Characterization of the murine homolog of C1qR(P): identical cellular expression pattern, chromosomal location and functional activity of the human and murine C1qR(P)

Short-Lived Cages Restrict Protein Diffusion in the Plasma Membrane

The plasma membrane is a heterogeneous environment characterized by anomalous diffusion and the presence of microdomains that are molecularly distinct from the bulk membrane. Using single particle tracking of the C-type lectin CD93, we have identified for the first time the transient trapping of transmembrane proteins in cage-like microdomains which restrict protein diffusion. These cages are stabilized by actin-dependent confinement regions, but are separate structures with sizes and lifespans uncorrelated to those of the underlying actin corral. These membrane cages require cholesterol for their strength and stability, with cholesterol depletion decreasing both. Despite this, cages are much larger in size and are longer lived than lipid rafts, suggesting instead that cholesterol-dependent effects on membrane fluidity or molecular packing play a role in cage formation. This diffusional compartment in the plasma membrane has characteristics of both a diffusional barrier and a membrane microdomain, with a size and lifespan intermediate between short-lived microdomains such as lipid rafts and long-lasting diffusional barriers created by the actin cytoskeleton.

Purification, Visualization, and Molecular Signature of Neural Stem Cells

Neural stem cells (NSCs) are isolated from primary brain tissue and propagated as a heterogeneous mix of cells, including neural progenitors. To date, NSCs have not been purified in vitro to allow study of their biology and utility in regenerative medicine. In this study, we identify C1qR1 as a novel marker for NSCs and show that it can be used along with Lewis-X (LeX) to yield a highly purified population of NSCs. Using time-lapse microscopy, we are able to follow NSCs forming neurospheres, allowing their visualization. Finally, using single-cell polymerase chain reaction (PCR), we determine the molecular signature of NSCs. The single-cell PCR data suggest that along with the Notch and Shh pathways, the Hippo pathway plays an important role in NSC activity.

Autoantibody-mediated arthritis in the absence of C3 and activating Fcγ receptors: C5 is activated by the coagulation cascade

Introduction

The effector functions of immunoglobulin G (IgG) are mediated by interaction of its Fc region with Fc receptors (FcγRs) and/or the complement system. The three main pathways of complement activation converge at C3. However, C3-independent pathways can activate C5 and other downstream complement components during IgG-initiated inflammatory responses. These C3-independent pathways of C5 activation are triggered by activating FcγRs in some systems or can be activated by factors of the coagulation cascade such as thrombin. Here we studied the interplay of C3, C5, and activating FcγRs in a model of spontaneous autoantibody-driven arthritis.

Methods

We utilized the K/BxN TCR transgenic mouse model of arthritis. We bred K/BxN mice bearing targeted or naturally-occurring mutations in one or more of the genes encoding complement components C3, C5, and FcRγ, the cytoplasmic signaling chain shared by the activating FcγRs. We measured arthritis development, the production of arthritogenic autoantibodies, T cell activation status and cytokine synthesis. In addition, we treated mice with anti-C5 monoclonal antibodies or with the thrombin inhibitor argatroban.

Results

We have previously shown that genetic deficiency of C5 protects K/BxN mice from the development of arthritis. We found here that C3-deficient K/BxN mice developed arthritis equivalent in severity to C3-sufficient animals. Arthritis also developed normally in K/BxN mice lacking both C3 and FcRγ, but could be ameliorated in these animals by treatment with anti-C5 monoclonal antibody or by treatment with argatroban. Production of arthritogenic autoantibodies, T cell activation, and T cell cytokine production were not affected by the absence of C3, C5, and/or FcRγ.

Conclusions

In K/BxN mice, C5-dependent autoantibody-driven arthritis can occur in the genetic absence of both complement C3 and activating FcγRs. Our findings suggest that in this setting, thrombin activates C5 to provoke arthritis.

