Characterization of a Receptor for C5a Anaphylatoxin on Human Eosinophils

Expression and function of the C5a receptor in rat alveolar epithelial cells

Although alveolar epithelial cells (AEC) form an important barrier for host defenses in the lung, there is limited information about ways in which AEC can directly participate in the lung inflammatory response. In the current studies, primary cultures of rat AEC (RAEC) have been shown to specifically bind recombinant rat C5a at high affinity and in a saturable manner. This binding was enhanced in a time-dependent manner by pre-exposure of RAEC to LPS, IL-6, or TNF-alpha, the increased binding of C5a being associated with increased levels of mRNA for the C5a receptor (C5aR). Exposure of RAEC to C5a also caused increased expression of mRNA for C5aR. As compared with exposure of RAEC to LPS or to C5a alone, exposure to the combination caused enhanced production of TNF-alpha, macrophage inflammatory protein-2, and cytokine-induced neutrophil chemoattractant-1, as well as increased intracellular levels of IL-1beta. These data indicate that RAEC, when activated, have enhanced binding of C5a in association with increased mRNA for C5aR. The functional outcome is enhanced release of proinflammatory mediators. These data underscore the phlogistic potential of RAEC and the ability of C5a to enhance the phlogistic responses of RAEC.

Functions of anaphylatoxin C5a in rat liver: direct and indirect actions on nonparenchymal and parenchymal cells

Growing evidence obtained in recent years indicates that anaphylatoxin C5a receptors (C5aR) are not restricted to myeloid cells but are also expressed on nonmyeloid cells in different tissues such as brain, lung, skin and liver. In contrast to its well-defined systemic functions, the actions of anaphylatoxins in these organs are poorly characterized. The liver can be a primary target organ for the C5a anaphylatoxin since the liver is directly connected to the gut, via the mesenteric veins and portal vein which is a main source of complement activating lipopolysaccharides (LPS). In the normal rat liver, the C5aR is only expressed by nonparenchymal cells, i.e. strongly by Kupffer cells (KC) and hepatic stellate cells (HSC) and weakly by sinusoidal endothelial cells (SEC), but not expressed by the parenchymal hepatocytes (HC). Accordingly, direct effects of C5a were only found in the C5aR-expressing KC and HSC: C5a induced the release of prostanoids from KC and HSC and enhanced the LPS-dependent release of interleukin-6 from KC. These soluble mediators indirectly influenced effector functions of the C5aR-free HC. C5a enhanced the glycogen phosphorylase activity and thus the glucose output from HC indirectly via prostanoids released from KC and HSC. Glucose can serve as an energy substrate as well as an electron donor for the synthesis of reactive oxygen intermediates by KC. Moreover, C5a also enhanced transcription of the gene for the type-2 acute phase protein alpha 2-macroglobulin in HC indirectly by increasing LPS-dependent IL-6 release from KC. Under pathological conditions, C5aR was found to be upregulated in various organs including the liver. Simulation of inflammatory conditions by treatment of rats with IL-6, a main inflammatory mediator in the liver, caused a de novo expression of functional C5aR in HC. In livers of IL-6-treated rats, C5a initiated glucose output from HC and perhaps other HC-specific defense reactions directly without the intervention of soluble mediators from nonparenchymal cells.

Expression of the complement anaphylatoxin C3a and C5a receptors on bronchial epithelial and smooth muscle cells in models of sepsis and asthma

The presence of the complement-derived anaphylatoxin peptides, C3a and C5a, in the lung can induce respiratory distress characterized by contraction of the smooth muscle walls in bronchioles and pulmonary arteries and aggregation of platelets and leukocytes in pulmonary vessels. C3a and C5a mediate these effects by binding to their specific receptors, C3aR and C5aR, respectively. The cells that express these receptors in the lung have not been thoroughly investigated, nor has their expression been examined during inflammation. Accordingly, C3aR and C5aR expression in normal human and murine lung was determined in this study by immunohistochemistry and in situ hybridization. In addition, the expression of these receptors was delineated in mice subjected to LPS- and OVA-induced models of inflammation. Under noninflamed conditions, C3aR and C5aR protein and mRNA were expressed by bronchial epithelial and smooth muscle cells of both human and mouse lung. C3aR expression increased significantly on both bronchial epithelial and smooth muscle cells in mice treated with LPS; however, in the OVA-challenged animals only the bronchial smooth muscle cells showed increased C3aR expression. C5aR expression also increased significantly on bronchial epithelial cells in mice treated with LPS, but was not elevated in either cell type in the OVA-challenged mice. These results demonstrate the expression of C3aR and C5aR by cells endogenous to the lung, and, given the participation of bronchial epithelial and smooth muscle cells in the pathology of diseases such as sepsis and asthma, the data suggest a role for these receptors during lung inflammation.

Intratracheal administration of anaphylatoxin C5a potentiates antigen-induced pulmonary reactions through the prolonged production of cysteinyl-leukotrienes

The effects of intratracheal administration of anaphylatoxin C5a on airway inflammation have been studied using two sources of material, zymosan activated serum (ZAS) and purified rat C5a des Arg, in order to determine the influence of complement activation on allergic airway disorders.The intratracheal administration of ovalbumin (OA) to OA-sensitized rats generated two phases of airway response, an immediate airway response (IAR) occurring within 15 min and a late airway response (LAR) beginning 4-6 h after the allergen challenge. The simultaneous administration of ZAS and OA into the trachea generated a sustained elevation of airway resistance (Raw) following IAR, while that of OA or ZAS alone resulted in Raw returning nearly to the baseline just after the IAR. The elevation of Raw after the combined challenge of OA and ZAS was significantly inhibited by pretreatment with a CysLT(1) receptor antagonist, pranlukast 30 mg/kg, but after that OA or ZAS alone was not significantly inhibited by pranlukast. The intratracheal administration of purified C5a produced an airway response that was similar to, but higher than, that evoked by ZAS. Namely, the challenge with OA plus C5a resulted in a higher IAR than OA plus ZAS, and also caused an early animal death up to 6 h, which was prevented by a combined pretreatment with pranlukast and the H(1) receptor antagonist, diphenhydramine.A histological examination at 6 h after the OA challenge identified an infiltration of inflammatory cells into the bronchial submucosal tissue, with a predominance of neutrophils and fewer eosinophils. On the other hand, a histological examination after the OA and ZAS challenge showed more severe infiltration of granulocytes into the bronchial submucosal tissue than that with OA or ZAS alone. The challenge with OA plus C5a was associated with severe perivascular leakage in the lungs and the combined pretreatment with both the antagonists led to a marked reduction in perivascular leakage. The quantitation of N-acetyl-leukotriene E(4) (N-Ac-LTE(4)), a major metabolite of cysteinyl-leukotrienes (cysLTs), in the bile indicated a significantly greater and longer excretion of cysLTs, from 1 to 6 h after the combined challenge, than that after either OA or ZAS alone. This suggested a prolonged generation of cysLTs in the lung by the combined challenge.In conclusion, our findings suggest that anaphylatoxin C5a may mediate the airway inflammatory response induced by a specific antigen challenge partly through a prolonged production of cysLTs and the release of histamine.

