Complement activation and regulation in rheumatic disease

Unveiling Actin Cytoskeleton Role in Mediating Chikungunya-Associated Arthritis: An Integrative Proteome-Metabolome Study

Background: Chikungunya is a zoonotic disease caused by the Chikungunya virus (CHIKV), primarily transmitted to humans through infected Aedes mosquitoes. The infection is characterized by symptoms such as high fever, musculoskeletal pain, polyarthritis, and a rash, which can lead to severe complications such as encephalitis, meningitis, and even fatalities. While many disease manifestations resemble those of other viral infections, chronic arthritis caused by CHIKV is unique, and its molecular mechanisms remain ill-defined. Materials and Methods: Proteomics data from both cellular and patient levels of CHIKV infection were curated from PubMed and screened using inclusion and exclusion criteria. Patient serum proteomics data obtained from PRIDE underwent reanalysis using Proteome Discoverer 2.2. Enrichment and protein-protein interaction network analysis were conducted on differentially expressed proteins from both serum and cellular datasets. Metabolite data from CHIKV-infected patients were further retrieved, and their protein binding partners were identified using BindingDB. The protein-metabolite interaction pathway was further developed using MetaboAnalyst. Results: The proteomics data analysis revealed differential expression of proteins involved in critical host mechanisms, such as cholesterol metabolism and mRNA splicing, during CHIKV infection. Consistent upregulation of two actin cytoskeleton proteins, TAGLN2 and PFN1, was noted in both serum and cellular datasets, and their upregulations are associated with arthritis. Furthermore, alterations in purine metabolism were observed in the integrative proteome-metabolome analysis, correlating with cytoskeletal remodelling. Conclusion: Collectively, this integrative view sheds light on the involvement of actin cytoskeleton remodeling proteins and purine metabolic pathways in the development of arthritis during CHIKV infection.

Human anti-C1q autoantibodies bind specifically to solid-phase C1q and enhance phagocytosis but not complement activation

Autoantibodies directed against complement component C1q are commonly associated with autoimmune diseases, especially systemic lupus erythematosus. Importantly, these anti-C1q autoantibodies are specific for ligand-bound, solid-phase C1q and do not bind to fluid-phase C1q. In patients with anti-C1q, C1q levels are in the normal range, and the autoantibodies are thus not depleting. To study these human anti-C1q autoantibodies at the molecular level, we isolated C1q-reactive B cells and recombinantly produced nine monoclonal antibodies (mAbs) from four different healthy individuals. The isolated mAbs were of the IgG isotype, contained extensively mutated variable domains, and showed high affinity to the collagen-like region of C1q. The anti-C1q mAbs exclusively bound solid-phase C1q in complex with its natural ligands, including immobilized or antigen-bound IgG, IgM or CRP, and necrotic cells. Competition experiments reveal that at least 2 epitopes, also targeted by anti-C1q antibodies in sera from SLE patients, are recognized. Electron microscopy with hexameric IgG-C1q immune complexes demonstrated that multiple mAbs can interact with a single C1q molecule and identified the region of C1q targeted by these mAbs. The opsonization of immune complexes with anti-C1q greatly enhanced Fc-receptor-mediated phagocytosis but did not increase complement activation. We conclude that human anti-C1q autoantibodies specifically bind neo-epitopes on solid-phase C1q, which results in an increase in Fc-receptor-mediated effector functions that may potentially contribute to autoimmune disease immunopathology.

Modelling and analysis of the complement system signalling pathways: roles of C3, C5a and pro-inflammatory cytokines in SARS-CoV-2 infection

