Activation of the complement system by antibody-antigen complexes: the classical pathway

The Role of the Complement in Clear Cell Renal Carcinoma (ccRCC)-What Future Prospects Are There for Its Use in Clinical Practice?

In recent years, the first-line available therapeutic options for metastatic renal cell carcinoma (mRCC) have radically changed with the introduction into clinical practice of new immune checkpoint inhibitor (ICI)-based combinations. Many efforts are focusing on identifying novel prognostic and predictive markers in this setting. The complement system (CS) plays a central role in promoting the growth and progression of mRCC. In particular, mRCC has been defined as an "aggressive complement tumor", which encompasses a group of malignancies with poor prognosie and highly expressed complement components. Several preclinical and retrospective studies have demonstrated the negative prognostic role of the complement in mRCC; however, there is little evidence on its possible role as a predictor of the response to ICIs. The purpose of this review is to explore more deeply the physio-pathological role of the complement in the development of RCC and its possible future use in clinical practice as a prognostic and predictive factor.

A nanobody-based complement inhibitor targeting complement component 2 reduces hemolysis in a complement humanized mouse model of autoimmune hemolytic anemia

C2 is an attractive therapeutic target for many complement-mediated diseases. We developed Nab1B10, a new anti-C2 nanobody that potently and selectively inhibits both the classical and lectin pathways of complement activation. Mechanistically, Nab1B10 binds to the C2a portion of C2 and inhibits the assembly of C3 convertase C4b2a. Nab1B10 cross-reacts with monkey but not rodent C2 and inhibits classical pathway-mediated hemolysis. Using a new complement humanized mouse model of autoimmune hemolytic anemia (AIHA), we demonstrated that Nab1B10 abolished classical pathway complement activation-mediated hemolysis in vivo. We also developed C2-neutralizing bi- and tetra-valent antibodies based on Nab1B10 and found these antibodies significantly more potent than the other anti-C2 monoclonal antibody that is already in clinical trials. These data suggest that these novel C2-neutralizing nanobodies could be further developed as new therapeutics for many complement-mediated diseases, in which pathogenesis is dependent on the classical and/or lectin pathway of complement activation.

Depletion of γδ T Cells Leads to Reduced Angiogenesis and Increased Infiltration of Inflammatory M1-like Macrophages in Ischemic Muscle Tissue

γδ T cells, a small subset of T cells in blood, play a substantial role in influencing immunoregulatory and inflammatory processes. The functional impact of γδ T cells on angiogenesis in ischemic muscle tissue has never been reported and is the topic of the present work. Femoral artery ligation (FAL) was used to induce angiogenesis in the lower leg of γδ T cell depleted mice and wildtype and isotype antibody-treated control groups. Gastrocnemius muscle tissue was harvested 3 and 7 days after FAL and assessed using (immuno-)histological analyses. Hematoxylin and Eosin staining showed an increased area of tissue damage in γδ T cell depleted mice 7 days after FAL. Impaired angiogenesis was demonstrated by lower capillary to muscle fiber ratio and decreased number of proliferating endothelial cells (CD31+/BrdU+). γδ T cell depleted mice showed an increased number of total leukocytes (CD45+), neutrophils (MPO+) and neutrophil extracellular traps (NETs) (MPO+/CitH3+), without changes in the neutrophils to NETs ratio. Moreover, the depletion resulted in a higher macrophage count (DAPI/CD68+) caused by an increase in inflammatory M1-like macrophages (CD68+/MRC1−). Altogether, we show that depletion of γδ T cells leads to increased accumulation of leukocytes and M1-like macrophages, along with impaired angiogenesis.

Plasma complement activation mechanisms differ in ornate (Terrapene ornata ornata) and eastern box turtles (Terrapene carolina carolina)

Eastern (Terrapene carolina carolina) and ornate (Terrapene ornata ornata) box turtles have robust plasma antibacterial activity, however, the mechanism behind this activity is unknown. We used sheep red blood cell (SRBC) hemolysis assays, mannan-affinity chromatography, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) to explore the mechanisms of complement activity in box turtles. Plasma from both species demonstrated volume, time, and temperature-dependent SRBC hemolysis, with significantly greater hemolytic activity in ornate box turtle plasma. Hemolytic activity was highly attenuated following treatment with heat, EDTA, and salicylaldoxime in both species, but was unchanged after treatment with methylamine and ammonium hydroxide. Two abundant mannan-binding proteins (presumed C-type lectins) were identified in eastern box turtle plasma using SDS-PAGE and MALDI-TOF, but ornate box turtles did not express either protein. Eastern box turtles appear to rely on the lectin pathway of complement activation while ornate box turtles utilize the alternative pathway. This study provides further evidence that mechanisms underlying immune function are not always conserved between closely related species. This finding may have important implications for explaining species differences in susceptibility to emerging threats such as disease, toxicants, and climate change.

Complement proteins detected through iTRAQ-based proteomics analysis of serum from black carp Mylopharyngodon piceus in response to experimentally induced Aeromonas hydrophila infection

The black carp Mylopharyngodon piceus is one of the culturally important '4 famous domestic fishes' in China. Recently, infectious diseases caused by Aeromonas hydrophila have drastically altered the operation of the black carp farming industry. In the present study, isobaric tags for relative and absolute quantitation (iTRAQ) were combined with mass spectrometry analysis to screen for differentially abundant black carp serum proteins in response to experimentally induced A. hydrophila infection. A total of 86 differentially abundant proteins were quantified at 24 h post-infection, including 78 down-regulated proteins and 8 up-regulated proteins. The down-regulated proteins included complement C1q subcomponent subunit C, complement factor B/C2A, complement factor B/C2B, complement C3-Q1, complement C3, and complement C4-2. Bioinformatic analysis indicated that the differentially abundant proteins were mainly associated with complement and coagulation cascades (27.9%). Moreover, real-time PCR (qPCR) analysis revealed changes in the gene expression of both C3 and B/C2A in blood cells, liver, kidney, gills, and intestines of the black carp infected with A. hydrophila. However, mRNA expression levels did not consistently correlate with the corresponding protein levels. A polyclonal antibody was prepared using a synthetic C3 peptide. Immunofluorescence analysis showed that the expression of C3 in the kidney was increased with A. hydrophila infection. This work provides a useful characterization of the impact of A. hydrophila infection on the complement system of the black carp.