The C5a receptor is expressed in normal renal proximal tubular but not in normal pulmonary or hepatic epithelial cells

C5a, a 74 amino acid peptide cleaved from the complement protein C5, is an extremely potent anaphylatoxin. Expression of the receptor for the anaphylatoxin C5a (C5aR) has been thought to be restricted to cells of myeloid origin. However, recent evidence suggests that the C5aR is also expressed in hepatocytes as well as in pulmonary epithelial, endothelial and smooth muscle cells. In the present study, we investigated the tissue distribution of C5aR by immunohistochemistry in normal human lung, liver, intestine and kidney using well-defined monoclonal antibodies (mAbs) directed against the extracellular N-terminus of the receptor. In all tissues examined, macrophages displayed an abundant expression of C5aR protein. However, in the normal human lung, C5aR expression was not detectable in bronchial and alveolar epithelial cells or in vascular smooth muscle or endothelial cells. In the normal human liver, no C5aR protein was detected in hepatocytes, whereas Kupffer cells strongly expressed the C5aR. In normal human kidney, the C5aR was detectable only in proximal tubular cells. C5aR gene transcription in Kupffer cells and proximal tubular cells was confirmed by in situ hybridization. Thus, our results point to an as yet unknown role of the C5aR in normal renal physiology. In the normal lung and liver, however, previous evidence for the ubiquitous expression of C5aR in epithelial, endothelial and smooth muscle cells in situ should be re-evaluated.

Expression of the anaphylatoxin C5a receptor in non-myeloid cells

C5, a 74 amino acid peptide cleaved from the complement protein C5, represents the most potent anaphylatoxin and possesses inflammatory as well as immunomodulatory activities. In the past, expression of the receptor for the anaphylatoxin C5a (C5aR) has been thought to be restricted to cells of myeloid origin. However, recent evidence suggests that the C5aR is constitutively expressed in non-myeloid cells including epithelial, endothelial and smooth muscle cells in the human liver and lung. These findings are contrasted by results from our laboratory which demonstrated that in the normal human liver and lung C5aR expression is detectable exclusively in macrophages and macrophage-derived cells (Kupffer cells). Interestingly, we found evidence that C5aR expression may be inducible in epithelial cells as C5aR mRNA was observed in vivo in human keratinocytes of the inflamed but not of the normal skin. Herein we review the work of our laboratory and others on the expression of the C5aR in various human non-myeloid cells types. A better understanding of the expression patterns of this important anaphylatoxin receptor may provide new insights in the pathophysiological role of C5a in vivo.

On the role of complement and Fc gamma-receptors in the Arthus reaction

The contribution of either the complement system or the activation of Fc receptors for IgG (FcyRs) to the inflammatory response in immune complex (IC) disease is puzzling. A series of studies has been performed in mice with engineered deficiencies of either FcgammaRs, the complement components C3, C4 or the C5a receptor. In addition, different C5-deficient mice strains have been evaluated. Mice with gene targeted disruption of the gamma-subunit, which mediates surface expression and signal transduction of the high affinity Fc receptor type I for IgG (FcgammaRI), the low affinity receptor Fc receptor type III for IgG (FcgammaRIII) and the high affinity receptor type I for IgE (IgepsilonRI), showed an impaired inflammatory response in the reverse passive Arthus reaction in skin, peritoneum and lung. These data suggest, that the activation of FgammaRs is the initial event triggering the inflammatory cascade in IC disease. On the other hand, C5aR deficient mice are either protected from tissue injury induced by ICs, as in the lung, or the degree of the inflammatory response is markedly attenuated, as in peritoneum and skin. A detailed analysis of data obtained with the different knock-out strains revealed that both the activation of the complement system as well as the activation of different effector cells via FcgammaRs contribute to the inflammatory sequelae leading to tissue destruction in IC disease. The relative contributions of FcgammaRI or FcgammaRIII and the main effector cells through which these receptors mediate their effector functions are tissue dependent. The activation of the C5a receptor pathway appears to be the prominent contribution of the complement system.

Human T Cells Express the C5a Receptor and Are Chemoattracted to C5a

The anaphylatoxin C5a is a potent mediator of inflammation that exerts a broad range of activity on cells of the myeloid lineage. In this study, we present the first evidence that human T cells express the C5a receptor (C5aR) and are chemotactic to C5a. Using FACS analysis, we found that the C5aR was expressed at a low basal level on unstimulated T cells and was strikingly up-regulated upon PHA stimulation in a time- and dose-dependent manner. CD3+ sorted T cells as well as Jurkat T cells were shown to express C5aR mRNA as assessed by RT-PCR. Moreover, semiquantitative RT-PCR analysis demonstrated that C5aR mRNA was down-regulated in purified T cells upon long-term PHA stimulation. To demonstrate that C5a was biologically active on T cells, we investigated the chemotactic activity of C5a and observed that purified CD3+ T cells are chemotactic to C5a at nanomolar concentrations. Finally, using a combination of in situ hybridization and immunohistochemistry, we showed that the T cells infiltrating the central nervous system during experimental allergic encephalomyelitis express the C5aR mRNA. In summary, these results suggest that C5a exerts direct effects on T cells and could be involved in the trafficking of T cells under physiological and pathological conditions, including inflammatory diseases of the central nervous system.

Eosinophil leukocyte degranulation in response to serum-opsonized beads: C5a and platelet-activating factor enhance ECP release, with roles for protein kinases A and C

BACKGROUND

Eosinophils have a typical content of granule-bound, cytotoxic, cationic proteins which may, when released to the external milieu, play roles in diseases such as asthma and parasitic infestation. Therefore, we have investigated possible mechanisms by which their release is regulated in eosinophils.

METHODS

The enzyme-linked immunosorbent assay (ELISA) was used to detect released eosinophil cationic protein (ECP). Release of ECP was induced by serum-opsonized, nonphagocytosable Sephadex beads (SOS).

