Molecular and cellular properties of the rat AA4 antigen, a C-type lectin-like receptor with structural homology to thrombomodulin

Inflammatory Mesenchymal Stem Cells Express Abundant Membrane-Bound and Soluble Forms of C-Type Lectin-like CD248

CD248 (endosialin) belongs to a glycoprotein family that also includes thrombomodulin (CD141), CLEC14A, and CD93 (AA4) stem cell markers. We analyzed the regulated expression of CD248 in vitro using skin (HFFF) and synovial (FLS) mesenchymal stem cell lines, and in fluid and tissue samples of rheumatoid arthritis (RA) and osteoarthritis (OA) patients. Cells were incubated with either rhVEGF₁₆₅, bFGF, TGF-β1, IL1-β, TNF-α, TGFβ1, IFN-γ, or PMA (Phorbol ester). There was no statistically significant change in membrane expression. A soluble (s) form of cleaved CD248 (sCD248) was detected after cell treatment with IL1-β and PMA. Matrix metalloprotease (MMP) MMP-1 and MMP-3 mRNAs were significantly up-regulated by IL1-β and PMA. A broad MMP inhibitor blocked the release of soluble CD248. In RA synovial tissue, we identified CD90⁺ perivascular MSCs double-stained for CD248 and VEGF. High sCD248 levels were detected in synovial fluid from RA. In culture, subpopulations of CD90⁺ CD14^- RA MSCs were either identified as CD248⁺ or CD141⁺ cells but CD93^-. CD248 is abundantly expressed by inflammatory MSCs and shed in an MMP-dependent manner in response to cytokines and pro-angiogenic growth factors. Both membrane-bound and soluble CD248 (acting as a decoy receptor) may contribute to RA pathogenesis.

Metoclopramide treatment blocks CD93-signaling-mediated self-renewal of chronic myeloid leukemia stem cells

Self-renewal is a key characteristic of leukemia stem cells (LSCs) responsible for the development and maintenance of leukemia. In this study, we identify CD93 as an important regulator of self-renewal and proliferation of murine and human LSCs, but not hematopoietic stem cells (HSCs). The intracellular domain of CD93 promotes gene transcription via the transcriptional regulator SCY1-like pseudokinase 1 independently of ligation of the extracellular domain. In a drug library screen, we identify the anti-emetic agent metoclopramide as an efficient blocker of CD93 signaling. Metoclopramide treatment reduces murine and human LSCs in vitro and prolongs survival of chronic myeloid leukemia (CML) mice through downregulation of pathways related to stemness and proliferation in LSCs. Overall, these results identify CD93 signaling as an LSC-specific regulator of self-renewal and proliferation and a targetable pathway to eliminate LSCs in CML.

Soluble CD93 is an apoptotic cell opsonin recognized by αx β2

Efferocytosis is essential for homeostasis and prevention of the inflammatory and autoimmune diseases resulting from apoptotic cell lysis. CD93 is a transmembrane glycoprotein previously implicated in efferocytosis, with mutations in CD93 predisposing patients to efferocytosis-associated diseases. CD93 is a cell surface protein, which is proteolytically shed under inflammatory conditions, but it is unknown how CD93 mediates efferocytosis or whether its efferocytic activity is mediated by the soluble or membrane-bound form. Herein, using cell lines and human monocytes and macrophages, we demonstrate that soluble CD93 (sCD93) potently opsonizes apoptotic cells but not a broad range of microorganisms, whereas membrane-bound CD93 has no phagocytic, efferocytic, or tethering activity. Using mass spectrometry, we identified α_x β₂ as the receptor that recognizes sCD93, and via deletion mutagenesis determined that sCD93 binds to apoptotic cells via its C-type lectin-like domain and to α_x β₂ by its EGF-like repeats. The bridging of apoptotic cells to α_x β₂ markedly enhanced efferocytosis by macrophages and was abrogated by α_x β₂ knockdown. Combined, these data elucidate the mechanism by which CD93 regulates efferocytosis and identifies a previously unreported opsonin-receptor system utilized by phagocytes for the efferocytic clearance of apoptotic cells.

