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PubMed1982

The interaction of human plasma fibronectin with a subunit of the first component of complement, C1q.

Fibronectin is a normal plasma protein that enhances reticuloendothelial system functioning, and may participate in immune complex clearance. The interaction of 125I-fibronectin with human C1 and C1q in vitro was investigated by employing a highly reproducible solid-phase binding assay in microtiter wells. We demonstrated that although fibronectin does not bind to antigen-antibody complex (BSA-anti-BSA) or immune complexes containing C1, a 20-fold increase in binding was obtained when the complexes contained C1q alone. In the absence of antigen-antibody complexes, fibronectin binds to the C1q fixed to the wells in a dose-response fashion but not to intact C1. C1q in the fluid phase inhibits 85% of the fibronectin binding to immobilized C1q. The amount of fibronectin bound by immobilized C1q or gelatin is approximately equal. The binding of fibronectin to C1q could be inhibited by the restoration of C1r + C1s to the C1 macromolecular complex before the addition of fibronectin. The inhibition was dependent on the concentration of C1r + C1s and achieved a maximum of 70% at 100 micrograms/ml. This inhibition could be reversed by the removal of C1r and C1s subunits with EDTA or C1 inhibitor. Digestion of C1q with pepsin resulted in an 85% loss of fibronectin binding. It therefore appears that at least one site of fibronectin binding to C1q is in the globular portion of this complement component.

Animals↗

PubMed1988

cDNA clones coding for the complete murine B chain of complement C1q: nucleotide and derived amino acid sequences.

We have isolated cDNA clones covering the complete B chain of the complement subunit C1q from mouse; this subunit initiates the classical complement pathway. Deoxynucleotide sequence analysis shows that these clones contain 156 nucleotides of the 5' untranslated region, followed by sequences coding for the 25 amino acids of the signal peptide, all of the 228 amino acids of the mature protein and 140 nucleotides of the 3' untranslated region, including a poly A addition signal. The coding region for the mature protein contains 261 nucleotides for the Gly-X-Y repeat and 408 nucleotides for the globular portion of C1q. By comparing the nucleotide sequence of the mouse B chain of C1q with the B chain from human (Reid, K. B. M., 1985, Biochem. J. 231, 729), we find a high homology (80%) within the mature protein, a lower homology within the signal peptide (59%) and the 3' untranslated region (47%) and no homology (26%) in the 5' untranslated region.

Amino Acid Sequence↗

PubMed1996

The binding protein for globular heads of complement C1q, gC1qR. Functional expression and characterization as a novel vitronectin binding factor.

A binding protein for the globular head domains of complement component C1q, designated gC1qR, recently described to be present on vascular and blood cells (Ghebrehiwet, B., Lim, B.-L., Peerschke, E. I. B., Willis, A. C., and Reid, K. B. M. (1994) J. Exp. Med. 179, 1809-1821 was expressed in recombinant form in bacteria to investigate its functional and structural properties. The recombinant gC1qR was found to be functional because tetramerization of the 24.3-kDa polypeptide occurred as described for the native protein, and the binding of the ligand C1q by recombinant gC1qR was indistinguishable from binding shown by gC1qR isolated from Raji cells. Recombinant gC1qR immobilized to microspheres was used to search for additional binding proteins unrelated to C1q. Surprisingly, it was found that vitronectin or complexes containing vitronectin were retained from plasma or serum, and subsequent analysis revealed the specific binding of the ternary vitronectin-thrombin-antithrombin complex to gC1qR. Because the thrombin-antithrombin complex was unable to interact with gC1qR, direct binding with vitronectin was investigated in a purified system. The heparin binding multimeric form of vitronectin but not the plasma form of vitronectin was found to bind specifically to gC1qR isolated from Raji cell membrane as well as to recombinant gC1qR. This interaction was saturable (KD approximately 20 nM) and inhibitable by glycosaminoglycans such as heparin but not by chondroitin sulfate. C1q and vitronectin did not compete with each other for binding to gC1qR, and both ligands seem to interact with different parts of the gC1qR because a truncated version of recombinant gC1qR lacking the N-terminal 22-amino acid portion hardly interacted with vitronectin but bound C1q as well as the intact gC1qR. These findings establish gC1qR as a novel vitronectin-binding protein that may participate in the clearance of vitronectin-containing complexes or opsonized particles or cooperate with vitronectin in the inhibition of complement-mediated cytolysis.

