C1q Receptor Research Articles

Human C1qRp is identical with CD93 and the mNI-11 antigen but does not bind C1q.

It has been suggested that the human C1qRp is a receptor for the complement component C1q; however, there is no direct evidence for an interaction between C1q and C1qRp. In this study, we demonstrate that C1q does not show enhanced binding to C1qRp-transfected cells compared with control cells. Furthermore, a soluble recombinant C1qRp-Fc chimera failed to interact with immobilized C1q. The proposed role of C1qRp in the phagocytic response in vivo is also unsupported in that we demonstrate that this molecule is not expressed by macrophages in a variety of human tissues and the predominant site of expression is on endothelial cells. Studies on the rodent homolog of C1qRp, known as AA4, have suggested that this molecule may function as an intercellular adhesion molecule. Here we show that C1qRp is the Ag recognized by several previously described mAbs, mNI-11 and two anti-CD93 Abs (clones X2 and VIMD2b). Interestingly, mNI-11 (Fab') has been shown to promote monocyte-monocyte and monocyte-endothelial cell adhesive interactions. We produced a recombinant C1qRp-Fc chimera containing the C-type lectin-like domain of C1qRp and found specific binding to vascular endothelial cells in sections of inflamed human tonsil, indicating the presence of a C1qRp ligand at this site. This interaction was Ca(2+) independent and was not blocked by our anti-C1qRp mAb BIIG-4, but was blocked by the proadhesive mAb mNI-11. Collectively, these data indicate that C1qRp is not a receptor for C1q, and they support the emerging role of C1qRp (here renamed CD93) in functions relevant to intercellular adhesion.

Cooperation of C1q receptors and integrins in C1q-mediated endothelial cell adhesion and spreading.

The interaction of C1q with endothelial cells elicits a multiplicity of biologic responses. Although these responses are presumed to be mediated by the interaction of C1q with endothelial cell surface proteins, the identity of the participants is not known. In this study we examined the roles of two C1q binding proteins, cC1q-R/calreticulin and gC1q-R/p33, in C1q-mediated adhesion and spreading of human dermal microvascular endothelial cells (HDMVEC). When HDMVEC were cultured in microtiter plate wells coated with concentrations of C1q ranging from 0 to 50 microg/ml, a specific and dose-dependent adhesion and spreading was observed. The extent of adhesion and spreading was similar to the adhesion seen on collagen-coated wells. Spreading (68 +/- 12%) and to a moderate extent adhesion (47 +/- 9%) were inhibited by anti-gC1q-R mAb 60.11. Similar effects were noted with polyclonal anti-cC1q-R but not with control nonimmune IgG. The two Abs had a slight additive effect (75 +/- 13% inhibition) when mixed together in the proportion of 100 microg/ml anti-gC1q-R and 30 microg/ml anti-cC1q-R. More importantly, a 100% inhibition of spreading, but not adhesion, to C1q-coated wells was observed when HDMVEC were cultured in the presence of 30 microM of the peptide GRRGDSP but not GRRGESP. Furthermore, while anti-beta(1) integrin Ab blocked both adhesion and spreading, anti-alpha(5) integrin blocked only spreading and not adhesion. Ag capture ELISA of endothelial cell membrane proteins using polyclonal anti-gC1q-R showed the presence of not only beta(1) and alpha(5) integrins but also CD44. Taken together these results suggest that endothelial cell adhesion and spreading require the cooperation of both C1qRs and beta(1) integrins and possibly other membrane-spanning molecules.

GC1q-R/p33: Structure-Function Predictions from the Crystal Structure

Human gC1q-R (p33) is a multicompartmental cellular protein expressed on various types of cells and tissues. Although originally isolated as a receptor for C1q by virtue of its specificity for the globular heads of that molecule, a large body of evidence has now been accumulated which shows that in addition to C1q, gC1q-R can serve as a receptor for diverse ligands including proteins of the intrinsic coagulation/bradykinin forming cascade, as well as antigens of cellular, bacterial, and viral origin. Furthermore, since gC1q-R has been shown to regulate the functions of protein kinase C (PKC), it is postulated that gC1q-R-induced signaling cascade may involve activation of PKC. These data collectively therefore suggest that gC1q-R plays an important role in blood coagulation, inflammation, and infection. However, although significant progress has been made in unraveling the molecular, biochemical, and structural features of this molecule, and data in support of its biological relevance is accumulating, it is still unclear as to how the molecule is anchored on the membrane since its sequence is devoid of a classical transmembrane domain or a glycosylphosphatidylinositol (GPI) anchor. Furthermore, while recombinant gC1q-R can bind to cell surfaces suggesting that it may bind directly to the phospholipid bilayer, our recent experiments show that, at least in vitro, gC1q-R does not bind to unilamellar vesicle preparations of either phosphatidylcholine (PC) or phosphatidylserine: phosphatidylcholine. This work was therefore undertaken to analyze the three-dimensional structure of gC1q-R in order to identify unique structural features that may serve not only to anchor the protein but also to explain its affinity for such a diversity of plasma as well as microbial and viral ligands.

