Human Signal-Regulatory Protein Is Expressed on Normal, But Not on Subsets of Leukemic Myeloid Cells and Mediates Cellular Adhesion Involving Its Counterreceptor CD47

Platelet homeostasis is regulated by platelet expression of CD47 under normal conditions and in passive immune thrombocytopenia

Interaction between target cell CD47 and the inhibitory macrophage receptor signal regulatory protein alpha (SIRPalpha) counteracts macrophage phagocytosis of CD47-expressing host cells. As platelets also express CD47, we asked whether inhibitory CD47/SIRPalpha signaling regulates normal platelet turnover and clearance of platelets in immune thrombocytopenic purpura (ITP). CD47(-/-) mice had a mild spontaneous thrombocytopenia, which was not due to a decreased platelet half-life as a result of increased expression of P-selectin, CD61, or phosphatidylserine. In contrast, CD47(-/-) platelets were rapidly cleared when transfused into CD47(+/+) recipients, whereas CD47(+/-) platelets had a nearly normal half-life in CD47(+/+) mice under nonautoimmune conditions. CD47(-/-) mice were more sensitive to ITP, as compared with CD47(+/+) mice. In vitro, macrophage phagocytosis of immunoglobulin G (IgG)-opsonized CD47(-/-) platelets was significantly higher than that for equally opsonized CD47(+/+) platelets. However, when SIRPalpha was blocked, phagocytosis of CD47(+/+) platelets increased to the level of CD47(-/-) platelets. Phagocytosis of opsonized CD47(+/-) platelets was higher than that for CD47(+/+) platelets, but lower than that for CD47(-/-) platelets, suggesting a gene-dose effect of CD47 in this system. In conclusion, we suggest that inhibitory CD47/SIRPalpha signaling is involved in regulating platelet phagocytosis in ITP, and that targeting SIRPalpha may be a new means of reducing platelet clearance in ITP.

Post-translational regulation of expression and conformation of an immunoglobulin domain in yeast surface display

Display of heterologous proteins on the surface of Saccharomyces cerevisiae is increasingly being exploited for directed evolution because of straightforward cell screens. However, yeast post-translationally modifies proteins in ways that must be factored into library engineering and refinement. Here, we express the extracellular immunoglobulin domain of an ubiquitous mammalian membrane protein, CD47, which is implicated in cancer, immunocompatibility, and motility. CD47 has multiple sites of glycosylation and a core disulfide bond. We assess the effects of both of these post-translational modifications on expression and antibody binding. CD47's extracellular domain is fused to the yeast mating protein Aga2p on the cell wall, and the resulting fusion protein binds several key antibodies, including a conformation-sensitive antibody. Site-by-site mutagenesis of CD47's five N-linked glycosylation sites progressively decreases expression levels on yeast, but folding appears stable. Cysteine mutations disrupt the expected core disulfide, and also decrease protein expression levels, though not to the extent seen with complete deglycosylation. However, with the core disulfide mutants, antibody binding proves to be lower than expected from expression levels and glycosylation is clearly reduced compared to wild-type. The results indicate that glycosylation regulates heterologous display on yeast more than core disulfides do and thus suggest bounds on directed evolution by post-translational processing.

Negative regulation of hepatocellular carcinoma cell growth by signal regulatory protein alpha1

Signal regulatory protein (SIRP) alpha1 is a member of the SIRP family that undergoes tyrosine phosphorylation and binds SHP-2 tyrosine phosphatase in response to various mitogens. The expression levels of SIRPalpha1 were decreased in HCC tissues, compared with the matched normal tissues. Exogenous expression of wild type SIRPalpha1, but not of a mutant SIRPalpha1 lacking the tyrosine phosphorylation sites, in SIRPalpha1-negative Huh7 human HCC cells resulted in suppression of tumor cell growth both in vitro and in vivo. Treatment of Huh7 transfectants with EGF or HGF induced tyrosine phosphorylation of SIRPalpha1 and its association with SHP-2, which were accompanied by reduced ERK1 activation. Expression of SIRPalpha1 significantly suppressed activation of NF-kappaB and also sensitized Huh7 cells to TNFalpha or cisplatin-induced cell death. In addition, SIRPalpha1-transfected Huh7 cells displayed reduced cell migration and cell spreading in a fashion that was dependent on SIRPalpha1/SHP-2 complex formation. In conclusion, a negative regulatory effect of SIRPalpha1 on hepatocarcinogenesis is exerted, at least in part, through inhibition of ERK and NF-kappaB pathways.

Human Lymphocytes Interact Directly with CD47 through a Novel Member of the Signal Regulatory Protein (SIRP) Family

Two closely related proteins, signal regulatory protein alpha (SIRPalpha; SHPS-1/CD172) and SIRPbeta, have been described in humans. The existence of a third SIRP protein has been suggested by cDNA sequence only. We show that this third SIRP is a separate gene that is expressed as a protein with unique characteristics from both alpha and beta genes and suggest that this gene should be termed SIRPgamma. We have expressed the extracellular region of SIRPgamma as a soluble protein and have shown that, like SIRPalpha, it binds CD47, but with a lower affinity (K(d), approximately 23 microM) compared with SIRPalpha (K(d), approximately 2 microM). mAbs specific to SIRPgamma show that it was not expressed on myeloid cells, in contrast to SIRPalpha and -beta, being expressed instead on the majority of T cells and a proportion of B cells. The short cytoplasmic tail of SIRPgamma does not contain any known signaling motifs, nor does it contain a characteristic lysine, as with SIRPbeta, that is required for DAP12 interaction. DAP12 coexpression is a requirement for SIRPbeta surface expression, whereas SIRPgamma is expressed in its absence. The SIRPgamma-CD47 interaction may therefore not be capable of bidirectional signaling as with the SIRPalpha-CD47, but, instead, use unidirectional signaling via CD47 only.

The phylogenetic origins of the antigen-binding receptors and somatic diversification mechanisms

The adaptive immune system arose in ancestors of the jawed vertebrates approximately 500 million years ago. Homologs of immunoglobulins (Igs), T-cell antigen receptors (TCRs), major histocompatibility complex I (MHC I) and MHC II, and the recombination-activating genes (RAGs) have been identified in all extant classes of jawed vertebrates; however, no definitive homolog of any of these genes has been identified in jawless vertebrates or invertebrates. RAG-mediated recombination and associated junctional diversification of both Ig and TCR genes occurs in all jawed vertebrates. In the case of Igs, somatic variation is expanded further through class switching, gene conversion, and somatic hypermutation. Although the identity of the 'primordial' receptor that was interrupted by the recombination mechanism in jawed vertebrates may never be established, many different families of genes that exhibit predicted characteristics of such a receptor have been described both within and outside the jawed vertebrates. Recent data from various model systems point toward a continuum of immune receptor diversity, encompassing many different families of recognition molecules whose functions are integrated in an organism's response to pathogenic invasion. Various approaches, including both genomic and protein-functional analyses, currently are being applied in jawless vertebrates, protochordates, and other invertebrate deuterostome systems and may yield definitive evidence regarding the presence or absence of adaptive immune homologs in species lacking adaptive immune systems. Such studies have the potential for uncovering previously unknown mechanisms of generating receptor diversity.

Src homology 2 domain-containing protein tyrosine phosphatase substrate 1 regulates the migration of Langerhans cells from the epidermis to draining lymph nodes

Src homology 2 domain-containing protein tyrosine phosphatase substrate 1 (SHPS-1) is a member of the signal regulatory protein family in which the extracellular region interacts with its ligand, CD47. Recent studies have demonstrated that SHPS-1 plays an important role in cell migration and cell adhesion. We demonstrate in this study, using immunohistochemical and flow cytometric analyses, that murine Langerhans cells (LCs) express SHPS-1. Treatment of mice ears with 2,4-dinitro-1-fluorobenzene significantly reduced the number of epidermal LCs, and that reduction could be reversed by pretreatment with mAb to SHPS-1 or the CD47-Fc fusion protein. Treatment with the SHPS-1 mAb in vivo reduced the number of FITC-bearing cells in the lesional lymph nodes after the application of FITC to the skin. The SHPS-1 mAb inhibited the in vivo TNF-alpha-induced migration of LCs. The emigration of dendritic cells expressing I-A(b+) from skin explants to the medium was also reduced by the SHPS-1 mAb. We further demonstrate that the chemotaxis of a murine dendritic cell line, XS52, by macrophage inflammatory protein-3beta was significantly inhibited by treatment with the SHPS-1 mAb or CD47-Fc recombinant protein. Finally, we show that migration of LCs was attenuated in mutant mice that lack the intracellular domain of SHPS-1. These observations show that the ligation of SHPS-1 with the SHPS-1 mAb or with CD47-Fc abrogates the migration of LCs in vivo and in vitro, which suggests that the SHPS-1-CD47 interaction may negatively regulate LC migration.

