An IgG primary sequence exposure theory for complement activation using synthetic peptides

A new methodology for polyvalent intravenous immunoglobulin solution production with a two-stage process of viral inactivation

Highly purified intravenous immunoglobulin G concentrate (IV IgG) was produced with the use of polyethylene glycol associated to a single-stage precipitation by ethanol, instead of the classic Cohn-Oncley process, which employs cold alcohol as the precipitating agent, in a three-stage process. Precipitation of crude fraction containing more than 95% of immunoglobulin G was performed by liquid chromatography with a cation exchanger, CM-Sepharose, as a stationary phase. During the process, the product was subjected to two-stage viral inactivation. The first stage was performed by the action of sodium caprylate, 30 mM at pH 5.1+/- 0.1, and the second stage was performed by the action of a solvent-detergent mixture. The finished product was formulated at 5% with 10% sucralose as the stabilizing agent. The process yields 3.3g of IgG/liter of plasma. The finished product analysis showed an anti-complementary activity lower than 1CH50. Polymer and aggregate percent levels were lower than 3% in the five batches studied. The analysis of neutralizing capacity showed the presence of antibacterial and antiviral antibodies in at least three times higher concentrations than the levels found in source plasma. The finished product fulfilled all purity requirements stated in the 4th edition of the European pharmacopeia.

Specific inhibition of the classical complement pathway with an engineered single-chain Fv to C1q globular heads decreases complement activation by apoptotic cells

Apoptotic cells are potent complement activators; and proposed mechanisms include IgM-mediated classical pathway activation, C-reactive protein (CRP)-mediated classical pathway activation, and IgM-mediated lectin pathway activation. While complement activation is beneficial in clearing apoptotic cells, the resulting complement-mediated inflammation may extend damage to the surrounding cells and tissues, as observed in ischemia/reperfusion injury. We previously engineered and characterized a single-chain Fv against C1q globular heads (scFv(QuVHVL)) that blocked C1q binding to immobilized IgG and to IgG-sensitized cells, and thereby inhibited IgG-mediated classical pathway activation [Hwang H.Y., Duvall M.R., Tomlinson S., Boackle R.J., 2008. Highly specific inhibition of C1q globular-head binding to human IgG: a novel approach to control and regulate the classical complement pathway using an engineered single-chain antibody variable fragment. Molecular Immunology 45, 2570-2580]. In the present study, this scFv(QuVHVL) was examined for its ability to restrict complement deposition on apoptotic cells in the presence of fresh normal human serum (NHS). Interestingly, the addition of scFv(QuVHVL) to NHS decreased C1-mediated C4b deposition on apoptotic cells by 60% as compared to appropriate buffer-treated control serum. By inhibiting initiation of the early complement components, the subsequent C3b and membrane attack complex depositions were inhibited by 70%. Apoptotic cells may acquire serum CRP, a known classical complement pathway activator. It was observed that scFv(QuVHVL) blocked C1 binding to CRP and blocked CRP-mediated classical pathway activation using an ELISA format. However, under the experimental conditions used, the addition of exogenous CRP to apoptotic cells did not further increase the levels of C4b, C3b, or MAC deposition significantly, suggesting predominance by other activation mechanisms, such as antibody-C1-mediated complement activation. In summary, the results indicated that C1-mediated classical pathway activation was a highly significant mechanism for complement activation by apoptotic cells. In the future, specific inhibition of classical complement pathway activation by a humanized form of scFv(QuVHVL) may be useful in reducing inadvertent damage to healthy bystander tissue in a variety of acute, complement-mediated inflammatory conditions, including ischemia/reperfusion injury.

Highly specific inhibition of C1q globular-head binding to human IgG: a novel approach to control and regulate the classical complement pathway using an engineered single chain antibody variable fragment

