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

Unexpected Off-Target Activities for Recombinant C5a in Human Macrophages

The anaphylatoxin C5a is core effector of complement activation. C5a exerts potent proinflammatory and immunomodulatory actions through interacting with its C5a receptors, C5aR1 and C5aR2, modulating multiple signaling and functional activities of immune cells. Native C5a contains a large N-linked glycosylation site at Asn⁶⁴, which accounts for up to 25% of its m.w. To date, the vast majority of published studies examining C5a are performed using Escherichia coli-generated recombinant C5a, which is readily available from numerous commercial suppliers, but lacks this glycosylation moiety. However, a plasma-purified "native" form of C5a is also commercially available. The different size and glycosylation of these two C5a versions could have functional implications. Therefore, the current study aimed to compare recombinant human C5a to purified plasma-derived human C5a in driving the signaling and functional activities of human primary macrophages. We found that both versions of C5a displayed similar potencies at triggering C5aR1- and C5aR2-mediated cell signaling, but elicited distinct functional responses in primary human monocyte-derived macrophages. Multiple commercial sources of recombinant C5a, but not the plasma-purified or a synthetic C5a version, induced human monocyte-derived macrophages to produce IL-6 and IL-10 in a C5a receptor-independent manner, which was driven through Syk and NF-κB signaling and apparently not due to endotoxin contamination. Our results, therefore, offer caution against the sole use of recombinant human C5a, particularly in functional/cytokine assays conducted in human primary immune cells, and suggest studies using recombinant human C5a should be paired with C5aR1 inhibitors or purified/synthetic human C5a to confirm relevant findings.

Arginine side-chain modification that occurs during copper-catalysed azide-alkyne click reactions resembles an advanced glycation end product

Dehydroascorbate is a by-product of copper-catalysed azide-alkyne click (CuAAC) reactions and also forms advanced glycation end products (AGEs) in tissues undergoing oxidative stress. Here we isolate and characterize an arginine-dehydroascorbate adduct formed during CuAAC reactions, investigate strategies for preventing its formation, and propose its biological relevance as an AGE.

Receptors for complement C5a. The importance of C5aR and the enigmatic role of C5L2

Complement component C5a is one of the most potent inflammatory chemoattractants and has been implicated in the pathogenesis of numerous inflammatory diseases. C5a binds two receptors, C5aR and C5L2. Most of the C5a functional effects occur through C5aR, and the pharmaceutical industry has focused on this receptor for the development of new anti-inflammatory therapies. We used a novel approach to generate and test therapeutics that target C5aR. We created human C5aR knock-in mice, and used neutrophils from these to immunize wild-type mice. This yielded high-affinity blocking mAbs to human C5aR. We tested these anti-human C5aR mAbs in mouse models of inflammation, using the human C5aR knock-in mice. These antibodies completely prevented disease onset and were also able to reverse established disease in the K/B x N arthritis model. The physiological role of the other C5a receptor, C5L2 is still unclear, and our studies with blocking mAbs to human C5L2 have failed to demonstrate a clear functional role in signaling to C5a. The development of effective mAbs to human C5aR is an alternative approach to drug development, for this highly attractive target.

C5a mutants are potent antagonists of the C5a receptor (CD88) and of C5L2: position 69 is the locus that determines agonism or antagonism

