A recombinant hybrid anaphylatoxin with dual C3a/C5a activity

Derivation of ligands for the complement C3a receptor from the C-terminus of C5a

The complement cascade is a highly sophisticated network of proteins that are well regulated and directed in response to invading pathogens or tissue injury. Complement C3a and C5a are key mediators produced by this cascade, and their dysregulation has been linked to a plethora of inflammatory and autoimmune diseases. Consequently, this has stimulated interest in the development of ligands for the receptors for these complement peptides, C3a receptor, and C5a1 (C5aR/CD88). In this study we used computational methods to design novel C5a1 receptor ligands. However, functional screening in human monocyte-derived macrophages using the xCELLigence label-free platform demonstrated altered specificity of our ligands. No agonist/antagonist activity was observed at C5a1, but we instead saw that the ligands were able to partially agonize the closely related complement receptor C3a receptor. This was verified in the presence of C3a receptor antagonist SB 290157 and in a stable cell line expressing either C5a1 or C3a receptor alone. C3a agonism has been suggested to be a potential treatment of acute neutrophil-driven traumatic pathologies, and may have great potential as a therapeutic avenue in this arena.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.

Site-Directed C3a Receptor Antibodies from Phage Display Libraries

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

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

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

Overexpression, purification, and characterization of third component of complement

The third component of complement (C3) plays a critical role in both pathways of complement activation by interacting with numerous other complement proteins. To elucidate the molecular features of C3 that relate to the functional activities of the molecule, we expressed the cDNA of human complement component C3 in cultured insect cells using a baculovirus expression vector system derived from the baculovirus Autographa california nuclear polyhedrosis virus (AcNPV). The expression of C3 was controlled by the promoter of the polyhedrin gene and, when recombinant baculovirus infected insect cells were cultured in serum-free medium, C3 was detected at a level of 10 micrograms/ml of culture medium. Characterization of the recombinant C3 (rC3) by SDS-PAGE revealed that the C3 gene product was translated as a 188 kDa protein comprised of two chains of 115 kDa and 73 kDa analogous to the alpha and beta chains of serum-derived human C3 (sC3). An analysis of the glycosylation pattern of purified rC3 revealed that, whereas both the alpha and beta chains were glycosylated as in sC3, the proC3 moiety of rC3 also was glycosylated. When rC3 was produced in the High Five cell line of insect cells and evaluated for reactivity with a panel of anti-C3 monoclonal antibodies (MoAb), the results suggested that the conformation of the baculovirus expressed C3 was similar to that of native C3. When the rC3 was purified by anion exchange column chromatography, it was able to react with several C3-binding proteins (CR1, P and H), reconstitute C3-deficient serum and support the activation of both complement pathways thus demonstrating that a baculovirus-expressed C3 can participate in the formation of and can be cleaved by both the classical and alternative pathway convertases. Incubation of rC3 with factor I and H revealed that both C3 and proC3 are susceptible to cleavage by factor I.

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.

Tryptophan mutants of human C5a anaphylatoxin: a fluorescence anisotropy decay and energy transfer study

Three mutants of the anaphylatoxin C5a were prepared with positions 2, 64 and 70, respectively, substituted by tryptophan. The last mutant was additionally labelled at Cys27 for fluorescence energy transfer (FET) measurements. The structural integrity and biological activity of the molecules were not affected. Fluorescence anisotropy decay (FAD) measurements showed that the rotational correlation time for tryptophan decreases in the order: [Trp2]rhC5a > [Trp64]rhC5a > [Trp70]rhC5a, indicating an increasing mobility of the side chain. Measurements of the fluorescence energy transfer from Trp70 to the 1,5-AEDANS group at Cys27 yielded a distance distribution of 2.4 +/- 0.8 nm. This value is compatible with the C-terminal chain being arranged as a slightly stretched helix pointing away from the body of the molecule.

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.