Blood Proteins↗

PubMed1997

Domain-switched mouse IgM/IgG2b hybrids indicate individual roles for C mu 2, C mu 3, and C mu 4 domains in the regulation of the interaction of IgM with complement C1q.

Although polymeric IgM and monomeric IgG are potent activators of the classical complement pathway, previous studies have indicated that monomeric IgM is inactive. To understand this and to examine the roles of the individual mu domains in complement activation, we created a set of IgM/IgG2b mouse chimeric Abs in which homologous domains of both Abs have been interchanged, either singly or together with adjacent domains. The monomer subunits (H2L2) of the resulting chimeras were analyzed for their capacities to bind C1q and to initiate complement-mediated lysis (CML) of haptenated erythrocytes. When C gamma 2 was flanked by C mu 4, the inherent C1q-binding activity of the C gamma 2 domain was lost. This demonstrates that C mu 4 can suppress the C1q-binding activity of the adjacent C gamma 2 domain, and suggests that C mu 4 may exert a similar effect on the C mu 3 domain in the IgM monomer subunit. When C mu 3 was located in an IgG2b background and potentially freed from the constraints imposed by the IgM background, the monomer was not able to bind C1q or initiate CML. This suggests that these activities are not expressed inherently in the C mu 3 domain. The transplantation of C mu 3 together with C mu 4 into the IgG background permitted polymer formation. This polymer was able to bind C1q, although neither the monomer nor the polymer forms were active in CML; conversely, all IgM polymers with a transplanted C gamma 2 domain were active in both C1q binding and CML, and demonstrated apparent Kd values similar to that of wild-type IgM.

Animals↗

PubMed2004

Complement C1q expression induced by Abeta in rat hippocampal organotypic slice cultures.

Amyloid beta peptide (Abeta) is a major component of senile plaques, one of the principle pathological features in Alzheimer's disease (AD) brains. Fibrillar Abeta has been shown to bind C1 via C1q, the recognition component of the classical complement pathway, resulting in the activation of the complement pathway, thereby initiating an inflammatory cascade in the brain. C1q has also been shown to enhance phagocytic activities of microglia, which could benefit in clearance of apoptotic cells or cellular debris. To begin to define the role of C1q in tissue injury mediated by Abeta, we assessed the appearance of C1q in hippocampal slice cultures treated with freshly solubilized or fibrillar Abeta 1-42. Here we demonstrate a dose- and time-dependent uptake of exogenously applied Abeta by pyramidal neurons in organotypic slice cultures from rat hippocampus. Importantly, when slices were immunostained with antibody against rat C1q, a distinct reactivity for C1q in cells within the neuronal cell layer of cornu ammonis (CA) of hippocampus, primarily the CA1/CA2, was observed in the Abeta-treated slices. No such immunoreactivity was detected in untreated cultures or upon addition of control peptides. ELISA assays also showed an increase in C1q in tissue extracts from slices of the treated group. Similarly, the mRNA level of C1q in slices was increased within 24 h after Abeta treatment. These data demonstrate that upon exposure to Abeta, C1q is expressed in neurons in this organotypic system. The induction of C1q may be an early, perhaps beneficial, tissue or cellular response to injury triggered by particular pathogenic stimuli.

Amyloid beta-Peptides↗

PubMed1989

Rapid identification and typing of herpes simplex virus by a new enzyme immunoassay with peroxidase-labeled complement C1q.

A new enzyme-linked immunoassay system (TC-ELISA) was developed for the rapid, specific detection and typing of herpes simplex virus (HSV) using anti-HSV immune sera and enzyme-labeled complement component C1q (P*-C1q). The method is based on viral antigen produced in HSV-infected cells being detected by simultaneous addition of immune serum and P*-C1q (ELISA-CF). With this assay, it was found that HSV was detected with anti-HSV immune serum and P*-C1q and that HSV type 1 and HSV type 2 could be differentiated with anti-HSV-1 and anti-HSV-2 immune sera and P*-C1q. The TC-ELISA test was HSV specific and not reactive with measles virus, cytomegalovirus, mumps virus or varicella-zoster virus. The whole assay procedure can be completed within 3 hours, as long as HSV-inoculated cell cultures with more than 10% cytopathic effect are used. These results suggest that TC-ELISA is useful in the diagnosis of viral infections.

Antibodies, Viral↗

PubMed1986

[Effect of the C1q complement subcomponent on blood coagulability and fibrinolysis].