The anaphylatoxin C5a exerts a plethora of biologic activities critical in the pathogenesis of systemic inflammatory diseases. Recently, we reported on a C5a mutant, jun/fos-A8, as a potent antagonist for the human and mouse C5a receptor (CD88). Addressing the molecular mechanism accounting for CD88 receptor antagonism by site-directed mutagenesis, we found that a positively charged amino acid at position 69 is crucial. Replacements by either hydrophobic or negatively charged amino acids switched the CD88 antagonist jun/fos-A8 to a CD88 agonist. In addition to CD88, the seven-transmembrane receptor C5L2 has recently been found to provide high affinity binding sites for C5a and its desarginated form, C5adesArg74. A jun/fos-A8 mutant in which the jun/ fos moieties and amino acids at positions 71-73 were deleted, A8Delta71-73, blocked C5a and C5adesArg74 binding to CD88 and C5L2. In contrast, the cyclic C5a C-terminal analog peptide AcF-[OP-d-ChaWR] inhibited binding of the two anaphylatoxins to CD88 but not to C5L2, suggesting that the C5a core segment is important for high affinity binding to C5L2. Both receptors are coexpressed on human monocytes and the human mast cell line HMC-1; however, C5L2 expression on monocytes is weaker as compared with HMC-1 cells and highly variable. In contrast, no C5L2 expression was found on human neutrophils. A8Delta71-73 is the first antagonist that blocks C5a and C5adesArg74 binding to both C5a receptors, CD88 and C5L2, making it a valuable tool for studying C5L2 functions and for blocking the biological activities of C5a and C5adesArg74 in mice and humans.

Inactivation of C3a and C5a octapeptides by carboxypeptidase R and carboxypeptidase N

Pro-carboxypeptidase R (proCPR), also known as thrombin-activatable fibrinolysis inhibitor (TAFI), precursor of carboxypeptidase U and plasma carboxypeptidase B is present in plasma and following activation by thrombin/thrombomodulin and/or plasmin can remove arginine from the carboxyterminal of C3a and C5a. We have shown that this enzyme can remove terminal arginine from the C5a octapeptide much more efficiently than the classical anaphylatoxin inactivator, carboxypeptidase N (CPN). Since we have previously demonstrated that proCPR is significantly upregulated in the inflammatory state, this enzyme would appear to significantly contribute to the inactivation of C5a, the most potent of the complement derived anaphylatoxins.

Selection of a C5a Receptor Antagonist from Phage Libraries Attenuating the Inflammatory Response in Immune Complex Disease and Ischemia/Reperfusion Injury

A C5a-receptor antagonist was selected from human C5a phage display libraries in which the C terminus of des-Arg74-hC5a was mutated. The selected molecule is a competitive C5a receptor antagonist in vitro and in vivo. Signal transduction is interrupted at the level of G-protein activation. In addition, the antagonist does not cause any C5a receptor phosphorylation. Proinflammatory properties such as chemotaxis or lysosomal enzyme release of differentiated U937 cells, as well as C5a-induced changes in intracellular Ca2+ concentration of murine peritoneal macrophages, are inhibited. The in vivo efficacy was evaluated in three different animal models of immune complex diseases in mice, i.e., the reverse passive Arthus reaction in the peritoneum, skin, and lung. The i.v. application of the C5a receptor antagonist abrogated polymorphonuclear neutrophil accumulation in peritoneum and markedly attenuated polymorphonuclear neutrophil migration into the skin and the lung. In a model of intestinal ischemia/reperfusion injury, i.v. administration of the C5a receptor antagonist decreased local and remote tissue injury: bowel wall edema and hemorrhage as well as pulmonary microvascular dysfunction. These data give evidence that C5a is an important mediator triggering the inflammatory sequelae seen in immune complex diseases and ischemia/reperfusion injury. The selected C5a receptor antagonist may prove useful to attenuate the inflammatory response in these disorders.

Small molecular probes for G-protein-coupled C5a receptors: conformationally constrained antagonists derived from the C terminus of the human plasma protein C5a

