Structure and function of C3a anaphylatoxin

In vivo safety evaluation and tracing of arginylglycylaspartic acid-engineered phage nanofiber in murine model

The engineered phage YSY184, mimicking the extracellular matrix nanofiber, effectively promotes stem cell differentiation and angiogenesis. This study evaluated its safety in a mouse model, monitoring weight, immunogenicity, spleen immune responses, and macrophage infiltration. Rapid clearance of YSY184 was observed, with peak tissue presence within three hours, significantly reduced by 24 hours, and negligible after one month. No adverse physiological or pathological effects were detected post-administration, affirming YSY184's safety and underscore its potential for therapeutic use, warranting further clinical exploration.

Molecular basis of anaphylatoxin binding, activation, and signaling bias at complement receptors

The complement system is a critical part of our innate immune response, and the terminal products of this cascade, anaphylatoxins C3a and C5a, exert their physiological and pathophysiological responses primarily via two GPCRs, C3aR and C5aR1. However, the molecular mechanism of ligand recognition, activation, and signaling bias of these receptors remains mostly elusive. Here, we present nine cryo-EM structures of C3aR and C5aR1 activated by their natural and synthetic agonists, which reveal distinct binding pocket topologies of complement anaphylatoxins and provide key insights into receptor activation and transducer coupling. We also uncover the structural basis of a naturally occurring mechanism to dampen the inflammatory response of C5a via proteolytic cleavage of the terminal arginine and the G-protein signaling bias elicited by a peptide agonist of C3aR identified here. In summary, our study elucidates the innerworkings of the complement anaphylatoxin receptors and should facilitate structure-guided drug discovery to target these receptors in a spectrum of disorders.

C3-dependent effector functions of complement

C3 is the central effector molecule of the complement system, mediating its multiple functions through different binding sites and their corresponding receptors. We will introduce the C3 forms (native C3, C3 [H₂ O], and intracellular C3), the C3 fragments C3a, C3b, iC3b, and C3dg/C3d, and the C3 expression sites. To highlight the important role that C3 plays in human biological processes, we will give an overview of the diseases linked to C3 deficiency and to uncontrolled C3 activation. Next, we will present a structural description of C3 activation and of the C3 fragments generated by complement regulation. We will proceed by describing the C3a interaction with the anaphylatoxin receptor, followed by the interactions of opsonins (C3b, iC3b, and C3dg/C3d) with complement receptors, divided into two groups: receptors bearing complement regulatory functions and the effector receptors without complement regulatory activity. We outline the molecular architecture of the receptors, their binding sites on the C3 activation fragments, the cells expressing them, the diversity of their functions, and recent advances. With this review, we aim to give an up-to-date analysis of the processes triggered by C3 activation fragments on different cell types in health and disease contexts.

Complement C3 and Activated Fragment C3a Are Involved in Complement Activation and Anti-Bacterial Immunity

In the complement system, C3 is a central component in complement activation, immune defense and immune regulation. In all pathways of complement activation, the pivotal step is conversion of the component C3 to C3b and C3a, which is responsible to eliminate the pathogen and opsonization. In this study, we examined the immunological properties of C3 and its activated fragment C3a from Japanese flounder (Paralichthys olivaceus) (PoC3 and PoC3a), a teleost species with important economic value. PoC3 is composed of 1655 amino acid residues, contains the six domains and highly conserved GCGEQ sequence of the C3 family. We found that PoC3 expression occurred in nine different tissues and was upregulated by bacterial challenge. In serum, PoC3 was able to bind to a broad-spectrum of bacteria, and purified native PoC3 could directly kill specific pathogen. When PoC3 expression in Japanese flounder was knocked down by siRNA, serum complement activity was significantly decreased, and bacterial replication in fish tissues was significantly increased. Recombinant PoC3a (rPoC3a) exhibited apparent binding capacities to bacteria and Japanese flounder peripheral blood leukocytes (PBL) and induce chemotaxis of PBL. Japanese flounder administered rPoC3a exhibited enhanced resistance against bacterial infection. Taken together, these results indicate that PoC3 is likely a key factor of complement activation, and PoC3 and PoC3a are required for optimal defense against bacterial infection in teleost.

Quantitative Modeling of the Alternative Pathway of the Complement System

The complement system is an integral part of innate immunity that detects and eliminates invading pathogens through a cascade of reactions. The destructive effects of the complement activation on host cells are inhibited through versatile regulators that are present in plasma and bound to membranes. Impairment in the capacity of these regulators to function in the proper manner results in autoimmune diseases. To better understand the delicate balance between complement activation and regulation, we have developed a comprehensive quantitative model of the alternative pathway. Our model incorporates a system of ordinary differential equations that describes the dynamics of the four steps of the alternative pathway under physiological conditions: (i) initiation (fluid phase), (ii) amplification (surfaces), (iii) termination (pathogen), and (iv) regulation (host cell and fluid phase). We have examined complement activation and regulation on different surfaces, using the cellular dimensions of a characteristic bacterium (E. coli) and host cell (human erythrocyte). In addition, we have incorporated neutrophil-secreted properdin into the model highlighting the cross talk of neutrophils with the alternative pathway in coordinating innate immunity. Our study yields a series of time-dependent response data for all alternative pathway proteins, fragments, and complexes. We demonstrate the robustness of alternative pathway on the surface of pathogens in which complement components were able to saturate the entire region in about 54 minutes, while occupying less than one percent on host cells at the same time period. Our model reveals that tight regulation of complement starts in fluid phase in which propagation of the alternative pathway was inhibited through the dismantlement of fluid phase convertases. Our model also depicts the intricate role that properdin released from neutrophils plays in initiating and propagating the alternative pathway during bacterial infection.

Complement activation in the context of stem cells and tissue repair

The complement pathway is best known for its role in immune surveillance and inflammation. However, its ability of opsonizing and removing not only pathogens, but also necrotic and apoptotic cells, is a phylogenetically ancient means of initiating tissue repair. The means and mechanisms of complement-mediated tissue repair are discussed in this review. There is increasing evidence that complement activation contributes to tissue repair at several levels. These range from the chemo-attraction of stem and progenitor cells to areas of complement activation, to increased survival of various cell types in the presence of split products of complement, and to the production of trophic factors by cells activated by the anaphylatoxins C3a and C5a. This repair aspect of complement biology has not found sufficient appreciation until recently. The following will examine this aspect of complement biology with an emphasis on the anaphylatoxins C3a and C5a.

Structural and functional characterization of human and murine C5a anaphylatoxins

Complement is an ancient part of the innate immune system that plays a pivotal role in protection against invading pathogens and helps to clear apoptotic and necrotic cells. Upon complement activation, a cascade of proteolytic events generates the complement effectors, including the anaphylatoxins C3a and C5a. Signalling through their cognate G-protein coupled receptors, C3aR and C5aR, leads to a wide range of biological events promoting inflammation at the site of complement activation. The function of anaphylatoxins is regulated by circulating carboxypeptidases that remove their C-terminal arginine residue, yielding C3a-desArg and C5a-desArg. Whereas human C3a and C3a-desArg adopt a canonical four-helix bundle fold, the conformation of human C5a-desArg has recently been described as a three-helix bundle. Here, the crystal structures of an antagonist version of human C5a, A8(Δ71-73), and of murine C5a and C5a-desArg are reported. Whereas A8(Δ71-73) adopts a three-helix bundle conformation similar to human C5a-desArg, the two murine proteins form a four-helix bundle. A cell-based functional assay reveals that murine C5a-desArg, in contrast to its human counterpart, exerts the same level of activition as murine C5a on its cognate receptor. The role of the different C5a conformations is discussed in relation to the differential activation of C5a receptors across species.

