C1q Receptor Research Articles

The Crystal Structure of the Bacillus anthracis Spore Surface Protein BclA Shows Remarkable Similarity to Mammalian Proteins

The lethal disease anthrax is propagated by spores of Bacillus anthracis, which can penetrate into the mammalian host by inhalation, causing a rapid progression of the disease and a mostly fatal outcome. We have solved the three-dimensional structure of the major surface protein BclA on B. anthracis spores. Surprisingly, the structure resembles C1q, the first component of complement, despite there being no sequence homology. Although most assays for C1q-like activity, including binding to C1q receptors, suggest that BclA does not mimic C1q, we show that BclA, as well as C1q, interacts with components of the lung alveolar surfactant layer. Thus, to better recognize and invade its hosts, this pathogenic soil bacterium may have evolved a surface protein whose structure is strikingly close to a mammalian protein.

Interaction of High Molecular Weight Kininogen with Endothelial Cell Receptors suPAR, gC1qR and Cytokeratin 1 by Surface Plasmon Resonance (BiaCore).

The physiologic activation of the plasma kallikrein kinin system (KKS) requires the assembly of these proteins on the cell membrane. High molecular weight kininogen (HK) binds to endothelial cells through an interaction with a multiprotein receptor complex that consist of: urokinase plasminogen activator receptor (uPAR), globular C1q receptor (gC1qR) and cytokeratin 1 (CK1). The affinity of HK and cleaved HK (Hka) for endothelial cells is KD=7–52 nM but the affinity for each of the three binding proteins is unknown. We first examined the direct binding of HK and Hka to the soluble receptor form of uPAR (suPAR), gC1qR and CK1 using surface plasmon resonance (BiaCore). We linked suPAR, gC1qR and CK1 (800–1100 pg/mm2) by amine coupling to a CM-5 chip and perfused HK and Hka at a concentration ranging from 50 to 400 nM in the presence or absence of 10 μM ZnCl2. A Langmuir binding model with local fit (stoichiometry of 1:1) was used to analyse kon (association rate constant), koff (dissociation rate constant) and KD (equilibrium dissociation constant) for HK and Hka.HKHkaKDkonkoffKDkonkoffImmobilized ProteinnMM-1s-1 x104s-1 x10-4nMM-1s-1 x104s-1 x10-4gC1qR0.8±0.712.3±5.00.8±0.50.7±0.524.4±10.91.4±0.5CK115.1±1.03.6±0.35.4±0.76.3±1.95.8±0.33.6±0.9suPAR2313.5±1465.31.0±0.7164.8±22.243.6±16.621.3±11.978.5±10.6The binding of HK and Hka to the three receptors was zinc dependent. The affinity for HK and Hka was gC1qR>CK1>suPAR. The high affinity for gC1qR and CK1 was similar for HK and Hka and was due to a very slow koff. The affinity for Hka compared to HK was 2.5-fold tighter for CK1 but 50-fold tighter for suPAR. A reversed immobiization was then performed. We immobilized HK, Hka and LK (800 pg/mm2) and suPAR, gC1qR and CK1 were flowed over the chip at a concentration ranging from 50 to 700 nM. Only gC1qR bound to immobilized HK and Hka, with a KD of 6.31± 2.8 pM for HK and 2.86±1.5 pM for Hka while the other 2 receptors did not bind. Receptor integrity was shown by preincubating HK or Hka with the pure receptors and observing competition with the immobilized receptors. Among the HK/Hka receptors, complex formation was only observed between gC1qR and immobiized CK1 with or without Hka indicating that a CK1-gC1qR complex can form independently of kininogen. This study indicates that although suPAR has the weakest affinity of the three receptors it is the only one that distinguishes between HK and Hka. Only one binary complex was revealed by this technique between CK1 and gC1qR. These results lay the foundation for anlayzing the affinity of these receptors in a cellular environment.

