Purification and characterization of human, guinea pig and mouse C1q by fast protein liquid chromatography (FPLC)

Complement Protein C1q Binds to Hyaluronic Acid in the Malignant Pleural Mesothelioma Microenvironment and Promotes Tumor Growth

C1q is the first recognition subcomponent of the complement classical pathway, which acts toward the clearance of pathogens and apoptotic cells. C1q is also known to modulate a range of functions of immune and non-immune cells, and has been shown to be involved in placental development and sensorial synaptic pruning. We have recently shown that C1q can promote tumor by encouraging their adhesion, migration, and proliferation in addition to angiogenesis and metastasis. In this study, we have examined the role of human C1q in the microenvironment of malignant pleural mesothelioma (MPM), a rare form of cancer commonly associated with exposure to asbestos. We found that C1q was highly expressed in all MPM histotypes, particularly in epithelioid rather than in sarcomatoid histotype. C1q avidly bound high and low molecular weight hyaluronic acid (HA) via its globular domain. C1q bound to HA was able to induce adhesion and proliferation of mesothelioma cells (MES) via enhancement of ERK1/2, SAPK/JNK, and p38 phosphorylation; however, it did not activate the complement cascade. Consistent with the modular organization of the globular domain, we demonstrated that C1q may bind to HA through ghA module, whereas it may interact with human MES through the ghC. In conclusion, C1q highly expressed in MPM binds to HA and enhances the tumor growth promoting cell adhesion and proliferation. These data can help develop novel diagnostic markers and molecular targets for MPM.

Classical complement pathway component C1q: purification of human C1q, isolation of C1q collagen-like and globular head fragments and production of recombinant C1q-derivatives. Functional characterization

The classical complement pathway (CCP) activation is a multimolecular complex, composed of three subcomponents namely C1q, C1r, and C1s. C1q is the recognition subunit of this complex and its binding to the specific targets leads to the formation of active C1, which in turn activates the CCP in an immunoglobulin-dependent or -independent manner. C1q is a hexameric glycoprotein composed of 18 polypeptide chains of three different types (A, B, and C), organized in two fragments-collagen-like (CLR) and globular head (gC1q) possessing different functional activity. The contemporary knowledge of the C1q structure allows the isolation and purification of a C1q molecule from serum by combination of different chromatography procedures including ion-exchange, size-exclusion, and affinity chromatography, as well as the isolation of CLR and gC1q by limited enzymatic hydrolysis of the native C1q molecule. In this chapter, we described methods for purification of human C1q and its CLR and gC1q fragments, as well as methods for their biochemical and functional characterization. The production and purification of recombinant C1q derivatives ghA, ghB, and ghC (globular fragments of the individual C1q chains) are also presented.

The complement system of the goat: haemolytic assays and isolation of major proteins

BACKGROUND

The aim of the present study was to develop a haemolytic assay for the study of the complement system in dairy goats (Capra aegagrus hircus) and to characterize the major goat complement system proteins.

RESULTS

The commonly used sheep erythrocyte sensitized with rabbit antibodies were not sensitive to lysis by goat serum, but the combination of human red blood cells (RBC) plus rabbit antibodies was the best option found for goat complement assay. A buffer based on HEPES instead of the classical veronal (barbitone) was developed. Three proteins were isolated: factor H, C1q and C3 and these were compared with the corresponding human proteins. A novel affinity chromatography technique was developed for isolation of factor H.

CONCLUSIONS

Human RBC plus rabbit antibodies were a suitable option for haemolytic assays. The isolated proteins are similar to the human counterparts.

Interaction of human C1q with IgG and IgM: revisited

The first step of activation of the classical complement pathway involves the binding of the globular C1q domain (gC1q) to the antigen-bound IgG or IgM. To improve our understanding of the mechanism of interaction of gC1q with IgG and IgM, we compared the immunoglobulin binding properties of single-residue mutants of individual globular modules of A and C chains. We found that Lys(A200) and Lys(C170) are significant for binding with both immunoglobulins. In addition, two C1q-specific scFv antibodies known as potent inhibitors of C1q-IgG and -IgM interactions were used in the epitope mapping analysis. A set of important residues, which participate in the C1q epitopes for scFv, were identified: Lys(C170) for the scFv3(V) epitope and Arg(B108) and Arg(B109) for the scFv10(V) epitope. The ability of scFv3(V) and scFv10(V) to bind preformed C1q-IgG or C1q-IgM complexes differed: scFv3(V) retained its ability to bind C1q, while scFv10(V) lost it. Given the different locations of the epitopes and the varying abilities of both antibodies to bind C1q-IgG and C1q-IgM complexes, we found that residues from the apical surface of C1q [where the scFv3(V) epitope was located] were involved in the initial recognition of IgG and IgM, while Arg(B108) and Arg(B109) are able to interact during the initial recognition as well as during the final binding of immunoglobulins. The reported results provide the first experimental evidence supporting the notion that apical and equatorial surfaces of gC1q have consecutive involvement following the gC1q reorientation during the interaction with specific C1q ligands.