Interplay between invertebrate C3a with vertebrate macrophages: functional characterization of immune activities of amphioxus C3a

Our current knowledge of the structure and function of C3a comes from the study of vertebrate C3a anaphylatoxins, virtually nothing is known about the structure and function of C3a molecules in invertebrates. Here we demonstrated that C3a from the invertebrate chordate Branchiostoma japonicum, BjC3a, was similar to vertebrate C3a possessing potential antibacterial activity, as revealed by sequence analysis and computational modeling. The antibacterial activity of BjC3a was definitely confirmed by both antibacterial assay and TEM observation showing that recombinant BjC3a was directly bactericidal. Additionally, recombinant BjC3a, like vertebrate C3a, was capable of inducing sea bass macrophage migration and enhancing macrophage phagocytosis and respiratory burst response. Moreover, recombinant BjC3a-desArg (generated by removal of the C-terminal arginine), like mammalian C3a-desArg, retained the immunological activities of BjC3a such as antibacterial and respiratory burst-stimulating activities, indicating that the immunological functions of C3a-desArg were conserved throughout chordate evolution. Altogether, our findings show that invertebrate (amphioxus) BjC3a is able to interact with vertebrate (sea bass) macrophages and mediate immune activities, suggesting the emergence of the inflammatory pathway of the complement system similar to that of vertebrates in the basal chordate amphioxus.

Human C3a and C3a desArg anaphylatoxins have conserved structures, in contrast to C5a and C5a desArg

Complement is a part of innate immunity that has a critical role in the protection against microbial infections, bridges the innate with the adaptive immunity and initiates inflammation. Activation of the complement, by specific recognition of molecular patterns presented by an activator, for example, a pathogen cell, in the classical and lectin pathways or spontaneously in the alternative pathway, leads to the opsonization of the activator and the production of pro-inflammatory molecules such as the C3a anaphylatoxin. The biological function of this anaphylatoxin is regulated by carboxypeptidase B, a plasma protease that cleaves off the C-terminal arginine yielding C3a desArg, an inactive form. While functional assays demonstrate strikingly different physiological effects between C3a and C3a desArg, no structural information is available on the possible conformational differences between the two proteins. Here, we report a novel and simple expression and purification protocol for recombinant human C3a and C3a desArg anaphylatoxins, as well as their crystal structures at 2.3 and 2.6 Å, respectively. Structural analysis revealed no significant conformational differences between the two anaphylatoxins in contrast to what has been reported for C5a and C5a desArg. We compare the structures of different anaphylatoxins and discuss the relevance of their observed conformations to complement activation and binding of the anaphylatoxins to their cognate receptors.

Sampling the N-terminal proteome of human blood

The proteomes of blood plasma and serum represent a potential gold mine of biological and diagnostic information, but challenges such as dynamic range of protein concentration have hampered efforts to unlock this resource. Here we present a method to label and isolate N-terminal peptides from human plasma and serum. This process dramatically reduces the complexity of the sample by eliminating internal peptides. We identify 772 unique N-terminal peptides in 222 proteins, ranging over six orders of magnitude in abundance. This approach is highly suited for studying natural proteolysis in plasma and serum. We find internal cleavages in plasma proteins created by endo- and exopeptidases, providing information about the activities of proteolytic enzymes in blood, which may be correlated with disease states. We also find signatures of signal peptide cleavage, coagulation and complement activation, and other known proteolytic processes, in addition to a large number of cleavages that have not been reported previously, including over 200 cleavages of blood proteins by aminopeptidases. Finally, we can identify substrates from specific proteases by exogenous addition of the protease combined with N-terminal isolation and quantitative mass spectrometry. In this way we identified proteins cleaved in human plasma by membrane-type serine protease 1, an enzyme linked to cancer progression. These studies demonstrate the utility of direct N-terminal labeling by subtiligase to identify and characterize endogenous and exogenous proteolysis in human plasma and serum.

Adenovirus activates complement by distinctly different mechanisms in vitro and in vivo: indirect complement activation by virions in vivo

Understanding innate immunity is key to improving the safety of adenovirus (Ad) vectors for systemic gene therapy. Ad has been shown to activate complement in vitro, but activation of complement after Ad injection in vivo has not been directly measured. Using complement protein C3a as a marker of complement activation, we show that types 2 and 5 human Ads cause rapid complement activation after intravenous injection in mice. Unexpectedly, the mechanisms in vivo were different than those in vitro. Antibodies were critical for the activation of complement by Ad in vitro, but antibodies were not required in vivo. The classical pathway was required in vitro, whereas complement activation in vivo involved both classical and nonclassical pathways as well as the reticuloendothelial system. Remarkably, the entry-deficient Ad mutant ts1 was completely unable to activate complement in vivo even though it was fully able to activate complement in vitro. This result demonstrates that the complement system senses intravenously injected Ad primarily by detecting the effects of Ad on cells rather than through direct interaction of complement with virions. Encouragingly, shielding Ad with polyethylene glycol was effective at reducing complement activation both in vitro and in vivo. In summary, intravenously injected Ad rapidly activates complement through multiple pathways, but these pathways are different than those identified by in vitro studies. In vitro studies are poorly predictive of in vivo mechanisms because Ad virions activate complement through indirect mechanisms in vivo.

Pathogenic role of antiphospholipid antibodies

The antiphospholipid antibody syndrome (APS) is characterized by recurrent arterial and venous thrombosis and/or pregnancy in association with antiphospholipid (aPL) antibodies. The pathogenic mechanisms in APS that lead to in vivo injury are incompletely understood. Recent evidence suggests that APL antibodies alter regulation of haemostasis and induce activation of complement. We will discuss the current knowledge on how aPL antibodies trigger increased inflammation and enhanced thrombotic tendency, and thereby lead to tissue damage.

Intravenous immunoglobulin (IVIG) protects the brain against experimental stroke by preventing complement-mediated neuronal cell death

Stroke is among the three leading causes of death worldwide and the most frequent cause of permanent disability. Brain ischemia induces an inflammatory response involving activated complement fragments. Here we show that i.v. Ig (IVIG) treatment, which scavenges complement fragments, protects brain cells against the deleterious effects of experimental ischemia and reperfusion (I/R) and prevents I/R-induced mortality in mice. Animals administered IVIG either 30 min before ischemia or after 3 h of reperfusion exhibited a 50-60% reduction of brain infarct size and a 2- to 3-fold improvement of the functional outcome. Even a single low dose of IVIG given after stroke was effective. IVIG was protective in the nonreperfusion model of murine stroke as well and did not exert any peripheral effects. Human IgG as well as intrinsic murine C3 levels were significantly higher in the infarcted brain region compared with the noninjured side, and their physical association was demonstrated by immuno-coprecipitation. C5-deficient mice were significantly protected from I/R injury compared with their wild-type littermates. Exposure of cultured neurons to oxygen/glucose deprivation resulted in increased levels of C3 associated with activation of caspase 3, a marker of apoptosis; both signals were attenuated with IVIG treatment. Our data suggest a major role for complement-mediated cell death in ischemic brain injury and the prospect of using IVIG in relatively low doses as an interventional therapy for stroke.

Structure and function of human plasma carboxypeptidase N, the anaphylatoxin inactivator

Human carboxypeptidase N (CPN) was discovered in the early 1960s as a plasma enzyme that inactivates bradykinin and was identified 8 years later as the major "anaphylatoxin inactivator" of blood. CPN plays an important role in protecting the body from excessive buildup of potentially deleterious peptides that normally act as local autocrine or paracrine hormones. This review summarizes the structure, enzymatic properties and function of this important human enzyme, including insights gained by the recent elucidation of the crystal structure of the CPN catalytic subunit and structural modeling of the non-catalytic regulatory 83 kDa subunit. We also discuss its physiological role in cleaving substrates such as kinins, anaphylatoxins, creatine kinase, plasminogen receptors, hemoglobin and stromal cell-derived factor-1alpha (SDF-1alpha).

