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

Inhibition of vertebrate complement system by hematophagous arthropods: inhibitory molecules, mechanisms, physiological roles, and applications

In arthropods, hematophagy has arisen several times throughout evolution. This specialized feeding behavior offered a highly nutritious diet obtained during blood feeds. On the other hand, blood-sucking arthropods must overcome problems brought on by blood intake and digestion. Host blood complement acts on the bite site and is still active after ingestion, so complement activation is a potential threat to the host's skin feeding environment and to the arthropod gut enterocytes. During evolution, blood-sucking arthropods have selected, either in their saliva or gut, anticomplement molecules that inactivate host blood complement. This review presents an overview of the complement system and discusses the arthropod's salivary and gut anticomplement molecules studied to date, exploring their mechanism of action and other aspects related to the arthropod-host-pathogen interface. The possible therapeutic applications of arthropod's anticomplement molecules are also discussed.

Recruitment of C4b-binding protein is not a complement evasion strategy employed by Staphylococcus aureus

Complement offers a first line of defence against infection through the opsonization of microbial pathogens, recruitment of professional phagocytes to the infection site and the coordination of inflammatory responses required for the resolution of infection. Staphylococcus aureus is a successful pathogen that has developed multiple mechanisms to thwart host immune responses. Understanding the precise strategies employed by S. aureus to bypass host immunity will be paramount for the development of vaccines and or immunotherapies designed to prevent or limit infection. To gain a better insight into the specific immune evasion mechanisms used by S. aureus we examined the pathogen's interaction with the soluble complement inhibitor, C4b-binding protein (C4BP). Previous studies indicated that S. aureus recruits C4BP using a specific cell-wall-anchored surface protein and that bound C4BP limits complement deposition on the staphylococcal surface. Using flow-cytometric-based bacterial-protein binding assays we observed no interaction between S. aureus and C4BP. Moreover, we offer a precautionary warning that C4BP isolated from plasma can be co-purified with minute quantities of human IgG, which can distort binding analysis between S. aureus and human-derived proteins. Combined our data indicates that recruitment of C4BP is not a complement evasion strategy employed by S. aureus.

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.

Evasion of the complement system by Leishmania through the uptake of C4bBP, a complement regulatory protein, and probably by the action of GP63 on C4b molecules deposited on parasite surface

The complement system is a primary component of the vertebrate innate immune system, and its activity is harmful to microorganisms and parasites. To evade complement attack, some pathogens, such as viruses, bacteria, and protozoa, can interact with complement regulatory proteins from their hosts. Our research group has described the ability of Leishmania species to bind Factor H from human serum and use it as a tool to evade the complement system. However, there is no description of the interaction of Leishmania with other complement regulatory proteins, such as the C4b-binding protein (C4bBP), a negative regulator of classical and lectins complement system pathways. The results presented in this manuscript suggest that Leishmania infantum, L. amazonensis, and L. braziliensis recruit C4bBP from human serum. The uptake of C4bBP by L. infantum was studied in detail to improve our understanding of this inhibitory mechanism. When exposed to this complement regulator, parasites with inactivated GP63 bind to C4bBP and inactivate C4b deposited on their surface after serum exposure. This inactivation occurs by the action of Factor I, a complement system protease. In addition to the C4bBP-Factor I inactivation mechanism, the surface parasite protease GP63 can also inactivate soluble C4b molecules and probably that C4b molecules deposited on the parasites surface. This manuscript shows that Leishmania has two independent strategies to inactivate C4b molecules, preventing the progress of classical and lectin pathways. The identification of theC4bBP receptor on the Leishmania membrane may provide a new vaccine target to fight leishmaniasis.

Neisseria gonorrhoeae co-opts C4b-binding protein to enhance complement-independent survival from neutrophils

Neisseria gonorrhoeae (Gc) is a human-specific pathogen that causes the sexually transmitted infection gonorrhea. Gc survives in neutrophil-rich gonorrheal secretions, and recovered bacteria predominantly express phase-variable, surface-expressed opacity-associated (Opa) proteins (Opa+). However, expression of Opa proteins like OpaD decreases Gc survival when exposed to human neutrophils ex vivo. Here, we made the unexpected observation that incubation with normal human serum, which is found in inflamed mucosal secretions, enhances survival of Opa+ Gc from primary human neutrophils. We directly linked this phenomenon to a novel complement-independent function for C4b-binding protein (C4BP). When bound to the bacteria, C4BP was necessary and sufficient to suppress Gc-induced neutrophil reactive oxygen species production and prevent neutrophil phagocytosis of Opa+ Gc. This research identifies for the first time a complement-independent role for C4BP in enhancing the survival of a pathogenic bacterium from phagocytes, thereby revealing how Gc exploits inflammatory conditions to persist at human mucosal surfaces.