A novel CD93 polymorphism in non-obese diabetic (NOD) and NZB/W F1 mice is linked to a CD4+ iNKT cell deficient state

In the present study, we characterize a polymorphism in the CD93 molecule, originally identified as the receptor for the C1q complement component (i.e., C1qRp, or AA4.1) in non-obese diabetic (NOD) mice. This allele carries a coding polymorphism in the first epidermal growth factor-like domain of CD93, which results in an amino acid substitution from Asn→His at position 264. This polymorphism does not appear to influence protein translation or ecto-domain cleavage, as CD93 is detectable in bone-marrow-derived macrophage and B-cell precursor lysates and in soluble form in the serum. The NOD CD93 isoform causes a phenotypic aberrancy in the early B-cell developmental stages (i.e., pro-, pre-, immature, and transitional), likely related to a conformational variation. Interestingly, the NZB/W F1 strain, which serves as a murine model of Lupus, also expresses an identical CD93 sequence polymorphism. Cd93 is located within the NOD Idd13 locus and is also tightly linked to the NZB/W F1 Wbw1 and Nkt2 disease susceptibility loci, which are thought to regulate natural killer T (NKT) cell homeostasis. Consistent with this genetic linkage, we found B6 CD93^−/− and B6.NOD^Idd13 mice to be susceptible to a profound CD4⁺ NKT cell deficient state. These data suggest that Cd93 may be an autoimmune susceptibility gene residing within the Idd13 locus, which plays a role in regulating absolute numbers of CD4⁺ NKT cells.

A novel role of complement factor C1q in augmenting the presentation of antigen captured in immune complexes to CD8+ T lymphocytes

Ag-IgG immune complexes (IC) are efficiently taken up, and Ag-derived peptides are subsequently processed and presented by APC. In vitro experiments indicate that IgG Fc Receptors (FcgammaR) facilitate the efficient uptake of IC by dendritic cells. Previous experiments showed that the cross-presentation of Ag-derived peptides after s.c. administration of IC is FcgammaR-dependent. To study the role of different FcgammaR and complement in MHC class I Ag presentation after i.v. administration, we used mice deficient for FcgammaRs and complement components. These mice were injected with CFSE-labeled OVA-specific CD8+ T cells followed by administration of IC composed of OVA and rabbit anti-OVA IgG i.v. to measure MHC class I presentation of OVA-derived peptides. The Ag presentation was partly reduced in FcRgamma-chain-deficient mice, but not affected in FcgammaRI/II/III-deficient mice, complement factor C3-deficient mice, or FcgammaRI/II/III x C3-deficient mice. Importantly, CD8+ T cell proliferation was significantly reduced in mice deficient for C1q. This proliferation could be restored when IC were incubated with purified human C1q before injection. Likewise, purified C1q could strongly enhance the uptake and presentation of IC by dendritic cells in vitro. Heat inactivation abrogated the C1q-mediated uptake of IC. In addition, in vivo uptake of OVA-IC in the spleen was significantly reduced in C1q-deficient mice compared with wild-type mice. Together, these results indicate a novel function of C1q, which is present in high levels in the bloodstream, by directly enhancing the uptake and MHC class I presentation of Ag captured in IC by APC to CD8+ T cells.

Complement proteins C1q and MBL are pattern recognition molecules that signal immediate and long-term protective immune functions

C1q and mannose binding lectin, members of the "defense collagen" family, are pattern recognition molecules that can trigger rapid enhanced phagocytosis resulting in efficient containment of pathogens or clearance of cellular debris, apoptotic cells and immune complexes. In addition, interaction of C1q and mannose binding lectin with the phagocyte alters subsequent phagocyte cytokine synthesis, and thus may have important implications in directing acute inflammation as well as long-term protective immunity. The importance of the role of defense collagens in phagocytosis of apoptotic cells is highlighted by studies in vivo of mice deficient in C1q, pulmonary surfactant D and mannose binding lectin in which there is delayed clearance of apoptotic cells. Indeed, deficiency of C1q is a risk factor for the development of autoimmunity in both humans and mice, consistent with the hypothesis that inefficient clearance of apoptotic cells results in release of autoantigens and contributes to the pathology associated with autoimmune diseases such as systemic lupus erythematosus. Further understanding of the importance of C1q and mannose binding lectin in the clearance of apoptotic cells and regulation of cytokine synthesis and identification of the receptors implicated in mediating these processes should provide novel targets for therapeutic intervention in the control and manipulation of the immune response in terms of both host defense against infectious disease and tissue repair and remodeling.