We sought to specifically regulate the binding of human C1q, and thus the activation of the first complement component, via the construction of a single chain antibody variable binding region fragment (scFv) targeting the C1q globular heads. Here we describe details of the construction, expression and evaluation of this scFv, which was derived from a high-affinity hybridoma (Qu) specific for the C1q globular heads. The scFv was comprised of the Qu variable heavy chain domain (VH) linked to the Qu variable light chain domain (VL) and was termed scFv-QuVHVL. When mixed with either purified C1q or with human serum as a source of C1, scFv-QuVHVL bound to C1q and competitively restricted the interaction of C1q or C1 with immobilized IgG or with IgG1 antibody-coated cells, and prevented the activation of native C1 in human serum as determined by analyses of C1-mediated C4 deposition and fluid-phase C4 conversion. However scFv-QuVHVL could be manipulated to become a C1 activator when it was irreversibly immobilized onto microtiter ELISA plates, prior to contact with human serum complement. This functional dichotomy can be a useful tool in selectively elucidating, differentiating, inducing or inhibiting specific roles of human C1q and the classical complement pathway in complement-mediated physiological processes. We project that once fully humanized, fluid-phase scFv-QuVHVL could become a useful therapeutic in limiting inadvertent host tissue damage elicited by the classical complement pathway.

Drug design using the example of the complement system inhibitors' development

Undesired activation of the complement system, a part of the immune system, is a major pathogenic factor contributing to various diseases, such as ischemia-reperfusion injury, sepsis, asthma, allergic reactions, rheumatoid arthritis, Alzheimer's disease, myasthenia, multiple sclerosis and others. The history of the development of complement system inhibitors, preventing its destructive action on the body, represents the evolution of the main methods of drug design. This review illustrates the main approaches of drug design, ranging from screening and modification of natural products to structure-based ligand design, on the basis of complement inhibitors' creation. The current status of the field of complement inhibitors is also discussed.

Lysine 322 in the human IgG3 C(H)2 domain is crucial for antibody dependent complement activation

The classical complement activation cascade of the immune system is initiated by multivalent binding of its first component, C1q, to the Fc region of immunoglobulins in immune complexes. The C1q binding site on mouse IgG2b has been shown to contain the amino acids Glu 318, Lys 320 and Lys 322 in the C(H)2 domain (Duncan, A.R., Winter, G.,1988. The binding site for C1q on IgG. Nature 322 738-740). Identical or closely related motifs are found on all IgGs in all species, and the binding site has therefore been thought to be universal. However, the results from another study indicate that the site is different in human IgG1 molecules (Morgan, A., Jones, N.D., Nesbitt, A.M., et al., 1995. The N-terminal end of the C(H)2 domain of chimeric human IgG1 anti-HLA-DR is necessary for C1q, Fc gamma RI and Fc gamma RIII binding. Immunology 86 319-324). To determine the site(s) responsible for complement activation in anti-NIP-mouse/human IgG3 antibodies, we have mutated amino acids Lys 276, Tyr 278, Asp 280, Glu 318, Lys 320 and Lys 322 in two beta-strands in the C(H)2 domains of human IgG3. In addition, we mutated the Glu 333, which resides in close proximity to the postulated C1q-binding site of mouse IgG2b, as well as Leu 235 in the lower hinge region. All mutants were tested in Antibody Dependent Complement Mediated Lysis (ADCML)(4) assays, where the antigen concentration on target cells was varied and human serum was complement source. Only the mutants that lacked the positively charged side chain of lysine in position 322 showed strong reduction in ADCML, particularly at low antigen density on target cells. Alanine scanning of positions 318 and 320 did not affect ADCML, contrary to what was observed for mouse IgG2b. Neither did a leucine to glutamic acid mutation in position 235 have the effect that has been reported for human IgG1. These results suggest that the complement binding site on human IgG3 molecules is different from that found on mouse IgG2b, and possibly on human IgG1 as well. Thus the contact site may not be conserved.

Polymerization of IgA and IgM: Roles of Cys309/Cys414 and the Secretory Tailpiece

We have investigated how the secretory tailpiece (tp), Cys414 and the amino acids flanking Cys414 or Cys309 are involved in regulating the different polymerization of IgM and IgA to pentamers and dimers/monomers, respectively. Whereas changing the tp of IgM to that of IgA has little effect on IgM polymerization, introducing the μtp to IgA leads to the formation of larger than wild-type IgA polymers, including pentamers and hexamer. This shows that the secretory tp can differentially regulate polymerization depending on the heavy chain context. Cys414, which is engaged in intermonomeric disulfide bonds in IgM, is not crucial for the difference in IgM and IgA polymerization; IgM with a C414S mutation forms more large polymers than IgA. Also, IgA with IgM-like mutations in the five amino acids flanking Cys309, which is homologous to Cys414, oligomerize similarly as IgA wild type. Thus, IgA appears to have an inherent tendency to form monomers and dimers that is partially regulated by the tp, while the Cys309 region has only a minor effect. We also show that complement activation by IgM is sensitive to alterations in the polymeric structure, while IgA is inactive in classical complement activation even for polymers such as pentamers and hexamers.