Binding and activation of human and mouse complement by Cryptosporidium parvum (Apicomplexa) and susceptibility of C1q- and MBL-deficient mice to infection

Cryptosporidium parvum is a protozoan parasite (Apicomplexa) that causes gastrointestinal disease in animals and humans. Whereas immunocompetent hosts can limit the infection within 1 or 2 weeks, immunocompromised individuals develop a chronic, life-threatening disease. The importance of the adaptive cellular immune response, with CD4+ T-lymphocytes being the major players, has been clearly demonstrated. Several non-adaptive immune mechanisms have been suggested to contribute to the host defence, such as interferon-gamma (IFN-gamma) from NK cells, certain chemokines, beta-defensins and pro-inflammatory cytokines, but the influence of the complement systems has been less well studied. We analysed the in vitro binding and activation of the human and mouse complement systems and tested the susceptibility to infection in complement-deficient mouse strains. We found that C. parvum can activate both the classical and lectin pathways, leading to the deposition of C3b on the parasite. Using real-time PCR, parasite development could be demonstrated in adult mice lacking mannan-binding lectin (MBL-A/C-/-) but not in mice lacking complement factor C1q (C1qA-/-) or in wild type C57BL/6 mice. The contribution of the complement system and the lectin pathway in particular to the host defence against cryptosporidiosis may become apparent in situations of immunodeficiency such as HIV infections or in early childhood.

Localization of ligand-binding sites on human C1q globular head region using recombinant globular head fragments and single-chain antibodies

As a charge pattern recognition molecule, human C1q can bind a range of immunoglobulin and non-immunoglobulin ligands via its carboxy-terminal globular domain and activate the classical complement pathway. Each globular domain has a heterotrimeric organization, composed of the carboxy-terminal halves of one A (ghA), one B (ghB), and one C (ghC) chain. Recently, we have found that the recombinant forms of individual ghA, ghB and ghC bind differentially to IgG, IgM, gp41 peptide 601-613 of human immunodeficiency virus-1 (HIV-1), gp21 peptide 400-429 of human T cell lymphotrophic virus-I (HTLV-I), beta-amyloid peptide, and apoptotic cells, suggesting a modular organization of the globular domain. This paper examines the interaction of ghA, ghB and ghC with two known C1q ligands: Klebsiella pneumoniae porin OmpK36 and salivary agglutinin. In addition, we have used a panel of recombinant single-chain antibodies (scFv) specific for ghA, ghB and ghC in order to map sites on the heterotrimeric globular domain which are likely to interact with IgG1, IgG3, IgM, OmpK36, salivary agglutinin and gp41 loop peptide. The combined use of recombinant ghA, ghB, ghC and single-chain antibodies has revealed at least three ligand-binding sites on the globular domain of C1q: one is IgG- and OmpK36-specific, the second (IgM-binding site) is most likely overlapping with IgG/OmpK36 binding site, and the third (the gp41-binding site) seems to be located at the junction between the collagen and globular domains.

Anti-inflammatory drugs modulate C1q secretion in human peritoneal macrophages in vitro

The complement system plays an important role in the humoral immune response. Activation of the classical complement pathway is mediated by its subcomponent, C1q, which is involved in the pathogenesis of several autoimmune disorders. Among the main C1q-synthesising tissues, macrophages have been attributed as the main source. We investigated the effects of anti-inflammatory drugs (methylprednisolone and acetylsalicylic acid (ASA)) on C1q secretion in human peritoneal macrophages in vitro. The macrophages were isolated from peritoneal lavage fluid of patients with end-stage renal disease undergoing continuous ambulatory peritoneal dialysis, and were maintained in culture for up to 6 days. ASA decreased while methylprednisolone increased C1q secretion from human peritoneal macrophages in vitro, which correlated well with the percentage of CD14 positive cells after treatment. We conclude that different response of the macrophages to treatment with methylprednisolone and ASA may point out to the importance of macrophage activation after treatment, as well as an increased abundance of membrane C1q accompanied by increased phagocytosis.