GC1q Receptor Ligation Selectively Down-Regulates Human IL-12 Production through Activation of the Phosphoinositide 3-Kinase Pathway

gC1qR, a complement receptor for C1q, plays a pivotal role in the regulation of inflammatory and antiviral T cell responses. Several pathogens, including hepatitis C virus, exploit gC1qR-dependent regulatory pathways to manipulate host immunity. However, the molecular mechanism(s) of gC1qR signaling involved in regulating inflammatory responses remains unknown. We report the selective inhibition of TLR4-induced IL-12 production after cross-linking of gC1qR on the surface of macrophages and dendritic cells. Suppression of IL-12 did not result from increased IL-10 or TGF-beta, but was dependent on PI3K activation. Activation of PI3K and subsequent phosphorylation of Akt define an intracellular pathway mediating gC1qR signaling and cross-talk with TLR4 signaling. This is the first report to identify signaling pathways used by gC1qR-mediated immune suppression, and it establishes a means of complement-mediated immune suppression to inhibit Th1 immunity crucial for clearing pathogenic infection.

Chemotaxis of human monocyte-derived dendritic cells to complement component C1q is mediated by the receptors gC1qR and cC1qR

Dendritic cells (DCs) are recruited to inflammatory sites where they phagocytose and process antigens for subsequent presentation to the T lymphocytes in the lymphoid tissue. Several leukocyte chemoattractants and their specific receptors have been shown to induce the migration of DC. The complement protein C1q has multiple immune functions including acting as a chemoattractant for neutrophils, eosinophils and mast cells. Therefore, the objective of this study was to determine if soluble C1q can induce chemotaxis of DC. Culturing cells in GM-CSF and IL-4 for 5 to 7 days generated human monocyte-derived DCs. In addition, LPS was added from day 5 to 7 to induce DC maturation. Cells were classified as either immature or mature DC by assessing the cell surface markers by flow cytometry, phagocytosis of dextran-FITC and T cell proliferation in an allogenic MLR. Immature DCs express the C1q receptors (C1qR), gC1qR and cC1qR/CR and, accordingly, display a vigorous migratory response to soluble C1q with maximal cell movement observed at 10–50 nM. In contrast, mature DCs neither express C1qR nor do move to a gradient of soluble C1q. Varying the concentration gradient of C1q (checkerboard assay) showed that the protein largely induces a chemotactic response. Finally, blocking gC1qR and cC1qR/CR by using specific antibodies abolished the chemotactic response to C1q but had no effect on a different chemoattractant C5a. These results clearly demonstrate that C1q functions as a chemotactic factor for immature DC, and migration is mediated through ligation of both gC1qR and cC1qR/CR.

Highly Immunogenic and Protective Recombinant Vaccine Candidate Expressed in Transgenic Plants

Vaccine development has been hampered by difficulties in developing new and safe adjuvants, so alternative technologies that offer new avenues forward are urgently needed. The goal of this study was to express a monoclonal recombinant immune complex in a transgenic plant. A recombinant protein consisting of a tetanus toxin C fragment-specific monoclonal antibody fused with the tetanus toxin C fragment was designed and expressed. Immune complex formation occurred between individual fusion proteins to form immune complex-like aggregates that bound C1q and FcgammaRIIa receptor and could be targeted to antigen-presenting cells. Unlike antigen alone, the recombinant immune fusion complexes were highly immunogenic in mice and did not require coadministration of an adjuvant (when injected subcutaneously). Indeed, these complexes elicited antibody titers that were more than 10,000 times higher than those observed in animals immunized with the antigen alone. Furthermore, animals immunized with only 1 mug of recombinant immune complex without adjuvant were fully protected against lethal challenge. This the first report on the use of a genetic fusion between antigen and antibody to ensure an optimal expression ratio between the two moieties and to obtain fully functional recombinant immune complexes as a new vaccine model.

