C4b-binding protein binds to necrotic cells and DNA, limiting DNA release and inhibiting complement activation

Joint Hypermobility Syndrome and Membrane Proteins: A Comprehensive Review

Ehlers-Danlos syndromes (EDSs) constitute a heterogeneous group of connective tissue disorders characterized by joint hypermobility, skin hyperextensibility, and tissue fragility. Asymptomatic EDSs, joint hypermobility without associated syndromes, EDSs, and hypermobility spectrum disorders are the commonest phenotypes associated with joint hypermobility. Joint hypermobility syndrome (JHS) is a connective tissue disorder characterized by extreme flexibility of the joints, along with pain and other symptoms. JHS can be a sign of a more serious underlying genetic condition, such as EDS, which affects the cartilage, bone, fat, and blood. The exact cause of JHS could be related to genetic changes in the proteins that add flexibility and strength to the joints, ligaments, and tendons, such as collagen. Membrane proteins are a class of proteins embedded in the cell membrane and play a crucial role in cell signaling, transport, and adhesion. Dysregulated membrane proteins have been implicated in a variety of diseases, including cancer, cardiovascular disease, and neurological disorders; recent studies have suggested that membrane proteins may also play a role in the pathogenesis of JHS. This article presents an exploration of the causative factors contributing to musculoskeletal pain in individuals with hypermobility, based on research findings. It aims to provide an understanding of JHS and its association with membrane proteins, addressing the clinical manifestations, pathogenesis, diagnosis, and management of JHS.

Extracellular vesicle biomarkers for complement dysfunction in schizophrenia

Schizophrenia, a complex neuropsychiatric disorder, frequently experiences a high rate of misdiagnosis due to subjective symptom assessment. Consequently, there is an urgent need for innovative and objective diagnostic tools. In this study, we used cutting-edge extracellular vesicles' (EVs) proteome profiling and XGBoost-based machine learning to develop new markers and personalized discrimination scores for schizophrenia diagnosis and prediction of treatment response. We analysed plasma and plasma-derived EVs from 343 participants, including 100 individuals with chronic schizophrenia, 34 first-episode and drug-naïve patients, 35 individuals with bipolar disorder, 25 individuals with major depressive disorder and 149 age- and sex-matched healthy controls. Our innovative approach uncovered EVs-based complement changes in patients, specific to their disease-type and status. The EV-based biomarkers outperformed their plasma counterparts, accurately distinguishing schizophrenia individuals from healthy controls with an area under curve (AUC) of 0.895, 83.5% accuracy, 85.3% sensitivity and 82.0% specificity. Moreover, they effectively differentiated schizophrenia from bipolar disorder and major depressive disorder, with AUCs of 0.966 and 0.893, respectively. The personalized discrimination scores provided a personalized diagnostic index for schizophrenia and exhibited a significant association with patients' antipsychotic treatment response in the follow-up cohort. Overall, our study represents a significant advancement in the field of neuropsychiatric disorders, demonstrating the potential of EV-based biomarkers in guiding personalized diagnosis and treatment of schizophrenia.

The Complement System and C4b-Binding Protein: A Focus on the Promise of C4BPα as a Biomarker to Predict Clopidogrel Resistance

The complement system plays a dual role in the body, either as a first-line defense barrier when balanced between activation and inhibition or as a potential driver of complement-associated injury or diseases when unbalanced or over-activated. C4b-binding protein (C4BP) was the first circulating complement regulatory protein identified and it functions as an important complement inhibitor. C4BP can suppress the over-activation of complement components and prevent the complement system from attacking the host cells through the binding of complement cleavage products C4b and C3b, working in concert as a cofactor for factor I in the degradation of C4b and C3b, and consequently preventing or reducing the assembly of C3 convertase and C5 convertase, respectively. C4BP, particularly C4BP α-chain (C4BPα), exerts its unique inhibitory effects on complement activation and opsonization, systemic inflammation, and platelet activation and aggregation. It has long been acknowledged that crosstalk or interplay exists between the complement system and platelets. Our unpublished preliminary data suggest that circulating C4BPα exerts its antiplatelet effects through inhibition of both complement activity levels and complement-induced platelet reactivity. Plasma C4BPα levels appear to be significantly higher in patients sensitive to, rather than resistant to, clopidogrel, and we suggest that a plasma C4BPα measurement could be used to predict clopidogrel resistance in the clinical settings.

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.

Urinary Protein-Biomarkers Reliably Indicate Very Early Kidney Damage in Children With Alport Syndrome Independently of Albuminuria and Inflammation

Introduction

Alport syndrome (AS) is a hereditary type IV collagen disease. It starts shortly after birth, without clinical symptoms, and progresses to end-stage kidney disease early in life. The earlier therapy starts, the more effectively end-stage kidney disease can be delayed. Clearly then, to ensure preemptive therapy, early diagnosis is an essential prerequisite.

Methods

To provide early diagnosis, we searched for protein biomarkers (BMs) by mass spectrometry in dogs with AS stage 0. At this very early stage, we identified 74 candidate BMs. Of these, using commercial enzyme-linked immunosorbent assays (ELISAs), we evaluated 27 in dogs and 28 in children, 50 with AS and 104 healthy controls.

Results

Most BMs from blood appeared as fractions of multiple variants of the same protein, as shown by their chromatographic distribution before mass spectrometry. Blood samples showed only minor differences because ELISAs rarely detect disease-specific variants. However, in urine , several proteins, individually or in combination, were promising indicators of very early and preclinical kidney injury. The BMs with the highest sensitivity and specificity were collagen type XIII, hyaluronan binding protein 2 (HABP2), and complement C4 binding protein (C4BP).

Conclusion

We generated very strong candidate BMs by our approach of first examining preclinical AS in dogs and then validating these BMs in children at early stages of disease. These BMs might serve for screening purposes for AS before the onset of kidney damage and therefore allow preemptive therapy.

Diverse Functions of C4b-Binding Protein in Health and Disease

C4b-binding protein (C4BP) is a fluid-phase complement inhibitor that prevents uncontrolled activation of the classical and lectin complement pathways. As a complement inhibitor, C4BP also promotes apoptotic cell death and is hijacked by microbes and tumors for complement evasion. Although initially characterized for its role in complement inhibition, there is an emerging recognition that C4BP functions in a complement-independent manner to promote cell survival, protect against autoimmune damage, and modulate the virulence of microbial pathogens. In this Brief Review, we summarize the structure and functions of human C4BP, with a special focus on activities that extend beyond the canonical role of C4BP in complement inhibition.

C4b-binding protein inhibits particulate- and crystalline-induced NLRP3 inflammasome activation

Dysregulated NLRP3 inflammasome activation drives a wide variety of diseases, while endogenous inhibition of this pathway is poorly characterised. The serum protein C4b-binding protein (C4BP) is a well-established inhibitor of complement with emerging functions as an endogenously expressed inhibitor of the NLRP3 inflammasome signalling pathway. Here, we identified that C4BP purified from human plasma is an inhibitor of crystalline- (monosodium urate, MSU) and particulate-induced (silica) NLRP3 inflammasome activation. Using a C4BP mutant panel, we identified that C4BP bound these particles via specific protein domains located on the C4BP α-chain. Plasma-purified C4BP was internalised into MSU- or silica-stimulated human primary macrophages, and inhibited MSU- or silica-induced inflammasome complex assembly and IL-1β cytokine secretion. While internalised C4BP in MSU or silica-stimulated human macrophages was in close proximity to the inflammasome adaptor protein ASC, C4BP had no direct effect on ASC polymerisation in in vitro assays. C4BP was also protective against MSU- and silica-induced lysosomal membrane damage. We further provide evidence for an anti-inflammatory function for C4BP in vivo, as C4bp^-/- mice showed an elevated pro-inflammatory state following intraperitoneal delivery of MSU. Therefore, internalised C4BP is an inhibitor of crystal- or particle-induced inflammasome responses in human primary macrophages, while murine C4BP protects against an enhanced inflammatory state in vivo. Our data suggests C4BP has important functions in retaining tissue homeostasis in both human and mice as an endogenous serum inhibitor of particulate-stimulated inflammasome activation.