Humoral autoreactivity directed against surfactant protein-A (SP-A) in rheumatoid arthritis synovial fluids

SP-A is found principally in the lung, and has been associated with lamellar bodies also found in the synovial joint. Both SP-A and C1q contain collagen-like regions, and SP-A and C1q have some structural similarities, both having a globular head region and a collagen-like tail. Here we are able to show that (i) autoreactivity to SP-A, as expressed by IgG and IgM autoantibodies, is present in synovial fluid (SF) isolated from patients with rheumatoid arthritis (RA); (ii) in absorption experiments only a limited degree of cross-reactivity between autoantibodies reactive with C1q and SP-A is observed; (iii) there is no cross-reactivity between autoantibodies reactive with type II collagen (CII) and those reactive with SP-A or C1q; (iv) autoantibodies react with polymeric (dimers and larger) SP-A, but not with monomeric SP-A subunits, indicating that a degree of quaternary structure is required for antibody binding. Unlike CII, which not accessible in the normal joint, both SP-A and C1q are available within the SF in patients with RA and may therefore provide antigens driving an autoimmune response directed against collagen-like structures.

Differential Binding of IgG and of a HIV gp41 Peptide by the B Chain and A Chain Globular Head Sequences of C1q, Respectively

Two individual globular head regions (ghA and ghB) of the heterotrimeric C1q molecule (containing A, B, and C chains) were expressed in a bacterial expression system using a coproduction with the bacterial chaperone GroESL. The purified proteins were soluble and monomeric, as shown by gel-filtration analysis. No association into homotrimers was seen, which indicates that the ability to form heterotrimers is coupled with the discrimination against homotrimeric self-association. The individual globular heads retained their binding activities toward two ligands bound by the whole C1q molecule, i.e., IgG and the peptide P(601–613) derived from the HIV envelope glycoprotein gp41. The differential binding activities displayed for these ligands indicated a degree of structural independence of the binding sites from the regions responsible for heterotrimerization. It was found, using single chain recombinant anti-C1q Abs, that the binding sites on C1q for IgG and gp41 do not overlap, and this observation is also consistent with the view that specialization between the C1q polypeptide chains takes place within the C1q molecule regarding their ligand-binding activities.

Isolation and characterization of the kininogen-binding protein p33 from endothelial cells. Identity with the gC1q receptor

Kininogens, the precursor proteins of the vasoactive kinins, bind specifically, reversibly, and saturably to platelets, neutrophils, and endothelial cells. Two domains of the kininogens expose major cell binding sites: domain D3 that is shared by H- and L-kininogen and domain D5H that is exclusively present in H-kininogen. Previously we have mapped the kininogen cell binding sites to 27 residues of D3 ("LDC27") and 20 residues of D5H ("HKH20"", respectively (Herwald, H., Hasan, A. A. K., Godovac-Zimmermann, J., Schmaier, A. H., and Müller-Esterl, W. (1995) J. Biol. Chem. 270, 14634-14642; Hasan, A. A. K., Cines, D. B., Herwald, H., Schmaier, A. H., and Müller-Esterl, W. (1995) J. Biol. Chem. 270, 19256-19261). The corresponding kininogen acceptor site(s) exposed by the cell surfaces are still poorly defined. Using a non-ionic detergent, Nonidet P-40, we have been able to solubilize kininogen binding sites from an endothelial cell line, EA.hy926, in their functionally active form. Affinity chromatography of the solubilized kininogen binding sites on HKH20, a synthetic peptide representing the D5H cell binding site, allowed us to isolate a 33-kDa protein ("p33") that binds specifically and reversibly to H-kininogen with a KD (apparent dissociation constant) of 9 +/- 2 nM. Preparative SDS electrophoresis followed by NH2-terminal amino acid sequence analysis identified the kininogen-binding protein p33 as the gC1q receptor ("gC1qR"), an extrinsic membrane protein that interacts with the globular domains of the complement component C1q. The purified p33 binds C1q with moderate affinity, KD = 240 +/- 10 nM. Recombinant expression of the corresponding cDNA in Escherichia coli demonstrated that p33 binds H-kininogen, but not L-kininogen. Peptide HKH20 but not peptide LDC27 inhibited binding of H-kininogen to the recombinant p33 in a concentration-dependent manner, indicating that H-kininogen binds to p33 via domain D5H. Recombinant p33 efficiently inhibited the binding of H-kininogen to EA.hy926 cells. Factor XII, but not prekallikrein, competed with H-kininogen binding to p33. These findings suggest that an endothelial binding protein mediates the assembly of critical components of the kinin-generating pathway on the surface of endothelial cells, thereby linking the early events of kinin formation and complement activation.

Autoreactivity to mouse C1q in a murine model of SLE

A large proportion of systemic lupus erythematosus (SLE) patients develop glomerulonephritis, coincident with the appearance of autoantibodies to C1q, the Fc-recognizing collagen-like subcomponent of the first component of complement, C1. The MRL/lpr/lpr mouse is an established model for SLE, developing both antinuclear and anti-type II collagen autoantibodies, and rheumatoid factors(s), exhibiting reduced complement levels and later on developing glomerulonephritis and often arthritis. We report here an age-dependent decrease in serum C1q levels coincident with the development of IgG2b autoantibodies reactive with mouse C1q in MRL/lpr/lpr mice. Unlike IgG2b, although high levels of IgM, IgG1 and IgG2a are present in these mice, few, if any, antibodies of these subclasses reactive with mouse C1q were observed in this study. This is the first report of autoantibodies against autologous C1q in an animal model, and the results should facilitate in clarification of the roles of C1q and autoantibodies reactive with C1q in SLE, as well as their potential connection with glomerulonephritis.