Positive Regulation of Phagocytosis by SIRPβ and Its Signaling Mechanism in Macrophages

SIRPbeta (signal-regulatory protein beta) is a transmembrane protein that is expressed in hematopoietic cells but whose functions are unknown. We have now cloned mouse SIRPbeta cDNA and have shown that the gene is expressed in various tissues in addition to cells of the macrophage lineage. Engagement of SIRPbeta by specific monoclonal antibodies promoted Fcgamma receptor-dependent or -independent phagocytosis in mouse peritoneal macrophages. It also induced marked activation of MAPK and the upstream kinase MEK but weak activation of Akt. MEK inhibitors markedly blocked the promotion of phagocytosis by SIRPbeta, whereas an inhibitor of phosphoinositide 3-kinase partly blocked such response. In addition, inhibitors of myosin light chain kinase or of myosin ATPase blocked the promotion of phagocytosis by SIRPbeta. Furthermore, SIRPbeta induced the formation of filopodia and lamellipodia in macrophages as well as the translocation of activated MAPK to these structures. It also elicited tyrosine phosphorylation of DAP12, Syk, and SLP-76, and a Syk inhibitor blocked the promotion of phagocytosis and activation of MAPK by SIRPbeta. Our results suggest that engagement of SIRPbeta promotes phagocytosis in macrophages by inducing the tyrosine phosphorylation of DAP12, Syk, and SLP-76 and the subsequent activation of a MEK-MAPK-myosin light chain kinase cascade.

Ectodomain Shedding of SHPS-1 and Its Role in Regulation of Cell Migration

SHPS-1 is a transmembrane protein whose cytoplasmic region undergoes tyrosine phosphorylation and then binds the protein-tyrosine phosphatase SHP-2. Formation of the SHPS-1-SHP-2 complex is implicated in regulation of cell migration. In addition, SHPS-1 and its ligand CD47 constitute an intercellular recognition system that contributes to inhibition of cell migration by cell-cell contact. The ectodomain of SHPS-1 has now been shown to be shed from cells in a reaction likely mediated by a metalloproteinase. This process was promoted by activation of protein kinase C or of Ras, and the released ectodomain exhibited minimal CD47-binding activity. Metalloproteinases catalyzed the cleavage of a recombinant SHPS-1-Fc fusion protein in vitro, and the primary cleavage site was localized to the juxtamembrane region of SHPS-1. Forced expression of an SHPS-1 mutant resistant to ectodomain shedding impaired cell migration, cell spreading, and reorganization of the actin cytoskeleton. It also increased the tyrosine phosphorylation of paxillin and FAK triggered by cell adhesion. These results suggest that shedding of the ectodomain of SHPS-1 plays an important role in regulation of cell migration and spreading by this protein.

Promotion of neurite and filopodium formation by CD47: roles of integrins, Rac, and Cdc42

Axon extension during development is guided by many factors, but the signaling mechanisms responsible for its regulation remain largely unknown. We have now investigated the role of the transmembrane protein CD47 in this process in N1E-115 neuroblastoma cells. Forced expression of CD47 induced the formation of neurites and filopodia. Furthermore, an Fc fusion protein containing the extracellular region of the CD47 ligand SHPS-1 induced filopodium formation, and this effect was enhanced by CD47 overexpression. SHPS-1-Fc also promoted neurite and filopodium formation triggered by serum deprivation. Inhibition of Rac or Cdc42 preferentially blocked CD47-induced formation of neurites and filopodia, respectively. Overexpression of CD47 resulted in the activation of both Rac and Cdc42. The extracellular region of CD47 was sufficient for the induction of neurite formation by forced expression, but the entire structure of CD47 was required for enhancement of filopodium formation by SHPS-1-Fc. Neurite formation induced by CD47 was also inhibited by a mAb to the integrin beta3 subunit. These results indicate that the interaction of SHPS-1 with CD47 promotes neurite and filopodium formation through the activation of Rac and Cdc42, and that integrins containing the beta3 subunit participate in the effect of CD47 on neurite formation.

Peptide-mediated inhibition of neutrophil transmigration by blocking CD47 interactions with signal regulatory protein alpha

CD47, a cell surface transmembrane Ig superfamily member, is an extracellular ligand for signal regulatory protein (SIRPalpha). Interactions between CD47 and SIRPalpha regulate many important immune cell functions including neutrophil (PMN) transmigration. Here we report identification of a novel function-blocking peptide, CERVIGTGWVRC, that structurally mimics an epitope on CD47 and binds to SIRPalpha. The CERVIGTGWVRC sequence was identified by panning phage display libraries on the inhibitory CD47 mAb, C5D5. In vitro PMN migration assays demonstrated that peptide CERVIGTGWVRC specifically inhibited PMN migration across intestinal epithelial monolayers and matrix in a dose-dependent fashion. Further studies using recombinant proteins indicated that the peptide specifically blocks CD47 and SIRPalpha binding in a dose-dependent fashion. Protein binding assays using SIRPalpha domain-specific recombinant proteins demonstrated that this peptide directly bound to the distal-most Ig loop of SIRPalpha, the same loop where CD47 binds. In summary, these findings support the relevance of CD47-SIRPalpha interactions in regulation of PMN transmigration and provide structural data predicting the key residues involved on the surface of CD47. Such peptide reagents may be useful for studies on experimental models of inflammation and provide a template for the design of anti-inflammatory agents.

Factors that affect the efficiency of cell transfection by immunoporation

Immunoporation is a recently discovered method that is able to transfect various human cell lines efficiently by targeting the cell surface antigens with antibody-coated beads. For this particular study, HL60, a cell line difficult to transfect by other methods, was used as a model to define the various parameters of the cell membrane that determine the efficiency of this method. The level of antigen expression on the cell surface was the first parameter to be analyzed and experiments indicated that there is a close correlation between the level of expression of surface antigens and the efficiency of immunoporation. The mixing speed, the bead to cell ratio, and the mixing time were all found to affect the ability of the antigen-coated beads to pull holes in the cells and it was found that for HL60 cells the optimum mixing speed was 40 rpm and the bead to cell ratio was 20:1 using a mixing time of 6 h.

Tyrosine phosphorylation of BIT on photic stimulation in the rat retina

BIT is a transmembrane glycoprotein with three immunoglobulin-like domains in its extracellular region and tyrosine phosphorylation sites in its cytosolic region. We have previously shown that BIT was tyrosine phosphorylated in the hypothalamic suprachiasmatic nucleus in response to light exposure during the dark period, and suggested that it was involved in the light entrainment of the circadian clock. To further investigate the function of BIT in the nervous system, we examined the effect of photic stimulation on its tyrosine phosphorylation in the rat retina. It was found that the tyrosine phosphorylation level of BIT in the retina was higher in the light period than in the dark period. In addition, a light stimulation during the dark period resulted in a rapid phosphorylation of BIT and a subsequent association of BIT with SHP-2. The phosphorylation state was quickly reverted when the light was turned off. The light-dependent phosphorylation of BIT was also observed in isolated cultured retinas, and this was blocked by a specific Src-family inhibitor, PP-2. Immunohistochemical study showed that BIT was highly enriched in the inner and outer plexiform layers in the retina, where the immunoreactivity to anti-SHP-2 antibody was also detected. These results suggest that tyrosine phosphorylation of BIT is involved in neuronal transmission in the retina.