Dysregulation of Protein S in COVID-19

Coronavirus Disease 2019 (COVID-19) has been widely associated with increased thrombotic risk, with many different proposed mechanisms. One such mechanism is acquired deficiency of protein S (PS), a plasma protein that regulates coagulation and inflammatory processes, including complement activation and efferocytosis. Acquired PS deficiency is common in patients with severe viral infections and has been reported in multiple studies of COVID-19. This deficiency may be caused by consumption, degradation, or clearance of the protein, by decreased synthesis, or by binding of PS to other plasma proteins, which block its anticoagulant activity. Here, we review the functions of PS, the evidence of acquired PS deficiency in COVID-19 patients, the potential mechanisms of PS deficiency, and the evidence that those mechanisms may be occurring in COVID-19.

The Hidden Side of Complement Regulator C4BP: Dissection and Evaluation of Its Immunomodulatory Activity

C4b-binding protein (C4BP) is a well-known regulator of the complement system that holds additional and important activities unrelated to complement inhibition. Recently, we have described a novel immunomodulatory activity in the minor C4BP(β-) isoform directly acting over inflammatory phagocytes. Here we show that incorporation of the β-chain to the C4BP α-chain oligomer interferes with this immunomodulatory activity of C4BP. Moreover, an oligomeric form including only the complement control protein 6 (CCP6) domain of the C4BP α-chain (PRP6-HO7) is sufficient to “reprogram” monocyte-derived DCs (Mo-DCs) from a pro-inflammatory and immunogenic phenotype to an anti-inflammatory and tolerogenic state. PRP6-HO7 lacks complement regulatory activity but retains full immunomodulatory activity over inflammatory Mo-DCs induced by TLRs, characterized by downregulation of relevant surface markers such as CD83, HLA-DR, co-stimulatory molecules such as CD86, CD80 and CD40, and pro-inflammatory cytokines such as IL-12 and TNF-α. Furthermore, PRP6-HO7-treated Mo-DCs shows increased endocytosis, significantly reduced CCR7 expression and CCL21-mediated chemotaxis, and prevents T cell alloproliferation. Finally, PRP6-HO7 shows also full immunomodulatory activity over Mo-DCs isolated from lupus nephritis patients with active disease, even without further pro-inflammatory stimulation. Therefore PRP6-HO7, retaining the immunomodulatory activity of C4BP(β-) and lacking its complement regulatory activity, might represent a promising and novel alternative to treat autoimmune diseases.

LC-MS/MS-Based Serum Protein Profiling for Identification of Candidate Biomarkers in Pakistani Rheumatoid Arthritis Patients

Rheumatoid arthritis is an autoimmune disorder of complex disease etiology. Currently available serological diagnostic markers lack in terms of sensitivity and specificity and thus additional biomarkers are warranted for early disease diagnosis and management. We aimed to screen and compare serum proteome profiles of rheumatoid arthritis serotypes with healthy controls in the Pakistani population for identification of potential disease biomarkers. Serum samples from rheumatoid arthritis patients and healthy controls were enriched for low abundance proteins using ProteoMinerTM columns. Rheumatoid arthritis patients were assigned to one of the four serotypes based on anti-citrullinated peptide antibodies and rheumatoid factor. Serum protein profiles were analyzed via liquid chromatography-tandem mass spectrometry. The changes in the protein abundances were determined using label-free quantification software ProgenesisQITM followed by pathway analysis. Findings were validated in an independent cohort of patients and healthy controls using an enzyme-linked immunosorbent assay. A total of 213 proteins were identified. Comparative analysis of all groups (false discovery rate < 0.05, >2-fold change, and identified with ≥2 unique peptides) identified ten proteins that were differentially expressed between rheumatoid arthritis serotypes and healthy controls including pregnancy zone protein, selenoprotein P, C4b-binding protein beta chain, apolipoprotein M, N-acetylmuramoyl-L-alanine amidase, catalytic chain, oncoprotein-induced transcript 3 protein, Carboxypeptidase N subunit 2, Apolipoprotein C-I and Apolipoprotein C-III. Pathway analysis predicted inhibition of liver X receptor/retinoid X receptor activation pathway and production of nitric oxide and reactive oxygen species pathway in macrophages in all serotypes. A catalogue of potential serum biomarkers for rheumatoid arthritis were identified. These biomarkers can be further evaluated in larger cohorts from different populations for their diagnostic and prognostic potential.

Neutrophil and remnant clearance in immunity and inflammation

Neutrophil-centred inflammation and flawed clearance of neutrophils cause and exuberate multiple pathological conditions. These most abundant leukocytes exhibit very high daily turnover in steady-state and stress conditions. Various armours including oxidative burst, NETs and proteases function against pathogens, but also dispose neutrophils to spawn pro-inflammatory responses. Neutrophils undergo death through different pathways upon ageing, infection, executing the intruder's elimination. These include non-lytic apoptosis and other lytic deaths including NETosis, necroptosis and pyroptosis with distinct disintegration of the cellular membrane. This causes release and presence of different intracellular cytotoxic, and tissue-damaging content as cell remnants in the extracellular environment. The apoptotic cells and apoptotic bodies get cleared with non-inflammatory outcomes, while lytic deaths associated remnants including histones and cell-free DNA cause pro-inflammatory responses. Indeed, the enhanced frequencies of neutrophil-associated proteases, cell-free DNA and autoantibodies in diverse pathologies including sepsis, asthma, lupus and rheumatoid arthritis, imply disturbed neutrophil resolution programmes in inflammatory and autoimmune diseases. Thus, the clearance mechanisms of neutrophils and associated remnants are vital for therapeutics. Though studies focused on clearance mechanisms of senescent or apoptotic neutrophils so far generated a good understanding of the same, clearance of neutrophils undergoing distinct lytic deaths, including NETs, are being the subjects of intense investigations. Here, in this review, we are providing the current updates in the clearance mechanisms of apoptotic neutrophils and focusing on not so well-defined recognition, uptake and degradation of neutrophils undergoing lytic death and associated remnants that may provide new therapeutic approaches in inflammation and autoimmunity.

Transcriptome analysis of the liver of Eospalax fontanierii under hypoxia

Hypoxia can induce cell damage, inflammation, carcinogenesis, and inhibit liver regeneration in non-adapted species. Because of their excellent hypoxia adaptation features, subterranean rodents have been widely studied to clarify the mechanism of hypoxia adaptation. Eospalax fontanierii, which is a subterranean rodent found in China, can survive for more than 10 h under 4% O₂ without observable injury, while Sprague-Dawley rats can survive for less than 6 h under the same conditions. To explore the potential mechanism of hypoxia responses in E. fontanierii, we performed RNA-seq analysis of the liver in E. fontanierii exposed to different oxygen levels (6.5% 6h, 10.5% 44h, and 21%). Based on the bioinformatics analysis, 39,439 unigenes were assembled, and 56.78% unigenes were annotated using public databases (Nr, GO, Swiss-Prot, KEGG, and Pfam). In total, 725 differentially expressed genes (DEGs) were identified in the response to hypoxia; six with important functions were validated by qPCR. Those DEGs were mainly involved in processes related to lipid metabolism, steroid catabolism, glycolysis/gluconeogenesis, and the AMPK and PPAR signaling pathway. By analyzing the expression patterns of important genes related to energy associated metabolism under hypoxia, we found that fatty acid oxidation and gluconeogenesis were increased, while protein synthesis and fatty acid synthesis were decreased. Furthermore, the upregulated expression of specific genes with anti-apoptosis or anti-oxidation functions under hypoxia may contribute to the mechanism by which E. fontanierii tolerates hypoxia. Our results provide an understanding of the response to hypoxia in E. fontanierii, and have potential value for biomedical studies.