Characterization of a monoclonal antibody to the capsule of Haemophilus influenzae type b, generated by in vitro immunization

Monoclonal antibodies to polyribosylribitolphosphate (PRP), the capsular polysaccharide of Haemophilus influenzae type b (Hib), are useful tools in the investigation of the molecular and cellular mechanisms causing Hib meningitis. A better understanding of these mechanisms may lead to improved therapeutic strategies. A number of different in vivo immunization techniques in BALB/c mice were used, which did not however reveal detectable serum levels of antibodies to PRP. Therefore a modified in vitro immunization technique, originally established for in vitro immunization of human B lymphocytes, was used for this weak immunogen in mice. After 5 days of in vitro stimulation with purified PRP the splenic lymphocytes of BALB/c mice were fused with the mouse myeloma line P3-X63-Ag8.653. One hybridoma produced an IgM antibody (12E7) which recognized the capsular polysaccharide in ELISA and specifically labelled all tested Hib strains in immune fluorescent microscopy. The blotted polysaccharide PRP was immunostained with monoclonal antibody 12E7. Preincubation of Hib with this antibody enhanced the oxygen radical metabolism of polymorphnuclear leucocytes in a chemiluminescence assay. There was no cross-reactivity with the supernatants of other Haemophilus influenzae serotypes and other bacterial species, as shown by counterimmunoelectrophoresis.

C1q, the collagen-like subcomponent of the first component of complement C1, is a membrane protein of guinea pig macrophages

C1q, a subcomponent of C1--the first component of complement, is synthesized by macrophages (M phi). Immunofluorescence and immunoperoxidase studies first indicated the presence of C1q on the surface of guinea pig (gp) and human peritoneal M phi (Loos, M., Storz, R., Müller, W. and Lemmel, E. M., Immunobiology 1981. 158: 213). In our study different methods for labeling of gp serum and gp M phi C1q were employed. The presence of C1q protein on the surface of gp peritoneal M phi is shown by cell surface labeling with the biotin derivative sulfosuccinimdyl-6-(biotinamido)-hexanoate and subsequent immunoprecipitation. The mechanism by which C1q is attached to the cell membrane was also investigated. Intact cells were treated with acid stripping-buffers or phosphatidylinositol-specific phospholipase C and separated membranes were extracted with a buffer containing 1 M KCl and 3 M urea. Regardless of which method was used, C1q remained attached to the membrane. When surface-labeled cells were cultured, they were found to release the C1q from their surface membrane into the culture medium. Lysates of biosynthetically labeled cells were used to show that, like secreted or serum C1q, cellular M phi C1q binds to immobilized homologous IgG. This implies that the globular regions of the cellular C1q are functionally active. The results reveal that (i) cellular M phi C1q is firmly located in the membrane throughout the biosynthetic pathway, such that it is comparable with an integral membrane protein, (ii) cellular M phi C1q is not reversibly bound to the cell surface via a receptor. We suggest that C1q, as a membrane protein of M phi, serves as an Fc binding factor that also is secreted into the environment.

Rapid isolation and biochemical characterization of rat C1 and C1q

Using a human IgG-Sepharose column to which rabbit anti-human IgG was bound (rabbit anti-human/human IgG-Sepharose), human and rat C1 or C1q were isolated from serum in a single step, and the C1q further purified to homogeneity by FPLC. This procedure allowed the rapid isolation of haemolytically active C1 or C1q, with a yield equal to or greater than published methods. The availability of human and rat C1q allowed comparison of the two molecules, revealing differences in their mobility on SDS-PAGE as well as on agarose gel electrophoresis. Amino terminal sequence analysis demonstrated greater than 78% residue identity between rat C1q A, B and C chains and the published human and mouse sequences. Similar amino acid compositions suggest that the homology extends throughout the molecules. In addition to the major A:B and C:C dimer bands, rat, unlike human C1q, contained minor dimer species. These may reflect heterogeneity in glycosylation and or lysine and proline hydroxylation.

