Identification of a gC1q-binding protein (gC1q-R) on the surface of human neutrophils. Subcellular localization and binding properties in comparison with the cC1q-R

The Globular C1q Receptor Is Required for Epidermal Growth Factor Receptor Signaling during Candida albicans Infection

During oropharyngeal candidiasis, Candida albicans activates the epidermal growth factor receptor (EGFR), which induces oral epithelial cells to endocytose the fungus and synthesize proinflammatory mediators. To elucidate EGFR signaling pathways that are stimulated by C. albicans, we used proteomics to identify 1,214 proteins that were associated with EGFR in C. albicans-infected cells. Seven of these proteins were selected for additional study. Among these proteins, WW domain-binding protein 2, Toll-interacting protein, interferon-induced transmembrane protein 3 (IFITM3), and the globular C1q receptor (gC1qR) were found to associate with EGFR in viable oral epithelial cells. Each of these proteins was required for maximal endocytosis of C. albicans, and all regulated fungus-induced production of interleukin-1β (IL-1β) and/or IL-8, either positively or negatively. gC1qR was found to function as a key coreceptor with EGFR. Interacting with the C. albicans Als3 invasin, gC1qR was required for the fungus to induce autophosphorylation of both EGFR and the ephrin type A receptor 2. The combination of gC1qR and EGFR was necessary for maximal endocytosis of C. albicans and secretion of IL-1β, IL-8, and granulocyte-macrophage colony-stimulating factor (GM-CSF) by human oral epithelial cells. In mouse oral epithelial cells, inhibition of gC1qR failed to block C. albicans-induced phosphorylation, and knockdown of IFITM3 did not inhibit C. albicans endocytosis, indicating that gC1qR and IFITM3 function differently in mouse versus human oral epithelial cells. Thus, this work provides an atlas of proteins that associate with EGFR and identifies several that play a central role in the response of human oral epithelial cells to C. albicans infection.

HCV Core Protein Induces Chemokine CCL2 and CXCL10 Expression Through NF-κB Signaling Pathway in Macrophages

HCV core protein is the first structural protein synthesized during hepatitis C virus (HCV) infection and replication. It is released from virus infected liver cells and mediates multiple functions to affect host cell response. The innate immune response is the first line of defense against viral infection. After HCV infection, Kupffer cells (KCs) which are liver macrophages play an important role in host innate immune response. Kupffer cells act as phagocytes and release different cytokines and chemokines to counter viral infection and regulate inflammation and fibrosis in liver. Earlier, we have demonstrated that HCV core protein interacts with gC1qR and activates MAPK, NF-κB and PI3K/AKT pathways in macrophages. In this study, we explored the effect of HCV core protein on CCL2 and CXCL10 expression in macrophages and the signaling pathways involved. Upon silencing of gC1qR, we observed a significant decrease expression of CCL2 and CXCL10 in macrophages in the presence of HCV core protein. Inhibiting NF-κB pathway, but not P38, JNK, ERK and AKT pathways greatly reduced the expression of CCL2 and CXCL10. Therefore, our results indicate that interaction of HCV core protein with gC1qR could induce CCL2 and CXCL10 secretion in macrophages via NF-κB signaling pathway. These findings may shed light on the understanding of how leukocytes migrate into the liver and exaggerate host-derived immune responses and may provide novel therapeutic targets in HCV chronic inflammation.

Anti gC1qR/p32/HABP1 Antibody Therapy Decreases Tumor Growth in an Orthotopic Murine Xenotransplant Model of Triple Negative Breast Cancer

gC1qR is highly expressed in breast cancer and plays a role in cancer cell proliferation. This study explored therapy with gC1qR monoclonal antibody 60.11, directed against the C1q binding domain of gC1qR, in a murine orthotopic xenotransplant model of triple negative breast cancer. MDA231 breast cancer cells were injected into the mammary fat pad of athymic nu/nu female mice. Mice were segregated into three groups (n = 5, each) and treated with the vehicle (group 1) or gC1qR antibody 60.11 (100 mg/kg) twice weekly, starting at day 3 post-implantation (group 2) or when the tumor volume reached 100 mm³ (group 3). At study termination (d = 35), the average tumor volume in the control group measured 895 ± 143 mm³, compared to 401 ± 48 mm³ and 701 ± 100 mm³ in groups 2 and 3, respectively (p < 0.05). Immunohistochemical staining of excised tumors revealed increased apoptosis (caspase 3 and TUNEL staining) in 60.11-treated mice compared to controls, and decreased angiogenesis (CD31 staining). Slightly decreased white blood cell counts were noted in 60.11-treated mice. Otherwise, no overt toxicities were observed. These data are the first to demonstrate an in vivo anti-tumor effect of 60.11 therapy in a mouse model of triple negative breast cancer.

Complement activation by IgG containing immune complexes regulates the interaction of C1q with its ligands

Classical pathway activation of the compl ement system is initiated by the binding of the globular head domains of glycoprotein C1q to its corresponding ligand leading to both C1 activation and C3 convertase formation. However, the whereabouts and function of C1q after complement activation have only been marginally investigated. This report presents two mechanisms of action that remove bound C1q from a complement activating IgG immune complex in concentrated serum. The first mechanism details that sequential activation of the classical and alternative pathways releases bound C1q from an immune complex and that the dissociated C1q is subsequently found in complex with complement fragment C3c. The second mechanism is the displacement of C1q from an immune complex by the addition of near physiologic concentrations of purified or serum C1q. This activity can also be demonstrated using serum depleted of C3, normal serum chelated in EDTA, or purified C1. Fresh C1q in C3-depleted serum was found to replace dissociated C1q on the immune complex. C1q dissociated from immune complexes by the mechanism of C1q displacement is able to bind B and T lymphoblastoid cells that express receptors and ligands for both the collagen like region and the globular head domains of C1q. C1q dissociated from immune complexes by the mechanism of C3 activation do not bind these cells. This result suggests that C3 bound to C1q during complement activation and dissociation interferes with the ability of released C1q to access C1q receptors and ligands, particularly receptors for the globular head domains. These underlying mechanisms that regulate the interaction of C1q with its ligands reveal a novel function for complement activation during the immune response.

Neutrophils in innate immunity and systems biology-level approaches

The innate immune system is the first line of host defense against invading microorganisms. Polymorphonuclear leukocytes (PMNs or neutrophils) are the most abundant leukocyte in humans and essential to the innate immune response against invading pathogens. Compared to the acquired immune response, which requires time to develop and is dependent on previous interaction with specific microbes, the ability of neutrophils to kill microorganisms is immediate, nonspecific, and not dependent on previous exposure to microorganisms. Historically, studies of PMN-pathogen interaction focused on the events leading to killing of microorganisms, such as recruitment/chemotaxis, transmigration, phagocytosis, and activation, whereas postphagocytosis sequelae were infrequently considered. In addition, it was widely accepted that human neutrophils possessed limited capacity for new gene transcription and thus, relatively little biosynthetic capacity. This notion has changed dramatically within the past 20 years. Further, there is now more effort directed to understand the events occurring in PMNs after killing of microbes. Herein, we give an updated review of the systems biology-level approaches that have been used to gain an enhanced view of the role of neutrophils during host-pathogen interaction and neutrophil-mediated diseases. We anticipate that these and future systems-level studies will continue to provide information important for understanding, treatment, and control of diseases caused by pathogenic microorganisms. This article is categorized under: Physiology > Organismal Responses to Environment Physiology > Mammalian Physiology in Health and Disease Biological Mechanisms > Cell Fates.

