Galectin-1, an endogenous lectin produced by thymic epithelial cells, induces apoptosis of human thymocytes.

Galectin-1, a beta-galactoside binding protein, is produced by thymic epithelial cells and binds to human thymocytes. We have previously reported that galectin-1 induces the apoptosis of activated T lymphocytes. Because the majority of thymocytes die via apoptosis while still within the thymus, we tested whether galectin-1 could induce the apoptosis of these cells. We now report that in vitro exposure to galectin-1 induced apoptosis of two subsets of CD4(lo) CD8(lo) thymocytes. The phenotypes of susceptible thymocytes were consistent with that of both negatively selected and nonselected cells. Galectin-1-induced apoptosis was enhanced by preexposure of thymocytes to antibody to CD3, suggesting that galectin-1 may be a participant in T-cell- receptor mediated apoptosis. In contrast, pretreatment of thymocytes with dexamethasone had no effect on galectin-1 susceptibility. We noted that 71% of the cells undergoing apoptosis after galectin-1 treatment had a DNA content greater than 2N, indicating that proliferating thymocytes were most sensitive to galectin-1. We propose that galectin-1 plays a role in the apoptosis of both negatively selected and nonselected thymocytes, and that the susceptibility of thymocytes to galectin-1 is regulated, in part, by entry or exit from the cell cycle.

Differentiating between memory and effector CD8 T cells by altered expression of cell surface O-glycans.

Currently there are few reliable cell surface markers that can clearly discriminate effector from memory T cells. To determine if there are changes in O-glycosylation between these two cell types, we analyzed virus-specific CD8 T cells at various time points after lymphocytic choriomeningitis virus infection of mice. Antigen-specific CD8 T cells were identified using major histocompatibility complex class I tetramers, and glycosylation changes were monitored with a monoclonal antibody (1B11) that recognizes O-glycans on mucin-type glycoproteins. We observed a striking upregulation of a specific cell surface O-glycan epitope on virus-specific CD8 T cells during the effector phase of the primary cytotoxic T lymphocyte (CTL) response. This upregulation showed a strong correlation with the acquisition of effector function and was downregulated on memory CD8 T cells. Upon reinfection, there was again increased expression of this specific O-glycan epitope on secondary CTL effectors, followed once more by decreased expression on memory cells. Thus, this study identifies a new cell surface marker to distinguish between effector and memory CD8 T cells. This marker can be used to isolate pure populations of effector CTLs and also to determine the proportion of memory CD8 T cells that are recruited into the secondary response upon reencounter with antigen. This latter information will be of value in optimizing immunization strategies for boosting CD8 T cell responses.

Human thymic epithelial cells express an endogenous lectin, galectin-1, which binds to core 2 O-glycans on thymocytes and T lymphoblastoid cells.

Thymic epithelial cells play a crucial role in the selection of developing thymocytes. Thymocyte-epithelial cell interactions involve a number of adhesion molecules, including members of the integrin and immunoglobulin superfamilies. We found that human thymic epithelial cells synthesize an endogenous lectin, galectin-1, which binds to oligosaccharide ligands on the surface of thymocytes and T lymphoblastoid cells. Binding of T lymphoblastoid cells to thymic epithelial cells was inhibited by antibody to galectin-1 on the epithelial cells, and by two antibodies, T305 and 2B11, that recognize carbohydrate epitopes on the T cell surface glycoproteins CD43 and CD45, respectively. T lymphoblastoid cells and thymocytes bound recombinant galectin-1, as demonstrated by flow cytometric analysis, and lectin binding was completely inhibited in the presence of lactose. The degree of galectin-1 binding to thymocytes correlated with the maturation stage of the cells, as immature thymocytes bound more galectin-1 than did mature thymocytes. Preferential binding of galectin-1 to immature thymocytes may result from regulated expression of preferred oligosaccharide ligands on those cells, since we found that the epitope recognized by the T305 antibody, the core 2 O-glycan structure on CD43, was expressed on cortical, but not medullary cells. The level of expression of the UDP-GlcNAc:Gal beta 1,3GalNAc-R beta 1, 6GlcNAc transferase (core 2 beta 1, 6 GlcNAc transferase, or C2GnT), which creates the core 2 O-glycan structure, correlated with the glycosylation change between cortical and medullary cells. Expression of mRNA encoding the C2GnT was high in subcapsular and cortical thymocytes and low in medullary thymocytes, as demonstrated by in situ hybridization. These results suggest that galectin-1 participates in thymocyte-thymic epithelial cell interactions, and that this interaction may be regulated by expression of relevant oligosaccharide ligands on the thymocyte cell surface.

A novel mammalian protein, p55CDC, present in dividing cells is associated with protein kinase activity and has homology to the Saccharomyces cerevisiae cell division cycle proteins Cdc20 and Cdc4.

