Studies on the complement-binding site of rabbit immunoglobulin G. I. Modification of tryptophan residues and their role in anticomplementary activity of rabbit IgG

Role of inter-heavy and light chain disulfide bonds in the effector functions of human immunoglobulin IgG1

The role of inter-heavy and light chain disulfide bonds in the effector functions of human IgG1 was investigated. This was accomplished by mutating appropriate sites in IgG1 such that the disulfide bond pattern now resembled that of IgG4. The effector functions of the mutant antibody were then compared to native IgG1 and IgG4. The antibody-dependent cell cytotoxicity activity was completely abolished in the mutant and the complement-dependent cytotoxicity assay was reduced fifteen-fold. The results suggest that the inter-heavy and light chain disulfide bond pattern of an antibody molecule play a role in its effector functions.

Fc receptors of liver sinusoidal endothelium in normal rats and humans. A histologic study with soluble immune complexes

Fc receptors for immunoglobulin G in the liver sinusoidal wall were studied in the normal rat and in humans by applying peroxidase-antiperoxidase immunoglobulin G complexes to the frozen sections. Fc receptors were found to exist continuously along the sinusoidal lining. The receptors showed no zonal distribution in the rat, and they were generally scarce near the central veins and portal areas in humans. To characterize the sinusoidal cells, carbon or latex was given intravenously and endogenous peroxidase was demonstrated for the rat, whereas factor VIII-related antigen and endogenous peroxidase were demonstrated for the humans. In the rat, Fc receptors were detected on Kupffer cells, which were characterized by an intense endogenous peroxidase activity and ingestion of latex or quantities of carbon. They were also detected on sinusoidal endothelial cells, which were characterized by undetectable peroxidase activity and no ingestion of latex nor of a small quantity of carbon. In humans, Fc receptors were also present on Kupffer cells as well as sinusoidal endothelial cells, as identified by endogenous peroxidase and factor VIII-related antigen, respectively.

Quantitative analysis of the interaction between immune complex and C1q complement subcomponent. The role of interdomain interactions in rabbit IgG in binding of C1q to immune precipitates

A novel method was developed for the analysis of the interaction of large multivalent ligands with surfaces (matrices) to analyse the binding of complement subcomponent C1q to immune precipitates. Our new evaluation method provides quantitative data characteristic of the C1q-immune-complex interaction and of the structure of the immune complex as well. To reveal the functional role of domain-domain interactions in the Fc part of IgG the binding of C1q to different anti-ovalbumin IgG-ovalbumin immune complexes was studied. Immune-complex precipitates composed of rabbit IgG in which the non-covalent or covalent bonds between the heavy chains had been eliminated were used. Non-covalent bonds were abolished by splitting off the CH3 domains, i.e. by using Facb fragments, and the covalent contact was broken by reduction and alkylation of the single inter-heavy-chain disulphide bond. The quantitative analysis of the binding curves provides a dissociation constant (K) of 200 nM for the interaction between C1q and immune precipitate formed from native IgG. Surprisingly, for immune precipitates composed of Facb fragments or IgG in which the inter-heavy-chain disulphide bond had been selectively reduced and alkylated, stronger binding (K = 30 nM) was observed. In this case, however, changes in the structure of the immune-complex matrix were also detected. These structural changes may account for the strengthening of the C1q-immune-complex interaction, which can be strongly influenced by the flexibility and the binding-site pattern of the immune-complex precipitates. These results suggest that domain-domain interactions in the Fc part of IgG affect the segmental mobility of IgG molecules and the spatial arrangement of the immune-complex matrix rather than the affinity of individual C1q-binding sites on IgG.

The interaction of 1-anilino-8-naphthalene sulphonate with human C1q

C1q has 12 binding sites for 1-anilino-8-naphthalene sulphonate (ANS), two per peripheral subunit. This number increases to 18 upon weak-acid-induced conformational transition in the globular heads. One ANS binding site is present in each C gamma 2 domain of human IgG. ANS is bound by C1q with a higher affinity (Ka = 2.07 X 10(6) M-1) than by the Fc fragment (Ka = 9.07 X 10(4) M-1) of human IgGl. Hence the inhibitory capacity of C1q binding to IgG immune complexes of ANS probably reflects its preferential binding to the globular heads of C1q. The characteristics of ANS-C1q binding may in part explain the hydrophobic component of the C1q-IgG interaction. It is suggested that an ionic-hydrophobic two-step process is involved in the contact between C1q and IgG.