The complement system is an essential part of innate immunity. It is activated by invading pathogens causing inflammation, opsonization, and lysis via complement anaphylatoxins, complement opsonin's and membrane attack complex (MAC), respectively. However, in SARS-CoV-2 infection overactivation of complement system is causing cytokine storm leading to multiple organs damage. In this study, the René Thomas kinetic logic approach was used for the development of biological regulatory network (BRN) to model SARS-CoV-2 mediated complement system signalling pathways. Betweenness centrality analysis in cytoscape was adopted for the selection of the most biologically plausible states in state graph. Among the model results, in strongly connected components (SCCs) pro-inflammatory cytokines (PICyts) oscillatory behaviour between recurrent generation and downregulation was found as the main feature of SARS-CoV-2 infection. Diversion of trajectories from the SCCs leading toward hyper-inflammatory response was found in agreement with in vivo studies that overactive innate immunity response caused PICyts storm during SARS-CoV-2 infection. The complex of negative regulators FI, CR1 and DAF in the inhibition of complement peptide (C5a) and PICyts was found desirable to increase immune responses. In modelling role of MAC and PICyts in lowering of SARS-CoV-2 titre was found coherent with experimental studies. Intervention in upregulation of C5a and PICyts by C3 was found helpful in back-and-forth variation of signalling pattern linked with the levels of PICyts. Moreover, intervention in upregulation of PICyts by C5a was found productive in downregulation of all activating factors in the normal SCCs. However, the computational model predictions require experimental studies to be validated by exploring the activation role of C3 and C5a which could change levels of PICyts at various phases of SARS-CoV-2 infection.

Overview on the Link Between the Complement System and Auto-Immune Articular and Pulmonary Disease

Complement system (CS) dysregulation is a key factor in the pathogenesis of different autoimmune diseases playing a central role in many immune innate and adaptive processes. Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by ta breach of self-tolerance leading to a synovitis and extra-articular manifestations. The CS is activated in RA and seems not only to mediate direct tissue damage but also play a role in the initiation of RA pathogenetic mechanisms through interactions with citrullinated proteins. Interstitial lung disease (ILD) represents the most common extra-articular manifestation that can lead to progressive fibrosis. In this review, we focused on the evidence of CS dysregulation in RA and in ILD, and highlighted the role of the CS in both the innate and adaptive immune responses in the development of diseases, by using idiopathic pulmonary fibrosis as a model of lung disease. As a proof of concept, we dissected the evidence that several treatments used to treat RA and ILD such as glucocorticoids, pirfenidone, disease modifying antirheumatic drugs, targeted biologics such as tumor necrosis factor (TNF)-inhibitors, rituximab, tocilizumab, and nintedanib may act indirectly on the CS, suggesting that the CS might represent a potential therapeutic target in these complex diseases.

Complosome - the intracellular complement system

The complement system is a recognized pillar of host defence against infection and noxious self-derived antigens. Complement is traditionally known as a serum-effective system, whereby the liver expresses and secretes most complement components, which participate in the detection of bloodborne pathogens and drive an inflammatory reaction to safely remove the microbial or antigenic threat. However, perturbations in normal complement function can cause severe disease and, for reasons that are currently not fully understood, the kidney is particularly vulnerable to dysregulated complement activity. Novel insights into complement biology have identified cell-autonomous and intracellularly active complement - the complosome - as an unexpected central orchestrator of normal cell physiology. For example, the complosome controls mitochondrial activity, glycolysis, oxidative phosphorylation, cell survival and gene regulation in innate and adaptive immune cells, and in non-immune cells, such as fibroblasts and endothelial and epithelial cells. These unanticipated complosome contributions to basic cell physiological pathways make it a novel and central player in the control of cell homeostasis and effector responses. This discovery, together with the realization that an increasing number of human diseases involve complement perturbations, has renewed interest in the complement system and its therapeutic targeting. Here, we summarize the current knowledge about the complosome across healthy cells and tissues, highlight contributions from dysregulated complosome activities to human disease and discuss potential therapeutic implications.

Effects of Cannabidiol on Innate Immunity: Experimental Evidence and Clinical Relevance

Cannabidiol (CBD) is the main non-psychotropic cannabinoid derived from cannabis (Cannabis sativa L., fam. Cannabaceae). CBD has received approval by the Food and Drug Administration (FDA) and European Medicines Agency (EMA) for the treatment of seizures associated with Lennox-Gastaut syndrome or Dravet syndrome. However, CBD also has prominent anti-inflammatory and immunomodulatory effects; evidence exists that it could be beneficial in chronic inflammation, and even in acute inflammatory conditions, such as those due to SARS-CoV-2 infection. In this work, we review available evidence concerning CBD's effects on the modulation of innate immunity. Despite the lack so far of clinical studies, extensive preclinical evidence in different models, including mice, rats, guinea pigs, and even ex vivo experiments on cells from human healthy subjects, shows that CBD exerts a wide range of inhibitory effects by decreasing cytokine production and tissue infiltration, and acting on a variety of other inflammation-related functions in several innate immune cells. Clinical studies are now warranted to establish the therapeutic role of CBD in diseases with a strong inflammatory component, such as multiple sclerosis and other autoimmune diseases, cancer, asthma, and cardiovascular diseases.

