Osteopontin binds and modulates functions of eosinophil-recruiting chemokines

BACKGROUND

Allergic asthma is characterized by eosinophilic inflammation and airway obstruction. There is also an increased risk of pulmonary infection caused by Streptococcus pneumoniae, in particular during severe asthma where high levels of the glycoprotein, osteopontin (OPN), are present in the airways. Eosinophils can be recruited by chemokines activating the receptor CCR3 including eotaxin-1/CCL11, eotaxin-2/CCL24, eotaxin-3/CCL26, RANTES/CCL5, and MEC/CCL28. In addition to inducing chemotaxis, several of these molecules have defensin-like antibacterial properties. This study set out to elucidate the functional consequences of OPN binding to eosinophil-recruiting chemokines.

METHODS

Antibacterial activities of the chemokines were investigated using viable count assays and electron microscopy. Binding studies were performed by means of surface plasmon resonance. The potential interference of OPN with antibacterial, receptor-activating, and lipopolysaccharide-neutralizing abilities of these chemokines was investigated.

RESULTS

We found that OPN bound all eosinophil-recruiting chemokines with high affinity except for CCL5. The eosinophil-recruiting chemokines all displayed bactericidal activity against S. pneumoniae, but only CCL26 and CCL28 retained high antibacterial activity in the presence of sodium chloride at physiologic concentrations. Preincubation of the chemokines with OPN strongly inhibited their antibacterial activity against S. pneumoniae but did not affect their ability to activate CCR3. All chemokines investigated showed LPS-neutralizing activity that was impaired by OPN only in the case of CCL24.

CONCLUSIONS

The data suggest that OPN may impair host defense activities of the chemokines without affecting their eosinophil-recruiting properties. This could be one mechanism explaining the increased vulnerability to acquire pneumococcal infection in parallel with sustained allergic inflammation in asthma.

Eotaxin-3 (CCL26) exerts innate host defense activities that are modulated by mast cell proteases

BACKGROUND

During bacterial infections of the airways, a Th1-profiled inflammation promotes the production of several host defense proteins and peptides with antibacterial activities including β-defensins, ELR-negative CXC chemokines, and the cathelicidin LL-37. These are downregulated by Th2 cytokines of the allergic response. Instead, the eosinophil-recruiting chemokines eotaxin-1/CCL11, eotaxin-2/CCL24, and eotaxin-3/CCL26 are expressed. This study set out to investigate whether these chemokines could serve as innate host defense molecules during allergic inflammation.

METHODS

Antibacterial activities of the eotaxins were investigated using viable count assays, electron microscopy, and methods assessing bacterial permeabilization. Fragments generated by mast cell proteases were characterized, and their potential antibacterial, receptor-activating, and lipopolysaccharide-neutralizing activities were investigated.

RESULTS

CCL11, CCL24, and CCL26 all showed potent bactericidal activity, mediated through membrane disruption, against the airway pathogens Streptococcus pneumoniae, Staphylococcus aureus, Nontypeable Haemophilus influenzae, and Pseudomonas aeruginosa. CCL26 retained bactericidal activity in the presence of salt at physiologic concentrations, and the region holding the highest bactericidal activity was the cationic and amphipathic COOH-terminus. Proteolysis of CCL26 by chymase and tryptase, respectively, released distinct fragments of the COOH- and NH2 -terminal regions. The COOH-terminal fragment retained antibacterial activity while the NH2 -terminal had potent LPS-neutralizing properties in the order of CCL26 full-length protein. An identical fragment to NH2 -terminal fragment generated by tryptase was obtained after incubation with supernatants from activated mast cells. None of the fragments activated the CCR3-receptor.

CONCLUSIONS

Taken together, the findings show that the eotaxins can contribute to host defense against common airway pathogens and that their activities are modulated by mast cell proteases.

Unravelling the mechanisms underpinning chemokine receptor activation and blockade by small molecules: a fine line between agonism and antagonism?