The in vitro experiments on human plasma have shown that C1q addition in a concentration of 120 micrograms/ml led to a substantial shortening of coagulation time of test-plasma, as well as kaolin- and cephalin time. The effect is preserved in plasma deficient in factors V, X and VII. It is assumed that C1q has properties similar to those of thromboplastin.

Blood Coagulation↗

PubMed1995

Gonococcal infection in a nonhuman host is determined by human complement C1q.

Human C1q displayed a dose-dependent protection of gonococcal cells (GC) from the bactericidal effect of newborn rat serum. All rat pups injected with C1q-preincubated GC developed bacteremia, while none of the animals injected with GC only were infected. After clearance of bacteremia at day 6, live GC could still be recovered from tested organs, including the liver. Preincubation of GC with higher concentrations of C1q was associated with increased morbidity. In contrast to human serum as a source of C1q, rat, rabbit, and mouse sera did not increase the in vivo virulence of Neisseria gonorrhoeae. C1q-deficient human serum, heat-inactivated C1q or human serum, type IV collagen, and complement C3 were inefficient in inducing infection. Experimental infection by C1q-preincubated GC was inhibited by anti-C1q antibodies in a dose-dependent fashion, demonstrating a causal effect of C1q function. This report demonstrates the novel finding that human C1q, a component of the human immune system with a general function for elimination of infection, may increase GC virulence and result in the development of disseminated infection in a nonhuman host.

Animals↗

PubMed1991

Precerebellin is a cerebellum-specific protein with similarity to the globular domain of complement C1q B chain.

The cerebellum contains a hexadecapeptide, termed cerebellin, that is conserved in sequence from human to chicken. Three independent, overlapping cDNA clones have been isolated from a human cerebellum cDNA library that encode the cerebellin sequence. The longest clone codes for a protein of 193 amino acids that we term precerebellin. This protein has a significant similarity (31.3% identity, 52.2% similarity) to the globular (non-collagen-like) region of the B chain of human complement component C1q. The region of relatedness extends over approximately 145 amino acids located in the carboxyl terminus of both proteins. Unlike C1q B chain, no collagen-like motifs are present in the amino-terminal regions of precerebellin. The amino terminus of precerebellin contains three possible N-linked glycosylation sites. Although hydrophobic amino acids are clustered at the amino terminus, they do not conform to the classical signal-peptide motif, and no other obvious membrane-spanning domains are predicted from the cDNA sequence. The cDNA predicts that the cerebellin peptide is flanked by Val-Arg and Glu-Pro residues. Therefore, cerebellin is not liberated from precerebellin by the classical dibasic amino acid proteolytic-cleavage mechanism seen in many neuropeptide precursors. In Northern (RNA) blots, precerebellin transcripts, with four distinct sizes (1.8, 2.3, 2.7, and 3.0 kilobases), are abundant in cerebellum. These transcripts are present at either very low or undetectable levels in other brain areas and extraneural structures. A similar pattern of cerebellin precursor transcripts are seen in rat, mouse, and human cerebellum. Furthermore, a partial genomic fragment from mouse shows the same bands in Northern blots as the human cDNA clone. During rat development, precerebellin transcripts mirror the level of cerebellin peptide. Low levels of precerebellin mRNA are seen at birth. Levels increase modestly from postpartum day 1 to 8, then increase more dramatically between day 5 and 15, and eventually reach peak values between day 21 and 56. Because cerebellin-like immunoreactivity is associated with Purkinje cell postsynaptic structures, these data raise interesting possibilities concerning the function of the cerebellin precursor in synaptic physiology.

Amino Acid Sequence↗

PubMed2002

Structure-function studies of the receptors for complement C1q.

C1q is an essential component of the phylogenetically ancient innate complement (C) system and is crucial to our natural ability to ward off infection and clear toxic cell debris (e.g. amyloid fibrils, apoptotic cells). Several candidate C1q receptors [C1q receptor for phagocytosis enhancement (C1qRp), complement receptor (CR) 1, calreticulin (CRT), binding protein for the globular head of C1q (gC1qbp)] have been described, and the aim of this review is to shed light on their structure-function relationships. One cell-surface molecule, C1qRp, has emerged as a defence collagen receptor for C1q, as well as mannose-binding lectin (MBL) and surfactant protein A. C1qRp (also known as the AA4 antigen in rodents) is the antigen recognized by a pro-adhesive monoclonal antibody called mNI-11 and antibodies against CD93, but recent results failed to confirm C1q binding activity. CR1 (CD35), a multifunctional receptor both in its ligand specificity and in its C regulation activities, is found on circulating monocytes and neutrophils, but the major site of expression is B-lymphocytes. As a receptor, CR1 binds to C1q, other C opsonins (C4b, C3b, iC3b) and MBL, and as such, has been involved in promoting phagocytosis. Several studies support a role for the cell surface receptor for the collagenous domains of C1q (cC1qR; also known as CRT). CRT belongs to the family of heat-shock proteins, the most abundant and ubiquitous soluble intracellular proteins. Though CRT does not have a transmembrane domain, it seems to mediate phagocytosis of the apoptotic cells through association with CD91. A 33 kDa protein interacts with the globular head of C1q and, logically, has been termed gC1qbp. This protein is located in mitochondria, suggesting that gC1qbp is not a cell-surface receptor itself.