RNA sequencing for global gene expression associated with muscle growth in a single male modern broiler line compared to a foundational Barred Plymouth Rock chicken line

Background

Modern broiler chickens exhibit very rapid growth and high feed efficiency compared to unselected chicken breeds. The improved production efficiency in modern broiler chickens was achieved by the intensive genetic selection for meat production. This study was designed to investigate the genetic alterations accumulated in modern broiler breeder lines during selective breeding conducted over several decades.

Methods

To identify genes important in determining muscle growth and feed efficiency in broilers, RNA sequencing (RNAseq) was conducted with breast muscle in modern pedigree male (PeM) broilers (n = 6 per group), and with an unselected foundation broiler line (Barred Plymouth Rock; BPR). The RNAseq analysis was carried out using Ilumina Hiseq (2 x 100 bp paired end read) and raw reads were assembled with the galgal4 reference chicken genome. With normalized RPM values, genes showing >10 average read counts were chosen and genes showing <0.05 p-value and >1.3 fold change were considered as differentially expressed (DE) between PeM and BPR. DE genes were subjected to Ingenuity Pathway Analysis (IPA) for bioinformatic functional interpretation.

Results

The results indicate that 2,464 DE genes were identified in the comparison between PeM and BPR. Interestingly, the expression of genes encoding mitochondrial proteins in chicken are significantly biased towards the BPR group, suggesting a lowered mitochondrial content in PeM chicken muscles compared to BPR chicken. This result is inconsistent with more slow muscle fibers bearing a lower mitochondrial content in the PeM. The molecular, cellular and physiological functions of DE genes in the comparison between PeM and BPR include organismal injury, carbohydrate metabolism, cell growth/proliferation, and skeletal muscle system development, indicating that cellular mechanisms in modern broiler lines are tightly associated with rapid growth and differential muscle fiber contents compared to the unselected BPR line. Particularly, PDGF (platelet derived growth factor) signaling and NFE2L2 (nuclear factor, erythroid 2-like 2; also known as NRF2) mediated oxidative stress response pathways appear to be activated in modern broiler compared to the foundational BPR line. Upstream and network analyses revealed that the MSTN (myostatin) –FST (follistatin) interactions and inhibition of AR (androgen receptor) were predicted to be effective regulatory factors for DE genes in modern broiler line. PRKAG3 (protein kinase, AMP-activated, gamma 3 non-catalytic subunit) and LIPE (lipase E) are predicted as core regulatory factors for myogenic development, nutrient and lipid metabolism.

Conclusion

The highly upregulated genes in PeM may represent phenotypes of subclinical myopathy commonly observed in the commercial broiler breast tissue, that can lead to muscle hardening, named as woody breast. By investigating global gene expression in a highly selected pedigree broiler line and a foundational breed (Barred Plymouth Rock), the results provide insight into cellular mechanisms that regulate muscle growth, fiber composition and feed efficiency.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3471-y) contains supplementary material, which is available to authorized users.

Myeloid 12/15-LOX regulates B cell numbers and innate immune antibody levels in vivo

Background. The myeloid enzyme 12/15-lipoxygenase (LOX), which generates bioactive oxidized lipids, has been implicated in numerous inflammatory diseases, with several studies demonstrating an improvement in pathology in mice lacking the enzyme. However, the ability of 12/15-LOX to directly regulate B cell function has not been studied.

Methods. The influence of 12/15-LOX on B cell phenotype and function, and IgM generation, was compared using wildtype (WT) and 12/15-LOX ( Alox15^-/-) deficient mice. The proliferative and functional capacity of splenic CD19 ⁺ B cells was measured in vitro in response to various toll-like receptor agonists.

Results. WT and Alox15^-/- displayed comparable responses. However in vivo, splenic B cell numbers were significantly elevated in Alox15^-/- mice with a corresponding elevation in titres of total IgM in lung, gut and serum, and lower serum IgM directed against the 12/15-LOX product, 12-hydroxyeicosatetraenoic acid-phosphatidylethanolamine (HETE-PE).

Discussion. Myeloid 12/15-LOX can regulate B cell numbers and innate immune antibody levels in vivo, potentially contributing to its ability to regulate inflammatory disease. Furthermore, the alterations seen in 12/15-LOX deficiency likely result from changes in the equilibrium of the immune system that develop from birth. Further studies in disease models are warranted to elucidate the contribution of 12/15-LOX mediated alterations in B cell numbers and innate immune antibody generation to driving inflammation in vivo.