The interaction between the complement system and hemostatic factors

PURPOSE OF REVIEW

To discuss the crosstalk between the complement system and hemostatic factors (coagulation cascade, platelet, endothelium, and Von Willebrand Factor), and the consequences of this interaction under physiologic and pathologic conditions.

RECENT FINDINGS

The complement and coagulation systems are comprised of serine proteases and are genetically related. In addition to the common ancestral genes, the complement system and hemostasis interact directly, through protein-protein interactions, and indirectly, on the surface of platelets and endothelial cells. The close interaction between the complement system and hemostatic factors is manifested both in physiologic and pathologic conditions, such as in the inflammatory response to thrombosis, thrombosis at the inflamed area, and thrombotic complications of complement disorders.

SUMMARY

The interaction between the complement system and hemostasis is vital for homeostasis and the protective response of the host to tissue injury, but also results in the pathogenesis of several thrombotic and inflammatory disorders.

Intracellular C4BPA Levels Regulate NF-κB-Dependent Apoptosis

The importance of innate immunity in cancer is increasingly being recognized with recent reports suggesting tumor cell-intrinsic intracellular functions for innate immunity proteins. However, such functions are often poorly understood, and it is unclear whether these are affected by patient-specific mutations. Here, we show that C4b-binding protein alpha chain (C4BPA), typically thought to reside in the extracellular space, is expressed intracellularly in cancer cells, where it interacts with the NF-κB family member RelA and regulates apoptosis. Interestingly, intracellular C4BPA expression is regulated in a stress- and mutation-dependent manner and C4BPA mutations are associated with improved cancer survival outcome. Using cell lines harboring patient-specific C4BPA mutations, we show that increasing intracellular C4BPA levels correlate with sensitivity to oxaliplatin-induced apoptosis in vitro and in vivo. Mechanistically, sensitive C4BPA mutants display increased IκBα expression and increased inhibitory IκBα-RelA complex stability. These data suggest a non-canonical intracellular role for C4BPA in regulating NF-κB-dependent apoptosis.

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C4BPA mutations are associated with improved overall survival in 23 tumor types

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C4BPA is found, for the first time, to interact with NF-κB family member RelA

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C4BPA expression is regulated in a mutation- and stress-responsive manner

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C4BPA has a non-canonical intracellular function in regulating NF-κB signaling

Interaction of the Factor H Family Proteins FHR-1 and FHR-5 With DNA and Dead Cells: Implications for the Regulation of Complement Activation and Opsonization

Complement plays an essential role in the opsonophagocytic clearance of apoptotic/necrotic cells. Dysregulation of this process may lead to inflammatory and autoimmune diseases. Factor H (FH), a major soluble complement inhibitor, binds to dead cells and inhibits excessive complement activation on their surface, preventing lysis, and the release of intracellular material, including DNA. The FH-related (FHR) proteins share common ligands with FH, due to their homology with this complement regulator, but they lack the domains that mediate the complement inhibitory activity of FH. Because their roles in complement regulation is controversial and incompletely understood, we studied the interaction of FHR-1 and FHR-5 with DNA and dead cells and investigated whether they influence the regulatory role of FH and the complement activation on DNA and dead cells. FH, FHR-1, and FHR-5 bound to both plasmid DNA and human genomic DNA, where both FHR proteins inhibited FH-DNA interaction. The FH cofactor activity was inhibited by FHR-1 and FHR-5 due to the reduced binding of FH to DNA in the presence of the FHRs. Both FHRs caused increased complement activation on DNA. FHR-1 and FHR-5 bound to late apoptotic and necrotic cells and recruited monomeric C-reactive protein and pentraxin 3, and vice versa. Interactions of the FHRs with pentraxins resulted in enhanced activation of both the classical and the alternative complement pathways on dead cells when exposed to human serum. Altogether, our results demonstrate that FHR-1 and FHR-5 are competitive inhibitors of FH on DNA; moreover, FHR-pentraxin interactions promote opsonization of dead cells.

Drivers and regulators of humoral innate immune responses to infection and cancer

The complement cascade consists of cell bound and serum proteins acting together to protect the host from pathogens, remove cancerous cells and effectively links innate and adaptive immune responses. Despite its usefulness in microbial neutralization and clearance of cancerous cells, excessive complement activation causes an immune imbalance and tissue damage in the host. Hence, a series of complement regulatory proteins present at a higher concentration in blood plasma and on cell surfaces tightly regulate the cascade. The complement cascade can be initiated by B-1 B cell production of natural antibodies. Natural antibodies arise spontaneously without any known exogenous antigenic or microbial stimulus and protect against invading pathogens, clear apoptotic cells, provide tissue homeostasis, and modulate adaptive immune functions. Natural IgM antibodies recognize microbial and cancer antigens and serve as an activator of complement mediated lysis. This review will discuss advances in complement activation and regulation in bacterial and viral infections, and cancer. We will also explore the crosstalk of natural antibodies with bacterial populations and cancer.

C4b Binding Protein Acts as an Innate Immune Effector Against Influenza A Virus

C4b Binding Protein (C4BP) is a major fluid phase inhibitor of the classical and lectin pathways of the complement system. Complement inhibition is achieved by binding to and restricting the role of activated complement component C4b. C4BP functions as a co-factor for factor I in proteolytic inactivation of both soluble and cell surface-bound C4b, thus restricting the formation of the C3-convertase, C4b2a. C4BP also accelerates the natural decay/dissociation of the C3 convertase. This makes C4BP a prime target for exploitation by pathogens to escape complement attack, as seen in Streptococcus pyogenes or Flavivirus. Here, we examined whether C4BP can act on its own in a complement independent manner, against pathogens. C4BP bound H1N1 and H3N2 subtypes of Influenza A Virus (IAV) most likely via multiple sites in Complement Control Protein (CCP) 1-2, 4-5, and 7-8 domains of its α-chain. In addition, C4BP CCP1-2 bound H3N2 better than H1N1. C4BP bound three IAV envelope proteins: Haemagglutinin (~70 kDa), Neuraminidase (~55 kDa), and Matrix protein 1 (~25kDa). C4BP suppressed H1N1 subtype infection into the lung epithelial cell line, A549, while it promoted infection by H3N2 subtype. C4BP restricted viral entry for H1N1 but had the opposite effect on H3N2, as evident from experiments using pseudo-typed viral particles. C4BP downregulated mRNA levels of pro-inflammatory IFN-α, IL-12, and NFκB in the case of H1N1, while it promoted a pro-inflammatory immune response by upregulating IFN- α, TNF-α, RANTES, and IL-6 in the case of H3N2. We conclude that C4BP differentially modulates the efficacy of IAV entry, and hence, replication in a target cell in a strain-dependent manner, and acts as an entry inhibitor for H1N1. Thus, CCP containing complement proteins such as factor H and C4BP may have additional defense roles against IAV that do not rely on the regulation of complement activation.

