Microparticles as antigenic targets of antibodies to DNA and nucleosomes in systemic lupus erythematosus

Yin-Yang: two sides of extracellular vesicles in inflammatory diseases

The concept of Yin-Yang, originating in ancient Chinese philosophy, symbolizes two opposing but complementary forces or principles found in all aspects of life. This concept can be quite fitting in the context of extracellular vehicles (EVs) and inflammatory diseases. Over the past decades, numerous studies have revealed that EVs can exhibit dual sides, acting as both pro- and anti-inflammatory agents, akin to the concept of Yin-Yang theory (i.e., two sides of a coin). This has enabled EVs to serve as potential indicators of pathogenesis or be manipulated for therapeutic purposes by influencing immune and inflammatory pathways. This review delves into the recent advances in understanding the Yin-Yang sides of EVs and their regulation in specific inflammatory diseases. We shed light on the current prospects of engineering EVs for treating inflammatory conditions. The Yin-Yang principle of EVs bestows upon them great potential as, therapeutic, and preventive agents for inflammatory diseases.

Circulating extracellular vesicles in Systemic Lupus Erythematosus: physicochemical properties and phenotype

OBJECTIVE

This study aimed to identify the physicochemical and phenotypic characteristics of circulating Extracellular Vesicles (EVs) in the plasma of patients with SLE, with or without Lupus Nephritis (LN), and their potential utility as disease biomarkers.

METHODS

Plasma-circulating EVs were concentrated using differential centrifugation from adult female patients (n=38) who met the 'American College of Rheumatology/European Alliance of Associations for Rheumatology 2019' criteria for SLE diagnosis with (LN) or without LN (nLN), confirmed by renal biopsy. Controls (n=18) were healthy volunteers matched by gender and similar age. The structure, size and Energy Dispersion Spectrum (EDS) of EVs were observed by electron microscopy. The surface charge and size distribution were evaluated using dynamic light scattering. The counts and phenotype of EVs from patients (SLE-EVs) and controls (Ctrl-EVs) were obtained using flow cytometry. Non-parametric statistical tests and exploratory analysis of multiple variables were performed. The discriminatory power of some variables as potential biomarkers of the disease was also evaluated.

RESULTS

Circulating EVs were heterogeneous in morphology and size, but SLE-EVs reached larger diameters than Ctrl-EVs (p<0.0001). Small SLE-EVs and large SLE-EVs were increased compared with Ctrl-EV (p<0.0001 and p<0.05, respectively). Likewise, patients with SLE (LN or nLN) had higher concentrations of large EVs compared with controls (p<0.001 and p<0.0001, respectively). SLE-EVs showed a different EDS (p<0.001) and were less electronegative (p<0.0001) than Ctrl-EVs. EV-CD45+, EV-CD14+ and EV-IgM+ were more frequent in patients with SLE compared with controls (p<0.001, p<0.05 and p<0.001, respectively). The concentrations of large EVs and EV-IgM+ allowed better discrimination of patients from controls.

CONCLUSIONS

Plasma-circulating EVs from patients with SLE with and without nephritis are increased in peripheral blood and have different physicochemical properties than controls. Characteristics of EVs such as larger size and the presence of IgM on the surface could help discriminate patients from controls.

B cells secrete functional antigen-specific IgG antibodies on extracellular vesicles

B cells and the antibodies they produce are critical in host defense against pathogens and contribute to various immune-mediated diseases. B cells responding to activating signals in vitro release extracellular vesicles (EV) that carry surface antibodies, yet B cell production of EVs that express antibodies and their function in vivo is incompletely understood. Using transgenic mice expressing the Cre recombinase in B cells switching to IgG1 to induce expression of fusion proteins between emerald green fluorescent protein (emGFP) and the EV tetraspanin CD63 as a model, we identify emGFP expression in B cells responding to foreign antigen in vivo and characterize the emGFP+ EVs they release. Our data suggests that emGFP+ germinal center B cells undergoing immunoglobulin class switching to express IgG and their progeny memory B cells and plasma cells, also emGFP+, are sources of circulating antigen-specific IgG+ EVs. Furthermore, using a mouse model of influenza virus infection, we find that IgG+ EVs specific for the influenza hemagglutinin antigen protect against virus infection. In addition, crossing the B cell Cre driver EV reporter mice onto the Nba2 lupus-prone strain revealed increased circulating emGFP+ EVs that expressed surface IgG against nuclear antigens linked to autoimmunity. These data identify EVs loaded with antibodies as a novel route for antibody secretion in B cells that contribute to adaptive immune responses, with important implications for different functions of IgG+ EVs in infection and autoimmunity.

Therapeutic potential of exosomes derived from mesenchymal stem cells for treatment of systemic lupus erythematosus

Autoimmune diseases are caused by an imbalance in the immune system, producing autoantibodies that cause inflammation leading to tissue damage and organ dysfunction. Systemic Lupus Erythematosus (SLE) is one of the most common autoimmune diseases and a major contributor to patient morbidity and mortality. Although many drugs manage the disease, curative therapy remains elusive, and current treatment regimens have substantial side effects. Recently, the therapeutic potential of exosomes has been extensively studied, and novel evidence has been demonstrated. A direct relationship between exosome contents and their ability to regulate the immune system, inflammation, and angiogenesis. The unique properties of extracellular vesicles, such as biomolecule transportation, biodegradability, and stability, make exosomes a promising treatment candidate for autoimmune diseases, particularly SLE. This review summarizes the structural features of exosomes, the isolation/purification/quantification method, their origin, effect, immune regulation, a critical consideration for selecting an appropriate source, and their therapeutic mechanisms in SLE.

Mitochondrial extracellular vesicles, autoimmunity and myocarditis

For many decades viral infections have been suspected as 'triggers' of autoimmune disease, but mechanisms for how this could occur have been difficult to establish. Recent studies have shown that viral infections that are commonly associated with viral myocarditis and other autoimmune diseases such as coxsackievirus B3 (CVB3) and SARS-CoV-2 target mitochondria and are released from cells in mitochondrial vesicles that are able to activate the innate immune response. Studies have shown that Toll-like receptor (TLR)4 and the inflammasome pathway are activated by mitochondrial components. Autoreactivity against cardiac myosin and heart-specific immune responses that occur after infection with viruses where the heart is not the primary site of infection (e.g., CVB3, SARS-CoV-2) may occur because the heart has the highest density of mitochondria in the body. Evidence exists for autoantibodies against mitochondrial antigens in patients with myocarditis and dilated cardiomyopathy. Defects in tolerance mechanisms like autoimmune regulator gene (AIRE) may further increase the likelihood of autoreactivity against mitochondrial antigens leading to autoimmune disease. The focus of this review is to summarize current literature regarding the role of viral infection in the production of extracellular vesicles containing mitochondria and virus and the development of myocarditis.