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.

Activity of Protein S-C4b Binding Protein and Total TFPI Levels in Egyptian SLE Patients: A Cross-Sectional Study

BACKGROUND: Systemic lupus erythematosus (SLE) is an immune disorder with alternating active and remission phases. Cardiovascular diseases and thrombosis are the major causes of mortality in SLE. The anticoagulant activity of Protein S (PS) is complemented by C4 binding protein (C4BP) and tissue factor pathway inhibitor (TFPI). AIM: This study aims to determine the extent of change in the levels of PS activity, C4BP, and total TFPI in active SLE in comparison to the SLE remission phase and their association with thrombosis during SLE flare. METHODS: The study included 180 Egyptian SLE patients who were classified into two groups: 100 SLE cases as the active group and 80 SLE cases as the remission group. The PS activity levels were processed on automated coagulation analyzers, whereas the C4BP and total TFPI levels were measured via enzyme-linked immunosorbent assay. RESULTS: The PS activity and C4BP levels were lower in the active SLE cases than in the remitted ones (p < 0.05). The levels of PS activity and C4BP were revealed to be independent predictors of SELENA-SLEDAI flare scores. In active SLE cases, the PS activity and C4BP levels were rated as excellent and fair classifiers of thrombotic risk in SLE flare, respectively. The total TFPI levels showed no association with SLE activity or its thrombotic consequences. CONCLUSIONS: The levels of PS activity and C4BP act as important biomarkers for SLE activity. Both can be implanted as predictive tools for thrombosis during activity.

Nontypeable Haemophilus influenzae P5 Binds Human C4b-Binding Protein, Promoting Serum Resistance

Exposure of P5 at the surface of NTHi positively correlates with C4BP binding.

C4BP bound to the bacterial surface retains its complement inhibitory capacity.

C4BP binding to P5 is important for NTHi serum resistance.

Nontypeable Haemophilus influenzae (NTHi) is a Gram-negative human pathogen that causes infections mainly in the upper and lower respiratory tract. The bacterium is associated with bronchitis and exacerbations in patients suffering from chronic obstructive pulmonary disease and frequently causes acute otitis media in preschool children. We have previously demonstrated that the binding of C4b binding protein (C4BP) is important for NTHi complement evasion. In this study, we identified outer membrane protein 5 (P5) of NTHi as a novel ligand of C4BP. Importantly, we observed significantly lower C4BP binding and decreased serum resistance in P5-deficient NTHi mutants. Surface expression of recombinant P5 on Escherichia coli conferred C4BP binding and consequently increased serum resistance. Moreover, P5 expression was positively correlated with C4BP binding in a series of clinical isolates. We revealed higher levels of P5 surface expression and consequently more C4BP binding in isolates from the lower respiratory tract of chronic obstructive pulmonary disease patients and tonsil specimens compared with isolates from the upper respiratory tract and the bloodstream (invasive strains). Our results highlight P5 as an important protein for protecting NTHi against complement-mediated killing.

How Does Complement Affect Hematological Malignancies: From Basic Mechanisms to Clinical Application

Complement, as a central immune surveillance system, can be activated within seconds upon stimulation, thereby displaying multiple immune effector functions. However, in pathologic scenarios (like in tumor progression), activated complement can both display protective effects to control tumor development and passively promotes the tumor growth. Clinical investigations show that patients with several hematological malignancies often display abnormal level of specific complement components, which in turn modulates complement activation or deregulated cascade. In the past decades, complement-dependent cytotoxicity and complement-dependent cell-mediated phagocytosis were fully approved to display vital roles in monoclonal antibody-based immunotherapies, especially in therapies against hematological malignancies. However, tumor-mediated complement evasion presents a big challenge for such a therapy. This review aims to provide an integrative overview on the roles of the complement in tumor promotion, highlights complement mediated effects on antibody-based immunotherapy against distinct hematological tumors, hopefully provides a theoretical basis for the development of complement-based cancer targeted therapies.