Risk of adverse pregnancy outcomes prior to the onset of an autoimmune rheumatic disease: a systematic review

OBJECTIVES

An increased risk of adverse maternal and foetal pregnancy complications (including pre-eclampsia, intrauterine growth restriction, and small for gestational age) is well described in women with autoimmune rheumatic disease (ARD) compared with the general population (GenPop). It is less clear, however, whether this risk of adverse pregnancy outcome (APO) also exists in women with 'preclinical ARD' (pre-ARD) before they are diagnosed with an ARD many years post-partum. Therefore, we have undertaken a systematic review of the available evidence on APO in patients who subsequently were diagnosed with a rheumatic disease to identify whether there is an increased risk in pre-ARD.

METHODS

The present study was reported in accordance with the guidance of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) standard. A systematic literature review was performed using the online PubMed database. Pre-SLE and pre-RA patients were defined as those who, over the subsequent years, developed SLE or RA according to international classification criteria.

RESULTS

A total of 176 articles were screened, and 27 original articles were selected for final analysis. Pre-RA was the most studied group, with 15 studies and a total of >1600 pregnancies, and pre-SLE was the second-most studied pre-ARD in pregnancy, with 14 studies and a total of >1000 pregnancies. We found that patients who subsequently developed SLE had an increased burden of poor pregnancy outcomes compared with pregnant women from the GenPop, but fewer APOs compared with pregnancies of women with SLE. In contrast, a similar rate of APOs was found when pre-RA pregnancies were compared with GenPop pregnancies.

CONCLUSION

Our findings of an increased risk of APO in certain pre-ARDs highlights the relevance of taking an obstetric history during the first rheumatology appointment and the need for novel screening strategies for the prediction of APOs. Further research is required to elucidate the immune basis of APOs in preclinical and clinical ARD.

The contribution of the alternative pathway in complement activation on cell surfaces depends on the strength of classical pathway initiation

Objectives

The complement system is an important component of innate immunity. The alternative pathway (AP) amplification loop is considered an essential feed forward mechanism for complement activation. However, the role of the AP in classical pathway (CP) activation has only been studied in ELISA settings. Here, we investigated its contribution on physiologically relevant surfaces of human cells and bacterial pathogens and in antibody-mediated complement activation, including in autoimmune haemolytic anaemia (AIHA) setting with autoantibodies against red blood cells (RBCs).

Methods

We evaluated the contribution of the AP to complement responses initiated through the CP on human RBCs by serum of AIHA patients and recombinant antibodies. Moreover, we studied complement activation on Neisseria meningitidis and Escherichia coli. The effect of the AP was examined using either AP-depleted sera or antibodies against factor B and factor D.

Results

We show that the amplification loop is redundant when efficient CP activation takes place. This is independent of the presence of membrane-bound complement regulators. The role of the AP may become significant when insufficient CP complement activation occurs, but this depends on antibody levels and (sub)class. Our data indicate that therapeutic intervention in the amplification loop will most likely not be effective to treat antibody-mediated diseases.

Conclusion

The AP can be bypassed through efficient CP activation. The AP amplification loop has a role in complement activation during conditions of modest activation via the CP, when it can allow for efficient complement-mediated killing.

