Biosynthesis of the collagen-like C1q molecule and its receptor functions for Fc and polyanionic molecules on macrophages

Monocyte Expressed Macromolecular C1 and C1q Receptors as Molecular Sensors of Danger: Implications in SLE

The ability of circulating blood monocytes to express C1q receptors (cC1qR and gC1qR) as well as to synthesize and secrete the classical pathway proteins C1q, C1r, and C1s and their regulator, C1-INH is very well established. What is intriguing, however, is that, in addition to secretion of the individual C1 proteins monocytes are also able to display macromolecular C1 on their surface in a manner that is stable and functional. The cell surface C1 complex is presumably formed by a Ca²⁺-dependent association of the C1r₂⋅C1s₂ tetramer to C1q, which in turn is anchored via a membrane-binding domain located in the N-terminus of its A-chain as shown previously. Monocytes, which circulate in the blood for 1–3 days before they move into tissues throughout the body, not only serve as precursors of macrophages and dendritic cells (DCs), but also fulfill three main functions in the immune system: phagocytosis, antigen presentation, and cytokine production. Since the globular heads of C1q within the membrane associated C1 are displayed outwardly, we hypothesize that their main function – especially in circulating monocytes – is to recognize and capture circulating immune complexes or pathogen-associated molecular patterns in the blood. This in turn may give crucial signal, which drives the monocytes to migrate into tissues, differentiate into macrophages or DCs, and initiate the process of antigen elimination. Unoccupied C1q on the other hand may serve to keep monocytes in a pre-dendritic phenotype by silencing key molecular players thus ensuring that unwarranted DC-driven immune response does not occur. In this paper, we will discuss the role of monocyte/DC-associated C1q receptors, macromolecular C1 as well as secreted C1q in both innate and acquired immune responses.

The C1q family of proteins: insights into the emerging non-traditional functions

Research conducted over the past 20 years have helped us unravel not only the hidden structural and functional subtleties of human C1q, but also has catapulted the molecule from a mere recognition unit of the classical pathway to a well-recognized molecular sensor of damage-modified self or non-self antigens. Thus, C1q is involved in a rapidly expanding list of pathological disorders – including autoimmunity, trophoblast migration, preeclampsia, and cancer. The results of two recent reports are provided to underscore the critical role C1q plays in health and disease. First is the observation by Singh et al. (2011) showing that pregnant C1q−/− mice recapitulate the key features of human preeclampsia that correlate with increased fetal death. Treatment of the C1q−/− mice with pravastatin restored trophoblast invasiveness, placental blood flow, and angiogenic balance and, thus, prevented the onset of preeclampsia. Second is the report by Hong et al. (2009) which showed that C1q can induce apoptosis of prostate cancer cells by activating the tumor suppressor molecule WW-domain containing oxydoreductase (WWOX or WOX1) and destabilizing cell adhesion. Downregulation of C1q on the other hand, enhanced prostate hyperplasia and cancer formation due to failure of WOX1 activation. C1q belongs to a family of structurally and functionally related TNF-α-like family of proteins that may have arisen from a common ancestral gene. Therefore C1q not only shares the diverse functions with the tumor necrosis factor family of proteins, but also explains why C1q has retained some of its ancestral “cytokine-like” activities. This review is intended to highlight some of the structural and functional aspects of C1q by underscoring the growing list of its non-traditional functions.

Human astrovirus coat protein inhibits serum complement activation via C1, the first component of the classical pathway

Human astroviruses (HAstVs) belong to a family of nonenveloped, icosahedral RNA viruses that cause noninflammatory gastroenteritis, predominantly in infants. Eight HAstV serotypes have been identified, with a worldwide distribution. While the HAstVs represent a significant public health concern, very little is known about the pathogenesis of and host immune response to these viruses. Here we demonstrate that HAstV type 1 (HAstV-1) virions, specifically the viral coat protein (CP), suppress the complement system, a fundamental component of the innate immune response in vertebrates. HAstV-1 virions and purified CP both suppress hemolytic complement activity. Hemolytic assays utilizing sera depleted of individual complement factors as well as adding back purified factors demonstrated that HAstV CP suppresses classical pathway activation at the first component, C1. HAstV-1 CP bound the A chain of C1q and inhibited serum complement activation, resulting in decreased C4b, iC3b, and terminal C5b-9 formation. Inhibition of complement activation was also demonstrated for HAstV serotypes 2 to 4, suggesting that this phenomenon is a general feature of these human pathogens. Since complement is a major contributor to the initiation and amplification of inflammation, the observed CP-mediated inhibition of complement activity may contribute to the lack of inflammation associated with astrovirus-induced gastroenteritis. Although diverse mechanisms of inhibition of complement activation have been described for many enveloped animal viruses, this is the first report of a nonenveloped icosahedral virus CP inhibiting classical pathway activation at C1.