RESULTS

The complement fragment C5a and platelet-activating factor (PAF) were found to enhance ECP release in response to SOS in a dose-dependent fashion, and, contrary to previous reports, they were not found to act as secretagogues themselves on eosinophils in suspension. The role of protein kinase C (PKC) in eosinophil degranulation has been controversial. We found that ECP release induced by SOS was inhibited by the PKC inhibitors staurosporine and calphostin C. Activation of protein kinase A (PKA), by raising cAMP, also inhibited ECP release. Furthermore, pertussis toxin decreased ECP release on opsonized beads, indicating the involvement of pertussis-toxin-sensitive G proteins.

CONCLUSIONS

C5a, and PAF enhance granule release, rather than acting as secretagogues themselves. PKC and PKA have opposing roles in the regulation of ECP release in response to SOS.

Complement C5a anaphylatoxin fragment causes apoptosis in TGW neuroblastoma cells

Human neuroblastoma TGW cells express a C5a anaphylatoxin receptor-like molecule termed neuronal C5a receptor. A C5a-receptor fragment peptide (termed PR226-multiple antigenic peptide) can induce rapid apoptosis in TGW cells via neuronal C5a receptor-associated signal transduction pathways. In order to analyse role of activated complement system in neurodegeneration, TGW cells were exposed to an oligomer form of a C5a fragment (amino acids: 37-53) peptide termed PL37-multiple antigenic peptide. Upon treatment with PL37-multiple antigenic peptide, an increased nuclear c-fos expression was shown within 30 min. DNA fragmentation, a hallmark of apoptosis, was noted within 4 h. Extracellular administration of 100 nM PL37-multiple antigenic peptide evoked inward calcium current pulses. At higher doses (0.5 microM-1 microM), PL37-multiple antigenic peptide evoked higher current pulses, followed by an irreversible, high inward current. To exert its apoptotic effect, PL37-multiple antigenic peptide utilizes a pertussis toxin-sensitive signal transduction pathway associated with the neuronal C5a receptor. Activation of the complement system and therefore release of C5a has already been reported in Alzheimer's disease. In addition, the presence of the Kunitz-type proteinase inhibitors indicates an impaired protease function and a possible abnormal fragmentation of C5a anaphylatoxin. Our data suggest that neurons expressing neuronal C5a receptor are more vulnerable to the apoptosis associated with the neuronal C5a receptor and the possibility that abnormal activation of C5a receptor and C5a anaphylatoxin fragments might be involved in the pathogenesis of Alzheimer's disease.

Molecular and biological characterization of the murine leukotriene B4 receptor expressed on eosinophils

The movement of leukocytes into tissues is regulated by the local production of chemical mediators collectively referred to as chemoattractants. Although chemoattractants constitute a diverse array of molecules, including proteins, peptides, and lipids, they all appear to signal leukocytes through a related family of seven transmembrane-spanning G protein-coupled receptors. The eosinophil is a potent proinflammatory cell that is attracted into tissues during allergic inflammation, parasitic infection, and certain malignancies. Since the molecular mechanisms controlling eosinophil recruitment are incompletely understood, we performed a degenerate polymerase chain reaction on cDNA isolated from murine eosinophils to identify novel chemoattractant receptors. We report the isolation of a cDNA that encodes a 351-amino acid glycoprotein that is 78% identical to a human gene that has been reported to be a purinoceptor (P2Y7) and a leukotriene B4 (LTB4) receptor (BLTR). Chinese hamster ovary (CHO) cells transfected with this cDNA specifically bound [3H]LTB4 with a dissociation constant of 0.6 +/- 0.1 nM. Furthermore, LTB4 induced a dose-dependent intracellular calcium flux in transfected CHO cells. In contrast, [35S]dATP did not specifically bind to these transfectants. This mRNA was expressed at high levels in interleukin 5-exposed eosinophils, elicited peritoneal macrophages and neutrophils, and to a lesser extent interferon gamma stimulated macrophages. Low levels of expression were detected in the lung, lymph node, and spleen of unchallenged mice. Western blot analysis detected the mBLTR protein in murine eosinophils and alveolar macrophages as well as human eosinophils. In addition, elevated levels of mBLTR mRNA were found in the lungs of mice in a murine model of allergic pulmonary inflammation in a time course consistent with the influx of eosinophils. Our findings indicate that this murine receptor is an LTB4 receptor that is highly expressed on activated leukocytes, including eosinophils, and may play an important role in mediating eosinophil recruitment into inflammatory foci.

Human ecalectin, a variant of human galectin-9, is a novel eosinophil chemoattractant produced by T lymphocytes

A 1.6-kilobase pair cDNA was isolated from a human T-cell-derived expression library that encodes a novel eosinophil chemoattractant (designated ecalectin) expressed during allergic and parasitic responses. Based on its deduced amino acid sequence, ecalectin is a 36-kDa protein consisting of 323 amino acids. Although ecalectin lacks a hydrophobic signal peptide, it is secreted from mammalian cells. Ecalectin is not related to any known cytokine or chemokine but rather is a variant of human galectin-9, a member of the large family of animal lectins that have affinity for beta-galactosides. Recombinant ecalectin, expressed in COS cells and insect cells, exhibited potent eosinophil chemoattractant activity and attracted eosinophils in vitro and in vivo in a dose-dependent manner but not neutrophils, lymphocytes, or monocytes. The finding that the ecalectin transcript is present in abundance in various lymphatic tissues and that its expression increases substantially in antigen-activated peripheral blood mononuclear cells suggests that ecalectin is an important T-cell-derived regulator of eosinophil recruitment in tissues during inflammatory reactions. We believe that this is the first report of the expression of an immunoregulatory galectin expressed by a T-cell line that is selective for eosinophils.

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.

A neuronal C5a receptor and an associated apoptotic signal transduction pathway

1. We report the first experimental evidence of a neuronal C5a receptor (nC5aR) in human cells of neuronal origin. Expression of nC5aR mRNA was demonstrated by the reverse transcriptase-polymerase chain reaction (RT-PCR) in TGW human neuroblastoma cells. 2. Expression of a functional C5aR was supported by the finding that C5a evoked a transient increase in the intracellular calcium level as measured by flow cytometry (FACS). 3. To analyse the function of the nC5aR, an antisense peptide fragment of the C5aR was used. Previous data showed that a C5aR fragment (a peptide termed PR226) has C5aR agonist and antagonist effects in U-937 cells depending on the concentration of the peptide. We found that a multiple antigenic peptide (MAP) form of the same peptide (termed PR226-MAP) induced rapid elevation of nuclear c-fos immunoreactivity and resulted in DNA fragmentation, a characteristic sign of apoptosis, in TGW cells. 4. Early electrophysiological events characteristic of apoptosis were also detected: intermittent calcium current pulses were recorded within 1-2 min of peptide administration. C5a pretreatment delayed the onset of this calcium influx. 5. We also demonstrated that the apoptotic pathway is linked to nC5aR via pertussis toxin-sensitive G-proteins. 6. Although the function of C5a and its receptor on neurons is unknown, these results suggest that an abnormal activation of this signal transduction pathway can result in apoptosis and, subsequently, in neurodegeneration.