Roles of the alternative complement pathway and C1q during innate immunity to Streptococcus pyogenes

Complement is important for innate immunity to the common bacterial pathogen Streptococcus pyogenes, but the relative importance of the alternative and classical pathways has not been investigated. Using mice and human serum deficient in either C1q, the first component of the classical pathway, or factor B, an important component of the alternative pathway, we have investigated the role of both pathways for innate immunity to S. pyogenes. C3b deposition on four different strains of S. pyogenes was mainly dependent on factor B. As a consequence opsonophagocytosis of S. pyogenes was reduced in serum from factor B-deficient mice, and these mice were very susceptible to S. pyogenes infection. In contrast, C3b deposition was not dependent on C1q for two of the strains investigated, H372 and H305, yet opsonophagocytosis of all four S. pyogenes strains was impaired in serum deficient in C1q. Furthermore, infection in C1q-deficient mice with strain H372 resulted in a rapidly progressive disease associated with large numbers of bacteria in target organs. These results demonstrate the important role of the alternative pathway and C1q for innate immunity to S. pyogenes and suggest that C1q-mediated innate immunity to at least some strains of S. pyogenes may involve mechanisms that are independent of C3b on the bacteria.

Modulated interaction of the ERM protein, moesin, with CD93

CD93 is a cell-surface glycoprotein that has been shown to influence defence collagen-enhanced Fc-receptor or CR1-mediated phagocytosis of suboptimally opsonized targets in vitro, and CD93-deficient mice are defective in the clearance of apoptotic cells in vivo. To investigate the mechanism of CD93 modulation of phagocytic activity, GST fusion proteins containing the 47 amino acid intracellular domain (GST-Cyto), or various mutants of the intracellular domain of CD93, were constructed and used to identify intracellular CD93-binding molecules. The intracellular protein moesin, well characterized for its role in linking transmembrane proteins to the cytoskeleton and in cytoskeletal remodelling, bound to GST-Cyto when either cell lysates or recombinant moesin were used as a source of interacting molecules. An association of moesin with CD93 within intact cells was confirmed by co-capping moesin with CD93 in human monocytes. The moesin-binding site on CD93 mapped to the first four positively charged amino acids in the juxtamembrane region of the CD93 cytoplasmic tail. Interestingly, deletion of the last 11 amino acids from the C terminus of CD93 (GST-Cyto-C11) dramatically increased moesin binding to the cytoplasmic tail of CD93 in the cell lysate assay, but not when the binding of purified recombinant moesin was assessed. Furthermore, moesin binding to CD93 was enhanced by the addition of phosphatidylinositol 4,5-bisphosphate (PIP(2)). Taken together, these data suggest that the interaction of moesin with the CD93 cytoplasmic domain is modulated by binding of other intracellular molecules to the C11 region and implies that a PIP(2) signalling pathway is involved in CD93 function.

CD93 interacts with the PDZ domain-containing adaptor protein GIPC: implications in the modulation of phagocytosis

CD93 was originally identified as a myeloid cell-surface marker and subsequently associated with an ability to modulate phagocytosis of suboptimally opsonized immunoglobulin G and complement particles in vitro. Recent studies using mice deficient in CD93 have demonstrated that this molecule modulates phagocytosis of apoptotic cells in vivo. To investigate signal transduction mechanisms mediated by CD93, CD93 cytoplasmic tail (CYTO)-binding proteins were identified in a yeast two-hybrid screen. Fifteen of 34 positive clones contained a splice variant or a partial cDNA encoding GIPC, a PSD-95/Dlg/ZO-1 (PDZ) domain-containing protein, shown previously to regulate cytoskeletal dynamics. A single clone of the N-terminal kinase-like protein p105 and an uncharacterized stem cell transcript also showed specificity for binding to the CYTO by yeast two-hybrid. Using the yeast two-hybrid system and an in vitro glutathione S-transferase fusion protein-binding assay, the binding of GIPC to the CYTO was shown to involve a newly identified class I PDZ-binding domain in the CD93 carboxyl terminus. Four positively charged amino acids in the juxtamembrane domain of CD93 were shown to be critical in stabilizing these interactions. Treatment of human monocytes with a cell-permeable peptide encoding the C-terminal 11 amino acids of CD93 resulted in an enhancement of phagocytosis, supporting the hypothesis that this protein-protein interaction domain is involved in the modulation of phagocytosis. These protein interactions may participate as molecular switches in modulating cellular phagocytic activity.