Antiphospholipid antibodies and pregnancy loss: a disorder of inflammation

The antiphospholipid syndrome (APS) is a leading cause of miscarriage and maternal and fetal morbidity. APS is characterized by thrombosis and pregnancy loss that occur in the presence of antiphospholipid (aPL) antibodies. Using a mouse model of APS induced by passive transfer of human aPL antibodies, we have shown that complement activation plays an essential and causative role in pregnancy loss and fetal growth restriction, and that blocking activation of the complement cascade rescues pregnancies. Conventional treatment for APS patients is sub-anticoagulant doses of heparin throughout pregnancy. Could heparin prevent pregnancy loss by inhibiting complement? In our experimental model of APS, heparin inhibits activation of complement on trophoblasts in vivo and in vitro, and anticoagulation in and of itself is not sufficient to prevent pregnancy complications. These studies underscore the importance of inflammation in fetal injury associated with aPL antibodies and raise the importance of developing and testing targeted complement inhibitory therapy for patients with APS.

Composition effect on peptide interaction with lipids and bacteria: variants of C3a peptide CNY21

The effect of peptide hydrophobicity and charge on peptide interaction with model lipid bilayers was investigated for the C3a-derived peptide CNY21 by fluorescence spectroscopy, circular dichroism, ellipsometry, z-potential, and photon correlation spectroscopy measurements. For both zwitterionic and anionic liposomes, the membrane-disruptive potency for CNY21 variants increased with increasing net positive charge and mean hydrophobicity and was completely lost on elimination of all peptide positive charges. Analogous effects of elimination of the peptide positive net charge in particular were found regarding bacteria killing for both Pseudomonas aeruginosa and Bacillus subtilis. The peptides, characterized by moderate helix content both in buffer and when attached to the liposomes, displayed high adsorption for the net positively charged peptide variants, whereas adsorption was non-measurable for the uncharged peptide. That electrostatically driven adsorption represents the main driving force for membrane disruption in lipid systems was also demonstrated by a drastic reduction in both liposome leakage and peptide adsorption with increasing ionic strength, and this salt inactivation can be partly avoided by increasing the peptide hydrophobicity. This increased electrolyte resistance translates also to a higher antibacterial effect for the hydrophobically modified variant at high salt concentration. Overall, our findings demonstrate the importance of the peptide adsorption and resulting peptide interfacial density for membrane-disruptive effects of these peptides.

Soluble Mediators as Therapeutic Targets in Systemic Lupus Erythematosus: Cytokines, Immunoglobulin Receptors, and the Complement System

After many years of anticipation, we have entered a period of promise for new lupus therapies; several clinical trials are planned or are in progress. The accelerated activity in systemic lupus erythematosus therapeutics has been driven by scientific advances. Enhanced understanding of the cells and mediators that drive autoimmunity and tissue damage has led to the identification of rational therapeutic targets. The conventional immunosuppressive therapies, including corticosteroids and cyclophosphamide, can be effective but at a high and unacceptable cost of adverse effects. There is high optimism that targeted therapies, including those that are specific for soluble mediators, will allow effective control of disease activity while sparing patients the damaging toxicities that are associated with traditional immunosuppressive agents.

Novel roles for acylation stimulating protein/C3adesArg: a review of recent in vitro and in vivo evidence

Recent experimental evidence is shedding more light on the physiological actions of acylation-stimulating protein (ASP)/C3adesArg. The role of ASP in regulating lipid metabolism has primarily focused on its participation in the stimulation of triglyceride synthesis (TGS) and glucose transport. Although there is no doubt that ASP, an adipocyte-produced hormone, plays a key physiological role, accumulating evidence suggests that the effects of ASP go beyond its acute effects on lipid metabolism. In this review, we present novel findings of ASP/C3adesArg effects on preadipocyte differentiation. In 3T3-L1 and 3T3-F442A cells, ASP can substitute for insulin and enhance differentiation as measured by intracellular lipid droplet accumulation, clonal expansion, and increased expression of differentiation markers. Specifically, ASP increased basal TGS by 250% after 9 days differentiation, with similar effects induced by insulin. With ASP treatment, expression of C/EBPdelta was up-regulated early in differentiation (day 2) and decreased thereafter. Expression of PPARgamma and late markers of differentiation, such as adipsin and diacylglycerol acyltransferase-1, were also increased. Effects on clonal expansion were indicated by a twofold increase in [(3)H] thymidine incorporation in 3T3-L1 cells compared to treatment with IBMX + DX alone. Further, the effects of ASP extended beyond adipose tissue to endocrine effects on hormone secretion of insulin (pancreatic cells); cytokines TNFalpha, IL-1beta, and IL-6 (myeloid cells); prolactin, growth hormone, and adrenocorticotropin (pituitary cells). Finally, the potential implication of C5L2, the newly discovered ASP receptor, and its expression profile in various tissues are discussed relative to ASP function.

Activation of the complement system generates antibacterial peptides

The complement system represents an evolutionary old and significant part of the innate immune system involved in protection against invading microorganisms. Here, we show that the anaphylatoxin C3a and its inactivated derivative C3a-desArg are antibacterial, demonstrating a previously unknown direct antimicrobial effect of complement activation. The C3a peptide, as well as functional epitopes in the sequence, efficiently killed the Gram-negative bacteria Escherichia coli, Pseudomonas aeruginosa, and the Gram-positive Enterococcus faecalis. In mice, a C3a-derived peptide suppressed infection by Gram-positive Streptococcus pyogenes bacteria. Fluorescence and electron microscopy demonstrated that C3a binds to and induces breaks in bacterial membranes. C3a was also found to induce membrane leakage of liposomes. These findings provide an interesting link between the complement system and antimicrobial peptides, which are two important branches of innate immunity.

The anaphylatoxin C3a downregulates the Th2 response to epicutaneously introduced antigen

Mechanical injury to the skin results in activation of the complement component C3 and release of the anaphylatoxin C3a. C3a binds to a seven-transmembrane G protein-coupled receptor, C3aR. We used C3aR(-/-) mice to examine the role of C3a in a mouse model of allergic inflammation induced by epicutaneous sensitization with OVA. C3aR(-/-) mice exhibited an exaggerated Th2 response to epicutaneous but not to intraperitoneal sensitization with OVA, as evidenced by significantly elevated levels of serum OVA-specific IgG1 and significantly increased secretion of the Th2 cytokines IL-4, IL-5, and IL-10 by antigen-stimulated splenocytes. Presentation of OVA peptide by C3aR(-/-) APCs caused significantly more IL-4 and IL-5 secretion by T cells from OVA-T cell receptor (OVA-TCR) transgenic mice compared with presentation by WT APCs. C3a inhibited the ability of splenocytes, but not of highly purified T cells, to secrete Th2 cytokines in response to TCR ligation. This inhibition was mediated by IL-12 secreted by APCs in response to C3a. These results suggest that C3a-C3aR interactions inhibit the ability of APCs to drive Th2 cell differentiation in response to epicutaneously introduced antigen and may have important implications for allergic skin diseases.