Animals↗

PubMed2006

Involvement of phosphatidylserine, alphavbeta3, CD14, CD36, and complement C1q in the phagocytosis of primary necrotic lymphocytes by macrophages.

OBJECTIVE: Uningested dead cells may be an important source of autoantigens and may trigger autoimmune diseases such as systemic lupus erythematosus (SLE). Multiple receptors involved in the clearance of apoptotic cells have been described; however, little is known about the receptors and ligands involved in uptake of necrotic cells that release autoantigens as well. METHODS: The uptake of autologous necrotic peripheral blood lymphocytes into human monocyte-derived macrophages was qualitatively and quantitatively monitored by confocal microscopy and 2-color flow cytometry, respectively. Blocking experiments were performed to examine the receptors and molecules involved in the phagocytosis of necrotic cells. Cytokine secretion by lipopolysaccharide-activated monocytes and macrophages was determined by enzyme-linked immunosorbent assay. RESULTS: Phosphatidylserine, which was exposed on necrotic as well as apoptotic cells, promoted the recognition and removal of primary necrotic lymphocytes. Several macrophage receptor systems, including the thrombospondin-CD36-alphavbeta3 complex, CD14, and the complement component C1q, contributed to the engulfment of necrotic cells. Necrotic peripheral blood lymphocytes slightly increased the lipopolysaccharide-induced secretion of interleukin-10 and reduced the secretion of tumor necrosis factor alpha in monocytes and macrophages. CONCLUSION: Our results indicate that at least some of the receptors and adaptors mediating the uptake of apoptotic cells are also involved in the clearance of necrotic cells. Hence, necrotic cells engage phagocyte receptors such as CD36, which mediate antiinflammatory signals from apoptotic cells. Necrotic cells consequently also have the potency to provide antiinflammatory signals to phagocytes; however, these signals may be overridden by proinflammatory factors released during necrosis. These findings have implications regarding the etiopathogenesis of autoimmune diseases such as SLE, in which impaired clearance of dead cells may foster autoimmunity by the release of potential autoantigens.

CD36 Antigens↗

PubMed2007

Complement C1q reduces early atherosclerosis in low-density lipoprotein receptor-deficient mice.

We explored the role of the classic complement pathway in atherogenesis by intercrossing C1q-deficient mice (C1qa-/-) with low-density lipoprotein receptor knockout mice (Ldlr-/-). Mice were fed a normal rodent diet until 22 weeks of age. Aortic root lesions were threefold larger in C1qa-/-/Ldlr-/- mice compared with Ldlr-/- mice (3.72 +/- 1.0% aortic root versus 1.1 +/- 0.4%; mean +/- SEM, P < 0.001). Furthermore, the cellular composition of lesions in C1qa-/-/Ldlr-/- was more complex, with an increase in vascular smooth muscle cells. The greater aortic root lesion size in C1qa-/-/Ldlr-/- mice occurred despite a significant reduction in C5b-9 deposition per lesion unit area, suggesting the critical importance of proximal pathway activity. Apoptotic cells were readily detectable by cleaved caspase-3 staining, terminal deoxynucleotidyl transferase dUTP nick-end labeling assay, and electron microscopy in C1qa-/-/Ldlr-/-, whereas apoptotic cells were not detected in Ldlr-/- mice. This is the first direct demonstration of a role for the classic complement pathway in atherogenesis. The greater lesion size in C1qa-/-/Ldlr-/- mice is consistent with the emerging homeostatic role for C1q in the disposal of dying cells. This study suggests the importance of effective apoptotic cell removal for containing the size and complexity of early lesions in atherosclerosis.