Noncanonical immunomodulatory activity of complement regulator C4BP(β-) limits the development of lupus nephritis

Lupus nephritis is a chronic autoimmune-inflammatory condition that can lead to end-stage kidney disease. Presently available immunosuppressive treatments for lupus nephritis are suboptimal and can induce significant side effects. Recently, we characterized a novel immunomodulatory activity of the minor isoform of the classical pathway complement inhibitor, C4BP(β-). We show here that C4BP(β-) treatment prevented the development of proteinuria and albuminuria, decreased significantly the formation of anti-dsDNA antibodies and, locally, mitigated renal glomerular IgG and C3 deposition and generation of apoptotic cells. There was a consequent histological improvement and increased survival in lupus-prone mice. The therapeutic efficacy of C4BP(β-) was analogous to that of the broad-acting immunosuppressant cyclophosphamide. Remarkably, a comparative transcriptional profiling analysis revealed that the kidney gene expression signature resulting from C4BP(β-) treatment turned out to be 10 times smaller than that induced by cyclophosphamide treatment. C4BP(β-) immunomodulation induced significant downregulation of transcripts relevant to lupus nephritis indicating immunopathogenic cell infiltration, including activated T cells (Lat), B cells (Cd19, Ms4a1, Tnfrsf13c), inflammatory phagocytes (Irf7) and neutrophils (Prtn3, S100a8, S100a9). Furthermore, cytokine profiling and immunohistochemistry confirmed that C4BP(β-), through systemic and local CXCL13 downregulation, was able to prevent ectopic lymphoid structures neogenesis in aged mice with lupus nephritis. Thus, due to its anti-inflammatory and immunomodulatory activities and high specificity, C4BP(β-) could be considered for further clinical development in patients with systemic lupus erythematosus.

The human serum protein C4b-binding protein inhibits pancreatic IAPP-induced inflammasome activation

AIMS/HYPOTHESIS

Inflammasome activation and subsequent IL-1β production is a driver of islet pathology in type 2 diabetes. Oligomers, but not mature amyloid fibrils, of human islet amyloid polypeptide (IAPP), which is co-secreted with insulin, trigger NOD-like receptor pyrin domain containing-3 (NLRP3) inflammasome activation. C4b-binding protein (C4BP), present in serum, binds to IAPP and affects transition of IAPP monomers and oligomers to amyloid fibrils. We therefore hypothesised that C4BP inhibits IAPP-mediated inflammasome activation and IL-1β production.

METHODS

Macrophages were exposed to IAPP in the presence or absence of plasma-purified human C4BP, and inflammasome activation was assessed by IL-1β secretion as detected by ELISA and reporter cell lines. IAPP fibrillation was assessed by thioflavin T assay. Uptake of IAPP-C4BP complexes and their effects on phagolysosomal stability were assessed by flow cytometry and confocal microscopy. The effect of C4BP regulation of IAPP-mediated inflammasome activation on beta cell function was assessed using a clonal rat beta cell line. Immunohistochemistry was used to examine the association of IAPP amyloid deposits and macrophage infiltration in isolated human and mouse pancreatic islets, and expression of C4BP from isolated human pancreatic islets was assessed by quantitative PCR, immunohistochemistry and western blot.

RESULTS

C4BP significantly inhibited IAPP-mediated IL-1β secretion from primed macrophages at physiological concentrations in a dose-dependent manner. C4BP bound to and was internalised together with IAPP. C4BP did not affect IAPP uptake into phagolysosomal compartments, although it did inhibit its formation into amyloid fibrils. The loss of macrophage phagolysosomal integrity induced by IAPP incubation was inhibited by co-incubation with C4BP. Supernatant fractions from macrophages activated with IAPP inhibited both insulin secretion and viability of clonal beta cells in an IL-1β-dependent manner but the presence of C4BP during macrophage IAPP incubation rescued beta cell function and viability. In human and mouse islets, the presence of amyloid deposits correlated with higher numbers of infiltrating macrophages. Isolated human islets expressed and secreted C4BP, which increased with addition of IL-1β.