Determination of monoclonal antibody specificity by immunoadsorption and western blotting

A rapid and simple method has been developed for identifying the specificity of monoclonal antibodies at an early stage in the production of hybridomas. The technique is a micro-method utilizing biotinylated crude antigen and the surface of microtiter plates as an immunoaffinity matrix. The monoclonal antibodies to be tested are adsorbed to the microtiter wells and incubated with the labeled antigen preparation. Non-specific binding can be reduced by blocking and repeated washing steps. The specific antigen is then eluted by SDS-containing buffers, subjected to SDS-PAGE, blotted onto nitrocellulose and detected by enzyme-labeled avidin in a Western blot assay. The amount of bound and removed antigen can be quantitated by developing eluted and non-eluted control wells by ELISA techniques. Since this ELISA can be performed rapidly, only samples which contain sufficient specific material can be selected for electrophoresis and blotting. The major advantages of the technique are (i) the use of a non-radioactive label resulting in an easy and time-saving procedure, (ii) the possibility of quantitating the amount of captured and detached antigen by ELISA, (iii) the need for only a minimal amount of antigen, (iv) the use of unpurified antibodies of all isotypes, (v) a high signal-to-noise ratio, and (vi) as with all immunoprecipitation techniques, the possibility of detecting SDS-sensitive epitopes and of using crude antigen preparations. Using this method we were able to characterize monoclonal antibodies against both soluble proteins (mouse and human C1q) and membrane determinants (human pan T cell CD5 and CD7).

Modulation of type II collagen-induced arthritis in DBA/1 mice by intravenous application of a peptide from the C1q-A chain

In this report we are able to show that intravenous (i.v.) application (day 0) of a nonapeptide (residues 26-34) from the human C1q A-chain (designated peptide A-C1q) prior to intradermal (i.d.) administration of chicken type II collagen (CII) in arthritis-susceptible DBA/1 mice (H2q), leads to abrogation of polymorphonuclear neutrophil (PMN) invasion into the joints. This nonapeptide exhibits epitope characteristics and high homology to residues 137-147 of CB11 (a cyanogen bromide fragment of chicken CII, known to contain both arthritis inducing and suppressing determinants). Arthritis index was lowest in animals pretreated i.v. with CII (as internal control), though animals pretreated i.v. with peptide K (residues 137-147 with an additional glycine residue from CB11) or peptide A-C1q exhibited comparative arthritic indices. Only in the arthritis-positive control group (day 0: PBS i.v.) did i.d. application of CII lead to invasion of PMN into the synovial layer and the joint space. Analysis of antibody (Ab) responses at day 48 after i.v. immunization (day 0) and CII challenge (day 7) revealed IgE-Abs to native CII and also to native C1q. IgG titers to CII were highest in animals pretreated with peptide A-C1q. Abs from this group, exhibiting activity to peptide A-C1q (immunizing antigen), were of mainly IgG1 and IgG3 isotypes. Evaluation of the immune response following i.v. application of peptide A-C1q or CII, prior to i.d. CII administration, in DBA/1 mice, revealed IgM responses to peptide A-C1q and peptide K, but not to CII. Intravenous application of peptide A-C1q led to generation of IgG3-Abs reacting only with peptide A-C1q and peptide K, but not with native CII. Additionally, i.v. application of peptide A-C1q elicited IgG responses to a pentapeptide, resembling amino acid residues 26-30 (K-G-E-Q-G) of the C1q A-chain. This five residue antigenic determinant is present in peptide K, in chicken and human CII as well as in human C1q. No specific IgE response to any of the antigens tested could be detected. Since a peptide from the C1q A-chain is both capable of eliciting immune responses and modulating CII-induced arthritis in mice, we postulate that the collagen-like complement component C1q is involved in the development of CII-induced inflammatory arthritic lesions, and may represent, in vivo, the early antigen responsible for inducing anticollagen antibodies prior to CII in hyaline cartilage becoming available as antigen.

C1q-bearing immune complexes detected by a monoclonal antibody to human C1q in rheumatoid arthritis sera and synovial fluids

Using a monoclonal antibody directed against the C-chain of human C1q, we detected C1q-bearing immune complexes (IC) in sera and synovial fluids of rheumatoid arthritis (RA) patients. In a sandwich-ELISA, C1q-bearing IC were captured by the solid-phase monoclonal antibody and then detected with peroxidase-labeled F(ab')2-antibodies to either human IgG or IgM. The results of this assay were compared to an ELISA-modification of the C1q-solid-phase binding assay (C1q-SPBA). C1q-bearing IC were detected in 81.1% of RA-sera and the 65.2% of RA-synovial fluids. IgG as well as IgM was present in 72.6% of the sera and 70% of the synovial fluids which were positive in both assays. Most RA sera that were only positive for C1q-bearing IC, contained IgG alone (81.5%). The corresponding synovial fluids showed IgG alone (53%) or both IgG and IgM (41.1%). IgM alone (25%) could be detected in sera, e.g. in juvenile forms of RA. The levels of IC were higher in synovial fluid than in paired serum. In comparison to normal human serum (NHS) and patients with osteoarthritis, complement activity (CH50 titers) and C1q-values in patients with RA were frequently elevated. Since the formation of C1q-bearing IC is an indicator for the classical complement pathway activation, an assay with monoclonal anti-C1q antibody may be a useful tool in the diagnosis of rheumatoid diseases.