Animals↗

PubMed1996

Stimulation of macrophage synthesis of complement C1q by interferon-gamma mediated by endogenous interferon-alpha/beta.

Both exogenous interferon-gamma (IFN-gamma) and interferon-alpha/beta (IFN-alpha/beta) stimulate C1q mRNA synthesis in murine macrophages. Previous studies suggested that IFN-gamma induced endogenous synthesis of IFN-alpha/beta by murine macrophages. In the present study, we determined the indirect effect of IFN-gamma on macrophage synthesis of C1q mRNA mediated by endogenous IFN-alpha/beta production. Upregulation of macrophage synthesis of C1q mRNA by both IFN-gamma and IFN-alpha/beta was reversed by neutralizing antibody to IFN-alpha/beta but not by nonimmune control serum. IFN-gamma was confirmed by ELISA to stimulate a six- to eightfold increase in macrophage secretion of IFN-alpha/beta compared with untreated control cells. We tentatively conclude that IFN-gamma stimulates macrophage synthesis and secretion of IFN-alpha/beta, which in turn promotes C1q mRNA synthesis in an autocrine or paracrine manner. Thus, exogenous IFN-gamma, derived from activated T lymphocytes, may act primarily on the macrophage to stimulate the endogenous synthesis of IFN-alpha/beta for autocrine modulation of a variety of biologic functions, including C1q synthesis.

Animals↗

PubMed2007

Differential chemokine response of fibroblast subtypes to complement C1q.

BACKGROUND AND OBJECTIVE: The pathogenesis of periodontitis includes an inappropriate activation of the classical complement cascade (C') with accumulation of inflammatory C' products in fluids and tissues. Our hypothesis is that in vivo the C' product, C1q, may act as a regulatory component of the innate immune response of distinct matrix fibroblasts to the inflammatory environment. This study analyzed the C1q induction of pro-inflammatory cytokine secretion in fibroblast subtypes derived from distinct periodontal tissues, and identified a mechanism of the cell response. MATERIAL AND METHODS: Primary human gingival fibroblast, periodontal ligament fibroblast, and granulation tissue fibroblast cultures were treated for 24 h with C1q. Protein arrays assessed the secretory profile of constitutive and C1q-inducible pro-inflammatory cytokines, and enzyme-linked immunosorbent assays were used to quantify the kinetics of each inducible cytokine. RESULTS: Granulation tissue fibroblast cultures were unresponsive to C1q challenge. In contrast, periodontal ligament fibroblasts responded with a release of monocyte chemoattractant protein (MCP)-1, interleukin-6, interleukin-8, and macrophage inflammatory protein (MIP)-1beta higher than the basal level by 8.2-, 7.0-, 3.8-, and 7.2-fold, respectively. Human gingival fibroblast cultures increased secretion of these chemokines by 5.2-, 4.5-, 3.0-, and 9.8-fold, respectively. Inhibitor studies revealed that C1q-inducible release of chemokines by the human gingival fibroblast and periodontal ligament cultures was contingent upon p38 mitogen-activated protein kinase activity. CONCLUSION: The ability of C1q to stimulate secretion of pro-inflammatory chemokines depends upon which specific fibroblast subtype is involved. Targeting C1q-activated intracellular signaling pathways may be an effective means to inhibit the production of chemokines that promote inflammatory cell infiltration into gingival and periodontal ligament tissues.

Cells, Cultured↗

PubMedDate not supplied

Expression and regulation of complement C1q by human THP-1-derived macrophages.

The regulation of C1q expression was examined in the human monocytic cell line THP-1. Since these cells can be differentiated into cells with macrophage properties and induced to express C1q, they were used as models for mature human monocyte/macrophages and indirectly microglia. Interferon-gamma (IFN-gamma) and the anti-inflammatory steroid agents dexamethasone and prednisone were powerful stimulators of C1q production, alone or in combination. Interleukin-6 (IL-6) and lipopolysaccharide (LPS) also had significant stimulatory activity. Phorbol myristate acetate, a protein kinase C activator, reduced C1q expression. Four additional classes of pharmacological agents were tested for their effect on C1q secretion. Tacrine, but not indomethacin, cimetidine, or propentofylline, showed activity in inhibiting C1q secretion by IFN-gamma treated THP-1-derived macrophages.

Brain↗

PubMed1994

[A stable conformer of IgG, prepared by an acidic influence: study by calorimetry, binding of the C1q complement component, and monospecific anti-IgG].