Expression and Function of C1q Receptors and C1q Binding Proteins at the Cell Surface

C1q is the recognition unit of the first component of complement that binds not only IgG and IgM containing immune complexes, but also recognizes foreign structures such as the lipid A of endotoxin, and molecules expressed at the surface of apoptotic cells. In this review, the plasma membrane receptors and binding proteins for C1q are discussed and new data are presented on calreticulin expression on human peripheral blood cells. Although much is known about C1q receptors and binding molecules there are still many questions regarding their role in vivo.

Identification of human CD93 as the phagocytic C1q receptor (C1qRp) by expression cloning

CD93 is a approximately 120 kDa O-sialoglycoprotein that within the hematopoietic system is selectively expressed on cells of the myeloid lineage. So far, its primary structure and function were unknown. We used retroviral-expression cloning to isolate the CD93 cDNA. Sequence analysis revealed that CD93 is identical to a protein on human phagocytes termed C1q receptor (C1qRp). C1qRp was shown previously to mediate enhancement of phagocytosis in monocytes and was suggested to be a receptor of C1q and two other structurally related molecules. When studying CD93 transductants and control cells, we found that cells expressing CD93 have enhanced capacity to bind C1q. Furthermore, we show that immature dendritic cells (DC) express CD93/C1qRp, and mature DC, known to have reduced capacity for antigen uptake and to have lost the ability to phagocytose, show weak-to-negative CD93/C1qRp expression.

C1q-bearing immune complexes induce IL-8 secretion in human umbilical vein endothelial cells (HUVEC) through protein tyrosine kinase- and mitogen-activated protein kinase-dependent mechanisms: evidence that the 126 kD phagocytic C1q receptor mediates immune complex activation of HUVEC.

Endothelial cells play a pivotal role in the initiation and perpetuation of inflammation. C1q, the first component of the classical pathway of complement, is a potent stimulus leading to endothelial cell activation and cytokine production. The specific cellular mechanisms through which endothelial cells are stimulated by C1q are not known. We stimulated human umbilical vein endothelial cells (HUVEC) with either monomeric C1q or C1q-bearing immune complexes (C1q-IC) in the presence or absence of inhibitors of protein tyrosine kinases (PTK) or mitogen-activated protein kinases (MAPK). C1q-IC, but not monomeric C1q, induced IL-8 production in dose- and time-dependent fashion. R3, a cross-linking monoclonal IgM antibody against the 126 kD phagocytic C1q receptor (C1qR), also stimulated IL-8 production. IL-8 mRNA accumulation was detected by Northern blot analysis within 2 h of stimulation by the immune complexes and was enhanced by the addition of cycloheximide. Secretion of IL-8 by C1q-IC stimulated HUVEC was completely blocked by the PTK inhibitor, genistein or the MAPK inhibitor, UO126. These experiments demonstrate that C1q-IC-induced production of IL-8 in HUVEC is dependent upon the activation of PTK and MAPK. These findings also support a role for the phagocytic C1qR as an important activator of HUVEC by immune complexes.

Immune complexes and IFN-γ decrease cholesterol 27-hydroxylase in human arterial endothelium and macrophages