Resistance of B16 Melanoma Cells to CD47-induced Negative Regulation of Motility as a Result of Aberrant N-Glycosylation of SHPS-1

The adhesion receptor SHPS-1 activates the protein-tyrosine-phosphatase SHP-2 and thereby promotes integrin-mediated reorganization of the cytoskeleton. SHPS-1 also contributes to cell-cell communication through association with CD47. Although functional alteration of SHPS-1 is implicated in cellular transformation, the role of the CD47-SHPS-1 interaction in carcinogenesis has been unclear. A soluble SHPS-1 ligand (CD47-Fc) has now been shown to bind to Melan-a non-tumorigenic melanocytes but not to syngeneic B16F10 melanoma cells. Treatment of B16F10 cells with 1-deoxymannojirimycin, which prevents N-glycan processing, restored the ability of SHPS-1 derived from these cells to bind CD47-Fc in vitro, indicating that aberrant N-glycosylation of SHPS-1 impairs CD47 binding in B16F10 cells. CD47-Fc inhibited the migration of Melan-a cells but not that of B16F10 cells. However, a monoclonal antibody that reacts with SHPS-1 on both Melan-a and B16F10 cells inhibited the migration of both cell types similarly. CD47 binding induced proteasome-mediated degradation of SHPS-1 in a tyrosine phosphorylation-independent manner. Furthermore, overexpression of SHPS-1 reduced the level of tyrosine phosphorylation of focal adhesion kinase, and this effect was reversed by CD47 binding. These results suggest that CD47 binds to and thereby down-regulates SHPS-1 on adjacent cells, resulting in inhibition of cell motility. Resistance to this inhibitory mechanism may contribute to the highly metastatic potential of B16 melanoma.

Ligation of CD47 During Monocyte Differentiation into Dendritic Cells Results in Reduced Capacity for Interleukin-12 Production

We have previously shown that the CD47-binding thrombospondin-1 (TSP-1)-derived peptide 4N1K induces a rapid apoptosis-like death of human monocytes and dendritic cells (DCs). However, not all cells were susceptible to the peptide-induced cell death and here, we have investigated whether surviving monocytes could differentiate into functionally normal DCs. We found that the cell-surface phenotype, the T-cell stimulatory capacity and the ability to undergo lipopolysaccharide (LPS)-induced maturation into CD83+ DCs were essentially identical in 4N1K-derived and control DCs. Interleukin-10 (IL-10) production was also normal, but a significant downregulation of the pro-inflammatory cytokines IL-12 and tumour necrosis factor-alpha (TNF-alpha) was observed in the 4N1K-derived DCs. To the contrary, simultaneous stimulation of control DCs with 4N1K and LPS + interferon-gamma did not alter IL-12 production. These results indicate that although activation of the TSP-1-binding region of CD47 on monocytes induces apoptosis in a large proportion of the cells, it does not hamper the overall capacity of the surviving cells to differentiate into DCs. Such DCs, however, have a reduced capacity for IL-12 and TNF-alpha production, and the possibility that this is linked to the uptake of apoptotic cells is discussed.

CD47 Ligation Induces a Rapid Caspase-Independent Apoptosis-Like Cell Death in Human Monocytes and Dendritic Cells

CD47 is a versatile cell-surface molecule expressed on nearly all haematopoietic cells. In its capacity as a thrombospondin-1 (TSP-1) receptor, CD47 has recently been shown to mediate cell death in certain cells, for example, activated but not resting T cells. Here, we have investigated the possibility that human monocytes and dendritic cells (DCs) undergo cell death, following CD47 ligation. Using the TSP-1-derived CD47-binding peptide 4N1K, we found that both freshly isolated monocytes and monocyte-derived DCs underwent a rapid, caspase-independent cell death. This was characterized by the simultaneous presence of phosphatidylserine exposure, plasma membrane permeability, reduced mitochondrial membrane potential and highly fragmented DNA. Not all cells were sensitive to 4N1K-induced apoptosis; a plateau of cell death reached at an average of 38% of the monocyte and DCs populations. The results presented here, thus, show that CD47 can mediate a rapid apoptosis-like cell death of human monocytes and DCs.

Tethering of Apoptotic Cells to Phagocytes through Binding of CD47 to Src Homology 2 Domain-Bearing Protein Tyrosine Phosphatase Substrate-1

Apoptotic cells are swiftly phagocytosed by macrophages and immature dendritic cells. In this study, we found that one mouse macrophage cell line (BAM3) engulfed apoptotic thymocytes, but not a lymphoma cell line (WR19L). mAbs that inhibited the phagocytosis of apoptotic thymocytes by BAM3 were identified. Purification of the Ag revealed that it was Src homology 2 domain-bearing protein tyrosine phosphatase substrate-1 (SHPS-1). CD47, the ligand for SHPS-1, was expressed in mouse thymocytes, but not in WR19L. When WR19L was transformed with CD47, the transformants, after induction of apoptosis, could be phagocytosed by BAM3. The WR19L transformants expressing CD47 were more efficiently engulfed in vivo by splenic dendritic cells than the parental WR19L. Masking of the phosphatidylserine exposed on apoptotic thymocytes inhibited the engulfment, whereas the anti-SHPS-1 mAb inhibited not only the engulfment, but also the binding of apoptotic cells to phagocytes. These results indicate that macrophages require CD47 and phosphatidylserine on apoptotic cells for engulfment, and suggest that the interaction between CD47 and SHPS-1 works as a tethering step in the phagocytosis.

Flow cytometric analysis of bone marrow leukocytes in neonatal dogs

Dogs represent both an important veterinary species and a convenient model for allogeneic hematopoietic stem cell transplantation. Even though anti-canine CD34 antibodies have recently become available, little is known about hematopoietic lineages in dogs, partially because CD34- cells have been ignored in all analyses performed so far. In this study, we have focused on the bone marrow mononuclear compartment to provide an additional piece of information on the phenotype of CD34+ progenitors and to identify the dominant CD34- population. We have shown that, in contrast to the adults, mature lymphocytes are scarce in neonatal dog bone marrow. Using cross-reactive antibodies against CD79alpha we have shown that the B lineage of hematopoiesis strongly prevails. CD34+ cells were shown to be positive for MHC class II and SWC3, a member of the signal regulatory protein family.

Characterization of nucleotide pyrophosphatase-5 as an oligomannosidic glycoprotein in rat brain

Membrane glycoproteins of neural cells play crucial roles in axon guidance, synaptogenesis, and neuronal transmission. We have here characterized membrane glycoproteins containing terminal alpha-mannose residues in rat brain membranes. Affinity purification using Galanthus nivalis agglutinin, that is highly specific for terminal alpha-mannose residues, revealed a 50-kDa protein as well as 80-kDa SHPS-1 and 45-kDa beta2 subunit of Na,K-ATPase in rat brain membranes. Combination of N-terminal peptide sequencing and mass spectrometry indicated that the 50-kDa protein was rat nucleotide pyrophosphatase-5 (NPP-5). In contrast to other NPPs, NPP-5 was a type-I transmembrane protein. Northern blot analysis showed that NPP-5 was highly expressed in brain, but also expressed in other peripheral tissues. However, we could not detect either the NPP activity or the lysophospholipase D activity in the immunoprecipitates with antibodies to NPP-5 from rat brain membranes. These data, therefore, suggest that NPP-5 is a neural oligomannosidic glycoprotein that may participate in neural cell communications.