Specific binding of L-ficolin and H-ficolin to apoptotic cells leads to complement activation

The serum lectins mannose-binding lectin (MBL), L-ficolin, and H-ficolin are recognition molecules in the lectin complement pathway, which play an important role in innate immunity. To assess involvement of the lectin pathway in the clearance of apoptotic cells, we used flow cytometry to quantify binding of MBL, L-ficolin, and H-ficolin to apoptotic HL60, U937, and Jurkat cells induced by actinomycin D. When apoptotic cells were incubated with normal human serum, MBL and L-ficolin bound to all three cell lines tested; moreover, H-ficolin bound to apoptotic Jurkat cells only. Subsequently, C4 and C3 were deposited on apoptotic cells of all three cell lines. MBL, L-ficolin, and H-ficolin binding to apoptotic cells was confirmed by the use of purified proteins. Purified C4 added to apoptotic cells that had bound pure L-ficolin was deposited on the cell surfaces. In L-ficolin-depleted serum, C3 deposition on HL60 or Jurkat cells decreased to approximately 50% or 70%, respectively, in comparison to the serum before L-ficolin depletion. We conclude that L-ficolin, in addition to MBL, recognizes apoptotic cells and activates complement via the lectin pathway. We also observed in vitro binding of L-ficolin and H-ficolin to cC1q receptor (C1q receptor specific for the collagenous region of C1q)/calreticulin, a candidate receptor for the collagenous region of MBL and C1q. Thus, L-ficolin and H-ficolin as well as MBL participate in the clearance of apoptotic cells through complement activation.

A role for lipid rafts in C1q-triggered O 2− generation by human neutrophils

Calreticulin, a candidate C1q receptor, was shown recently to be present on the surface of human neutrophils in association with glycosylphosphatidylinositol (GPI) anchored proteins, particularly CD59. In this study, we show that antibodies to CD59, as well as to every other GPI-anchored protein tested, inhibited the C1q-triggered release of O 2 − from PMN. Methyl β cyclodextrin (MβCD) treatment of the cells to disrupt lipid rafts also prevented C1q-triggered O 2 − production. β 2 integrin-dependent co-stimulation is required for O 2 − production from PMN, however MβCD had no effect on LFA-1 or Mac-1-mediated adhesion, soluble iC3b binding to PMN, or spreading and migration, all of which suggested that PMN integrin function remained intact. Flow cytometric analysis of PMN treated with MβCD showed upregulation of PMN granule-associated integrins and a corresponding increase in integrin activation-reporter epitopes, in contrast to the decreased expression of GPI-anchored antigens. These data support a model where lipid rafts and their associated GPI-anchored proteins are critical for C1q-triggered O 2 − production, consistent with a model where calreticulin serves as the C1q receptor for O 2 − production from PMN.

CC1q-R (calreticulin) and gC1q-R/p33: ubiquitously expressed multi-ligand binding cellular proteins involved in inflammation and infection

The first component of complement, C1, is a multi-molecular complex comprising of C1q and the Ca 2+-dependent tetramer C1r 2–C1s 2. The traditional role of C1q within the complex is that of recognition signal—a signal, which is instantly converted into a highly specific intramolecular proteolytic activation of the C1r 2–C1s 2 tetramer thereby triggering activation of the classical pathway. Another important function of C1q is its ability to bind to a wide range of cell types resulting in the induction of cell-specific biological responses. These cells include polymorphonuclear leukocytes, monocytes, lymphocytes, dendritic cells, endothelial cells and platelets. Interaction of C1q with endothelial cells and platelets, for example, leads to cellular activation followed by release of biological mediators and/or expression of adhesion molecules, all of which contribute, directly or indirectly to the inflammatory process. These specific responses are mediated by the interaction of C1q with C1q binding proteins or receptors on the cell surface. To date, four types of putative C1q binding cell surface expressed proteins/receptors have been described. These include cC1q-R/CR, or calreticulin (CR), a 60 kDa protein, which is also known as collectin receptor; gC1q-R/p33, a 33 kDa homotrimeric protein; C1q-Rp (CD93), a 120 kDa, O-sialoglycoprotein; and CR1 (CD35), the receptor for C3b. Although the specific role of each of these molecules in a given C1q-mediated cellular response is yet to be worked out, all of them may, in one form or another, participate in the inflammatory processes associated with vascular or atherosclerotic lesions, autoimmune diseases, or infections. The main focus of our laboratory for the past 20 years has been to elucidate the structure and function of cC1q-R/CR and gC1q-R/p33, both of which have been isolated and characterized on the basis of their ability to bind C1q. The purpose of this article is therefore to provide an up to date overview of these two proteins with particular emphasis on their unique structural and functional features, their multi-faceted nature and most importantly their role in infection and inflammation.