The enzyme cholesterol 27-hydroxylase, expressed by arterial endothelium and monocytes/macrophages, is one of the first lines of defense against the development of atherosclerosis. By catalyzing the hydroxylation of cholesterol to 27-hydroxycholesterol, which is more soluble in aqueous medium, the enzyme promotes the removal of cholesterol from the arterial wall. Prior studies have suggested that immune reactants play a role in the pathogenesis of atherosclerosis; we report here that immune reactants, IFN-γ and immune complexes bound to C1q, but not interleukin-1 and tumor necrosis factor, diminish the expression of cholesterol 27-hydroxylase in human aortic endothelial cells, peripheral blood mononuclear cells, monocyte-derived macrophages, and the human monocytoid cell line THP-1. In addition, our studies demonstrate that immune complexes down-regulate cholesterol 27-hydroxylase only after complement fixation via interaction with the 126-kD C1qRp protein on endothelial cells and THP-1 cells. These results are consistent with the prior demonstration that IFN-γ contributes to the pathogenesis of atherosclerosis and suggest a role for C1q receptors in the atherogenic process. Moreover, these observations suggest that one mechanism by which immune reactants contribute to the development of atherosclerosis is by down-regulating the expression of the enzymes required to maintain cholesterol homeostasis in the arterial wall.—Reiss, A. B., N. W. Awadallah, S. Malhotra, M. C. Montesinos, E. S. L. Chan, N. B. Javitt, and B. N. Cronstein. Immune complexes and IFN-γ decrease cholesterol 27-hydroxylase in human arterial endothelium and macrophages.

Hepatitis C virus core protein inhibits human T lymphocyte responses by a complement-dependent regulatory pathway.

Complement proteins are involved in early innate immune responses against pathogens and play a role in clearing circulating viral Ags from the blood of infected hosts. We have previously demonstrated that hepatitis C virus (HCV) core, the first protein to be expressed and circulating in the blood of infected individuals, inhibited human T cell proliferative response through interaction with the complement receptor, globular domain of C1q receptor (gC1qR). To investigate the mechanisms of HCV core/gC1qR-induced inhibition of T cell proliferation, we examined the effect of core protein on the early events in T cell activation. We found that HCV core inhibited phosphorylation of extracellular signal-regulated kinase (ERK) and mitogen-activated ERK kinase (MEK). HCV core-induced impairment of ERK/MEK mitogen-activated protein kinase resulted in the inhibition of IL-2 and IL-2Ralpha gene transcription, which led to the inhibition of IL-2 production and high-affinity IL-2R expression. Importantly, the ability of anti-gC1qR Ab treatment to reverse HCV core-induced inhibition of ERK/MEK phosphorylation reveals that the interaction between HCV core and gC1qR is linked to the interference of ERK/MEK mitogen-activated protein kinase activation. These results imply that HCV core-induced blockage of intracellular events in T cell activation by a complement-dependent regulatory pathway may play a critical role in the establishment of HCV persistence during the acute phase of viral infection.

In vivo pharmacokinetics of calreticulin S-domain, an inhibitor of the classical complement pathway

Inhibition of the complement system is potentially therapeutic in diseases where uncontrolled or overshooting complement activation plays a significant role in the pathogenesis of the disorder. Calreticulin (CRT) is a multifunctional protein whose cell-surface form (ectocalreticulin) is reported to be a C1q receptor. A 124-residue domain within CRT, the S-domain, binds to C1q, prevents the formation of C1 and so inhibits activation of the classical pathway. To assess the usefulness of CRT S-domain as a complement inhibitor, recombinant S-domain was expressed, radiolabeled, and the fate of the radiolabeled peptide followed in vivo. In rats, CRT-S-domain shows a half-life of 1.21±0.34 and 40.5±2.7 min in the distribution and elimination phases from plasma, respectively. The peptide remains largely intact, and is cleared from the circulation by the kidneys, where it accumulates in the proximal convoluted tubules, but is not excreted. Much smaller amounts of the peptide accumulate in other tissues, and essentially none crosses the blood–brain barrier.

Exposure of human epidermal keratinocyte cell cultures to sulfur mustard promotes binding of complement C1q: implications for toxicity and medical countermeasures.

Sulfur mustard (HD)-increased proteolytic activity, HD-enhanced expression of Fc receptor (FcR) on human epidermal keratinocytes (HEK) and associated inflammatory responses may contribute to HD pathology. Like the FcR, the first component of the classical complement (C') cascade, C1q, binds to the Fc region of antibody to mediate inflammatory responses. Complement C1q binds specifically to the C1q receptor (C1qR) on the blebs of apoptotic human keratinocytes and is proposed as a cell surface marker for apoptosis. Assays by fluorescent antibodies demonstrated significantly enhanced binding of C1q to HEK cell cultures exposed to HD. The cell populations of HEK that showed enhanced C1q binding also demonstrated an intermediate uptake of propidium iodide that was greater than in viable unexposed cells but less than in dead cells. The HD-enhanced C1q binding was concentration-dependent, negative by flow cytometry or weakly positive by digital scanning microscopy at 100 microM and positive by both methods at 300 microM. Binding of C1q was also time-dependent, weakly positive at 8 h, and positive at 16 and 24 h after HD exposure. The HD-increased C1qR that binds C1q to the surface of HEK might be a contributing mechanism or a marker for the inflammation and vesication associated with HD exposure.