A case of episodic angioedema associated with blood eosinophilia: upregulated C5a receptor expression on eosinophils

A 47-year-old woman was admitted to hospital complaining of swelling and pain of the extremities, accompanied by high fever and generalized erythema. Laboratory examination showed marked blood eosinophilia with elevation of IgM, IgE, and C-reactive protein. All autoantibodies examined were negative. The heart and lungs showed no untoward findings. Biopsies of the skin and muscle revealed cellular infiltration of eosinophils around small blood vessels. Quantitation of C5a receptor (C5aR) expression by flow cytometry using anti-C5aR antibody showed upregulated expression of C5aR on blood eosinophils but downregulated expression on neutrophils. The abnormal C5aR expression on eosinophils and neutrophils became normal after spontaneous resolution of symptoms and blood eosinophilia. The possibility that C5aR expression on granulocytes is related to the pathogenesis of this syndrome may be considered.

Characterization of rat C5a anaphylatoxin receptor (C5aR): cloning of rat C5aR cDNA and study of C5aR expression by rat astrocytes

Complement system activation within the central nervous system (CNS) is involved in demyelinating and neurodegenerative disorders, but the role of complement in the pathogenic process or in the repair remains unclear. Besides the direct lytic effects of complement on target cells (oligodendrocytes or neurons), complement can exert other functions through interaction of complement fragments with specific receptors. The C5a anaphylatoxin, an inflammatory peptide which is formed during complement activation, might play a role in the CNS pathogenesis, and activation and recruitment of glial cells by binding to its receptor (C5aR) on CNS cells. Using degenerate primers corresponding to homologous regions between human and mouse C5aR cDNAs, we have cloned a rat C5aR cDNA probe from rat monocytes RNAs after RT-PCR experiment. The rat C5aR probe isolated by this procedure allowed us to clone the rat C5aR cDNA-coding sequence using a library screening cloning strategy. This probe was also used to study the expression of the C5aR mRNA in the rat CNS. Northern blotting and RT-PCR experiments demonstrated the constitutive expression of C5aR mRNA in brain, spleen, kidney and lung. This transcript was also observed in primary culture of rat astrocytes. Microfluorimetry experiments demonstrated that C5aR expressed by astrocytes in culture is functional since the addition of C5a induced a dose-dependent increase of intracellular calcium concentration. The expression of the C5aR by astrocytes suggests new roles for the C5a anaphylatoxin in reactive astrogliosis to CNS injuries.

C3a and C5a Stimulate Chemotaxis of Human Mast Cells

The factors that control migration of mast cells to sites of inflammation and tissue repair remain largely undefined. Whereas several recent studies have described chemotactic factors that induce migration of murine mast cells, only stem cell factor (SCF ) is known to induce migration of human mast cells. We report here that the anaphylatoxins C3a and C5a are chemotactic factors for the human mast cell line HMC-1, human cord blood-derived mast cells (CBMC) and cutaneous mast cells in vitro. The presence of an extracellular matrix protein, laminin, was required for chemotaxis in response to complement peptides. Migration of mast cells towards C3a and C5a was dose-dependent, peaking at 1 μg/mL (100 nmol/L), and was inhibited by specific antibodies. Pretreatment with pertussis toxin inhibited the anaphylatoxin-mediated migration of HMC-1 cells, indicating that Gi proteins are involved in complement-activated signal transduction pathways in human mast cells. Both C3a and C5a also induced a rapid and transient mobilization of intracellular free calcium ([Ca2+]i ) in HMC-1 cells. Besides SCF, other chemotactic factors tested, such as interleukin-3, nerve growth factor, transforming growth factor β, RANTES (regulated upon activation, normal T cell expressed and secreted), monocyte chemotactic protein-1 (MCP-1), MCP-2, MCP-3, macrophage inflammatory protein-1α (MIP-1α), and MIP-1β, failed to stimulate migration of human mast cells. In summary, these findings indicate that C3a and C5a serve as chemotaxins for human mast cells. Anaphylatoxin-mediated recruitment of mast cells might play an important role in hypersensitivity and inflammatory processes.

Expression and regulation of monocyte chemoattractant protein-1 by human eosinophils

Several recent studies have identified eosinophils as a cellular source of various cytokines, indicating that eosinophils play not only an effector role, but also a regulatory role within the allergic inflammatory cell network. In this study, we demonstrate that eosinophils can generate and secrete monocyte chemoattractant protein-1 (MCP-1), a prototype of C-C chemokines. Eosinophils generated immunoreactive MCP-1 in response to such diverse stimuli as C5a, formyl-methionyl-leucyl-phenylalanine (FMLP) and ionomycin, but MCP-1 production was not induced by interleukin (IL)-1 or tumor necrosis factor-alpha. C5a- and FMLP-induced eosinophil MCP-1 production was absolutely dependent on pretreatment with cytochalasin B. Eosinophils elaborated significantly more MCP-1 than neutrophils. Immunoreactive MCP-1 was detected at 6 h of incubation with C5a or FMLP. Expression of MCP-1 mRNA reached a maximum within the first 3 h after stimulation and then declined rapidly to a very low and stable level by 18 h. Pretreatment with IL-5 markedly amplified C5a-induced MCP-1 production, and the enhancement occurred at the pretranslational level. Eosinophil-active chemokines such as eotaxin failed to induce MCP-1 generation, even when eosinophils were primed by IL-5. Since MCP-1 exerts a potent histamine-releasing effect on human basophils, our results indicate that eosinophils may regulate basophil mediator release with possible consequent contribution to the pathogenesis of allergic inflammation via a paracrine mechanism.