Studies on the interaction of C1q, a subcomponent of the first component of complement, with porins from Salmonella minnesota incorporated into artificial membranes

Purified outer membrane proteins (OMP) of Salmonella minnesota, Re-form, were incorporated into liposomes. These induced in macrophages a chemiluminescence signal identical to that of the intact Re-form. This signal was abolished by preincubation of porin-containing liposomes with purified C1q. Incorporation of isolated OMP into black lipid membranes (BLM) resulted in channel-formation which could not be inhibited by isolated C1q. Additionally, incubation of OMP-containing liposomes with BLM resulted in pore-formation within the BLM. This was amplified when lipid A was present within the liposomes. Preincubation of OMP-containing liposomes with purified C1q abolished pore-formation within the BLM.

[Autoantibodies against the complement component C1q in systemic lupus erythematosus]

Autoantibodies against C1q, a subcomponent of the first complement component C1, could be detected in 49.4% of sera from patients with systemic lupus erythematosus (SLE). They are directed against the collagen-like portion of the C1q molecule and recognize only bound, but not fluid-phase C1q. The appearance of these autoantibodies in the course of SLE correlates with the detection of IgG in the C1q-Solid-Phase-Binding-assay, with high titres of dsDNA-antibodies and with depressed total complement activity (CH50) and C1q-values. Our investigations show that autoantibodies against the collagen-like portion of bound C1q but not immune complexes are the main constituent of C1q-binding IgG in SLE.

Disulfide bridge formation between C1q and IgG in vitro

The globular heads of C1q are known to possess free-SH groups. Here we show that these groups, which are concealed in the native molecule, are exposed by interaction of C1q with dialysis membrane. During iodination, I+ and I2 oxidize these sulfhydryls to produce disulfide-linked C1q aggregates. Approximately 15% of C1q bound to immunoglobulin aggregates is resistant to high conductivity elution and reducing agent is required to release it. These data show that dialysis, adsorption to Ig and iodination of C1q result in structural and functional changes in the molecule, and suggest a mechanism by which these changes occur. Disulfide bridging between C1q and IgG in vitro suggests that this may be a normal physiological function of C1q for which the free cysteines of human, mouse and guinea pig C1q have been conserved.

Monoclonal antibodies to complement components without the need of their prior purification. I. Antibodies to mouse C1q

Rat monoclonal antibodies (MAbs) reactive with mouse complement subcomponent C1q were raised applying a principle that requires minute amounts of serum and circumvents purification of serum-derived C1q. The principle involves a) using the high affinity of certain cell-bound antibody isotypes for intercalating C1q from serum of various species, b) selecting such antibodies as are syngeneic to the immunized animal species, thus avoiding the production of antibodies against the intercalating antibodies and c) screening for the anti-C1q MAb in microtiter plates coated with C1q-intercalating MAb isotypes that are heterogeneic to the immunized animal species. We could establish 3 MAbs of IgM subclass, whose reactivity to mouse C1q was shown by ELISA techniques. One of them (RmC13C9) crossreacted with human C1q and was shown by SDS-PAGE and subsequent immunoblotting to recognize the C-chain of mouse and human C1q. The other two MAbs are directed against SDS-sensitive epitopes on mouse C1q and were, therefore, further characterized by native agarose gel electrophoresis and immunohistochemical studies.

Molecular cloning and characterization of the complementary DNA coding for the B-chain of murine Clq

cDNA clones coding for the B-chain of murine Clq were isolated from a mouse macrophage library. The characterized clones include the total coding region plus a leader sequence. High homology was found with human Clq B-chain in the coding region (81%). Northern blot analysis of total RNA from different tissues of Balb/c mice showed one band of approximately 1.2 kb. The highest signal was found in RNA preparations of thioglycolate-activated peritoneal macrophages. The probe also hybridized with mRNA from spleen, thymus and heart. Extremely weak signals were found in liver, kidney, lung and intestine tissues.

Common epitopes in Clq and collagen type II

An epitope common for collagen type II and Clq was demonstrated by specific binding of a monoclonal anti-collagen type II antibody, MAb B1, to purified Clq. This was further substantiated by the affinity shown between F(ab')2 fragments of anti-Clq antibodies and rat chondrosarcoma collagen type II. The interaction between MAb B1 and Clq was demonstrated in hemolytic assays, in an enzyme-linked biotin-avidin assay and by the binding of Clq to MAb B1 immobilized on Sepharose 4B beads. MAb B1 recognized only purified Clq and not the macromolecular Cl complex, indicating that the epitope for MAb B1 was situated in the collagen-like region in Clq, where Clq and Cls are anchored. The binding of the purified collagen-like fragment of Clq to radiolabelled MAb B1 confirmed these findings. The affinity between MAb B1 and Clq was significantly increased if Clq was first reacted with heat aggregated IgG, indicating a demasking of the reactive epitope on binding to the aggregated IgG. The present findings raise the question of the pathogenetic significance of the presence of anti-collagen type II antibodies and free Clq, both of which are frequently seen in high amounts in rheumatoid arthritis.