Chemokines are a family of small basic proteins which induce the directed migration of cells, notably leucocytes, by binding to specific GPCRs (G-protein-coupled receptors). Both chemokines and their receptors have been implicated in a host of clinically important diseases, leading to the notion that antagonism of the chemokine–chemokine receptor network may be therapeutically advantageous. Consequently, considerable effort has been put into the development of small-molecule antagonists of chemokine receptors and several such compounds have been described in the literature. One curious by-product of this activity has been the description of several small-molecule agonists of the receptors, which are typically discovered following the optimization of lead antagonists. In this review we discuss these findings and conclude that these small-molecule agonists might be exploited to further our understanding of the molecular mechanisms by which chemokine receptors are activated.

Tails of the unexpected - an atypical receptor for the chemokine RANTES/CCL5 expressed in brain

Chemokines and their receptors play a central role in the trafficking of leukocytes within the body, a process which is amenable to antagonism by small molecules and which holds promise as a treatment for clinically important diseases. In the issue of the British Journal of Pharmacology accompanying this commentary, Ignatov and colleagues describe an unexpected role for the chemokine RANTES/CCL5, namely an ability to signal via the orphan G protein-coupled receptor named GPR75. This receptor bears little homology to other chemokine receptors, most strikingly within the putative intracellular domains, with the third loop and C-terminal tail dwarfing those of other known chemokine receptors. This most likely accounts for the atypical pertussis toxin-insensitive signalling induced by RANTES. Intriguingly, this signalling is neuro-protective, inducing the survival of a hippocampal cell line following insult with the neurotoxic amyloid-beta peptide. Since this peptide is implicated in the pathogenesis of Alzheimer's disease, it may be that exploitation of this signalling pathway presents itself as a future therapeutic treatment.

Chemokines, innate and adaptive immunity, and respiratory disease

Selective leukocyte trafficking and recruitment is primarily regulated by a specific family of small proteins called "chemokines". This extended family shepherds and guides leukocytes through their lives, facilitating their development, regulating their interactions with other leukocyte types, and guiding their recruitment to sites of inflammation. Through the actions of chemokines, allergen sensitization is regulated in atopic asthma, through the controlled migration of dendritic cells, T- and B-lymphocytes, mast cells and basophils. Subsequently, atopic inflammation is driven by chemokine-directed recruitment of eosinophils, basophils and lymphocytes. Diseases from cancer to chronic obstructive pulmonary disease to interstitial fibrosis are all potential targets for chemokine receptor antagonism. Innate immunity (the early pattern-recognition responses to stimuli such as lipopolysaccharide, viral proteins and bacterial DNA) needs to bridge the gap to specific immunity and antibody production and immunological memory. Again, chemokines are likely to be fundamental mediators of these responses. Chemokines are fundamental regulators of leukocyte homeostasis and inflammation, and their antagonism by small molecule chemokine receptor antagonists may be of enormous importance in the future treatment of human respiratory disease.

The CC chemokine eotaxin (CCL11) is a partial agonist of CC chemokine receptor 2b

Despite sharing considerable homology with the members of the monocyte chemoattractant protein (MCP) family, the CC chemokine eotaxin (CCL11) has previously been reported to signal exclusively via the receptor CC chemokine receptor 3 (CCR3). Using the monocyte cell line THP-1, we investigated the relative abilities of eotaxin and MCPs 1-4 to induce CCR2 signaling, employing assays of directed cell migration and intracellular calcium flux. Surprisingly, 1 microm concentrations of eotaxin were able to recruit THP-1 cells in chemotaxis assays, and this migration was sensitive to antagonism of CCR2 but not CCR3. Radiolabeled eotaxin binding assays performed on transfectants bearing CCR2b or CCR3 confirmed eotaxin binding to CCR2 with a K(d) of 7.50 +/- 3.30 nm, compared with a K(d) of 1.68 +/- 0.91 nm at CCR3. In addition, whereas 1 microm concentrations of eotaxin were able to recruit CCR2b transfectants, substimulatory concentrations of eotaxin inhibited MCP-1-induced chemotaxis of CCR2b transfectants and also inhibited MCP-1-induced intracellular calcium flux of THP-1 cells. Collectively, these findings suggest that eotaxin is a partial agonist of the CCR2b receptor. A greater understanding of the interaction of CCR2 with all of its ligands, both full and partial agonists, may aid the rational design of specific antagonists that hold great promise as future therapeutic treatments for a variety of inflammatory disorders.