Temporal pattern of C1q deposition after transient focal cerebral ischemia

Recent studies have focused on elucidating the contribution of individual complement proteins to post-ischemic cellular injury. As the timing of complement activation and deposition after cerebral ischemia is not well understood, our study investigates the temporal pattern of C1q accumulation after experimental murine stroke. Brains were harvested from mice subjected to transient focal cerebral ischemia at 3, 6, 12, and 24 hr post reperfusion. Western blotting and light microscopy were employed to determine the temporal course of C1q protein accumulation and correlate this sequence with infarct evolution observed with TTC staining. Confocal microscopy was utilized to further characterize the cellular localization and characteristics of C1q deposition. Western Blot analysis showed that C1q protein begins to accumulate in the ischemic hemisphere between 3 and 6 hr post-ischemia. Light microscopy confirmed these findings, showing concurrent C1q protein staining of neurons. Confocal microscopy demonstrated co-localization of C1q protein with neuronal cell bodies as well as necrotic cellular debris. These experiments demonstrate the accumulation of C1q protein on neurons during the period of greatest infarct evolution. This data provides information regarding the optimal time window during which a potentially neuroprotective anti-C1q strategy is most likely to achieve therapeutic success.

Human macrophages simultaneously express membrane-C1q and Fc-receptors for IgG

Membrane C1q (mC1q) of macrophages (MPhi) is a precursor of the IgG-binding serum protein C1q. Thus, mC1q potentially provides one of several Fcgamma binding sites of mature MPhi and we analyzed whether simultaneous expression occurs of established receptors for IgG, FcgammaRI, II, and III, and mC1q during in vitro differentiation of MPhi. Using flow cytometry, immunoprecipitation combined with Western blotting and Northern blot analysis mC1q was hardly detected in freshly isolated blood monocytes, but increasingly in developing monocyte-derived MPhi. Laser scanning fluorescence microscopy confirmed the membrane localization of mC1q. Two-color-staining flow cytometry experiments indicated that mC1q and all three types of FcgammaRs are simultaneously expressed on mature monocyte-derived MPhi. A high correlation was found for the expression of mC1q and FcgammaRs, in particular FcgammaRII, but not mC1q and CD14, another marker of monocytes/MPhi.

Predominant role of IgM-dependent activation of the classical pathway in the clearance of dying cells by murine bone marrow-derived macrophages in vitro

Soluble molecules including complement components have been shown to facilitate the clearance of dying cells by phagocytes, a process that is important in preventing tissue damage and autoimmunity. However, the extent to which complement is involved in this process and the relative contribution of each of the complement activation pathways is not fully understood. We examined the role of complement in the recognition/uptake of apoptotic thymocytes by murine bone marrow-derived macrophages (BMDM) in vitro using sera from gene-targeted mice. We found this process to be IgM- and complement-dependent, especially when the apoptotic cell-to-BMDM ratio was low, and the level of C3 deposition on apoptotic cells correlated closely with their uptake. The addition of C1q rectified the phagocytic defect seen in the presence of C1q-deficient serum in vitro but had no effect on the phagocytic defect observed with serum deficient in both IgM antibodies and C1q. Similarly, complement activation by IgM antibodies was essential for in vivo C3 deposition on apoptotic cells and their uptake by peritoneal macrophages. Hence, the efficient uptake of dying cells by BMDM requires IgM antibodies and complement.