Human monocyte-derived dendritic cells are chemoattracted to C3a after up-regulation of the C3a receptor with interferons

The anaphylatoxin C3a is an important inflammatory mediator in the innate and adaptive immune systems. Recent reports in various animal models have fostered the role of C3a in mediating allergic reactions such as pulmonary allergies. However, data in humans are limited and the cellular targets for C3a are not fully understood. We sought to explore human dendritic cells as a new target for C3a, because C3a receptor (C3aR) expression has been described on myeloid cells, and dendritic cells are likely make contact with C3a at sites of inflammatory reactions. In this study, we demonstrated the expression of the C3aR on human monocyte-derived dendritic cells (MoDC) and its up-regulation by interferon (IFN)-alpha, IFN-gamma and prostaglandin E2 (PGE2). The strongest up-regulation was yielded by the combination of IFN-alpha+ IFN-gamma. Tumour necrosis factor-alpha (TNF-alpha) down-regulated the C3aR. After up-regulation of the C3aR by IFN-alpha+ IFN-gamma, C3a significantly up-regulated the surface expression of CD54, CD83 and CD86, but not of CD40, CD80 or human leucocyte antigen (HLA)-DR. C3a had no effect on the production of interleukin (IL)-10 or IL-12p70, or on the capacity of MoDC to stimulate autologous T-cell proliferation. However, C3a had a direct migratory effect on MoDC, as indicated by the induction of F-actin polymerization and migration in Boyden chamber experiments, which was pronounced after up-regulation of the C3aR with IFN-alpha+ IFN-gamma. Therefore, dendritic cells represent another group of target cells that might be recruited by C3a to areas of inflammation, in particular under conditions where IFNs are increased in the surrounding environment.

Complement C3a receptors in the pituitary gland: a novel pathway by which an innate immune molecule releases hormones involved in the control of inflammation

Two-way communication exists between the endocrine and immune systems using molecules such as hormones and cytokines. Here we describe a new pathway by which C3a, a complement-derived cytokine, stimulates anterior pituitary hormone release and activates the hypothalamic-pituitary-adrenal axis, a reflex central to the stress response and to the control of inflammation. We show that C3a receptors are expressed in pituitary hormone secreting and non-hormone secreting (folliculostellate) cells and that both C3a and C3adesArg (a non-inflammatory metabolite) stimulate pituitary cell cultures to release prolactin, growth hormone, and adrenocorticotropin. Serum levels of these hormones, together with adrenal corticosterone, increase dose dependently with recombinant C3a and C3adesArg administration in vivo. Pertussis toxin blocks the response to C3a but not C3adesArg, which indicates the presence of two receptors, only one of which is coupled to Galphai-proteins. We propose that the complement innate immune molecules (cytokines) modulate tissue-specific and systemic inflammatory responses through communication with the endocrine pituitary gland.

Introduction of a low molecular weight agonist peptide for complement C3a receptor into soybean proglycinin A1aB1b subunit by site-directed mutagenesis

LPLPR, a complement C3a agonist peptide, with hypocholesterolemic activity was introduced into the homologous site of soybean proglycinin A1aB1b subunit by site-directed mutagenesis. This modified proglycinin was expressed in E. coli and recovered from the insoluble fraction. LPLPR was released by the action of chymotrypsin and trypsin as expected. Furthermore, two peptides (RPSYLPLPR and PSYLPLPR) with extended sequence at the amino-terminus of LPLPR were obtained. Their ileum-contracting activity was 9 to approximately 13 times stronger than that of LPLPR. The overall yields of purified LPLPR, RPSYLPLPR and PSYLPLPR were 25%, 12%, and 0.7% respectively.

Plasma C3a and C4a levels in liver transplant recipients: a longitudinal study

Liver transplant patients were enrolled in a study designed to investigate correlations between plasma complement C3a or C4a levels and various postoperative complications. Longitudinal EDTA-plasma levels of C3a and C4a were measured by quantitative radioimmunoassay. Acute rejection gave a characteristic and marked increase in blood C3a, C4a and gamma-glutamyl transferase (gammaGT) levels, which rapidly resolved after high dose steroid treatment. Cytomegalovirus (CMV) infections in two of three patients gave an initial small increase only in C3a levels (i.e., alternative pathway activation) followed approximately 6 weeks later by a marked increase in C4a levels (i.e., classical or lectin pathway activation). In a third patient diagnosed for CMV infection, the complement activation profile was complicated by a coincident minor rejection episode. However, a late stage elevation in C4a was also noted. Two patients experiencing biopsy proven recurrent hepatitis C infections following transplantation exhibited increases in both gammaGT and C4a levels, without a significant increase in the level of C3a. Several hepatitis C and one hepatitis B patient had multiple late activation episodes involving marked elevation in both plasma C3a and C4a levels without detectable increases in the liver enzymes conventionally used to monitor organ function. We also showed that ex vivo activation of complement in EDTA plasma from all transplant patients was abnormally high. The classical or lectin pathway is believed to be responsible for this excessive ex vivo complement activation in the plasma of these patients. Therefore, subclinical rejection episodes and/or viral infections may be effectively detected or monitored by measuring C3a and C4a levels in plasma samples from liver transplant patients. Routine measurement of plasma complement products may provide an early non-invasive mode for detecting infections and also serve to monitor chronic or acute changes in the patient's immune system.

Ligand-induced phosphorylation of anaphylatoxin receptors C3aR and C5aR is mediated by "G protein-coupled receptor kinases

Continuous stimulation of anaphylatoxin receptors C3aR and C5aR with their cognate ligands engenders, within minutes, diminished responsiveness of these receptors. We tested the hypothesis that agonist-induced desensitization involves C3aR and C5aR phosphorylation by G protein-coupled receptor kinases (GRK). When expressed in rat basophilic leukemia cells and exposed to C3a, the C3aR underwent rapid (t(1/2) approximately 15 s), dose-dependent (EC50 approximately 10 nM) and reversible phosphorylation by a kinase refractory to the effects of PKC inhibitors. Phosphoamino acid analysis revealed that the C3aR is phosphorylated on serine and threonine, but not on tyrosine residues. Overexpression of GRK2, GRK3, GRK5 or GRK6 together with C3aR in COS-7 cells enhanced the C3a-induced C3aR phosphorylation 1.5 - 1.9-fold (p < 0.05), but each kinase reduced ligand-stimulated phospholipase C activity differently. Conversely, antibody-mediated inhibition of endogenous GRK2 and GRK3 significantly inhibited C3aR phosphorylation in permeabilized cells. GRK overexpression in cells which co-expressed C5aR and were exposed to C5a resulted in the hyperphosphorylation of the C5aR. These findings are of physiological relevance, since we observed anaphylatoxin-induced phosphorylation of C3aR and C5aR endogenously expressed in human mast cells (HMC-1) which contain significant intracellular levels of GRK2 and GRK3.

Plasma acylation stimulating protein, adipsin and lipids in non-obese and obese populations

BACKGROUND

Acylation stimulating protein (ASP) is a potent stimulator of TG synthesis in human adipocytes.

DESIGN

In the present study, we have analysed plasma ASP and adipsin levels and their relationships to plasma lipids in non-obese and obese groups.

RESULTS

The results show that the frequency distribution of ASP is skewed but that of adipsin is normal in both groups. In the non-obese population, the mean levels of plasma ASP and adipsin were 20.2 nmol L-1 (median) and 66.6 +/- 19 nmol L-1 (mean) respectively. No difference was observed between men and women for each of the parameters. In the obese population, the median plasma ASP was increased by 246% (69.9 nmol L-1) and adipsin by 31% (87.0 +/- 22.7 nmol L-1) above that of the control group. Although the levels for men and women were not statistically different for adipsin, the median ASP plasma concentration was 1.9-fold higher in obese women than in obese men (71.8 nmol L-1 vs. 37.6 nmol L-1, P < 0.05). Best subset regression analysis provided a model with variables that best predict plasma ASP [r2 = 0.160, P < 0.008 for body mass index (BMI), P < 0.05 for triacylglycerol (TG), P < 0.03 for free fatty acid (FFA)] and plasma adipsin (r2 = 0.057, P < 0.017 for BMI) in a non-obese population. In obese subjects, the model was different for plasma ASP (P = NS for any of the variables) and plasma adipsin (r2 = 0.356, P < 0.008 for FFA, P < 0.0002 for BMI, P < 0.02 for age). There was no correlation between ASP and adipsin in either the non-obese or the obese group.