The association between integrin-associated protein and SHPS-1 regulates insulin-like growth factor-I receptor signaling in vascular smooth muscle cells

Growth factor signaling is usually analyzed in isolation without considering the effect of ligand occupancy of transmembrane proteins other than the growth factor receptors themselves. In smooth muscle cells, the transmembrane protein Src homology 2 domain containing protein tyrosine phosphatase substrate-1 (SHPS-1) has been shown to be an important regulator of insulin-like growth factor-I (IGF-I) signaling. SHPS-1 is phosphorylated in response to IGF-I, leading to recruitment of Src homology 2 domain tyrosine phosphatase (SHP-2). Subsequently, SHP-2 is transferred to IGF-I receptor and regulates the duration of IGF-I receptor phosphorylation. Whether ligand occupancy of SHPS-1 influences SHPS-1 phosphorylation or SHP-2 recruitment, thereby altering growth factor signaling, is unknown. Previous studies have shown that integrin associated protein (IAP) associates with SHPS-1. We undertook these studies to determine whether this interaction controlled SHPS-1 phosphorylation and/or SHP-2 recruitment and thereby regulated IGF-I signaling. Disruption of IAP-SHPS-1 binding, by using an IAP monoclonal antibody or cells expressing mutant forms of IAP that did not bind to SHPS-1, inhibited IGF-I-stimulated SHPS-1 phosphorylation and SHP-2 recruitment. This was associated with a lack of SHP-2 transfer to IGF-I receptor and sustained receptor phosphorylation. This resulted in an inability of IGF-I to stimulate sustained mitogen-activated protein kinase activation, cell proliferation, and cell migration. The effect was specific for IGF-I because disruption of the IAP-SHPS-1 interaction had no effect on platelet-derived growth factor-stimulated SHPS-1 phosphorylation or cell migration. In summary, our results show that 1) ligand occupancy of SHPS-1 is a key determinant of its ability to be phosphorylated after IGF-I stimulation, and 2) the interaction between IAP and SHPS-1 is an important regulator of IGF-I signaling because disruption of the results in impaired SHP-2 recruitment and subsequent inhibition of IGF-I-stimulated cell proliferation and migration.

Intestinal epithelial pathobiology: past, present and future

The intestinal epithelium serves as one of man's primary interfaces with the outside world. Its importance is illustrated by the fact that the proper functioning of this interface is absolutely essential for human health, and even modest perturbations in its function may lead to diarrhoea, constipation, malnutrition, dehydration, infectious disease or chronic intestinal inflammatory diseases such as Crohn's disease and ulcerative colitis, collectively referred to as inflammatory bowel disease. Both pathogen-induced intestinal inflammation and the active flares of inflammatory bowel disease are histopathologically defined, their sequellae being mediated by neutrophils that migrate across the intestinal epithelium, forming a crypt abscess. Classically, the intestinal epithelium has been thought of primarily as a barrier, and indeed this is a very important aspect of its function, but the intestinal epithelium is also a highly interactive barrier. This chapter will summarize some of the basic research conducted over the past 15 years that has revealed basic insights into how the epithelium participates in the formation of a crypt abscess and how it plays a role in causing the characteristic clinical manifestations that ensue. In addition, the chapter will discuss how this research has resurrected the 'old', yet newly emerging, concept that physiological malfunction of the intestinal epithelium can be the primary defect that leads to the innate and adaptive immune dysregulation mediating inflammatory bowel disease.

Gene for integrin-associated protein (IAP, CD47): physical mapping, genomic structure, and expression studies in skeletal muscle

Integrin-associated protein (IAP) is a widely expressed membrane protein with multiple functions in immunological and neuronal processes. Having physically mapped the IAP gene into a BAC/PAC contig covering approximately 1 Mb on human chromosome 3ql3.1-q13.2, we determined the genomic organization of the gene, established its expression in skeletal muscle, and identified a novel splice variant. Our expression studies demonstrate expression of integrin-associated protein in the t-tubular system and the euchromatin of skeletal muscle cells where its function thus far is not known.

Regulation of integrin function by CD47 ligands. Differential effects on alpha vbeta 3 and alpha 4beta1 integrin-mediated adhesion

We examined the regulation of alpha4beta1 integrin function in melanoma cells and T cells by ligands of CD47. A CD47 antibody (B6H12) that inhibited alphavbeta3-mediated adhesion of melanoma cells induced by CD47-binding peptides from thrombospondin-1 directly stimulated alpha4beta1-mediated adhesion of the same cells to vascular cell adhesion molecule-1 and N-terminal regions of thrombospondin-1 or thrombospondin-2. B6H12 also stimulated alpha4beta1- as well as alpha2beta1- and alpha5beta1-mediated adhesion of CD47-expressing T cells but not of CD47-deficient T cells. alpha4beta1 and CD47 co-purified as a detergent-stable complex on a CD47 antibody affinity column. CD47-binding peptides based on C-terminal sequences of thrombospondin-1 also specifically enhanced adhesion of melanoma cells and T cells to alpha4beta1 ligands. Unexpectedly, activation of alpha4beta1 function by the thrombospondin-1 CD47-binding peptides also occurred in CD47-deficient T cells. CD47-independent activation of alpha4beta1 required the Val-Val-Met (VVM) motif of the peptides and was sensitive to inhibition by pertussis toxin. These results indicate that activation of alpha4beta1 by the CD47 antibody B6H12 and by VVM peptides occurs by different mechanisms. The antibody directly activates a CD47-alpha4beta1 complex, whereas VVM peptides may target an unidentified Gi-linked receptor that regulates alpha4beta1.

Negative regulation of platelet clearance and of the macrophage phagocytic response by the transmembrane glycoprotein SHPS-1

SHPS-1 is a receptor-type glycoprotein that binds and activates the protein-tyrosine phosphatases SHP-1 and SHP-2, and thereby negatively modulates intracellular signaling initiated by various cell surface receptors coupled to tyrosine kinases. SHPS-1 also regulates intercellular communication in the neural and immune systems through its association with CD47 (integrin-associated protein) on adjacent cells. Furthermore, recent studies with fibroblasts derived from mice expressing an SHPS-1 mutant that lacks most of the cytoplasmic region suggested that the intact protein contributes to cytoskeletal function. Mice homozygous for this SHPS-1 mutation have now been shown to manifest thrombocytopenia. These animals did not exhibit a defect in megakaryocytopoiesis or in platelet production. However, platelets were cleared from the bloodstream more rapidly in the mutant mice than in wild-type animals. Furthermore, peritoneal macrophages from the mutant mice phagocytosed red blood cells more effectively than did those from wild-type mice; in addition, they exhibited an increase both in the rate of cell spreading and in the formation of filopodia-like structures at the cell periphery. These results indicate that SHPS-1 both contributes to the survival of circulating platelets and down-regulates the macrophage phagocytic response.

Molecular cloning and functional characterization of the pig homologue of integrin-associated protein (IAP/CD47)

We report the cloning of cDNA encoding the pig homologue of human integrin-associated protein (IAP or CD47). A pig CD47-specific probe was generated by polymerase chain reaction (PCR) amplification of pig leucocyte cDNA, using primers based on consensus regions among the known sequences of CD47 from different species. Screening of a pig aorta smooth muscle cDNA library identified seven clones, all containing identical sequences. The clones contained an open reading frame (ORF) that encoded an 18 amino acid putative signal peptide, a 122 amino acid sequence consisting of a single extracellular immunoglobulin variable (IgV)-like domain followed by a 147 amino acid region containing five membrane-spanning domains and a 16 amino acid cytoplasmic tail. The amino acid sequence of the clones was 73% homologous to human IAP and therefore it was termed pig IAP or CD47. Reverse transcription-polymerase chain reaction (RT-PCR) showed that pig CD47 was expressed in a wide range of tissues and detected different alternatively spliced forms. The monoclonal antibody (mAb) BRIC 126, anti-human CD47, was shown, by flow cytometry, to stain pig platelets as well as Chinese hamster ovary (CHO) cells transfected with the cDNA encoding pig CD47. Western blot analysis of pig erythocytes and platelets showed a molecular weight (MW) of 43 000-50 000 and of 55 000-65 000, respectively, under non-reducing conditions. Pig CD47 was stably expressed on CHO cells and shown to bind human thrombospondin (TSP). BRIC126 antibody inhibited the binding of platelets and of CD47-transfected cells to human TSP and to pig fibrinogen, whereas no effect was observed on control CHO cells.