A newly discovered function for C1 inhibitor, removal of the entire C1qr2s2 complex from immobilized human IgG subclasses

A new function for C1 inhibitor (C1 INH) is reported. C1 inhibitor dislodged the entire activated C1 complex (C1qr2s2) from immobilized human IgG. C1 binding to doses of immobilized human IgG3, IgG1, or IgG2 was quantified as a function of time. When human serum, as a source of C1qr2s2, was added to relatively low doses of immobilized IgG, C1q binding peaked at 1.0 min then gradually decreased. However when purified C1q was applied to immobilized IgG, C1q binding did not diminish with time. The removal of C1q was duplicated by adding purified C1 INH to C1qr2s2 which had been bound to immobilized IgG. The dislodgement of C1q from immobilized IgG required the presence of intact C1qr2s2 and of C1 INH. This removal of C1q by purified C1 INH was prevented when activated C1s was used to neutralize C1 INH function or when relatively high levels of IgG were immobilized.

Identification and characterisation of C1q-binding phage displayed peptides

Five phage displayed peptide libraries were screened for binders to C1q, the recognition subunit of the classical complement pathway. Two rounds of panning resulted in the isolation and characterisation of several different phage displayed C1q-binding peptides from all five libraries. Two groups of the characterised peptides show sequence similarity with part of the metal ion dependent adhesion site (MIDAS) of integrin A-domains, and the site 187LRNPCPNKEKECQPPF of CD18 (integrin beta2), respectively. These results support binding of complement receptor 3 (CR3, CD11b/CD18, Mac1) to C1q and further suggest C1q binding sites in CR3. We also discuss sequence matches between the characterised peptides and proteins known to interact with C1q, as well as other proteins listed in the SwissProt databank. These findings are of interest for the study of the complement system and may lead to the development of peptides, fusion products or peptido-mimetics with C1q modulating potential.

The F(ab')2 fragment of an anti-ICAM-1 monoclonal antibody attenuates liver injury after orthotopic liver transplantation

Long-term storage of liver grafts results in increased adhesion of leukocytes onto the sinusoidal walls. This eventually leads to posttransplant graft damage through disturbances of hepatic microcirculation. Intracellular adhesion molecule-1 (ICAM-1) is known to be involved in attachment of leukocytes. This study was designed to examine whether ICAM-1 participated in the pathogenesis of posttransplant liver injury. Inbred Lewis rats were used as both donors and recipients to avoid immunoreactivity. Donor livers were stored for either 1 or 6 hr in ice-cold Euro-Collins solution and subsequently implanted. Expression of ICAM-1 was examined immunohistochemically. In some rats that received livers stored for 6 hr, the intact IgG (1.0 mg/kg) or the F(ab')2 fragment (0.5 mg/kg) of an anti-ICAM-1 mAb (1A29) was administered via the tail vein immediately after reperfusion of portal blood. In the group receiving livers stored for 6 hr, ICAM-1 began to be expressed on the sinusoidal endothelial cells as early as 15 min after reperfusion of the portal blood. Strong ICAM-1 expression was observed from 2 hr up to 24 hr after reperfusion. In contrast, expression of ICAM-1 was not evident at any time point after surgery in the 1-hr storage group as well as in untransplanted, normal livers. Serum alanine aminotransferase (ALT) levels were significantly higher in the 6-hr storage group compared with those of the 1-hr storage group (1-hr: 171 +/- 9 IU/L; 6-hr: 825 +/- 109 IU/L, P < 0.05; mean +/- SEM) 24 hr after transplantation. Serum ALT levels were markedly reduced by treatment with the F(ab')2 fragment of 1A29 (247 +/- 34 IU/L, P < 0.05 vs. 6-hr storage group). This was associated with reduced accumulation of leukocytes in the liver. In marked contrast, treatment with the intact IgG of 1A29 increased serum ALT levels dramatically (5297 +/- 634 IU/L, P < 0.05 vs. 6-hr storage group) and reduced serum complement. Histological examination revealed focal hepatocellular necrosis 24 hr after surgery in the 6-hr storage group. Treatment with the F(ab')2 fragment decreased the liver damage; in marked contrast, treatment with the intact IgG strikingly aggravated the injury, as characterized by massive necrosis throughout the liver. Liver damage caused by the intact IgG might be related to activation of the complement system by the Fc portion of the antibody. Taken together, these results indicate that ICAM-1 is involved in the mechanism of postoperative liver injury following liver transplantation.