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.

Regulation of CD93 Cell Surface Expression by Protein Kinase C Isoenzymes

Human CD93, also known as complement protein 1, q subcomponent, receptor (C1qRp), is selectively expressed by cells with a myeloid lineage, endothelial cells, platelets, and microglia and was originally reported to be involved in the complement protein 1, q subcomponent (C1q)-mediated enhancement of phagocytosis. The intracellular molecular events responsible for the regulation of its expression on the cell surface, however, have not been determined. In this study, the effect of protein kinases in the regulation of CD93 expression on the cell surface of a human monocyte-like cell line (U937), a human NK-like cell line (KHYG-1), and a human umbilical vein endothelial cell line (HUV-EC-C) was investigated using four types of protein kinase inhibitors, the classical protein kinase C (cPKC) inhibitor Go6976, the novel PKC (nPKC) inhibitor Rottlerin, the protein kinase A (PKA) inhibitor H-89 and the protein tyrosine kinase (PTK) inhibitor herbimycin A at their optimum concentrations for 24 hr. CD93 expression was analyzed using flow cytometry and glutaraldehyde-fixed cellular enzyme-linked immunoassay (EIA) techniques utilizing a CD93 monoclonal antibody (mAb), mNI-11, that was originally established in our laboratory as a CD93 detection probe. The nPKC inhibitor Rottlerin strongly down-regulated CD93 expression on the U937 cells in a dose-dependent manner, whereas the other inhibitors had little or no effect. CD93 expression was down-regulated by Go6976, but not by Rottlerin, in the KHYG-1 cells and by both Rottlerin and Go6976 in the HUV-EC-C cells. The PKC stimulator, phorbol myristate acetate (PMA), strongly up-regulated CD93 expression on the cell surface of all three cell-lines and induced interleukin-8 (IL-8) production by the U937 cells and interferon-gamma (IFN-gamma) production by the KHYG-1 cells. In addition, both Go6976 and Rottlerin inhibited the up-regulation of CD93 expression induced by PMA and IL-8 or IFN-gamma production in the respective cell-lines. Whereas recombinant tumor necrosis factor-alpha (rTNF-alpha) slightly up-regulated CD93 expression on the U937 cells, recombinant interleukin-1beta (rIL-1beta), recombinant interleukin-2 (rIL-2), recombinant interferon-gamma (rIFN-gamma) and lipopolysaccharide (LPS) had no effect. Taken together, these findings indicate that the regulation of CD93 expression on these cells involves the PKC isoenzymes.

The epidermal growth factor-like domain of CD93 is a potent angiogenic factor

Human CD93, an epidermal growth factor (EGF)-like domain containing transmembrane protein, is predominantly expressed in the vascular endothelium. Studies have shown that AA4, the homolog of CD93 in mice, may mediate cell migration and angiogenesis in endothelial cells. Soluble CD93 has been detected in the plasma of healthy individuals. However, the role of soluble CD93 in the endothelium remains unclear. Recombinant soluble CD93 proteins with EGF-like domains (rCD93D123, with domains 1, 2, and 3; and rCD93D23, with domains 2 and 3) were generated to determine their functions in angiogenesis. We found that rCD93D23 was more potent than rCD93D123 in stimulating the proliferation and migration of human umbilical vein endothelial cells (HUVECs). Production of matrix-metalloproteinase 2 increased after the HUVECs were treated with rCD93D23. Further, in a tube formation assay, rCD93D23 induced cell differentiation of HUVECs through phosphoinositide 3-kinase/Akt/endothelial nitric oxide synthase and extracellular signal-regulated kinases-1/2 signaling. Moreover, rCD93D23 promoted blood vessel formation in a Matrigel-plug assay and an oxygen-induced retinopathy model in vivo. Our findings suggest that the soluble EGF-like domain containing CD93 protein is a novel angiogenic factor acting on the endothelium.