Signal-regulatory protein alpha-CD47 interactions are required for the transmigration of monocytes across cerebral endothelium

Monocyte infiltration into inflamed tissue requires their initial arrest onto the endothelial cells (ECs), followed by firm adhesion and subsequent transmigration. Although several pairs of adhesion molecules have been shown to play a role in the initial adhesion of monocytes to ECs, the mechanism of transendothelial migration is poorly defined. In this study, we have investigated the role of signal-regulatory protein (SIRP)alpha-CD47 interactions in monocyte transmigration across brain ECs. CD47 expression was observed in vivo on cerebral endothelium of both control animals and animals suffering from experimental allergic encephalomyelitis. To investigate whether SIRPalpha-CD47 interactions are instrumental in the trafficking of monocytes across cerebral EC monolayers, in vitro assays were conducted in which the migration of monocytes, but not adhesion, was found to be effectively diminished by blocking SIRPalpha and CD47 on monocytes and ECs, respectively. In this process, SIRPalpha was found to interact solely with its counterligand CD47 on ECs. Overexpression of the CD47 molecule on brain ECs significantly enhanced monocytic transmigration, but did not affect adhesion. SIRPalpha-CD47-mediated transendothelial migration involved Gi protein activity, a known signaling component of CD47. Finally, cross-linking of CD47 on brain ECs induced cytoskeletal reorganization of the endothelium, a process that was Gi protein independent. These data provide the first evidence that the interaction of CD47 with its monocytic counterligand SIRPalpha is of importance in the final step of monocyte trafficking into the brain, a critical event in the development of neuroinflammatory diseases.

SHPS-1, a multifunctional transmembrane glycoprotein

Src homology 2 (SH2) domain-containing protein tyrosine phosphatase substrate 1 (SHPS-1) is a member of the signal regulatory protein (SIRP) family. The amino-terminal immunoglobulin-like domain of SHPS-1 is necessary for interaction with CD47, a ligand for SHPS-1, which plays an important role in cell-cell interaction. The intracellular region of SHPS-1, on the other hand, may act as a scaffold protein, binding to various adapter proteins. Interestingly, increasing evidence has shown that SHPS-1 is involved in various biological phenomena, including suppression of anchorage-independent cell growth, negative regulation of immune cells, self-recognition of red blood cells, mediation of macrophage multinucleation, skeletal muscle differentiation, entrainment of circadian clock, neuronal survival and synaptogenesis. Recent progress has been made in attributing these novel exciting functions. Here we discuss how this interesting molecule works and consider its true role in biology.

Interaction between Src homology 2 domain bearing protein tyrosine phosphatase substrate-1 and CD47 mediates the adhesion of human B lymphocytes to nonactivated endothelial cells

CD47 modulates a variety of cell functions such as adhesion, spreading, and migration. Using a fusion protein consisting of the extracellular region of Src homology 2 domain bearing protein tyrosine phosphatase substrate-1 (SHPS-1) and the Fc portion of human Ig (SHPS-1-Ig) we investigated the effects of SHPS-1 as a ligand for CD47 on B lymphocytes. Although SHPS-1-Ig binding to human B cell lines was solely mediated via CD47, their binding capacity for soluble and immobilized SHPS-1-Ig varied among cell lines irrespective of the similar expression levels of CD47, suggesting that distinctive affinity/avidity states exist during B cell maturation. Nalm6 cell line and tonsilar B lymphocytes adhered to immobilized SHPS-1-Ig and showed polarization-like morphology. These effects of SHPS-1-Ig were blocked by anti-CD47 mAbs (B6H12 and SE5A5). Wortmannin, a phosphatidylinositol-3 kinase inhibitor, but not pertussis toxin significantly inhibited the polarization induced by the immobilized SHPS-1-Ig. Thus, SHPS-1 acts as an adhesive substrate via CD47 in human B lymphocyte. Immunohistochemical analyses indicated that SHPS-1 is expressed on high endothelial venule as well as macrophages in human tonsils. HUVECs also express SHPS-1 in the absence of any stimuli, and the adhesion of tonsilar B lymphocytes to nonactivated HUVECs was significantly inhibited by SE5A5, indicating that SHPS-1/CD47 interaction is involved in the adhesion. Our findings suggest that SHPS-1/CD47 interaction may contribute to the recruitment of B lymphocytes via endothelial cells under steady state conditions.

Signal regulatory protein (SIRPalpha), a cellular ligand for CD47, regulates neutrophil transmigration

Recent studies have demonstrated that CD47 plays an important role in regulating human neutrophil (PMN) chemotaxis. Two ligands for CD47, thrombospondin and SIRPalpha, have been described. However, it is not known if SIRP-CD47 interactions play a role in regulating PMN migration. In this study, we show that SIRPalpha1 directly binds to the immunoglobulin variable domain loop of purified human CD47 and that such SIRP-CD47 interactions regulate PMN transmigration. Specifically, PMN migration across both human epithelial monolayers and collagen-coated filters was partially inhibited by anti-SIRP monoclonal antibodies. Similar kinetics of inhibition were observed for PMN transmigration in the presence of soluble, recombinant CD47 consisting of the SIRP-binding loop. In contrast, anti-CD47 monoclonal antibodies inhibited PMN transmigration by markedly different kinetics. Results of signal transduction experiments suggested differential regulation of PMN migration by SIRP versus CD47 by phosphatidylinositol 3-kinase and tyrosine kinases, respectively. Immunoprecipitation followed by Western blotting after SDS-PAGE under nonreducing conditions suggested that several SIRP protein species may be present in PMN. Stimulation of PMN with fMLP resulted in increased surface expression of these SIRP proteins, consistent with the existence of intracellular pools. Taken together, these results demonstrate that PMN migration is regulated by CD47 through SIRPalpha-dependent and SIRPalpha-independent mechanisms.

The role of CD47 in neutrophil transmigration. Increased rate of migration correlates with increased cell surface expression of CD47

CD47, a cell surface glycoprotein, plays an important role in modulating neutrophil (PMN) migration across endothelial and epithelial monolayers. Here we show that anti-CD47 monoclonal antibodies (mAbs) delay PMN migration across collagen-coated filters or T84 epithelial monolayers toward the chemoattractant formylmethionylleucylphenylalanine (fMLP). Despite delayed transmigration by anti-CD47 mAbs, the numbers of PMN migrating across in either condition were the same as in the presence of control non-inhibitory mAbs. Cell surface labeling and immunoprecipitation demonstrated upregulation of CD47 to the PMN cell surface with kinetics similar to those of the transmigration response. Subcellular fractionation studies revealed redistribution of CD47 from intracellular compartments that co-sediment with secondary granules to plasma membrane-containing fractions after fMLP stimulation. Experiments performed to investigate potential signaling pathways revealed that inhibition of tyrosine phosphorylation with genistein reversed the anti-CD47-mediated PMN migration delay, whereas inhibition of phosphatidylinositol 3-kinase only partially reversed anti-CD47 effects that correlated with a rapid increase in PMN cell surface CD47. Analysis of the contribution of epithelial-expressed CD47 to PMN transmigration revealed that PMN migration across CD47-deficient epithelial monolayers (CaCO2) was significantly increased after stable transfection with CD47. These results suggest that cell surface CD47 and downstream tyrosine phosphorylation signaling events regulate, in part, the rate of PMN migration during the inflammatory response.

Gain- and loss-of-function Lyn mutant mice define a critical inhibitory role for Lyn in the myeloid lineage

To investigate the role of the Lyn kinase in establishing signaling thresholds in hematopoietic cells, a gain-of-function mutation analogous to the Src Y527F-activating mutation was introduced into the Lyn gene. Intriguingly, although Lyn is widely expressed within the hematopoietic system, these mice displayed no propensity toward hematological malignancy. By contrast, analysis of aging cohorts of both loss- and gain-of-function Lyn mutant mice revealed that Lyn(-/-) mice develop splenomegaly, increased numbers of myeloid progenitors, and monocyte/macrophage (M phi) tumors. Biochemical analysis of cells from these mutants revealed that Lyn is essential in establishing ITIM-dependent inhibitory signaling and for activation of specific protein tyrosine phosphatases within myeloid cells. Loss of such inhibitory signaling may predispose mice lacking this putative protooncogene to tumorigenesis.

