents Against Infectious Disease

Immunotherapies that can either activate or suppress innate immune responses are being investigated as treatments against infectious diseases and the pathology they can cause. The objective of these therapies is to elicit protective immune responses thereby limiting the harm inflicted by the pathogen. The Toll-like receptor (TLR) signaling pathway plays critical roles in numerous host immune defenses and has been identified as an immunotherapeutic target against the consequences of infectious challenge. This review focuses on some of the recent advances being made in the development of TLR-ligands as potential prophylactic and/or therapeutic agents.

Neutrophil-mediated IFN activation in the bone marrow alters B cell development in human and murine systemic lupus erythematosus

Inappropriate activation of type I IFN plays a key role in the pathogenesis of autoimmune disease, including systemic lupus erythematosus (SLE). In this study, we report the presence of IFN activation in SLE bone marrow (BM), as measured by an IFN gene signature, increased IFN regulated chemokines, and direct production of IFN by BM-resident cells, associated with profound changes in B cell development. The majority of SLE patients had an IFN signature in the BM that was more pronounced than the paired peripheral blood and correlated with both higher autoantibodies and disease activity. Pronounced alterations in B cell development were noted in SLE in the presence of an IFN signature with a reduction in the fraction of pro/pre-B cells, suggesting an inhibition in early B cell development and an expansion of B cells at the transitional stage. These B cell changes strongly correlated with an increase in BAFF and APRIL expression in the IFN-high BM. Furthermore, we found that BM neutrophils in SLE were prime producers of IFN-α and B cell factors. In NZM lupus-prone mice, similar changes in B cell development were observed and mediated by IFN, given abrogation in NZM mice lacking type-I IFNR. BM neutrophils were abundant, responsive to, and producers of IFN, in close proximity to B cells. These results indicate that the BM is an important but previously unrecognized target organ in SLE with neutrophil-mediated IFN activation and alterations in B cell ontogeny and selection.

The function of Fcγ receptors in dendritic cells and macrophages

Dendritic cells (DCs) and macrophages use various receptors to recognize foreign antigens and to receive feedback control from adaptive immune cells. Although it was long believed that all immunoglobulin Fc receptors are universally expressed by phagocytes, recent findings indicate that only monocyte-derived DCs and macrophages express high levels of activating Fc receptors for IgG (FcγRs), whereas conventional and plasmacytoid DCs express the inhibitory FcγR. In this Review, we discuss how the uptake, processing and presentation of antigens by DCs and macrophages is influenced by FcγR recognition of immunoglobulins and immune complexes in the steady state and during inflammation.

Molecular determinants of sterile inflammation

Necrotic cell death alerts the acquired immune system to activate naïve T cells even in the absence of non-self derived molecules (e.g. pathogens). In addition, sterile necrosis leads to innate immune-mediated acute inflammation. The dying cells still represent a threat to the body that should be eliminated by the host immune response. Although the inflammatory response plays important roles in protecting the host and repairing tissues, it can also cause the collateral damage to normal tissues that underlies disease pathogenesis. Tissue resident macrophages recognize the danger signals released from necrotic cells via the pattern recognition receptors and secrete IL-1 that results in acute neutrophilic inflammation. This article will review our current knowledge especially focusing on the role of IL-1 in the sterile necrotic cell death induced inflammation.

TLR and NKG2D signaling pathways mediate CS-induced pulmonary pathologies

Long-term exposure to cigarette smoke (CS) can have deleterious effects on lung epithelial cells including cell death and the initiation of inflammatory responses. CS-induced cell injury can elaborate cell surface signals and cellular byproducts that stimulate immune system surveillance. Our previous work has shown that the expression of ligands for the cytotoxic lymphocyte activating receptor NKG2D is enhanced in patients with COPD and that the induction of these ligands in a mouse model can replicate COPD pathologies. Here, we extend these findings to demonstrate a role for the NKG2D receptor in CS-induced pathophysiology and provide evidence linking nucleic acid-sensing endosomal toll-like receptor (TLR) signaling to COPD pathology through NKG2D activation. Specifically, we show that mice deficient in NKG2D exhibit attenuated pulmonary inflammation and airspace enlargement in a model of CS-induced emphysema. Additionally, we show that CS exposure induces the release of free nucleic acids in the bronchoalveolar lavage and that direct exposure of mouse lung epithelial cells to cigarette smoke extract similarly induces functional nucleic acids as assessed by TLR3, 7, and 9 reporter cell lines. We demonstrate that exposure of mouse lung epithelial cells to TLR ligands stimulates the surface expression of RAET1, a ligand for NKG2D, and that mice deficient in TLR3/7/9 receptor signaling do not exhibit CS-induced NK cell hyperresponsiveness and airspace enlargement. The findings indicate that CS-induced airway injury stimulates TLR signaling by endogenous nucleic acids leading to elevated NKG2D ligand expression. Activation of these pathways plays a major role in the altered NK cell function, pulmonary inflammation and remodeling related to long-term CS exposure.

Oxidative damage of DNA confers resistance to cytosolic nuclease TREX1 degradation and potentiates STING-dependent immune sensing

Immune sensing of DNA is critical for antiviral immunity but can also trigger autoimmune diseases such as lupus erythematosus (LE). Here we have provided evidence for the involvement of a damage-associated DNA modification in the detection of cytosolic DNA. The oxidized base 8-hydroxyguanosine (8-OHG), a marker of oxidative damage in DNA, potentiated cytosolic immune recognition by decreasing its susceptibility to 3' repair exonuclease 1 (TREX1)-mediated degradation. Oxidizative modifications arose physiologically in pathogen DNA during lysosomal reactive oxygen species (ROS) exposure, as well as in neutrophil extracellular trap (NET) DNA during the oxidative burst. 8-OHG was also abundant in UV-exposed skin lesions of LE patients and colocalized with type I interferon (IFN). Injection of oxidized DNA in the skin of lupus-prone mice induced lesions that closely matched respective lesions in patients. Thus, oxidized DNA represents a prototypic damage-associated molecular pattern (DAMP) with important implications for infection, sterile inflammation, and autoimmunity.