The general law of plasma proteome alterations occurring in the lifetime of Chinese individuals reveals the importance of immunity

Background: Aging is characterized by a continuous loss of protein homeostasis. A closer examination of peripheral blood, which houses proteins from nearly all tissues and cells, helped identify several biomarkers and other aspects of aging biology. To further explore the general law of aging and identify key time nodes and associated aging biology, we collected 97 plasma samples from 253 healthy individuals aged 0-100 years without adverse outcomes to conduct nano-Ultra High Performance Liquid Chromatography-Mass Spectrometry/Mass Spectrometry (nano-UHPLC-MS/MS) and weighted gene co-expression network analysis (WGCNA).

Results: Through biological processes and key biological pathways identified in discrete age group modules, our analyses highlighted a strong correlation between alterations in the immune system and aging process. We also identified hub genes associated with distinct age groups that revealed alterations not only in protein expression but also in signaling cascade. Among them, hub genes from age groups of 0-20 years old and 71-100 years old are mostly involved in infectious diseases and the immune system. In addition, CDC5L and HMGB2 were the key transcription factors (TFs) regulating genes expression in people aged between 51-60 and 71-100 years of age. They were shown to not only be independent but also mutually regulate certain hub gene expressions.

Conclusions: This study reveals that the plasma proteome undergoes a complex alteration over the lifetime of a human. In this process, the immune system is crucial throughout the lifespan of a human being. However, the underlying mechanism(s) regulating differential protein expressions at distinct ages remains to be elucidated.

The complement system and autoimmune diseases

The complement system plays a key role in the pathogenesis of autoimmune diseases, which usually injures the kidney. More and more studies have shown the pathogenic role and indicated that abnormal activation of the complement system was highly involved in the outbreak of autoimmune diseases. This review mainly introduced recent studies of complement system activation contributing to the pathogenesis of autoimmune diseases, including systemic lupus erythematosus, antiphospholipid syndrome, antineutrophil cytoplasmic antibody‐associated vasculitides, and so on. Understanding the pathogenic roles of complement activation in various autoimmune diseases will identify potential novel therapeutic targets on complement systems.

The complement system is important in the pathogenesis of a group of autoimmune diseases.

Targeting the complement system might provide us with novel target therapies in autoimmune diseases.

Patients with MPNs and retinal drusen show signs of complement system dysregulation and a high degree of chronic low-grade inflammation

BACKGROUND

The hematopoietic stem cell disorders, myeloproliferative neoplasms (MPNs), are characterised by chronic low-grade inflammation (CLI). Recently, we showed that patients with MPNs have an increased prevalence of drusen and age-related macular degeneration (AMD), and drusen prevalence seemed associated with higher CLI. Studying MPNs may reveal more about drusen pathophysiology. This study investigated CLI further by measuring cytokine levels and complement system markers, comparing these between patients with MPNs and AMD.

METHODS

This cross-sectional study, between July 2018 and November 2020 conducted at Zealand University Hospital (ZUH) - Roskilde, Denmark, included 29 patients with neovascular AMD (nAMD), 28 with intermediate-stage AMD (iAMD), 62 with MPNs (35 with drusen - MPNd and 27 with healthy retinas - MPNn). With flow cytometry, we measured complement-regulatory-proteins (Cregs). With immunoassays, we investigated cytokine levels combined into a summary-inflammation-score (SIS).

FINDINGS

The MPNd and nAMD groups had similar SIS, significantly higher than the MPNn and iAMD groups. Additionally, we found SIS to increase over the MPN biological continuum from early cancer stage, essential thrombocytaemia (ET), over polycythaemia vera (PV) to the late-stage primary myelofibrosis (PMF). MPNs showed signs of complement dysregulation, with Cregs expression lower in PV than ET and PMF and even lower in PV patients with drusen.

INTERPRETATION

This study suggests that MPNd have a higher CLI than MPNn and may indicate systemic CLI to play a greater part in, and even initiate drusen formation. We suggest using MPNs as a "Human Inflammation Model" of drusen development. The CLI in MPNs elicits drusen formation, triggering more CLI creating a vicious cycle, increasing the risk of developing AMD.

FUNDING

Fight for Sight, Denmark, and Region Zealand's research promotion fund.

Proteomic analyses of plasma-derived exosomes in immunoglobulin (Ig) G4-related disease and their potential roles in B cell differentiation and tissue damage

OBJECTIVE

To investigate the proteomic profiles of plasma exosomes isolated from patients with immunoglobulin (Ig) G4-related disease (IgG4-RD) and to determine their potential roles in B cell differentiation and tissue damage.