Kaposi's Sarcoma-Associated Herpesvirus and Host Interaction by the Complement System

Kaposi’s sarcoma-associated herpesvirus (KSHV) modulates the immune response to allow the virus to establish persistent infection in the host and facilitate the development of KSHV-associated cancer. The complement system has a central role in the defense against pathogens. Hence, KSHV has adopted an evasion strategy for complement attack using the viral protein encoded by KSHV open reading frame 4. However, despite this defense mechanism, the complement system appears to become activated in KSHV-infected cells as well as in the region surrounding Kaposi’s sarcoma tumors. Given that the complement system can affect cell fate as well as the inflammatory microenvironment, complement activation is likely associated with KSHV pathogenesis. A better understanding of the interplay between KSHV and the complement system may, therefore, translate into the development of novel therapeutic interventions for KSHV-associated tumors. In this review, the mechanisms and functions of complement activation in KSHV-infected cells are discussed.

Different mechanisms of serum complement activation in the plasma of common (Chelydra serpentina) and alligator (Macrochelys temminckii) snapping turtles

Reptiles are declining worldwide yet our understanding of their immune function lags far behind other taxa. The innate immune system is the primary mode of defense in reptiles, and the serum complement cascade is its major component. We assessed serum complement activity of plasma in two closely related aquatic turtle species, the common snapping turtle (CST; Chelydra serpentina) and alligator snapping turtle (AST; Macrochelys temminckii). We used a sheep red blood cell (SRBC) hemolysis assay to assess serum complement activity. Although the antibacterial activities of the plasma of these turtle species are similar, the hemolytic activity was much stronger in CST than AST. Treatment with inhibitors of the serum complement cascade indicated differences in the mechanisms of complement activation between the turtle species. We subjected plasma from both turtle species to mannan affinity chromatography and analyzed the eluate with SDS-PAGE, which revealed that plasma from the CSTs contained only small amounts of one C-type lectin protein while the AST plasma contained high concentrations of two C-type lectins (31.0 and 35.9 kDa). Edman degradation analyses confirmed that the two AST proteins contained identical N-terminal sequences. Thus, the CST appears to rely more heavily on the alternative mechanism of serum complement activation, while the AST appears to rely more on the lectin-mediated pathway, which is a pattern recognition response to prokaryotes not activated by the SRBCs. These results are unique in that the use of serum complement pathways are generally assumed to be conserved within clades.

Anti-Toxoplasma gondii antibodies attach to mouse cancer cell lines but not normal mouse lymphocytes

Toxoplasma gondii (T. gondii) is prevalent intracellular parasite and a cause of worldwide infection in the human population. An inhibitory effect of this parasite on cancer growth has been demonstrated in cell culture and animal models. To determine whether the anticancer activities of T. gondii are associated with host immune response, in the current study the reactivity of anti-T. gondii antiserum with the surface of cancer cell lines was investigated. Anti-T. gondii antibodies were raised in rabbit and the reaction of this antiserum in comparison with other anti-parasite antisera (anti-T. vaginalis, anti-hydatid cyst fluid, anti-protoscolices antigens) with mouse melanoma or breast cancer cells lines was investigated using flow cytometry. Anti-T. gondii antiserum reacted markedly with the surface of mouse melanoma and breast cancer cells, and less so with the normal mouse spleen lymphocytes. Meanwhile, the other anti-parasite antisera did not react strongly with the surface of cancer cells compared with normal mouse spleen lymphocytes. In summary, it has been demonstrated herein that anti-T. gondii antiserum may selectively react with the surface of mouse cancer cells but not with normal mouse spleen lymphocytes. Therefore, further study on anti-Toxoplasma antibodies may be useful for directing the application of selective drug delivery in cancer treatment.

Fc microparticles can modulate the physical extent and magnitude of complement activity

The complement system is an integral component of the humoral immune system, and describes a cascade of interacting proteins responsible for the opsonization and lysis of foreign pathogens, in addition to the recruitment of immune cells. However, complement activation is also implicated in the progression and complication of immune dysfunctions such as sepsis. Microparticle (MP) biomaterials capable of tuning the local magnitude of serum complement activation could improve complement-mediated cytotoxicity to serum-resistant bacteria or calm an overactive immune response during sepsis. We demonstrate that model Fc-functionalized microparticles can be designed to either enhance or diminish the local cytotoxic effect of complement activation in human serum. The particles were formed with either the antibody Fc domains oriented outward from the particle surface or randomly adsorbed in a non-oriented fashion. In the oriented Fc form, complement products were directly sequestered to the particle surface, including C5a, a potent anaphylatoxin that, when elevated, is associated with poor sepsis prognosis. The oriented particle also lowered the cytotoxicity of serum and thus decreased the antibiotic effect when compared to serum alone. Conversely, the non-oriented microparticles were found to sequester similar levels of C5a, but much lower levels of iC3b and TCC on the microparticle surface, thereby increasing the amount of the soluble terminal complement complex. In addition, the non-oriented microparticles extend the distance over which TCC forms and enhance serum cytotoxicity to bacteria. Together, these two types of complement-modulating particles provide the first biomaterial that can functionally modify the range of complement activation at sites distant from the particle surface. Thus, biomaterials that exploit Fc presentation provide new possibilities to functionally modulate complement activation to achieve a desired clinical result.