Complement component 1q subcomponent binding protein in the brain of the rat

Complement component 1q subcomponent binding protein (C1qbp) is a multifunctional protein involved in immune response, energy homeostasis of cells as a plasma membrane receptor, and a nuclear, cytoplasmic or mitochondrial protein. Recent reports suggested its neuronal function, too, possibly in axon maintenance, synaptic function, and neuroplasticity. Therefore, we addressed to identify C1qbp in the rat brain using in situ hybridization histochemistry and immunolabelling at light and electron microscopic level. C1qbp has a topographical distribution in the brain established by the same pattern of C1qbp mRNA-expressing and protein-containing neurons with the highest abundance in the cerebral cortex, anterodorsal thalamic nucleus, hypothalamic paraventricular (PVN) and arcuate nuclei, spinal trigeminal nucleus. Double labelling of C1qbp with the neuronal marker NeuN, with the astrocyte marker S100, and the microglia marker Iba1 demonstrated the presence of C1qbp in neurons but not in glial cells in the normal brain, while C1qbp appeared in microglia following their activation induced by focal ischemic lesion. Only restricted neurons expressed C1qbp, for example, in the PVN, magnocellular neurons selectively contained C1qbp. Further double labelling by using the mitochondria marker Idh3a antibody suggested the mitochondrial localization of C1qbp in the brain, confirmed by correlated light and electron microscopy at 3 different brain regions. Post-embedding immunoelectron microscopy also suggested uneven C1qbp content of mitochondria in different brain areas but also heterogeneity within single neurons. These data suggest a specific function of C1qbp in the brain related to mitochondria, such as the regulation of local energy supply in neuronal cells.

Analysis of the Interaction between Globular Head Modules of Human C1q and Its Candidate Receptor gC1qR

The heterotrimeric globular head (gC1q) domain of human C1q is made up of the C-terminal ends of the three individual chains, ghA, ghB, and ghC. A candidate receptor for the gC1q domain is a multi-functional pattern recognition protein, gC1qR. Since understanding of gC1qR and gC1q interaction could provide an insight into the pleiotropic functions of gC1qR, this study was undertaken to identify the gC1qR-binding site on the gC1q domain, using the recombinant ghA, ghB, and ghC modules and their substitution mutants. Our results show that ghA, ghB, and ghC modules can interact with gC1qR independently, thus reinforcing the notion of modularity within the gC1q domain of human C1q. Mutational analysis revealed that while Arg162 in the ghA module is central to interaction between gC1qR and C1q, a single amino acid substitution (arginine to glutamate) in residue 114 of the ghB module resulted in enhanced binding. Expression of gC1qR and C1q in adherent monocytes with or without pro-inflammatory stimuli was also analyzed by qPCR; it showed an autocrine/paracrine basis of C1q and gC1qR interaction. Microscopic studies revealed that C1q and gC1qR are colocalized on PBMCs. Cell proliferation assays indicated that ghA, ghB, and ghC modules were able to attenuate phytohemagglutinin-stimulated proliferation of PBMCs. Addition of gC1qR had an additive effect on the anti-proliferative effect of globular head modules. In summary, our results identify residues involved in C1q-gC1qR interaction and explain, to a certain level, their involvement on the immune cell surface, which is relevant for C1q-induced functions including inflammation, infection, and immunity.

A calreticulin/gC1qR complex prevents cells from dying: a conserved mechanism from arthropods to humans

The crossroad between cell death and proliferation is a general target for viral infections because viruses need to obstruct apoptosis to use cells for their own replication. Inducing immunogenic cell death in proliferating cells is also an important aim of anticancer chemotherapy. The C1q-binding proteins calreticulin (CRT) and gC1qR are highly conserved ubiquitous proteins, which are putative targets for viral manipulation and are associated with cancer. Here we show that these proteins form a complex in the cytoplasm as a response to viral infection resulting in apoptosis prevention. The formation of a cytosolic CRT/gC1qR complex prevents cell death by reducing gC1qR translocation into the mitochondria, and we provide evidence that this mechanism is conserved from arthropods to human cancer cells. Furthermore, we show that it is possible to prevent this complex from being formed in cancer cells. When the peptides of the complex proteins are overexpressed in these cells, the cells undergo apoptosis. This finding shows a causal link between virus and cancer and may be used to develop new tools in anticancer or antiviral therapy.

Interaction of HmC1q with leech microglial cells: involvement of C1qBP-related molecule in the induction of cell chemotaxis

Background

In invertebrates, the medicinal leech is considered to be an interesting and appropriate model to study neuroimmune mechanisms. Indeed, this non-vertebrate animal can restore normal function of its central nervous system (CNS) after injury. Microglia accumulation at the damage site has been shown to be required for axon sprouting and for efficient regeneration. We characterized HmC1q as a novel chemotactic factor for leech microglial cell recruitment. In mammals, a C1q-binding protein (C1qBP alias gC1qR), which interacts with the globular head of C1q, has been reported to participate in C1q-mediated chemotaxis of blood immune cells. In this study, we evaluated the chemotactic activities of a recombinant form of HmC1q and its interaction with a newly characterized leech C1qBP that acts as its potential ligand.

Methods

Recombinant HmC1q (rHmC1q) was produced in the yeast Pichia pastoris. Chemotaxis assays were performed to investigate rHmC1q-dependent microglia migration. The involvement of a C1qBP-related molecule in this chemotaxis mechanism was assessed by flow cytometry and with affinity purification experiments. The cellular localization of C1qBP mRNA and protein in leech was investigated using immunohistochemistry and in situ hybridization techniques.

Results

rHmC1q-stimulated microglia migrate in a dose-dependent manner. This rHmC1q-induced chemotaxis was reduced when cells were preincubated with either anti-HmC1q or anti-human C1qBP antibodies. A C1qBP-related molecule was characterized in leech microglia.

Conclusions

A previous study showed that recruitment of microglia is observed after HmC1q release at the cut end of axons. Here, we demonstrate that rHmC1q-dependent chemotaxis might be driven via a HmC1q-binding protein located on the microglial cell surface. Taken together, these results highlight the importance of the interaction between C1q and C1qBP in microglial activation leading to nerve repair in the medicinal leech.

Neutrophils in innate host defense against Staphylococcus aureus infections

Staphylococcus aureus has been an important human pathogen throughout history and is currently a leading cause of bacterial infections worldwide. S. aureus has the unique ability to cause a continuum of diseases, ranging from minor skin infections to fatal necrotizing pneumonia. Moreover, the emergence of highly virulent, drug-resistant strains such as methicillin-resistant S. aureus in both healthcare and community settings is a major therapeutic concern. Neutrophils are the most prominent cellular component of the innate immune system and provide an essential primary defense against bacterial pathogens such as S. aureus. Neutrophils are rapidly recruited to sites of infection where they bind and ingest invading S. aureus, and this process triggers potent oxidative and non-oxidative antimicrobial killing mechanisms that serve to limit pathogen survival and dissemination. S. aureus has evolved numerous mechanisms to evade host defense strategies employed by neutrophils, including the ability to modulate normal neutrophil turnover, a process critical to the resolution of acute inflammation. Here we provide an overview of the role of neutrophils in host defense against bacterial pathogens and discuss strategies employed by S. aureus to circumvent neutrophil function.

Cofactor regulation of C5a chemotactic activity in physiological fluids. Requirement for the vitamin D binding protein, thrombospondin-1 and its receptors

Factors in physiological fluids that regulate the chemotactic activity of complement activation peptides C5a and C5a des Arg are not well understood. The vitamin D binding protein (DBP) has been shown to significantly enhance chemotaxis to C5a/C5a des Arg. More recently, platelet-derived thrombospondin-1 (TSP-1) has been shown to facilitate the augmentation of C5a-induced chemotaxis by DBP. The objective of this study was to better characterize these chemotactic cofactors and investigate the role that cell surface TSP-1 receptors CD36 and CD47 may play in this process. The chemotactic activity in C-activated normal serum, citrated plasma, DBP-depleted serum or C5 depleted serum was determined for both normal human neutrophils and U937 cell line transfected with the C5a receptor (U937-C5aR). In addition, levels of C5a des Arg, DBP and TSP-1 in these fluids were measured by RIA or ELISA. Results show that there is a clear hierarchy with C5a being the essential primary signal (DBP or TSP-1 will not function in the absence of C5a), DBP the necessary cofactor and TSP-1 a dependent tertiary factor, since it cannot function to enhance chemotaxis to C5a without DBP. Measurement of the C5a-induced intracellular calcium flux confirmed the same hierarchy observed with chemotaxis. Moreover, analysis of bronchoalveolar lavage fluid (BALF) from patients with the adult respiratory distress syndrome (ARDS) demonstrated that C5a-dependent chemotactic activity is significantly decreased after anti-DBP treatment. Finally, results show that TSP-1 utilizes cell surface receptors CD36 and CD47 to augment chemotaxis, but DBP does not bind to TSP-1, CD36 or CD47. The results clearly demonstrate that C5a/C5a des Arg needs both DBP and TSP-1 for maximal chemotactic activity and suggest that the regulation of C5a chemotactic activity in physiological fluids is more complex than previously thought.