Complement activation by whole endotoxin is blocked by a monoclonal antibody to factor B

Sepsis is both a common and, despite present-day therapy, a serious disease. The pathophysiology of the septic response is a complex, multifactorial phenomenon which in part involves the activation of complement by bacterial endotoxin. A monoclonal antibody to human complement factor B, code-named 1H5, which was capable of specifically inhibiting the alternative pathway of complement activation at concentrations as low as 1 microgram/ml, is described. This agent had no effect on the classical pathway of complement activation. It was capable of preventing the activation of complement by even high concentrations (0.1 mg/ml) of whole endotoxin; however, it was ineffective in preventing activation of complement by endotoxin derived from a rough mutant. This agent could potentially be used in the treatment of sepsis.

Regulation of antibody production mediated by Fc gamma receptors, IgG binding factors, and IgG Fc-binding autoantibodies

Fc receptors (FcRs) are immunoglobulin-binding structures that enable antibodies to perform a variety of functions by forming connections between specific recognition and effector cells. Besides eliciting cytotoxicity, inducing secretion of mediators and endocytosis of opsonized particles, FcRs are involved in the regulation of antibody production, both as integral membrane proteins and as soluble molecules released from the cell surface. Most FcRs belong to the same family of proteins as their ligands (immunoglobulin superfamily). This review contains recent data obtained by use of monoclonal antibodies and cloning studies on FcRs and FcR-like molecules. The importance of fine specificity of receptor binding site(s)--that of the conformation of FcRs and their ligands in triggering signaling mechanisms--is analyzed. The regulatory function of membrane-bound and -released FcRs; the correlation between cell cycle, FcR expression, and release; as well as the possible mechanisms of these phenomena are discussed.

The interaction of salivary secretions with the human complement system--a model for the study of host defense systems on inflamed mucosal surfaces

When complement first contacts salivary secretions, as when gingival crevicular fluid first meets saliva at the gingival margin, complement function is enhanced. The immediate potentiation of the complement system at equal volume ratios of serum to saliva is due to several factors, including the lower ionic strength of saliva when compared with serum and the presence of certain salivary glyproteins such as the nonimmunoglobulin agglutinins that appear to simultaneously activate C1 and affect (sequester) certain complement control proteins, such as Factor H. This initial potentiation of the complement cascade by saliva may aid in defending the area immediately above the gingival crevice from oral microbiota that are being coated with a combination of serous exudate components and salivary components. As serum becomes much more diluted with saliva (i.e., crevicular fluid moves away from the supragingival area), the acidic proline-rich salivary proteins (APRP) begin to disrupt the unbound C1q-C1r2-C1s2 macromolecular complexes. Thus, the APRP along with other C1 fixing substances in saliva appear to restrict complement function, but only when the ratios of saliva to serum exceed 250:1. Since certain salivary glycoproteins bind to viruses, the potentiation of the complement system by saliva may also play a role in neutralizing certain viral infections on mucosal surfaces where tissue transudates containing complement begin to contact mucosal secretions such as saliva. Again, the ratio of serous fluid to mucosal secretion appears to be an important factor. This article also discusses some of our preliminary data and speculations concerning the binding of the self-associating high-molecular-weight nonimmunoglobulin salivary agglutinins (NIA) with the envelope of the human immunodeficiency virus (HIV) and the possible cooperative role of C1q and fibronectin in aiding neutralization of HIV infectivity.

