Heymann antibodies induce complement-dependent injury of rat glomerular visceral epithelial cells

The aim of this study was to investigate the in vitro role of the complement membrane attack complex (MAC) in the injury induced by nephritogenic anti-brush border vesicle (Fx1A) antibodies on rat glomerular visceral epithelial cells (GEC). Both sheep and rabbit anti-rat brush border vesicle IgG-induced complement-dependent lysis of cultured GEC. Fab fragments of sheep anti-rat brush border vesicles and polyclonal or monoclonal gp330 IgG were devoid of lytic activity. Shedding of cell-surface antigens induced by sheep or rabbit anti-rat brush border vesicle IgG protected GEC from subsequent exposure to lytic antibodies and complement, an effect that was not obtained with Fab fragments. When GEC were incubated with sheep or rabbit anti-rat brush border vesicle IgG in capping conditions, the C3 component was co-redistributed with Heymann immune complexes; in contrast, the MAC remained diffusely bound to the cell surface, indicating that it was not associated with the antigen-antibody complexes. The MAC was demonstrated on the surface of GEC by immunofluorescence staining with anti-MAC neoantigen and by electron microscopy of negatively stained membranes showing focal clusters of 110 A MAC lesions. When GEC were treated with sheep IgG or rabbit IgG plus C6-deficient sera, the cells were not lysed and MAC was not demonstrable on the surface; however, lytic activity was restored when C6-deficient sera were reconstituted with purified C6. The results are consistent with the interpretation that injury induced by Heymann antibodies on GEC is MAC-dependent.     

The Staphylococcus aureus Protein Sbi Acts as a Complement Inhibitor and Forms a Tripartite Complex with Host Complement Factor H and C3b

The Gram-positive bacterium Staphylococcus aureus, similar to other pathogens, binds human complement regulators Factor H and Factor H related protein 1 (FHR-1) from human serum. Here we identify the secreted protein Sbi (Staphylococcus aureus binder of IgG) as a ligand that interacts with Factor H by a—to our knowledge—new type of interaction. Factor H binds to Sbi in combination with C3b or C3d, and forms tripartite Sbi∶C3∶Factor H complexes. Apparently, the type of C3 influences the stability of the complex; surface plasmon resonance studies revealed a higher stability of C3d complexed to Sbi, as compared to C3b or C3. As part of this tripartite complex, Factor H is functionally active and displays complement regulatory activity. Sbi, by recruiting Factor H and C3b, acts as a potent complement inhibitor, and inhibits alternative pathway-mediated lyses of rabbit erythrocytes by human serum and sera of other species. Thus, Sbi is a multifunctional bacterial protein, which binds host complement components Factor H and C3 as well as IgG and β₂-glycoprotein I and interferes with innate immune recognition.     

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.     

Surfactant protein A regulates complement activation.

Complement proteins aid in the recognition and clearance of pathogens from the body. C1, the first protein of the classical pathway of complement activation, is a calcium-dependent complex of one molecule of C1q and two molecules each of C1r and C1s, the serine proteases that cleave complement proteins. Upon binding of C1q to Ag-bound IgG or IgM, C1r and C1s are sequentially activated and initiate the classical pathway of complement. Because of structural and functional similarities between C1q and members of the collectin family of proteins, including pulmonary surfactant protein A (SP-A), we hypothesized that SP-A may interact with and regulate proteins of the complement system. Previously, SP-A was shown to bind to C1q, but the functional significance of this interaction has not been investigated. Binding studies confirmed that SP-A binds directly to C1q, but only weakly to intact C1. Further investigation revealed that the binding of SP-A to C1q prevents the association of C1q with C1r and C1s, and therefore the formation of the active C1 complex required for classical pathway activation. This finding suggests that SP-A may share a common binding site for C1r and C1s or Clq. SP-A also prevented C1q and C1 from binding to immune complexes. Furthermore, SP-A blocked the ability of C1q to restore classical pathway activity to C1q-depleted serum. SP-A may down-regulate complement activity through its association with C1q. We hypothesize that SP-A may serve a protective role in the lung by preventing C1q-mediated complement activation and inflammation along the delicate alveolar epithelium.     