The multicompartmental p32/gClqR as a new target for antibody-based tumor targeting strategies

Tumor-associated cell surface antigens and tumor-associated vascular markers have been used as a target for cancer intervention strategies. However, both types of targets have limitations due to accessibility, low and/or heterogeneous expression, and presence of tumor-associated serum antigen. It has been previously reported that a mitochondrial/cell surface protein, p32/gC1qR, is the receptor for a tumor-homing peptide, LyP-1, which specifically recognizes an epitope in tumor cells, tumor lymphatics, and tumor-associated macrophages/myeloid cells. Using antibody phage technology, we have generated an anti-p32 human monoclonal antibody (2.15). The 2.15 antibody, expressed in single-chain fragment variable and in trimerbody format, was then characterized in vivo using mice grafted subcutaneously with MDA-MB-231 human breast cancers cells, revealing a highly selective tumor uptake. The intratumoral distribution of the antibody was consistent with the expression pattern of p32 in the surface of some clusters of cells. These results demonstrate the potential of p32 for antibody-based tumor targeting strategies and the utility of the 2.15 antibody as targeting moiety for the selective delivery of imaging and therapeutic agents to tumors.

HIV gp41 engages gC1qR on CD4+ T cells to induce the expression of an NK ligand through the PIP3/H2O2 pathway

CD4⁺ T cell loss is central to HIV pathogenesis. In the initial weeks post-infection, the great majority of dying cells are uninfected CD4⁺ T cells. We previously showed that the 3S motif of HIV-1 gp41 induces surface expression of NKp44L, a cellular ligand for an activating NK receptor, on uninfected bystander CD4⁺ T cells, rendering them susceptible to autologous NK killing. However, the mechanism of the 3S mediated NKp44L surface expression on CD4⁺ T cells remains unknown. Here, using immunoprecipitation, ELISA and blocking antibodies, we demonstrate that the 3S motif of HIV-1 gp41 binds to gC1qR on CD4⁺ T cells. We also show that the 3S peptide and two endogenous gC1qR ligands, C1q and HK, each trigger the translocation of pre-existing NKp44L molecules through a signaling cascade that involves sequential activation of PI3K, NADPH oxidase and p190 RhoGAP, and TC10 inactivation. The involvement of PI3K and NADPH oxidase derives from 2D PAGE experiments and the use of PIP3 and H2O2 as well as small molecule inhibitors to respectively induce and inhibit NKp44L surface expression. Using plasmid encoding wild type or mutated form of p190 RhoGAP, we show that 3S mediated NKp44L surface expression on CD4⁺ T cells is dependent on p190 RhoGAP. Finally, the role of TC10 in NKp44L surface induction was demonstrated by measuring Rho protein activity following 3S stimulation and using RNA interference. Thus, our results identify gC1qR as a new receptor of HIV-gp41 and demonstrate the signaling cascade it triggers. These findings identify potential mechanisms that new therapeutic strategies could use to prevent the CD4⁺ T cell depletion during HIV infection and provide further evidence of a detrimental role played by NK cells in CD4⁺ T cell depletion during HIV-1 infection.

HIV infected individuals suffer from a loss of CD4⁺ lymphocytes. Initially, dying CD4⁺ lymphocytes are mainly infected ones. Afterward, the great majority of dying CD4⁺ lymphocytes are uninfected. The cause of uninfected CD4⁺ lymphocyte death during HIV infection is still under debate. We previously showed that one of the HIV-1 envelop proteins, gp41, induces the expression of a stress molecule called NKp44L on the surface of uninfected CD4⁺ lymphocytes. Uninfected CD4⁺ lymphocytes expressing NKp44L are killed, in vitro and in vivo, by cells of the immune system called NK cells. In this report, we study the CD4⁺ lymphocyte's proteins involved in the expression of NKp44L. To do so, we used several techniques to identify interacting or differentially expressed proteins and to inhibit or monitor enzymes activity. We also induce NKp44L using the product of some of the proteins involved in NKp44L expression. We found that HIV-1 gp41 binds to its receptor gC1qR on CD4⁺ lymphocytes. This interaction respectively activates the PI3K, the NADPH oxidase and p190 RhoGAP which inactivates TC10. Using the obtained data we build a model of the protein cascade involved in NKp44L surface expression.

C1q induces a rapid up-regulation of P-selectin and modulates collagen- and collagen-related peptide-triggered activation in human platelets

Blood platelets are emerging as important immunomodulatory cells, but complement interaction with platelets is not well understood. Several platelet structures have been described as complement protein 1q (C1q) binding receptors, such as C1qRp/CD93 and gC1qR. However, there are conflicting results whether these receptors are C1q binding structures, or even at all expressed on the cell surface. Recently, the collagen-binding integrin αIIβI was reported to bind C1q on mast cells, and this receptor is also present on platelets. The aim of this study was to further characterize the effects of C1q on platelets, by quantifying the platelet surface expression of P-selectin (CD62P) and monitoring the formation of platelet-neutrophil aggregates. Using flow cytometry, we found that C1q dose-dependently triggered a rapid but moderate and transient up-regulation of P-selectin already within 5s of C1q exposure. Pre-incubation with an antibody directed against gC1qR significantly inhibited (with 57% compared to control) the up-regulation, whereas an antibody towards the αIIβI-integrin showed no effect. Stimulation with C1q did not change the cytosolic calcium-levels, as measured with the fluorescent ratiometric probe Fura-2, however, a protein kinase C inhibitor (GF109203x) blocked the C1q-induced P-selectin expression. Furthermore, pre-incubation of platelets with C1q diminished both the collagen as well as the collagen-related peptide-induced up-regulation of P-selectin, most evident after 90s of stimulation. This indicates that C1q may regulate platelet activation via the GPVI receptor, which is a novel finding. Moreover, C1q antagonized the collagen-induced formation of platelet-neutrophil aggregates, indicating a reduced interaction between platelet P-selectin and neutrophil P-selectin glycoprotein ligand-1(PSGL-1/CD162). In summary, C1q induces a moderate rapid platelet P-selectin expression, modulates subsequent collagen and collagen-related peptide stimulation of platelets, and inhibits the formation of platelet-neutrophil aggregates. These immuno-regulatory effects of C1q may have a crucial role in innate immunity and inflammation.

Role of neutrophils in innate immunity: a systems biology-level approach

The innate immune system is the first line of host defense against invading microorganisms. Polymorphonuclear leukocytes (PMNs or neutrophils) are the most abundant leukocyte in humans and essential to the innate immune response against invading pathogens. Compared with the acquired immune response, which requires time to develop and is dependent on previous interaction with specific microbes, the ability of neutrophils to kill microorganisms is immediate, non-specific, and not dependent on previous exposure to microorganisms. Historically, studies on PMN-pathogen interaction focused on the events leading to killing of microorganisms, such as recruitment/chemotaxis, transmigration, phagocytosis, and activation, whereas post-phagocytosis sequelae were infrequently considered. In addition, it was widely accepted that human neutrophils possessed limited capacity for new gene transcription and thus, relatively little biosynthetic capacity. This notion has changed dramatically within the past decade. Further, there is now more effort directed to understand the events occurring in PMNs after killing of microbes. Herein we review the systems biology-level approaches that have been used to gain an enhanced view of the role of neutrophils during host-pathogen interaction. We anticipate that these and future systems-level studies will ultimately provide information critical to our understanding, treatment, and control of diseases caused by pathogenic microorganisms.