Synthetic peptide inhibitors of complement serine proteases--III. Significant increase in inhibitor potency provides further support for the functional equivalence hypothesis

Synthetic peptides based on functionally equivalent (as defined by similar patterns of chemically equivalent amino acids) serine protease inhibitor (serpin) C-terminal sequences inhibit both classical and alternative pathways of complement activation. Inhibition was also found with hybrid peptides consisting of the cleavage site of one serpin (antithrombin III, alpha-1-antitrypsin, or antichymotrypsin) attached to the short and long functionally equivalent protease binding cores of the other two serpins. A hybrid peptide composed of the sequence at the site of cleavage of C4 by C1s attached to the long binding core of antithrombin III was selective in inhibiting the classical pathway with no effect on the alternative pathway at a concn of 10 microM. Extension of the functional equivalence hypothesis has produced inhibitors of complement activation named generic and generic +, whose sequences differ by 77% or 87%, respectively, from those of all three serpin sequences. A hybrid peptide composed of the antithrombin III cleavage site attached to the generic peptide is an inhibitor of complement activation at 500 nM, the most potent inhibitor found in this study.

Substitution of asparagine324 with aspartic acid in the Fc portion of mouse antibodies reduces their capacity for C1q binding

The sequence of the heavy chain C region of mouse mutant IgG2a antibodies with reduced capacity for C1q binding but with retained ability for Fc receptor-mediated functions was determined by cDNA cloning and by mRNA sequencing. The specific mutation was found to be the substitution of asparagine324 with aspartic acid. Asparagine324 represents a new residue relating to the C1q-binding sites previously described.

Comparison of the role of tyrosine residues in human IgG and rabbit IgG in binding of complement subcomponent C1q

Treatment of covalently cross-linked or heat-aggregated oligomers of human IgG with 4 mM-tetranitromethane abrogated their C1q-binding activity. In contrast, tetranitromethane modification of rabbit IgG oligomers, under identical conditions, had no effect upon their C1q-binding activity. The tetranitromethane treatment led to nitration of about ten tyrosine residues per IgG molecule in both species, and the modification was specific for tyrosine residues. Reduction of the nitrated protein with Na2S2O4 did not lead to recovery of C1q-binding activity in human IgG oligomers or to loss of activity in rabbit IgG oligomers. Tryptic peptides from the nitrated proteins were isolated and a peptide containing nitrotyrosine-319 was recovered from human IgG, as well as peptides from both species corresponding to the region around nitrotyrosine-278. These data are consistent with the inactivation of C1q-binding activity in human IgG being the result of nitration of tyrosine-319; the rabbit IgG is unaffected by nitration because position 319 is phenylalanine. The evidence supports the C1q-receptor site proposed by Burton, Boyd, Brampton, Easterbrook-Smith, Emanuel, Novotny, Rademacher, van Schravendijk, Sternberg & Dwek [(1980) Nature (London) 288, 338-344]: residues 316-338.

Synthetic peptide inhibitors of complement serine proteases--II. Effects on hemolytic activity and production of C3a and C4a

Synthetic peptides based on the amino acid sequence at the site of cleavage of C3 by classical and alternative pathway convertases were found to be poor inhibitors of hemolysis except at concns of 1 mM and higher. Synthetic peptides of a second type, based on the C-terminal sequence of antithrombin III, were more effective; the best among them caused significant inhibition of hemolysis at a concn of 5 microM. A hybrid peptide composed of the sequence at the site of cleavage of C4 by C1s attached to an antithrombin III sequence was selective, inhibiting the classical pathway with no effect on the alternative pathway at a concentration of 25 microM. Several of the antithrombin III peptides that inhibited hemolysis did not inhibit C4 activation by the classical pathway or activation of C3 by the classical and alternative pathways suggesting that these peptides affect hemolysis by inhibiting enzymes other than C1s and C4b2a of the classical pathway and C3bBb of the alternative pathway.

The binding site for C1q on IgG

In humoral defence, pathogens are cleared by antibodies acting as adaptor molecules: they bind to antigen and trigger clearance mechanisms such as phagocytosis, antibody-dependent cell-mediated cytotoxicity and complement lysis. The first step in the complement cascade is the binding of C1q to the antibody. There are six heads on C1q, connected by collagen-like stems to a central stalk, and the isolated heads bind to the Fc portion of antibody rather weakly, with an affinity of 100 microM (ref. 3). Binding of antibody to multiple epitopes on an antigenic surface, aggregates the antibody and this facilitates the binding of several C1q heads, leading to an enhanced affinity of about 10 nM (ref. 1). Within the Fc portion of the antibody, C1q binds to the CH2 domain. The interaction is sensitive to ionic strength, and appears to be highly conserved throughout evolution as C1q reacts with IgG from different species (for example see ref. 8). By systematically altering surface residues in the mouse IgG2b isotype, we have localized the binding site for C1q to three side chains, Glu 318, Lys 320 and Lys 322. These residues are relatively conserved in other antibody isotypes, and a peptide mimic of this sequence is able to inhibit complement lysis. We propose that this sequence motif forms a common core in the interactions of IgG and C1q.