Structural and functional studies in C1q deficiency

The sera of two brothers were found totally lacking hemolytic C activity. One of them, a 16-yr-old male, presented a severe lupus-like syndrome, whereas the other was apparently healthy. Immunochemical quantitation of C components in both sera showed depressed levels of C1q, whereas the levels of C1r, C1s, and C1 inhibitor were elevated. C4, C3, C5, factor B, and beta 1H levels were in the normal range. Hemolytic C1 activity was totally lacking. C4 titers were elevated (150% of normal). C2 hemolytic activity was about one-third of normal, and the titers of the terminal components C3-C9 were also reduced in the two siblings. Double immunodiffusion against anti-C1q antiserum showed a partial loss of C1q antigenic determinants in the two siblings. Furthermore, the C1q of both siblings was unable to interact with immunoglobulins or to associate with C1r and C1s. Addition of purified human C1q to the sera restored their total C and C1 hemolytic activity. The dose response to the C1q addition was linear, indicating that the functional deficiency was not due to the presence of a serum inhibitor. Although antigenically deficient in comparison with normal C1q, the abnormal C1q appeared to have a larger m.w., as determined by gel chromatography. Investigation of other members of this family suggests a genetically linked disorder, because four out of six siblings had the same dysfunctional C1q in their serum.     

The serology and immunochemistry of HIV-induced platelet-bound immunoglobulin

A study was carried out on the presence of platelet-bound immunoglobulins, platelet-bound complement and serum immunoglobulin reactive with platelets in the blood of persons infected with HIV and those at risk of HIV infection. Platelet-bound immunoglobulins, predominantly IgG and IgM, but not complement, were demonstrated by immunofluorescence in 16 out of 16 patients with AIDS, in 5 out of 7 with AIDS-related complex/persistent generalized lymphadenopathy and in 7 out of 10 apparently healthy sexually active homosexual men, of whom 2 were anti-HIV1 seropositive. There was no correlation between the presence of platelet-bound immunoglobulins and either the platelet count or the level of circulating immune complexes. The specificity of the platelet-bound immunoglobulins and platelet-reactive immunoglobulins in the corresponding sera was studied. Platelet-bound immunoglobulins were eluted and then investigated for cross-reactivity with HIV. Both sera and eluates were tested for reactivity with cardiolipin and reactivity with the major target antigen in classical autoimmune thrombocytopenia, the GP IIb/IIIa complex. Of 17 eluates containing platelet-reactive immunoglobulins, 5 reacted with HIV-determinants but 2 out of 5 eluates that did not contain platelet-reactive immunoglobulins also reacted. Although anti-cardiolipin antibodies were detected in all sera, none of the 17 eluates reacted with cardiolipin. Moreover, sera and eluates, reactive with normal platelets, did not react with type-1-Glanzmann disease platelets. This indicates that the antibodies are directed against the glycoprotein IIb/IIIa complex of platelets. This could not be confirmed by immunoprecipitation or by immunoblotting, however. We conclude that the presence of platelet-bound immunoglobulins is common in HIV-infection but may also occur in persons at risk and that the nature of the auto-antibodies is not different from that of the auto-antibodies observed in classical ITP.     

In vitro effects of organic solvents on immunity indicators in serum

Serum treatment in vitro with organic solvents (chloroform, ether, toluene) failed to produce an effect on immunoglobulin levels and activity. After chloroform and ether treatment, no complement activity could be determined, with chloroform-treated serum beginning to express anticomplement activity against autologous, allogenic and xenogenic sera. The classical pathway of complement activation (C1, C4, C2, C3) was primarily inhibited, whereas the alternative pathway remained unaffected. Chloroform-treated sera exhibited significantly declined levels of C1-INH, C3 and C4 as well as of circulating immune complexes. Toluene did not influence any of the parameters tested, while ether blocked complement activity without affecting either the concentration or activity of the other components under investigation. The obtained findings are discussed from the aspect of organic solvent applications in preparing immune products and determining immunity indicators in the serum or other biological fluids.     