Activation of the human complement system of plasma proteins in response to infection or injury produces a 4-helix bundle glycoprotein (74 amino acids) known as C5a. C5a binds to G-protein-coupled receptors on cell surfaces triggering receptor-ligand internalization, signal transduction, and powerful inflammatory responses. Since excessive levels of C5a are associated with autoimmune and chronic inflammatory disorders, inhibitors of receptor activation may have therapeutic potential. We now report solution structures and receptor-binding and antagonist activities for some of the first small molecule antagonists of C5a derived from its hexapeptide C terminus. The antagonist NMe-Phe-Lys-Pro-D-Cha-Trp-D-Arg-CO2H (1) surprisingly shows an unusually well-defined solution structure as determined by 1H NMR spectroscopy. This is one of the smallest acyclic peptides found to possess a defined solution conformation, which can be explained by the constraining role of intramolecular hydrogen bonding. NOE and coupling constant data, slow deuterium exchange, and a low dependence on temperature for the chemical shift of the D-Cha-NH strongly indicate an inverse gamma turn stabilized by a D-Cha-NH. OC-Lys hydrogen bond. Smaller conformational populations are associated with a hydrogen bond between Trp-NH.OC-Lys, defining a type II beta turn distorted by the inverse gamma turn incorporated within it. An excellent correlation between receptor-affinity and antagonist activity is indicated for a limited set of synthetic peptides. Conversion of the C-terminal carboxylate of 1 to an amide decreases antagonist potency 5-fold, but potency is increased up to 10-fold over 1 if the amide bond is made between the C-terminal carboxylate and a Lys/Orn side chain to form a cyclic analogue. The solution structure of cycle 6 also shows gamma and beta turns; however, the latter occurs in a different position, and there are clear conformational changes in 6 vs 1 that result in enhanced activity. These results indicate that potent C5a antagonists can be developed by targeting site 2 alone of the C5a receptor and define a novel pharmacophore for developing powerful receptor probes or drug candidates.

Complement factors and their receptors

In summary, recent advances in molecular cloning of anaphylatoxins and the anaphylatoxin receptors add new dimensions to our investigations and understanding of the molecular mechanisms involved in anaphylatoxin action. Combining knowledge accumulated from peptide modeling of the ligands with mutagenesis studies of these ligands and their receptors makes it possible to more accurately model interactive sites and understand the sequence of molecular interactions required for cellular activation. In addition, these new developments provide valuable tools for investigating, yet unknown, activities and cellular targets of the anaphylatoxin molecules.

C5aR ligand peptide 3D QSAR study performed with an applied linear conformation

A 3D quantitative structure-activity relationship study (QSAR) of binding and activation of the human C5a receptor by peptide analogs of the C-terminal binding domain of C5a anaphylatoxin is reported. Using published C5a analog affinity and activity data, this paper seeks to elucidate the pharmacophore for the high affinity C-terminal binding domain of the C5a peptide with the molecular modeling technique of comparative molecular field analysis (CoMFA). In order to model peptides for which there was incomplete conformational data, an arbitrary linear conformation was imposed upon the highly flexible C5a analogs. The resulting models yield a crossvalidated q2 of 0.889 and 0.787, for receptor-ligand affinity and EC50 calcium release activity, respectively, suggesting these models have good predictive ability for other test peptides.

A selection system to study C5a-C5a-receptor interactions: phage display of a novel C5a anaphylatoxin, Fos-C5aAla27

Binding and effector domains of the human anaphylatoxin C5a have been determined by either site directed mutagenesis or synthetic peptide studies. However, the lack of specific selection methods, which allow direct investigation of C5a-C5a-receptor interaction made these studies laborious. To overcome these limitations we have constructed a novel Fos-C5a expressed on the tip of a filamentous phage. To guarantee for a free C-terminus which is required for C5a activity C5a cDNA was cloned into the phagemid vector pJuFo. Helper phage infection of pJuFc-C5a transformed cells resulted in a mutant phage displaying Fos-C5a on its surface. However studies with Bt2cAMP differentiated U937 cells revealed that phage displayed Fos-C5a is functional inactive. Subsequently we replaced a nonconserved cysteine residue at position 27 by alanine and obtained Fos-C5aAla27. Both the purified and the phage displayed Fos-C5aAla27 proteins were functional active and induced enzyme release from differentiated U937 cells. In addition, purified Fos-C5aAla27 exhibited the same binding profile as compared to rhC5a. Fos-C5aAla27 displaying phages were mixed with phage harboring only the pJuFo plasmid at a ratio of 10(6). After four successive rounds of panning on differentiated U937 cells Fos-C5aAla27 phages were enriched to 100% as shown by C5a-specific ELISA. We expect this approach to prove helpful for studying C5a-C5a-receptor interactions. i.e. to screen C5a libraries for high affinity binders with agonistic or antagonistic properties directly on cells.