Cord blood and adult T cells show different responses to C1q-bearing immune complexes

We have previously shown that T cells can be activated through cell-surface C1q receptors, resulting in secretion of interferon-γ (IFN-γ) and tumor necrosis factor α (TNFα), further demonstrating the intimate linkage between innate and adaptive immunity. In this current report, we sought to determine whether: (1) T cell responses to C1q-bearing immune complexes are dependent on the maturational status of the T cells and (2) whether signaling through the C1q receptor on T cells modulates conventional activation mediated through the conventional T cell receptor (TCR)/CD3 signaling complex. We first examined the capacity of neonatal T cells to respond to C1q-bearing immune complexes using IFNγ, IL-2, and MIF secretion as measures of activation (MIF was chosen because of its crucial role in coordinating innate and adaptive immunity). Neonatal T cells produced significantly less IFNγ but not IL-2, when stimulated by C1q immune complexes compared with adult T cells. MIF levels did not exceed background levels in these experiments. Next, we examined the capacity of C1q-bearing immune complexes to regulate signaling through the conventional TCR/CD3 signaling complex. Pre-incubating adult T cells with C1q-bearing immune complexes significantly reduced IFNγ secretion when those same cells were subsequently stimulated with anti-CD3 and anti-CD28 monoclonal antibodies. Pre-incubation of neonatal T-cells with C1q-bearing immune complexes had no effect on IFNγ secretion, although IFNγ secretion was lower than that found in adult T cells for each experimental condition. We speculate that reduced IFNγ secretion after pre-incubation with C1q immune complexes may be due to IL-10 secretion, which was observed in C1q-stimulated adult (but not neonatal) T cells. Conclusions: C1q-bearing immune complexes exert complex effects on mature T cells that include both pro- and anti-inflammatory responses. Immunologic maturation is required for these effects, as cord blood T cells are relatively hyporesponsive to C1q-bearing immune complexes compared with adult T cells.

The C4b-binding Protein-Protein S Complex Inhibits the Phagocytosis of Apoptotic Cells

The phagocytosis of apoptotic cells is a complex process involving numerous interactions between the target cell and the macrophage. We have examined a role of the major soluble inhibitor of the classic and lectin complement pathways, C4b-binding protein (C4BP), in the clearance of apoptotic cells. The major form of C4BP present in blood is composed of seven alpha-chains and one beta-chain, which binds protein S (PS). Approximately 70% of all PS in human plasma is trapped in such a complex and is able to localize C4BP to the surface of apoptotic cells due to the high affinity to phosphatidylserine. Free PS has recently been shown to enhance phagocytosis of apoptotic cells by macrophages. We observed a stimulatory effect of free PS on the engulfment of apoptotic cells (BL-41 and Jurkat) by primary human macrophages or THP-1 cells and a decrease of activity in serum depleted of PS in agreement with previous results. However, we also show that the process is strongly inhibited in the presence of the C4BP-PS complex. Addition of the C4BP-PS complex to serum deficient in both molecules abolished the enhancing effect of serum on phagocytosis. The effect of both free PS and the C4BP-PS complex could be inhibited with monoclonal antibody directed against the Gla domain of PS. Although the presence of the C4BP-PS complex on apoptotic cells may lead to decreased phagocytosis, it may still be beneficial to the host, since it could prevent secondary necrosis because it inhibits further complement attack.