A novel protein, p55CDC, has been identified in cycling mammalian cells. This transcript is readily detectable in all exponentially growing cell lines but disappears when cells are chemically induced to fall out of the cell cycle and differentiate. The p55CDC protein appears to be essential for cell division, since transfection of antisense p55CDC cDNA into CHO cells resulted in isolation of only those cells which exhibited a compensatory increase in p55CDC transcripts in the sense orientation. Immunoprecipitation of p55CDC yielded protein complexes with kinase activity which fluctuated during the cell cycle. Since p55CDC does not have the conserved protein kinase domains, this activity must be due to one or more of the associated proteins in the immune complex. The highest levels of protein kinase activity were seen with alpha-casein and myelin basic protein as substrates and demonstrated a pattern of activity distinct from that described for the known cyclin-dependent cell division kinases. The p55CDC protein was also phosphorylated in dividing cells. The amino acid sequence of p55CDC contains seven repeats homologous to the beta subunit of G proteins, and the highest degree of homology in these repeats was found with the Saccharomyces cerevisiae Cdc20 and Cdc4 proteins, which have been proposed to be involved in the formation of a functional bipolar mitotic spindle in yeast cells. The G beta repeat has been postulated to mediate protein-protein interactions and, in p55CDC, may modulate its association with a unique cell cycle protein kinase. These findings suggest that p55CDC is a component of the mammalian cell cycle mechanism.

Galectin-1 induces nuclear translocation of endonuclease G in caspase- and cytochrome c-independent T cell death.

Galectin-1, a mammalian lectin expressed in many tissues, induces death of diverse cell types, including lymphocytes and tumor cells. The galectin-1 T cell death pathway is novel and distinct from other death pathways, including those initiated by Fas and corticosteroids. We have found that galectin-1 binding to human T cell lines triggered rapid translocation of endonuclease G from mitochondria to nuclei. However, endonuclease G nuclear translocation occurred without cytochrome c release from mitochondria, without nuclear translocation of apoptosis-inducing factor, and prior to loss of mitochondrial membrane potential. Galectin-1 treatment did not result in caspase activation, nor was death blocked by caspase inhibitors. However, galectin-1 cell death was inhibited by intracellular expression of galectin-3, and galectin-3 expression inhibited the eventual loss of mitochondrial membrane potential. Galectin-1-induced cell death proceeds via a caspase-independent pathway that involves a unique pattern of mitochondrial events, and different galectin family members can coordinately regulate susceptibility to cell death.

Galectins: versatile modulators of cell adhesion, cell proliferation, and cell death.

Lectins, or carbohydrate binding proteins, recognize specific oligosaccharide structures on glycoproteins and glycolipids. Several families of animal lectins have been identified; for some of these lectins, functions such as leukocyte adhesion and microbial opsonization have been described. The galectins are a family of lectins found in species ranging from sponges and nematodes to humans. Members of the galectin family have been proposed to mediate cell adhesion, to regulate cell growth, and to trigger or inhibit apoptosis. The expression pattern of different galectins changes during development, and this pattern is also altered at sites of inflammation and in breast, colon, prostate, and thyroid carcinomas. In addition, the level of expression of some galectins by tumor cells has been shown to be correlated with metastatic potential. The mechanisms by which galectins exert these diverse effects remain largely unknown. Some glycoprotein counterreceptors recognized by certain galectins have been identified; this is an important first step in understanding the cell-type specific effects of different galectins. This review discusses the way in which the modulation of galectin activity may affect strategies for treatment of a variety of human diseases, including autoimmunity and cancer.

Influenza virus strains selectively recognize sialyloligosaccharides on human respiratory epithelium; the role of the host cell in selection of hemagglutinin receptor specificity.

The complement of sialyloligosaccharides present on the surface of human tracheal epithelium has been implicated as an important factor in the selection of hemagglutinin receptor specificity of human influenza A virus. Human strains of influenza A virus preferentially recognize host cell receptors bearing SA alpha 2,6Gal sequences, a sequence which is found on the surface of ciliated tracheal epithelium. A fluorescently-labelled H3 human virus strain bound avidly to the apical surface of human tracheal epithelium, while a fluorescently-labelled receptor variant strain, which preferentially binds SA alpha 2,3Gal sequences, showed little binding to the epithelial surface and localized primarily to intracellular mucin droplets. Extracts of human bronchial mucin, which is known to contain sialic acid primarily in the SA alpha 2,3Gal linkage, was a potent inhibitor of the binding of the receptor variant strain to trachea sections, while the binding of the parent strain was unaffected by the presence of mucin. Human bronchial mucin also inhibited the binding of the receptor variant strains, but not the parent virus strains, to human erythrocytes derivatized to contain SA alpha 2,6Gal sequences. These results suggest that a combination of selection pressures present in the respiratory tract environment have resulted in the evolution of a hemagglutinin receptor specificity in human influenza A virus strains which optimizes recognition of, binding to and infection of host cells.