Neutralization of influenza virus by normal human sera: mechanisms involving antibody and complement

All normal human sera examined neutralized WS/33 H1N1 influenza virus efficiently by one of two antibody-dependent mechanisms. A minority of the sera contained moderate levels of IgG antibody directed against the viral hemagglutinin that had the ability to directly neutralize the virus. The majority of sera tested contained very low levels of IgG anti-hemagglutinin antibody, which was detectable with a specific ELISA but not by conventional HAI assays. Such IgG antibody was unable to directly neutralize the virus. Studies with agammaglobulinemic serum and with sera depleted of and reconstituted with complement components established essential roles for IgG and the components of the classical complement pathway through C3 for neutralization. The components of the alternative and membrane attack pathways were not needed for neutralization. As anticipated from the requirement for IgG and exclusive mediation of neutralization by the classical pathway, the virus-IgG immune complex activated purified C1. Binding of C3 and C4 to the virus was demonstrated, as was classical pathway-mediated triggering of the alternative pathway, with recruitment of properdin. In addition, the H1N1 influenza virus also directly activated the alternative complement pathway in human serum, leading to C3 and properdin deposition on the viral envelope. Such direct alternative pathway activation also required immunoglobulin. However, the alternative pathway alone was unable to neutralize the virus. Thus, most normal sera examined contain low levels of IgG anti-hemagglutinin antibody, which activate the classical pathway of the complement system and neutralize WS/33 influenza virus by deposition of C3 and C4 on the viral envelope.     

Functional Characterization of Autoantibodies against Complement Component C3 in Patients with Lupus Nephritis

Lupus nephritis (LN) is a complication of the autoimmune disease systemic lupus erythematosus. Because the complement system plays a critical role in orchestrating inflammatory and immune responses as well as in the clearance of immune complexes, autoreactivity to complement components may have considerable pathological consequences. Autoantibodies against the central complement component C3 have been reported in systemic lupus erythematosus, but their molecular mechanism and functional relevance are not well understood. The objective of this study was to evaluate the frequency and the functional properties of the anti-C3 autoantibodies. Anti-C3 autoantibodies were measured in plasma of 39 LN patients, and identification of their epitopes on the C3 molecule was performed. By using surface plasmon resonance, we analyzed the influence of patient-derived IgG antibodies on the interaction of C3b with Factor B, Factor H, and complement receptor 1. The capacity of these antibodies to dysregulate the C3 convertase on the surface of endothelial cell was measured by flow cytometry. Here we report that the frequency of anti-C3 autoantibodies in LN is ∼30%. They inhibited interactions of the negative complement regulators Factor H and complement receptor 1 with C3b. An enhanced C3 deposition was also observed on human endothelial cells in the presence of C3 autoantibodies. In addition, anti-C3 autoantibody levels correlated with disease activity. In conclusion, the anti-C3 autoantibodies in LN may contribute to the autoimmune pathology by their capacity to overactivate the complement system.     

Activation of C1

The first component of complement, C1, is a calcium-dependent complex of two loosely interacting subunits: C1q, responsible for the binding of activators to C1; C1r2-C1s2, which supports the autoactivation potential of C1, together with the proteolytic activity of activated C1- on its two substrates, C4 and C2. Isolated dimeric C1r2 is able to autoactivate through an intradimer cross-proteolysis; this capacity is lost when C1r2 is associated with two molecules of C1s inside the calcium-dependent C1r2-C1s2 subunit; this capacity is again observed in reconstituted C1. A model for reconstituted soluble C1 is proposed, based on electron microscopy, neutron diffraction, ultra-centrifugation, various biochemical findings, as well as functional properties of C1 or of its subcomponents. The flexible rod-like structure of C1r2-C1s2 is folded around two arms of C1q, with the catalytic domains of C1r and C1s inserted inside the cone defined by the C1q stalks. Activation of C1 which, in vivo, is controlled by C1 inhibitor, can be achieved by various activators, such as immune complexes; it appears to result from the suppression of a negative control and resides in a positive modulation of the intrinsic autocatalytic potential of C1r inside C1.     