Immunomodulating activities of soluble synthetic polymer-bound drugs

The introduction of a synthetic material into the body always affects different body systems, including the defense system. Synthetic polymers are usually thymus-independent antigens with only a limited ability to elicit antibody formation or to induce a cellular immune response against them. However, there are many other ways that they influence or can be used to influence the immune system of the host. Low-immunogenic water-soluble synthetic polymers sometimes exhibit significant immunomodulating activity, mainly concerning the activation/suppression of NK cells, LAK cells and macrophages. Some of them, such as poly(ethylene glycol) and poly[N-(2-hydroxypropyl)methacrylamide], can be used as effective protein carriers, as they are able to reduce the immunogenicity of conjugated proteins and/or to reduce non-specific uptake of liposome/nanoparticle-entrapped drugs and other therapeutic agents. Recently, the development of vaccine delivery systems prepared from biodegradable and biocompatible water-soluble synthetic polymers, microspheres, liposomes and/or nanoparticles has received considerable attention, as they can be tailored to meet the specific physical, chemical, and immunogenic requirements of a particular antigen and some of them can also act as adjuvants.

Increased susceptibility of C1q-deficient mice to Salmonella enterica serovar Typhimurium infection

The role of the complement system in host defense against Salmonella infection is poorly defined. Bacterial cell wall O-antigen polysaccharide can activate the alternative pathway in vitro. No studies, however, have elucidated the role of the classical pathway in immunity to Salmonella spp. in vivo. C1q-deficient mice (C1qa(-/-)) on a 129/Sv genetic background and strain-matched controls were infected intraperitoneally and intravenously with Salmonella enterica serovar Typhimurium and monitored over a 14-day period. After inoculation by either route, the C1qa(-/-) mice were found to be significantly more susceptible to Salmonella infection. Hepatic and splenic bacterial counts, performed at various time points, showed increased numbers of colonies in complement-deficient mice compared to controls. Analysis of blood clearance showed no difference between the two experimental groups during the first 15 min. However, after 20 min and until 6 h postinfection, numbers of circulating bacteria were significantly higher in complement-deficient mice. In vitro experiments using either resident or thioglycolate-elicited peritoneal macrophages showed a significant increase in the number of bacteria inside C1q-deficient macrophages compared to controls irrespective of the serum used for opsonizing the bacteria. These findings could not be explained either by an increased bacterial uptake, analyzed in vitro and in vivo using green fluorescent protein-tagged salmonellae, or by a defect in the respiratory burst or in NO production. The data presented here suggest the possibility of novel pathways by which C1q may modulate the pathogenesis of infectious diseases caused by intracellular pathogens.

Expression of membrane C1q in human monocyte-derived macrophages is developmentally regulated and enhanced by interferon-gamma

The present study investigated when during "in vitro" maturation macrophages (MPhi) express membrane C1q (mC1q), and whether cell activation affects expression and function of mC1q. Although C1q mRNA was repeatedly detected in freshly isolated monocytes using reverse transcriptase-polymerase chain reaction, C1q protein was observed only in developing MPhi from day 1 to 4 on using immunodetection and flow cytometry. However, the quantity of mC1q and other MPhi membrane proteins differed strikingly in cells from different donors. We report here for the first time that CD14(+) and CD14(-) mC1q-bearing MPhi can develop, and that interferon-gamma increases mC1q display at the cell surface, and mC1q-mediated phagocytosis.

Cytokines associated with amyloid plaques in Alzheimer's disease brain stimulate human glial and neuronal cell cultures to secrete early complement proteins, but not C1-inhibitor