Endothelial cells, megakaryoblasts, platelets and alveolar epithelial cells express abundant levels of the mouse AA4 antigen, a C-type lectin-like receptor involved in homing activities and innate immune host defense.

Phagocytic complement C1q receptor (C1qRp) and thrombomodulin (TM, CD141), are two key members of a novel family of lectin receptors involved in cell-cell interactions and innate immune host defense. We report here that the mouse homologue of human C1qRp (AA4) and TM are derived from a common ancestor on the basis that both genes co-localized to the same region of the chromosome 2 and also because they share similar domain composition and organization. Moreover, we found that mouse AA4 was localized to sites known to express TM. Mouse AA4 was identified by Western blot as a heavily glycosylated protein (110 kDa) expressed in all tissues tested. Brain and spleen expressed an additional 130-kDa band. Immunoperoxidase and double-immunofluorescence experiments showed that, surprisingly, the major site of AA4 expression in all tissue tested is on endothelial cells and that despite the apparent involvement of AA4 in the phagocytic response, it is not expressed by tissue macrophages. In contrast, megakaryoblasts (in bone marrow and spleen) and platelets expressed abundant levels of AA4 stored in cytoplasmic vesicles. Interestingly, alveolar epithelial cells (lung) but not other epithelia (e.g. uterus) were strongly stained for AA4. Although AA4 has been described on all hematopoietic progenitors, we found that only circulating immature B cells, monocytes and NK cells but not T cells and neutrophils expressed AA4.

C1qRp elicits a Ca++ response in rat NK cells but does not influence NK-mediated cytotoxicity.

The cell surface receptor C1qRp (receptor for C1q, regulating phagocytosis) present on macrophages and neutrophils, is presumed to stimulate phagocytosis in these cells. However, C1qRp is also present on natural killer (NK) cells, and in these cells its physiological function is not currently known. We have investigated putative functions of this cell surface molecule in rat NK cells with the aid of two novel monoclonal antibodies (MoAb) LOV3 and LOV8 against rat C1qRp. NK cells are known to be potent cytotoxic effector cells, both through specific recognition of ligands on a target cell and killing of antibody-coated target cells (antibody-dependent cellular cytotoxicity, ADCC). NK cells prestimulated with MoAbs LOV3 or LOV8 did not exhibit altered ADCC. Furthermore, the addition of MoAb LOV3 or LOV8 to cytotoxic cultures of NK cells and Fc-receptor positive tumour cells did not affect killing in a redirected killing assay, indicating that the receptor did not influence NK cytotoxicity. However, this is the first paper to show that an intracellular Ca++-response is induced in rat NK cells upon stimulation of C1qRp with LOV3 and LOV8. The response induced by the antibodies was only minimally reduced in the presence of EGTA, indicating that most of the response is owing to the Ca++ mobilization from intracellular calcium stores.

Complement facilitates early prion pathogenesis.

New-variant Creutzfeldt-Jakob disease and scrapie are typically initiated by extracerebral exposure to the causative agent, and exhibit early prion replication in lymphoid organs. In mouse scrapie, depletion of B-lymphocytes prevents neuropathogenesis after intraperitoneal inoculation, probably due to impaired lymphotoxin-dependent maturation of follicular dendritic cells (FDCs), which are a major extracerebral prion reservoir. FDCs trap immune complexes with Fc-gamma receptors and C3d/C4b-opsonized antigens with CD21/CD35 complement receptors. We examined whether these mechanisms participate in peripheral prion pathogenesis. Depletion of circulating immunoglobulins or of individual Fc-gamma receptors had no effect on scrapie pathogenesis if B-cell maturation was unaffected. However, mice deficient in C3, C1q, Bf/C2, combinations thereof or complement receptors were partially or fully protected against spongiform encephalopathy upon intraperitoneal exposure to limiting amounts of prions. Splenic accumulation of prion infectivity and PrPSc was delayed, indicating that activation of specific complement components is involved in the initial trapping of prions in lymphoreticular organs early after infection.

Characterization and molecular cloning of rat C1qRp, a receptor on NK cells.