Eotaxin and asthma

Eotaxin is a small protein that is produced in the lungs of asthmatic patients and is a potent chemoattractant for eosinophils. Eotaxin, a CC chemokine, stimulates the migration of eosinophils from the small blood vessels in the lungs by acting on the CC chemokine receptor CCR3, which is located on the leukocyte cell surface. In the past year, three low molecular weight compounds have been developed that can block this receptor. Such compounds may be developed into orally available drugs aimed at preventing eosinophil recruitment and, hence, the pathogenesis associated with the activation of these cells within the lung tissue.

Roles of chemokines in the regulation of leucocyte recruitment

In the search for new treatments for human inflammatory disease, antagonism of chemokine receptors by small molecules is an attractive goal. Although there are overlapping patterns of expression of chemokine receptors between leucocyte types, an investigation of the chemokine responsiveness of cells important in allergic inflammation, such as the eosinophil and the basophil, is beginning to uncover how selective recruitment may be regulated. The story of the eotaxin receptor, CCR3, and its central role in allergic inflammation illustrates that therapeutic antagonism of these pathways is imminently achievable.

Delta 32 deletion of CCR5 gene and association with asthma or atopy

The CCR5-delta32 deletion polymorphism (CCR5-delta32) was investigated for linkage and association to asthma and atopy using two panels of nuclear families containing 1284 individuals. No statistically significant linkage to asthma/wheeze or atopy was observed in either of the two panels of families. Multiallelic transmission disequilibrium tests (TDT) of the combined data found no significant association for atopy (52 independent alleles transmitted, 51 non-transmitted) or asthma/wheeze (39 transmitted, 44 non-transmitted). Although functional evidence might suggest that CCR5 is a good candidate gene for atopic asthma, this study provides no genetic evidence from CCR5-delta32 polymorphism to support this hypothesis.

A small molecule antagonist of chemokine receptors CCR1 and CCR3. Potent inhibition of eosinophil function and CCR3-mediated HIV-1 entry

We describe a small molecule chemokine receptor antagonist, UCB35625 (the trans-isomer J113863 published by Banyu Pharmaceutical Co., patent WO98/04554), which is a potent, selective inhibitor of CCR1 and CCR3. Nanomolar concentrations of UCB35625 were sufficient to inhibit eosinophil shape change responses to MIP-1alpha, MCP-4, and eotaxin, while greater concentrations could inhibit the chemokine-induced internalization of both CCR1 and CCR3. UCB35625 also inhibited the CCR3-mediated entry of the human immunodeficiency virus-1 primary isolate 89.6 into the glial cell line, NP-2 (IC(50) = 57 nm). Chemotaxis of transfected cells expressing either CCR1 or CCR3 was inhibited by nanomolar concentrations of the compound (IC(50) values of CCR1-MIP-1alpha = 9.6 nm, CCR3-eotaxin = 93.7 nm). However, competitive ligand binding assays on the same transfectants revealed that considerably larger concentrations of UCB35625 were needed for effective ligand displacement than were needed for the inhibition of receptor function. Thus, it appears that the compound may interact with a region present in both receptors that inhibits the conformational change necessary to initiate intracellular signaling. By virtue of its potency at the two major eosinophil chemokine receptors, UCB35625 is a prototypic therapy for the treatment of eosinophil-mediated inflammatory disorders, such as asthma and as an inhibitor of CCR3-mediated human immunodeficiency virus-1 entry.