CD93/AA4.1: a novel regulator of inflammation in murine focal cerebral ischemia

The stem-cell marker CD93 (AA4.1/C1qRp) has been described as a potential complement C1q-receptor. Its exact molecular function, however, remains unknown. By using global expression profiling we showed that CD93-mRNA is highly induced after transient focal cerebral ischemia. CD93 protein is upregulated in endothelial cells, but also in selected macrophages and microglia. To elucidate the potential functional role of CD93 in postischemic brain damage, we used mice with a targeted deletion of the CD93 gene. After 30 min of occlusion of the middle cerebral artery and 3 d of reperfusion these mice displayed increased leukocyte infiltration into the brain, increased edema, and significantly larger infarct volumes (60.8 +/- 52.2 versus 23.9 +/- 16.6 mm(3)) when compared with wild-type (WT) mice. When the MCA was occluded for 60 min, after 2 d of reperfusion the CD93 knockout mice still showed more leukocytes in the brain, but the infarct volumes were not different from those seen in WT animals. To further explore CD93-dependent signaling pathways, we determined global transcription profiles and compared CD93-deficient and WT mice at various time points after induction of focal cerebral ischemia. We found a highly significant upregulation of the chemokine CCL21/Exodus-2 in untreated and treated CD93-deficient mice at all time points. Induction of CCL21 mRNA and protein was confirmed by PCR and immunohistochemistry. CCL21, which was formerly shown to be released by damaged neurons and to activate microglia, contributes to neurodegeneration. Thus, we speculate that CD93-neuroprotection is mediated via suppression of the neuroinflammatory response through downregulation of CCL21.

The multiple roles of the innate immune system in the regulation of apoptosis and inflammation in the brain

Central nervous system (CNS) tissues contain cells (i.e. glia and neurons) that have innate immune functions. These cells express a range of receptors that are capable of detecting and clearing apoptotic cells and regulating inflammatory responses. Phagocytosis of apoptotic cells is a nonphlogistic (i.e. noninflammatory) process that provides immune regulation through anti-inflammatory cytokines andregulatory T cells. Neurons and glia express cellular death signals, including CD95Fas/CD95L, FasL, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and tumor necrosis factor receptor 1 (TNFR), through which they can trigger apoptosis in T cells and other infiltrating cells. Microglia, astrocytes, ependymal cells, and neurons express defense collagens and scavenger and phagocytic receptors that recognize apoptotic cells displaying apoptotic cell-associated molecular patterns, which serve as markers of "altered self." Glia also express pentraxins and complement proteins (C1q, C3b, and iC3b) that opsonize apoptotic cells, making them targets for the phagocytic receptors CR3 and CR4. Immunoregulatory molecules such as the complement regulator CD46 are lost from apoptotic cells and stimulate phagocytosis, whereas the expression of CD47 and CD200 is upregulated during apoptosis; this inhibits proinflammatory microglial cytokine expression, thereby reducing the severity of inflammation. This review outlines the cellular pathways used for the detection and phagocytosis of apoptotic cells in vitro and in experimental models of CNS inflammation.

Complement factor H, a marker of self protects against experimental autoimmune encephalomyelitis

The CNS innate immune response is a "double-edged sword" representing a fine balance between protective antipathogen responses and detrimental neurocytotoxic effects. Hence, it is important to identify the key regulatory mechanisms involved in the control of CNS innate immunity and which could be harnessed to explore novel therapeutic avenues. In analogy to the newly described neuroimmune regulatory proteins also known as "don't eat me" signals (CD200, CD47, CD22, fractalkine, semaphorins), we herein identify the key role of complement regulator factor H (fH) in controlling neuroinflammation initiated in an acute mouse model of Ab-dependent experimental autoimmune encephalomyelitis. Mouse fH was found to be abundantly expressed by primary cultured neurons and neuronal cell lines (N1E115 and Neuro2a) at a level comparable to BV2 microglia and CLTT astrocytes. Mouse neurons expressed other complement regulators crry and low levels of CD55. In the brain, the expression of fH was localized to neuronal bodies and axons, endothelial cells, microglia but not oligodendrocytes and myelin sheaths and was dramatically reduced in inflammatory experimental autoimmune encephalomyelitis settings. When exogenous human fH was administered to disease Ab-dependent experimental autoimmune encephalomyelitis animals, there was a significant decrease in clinical score, inflammation, and demyelination, as compared with PBS-injected animals. We found that the accumulation of human fH in the brain parenchyma protected neurons from complement opsonization, axonal injury, and leukocyte infiltration. Our data argue for a key regulatory activity of fH in neuroprotection and provide novel therapeutic avenues for CNS chronic inflammatory diseases.