Extracellular vesicles as tools and targets in therapy for diseases

Extracellular vesicles (EVs) are nano-sized, membranous structures secreted into the extracellular space. They exhibit diverse sizes, contents, and surface markers and are ubiquitously released from cells under normal and pathological conditions. Human serum is a rich source of these EVs, though their isolation from serum proteins and non-EV lipid particles poses challenges. These vesicles transport various cellular components such as proteins, mRNAs, miRNAs, DNA, and lipids across distances, influencing numerous physiological and pathological events, including those within the tumor microenvironment (TME). Their pivotal roles in cellular communication make EVs promising candidates for therapeutic agents, drug delivery systems, and disease biomarkers. Especially in cancer diagnostics, EV detection can pave the way for early identification and offers potential as diagnostic biomarkers. Moreover, various EV subtypes are emerging as targeted drug delivery tools, highlighting their potential clinical significance. The need for non-invasive biomarkers to monitor biological processes for diagnostic and therapeutic purposes remains unfulfilled. Tapping into the unique composition of EVs could unlock advanced diagnostic and therapeutic avenues in the future. In this review, we discuss in detail the roles of EVs across various conditions, including cancers (encompassing head and neck, lung, gastric, breast, and hepatocellular carcinoma), neurodegenerative disorders, diabetes, viral infections, autoimmune and renal diseases, emphasizing the potential advancements in molecular diagnostics and drug delivery.

Extracellular histones as damage-associated molecular patterns in neuroinflammatory responses

The four core histones H2A, H2B, H3, H4, and the linker histone H1 primarily bind DNA and regulate gene expression within the nucleus. Evidence collected mainly from the peripheral tissues illustrates that histones can be released into the extracellular space by activated or damaged cells. In this article, we first summarize the innate immune-modulatory properties of extracellular histones and histone-containing complexes, such as nucleosomes, and neutrophil extracellular traps (NETs), described in peripheral tissues. There, histones act as damage-associated molecular patterns (DAMPs), which are a class of endogenous molecules that trigger immune responses by interacting directly with the cellular membranes and activating pattern recognition receptors (PRRs), such as toll-like receptors (TLR) 2, 4, 9 and the receptor for advanced glycation end-products (RAGE). We then focus on the available evidence implicating extracellular histones as DAMPs of the central nervous system (CNS). It is becoming evident that histones are present in the brain parenchyma after crossing the blood-brain barrier (BBB) or being released by several types of brain cells, including neurons, microglia, and astrocytes. However, studies on the DAMP-like effects of histones on CNS cells are limited. For example, TLR4 is the only known molecular target of CNS extracellular histones and their interactions with other PRRs expressed by brain cells have not been observed. Nevertheless, extracellular histones are implicated in the pathogenesis of a variety of neurological disorders characterized by sterile neuroinflammation; therefore, detailed studies on the role these proteins and their complexes play in these pathologies could identify novel therapeutic targets.

Extracellular vesicles as personalized medicine

Extracellular vesicles (EVs) are released from all cells in the body, forming an important intercellular communication network that contributes to health and disease. The contents of EVs are cell source-specific, inducing distinct signaling responses in recipient cells. The specificity of EVs and their accumulation in fluid spaces that are accessible for liquid biopsies make them highly attractive as potential biomarkers and therapies for disease. The duality of EVs as favorable (therapeutic) or unfavorable (pathological) messengers is context dependent and remains to be fully determined in homeostasis and various disease states. This review describes the use of EVs as biomarkers, drug delivery vehicles, and regenerative therapeutics, highlighting examples involving viral infections, cancer, and neurological diseases. There is growing interest to provide personalized therapy based on individual patient and disease characteristics. Increasing evidence suggests that EV biomarkers and therapeutic approaches are ideal for personalized medicine due to the diversity and multifunctionality of EVs.

Moonlighting chromatin: when DNA escapes nuclear control

Extracellular chromatin, for example in the form of neutrophil extracellular traps (NETs), is an important element that propels the pathological progression of a plethora of diseases. DNA drives the interferon system, serves as autoantigen, and forms the extracellular scaffold for proteins of the innate immune system. An insufficient clearance of extruded chromatin after the release of DNA from the nucleus into the extracellular milieu can perform a secret task of moonlighting in immune-inflammatory and occlusive disorders. Here, we discuss (I) the cellular events involved in the extracellular release of chromatin and NET formation, (II) the devastating consequence of a dysregulated NET formation, and (III) the imbalance between NET formation and clearance. We include the role of NET formation in the occlusion of vessels and ducts, in lung disease, in autoimmune diseases, in chronic oral disorders, in cancer, in the formation of adhesions, and in traumatic spinal cord injury. To develop effective therapies, it is of utmost importance to target pathways that cause decondensation of chromatin during exaggerated NET formation and aggregation. Alternatively, therapies that support the clearance of extracellular chromatin are conceivable.

Structural features of Dnase1L3 responsible for serum antigen clearance

Autoimmunity develops when extracellular DNA released from dying cells is not cleared from serum. While serum DNA is primarily digested by Dnase1 and Dnase1L3, Dnase1 cannot rescue autoimmunity arising from Dnase1L3 deficiencies. Dnase1L3 uniquely degrades antigenic forms of cell-free DNA, including DNA complexed with lipids and proteins. The distinct activity of Dnase1L3 relies on its unique C-terminal Domain (CTD), but the mechanism is unknown. We used multiple biophysical techniques and functional assays to study the interplay between the core catalytic domain and the CTD. While the core domain resembles Dnase1, there are key structural differences between the two enzymes. First, Dnase1L3 is not inhibited by actin due to multiple differences in the actin recognition site. Second, the CTD augments the ability of the core to bind DNA, thereby facilitating the degradation of complexed DNA. Together, these structural insights will inform the development of Dnase1L3-based therapies for autoimmunity.

Microparticles in Autoimmunity: Cause or Consequence of Disease?

Microparticles (MPs) are small (100 nm - 1 um) extracellular vesicles derived from the plasma membrane of dying or activated cells. MPs are important mediators of intercellular communication, transporting proteins, nucleic acids and lipids from the parent cell to other cells. MPs resemble the state of their parent cells and are easily accessible when released into the blood or urine. MPs also play a role in the pathogenesis of different diseases and are considered as potential biomarkers. MP isolation and characterization is technically challenging and results in different studies are contradictory. Therefore, uniform guidelines to isolate and characterize MPs should be developed. Our understanding of MP biology and how MPs play a role in different pathological mechanisms has greatly advanced in recent years. MPs, especially if derived from apoptotic cells, possess strong immunogenic properties due to the presence of modified proteins and nucleic acids. MPs are often found in patients with autoimmune diseases where MPs for example play a role in the break of immunological tolerance and/or induction of inflammatory conditions. In this review, we describe the main techniques to isolate and characterize MPs, define the characteristics of MPs generated during cell death, illustrate different mechanism of intercellular communication via MPs and summarize the role of MPs in pathological mechanisms with a particular focus on autoimmune diseases.

In sickness and in health: The functional role of extracellular vesicles in physiology and pathology in vivo: Part II: Pathology: Part II: Pathology

It is clear from Part I of this series that extracellular vesicles (EVs) play a critical role in maintaining the homeostasis of most, if not all, normal physiological systems. However, the majority of our knowledge about EV signalling has come from studying them in disease. Indeed, EVs have consistently been associated with propagating disease pathophysiology. The analysis of EVs in biofluids, obtained in the clinic, has been an essential of the work to improve our understanding of their role in disease. However, to interfere with EV signalling for therapeutic gain, a more fundamental understanding of the mechanisms by which they contribute to pathogenic processes is required. Only by discovering how the EV populations in different biofluids change-size, number, and physicochemical composition-in clinical samples, may we then begin to unravel their functional roles in translational models in vitro and in vivo, which can then feedback to the clinic. In Part II of this review series, the functional role of EVs in pathology and disease will be discussed, with a focus on in vivo evidence and their potential to be used as both biomarkers and points of therapeutic intervention.