Normal ligand binding and signaling by CD47 (integrin-associated protein) requires a long range disulfide bond between the extracellular and membrane-spanning domains

CD47 is a unique member of the Ig superfamily with a single extracellular Ig domain followed by a multiply membrane-spanning (MMS) domain with five transmembrane segments, implicated in both integrin-dependent and -independent signaling cascades. Essentially all functions of CD47 require both the Ig and MMS domains, raising the possibility that interaction between the two domains is required for normal function. Conservation of Cys residues among CD47 homologues suggested the existence of a disulfide bond between the Ig and MMS domains that was confirmed by chemical digestion and mapped to Cys(33) and Cys(263). Subtle changes in CD47 conformation in the absence of the disulfide were suggested by decreased binding of two anti-Ig domain monoclonal antibodies, decreased SIRPalpha1 binding, and reduced CD47/SIRPalpha1-mediated cell adhesion. Mutagenesis to prevent formation of this disulfide completely disrupted CD47 signaling independent of effects on ligand binding, as assessed by T cell interleukin-2 secretion and Ca(2+) responses. Loss of the disulfide did not affect membrane raft localization of CD47 or its association with alpha(v)beta(3) integrin. Thus, a disulfide bond between the Ig and MMS domains of CD47 is required for normal ligand binding and signal transduction.

Role of CD47 in the induction of human naive T cell anergy

We recently reported that CD47 ligation inhibited IL-2 release by umbilical cord blood mononuclear cells activated in the presence of IL-12, but not IL-4, preventing the induction of IL-12Rbeta(2) expression and the acquisition of Th1, but not the Th2 phenotype. Here we show that in the absence of exogenous cytokine at priming, CD47 ligation of umbilical cord blood mononuclear cells promotes the development of hyporesponsive T cells. Naive cells were treated with CD47 mAb for 3 days, expanded in IL-2 for 9-12 days, and restimulated by CD3 and CD28 coengagement. Effector T cells generated under these conditions were considered to be anergic because they produced a reduced amount of IL-2 at the single-cell level and displayed an impaired capacity 1) to proliferate, 2) to secrete Th1/Th2 cytokines, and 3) to respond to IL-2, IL-4, or IL-12. Moreover, CD47 mAb strongly suppressed IL-2 production and IL-2Ralpha expression in primary cultures and IL-2 response of activated naive T cells. Induction of anergy by CD47 mAb was IL-10 independent, whereas inclusion of IL-2 and IL-4, but not IL-7, at priming fully restored T cell activation. Furthermore, CD28 costimulation prevented induction of anergy. Thus, CD47 may represent a potential target to induce anergy and prevent undesired Th0/Th1 responses such as graft vs host diseases, allograft rejection, or autoimmune diseases.

Bidirectional negative regulation of human T and dendritic cells by CD47 and its cognate receptor signal-regulator protein-alpha: down-regulation of IL-12 responsiveness and inhibition of dendritic cell activation

Proinflammatory molecules, including IFN-gamma and IL-12, play a crucial role in the elimination of causative agents. To allow healing, potent anti-inflammatory processes are required to down-regulate the inflammatory response. In this study, we first show that CD47/integrin-associated protein, a ubiquitous multispan transmembrane protein highly expressed on T cells, interacts with signal-regulator protein (SIRP)-alpha, an immunoreceptor tyrosine-based inhibition motif-containing molecule selectively expressed on myelomonocytic cells, and next demonstrate that this pair of molecules negatively regulates human T and dendritic cell (DC) function. CD47 ligation by CD47 mAb or L-SIRP-alpha transfectants inhibits IL-12R expression and down-regulates IL-12 responsiveness of activated CD4(+) and CD8(+) adult T cells without affecting their response to IL-2. Human CD47-Fc fusion protein binds SIRP-alpha expressed on immature DC and mature DC. SIRP-alpha engagement by CD47-Fc prevents the phenotypic and functional maturation of immature DC and still inhibits cytokine production by mature DC. Finally, in allogeneic MLR between mDC and naive T cells, CD47-Fc decreases IFN-gamma production after priming and impairs the development of a Th1 response. Therefore, CD47 on T cells and its cognate receptor SIRP-alpha on DC define a novel regulatory pathway that may be involved in the maintenance of homeostasis by preventing the escalation of the inflammatory immune response.

Siglecs, sialic acids and innate immunity

Siglecs are members of the Ig superfamily that bind to sialic acid (Sia) and are mainly expressed by cells of the hematopoietic system. Until three years ago, only four Siglecs were known, namely sialoadhesin, CD22, myelin-associated glycoprotein and CD33. Since then, a further six human CD33-related Siglecs with features of inhibitory receptors have been identified and shown to be expressed by discrete subsets of leukocytes. Recognition of Sia by these Siglecs could play a role in the regulation of the innate immune system.

The monoclonal antibody W7C5 defines a novel surface antigen on hematopoietic stem cells

We recently raised a monoclonal antibody, termed W7C5, against a surface antigen that is expressed at low levels on bone marrow and peripheral blood CD34+ stem/progenitor cells but at high levels on fetal liver CD34+ cells. A reasonable staining intensity was achieved using magnetofluorescent liposome conjugates to analyze expression of W7C5 antigen on CD34+CD38- bone marrow (BM) cells. Flow cytometric analyses revealed that W7C5 detects about 50% of immature CD34+CD38- BM cells that coexpressed the differentiation antigens CD164, CD133, and CD172a (SIRP alpha). In addition, W7C5 also recognized a CD34- BM fraction. These cells were negative for CD117 and CD133, but expressed CD45 and moderate levels of CD164. Injection of selected CD34+W7C5+ and CD34-W7C5+ cells into 55-60-day-old fetal sheep resulted in an engraftment of both fractions. Partial amino acid sequence analysis of affinity-purified lysates of KU-812 cells revealed that W7C5 detects a novel membrane protein. Together, W7C5 defines a novel molecule that is expressed on CD34+ as well as on CD34- stem cell subsets.

Carcinoembryonic Antigen-Related Cell Adhesion Molecule 1 on Murine Dendritic Cells Is a Potent Regulator of T Cell Stimulation

Dendritic cells (DC) are important APCs that play a key role in the induction of an immune response. The signaling molecules that govern early events in DC activation are not well understood. We therefore investigated whether DC express carcinoembryonic Ag-related cell adhesion molecule 1 (CEACAM1, also known as BGP or CD66a), a well-characterized signal-regulating cell-cell adhesion molecule that is expressed on granulocytes, monocytes, and activated T cells and B cells. We found that murine DC express in vitro as well as in vivo both major isoforms of CEACAM1, CEACAM1-L (having a long cytoplasmic domain with immunoreceptor tyrosine-based inhibitory motifs) and CEACAM1-S (having a short cytoplasmic domain lacking phosphorylatable tyrosine residues). Ligation of surface-expressed CEACAM1 on DC with the specific mAb AgB10 triggered release of the chemokines macrophage inflammatory protein 1alpha, macrophage inflammatory protein 2, and monocyte chemoattractant protein 1 and induced migration of granulocytes, monocytes, T cells, and immature DC. Furthermore, the surface expression of the costimulatory molecules CD40, CD54, CD80, and CD86 was increased, indicating that CEACAM1-induced signaling regulates early maturation and activation of dendritic cells. In addition, signaling via CEACAM1 induced release of the cytokines IL-6, IL-12 p40, and IL-12 p70 and facilitated priming of naive MHC II-restricted CD4(+) T cells with a Th1-like effector phenotype. Hence, our results show that CEACAM1 is a signal-transducing receptor that can regulate early maturation and activation of DC, thereby facilitating priming and polarization of T cell responses.