Chemotactic 5-oxo-eicosatetraenoic acids induce oxygen radical production, Ca2+-mobilization, and actin reorganization in human eosinophils via a pertussis toxin-sensitive G-protein

The arachidonic acid metabolites 5-oxo-[6E,8Z,11Z,14Z]-eicosatetraen oic acid (5oETE) and 5-oxo-15-hydroxy-[6E,8Z,11Z,13E]-eicosatetrae noi c acid (5oHETE) are potent eosinophil chemotaxins. Here, the activation profile of 5-oxo-eicosanoids in eosinophils was further characterized and compared to other eosinophil activators such as complement fragment C5a (C5a), platelet-activating factor (PAF), interleukin-5 (IL-5), and phorbol ester (PMA). Flow cytometric studies revealed a rapid and transient actin polymerization upon stimulation by both 5-oxo-eicosanoids. Desensitization studies using actin polymerization as the parameter indicated cross-desensitization between the two 5-oxo-eicosanoids but revealed no interference with the response to other chemotaxins. Fluorescence measurements with Fura-2-labeled eosinophils in the presence of EGTA indicated Ca2+-mobilization from intracellular stores by 5oETE and 5oHETE. Both 5-oxo-eicosanoids stimulated the production of reactive oxygen metabolites as demonstrated by lucigenin-dependent chemiluminescence, superoxide dismutase-inhibitable cytochrome C reduction, and flow cytometric dihydrorhodamine-123 analysis. At optimal concentrations the changes induced by 5-oxo-eicosanoids were comparable to those obtained by C5a and PAF, whereas IL-5 and PMA induced only a restricted pattern of cell responses. Cell responses elicited by 5-oxo-eicosanoids were inhibited by pertussis toxin, indicating coupling of the putative 5-oxo-eicosanoid-receptor to G-proteins. These results indicate that 5-oxo-eicosanoids are stong activators of eosinophils with comparable biologic activity to the eosinophil chemotaxins C5a and PAF. These findings point to a role of 5-oxo-eicosanoids in the pathogenesis of eosinophilic inflammation as chemotaxins as well as activators of pro-inflammatory activities.

CD4-induced interaction of primary HIV-1 gp120 glycoproteins with the chemokine receptor CCR-5

For efficient entry into target cells, primary macrophage-tropic and laboratory-adapted human immunodeficiency viruses type 1 (HIV-1) require particular chemokine receptors, CCR-5 and CXCR-4, respectively, as well as the primary receptor CD4 (refs 1-6). Here we show that a complex of gp120, the exterior envelope glycoprotein, of macrophage-tropic primary HIV-1 and soluble CD4 interacts specifically with CCR-5 and inhibits the binding of the natural CCR-5 ligands, macrophage inflammatory protein (MIP)-1alpha and MIP-1beta (refs 7, 8). The apparent affinity of the interaction between gp120 and CCR-5 was dramatically lower in the absence of soluble CD4. Additionally, in the absence of gp120, an interaction between a two-domain CD4 fragment and CCR-5 was observed. A gp120 fragment retaining the CD4-binding site and overlapping epitopes was able to interact with CCR-5 only if the V3 loop, which can specify HIV-1 tropism and chemokine receptor choice, was also present on the molecule. Neutralizing antibodies directed against either CD4-induced or V3 epitopes on gp120 blocked the interaction of gp12O-CD4 complexes with CCR-5. These results suggest that HIV-1 attachment to CD4 creates a high-affinity binding site for CCR-5, leading to membrane fusion and virus entry.

Detection of C5a receptors on human eosinophils and inhibition of eosinophil effector functions by anti-C5a receptor (CD88) antibodies

Eosinophils and complement activation are reported to play a crucial role in the pathogenesis of connective tissue diseases. Depositions of antigens and antigen-antibody complexes lead to complement activation with the generation of anaphylatoxins, particularly C5a, which is thought to be responsible for the infiltration and activation of eosinophils in the tissue. Previous studies suggested that the eosinophil C5a receptor differs structurally from the receptor expressed on neutrophils. In this study, we investigated the expression and functional properties of C5a receptors on human eosinophils using the C5a receptor monoclonal antibody S5/1 (anti-CD88 mAb). Flow cytometric analysis demonstrated that the anti-CD88 mAb bound homogeneously on the surface of human eosinophils from nonatopic healthy donors. In addition, no subpopulations with respect to C5a receptor expression were identified in normodense or hypodense eosinophils of patients with hypereosinophilia. Pre-incubation of eosinophils with anti-CD88 specifically inhibited C5a-induced intracellular calcium concentration transients. C5a-induced chemotactic activity of eosinophils was significantly inhibited after pre-incubation of cells with anti-CD88 mAb in a dose-dependent manner. Furthermore, anti-CD88 mAb inhibited dose-dependently the release of reactive oxygen species by eosinophils following stimulation with C5a. Thus, the human eosinophil C5a receptor is homogeneously expressed on normal eosinophils from healthy donors as well as on hypodense and normodense eosinophil subpopulations from patients with hypereosinophilia. Based on the inhibitory effect of the S5/1 mAb on C5a-stimulated eosinophil effector functions, we conclude that a single C5a receptor type exists on human eosinophils. In addition, the inhibitory effect of the S5/1 mAb on C5a functions may enable a new experimental approach to the treatment of diseases that have been associated with C5a-mediated activation.

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.

Molecular and cellular interactions mediating granulocyte accumulation in vivo

An inflammatory response, either beneficial in host defence or detrimental resulting in an inflammatory disease, is associated with alterations in vascular tone and blood flow, enhanced vascular permeability to macromolecules and the extravasation of leucocytes from the vascular lumen into extravascular tissue. The adhesive interaction of granulocytes with venular endothelial cells is an essential step in the process of granulocyte accumulation at sites of inflammation in vivo. Recent advances made in the field of chemokines and adhesion molecules have led to a better understanding of the molecular events mediating this important component of the inflammatory response. The defined molecular interactions that mediate and regulate these events, in the process of neutrophil accumulation, will be discussed in this article. Recently, the eosinophil has come into focus because of its prominence in allergic reactions. This cell will be discussed in comparison.