The Complement System in Neuropathic and Postoperative Pain

Certain types of pain are major unmet medical needs that affect more than 8 percent of the population. Neuropathic pain can be caused by many pathogenic processes including injury, autoimmune disease, neurological disease, endocrine dysfunction, infection, toxin exposure, and substance abuse and is frequently resistant to available pain therapies. The same can be said of postsurgical pain, which can arise from uncontrolled inflammation around the wound site. The complement system is part of the innate immune system and can both initiate and sustain acute and chronic inflammatory pain. Here we review the complement system and original investigations that identify potential drug targets within this system. Drugs that act to inhibit the complement system could fill major gaps in our current standard of care for neuropathic pain states.

An update on ABO-incompatible kidney transplantation

ABO-incompatible kidney transplantation is nowadays a well-established procedure to expand living donor transplantation to blood group incompatible donor/recipient constellations. In the last two decades, transplantation protocols evolved to more specific isohaemagglutinin elimination techniques and established competent antirejection protection protocols without the need of splenectomy. ABOi kidney transplantation associated accommodation despite isohaemagglutinin reappearance, C4d positivity of peritubular capillaries as well as the increased incidence of bleeding complications is currently under intense investigation. However, most recent data show excellent graft survival rates equivalent to ABO-compatible kidney transplantation outcome.

TUNABLE COMPLEMENT ACTIVATION BY PARTICLES WITH VARIABLE SIZE AND Fc DENSITY

The complement system is an integral innate immune component that is made up of a cascade of enzymatic proteins that, once activated, results in lysis of invading pathogens, opsonization or recruitment of other innate and/or acquired immune responders, or some combination of the three. Due to the importance of the signal amplification and control points present in the cascade, complement is highly sensitive to subtle variations in initiation conditions, including nanoscale changes to molecular spacing. Using Fc-functionalized microparticles and nanoparticles, we find that activation requires a minimum threshold surface concentration of Fc of at least 20% surface coverage. This result indicates that a high surface density Fc is necessary for micro/nanoparticle complement activation through the classical pathway. In addition, the magnitude of the response was dependent on the size of the particle, with larger particles causing decreased activation. We hypothesize that a high density of Fc is needed to efficiently bind and closely appose molecular initiators of the complement cascade, from initiation to terminal complement complex formation. These fundamental studies of the interaction of microparticles and nanoparticles with the immune system suggest design rules for particle size and molecular density that impact immunostimulation through the complement system. Providing a therapeutic agent to modulate the complement response could aid a variety of treatment strategies. Engineered nanoparticles with controlled gaps between molecular activators could lead to new types of immunomodulatory agents.

Innate immunity and neuroinflammation

Inflammation of central nervous system (CNS) is usually associated with trauma and infection. Neuroinflammation occurs in close relation to trauma, infection, and neurodegenerative diseases. Low-level neuroinflammation is considered to have beneficial effects whereas chronic neuroinflammation can be harmful. Innate immune system consisting of pattern-recognition receptors, macrophages, and complement system plays a key role in CNS homeostasis following injury and infection. Here, we discuss how innate immune components can also contribute to neuroinflammation and neurodegeneration.

The Complement System in Pregnancy

Maternal recognition of fetal antigens is well-documented. Antibodies directed against fetal antigens are the rule rather than the exception. Maternal antibodies seem to bind fetal antigens at the placental level and apparently activate the complement system even in normal pregnancy, yet multiple studies confirm an increase in C1q, C4, C3 and CH50 levels in pregnancy and an absence of complement activation. More sensitive assays and a broadened concept of the timing of crucial immunologic events may lead to a greater understanding of the importance of the complement system in pregnancy.

Preformed complement-activating low-level donor-specific antibody predicts early antibody-mediated rejection in renal allografts

BACKGROUND.: Donor-specific anti-HLA antibodies (DSA) are a major cause of alloimmune injury. Transplant recipients with negative complement-dependent cytotoxic crossmatch (CDC-XM) and donor cell-based flow cytometric crossmatch (flow-XM) but low level DSA (i.e., by Luminex) have worse outcomes compared with nonsensitized patients. The aim of this study was to establish whether complement-activating ability in this low-level DSA, present before transplantation, as determined by this technique is important in dictating pathogenicity. METHODS.: We retrospectively studied 52 patients with preformed DSA detected by single-antigen flow cytometric fluorescent beads (SAFBs). Patients were transplanted using a steroid-sparing regimen consisting of alemtuzumab induction, 1 week of corticosteroids and tacrolimus monotherapy.Fifteen (29%) of 52 patients experienced antibody-mediated rejection (AMR), whereas 37 (71%) patients did not. There were no demographic differences between patients with AMR and those without. Pretransplant sera were retested using a modified (SAFB) assay, which detects the presence of the complement fragment C4d as a result of DSA-induced complement activation. RESULTS.: C4d+DSA were detected in 10 (19%) of 52 patients. Biopsy-proven AMR occurred in 7 (70%) of the 10 patients with C4d+DSA and in 8 (19%) of 42 patients with C4d-DSA. AMR-free survival was worse in patients with C4d+DSA (P<0.001). CONCLUSIONS.: The ability of preformed, low-level, DSA to trigger C4d fixation in vitro in patients with negative conventional crossmatch tests is predictive for AMR. C4d SAFB is potentially a powerful tool for risk stratification prior to transplantation and may allow identification of unacceptable donor antigens, or patients who may require enhanced immunosuppression.

Familial atypical hemolytic uremic syndrome: a review of its genetic and clinical aspects

Atypical hemolytic uremic syndrome (aHUS) is a rare renal disease (two per one million in the USA) characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. Both sporadic (80% of cases) and familial (20% of cases) forms are recognized. The study of familial aHUS has implicated genetic variation in multiple genes in the complement system in disease pathogenesis, helping to define the mechanism whereby complement dysregulation at the cell surface level leads to both sporadic and familial disease. This understanding has culminated in the use of Eculizumab as first-line therapy in disease treatment, significantly changing the care and prognosis of affected patients. However, even with this bright outlook, major challenges remain to understand the complexity of aHUS at the genetic level. It is possible that a more detailed picture of aHUS can be translated to an improved understanding of disease penetrance, which is highly variable, and response to therapy, both in the short and long terms.