Signal-regulatory protein alpha (SIRPalpha) but not SIRPbeta is involved in T-cell activation, binds to CD47 with high affinity, and is expressed on immature CD34(+)CD38(-) hematopoietic cells

Signal-regulatory proteins (SIRPs) represent a new family of inhibitory/activating receptor pairs. They consist of 3 highly homologous immunoglobulin (Ig)-like domains in their extracellular regions, but differ in their cytoplasmic regions by the presence (SIRPalpha) or absence (SIRPbeta) of immunoreceptor tyrosine-based inhibitory motifs (ITIMs). To analyze the differential expression on hematopoietic cells, function and ligand binding capacity of SIRPalpha and SIRPbeta molecules, soluble fusion proteins consisting of the extracellular domains of SIRPalpha1, SIRPalpha2, and SIRPbeta1, as well as SIRPalpha/beta-specific and SIRPbeta-specific monoclonal antibodies (MoAbs) were generated. In contrast to SIRPalpha1 and SIRPalpha2, no adhesion of SIRPbeta1 to CD47 could be detected by cell attachment assays and flow cytometry. Using deletion constructs of SIRPalpha1, the epitope responsible for SIRPalpha1 binding to CD47 could be confined to the N-terminal Ig-like loop. Flow cytometry analysis with SIRPalpha/beta- and SIRPbeta-specific MoAbs revealed that SIRPalpha but not SIRPbeta is expressed on CD34(+)CD38(-) hematopoietic cells. In addition, a strong SIRPalpha expression was also observed on primary myeloid dendritic cells (DCs) from peripheral blood as well as on in vitro generated DCs. Analysis of the T-cell stimulatory capacity of in vitro generated DCs in the presence of soluble SIRPalpha1 fusion proteins as well as SIRPalpha/beta-specific and CD47-specific MoAbs revealed a significant reduction of T-cell proliferation in mixed lymphocyte reaction and inhibition of induction of primary T-cell responses under these conditions. In contrast, soluble SIRPalpha or SIRPbeta-specific antibodies had no effect. The data suggest that the interaction of SIRPalpha with CD47 plays an important role during T-cell activation and induction of antigen-specific cytotoxic T-lymphocyte responses by DCs.

Differential galactosylation of neuronal and haematopoietic signal regulatory protein-(α) determines its cellular binding-specificity

Signal regulatory protein-(α) (SIRP(α)) is a member of the Ig superfamily selectively expressed by neuronal and myeloid cells. The molecule mediates functional interactions with CD47/integrin-associated protein. Here we provide evidence for the tissue-specific glycosylation of neuronal and haematopoietic SIRP(α). We demonstrate a major difference in the galactosylation of N-linked glycans isolated from neuronal (i.e. brain-derived) SIRP(α) as compared to myeloid (i.e. spleen-derived) SIRP(α), with neuronal SIRP(α) almost completely lacking galactose. (β)4-galactosyltransferase assays demonstrated that this is most likely due to a low galactosylation capacity of the brain. In order to investigate the role of galactosylation of SIRP(α) in cellular interactions, soluble recombinant SIRP(α) glycoforms containing galactose (SIRP(α)-Fc) or lacking galactose (SIRP(α)((Δ)Gal)-Fc) were produced. Binding studies demonstrated superior binding of SIRP(α)((Δ)Gal)-Fc to cerebellar neurons and isolated lymphocytes. In contrast, SIRP(α)-Fc bound relatively strong to macrophages. These data show that the galactosylation of SIRP(α) determines its cellular binding specificity.

Membrane raft association of CD47 is necessary for actin polymerization and protein kinase C theta translocation in its synergistic activation of T cells

CD47 is a ubiquitously expressed membrane protein with an extracellular Ig domain and a multiple membrane-spanning domain that can synergize with antigen to induce interleukin (IL)-2 secretion by T lymphocytes. Ligation of CD47 induced actin polymerization and increased protein kinase Ctheta (PKCtheta) association with the cytoskeleton independent of antigen receptor ligation, but ligation of mutant forms of the molecule missing either the Ig domain or the multiple membrane-spanning domain did not. Simultaneous ligation of CD47 and CD3 led to additive effects on F-actin and synergistic effects on PKCtheta cytoskeletal association. Disruption of membrane rafts by removal of cholesterol with cyclodextrin blocked CD47-induced actin polymerization, and mutant forms of CD47 that localized poorly to rafts failed to effect cytoskeletal rearrangement. However, raft association alone was not sufficient, because a raft-localized CD47 Ig domain bound to the membrane by a glycan phosphoinositol anchor was unable to induce actin polymerization. A mutant form of CD47 without its Ig domain that did not induce actin polymerization or localize to rafts still enhanced T cell receptor (TCR)-dependent tyrosine phosphorylation of PLCgamma and associated Ca(2+) signaling but did not augment IL-2 secretion. Thus, CD47 synergy with TCR to increase [Ca(2+)](i) is independent of actin and rafts but is insufficient to explain CD47 cooperation with TCR in IL-2 synthesis. Full synergy with TCR requires CD47 localization to membrane rafts where ligation leads to TCR-independent signals causing actin polymerization and PKCtheta translocation.

Thrombospondin-1 inhibits TCR-mediated T lymphocyte early activation

Biological activities of the matrix glycoprotein thrombospondin-1 (TSP1) are cell type specific and depend on the relative expression or activation of several TSP1 receptors. Although engaging individual TSP1 receptors in T lymphocytes can elicit costimulating signals, in this study we show that intact TSP1 inhibits TCR-mediated T cell activation, assessed globally using cDNA microarrays. TSP1 signaling suppressed expression of several genes induced in Jurkat T cells, including the T cell activation markers CD69, early growth response gene-1 (Egr-1), and phosphatase of activated cells (PAC-1). TCR-stimulated and CD47-costimulated IL-2 secretion and cell surface CD69 expression were also inhibited by TSP1. The specific inhibitory effect of TSP1 was verified in freshly isolated human PBMCs. TSP1 inhibited TCR-mediated but not protein kinase C-mediated T cell activation. Using CD69 expression as a marker, we demonstrated that the inhibitory activity of TSP1 depended on two TSP1 receptors, CD47 and integrin-associated protein heparan sulfate proteoglycans. Signals from these receptors inhibited TCR signaling downstream of ZAP70, but upstream of NF-AT. Therefore, the expression of TSP1 induced during wound repair and in tumor stroma may limit T cell activation at these sites.

Integrin-associated protein (CD47/IAP) contributes to T cell arrest on inflammatory vascular endothelium under flow

Integrin-associated protein (CD47/IAP) is a pentaspan molecule that regulates integrin functions. We prepared a CD47-deficient Jurkat T cell line to assess its role in the arrest of T cells on inflammatory endothelium. Under flow conditions, constitutive arrest of CD47-deficient cells is strongly decreased as compared to the original cell line, whereas reexpression of CD47 reestablishes their ability to stop. Moreover, cells transfected with a chimera made with the extracellular portion of CD47 and the transmembrane domain of CD7 or several truncated forms of CD47 show that the first transmembrane domain and a short cytoplasmic loop are sufficient for this process. CD47 effect is indirect and depends mainly on the alpha4beta1/VCAM-1 pathway, as shown by blocking antibodies. We detected on endothelium the two CD47 counter receptors known to date: thrombospondin and SIRP1alpha. Blocking experiments show that both are involved. Overall, CD47 participates in the constitutive arrest of T lymphocytes on inflamed vascular endothelium by up-regulating alpha 4beta1 integrins.

Revelations of the RYK receptor

Significant progress has been made over the last decade in elucidating the mechanisms employed by receptor protein tyrosine kinases (RTKs) in transducing extracellular signals critical for the regulation of diverse cellular activities. Nevertheless, revealing the biological significance of a subset of the RTKs that contain catalytically inactive protein tyrosine kinase domains has proven more elusive. ErbB3 has served as the prototype for models of catalytically inactive RTK function, performing the role of signal diversification in heterodimeric receptor complexes with other ErbB subfamily members. The receptor related to tyrosine kinases (RYK) is unique amongst the catalytically inactive RTKs. Based on structural or functional properties of the extracellular domain, RYK cannot be classified into an existing RTK subfamily. Recent genetic analyses of mouse Ryk and its Drosophila orthologue derailed have defined a role for this novel subfamily of receptors in the control of craniofacial development and neuronal pathway selection, respectively. Recent biochemical data lead us to propose a model that involves RYK in signal crosstalk and scaffold assembly with Eph receptors. This model is consistent with the established roles of Eph receptors and ephrins in craniofacial and nervous system morphogenesis. BioEssays 23:34-45, 2001.