Mechanisms of human eosinophil activation by complement protein C5a and platelet-activating factor: similar functional responses are accompanied by different morphologic alterations

The complement system is an important amplification system for the propagation of allergic as well as pseudoallergic inflammatory reactions. In the present study, the effect of the major anaphylatoxin C5a was compared with that of platelet-activating factor (PAF) on highly purified eosinophils (> or = 95%) by functional as well as morphologic criteria. Upon stimulation with C5a, eosinophils maintained their spheric structure, developing short, pseudopodia-like protrusions, whereas PAF induced the generation of a number of digitating protrusions. As shown by functional and ultrastructural assay systems, both stimuli provoked significant extracellular and intracellular H2O2 production in eosinophils, which was inhibited by cytochalasin B. With C5a, a pronounced H2O2 production was detected within the small cytoplasmic vesicles, whereas PAF-induced H2O2 production was observed on the outer surface of the plasma membrane in the contact zones between adjacent cells. Morphologic signs of degranulation induced by C5a and PAF were accompanied by the significantly increased release of eosinophil cationic protein and eosinophil peroxidase in the presence of cytochalasin B. Like PAF, C5a induced a significant production of reactive oxygen species in eosinophils, as measured by lucigenin-dependent chemiluminescence (CL) responses in eosinophils. Maximal responses, comparable with those of interleukin-5 (100 U/ml), were observed at concentrations of 10(-5)-10(-6) and 10(-7)-10(-8) M for PAF and C5a, respectively. Separation of eosinophils by discontinuous density gradients revealed the existence of two hypodense eosinophil populations, one of them showing significantly reduced CL responses upon stimulation with C5a and PAF. In addition, CL responses upon stimulation with C5a and PAF were abrogated by cytochalasin B, staurosporine, and wortmannin, and were almost completely blocked by pertussis toxin. In conclusion, these data indicate that C5a induces events in human eosinophils comparable to those induced by PAF in the assay systems tested. Thus, C5a, generated after activation of the complement system, may be of major importance for the eosinophil activation observed in eosinophil-related disease.

Regulation of neutrophil activation by oleic acid

Physiological concentrations of oleic acid inhibited C5a-induced myeloperoxidase release from neutrophils. The inhibition occurred promptly following the addition of oleic acid, was dose-dependent and saturable, and was greater at low concentrations of C5a. Kinetic analysis of the curve for 1/myeloperoxidase release against 1/[C5a] in the presence of oleic acid, was compatible with a cooperative pattern of interaction. The inhibitory effect persisted after repeated washings of cells preincubated with oleic acid. The effect of oleic acid was not specific for C5a-induced neutrophil activation; oleic acid also inhibited myeloperoxidase release induced by the Ca2+ ionophore A-23187, as well as zymosan activated serum-induced chemotaxis. In experiments designed to localize the site of action of oleic acid on neutrophil activation, an intracellular oleic acid-binding protein M(r) = 13,000) was isolated. The results suggest that acute changes in plasma fatty acid levels may have significant effects on neutrophil function under physiological conditions.

Effects of aprotinin on complement and granulocyte activation during ex vivo hemodialysis

Hemodialysis with cellulosic membranes results in complement activation, granulocytopenia, and granulocyte activation. To further investigate the relationship between complement activation and granulocyte activation, we developed a model of ex vivo hemodialysis with blood flow, dialysate flow, and dialysate composition similar to in vivo hemodialysis. We used this model to investigate the effects of aprotinin, a potent serine protease inhibitor frequently used as an anti-inflammatory agent during cardiopulmonary bypass surgery, on both complement and granulocyte activation. Seven normal human volunteers were phlebotomized for ex vivo hemodialysis on two occasions each, one with and once without 800,000 kallikrein inhibitor units of aprotinin added to the circuit. Measurements were made of complement activation (radioimmunoassay for C3a desArg and C5a desArg), as well as granulocyte activation (flow cytometric measurements of reactive oxygen species (ROS) production, granulocyte CD11b-CD18 [MAC-1, CR3] expression, and CD62-L [L-selectin] expression). Statistically significant elevations in C3a desArg levels occurred by 10 minutes and reached a maximum of 5,367 +/- 712 ng/mL by 60 minutes after the initiation of ex vivo hemodialysis. Plasma C5a levels were elevated to 236 +/- 32 ng/mL at 60 minutes compared with 45 +/- 15 ng/mL predialysis. Aprotinin was able to significantly inhibit dialysis-induced C3a generation (peak 2,456 +/- 572 ng/mL at 60 minutes) as well as C5a generation (86 +/- 23 ng/mL at 60 minutes). During ex vivo hemodialysis, there was also a significant increase in granulocyte ROS production, MAC-1 upregulation, and L-selectin downregulation. Changes in granulocyte activation were not affected by the administration of aprotinin.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of human neutrophils by C3a and C5A. Comparison of the effects on shape changes, chemotaxis, secretion, and respiratory burst

The effects of anaphylatoxin C3a on human neutrophils were studied in comparison with C5a. Both peptides induced a transient shape change response and a respiratory burst. In both cases C3a was 50- to 100-times less potent than C5a. A marked chemotactic response with bimodal concentration dependence was obtained with C5a, but no neutrophil chemotaxis was observed with C3a. Repeated stimulation led to homologous desensitization of shape changes and respiratory burst but no cross-desensitization, indicating that the two anaphylatoxins act through separate receptors. The lack of chemotactic activity suggests that C3a is not involved in neutrophil recruitment into infected or inflamed tissues.

Site-specific mutagenesis of residues in the human C5a anaphylatoxin which are involved in possible interaction with the C5a receptor

To check and clarify existing data on receptor-interacting residues in the human C5a anaphylatoxin, we tested mutant C5a proteins obtained by site-directed mutagenesis of a recombinant human C5a (rhC5a) cDNA clone for structural and functional integrity. Amino acid positions in three different regions of the molecule were investigated: Arg74 at the C-terminus, Arg40 and Pro45 located in the core region, and Lys14 and Lys19, Lys20 in the N-terminus. Des-Arg74-rhC5a displayed only a residual 3-4% functional activity in the myeloperoxidase-release assay from human granulocytes while retaining the three-dimensional solution structure of wild-type (wt)-rhC5a as shown by circular dichroism (CD) spectroscopy. Des-Arg74-rhC5a was able to activate the human C5a receptor transiently expressed in Xenopus oocytes, but was inactive in the heterologous guinea pig (gp) ileum-contraction assay. These results reveal profound differences between the guinea pig and human C5a-receptor ligand-binding characteristics. Exchange of the core residue Arg40 by a glycine did not significantly affect functional C5a activity, in contrast to a previous observation [Mollison, K. W., Mandecki, W., Zuiderweg, E. P., Fayer, L., Fey, T. A., Krause, R. A., Conway, R. G., Miller, L., Edalji, R. P., Shallcross, M. A., Lane, B., Fox, J. L., Greer, J. & Carter, G. W. (1989) Identification of receptor-interacting residues in the inflammatory complement protein C5a by site-directed mutagenesis, Proc. Natl Acad. Sci. USA 86, 292-296], nor did exchange of the conserved Pro45 residue by the C3a analogue glutamic acid, a mutation expected to alter the whole geometry of the loop connecting helix III-helix IV (including Arg40) of the C5a molecule. Thus, participation of this loop in receptor interaction appears unlikely. While exchange of the N-terminal Lys14 residue by alanine did not significantly affect functional activity, a double replacement of Lys19 and Lys20 by alanine residues reduced activity more than 30-fold. These results confirm Lys19 and/or Lys20 as a putative receptor-interacting site, although we could not obtain a CD spectrum of this important mutant due to poor expression.