FCRL3 promotes TLR9-induced B-cell activation and suppresses plasma cell differentiation

Fc receptor-like (FCRL) molecules are preferentially expressed by B lymphocytes and possess tyrosine-based immunoregulatory function. Although they generally inhibit B-cell receptor signaling, their influence on other activation pathways remains largely unexplored. In humans, FCRL3 encodes a type I transmembrane protein harboring both cytoplasmic ITAM and ITIM elements that can repress B-cell receptor activation. Despite this inhibitory property, mounting associations for FCRL3 with autoimmune and lympho-proliferative disorders imply a role for it in promoting B-cell pathogenesis. Here, we explore the influence of FCRL3 on B-cell responses to innate TLR9 stimulation. A detailed survey of blood B-cell populations found that FCRL3 expression increased as a function of differentiation and was higher among memory subsets with innate-like features. FCRL3 ligation augmented CpG oligodeoxynucleotide TLR9-mediated B-cell proliferation, activation, and survival, but surprisingly, abrogated plasma cell differentiation and antibody production. Although FCRL3 amplified the NF-κB and mitogen-activated protein kinase signaling cascades, it halted CpG triggered BLIMP1 induction in an ERK-dependent fashion. These findings indicate that FCRL3 differentially modulates innate signaling in B cells and provide new insight into the potential of this disease-associated receptor to counter-regulate adaptive and innate immunity.

Sterile inflammation in the liver and pancreas

The ability of tissue injury to result in inflammation is a well-recognized phenomenon and is central to a number of common liver and pancreatic diseases including alcoholic steatohepatitis and pancreatitis, as well as drug-induced liver injury, non-alcoholic steatohepatitis, and pancreatitis from other causes. The requirements of extracellular damage-associated molecules and a cytosolic machinery labeled the inflammasome have been established in in vitro culture systems and in vivo disease models. This has provided a generic insight into the pathways involved, and the challenge now is to understand the specifics of these mechanisms in relation to the particular insults and organs involved. One reason for the excitement in this field is that a number of therapeutic candidates such a toll-like receptor antagonists and interleukin-1R antagonists are either approved or in clinical trials for other indications.

Interferon regulatory factor 5 and autoimmune lupus

Systemic lupus erythematosus (SLE) is a severe multi-system autoimmune disease, whereas interferon regulatory factor (IRF) 5 belongs to the family of transcription factors that modulate immune system activities. Recently, many lines of investigations suggested that IRF5 gene polymorphisms are closely associated with the disease onset of SLE. Indeed, expressed in B cells, dendritic cells (DCs), monocytes and macrophages, IRF5 could significantly affect these immune cells participating in the pathogenesis of SLE, and numerous studies implied that this transcription factor is mechanistically linked to the disease progression. Here, we comprehensively review the updated evidence indicating the roles of IRF5 in autoimmune lupus. Hopefully, the information obtained will lead to a better understanding of the pathogenesis and development of novel therapeutic strategies for the systemic autoimmune disease.

Toll-like receptors in atherosclerosis

Atherosclerosis, the leading cause of cardiovascular disease (CVD), is driven by inflammation. Increasing evidence suggests that toll-like receptors (TLRs) are key orchestrators of the atherosclerotic disease process. Interestingly, a distinct picture is being revealed for individual receptors in atherosclerosis. TLRs exhibit a complex nature enabling the detection of multiple motifs named danger-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). Activation of these receptors triggers an intracellular signalling cascade mediated through MyD88 or TRIF, leading to the production of pro- and anti-inflammatory cytokines. In this review we explore key novel findings pertaining to TLR signalling in atherosclerosis, including recently described endosomal TLRs and future directions in TLR research.

Adaptive immunity after cell death

We understand much about the agents, receptors, and signalling pathways that lead to immunity to pathogens. Less is known about how the process is initiated in apparently sterile conditions such as spontaneous immunity to certain tumours, tissue grafts, or autoimmune disorders. Proinflammatory molecules released by dying cells, termed damage-associated molecular patterns (DAMPs), have been proposed to activate dendritic cells (DCs) to promote T cell responses to antigens present in cell corpses. Surprisingly, rather than affecting activation, some recently identified DAMP receptors control specialised DC functions such as antigen acquisition and presentation. This selectivity reveals a new point of control in the regulation of adaptive immunity and, potentially, tolerance that renders DAMPs nonredundant players in responses to both sterile and nonsterile insults.

Selectively targeting the toll-like receptor 9 (TLR9)--IRF 7 signaling pathway by polymer blend particles

Signaling through toll-like receptor 9 (TLR9) has been exploited for cancer therapy. The stimulation of TLR9 leads to two bifurcating signaling pathways - NF-κB-dependent pro-inflammatory cytokines pathway and IRF-7-dependent type I interferons (IFNs) pathway. In this study, we employ polymer blend particles to present the synthetic ligand, CpG oligonucleotides (CpG ODNs), to TLR9. The polymer blend particles are made from the blend of pH-insensitive and pH-sensitive copolymer. By tailoring the composition of the pH-sensitive polymer, CpG ODNs are presented to TLR9 in a way that only activates the IRF-7 signaling pathway. CpG ODNs have been used for cancer therapy in both preclinical and clinical studies. The selective activation of IRF-7 could potentially enhance the apoptosis of tumor cells and immunological control of tumor progression without inadvertently activating NF-κB-dependent oncogenesis.

Characterization of the role of distinct plasma cell-free DNA species in age-associated inflammation and frailty

Plasma cell-free DNA (cf-DNA) has recently emerged as a potential biomarker of aging, reflecting systemic inflammation, and cell death. In addition, it has been suggested that cf-DNA could promote autoinflammation. Because the total cf-DNA pool comprises different cf-DNA species, we quantified the plasma levels of gene-coding cf-DNA, Alu repeat cf-DNA, mitochondrial DNA (mtDNA) copy number, and the amounts of unmethylated and total cf-DNAs. We identified the relationships between these cf-DNA species and age-associated inflammation, immunosenescence, and frailty. Additionally, we determined the cf-DNA species-specific transcriptomic signatures in blood mononuclear cells to elucidate the age-linked leukocyte responses to cf-DNA. The study population consisted of n = 144 nonagenarian participants of the Vitality 90+ Study and n = 30 young controls. In the nonagenarians, higher levels of total and unmethylated cf-DNAs were associated with systemic inflammation and increased frailty. The mtDNA copy number was also directly correlated with increased frailty but not with inflammation. None of the cf-DNA species were associated with immunosenescence. The transcriptomic pathway analysis revealed that higher levels of total and unmethylated cf-DNAs were associated with immunoinflammatory activation in the nonagenarians but not in the young controls. The plasma mtDNA appeared to be inert in terms of inflammatory activation in both the nonagenarians and young controls. These data demonstrate that the plasma levels of total and unmethylated cf-DNA and the mtDNA copy number could serve as biomarkers of frailty. In addition, we suggest that circulating self-DNA, assessed as total or unmethylated cf-DNA, might aggravate immunoinflammatory reactivity in very old individuals.