Regulation of the complement system and immunological tolerance in pregnancy

Preeclampsia is a serious vascular complication of the human pregnancy, whose etiology is still poorly understood. In preeclampsia, exacerbated apoptosis and fragmentation of the placental tissue occurs due to developmental qualities of the placental trophoblast cells and/or mechanical and oxidative distress to the syncytiotrophoblast, which lines the placental villi. Dysregulation of the complement system is recognized as one of the mechanisms of the disease pathology. Complement has the ability to promote inflammation and facilitate phagocytosis of placenta-derived particles and apoptotic cells by macrophages. In preeclampsia, an overload of placental cell damage or dysregulated complement system may lead to insufficient clearance of apoptotic particles and placenta-derived debris. Excess placental damage may lead to sequestration of microparticles, such as placental vesicles, to capillaries in the glomeruli of the kidney and other vulnerable tissues. This phenomenon could contribute to the manifestations of typical diagnostic symptoms of preeclampsia: proteinuria and new-onset hypertension. In this review we propose that the complement system may serve as a regulator of the complex tolerance and clearance processes that are fundamental in healthy pregnancy. It is therefore recommended that further research be conducted to elucidate the interactions between components of the complement system and immune responses in the context of complicated and healthy pregnancy.

The role of TAM family receptors and ligands in the nervous system: From development to pathobiology

Tyro3, Axl, and Mertk, referred to as the TAM family of receptor tyrosine kinases, are instrumental in maintaining cell survival and homeostasis in mammals. TAM receptors interact with multiple signaling molecules to regulate cell migration, survival, phagocytosis and clearance of metabolic products and cell debris called efferocytosis. The TAMs also function as rheostats to reduce the expression of proinflammatory molecules and prevent autoimmunity. All three TAM receptors are activated in a concentration-dependent manner by the vitamin K-dependent growth arrest-specific protein 6 (Gas6). Gas6 and the TAMs are abundantly expressed in the nervous system. Gas6, secreted by neurons and endothelial cells, is the sole ligand for Axl. ProteinS1 (ProS1), another vitamin K-dependent protein functions mainly as an anti-coagulant, and independent of this function can activate Tyro3 and Mertk, but not Axl. This review will focus on the role of the TAM receptors and their ligands in the nervous system. We highlight studies that explore the function of TAM signaling in myelination, the visual cortex, neural cancers, and multiple sclerosis (MS) using Gas6^-/- and TAM mutant mice models.

HpARI Protein Secreted by a Helminth Parasite Suppresses Interleukin-33

Infection by helminth parasites is associated with amelioration of allergic reactivity, but mechanistic insights into this association are lacking. Products secreted by the mouse parasite Heligmosomoides polygyrus suppress type 2 (allergic) immune responses through interference in the interleukin-33 (IL-33) pathway. Here, we identified H. polygyrus Alarmin Release Inhibitor (HpARI), an IL-33-suppressive 26-kDa protein, containing three predicted complement control protein (CCP) modules. In vivo, recombinant HpARI abrogated IL-33, group 2 innate lymphoid cell (ILC2) and eosinophilic responses to Alternaria allergen administration, and diminished eosinophilic responses to Nippostrongylus brasiliensis, increasing parasite burden. HpARI bound directly to both mouse and human IL-33 (in the cytokine’s activated state) and also to nuclear DNA via its N-terminal CCP module pair (CCP1/2), tethering active IL-33 within necrotic cells, preventing its release, and forestalling initiation of type 2 allergic responses. Thus, HpARI employs a novel molecular strategy to suppress type 2 immunity in both infection and allergy.

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HpARI is a suppressor of IL-33 release and consequent allergic sensitization

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HpARI binds active IL-33 and nuclear DNA, tethering IL-33 within necrotic cells

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HpARI is active against both human and murine IL-33

Protection of host cells by complement regulators

The complement cascade is an ancient immune-surveillance system that not only provides protection from pathogen invasion but has also evolved to participate in physiological processes to maintain tissue homeostasis. The alternative pathway (AP) of complement activation is the evolutionarily oldest part of this innate immune cascade. It is unique in that it is continuously activated at a low level and arbitrarily probes foreign, modified-self, and also unaltered self-structures. This indiscriminate activation necessitates the presence of preformed regulators on autologous surfaces to spare self-cells from the undirected nature of AP activation. Although the other two canonical complement activation routes, the classical and lectin pathways, initiate the cascade more specifically through pattern recognition, their activity still needs to be tightly controlled to avoid excessive reactivity. It is the perpetual duty of complement regulators to protect the self from damage inflicted by inadequate complement activation. Here, we review the role of complement regulators as preformed mediators of defense, explain their common and specialized functions, and discuss selected cases in which alterations in complement regulators lead to disease. Finally, rational engineering approaches using natural complement inhibitors as potential therapeutics are highlighted.

Complement in removal of the dead - balancing inflammation

Recognition and removal of apoptotic and necrotic cells must be efficient and highly controlled to avoid excessive inflammation and autoimmune responses to self. The complement system, a crucial part of innate immunity, plays an important role in this process. Thus, apoptotic and necrotic cells are recognized by complement initiators such as C1q, mannose binding lectin, ficolins, and properdin. This triggers complement activation and opsonization of cells with fragments of C3b, which enhances phagocytosis and thus ensures silent removal. Importantly, the process is tightly controlled by the binding of complement inhibitors C4b-binding protein and factor H, which attenuates late steps of complement activation and inflammation. Furthermore, factor H becomes actively internalized by apoptotic cells, where it catalyzes the cleavage of intracellular C3 to C3b. The intracellularly derived C3b additionally opsonizes the cell surface further supporting safe and fast clearance and thereby aids to prevent autoimmunity. Internalized factor H also binds nucleosomes and directs monocytes into production of anti-inflammatory cytokines upon phagocytosis of such complexes. Disturbances in the complement-mediated clearance of dying cells result in persistence of autoantigens and development of autoimmune diseases like systemic lupus erythematosus, and may also be involved in development of age-related macula degeneration.

Identification of genes differentially expressed in menstrual breakdown and repair

STUDY QUESTION

Does the changing molecular profile of the endometrium during menstruation correlate with the histological profile of menstruation.

SUMMARY ANSWER

We identified several genes not previously associated with menstruation; on Day 2 of menstruation (early-menstruation), processes related to inflammation are predominantly up-regulated and on Day 4 (late-menstruation), the endometrium is predominantly repairing and regenerating.

WHAT IS KNOWN ALREADY

Menstruation is induced by progesterone withdrawal at the end of the menstrual cycle and involves endometrial tissue breakdown, regeneration and repair. Perturbations in the regulation of menstruation may result in menstrual disorders including abnormal uterine bleeding.

STUDY DESIGN, SIZE DURATION

Endometrial samples were collected by Pipelle biopsy on Days 2 (n = 9), 3 (n = 9) or 4 (n = 6) of menstruation.

PARTICIPANTS/MATERIALS, SETTING, METHODS

RNA was extracted from endometrial biopsies and analysed by genome wide expression Illumina Sentrix Human HT12 arrays. Data were analysed using 'Remove Unwanted Variation-inverse (RUV-inv)'. Ingenuity pathway analysis (IPA) and the Database for Annotation, Visualization and Integrated Discovery (DAVID) v6.7 were used to identify canonical pathways, upstream regulators and functional gene clusters enriched between Days 2, 3 and 4 of menstruation. Selected individual genes were validated by quantitative PCR.