Generation of chimeric C5a/formyl peptide receptors: towards the identification of the human C5a receptor binding site

We employed the polymerase chain reaction to produce a series of chimeric C5a/formyl peptide receptors. Chinese hamster ovary cells transfected with these constructs were tested for their ability to bind C5a. Substitution of three of the extracellular domains of the C5a receptor with the corresponding domains of the formyl peptide receptor abolished C5a binding, whilst replacement of the first extracellular loop of the C5a receptor with that of the formyl peptide receptor had little effect on the affinity of the receptor for C5a. We therefore conclude that this first outer loop of the C5a receptor does not participate in ligand binding, whilst involvement of the other extracellular domains of the receptor cannot be ruled out.

Mechanisms of neutrophil and eosinophil accumulation in vivo

The accumulation of leukocytes into tissues is a characteristic feature of inflammatory reactions. This process is triggered by chemical signals generated in a tissue in response to an inflammatory stimulus e.g., invading microbes, other foreign organisms, allergens, or damaged tissue cells. The mechanisms involved in neutrophil and eosinophil accumulation in vivo are complex and dependent on an initial interaction between the leukocytes and the microvascular endothelial cells. This response is regulated by the coordinated expression and/or activation of leukocyte and endothelial cell adhesion molecules. The precise mechanisms that control the selective accumulation of eosinophils, as opposed to neutrophils, in certain inflammatory reactions (e.g., in IgE-mediated allergic reactions) remain unclear. This may be explained partly by the generation of eosinophil-specific inflammatory mediators and activation of selective adhesion pathways such as the VLA-4/VCAM-1 interaction. Although the neutrophil and eosinophil have distinct roles in host defense, they have been implicated in the pathogenesis of a number of inflammatory disorders. Thus, a better understanding of the events mediating and regulating neutrophil and eosinophil accumulation in vivo will be of considerable value in the development of therapeutic strategies for inflammatory disease states.

Complement 5a induces in vivo synthesis of cysteinyl leukotrienes in rats

The complement derived anaphylatoxin complement 5a (C5a) is suggested to be involved in the pathogenesis of various types of diseases including endotoxic or anaphylactic shock. Studies in our laboratory demonstrated a marked and sustained reduction in renal blood flow and glomerular filtration rate after infusion of a low dose of recombinant C5a (rC5a). Renal rC5a effects were inhibited by leukotriene (LT) and thromboxane antagonists suggesting that the effects were mediated by LT. To elucidate the mechanisms of C5a effects, we monitored the biliary excretion rate of the stable metabolite, N-acetyl-LTE4, by reversed phase high performance liquid chromatography (HPLC). Rats in the experimental group were administered rC5a intravenously at 0.5 micrograms/min for 10 min. Biliary N-acetyl-LTE4 excretion was significantly increased following rC5a infusion, 0.03 ng/microliters bile to 0.129 ng/microliters. The bile flow in the experimental group was reduced about 39% by rC5a, while bile flow of the control group increased by 20% during the observation period. Infusion of rC5a resulted in an increase of arterial hematocrit from 44.7% to 48.7%, whereas blood pressure was not significantly altered in experimental and control groups. Our results suggest the in vivo effects of C5a to be mediated by cysteinyl leukotrienes, which may be important in the pathogenesis of septic, anaphylactic or traumatic shock.

Evaluation of the C-terminal C5a effector site with short synthetic C5a analog peptides

Biological activities have been determined for a series of 18 peptides based on the C-terminal sequence of human or rat C5a. Lysosomal enzyme release was tested in two cell types, the promyelotic leukemia cell line U937 and polymorphonuclear leukocytes. In addition, an ATP-release assay with guinea pig platelets was performed. It was demonstrated that the C-terminal octapeptide 67-74 of human C5a represents the minimal sequence required to induce a measurable biological signal in all assays. Extending this peptide to a length of 21 amino acids produced at best only a slight enhancement of potency. Amino acid replacements with either tryptophanyl or phenylalanyl residues in positions between 65-69 either increased potency (at position 67), or abrogated potency (at position 66) in the two lysosomal enzyme assays. N-terminal acylation with the fluorenylmethoxy-carbonyl-aminohexanoyl group slightly enhanced C5a potency. In desensitization experiments with guinea pig platelets all peptides with a C5a activity were able to desensitize not only the C5a but also the C3a responses.

Human chemotaxis receptor genes cluster at 19q13.3-13.4. Characterization of the human C5a receptor gene

The human C5a anaphylatoxin and formyl peptide receptor genes, as well as two genes with high sequence identity to the formyl peptide receptor, FPRH1 and FPRH2, have been mapped to chromosome 19 (Lu et al., 1992). Further analysis reveals that these genes are present in the 19q13.3 band adjacent to the 13.3-13.4 interface. MRNAs for the C5a and formyl peptide receptors, as well as for FPRH1, are expressed in cAMP differentiated U937 cells and human eosinophils, while all four transcripts are expressed in human lung. This observation opens the possibility for coordinate regulation of these genes. In order to initiate the mapping of fine structure at this locus, genomic clones have been analyzed. All four of the genes have a similar structure, with the receptor protein encoded in a single exon. Detailed characterization of the C5a receptor gene reveals a two exon structure, with the 5' untranslated sequence and initiating methionine located in the first exon. An intron of approximately 9 kb separates exon 1 from the receptor-encoding exon 2. The region of genomic DNA flanking the 5' untranslated sequence possesses promoter activity when transfected into the myeloid-derived rat basophilic leukemia RBL-1 cells, but the same region is inactive when transfected into nonmyeloid cells. Deletional analyses indicate that C5a receptor 5' flanking region contains both cell-specific suppressor and promoter regions.

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.