The presence of histidine residues at or near the C1q binding site of rabbit immunoglobulin G

Treatment of covalently crosslinked rabbit IgG oligomers with diethylpyrocarbonate resulted in the loss of their C1q binding activity. The inactivation was a first-order process with respect to time in the range 0-8 min, and modifier concentration from 0 to 2.39 mM. Hydroxylamine treatment of diethylpyrocarbonate-treated IgG oligomers led to 80% recovery of their C1q binding activity. Diethylpyrocarbonate treatment of IgG oligomers had little effect on their absorbance at 278 nm, but led to an increase in their absorbance at 242 nm. The apparent pKa of the modified residues was 6.91 +/- 0.12. These data are consistent with diethylpyrocarbonate modification of histidine residues leading to loss of C1q binding activity in rabbit IgG oligomers. Modification of four histidine residues per IgG molecule was associated with the loss of C1q binding activity. Thus, there may be two histidine residues at or near the C1q binding sites in the CH2 domains of rabbit IgG.

Evidence for the binding of human serum amyloid P component to Clq and Fab gamma

In order to clarify the mechanism of interaction of serum amyloid P component (SAP) with complement, the interaction of SAP with C1q and with IgG was studied. It is known that SAP binds Sepharose in the presence of calcium. When purified 125I-C1q was incubated with SAP prior to Sepharose affinity chromatography, 125I-C1q was retained. However, in the absence of SAP, the 125I-C1q was not retained. To further examine the interaction of SAP with C1q, isolated SAP was incubated at varying ratios with C1q in the presence of 1.5 mM Ca2+. These mixtures were subsequently examined via crossed immunoelectrophoresis against goat anti-SAP. A change in the electrophoretic behavior of SAP was observed in the presence of C1q. In other studies, it was observed that SAP might interact with the collagen-like stem of C1q. In these latter studies, 125I-SAP was incubated with pepsin digests of C1q in a microtitre solid-phase binding assay. In addition, a microtitre solid-phase binding assay was utilized in order to investigate the possible binding of isolated 125I-SAP with IgG. Interestingly in the presence of Ca2+, human IgG and Fab gamma, but not Fc gamma, were found to bind 125I-SAP.

Inhibition by various peptides of the activation of C1, the first component of complement, and the interaction of C gamma 2 domain of IgG with C1q

Three groups of peptides were synthesized, each of which was proposed to be a part of the C1q binding sites of the C gamma 2 domain of IgG. They were: Trp(277)-Tyr-Val-Asp-Gly (WYVDG), Thr(289)-Lys-Pro-Arg (tuftsin) and Gly(316)-Lys-Glu-Tyr-Lys (GKEYK) or portions of these peptides. Assays included CH50, consumption of serum complement induced by heat-aggregated IgG, C1 hemolysis and an enzyme immunoassay that directly measures interaction between C1q and IgG. Peptides near Gly(316) such as GKEY, GKE or EYK inhibited CH50 and heat-aggregated IgG-induced consumption of serum complement. WYVDG also inhibited CH50, with 50% inhibition at 2.05 mM, which was more than the concentrations of peptides near Gly(316) at 50% inhibition. Tuftsin was only slightly inhibitory in both systems. Results of C1 hemolysis indicated that dipeptides composed of two aromatic amino acids, especially Trp-Tyr, were more inhibitory than dipeptides of which one residue was an aromatic amino acid. Peptides such as EYK, GKEY or GKE were very inhibitory, and tuftsin was far less inhibitory than these peptides in C1 hemolysis. Results of enzyme immunoassay also showed that dipeptides composed of two aromatic amino acids were more inhibitory than dipeptides of which one residue was aromatic amino acid. WYVDG was most inhibitory in enzyme immunoassay, but tuftsin, EYK, GKEY GKE and KE were less effective.