Hepatitis C virus core and nonstructural proteins induce fibrogenic effects in hepatic stellate cells

Background & Aims : The mechanisms by which hepatitis C virus (HCV) induces liver fibrosis are unknown. Hepatocytes secrete HCV proteins, which may interact with hepatic stellate cells (HSCs). Our aims were to investigate whether HCV proteins induce fibrogenic effects on HSCs. Methods & Results : Human-activated HSCs expressed messenger RNA (mRNA) for the putative HCV receptors CD81, LDL receptor, and C1q receptor as assessed by RT-PCR. Incubation of activated but not quiescent human HSCs with recombinant core and NS3 protein increased intracellular calcium concentration and reactive oxygen species production, as well as stimulated intracellular signaling pathways. Adenoviruses encoding core and nonstructural proteins (NS3-NS5) were used to express HCV proteins in HSCs. Expression of core protein increased cell proliferation in a Ras/ERK and PI3K/AKT dependent manner. In contrast, NS3-NS5 protein expression preferentially induced proinflammatory actions, such as increased chemokine secretion and expression of intercellular cell adhesion molecule type 1 (ICAM-1) through the NF-κB and c-Jun N-terminal kinase pathways. These effects were attenuated by antioxidants. Infection of freshly isolated rat HSCs with adenovirus-encoding core protein resulted in accelerated cell activation, as assessed by α-smooth muscle actin expression. Moreover, adenovirus-encoding core and NS3-NS5 proteins increased the secretion of bioactive TGFβ1 and the expression of procollagen α1(I) in early cultured rat HSCs, as assessed by ELISA and RNase protection assay, respectively. Conclusions : HCV core and nonstructural proteins regulate distinct biologic functions in HSCs. A direct interaction between HCV proteins and HSCs may contribute to HCV-induced liver fibrosis.

Effect of His-Gly-Lys Motif Derived from Domain 5 of High Molecular Weight Kininogen on Suppression of Cancer Metastasis Both in Vitro and in Vivo

We have demonstrated previously that kinin-free high molecular weight kininogen, its domain 5 (D5H, Gly402-Lys502), and peptides derived from D5H inhibited vitronectin-mediated migration and invasion of cancer cells in vitro (Kamiyama, F., Maeda, T., Yamane, T., Li, Y. H., Ogikubo, O., Otsuka, T., and Ohkubo, I. (2001) Biochem. Biophys. Res. Commun. 288, 975-980). In this study, we found that the amino acid sequence His-Gly-Lys (HGK) in D5H is the core motif for inhibition of adhesion and invasion of MDA-MB-231 cells in vitro. P-5m (484GHGKHKNK491, Gly484-Lys491), an octapeptide including the HGK motif derived from D5H, and HGK, a tripeptide, inhibited both cell adhesion and invasion in vitro. However, an octapeptide designated P-5m (K487R), in which Lys487 was changed to Arg, did not inhibit either cell adhesion or invasion, and peptides HGR and HGG also had no inhibitory effect. Recombinant GST-D5H expressed in Escherichia coli had a stronger inhibitory effect on cell adhesion and invasion in vitro than did GST-D5H (K487R) in which Lys487 was changed to Arg. Furthermore, P-5m (Gly484-Lys491) peptide clearly suppressed lung metastasis in mice experimentally induced by using B16-F10 cells, but P-5m (G487R) had no effect. These data strongly indicate that both the HGK motif and lysine residue (Lys487) play essential roles in inhibition of cell adhesion and invasion in vitro and in prevention of metastasis of cancer cells in vivo. We tried to identify the HGK motif binding protein on the surface of cancer cells. A 95-kDa surface biotin-labeled membrane protein was specifically detached from GST-D5H by P-5 (His479-Lys493) peptide but not by P-1 (Gly402-Lys420) peptide originating from the N-terminal region of D5H.

Ligation of the C1q receptor, gC1qR/p33, associates with both the α vβ 3 integrin and vitronectin and causes protein phosphorylation

Objective: Recent evidence indicates that certain complement components may play a specific role in mammalian fertilization. C1q, the first molecule in the classical complement cascade, has the ability to promote sperm agglutination as well as altering sperm-oolemma binding and fusion. We have previously detected an increase in the expression of its receptor, gC1qR, over the rostral portion of the sperm head, occurring after capacitation, and have shown that its ligation leads to an increase in intracellular calcium and acrosomal exocytosis.