MAIN RESULTS AND THE ROLE OF CHANCE

Overall, 1753 genes were differentially expressed in one or more comparisons. Significant canonical pathways, gene clusters and upstream regulators enriched during menstrual bleeding included those associated with immune cell trafficking, inflammation, cell cycle regulation, extracellular remodelling and the complement and coagulation cascade. We provide the first evidence for a role for glutathione-mediated detoxification (glutathione-S-transferase mu 1 and 2; GSTM1 and GSTM2) during menstruation. The largest number of differentially expressed genes was between Days 2 and 4 of menstruation (n = 1176). We identified several genes not previously associated with menstruation including lipopolysaccharide binding protein, serpin peptidase inhibitor, clade B (ovalbumin), member 3 (SERPINB3) and -4 (SERPINB4), interleukin-17C (IL17C), V-set domain containing T-cell activation inhibitor 1 (VTCN1), proliferating cell nuclear antigen factor (KIAA0101/PAF), trefoil factor 3 (TFF3), laminin alpha 2 (LAMA2) and serine peptidase inhibitor, Kazal type 1 (SPINK1). Genes related to inflammatory processes were up-regulated on Day 2 (early-menstruation), and those associated with endometrial repair and regeneration were up-regulated on Day 4 (late-menstruation).

LIMITATIONS, REASONS FOR CAUTION

Participants presented with a variety of endometrial pathologies related to bleeding status and other menstrual characteristics. These variations may also have influenced the menstrual process.

WIDER IMPLICATIONS OF THE FINDINGS

The temporal molecular profile of menstruation presented in this study identifies a number of genes not previously associated with the menstrual process. Our findings provide valuable insight into the menstrual process and may present novel targets for therapeutic intervention in cases of endometrial dysfunction.

LARGE SCALE DATA

All microarray data have been deposited in the public data repository Gene Expression Omnibus (GSE86003).

STUDY FUNDING AND COMPETING INTERESTS

Funding for this work was provided by a National Health and Medical Research Council of Australia (NHMRC) Project Grant APP1008553 to M.H., P.R. and J.G. M.H. is supported by an NHMRC Practitioner Fellowship. P.P. is supported by a NHMRC Early Career Fellowship. The authors have no conflict of interest to declare.

C4b-binding Protein Protects β-Cells from Islet Amyloid Polypeptide-induced Cytotoxicity

C4BP (C4b-binding protein) is a polymer of seven identical α chains and one unique β chain synthesized in liver and pancreas. We showed previously that C4BP enhances islet amyloid polypeptide (IAPP) fibril formation in vitro Now we report that polymeric C4BP strongly inhibited lysis of human erythrocytes incubated with monomeric IAPP, whereas no lysis was observed after incubation with preformed IAPP fibrils. In contrast, incubation with the monomeric α-chain of C4BP was less effective. These data indicate that polymeric C4BP with multiple binding sites for IAPP neutralizes lytic activity of IAPP. Furthermore, addition of monomeric IAPP to a rat insulinoma cell line (INS-1) resulted in decreased cell viability, which was restored in the presence of physiological concentrations of C4BP. Treatment of INS-1 cells and primary rat islets with IAPP also diminished their ability to secrete insulin upon stimulation with glucose, which was reversed in the presence of C4BP. Further, C4BP was internalized together with IAPP into INS-1 cells. Pathway analyses of mRNA expression microarray data indicated that cells exposed to C4BP and IAPP in comparison with IAPP alone increased expression of genes involved in cholesterol synthesis. Depletion of cholesterol through methyl-β-cyclodextrin or cholesterol oxidase abolished the protective effect of C4BP on IAPP cytotoxicity of INS-1 cells. Also, inhibition of phosphoinositide 3-kinase but not NF-κB had a similar effect. Taken together, C4BP protects β-cells from IAPP cytotoxicity by modulating IAPP fibril formation extracellularly and also, after uptake by the cells, by enhancing cholesterol synthesis.

C4b-binding protein: The good, the bad and the deadly. Novel functions of an old friend

C4b-binding protein (C4BP) is best known as a potent soluble inhibitor of the classical and lectin pathways of the complement system. This large 500 kDa multimeric plasma glycoprotein is expressed mainly in the liver but also in lung and pancreas. It consists of several identical 75 kDa α-chains and often also one 40 kDa β-chain, both of which are mainly composed of complement control protein (CCP) domains. Structure-function studies revealed that one crucial binding site responsible for inhibition of complement is located to CCP1-3 of the α-chain. Binding of anticoagulant protein S to the CCP1 of the β-chain provides C4BP with the ability to strongly bind apoptotic and necrotic cells in order to prevent inflammation arising from activation of complement by these cells. Further, C4BP interacts strongly with various types of amyloid and enhances fibrillation of islet amyloid polypeptide secreted from pancreatic beta cells, which may attenuate pro-inflammatory and cytotoxic effects of this amyloid. Full deficiency of C4BP has not been identified but non-synonymous alterations in its sequence have been found in haemolytic uremic syndrome and recurrent pregnancy loss. Furthermore, C4BP is bound by several bacterial pathogens, notably Streptococcus pyogenes, which due to inhibition of complement and enhancement of bacterial adhesion to endothelial cells provides these bacteria with a survival advantage in the host. Thus, depending on the context, C4BP has a protective or detrimental role in the organism.

Endogenous C1-inhibitor production and expression in the heart after acute myocardial infarction

BACKGROUND

Complement activation contributes significantly to inflammation-related damage in the heart after acute myocardial infarction. Knowledge on factors that regulate postinfraction complement activation is incomplete however. In this study, we investigated whether endogenous C1-inhibitor, a well-known inhibitor of complement activation, is expressed in the heart after acute myocardial infarction.

MATERIALS AND METHODS

C1-inhibitor and complement activation products C3d and C4d were analyzed immunohistochemically in the hearts of patients who died at different time intervals after acute myocardial infarction (n=28) and of control patients (n=8). To determine putative local C1-inhibitor production, cardiac transcript levels of the C1-inhibitor-encoding gene serping1 were determined in rats after induction of acute myocardial infarction (microarray). Additionally, C1-inhibitor expression was analyzed (fluorescence microscopy) in human endothelial cells and rat cardiomyoblasts in vitro.

RESULTS

C1-inhibitor was found predominantly in and on jeopardized cardiomyocytes in necrotic infarct cores between 12h and 5days old. C1-inhibitor protein expression coincided in time and colocalized with C3d and C4d. In the rat heart, serping1 transcript levels were increased from 2h up until 7days after acute myocardial infarction. Both endothelial cells and cardiomyoblasts showed increased intracellular expression of C1-inhibitor in response to ischemia in vitro (n=4).

CONCLUSIONS

These observations suggest that endogenous C1-inhibitor is likely involved in the regulation of complement activity in the myocardium following acute myocardial infarction. Observations in rat and in vitro suggest that C1-inhibitor is produced locally in the heart after acute myocardial infarction.