Complement Activation in Renal Disease

Perturbation of the serum complement system in glomerulonephritis can be the result of classical pathway activation by immune complexes or of C3 activation by the C3-C5 convertase stabilized by the C3 nephritic factor. Low C3 levels can also be the result of diminished C3 synthesis and possibly, in certain circumstances, of C3 convertases deposited on capillary walls. In glomerulonephritis the complement profile is helpful in diagnosis, in following the course of therapy and in providing insights into pathogenesis. Complement profiles must be interpreted recognizing that a pattern resembling classical pathway activation can be produced by idiopathic nephrotic syndrome, that hypogammaglobulinemia can reduce Clq levels, and that a primary deficiency of factor H or I or both, will secondarily produce subnormal levels of C3 and factor B. With these caveats, the complement profiles typical of systemic lupus erythematosus, membranoproliferative glomerulonephritis, acute glomerulonephritis, acquired Cl inhibitor deficiency, and hypocomplementemic vasculitis syndrome are described.

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

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

On-chip Complement Activation Adds an Extra Dimension to Antigen Microarrays

Antibody profiling on antigen microarrays helps us in understanding the complexity of responses of the adaptive immune system. The technique, however, neglects another, evolutionarily more ancient apparatus, the complement system, which is capable of both recognizing and eliminating antigen and serves to provide innate defense for the organism while cooperating with antibodies on multiple levels. Complement components interact with both foreign substances and self molecules, including antibodies, and initiate a cascade of proteolytic cleavages that lead to the covalent attachment of complement components to molecules in nanometer proximity. By refining the conditions of antibody profiling on antigen arrays we made use of this molecular tagging to identify antigens that activate the complement system. Antigen arrays were incubated with serum under conditions that favor complement activation, and the deposited complement C3 fragments were detected by fluorescently labeled antibodies. We used genetically C3-deficient mice or inhibition of the complement cascade to prove that the technique requires complement activation for the binding of C3 to features of the array. We demonstrate that antigens on the array can initiate complement activation both by antibody-dependent or -independent ways. Using two-color detection, antibody and complement binding to the relevant spots was measured simultaneously. The effect of adjuvants on the quality of the immune response and binding of autoantibodies to DNA with concomitant complement activation in the serum of mice suffering from systemic autoimmune disease was readily measurable by this new method. We propose that measurement of complement deposition on antigen microarrays supplements information from antibody binding measurements and provides an extra, immune function-related fingerprint of the tested serum.

Effect of cholesterol content in activation of the classical versus the alternative pathway of rat complement system induced by hydrogenated egg phosphatidylcholine-based liposomes

Liposomes composed of hydrogenated egg phosphatidylcholine (HEPC) and cholesterol (CHOL) were found to activate the rat complement (C) system in a CHOL content-dependent manner. Liposomes containing 22 or 33 mol% CHOL activated the C system in a Ca(2+)-dependent manner, suggesting that C activation occurred via the classical pathway. Liposomes containing 44 mol% CHOL activated the C system in a Ca(2+) independent manner, suggesting that C activation occurred via the alternative pathway. The CHOL content appeared to dictate the pathway by which the C system was activated. This C activation was inhibited by removal of serum component(s), which adsorb to the liposomes. Activation of the alternative pathway, induced by the liposomes, was reduced by the depletion of IgG and IgM, whereas the classical pathway activation was reduced by the depletion of IgG, but not IgM. In addition, the removal of adsorbed serum component(s) by treatment with 44 mol% CHOL-containing liposomes decreased serum IgG and IgM levels that adsorb to the same liposomes, whereas the removal of adsorbed serum component(s) by treatment with 22 mol% CHOL-containing liposomes only slightly decreased serum IgG levels, which adsorbs to the same liposomes. Collectively, both IgG and IgM, which are specifically adsorbed to the liposomes in a CHOL-content dependent manner, were responsible for C activation via the alternative pathway induced by the 44 mol% CHOL containing liposomes. IgG alone would be partially responsible for C activation via the classical pathway induced by 22 or 33 mol% CHOL-containing liposomes. The discovery of this unique C-activating property of liposomes will be of value in attempts to decipher the underlying mechanism of C activation by providing a useful model membrane system.

Biodistribution of liposomes and C3 fragments associated with liposomes: evaluation of their relationship

The biodistribution of liposomes with two different kind phospholipids (hydrogenated egg phosphatidylcholine and egg phosphatidylcholine) plus cholesterol (CHOL) were investigated after intravenous administration to rats. Elimination of liposomes from blood circulation was affected by the lipid composition. It appeared that the inclusion of CHOL in liposomes accelerates the rate of liposome uptake by liver, resulting in rapid elimination of liposomes. The amount of C3 fragments bound to liposomes was quantitatively determined to assess the contribution of the complement system to liposome accumulation into organs and liposome destabilization in vivo and in vitro. The amount of bound C3 fragments was directly proportional to CHOL content, and the amount was also proportional to the CLh, CLs as well as CLrel. This relationship suggests that the complement system is responsible for the elimination of liposomes from blood circulation, presumably as a consequence of opsonization by C3 fragments and assembly of membrane attack complex (MAC) onto liposomes. In addition, substitution of cholesteryl methyl ether into the liposome formulation for CHOL significantly diminished not only the binding of C3 fragments but also the CLh, CLs and CLrel, resulting in increased mean resident time (MRT) of the liposomes. This result suggests that the hydroxyl-group on CHOL is a binding site for C3 fragments on the liposomes and that CHOL in a liposome formulation promotes the accumulation of liposomes into the liver and spleen, probably due to their uptake by phagocytic cells, and impairs the stability of the liposomes in blood circulation, via a mechanism involving the complement system.