Characterization of the binding of Bolton-Hunter labeled [125I]C5a to human neutrophil, monocyte and U-937 cell membranes

The fifth component of the complement cascade, C5a, was iodinated using the Bolton-Hunter reagent. Results from the present study, using the high affinity ligand, [125I]Bolton-Hunter-labeled C5a ([125I]BH-C5a), revealed a single binding site on membranes prepared from human neutrophils, U-937 cells and human monocytes. Saturation studies demonstrated Bmax values in these cells of 11.5, 47.3 and 16.6 fmol/10(6) cells, respectively. The C5a receptor demonstrated a very high affinity for [125I]BH-C5a of approximately 4 pM in each cell type. Competition studies using analogs of C5a generated a similar order of potency in each of the cell types of C5a > or = C5a(1-74), Ser66Ala > C5a(1-73) > C5a(1-69). These studies indicate that [125I]BH-C5a labels a similar receptor in neutrophil, U-937 cell and monocyte membranes. Furthermore, C5a(1-73) produced shallow inhibition curves in competition experiments in each cell type. Computer analysis of the binding data revealed two components of binding. When 10 nM unlabeled C5a was used to initiate dissociation of [125I]BH-C5a binding in neutrophil membranes, two binding components were observed. In addition, dissociation of [125I]BH-C5a binding by 10 nM unlabeled C5a in the presence of 1 mM GppNHp decreased the percentage of binding to the slowly dissociating, high affinity binding component from 84 to 58%. These results suggest that multiple states of the C5a receptor exist.

Production of monoclonal antibodies against the human anaphylatoxin C5a des Arg and their application in the neoepitope-specific sandwich-ELISA for the quantification of C5a des Arg in plasma

A panel of ten murine monoclonal antibodies (MAbs) was raised against the human anaphylatoxin C5a des Arg. The MAbs were shown to abrogate or significantly inhibit the chemotactic activity in zymosan activated serum. MAb 4A2E10E2 and MAb 3G3C4 were used as capture and detecting antibody, respectively, in a sandwich enzyme-linked immunosorbent assay (ELISA) for the quantification of C5a des Arg. This ELISA was shown to be very sensitive (detection limit 20 pg/ml) and could be applied directly to plasma/serum samples. The lack of interference by plasma components, in particular C5, suggested specificity for an epitope on C5a (des Arg) which is concealed in native C5 and exposed on the activation fragment only, i.e., a 'neoepitope'. The mean C5a des Arg level in EDTA-plasma from 25 healthy individuals assessed at a 1/20 dilution was 11.2 ng/ml (SD 3.4; range 6.4-16.8 ng/ml). The applicability of the assay was investigated in patients treated with haemodialysis using different membranes. Markedly elevated plasma levels of C5a des Arg were found in blood returning from the dialyzer following contact with cuprophane membranes.

Human C5a anaphylatoxin: gene cloning and expression in Escherichia coli

A gene coding for the human anaphylatoxin C5a was cloned and expressed in Escherichia coli. A combination of reverse transcription of mRNA of the U937 cell line with subsequent preparative polymerase chain reaction was employed to obtain the gene. The sequence was cloned into the plasmid vector pKK 233-2 behind an ATG initiation codon under the control of a trc promotor. After purification by ion exchange chromatography and reversed phase FPLC a mixture of predominantly non-glycosylated recombinant human C5a with a beta-mercaptoethanol adduct at cysteine 27 and the N-methionyl derivative was obtained which was homogeneous on silver-stained gels, immunoreactive with C5a-specific monoclonal antibodies and functionally active in releasing myeloperoxidase from human granulocytes and ATP from guinea pig platelets. The final yield was about 0.4-0.8 mg purified recombinant C5a per liter bacterial culture.

Characterization of the C5a receptor on guinea pig platelets

Guinea pig (gp) platelets react to nanomolar doses of the complement-derived anaphylatoxin C5a with a shape change, aggregation and release of biogenic amines and nucleotides from their granules. We have investigated the specific receptor for C5a on gp platelets which mediates these biological effects. Competitive binding studies with 125I-labeled guinea pig C5a (125I-gpC5a) revealed approx. 4000 binding sites/cell with Kd = 6 x 10(-9) M. The more than 60-fold higher biological activity (ATP-release from gp platelets) of gpC5a versus recombinant human C5a (rhuC5a) and the different binding behavior of gpC5a and rhuC5a point to a species restriction in the gp platelet system. Cross-linking of 125I-gpC5a to gp platelets (250 microM DSS) and analysis by SDS-PAGE under reducing conditions resulted in labeling of a single band with a molecular mass of 32 kDa (ligand-receptor complex). Because of these characteristics, the C5a receptor on gp platelets clearly differs from all previously described C5a receptors.

Functional expression of a human C5a receptor clone in Xenopus oocytes requires additional RNA

cRNA from a PCR-generated C5aR clone was prepared by in vitro transcription and microinjected into Xenopus laevis oocytes. Ligand-induced whole cell current could be detected after co-injection of cRNA for the C5aR with total RNA of the unstimulated U937 cell line, but not with either of the components injected alone. These data clearly demonstrate an absolute requirement of the C5aR for an additional human factor to become functionally expressed in Xenopus oocytes.

Pitfalls in the quantitative estimation of the secretion of granule proteins by eosinophils

The secretion of preformed granule proteins by eosinophils is an important correlate of eosinophil activation. However, a review of the literature reveals large disparities in the amounts of these substances which were reportedly secreted when eosinophils were activated. In the present study we report that our attempts to quantitate the secretion of eosinophil peroxidase and eosinophil-derived neurotoxin from activated eosinophils by measuring these substances in the incubation supernatants were uniformly unsuccessful. We found that, once they were secreted, both eosinophil peroxidase and eosinophil-derived neurotoxin were promptly lost to assay and presumably destroyed. Thus the measurement of the difference in the concentration of these substances in eosinophils prior to and after activation, revealed that as much as 65% of the eosinophil-derived neurotoxin and 62% of the peroxidase in the eosinophils were lost to assay during activation of the cells whereas the largest amount of these substances which could be measured in the incubation supernatants never exceeded 2%. Evidence is presented that the destruction of eosinophil-derived neurotoxin must occur prior to the release of this substance into the medium. Attempts to inhibit the destruction of eosinophil peroxidase and of eosinophil-derived neurotoxin by incorporating various inhibitors into the incubations were unsuccessful. These results emphasize the need to monitor the overall recoveries of secreted products from activated eosinophils and suggest that meaningful estimates of the secretion of these granule proteins from activated eosinophils can only be obtained by measuring the residual content of these substances in eosinophils after they have been activated and comparing these values to the contents of eosinophils prior to activation.

Complement C5a receptor assay for high throughput screening

The complement C5a receptor on U937 cells, a human histiocytic lymphoma cell line, stimulated with dibutyryl-cAMP have been stabilized for at least 3 months at a dilute, ready to use concentration. [125I]-Bolton Hunter labeled C5a, (recombinant, human) has been prepared by reverse phase HPLC to 2200 Ci/mmol. Using a filtration binding assay the Kd from receptor saturation analysis is 10-40 pM and there are 50,000-100,000 receptor sites per cell. These reagents have permitted the development of a reliable, reproducible and convenient drug screening assay, in kit format, for compounds acting at the C5a receptor.