Calreticulin Is at the Surface of Circulating Neutrophils and Uses CD59 as an Adaptor Molecule

Calreticulin, which has been proposed to be a C1q receptor on neutrophils, has neither a transmembrane domain nor a GPI-anchor attachment site and must utilize an adaptor molecule to attach to the plasma membrane. The expression of ecto-calreticulin on purified human neutrophils did not result from contamination by soluble or intracellular calreticulin released during cell fractionation because it was expressed on circulating neutrophils, and the expression did not increase significantly with neutrophil isolation. All neutrophils expressed calreticulin with a unimodal distribution. Treatment of neutrophils with either a cholesterol-binding agent or phosphatidylinositol-specific phospholipase C dramatically decreased ecto-calreticulin expression indicating that the adaptor molecule(s) are located in lipid rafts and have a GPI-anchor. Analysis for the co-expression of specific GPI-anchored proteins and ecto-calreticulin in cells that were deficient in specific GPI-anchored proteins, indicated that ecto-calreticulin was best associated with CD59. Calreticulin reciprocally immunoprecipited with CD59, which provided direct evidence that CD59 is an adaptor for ecto-calreticulin. Immunofluorescence and confocal microscopy demonstrated that ecto-calreticulin co-localized with a fraction of CD59 at the cell surface. Cross-linking ecto-calreticulin with antibodies induced a Ca2+ flux, which suggests that ecto-calreticulin is capable of signaling following ligand binding. Ecto-calreticulin has been associated with diverse cellular functions. An appreciation that the adaptors for ecto-calreticulin are GPI-anchored will provide a framework for understanding any common features underlying ecto-calreticulin ligation.

Role of C1q and C1q receptors in the pathogenesis of systemic lupus erythematosus.

The association between C1q and autoimmune diseases such as rheumatoid arthritis and systemic lupus erythematosus (SLE) is well established. Deficiency in C1q is considered to be a strong susceptibility factor and is corroborated by the fact that > or = 92% of the known cases of hereditary deficiency in C1q develop rheumatic disease. Furthermore, the observation of the presence of high-affinity autoantibodies against C1q antibodies in patients with SLE provides a strong correlation between these antibodies and the inflammatory processes that occur in this disease. Recent evidence using C1q-deficient mice has shown the presence of glomerulonephritis with immune deposits and a large number of apoptotic bodies in the diseased glomeruli suggesting a defect in the clearance of apoptotic cell by macrophages and dendritic cells (DCs). Although these data are consistent with the hypothesis that C1q deficiency may induce a generalized failure to clear immune complexes and apoptotic cells, this concept alone cannot wholly explain why individuals with C1q deficiency are prone to develop SLE. Therefore, C1q alone or in conjunction with other surface molecules must play a much more fundamental role in immunoregulation, especially those processes that regulate T cell function and tolerance. In support of this hypothesis is the finding that C1q causes inhibition of mitrogen-induced T cell-proliferative response by interaction with C1q receptors. Furthermore, macrophages and possibly DCs not only synthesize but also display C1q as a type II cell surface molecule, especially at sites of inflammation. Although it is not yet known what role the surface-expressed C1q plays, it is tempting to assume that it plays a role in the priming of naïve T cells by DCs. This work will review the current concepts of the role of C1q and C1q receptors in autoimmunity.

Complement component C1q induces endothelial cell adhesion and spreading through a docking/signaling partnership of C1q receptors and integrins