Extracellular Vesicles and Their Relationship with the Heart-Kidney Axis, Uremia and Peritoneal Dialysis

Cardiorenal syndrome (CRS) is described as primary dysfunction in the heart culminating in renal injury or vice versa. CRS can be classified into five groups, and uremic toxin (UT) accumulation is observed in all types of CRS. Protein-bound uremic toxin (PBUT) accumulation is responsible for permanent damage to the renal tissue, and mainly occurs in CRS types 3 and 4, thus compromising renal function directly leading to a reduction in the glomerular filtration rate (GFR) and/or subsequent proteinuria. With this decrease in GFR, patients may need renal replacement therapy (RRT), such as peritoneal dialysis (PD). PD is a high-quality and home-based dialysis therapy for patients with end-stage renal disease (ESRD) and is based on the semi-permeable characteristics of the peritoneum. These patients are exposed to factors which may cause several modifications on the peritoneal membrane. The presence of UT may harm the peritoneum membrane, which in turn can lead to the formation of extracellular vesicles (EVs). EVs are released by almost all cell types and contain lipids, nucleic acids, metabolites, membrane proteins, and cytosolic components from their cell origin. Our research group previously demonstrated that the EVs can be related to endothelial dysfunction and are formed when UTs are in contact with the endothelial monolayer. In this scenario, this review explores the mechanisms of EV formation in CRS, uremia, the peritoneum, and as potential biomarkers in peritoneal dialysis.

Serum IgG2 antibody multi-composition in systemic lupus erythematosus and in lupus nephritis (Part 2): prospective study

OBJECTIVES

Circulating anti-ENO1 and anti-H2A IgG2 have been identified as specific signatures of LN in a cross-over approach. We sought to show whether the same antibodies identify selected population of patients with LN with potentially different clinical outcomes.

METHODS

Here we report the prospective analysis over 36 months of circulating IgG2 levels in patients with newly diagnosed LN (n=91) and SLE (n=31) and in other patients with SLE recruited within 2 years from diagnosis (n=99). Anti-podocyte (ENO1), anti-nucleosome (DNA, histone 2 A, histone 3) and anti-circulating proteins (C1q, AnnexinA1-ANXA1) IgG2 antibodies were determined by home-made techniques.

RESULTS

LN patients were the main focus of the study. Anti-ENO1, anti-H2A and anti-ANXA1 IgG2 decreased in parallel to proteinuria and normalized within 12 months in the majority of patients while anti-dsDNA IgG2 remained high over the 36 months. Anti-ENO1 and anti-H2A had the highest association with proteinuria (Heat Map) and identified the highest number of patients with high proteinuria (68% and 71% respectively) and/or with reduced estimated glomerula filtration rate (eGFR) (58% for both antibodies) compared with 23% and 17% of anti-dsDNA (agreement analysis). Anti-ENO1 positive LN patients had higher proteinuria than negative patients at T0 and presented the maximal decrement within 12 months.

CONCLUSIONS

Anti-ENO1, anti-H2A and anti-ANXA1 antibodies were associated with high proteinuria in LN patients and Anti-ENO1 also presented the maximal reduction within 12 months that paralleled the decrease of proteinuria. Anti-dsDNA were not associated with renal outcome parameters. New IgG2 antibody signatures should be utilized as tracers of personalized therapies in LN.

TRIAL REGISTRATION

The Zeus study was registered at https://clinicaltrials.gov (study number: NCT02403115).

Extracellular Vesicles in Immune System Regulation and Type 1 Diabetes: Cell-to-Cell Communication Mediators, Disease Biomarkers, and Promising Therapeutic Tools

Extracellular vesicles (EVs) are generated by cells of origin through complex molecular mechanisms and released into extracellular environment. Hence, the presence of EVs has been described in multiple biological fluids and in most cases their molecular cargo, which includes non-coding RNAs (ncRNA), messenger RNAs (mRNA), and proteins, has been reported to modulate distinct biological processes. EVs release and their molecular cargo have been demonstrated to be altered in multiple diseases, including autoimmune diseases. Notably, numerous evidence showed a relevant crosstalk between immune system and interacting cells through specific EVs release. The crosstalk between insulin-producing pancreatic β cells and immune system through EVs bidirectional trafficking has yet started to be deciphered, thus uncovering an intricate communication network underlying type 1 diabetes (T1D) pathogenesis. EVs can also be found in blood plasma or serum. Indeed, the assessment of circulating EVs cargo has been shown as a promising advance in the detection of reliable biomarkers of disease progression. Of note, multiple studies showed several specific cargo alterations of EVs collected from plasma/serum of subjects affected by autoimmune diseases, including T1D subjects. In this review, we discuss the recent literature reporting evidence of EVs role in autoimmune diseases, specifically focusing on the bidirectional crosstalk between pancreatic β cells and immune system in T1D and highlight the relevant promising role of circulating EVs as disease biomarkers.

Extracellular Vesicles as Potential Therapeutics for Inflammatory Diseases

Extracellular vesicles (EV) deliver cargoes such as nucleic acids, proteins, and lipids between cells and serve as an intercellular communicator. As it is revealed that most of the functions associated to EVs are closely related to the immune response, the important role of EVs in inflammatory diseases is emerging. EVs can be functionalized through EV surface engineering and endow targeting moiety that allows for the target specificity for therapeutic applications in inflammatory diseases. Moreover, engineered EVs are considered as promising nanoparticles to develop personalized therapeutic carriers. In this review, we highlight the role of EVs in various inflammatory diseases, the application of EV as anti-inflammatory therapeutics, and the current state of the art in EV engineering techniques.

Autoantibody-mediated impairment of DNASE1L3 activity in sporadic systemic lupus erythematosus

Antibodies to double-stranded DNA (dsDNA) are prevalent in systemic lupus erythematosus (SLE), particularly in patients with lupus nephritis, yet the nature and regulation of antigenic cell-free DNA (cfDNA) are poorly understood. Null mutations in the secreted DNase DNASE1L3 cause human monogenic SLE with anti-dsDNA autoreactivity. We report that >50% of sporadic SLE patients with nephritis manifested reduced DNASE1L3 activity in circulation, which was associated with neutralizing autoantibodies to DNASE1L3. These patients had normal total plasma cfDNA levels but showed accumulation of cfDNA in circulating microparticles. Microparticle-associated cfDNA contained a higher fraction of longer polynucleosomal cfDNA fragments, which bound autoantibodies with higher affinity than mononucleosomal fragments. Autoantibodies to DNASE1L3-sensitive antigens on microparticles were prevalent in SLE nephritis patients and correlated with the accumulation of cfDNA in microparticles and with disease severity. DNASE1L3-sensitive antigens included DNA-associated proteins such as HMGB1. Our results reveal autoantibody-mediated impairment of DNASE1L3 activity as a common nongenetic mechanism facilitating anti-dsDNA autoreactivity in patients with severe sporadic SLE.