Complement activation promotes muscle inflammation during modified muscle use

Modified muscle use can result in muscle inflammation that is triggered by unidentified events. In the present investigation, we tested whether the activation of the complement system is a component of muscle inflammation that results from changes in muscle loading. Modified rat hindlimb muscle loading was achieved by removing weight-bearing from the hindlimbs for 10 days followed by reloading through normal ambulation. Experimental animals were injected with the recombinant, soluble complement receptor sCR1 to inhibit complement activation. Assays for complement C4 or factor B in sera showed that sCR1 produced large reductions in the capacity for activation of the complement system through both the classical and alternative pathways. Analysis of complement C4 concentration in serum in untreated animals showed that the classical pathway was activated during the first 2 hours of reloading. Analysis of factor B concentration in untreated animals showed activation of the alternative pathway at 6 hours of reloading. Administration of sCR1 significantly attenuated the invasion of neutrophils (-49%) and ED1(+) macrophages (-52%) that occurred in nontreated animals after 6 hours of reloading. The presence of sCR1 also reduced significantly the degree of edema by 22% as compared to untreated animals. Together, these data show that increased muscle loading activated the complement system which then briefly contributes to the early recruitment of inflammatory cells during modified muscle loading.

Inhibitory effect of bilirubin on complement-mediated hemolysis

We investigated the in vitro action of the bile pigments, unconjugated bilirubin (UB) and bilirubin monoglucuronide (BMG) on complement (C) cascade reaction. Both UB and BMG inhibited hemolysis in the classical pathway (CP) in a dose-dependent manner at low micromolar concentrations, UB showing a stronger effect than BMG. The analysis of the action of UB on the hemolytic activity of the C1, C4, C2 and C-EDTA components of the C cascade revealed that the C1 step was the most inhibited. An enzyme immunoassay was developed to evaluate the effect of UB on the binding of C1q, one of the subcomponents of C1, to human IgM and IgG. The study demonstrated that the unconjugated pigment interferes both the C1q-IgM and -IgG interactions, thus tentatively explaining the inhibitory action of UB on hemolytic activity of C1. We conclude that the anti-complement effect of UB is mainly exerted on the C1 component, the recognition unit of CP. The potential clinical implication of the reported effects in hyperbilirubinemia is discussed.

C1 inhibitor removes the entire C1qr2s2 complex from anti-C1Q monoclonal antibodies with low binding affinities

Evidence is presented for a new C1 Inhibitor (C1 INH) function. C1 INH was capable of dislodging the entire C1qr2s2 complex from C1-activating substances that bound weakly to the globular heads of C1q. Two different mouse IgG1 monoclonal antibodies with different affinities for C1q globular heads were compared for their complement-activating properties in the presence of normal human serum. As expected the higher affinity monoclonal antibody (Qu) was more effective in binding C1q and causing C1-mediated C4b deposition. Unexpectedly, time responses of C1 (C1q) binding to immobilized 3C7 reached a peak then gradually decreased. However, C1q remained constantly bound to immobilized Qu. These results indicated that after C1 activation in human serum, the entire C1 complex (including C1q) was dislodged from 3C7, but not from immobilized Qu. The addition of purified C1 INH to purified C1, which had bound to immobilized 3C7, resulted in removal of C1 (C1q). Removal of the entire C1qr2s2 did not occur when C1 INH preparations were first neutralized by the addition of purified activated C1s. In summary, it is suggested that C1 INH plays a prominent role in dislodging the entire C1qr2s2 from immunoglobulin preparations which have a low binding affinity for the globular heads of C1q.

C1q-mediated chemotaxis by human neutrophils: involvement of gClqR and G-protein signalling mechanisms

C1q, the first component of the classical pathway of the complement system, interacts with various cell types and triggers a variety of cell-specific cellular responses, such as oxidative burst, chemotaxis, phagocytosis, etc. Different biological responses are attributed to the interaction of C1q with more than one putative cell-surface C1q receptor/C1q-binding protein. Previously, it has been shown that C1q-mediated oxidative burst by neutrophils is not linked to G-protein-coupled fMet-Leu-Phe-mediated response. In the present study, we have investigated neutrophil migration brought about by C1q and tried to identify the signal-transduction pathways involved in the chemotactic response. We found that C1q stimulated neutrophil migration in a dose-dependent manner, primarily by enhancing chemotaxis (directed movement) rather than chemokinesis (random movement). This C1q-induced chemotaxis could be abolished by an inhibitor of G-proteins (pertussis toxin) and PtdIns(3,4,5)P3 kinase (wortmannin and LY294002). The collagen tail of C1q appeared to mediate chemotaxis. gC1qR, a C1q-binding protein, has recently been reported to participate in C1q-mediated chemotaxis of murine mast cells and human eosinophils. We observed that gC1qR enhanced binding of free C1q to adherent neutrophils and promoted C1q-mediated chemotaxis of neutrophils by nearly seven-fold. Our results suggests C1q-mediated chemotaxis involves gC1qR as well as G-protein-coupled signal-transduction mechanisms operating downstream to neutrophil chemotaxis.