C1q and C4b bind simultaneously to CR1 and additively support erythrocyte adhesion.

Previously, we showed that soluble C1q bound specifically to CR1 on transfected cells. If the CR1-C1q interaction were to participate in immune complex clearance, then this interaction should support E adhesion. Using a tip plate adhesion assay, we found that immobilized C1q mediated adhesion of human E. E binding to C1q was specifically inhibited by polyclonal anti-CR1 Fab fragments. Intact C1 was not efficient as an adherence ligand until it was treated with EDTA or the C1 inhibitor to remove the C1r2C1s2 complex from C1, leaving C1q. Titration of C1q alone, C4b alone, and C1q + C4b indicated that the two complement ligands were additive in their ability to support CR1-mediated adhesion of E. Analysis of binding to immobilized CR1 using a BIAcore instrument documented that C1q, C4b, and C3b binding were independent events. Additionally, C1q-dependent binding of immune complexes and heat-aggregated IgG to E was documented. These experiments confirm that the immune adherence receptor in humans, CR1, is the single receptor for all of the opsonic ligands of complement, provide evidence for a single C1q binding site on LHR-D of CR1, and suggest that C1q may participate in immune clearance.     

Nature of double-stranded DNA binding activity in seropositive rheumatoid arthritis: formation of low avidity DNA/rheumatoid factor/IgG/low density lipoprotein complexes

Sera from majority of patients with seropositive rheumatoid arthritis, which generally lacked detectable anti-double stranded DNA in Farr, Crithidia luciliae, and microcomplement fixation assays, exhibited high levels of dsDNA binding in the presence of 3.5% polyethylene glycol when using intrinsically labeled 3H-PM2 DNA as antigen. Except for SLE, such increased dsDNA binding was absent in normal and a variety of other disease sera, including those from patients with seronegative rheumatoid arthritis. In contrast to the situation in SLE, in which dsDNA binding is mediated by specific anti-DNA antibody, the increased dsDNA binding activity in seropositive rheumatoid arthritis was shown to be dependent upon complex low avidity interactions involving DNA, IgG, IgM rheumatoid factor, and low density lipoproteins. Analysis of the composition of the polyethylene glycol serum precipitates by 2-dimensional gel diffusion, immunoelectrophoresis, and sodium dodecyl sulfate polyacrylamide gel electrophoresis failed to reveal the presence of additional DNA-binding proteins unique to seropositive rheumatoid arthritis. The only feature distinguishing high DNA binding sera from those with low DNA binding activity was an increased amount of polyethylene glycol-insoluble IgG in the former, presumably reflecting IgG/IgG and/or IgG/IgM complexes. The significance of these unusual DNA/low density lipoprotein/IgG/rheumatoid factor complexes with respect to the diagnostic specificity and pathophysiology of the DNA/anti-DNA system is discussed.     

Immune immobilization of Treponema pallidum: antibody and complement interactions revisited

The Treponema pallidum immobilization test was designed for serodiagnosis of syphilis and is dependent upon specific antibody and a heat labile component of normal serum. Investigators have shown the component to be dependent upon divalent cations and it is presumed to be complement. Experiments were performed to reevaluate the interactions of antibody and complement and the mechanism of immobilization. The loss of treponemal motility was correlated to the loss of complement activity in the reaction mixture. When motility of treponemes incubated with immune serum IgG and complement had dropped to 50% (3.4 h), 72% of the available complement had been consumed. At the same time, treponemes incubated with normal serum IgG and complement were 82% motile and only 51% of the complement had been consumed. C6 deficient rabbit serum and C4 deficient guinea pig serum were used in conjunction with immune serum IgG to determine which components of the complement cascade were necessary for immobilization. Treponemes were not immobilized by either sera. Results suggest that the heat labile factor in normal sera is complement, that both early and late components of the complement cascade are necessary, and that the reaction proceeds via the classical complement pathway. Although T. pallidum is susceptible to the actions of antibody and complement, the organisms must interact with these components for at least 2 h before immobilization will result.     