Receptor activation by human C5a des Arg74 but not intact C5a is dependent on an interaction between Glu199 of the receptor and Lys68 of the ligand

Despite the expression of only one type of receptor, there is great variation in the ability of different cell types to discriminate between C5a and its more stable metabolite, C5a des Arg74. The mechanism that underlies this phenomenon is not understood but presumably involves differences in the interaction with the C5a receptor. In this paper, we have analyzed the effects of a substitution mutation of the receptor (Glu199 --> Lys199) and the corresponding reciprocal mutants (Lys68 --> Glu68) of C5a, C5a des Arg74 and peptide analogues of the C-terminus of C5a on the ability of the C5a receptor to discriminate between ligands with and without Arg74. The use of these mutants indicates that the Lys68/Glu199 interaction is essential for activation of receptor by C5a des Arg74 but not for activation by intact C5a. The substitution of Asp for Arg74 of C5a [Lys68] produces a ligand with equal potency on both the wild-type and mutant receptors, suggesting that it is the C-terminal carboxyl group rather than the side chain of Arg74 that controls the responsiveness of the receptor to Lys68. In contrast, the mutation of Lys68 to Glu(68) has little effect on the ability of either C5a or C5a des Arg(74) to displace [(125)I]C5a from the receptors, indicating that binding of ligand and receptor activation are distinct but interdependent events. C5a and the truncated ligand, C5a des Arg74, appear to have different modes of interaction with the receptor and the ability of the human C5a receptor to discriminate between these ligands is at least partly dependent on an interaction with the receptor residue, Glu199.

Differential regulation of eosinophil chemokine signaling via CCR3 and non-CCR3 pathways

To investigate eosinophil stimulation by chemokines we developed a sensitive assay of leukocyte shape change, the gated autofluorescence/forward scatter assay. Leukocyte shape change responses are mediated through rearrangements of the cellular cytoskeleton in a dynamic process typically resulting in a polarized cell and are essential to the processes of leukocyte migration from the microcirculation into sites of inflammation. We examined the actions of the chemokines eotaxin, eotaxin-2, monocyte chemoattractant protein-1 (MCP-1), MCP-3, MCP-4, RANTES, macrophage inflammatory protein-1alpha (MIP-1alpha), and IL-8 on leukocytes in mixed cell suspensions and focused on the responses of eosinophils to C-C chemokines. Those chemokines acting on CCR3 induced a rapid shape change in eosinophils from all donors; of these, eotaxin and eotaxin-2 were the most potent. Responses to MCP-4 were qualitatively different, showing marked reversal of shape change responses with agonist concentration and duration of treatment. In contrast, MIP-1alpha induced a potent response in eosinophils from a small and previously undescribed subgroup of donors via a non-CCR3 pathway likely to be CCR1 mediated. Incubation of leukocytes at 37 degrees C for 90 min in the absence of extracellular calcium up-regulated responses to MCP-4 and MIP-1alpha in the majority of donors, and there was a small increase in responses to eotaxin. MIP-1alpha responsiveness in vivo may therefore be a function of both CCR1 expression levels and the regulated efficiency of coupling to intracellular signaling pathways. The observed up-regulation of MIP-1alpha signaling via non-CCR3 pathways may play a role in eosinophil recruitment in inflammatory states such as occurs in the asthmatic lung.