Complement activation products C1q, C4c/d, and C3c/d in amyloid plaques in Alzheimer's disease probably result from direct binding and activation of C1 by amyloid beta peptides. RT-PCR and in situ hybridization studies have shown that several complement factors are produced in the brain parenchyma. In the present study, cytokines that can be detected in amyloid plaques (i.e., interleukin (IL)-1, IL-6, and tumor necrosis factor (TNF)-alpha) were found to differentially stimulate the expression of C1 subcomponents, C1-Inhibitor (C1-Inh), C4, and C3, by astrocyte and microglial cell cultures derived from postmortem adult, human brain specimens and by neuroblastoma cell lines in culture. C1r and C1s were secreted at low levels by astrocytes and neuroblastoma cell lines. Exposure of cells to IL-1 alpha, IL-1 beta, TNF-alpha and to a far lesser extent IL-6, markedly upregulated C1r, C1s, and C3 production. C4 synthesis increased in response to interferon (IFN)-gamma and IL-6, whereas that of C1-Inh could be stimulated only by IFN-gamma. Thus, C1-Inh production is refractory to stimulation by plaque-associated cytokines, whereas these cytokines do stimulate C1r, C1s, and also C4 and C3 secretion by astrocytes and neuronal cells in culture. In contrast to the amyloid plaque associated cytokines IL-1 beta, IL-1 alpha, and TNF-alpha, the amyloid peptide A beta 1-42 itself did not stimulate C1r and C1s synthesis by astrocytes, microglial cells, or neuroblastoma cell lines. Microglial cells were the only cell type that constitutively expressed C1q. The ability of C1q to reassociate with newly formed C1r and C1s upon activation of C1 and subsequent inactivation by C1-Inh, may enable ongoing complement activation at sites of amyloid deposition, especially when C1-Inh is consumed and not replaced.

Fc receptors in liver sinusoidal endothelial cells in NZB/W F1 lupus mice: a histological analysis using soluble immunoglobulin G-immune complexes and a monoclonal antibody (2.4G2)

In systemic lupus erythematosus accompanied by the abnormal appearance of circulating immune complexes (ICs), Fc gamma receptor (FcR)-mediated IC handling in macrophages including Kupffer cells has been shown previously. However, sinusoidal endothelial cells (SECs) largely ingest soluble immunoglobulin (Ig) G-ICs through FcRs. In this study, the character, antigenic expression, and activity (i.e., ligand-binding capacity of SEC FcRs in NZB/NZW F1 lupus and NZW nonautoimmune mice) were immunohistochemically analyzed using monoclonal antibody (MAb) 2.4G2 to FcRs and peroxidase-antiperoxidase IgG as a ligand on cryosections. MAb 2.4G2 stained SECs and blocked the ligand binding of SEC FcRs in both mice strains. The staining intensities with MAb 2.4G2 in SECs and the FcR activities in SECs alone and all sinusoidal cells in both mice strains reached their maximum values at the age of 5 months. Staining intensities in NZB/W F1 were significantly higher at 1 and 2 months and lower at 9 months than those in NZW. The number of Kupffer cells detected by MAb F4/80 to macrophages in both mice strains gradually increased until 5 months, but their number in NZB/W F1 at 9 months was twice as large as that in NZW. In conclusion, SEC FcRs in mice are low-affinity FcRs that react with MAb 2.4G2. The data of FcR activity suggest no impairment of the FcR-mediated IgG-IC binding on SECs in NZB/W F1 in early life.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Complement biosynthesis by mononuclear phagocytes

Mononuclear phagocytes are an important in vivo source of a wide range of complement components. They are able to rapidly up-regulate or down-regulate complement synthesis in response to many different pharmacological and biological stimuli. This ability is likely to make a significant contribution to maintaining host defences particularly in peripheral tissues. The important role of molecular biology in the study of complement biosynthesis by mononuclear phagocytes will be emphasised.

Inhibitor of C1q secretion suppresses the macrophage response to lipid A for nitric oxide but not for TNF production: evidence for a role of C1q in autocrine binding of TNF