Extracellular Vesicles Tune the Immune System in Renal Disease: A Focus on Systemic Lupus Erythematosus, Antiphospholipid Syndrome, Thrombotic Microangiopathy and ANCA-Vasculitis

Extracellular vesicles (EV) are microparticles released in biological fluids by different cell types, both in physiological and pathological conditions. Owing to their ability to carry and transfer biomolecules, EV are mediators of cell-to-cell communication and are involved in the pathogenesis of several diseases. The ability of EV to modulate the immune system, the coagulation cascade, the angiogenetic process, and to drive endothelial dysfunction plays a crucial role in the pathophysiology of both autoimmune and renal diseases. Recent studies have demonstrated the involvement of EV in the control of renal homeostasis by acting as intercellular signaling molecules, mediators of inflammation and tissue regeneration. Moreover, circulating EV and urinary EV secreted by renal cells have been investigated as potential early biomarkers of renal injury. In the present review, we discuss the recent findings on the involvement of EV in autoimmunity and in renal intercellular communication. We focused on EV-mediated interaction between the immune system and the kidney in autoimmune diseases displaying common renal damage, such as antiphospholipid syndrome, systemic lupus erythematosus, thrombotic microangiopathy, and vasculitis. Although further studies are needed to extend our knowledge on EV in renal pathology, a deeper investigation of the impact of EV in kidney autoimmune diseases may also provide insight into renal biological processes. Furthermore, EV may represent promising biomarkers of renal diseases with potential future applications as diagnostic and therapeutic tools.

Breaching self-tolerance by targeting the gatekeeper

Loss-of-function mutations in DNaseL13, the enzyme that restricts the amount of microparticle-associated DNA, cause SLE in humans and mice. In this issue of JEM, Hartl et al. (2021. J. Exp. Med.https://doi.org/10.1084/jem.20201138) uncover a reduction in plasma DNASE1L3 enzymatic activity due to the presence of autoantibodies in patients with nonfamilial SLE.

Microvesicles in active lupus nephritis show Toll-like receptor 9-dependent co-expression of galectin-3 binding protein and double-stranded DNA

Circulating microvesicles (MVs) from patients with systemic lupus erythematosus (SLE) express the type 1 interferon (IFN)-inducible protein galectin-3 binding protein (G3BP), which may enhance their deposition in the glomerular basement membrane. The release of G3BP-expressing MVs from normal peripheral blood mononuclear cells (PBMCs) is induced by Toll-like receptor 9 (TLR-9) ligands, and these vesicles contain autoantibody-accessible double-stranded DNA (dsDNA). This study compares the release of MVs expressing G3BP and dsDNA from PBMCs derived from SLE patients with or without active lupus nephritis (LN) and from healthy donors, and taps further into the potential dependency on IFN-α for their generation and impacts of TLR-7/TLR-9 co-stimulation. PBMCs from 10 healthy donors and 12 SLE patients, six of whom had active LN at study inclusion, were stimulated in-vitro with recombinant human IFN-α and the TLR-9 agonists oligodeoxynucleotide (ODN)2216 or ODN2395 alone or in combination with the TLR-7 agonist gardiquimod. MVs in the supernatants were subsequently isolated by differential centrifugation and their expression of G3BP and dsDNA was quantified by flow cytometry. Stimulation with ODN2395 significantly increased the release of MVs co-expressing G3BP and dsDNA from PBMCs isolated from healthy donors and SLE patients. The expression of G3BP on individual MVs and the proportion of G3BP and dsDNA double-positive MVs released were increased in active LN patients. Neither co-stimulation with gardiquimod nor with the IFN-α inhibitor IN-1 had any effect on the MV release induced by ODN2395. In conclusion, the TLR-9-mediated inducibility of MVs co-expressing G3BP and dsDNA is increased in SLE patients with active LN.

Neutrophil Extracellular Traps in the Autoimmunity Context

The formation of neutrophil extracellular traps (NETs) is a strategy utilized by neutrophils for capturing infective agents. Extracellular traps consist in a physical net made of DNA and intracellular proteins externalized from neutrophils, where bacteria and viruses are entrapped and killed by proteolysis. A complex series of events contributes to achieving NET formation: signaling from infectious triggers comes first, followed by decondensation of chromatin and extrusion of the nucleosome components (DNA, histones) from the nucleus and, after cell membrane breakdown, outside the cell. NETs are composed of either DNA or nucleosome proteins and hundreds of cytoplasm proteins, a part of which undergo post-translational modification during the steps leading to NETs. There is a thin balance between the production and the removal of circulating NETs from blood where digestion of DNA by circulating DNases 1 and IL3 has a critical role. A delay in NET removal may have consequences for autoimmunity. Recent studies have shown that circulating NET levels are increased in systemic lupus erythematosus (SLE) for a functional block of NET removal mediated by anti-DNase antibodies or, in rare cases, by DNase IL3 mutations. In SLE, the persistence in circulation of NETs signifies elevated concentrations of either free DNA/nucleosome components and oxidized proteins that, in some cases, are recognized as non-self and presented to B-cells by Toll-like receptor 9 (TLR9). In this way, it is activated as an immunologic response, leading to the formation of IgG2 auto-antibody. Monitoring serum NET levels represents a potential new way to herald the development of renal lesions and has clinical implications. Modulating the balance between NET formation and removal is one of the objectives of basic research that are aimed to design new drugs for SLE. Clinical Trial Registration Number: The Zeus study was registered at https://clinicaltrials.gov (study number: NCT02403115).

Exosomes as Drug Delivery Systems: Endogenous Nanovehicles for Treatment of Systemic Lupus Erythematosus

Exosomes, nanometer-sized lipid-bilayer-enclosed extracellular vesicles (EVs), have attracted increasing attention due to their inherent ability to shuttle proteins, lipids and genes between cells and their natural affinity to target cells. Their intrinsic features such as stability, biocompatibility, low immunogenicity and ability to overcome biological barriers, have prompted interest in using exosomes as drug delivery vehicles, especially for gene therapy. Evidence indicates that exosomes play roles in both immune stimulation and tolerance, regulating immune signaling and inflammation. To date, exosome-based nanocarriers delivering small molecule drugs have been developed to treat many prevalent autoimmune diseases. This review highlights the key features of exosomes as drug delivery vehicles, such as therapeutic cargo, use of targeting peptide, loading method and administration route with a broad focus. In addition, we outline the current state of evidence in the field of exosome-based drug delivery systems in systemic lupus erythematosus (SLE), evaluating exosomes derived from various cell types and engineered exosomes.