Complement activation and regulation in preeclamptic placenta

Preeclampsia (PE) is a common disorder of pregnancy originating in the placenta. We examined whether excessive activation or poor regulation of the complement system at the maternal–fetal interface could contribute to the development of PE. Location and occurrence of complement components and regulators in placentae were analyzed. Cryostat sections of placentae were processed from 7 early-onset PE (diagnosis <34 weeks of gestation), 5 late-onset PE, 10 control pregnancies, and immunostained for 6 complement activators and 6 inhibitors. Fluorescence was quantified and compared between PE and control placentae. Gene copy numbers of complement components C4A and C4B were assessed by a quantitative PCR method. Maternal C4 deficiencies (≥1 missing or non-functional C4) were most common in the early-onset PE group (71%), and more frequent in late-onset PE compared to healthy controls (60 vs. 38%). Complement C1q deposition differed significantly between control and patient groups: controls and early-onset PE patients had more C1q than late-onset PE patients (mean p = 0.01 and p = 0.005, respectively). C3 activation was analyzed by staining for C3b/iC3b and C3d. C3d was mostly specific to the basal syncytium and C3b/iC3b diffuse in other structures, but there were no clear differences between the study groups. Activated C4 and membrane-bound regulators CD55, CD46, and CD59 were observed abundantly in the syncytiotrophoblast. Syncytial knots, structures enriched in PE, stained specifically for the classical pathway inhibitor C4bp, whereas the key regulator alternative pathway, factor H (FH) showed a wider distribution in the placenta. Differences in C1q deposition between late- and early-onset PE groups may be indicative of the different etiology of PE symptoms in these patients. Irregular distribution of the complement regulators C4bp and FH in the PE placenta and a higher frequency of C4A deficiencies suggest a disturbed balance between complement activation and regulation in PE.

Purification and functional characterization of C4b-binding protein (C4BP)

C4b-binding protein (C4BP) is a soluble, 570 kDa large glycoprotein, present in plasma at a concentration of approximately 200 mg/L. C4BP is the main inhibitor of the classical and lectin pathways of complement, where it controls C4b-mediated reactions. Here, we describe a method for purification of C4BP from human plasma, which is based on barium chloride precipitation, anion exchange chromatography, and gel filtration. We also describe a functional assay, in which C4BP's cofactor activity to factor I, in the degradation of C4b, can be assessed.

Anti-cancer mechanisms of clinically acceptable colchicine concentrations on hepatocellular carcinoma

AIMS

This study was to investigate whether the clinically acceptable colchicine concentrations had anti-cancer effects on hepatocellular carcinoma (HCC) and their anti-cancer mechanisms.

MAIN METHODS

Two human HCC cell lines (HCC24/KMUH, HCC38/KMUH) and two human cancer-associated fibroblast (CAF) cell lines (F28/KMUH, F59/KMUH) were investigated by proliferative assay, microarray, quantitative reverse transcriptase-polymerase chain reaction, and nude mouse study using clinically acceptable colchicine concentrations.

KEY FINDINGS

Both 2 and 6ng/mL colchicine significantly inhibited the cellular proliferation of all cell lines tested (P<0.05). The anti-proliferative effects of colchicine on F28/KMUH, HCC24/KMUH and HCC38/KMUH cells were dose-dependent. The anti-proliferative effects of 6ng/mL colchicine on both HCC cell lines were similar to the effects of 1μg/mL epirubicin. The anti-proliferative effects of colchicine on HCC cells could be partially explained by dose-dependent up-regulations of 2 anti-proliferative genes (AKAP12, TGFB2) in these cells. TGFB2 was also up-regulated in CAFs but was not dose-dependent. Up-regulation of MX1 which can accelerate cell death was a common effect of 6ng/mL colchicine on both CAF cell lines, but 2ng/mL colchicine down-regulated MX1 in F28/KMUH cells. Nude mouse (BALB/c-nu) experiment showed that colchicine-treated mice (0.07mgcolchicine/kg/day×14days) had lower increased tumor volume ratios, slower tumor growth rates and larger percentages of tumor necrotic areas than control mice (all P<0.05).

SIGNIFICANCE

Clinically acceptable colchicine concentrations have anti-cancer effects on HCC. This drug has potential for the palliative treatment of HCC.

Analysis of genes coding for CD46, CD55, and C4b-binding protein in patients with idiopathic, recurrent, spontaneous pregnancy loss

Since a tightly regulated complement system is needed for a successful pregnancy, we hypothesized that alterations in complement inhibitors may be associated with idiopathic, recurrent miscarriage. We sequenced all exons coding for three complement inhibitors: C4b-binding protein (C4BP), CD46, and CD55 in 384 childless women with at least two miscarriages that could not be explained by known risk factors. Several alterations were found in C4BPA, of which the R120H, I126T, and the G423T mutations affected the expression level and/or the ability of recombinant C4BP to serve as cofactor for factor I. The only variant in C4BPB was located in the C-terminal part, and did not impair the polymerization of the molecule. Our results identify for the first time alterations in C4BP in women experiencing recurrent miscarriages. We also found four CD46 alterations in individual patients that were not found in healthy controls. One of the rare variants, P324L, showed decreased expression, whereas N213I resulted in deficient protein processing as well as an impaired cofactor activity in the degradation of both C4b and C3b. The identified alterations may result in in vivo consequences and contribute to the disorder but the degree of association must be evaluated in larger cohorts.

Interactions of the complement system with molecules of extracellular matrix: relevance for joint diseases

Rheumatoid arthritis (RA) is a highly disabling disease affecting all structures of the joint. Understanding the pathology behind the development of RA is essential for developing targeted therapeutic strategies as well as for developing novel markers to predict disease onset. Several molecules normally hidden within the cartilage tissue are exposed to complement components in the synovial fluid upon cartilage breakdown. Some of these have been shown to activate complement and toll-like receptors, which may enhance an already existing inflammatory response, thereby worsening the course of disease. Other cartilage-resident molecules have in contrast shown to possess complement-inhibitory properties. Knowledge about mechanisms behind pathological complement activation in the joints will hopefully lead to methods which allow us to distinguish patients with pathological complement activation from those where other inflammatory pathways are predominant. This will help to elucidate which patients will benefit from complement inhibitory therapies, which are thought to aid a specific subset of patients or patients at a certain stage of disease. Future challenges are to target the complement inhibition specifically to the joints to minimize systemic complement blockade.

The α7β0 isoform of the complement regulator C4b-binding protein induces a semimature, anti-inflammatory state in dendritic cells

The classical pathway complement regulator C4b-binding protein (C4BP) is composed of two polypeptides (α- and β-chains), which form three plasma oligomers with different subunit compositions (α7β1, α7β0, and α6β1). We show in this article that the C4BP α7β0 isoform (hereafter called C4BP[β(-)] [C4BP lacking the β-chain]), overexpressed under acute-phase conditions, induces a semimature, tolerogenic state on human monocyte-derived dendritic cells (DCs) activated by a proinflammatory stimulus. C4BP isoforms containing β-chain (α7β1 and α6β1; C4BP[β(+)]) neither interfered with the normal maturation of DCs nor competed with C4BP(β(-)) activity on these cells. Immature DCs (iDCs) treated with C4BP(β(-)) retained high endocytic activity, but, upon LPS treatment, they did not upregulate surface expression of CD83, CD80, and CD86. Transcriptional profiling of these semimature DCs revealed that treatment with C4BP(β(-)) prevented the induction of IDO and BIC-1, whereas TGF-β1 expression was maintained to the level of iDCs. C4BP(β(-))-treated DCs were also unable to release proinflammatory Th1 cytokines (IL-12, TNF-α, IFN-γ, IL-6, IL-8) and, conversely, increased IL-10 secretion. They prevented surface CCR7 overexpression and, accordingly, displayed reduced chemotaxis, being morphologically indistinguishable from iDCs. Moreover, C4BP(β(-))-treated DCs failed to enhance allogeneic T cell proliferation, impairing IFN-γ production in these cells and, conversely, promoting CD4(+)CD127(low/neg)CD25(high)Foxp3(+) T cells. Deletion mutant analysis revealed that the complement control protein-6 domain of the α-chain is necessary for the tolerogenic activity of C4BP(β(-)). Our data demonstrate a novel anti-inflammatory and immunomodulatory function of the complement regulator C4BP, suggesting a relevant role of the acute-phase C4BP(β(-)) isoform in a number of pathophysiological conditions and potential applications in autoimmunity and transplantation.