Role of nucleic acid-sensing TLRs in diverse autoantibody specificities and anti-nuclear antibody-producing B cells

Nucleic acid (NA)-sensing TLRs (NA-TLRs) promote the induction of anti-nuclear Abs in systemic lupus erythematosus. However, the extent to which other nonnuclear pathogenic autoantibody specificities that occur in lupus and independently in other autoimmune diseases depend on NA-TLRs, and which immune cells require NA-TLRs in systemic autoimmunity, remains to be determined. Using Unc93b1(3d) lupus-prone mice that lack NA-TLR signaling, we found that all pathogenic nonnuclear autoantibody specificities examined, even anti-RBC, required NA-TLRs. Furthermore, we document that NA-TLRs in B cells were required for the development of antichromatin and rheumatoid factor. These findings support a unifying NA-TLR-mediated mechanism of autoantibody production that has both pathophysiological and therapeutic implications for systemic lupus erythematosus and several other humoral-mediated autoimmune diseases. In particular, our findings suggest that targeting of NA-TLR signaling in B cells alone would be sufficient to specifically block production of a broad diversity of autoantibodies.

Toll-like receptors as therapeutic targets for autoimmune connective tissue diseases

Autoimmune connective tissue diseases (ACTDs) are a family of consistent systemic autoimmune inflammatory disorders, including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), systemic sclerosis (SSc) and Sjögren's syndrome (SS). IL-1R-like receptors (TLRs) are located on various cellular membranes and sense exogenous and endogenous danger-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs), playing a critical role in innate immune responses. During the past decade, the investigation of TLRs in inflammatory autoimmune diseases has been fruitful. In this report, we review the significant biochemical, physiological and pathological studies of the key functions of TLRs in ACTDs. Several proteins in the TLR signaling pathways (e.g., IKK-2 and MyD88) have been identified as potential therapeutic targets for the treatment of ACTDs. Antibodies, oligodeoxyribonucleotides (ODNs) and small molecular inhibitors (SMIs) have been tested to modulate TLR signaling. Some drug-like SMIs of TLR signaling, such as RDP58, ST2825, ML120B and PHA-408, have demonstrated remarkable potential, with promising safety and efficacy profiles, which should warrant further clinical investigation. Nonetheless, one should bear in mind that all TLRs exert both protective and pathogenic functions; the function of TLR4 in inflammatory bowel disease represents such an example. Therefore, an important aspect of TLR modulator development involves the identification of a balance between the suppression of disease-inducing inflammation, while retaining the beneficiary host immune response.

Protective autoantibodies in the rheumatic diseases: lessons for therapy

The adaptive immune system augments host defenses against diverse infectious threats, yet also carries intertwined risks for the development of autoimmune disease. The immune system incorporates homeostatic pathways for essential housekeeping functions that involve recognition of oxidation-modified endogenous molecules. Now, the properties of a physiological class of natural autoantibodies, which seem to modulate the severity or even prevent the onset of autoimmune disease, are beginning to be defined. Whereas disease-associated IgG autoantibodies to nuclear antigens and citrulline-modified self-proteins have been shown to activate innate pattern recognition receptors leading to increased cell death and tissue injury, a class of IgM autoantibodies to oxidation-associated neo-antigens can oppose these pathogenic effects. These naturally arising regulatory IgM autoantibodies enhance the capacity for the phagocytic clearance of host cells affected by programmed death pathways. These antibodies can also suppress key signalling pathways in the innate immune system involved in the control and resolution of inflammatory responses to Toll-like receptor agonists and disease-associated IgG autoantibodies.

Mechanistic biomarkers for clinical decision making in rheumatic diseases

The use of biomarkers is becoming increasingly intrinsic to the practice of medicine and holds great promise for transforming the practice of rheumatology. Biomarkers have the potential to aid clinical diagnosis when symptoms are present or to provide a means of detecting early signs of disease when they are not. Some biomarkers can serve as early surrogates of eventual clinical outcomes or guide therapeutic decision making by enabling identification of individuals likely to respond to a specific therapy. Using biomarkers might reduce the costs of drug development by enabling individuals most likely to respond to be enrolled in clinical trials, thereby minimizing the number of participants required. In this Review, we discuss the current use and the potential of biomarkers in rheumatology and in select fields at the forefront of biomarker research. We emphasize the value of different types of biomarkers, addressing the concept of 'actionable' biomarkers, which can be used to guide clinical decision making, and 'mechanistic' biomarkers, a subtype of actionable biomarker that is embedded in disease pathogenesis and, therefore, represents a potentially superior biomarker. We provide examples of actionable and mechanistic biomarkers currently available, and discuss how development of such biomarkers could revolutionize clinical practice and drug development.

Danger-associated molecular patterns (DAMPs) in acute lung injury

Danger-associated molecular patterns (DAMPs) are host-derived molecules that can function to regulate the activation of pathogen recognition receptors (PRRs). These molecules play a critical role in modulating the lung injury response. DAMPs originate from multiple sources, including injured and dying cells, the extracellular matrix, or exist as immunomodulatory proteins within the airspace and interstitium. DAMPs can function as either toll-like receptor (TLR) agonists or antagonists, and can modulate both TLR and nod-like receptor (NLR) signalling cascades. Collectively, this diverse group of molecules may represent important therapeutic targets in the prevention and/or treatment of acute lung injury (ALI) and its more severe form, acute respiratory distress syndrome (ARDS).

Fundamental roles of the innate-like repertoire of natural antibodies in immune homeostasis

The composition of the early immune repertoire is biased with prominent expression of spontaneously arising B cell clones that produce IgM with recurrent and often autoreactive binding specificities. Amongst these naturally arising antibodies (NAbs) are IgM antibodies that specifically recognized amaged and senescent cells, often via oxidation-associated neo-determinants. These NAbs are present from birth and can be further boosted by apoptotic cell challenge. Recent studies have shown that IgM NAb to apoptotic cells can enhance phagocytic clearance, as well as suppress proinflammatory responses induced via Toll-like receptors, and block pathogenic IgG-immune complex (IC)-mediated inflammatory responses. Specific antibody effector functions appear to be involved, as these anti-inflammatory properties are dependent on IgM-mediated recruitment of the early recognition factors of complement. Clinical surveys have suggested that anti-apoptotic cell (AC) IgM NAbs may modulate disease activity in some patients with autoimmune disease. In mechanistic studies, anti-AC NAbs were shown to act in dendritic cells by inhibition of the mitogen-activated protein kinase (MAPK) pathway, a primary signal transduction pathway that controls inflammatory responses. This immunomodulatory pathway has an absolute requirement for the induction of MAPK phosphatase-1. Taken together, recent studies have elucidated the novel properties of a class of protective NAbs, which may directly blunt inflammatory responses through a primitive pathway for regulation of the innate immune system.