Extracellular vesicles and lupus nephritis - New insights into pathophysiology and clinical implications

Lupus nephritis (LN) is a major cause for overall morbidity and mortality in patients with systemic lupus erythematosus (SLE), while its pathogenic mechanisms are still not well understood. Extracellular vesicles (EVs) are membrane vesicles that are released from almost all cell types. EVs can be subdivided into exosomes, microvesicles, and apoptotic bodies. Latest studies have shown that EVs can be released during several cellular events, including cell activation, autophagy, and several types of programed cell death, i.e. apoptosis, necroptosis, pyroptosis, and NETosis. Emerging evidence demonstrates that EVs harbor different bioactive molecules, including nucleic acids, proteins, lipids, cytokines, immune complexes (ICs), complements, and other molecules, some of which may contribute to pathogenesis of autoimmune diseases. EVs can serve as novel information shuttle to mediate local autocrine or paracrine signals to nearby cells, and distant endocrine signals to cells located far away. In LN, EVs may have pathogenic effects by transportation of autoantigens or complements, promotion of IC deposition or complement activation, and stimulation of inflammatory responses, renal tissue injury, or microthrombus formation. Additionally, EVs released from kidney cells may serve as specific biomarkers for diagnosis or monitoring of disease activity and therapeutic efficacy. In this review, we will summarize the latest progress about EV generation from basic research, their potential pathologic effects on LN, and their clinical implications. The cutting-edge knowledge about EV research provides insights into novel therapeutic strategy, new tools for diagnosis or prognosis, and evaluation approaches for treatment effectiveness in LN.

Extracellular vesicles as potential biomarkers and therapeutic approaches in autoimmune diseases

Extracellular vesicles are heterogeneous populations of naturally occurring secreted small vesicles. EVs function as signaling platforms to facilitate intracellular communication, which indicates the physiological or pathophysiological conditions of cells or tissues. Considering that EVs can be isolated from most body fluids and that molecular constituents could be reprogrammed according to the physiological status of the secreting cells, EVs are regarded as novel diagnostic and prognostic biomarkers for many diseases. The ability to protect encapsulated molecules from degradation in body fluids suggests the potential of EVs as biological medicines or drug delivery systems. This article focuses on the EV-associated biomarkers and therapeutic approaches in autoimmune diseases.

DNA-nanoparticle interactions: Formation of a DNA corona and its effects on a protein corona

There has been much recent interest in the protein "corona," the nonspecific adsorption of proteins on the surface of nanoparticles used in biological applications. This research investigates an analogous DNA corona. We find that particles (200 nm and 1 μm) incubated with DNA form a DNA corona, with a higher concentration of DNA adsorbed on the surface of cationic nanoparticles. With protein present, a combined DNA and protein corona is formed although DNA in solution displaces protein from the nanoparticle surface. Displacement of protein from the nanoparticle surface is dependent on the concentration of DNA in solution and was also observed for planar surfaces. Overall, we expect this investigation of the DNA corona to be important for nanomedicine applications, as well as disease states, especially systemic lupus erythematosus, in which biological particles with bound DNA are important mediators of inflammation and thrombosis.

Role of Extracellular Vesicles in Autoimmune Pathogenesis

Autoimmune diseases are conditions that emerge from abnormal immune responses to natural parts of the body. Extracellular vesicles (EVs) are membranous structures found in almost all types of cells. Because EVs often transport “cargo” between cells, their ability to crosstalk may be an important communication pathway within the body. The pathophysiological role of EVs is increasingly recognized in autoimmune diseases, including multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus, Sjogren's syndrome, Type 1 diabetes, and autoimmune thyroid disease. EVs are considered as biomarkers of these diseases. This article outlines existing knowledge on the biogenesis of EVs, their role as messegers in cellular communication and the function in T/B cell differentiation and maturation, and focusing on their potential application in autoimmune diseases.

Cell-free DNA profiling in patients with lupus nephritis

Background

Increased level of cell-free DNA (cf-DNA) is associated with systemic lupus erythematosus (SLE) and might be related to disease activity. The aim of this study was to evaluate whether cfDNA integrity, size distribution and concentration of different cfDNA fractions is associated with lupus activity and kidney involvement.

Methods

Blood samples were collected from 43 SLE patients and 50 healthy controls. Nuclear and mitochondrial fractions of cfDNA and intracellular DNA were quantified by real-time qPCR. Sizing and quantification of total cfDNA level was performed on Bioanalyzer.

Results

We determined four parameters that characterized cfDNA profile: fragmentation index, ratio of intra- to extracellular mtDNA copy number, cfDNA concentration, and presence of 54–149 bp and 209–297 bp fragments. Patients with healthy-like cfDNA profile had higher eGFR ( P = 0.009) and more often no indications for kidney biopsy or less advanced lupus nephritis (LN) ( P = 0.037). In contrary, SLE patients with distinct cfDNA profile (characterized by increased cfDNA concentration and fragmentation, higher discrepancy between intra- to extracellular mtDNA copy number, and the presence of 54–149 bp and 209–297 bp fragments) had lower eGFR ( P = 0.005) and more often advanced LN or history of renal transplantation ( P = 0.001).

Conclusions

We showed that cfDNA profiling may help to distinguish SLE patients with renal involvement and severe disease course from patients with more favorable outcomes. We suggest cfDNA profile a promising SLE biomarker.

Extracellular vesicles in Inflammatory Skin Disorders: from Pathophysiology to Treatment

Extracellular vesicles (EVs), naturally secreted by almost all known cell types into extracellular space, can transfer their bioactive cargos of nucleic acids and proteins to recipient cells, mediating cell-cell communication. Thus, they participate in many pathogenic processes including immune regulation, cell proliferation and differentiation, cell death, angiogenesis, among others. Cumulative evidence has shown the important regulatory effects of EVs on the initiation and progression of inflammation, autoimmunity, and cancer. In dermatology, recent studies indicate that EVs play key immunomodulatory roles in inflammatory skin disorders, including psoriasis, atopic dermatitis, lichen planus, bullous pemphigoid, systemic lupus erythematosus, and wound healing. Importantly, EVs can be used as biomarkers of pathophysiological states and/or therapeutic agents, both as carriers of drugs or even as a drug by themselves. In this review, we will summarize current research advances of EVs from different cells and their implications in inflammatory skin disorders, and further discuss their future applications, updated techniques, and challenges in clinical translational medicine.

Evolving story of autoantibodies in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by antinuclear antibody (ANA) production. ANAs bind to DNA, RNA and complexes of proteins and nucleic acids and are important markers for diagnosis and activity. According to current models, ANAs originate from antigen-driven processes; nevertheless, antibody responses to both DNA and RNA binding proteins display features unexpected in terms of current paradigms for antigenicity. These differences may reflect disturbances in both B and T cells critical for autoreactivity. Clinically, ANA testing has new uses for determining classification as well as assessing eligibility for clinical trials. Studies of patients with established disease show frequent seronegativity. In this setting, seronegativity may indicate a stage of disease called post-autoimmunity in which the natural history of disease or effects of immunosuppressive therapies modifies responses. The new uses of ANA testing highlight the importance of understanding autoantigenicity and developing sensitive and informative assays for clinical assessments.