Role of the receptor for the globular domain of C1q protein in the pathogenesis of hepatitis C virus-related cryoglobulin vascular damage

Mixed cryoglobulinemia (MC) is a lymphoproliferative disorder observed in approximately 10 to 15% of hepatitis C virus (HCV)-infected patients. Circulating, nonenveloped HCV core protein, which has been detected in cryoprecipitable immune complexes, interacts with immunocytes through the receptor for the globular domain of C1q protein (gC1q-R). In this study, we have evaluated circulating gC1q-R levels in chronically HCV-infected patients, with and without MC. These levels were significantly higher in MC patients than in those without MC and in healthy controls and paralleled specific mRNA expression in PBL. Soluble gC1q-R circulates as a complexed form containing both C1q and HCV core proteins. Higher serum gC1q-R levels negatively correlated with circulating concentrations of the C4d fragment. The presence of sequestered C4d in the vascular bed of skin biopsies from MC patients was indicative of in situ complement activation. In vitro studies showed that release of soluble gC1q-R is regulated by HCV core-mediated inhibition of cell proliferation. Our results indicate that up-regulation of gC1q-R expression is a distinctive feature of MC, and that dysregulated shedding of C1q-R molecules contributes to vascular cryoglobulin-induced damage via the classic complement-mediated pathway.

A hantavirus causing hemorrhagic fever with renal syndrome requires gC1qR/p32 for efficient cell binding and infection

Hantaan virus (HTNV) is a pathogenic hantavirus that causes hemorrhagic fever with renal syndrome (HFRS). HTNV infection is mediated by alpha v beta3 integrin. We used protein blots of Vero E6 cell homogenates to demonstrate that radiolabeled HTNV virions bind to gC1qR/p32, the acidic 32-kDa protein known as the receptor for the globular head domain of complement C1q. RNAi-mediated suppression of gC1qR/p32 markedly reduced HTNV binding and infection in human lung epithelial A549 cells. Conversely, transient expression of either simian or human gC1qR/p32 rendered non-permissive CHO cells susceptible to HTNV infection. These results suggest an important role for gC1qR/p32 in HTNV infection and pathogenesis.

Crosstalk between the alpha2beta1 integrin and c-met/HGF-R regulates innate immunity

Data from several investigators suggest that the alpha2beta1 integrin, a receptor for collagens, laminins, decorin, E-cadherin, matrix metalloproteinase-1, endorepellin, and several viruses, is required for innate immunity and regulation of autoimmune/allergic disorders. We demonstrated that the innate immune response to Listeria monocytogenes required alpha2beta1 integrin expression by peritoneal mast cells (PMCs). Ligation of the alpha2beta1 integrin by C1q contained in immune complexes comprised of Listeria and antibody was required for PMC activation in vitro and in vivo. However, ligation of the alpha2beta1 integrin alone was insufficient to activate cytokine secretion, suggesting that one or more additional signals emanating from a coreceptor were required for PMC activation. Here, we demonstrate that C1q, but neither other complement proteins nor FcRgamma, is required for early innate immune response to Listeria. The binding of Listeria's Internalin B (InlB) to hepatocyte growth factor receptor (HGF-R)/c-met provides the costimulatory function required for PMC activation. Either HGF or Listeria InlB bound to c-met and either C1q or type I collagen bound to alpha2beta1 integrin stimulates PMC activation. These findings suggest that crosstalk between c-met and the alpha2beta1 integrin may contribute to mast-cell activation in autoimmune and inflammatory disorders.

The alpha2beta1 integrin: a novel collectin/C1q receptor

Our laboratory focuses on the alpha2beta1 integrin, a receptor for a number of matrix and non-matrix ligands, including collagens, laminins, decorin, E-cadherin, matrix metalloproteinase-1 (MMP-1), endorepellin, and several viruses. The alpha2beta1 integrin is expressed on numerous different cell types, including epithelial cells, endothelial cells, fibroblasts, and hematopoietic elements, including platelets and specific subsets of leukocytes. Although alpha2beta1 integrin expression is widespread, it is not ubiquitous. Rather, it is expressed in a differentiation-dependent and activation-dependent manner. Interactions between the alpha2beta1 integrin and extracellular matrix ligands have been implicated in important biological processes including inflammation and immunity. Studies from a number of laboratories have demonstrated a role for the alpha2beta1 integrin during the immune response. Our laboratory generated an alpha2beta1 integrin-deficient mouse to define the role of the alpha2beta1 integrin in vivo. Our studies demonstrated that the alpha2-null mice have a profound defect in the innate immune response. We have recently reported the identification of a novel family of ligands for the alpha2beta1 integrin, which include C1q and the collectins. The goal of this article is to review the important role that the interaction between the alpha2beta1 integrin and C1q plays in the innate immune response. The identification of C1q and the collectins as ligands for the alpha2beta1 integrin suggests that the integrin may play important roles in a number of immunological responses.

Protruding disordered loop of gC1qR is specifically exposed and related to antiapoptotic property in germ cell lineage

We established a monoclonal antibody (MAb), 5G9, with the use of a fixed seminoma tissue from an archival paraffin-embedded specimen, as an immunogen. Without antigen retrieval, positive 5G9-immunohistochemical staining was confined mostly to primordial germ cells, spermatogonia and various germ cell tumors. 5G9 recognized a mitochondrial 32-kD protein with an isoelectric point of pH 4.2, identified as a multifunctional ubiquitous protein, receptor for globular head of C1q (gC1qR), whose epitope was mapped in a disordered loop connecting the beta3 and the beta4 strands. Reflecting the ubiquitous distribution of gC1qR, with antigen retrieval, 5G9 was found reactive to a wide range of normal and tumor tissues. Since several co-precipitated and phosphorylated bands were observed in various human cell lines but not in germ cell tumor cell lines by in vitro phosphorylation assay, we speculate that the epitope of gC1qR is specifically unmasked in the germ cell lineage. By reducing gC1qR by siRNA, a significant increase was observed in the number of apoptotic cells in ITO-II and TCam-2 cell lines, but to a lesser extent in the Colo201 colon cancer cell line, showing an antiapoptotic property of gC1qR in the germ cells. Since protein-protein interaction is partially preserved by fixation, archival paraffin-embedded specimens can be a valuable source of immunogens for generating monoclonal antibodies (MAbs) that recognize tissue-specific protein conformation.

Recognition ligands on apoptotic cells: a perspective

The process of apoptosis includes critically important changes on the cell surface that lead to its recognition and removal. The recognition also generates a number of other local tissue responses including suppression of inflammation and immunity. It is surprising that the ligands generated on the apoptotic cell, which mediates these effects, have received relatively little attention. Some of these candidate molecules and possible mechanisms for their surface expression are addressed herein, with particular emphasis on phosphatidylserine and calreticulin. However, exposure of such ligands is exclusive to apoptosis and may, in fact, occur on viable cells. To partially explain the lack of response to such potential stimuli, the presence on viable cells of "don't eat me" signals, in this case, CD47 is suggested to prevent such unwarranted actions. Loss or inactivation of the don't eat me CD47 effects accompanies apoptosis and now allow the cells to be recognized and cleared.

Vitronectin in atherosclerotic disease

Atherosclerosis is characterized by the development of an intimal thickening that contains monocytes, T lymphocytes, and smooth muscle cells within an accumulation of lipid and extracellular matrix proteins. Vitronectin is a plasma glycoprotein implicated as a regulator of diverse physiological process, including blood coagulation, fibrinolysis, pericellular proteolysis, complement dependent immune responses, and cell attachment and spreading. Because of its ability to bind platelet glycoproteins and mediate platelet adhesion and aggregation at sites of vascular injury, vitronectin has become an important mediator in the pathogenesis of coronary atherosclerosis.

Novel collectin/C1q receptor mediates mast cell activation and innate immunity

Mast cells play a critical role in innate immunity, allergy, and autoimmune diseases. The receptor/ligand interactions that mediate mast cell activation are poorly defined. The alpha2beta1 integrin, a receptor for collagens, laminins, decorin, E-cadherin, matrix metalloproteinase-1 (MMP-1), endorepellin, and several viruses, has been implicated in normal developmental, inflammatory, and oncogenic processes. We recently reported that alpha2 integrin subunit-deficient mice exhibited markedly diminished neutrophil and IL-6 responses during Listeria monocytogenes- and zymosan-induced peritonitis. Peritoneal mast cells require alpha2beta1 integrin expression for activation in response to pathogens, yet the ligand and molecular mechanisms by which the alpha2beta1 integrin induces activation and cytokine secretion remain unknown. We now report that the alpha2beta1 integrin is a novel receptor for multiple collectins and the C1q complement protein. We demonstrate that the alpha2beta1 integrin provides a costimulatory function required for mast cell activation and cytokine secretion. This finding suggests that the alpha2beta1 integrin is not only important for innate immunity but may serve as a critical target for the regulation of autoimmune/allergic disorders.