The interaction of C1q with endothelial cells elicits a multiplicity of biologic responses. Although these specific responses are thought to be mediated by the interaction of C1q with proteins of the endothelial cell surface, the molecular identity of the participant(s) has not been clearly defined. In this study, we examined the role of two C1q-binding proteins, cC1q-R/CR and gC1q-R/p33, on C1q-mediated adhesion and spreading of human dermal microvascular endothelial cells (HDMVECs). A specific and dose-dependent adhesion and spreading was observed when HDMVECs were cultured in microtiter plate wells coated with concentrations of C1q ranging from 0 to 50 μg/ml. The extent of adhesion and spreading was similar to the adhesion seen on collagen-coated wells. Furthermore, the effect of C1q was mimicked by either polyclonal anti-cC1q-R or mAb 60.11, but not with isotype- and species-matched control IgG. More importantly, however, a 100% inhibition of spreading but not adhesion to C1q-coated wells was observed when HDMVECs were cultured in the presence of 30 mM of the peptide GRRGDSP but not GRRGESP. Furthermore, while anti-β1 integrin antibody blocked adhesion and spreading, antiα5 integrin only blocked spreading. Since earlier studies have shown that zinc induces the exposure of hydrophobic sites in the C-terminus of gC1q-R including the putative high-molecular weight kininogen (HK)-binding site corresponding to residues 204–218, we also examined the effect of zinc on antibody binding to cell surface gC1q-R. Flow cytometric data show that the binding of mAb 74.5.2, which recognizes residues 204–218, is greatly enhanced when endothelial cells were incubated in the presence of 50 μM zinc. In summary, our data show that: (a) C1q-mediated endothelial cell adhesion and spreading requires the cooperation of both C1q receptors and 1 integrins, and possibly other membrane-spanning molecules, and (b) zinc can induce the exposure of hydrophobic sites in the C-terminal domain of gC1q-R allowing a more efficient binding of mAb 74.5.2 and HK.

Structure-function studies of the receptors for complement C1q

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

Maturation-dependent expression of C1q binding proteins on the cell surface of human monocyte-derived dendritic cells

The expression and cell surface levels of many important receptors are dependent on the maturation stage of dendritic cells (DCs), and related to the unique function of immature and mature DCs. In this report, we show, for the first time, that human monocyte-derived DCs express two types of C1q receptors, gC1qR and cC1qR. Furthermore, immature DCs secrete detectable amount of C1q into the culture supernatant. Immature DCs express higher cell surface levels of both C1qRs than mature ones, while the total C1qR protein and mRNA levels remain the same. The following experimental evidence support this conclusion: (1) Inflammatory cytokines and LPS, which induce maturation of DCs, downregulate surface expression of both C1qR molecules. (2) Cytokines and drugs (IL-10, IFN-α, Dexamethasone), which keep DCs phenotypically and functionally immature, significantly upregulate the cell surface expression of both C1qRs. (3) Neither of these treatments changed the intracellular gC1qR level nor the gC1qR mRNA levels measured by real time RT-PCR. The elevated surface expression of C1qRs on DCs has been found to be not due to increased apoptosis or cell death as the result of DC treatment. Taken together, these data show that human monocyte-derived DCs express gC1qR and cC1qR, their expression on the cell surface is maturation dependent, and immature DCs secrete C1q. These data strongly suggest the role of C1qRs in immature DC function and in the regulation of immune processes.

Structure-function studies of the receptors for complement C1q

Structure-function studies of the receptors for complement C1q

C1qRp defines a new human stem cell population with hematopoietic and hepatic potential.

The characterization of two distinct classes of hematopoietic stem cells based on CD34 expression and the ability of human bone marrow (BM) cells to differentiate into nonhematopoietic cells introduced new levels of complexity within the stem cell compartment. Here we report the identification and purification of a rare human stem cell population with hematopoietic and hepatic potential based on the expression of a receptor for the complement molecule C1q (C1qR(p)). We show that C1qR(p) is a positive marker of all BM-repopulating stem cells because it is expressed on both CD34(-) and CD34(+) stem cells from umbilical cord blood and adult BM. In addition, we show that highly purified lineage-negative CD45(+)CD38(-)CD34(+or-)C1qR(p)(+) cells not only have BM-repopulating capacity but also can differentiate into human hepatocytes in vivo. The identification of human hepatocytes in mouse livers indicates that the NOD/SCID (nonobese diabetic/severe combined immunodeficient) mouse model can be a valuable tool to study the differentiation potential of adult human stem cells. These findings may have important scientific and clinical implications in the field of human stem cell biology and transplantation.