Administration of low-dose endotoxin to healthy humans increases C5a binding to circulating neutrophils.
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Adult↗
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Results for “Complement C5a”
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Identification of an antigenic epitope and receptor binding domain of human C5a.
Nine different murine anti-human C5a monoclonal antibodies have been produced and characterized. They exhibit Kas for the 125I-labeled ligand that range from 0.4 to 48 X 10(8) M-1, and they display limited cross-reactivity with C5a from other species. Each of these antibodies has been found to compete with the granulocyte C5a receptor for binding site(s) on the C5a polypeptide. Exploration of the antigenic topography of C5a revealed that the immunodominant portion of this glycopolypeptide resides between residues Lys20 and Arg37, with the area surrounding Cys27 being particularly important. In addition, a specific C5a derived tryptic peptide containing these amino acid residues competes with 125I-C5a for binding to the receptor. These observations are consistent with previously published data and suggest that this area of the C5a molecule is an important part of the receptor "recognition domain", and thus plays a critical role in the C5a receptor interaction.
Antibodies, Monoclonal↗
Different endocytosis pathways of the C5a receptor and the N-formyl peptide receptor.
Two chemoattractant receptors, C5aR (the complement fragment C5a receptor) and FPR (the N-formyl peptide receptor), are involved in neutrophil activation at sites of inflammation. In this study, we found major differences in the intracellular trafficking of the receptors in transfected Chinese hamster ovary (CHO) cells. Western blot analysis showed that FPR was stable during a 3 h stimulation with ligand, but C5aR was reduced in quantity by 50%. Not all C5aR was targeted directly for degradation however; a small, but visible fraction of the receptor became re-phosphorylated upon subsequent addition of ligand, suggesting that some of the receptor had cycled to the cell surface. Light membrane fractions isolated from activated cells showed C5aR distribution at the bottom of a glycerol gradient, colocalizing with the main distribution of the late endosomal/lysosomal marker LAMP2, whereas FPR was found at the bottom of the gradient as well as in the middle of the gradient, where it cofractionated with the early/sorting endosomal marker Rab5. Using fluorescence microscopy, we observed ligand-dependent redistribution of C5aR-EGFP from the plasma membrane to LAMP2-positive compartments, whereas FPR-EGFP showed significant colocalization with the early/sorting endosomes. Analysis of endogenous C5aR and FPR in neutrophils revealed a pattern similar to the CHO transfectants: C5aR underwent degradation after prolonged ligand stimulation, while FPR did not. Finally, we confirmed the down-regulation of C5aR in a functional assay by showing reduced chemotaxis toward C5a in both CHO transfectants and neutrophils after preincubation with C5a. A similar decrease in FPR-mediated chemotaxis was not observed.
Animals↗
Reduced expression of C5a receptors on neutrophils from cord blood.
AIM: To describe further functional deficiencies of neonatal neutrophils by measuring the expression of C5a receptors. METHODS: C5a uptake was measured using flow cytometry with fluorescein isothiocynate labelled recombinant C5a. The response of neutrophils to stimulation with C5a and fMLP was tested by measuring migration and exocytosis of myeloperoxidase and lactoferrin. RESULTS: C5a mean fluorescence on neutrophils from neonates was significantly lower (22.4 (SD 3.5)) than in adult controls (31.5 (3.1)). Neutrophils from neonates migrated poorly towards both C5a and fMLP compared with those from adult controls. Exocytosis of myeloperoxidase, but not lactoferrin from neonatal neutrophils stimulated with C5a, was significantly lower than in adult controls. fMLP stimulation, on the other hand, resulted in significantly higher exocytosis in neonates. CONCLUSION: The lower expression of C5a receptors on neutrophils from neonates could be related to reduced C5a mediated exocytosis of myeloperoxidase.
Antigens, CD↗
N-linked glycosylation of the C5a receptor.