Studies were designed to further define the modulatory role of complement subcomponent C1q in macrophage activation by Lipid A to mediate production of TNF and cytotoxic nitric oxide (NO). Pretreatment of macrophages for 24 h with 2.5 mM 3,4,dehydro-D,L-proline (DHP), an inhibitor of C1q secretion, suppressed their response to Lipid A activation for cytotoxicity of P815 tumor targets which correlated with a corresponding decrease in NO production. In contrast, DHP-pretreated macrophages displayed an increase in the release of TNF in response to Lipid A as compared to untreated controls. Time kinetic studies indicated that DHP-pretreated macrophages produced higher sustained levels of TNF activity during 1 to 24 h culture with Lipid A than did untreated control macrophages. This was confirmed by increased TNF mRNA expression in response to Lipid A by DHP-treated cells. DHP-pretreated macrophages had reduced levels of cell surface C1q as determined by cytofluorometric analysis of the binding of FITC-labeled anti-C1q, F(ab')2. Macrophages were also found to have reduced binding capacity for phycoerythrin-labeled rTNF (PE-TNF) by cytofluorometric analysis following DHP treatment. Exposure of DHP-pretreated macrophage to soluble C1q at 4 degrees C restored their reduced binding of PE-TNF. C1q was confirmed to bind to macrophages at 4 degrees C as detected by FITC anti-C1q, F(ab')2 and such C1q binding promoted a corresponding increased binding of PE-TNF. Macrophages which were plated over immobilized C1q were also markedly enhanced in their binding of PE-TNF probe. Our results indicate that the inhibition of macrophage secretion of C1q by DHP pretreatment, was accompanied by an increased TNF mRNA expression and release with a decrease in NO generation following Lipid A activation. Since TNF binding to DHP-treated macrophages was reconstituted by the binding of exogenous C1q to the cells, it appears that C1q may be involved in the modulation of autocrine binding of TNF for subsequent generation of cytotoxic NO.

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.

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.

Reconstitution of murine resident peritoneal macrophages for antibody-dependent cellular cytotoxicity by homologous serum Clq

Mouse resident peritoneal macrophages (PM) were reconstituted in their response to activation for antibody-dependent cellular cytotoxicity (ADCC) for sheep erythrocyte targets (SRBC) by subhemolytic dilutions of homologous or autologous sera. ADCC-responsive inflammatory PM were largely unaffected in their activation by exogenous serum. Augmentation of resident PM for ADCC by homologous serum was correlated with the complement-activating potential of the mouse monoclonal anti-SRBC IgG isotype in that serum augmented IgG gamma 2a greater than IgG gamma 2b much greater than IgG gamma 1. The active component of mouse serum was heat-labile at 56 degrees C for 30 min and was present in both C5-deficient AKR and C5-sufficient homologous C3H mouse sera. Western blot analysis of the cell lysates for Clq confirmed that oil-elicited and thioglycollate-elicited inflammatory PM had greater levels of endogenous Clq than did resident PM which correlated with their innate responsiveness for ADCC activation. Depletion of Clq from serum by immunoprecipitation with IgG antibody to Clq or by ion exchange chromatography removed the active reconstituting activity for ADCC. Purified mouse Clq (0.4 microgram) partially replenished the ADCC augmenting activity of Clq-depleted AKR mouse serum. SRBC targets preopsonized with IgG gamma 2a and purified mouse Clq (0.075-5.0 microgram/ml) fully reconstituted the ADCC response of resident PM similar to homologous serum indicating that the major active component of serum was Clq. Thus resident PM with low endogenous levels of Clq were reconstituted for ADCC by the addition of exogenous Clq, whereas inflammatory PM with sufficiently high endogenous levels of Clq were not further enhanced by exogenous Clq. Our findings indicate that Clq may provide an essential second signal in concert with Fc receptor binding of IgG to initiate ADCC activation of macrophages.

Guinea pig macrophages synthesize a low molecular weight form of C1q with affinity for the C1r2C1s2-complex but which does not bind to Fc in immunoglobulin aggregates

Biosynthetically labelled C1q secreted by guinea pig peritoneal macrophages was analysed by sedimentation through sucrose gradients followed by SDS-PAGE. In addition to the haemolytically active C1q of mol. wt 460,000 Da a low mol. wt (LMW) form of C1q was identified which had no detectable affinity for Fc of aggregated immunoglobulin, but which retained the ability to associate with the C1r2s2-complex. This LMW-C1q was covalently associated with two additional polypeptides of mol. wt 46 and 50 kDa.