Interferon regulatory factor 5 is critical for the development of lupus in MRL/lpr mice

OBJECTIVE

Interferon regulatory factor 5 (IRF-5) is a transcription factor that mediates intracellular signals activated by engagement of Toll-like receptors (TLRs). IRF5 polymorphisms are associated with an increased or decreased risk of systemic lupus erythematosus (SLE) in various human populations, but the precise role of IRF5 in SLE development is not fully understood. This study was undertaken to examine the role of IRF5 in the development of murine lupus.

METHODS

We crossed gene-targeted IRF5-deficient (IRF5(-/-) ) mice with MRL/MpJ-lpr/lpr (MRL/lpr) mice and examined the progeny for survival, glomerulonephritis, autoantibody levels, immune system cell populations, and dendritic cell function.

RESULTS

IRF5(-/-) MRL/lpr mice survived longer than control IRF5(+/+) MRL/lpr mice and displayed only very mild glomerulonephritis. Autoantibodies to SLE-related nuclear antigens were lower in IRF5(-/-) MRL/lpr mouse serum, and numbers of activated CD4+ T cells were reduced in the spleen. Splenic DCs from IRF5(-/-) MRL/lpr mice produced lower levels of inflammatory cytokines when treated in vitro with TLR-7 or TLR-9 ligands or immune complexes. Interferon-α production in response to CpG was also decreased.

CONCLUSION

Our results show that IRF5 is a crucial driver of lupus development in mice, and indicate that IRF5 may be an attractive new target for therapeutic intervention to control disease in SLE patients.

Natural antibody to apoptotic cell membranes inhibits the proinflammatory properties of lupus autoantibody immune complexes

OBJECTIVE

Naturally arising IgM antibodies (NAb) to apoptotic cell (AC) determinants are present from birth and can be further induced by AC challenge. In systemic lupus erythematosus, lower anti-AC NAb levels have been associated with higher disease activity. We have recently shown that a prototypical AC-specific IgM NAb can suppress proinflammatory responses to purified agonists of Toll-like receptors and block the in vivo induction of IgG immune complex (IC)-induced arthritis. Nuclear antigens, which activate dendritic cells (DCs), form complexes with IgG autoantibody, and these have been implicated in the pathogenesis of autoimmune disease. In this study, we sought to investigate potential roles of such NAb for regulating IC-mediated activation of DCs, which is believed to be involved in disease initiation and perpetuation.

METHODS

Bone marrow-derived myeloid DCs were stimulated with ICs composed of IgG autoantibody and chromatin or IgG autoantibody and RNA. Outcome was evaluated according to the production of inflammatory cytokines, as determined by enzyme-linked immunosorbent assay, and the expression of costimulatory molecules (markers of DC activation), as determined by flow cytometry. MAPK activation was evaluated by phospho-flow analysis and immunofluorescence microscopy.

RESULTS

IgM anti-AC NAb dose-dependently suppressed the production of DNA IC- and RNA IC-induced interleukin-6 and DNA IC-induced tumor necrosis factor α, as well as the RNA IC-induced up-regulation of CD86 and CD40 on DCs. IgM NAb-mediated inhibition was associated with suppression of IC-mediated p38 MAPK activation and nuclear localization.

CONCLUSION

We demonstrated a direct in vitro inhibitory effect of IgM NAb on inflammatory responses induced by IgG-nucleic acid ICs. These findings contribute to emerging evidence that regulatory NAb to AC determinants may oppose the influence of pathogenic lupus autoantibody ICs and thereby play roles in the maintenance of immune homeostasis.

TLR7 and TLR9 in SLE: when sensing self goes wrong

Autoreactive B and T cells are present in healthy, autoimmunity-free individuals, but they are kept in check by various regulatory mechanisms. In systemic lupus erythematosus (SLE) patients, however, autoreactive cells are expanded, activated, and produce large quantities of autoantibodies, directed especially against nuclear antigens. These antibodies form immune complexes with self-nucleic acids present in SLE serum. Since self-DNA and self-RNA in the form of protein complexes can act as TLR9 and TLR7 ligands, respectively, TLR stimulation is suggested as an additional signal contributing to activation and/or modulation of the aberrant adaptive immune response. Data from mouse models suggest a pathogenic role for TLR7 and a protective role for TLR9 in the pathogenesis of SLE. Future investigations are needed to elucidate the underlying modulatory mechanisms and the role of TLR7 and TLR9 in the complex pathogenesis of human SLE.

Sterile inflammation in the liver

Inflammation In the absence of pathogens occurs in all tissues in response to a wide range of stimuli that cause tissue stress and injury. Such sterile inflammation (SI) is a key process in drug-induced liver injury, nonalcoholic steatohepatitis, and alcoholic steatohepatitis and is a major determinant of fibrosis and carcinogenesis. In SI, endogenous damage-associated molecular patterns (DAMPS), which are usually hidden from the extracellular environment, are released on tissue injury and activate receptors on immune cells. More than 20 such DAMPS have been identified and activate cellular pattern recognition receptors, which were originally identified as sensors of pathogen-associated molecular patterns. Activation of pattern recognition receptors by DAMPS results in a wide range of immune responses, including production of proinflammatory cytokines and localization of immune cells to the site of injury. DAMPS result in the assembly of a cytosolic protein complex termed the inflammasome, which activates the serine protease caspase-1, resulting in activation and secretion of interleukin-1β and other cytokines. SI-driven liver diseases are responsible for the majority of liver pathology in industrially developed countries and lack specific therapy. Identification of DAMPS, their receptors, signaling pathways, and cytokines now provides a wide range of therapeutic targets for which many antagonists are already available.