The recent cloning of the cDNA encoding the human C5a receptor reveals a single potential site for N-linked glycosylation. Previous studies have suggested the presence of at least one carbohydrate moiety in the C5a receptor. Enzymatic digestion with Endoglycosidase F confirmed this presence and a point mutation at the predicted site of glycosylation was sufficient to lower the apparent molecular weight of the receptor as determined by SDS-PAGE. Removal of the carbohydrate moiety failed to abolish expression of the receptor and brought about only a slight reduction in the dissociation constant of the C5a receptor suggesting that the role of glycosylation in the binding of C5a by its receptor is limited.
Animals↗
Anaphylatoxin generation and distribution during in vitro LDL apheresis.
The extent of anti-apo-B IgG-Sepharose-induced complement activation in serum and plasma (heparin 2 U/ml and ACD-B 1:20) was investigated using an in vitro model of LDL apheresis. The total volume of serum or plasma loaded to the chromatography column was collected in defined aliquots. The washing, desorption and regeneration fluids were processed in the same way. From the obtained values of generated complement split products C3a (desarg), C4a (desarg), C5a (desarg) and complement proteins C3, C4, C5, the conversion rates of the precursor were calculated. In the experiments with serum, 19% of C3, 8% of C4 and 2.3% of C5 were converted by the immunoadsorbent, whereas with plasma 7, 6, and 0.6%, respectively, were found. Furthermore, only 60-74% of total anaphylatoxins were found in the effluent during the loading process. The residual 26-40% was removed from the column with the subsequent washing fluids. Therefore, in the clinical routine, only a reduced part of generated anaphylatoxins will be retransfused to the patient. The fact that C5 is converted to the most limited extent to its biologically active fragment additionally contributes to the understanding of the good clinical tolerability of the LDL apheresis.
Anaphylatoxins↗
Characterization of a C5a receptor on human polymorphonuclear leukocytes (PMN).
Elucidation of the interactions between C5a and granulocytes is central to understanding the role of C5a in inflammation. In this study, interactions between C5a and PMN have been studied at two levels. Binding of human C5a to intact human cells has been characterized by using the radiolabeled ligand 125I-C5a. Binding is shown to be reversible, saturable, and to reach equilibrium in 60 to 90 min at 0 degrees C. Results show high affinity C5a binding sites with Kd = 2 X 10(-9) M and a range of 50,000 to 113,000 binding sites per PMN. These values for C5a receptors are comparable with the number of fMLP and LTB4 receptors on PMN. Binding of C5a to PMN fails to reach equilibrium at 37 degrees C because there is an irreversible loss of available surface receptors caused by an active internalization of the ligand-receptor complex. Interactions between C5a and human PMN were characterized further by cross-linking experiments, with the use of ethylene glycol bis succinimidylsuccinate (EGS). Cross-linking of 125I-C5a to intact PMN followed by subcellular fractionation revealed a single radioactive band present only in the plasma membrane fraction and visualized by autoradiography. Similar experiments resulted in a covalent linkage between 125I-C5a and a component in the isolated plasma membrane of PMN. The covalent complex containing C5a and a putative receptor has been visualized by autoradiography as a single 60,000 Mr complex on SDS-PAGE. The complex is not present when experiments are performed in the presence of excess unlabeled C5a or in the absence of EGS. Therefore, the putative receptor for C5a on human neutrophils is estimated to be approximately 48,000 Mr, assuming contribution of 12,000 to 13,000 daltons by the ligand 125I-C5a.
Cell Membrane↗
Contractile effect of anaphylatoxin C5a and of a mimetic peptide on the human umbilical artery: further evidence for leukocyte-dependent vasomotion.