NETs: the missing link between cell death and systemic autoimmune diseases?

For almost 20 years, apoptosis and secondary necrosis have been considered the major source of autoantigens and endogenous adjuvants in the pathogenic model of systemic autoimmune diseases. This focus is justified in part because initial evidence in systemic lupus erythematosus (SLE) guided investigators toward the study of apoptosis, but also because other forms of cell death were unknown. To date, it is known that many other forms of cell death occur, and that they vary in their capacity to stimulate as well as inhibit the immune system. Among these, NETosis (an antimicrobial form of death in neutrophils in which nuclear material is extruded from the cell forming extracellular traps), is gaining major interest as a process that may trigger some of the immune features found in SLE, granulomatosis with polyangiitis (formerly Wegener’s granulomatosis) and Felty’s syndrome. Although there have been volumes of very compelling studies published on the role of cell death in autoimmunity, no unifying theory has been adopted nor have any successful therapeutics been developed based on this important pathway. The recent inclusion of NETosis into the pathogenic model of autoimmune diseases certainly adds novel insights into this paradigm, but also reveals a previously unappreciated level of complexity and raises many new questions. This review discusses the role of cell death in systemic autoimmune diseases with a focus on apoptosis and NETosis, highlights the current short comings in our understanding of the vast complexity of cell death, and considers the potential shift in the cell death paradigm in autoimmunity. Understanding this complexity is critical in order to develop tools to clearly define the death pathways that are active in systemic autoimmune diseases, identify drivers of disease propagation, and develop novel therapeutics.

Annexin A2 and A5 serve as new ligands for C1q on apoptotic cells

C1q is the initiator of the classical complement pathway and opsonizes apoptotic cells to facilitate phagocytosis. Deficiency of C1q is the strongest known risk factor for development of systemic lupus erythematosus (SLE), which appears to be related to ensuing impaired clearance of apoptotic material. The objective of the current study was to investigate new ligands for C1q on the surface of apoptotic cells. We revealed that the two phospholipid-binding proteins annexin A2 and A5 are, beside DNA, significant C1q ligands. We furthermore, demonstrated that C1q binds directly to histones exposed on the surface of dying cells but we did not detect significant interaction with phosphatidylserine. The complement inhibitors C4b-binding protein and factor H also interact with dying cells, most likely to decrease complement activation beyond the level of C3 to allow noninflammatory clearance. Despite the fact that C4b-binding protein, factor H, and C1q share some ligands on dying cells, we showed that these three proteins did not compete with one another for binding to apoptotic cells. We additionally demonstrated that the way in which apoptosis is induced influenced both the degree of apoptosis and the binding of C1q. The knowledge, that annexin A2 and A5 act as ligands for C1q on apoptotic cells, sheds new light on the pathophysiology of autoimmune diseases.

Neutrophil extracellular traps that are not degraded in systemic lupus erythematosus activate complement exacerbating the disease

Ongoing inflammation including activation of the complement system is a hallmark of systemic lupus erythematosus (SLE). Antimicrobial neutrophil extracellular traps (NETs) are composed of secreted chromatin that may act as a source of autoantigens typical for SLE. In this study, we investigated how complement interacts with NETs and how NET degradation is affected by complement in SLE patients. We found that sera from a subset of patients with active SLE had a reduced ability to degrade in vitro-generated NETs, which was mostly restored when these patients were in remission. Patients that failed to degrade NETs had a more active disease and they also displayed lower levels of complement proteins C4 and C3 in blood. We discovered that NETs activated complement in vitro and that deposited C1q inhibited NET degradation including a direct inhibition of DNase-I by C1q. Complement deposition on NETs may facilitate autoantibody production, and indeed, Abs against NETs and NET epitopes were more pronounced in patients with impaired ability to degrade NETs. NET-bound autoantibodies inhibited degradation but also further increased C1q deposition, potentially exacerbating the disease. Thus, NETs are a potent complement activator, and this interaction may play an important role in SLE. Targeting complement with inhibitors or by removing complement activators such as NETs could be beneficial for patients with SLE.

The role of higher-order protein structure in supporting binding by heteroclitic monoclonal antibodies: the monoclonal antibody KIM185 to CD18 also binds C4-binding protein

Heteroclitic monoclonal antibodies are characterized by the ability to bind multiple epitopes with little or no similarity. Such antibodies have been reported earlier, but insight into to the molecular basis of this propensity is limited. Here we report that the KIM185 antibody to human CD18 reacts with the plasma protein C4b-binding protein (C4BP). This was revealed during affinity purification procedures where human serum was incubated with surfaces coated with monoclonal antibodies to CD18. Other monoclonal antibodies to CD18 (KIM127 and TS1/18) showed no such interaction with C4BP. We constructed a sandwich-type time-resolved immunofluorometric assay using KIM185 both as capture and developing antibody. By use of proteolytic fragments of KIM185 and recombinant deletion mutants of C4BP the interaction sites were mapped to the variable region of KIM185 and the oligomerization domain of C4BP, respectively. C4BP is a large oligomeric plasma protein that binds activated complement factor C4b and other endogenous ligands as well as microorganisms. By use of the recent crystallographic data on the structure of CD11c/CD18 and prediction of the secondary structure of the C4BP oligomerization domain, we show that epitopes bound by KIM185 in these proteins are unlikely to share any major structural similarity. However, both antigens may form oligomers that would enable avid binding by the antibody. Our report points to the astonishing ability of heteroclitic antibodies to accommodate the binding of multiple proteins with no or little structural similarity within the confined space of the variable regions.

Factor H as a regulator of the classical pathway activation

C1q, the first subcomponent of the classical pathway, is a charge pattern recognition molecule that binds a diverse range of self, non-self and altered self ligands, leading to pro-inflammatory complement activation. Although complement is required for tissue homeostasis as well as defence against pathogens, exaggerated complement activation can be damaging to the tissue. Therefore, a fine balance between complement activation and inhibition is necessary. We have recently found that factor H, a polyanion recognition molecule and soluble regulator of alternative pathway activation in blood and on cell surfaces, can directly compete with C1q in binding to anionic phospholipids (cardiolipin), lipid A and Escherichia coli (three known activators of the classical pathway) and acts as a direct down regulator of the complement classical pathway. This ability of factor H to dampen classical pathway activation is distinct from its role as an alternative pathway down-regulator. Thus, by directly competing for specific C1q ligands (exogenous as well as endogenous), factor H is likely to be involved in fine-tuning and balancing the C1q-driven inflammatory processes in autoimmunity and infection. However, in the case of apoptotic cells, C1q-mediated enhancement of uptake/adhesion of the apoptotic cells by monocytes was reduced by factor H. Thus, factor H may be important in controlling the inflammation, which might arise from C1q deposition on apoptotic cells.

Binding of flavivirus nonstructural protein NS1 to C4b binding protein modulates complement activation

The complement system plays a pivotal protective role in the innate immune response to many pathogens including flaviviruses. Flavivirus nonstructural protein 1 (NS1) is a secreted nonstructural glycoprotein that accumulates in plasma to high levels and is displayed on the surface of infected cells but absent from viral particles. Previous work has defined an immune evasion role of flavivirus NS1 in limiting complement activation by forming a complex with C1s and C4 to promote cleavage of C4 to C4b. In this study, we demonstrate a second mechanism, also involving C4 and its active fragment C4b, by which NS1 antagonizes complement activation. Dengue, West Nile, or yellow fever virus NS1 directly associated with C4b binding protein (C4BP), a complement regulatory plasma protein that attenuates the classical and lectin pathways. Soluble NS1 recruited C4BP to inactivate C4b in solution and on the plasma membrane. Mapping studies revealed that the interaction sites of NS1 on C4BP partially overlap with the C4b binding sites. Together, these studies further define the immune evasion potential of NS1 in reducing the functional capacity of C4 in complement activation and control of flavivirus infection.