Inhibition of the activation of the first component of complement, C1, by various amino acids or peptides

The effects of various amino acids (or their analogues) and peptides on the activation or consumption of human complement by erythrocytes bound with hemolysin or heat-aggregated immunoglobulin G (aggIgG) were studied by using the hemolysis of hemolysin-bound erythrocytes, the consumption of complement by aggIgG in the serum, the hydrolysis of acetyl tyrosine ethyl ester by activated Cl (Cls), Cl hemolysis and a newly developed enzyme immunoassay, which directly measures interaction between Clq and aggIgG. Amino acids or peptides which were proposed to comprise Clq binding sites of the C2 region of IgG or their analogues were used. CH50 was inhibited by lysine or arginine to the largest extent, but other amino acids, including tranexamic acid and epsilon-amino caproic acid were not inhibitory up to 60 mM. The consumption of serum complement by aggIgG was prevented by arginine or lysine (about 60% inhibition at 60 mM) and by histidine to a lesser extent. The activation of Cls in the Cl complex by aggIgG precipitated at pH 5.5 was most inhibited by lysine, and to a lesser extent by tranexamic acid, arginine and epsilon-aminocaproic acid, but not glutamic acid or glycine. The results of Cl hemolysis indicated that, of all amino acids soluble at neutral pH, lysine and arginine were most effective in the inhibition of Cl hemolysis. Tranexamic acid and epsilon-aminocaproic acid were less effective, and glycine and norleucine were hardly effective. Among the dipeptides used, those that are composed of aromatic amino acids were very effective in the inhibition of Cl hemolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Ligand inhibition studies on the role of Fc receptors in antibody-dependent cell-mediated cytotoxicity

Subjection of human peripheral blood lymphocytes to a temp shift from 4 to 37 degrees C resulted in a shedding of Fc receptors (termed FcRI) from 40-50% of FcR-positive cells followed by their re-expression within 4 hr; a phenomenon which had no effect on the cells' antibody-dependent killing capacity. Removal of lymphocytes having an immobile form of the Fc receptor resistant to the effects of the temp shift (termed FcRII), or removal of lymphocytes bearing both FcRI and FcRII, resulted in a similar amount of reduction in ADCC activity. This was attributed, therefore, to the loss of FcRII-positive cells. The influence of isolated (shedded) FcRI and Clq on ADCC activity was investigated. Soluble FcRI was shown to inhibit ADCC mediated through the immobile Fc receptors (FcRII), despite its lack of an ability to block EA rosette formation through these receptors. Clq also had a dose-dependent inhibitory effect on ADCC. These observations are consistent with earlier findings that FcRII possesses two active binding sites; and suggest that a prerequisite for killing in ADCC is the interaction of these with the C gamma 2 and C gamma 3 domains. The ability of synthetic peptides representative of human gamma 1-chain sequences to inhibit ADCC was determined, in an attempt to locate those sites within the IgG antibody Fc region involved in interaction with two FcR binding sites. Preliminary evidence was obtained to suggest that one of these is situated within the C gamma 2 domain, in the region of residues 274 (Lys)-294 (Glu).

The first component of human complement (C1): activation and control

The first component of human complement (C1) is a 750 000 dalton glycoprotein that requires calcium or other specific metal ions to maintain its native structure and function. Under physiologic conditions, C1 comprises two weakly interacting subunits, C1q and C1r2s2, with C1q containing the binding site(s) for activators and C1r2s2 possessing enzymatic potential. C1 circulates in a precursor state and only after "activation" does it acquire functional activity, manifested as enzymatic activity specific for its natural substrates C2 and C4. C1 activation, which is accompanied by limited proteolysis and conformational changes, can be induced by immune complexes or certain nonimmune substances. With C1 binding to an immune complex, the strength of interaction between C1q and C1r2s2 increases. C1 also spontaneously activates at 37 degrees C by an intramolecular autocatalytic mechanism although at a slower rate than that induced by activators. C1 functions are controlled by the serum glycoprotein C1-inhibitor (C1-In) which blocks the enzymatic activities of activated C1 (C1). Under physiologic conditions, C1 has a half-life of only 13 seconds in the presence of C1-In. C1 is efficiently disassembled by C1-In, thereby releasing two inactive C1rC1s(C1-In)2 complexes per C1 molecule, leaving C1q activator-bound with biologically reactive sites uncovered that are not expressed in macromolecular C1. The most recently recognized function of C1-In is that of controlling the C1 activation process itself. While having only limited effect on immune complex-induced C1 activation, C1-In effectively controls certain nonimmune-induced as well as spontaneous C1 activation. Thus C1-In plays an important role in regulating nonspecific complement activation. The latter observation is relevant for the understanding of the human disease hereditary angioedema. An overabundance of spontaneous C1 autoactivation, due to low C1-In levels, might underlie the abnormal activation of complement via the classical pathway detected in the sera of these patients. Finally, recent studies indicate that C1 may have other important biologic functions in addition to initiating the complement cascade.