CONCLUSIONS/INTERPRETATION

IAPP deposition is associated with inflammatory cell infiltrates in pancreatic islets. C4BP blocks IAPP-induced inflammasome activation by preventing the loss of macrophage phagolysosomal integrity required for NLRP3 activation. The consequence of this is the preservation of beta cell function and viability. C4BP is secreted directly from human pancreatic islets and this increases in response to inflammatory cytokines. We therefore propose that C4BP acts as an extracellular chaperone protein that limits the proinflammatory effects of IAPP.

Hepatitis B virus X protein up-regulates C4b-binding protein α through activating transcription factor Sp1 in protection of hepatoma cells from complement attack

Hepatitis B virus X protein (HBx) plays crucial roles in the development of hepatocellular carcinoma (HCC). We previously showed that HBx protected hepatoma cells from complement attack by activation of CD59. Moreover, in this study we found that HBx protected hepatoma cells from complement attack by activation of C4b-binding protein α (C4BPα), a potent inhibitor of complement system. We observed that HBx were positively correlated with those of C4BPα in clinical HCC tissues. Mechanistically, HBx activated the promoter core region of C4BPα, located at -1199/-803nt, through binding to transcription factor Sp1. In addition, chromatin immunoprecipitation (ChIP) assays showed that HBx was able to bind to the promoter of C4BPα, which could be blocked by Sp1 silencing. Functionally, knockdown of C4BPα obviously increased the deposition of C5b-9, a complex of complement membrane attack, and remarkably abolished the HBx-induced resistance of hepatoma cells from complement attack in vitro and in vivo. Thus, we conclude that HBx up-regulates C4BPα through activating transcription factor Sp1 in protection of liver cancer cells from complement attack. Our finding provides new insights into the mechanism by which HBx enhances protection of hepatoma cells from complement attack.

Local expression of complement factor I in breast cancer cells correlates with poor survival and recurrence

Tumor cells often evade killing by the complement system by overexpressing membrane-bound complement inhibitors. However, production of soluble complement inhibitors in cells other than hepatocytes was rarely reported. We screened several breast cancer cell lines for expression of soluble complement inhibitor, complement factor I (FI). We also analyzed local production of FI in tissue microarrays with tumors from 130 breast cancer patients by in situ hybridization and immunohistochemistry. We found expression of FI in breast adenocarcinoma cell line MDA-MB-468 and confirmed its functional activity. Expression of FI at mRNA and protein levels was also confirmed in tumor cells and tumor stroma, both in fibroblasts and infiltrating immune cells. Multivariate Cox regression analyses revealed that high expression of FI protein in tumor cells was correlated with significantly shorter cancer-specific survival (HR 2.8; 95 % CI 1.0-7.5; p = 0.048) and recurrence-free survival (HR 3.4; 95 % CI 1.5-7.4; p = 0.002). High FI expression was positively correlated with tumor size (p < 0.001), and Nottingham histological grade (p = 0.015) and associated with estrogen and progesterone receptor status (p = 0.03 and p = 0.009, respectively). Our data show that FI is expressed in breast cancer and is associated with unfavorable clinical outcome.

Systematic toxicity mechanism analysis of proton pump inhibitors: an in silico study

Proton pump inhibitors (PPIs) are extensively used for the treatment of gastric acid-related disorders. PPIs appear to be well tolerated and almost have no short-term side effects. However, the clinical adverse reactions of long-term PPI usage are increasingly reported in recent years. So far, there is no study that elucidates the side effect mechanisms of PPIs comprehensively and systematically. In this study, a well-defined small molecule perturbed microarray data set of 344 compounds and 1695 samples was analyzed. With this high-throughput data set, a new index (Identity, I) was designed to identify PPI-specific differentially expressed genes. Results indicated that (1) up-regulated genes, such as RETSAT, CYP1A1, CYP1A2, and UGT, enhanced vitamin A's metabolism processes in the cellular retinol metabolism pathway; and that (2) down-regulated genes, such as C1QA, C1QC, C4BPA, C4BPB, CFI, and SERPING1, enriched in the complement and coagulation cascades pathway. In addition, strong association was observed between these PPI-specific differentially expressed genes and the reported side effects of PPIs by the gene-disease association network analysis. One potential toxicity mechanism of PPIs as suggested from this systematic PPI-specific gene expression analysis is that PPIs are enriched in acidic organelles where they are activated and inhibit V-ATPases and acid hydrolases, and consequently block the pathways of antigen presentation, the synthesis and secretion of cytokines, and complement component proteins and coagulation factors. The strategies developed in this work could be extended to studies on other drugs.