Identification of casoxin C, an ileum-contracting peptide derived from bovine kappa-casein, as an agonist for C3a receptors

Casoxin C (Tyr-Ile-Pro-Ile-Gln-Tyr-Val-Leu-Ser-Arg) is a bioactive peptide that was isolated from a tryptic digest of bovine kappa-casein as an anti-opioid peptide in longitudinal strips of guinea pig ileum. Casoxin C also evokes contraction of the ileal strips, and we found that this process was biphasic with rapid and slow components. The contractile profile was very similar to that of human complement C3a(70-77), which is the COOH-terminal octapeptide of C3a and has, although less potent, qualitatively the same biological activities as C3a. Casoxin C also has homology with C3a(70-77). The rapid contraction was mediated by histamine release and the slow contraction was mediated by a prostaglandin E2-like substance, judging from the effects of various pharmacological inhibitors and antagonists on the ileal contraction. Casoxin C had affinity for C3a receptors (IC50 = 40 microM) in the radioreceptor assay. In addition, casoxin C showed phagocyte-stimulating activities. Casoxin C is therefore the first milk-derived peptide identified, that acts through complement C3a receptors.

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.

Studies on the ileum-contracting mechanisms and identification as a complement C3a receptor agonist of oryzatensin, a bioactive peptide derived from rice albumin

Oryzatensin (Gly-Tyr-Pro-Met-Tyr-Pro-Leu-Pro-Arg) is an ileum-contracting and immunostimulating peptide derived from rice albumin. The mechanisms for the ileal contraction that it induces, consisting of rapid and slow components, were examined. The rapid contraction was mediated by histamine release and the slow contraction by a prostaglandin E2-like substance, judging from the effects of various pharmacological inhibitors and antagonists on ileal contraction and titration of histamine release. The contractile profile was very similar to that of human complement C3a(70-77), which is the COOH-terminal octapeptide of C3a and has, although less potent, qualitatively the same biological activities as C3a. Oryzatensin showed homology with C3a(70-77) and affinity for C3a receptors (IC50 = 44 microM) by radioreceptor assay. This is the first report of a food-derived bioactive peptide acting through complement C3a receptors.

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.

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.

Evidence that the extracellular N-terminal domain of C5aR contains amino-acid residues crucial for C5a binding

The human C5a anaphylatoxin is a cationic 74 amino-acid long glycopeptide which derives from proteolysis of the fifth component of complement. It interacts with high affinity with a receptor that belongs to the G protein-coupled receptor superfamily. Several studies have previously suggested that multiple contact points between C5a and the receptor are required to achieve high-affinity interaction. However, at the receptor level little is known about the sites of interaction with C5a. We have investigated by in vitro mutagenesis whether the N-terminal extracellular sequence of the C5a receptor, which is rich in aspartic acid residues, could play some role in C5a binding. Conversion of Asp10 into asparagine did not impair the level of expression at the plasma membrane, nor did it alter the affinity for C5a. However, we consistently observed a discrepancy between an apparent high level of surface expression and a weak capacity to bind C5a with high affinity, suggesting that many receptor molecules, although present on the cell surface, might be misfolded and unable to bind C5a. Replacement of Pro9 by an isoleucine had little effect, if any, on either the affinity or the C5a-binding capacity, whereas the conversion of Pro36 into leucine dramatically reduced the expression of high-affinity receptor at the cell surface. N-glycosylation of human C5a receptor was found to be dispensable for the function of the receptor.