Detection of serum antibodies against Chlamydia pneumoniae by ELISA

Abstract Chlamydia pneumoniae causes pneumonia and other respiratory infections in children, adolescents and adults. We tried to evaluate the diagnostic value of detection of serum antibodies by ELISA for C. pneumoniae infections in Japanese children. Serum IgG, IgA and IgM antibodies to C. pneumoniae were determined by the microimmunofluorescence (MIF) test. Serum IgG and IgA antibodies were also determined by ELISA test kits. Results obtained by ELISA were compared with those obtained by MIF test. IgG antibody to C. pneumoniae was detected in 135 (39.5%) by ELISA and in 125 (36.5%) by MIF out of 342 sera from Japanese infants and children without respiratory infections (aged from 2 months old to 15 years old). IgA antibody to C. pneumoniae was detected in 129 (37.7%) by ELISA and in 117 (34.2%) by MIF out of 342 sera tested. Of 342 specimens 113 were IgG-positive by ELISA and MIF (sensitivity: 90.4%, specificity: 89.9%, r = 0.853). Of 342 sera 28 had IgG antibody titers of 1:256 and none had titers 1:512 or higher by MIF. Of 28 infants and children a total of nine were less than 4 years of age. On the other hand, of 342 specimens 99 were IgA-positive by ELISA and MIF (sensitivity: 84.6%, specificity: 86.7%, r = 0.769). Of 342 sera 16 had IgA antibody titers of 1:256 or higher by MIF. Of 16 infants and children, ten were less than 4 years of age. ELISA had excellent sensitivity and specificity relative to MIF test for detection of IgC and IgA antibodies to C. pneumoniae . It was suggested that C. pneumoniae infection in Japanese infants and children under 4 years of age was not infrequent.     

Circulating immune complexes as possible cause for anticomplementary activity in humans with malignant melanoma

Summary Sera from 98 melanoma patients, 20 noncancer patients with immune complex-associated diseases, and 90 normal donors were analyzed for anticomplementary (AC) activity by the complement consumption method. Some of these sera were also tested for immune complex-like materials by the Raji cell radioimmune assay. In addition, serum samples from ten melanoma patients were analyzed serially to correlate the AC activity with clinical course. Significant levels of Ac activity were found in 45% of melanoma sera, 75% of nonmalignant immune complex-associated disease sera, and 10% of normal donors' sera. In some patients, AC activity decreased and became undetectable as their disease progressed. AC-negative serum samples taken from melanoma patients late in the course of disease when the tumor burden was large became anticomplementary when mixed with autologous or allogeneic serum samples taken earlier at the time of little or no tumor burden. The early serum samples contained antibodies against autologous tumor extracts, as shown by a complement fixation test. Absorption of early serum samples with cultured allogeneic melanoma cells reduced their ability to consume complement when mixed with autologous late serum samples, suggesting the presence of free antigen in the latter. The mixed samples of early and late sera and the sera positive in the complement consumption test contained heavy nonmonomeric IgG. Therefore, the AC activity of melanoma sera could be due to tumor-associated antigen-antibody complexes.     

Use of an IgG fragment prepared with particulate plasmin to study the C1 binding and activation.

Facb, fragment antigen and complement binding (this last property is shown when the fragment is involved in an immune complex); Fc, C-terminal half of the heavy-chain dimer; pFc′, major C-terminal fragment released from IgG by pepsin or plasmin digestion; DFP, diisopropylphosphofluoridate; ATEE, N-acetyl-L-tyrosine ethyl ester; BAEE, N-benzoyl-L-arginine ethyl ester; TAME, p-toluene sulfonyl-L-arginine methyl ester; SDS, sodium dodecyl sulfate; SBTI, soybean trypsin inhibitor.The nomenclature of the complement components (C1, C1q, C1r, C1s) follows the W.H.O. recommendations. Enzymatic activities are expressed in nanokatals (nkat.) as recommended in the Enzyme Nomenclature (1973).     