The N-terminal extracellular segments of the chemokine receptors CCR1 and CCR3 are determinants for MIP-1alpha and eotaxin binding, respectively, but a second domain is essential for efficient receptor activation

CCR1 and CCR3 are seven-transmembrane domain G protein-coupled receptors specific for members of the CC chemokine subgroup of leukocyte chemoattractants. Both have been implicated in the inflammatory response, and CCR3, through its expression on eosinophils, basophils, and Th2 lymphocytes, may be especially important in allergic inflammation. CCR1 and CCR3 are 54% identical in amino acid sequence and share some ligands but not others. In particular, macrophage inflammatory protein 1alpha (MIP-1alpha) is a ligand for CCR1 but not CCR3, and eotaxin is a ligand for CCR3 but not CCR1. To map ligand selectivity determinants and to guide rational antagonist design, we analyzed CCR1:CCR3 chimeric receptors. When expressed in mouse pre-B cells, chimeras in which the N-terminal extracellular segments were switched were both able to bind both MIP-1alpha and eotaxin, but in each case, binding occurred via separate sites. Nevertheless, neither MIP-1alpha nor eotaxin were effective agonists at either chimeric receptor in either calcium flux or chemotaxis assays. These data are consistent with a multi-site model for chemokine-chemokine receptor interaction in which one or more subsites determine chemokine selectivity, but others are needed for receptor activation. Agents that bind to the N-terminal segments of CCR1 and CCR3 may be useful in blocking receptor function.

Microbial corruption of the chemokine system: an expanding paradigm

The chemokine signaling system includes more than 40 secreted pro-inflammatory peptides and 12 G protein-coupled receptors that together orchestrate specific leukocyte trafficking in the mammalian immune system, ideally for anti- microbial defense and tissue repair processes. Paradoxically and perversely, some chemokines and chemokine receptors are also promicrobial factors and facilitate infectious disease, the result of either exploitation or subversion by specific microbes. Two modes of exploitation are known: usage of cellular chemokine receptors for cell entry by intracellular pathogens, including HIV, and usage of virally-encoded chemokine receptors for host cell proliferation. Likewise, two modes of subversion are known: virally-encoded chemokine antagonists and virally-encoded chemokine scavengers. Understanding how microbes turn the tables on the chemokine system may point to new methods to prevent or treat infection, or, more generally, to treat inappropriate chemokine-mediated inflammation.

Molecular cloning of leukotactin-1: a novel human beta-chemokine, a chemoattractant for neutrophils, monocytes, and lymphocytes, and a potent agonist at CC chemokine receptors 1 and 3

A new member of human beta-chemokine cDNA was isolated and named leukotactin-1 (Lkn-1). Lkn-1, along with murine macrophage inflammatory protein-related protein-1 and -2, defines a subgroup of beta-chemokines based on two conserved cysteines in addition to the four others conserved in all beta-chemokines. The putative mature Lkn-1 is composed of 92 amino acids with a calculated m.w. of 10,162. The Lkn-1 gene was mapped to human chromosome 17, region q12. Recombinant Lkn-1 was a potent chemoattractant for neutrophils, monocytes, and lymphocytes and induced calcium flux in these cells. Lkn-1 specifically induced calcium flux in CCR1- and CCR3-expressing HOS cell lines. Lkn-1 suppressed colony formation by human granulocyte-macrophage, erythroid, and multipotential progenitor cells stimulated by combinations of growth factors. Hence, we have isolated and characterized a human C6 beta-chemokine that is a potent agonist at CCR1 and CCR3 and shows broad biologic activities, including leukocyte chemoattraction.

Molecular cloning of leukotactin-1: a novel human beta-chemokine, a chemoattractant for neutrophils, monocytes, and lymphocytes, and a potent agonist at CC chemokine receptors 1 and 3

A new member of human beta-chemokine cDNA was isolated and named leukotactin-1 (Lkn-1). Lkn-1, along with murine macrophage inflammatory protein-related protein-1 and -2, defines a subgroup of beta-chemokines based on two conserved cysteines in addition to the four others conserved in all beta-chemokines. The putative mature Lkn-1 is composed of 92 amino acids with a calculated m.w. of 10,162. The Lkn-1 gene was mapped to human chromosome 17, region q12. Recombinant Lkn-1 was a potent chemoattractant for neutrophils, monocytes, and lymphocytes and induced calcium flux in these cells. Lkn-1 specifically induced calcium flux in CCR1- and CCR3-expressing HOS cell lines. Lkn-1 suppressed colony formation by human granulocyte-macrophage, erythroid, and multipotential progenitor cells stimulated by combinations of growth factors. Hence, we have isolated and characterized a human C6 beta-chemokine that is a potent agonist at CCR1 and CCR3 and shows broad biologic activities, including leukocyte chemoattraction.