Human endothelial precursor cells express tumor endothelial marker 1/endosialin/CD248

Angiogenesis occurs during normal physiologic processes as well as under pathologic conditions such as tumor growth. Serial analysis of gene expression profiling revealed genes [tumor endothelial markers (TEM)] that are overexpressed in tumor endothelial cells compared with normal adult endothelial cells. Because blood vessel development of malignant tumors under certain conditions may include endothelial precursor cells (EPC) recruited from bone marrow, we investigated TEM expression in EPC. The expression of TEM1 or endosialin (CD248) and other TEM has been discovered in a population of vascular endothelial growth factor receptor 2+/CD31+/CD45-/VE-cadherin+ EPC derived from human CD133+/CD34+ cells. EPC share some properties with fully differentiated endothelial cells from normal tissue, yet reverse transcription-PCR and flow cytometry reveal that EPC express higher levels of endosialin at the molecular and protein levels. The elevated expression of endosialin in EPC versus mature endothelial cells suggests that endosialin is involved in the earlier stages of tumor angiogenesis. Anti-endosialin antibodies inhibited EPC migration and tube formation in vitro. In vivo, immunohistochemistry indicated that human EPC continued to express endosialin protein in a Matrigel plug angiogenesis assay established in nude mice. Anti-endosialin antibodies delivered systemically at 25 mg/kg were also able to inhibit circulating murine EPC in nude mice bearing s.c. SKNAS tumors. EPC and bone marrow-derived cells have been shown previously to incorporate into malignant blood vessels in some instances, yet they remain controversial in the field. The data presented here on endothelial genes that are up-regulated in tumor vasculature and in EPC support the hypothesis that the angiogenesis process in cancer can involve EPC.

Human endosialin (tumor endothelial marker 1) is abundantly expressed in highly malignant and invasive brain tumors

Endosialin (tumor endothelial marker 1) is expressed preferentially by tumor endothelial cells but not by normal endothelium. Its protein domain architecture is homologous to that of CD93 and thrombomodulin (CD141), suggesting a similar function in mediating cell-cell interactions. The aim of this study was to investigate the expression pattern of endosialin in human brain tumors in a bid to decipher its contribution to tumor angiogenesis. We generated an antibody specifically recognizing human endosialin and used it to study endosialin expression in 30 human brain tumor specimens by immunoblotting and immunohistochemistry. Twenty of 30 tumors expressed endosialin in a heterogeneous manner. The largest proportion of endosialin-expressing tumors was found in highly invasive glioblastoma multiforme, anaplastic astrocytomas, and metastatic carcinomas. Endosialin was localized to the endothelium of small and large vessels strongly stained for CD31 and was also expressed by Thy-1-positive fibroblast-like cells close to the meninges and alpha-smooth muscle actin-positive cells in some vessels. Endosialin colocalized with thrombomodulin, suggesting the proteins may have complementary functions in tumor progression.

The antimetastatic role of thrombomodulin expression in islet cell-derived tumors and its diagnostic value

Islet cell tumors, endocrine neoplasm arising from pancreatic islets of Langerhans, are histologically difficult to diagnose as benign or malignant. Molecular markers are associated with the clinical characteristics that most of insulinoma are usually benign tumors, whereas other islet cell tumors are malignant but have not been identified. In this context, we newly found that an endothelial anticoagulant thrombomodulin was expressed in the normal islet beta cells and insulinoma, but not of other islet components or noninsulinoma islet cell tumors. Clinically, all of the subjects (n=15) of the insulinoma group showed no metastasis together with thrombomodulin expression in the lesions, whereas the other islet cell tumor groups showed a high incidence of metastasis (82%) and a low expression rate of thrombomodulin (6%). To examine the functional role of thrombomodulin, especially regarding the clinical characteristics of islet cell tumors, we tested the effect of exogenous thrombomodulin overexpression on cell adhesiveness and proliferation using MIN6 insulinoma cell line. In cell-based experiments, thrombomodulin overexpression reduced cell proliferation and enhanced Ca2+-independent cell aggregation, possibly through direct interaction with neural cell adhesion molecule. Taken together, these results are suggesting that thrombomodulin may act as antimetastatic molecule of insulinomas. In addition, thrombomodulin is a clinically useful molecular marker not only for identifying beta-cell-origin islet cell tumors (i.e., insulinomas) but also for predicting disease prognosis of islet cell tumors.