Complement activation by cell-associated immune complexes in contact sensitivity

Lymph node cells collected 4 days after painting the skin with picryl chloride activate the first components of the classical pathway of complement cascade, as shown by consumption of C4 of rabbit complement with total sparing of C5 and factor B activity. In contrast, lymph node cells collected 1 or 6 days after sensitization fail to do so. The ability of “4-day” cells to activate complement is inhibited by treating the cells with specific low-molecular-weight hapten, which is known to dissociate the immune complex present on the cell surface. When mouse serum was used as source of complement, a different behavior in complement activation between CBA/J and B.10.D2-New/SnJ serum was observed: “4-day” cells failed to consume CBA/J serum whereas a normal complement activation was detected when B.10.D2-New/SnJ serum was used. Using these two sera which differ in the level of C4, an inverse relationship between the ability of “4-day” cells to activate complement and their capacity to induce contact sensitivity when injected into the footpad of normal recipients was reported. Experiments performed using sera from C5 genetically deficient mice demonstrate that only the early complement components are involved, suggesting that membrane immune complexes are solubilized as a result of complement activation; on the other hand, membrane bound activated complement components could alter the immunizing potential of “4-day” cells.     

Presence of IgM Antibodies Which Sensitize HIV-1-Infected Cells to Cytolysis by Homologous Complement in Long-Term Survivors of HIV Infection

Although human cells are resistant to homologous human complement due to the presence of species-specific membrane inhibitors, a naturally occurring IgM antibody which recognizes an asialo-oligosaccharide can sensitize HIV-1-infected cells for complement-mediated cytolysis. Therefore, we investigated whether long-term survivors of HIV-1 infection harbor such antibodies in their sera. Thirty of 31 sera from HIV-1 seropositive hemophilia patients who have survived HIV-1 infection 10 years or more showed appreciable cytolytic activity, while only 2 sera of 10 seropositive patients presumed to have been infected with HIV-1 (due to sexual contact) more recently showed cytolytic activity. On the other hand, only 7 out of 43 sera from seronegative hemophilia patients showed cytolytic activity. Immunofluorescence staining for IgM on HIV-L -infected cells essentially correlated with the cytolytic capacity of the sera. Therefore, naturally occurring IgM antibodies and/or generated IgM antibodies reactive with the HIV-L -infected cells in patients might have been responsible for long-term survival due to complement-mediated immune cytolysis which may, in conjunction with cytotoxic T lymphocytes, synergistically suppress the infected cells in vivo. Therefore, the transfusion of such IgM antibodies could be effective for the treatment of HIV-L -infected individuals.     

High incidence of antinuclear antibodies that recognize the matrix attachment region.

The matrix attachment region (MAR) is a distinctive genomic DNA involved in a variety of nuclear processes through association with the nuclear matrix. Recent studies suggest that nuclear matrix is altered in the process of apoptosis and presented to the immune system, leading to the production of autoantibodies against its protein components. To see whether MARs are also recognized by autoantibodies, a collection of human sera containing antinuclear antibodies was screened for the presence of binding activities against cloned MARs. We found that MAR-binding activities are quite common in these sera. There was a positive correlation among the MAR-binding titers for three different MAR probes. As expected, the MAR-binding activity was copurified with serum IgG, and subclass analysis with affinity-purified IgG on MAR-Sepharose showed a predominance of IgG2 isotype. Several lines of evidence implied that the anti-MAR antibodies detected here is distinct from the ordinary anti-DNA antibodies that are reactive to bulk DNA.     

C1 inhibitor-dependent dissociation of human complement component C1 bound to immune complexes.

The interaction of C1 inhibitor with complement component C1 bound to immune complexes was examined by using 125I-labelled C1 subcomponents. The inhibitor binds rapidly to subcomponent C1s, and more slowly to subcomponent C1r. Formation of the C1r-C1 inhibitor complex causes rapid dissociation of subcomponents C1r and C1s from the antibody-antigen-component C1 aggregate. The rate and extent of this release are proportional to C1 Inhibitor concentration and are also dependent on ionic strength. Results obtained with purified C1 Inhibitor, plasma or serum as source of C1 Inhibitor are all closely comparable. Only slight dissociation of subcomponent C1q is observed under the same range of conditions. The implications of the release phenomenon are discussed in relation to the structure of component C1 and the possibility of differential turnover of C1 subcomponents.