Determinants of HIV-1 coreceptor function on CC chemokine receptor 3. Importance of both extracellular and transmembrane/cytoplasmic regions

The chemokine receptors CXCR4, CCR2b, CCR3, and CCR5 are cell entry coreceptors for HIV-1. Using an HIV-1 envelope (Env)-dependent cell-cell fusion model of entry, we show that CCR3 can interact with Envs from certain macrophage (M)-tropic strains (which also use CCR5), T cell line (TCL)-tropic laboratory-adapted strains (which also use CXCR4), and a dual-tropic primary isolate (which also uses CCR2b, CCR5, and CXCR4). Paradoxically, CCR1 is the closest homologue to CCR3 (63% amino acid identity), but lacked HIV-1 coreceptor activity. These results confirm and extend previous reports. Replacing the N-terminal segment of CCR3 with that of CCR1 abolished activity of the resulting chimera for M-tropic and TCL-tropic Envs, but not for the dual-tropic Env. Replacing extracellular loop 2 of CCR3 with that of CCR1 abolished activity for TCL-tropic Envs, but not for M- and dual-tropic Envs. A chimera containing all four extracellular regions of CCR3 on a backbone of CCR1 lacked any activity. Env-CCR3 interactions were strongly inhibited by the major CCR3 ligand eotaxin, but weakly or not at all by other CCR3 ligands. With primary macrophages, eotaxin induced transient calcium flux and partially inhibited fusion with cells expressing M-tropic Envs. We conclude that specificity determinants for different Envs are located in shared and distinct extracellular regions of CCR3, the transmembrane/cytoplasmic domains make major contributions to coreceptor function, and CCR3 may be used by certain HIV-1 strains as a cell fusion factor on macrophages.

Mutation of glutamate 199 of the human C5a receptor defines a binding site for ligand distinct from the receptor N terminus

C5a, a potent chemoattractant for monocytes, neutrophils, and other leukocytes, binds to a cell surface receptor of the seven-transmembrane superfamily. Here we report the effects of substituting Gln for Glu199 of the human C5a receptor (hC5aR) expressed in a model cell system for chemoattractant receptor signaling, the rat basophilic leukemia cell line RBL-2H3. Both the binding affinity for hC5a and the EC50 for subsequent cellular signals are reduced 5-10-fold by this substitution. A peptide mimic of the C terminus of C5a also binds to, and activates, hC5aR. The response to this peptide is reduced in cells bearing mutated hC5aR, indicating that the mutation affects interactions with the C terminus of hC5a. The C-terminal peptide contains only two basic residues, a Lys and an Arg (assumed to be analogous to Lys68 and Arg74 of hC5a), which could act as counter-ions for Glu199 of the receptor. If the counter-ion on hC5a was Arg74, then it would be expected that intact hC5a and hC5a des-Arg74 would have identical affinities and potencies when interacting with mutant hC5aR. It was found, however, that the binding affinity and potency (for receptor signaling events) of hC5a des-Arg74 was always lower than for intact hC5a. Furthermore, the equivalent C-terminal peptide to hC5a des-Arg74 (i.e. lacking the C-terminal Arg) could partially activate the wild type but not the mutant receptor, whereas the converse peptide, containing Arg but containing Met instead of Lys, had equal potencies for both wild type and mutant receptors. Taken together these data indicate that Glu199 of hC5aR is not involved in an interaction with Arg74 of hC5a, but may interact with Lys68 of hC5a. Mutation of Glu199 defines a second ligand binding site on hC5aR, distinct from the previously characterized site on the receptor N terminus. Unlike the N-terminal binding site, this second site is associated not just with the interaction with hC5a, but also with receptor activation.