Monoclonal antibodies against unfertilized zona-free mouse oocytes: characterization and effects on fertilization

A panel of anti-oocyte antibodies was raised against unfertilized zona-free mouse oocytes by intrasplenic immunization and checked for their effects on in vitro fertilization. Four antibodies decreased the fertilization rate from about 90% in controls to 8% (B5-2 F7), 12% (A2-2 A7), 13% (4-G1), and 25% (A2-2 F2), when the sperm cell concentration was 1 x 10(5) to 1 x 10(6). Antigen localization: All the antibodies labelled components in the cell membrane of zona-free oocytes as demonstrated by indirect immunofluorescence and/or by complement-mediated oocyte lysis. In various patterns, the ooplasm and zona pellucida were also labelled with different intensities. Western blotting: A2-2 A7 and A2-2 F2 recognized a protein with a molecular weight of approximately 65 kDa, while antibody B5-2 F7 bound a 97 kDa protein. Complement activation and complement-mediated oocyte lysis: Systemically injected antibodies, C3 and C4 were detected on zona-free oocytes recovered from the mouse oviduct indicating the activation of C3 and C4 by antigen-antibody complexes. The recovered oocytes were not damaged, suggesting a presence of complement-regulating factors. In vitro, however, a large number of zona-free oocytes preincubated with antibodies were lysed or protruded ooplasma vesicles in complement-active serum. Stage, tissue, and species specificity: None of the antibodies, except A2-2 A7, showed a positive immunolabelling to the pronuclear stage. Antibodies 4-G1 and A2-2 F2 cross-reacted with the ovarian oocytes. No antibodies bound to any of the tissues tested, indicating that the corresponding antigen epitopes are not commonly expressed. A2-2 A7, A2-2 F2, and B5-2 F7 cross-reacted with hamster and human unfertilized oocytes, suggesting the presence of developmentally conserved molecules and the possibility to apply these antibodies in hamster and human in vitro fertilization. It is concluded that the approach used could be a useful strategy in searching for anti-fertilization antibodies for human contraception.

Atypical Gly-X-Y sequences surround interruptions in the repeating tripeptide pattern of basement membrane collagen

The triple-helical domains of type IV collagen chains have more than 20 sites at which the repeating (Gly-X-Y)n pattern is interrupted. Analysis of alpha 1 (IV) and alpha 2 (IV) chains indicates the residues in the three Gly-X-Y triplets preceding or following interruptions differ statistically from the rest of the chain. Unusually high frequencies of charged residues are seen at a number of X and Y sites, with the charge density being particularly high C-terminal to the interruption site. Analyses were carried out on individual categories of interruptions, classified as insertions or deletions in the Y position. All of the residues in the X and Y positions of the triplets flanking insertion sites are atypical, with a high concentration of charged residues. Triplets flanking sites where there has been a deletion in the Y position show unusually high frequencies of charged residues at some sites, hydrophobic residues at other sites, and an invariant imino acid N-terminal to the interruption. The presence of atypical sequences surrounding interruptions could be important at a molecular level, related to triple-helix stability, or at a supramolecular level, related to the association of molecules to form networks in basement membranes.

Complement components C1q, C1r/C1s, and C1INH in rheumatoid arthritis. Correlation of in situ hybridization and northern blot results with function and protein concentration in synovium and primary cell cultures

OBJECTIVE

To analyze the synovial site and the cell types expressing C1q, C1r/C1s, and C1-esterase inhibitor (C1INH) and to characterize newly synthesized C1q in patients with rheumatoid arthritis (RA).

METHODS

Tissue and primary cell cultures of synovium from RA patients were analyzed for C1q, C1r/C1s, and C1INH by Northern blotting, in situ hybridization, and pulse-chase experiments for C1q.

RESULTS

The de novo synthesis of C1q, C1r/C1s, and C1INH in synovium and primary cell cultures was proven by Northern blot and by antigenic and functional analysis. In in situ hybridization experiments, the synovial lining cell layer was identified as the site of C1q, C1r, and C1INH expression. In contrast, immunohistologic analysis showed that C1q, C1s, and C1INH proteins were present in a thin film covering the synovial lining cells. In situ hybridization performed on primary cell cultures provided evidence that only macrophages were able to express C1q, whereas fibroblasts and stellate cells synthesized C1r.

CONCLUSION

The synovium is important for the synthesis and secretion of C1q and C1r/C1s, as well as the control protein C1INH, which supports the idea of a locally occurring inflammatory process in RA patients.

Isolation of the C3 complement component and its C3d subunit from IY-1 fraction of Cohn's fractionation of human plasma

C3 complement component and its C3d subunit were isolated from the IY-1 Cohn's fraction, which is the waste of industrially produced albumin and immunoglobulins. The first step was the fractionation of precipitate IY-1 by polyethylene glycol (PEG) 4000 to a final concentration of 16% PEG. The precipitate formed was separated by centrifugation. The supernatant contained the C3d subunit of C3, and the redissolved 16% PEG precipitate contained the C3 component. Then the supernatant and the dissolved precipitate were subjected to anion-exchange chromatography on DEAE-Toyopearl 650 M. In the last step fractions containing C3 and C3d concentrated by ultrafiltration were chromatographed on Sephacryl S-200.

Characterization of collagen-like peptides containing interruptions in the repeating Gly-X-Y sequence

Glycine is found as every third residue along the entire length of triple helices in fibrillar collagens, but the triple-helix regions of nonfibrillar collagens and other proteins usually contain one or more interruptions in this repeating pattern. A set of four peptides was designed to model the effect of interruptions in the (Gly-X-Y)n repeating pattern on triple-helix formation, stability, and folding. Into the middle of the stable triple-helical peptide (Pro-Hyp-Gly)10, an interruption was introduced representing one of the four possible categories: a glycine deletion, a deletion of a hydroxyproline (Y position), an alanine insertion, or a glycine to alanine substitution. As shown by sedimentation equilibrium, NMR, and CD studies, the introduction of an interruption still allowed formation of trimers in solution, but with marked decrease in stability. The degree of destabilization and the thermodynamic basis for the loss of stability depended on the type of interruption. The glycine substitution and alanine insertion were the least disruptive, followed by the hydroxyproline deletion, with the glycine deletion being the most destabilizing. Our results suggest that the breaks in these peptides affect both the triple-helical conformation and the monomer conformation. These studies provide a basis for considering the structural and functional consequences of different kinds of interruptions in collagen.