Antinuclear antibodies in rheumatic disease: a proposal for a function-based classification

Antinuclear antibodies (ANAs) are a diverse group of autoantibodies that bind macromolecular components of the cell nucleus. While some ANAs occur in normal individuals, others are expressed almost exclusively in patients with rheumatic disease and serve as markers for diagnosis and prognosis. Despite the clinical associations of ANAs, the relationship of these antibodies to specific disease manifestations is often unknown because the target antigens are intracellular molecules that are ubiquitously expressed. In systemic lupus erythematosus, the role of ANAs in disease manifestations is better understood, especially for antibodies to DNA and related nucleosomal antigens. These antibodies can promote nephritis by the formation of immune complexes that are deposited in the kidney. In addition, anti-DNA, along with antibodies to RNA-binding proteins such as anti-Sm, can induce non-specific immune abnormalities based on the induction of type interferon 1 by plasmacytoid dendritic cells. Despite ANA expression in rheumatic disease, studies in animal models of inflammation and tissue injury indicate that antibodies to certain nuclear molecules such as HMGB1 have protective effects. Together, these considerations suggest a function-based classification of ANAs based on their expression in normal and autoimmune individuals as well as their capacity to induce or attenuate immunological disturbances. This classification provides a framework to elucidate the serological features of rheumatic disease and the often uncertain relationship between ANA expression and disease manifestations.

The Yaa locus and IFN-α fine-tune germinal center B cell selection in murine systemic lupus erythematosus

Male NZW/BXSB.Yaa (W/B) mice express two copies of TLR7 and develop pathogenic autoantibodies, whereas females with only one copy of TLR7 have attenuated disease. Our goal was to analyze the regulation of the autoantibody response in male and female W/B mice bearing the autoreactive site-directed H chain transgene 3H9. Serum anti-dsDNA Abs appeared in males at 12 wk, and most had high-titer IgG anti-dsDNA and anti-cardiolipin Abs and developed >300 mg/dl proteinuria by 8 mo. Females had only low-titer IgG anti-cardiolipin Abs, and none developed proteinuria by 1 y. Males had a smaller marginal zone than females with a repertoire that was distinct from the follicular repertoire, indicating that the loss of marginal zone B cells was not due to diversion to the follicular compartment. Vk5-43 and Vk5-48, which were rare in the naive repertoire, were markedly overrepresented in the germinal center repertoire of both males and females, but the VJ junctions differed between males and females with higher-affinity autoreactive B cells being selected into the germinal centers of males. Administration of IFN-α to females induced anti-cardiolipin and anti-DNA autoantibodies and proteinuria and was associated with a male pattern of junctional diversity in Vk5-43 and Vk5-48. Our studies are consistent with the hypothesis that presence of the Yaa locus, which includes an extra copy of Tlr7, or administration of exogenous IFN-α relaxes the stringency for selection in the germinal centers resulting in increased autoreactivity of the Ag-driven B cell repertoire.

Apoptosis and NET formation in the pathogenesis of SLE

Systemic Lupus Erythematosus is an autoimmune disease characterized by the formation of anti-nuclear autoantibodies, particularly anti-chromatin. Although the aetiology of the disease has not yet been fully elucidated, several mechanisms have been proposed to be involved. Due to an aberrant apoptosis or decreased removal of apoptotic cells, apoptotic blebs containing chromatin are released. During apoptosis, chromatin is modified that increases its immunogenicity. Myeloid dendritic cells (myDC) can take up apoptotic blebs and stimulate autoreactive T helper cells, and subsequently the formation of autoantibodies by autoreactive B cells. Immune complexes formed by anti-chromatin autoantibodies and modified chromatin deposit on basal membranes, and incite a local inflammation, but can also stimulate plasmacytoid dendritic cells to produce IFN-α. In addition to apoptotic blebs, neutrophil extracellular traps released by dying neutrophils, in a process called NETosis, may serve as a source of autoantigens as well. In this review, we describe the role of both apoptosis and NETosis in the pathogenesis of SLE, and show how both processes may interact with each other.

Receptor cross-talk spatially restricts p-ERK during TLR4 stimulation of autoreactive B cells

To maintain tolerance, autoreactive B cells must regulate signal transduction from the BCR and TLRs. We recently identified that dendritic cells and macrophages regulate autoreactive cells during TLR4 activation by releasing IL-6 and soluble CD40 ligand (sCD40L). These cytokines selectively repress Ab secretion from autoreactive, but not antigenically naive, B cells. How IL-6 and sCD40L repress autoantibody production is unknown. In this work, we show that IL-6 and sCD40L are required for low-affinity/avidity autoreactive B cells to maintain tolerance through a mechanism involving receptor cross-talk between the BCR, TLR4, and the IL-6R or CD40. We show that acute signaling through IL-6R or CD40 integrates with chronic BCR-mediated ERK activation to restrict p-ERK from the nucleus and represses TLR4-induced Blimp-1 and XBP-1 expression. Tolerance is disrupted in 2-12H/MRL/lpr mice where IL-6 and sCD40L fail to spatially restrict p-ERK and fail to repress TLR4-induced Ig secretion. In the case of CD40, acute signaling in B cells from 2-12H/MRL/lpr mice is intact, but the chronic activation of p-ERK emanating from the BCR is attenuated. Re-establishing chronically active ERK through retroviral expression of constitutively active MEK1 restores tolerance upon sCD40L, but not IL-6, stimulation, indicating that regulation by IL-6 requires another signaling effector. These data define the molecular basis for the regulation of low-affinity autoreactive B cells during TLR4 stimulation; they explain how autoreactive but not naive B cells are repressed by IL-6 and sCD40L; and they identify B cell defects in lupus-prone mice that lead to TLR4-induced autoantibody production.

Lupus nephritis: a critical review

Lupus nephritis remains one of the most severe manifestations of systemic lupus erythematosus associated with considerable morbidity and mortality. A better understanding of the pathogenesis of lupus nephritis is an important step in identifying more targeted and less toxic therapeutic approaches. Substantial research has helped define the pathogenetic mechanisms of renal manifestations and, in particular, the complex role of type I interferons is increasingly recognized; new insights have been gained into the contribution of immune complexes containing endogenous RNA and DNA in triggering the production of type I interferons by dendritic cells via activation of endosomal toll-like receptors. At the same time, there have been considerable advances in the treatment of lupus nephritis. Corticosteroids have long been the cornerstone of therapy, and the addition of cyclophosphamide has contributed to renal function preservation in patients with severe proliferative glomerulonephritis, though at the cost of serious adverse events. More recently, in an effort to minimize drug toxicity and achieve equal effectiveness, other immunosuppressive agents, including mycophenolate mofetil, have been introduced. Herein, we provide a detailed review of the trials that established the equivalency of these agents in the induction and/or maintenance therapy of lupus nephritis, culminating in the recent publication of new treatment guidelines by the American College of Rheumatology. Although newer biologics have been approved and continue to be a focus of research, they have, for the most part, been relatively disappointing compared to the effectiveness of biologics in other autoimmune diseases. Early diagnosis and treatment are essential for renal preservation.