METHODS

One hundred untreated IgG4-RD patients and 135 sex- and age-matched healthy controls (HCs) were enrolled in this study. A combination of liquid chromatography-tandem mass spectrometry (LC-MS/MS) and tandem mass tag (TMT)-label quantitation was used for proteomic profiling. Differentially expressed proteins were validated by Western blot, enzyme-linked immunosorbent assay (ELISA) and real-time quantitative PCR (RT-qPCR) analyses. B cell activation, apoptosis, differentiation and reactive oxygen species (ROS) production were analyzed by flow cytometry. We also analyzed the correlations between differentially expressed complement proteins and laboratory parameters.

RESULTS

A total of 178 differentially expressed proteins were identified in plasma exosomes in IgG4-RD patients compared with HCs, and these proteins were enriched predominantly in the complement cascade pathway. Furthermore, reduced expression levels of complement components C3 and C5 in IgG4-RD were correlated with clinical parameters. Following stimulation with IgG4-RD plasma exosomes, the percentages of naïve B cells decreased, while those of memory B cells and plasmablasts increased; the levels of cytochrome c, somatic (CYCS) and downstream complement system activation also increased. Moreover, ROS production was greater in B cells of IgG4-RD patients than in those of HCs. In affected submandibular glands, the BCR signalling pathway was activated, and exosomes were enriched.

CONCLUSION

Proteomic profiling revealed that plasma exosome proteins may participate in the pathogenesis of IgG4-RD through complement activation and may be involved in B cell differentiation and activation of the B cell auto-oxidative damage pathway.

Ex Vivo Complement Activation on Endothelial Cells: Research and Translational Value

The spectrum of human diseases with complement contribution is ever increasing. Tools to study the complement contribution and the potential interest of novel complement inhibitors in clinical practice are lacking. Here we discuss a functional ex vivo assay to monitor complement activation on endothelial cells, which can answer to this need.

Therapeutic Efficacy of a Trichinella Spiralis Paramyosin-Derived Peptide Modified With a Membrane-Targeting Signal in Mice With Antigen-Induced Arthritis

Helminth-derived molecules have the ability to modulate the host immune system. Our previous study identified a tetradecapeptide derived from Trichinella spiralis paramyosin (Ts-pmy) that could bind to human complement component C9 to inhibit its polymerization, making the peptide a candidate therapeutic agent for complement-related immune disorders. Here, the peptide underwent an N-terminal modification with a membrane-targeting signal (a unique myristoylated peptide) to improve its therapeutic efficacy. We found that the modified peptide had a binding affinity to human C9 that was similar to that of the original peptide, as confirmed by microscale thermophoresis assays. The binding of the modified peptide to human C9 resulted in the inhibition of C9-related complement activation, as reflected by the decreased Zn²⁺-induced C9 polymerization and the decreased C9-dependent lysis of rabbit erythrocytes. In addition, the original and modified peptides could both bind to recombinant mouse C9 and inhibit the C9-dependent lysis of rabbit erythrocytes in normal mouse serum (NMS), which meant that the peptides could cross the species barrier to inhibit complement activity in mice. Further in vitro and in vivo analyses confirmed that the peptide modification increased the retention time of the peptide. Furthermore, intraarticular injection of the modified peptide markedly ameliorated knee swelling and joint damage in mice with antigen-induced arthritis (AIA), as assessed histologically. These results suggested that the Ts-pmy-derived peptide modified with a membrane-targeting signal was a reasonable candidate therapeutic agent for membrane attack complex (MAC)-related diseases [such as rheumatoid arthritis (RA)] and the study presented a new modification method to improve the potential therapeutic effects of the peptide.