C1q enhancement of antibody-dependent granulocyte-mediated killing of nonphagocytosable targets in vitro

A possible role for C1q in antibody-dependent granulocyte-mediated killing of nonphagocytosable targets was investigated utilizing IgG-dependent granulocyte cytotoxicity directed against microfilariae of Dirofilaria immitis. Granulocyte-mediated killing of microfilariae is enhanced by addition of fresh serum. Lack of C4 did not significantly reduce the observed increase in cytotoxicity. The addition of highly purified monomeric human Clq (0.2 microgram/ml) in the presence of immune IgG resulted in a two- to fivefold enhancement of killing (P less than 0.025). C1q enhancement of killing occurred in the absence of fluid-phase IgG, but killing was significantly less than when both fluid-phase IgG and C1q were present. The effect of C1q was inhibited by the addition of solubilized type I collagen (44-92% inhibition of killing, P less than 0.05). Significant 125I-Clq binding to microfilariae occurred only in the presence of immune IgG. In addition, C1q in concentrations ranging from 0.5 to 2.0 micrograms/ml resulted in a dose-dependent increase in binding of 125I-immune IgG to microfilariae. Finally, when purified C1q was added to preopsonized, washed microfilariae, granulocyte production of superoxide was increased from 0.25 +/- 0.07 to 0.68 +/- 0.07 nm/10(6) cells.10 min (P less than 0.01). These results describe a novel functional role for C1q in enhancement of antibody-dependent cellular cytotoxicity towards nonphagocytosable targets.

Inhibition of the activation of Hageman factor (factor XII) by complement subcomponent C1q

Hageman factor (HF, Factor XII) is activated by glass, collagen, and ellagic acid, and initiates blood coagulation via the intrinsic pathway. C1q inhibits collagen-induced platelet aggregation and adherence of platelets to glass, effects attributable to the collagen-like region of C1q. We examined the actions of C1q on HF activation. Incubation of C1q with HF before addition of HF-deficient plasma extended the activated partial thromboplastin time. Similarly, when glass tubes were coated with C1q before testing, the partial thromboplastin time of normal plasma was increased. C1q reduced the activation of HF by ellagic acid, as measured by the release of p-nitroaniline from the synthetic substrate H-D-prolyl-L-phenylalanyl-L-arginine-p-nitroanilide dihydrochloride, an effect inhibited by monoclonal anti-human C1q murine IgG and by digestion of C1q by collagenase. Thus, C1q inhibits activation of HF in vitro in clot-promoting and amidolytic assays and suggests a regulatory mechanism for the inhibition of coagulation.

Selective inhibition of human natural killing and antibody-dependent cellular cytotoxicity by a polyanion

A high molecular polyanion, Liquoid, was found to inhibit at nontoxic concentrations (12-50 micrograms/ml) the natural killing (NK) and the antibody-dependent cellular cytotoxic (ADCC) activity of human peripheral blood mononuclear cells selectively. Whereas NK of the K 562 target cell was slightly or not at all affected, the spontaneous lysis of PDe-B-1, an EBV-transformed B-cell line, was strongly inhibited or even completely abolished. ADCC activity could only be inhibited by Liquoid if the target cells were mycoplasma-free, while the polyanion had no effect when mycoplasma-contaminated target cells were used. Liquoid did not alter the target binding capacity of the NK effector cells and did not activate monocytes or induce other suppressive cells. Alpha interferon, but neither beta nor gamma interferon, was able to neutralize the NK reduction. These results suggest that Liquoid inhibits a target cell-related, selective process in the post-binding stage of NK cell lysis.

Biosynthesis of the subcomponents C1q, C1r and C1s of the first component of complement (C1) by guinea pig hepatocyte primary cultures

Thus far, the synthesis of C1q by liver cells has not been demonstrated. To investigate this possibility, viable hepatocytes were isolated from the liver of guinea pigs and primary cultures were established. The cells (10(6) cells/ml) were cultured under serum-free conditions for 8 days and the culture medium was changed every 24 h. The few contaminating Kupffer cells were lysed by preincubating the cell cultures with a monoclonal (22C4-8) antibody directed against a nonpolymorphic Ia determinant and preabsorbed rabbit serum. The hemolytic activity of C1 and its subcomponents C1q and C1r/C1s was tested in the supernatants. Guinea pig hepatocyte primary cultures synthesize and secrete up to 3 X 10(3) effective C1q molecules/cell/24 h and 34 X 10(3) effective C1r/C1s molecules/cell/24 h. The synthesis of C1q and C1r/C1s could be reversibly inhibited by cycloheximide (50 micrograms/ml). Furthermore, to demonstrate de novo synthesis of the C1q subcomponent, endogeneous labeling with 3H-proline (or 14C-proline) was performed. The immunoprecipitated C1q from cellular lysates and culture medium was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. Compared to biosynthetically labeled guinea pig C1q from peritoneal macrophages, three corresponding bands (30, 28 and 24 kDa, respectively) were detectable in the fluorograph. The data show that guinea pig hepatocytes are able to synthesize C1 subcomponents, whereby the synthesis of C1q and C1r/C1s occurs independently.