Effector functions of a monoclonal aglycosylated mouse IgG2a: binding and activation of complement component C1 and interaction with human monocyte Fc receptor

Aglycosylated monoclonal anti-DNP mouse IgG2a produced in the presence of tunicamycin was compared with the native monoclonal IgG2a with respect to its ability to interact with the first component of complement, C1, and to compete with human IgG for binding to human monocyte Fc receptors. The aglycosylated IgG2a was found to bind subcomponent C1q with an equivalent capacity to the native IgG2a, but the dissociation constant was found to be increased three-fold. When activation of C1 by the glycosylated and aglycosylated IgG2a was compared, the rate of C1 activation by the aglycosylated IgG2a was reduced approximately three-fold. In contrast aglycosylation was accompanied by a large decrease (greater than or equal to 50-fold) in the apparent binding constant of monomeric IgG2a to human monocytes. The data suggest that the aglycosylated IgG2a has a structure which differs in the CH2 domain from the native IgG2a, and that the heterogeneous N-linked oligosaccharides of this monoclonal IgG2a which occur at a conserved position in the CH2 domain play a role in maintaining the integrity of its monocyte-binding site. This lack of monocyte binding may result either from a localized conformational change occurring in a single CH2 domain or from an alteration in the CH2-CH2 cross-domain architecture which is normally structured by a pair of opposing and interacting oligosaccharides. The minimal changes in C1q binding and C1 activation suggest that the oligosaccharides are, at most, indirectly involved in these events.

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.

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].

Expression of biological effector functions by immunoglobulin G molecules lacking the hinge region

Several biological effector functions mediated by sites on the Fc region of human IgG1 have been studied in two variant IgG1 kappa monoclonal proteins (Dob and Lec) which contain deletions corresponding to the entire hinge region of the heavy chains. Neither Dob nor Lec protein in aggregated form was able to activate the classical complement pathway, and this was shown to be due to an inability to bind the first component of complement (C1). By rosette inhibition assays, Dob and Lec proteins were shown to have no measurable affinity for Fc receptors on human B cells or neutrophils. Dob and Lec proteins had a much reduced affinity for Fc receptors on the murine macrophage-like cell line P388D1 when compared to normal human IgG1. Furthermore, the hinge-deleted proteins were able to compete with murine IgG2b for P388D1 receptors but not with murine IgG2a. In contrast, the binding of Dob and Lec proteins to protein A from Staphylococcus aureus was entirely normal. The functional consequences of the hinge deletion were parallel to those seen when normal IgG1 was reduced and alkylated. It was concluded that the functional impotency of Dob and Lec proteins was related to the close association between the Fab and Fc regions in these molecules and the limited degree of segmental flexibility permitted in the absence of the hinge region. The data also suggest a major role for the C gamma 2 domain (C is the constant region) in mediating effector functions in normal IgG1.

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.

Studies on the occurrence of circulating immune complexes in vascular diseases

The presence of circulating immune complexes was studied in 347 samples of serum from 212 patients with various vascular diseases. Two quantitative methods (complement-consumption assay and C1q-solubility test) were used for the measurement of the concentration of the complexes. Immune complexes were detected in each group of patients tested (coronary arteriosclerosis, myocardial infarction, cerebral artery sclerosis, arteriosclerosis obliterans, phlebothrombosis, pulmonary infarction). A high proportion of positivity was recorded in myocardial infarction (in 43 patients out of the 94 tested) and in arteriosclerosis obliterans (7 out of 11 cases). The possible pathogenic role of the circulating immune complexes is discussed.

Activation of the classical and alternative pathways of complement fixation by immune complexes containing normal and tryptophan-modified immunoglobulin G

Koshland's reagent (2-hydroxy-5-nitrobenzyl bromide (NBB)) has been shown to modify tryptophanyl residues in anti-ovalbumin IgG. As little as 2 moles NBB/mole IgG antibody are sufficient to block the classical pathway of complement activation when the antibody is complexed to antigen (ovalbumin). In contrast, immune complexes containing antibody with the same degree of tryptophanyl substitution will activate the alternative pathway of complement fixation. Immune complexes containing F(ab')2 fragments derived from anti-ovalbumin IgG do not activate the classical pathway. When measuring the percentage activation of C3 using the method of Laurell, NBB does not affect the alternative pathway of the complement system up to a molar ratio of 2 NBB/F(ab')2. The above findings, provide a means to evaluate the relative contribution of complement activation by the different pathways.

Complement fixing site of human IgG1

A number of chemical modifications were made to purified human IgG1. The effects of these modifications on the complement activity of the immunoglobulin were studied using aggregation on latex and measuring the consumption of the complement by CH50-quantitation. Tryptophan and tyrosine are implicated in the complement fixing site of this human immunoglobulin, and an arginine moiety probably provides a binding site for the complement.