Thermal stability and functional activity of rabbit IgG in its native conformation and after incubation at pH 2.0 were studied using differential scanning calorimetry and binding of conformational probes, i.e., the C1q component of the complement and two monospecific anti-IgG antibodies. The latter reacted selectively with the "hinge" region joining the Fab and Fc fragments of IgG or with the CH2 domain in the Fc fragment. At pH 2.0 complete unfolding of rabbit IgG did not occur: the protein demonstrated the presence of secondary and compact tertiary structures but differed from the native conformation by decreased overall enthalpy and TM of thermal denaturation as well as by changed secondary structure parameters as could be evidenced from CD spectroscopy and scanning calorimetry data. Incubation at pH 2.0 followed by renaturation at neutral pH led to irreversible conformational changes in IgG. The most significant differences between the two IgG conformers were demonstrated by calorimetry at pH 3.5 which revealed that the acid-treated conformer differs from the native one by enhanced thermal stability of the CH2 domain. Using the combination of thermodynamic and functional studies, it was shown that the origin of stabilization was the increase in the extent of interaction between the CH2 domain in the Fc fragment and the CHI domain in the Fab fragment. This resulted in the increase of the functional link between the antigen-binding domain and the C1q binding site in the CH2 domain of the acid-induced IgG conformer. In parallel with the increase in stability of the CH2 domain, conformational changes in the "hinge" region were found, together with the absence of intrinsic conformational changes in the CH2 domain proper as could be judged from C1q and monospecific anti-IgG binding assays. The results obtained demonstrate one of possible mechanisms whereby functionally significant rearrangements in the IgG molecule can be induced by changes in the interactions between invariably folded domains rather than by intrinsic changes in the domain conformation.

Animals↗

PubMed1998

Complement C1q inhibits cellular spreading and stimulates adenylyl cyclase activity of fibroblasts.

C1q selectively localizes at injured tissues, where it may function as a regulator of cell-matrix interactions. We show here that purified C1q, added to the culture medium of human gingival fibroblasts (HF) spread onto fibronectin substrates, elicited a round morphology that was accompanied by altered F-actin and correlated with inhibition of cellular spreading. Shape modification required integrity of the molecule and was specific, dose dependent, nontoxic, and reversible. Antispreading activity was mediated, at least in part, by specific cell-surface C1q receptors. We hypothesized that ligand occupancy of C1q receptors could influence shape by affecting intracellular levels of cyclic AMP (cAMP). Within 20 min of exposure of adhering HF to C1q, we detected an increase in adenylyl cyclase activity (six- to ninefold) in cAMP accumulation (by 20%) and in cAMP-dependent protein kinase activity (by 20%). These changes suggested that the rounding effect of C1q may be associated with activation of the adenylyl cyclase pathway.

Adenylyl Cyclases↗

PubMed1984

Complement C1q binding affects spin-labeled heterosaccharides of rabbit antibodies in immune but not artificial immunoglobulin G aggregates.

IgG anti-hapten antibodies were purified from the sera of rabbits homozygous for allotypic determinants d11 and d12 in the constant region of the heavy chain. Correlative with this determinant is the absence (d11) or presence (d12) of an oligosaccharide chain just below the hinge region of the IgG molecule. Both d11 and d12 molecules contain a complex heterosaccharide chain located near the carboxyl terminus of the second constant region domain. The two populations of IgG antibodies were thus selectively labeled with the spin probe Tempamine in their second constant region domains by reductive amination primarily of terminal N-acetylneuraminic acid residues. Chemical and enzymatic cleavages showed about 80% of the attached spin labels were N-acetylneuraminic acid-associated. Analysis of probe adducts by ESR spectrometry showed the presence of slower and faster moving subcomponents. Formation of immune complexes by antigen induces slight but significant restrictions of spin label mobility for both d11 and d12 IgG molecules. This restriction is qualitatively different from that seen in glutaraldehyde-, carbodiimide-, or ethanol-induced aggregates of the same IgG antibodies. The addition of purified complement C1 subcomponent C1q to immune aggregates resulted in marked immobilization of spin labels, the rotational correlation time of which was 30-40 mu s for both d11 and d12 molecules (evaluated by saturation transfer spectroscopy). A similar spin probe immobilizing effect is not seen when C1q binds to chemically aggregated IgG antibodies (which also do not activate C1). A novel model is proposed in which C1q is hypothesized to juxtapose Fc moieties in a discrete fashion required for subsequent C1 activation processes mediated by immune complexes.

Animals↗