Modulating Cytokine Production via Select Packaging and Secretion From Extracellular Vesicles

Cytokines are soluble factors that play vital roles in systemic function due to their ability to initiate and mediate cell-to-cell communication. Another important mechanism of intercellular communication that has gained significant attention in the past 10 years is the release of extracellular vesicles (EVs). EVs are released by all cells during normal physiology, in states of resting and activation, as well as during disease. Accumulating evidence indicates that cytokines may be packaged into EVs, and the packaging of cytokines into EVs, along with their ultimate secretion, may also be regulated by cytokines. Importantly, the repertoire of biomolecules packaged into EVs is shaped by the biological state of the cell (resting vs. activated and healthy vs. disease) and the EV biogenesis pathway involved, thus providing mechanisms by which EV packaging and secretion may be modulated. Given the critical role of cytokines in driving acute and chronic inflammatory and autoimmune diseases, as well as their role in establishing the tumor immune microenvironment, in this review, we will focus on these disease settings and summarize recent progress and mechanisms by which cytokines may be packaged within and modulated by EVs, as a therapeutic option for regulating innate and adaptive immunity.

The binding of SLE autoantibodies to mitochondria

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by immune complexes. Because these complexes contain mitochondrial components, we assessed the presence of antibodies to whole mitochondria (wMITO) using an ELISA in which mitochondria from mouse liver are bound to microtiter plates pre-coated with poly-l-lysine. Studies with this ELISA demonstrated that SLE plasmas contain abundant anti-wMITO activity. While digestion with DNase 1 did not affect anti-wMITO activity, adsorption of plasma on DNA affinity columns could reduce binding activity. Assay for anti-mitochondrial antibodies (AMA) by immunofluorescence and an ELISA with the M2 antigen (2-oxo-acid dehydrogenase protein complex) showed a low frequency of positivity, indicating that AMA and anti-wMITO are distinct specificities. In the study of 204 patients with SLE, the levels of anti-wMITO were higher in active SLE and correlated with levels of anti-DNA. These findings suggest that anti-wMITO can form immune complexes with mitochondria which may drive pathogenesis.

Chicken-or-egg question: Which came first, extracellular vesicles or autoimmune diseases?

Extracellular vesicles (EVs) have attracted great interest as contributors to autoimmune disease (AD) pathogenesis, owing to their immunomodulatory potential; they may also play a role in triggering tolerance disruption, by delivering auto‐antigens. EVs are released by almost all cell types, and afford paracrine or distal cell communication, functioning as biological carriers of active molecules including lipids, proteins, and nucleic acids. Depending on stimuli from the external microenvironment or on their cargo, EVs can promote or suppress immune responses. ADs are triggered by inappropriate immune‐system activation against the self, but their precise etiology is still poorly understood. Accumulating evidence indicates that lifestyle and diet have a strong impact on their clinical onset and development. However, to date the mechanisms underlying AD pathogenesis are not fully clarified, and reliable markers, which would provide early prediction and disease progression monitoring, are lacking. In this connection, EVs have recently been indicated as a promising source of AD biomarkers. Although EV isolation is currently based on differential centrifugation or density‐gradient ultracentrifugation, the resulting co‐isolation of contaminants (i.e., protein aggregates), and the pooling of all EVs in one sample, limit this approach to abundantly‐expressed EVs. Flow cytometry is one of the most promising methods for detecting EVs as biomarkers, and may have diagnostic applications. Furthermore, very recent findings describe a new method for identifying and sorting EVs by flow cytometry from freshly collected body fluids, based on specific EV surface markers.

Triggers of Autoimmunity: The Role of Bacterial Infections in the Extracellular Exposure of Lupus Nuclear Autoantigens

Infections are considered important environmental triggers of autoimmunity and can contribute to autoimmune disease onset and severity. Nucleic acids and the complexes that they form with proteins—including chromatin and ribonucleoproteins—are the main autoantigens in the autoimmune disease systemic lupus erythematosus (SLE). How these nuclear molecules become available to the immune system for recognition, presentation, and targeting is an area of research where complexities remain to be disentangled. In this review, we discuss how bacterial infections participate in the exposure of nuclear autoantigens to the immune system in SLE. Infections can instigate pro-inflammatory cell death programs including pyroptosis and NETosis, induce extracellular release of host nuclear autoantigens, and promote their recognition in an immunogenic context by activating the innate and adaptive immune systems. Moreover, bacterial infections can release bacterial DNA associated with other bacterial molecules, complexes that can elicit autoimmunity by acting as innate stimuli of pattern recognition receptors and activating autoreactive B cells through molecular mimicry. Recent studies have highlighted SLE disease activity-associated alterations of the gut commensals and the expansion of pathobionts that can contribute to chronic exposure to extracellular nuclear autoantigens. A novel field in the study of autoimmunity is the contribution of bacterial biofilms to the pathogenesis of autoimmunity. Biofilms are multicellular communities of bacteria that promote colonization during chronic infections. We review the very recent literature highlighting a role for bacterial biofilms, and their major components, amyloid/DNA complexes, in the generation of anti-nuclear autoantibodies and their ability to stimulate the autoreactive immune response. The best studied bacterial amyloid is curli, produced by enteric bacteria that commonly cause infections in SLE patients, including Escherichia coli and Salmonella spps. Evidence suggests that curli/DNA complexes can trigger autoimmunity by acting as danger signals, molecular mimickers, and microbial chaperones of nucleic acids.

Stimulation of Mononuclear Cells Through Toll-Like Receptor 9 Induces Release of Microvesicles Expressing Double-Stranded DNA and Galectin 3-Binding Protein in an Interferon-α-Dependent Manner

Background: Microvesicles (MVs) expressing the type 1 interferon (IFN)-inducible protein galectin-3 binding protein (G3BP) may play a pathogenic role in systemic lupus erythematosus (SLE). Co-expression of double-stranded DNA (dsDNA) on such MVs may render them immunogenic and targets for anti-dsDNA antibodies. Little is known about the mechanisms underlying generation of this MV population. In this study, we investigated how Toll-like receptors (TLRs), IFN-α, and T cells are involved in this process in healthy subjects.

Methods: Peripheral blood mononuclear cells (PBMCs) isolated from 12 healthy donors were stimulated in-vitro for 24 h with a series of TLR-agonists or the T cell activating antibody OKT3 or were subjected to apoptosis by incubation with staurosporine. MVs in the supernatants were subsequently isolated by differential centrifugation and were quantified and characterized with respect to expression of G3BP and dsDNA by flow cytometry.

Results: Stimulation of PBMCs with the TLR9-agonist and strong IFN-α inducer ODN2395 significantly increased the release of MVs expressing G3BP. The production of MVs with this phenotype was markedly enhanced by co-stimulation of T cells. Furthermore, dependency on IFN-α in the generation of G3BP-expressing MVs was indicated by a marked reduction following addition of the IFN-α inhibitor IFN alpha-IFNAR-IN-1 hydrochloride.

Conclusion: Release of G3BP-expressing MVs from healthy donor PBMCs is induced by stimulation of TLR9 in an IFN-α-dependent manner and is enhanced by co-stimulation of T cells.

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.