C1q: Its Functions within the Innate and Adaptive Immune Responses and its Role in Lupus Autoimmunity

The complement cascade is a multi-faced effector component of the innate immune response. C1q is the recognition component of the classical pathway of complement activation. In addition, C1q has been recognized to serve a number of other biological functions including a modulating role on cellular functions within the adaptive immune response. The importance of C1q to normal immune regulation is reflected by the fact that greater than 90% of individuals who have complete congenital deficiency of C1q have been observed to develop early-onset photosensitive systemic lupus erythematosus (SLE). As a number of single nucleotide polymorphisms have been identified in three C1q genes, it is possible that more subtle variations in C1q expression could be a risk factor for cutaneous LE and SLE. Thus, a more comprehensive delineation of complotype could be of increasing clinical importance in the future.

Non-enveloped HCV core protein as constitutive antigen of cold-precipitable immune complexes in type II mixed cryoglobulinaemia

Hepatitis C virus (HCV) infection has been detected in a large proportion of patients with mixed cryoglobulinaemia (MC). Circulating 'free' non-enveloped HCV core protein has been demonstrated in HCV-infected patients, and this suggests its possible involvement in the formation of cryoprecipitable immune complexes (ICs). Thirty-two anti-HCV, HCV RNA-positive patients with type II MC were evaluated. Non-enveloped HCV core protein, HCV RNA sequences, total IgM, rheumatoid factor (RF) activity, IgG and IgG subclasses, C3 and C4 fractions, C1q protein and C1q binding activity were assessed in both cryoprecipitates and supernatants. Non-enveloped HCV core protein was demonstrated in 30 of 32 (93.7%) type II MC patients. After separation of cold-precipitable material, the protein was removed completely from supernatant in 12 patients (40%), whereas it was enriched in the cryoprecipitates of the remaining 18. In addition, HCV RNA and IgM molecules with RF activity were concentrated selectively in the cryoprecipitates. Differential precipitation was found for both total IgG and IgG subclasses, as they were less represented in the cryoglobulins and no selective enrichment was noted. Immunological characterization of HCV core protein-containing cryoprecipitating ICs after chromatographic fractionation showed that the IgM monoclonal component had RF activity, whereas anti-HCV core reactivity was confined to the IgG fraction. C1q enrichment in addition to high avidity of ICs for C1q binding in the cryoprecipitates suggest that complement activation may occur through the C1q protein pathway. The present data demonstrate that non-enveloped HCV core protein is a constitutive component of cryoprecipitable ICs in type II MC patients.

C1q and mannose binding lectin engagement of cell surface calreticulin and CD91 initiates macropinocytosis and uptake of apoptotic cells

Removal of apoptotic cells is essential for maintenance of tissue homeostasis, organogenesis, remodeling, development, and maintenance of the immune system, protection against neoplasia, and resolution of inflammation. The mechanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we provide evidence that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the alpha-2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune system. Ingestion of the apoptotic cells through calreticulin/CD91 stimulation is further shown to involve the process of macropinocytosis, implicated as a primitive and relatively nonselective uptake mechanism for C1q- and MBL-enhanced engulfment of whole, intact apoptotic cells, as well as cell debris and foreign organisms to which these molecules may bind.

C1q and mannose binding lectin engagement of cell surface calreticulin and CD91 initiates macropinocytosis and uptake of apoptotic cells

Removal of apoptotic cells is essential for maintenance of tissue homeostasis, organogenesis, remodeling, development, and maintenance of the immune system, protection against neoplasia, and resolution of inflammation. The mechanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we provide evidence that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the alpha-2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune system. Ingestion of the apoptotic cells through calreticulin/CD91 stimulation is further shown to involve the process of macropinocytosis, implicated as a primitive and relatively nonselective uptake mechanism for C1q- and MBL-enhanced engulfment of whole, intact apoptotic cells, as well as cell debris and foreign organisms to which these molecules may bind.

Surfactant proteins a and d and pulmonary host defense

The lung collectins, SP-A and SP-D, are important components of the innate immune response to microbial challenge and participate in other aspects of immune and inflammatory regulation within the lung. Both proteins bind to surface structures expressed by a wide variety of microorganisms and have the capacity to modulate multiple leukocyte functions, including the enhanced internalization and killing of certain microorganisms in vitro. In addition, transgenic mice with deficiencies in SP-A and SP-D show defective or altered responses to challenge with bacterial, fungal, and viral microorganisms and to bacterial lipopolysaccharides in vivo. Thus collectins could play particularly important roles in settings of inadequate or impaired specific immunity, and acquired alterations in the levels of active collectins within the airspaces and distal airways may increase susceptibility to infection.

gC1q-R/p32, a C1q-binding protein, is a receptor for the InlB invasion protein of Listeria monocytogenes

InlB is a Listeria monocytogenes protein that promotes entry of the bacterium into mammalian cells by stimulating tyrosine phosphorylation of the adaptor proteins Gab1, Cbl and Shc, and activation of phosphatidyl- inositol (PI) 3-kinase. Using affinity chromatography and enzyme-linked immunosorbent assay, we demonstrate a direct interaction between InlB and the mammalian protein gC1q-R, the receptor of the globular part of the complement component C1q. Soluble C1q or anti-gC1q-R antibodies impair InlB-mediated entry. Transient transfection of GPC16 cells, which are non-permissive to InlB-mediated entry, with a plasmid-expressing human gC1q-R promotes entry of InlB-coated beads. Furthermore, several experiments indicate that membrane recruitment and activation of PI 3-kinase involve an InlB-gC1q-R interaction and that gC1q-R associates with Gab1 upon stimulation of Vero cells with InlB. Thus, gC1q-R constitutes a cellular receptor involved in InlB-mediated activation of PI 3-kinase and tyrosine phosphorylation of the adaptor protein Gab1. After E-cadherin, the receptor for internalin, gC1q-R is the second identified mammalian receptor promoting entry of L. monocytogenes into mammalian cells.

Structure of the lnlB leucine-rich repeats, a domain that triggers host cell invasion by the bacterial pathogen L. monocytogenes

The L. monocytogenes protein lnlB activates phosphoinositide 3-kinase and induces phagocytosis in several mammalian cell types. The 1.86 A resolution X-ray crystal structure of the leucine-rich repeat domain of lnlB that is both necessary and sufficient to induce phagocytosis is presented here. The structure supports a crucial role for calcium in host cell invasion by L. monocytogenes and supplies a rationale for its function. Calciums are bound to the protein in an unusually exposed manner that suggests that the metals may act as a bridge between lnlB and mammalian cell surface receptors. The structure also identifies surfaces on the curved and elongated molecule that may constitute additional interaction sites in forming a bacterial-mammalian signaling complex.

C1q-Containing Immune Complexes Purified from Sera of Juvenile Rheumatoid Arthritis Patients Mediate IL-8 Production by Human Synoviocytes: Role of C1q Receptors

Immune complexes that vary in size and composition are present in the sera and synovial fluid of juvenile rheumatoid arthritis (JRA) patients. They are believed to be potent inducers of the ongoing inflammatory process in JRA. However, the precise composition and role of these complexes in the pathophysiology of JRA remain unclear. We hypothesized that circulating ICs have the potential to interact with resident joint synovial fibroblasts (synoviocytes) and induce the expression of inflammatory cytokines. To test this hypothesis, cultures of synoviocytes from healthy individuals were treated with ICs isolated from the sera of JRA patients. Studies reported in this work demonstrate that IgM affinity-purified ICs from the sera of JRA patients contain IgM, C1q, IgG, and C3 to a variable extent. These ICs induce IL-8 mRNA and protein production in normal synoviocytes. Our data indicate that C1q in these ICs mediates, in part, IL-8 induction in synoviocytes. This is based on our findings of C1q-binding proteins for collagen stalks (cC1qR) and globular heads (gC1q-binding protein) of C1q in synoviocytes. In addition, collagen stalk and to some extent globular head fragments of C1q inhibit IC-mediated IL-8 induction in synoviocytes. Together, these findings provide evidence for a novel mechanism of IL-8 production by synoviocytes, which could play a key role in inflammation by recruiting leukocytes to synovial tissue and fluid—and subsequently contributing to joint disease.