CONCLUSION

The present data suggest involvement of the ASP/adipsin pathway in the pathogenesis of obesity.

Evaluation of C3a receptor expression on human leucocytes by the use of novel monoclonal antibodies

Varying results have been published in the past regarding the reactivity of different leucocyte subpopulations, including neutrophils, monocytes and B lymphocytes, to the anaphylatoxin C3a and its degradation product C3a(desArg). To better characterize the cellular distribution of C3a receptor (C3aR) expression, monoclonal antibodies against two different epitopes on the third extracellular domain of the human C3aR were generated. Quantification of C3aR as compared with C5aR densities was performed on peripheral blood leucocytes by quantitative indirect immunofluorescence. Eosinophils and basophils expressed similar numbers of C3aR and C5aR molecules/cell. On eosinophils 10 700+/-4500 (mean+/-SD) C3aR and 14 700+/-4100 C5aR were found, whereas basophils carried 8100+/-2100 C3aR and 13 500+/-3800 C5aR. Monocytes expressed approximately six times more C5aR than C3aR molecules on their surface (6000+/-2500 C3aR versus 34 100+/-9300 C5aR molecules) whereas on neutrophils, the expression of C5aR was more than 20 times higher than the expression of C3aR (3100+/-1000 C3aR versus 63 500+/-12 200 C5aR). No C3aR expression was detectable on peripheral blood-derived B lymphocytes and on tonsillar B cells before and after stimulation with interleukin-2/Staphylococcus aureus Cowan strain I. Our findings correspond well with the paucity of data on C3a-induced functional activities in monocytes and neutrophils and suggest that eosinophilic and basophilic granulocytes represent the primary effector cells in the peripheral blood which can be stimulated by C3a.

Genetic disruption of the murine complement C3 promoter region generates deficient mice with extrahepatic expression of C3 mRNA

Genetic deficiencies of the complement protein C3 occur naturally in humans and animal models and have been induced in mice by targeted deletion of the C3 gene. The study of these deficiencies has provided evidence for C3 functions in immune responses. C3 deficient mice were generated by replacing the 5'-flanking region of the C3 gene with the neomycin-resistance (neo) gene. Serum from these mice had no detectable C3 protein or complement activity. Challenge with Streptococcus pneumoniae revealed approximately 2000-fold increase in bacteremia as compared to littermate controls. C3 mRNA was absent in the liver, but it was detected in the lung, kidney, fat tissue, heart and spleen. Metabolic labeling of the lung tissue and peritoneal macrophages showed synthesis of pro-C3, but no post-synthetic intracellular processing of the protein and no secretion of mature C3. cDNA analysis at the cap site indicated that extrahepatic transcription of the targeted gene was initiated in the neo cassette, close to the C3/neo junction and predicted a primary translation product lacking the leader peptide. The data indicate that these mice provide a good animal model for the study of complete C3 deficiencies and a potential probe for tissue-specific C3 gene regulatory elements.

Deposition of complement C3 and factor H in tissue traumatized by burn injury

Activation of complement is known to accompany burn injury. To study deposition of complement proteins within tissue traumatized by burn we employed the technique of intravital microscopy using a murine dorsal skinfold chamber model. C3, factor H, factor B, HSA, and transferrin were labeled fluorescently and injected into the tail vein of mice which had been subjected to a small third degree burn within the skin fold. Only C3 and factor H deposited within blood vessels of the traumatized tissue. Binding was specific because it occurred only in and proximal to burn sites, and neither C3 nor factor H was observed to accumulate in blood vessels of healthy tissue. Furthermore, fluorescently labelled HSA, factor B, and transferrin all failed to deposit at or around burn loci. The deposition of C3 and factor H occurred within 10 min of injury and was intravascular occurring in major blood vessels, capillaries, and post-capillary venules, with little evidence of accumulation in the interstitium. Since both C3 fragments and factor H are recognized as adhesion molecules by granulocyte receptors, these deposited proteins could promote leukocyte accumulation, thereby contributing to an initiation of an inflammatory cascade at a site of burn injury.

A Comparison of C3a and C5a-Mediated Stable Adhesion of Rolling Eosinophils in Postcapillary Venules and Transendothelial Migration In Vitro and In Vivo

The comparative ability of the complement anaphylatoxins C3a and C5a to mediate leukocyte adhesion and transendothelial migration in vivo and in vitro was investigated. Superfusion of IL-1β-stimulated rabbit mesentery with C3a resulted in a rapid and stable adhesion of rolling eosinophils, but not neutrophils, to postcapillary venules. However, C3a failed to evoke subsequent transmigration of the adherent eosinophils. In contrast, C5a induced both the rapid activation-dependent firm adhesion and transmigration of eosinophils and neutrophils through venular endothelium. C3a induced selective shedding of L-selectin and an increase in αMβ2 integrin expression on eosinophils but not neutrophils, while C5a induced shedding of L-selectin and up-regulation of αMβ2 integrin on both eosinophils and neutrophils. Both C3a- and C5a-dependent adhesion to venular endothelium was blocked by ex vivo treatment of eosinophils with anti-α4 and anti-β2 integrin mAbs. In vitro, both C3a (but not C3adesArg) and C5a (including C5adesArg)-dependent transmigration of eosinophils across IL-1β-stimulated endothelial monolayer was mediated by α4β1 and αMβ2 integrins. Overall these studies suggest that C3a is eosinophil-specific chemotactic mediator that influences selectively eosinophil adhesion but not transmigration in vivo. C5a in contrast is a complete activator of integrin-dependent adhesion as well as transmigration of eosinophils and neutrophils.

Regulation of IL-6 Synthesis in Human Peripheral Blood Mononuclear Cells by C3a and C3adesArg

The anaphylatoxin C3a has been reported to have immunomodulatory effects on a number of different cell types. In this study we investigated the effects of C3a and C3adesArg on gene expression and protein secretion of IL-6 in human PBMCs, either alone or in combination with LPS or IL-1β. C3a or C3adesArg alone exhibited no effect on the expression or secretion of IL-6. However, when PBMC were stimulated with LPS or IL-1β, both C3a and C3adesArg were found to enhance IL-6 release by PBMC in a dose-dependent manner. Since C3a has been shown to induce PGE2 production by monocytes, and PGE2 has been shown to influence cytokine production, we investigated the potential role of PGE2 in C3a-mediated enhancement of LPS- and IL-1β-induced IL-6 production. Indomethacin blocked PGE2 release, but had no influence on the observed effects of C3a, suggesting that the effects of C3a on IL-6 production are independent of PGE2 formation by monocytes. Northern blot analysis showed that C3a as well as C3adesArg enhanced LPS-induced mRNA levels for IL-6. Pretreatment of PBMCs with pertussis toxin blocked the functions of C3a and C3adesArg, indicating that the actions of these two molecules are mediated by a G protein-coupled pathway. Furthermore, we investigated the effects of C3a and C3adesArg on induction of NF-κB and activating protein-1 binding. Both molecules enhanced LPS-induced NF-κB and activating protein-1 binding activity. These results demonstrate the capacity of intact C3a and its circulating des-Arg form to exert immunmodulatory effects in vitro.

Structural and functional identification of complement components of the bony fish, carp (Cyprinus carpio)

Complement is a humoral factor of innate immunity and plays important roles in immune surveillance and clearance of invading pathogens. Mammalian complement consists of the classical (antibody-dependent), the alternative (antibody-independent) and the lectin (triggered by mannose-binding lectin) pathways of activation, and of the lytic pathway. Phylogenetically, bony fish are one of the lowest groups of vertebrates with serum that shows hemolytic activity indistinguishable from that of mammalian complement. In our series of trials to address the question "How many components constitute the bony fish complement?" Functional analyses, protein isolation, and molecular cloning revealed the presence of major components constituting each pathway, the similarity between bony fish and mammalian complement being considerable. Recent investigations on carp (Cyprinus carpio) and other fish species revealed striking features unique to bony fish complement, including a remarkable diversity in structure and function of the third component, C3, and the presence of a newly identified lineage in evolution of the factor B and C2 family. For a significant insight into the evolution of complement systems and clinical applications to aquaculture industry, further extended studies are warranted.