Cytosolic sensing of extracellular self-DNA transported into monocytes by the antimicrobial peptide LL37

The intracellular location of nucleic acid sensors prevents recognition of extracellular self-DNA released by dying cells. However, on forming a complex with the endogenous antimicrobial peptide LL37, extracellular DNA is transported into endosomal compartments of plasmacytoid dendritic cells, leading to activation of Toll-like receptor-9 and induction of type I IFNs. Whether LL37 also transports self-DNA into nonplasmacytoid dendritic cells, leading to type I IFN production via other intracellular DNA receptors is unknown. Here we found that LL37 very efficiently transports self-DNA into monocytes, leading the production of type I IFNs in a Toll-like receptor-independent manner. This type I IFN induction was mediated by double-stranded B form DNA, regardless of its sequence, CpG content, or methylation status, and required signaling through the adaptor protein STING and TBK1 kinase, indicating the involvement of cytosolic DNA sensors. Thus, our study identifies a novel link between the antimicrobial peptides and type I IFN responses involving DNA-dependent activation of cytosolic sensors in monocytes.

Cross-presentation of IgG-containing immune complexes

IgG is a molecule that functionally combines facets of both innate and adaptive immunity and therefore bridges both arms of the immune system. On the one hand, IgG is created by adaptive immune cells, but can be generated by B cells independently of T cell help. On the other hand, once secreted, IgG can rapidly deliver antigens into intracellular processing pathways, which enable efficient priming of T cell responses towards epitopes from the cognate antigen initially bound by the IgG. While this process has long been known to participate in CD4(+) T cell activation, IgG-mediated delivery of exogenous antigens into a major histocompatibility complex (MHC) class I processing pathway has received less attention. The coordinated engagement of IgG with IgG receptors expressed on the cell-surface (FcγR) and within the endolysosomal system (FcRn) is a highly potent means to deliver antigen into processing pathways that promote cross-presentation of MHC class I and presentation of MHC class II-restricted epitopes within the same dendritic cell. This review focuses on the mechanisms by which IgG-containing immune complexes mediate such cross-presentation and the implications that this understanding has for manipulation of immune-mediated diseases that depend upon or are due to the activities of CD8(+) T cells.

Toll/interleukin-1 receptor domain dimers as the platform for activation and enhanced inhibition of Toll-like receptor signaling

TIR (Toll/IL-1 receptor) domains mediate interactions between TLR (Toll-like) or IL-1 family receptors and signaling adapters. While homotypic TIR domain interactions mediate receptor activation they are also usurped by microbial TIR domain containing proteins for immunosuppression. Here we show the role of a dimerized TIR domain platform for the suppression as well as for the activation of MyD88 signaling pathway. Coiled-coil dimerization domain, present in many bacterial TCPs, potently augments suppression of TLR/IL-1R signaling. The addition of a strong coiled-coil dimerization domain conferred the superior inhibition against the wide spectrum of TLRs and prevented the constitutive activation by a dimeric TIR platform. We propose a molecular model of MyD88-mediated signaling based on the dimerization of TIR domains as the limiting step.

The inhibition of anti-DNA binding to DNA by nucleic acid binding polymers

Antibodies to DNA (anti-DNA) are the serological hallmark of systemic lupus erythematosus (SLE) and can mediate disease pathogenesis by the formation of immune complexes. Since blocking immune complex formation can attenuate disease manifestations, the effects of nucleic acid binding polymers (NABPs) on anti-DNA binding in vitro were investigated. The compounds tested included polyamidoamine dendrimer, 1,4-diaminobutane core, generation 3.0 (PAMAM-G3), hexadimethrine bromide, and a β-cylodextrin-containing polycation. As shown with plasma from patients with SLE, NABPs can inhibit anti-DNA antibody binding in ELISA assays. The inhibition was specific since the NABPs did not affect binding to tetanus toxoid or the Sm protein, another lupus autoantigen. Furthermore, the polymers could displace antibody from preformed complexes. Together, these results indicate that NABPs can inhibit the formation of immune complexes and may represent a new approach to treatment.

Toll-like receptors as modulators of mesenchymal stem cells

Mesenchymal stem cells (MSCs) have differentiation and immunomodulatory properties that make them interesting tools for the treatment of degenerative disorders, allograft rejection, or inflammatory and autoimmune diseases. Biological properties of MSCs can be modulated by the inflammatory microenvironment they face at the sites of injury or inflammation. Indeed, MSCs do not constitutively exert their immunomodulating properties but have to be primed by inflammatory mediators released from immune cells and inflamed tissue. A polarization process, mediated by Toll-like receptors (TLRs), toward either an anti-inflammatory or a pro-inflammatory phenotype has been described for MSCs. TLRs have been linked to allograft rejection and the perpetuation of chronic inflammatory diseases (e.g., Crohn’s disease, rheumatoid arthritis) through the recognition of conserved pathogen-derived components or endogenous ligands (danger signals) produced upon injury. Interest in understanding the effects of TLR activation on MSCs has greatly increased in the last few years since MSCs will likely encounter TLR ligands at sites of injury, and it has been proven that the activation of TLRs in MSCs can modulate their function and therapeutic effect.

Hydroxychloroquine is associated with impaired interferon-alpha and tumor necrosis factor-alpha production by plasmacytoid dendritic cells in systemic lupus erythematosus

Introduction

Plasmacytoid dendritic cells (pDCs) constitutively express two members of the Toll-like receptor (TLR) family, TLR-9 and TLR-7, through which they can be stimulated to produce high levels of interferon (IFN)-α, a key mediator of the pathogenesis of systemic lupus erythematosus (SLE). Given the known efficacy of hydroxychloroquine (HCQ) in the treatment of SLE, we examined its ability to inhibit such pDC function in vivo.

Methods

Peripheral blood mononuclear cells (PBMCs) from SLE subjects treated or not with HCQ and from healthy controls were stimulated with the TLR-9 agonist, CpG oligodeoxynucleotides (CpG-A ODN)-2216, and the TLR-7 agonist, imiquimod. The proportion of monocytes, B cells, myeloid dendritic cells, pDCs, and natural killer (NK) cells producing IFN-α and tumor necrosis factor alpha (TNF-α) was then analyzed by multiparameter flow cytometry.