Interaction of factor XII and high molecular weight kininogen with cytokeratin 1 and gC1qR of vascular endothelial cells and with aggregated Abeta protein of Alzheimer's disease

High molecular weight kininogen (HK) attaches to endothelial cells at separate sites on the heavy and light chains by a process which requires 15-50 microM zinc. Previously identified binding proteins include gClqR, cytokeratin 1, and the urokinase plasminogen activator receptor (U-par), however, their relative contribution to binding are not yet clarified. We have purified the binding proteins by affinity chromatography, in the presence of zinc ion, and identified cytokeratin 1 and gC1qR by amino acid sequencing of an internal peptide and by immunoblot as heavy chain and light chain binding proteins, respectively. Antibody to cytokeratin 1 inhibited HK binding to endothelial cells by 30%, antibody to gClqR inhibited HK binding to endothelial cells by 72%, and a mixture of both inhibited binding by 86%. The binding and activation of the proteins of the kinin-forming cascade along the cell surface is zinc-dependent. Similarly, proteins of the plasma kinin-forming cascade can be activated by binding to aggregated A(beta) protein of Alzheimer's disease. Activation of the cascade using purified proteins or upon addition of Abeta to plasma requires aggregation of A(beta) and the reactions are zinc-dependent. In plasma, HK is cleaved and bradykinin is liberated. The data demonstrate that aggregated A(beta) can bind and activate proenzymes of the plasma kinin-forming cascade to release bradykinin and these reactions are dependent on zinc ion.

Cytokeratin 1 and gC1qR mediate high molecular weight kininogen binding to endothelial cells

High molecular weight kininogen (HK) attaches to endothelial cells by separate sites on the heavy and light chains and requires 15-50 microM zinc. Previously identified binding proteins include gC1qR, cytokeratin 1, and the urokinase plasminogen activator receptor; however, their relative contributions to binding are not yet clarified. We have prepared affinity columns to which were coupled either cleaved HK or peptide LDCNAEVYVVPWEKKIYPTVNCQPLGM derived from heavy-chain domain 3. Endothelial cell membranes were solubilized and chromatographed in the presence or absence of zinc ion, the bound proteins were eluted, and active fractions were identified by dot blot using biotinylated HK, SDS/PAGE, and Western blot analysis. The peptide containing column eluate revealed but one band at 68 kDa if zinc ion was present which was identified as cytokeratin 1 by amino acid sequencing of an internal peptide. The HK affinity column revealed bands at 68 kDa (cytokeratin 1), 33 kDa (gC1qR), and 66 kDa (unidentified). HK or domain 3-derived peptide bound to the 68 kDa band; prekallikrein and Factor XII did not. HK or Factor XII bound to the 33-kDa band if zinc was present while no binding to the 66 kDa band was observed. Antibody to cytokeratin 1 inhibited HK binding to endothelial cells by 30%, antibody to gC1qR inhibited HK binding to endothelial cells by 72%, and a mixture of both inhibited binding by 86%. Our data suggest HK binding by interaction of the heavy-chain domain 3 with cytokeratin 1 and the light chain with gC1qR.

The receptor for the globular "heads" of C1q, gC1q-R, binds to fibrinogen/fibrin and impairs its polymerization

The 33-kDa cellular C1q binding protein, designated gC1q-R was previously shown to bind a number of plasma proteins involved in the coagulation and kinin systems. This study demonstrates the interaction between recombinant gC1q-R and fibrinogen. Using enzyme-linked immunosorbent assays, biotinylated gC1q-R was found to bind to microplate-immobilized fibrinogen in a manner which was specific and inhibited by excess soluble fibrinogen or polyclonal antibodies directed against either gC1q-R or fibrinogen. Moreover, gC1q-R inhibited fibrin polymerization in a dose-dependent manner. Reptilase induced fibrin clot formation was completely inhibited by gC1q-R at a 2:1 molar ratio (gC1q-R:fibrinogen), and repolymerization of thrombin induced fibrin monomers was similarly abrogated. At equivalent molar concentrations, gC1q-R appeared to be a more potent inhibitor of fibrin polymerization than fibrinogen, a well-known inhibitor. Moreover, in the presence of both gC1q-R and soluble fibrinogen, the effect of each inhibitor on fibrin polymerization was additive. When plasmin derived fibrinogen degradation products, including the C-terminal D domain (D-100) or the N-terminal E domain, were immobilized on microtiter plates, gC1q-R bound to fibrinogen fragment D-100, but not to fragment E. Further digestion of fibrinogen fragment D-100 by plasmin to fragment D-60 resulted in loss of gC1q-R binding. Thus, gC1q-R binds to the D domain of fibrinogen/fibrin, and the carboxyterminal segment of at least the fibrinogen/fibrin gamma chain appears important for this interaction. These observations may suggest a potential role for gC1q-R in modulating fibrin formation particularly at local sites of immune injury or inflammation.

The splicing factor-associated protein, p32, regulates RNA splicing by inhibiting ASF/SF2 RNA binding and phosphorylation

The cellular protein p32 was isolated originally as a protein tightly associated with the essential splicing factor ASF/SF2 during its purification from HeLa cells. ASF/SF2 is a member of the SR family of splicing factors, which stimulate constitutive splicing and regulate alternative RNA splicing in a positive or negative fashion, depending on where on the pre-mRNA they bind. Here we present evidence that p32 interacts with ASF/SF2 and SRp30c, another member of the SR protein family. We further show that p32 inhibits ASF/SF2 function as both a splicing enhancer and splicing repressor protein by preventing stable ASF/SF2 interaction with RNA, but p32 does not block SRp30c function. ASF/SF2 is highly phosphorylated in vivo, a modification required for stable RNA binding and protein-protein interaction during spliceosome formation, and this phosphorylation, either through HeLa nuclear extracts or through specific SR protein kinases, is inhibited by p32. Our results suggest that p32 functions as an ASF/SF2 inhibitory factor, regulating ASF/SF2 RNA binding and phosphorylation. These findings place p32 into a new group of proteins that control RNA splicing by sequestering an essential RNA splicing factor into an inhibitory complex.

The First Subcomponent of Complement, C1q, Triggers the Production of IL-8, IL-6, and Monocyte Chemoattractant Peptide-1 by Human Umbilical Vein Endothelial Cells

We and others have demonstrated previously the occurrence of cC1qR/CaR, a receptor for the collagen-like stalks of complement component C1q, on endothelial cells. In the present study we investigated whether binding of C1q to endothelial cells resulted in enhancement of cytokine or chemokine production. HUVEC produced 82 ± 91 pg/ml of IL-8, 79 ± 113 pg/ml of IL-6, and 503 ± 221 pg/ml of monocyte chemoattractant peptide-1 (MCP-1) under basal conditions. Incubation with C1q resulted in a time- and dose-dependent up-regulation of IL-8 (1012 ± 43 pg/ml), IL-6 (392 ± 20 pg/ml), and MCP-1 (2450 ± 101 pg/ml). This production is dependent on de novo protein synthesis, as demonstrated by the detection of specific mRNA after C1q stimulation, and inhibition of peptide production in the presence of cycloheximide. The production of all factors was inhibited (69 ± 7%) by the collagenous fragments of C1q, while the C1q globular heads only induced 13 ± 11% inhibition. When HUVEC were incubated with C1q in the presence of aggregated IgM, enhanced production of IL-8 (2500 ± 422 pg/ml), IL-6 (997 ± 21 pg/ml), and MCP-1 (5343 ± 302 pg/ml) was found. Furthermore, F(ab′)2 anti-calreticulin partially inhibited the production of IL-8, confirming at least the involvement of cC1qR/CaR. These experiments suggest that in an inflammatory response C1q not only is able to activate the complement pathway, but when presented in a proper fashion also might induce the production of factors that contribute to acute phase responses and recruitment of inflammatory cells.