Mutation of aspartate 82 of the human C5a receptor abolishes the secretory response to human C5a in transfected rat basophilic leukemia cells

C5a is a potent chemoattractant for monocytes, neutrophils and other leukocytes. The receptor for human C5a is a member of the rhodopsin superfamily of G protein-coupled receptors and contains an aspartate residue (Asp82) within the putative second transmembrane domain conserved in all other G protein-linked receptors. We investigated the role of this residue and also the carboxy-terminal 23 residues of the C5a receptor in ligand binding and signal transduction by expressing wild-type and mutant receptors in the rat basophilic leukemia cell line RBL-2H3. Wild-type and truncated receptors coupled efficiently to effector systems, resulting in the C5a-dependent discharge of granule contents. In contrast RBL cells transfected with receptors in which Asp82 had been mutated to asparagine did not respond to human C5a by secretion despite binding human C5a with high affinity. We conclude therefore that Asp82 is not involved in the interaction with ligand but is essential for the proper transduction of the ligand binding signal.

Site directed mutagenesis of the complement C5a receptor--examination of a model for its interaction with the ligand C5a

C5a is a 74 amino acid peptide cleaved from the fifth component of the complement system after activation of either the alternative or classical pathways. It is a potent chemoattractant for neutrophils and monocytes binding to identical receptors on the cell surface. Following the cloning of the cDNA encoding for the human complement C5a receptor, revealing it to be a member of the rhodopsin superfamily of G-protein coupled receptors, a model for the interaction of the C5a receptor with its ligand was proposed, the structure for the receptor being modelled on that of the well defined receptor bacteriorhodopsin. In this model two key residues of the receptor, aspartate82 and either glutamate179 or glutamate 180 were proposed to make up part of the binding site for C5a, acting as counter ions for arginine74 and arginine40, respectively of the C5a molecule. Replacement of aspartate82, glutamate179 and glutamate180 of the C5a receptor with asparagine and glutamine, respectively was shown to have little effect on the dissociation constant of the receptor as detected by Scatchard analysis and competitive binding assays. Hence this modus operandi for the interaction of C5a with its receptor can be rejected.

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.

C5a stimulus-secretion coupling in rat basophilic leukaemia (RBL-2H3) cells transfected with the human C5a receptor is mediated by pertussis and cholera toxin-sensitive G proteins

Rat basophilic leukaemia cells (RBL-2H3) were transfected with either the wild type human C5a receptor or a truncated form lacking the last 23 C-terminal residues. Transfected cells bound human C5a specifically, with affinities in the range 3-20nM, and 12-166,000 receptors per cell, similar values to those obtained on human neutrophils and monocytic cells. The stimulation of secretion by human C5a was completely inhibited by pertussis toxin and partially sensitive to cholera toxin, indicating that both wild-type and mutated receptors are coupled to G proteins. Cells transfected with the mutated receptor were equally sensitive to hC5a, suggesting that this portion of the C terminus is not an absolute requirement for signal transduction.

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.

The generation of stable CHO cell lines expressing very high levels of complement C5A receptors and subsequent modulation of binding affinity for C5A

A single gene fragment encoding for the C5a receptor was produced by reverse transcription of mRNA isolated from differentiated U937 cells and subsequently amplified by means of the polymerase chain reaction. This fragment was introduced into the mammalian expression vector pEE6hCMV.neo and used to transfect a CHO cell line constitutively expressing a viral transactivator protein. Binding characteristics identical to the native neutrophil receptor were observed. A combination of antibiotic selection and cell sorting using anti-C5a receptor antiserum were then used to generate stable cell lines expressing up to 1.2 x 10(7) functional C5a receptors/cell with a lower affinity for C5a. It is postulated that this modulation of receptor affinity is dependent on coupling to native G-proteins in the host cells.