The use of dye-ligand affinity chromatography for the purification of non-enzymatic human plasma proteins

Literature data are analysed in this review on the use of immobilized triazine dyes for the characterization, isolation and purification of non-enzymatic human plasma proteins in both conventional and high-pressure liquid chromatography systems. Attention is focused on the mode of interaction between the dyes and these proteins, as well as on the advantages over previously reported techniques. Future developments are discussed.

Evidence for the presence of autoantibodies to the collagen-like portion of C1q in systemic lupus erythematosus

We investigated the connection between the C1q solid-phase binding assay (C1q SPBA) and double-stranded DNA antibodies, and analyzed the immune complex material in systemic lupus erythematosus (SLE) sera. Comparison with a new monoclonal assay for C1q-bearing immune complexes (the 242G3 assay) revealed that the immune complexes in SLE bind specifically to solid-phase C1q, and not to fluid-phase C1q. The C1q solid-phase binding activity sedimented as 7S IgG, was insensitive to DNase treatment, and could be selectively absorbed by C1q-coupled beads and by bovine serum albumin-anti-bovine serum albumin C1q beads, but not by DNA. Thus, antibodies to double-stranded DNA do not interfere in the C1q SPBA. Isolated IgG from SLE serum precipitated the collagen-like portions, and not the globular, Fc-recognizing portions, of C1q. F(ab')2 fragments of IgG from SLE patient serum were able to bind C1q. These data show that in SLE sera, especially in those with low levels of CH50 and C1q, autoantibodies that react with the collagen-like part of C1q are detectable. Since in the C1q SPBA, the C1q molecule is randomly fixed to the solid phase, we can detect not only immune complexes, but also antibodies that react with the collagen part of C1q; this may explain the high percentage of positive results for SLE sera in the C1q SPBA, in contrast to results of other immune complex assays.

Enzymatic alteration of C1q, the collagen-like subcomponent of the first component of complement, leads to cross-reactivity with type II collagen

Native serum C1q, the collagenous-like subcomponent of the first component of complement, is not recognized by polyclonal anti-collagen type II antibodies. However, when purified C1q was subjected to limited proteolysis by collagenase it showed antigenic cross-reactivity with collagen type II. The same cross-reactivity was observed with hemolytically active C1q in synovial fluids of patients with rheumatoid arthritis (RA), whereas C1q from synovial fluids of patients with osteoarthritis (OA), villo-nodular synovitis and ankylosing spondylitis was not recognized by this antibody. However, incubation of synovial fluid C1q of OA patients with synovial fluid leucocytes from RA patients led to an alteration of OA-C1q which was now recognized by the anti-collagen type II antibody.

Detection of C1q-bearing immune complexes by a monoclonal anti-C1q ELISA system

A monoclonal antibody directed against the collagenous portion of human C1q was used to detect C1q-bearing immune complexes in patients with rheumatic disorders. Sera of patients with rheumatoid arthritis, systemic lupus erythematosus (SLE), osteoarthritis, as well as normal human sera (NHS) used as controls were tested in an ELISA system. C1q-bearing immune complexes were bound to a solid-phase monoclonal anti-C1q antibody, and detected with F(ab')2 antibodies to human IgG. Heat-aggregated human IgG was adjusted to the same concentration as the WHO standard for immune complexes and used for the standard curve in NHS. The mean value in NHS was 19.5 micrograms/ml equivalents of aggregated IgG. Using 2 SD over the mean as the upper limit for normal values, samples greater than 43 micrograms/ml were considered positive. Patients with osteoarthritis were negative; high levels of C1q-bearing immune complexes were detected in patients with rheumatoid arthritis (up to 800 micrograms/ml equivalents of aggregated IgG). With our assay C1q-bearing immune complexes were detected with high frequency (81%) in the sera of patients with rheumatoid arthritis, while a C1q solid-phase binding assay (C1q SPBA) revealed positive results only in 67% of rheumatoid arthritis sera. Compared to NHS, CH50 titers and C1q values of sera from patients with rheumatoid arthritis were frequently high. In contrast, the sera of SLE patients with low CH50 titers and low C1q levels had IgG immune complexes which could be detected only in the C1q-SPBA. C1q-bearing immune complexes were not detectable in the sera of patients with SLE. Since C1q triggers activation of the classical C pathway, this assay with monoclonal anti-C1q antibody appears to be useful for detecting immune complexes in rheumatoid arthritis patients with normal or elevated CH50 and C1q values, especially in the early stage of the disease.