Molecular Cloning of Two Isoforms of the Guinea Pig C3a Anaphylatoxin Receptor: Alternative Splicing in the Large Extracellular Loop

The anaphylatoxin C3a is released from C3 during complement activation. C3a is a potent spasmogen and has recently been described as an eosinophil and mast cell chemotactic factor that mediates a number of inflammatory reactions. Previously, we demonstrated the presence of a specific C3a receptor (C3aR) on guinea pig platelets. We report here the isolation of cDNA clones encoding for two isoforms of guinea pig C3aR (gpC3aR). Hydropathy analysis of the deduced amino acid sequence of both gpC3aR clones indicated seven transmembrane domains with a large extracellular (EC) loop between the fourth and fifth transmembrane domains, which is a known characteristic of the human C3aR. Northern blot analysis revealed that the gpC3aR was abundantly expressed on macrophages and in the spleen. A comparison of the deduced amino acid sequence of the larger gpC3aR (gpC3aR-L) with the recently cloned human C3aR indicated a 59.5% identity. The deduced amino acid sequence of the second, smaller cDNA clone was identical with gpC3aR-L, except that it lacked 35 amino acids in the large EC loop. Our evidence indicates that alternative splicing occurred in the large EC loop that accounts for these two isoforms. L cells separately expressing one of these two isoforms of the gpC3aR showed similar high-affinity C3a binding. An RT-PCR analysis documented that both forms of the C3aR were expressed in a variety of guinea pig tissues. The cloning and expression of these two natural forms of gpC3aR cDNA indicated that the deletion of the 35-residue portion of the large EC loop of gpC3aR-L did not alter C3a binding.

Biological activities of C3 beta c, a novel neutrophil chemoattractant derived from the beta-chain of rat complement C3

Biological activities of C3 beta c, which is a C-terminal fragment of the beta-chain of rat complement C3, have been studied by in vivo and in vitro experiments. C3 beta c was purified as a novel neutrophil chemoattractant from the exudate of the chronic phase of rat carrageenin-induced inflammation. The purified C3 beta c induced neutrophil chemotaxis in vivo when C3 beta c was injected into the preformed air-pouch on the back of rats. C3 beta c transiently increased the intracellular free Ca2+ concentration of neutrophils and enhanced the adhesion of neutrophils to fibrinogen in vitro, suggesting that C3 beta c has the ability to express an adhesion molecule of rat neutrophils. In addition, C3 beta c at low concentrations (10(-10)-10(-11) M) stimulated rat macrophages to produce cytokine-induced neutrophil chemoattractant-2, a member of the interleukin-8 family. Furthermore, C3 beta c enhanced vascular permeability in vivo, which is suppressed by cyproheptadine, suggesting that C3 beta c may have the characteristics of an anaphylatoxin. Our results suggest that C3 beta c contributes to oedema formation and neutrophil accumulation at inflammatory sites in rats.

Anaphylatoxin C3a but not C3a(desArg) is a chemotaxin for the mouse macrophage cell line J774

Varying results have been published regarding the functional reactivity of different cell types, including human monocytes, to the anaphylatoxin C3a and its degradation product C3a(desArg). To further delineate the functions of C3a and C3a(desArg) on this cell type we used the murine macrophage (Mø) cell line J774A.1 which is known to respond to the anaphylatoxin C5a. J774 cells specifically bound fluorescein isothiocyanate-labeled recombinant human C3a (rC3a). The cells migrated along rC3a concentration gradients in a dose-dependent manner with an optimal concentration of about 3 nM (rC5a:7 nM) and a half-maximal effective concentration (EC50) of about 1.2 nM (rC5a: 2 nM). The degradation product rC3a(desArg) was devoid of chemotactic activity. mRNA for the recently cloned G protein-coupled mouse high-affinity C3a receptor (C3aR) was detected in J774 cells, suggesting that this receptor represents the binding site for C3a on J774 Mø. In support of the specific nature of C3a-stimulated cellular mobility, RBL-2H3 transfectants expressing the human C3aR were also shown to migrate along gradients of rC3a (optimal concentration about 8 nM; EC50 about 3.5 nM) whereas rC3a(desArg) was again inactive. In summary, our findings demonstrate for the first time a specific, receptor-mediated chemoattraction of cells of the monocytic lineage to the anaphylatoxin C3a which may contribute to the accumulation of Mø at sites of inflammation.

Site-Directed C3a Receptor Antibodies from Phage Display Libraries

Recent cloning of the human C3a receptor (C3aR) revealed that this receptor belongs to the large family of rhodopsin-type receptors. A unique feature of the C3aR is the large second extracellular loop comprising about 175 amino acid residues. We constructed combinatorial phage Ab libraries expressing single chain Fv Abs from BALB/c mice immunized with the affinity-purified second extracellular loop of the C3aR, fused to glutathione-S-transferase. A panel of anti-C3aR single chain Fv fragments (scFvs) was selected after four rounds of panning using the second extracellular loop of the C3aR, fused to the maltose binding protein as Ag. Sequencing of the clones obtained revealed three different groups of scFvs, the epitopes of which were mapped to two distinct regions within the loop, i.e., positions 185 to 193 and 218 to 226, representing the immunodominant domains of the loop. By flow cyotmetric analyses, the scFvs bound to RBL-2H3 cells transfected with the C3aR, but not to cells transfected with the C5aR or to nontransfected RBL-2H3 cells. In addition, the scFvs bound to the human mast cell line HMC-1. Immunofluorescence studies showed C3aR expression on polymorphonuclear granulocytes and monocytes, but not on lymphocytes. In addition, no C3aR expression was observed on human erythrocytes or platelets. Surprisingly, none of the scFvs alone or in combination inhibited C3a-induced Ca2+ mobilization from RBL-2H3 cells transfected with the C3aR. In addition, C3a did not displace binding of the scFvs to the receptor, strongly suggesting that the N-terminal part of the second extracellular loop is not involved in ligand binding.

Solution structure of Compstatin, a potent complement inhibitor

The third component of complement, C3, plays a central role in activation of the classical, alternative, and lectin pathways of complement activation. Recently, we have identified a 13-residue cyclic peptide (named Compstatin) that specifically binds to C3 and inhibits complement activation. To investigate the topology and the contribution of each critical residue to the binding of Compstatin to C3, we have now determined the solution structure using 2D NMR techniques; we have also synthesized substitution analogues and used these to study the structure-function relationships involved. Finally, we have generated an ensemble of a family of solution structures of the peptide with a hybrid distance geometry-restrained simulated-annealing methodology, using distance, dihedral angle, and 3J(NH-Halpha)-coupling constant restraints. The Compstatin structure contained a type I beta-turn comprising the segment Gln5-Asp6-Trp7-Gly8. Preference for packing of the hydrophobic side chains of Val3, Val4, and Trp7 was observed. The generated structure was also analyzed for consistency using NMR parameters such as NOE connectivity patterns, 3J(NH-Halpha)-coupling constants, and chemical shifts. Analysis of Ala substitution analogues suggested that Val3, Gln5, Asp6, Trp7, and Gly8 contribute significantly to the inhibitory activity of the peptide. Substitution of Gly8 caused a 100-fold decrease in inhibitory potency. In contrast, substitution of Val4, His9, His10, and Arg11 resulted in minimal change in the activity. These findings indicate that specific side-chain interactions and the beta-turn are critical for preservation of the conformational stability of Compstatin and they might be significant for maintaining the functional activity of Compstatin.