Cascade system activation in shock

Trauma and sepsis activate different cascade systems. Activation of the coagulation and clotting systems, of the kinin-kallikrein system and of the complement system are important etiological mechanisms behind development of the adult respiratory distress syndrome (ARDS) and multisystem organ failure (MOF) after extensive trauma or severe septic situations. Activation of complement with the release of anaphylatoxins and terminal complement complexes is associated with increased mortality and development of ARDS and MOF after major surgery and in situations of septic shock. The anaphylatoxins have potent vascular properties and they activate leukocytes. Their effects on the leukocytes lead to the release of free oxygen radicals, different lysosomal enzymes and cytokines, leukotrienes and histamine. All these inflammatory mediators may, if released in extensive amounts, induce microvascular injury and interstitial edema. If this process takes place in the lung, ARDS may develop and if other organs, i.e the liver and the kidneys, are involved, MOF may be the result.

The region Ser333-Arg356 of the alpha-chain of human C4b-binding protein is involved in the binding of complement C4b

Human C4b-binding protein (C4BP) functions as a cofactor to factor I in the degradation of C4b and accelerates the decay rate of the C4b2a complex. In this study we describe a monoclonal antibody directed against the alpha-chain of C4BP that inhibits the binding of C4b to C4BP. In order to identify the structural domain of the alpha-chain of C4BP that interacts with C4b, tryptic fragments of C4BP were generated. Amino acid sequence analysis of the fragments revealed that the residues Ser333-Arg356 of the alpha-chain of C4BP contain the epitope of this antibody, and as a consequence, that this part of the alpha-chain of C4BP is likely to be involved in the interaction with C4b.

Separation of different forms of the fourth component of human complement by fast protein liquid chromatography

Disruption of the thiolester in native C4 yields a 'C4b-like C4' molecule (iC4) that functionally resembles C4b and is therefore probably accompanied by conformational changes in the C4 molecule. In most purified C4 preparations, iC4 and C4b are present to a variable extent. In this study we evaluated the use of fast protein liquid chromatography (FPLC) to resolve and isolate these various forms of C4. C4 was purified from fresh human plasma in a 4-step procedure that included barium citrate adsorption, polyethylene glycol 6000 (PEG) precipitation, Q-Sepharose Fast Flow and mono Q ion exchange chromatography. The final preparation appeared to be homogeneous on SDS-PAGE and under reducing conditions consisted of three bands that corresponded to the intact alpha, beta and gamma chains of C4. In some preparations the alpha' chain of C4b was also observed. On a Mono Q column the purified C4 preparations could be separated into three peaks that by hemolytic assay and SDS-PAGE were characterized as representing native C4, and monomeric and dimeric iC4 (or monomeric and dimeric C4b). Finally, the apparent KA of the various forms of C4 for C4b-binding protein (C4BP) was investigated. The monomeric iC4 and C4b species demonstrated similar C4BP binding affinity with an apparent KA of 5.6-6.4 x 10(8) M-1, whereas their dimeric forms demonstrated a higher affinity for C4BP with an apparent KA: 0.9-2.3 x 10(9) M-1. Binding of native C4 to C4BP was undetectable.

Imaging of proteins by scanning tunnelling microscopy

Scanning tunnelling microscopy has been used to examine the structure of proteins deposited on a graphite surface. Three molecules have been studied; immunoglobulin G (IgG), Complement component 1q (C1q) and ATP-citrate lyase (ACL). The images show IgG as a tri-lobed molecule, consistent with the known 3D structure as determined by X-ray crystallography. The C1q images differ from the well known "tulip bunch" model derived by electron microscopy, but are consistent with the model if it is assumed that the six globular heads have aggregated. Molecules of ACL are visible as discrete units, with some hints of substructure. These results highlight the potential of STM in studying protein structures, but also illustrate the difficulties of interpreting micrographs of proteins whose structure is currently unknown.

A functionally active complement system is present in uterine secretion of the mouse prior to implantation

Sephadex beads were placed carefully in the uterus on days 2 and 3 and left for 6 to 8 h to absorb uterine secretion. The beads were then removed with volatile silicon oil and mounted on small pieces of nitrocellulose paper. Immuno-staining of these bead blots showed they contained the complement components C1q, C3, C4, and C5. We demonstrated that complement component C3 in the uterine secretion could be activated and deposited on model immune complexes, and also that antibody-coated erythrocytes were lysed in utero, that is, a membrane attack complex was produced. Thus, the mouse uterine secretion at the preimplantation stage contains a functionally active complement system.

The interaction between complement component C4b-binding protein and the vitamin K-dependent protein S forms a link between blood coagulation and the complement system

Images

C1q, a collagen-like complement subcomponent, in dermatosparactic cattle: its extracellular modification is not affected by lack of procollagen N-terminal proteinase (pN-proteinase)

C1q, a collagen-like complement protein, was purified from the serum of a dermatosparactic calf which lacks procollagen N-terminal proteinase (pN-proteinase). The specific hemolytic activity of the serum C1q from the dermatosparactic animal was identical to that of C1q from a normal calf. Gel-filtration of serum from the dermatosparactic calf, on Sepharose 6B, showed the presence of C1q-antigenic material at only one position which was identical to the elution position of normal bovine C1q. No difference, under dissociating conditions, could be seen in the size of the chains of C1q in specific immunoprecipitates isolated from the sera of dermatosparactic and normal animals, as judged by polyacrylamidegel electrophoresis (PAGE) in the presence of sodium dodecyl sulfate (SDS). The C1q from the dermatosparactic animal showed the same N-terminal amino acid and tryptic-digest peptide pattern on HPLC as C1q from the normal calf. These results strongly suggest that pN-proteinase is not involved in the extracellular processing of C1q.