Results

After TLR-9/7 stimulation in both SLE and healthy subjects, significant production of IFN-α and TNF-α was only observed in pDCs. TLR-7 and TLR-9 induced IFN-α and TNF-α production by pDCs from subjects with SLE was decreased relative to that found in controls (TLR-9/IFN-α, P < 0.0001; TLR-9/TNF-α P < 0.0001; TLR-7/TNF-α P = 0.01). TLR-9 and TLR-7 induced IFN-α and TNF-α production by pDCs was severely impaired in 36% (TLR-9) and 33% (TLR-7) of SLE subjects. In almost all cases, these subjects were being treated with HCQ (HCQ vs. no HCQ: impaired TLR-9/IFN-α, P = 0.0003; impaired TLR-7/IFN-α, P = 0.07; impaired TLR-9/TNF-α, P < 0.009; impaired TLR-7/TNF-α, P < 0.01).

Conclusions

Treatment with HCQ is associated with impaired ability of pDCs from subjects with SLE to produce IFN-α and TNF-α upon stimulation with TLR-9 and TLR-7 agonists.

New insights into the mechanisms of innate immune receptor signalling in fibrosis

Recent advances in our understanding of innate immunity and inflammation have direct bearing on how we understand autoimmunity, and fibrosis, and how innate immune sensors might stimulate both of these key features of several fibrotic diseases. Toll-like receptors (TLRs) are the major receptors for recognizing pathogen associated molecular patterns present on bacterial cell walls, such as LPS, and nucleic acids (RNA and DNA). Several intracellular pathways mediate TLR effects and initiate various pro-inflammatory programs. Mechanisms for control of inflammation, matrix remodeling, and ultimately fibrosis are also activated. Transforming growth factor-beta (TGF-β), Interleukin-1 (IL-1), interleukin-4 (IL-4), interleukin-6 (IL-6), interleukin-13 (IL-13), and interferon (IFNs) appear particularly important in regulating pro-fibrotic aspects of innate immune activation. These mechanisms appear important in fibrotic disease affecting multiple organ-systems, including lung, liver, kidney, and skin. These observations provide new paradigms for understanding the relationship between immunity/inflammation and fibrosis, however, the precise ligand and mechanism linking innate immune sensor(s) to fibrosis remain uncertain in most illnesses.

Taming lupus-a new understanding of pathogenesis is leading to clinical advances

Systemic lupus erythematosus (SLE) is an autoimmune disease that is characterized by the loss of tolerance to nuclear self antigens, the production of pathogenic autoantibodies and damage to multiple organ systems. Over the years, patients with SLE have been managed largely with empiric immunosuppressive therapies, which are associated with substantial toxicities and do not always provide adequate control of the disease. The development of targeted therapies that specifically address disease pathogenesis or progression has lagged, largely because of the complex and heterogeneous nature of the disease, as well as difficulties in designing uniform outcome measures for clinical trials. Recent advances that could improve the treatment of SLE include the identification of genetic variations that influence the risk of developing the disease, an enhanced understanding of innate and adaptive immune activation and regulation of tolerance, dissection of immune cell activation and inflammatory pathways and elucidation of mechanisms and markers of tissue damage. These discoveries, together with improvements in clinical trial design, form a platform from which to launch the development of a new generation of lupus therapies.

Myeloid dendritic cells from B6.NZM Sle1/Sle2/Sle3 lupus-prone mice express an IFN signature that precedes disease onset

Patients with systemic lupus erythematosus show an overexpression of type I IFN-responsive genes that is referred to as "IFN signature." We found that B6.NZMSle1/Sle2/Sle3 (Sle1,2,3) lupus-prone mice also express an IFN signature compared with non-autoimmune C57BL/6 mice. In vitro, myeloid dendritic cells (mDCs) (GM-CSF bone marrow-derived dendritic cells; BMDCs) from Sle1,2,3 mice constitutively overexpressed IFN-responsive genes such as IFN-β, Oas-3, Mx-1, ISG-15, and CXCL10 and members of the IFN signaling pathway STAT1, STAT2, and IRF7. The IFN signature was similar in Sle1,2,3 BMDCs from young, pre-autoimmune mice and from mice with high titers of autoantibodies, suggesting that the IFN signature in mDCs precedes disease onset and is independent from the autoantibodies. Sle1,2,3 BMDCs hyperresponded to stimulation with IFN-α and the TLR7 and TLR9 agonists R848 and CpGs. We propose that this hyperresponse is induced by the IFN signature and only partially contributes to the signature, as oligonucleotides inhibitory for TLR7 and TLR9 only partially suppressed the constitutive IFN signature, and pre-exposure to IFN-α induced the same hyperresponse in wild-type BMDCs as in Sle1,2,3 BMDCs. In vivo, mDCs and to a lesser extent T and B cells from young prediseased Sle1,2,3 mice also expressed the IFN signature, although they lacked the strength that BMDCs showed in vitro. Sle1,2,3 plasmacytoid DCs expressed the IFN signature in vitro but not in vivo, suggesting that mDCs may be more relevant before disease onset. We propose that Sle1,2,3 mice are useful tools to study the role of the IFN signature in lupus pathogenesis.

Autoantibody epitope spreading in the pre-clinical phase predicts progression to rheumatoid arthritis

Rheumatoid arthritis (RA) is a prototypical autoimmune arthritis affecting nearly 1% of the world population and is a significant cause of worldwide disability. Though prior studies have demonstrated the appearance of RA-related autoantibodies years before the onset of clinical RA, the pattern of immunologic events preceding the development of RA remains unclear. To characterize the evolution of the autoantibody response in the preclinical phase of RA, we used a novel multiplex autoantigen array to evaluate development of the anti-citrullinated protein antibodies (ACPA) and to determine if epitope spread correlates with rise in serum cytokines and imminent onset of clinical RA. To do so, we utilized a cohort of 81 patients with clinical RA for whom stored serum was available from 1–12 years prior to disease onset. We evaluated the accumulation of ACPA subtypes over time and correlated this accumulation with elevations in serum cytokines. We then used logistic regression to identify a profile of biomarkers which predicts the imminent onset of clinical RA (defined as within 2 years of testing). We observed a time-dependent expansion of ACPA specificity with the number of ACPA subtypes. At the earliest timepoints, we found autoantibodies targeting several innate immune ligands including citrullinated histones, fibrinogen, and biglycan, thus providing insights into the earliest autoantigen targets and potential mechanisms underlying the onset and development of autoimmunity in RA. Additionally, expansion of the ACPA response strongly predicted elevations in many inflammatory cytokines including TNF-α, IL-6, IL-12p70, and IFN-γ. Thus, we observe that the preclinical phase of RA is characterized by an accumulation of multiple autoantibody specificities reflecting the process of epitope spread. Epitope expansion is closely correlated with the appearance of preclinical inflammation, and we identify a biomarker profile including autoantibodies and cytokines which predicts the imminent onset of clinical arthritis.