Reproducible and concentration-dependent contractile effects were recorded when the human umbilical artery was exposed to recombinant C5a (>or=1 nM). The synthetic mimetic peptide Ac-YSFKPMPLaR was about 100-fold less potent in this respect, and its effect was largely prevented by treatment with the TP prostanoid receptor antagonist SQ 29548. The selective cyclooxygenase-1 inhibitor flurbiprofen, but not by the cyclooxygenase-2 inhibitor L-745337, prevented the contractile effect of both C5a and Ac-YSFKPMPLaR. Cells positive for C5a receptor (CD 88) immunoreactivity were scattered in the umbilical artery structure and were apparently more abundant in the periphery. The macrophage marker CD 68 was also expressed by dispersed cells, with a distribution similar to CD 88; both markers were co-expressed in some cells represented in consecutive sections. Immunoblot for C5a receptors has been applied to tissue or cells extracts: a specific approximately 42-kd major band observed in peripheral blood leukocytes was also expressed by the fresh umbilical artery, but not by cultured smooth muscle cells derived from the umbilical artery. Macrophages dispersed in the connective structure of the vessel wall may, in response to C5a receptor agonists, release prostanoids formed by cyclooxygenase-1 that indirectly contract smooth muscle cells. Leukocyte-dependent vasospasm may be relevant in situations in which complement is activated and may trigger thromboembolic complications via the secretion of thromboxane-like eicosanoids also active on circulatory platelets.
Antigens, CD↗
Functional expression and modulation of C5a receptor (CD88) on skin dendritic cells.
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Antigens, CD↗
Characterization of a polymorphism in the coding region of the human C5a anaphylatoxin receptor.
A polymorphism was identified in the coding region of the human C5a anaphylatoxin receptor gene leading to C to T transition at nucleotide position 450 (a silent substitution in the Ala150 codon, GCC to GCT). Its distribution was studied in a population of healthy volunteers from the Québec city region (prevalence of 2.8%) and among patients with end-stage renal failure who had previously undergone renal graft (prevalence 1.4%, not significantly different from that of the control group). This new marker provides a valuable tool to assess the risk for putative C5a-associated disorders with genetic determinism.
Alleles↗
Interaction between the C5a receptor and Gi in both the membrane-bound and detergent-solubilized states.
C5a elicits a variety of responses from the polymorphonuclear leukocyte all of which utilize G proteins as transducing elements. In the present study, we report the consequences of the interaction between the C5a receptor and the G proteins and describe a system which may allow identification of the transducing proteins. C5a binding to polymorphonuclear leukocyte membranes is inhibited by pertussis, but not cholera, toxin and by a variety of guanine nucleotides. In the absence of nucleotide, we observed a single class of sites with a Kd of 17 pM. The presence of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) did not alter this affinity but did result in a concentration-dependent decrease in the number of binding sites. Surprisingly, we did not observe the concomitant appearance of a low affinity state implying that, if such a state exists, its affinity is below our limit of detection (5 nM). The receptor and G protein retained their functional interaction following solubilization of the membrane in digitonin. In the absence of nucleotide, we observed a single class of sites with a Kd of 28 pM. Addition of GTP gamma S suppressed binding, and, as was found in membranes, this inhibition is due almost entirely to a decrease in the number of sites. Again we failed to detect the appearance of a lower affinity state. Gel filtration studies of the detergent-solubilized receptor and receptor-C5a complexes indicate that the receptor is precoupled to G protein in the absence of ligand (C5a).
Binding Sites↗
Identification of the polymorphonuclear leukocyte C5a receptor.
The peptide C5a is thought to play an important role in the inflammatory response primarily through its action on the polymorphonuclear leukocyte (PMN). The receptor for C5a on human PMN has now been identified by affinity labeling. Cross-linking 125I-C5a to intact PMN with disuccinimidyl suberate produced a species that had a molecular mass on sodium dodecyl sulfate gels of 5.2 X 10(4) daltons. We believe this species represents a complex between C5a and its receptor for the following reasons. The band is eliminated if the cross-linking experiment is performed in the presence of a large excess of unlabeled C5a, but is unaffected by the presence of nonspecific protein or the chemotactic factors N-formyl-Met-Leu-Phe and leukotriene B4. The 5.2 X 10(4)-dalton species is not observed if the cross-linker is omitted. Finally, the dose-response curves for the inhibition of binding of 125I-C5a by unlabeled C5a and the inhibition of cross-linking are similar. Subtraction of the molecular mass of C5a from that of the complex gives a molecular mass for the binding moiety of the C5a receptor of 4.0 X 10(4) daltons.