[Thrombotic microangiopathy in rheumatology: a link between thrombosis and autoimmunity]

Uncontrolled hypercoagulation and inflammation (thromboinflammation), which are both independent and closely related and amplifying each other pathological processes, form the basis for pathogenesis of a wide range of diseases and complications, including immuno-inflammatory (autoimmune) rheumatic diseases, with the development of potentially fatal injuries of internal organs. Thrombotic microangiopathy is one of the most prominent prototypes of thromboinflammatory pathological conditions. The close link between environmental factors, hemostasis genetic defects and the complement system, inflammation and autoimmunity as pathogenetic mechanisms of microthrombosis draws particular attention to studying thrombotic microangiopathy in immuno-inflammatory rheumatic diseases, primarily systemic lupus erythematosus, antiphospholipid syndrome and scleroderma renal crisis. In future, these studies may be important for expanding the idea of the role of autoimmune mechanisms in pathogenesis of critical hemostasis disorders in human diseases, and for developing new approaches to therapy. Recently, special attention has been paid to the treatment of systemic lupus erythematosus and antiphospholipid syndrome with eculizumab, which is humanized monoclonal IgG2/4k antibody that blocks the complement component C5a and the membrane attack complex (C5b-9) formation, and which is registered for the treatment of atypical hemolytic uremic syndrome, paroxysmal nocturnal hemoglobinuria, as well as severe forms of myasthenia gravis and neuromyelitis optica. Further studies in this direction will create prerequisites for improving the prognosis not only in patients with orphan disorders, but also for widespread human diseases.

Complement Receptor 2 Based Immunoassay Measuring Activation of the Complement System at C3-Level in Plasma Samples From Mice and Humans

We aimed at establishing a sensitive and robust assay for estimation of systemic complement activation at complement component C3 level in mouse and human plasma samples. In order to capture the activation products iC3b and C3dg in a specific and physiological relevant manner we utilized a construct consisting of the iC3b/C3dg-binding site of human complement receptor 2 (CR2) attached to an Fc-part of mouse IgG. This construct binds C3dg and iC3b from both mice and humans. We purified the CR2-IgG construct from mouse B myeloma cell line supernatants, J558L-CR2-IgG, by protein G affinity chromatography. The CR2-IgG construct was used for capturing C3 fragments in microtiter wells and an anti-mouse or an anti-human-C3 antibody was used for detection of bound C3 fragments. Initially we tested the specificity of the assays with the use of purified C3 fragments. Further, with the use of the CR2-based assay, we measured an up to three-fold higher signal in activated mouse serum as compared to non-activated mouse serum, whereas activated serum from a C3 knock-out mouse gave no signal. We tested in vivo generated samples from a mouse experiment; complement activation was induced by injecting cobra venom factor or heat aggregated IgG into C57bl6 mice, followed by withdrawal of EDTA blood samples at different time points and measurement of iC3b/C3dg. We observed a clear time-dependent distinction in signals between samples with expected high and low complement activation. Furthermore, with the use of the assay for human C3 fragments, we observed that patients with systemic lupus erythematosus (SLE) (n = 144) had significantly higher iC3b/C3dg levels as compared to healthy individuals (n = 144) (p < 0.0001). We present two functional immunoassays, that are able to measure systemic levels of the C3-activation products iC3b and C3dg in mice and humans. To our knowledge, these are the first assays for complement activation that use a physiological relevant capture construct such as CR2. These assays will be a relevant tool when investigating mouse models and human diseases involving the complement system.

Inflammation in Hypertension

For more than 50 years, evidence has accumulated that inflammation contributes to the pathogenesis of hypertension. Immune cells have been observed in vessels and kidneys of hypertensive humans. Biomarkers of inflammation, including high sensitivity C-reactive protein, various cytokines, and products of the complement pathway are elevated in humans with hypertension. Emerging evidence suggests that hypertension is accompanied and indeed initiated by activation of complement, the inflammasome, and by a change in the phenotype of circulating immune cells, particularly myeloid cells. High-dimensional transcriptomic analyses are providing insight into new subclasses of immune cells that are likely injurious in hypertension. These inflammatory events are interdependent and there is ultimately engagement of the adaptive immune system through mechanisms involving oxidative stress, modification of endogenous proteins, and alterations in antigen processing and presentation. These observations suggest new therapeutic opportunities to reduce end organ damage in hypertension might be used and guided by levels of inflammatory biomarkers.