Blood- and skin-derived monocytes/macrophages respond to C3a but not to C3a(desArg) with a transient release of calcium via a pertussis toxin-sensitive signal transduction pathway

Controversial results have been published in the past regarding the functional reactivity of monocytes (Mo) and macrophages (M phi) to the anaphylatoxin C3a and its degradation product C3a(desArg). In this study we performed binding and calcium mobilization experiments with recombinant human C3a (rC3a) and rC3a(desArg). Blood Mo displayed non-inhibitable binding of FITC-labeled rC3a (rC3aFITC) but responded to rC3a with a transient release of the intracellular calcium concentration ([Ca2+]i), whereas rC3a(desArg) was completely inactive. In contrast, binding of rC3aFITC to eosinophilic granulocytes and the mast cell line HMC-1 which have been shown previously to express C3a binding sites could be blocked by a monoclonal anti-C3a antibody. The rC3a-induced [Ca2+]i release in blood Mo was pertussis toxin (PTX)-sensitive suggesting the involvement of G-proteins in the signal transduction pathway. Skin-derived Mo/M phi reacted similarly to blood Mo as no specific binding of rC3aFITC to these cells could be demonstrated, whereas an intracellular release of calcium ions in response to the anaphylatoxin was observed. Homologous desensitization to rC3a but not heterologous desensitization to rC5a was detected in further experiments. The functional effect of C3a, but not the unspecific binding of rC3aFITC to blood Mo and skin-derived Mo/M phi could be blocked by the monoclonal anti-C3a antibody. These results suggest the expression of the recently cloned G-protein-coupled receptor for C3a on human blood Mo and skin-derived Mo/M phi. However, the total number of specific C3a binding sites on these cells is distinctly lower as compared to eosinophilic granulocytes and cells of the mast cell line HMC-1. The small number of C3a receptors on Mo/M phi may be masked by a pronounced non-inhibitable binding of rC3aFITC. This binding, however, may contribute to the recently described biological effects of C3a(desArg) on Mo.

Expression cloning of the human C3a anaphylatoxin receptor (C3aR) from differentiated U-937 cells

A cDNA clone encoding the human C3a anaphylatoxin receptor (C3aR) was isolated from a pcDNAI/Amp expression library prepared from U-937 cells which had been differentiated with dibutyryl cAMP to a macrophage-like phenotype. The cDNA clone contained an insert of 4.3 kbp and was able to confer to transfected human HEK-293 cells the capacity to bind specifically iodinated human C3a. Chinese hamster ovary cells co-transfected with this cDNA clone and a G-protein alpha subunit (G alpha-16) became functionally responsive to C3a and a C3a analog synthetic peptide, as measured by increased phosphoinositide hydrolysis. As inferred from the cDNA sequence, the clone encodes a 482-residue polypeptide with seven hydrophobic membrane-spanning helices and a high homology to the human C5a and formyl-Met-Leu-Phe receptors. Uniquely among the family of G-protein coupled receptors, the C3aR contains an exceptionally large second extracellular loop of approximately 175 residues. Northern hybridizations revealed an approximately 2.3-kb transcript as the major and an additional approximately 3.9 kb-transcript as a minor transcription product of the C3aR. The C3aR appears to be widely expressed in different lymphoid tissues, as shown by Northern hybridizations, providing evidence for a central role of the C3a anaphylatoxin in inflammatory processes.

Epitope mapping of a C5a neutralizing mAb using a combined approach of phage display, synthetic peptides and site-directed mutagenesis

BACKGROUND

The anaphylatoxin C5a is a powerful proinflammatory protein generated on activation of the complement system. Recently, we described an anti-hC5a neoepitope specific mAb, mAb 2925, which was raised against the nonapeptide ISHKDMQLG (C5a-(65-73). This mAb is unique in that it recognizes both hC5a and hC5adesArg, even when it is denatured. It inhibits binding of [125I]C5a to its receptor on Bt2-cAMP differentiated U937 cells.

OBJECTIVES

To define the epitope of mAb 2925, we used a combined approach of a bacteriophage random octapeptide library, synthetic peptides and site-directed mutagenesis.

STUDY DESIGN

First a phage peptide library was screened with the anti C5a mAb 2925. Then synthetic peptides were synthesized with respect to the sequence information yielded from the phage approach, and used for binding studies. Site-directed mutagenesis was performed to confirm the results from the mapping experiments.

RESULTS AND CONCLUSION

Most phages selected by biotinylated Fab 2925 displayed sequences on the minor coat protein which correspond to residues within the C-terminus of human C5a. A first consensus motif comprised amino acids His-Lys or His-Arg, which allowed us to define position 67 and 68 as part of the epitope. A second consensus motif was selected, comprising Arg/Lys-Trp-Trp. This motif did not match any residues within the C5a C-terminus. However, when expressed together with the consensus motif His-Arg, as in HRWWXXXX or in HRXKWWXX, binding of these peptides to Fab 2925 increased as compared to peptides expressing the His-Arg motif only. Thus, the Arg/Lys-Trp-Trp motif serves to stabilize the binding of His-Arg to mAb 2925. Synthetic peptide studies revealed further N-terminal residues Ile65 and Ser66 as part of the epitope. A C5a mutant with an exchange Lys68Glu (C5aGlu68) confirmed the participation of Lys68 as a contact residue within the epitope of mAb 2925. Hence, the epitope recognized by mAb 2925 is linear and comprises residues Ile65, Ser66, His67, and Lys68. Thus, we could demonstrate for the first time that a mAb inhibits C5a receptor binding through specific interaction with receptor binding residues of the ligand.

Expression of high- and low-affinity receptors for C3a on the human mast cell line, HMC-1

The proteolytic cleavage product of complement component 3, (C3a), is like C4a and C5a, is a potent anaphylatoxin and induces the production of inflammatory mediators in phagocytes. Notably, mast cells respond to C3a with the release of vasoactive substances, including histamine. We have examined the function and receptor binding of C3a in a human leukemic mast cell line, HMC-1. Similar to chemoattractant agonists in leukocytes, C3a induced rapid cytosolic free calcium concentration increases in HMC-1 cells. EGTA did not diminish this response, indicating that mobilizable Ca2+ was from intracellular stores. Receptors of C3a in HMC-1 cells couple in part to Bordetella pertussis toxin-sensitive G-proteins and, therefore, appear to belong to the family of serpentine receptors that require G-proteins for signal transduction. HMC-1 cells express two types of C3a receptors, C3aR1 and C3aR2, that were shown to bind 125I-C3a with high-(Kd1 = 2.1-4.8 nM) or low-affinity (Kd2 = 30-150 nM), and both receptors are expressed at high level: 3 x 10(5)-6 x 10(5) C3aR1/cell and 5 x 10(5)-2.3 x 10(6) C3aR2/cell. Results from cross-linking experiments with 125I-C3a fully agree with the presence of two different classes of C3a receptors in HMC-1 cells. Two membrane proteins with apparent molecular masses of 54-61 kDa (p57) and 86-107 kDa (p97) could be covalently modified with 125I-C3a, and this cross-linking was inhibited with an excess of unlabeled C3a. Many of the known agonists for leukocytes including 13 chemokines (IL-8, NAP-2, GRO alpha, ENA-78, IP10, PF4, MCP-1, 2 and 3, RANTES, MIP-1 alpha, MIP-1 beta and I309), three neuropeptides (neuropeptide Y, somatostatin and calcitonin), as well as C5a, did not activate HMC-1 cells, indicating that C3a is one of a few protein ligands for which this cell line expresses specific receptors. The apparent selectivity for C3a and the abundant expression of C3a receptors make the HMC-1 cell line an excellent choice for the cloning of the receptor genes.