Here we report the generation of monoclonal antibodies (mAb) LOV3 and LOV8 to a 110-130-kDa membrane glycoprotein expressed by rat NK cells. This NK surface molecule was identified by eucaryotic expression cloning as the structural orthologue of the phagocytosis-stimulating receptor for complement factor C1q and mannose-binding lectin on human macrophages, C1qRp. Rat C1qRp is a monomeric type I integral membrane protein consisting of 643 amino acids with an N-terminal lectin-like domain, five epidermal growth factor-like domains, a transmembrane domain and a 45-residue cytoplasmic domain. It is encoded by a single gene on rat chromosome 3q41-q42 and is 67% and 87.5% identical at the amino acid level to human and mouse C1qRp, respectively. Rat C1qRp is expressed by resting and by activated NK cells, on subpopulations of NKR-P1(+) T cells (NK/T cells), dendritic cells, macrophages and granulocytes, but not by B cells or NKR-P1(-) T cells. Expression of this innate immune receptor is therefore not restricted to hematopoietic cells of the myeloid lineage, but is also expressed on subsets of cells of lymphoid origin. The mAb did not affect the cytotoxic function of NK cells, and C1qRp on NK cells may have functions not related to NK killing.

Molecular and Cellular Properties of the Rat AA4 Antigen, a C-type Lectin-like Receptor with Structural Homology to Thrombomodulin

The murine fetal stem cell marker AA4 has recently been cloned and is known to be the homolog of the human phagocytic C1q receptor involved in host defense. We herein report the molecular cloning and the cellular expression pattern of the rat AA4 antigen. Modular architecture analysis indicated that the rat AA4 is a member of C-type lectin-like family and, interestingly, displays similar domain composition and organization to thrombomodulin. Northern blot and reverse transcriptase-polymerase chain reaction analyses indicated that rat AA4 was encoded by a single transcript of 7 kilobases expressed constitutively in all tissues. In situ hybridization showed that AA4 was expressed predominantly by pneumocytes and vascular endothelial cells. Using an affinity purified polyclonal antibody raised against a rat AA4-Fc fusion protein, AA4 was identified as a glycosylated protein of 100 kDa expressed by endothelial cells > platelets > NK cells and monocytes (ED1+ cells). The staining was associated to the cell surface and intracytoplasmic vesicles. Conversely, erythrocytes, T and B lymphocytes, neutrophils, and macrophages (ED2+ cells) were consistently negative for AA4. As expected, the macrophage cell line NR8383 expressed weak levels of AA4. Taken together, our results support the idea that AA4/C1qRp is involved in some cell-cell interactions.

Localization of P32 protein (gC1q-R) in mitochondria and at specific extramitochondrial locations in normal tissues.

P32 protein, also known as the gC1q receptor for complement component C1q, is a binding protein for nuclear pre-mRNA splicing factor SF2/ASF and numerous other nuclear and cell surface proteins, yet is targeted to the mitochondrial matrix compartment where these proteins are not present. In the present study, we use immunogold electron microscopy to evaluate the subcellular distribution of P32 protein (gC1q-R) in cultured cell lines and in rat tissues embedded in the acrylic resin LR Gold. Immunogold labeling of Raji lymphoma, CHO, human fibroblasts, HeLa and B-SC-1 cells shows reactivity primarily within mitochondria. Highly specific labeling of mitochondria is also obtained in rat tissues, including adrenal gland, cerebellum, cerebral cortex, heart, kidney, liver, pituitary, pancreas, skeletal muscle, spleen, testes and thyroid. However, strong P32 (gClq-R) reactivity is also present in (i) zymogen granules, condensing vacuoles, endoplasmic reticulum, and on the cell surface of pancreatic acinar cells, (ii) on the cell surface of microvascular endothelial cells in pancreas and kidney, (iii) on the cell surface and in nuclei of splenic lymphocytes, and (iv) in the acrosome of developing spermatids in testes. Western immunoblots show that the polyclonal antibody to P32 (gC1q-R) used in this study reacts specifically with a 32-kDa protein in both purified pancreatic zymogen granules and in mitochondria, and no other proteins are reactive. These results provide evidence that P32 (gC1q-R) is a mitochondrial protein that also localizes outside mitochondria in certain cells and tissues under normal physiological conditions.