Isolation and characterization of macrophage-derived C1q and its similarities to serum C1q

Recently, we have shown that the collagen-like, Fc-recognizing subcomponent C1q of the first complement component is synthesized by human, guinea pig and mouse peritoneal macrophages. To test whether macrophages may contribute to the serum pool of C1q, C1q was purified from guinea pig serum and from guinea pig peritoneal macrophage supernatants and compared for similarities. Both molecules had a similar sedimentation rate (macrophage C1q: 11.3 S, serum C1q: 11.2 S) and showed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions three identical bands with molecular weights of Mr, 29 000, Mr, 27 000 and Mr 23 000 for the A, B and C chains, respectively. Both C1q molecules migrated by immunoelectrophoresis in the gamma region and, in Ouchterlony analysis, showed complete antigenic identity with rabbit anti-serum C1q. These experiments demonstrate the antigenic and protein chemical similarities between serum C1q and C1q secreted by macrophages supporting the idea that macrophages have to be considered as one potential source of serum C1q. Furthermore, macrophage-derived C1q may be of importance in the local microenvironment at an inflammatory site involving macrophages.

C1q content of serum and lung lavage fluid of rats exposed to toxic levels of oxygen

We used a sensitive hemolytic assay to measure the level of C1q, a subcomponent of the first complement protein in the classic pathway, in bronchoalveolar lavage fluid and serum of rats exposed to air and hyperoxia. The serum level was 125 +/- 5 micrograms/ml and the lavage level was 41 +/- 17 ng/ml in rats breathing air. In rats exposed to acute oxygen toxicity (95% O2 for 66 hr), the serum level was at 107 +/- 10 micrograms/ml, but the level in lavage fluid increased to 2457 +/- 400 ng/ml (p less than 0.05 compared to air). Administration of the proline analogue cis-4-hydroxy-L-proline to air- and O2-exposed rats reduced the serum C1q level by 28% and 34%, respectively (both p less than 0.05), presumably by interfering with metabolism of the collagen-like sequence of C1q. The level of C1q in bronchoalveolar lavage fluid is a sensitive marker of acute lung injury.

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.

Molecular cloning and characterization of the complementary DNA and gene coding for the B-chain of subcomponent C1q of the human complement system

Plasmid clones containing cDNA coding for the B-chain of human Clq were isolated from a liver cDNA library. The longest cDNA insert isolated contained all the coding sequence for amino acid residues B1 to B226 plus a 3' non-translated region of 264 nucleotides that extended into the poly(A) tail, thus accounting for 950 nucleotides of the mRNA. The B-chain mRNA was estimated by Northern-blot analysis to be 1.46 kb (kilobases) long, which indicated that approx. 500 bases were not accounted for in the cDNA clone. A cosmid clone containing the C1q-B chain gene was isolated from a human genomic DNA library. The precise 5' limit of gene was not established, but from the data available it appears that the gene is approx. 2.6 kb long. The coding sequence for residues B1 to B226 in the gene is interrupted by one intron, of 1.1 kb, which is located within the codon coding for glycine at position B36. This glycine residue is located in the middle of the triple-helical regions found in C1q at exactly the position where there is an unusual structural feature, i.e. a bend in each of the helical regions brought about by the interruption of the Gly-Xaa-Yaa repeating triplet sequences in the A- and C-chains and the presence of an 'extra' triplet in the B-chain. Nucleotide sequencing of the 5' end of the gene indicates the presence of a predominantly hydrophobic stretch of 29 amino acids, immediately before residue B1, which could serve as a signal peptide.