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 .

Thrombomodulin

Since its discovery as a critical cofactor in the initiation of the protein C (PC) anticoagulant pathway [1,2], biochemical and structural investigations, combined with in vivo analyses of genetically engineered mice have revealed new, and in part PC- and thrombin-independent aspects of thrombomodulin (TM) function in fibrinolysis and inflammation, and in embryogenesis. This review summarizes more recent structural and functional investigations of TM, gives an overview of the association of TM gene polymorphisms with human disease, and provides a synopsis of what is know about TM function in disease states of thrombosis, stroke, arteriosclerosis, and cancer. Newly emerging aspects of TM function in inflammation and embryogenesis are presented and discussed in detail.

The lectin-like domain of thrombomodulin confers protection from neutrophil-mediated tissue damage by suppressing adhesion molecule expression via nuclear factor kappaB and mitogen-activated protein kinase pathways

Thrombomodulin (TM) is a vascular endothelial cell (EC) receptor that is a cofactor for thrombin-mediated activation of the anticoagulant protein C. The extracellular NH(2)-terminal domain of TM has homology to C-type lectins that are involved in immune regulation. Using transgenic mice that lack this structure (TM(LeD/LeD)), we show that the lectin-like domain of TM interferes with polymorphonuclear leukocyte (PMN) adhesion to ECs by intercellular adhesion molecule 1-dependent and -independent pathways through the suppression of extracellular signal-regulated kinase (ERK)(1/2) activation. TM(LeD/LeD) mice have reduced survival after endotoxin exposure, accumulate more PMNs in their lungs, and develop larger infarcts after myocardial ischemia/reperfusion. The recombinant lectin-like domain of TM suppresses PMN adhesion to ECs, diminishes cytokine-induced increase in nuclear factor kappaB and activation of ERK(1/2), and rescues ECs from serum starvation, findings that may explain why plasma levels of soluble TM are inversely correlated with cardiovascular disease. These data suggest that TM has antiinflammatory properties in addition to its role in coagulation and fibrinolysis.

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.

Identification of a site on mannan-binding lectin critical for enhancement of phagocytosis

Mannan-binding lectin (MBL) constitutes an important part of the human innate immune defense system. It has been shown to mediate the activation of complement upon binding to specific microbial carbohydrate motifs, to directly opsonize organisms, and to enhance the phagocytosis of targets suboptimally opsonized with IgG or complement components C3b or C4b. This enhancement of phagocytic activity induced by MBL and other molecules that contain a collagen-like region contiguous with a pattern recognition domain is mediated by a 126,000 M(r) surface glycoprotein, designated C1qR(P). Although it has been known that the collagen-like domain of these "defense collagens" contains the interaction site(s) that triggers this enhancement of uptake, the specific interaction site has not been identified. To address this issue, wild type and mutant MBL constructs were generated, inserted into baculovirus, expressed in Sf9 cells, and the recombinant MBL (rMBL) proteins purified by mannan affinity chromatography. The effect of wild type and mutant rMBL on the phagocytosis of targets suboptimally opsonized with IgG or with IgM and C4b by human peripheral blood monocytes was then assessed. Two mutants, one of which has five GXY triplets deleted below the kink region of MBL and the other one having only two of the GXY triplets deleted below the kink, failed to enhance phagocytosis, suggesting the importance of the specific sequence GEKGEP in stimulating phagocytic activity. Similar sequences were detected in other defense collagens, implicating the consensus motif GE(K/Q/R)GEP as critical in mediating the enhancement of phagocytosis through C1qR(P.) Clarification of specific ligand-C1qR(P) interactions should facilitate the investigation of the signal transduction processes involved in the cell activation, as well as provide the basis for the design of specific modulators of the functions mediated by this receptor.

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.

The mannan-binding-lectin pathway of the innate immune response

Dramatic progress has been achieved during the past year in our understanding of how the complement system is activated via the mannan-binding-lectin pathway. Surprising discoveries have changed our concepts of the complexes that are formed upon engagement of mannan-binding lectin with its serine proteases.