Simplified methods for the purification, quantitation, and functional estimation of human complement C-1-inhibitor (C-1-INH) with a monoclonal anti-C-1-INH antibody

New methods have been developed for the isolation, quantitative detection, and functional measurement of human complement C-1-inhibitor (C-1-INH). The two-step purification procedure for C-1-INH from human plasma or serum employs affinity chromatography with a monoclonal anti-C-1-INH antibody coupled to CNBr-activated Sepharose 4B followed by fractionation on a FPLC Mono Q HR 5/5 column. It yields functionally active, homogeneous C-1-INH with about 40% recovery. For quantitative estimation of C-1-INH an ELISA was performed. ELISA plates were coated with a polyclonal anti-C-1-INH antibody, serum or plasma was added and bound C-1-INH was detected with the monoclonal anti-C-1-INH antibody. The method has a sensitivity of 0.4 ng C-1-INH per assay corresponding to 20 ng/ml. For the detection of functionally active C-1-INH an ELISA was developed using C1-s-coated microtiter plates. After incubation with serum or plasma, C1-s-bound C-1-INH was monitored with the monoclonal anti-C-1-INH antibody. With this method it is possible to measure as little as 0.3 ng of functionally active C-1-INH in 20 microliter of a biological sample. All methods described in the present paper are easy to perform, rapid, sensitive, and highly reproducible.

Interaction of purified lipoteichoic acid with the classical complement pathway

Glycerophosphate-containing lipoteichoic acids (LTAs) interact with the first component of the classical complement pathway (C1). This resulted in the activation of the classical complement pathway in serum, shown by the consumption of C1, C2, and C4. The dose-dependent interaction of LTAs with purified C1 and C1q was dependent on the negative charges of the phosphate groups of LTA. It was reduced by charge compensation through D-alanine ester substituents and by sterical hindrance through di- and trihexosyl residues linked to position 2 of the glycerol moieties. The charge density of LTA may also play a role: poly(digalactosylglycerophosphate) LTAs, in which the phosphate groups are in a greater distance from each other, were less effective, and the loss of micellar organization by deacylation of LTA drastically reduced the complement activation capacity.

Contributions of C1q, bacterial lipopolysaccharide, and porins during attachment and ingestion phases of phagocytosis by murine macrophages

In contrast to the S-form of Salmonella minnesota, its Re mutant binds to mouse peritoneal macrophages. The binding reaction triggers an oxidative burst, measured by a chemiluminescent reaction. The oxidative burst was abolished in the presence of either purified lipopolysaccharide or porins (outer membrane proteins) extracted from the Re mutant, suggesting that both components are involved in binding of the Re mutant to macrophages. In addition, Fc-recognizing membrane structures on the macrophage surface bind the Re mutant. Preincubation of macrophages with the Re mutant abolishes immunoglobulin G-sensitized erythrocyte-induced chemiluminescence. Macrophages preincubated with immunoglobulin G-sensitized erythrocytes had a low chemiluminescent signal, and after treatment of the cells with the Re mutant, there was an additional, higher signal. Binding of purified C1q to the Re mutant decreased the adherence of the Re mutant to macrophages, resulting in a diminished chemiluminescent signal. Blocking of endogenous macrophage membrane-associated C1q with a monoclonal antibody [F(ab')2 fragment] directed against mouse macrophages (recognizes the A and B chains of C1q) diminished the oxidative burst. Therefore, the endogenous C1q of macrophages also appears to be involved in attachment of the S. minnesota Re mutant.

Enzyme-linked immunosorbent assay for C1q in human serum by use of monoclonal antibodies

A sandwich ELISA system has been developed for the detection of C1q in human serum. It is specific, uses monoclonal antibodies, is sensitive into the nanogram range and is rapidly performed. Therefore, it may be a helpful tool for clinical routine diagnosis, e.g., detecting abnormal C1q levels in patients with rheumatic disorders. Various combinations of poly- and monoclonal antibodies were tested in a sandwich assay. One of these combinations, in particular, resulted in a highly reproducible standard curve: C1q bound to solid-phase polyclonal anti-C1q was detected by the monoclonal antibody 242 G3. In this assay, the C1q concentration in sera of normal individuals was found to be 160 micrograms/ml (mean value of 70 normal human sera). This ELISA detected nanogram levels of C1q and gave results comparable to those obtained by haemolytic C1q titration. One nanogram of C1q corresponded to ca. 2.6 X 10(10) effective C1q molecules. With this technique, selective C1q deficient sera as well as sera from patients with rheumatoid diseases were analysed.