Sources of Pathogenic Nucleic Acids in Systemic Lupus Erythematosus

A hallmark of systemic lupus erythematosus (SLE), and several related autoimmune diseases, is the presence of autoantibodies against nucleic acids and nucleic acid-binding proteins, as well as elevated type I interferons (IFNs), which appear to be instrumental in disease pathogenesis. Here we discuss the sources and proposed mechanisms by which a range of cellular RNA and DNA species can become pathogenic and trigger the nucleic acid sensors that drive type I interferon production. Potentially SLE-promoting DNA may originate from pieces of chromatin, from mitochondria, or from reverse-transcribed cellular RNA, while pathogenic RNA may arise from mis-localized, mis-processed, ancient retroviral, or transposable element-derived transcripts. These nucleic acids may leak out from dying cells to be internalized and reacted to by immune cells or they may be generated and remain to be sensed intracellularly in immune or non-immune cells. The presence of aberrant DNA or RNA is normally counteracted by effective counter-mechanisms, the loss of which result in a serious type I IFN-driven disease called Aicardi-Goutières Syndrome. However, in SLE it remains unclear which mechanisms are most critical in precipitating disease: aberrant RNA or DNA, overly sensitive sensor mechanisms, or faulty counter-acting defenses. We propose that the clinical heterogeneity of SLE may be reflected, in part, by heterogeneity in which pathogenic nucleic acid molecules are present and which sensors and pathways they trigger in individual patients. Elucidation of these events may result in the recognition of distinct "endotypes" of SLE, each with its distinct therapeutic choices.

Extracellular vesicles in autoimmune vasculitis - Little dirts light the fire in blood vessels

Systemic vasculitis is diverse group of autoimmune disorders which are characterized by inflammation of blood vessel walls with deep aching and burning pain. Their underlying etiology and pathophysiology still remain poorly understood. Extracellular vesicles (EVs), including exosomes, microvesicles (MVs), and apoptotic bodies, are membrane vesicular structures that are released either during cell activation, or when cells undergo programmed cell death, including apoptosis, necroptosis, and pyroptosis. Although EVs were thought as cell dusts, but now they have been found to be potently active since they harbor bioactive molecules, such as proteins, lipids, nucleic acids, or multi-molecular complexes. EVs can serve as novel mediators for cell-to-cell communications by delivery bioactive molecules from their parental cells to the recipient cells. Earlier studies mainly focused on MVs budding from membrane surface. Recent studies demonstrated that EVs may also carry molecules from cytoplasm or even from nucleus of their parental cells, and these EVs may carry autoantigens and are important in vasculitis. EVs may play important roles in vasculitis through their potential pathogenic involvements in inflammation, autoimmune responses, procoagulation, endothelial dysfunction/damage, angiogenesis, and intimal hyperplasia. EVs have also been used as specific biomarkers for diagnostic use or disease severity monitoring. In this review, we have focused on the aspects of EV biology most relevant to the pathogenesis of vasculitis, discussed their perspective insights, and summarized the exist literature on EV relevant studies in vasculitis, therefore provides an integration of current knowledge regarding the novel role of EVs in systemic vasculitis.

The central role of nucleic acids in the pathogenesis of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease whose pathogenesis can be conceptualized by a model based on a central role for immune complexes (ICs) between antinuclear antibodies and nucleic acids. According to this model, ICs can promote pathogenesis by two main mechanisms: deposition in the tissue to incite local inflammation and interaction with cells of the innate immune system to stimulate the production of cytokines, most prominently type 1 interferon. The latter stimulation results from the uptake of DNA and RNA in the form of ICs into cells and subsequent signaling by internal nucleic acid sensors for DNA and RNA. These sensors are likely important for the response to intracellular infection, although they may also be triggered during cell stress or injury by DNA or RNA aberrantly present in the cytoplasm. For IC formation, a source of extracellular DNA and RNA is essential. The current model of SLE posits that cell death is the origin of the nucleic acids in the ICs and that impairment of clearance mechanisms increases the amount of nuclear material in the extracellular space. This model of SLE is important since it points to new approaches to therapy; agents targeting interferon or the interferon receptor are examples of therapeutic approaches derived from this model. Future studies will explore novel biomarkers to monitor the operation of these mechanisms and to elucidate other steps in pathogenesis that can be targeted for therapy.

Anti-mitochondrial autoantibodies in systemic lupus erythematosus and their association with disease manifestations

Mitochondria are organelles that govern energy supply and control cell death. Mitochondria also express bacterial features, such as the presence of inner membrane cardiolipin and a circular genome rich in hypomethylated CpG motifs. While mitochondrial extrusion by damaged organs or activated cells is thought to trigger innate immunity, it is unclear whether extracellular mitochondria also stimulate an adaptive immune response. We describe the development of novel assays to detect autoantibodies specific to two distinct components of the mitochondrion: the mitochondrial outer membrane and mitochondrial DNA. Antibodies to these two mitochondrial constituents were increased in both human and murine systemic lupus erythematosus (SLE), compared to controls, and were present at higher levels than in patients with antiphospholipid syndrome or primary biliary cirrhosis. In both bi- and multi-variate regression models, antibodies to mitochondrial DNA, but not whole mitochondria, were associated with increased anti-dsDNA antibodies and lupus nephritis. This study describes new and optimized methods for the assessment of anti-mitochondrial antibodies, and demonstrates their presence in both human and murine SLE. These findings suggest that different mitochondrial components are immunogenic in SLE, and support the concept that extracellular mitochondria may provide an important source of circulating autoantigens in SLE.

Microparticles as autoantigens in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by the production of antibodies to components of the cell nucleus (antinuclear antibodies or ANAs) and the formation of immune complexes with nuclear antigens. These complexes can drive pathogenesis by depositing in the tissue to incite inflammation or induce cytokine production by cells of the innate immune system. While ANAs can bind to purified nuclear molecules, nuclear autoantigens in vivo most likely exist attached to other molecules or embedded in larger structures. Among these structures, microparticles (MPs) are membrane bound vesicles that are released from dead and dying cells by a blebbing process; MPs can also be released during activation of platelets. The presence of MPs in the blood or tissue culture media can be assayed by flow cytometry on the basis of light scattering as well as binding of marker antibodies to identify the cell of origin. As shown by biochemical analyses, MPs contain an ensemble of intracellular components including nuclear, cytoplasmic and membrane molecules. Because of the display of these molecules on the particle surface or in an otherwise accessible form, ANAs, including anti-DNA, can bind to particles. Levels of MPs are increased in the blood of patients with SLE, with flow cytometry demonstrating the presence of IgG-containing particles. In addition to forming immune complexes, MPs can directly stimulate immune responses. Together, these findings suggest an important role of particles in the pathogenesis of SLE and their utility as biomarkers.

The characteristics and pivotal roles of triggering receptor expressed on myeloid cells-1 in autoimmune diseases

Triggering receptor expressed on myeloid cells-1 (TREM-1) engagement can directly trigger inflammation or amplify an inflammatory response by synergizing with TLRs or NLRs. Autoimmune diseases are a family of chronic systemic inflammatory disorders. The pivotal role of TREM-1 in inflammation makes it important to explore its immunological effects in autoimmune diseases. In this review, we summarize the structural and functional characteristics of TREM-1. Particularly, we discuss recent findings on TREM-1 pathway regulation in various autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), inflammatory bowel disease (IBD), type 1 diabetes (T1D), and psoriasis. This receptor may potentially be manipulated to alter the inflammatory response to chronic inflammation and possible therapies are explored in this review.