Formation of complement subcomponent C1q-immunoglobulin G complex. Thermodynamic and chemical-modification studies

The interaction between the complement subcomponent C1q and immunoglobulin G was investigated under a variety of experimental conditions. Formation of the subcomponent C1q--immunoglobulin G complex was shown to be an equilibrium process. Thermodynamic studies of the effect of varying the ionic strength indicate that over the salt range 0.15--0.225 M-NaCl the binding of subcomponent C1q to immunoglobulin aggregates releases 9--12 salt ions (Na+ and/or Cl-), illustrating the importance of ionic interactions for the formation of the complex. The effects of small peptide and organic ion inhibitors support this conclusion. Chemical modifications of carboxylate residues on immunoglobulin G by glycine ethyl ester/water-soluble carbodi-imide (up to 12 residues modified per whole molecule of immunoglobulin G) and of lysine residues by acetic anhydride (3 residues per whole molecule of immunoglobulin G) or methyl acetimidate (19 residues per whole molecule of immunoglobulin G) lowered the binding affinity of immunoglobulin for subcomponent C1q. Modification of arginine residues by cyclohexane-1,2-dione-1,2 (14 residues per whole molecule of immunoglobulin G) and of tryptophan by hydroxynitrobenzyl bromide (2 residues per whole molecule of immunoglobulin G), however, had little or no effect. The results are consistent with the proposal that the subcomponent-C1q-binding site on immunoglobulin G is to be found on the last two beta-strands of the Cv2 domain [Burton, Boyd, Brampton, Easterbrook-Smith, Emanuel, Novotny, Rademacher, van Schravendijk, Sternberg & Dwek (1980) Nature (London) 288, 338--344].

Linkage and sequence homology of two human immunoglobulin gamma heavy chain constant region genes

We report the nucleotide sequence of a gene encoding a human immunoglobulin C gamma 2 region. Comparison with the previously determined C gamma 4 sequence reveals that these two genes share extensive (approximately 95%) homology in the three CH domain exons and adjacent noncoding regions. In contrast, hinge exons have diverged to a much greater degree, implying that natural selection has favored the generation of diversity in these coding regions. We have used the noncoding nucleotide differences to estimate that approximately 6-7 million years have elapsed since the occurrence of the gene duplication or correction event which generated the two identical ancestral genes. In addition we show that the two C gamma genes are arranged in human chromosomal DNA in the configuration 5'-C gamma 2-17 kilobase pairs -C gamma 4-3'.

Functional affinity constants of subfragments of immunoglobulin G for Clq

The functional affinity constants for Clq of subfragments if IgG1 representing the C gamma 2 (c gamma 2III) region or the whole C gamma 3 region of Fc (pFc'), have been measured by examining the ability of these fragments to inhibit the interaction between radiolabelled Clq and glutaraldehyde-treated human erythrocytes or aggregated human IgG. The value of the functional affinity constant for the C gamma 2III fragment was the same as that for Fc and that determined previously for monomeric IgG, indicating that all the elements necessary for Clq binding are contained in a single C gamma 2 domain. The pFc' fragment was inactive but a more degraded trypsin fragment from this region, at C gamma 3, showed the same affinity of binding for Clq as the C gamma 2III and Fc. These results confirm earlier findings that it is not combination of residues in the C gamma 2 which bind Clq which is responsible for their activity but their accessibility.

The Clq receptor site on immunoglobulin G

We propose that, the binding site for the complement subcomponent Clq on immunoglobulin G involves the last two (C-terminal) beta-strands of the C gamma 2 domain. This region contains a large number of accessible and highly conserved charged residues and charge is postulated as an important component of the Clq-IgG interaction. The conclusions are reached on the basis of accessibility and sequence conservation analyses of C gamma 2 amino acid residues, the use of specific inhibitors and chemical modification studies.