Characterization of C1q, C1s and C-1 Inh synthesized by stimulated human monocytes in vitro

C1q, C1s and C1 Inh synthesized and secreted by human monocytes were characterized by SDS-PAGE. C1q is formed of three chains A (Mr approximately 35 000), B (Mr approximately 33 000) and C (Mr approximately 25 000) which are associated in two subunits A-B and C-C. It appears identical to C1q purified from plasma. C1s is secreted as a non-activated, monocatenar protein of Mr approximately 87 000 identical to proenzymic C1s from plasma. Secreted C1 Inh (Mr approximately 100 000) has a slightly higher Mr than purified plasmatic C1 Inh. Monensin treatment of the cells favours the intracytoplasmic accumulation of products at various glycosylation stages.

Effects of soluble aggregates of IgG on the binding, uptake and degradation of the C1q subcomponent of complement by adherent guinea pig peritoneal macrophages

Earlier studies have indicated that C1q, the first subcomponent of complement component C1, is bound to lymphocytes via specific C1q receptor sites. We have recently shown that adherent guinea pig peritoneal exudate macrophages express specific receptors for C1q (Veerhuis, R. et al., Immunology 1985. 54: 801). The present studies were performed to determine whether binding of 125I-labeled human C1q (125I-C1qhu) to adherent guinea pig peritoneal exudate macrophages would also result in ingestion and subsequent degradation of 125I-C1qhu. The binding of 125I-C1qhu to adherent peritoneal macrophages at 4 degrees C is inhibited fully not only by C1qhu and guinea pig C1q (C1qgp) but also by pepsin fragments of C1qhu. The amount of trichloroacetic acid nonprecipitable radioactivity that appeared in the supernatant was used as a measure for the degradation of 125I-C1qhu. 125I-C1qhu is degraded initially into fragments of 25 kDa, after which it is degraded further into small molecular weight peptides. Ingestion of 125I-C1q by the macrophages occurs before the 125I-C1q is degraded. In the presence of limited amounts of soluble aggregates of guinea pig IgG2 (AIgG), a known activator of C1, part of the C1q is bound to the AIgG and all of the AIgG in turn is bound to the cellular Fc receptors leading to an enhanced binding of 125I-C1q to the cells, a binding that was maximal at near equimolar concentrations of 125I-C1qhu and 131I-AIgG. In the presence of a 30-fold excess of AIgG, however, only a small percentage of the AIgG binds to cellular Fc receptors and the interaction of C1q with its receptor is decreased due to competitive inhibition. The results presented in this report thus suggest that free C1q may be eliminated by specific interaction with C1q receptors present on circulating and tissue phagocytoses and, in addition, that in the presence of immune complexes modulation of elimination of C1q may be encountered.

Synthesis of complement by macrophages and modulation of their functions through complement activation

During the last decade considerable progress has been made to characterize intimate functional links between macrophages, a major cellular component of immunoinflammatory responses, and the complement system representing the major humoral mediator of inflammation. Macrophages of various species and tissue sites have been shown to synthesize and release most of the complement components providing these cells with their own "pericellular" complement system. Circumstantial evidence for the assembly of both classical and alternative pathway convertases has been adduced. An intricate network of feedback loops involving endogenous and extrinsic factors operates to adjust complement production to acute requirements, for example augmenting production in the face of accelerated turnover at sites of inflammation, and returning it to baseline levels once the inflammatory stimulus has subsided, in order to maintain a fine-tuned balance. The molecular mechanisms underlying regulation of complement synthesis by macrophages are beginning to be elucidated by use of gene technology. On the other hand, complement activation products exert a number of effects on macrophages via specific surface receptors causing internalization of offending agents, microbes, and immune complexes, promotion of intracellular killing, controlling migration behavior, inducing release of potent biologic substances such as lysosomal enzymes, arachidonic acid metabolites, and interleukin 1. In these interactions, two important humoral mediator systems of inflammation, the complement system and the arachidonic acid cascade, are functionally linked at the level of the macrophage. Stimulation of the release of immunomodulating compounds from macrophages invoke a role for complement in immune regulation. This multifaceted interplay is of particular importance considering the mobility of macrophages that allows them to gain almost unrestricted access to sites of ongoing immunoinflammatory responses. The time seems to have come to abandon the petrified thinking in socalled systems as, for instance, humoral versus cellular, specific versus unspecific, and to proceed to interlocking functions guided by physiology proper.