Binding, Competitive↗
Analysis of the C5a anaphylatoxin core domain using a C5a phage library selected on differentiated U937 cells.
The human anaphylatoxin C5a is a 74-amino acid comprising polypeptide with a plethora of biological functions. Site directed mutagenesis studies suggest that several residues within the core and the C-terminus mediate the interaction with the C5a receptor. However, the contribution of particular core residues to receptor binding remained to be clarified. By means of the phage display technique, the loop between positions 35-40 was randomly mutated and the resulting C5a[35-40] fusion phage library affinity selected on C5a receptor expressing U937 cells. After five rounds of affinity enrichment, residues Arg37 and Arg40 were preferably selected. Enrichment was as high as 100% for Arg37 and 79% for Arg40. No significant enrichment of consensus residues could be obtained for positions 35, 36, 38 and 39. The core mutant C5a[A35E36R37A38S39R40], in which only Arg37/40 and Ala38 are of the native C5a sequence, was as potent as native C5a in both receptor binding and enzyme release examined on U937 cells. In contrast, replacement of Arg40 as in the mutant C5a[Q35E36R37I38L39N40] resulted in a 10-fold decrease in both binding and functional activities. Thus, selected out of a multiplicity of possibilities by the natural binding partner, Arg37 as well as Arg40 appear to be anchor residues in binding to the C5a receptor.
Antigens, CD↗
Identification of novel adhesins from Group B streptococci by use of phage display reveals that C5a peptidase mediates fibronectin binding.
Group B streptococci (GBS) are a major cause of pneumonia, sepsis, and meningitis in newborns and infants. GBS initiate infection of the lung by colonizing mucosal surfaces of the respiratory tract; adherence of the bacteria to host cells is presumed to be the initial step in and prerequisite for successful colonization (G. S. Tamura, J. M. Kuypers, S. Smith, H. Raff, and C. E. Rubens, Infect. Immun. 62:2450-2458, 1994). We have performed a genome-wide screen to identify novel genes of GBS that mediate adherence to fibronectin. A shotgun phage display library was constructed from chromosomal DNA of a serotype Ia GBS strain and affinity selected on immobilized fibronectin. DNA sequence analysis of different clones identified 19 genes with homology to known bacterial adhesin genes, virulence genes, genes involved in transport or metabolic processes, and genes with yet-unknown function. One of the isolated phagemid clones showed significant homology to the gene (scpB) for the GBS C5a peptidase, a surface-associated serine protease that specifically cleaves the complement component C5a, a chemotaxin for polymorphonuclear leukocytes. In this work we have demonstrated that affinity-purified recombinant ScpB and a peptide ScpB fragment (ScpB-PDF), similar to the peptide identified in the phagemid, bound fibronectin in a concentration-dependent manner. Adherence assays to fibronectin were performed, comparing an isogenic scpB mutant to the wild-type strain. Approximately 50% less binding was observed with the mutant than with the wild-type strain. The mutant phenotype could be fully restored by in trans complementation of the mutant with the cloned wild-type scpB gene, providing further evidence for the role of ScpB in fibronectin adherence. Our results suggest that C5a peptidase is a bifunctional protein, which enzymatically cleaves C5a and mediates adherence to fibronectin. Since binding of fibronectin has been implicated in attachment and invasion of eukaryotic cells by streptococci, our results may imply a second important role for this surface protein in the pathogenesis of GBS infections.
Adhesins, Bacterial↗
The chemoattractant des-Arg74-C5a regulates the expression of its own receptor on a monocyte-like cell line.
The interaction of the chemoattractant des-Arg74-C5a (C5a des Arg) with its receptor on a human monocyte-like cell line, U-937, was examined. The data obtained suggest that C5a des Arg receptor expression is regulated by the extracellular concentration of C5a des Arg itself.