Adiponectin, a new member of the family of soluble defense collagens, negatively regulates the growth of myelomonocytic progenitors and the functions of macrophages

AbstractWe investigated the functions of adiponectin, an adipocyte-specific secretory protein and a new member of the family of soluble defense collagens, in hematopoiesis and immune responses. Adiponectin suppressed colony formation from colony-forming units (CFU)—granulocyte-macrophage, CFU-macrophage, and CFU-granulocyte, whereas it had no effect on that of burst-forming units—erythroid or mixed erythroid-myeloid CFU. In addition, adiponectin inhibited proliferation of 4 of 9 myeloid cell lines but did not suppress proliferation of erythroid or lymphoid cell lines except for one cell line. These results suggest that adiponectin predominantly inhibits proliferation of myelomonocytic lineage cells. At least one mechanism of the growth inhibition is induction of apoptosis because treatment of acute myelomonocytic leukemia lines with adiponectin induced the appearance of subdiploid peaks and oligonucleosomal DNA fragmentation. Aside from inhibiting growth of myelomonocytic progenitors, adiponectin suppressed mature macrophage functions. Treatment of cultured macrophages with adiponectin significantly inhibited their phagocytic activity and their lipopolysaccharide-induced production of tumor necrosis factor α. Suppression of phagocytosis by adiponectin is mediated by one of the complement C1q receptors, C1qRp, because this function was completely abrogated by the addition of an anti-C1qRp monoclonal antibody. These observations suggest that adiponectin is an important negative regulator in hematopoiesis and immune systems and raise the possibility that it may be involved in ending inflammatory responses through its inhibitory functions.

Adiponectin, a new member of the family of soluble defense collagens, negatively regulates the growth of myelomonocytic progenitors and the functions of macrophages

We investigated the functions of adiponectin, an adipocyte-specific secretory protein and a new member of the family of soluble defense collagens, in hematopoiesis and immune responses. Adiponectin suppressed colony formation from colony-forming units (CFU)—granulocyte-macrophage, CFU-macrophage, and CFU-granulocyte, whereas it had no effect on that of burst-forming units—erythroid or mixed erythroid-myeloid CFU. In addition, adiponectin inhibited proliferation of 4 of 9 myeloid cell lines but did not suppress proliferation of erythroid or lymphoid cell lines except for one cell line. These results suggest that adiponectin predominantly inhibits proliferation of myelomonocytic lineage cells. At least one mechanism of the growth inhibition is induction of apoptosis because treatment of acute myelomonocytic leukemia lines with adiponectin induced the appearance of subdiploid peaks and oligonucleosomal DNA fragmentation. Aside from inhibiting growth of myelomonocytic progenitors, adiponectin suppressed mature macrophage functions. Treatment of cultured macrophages with adiponectin significantly inhibited their phagocytic activity and their lipopolysaccharide-induced production of tumor necrosis factor α. Suppression of phagocytosis by adiponectin is mediated by one of the complement C1q receptors, C1qRp, because this function was completely abrogated by the addition of an anti-C1qRp monoclonal antibody. These observations suggest that adiponectin is an important negative regulator in hematopoiesis and immune systems and raise the possibility that it may be involved in ending inflammatory responses through its inhibitory functions.

Molecular cloning and tissue expression of the phagocytic complement C1q receptor in rat

Molecular cloning and tissue expression of the phagocytic complement C1q receptor in rat

Structural and functional evidence for microglial expression of C1qR(P), the C1q receptor that enhances phagocytosis.

Microglial activation has been associated with several degenerative diseases of the central nervous system (CNS). One consequence of activation is the induction of a more efficient phagocytic response, and it is therefore important to determine what factors regulate microglial phagocytosis and whether this capacity influences the progression of neurodegenerative changes. Previous studies have demonstrated that complement component C1q enhances Fc receptor- and CR1-mediated phagocytosis in cells of the myeloid lineage via a cell surface receptor, C1qRp. Because C1q has been found in the area of lesions in several degenerative CNS diseases, the current investigations were carried out to characterize the effects of C1q on microglial phagocytosis. Neonatal rat microglia were shown to express C1qRp, as assessed by flow cytometry and immunocytochemistry. Interaction of these cells with substrate-bound C1q was shown to enhance both FcR-and CR1-mediated phagocytosis two- to fourfold. In addition, introduction of an antibody raised against the carboxy-terminal, cytoplasmic domain of C1qRp into microglia by electroporation markedly diminished the ability of C1q to enhance uptake of IgG-coated targets, whereas nonspecific IgG had no such effect. These results suggest that C1q in areas of active degeneration may promote the phagocytic capacity of microglia via interaction with microglial C1qRp.