Blocking toll-like receptors 7 and 9 reduces postinterventional remodeling via reduced macrophage activation, foam cell formation, and migration

OBJECTIVE

The role of toll-like receptors (TLRs) in vascular remodeling is well established. However, the involvement of the endosomal TLRs is unknown. Here, we study the effect of combined blocking of TLR7 and TLR9 on postinterventional remodeling and accelerated atherosclerosis.

METHODS AND RESULTS

In hypercholesterolemic apolipoprotein E*3-Leiden mice, femoral artery cuff placement led to strong increase of TLR7 and TLR9 presence demonstrated by immunohistochemistry. Blocking TLR7/9 with a dual antagonist in vivo reduced neointimal thickening and foam cell accumulation 14 days after surgery by 65.6% (P=0.0079). Intima/media ratio was reduced by 64.5% and luminal stenosis by 62.8%. The TLR7/9 antagonist reduced the arterial wall inflammation, with reduced macrophage infiltration, decreased cytoplasmic high-mobility group box 1 expression, and altered serum interleukin-10 levels. Stimulation of cultured macrophages with TLR7 and TLR9 ligands enhanced tumor necrosis factor-α expression, which is decreased by TLR7/9 antagonist coadministration. Additionally, the antagonist abolished the TLR7/9-enhanced low-density lipoprotein uptake. The antagonist also reduced oxidized low-density lipoprotein-induced foam cell formation, most likely not via decreased influx but via increased efflux, because CD36 expression was unchanged whereas interleukin-10 levels were higher (36.1 ± 22.3 pg/mL versus 128.9 ± 6.6 pg/mL; P=0.008).

CONCLUSIONS

Blocking TLR7 and TLR9 reduced postinterventional vascular remodeling and foam cell accumulation indicating TLR7 and TLR9 as novel therapeutic targets.

TLR tolerance reduces IFN-alpha production despite plasmacytoid dendritic cell expansion and anti-nuclear antibodies in NZB bicongenic mice

Genetic loci on New Zealand Black (NZB) chromosomes 1 and 13 play a significant role in the development of lupus-like autoimmune disease. We have previously shown that C57BL/6 (B6) congenic mice with homozygous NZB chromosome 1 (B6.NZBc1) or 13 (B6.NZBc13) intervals develop anti-nuclear antibodies and mild glomerulonephritis (GN), together with increased T and B cell activation. Here, we produced B6.NZBc1c13 bicongenic mice with both intervals, and demonstrate several novel phenotypes including: marked plasmacytoid and myeloid dendritic cell expansion, and elevated IgA production. Despite these changes, only minor increases in anti-nuclear antibody production were seen, and the severity of GN was reduced as compared to B6.NZBc1 mice. Although bicongenic mice had increased levels of baff and tnf-α mRNA in their spleens, the levels of IFN-α-induced gene expression were reduced. Splenocytes from bicongenic mice also demonstrated reduced secretion of IFN-α following TLR stimulation in vitro. This reduction was not due to inhibition by TNF-α and IL-10, or regulation by other cellular populations. Because pDC in bicongenic mice are chronically exposed to nuclear antigen-containing immune complexes in vivo, we examined whether repeated stimulation of mouse pDC with TLR ligands leads to impaired IFN-α production, a phenomenon termed TLR tolerance. Bone marrow pDC from both B6 and bicongenic mice demonstrated markedly inhibited secretion of IFN-α following repeated stimulation with a TLR9 ligand. Our findings suggest that the expansion of pDC and production of anti-nuclear antibodies need not be associated with increased IFN-α production and severe kidney disease, revealing additional complexity in the regulation of autoimmunity in systemic lupus erythematosus.

The origin and properties of extracellular DNA: from PAMP to DAMP

DNA is a polymeric macromolecule whose biological activities depend on location as well as binding to associated molecules. Inside the cell, DNA is the source of genetic information and binds histones to form nucleosomes. DNA can exit the cell, however, to enter the extracellular space primarily during cell death, either apoptosis or necrosis, as well as NETosis. While bacterial DNA is a potent immune stimulant by virtue of its CpG motifs, mammalian DNA, which is ordinarily inactive, can acquire activity by associating with nuclear, cytoplasmic and serum proteins which promote its uptake into cells to stimulate internal DNA sensors, including Toll-like receptor 9. Among these proteins, anti-DNA autoantibodies can form immune complexes with DNA to stimulate plasmacytoid dendritic cells to produce type 1 interferon. Together, these findings suggest that the immune properties of DNA are mutable and diverse, reflecting its context and the array of attached molecules.

Role of type I interferons in the activation of autoreactive B cells

Type I interferons (IFNs) are a family of cytokines involved in the defense against viral infections that play a key role in the activation of both the innate and adaptive immune system. IFNs both directly and indirectly enhance the capacity of B lymphocytes to respond to viral challenge and produce cytotoxic and neutralizing antibodies. However, prolonged type I IFN exposure is not always beneficial to the host. If not regulated properly IFN can drive autoantibody production as well as other parameters of systemic autoimmune disease. Type I IFNs impact B-cell function through a variety of mechanisms, including effects on receptor engagement, Toll-like receptor expression, cell migration, antigen presentation, cytokine responsiveness, cytokine production, survival, differentiation and class-switch recombination. Type I IFNs are also cytotoxic for a variety of cell types and thereby contribute to the accumulation of cell debris that serves as a potential source for autoantigens. Type I IFN engagement of a variety of accessory cells further promotes B-cell survival and activation, as exemplified by the capacity of type I IFNs to increase the level of B-cell survival factors, such as B lymphocyte stimulator, produced by dendritic cells. Therefore, it is not surprising that the loss of expression of the type I IFN receptor can have dramatic effects on the production of autoantibodies and on the clinical features of systemic autoimmune diseases such as systemic lupus erythematosus.