Serum amyloid P component facilitates DNA clearance and inhibits plasmid transfection: implications for human DNA vaccine

The demonstration that naked plasmid DNA can induce strong immune responses in mice has attracted considerable attention in the vaccine community. However, similar immunizations have been less/not effective in clinical trials during the past decade, and the underlying mechanisms remain unknown. In this study, we hypothesized that some DNA-binding proteins in human serum may serve as host barriers, responsible for the low efficiency of plasmids' transfection in vivo. Using proteomics, we showed that human serum amyloid P component (hSAP) is specifically present in human DNA-protein complexes. Further analysis indicated that hSAP effectively binds plasmid DNA, inhibits DNA transfection into somatic cells and facilitates the endocytosis of DNA by macrophages, whereas mouse SAP (mSAP) has similar, but much weaker, activities. In the presence of hSAP, the plasmid DNA expression in vivo and plasmid DNA-induced immune responses also significantly decreased. Therefore, our results suggest that hSAP contributes to extracellular DNA clearance and the inhibition of plasmid DNA transfection in vivo. This mechanism may be partly responsible for the insufficient immune responses to DNA vaccination in human beings; therefore, it may serve as a novel target for the improvement of DNA vaccines and DNA-based gene therapy.

Human serum amyloid P functions as a negative regulator of the innate and adaptive immune responses to DNA vaccines

The utility of DNA vaccines has been limited by their failure to elicit sufficiently potent immune responses in many human applications, whereas DNA vaccinations in mice have been very successful. However, the underlying mechanisms remain unknown. We hypothesize that serum amyloid P component (SAP), which has a species-specific, DNA-binding ability, contributes to the differences between human and mice and then limits DNA vaccine's efficacy in vivo. In our study, DNA vaccine-induced adaptive immune responses were also significantly decreased in the human SAP (hSAP) transgenic mice. Using human promonocytic cell line THP-1-derived macrophages as a cell model, we found that cells incubated with a hSAP-DNA complex showed significant defects in innate immune activations, whereas mouse SAP had similar, albeit very weak, activities. hSAP also significantly inhibited the functions of two identified DNA sentinels, high-mobility group B protein 1 and antimicrobial peptide LL37, and redirected DNA update to FcRs leading to endocytosis and endosomal degradation. We also found that a chemical SAP inhibitor strongly recovered the suppressed innate immune responses to DNA in the presence of human serum and enhanced the immunogenicity of DNA vaccines in vivo. Our data indicated that SAP is a key negative regulator for innate immune responses to DNA and may be partly responsible for the insufficient immune responses after DNA vaccinations in humans. SAP suppression may be a novel strategy for improving efficacy of human DNA vaccines and requires further clinical investigations.

Review: Soluble innate immune pattern-recognition proteins for clearing dying cells and cellular components: implications on exacerbating or resolving inflammation

Soluble innate immune pattern-recognition proteins (sPRPs) identify non-self or altered-self molecular patterns. Dying cells often display altered-self arrays of molecules on their surfaces. Hence, sPRPs are ideal for recognizing these cells and their components. Dying cell surfaces often contain, or allow the access to different lipids, intracellular glycoproteins and nucleic acids such as DNA at different stages of cell death. These are considered as 'eat me' signals that replace the native 'don't eat me' signals such as CD31, CD47 present on the live cells. A programmed cell death process such as apoptosis also generates cell surface blebs that contain intracellular components. These blebs are easily released for effective clearance or signalling. During late stages of cell death, soluble components are also released that act as 'find me' signal (e.g. LysoPC, nucleotides). The sPRPs such as collectins, ficolins, pentraxins, sCD14, MFG-E8, natural IgM and C1q can effectively identify some of these specific molecular patterns. The biological end-point is different depending on sPRP, tissue, stage of apoptosis and the type of cell death. The sPRPs that reside in the immune-privileged surfaces such as lungs often act as opsonins and enhance a silent clearance of dying cells and cellular material by macrophages and other phagocytic cells. Although the recognition of these materials by complement-activating proteins could amplify the opsonic signal, this pathway may aggravate inflammation. Clear understanding of the involvement of specific sPRPs in cell death and subsequent clearance of dying cell and their components is essential for devising appropriate treatment strategies for diseases involving infection, inflammation and auto-antibody generation.

Progression of lupus-like disease drives the appearance of complement-activating IgG antibodies in MRL/lpr mice

OBJECTIVES

Nucleic acids are known to induce complement activation, which results in the masking and removal of apoptotic cells exposing nuclear components. Dysregulation of these events is characteristic of SLE, a systemic autoimmune disease characterized by the appearance of ANAs. In this study, we aimed to investigate the relationship between development of ANAs and their effect on complement activation by nucleic acids.

METHODS

We used protein array technology to characterize complement activation by murine mAbs and polyclonal antibodies against various forms of nucleic acid. Serum samples from MRL/lpr mice were collected, starting before the onset of the disease till 6 months of age. Binding of IgG and its subclasses to dsDNA, ssDNA, RNA, plasmid DNA and nucleosome complexes was determined, along with C3 fixation.

RESULTS

We show that complement C3 binding to various forms of nucleic acid that serve as targets in lupus is absent in normal serum. The addition of dsDNA-specific mAbs to normal serum results in the deposition of complement C3 to nucleic acids. In MRL/lpr mice, IgG antibodies against various nuclear antigens appear with ageing and disease progression. C3 binding to the antigens is somewhat delayed and suggests that accumulation or maturation of pathogenic antibodies is required for inducing C3 binding to ICs containing nucleic acids.

CONCLUSIONS

C3 deposition on nuclear antigens, therefore, reflects the state of disease progression in this murine model of SLE.

Annexin-II, DNA, and histones serve as factor H ligands on the surface of apoptotic cells

Apoptotic cells are opsonized by complement components such as C1q and C3b, which increases their susceptibility to phagocytosis. Soluble complement inhibitors such as factor H (fH) also recognize apoptotic cells to minimize the pro-inflammatory effects of downstream complement activation. We used four radiolabeled protein constructs that span different regions of the 20 complement control protein (CCP) modules that make up fH and found that fragments comprising CCPs 6-8, CCPs 8-15, and CCPs 19-20 but not CCPs 1-4, bound to apoptotic Jurkat T cells. There are four possible ligand types on apoptotic cells that could recruit fH: proteins, carbohydrates, lipids, and DNA. We found that CCPs 6-8 of fH bind to annexin-II, a trypsin-insensitive protein that becomes exposed on surfaces of apoptotic cells. The second ligand of fH, which interacts with CCPs 6-8 and 19-20, is DNA. Confocal microscopy showed co-localization of fH with antibodies specific for DNA. fH also binds to histones devoid of DNA, and CCPs 1-4, 6-8, and 8-15 mediate this interaction. Treatment of apoptotic cells with neuraminidase, chondroitinase, heparitinase, and heparinase did not change fH binding. Treatment of apoptotic cells with phospholipase A(2) dramatically increased both binding of fH and cell-surface DNA. We also excluded the possibility that fH interacts with lysophospholipids using surface plasmon resonance and flow cytometry with lipid-coated beads. Identification of annexin-II as one of the fH ligands on apoptotic cells together with the fact that autoantibodies against annexin-II are found in systemic lupus erythematosus provides further insight into understanding the pathogenesis of this disease.