Bucladesine↗
Unidirectional heterologous receptor desensitization between both the fMLP and C5a receptor and the IL-8 receptor.
During inflammation neutrophils receive multiple signals that are integrated, allowing a single modified response. One mechanism for this discrimination is receptor desensitization, a process whereby ligand-receptor binding is disassociated from cell activation. We examined the effect of heterologous receptor desensitization on neutrophil chemotaxis, calcium mobilization, and arachidonic acid production, using interleukin-8 (IL-8), C5a, and N-formyl-methionyl-leucyl-phenylalanine (fMLP). We observed reciprocal inhibition with respect to chemotaxis. We demonstrated that homologous desensitization, with respect to the mobilization of intracellular calcium stores, lasted approximately 15 min. Heterologous desensitization between the fMLP receptor and the C5a receptor was reciprocal; either stimulant would diminish the cells' response to stimulation by the other for approximately 3-5 min. However, we observed a unidirectional heterologous desensitization of the IL-8 receptor by both the fMLP and the C5a receptor. This unidirectional heterologous desensitization was observed with respect to both calcium mobilization and arachidonic acid production (i.e., prestimulation of the IL-8 receptor had no effect on subsequent stimulation by either fMLP or C5a).
Antigens, CD↗
Functional reconstitution of a receptor-activated signal transduction pathway in Xenopus laevis oocytes using the cloned human C5a receptor.
We have used the polymerase chain reaction to isolate and clone the cDNA encoding the human C5a receptor, and have injected the cDNA-derived receptor cRNA into Xenopus laevis oocytes for functional characterization of the receptor protein. Receptor activity was determined either electrophysiologically by measuring the agonist-dependent opening of [Ca2+]i-dependent Cl- channels, or by analysing the agonist-dependent efflux of 45Ca2+ from the oocytes. Using both methodologies, injection of pure C5a receptor cRNA failed to confer C5a sensitivity on the oocytes. In contrast, marked responses to C5a were observed when the receptor cRNA was supplemented with poly(A)+ RNA isolated from undifferentiated HL-60 cells, which is devoid of C5a receptor mRNA. Binding studies using radioiodinated C5a revealed that the C5a receptor polypeptide was in fact synthesized and targeted to the oocyte plasma membrane in oocytes injected with receptor cRNA alone, and that the level of receptor expression was not influenced by coinjection of poly(A)+ RNA from undifferentiated HL-60 cells. These results strongly suggest that the human C5a receptor requires a specific cofactor(s) lacking in Xenopus oocytes but present in undifferentiated HL-60 cells, to generate intracellular signals in oocytes. Identification and characterization of this factor will provide important information about the molecular mechanisms by which G-protein-coupled receptors activate phospholipase C.
Animals↗
C5a receptor-mediated production of IL-8 by the human retinal pigment epithelial cell line, ARPE-19.
PURPOSE: C5a anaphylatoxin is a potent inflammatory mediator that acts on polymorphonuclear cells (PMN) and monocytes via C5a receptors (C5aR). It mediates chemotaxis of both cell types and stimulates cytokine release from the latter. To investigate whether C5a can act on retinal pigment epithelial (RPE) cells, we examined ARPE-19 cells for the presence of C5aR and the effect of C5a stimulation. METHODS: C5aR expression was measured by flow cytometry using specific anti-C5aR antibody and by RT-PCR analyses. Cells were stimulated with 50 nM C5a and the induction of IL-8 mRNA expression was measured by semi-quantitative RT-PCR. RESULTS: Surface levels of C5aR on ARPE-19 cells were found to be comparable to those on human PMN. Stimulation with C5a induced a dose- and time-dependent increase of IL-8 mRNA expression. CONCLUSION: The findings of C5aR on ARPE-19 cells and induction of IL-8 mRNA upon C5a stimulation suggests that C5a may participate in the defense of choroidal and retinal tissue during inflammation.
Antigens, CD↗