Murine CD93 (C1qRp) Contributes to the Removal of Apoptotic Cells In Vivo but Is Not Required for C1q-Mediated Enhancement of Phagocytosis

Human CD93 (known as C1qRp) has been shown to be a phagocytic receptor involved in the in vitro C1q-dependent enhancement of phagocytosis. However, binding of CD93 to C1q and its function remain controversial. In this study, we have generated CD93-deficient mice (CD93(-/-)) to investigate its biological role(s). The CD93(-/-) mice were viable and showed no gross abnormalities in their development. Thioglycolate-elicited peritoneal macrophages deficient in CD93 showed a similar enhancement in complement- and FcgammaR-dependent uptake of RBC to the wild-type macrophages when plated on C1q-coated surfaces suggesting that the lack of this receptor had no effect on these C1q-mediated events. There was no impairment in either complement- or FcgammaR-dependent phagocytic assays in vivo. By contrast, the CD93(-/-) mice had a significant phagocytic defect in the clearance of apoptotic cells in vivo (human Jurkat T cells and murine thymocytes: p=0.0006 and p=0.0079, respectively) compared with strain-matched controls. However, in vitro, the CD93(-/-) macrophages showed similar engulfment of apoptotic cells to wild-type macrophages. Furthermore, no supporting evidence for a role of CD93 as an adhesion molecule was found using intravital microscopy or analyzing peritoneal cell recruitment in response to three different inflammatory stimuli (thioglycolate, zymosan A, and IL-1beta). Thus, our findings indicate that murine CD93 is expressed on the peritoneal macrophage, especially on thioglycolate-elicited cells, but does not appear to play a key role in C1q-mediated enhancement of phagocytosis or in the intercellular adhesion events tested. However, our results suggest that it may contribute to the in vivo clearance of dying cells.

Cell surface expression of C1qRP/CD93 is stabilized by O-glycosylation

C1qRP/CD93 is a cell surface receptor predominantly expressed on monocytes, neutrophils, endothelial cells, and early stem cell precursors. In phagocytic cells, it has been characterized as contributing to the enhancement of FcR- and CR1-induced phagocytosis triggered by innate immune system defense collagens such as C1q and mannose binding lectin (MBL). Previously, we demonstrated a high level of glycosylation on C1qRP/CD93 that was predominantly O-linked. In this study, we investigate the role of glycosylation in C1qRP/CD93 stability first by inhibiting O-glycosylation by addition of benzyl 2-acetamido-2-deoxy-alpha-D-galactopyranoside (BAG) to the human histiocytic cell line U937, and secondly, by expression of C1qRP/CD93 in the CHO-derived cell line ldlD which has a reversible defect in protein glycosylation. In both U937 cells and in ldlD cells transfected to express C1qRP/CD93, glycosylation deficiency caused cell surface expression levels of C1qRP/CD93 to decrease, concomitant with the detection of C1qRP/CD93 reactivity in the culture media. Metabolic labeling studies show that when glycosylation is absent, C1qRP/CD93 is synthesized and rapidly released into the culture supernatant or degraded. These studies demonstrate that O-glycosylation is important in the stable cell surface expression of C1qRP/CD93 .

Constitutive versus activation-dependent cross-presentation of immune complexes by CD8(+) and CD8(-) dendritic cells in vivo

Murine splenic dendritic cells (DCs) can be divided into two subsets based on CD8alpha expression, but the specific role of each subset in stimulation of T cells is largely unknown. An important function of DCs is the ability to take up exogenous antigens and cross-present them in the context of major histocompatibility complex (MHC) class I molecules to CD8(+) T cells. We previously demonstrated that, when cell-associated ovalbumin (OVA) is injected into mice, only the CD8(+) DC subset cross-presents OVA in the context of MHC class I. In contrast to this selectivity with cell-associated antigen, we show here that both DC subsets isolated from mice injected with OVA/anti-OVA immune complexes (OVA-IC) cross-present OVA to CD8(+) T cells. The use of immunoglobulin G Fc receptor (Fc(gamma)R) common gamma-chain-deficient mice revealed that the cross-presentation by CD8(-) DCs depended on the expression of gamma-chain-containing activating FcgammaRs, whereas cross-presentation by CD8(+) DCs was not reduced in gamma-chain-deficient mice. These results suggest that although CD8(+) DCs constitutively cross-present exogenous antigens in the context of MHC class I molecules, CD8(-) DCs only do so after activation, such as via ligation of Fc(gamma)Rs. Cross-presentation of immune complexes may play an important role in autoimmune diseases and the therapeutic effect of antitumor antibodies.

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

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

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.