DNA as a self-antigen: nature and regulation

High-affinity antibodies to double-stranded DNA are a hallmark of systemic lupus erythematosus (SLE) and are thought to contribute to disease flares and tissue inflammation such as nephritis. Notwithstanding their clinical importance, major questions remain about the development and regulation of these pathogenic anti-DNA responses. These include the mechanisms that prevent anti-DNA responses in healthy subjects, despite the constant generation of self-DNA and the abundance of DNA-reactive B cells; the nature and physical form of antigenic DNA in SLE; the regulation of DNA availability as an antigen; and potential therapeutic strategies targeting the pathogenic DNA in SLE. This review summarizes current progress in these directions, focusing on the role of secreted DNases in the regulation of antigenic extracellular DNA.

Antibodies targeting circulating protective molecules in lupus nephritis: Interest as serological biomarkers

Lupus nephritis (LN) is one of the most frequent and severe manifestations of systemic lupus erythematosus (SLE), considered as the major predictor of poor prognosis. An early diagnosis of LN is a real challenge in the management of SLE and has an important implication in guiding treatments. In clinical practice, conventional parameters still lack sensitivity and specificity for detecting ongoing disease activity in lupus kidneys and early relapse of nephritis. LN is characterized by glomerular kidney injury, essentially due to deposition of immune complexes involving autoantibodies against cellular components and circulating proteins. One of the possible mechanisms of induction of autoantibodies in SLE is a defect in apoptotic cells clearance and subsequent release of intracellular autoantigens. Autoantibodies against soluble protective molecules involved in the uptake of dying cells, including complement proteins and pentraxins, have been described. In this review, we present the main autoantibodies found in LN, with a focus on the antibodies against these protective molecules. We also discuss their pathogenic role and conclude with their potential interest as serological biomarkers in LN.

Potential Involvement of Platelet-Derived Microparticles and Microparticles Forming Immune Complexes during Monocyte Activation in Patients with Systemic Lupus Erythematosus

Microparticles (MPs) are vesicles derived from the plasma membrane of different cells, are considered a source of circulating autoantigens, and can form immune complexes (MPs-ICs). The number of MPs and MPs-ICs increases in patients with systemic lupus erythematosus (SLE). MPs activate myeloid cells by inducing IL-6 and TNF-α in both SLE and other diseases. Therefore, we propose that the recognition of MPs-ICs by monocytes rather that MPs may define their phenotype and contribute to the inflammatory process in patients with SLE. Thus, the aims of this study were to evaluate the association among circulating MPs-ICs from different cell sources, alterations observed in monocyte subsets, and disease activity in patients with SLE and to establish whether monocytes bind and respond to MPs-ICs in vitro. Circulating MPs and monocyte subsets were characterized in 60 patients with SLE and 60 healthy controls (HCs) using multiparametric flow cytometry. Patients had higher MP counts and frequencies of MPs-CD41a + (platelet-derived) compared with HCs, regardless of disease activity. MPs from patients with SLE were C1q + and formed ICs with IgM and IgG. MPs-IgG + were positively correlated with active SLE (aSLE), whereas MPs-IgM + were negatively correlated. Most of the circulating total ICs-IgG + were located on MPs. The proportion and number of non-classical monocytes were significantly decreased in patients with SLE compared with HCs and in patients with aSLE compared with patients with the inactive disease. Non-classical monocytes obtained from patients with SLE exhibited increased levels of CD64 associated with MPs-IgG +, MPs-C1q +, total circulating ICs-IgG +, and disease activity. The direct effects of MPs and MPs-IgG + on monocytes were evaluated in cell culture. Monocytes from both HCs and patients bound to and internalized MPs and MPs-IgG + independent of CD64. These vesicles derived from platelets (PMPs), mainly PMPs-IgG +, activated monocytes in vitro and increased the expression of CD69, CD64, and pro-inflammatory cytokines such as IL-1β, TNF-α, and IFN-α. Therefore, MPs are one of the most representative sources of the total amount of circulating ICs-IgG + in patients with SLE. MPs-IgG + are associated with SLE activity, and PMPs-IgG + stimulate monocytes, changing their phenotype and promoting pro-inflammatory responses related to disease activity.

From blood coagulation to innate and adaptive immunity: the role of platelets in the physiology and pathology of autoimmune disorders

Thrombosis and cardiovascular complications are common manifestations of a variety of pathological conditions, including infections and chronic inflammatory diseases. Hence, there is great interest in determining the hitherto unforeseen immune role of the main blood coagulation executor-the platelet. Platelets store and release a plethora of immunoactive molecules, generate microparticles, and interact with cells classically belonging to the immune system. The observed effects of platelet involvement in immune processes, especially in autoimmune diseases, are conflicting-from inciting inflammation to mediating its resolution. An in-depth understanding of the role of platelets in inflammation and immunity could open new therapeutic pathways for patients with autoimmune disorders. This review aims to summarize the current knowledge on the role of platelets in the patomechanisms of autoimmune disorders and suggests directions for future research.

Polymer-Mediated Inhibition of Pro-invasive Nucleic Acid DAMPs and Microvesicles Limits Pancreatic Cancer Metastasis

Nucleic acid binding polymers (NABPs) have been extensively used as vehicles for DNA and RNA delivery. More recently, we discovered that a subset of these NABPs can also serve as anti-inflammatory agents by capturing pro-inflammatory extracellular nucleic acids and associated protein complexes that promote activation of toll-like receptors (TLRs) in diseases such as lupus erythematosus. Nucleic-acid-mediated TLR signaling also facilitates tumor progression and metastasis in several cancers, including pancreatic cancer (PC). In addition, extracellular DNA and RNA circulate on or within lipid microvesicles, such as microparticles or exosomes, which also promote metastasis by inducing pro-tumorigenic signaling in cancer cells and pre-conditioning secondary sites for metastatic establishment. Here, we explore the use of an NABP, the 3^rd generation polyamidoamine dendrimer (PAMAM-G3), as an anti-metastatic agent. We show that PAMAM-G3 not only inhibits nucleic-acid-mediated activation of TLRs and invasion of PC tumor cells in vitro, but can also directly bind extracellular microvesicles to neutralize their pro-invasive effects as well. Moreover, we demonstrate that PAMAM-G3 dramatically reduces liver metastases in a syngeneic murine model of PC. Our findings identify a promising therapeutic application of NABPs for combating metastatic disease in PC and potentially other malignancies.

Antigen Presentation by Extracellular Vesicles from Professional Antigen-Presenting Cells

The initiation and maintenance of adaptive immunity require multifaceted modes of communication between different types of immune cells, including direct intercellular contact, secreted soluble signaling molecules, and extracellular vesicles (EVs). EVs can be formed as microvesicles directly pinched off from the plasma membrane or as exosomes secreted by multivesicular endosomes. Membrane receptors guide EVs to specific target cells, allowing directional transfer of specific and complex signaling cues. EVs are released by most, if not all, immune cells. Depending on the type and status of their originating cell, EVs may facilitate the initiation, expansion, maintenance, or silencing of adaptive immune responses. This review focusses on EVs from professional antigen-presenting cells, their demonstrated and speculated roles, and their potential for cancer immunotherapy.