Inhibition of Activation of the Classical Pathway of Complement by Human Neutrophil Defensins

Defensins are small, cationic antimicrobial peptides that are present in the azurophilic granules of neutrophils. Earlier studies have shown that defensins may influence complement activation by specific interaction with activated C1, C1q, and C1-inhibitor. In the present study, we show that the defensin human neutrophil peptide-1 (HNP-1) is able to inhibit activation of the classical complement pathway by inhibition of C1q hemolytic activity. The binding site for HNP-1 on C1q is most likely located on the collagen-like stalks, as a clear, dose-dependent binding of HNP-1 to either intact C1q or to the collagen-like stalks of C1q was demonstrated using enzyme-linked immunosorbent assay (ELISA). Besides binding of HNP-1 to C1q, also a limited binding to C1 and to a mixture of C1r and C1s was observed, whereas no binding to C1-inhibitor was found. Because binding of HNP-1 to C1-inhibitor has been suggested in earlier studies, we also assessed the binding of HNP-1 to mixtures of C1-inhibitor with either C1r/ C1s or C1. No binding was found. Using a competition ELISA, it was found that HNP-1, but not protamine, inhibited binding of biotin-labeled HNP-1 to C1q in a dose-dependent fashion. In the fluid phase, preincubation of HNP-1 with C1q resulted in complex formation of HNP-1 and C1q and generation of stable complexes. In conclusion, HNP-1 is able to bind to C1q in the fluid phase and inhibits the classical complement pathway. This mechanism may be involved in the control of an inflammatory response in vivo.

Inhibition of Activation of the Classical Pathway of Complement by Human Neutrophil Defensins

Defensins are small, cationic antimicrobial peptides that are present in the azurophilic granules of neutrophils. Earlier studies have shown that defensins may influence complement activation by specific interaction with activated C1, C1q, and C1-inhibitor. In the present study, we show that the defensin human neutrophil peptide-1 (HNP-1) is able to inhibit activation of the classical complement pathway by inhibition of C1q hemolytic activity. The binding site for HNP-1 on C1q is most likely located on the collagen-like stalks, as a clear, dose-dependent binding of HNP-1 to either intact C1q or to the collagen-like stalks of C1q was demonstrated using enzyme-linked immunosorbent assay (ELISA). Besides binding of HNP-1 to C1q, also a limited binding to C1 and to a mixture of C1r and C1s was observed, whereas no binding to C1-inhibitor was found. Because binding of HNP-1 to C1-inhibitor has been suggested in earlier studies, we also assessed the binding of HNP-1 to mixtures of C1-inhibitor with either C1r/ C1s or C1. No binding was found. Using a competition ELISA, it was found that HNP-1, but not protamine, inhibited binding of biotin-labeled HNP-1 to C1q in a dose-dependent fashion. In the fluid phase, preincubation of HNP-1 with C1q resulted in complex formation of HNP-1 and C1q and generation of stable complexes. In conclusion, HNP-1 is able to bind to C1q in the fluid phase and inhibits the classical complement pathway. This mechanism may be involved in the control of an inflammatory response in vivo.

Collectins and pulmonary host defense

The surfactant-associated proteins SP-A and SP-D are members of a family of collagenous host defense lectins, designated collectins. There is increasing evidence that these pulmonary epithelial-derived proteins are important components of the innate immune response to microbial challenge, and that they participate in other aspects of immune and inflammatory regulation within the lung. The collectins bind to glycoconjugates and/or lipid moieties expressed by a wide variety of microorganisms and certain other organic particles in vitro. Although binding may facilitate microbial clearance through aggregation or other direct effects on the organism, SP-A and SP-D also have the capacity to modulate leukocyte function and, in some circumstances, to enhance their killing of microorganisms. The biologic activity of cell wall components, such as gram-negative bacterial polysaccharides, may be altered by interactions with collectins. Complementary or cooperative interactions between SP-A and SP-D could contribute to the efficiency of this defense system. Collectins may play particularly important roles in settings of inadequate or impaired specific immunity. Acquired or genetic alterations in the levels of active proteins within the airspaces and distal airways may increase susceptibility to infection.

The Multiligand-Binding Protein gC1qR, Putative C1q Receptor, Is a Mitochondrial Protein

A protein of 33 kDa (p33) that tightly binds to the globular domains of the first complement component, C1q, is thought to serve as the major C1q receptor (gC1qR) on B cells, neutrophils, and mast cells. However, the cellular routing and the subcellular localization of p33/gC1qR are unknown. We have performed confocal laser-scanning microscopy and found that p33/gC1qR is present in intracellular compartments, where it colocalizes with the mitochondrial marker protein, pyruvate dehydrogenase. No surface staining for p33/gC1qR on endothelial EA.hy926 cells was observed. A fusion protein of the p33/gC1qR presequence with green fluorescent protein translocated to the mitochondria of transfected COS-7 cells. Concomitantly, a 6-kDa portion of the fusion protein was proteolytically removed. The 33 amino-terminal residues of the presequence proved sufficient to direct reporter constructs to mitochondria. Association of p33/gC1qR with mitoplasts indicated that the mature protein of 209 residues resides in the matrix and/or the inner membrane of mitochondria. Immunocytochemistry of fetal mice tissues revealed a ubiquitous expression of p33/gC1qR, most prominently in tissues that are rich in mitochondria. Thus, the candidate complement receptor p33/gC1qR of intact cells cannot interact with plasma C1q due to mutually exclusive localizations of the components. The functional role of p33/gC1qR needs to be reconsidered.

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.

C1qRP, the C1q Receptor That Enhances Phagocytosis, Is Detected Specifically in Human Cells of Myeloid Lineage, Endothelial Cells, and Platelets

The complement component C1q can interact with a variety of different cells, resulting in multiple functional consequences depending on the cell type. mAbs R3 and R139, which recognize a 126,000 Mr (reduced) cell surface protein, are able to abrogate the C1q-mediated enhancement of monocyte phagocytosis. The cDNA encoding this C1q receptor that modulates phagocytosis, C1qRP, has recently been cloned. Using a DNA probe based on the coding region of the receptor, Northern blot and RT-PCR analysis of RNA isolated from different cell types showed C1qRP expression in cells of myeloid origin and in endothelial cells, but not in cells of lymphoid origin nor in the HeLa epithelial-like cell line or iliac artery smooth muscle cells. FACS analysis of cell surface expression of C1qRP, as detected by mAb R139 and R3, corresponded in all cases to the mRNA levels detected. Using the anti-C1qRP mAb, the 126,000 Mr receptor was also detected in lysates of human platelets. Interestingly, C1qRP is not expressed by the promyelocytic leukemia cell line HL-60, and differentiation of these cells with various chemical compounds did not induce C1qRP expression. It has been reported that C1q can induce specific receptor-mediated responses in fibroblasts. However, RNA and cell surface expression analysis for C1qRP indicate that this particular C1q receptor is not expressed by either human gingival or human skin fibroblasts. These data demonstrate selective expression of C1qRP in specific cell types and support the hypothesis that there is more than one C1q receptor mediating the diverse responses triggered by C1q.