Importance of the alpha 3-fragment of complement C4 for the binding with C4b-binding protein

The human regulatory complement component C4b-binding protein (C4BP) is a multimeric plasma protein, which regulates the classical pathway of the complement system. C4BP functions as a cofactor to factor 1 in the degradation of C4b and accelerates the decay rate of the C4b2a complex. Previously, we have demonstrated that monoclonal antibodies (C4-2 and 9) directed against the alpha'-chain of C4b inhibit the binding of C4b to C4BP. In order to identify the structural domain of C4b that binds C4BP, proteolytic fragments of C4 were generated with trypsin and Staphylococcus aureus V8 protease. Sodium dodecyl sulfate polyacrylamide gel electrophoresis, immunoblotting and amino acid sequence analysis of the proteolytic fragments reactive with the anti-C4 mAb's revealed that the residues Ala738-Arg826 of the alpha 3-fragment of C4b are important for the interaction with C4BP.

Accumulation of anaphylatoxins and terminal complement complexes in inflammatory fluids

The presence of anaphylatoxins (C3a and C5a) and terminal complement complexes (TCC) in different inflammatory fluids and plasma was studied in 33 patients. Anaphylatoxins were assayed using a radioimmunoassay technique, and the terminal complement complexes were determined by an ELISA method. Patients with peritoneal (n = 14), pleural (n = 7), pericardial (n = 6) or burn bullae fluid (n = 6) were studied. High C3a and TCC concentrations were found in all these fluids. Elevated C3a and TCC concentrations in inflammatory fluids were found not only in patients with elevated plasma C3a and TCC concentrations, but also in patients with normal plasma levels. No increases in C5a concentration were observed in pleural or burn bullae fluid. In one patient with pericarditis, and in subjects with acute pancreatitis with ascites, high C5a levels were found in the fluid. However, the high TCC concentration in the fluids suggests that C5a had been formed but was probably removed by leucocytes present in the fluid.

Ultracytochemical study of lytic complex insertion in the glycocalyx of red cells during immune hemolysis mediated by complement

Ruthenium red (RR), a cationic dye and an ultrastructural tracer of cell membrane permeability, was used on sheep red blood cells after lysis produced by a specific antibody and guinea pig complement. In addition to the opacification of the glycocalyx, RR stained structures related to lytic complexes, which appeared as rod-like structures with variable dimensions (generally 45 nm in width, 75 nm in height) inserted in the glycocalyx of red cells. They extended across the external layer of the trilaminar plasma membrane without reaching the internal layer or the cytoplasm. RR staining visualized the internal configuration of the lytic complexes and revealed small channels measuring 10 nm in diameter localized within the complexes. These lytic complexes are thought to correspond to membrane attack complex of complement. To the best of our knowledge, this is the first report of ultrastructural positive staining of lytic complexes in thin sections, allowing visualization of their internal configuration and their insertion in the plasma membrane glycocalyx.

Characterization of C1q found in a patient with hypocomplementemic vasculitis-urticaria syndrome

C1q, a subcomponent of the first complement component, of a 60-year-old female patient with hypocomplementemic vasculitis-urticaria syndrome (HVUS) was characterized. The C1q-precipitin activity (C1q-p) could not be detected by a routine method with 0.6% agarose in 10 mM Na-phosphate buffer containing 10 mM EDTA (pH 7.2). Hemolytic activity of her serum complement (CH50) and levels of C1 and C4 were significantly reduced at the exacerbation stage, but levels of other complement components were almost within the normal range throughout her clinical course. The specific activity of C1q at her exacerbation stage was significantly low, and its elution position on Sephacryl S-300 column was spread toward the low molecular weight in comparison with that of normal plasma. Molecular weights of the delayed fraction of C1q were estimated to be approximately 300,000 on the Sephacryl and 440,000 by the polyacrylamide gel electrophoresis (PAGE) containing sodium dodecyl sulfate (SDS) followed by immunoblotting, respectively. On reduction of her plasma, two bands with molecular weights equivalent to those of B and C chains of the normal C1q in an approximate molar ratio of 2:1 were immunostained. Plasma at her exacerbation stage showed only one precipitation line against anti-human C1q-antiserum which was completely fused with that formed between purified normal human C1q and the same antiserum. The probable structural change of the hypofunctional C1q in the case of this HVUS is discussed.

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.

Interaction between complement subcomponent C1q and bacterial lipopolysaccharides

The heptose-less mutant of Escherichia coli, D31m4, bound complement subcomponent C1q and its collagen-like fragments (C1qCLF) with Ka values of 1.4 x 10(8) and 2.0 x 10(8) M-1 respectively. This binding was suppressed by chemical modification of C1q and C1qCLF using diethyl pyrocarbonate (DEPC). To investigate the role of lipopolysaccharides (LPS) in this binding, biosynthetically labelled [14C]LPS were purified from E. coli D31m4 and incorporated into liposomes prepared from phosphatidylcholine (PC) and phosphatidylethanolamine (PE) [PC/PE/LPS, 2:2:1, by wt.]. Binding of C1q or its collagen-like fragments to the liposomes was estimated via a flotation test. These liposomes bound C1q and C1qCLF with Ka values of 8.0 x 10(7) and 2.0 x 10(7) M-1; this binding was totally inhibited after chemical modification of C1q and C1qCLF by DEPC. Liposomes containing LPS purified from the wild-strain E. coli K-12 S also bound C1q and C1qCLF, whereas direct binding of C1q or C1qCLF to the bacteria was negligible. Diamines at concentrations which dissociate C1 into C1q and (C1r, C1s)2, strongly inhibited the interaction of C1q or C1qCLF with LPS. Removal of 3-deoxy-D-manno-octulosonic acid (2-keto-3-deoxyoctonic acid; KDO) from E. coli D31m4 LPS decreases the binding of C1qCLF to the bacteria by 65%. When this purified and modified LPS was incorporated into liposomes, the C1qCLF binding was completely abolished. These results show: (i) the essential role of the collagen-like moiety and probably its histidine residues in the interaction between C1q and the mutant D31m4; (ii) the contribution of LPS, particularly the anionic charges of KDO, to this interaction.