CD47, a ligand for the macrophage fusion receptor, participates in macrophage multinucleation

The macrophage fusion receptor (MFR), also called P84/BIT/SIRPalpha/SHPS-1, is a transmembrane glycoprotein that belongs to the superfamily of immunoglobulins. Previously, we showed that MFR expression is highly induced at the onset of fusion in macrophages, and that MFR appears to play a role in macrophage-macrophage adhesion/fusion leading to multinucleation. The recent finding that IAP/CD47 acts as a ligand for MFR led us to hypothesize that it interacts with CD47 at the onset of cell-cell fusion. CD47 is a transmembrane glycoprotein, which, like MFR, belongs to the superfamily of immunoglobulins. We show that macrophages express the hemopoietic form of CD47, the expression of which is induced at the onset of fusion, but to a lower level than MFR. A glutathione S-transferase CD47 fusion protein engineered to contain the extracellular domain of CD47, binds macrophages, associates with MFR, and prevents multinucleation. CD47 and MFR associate via their amino-terminal immunoglobulin variable domain. Of the nine monoclonal antibodies raised against the extracellular domain of CD47, three block fusion, as well as MFR-CD47 interaction, whereas the others have no effect. Together, these data suggest that CD47 is involved in macrophage multinucleation by virtue of interacting with MFR during adhesion/fusion.

Dendritic cells, implications on function from studies of the afferent lymph veiled cell

Studies of afferent lymph veiled cells (ALVC) show that the full biological function of dendritic cells in peripheral tissue is not explained by a simple model in which immature dendritic cells at the body surface take up antigen, migrate via the afferent lymph ducts, mature and then effectively present antigens to T-cells in the draining lymph node. Furthermore, it is evident from various investigations that the dendritic cells in afferent lymph draining from the body surfaces are not a homogeneous population of cells. They comprise a mixture of cell phenotypes defined by staining with monoclonal antibodies, and the different sub-populations have distinct biological functions and roles in vivo. The molecular basis for differences between the function of afferent lymph dendritic cell subsets is only now being explored and defined but some progress has been made in understanding the role of co-stimulatory molecules. It should be possible to exploit knowledge of the functions of these cells and aid future vaccination strategies in domesticated animals thereby improving animal health and reducing economic loss, and, as a consequence, improving human health. By deliberately targeting functionally distinct subsets of either precursor or mature dendritic cells in vivo, it should become feasible to achieve an appropriately biased immune response.

CD47 ligation selectively inhibits the development of human naive T cells into Th1 effectors

The CD47 Ag, also named integrin-associated protein, was recently reported to regulate the production of IL-12 by human monocytes and dendritic cells. The present study shows that CD47 ligation by CD47 mAb in primary cultures of cord blood mononuclear cells inhibits IL-12-driven Th1 cell development, as revealed by the cytokine secretion profile at restimulation and IFN-gamma production at the single-cell level. F(ab')(2) fragments of CD47 mAb or the synthetic peptide 4N1K, corresponding to the CD47 binding site of thrombospondin, display the same activity. CD47 engagement does not change the phenotype of IL-12-primed cells from Th1 to Th2 or affect IL-4-induced Th2 cell development. Moreover, CD47 mAb inhibits IL-12- but not IL-4-induced IL-2 production as well as IFN-gamma in primary cultures, which was correlated with a decrease of the IL-12Rbeta2 chain expression. Inclusion of exogenous IL-2 at priming corrects IL-12R expression as well as the inhibition of Th1 cell development. The data thus underline the role of IL-2 in Th1 cell development and further suggest that targeting IL-2 and IL-12 simultaneously may have some therapeutic advantage in Th1 autoimmune diseases.

Osteoclasts and giant cells: macrophage-macrophage fusion mechanism

Membrane fusion is a ubiquitous event that occurs in a wide range of biological processes. While intracellular membrane fusion mediating organelle trafficking is well understood, much less is known about cell-cell fusion mediating sperm cell-oocyte, myoblast-myoblast and macrophage-macrophage fusion. In the case of mononuclear phagocytes, their fusion is not only associated with the differentiation of osteoclasts, cells which play a key role in the pathogenesis of osteoporosis, but also of giant cells that are present in chronic inflammatory reactions and in tumours. Despite the biological and pathophysiological importance of intercellular fusion events, the actual molecular mechanism of macrophage fusion is still unclear. One of the main research themes in my laboratory has been to investigate the molecular mechanism of mononuclear phagocyte fusion. Our hypothesis has been that macrophage-macrophage fusion, similar to virus-cell fusion, is mediated by specific cell surface proteins. But, in contrast with myoblasts and sperm cells, macrophage fusion is a rare event that occurs in specific instances. To test our hypothesis, we established an in vitro cell-cell fusion assay as a model system which uses alveolar macrophages. Upon multinucleation, these macrophages acquire the osteoclast phenotype. This indicates that multinucleation of macrophages leads to a specific and novel functional phenotype in macrophages. To identify the components of the fusion machinery, we generated four monoclonal antibodies (mAbs) which block the fusion of alveolar macrophages and purified the unique antigen recognized by these mAbs. This led us to the cloning of MFR (Macrophage Fusion Receptor). MFR was cloned simultaneously as P84/SHPS-1/SIRPalpha/BIT by other laboratories. We subsequently showed that the recombinant extracellular domain of MFR blocks fusion. Most recently, we identified a lower molecular weight form of MFR that is missing two extracellular immunoglobulin (Ig) C domains. Shortly after we cloned MFR, CD47 was reported to be a ligand for P84/SIRPalpha. We have since generated preliminary results which suggest that CD47 interacts with MFR during adhesion/fusion and is a member of the fusion machinery. We also identified CD44 as a plasma membrane protein which, like MFR, is highly expressed at the onset of fusion. The recombinant soluble extracellular domain of CD44 blocks fusion by interacting with a cell-surface binding site. We now propose a model in which both forms of MFR, CD44, and CD47 mediate macrophage adhesion/fusion and therefore the differentiation of osteoclasts and giant cells.

Characteristics and analysis of normal and leukemic stem cells: current concepts and future directions

Acute myeloid leukemias (AML) are considered to be clonal disorders involving early hematopoietic progenitor cells. The recent advances in characterization of early stem cells give rise to the question whether it is possible to distinguish healthy progenitors from cells of the leukemic clone in leukemia patients. Differences and similarities in phenotype, genotype and biology are described for leukemic cells and normal hematological progenitors. Recent new insights into human stem cell development offer the perspective that distinction between benign and malignant progenitors might be possible in the future at a very early stage of maturation.

Negative regulation of growth hormone receptor/JAK2 signaling by signal regulatory protein alpha

Signal regulatory proteins (SIRPs) are receptor-like transmembrane proteins, the majority of which contain a cytoplasmic proline-rich region and four cytoplasmic tyrosines that, when phosphorylated, bind SH2 domain-containing protein tyrosine phosphatases (SHP). We demonstrated previously that growth hormone (GH) induces tyrosyl phosphorylation of SIRPalpha and association of SIRPalpha with SHP-2. The GH-activated tyrosine kinase JAK2 associates with and tyrosyl-phosphorylates SIRPalpha1. Here we show that JAK2-SIRPalpha1 association does not require phosphotyrosines in SIRPalpha1 or JAK2 or the proline-rich region of SIRPalpha1. However, when the C-terminal 30 amino acids of SIRPalpha1 containing the proline-rich region and tyrosine 495 are deleted, tyrosyl phosphorylation of SIRPalpha1 by JAK2 and association of SHP-2 with SIRPalpha1 are reduced. GH-dependent tyrosyl phosphorylation of JAK2 is reduced when wild-type SIRPalpha1 compared with SIRPalpha1 lacking the four cytoplasmic tyrosines (SIRP 4YF) is expressed in cells, suggesting that SIRPalpha1 negatively regulates GHR/JAK2 signaling. Consistent with reduced JAK2 activity, overexpression of wild-type SIRPalpha1 but not SIRP 4YF reduces GH-induced phosphorylation of ERKs 1 and 2, STAT3, and STAT5B. These results suggest that SIRPalpha1 is a negative regulator of GH signaling and that the ability of SIRPalpha1 mutants to negatively regulate GHR-JAK2 signaling correlates with their ability to bind SHP-2.