Anti-C1q receptor/calreticulin autoantibodies in patients with systemic lupus erythematosus (SLE)

SLE is a disease characterized by the presence of multiple autoantibodies and high levels of circulating immune complexes. We studied the presence and functional relevance of autoantibodies directed against a receptor for the collagen-like stalks of the first subcomponent of complement, also known as calreticulin (cC1qR/CaR), in patients with SLE. In a cross-sectional study it was found that higher titres of antibodies against cC1qR/CaR are present in sera of SLE patients compared with normal donors. No association between anti-cC1qR/CaR titres and SLE disease activity was found. Following gel filtration of SLE serum it was found that anti-cC1qR/CaR reactivity is associated with the peak of monomeric IgG. Purified IgG from patients was able to specifically immunoprecipitate cC1qR/CaR. Since we have shown previously that cC1qR/CaR is able to inhibit the haemolytic activity of Clq, we determined a possible pathogenic role for anti-cC1qR/CaR on complement regulation. IgG derived from SLE serum reversed the inhibitory capacity of cC1qR/CaR in a dose-dependent fashion up to 63%, whereas IgG from normal donors had no significant effect. With respect to the capacity of anti-cC1qR/CaR antibodies to activate neutrophils, it was found that incubation of normal neutrophils with F(ab')2 anti-cC1qR/CaR resulted in a very limited oxidative burst. However, cross-linking of F(ab')2 anti-cC1qR/CaR on the neutrophils clearly induced neutrophil activation. Pre-incubation of the SLE-derived F(ab')2 with cC1qR/CaR prevented activation of neutrophils up to 81+/-5%. These results suggest that the presence of anti-cC1qR/CaR antibodies in patients with SLE may modulate complement and neutrophil activation.

Neutrophil Fc gamma and complement receptors involved in binding soluble IgG immune complexes and in specific granule release induced by soluble IgG immune complexes

We examined the effect of soluble IgG immune complex (IC) characteristics on the binding of IC to human neutrophils and IC-induced specific granule release of neutrophils via Fc gamma receptors (CD16 and CD32) and complement receptors (CR1 and CR3). A set of soluble IgG IC varying in size, IgG subclass, antigen epitope density and complement (C) incorporation were formed between 5-iodo-4-hydroxy-3-nitrophenacetyl (NIP) coupled to bovine serum albumin (BSA) and chimeric mouse-human anti-NIP monoclonal antibodies (mAb) of all four IgG subclasses. High and low epitope density IC of all four IgG subclasses induced specific granule release with C, but in the absence of C only IgG1 and IgG3 IC were functionally active. The Fc gamma and C receptors responsible for IgG IC-induced specific granule release and IC binding were determined using mAb specific for the ligand binding sites of CD16, CD32 and CR3, and recombinant soluble CR1. Each defined IC displayed a unique pattern of receptor preference, dependent upon subclass and antigenic epitope density. IC binding and IC-induced specific granule release was not mediated by the same receptor, or combination of receptors. High and low epitope density IgG3 IC binding and induction of specific granule release was mediated predominantly via CD16. Other IC subclasses bound differently, i.e. IgG1 IC used CD16 and CR3; IgG2 and IgG4 predominantly used complement receptors; but all three induced specific granule release via CD32. In vivo these results may translate into differential activation of neutrophils by soluble IC dependent upon their characteristics, leading to subtle nuances in the etiology, pathology and control of the immune response in IC-related diseases.

P32/TAP, a cellular protein that interacts with EBNA-1 of Epstein-Barr virus

The Epstein-Barr virus (EBV) EBNA-1 protein has a central role in the maintenance of a latent EBV infection and is the only virus-encoded protein expressed in all EBV-associated tumors. EBNA-1 is required for replication of the episomal form of the latent viral genome and transactivates the latency C and LMP-1 promoters. The mechanisms by which EBNA-1 performs these functions are not known. Here we describe the cloning, expression, and characterization of a cellular protein, P32/TAP, which strongly interacts with EBNA-1. We show that P32/TAP is expressed at high levels in Raji cells and is synthesized as a proprotein of 282 amino acids (aa) that is posttranslationally processed by a two-step cleavage process to yield a mature protein of 209 aa. It has been previously reported that P32/TAP is expressed on the cell surface. Our transient expression assays detected full-length P32/TAP (1-282 aa) in the cytoplasm while mature P32/TAP protein localized to the nucleus. Three lines of evidence support P32/TAP interaction with EBNA-1. First, in the yeast two-hybrid system we mapped two interactive N-terminal regions of EBNA-1, aa 40-60 and aa 325-376, each of which contains arginine-glycine repeats. These regions interact with the C-terminal half of P32/TAP. Second, the full-length cytoplasmic P32/TAP protein can translocate nuclear EBNA-1 into the cytoplasm. Third, P32/TAP co-immunoprecipitated with EBNA-1. We have confirmed that a Gal4 fusion protein containing the C-terminal region of P32/TAP (aa 244-282) transactivates expression from a reporter containing upstream Gal4-binding sites. Deletion of the P32/TAP interactive regions of EBNA-1 severely diminished EBNA-1 transactivation of FRTKCAT in transient expression assays. Our data suggest that interaction with P32/TAP may contribute to EBNA-1-mediated transactivation.

Distribution of receptors of collagen and globular domains of C1q in human lung fibroblasts

Fibroblasts are the predominant cell type responsible for the synthesis of collagen and other matrix elements in normal and fibrotic lungs. We have previously reported that human lung fibroblasts are heterogeneous in C1q binding and that subpopulations differing in C1q binding can be isolated and subcultured. We have investigated the distribution of receptors for C1q-collagen domain (cC1q-R) and globular domain (gC1q-R) in adult human lung fibroblasts. Fibroblasts were isolated from cultures of adult human lung explants in medium containing fresh- or heated plasma-derived human sera and separated by FACS-cell sorting into populations binding to C1q with high- (HF) and low- (LF) fluorescence. The cC1q-R was obtained from fibroblast membrane preparations by affinity chromatography through an anti-cC1q-R antibody column and its distribution was determined by Western analysis. The presence of gC1q-R was determined by immunoblots using an anti-gC1q-R antibody raised against a synthetic peptide. The results showed that a 54 kD protein crossreacting with anti-cC1q-R antibody was produced by LF cells, but it was barely detectable in HF cultures. Immunostaining with anti-cC1q-R antibody revealed that most of the cells in LF cultures were positive while the HF cells were negative. A 38 kD protein recognized by anti-gC1q-R antibody was produced by lung fibroblasts; however, no differences were detected in its distribution between LF and HF cultures. SDS-polyacrylamide gel electrophoresis of membrane proteins binding to an affinity column of C1q-globular fragment showed that the HF cultures contain a approximately 51 kD protein, which was a minor component in LF membranes. These data show that cC1q-R is expressed predominantly by a population of human lung fibroblasts, while the 38 kD gC1q-R is produced by all cells. Another 51 kD protein appears to be produced by a separate population of fibroblasts which does not express cC1q-R. Our results indicate that two lung fibroblast subtypes may be distinguished based on production of the 54 kD putative cC1q-R and another 51 kD protein which binds to C1q-globular domain.

The C1q-binding cell membrane proteins cC1q-R and gC1q-R are released from activated cells: subcellular distribution and immunochemical characterization

Two types of widely coexpressed cell surface C1q-binding proteins (C1q-R): a 60-kDa calreticulin-homolog which binds to the collagen-like "stalk" of C1q and a 33-kDa protein with affinity for the globular "heads" of the molecule, have been described. In this report, we show that the two molecules are also secreted by Raji cells and peripheral blood lymphocytes and can be isolated in soluble form from serum-free culture supernatant by HPLC purification using a Mono-Q column. The two purified soluble proteins had immunochemical and physical characteristics similar to their membrane counterparts in that both bound to intact C1q and to their respective C1q ligands, cC1q and gC1q. In addition, N-terminal amino acid sequence analyses of the soluble cC1q-R and gC1q-R were found to be identical to the reported sequences of the respective membrane-isolated proteins. Ligand blot analyses using biotinylated membrane or soluble cC1q-R and gC1q-R showed that both bind to the denatured and nondenatured A-chain and moderately to the C-chain of C1q. Moreover, like their membrane counterparts, the soluble proteins were found to inhibit serum C1q hemolytic activity. Although cC1q-R was released when both peripheral blood lymphocytes and Raji cells were incubated in phosphate-buffered saline for 1 hr under tissue culture conditions, gC1q-R was releasable only from Raji cells, suggesting that perhaps activation or transformation leading to immortalization is required for gC1q-R release. Subcellular fractionation of Raji cells and analyses by enzyme-linked immunosorbent assay and Western blotting showed that the two molecules are present in the cytosolic fractions as well as on the membrane. The data suggest that soluble forms of both C1q-binding molecules are released from cells and that these molecules may play important roles in vivo as regulators of complement activation.