C3a is a chemotaxin for human eosinophils but not for neutrophils. I. C3a stimulation of neutrophils is secondary to eosinophil activation

Inflammatory action of the potent chemotaxin C5a has been well characterized on a variety of human cell types, including neutrophils, monocytes, basophils, and eosinophils. The cellular effects of C3a are less well defined. Contradictory reports have been published for C3a activation of neutrophils. Recent reports that C3a activates both basophils and eosinophils prompted us to reinvestigate the effects of C3a stimulation on eosinophils. We hypothesized that C3a activation of eosinophils, cells that are present in most neutrophil preparations, might lead to neutrophil activation. Using neutrophils of 98% purity, we observed no evidence of cellular activation after stimulation with either C3a, recombinant human C3a (rhC3a), or the synthetic C3a analogue C3a 57-77, Y57. Eosinophils purified to > 98% purity displayed concentration-dependent polarization, chemotaxis, and enzyme release by stimulation with C3a, rhC3a, and the synthetic C3a analogue. An inactive form of C3a, C3adesArg, failed to stimulate either eosinophils or neutrophils. Using neutrophil preparations containing 5-9% eosinophils, up to 20% of neutrophils became polarized after exposure to C3a. Likewise, we demonstrated that supernatant from C3a-stimulated eosinophils promotes neutrophil chemotaxis. Eosinophil polarization experiments were repeated in the presence of antibody to the C5a receptor (C5aR) to show that C3a and C5a interact with different receptors. C3a activates eosinophils in the presence of anti-C5aR antibody at concentrations that fully block C5a activation. We conclude that eosinophils are directly activated by either C3a or C5a, whereas C3a failed to activate neutrophils. C3a acts on eosinophils via a receptor that is distinct from C5aR. Since neutrophils are indirectly stimulated by C3a, eosinophils contaminating neutrophil preparations may explain earlier reports that C3a activates human neutrophils.

C3a activates reactive oxygen radical species production and intracellular calcium transients in human eosinophils

Whereas C5a is a well-established potent activator of eosinophils, the functional role of C3a in the activation of eosinophils is, so far, poorly understood. Here, the activation of human eosinophils stimulated with C3a was analyzed and compared to C5a activation. Flow-cytometrical measurements revealed that stimulation of eosinophils by C3a resulted in a transient elevation of the intracellular calcium concentration ([Ca2+]i) in a dose-dependent manner. In addition, the production of reactive oxygen radical species (ROS) of eosinophils after C3a and C5a stimulation was measured by lucigenin-dependent chemiluminescence and quantified by superoxide dismutase-inhibitable reduction of ferricytochrome C. Half maximal and maximal ROS production in response to C3a was observed at 50 ng/ml and 1000 ng/ml, respectively, whereas C3a-desArg was inactive. To ensure that C3a stimulation was not caused by contamination with C5a, monoclonal antibodies were used to demonstrate the specificity of C3a. The effect of C3a was completely abolished in the presence of monovalent antigen-binding fragments of a functionally blocking anti-C3a monoclonal antibody. In addition, blockade of the C5a receptor by the monoclonal anti-C5a receptor antibody S5/1 totally inhibited the C5a-evoked ROS production, whereas the C3a response in the presence of S5/1 was unaffected. Finally, desensitization experiments revealed a homologous desensitization of C3a after restimulation with C3a. In contrast, no cross-desensitization was observed upon stimulation with C5a. Furthermore, the C3a- and C5a-induced production of ROS of eosinophils was totally inhibited by pertussis toxin, indicating the involvement of guanine nucleotide-binding proteins (Gi-proteins). In summary, these results demonstrate that C3a is a potent activator for eosinophils initiating transient [Ca2+]i changes and production of reactive oxygen species. C3a therefore may play a part in the pathophysiology of diseases with eosinophil and complement activation.

Chronic myelogenous leukemia-derived basophilic granulocytes express a functional active receptor for the anaphylatoxin C3a

The receptor for the inflammatory peptide C3a has scarcely been examined on human cells. This work demonstrates that human tumor-derived basophilic granulocytes express C3a receptors, and presents parts of the hitherto unknown C3a-signal transduction. When incubated with IL-3, these cells specifically liberated histamine on C3a stimulation. Independent from IL-3, 240,000 +/- 100,000 receptors per cell with a Kd of 5.6 +/- 0.9 nM were determined. [Ca2+]i increased from 120 +/- 35 nM to 300 +/- 80 nM after a C3a challenge, as measured by digital imaging fluorescence microscopy, and rested at its basal level in the presence of C3a-desArg, the immediate catabolic product of C3a in vivo. This [Ca2+]i increase could be completely desensitized homologously by C3a as well as inhibited by up to 75% by pertussis toxin. Thus, tumor-derived basophils are suitable for cloning of the human C3a receptor.

Synthetic peptides as antagonists of the anaphylatoxin C3a

Peptide compounds resembling the receptor-binding C-terminal domain of the anaphylatoxic peptide C3a were synthesized to examine two kinds of C3a antagonism: (a) specific desensitization of C3a-sensitive cells and (b) competitive binding to the C3a receptor. We used guinea-pig platelets, which express a C3a receptor and specifically release ATP upon stimulation, to evaluate the actions of the C3a analogues. The ATP liberation can be inhibited by pretreatment (i.e. desensitization) of the guinea-pig platelets with substimulatory concentrations of C3a or its analogues. Compared to C3a, several peptides were found with at least a tenfold greater difference between the required concentrations for C3a-specific half-maximal desensitization (DD50) and half-maximal platelet activation (ED50). The most potent compounds were YAAALKLAR and Fmoc-EAALKLAR (Fmoc: 9-fluorenylmethoxycarbonyl) with an ED50/DD50 of 140 +/- 28 and 80 +/- 17, respectively (mean +/- standard deviation). The ED50/DD50 of human C3a was found to be only 6 +/- 2. Some C3a derivatives were also tested in competitive binding studies for their ability to compete with C3a for receptor sites on guinea-pig platelets. Three of them were considered partial antagonists [YRRGRCGGLCLAR, YRRGRXCGGLCLAR and YRRGRXCGALCLAR (X = 6-aminohexanoyl)] because their Ki were smaller than their ED50 (Ki/ED50 = 0.6 +/- 0.3, 0.5 +/- 0.1 and 0.4 +/- 0.2, respectively). Interestingly, the last two compounds also had ED50/DD50 values greater than 60. Common to all three peptides are N-terminal arginine-rich sequences and intramolecular disulfide bridges which introduce conformational constraint.

Functional activities of synthetic anaphylatoxic peptides in widely used biological assays

A comparison study was carried out between the modern ATP release assay (ARA) with guinea-pig platelets and the traditional guinea-pig ileum contraction assay (ICA). The biological activities of the anaphylatoxin C3a and synthetic C3a analogue peptides were determined in both assays. In dose-response curves with C3a, a human C3a peptide with the last 21 amino acids of the C terminus (C3a 56-77) and a peptide with 13 amino acids which was acylated N-terminal with the aromatic fluorenylmethoxycarbonyl group and an aminohexanoyl group (Fmoc-Ahx YRRGRAAALGLAR) were tested. The ARA turned out to be 100 times more sensitive than the ICA. In contrast to previous reports the 21 amino acid long C3a analogue peptide did not exhibit full C3a activity but only 7% (ARA) or 12% (ICA). The potentiation of biological activity in the ARA by coupling non-peptide acyl-residues N terminal to peptide C3a analogues could be confirmed with Fmoc-Ahx-YRRGRAAALGLAR in the ICA. In addition, the tri-peptide Fmoc-Ahx-LAR displayed C3a specific activity in the ICA demonstrated by desensitization experiments.