Small intestinal submucosa induces loss of mitochondrial integrity and caspase-dependent apoptosis in human T cells.

Porcine small intestinal submucosa (SIS) is a cell-free biomaterial used in humans for wound healing and as scaffold material for constructive remodeling of damaged or missing tissue. We have previously shown that SIS contains a factor that suppresses human helper T cell subset differentiation and expansion by inducing programmed cell death. Our aims here were to identify in detail the processes involved in SIS-induced T cell apoptosis and to perform the first characterization of the apoptosis-inducing factor present in SIS. In in vitro experiments, we utilized human T cell lines, Jurkat and CEM, to identify the processes involved in SIS-induced T cell apoptosis. Two types of sterile SIS material were used: hydrated sheets and rehydrated clinical-grade sheets. We found that SIS-mediated apoptosis as detected by induction of membrane annexin V staining involved the loss of mitochondrial membrane potential and was dependent on caspase activation. We eliminated transforming growth factor beta (TGF-beta), Fas ligand (FasL), and galectin family members as factors in SIS-mediated T cell apoptosis. We further established that processes required to prepare SIS for clinical use, freeze-drying, and gas sterilization destroyed the apoptosis-inducing factor. SIS contains a factor that induces loss of mitochondrial integrity and caspase-dependent apoptosis in human T cells. This factor is destroyed by freeze-drying and gas sterilization and is not TGF-beta, FasL, or a galectin family member. Normal T cell homeostasis in gut-associated tissues may be regulated in part by this unknown factor.

Application of nonisotopic RNA in situ hybridization to detect endogenous gene expression in pathologic specimens.

Detection of endogenous messenger RNA in surgical pathology tissue samples is a useful technique for identifying or characterizing populations of normal or neoplastic cells. Potential problems with this technique include degradation of target RNA in routinely processed specimens, and low concentration of target RNA sequences in the cells, necessitating the use of radiolabeled probes for detection. In this report, we demonstrate detection of nonmuscle actin messenger RNA in fixed, paraffin-embedded lymphoid tissues and bone marrow aspirate smears, using routinely obtained surgical and hematologic specimens. In addition, increased sensitivity was achieved by hybridization with full-length RNA probes, rather than oligonucleotide DNA probes; these probes were labeled with digoxigenin-11-UTP, allowing colorimetric detection of hybridization and avoiding the use of isotopes. The advantages and potential applications of this technique as an aid in pathologic diagnosis are discussed.

Synthesis of linkage-specific sialoside substrates for colorimetric assay of neuraminidases.

Neuraminidase substrates suitable for analysis of linkage specificity were enzymically synthesized in good yield by linking N-acetylneuraminic acid (Neup5Ac) to O-6 and O-3 of 4-nitrophenyl beta-D-galactopyranoside with beta-D-galactoside-alpha-(2----6)-sialyltransferase and beta-D-galactoside-alpha-(2----3)-sialyltransferase, respectively. By use of these substrates, a convenient colorimetric assay method was developed for the determination of linkage specificity of bacterial and viral neuraminidases. The substrates are incubated with viral or bacterial neuraminidase and subsequently treated with beta-D-galactosidase to convert the liberated 4-nitrophenyl beta-D-galactopyranoside to 4-nitrophenol. The amount of liberated 4-nitrophenol is equivalent to the amount of Neup5Ac released from the substrate, thus allowing measurement of neuraminidase activity. The results showed that bacterial and viral neuraminidases can discriminate between these two compounds, making them useful substrates for the rapid determination of neuraminidase linkage specificity.

CD45 glycosylation controls T-cell life and death.

CD45, an abundant and highly glycosylated cell-surface protein, is a critical regulator of T-cell development. CD45 is differentially glycosylated throughout the life of a T cell, and the glycosylation state of CD45 controls recognition by various binding partners, affects intracellular signaling by the cytoplasmic tyrosine phosphatase domain and modulates the response of the T cell to antigen. Although the importance of CD45 during T-cell development has been established, it is becoming increasingly clear that glycosylation of CD45 is a dynamic process that modifies T-cell survival, activation and immune function. In this review, we address changes that occur in CD45 glycosylation during T-cell development and differentiation, describe carbohydrate-binding proteins that recognize differentially glycosylated forms of CD45, and discuss how differential glycosylation alters the T-cell response to a variety of signals involved in selection, activation and apoptosis.

The N2 neuraminidase of human influenza virus has acquired a substrate specificity complementary to the hemagglutinin receptor specificity.

A survey of 10 human influenza A viruses of the N2 serotype, isolated between 1957 and 1987, has revealed a drift in neuraminidase linkage specificity. While the earliest N2 strains examined exhibit strict specificity for cleavage of the NeuAc alpha 2,3Gal sequence, N2 isolates from 1967 to 1968 also show limited activity towards the NeuAc alpha 2,6Gal linkage. In strains isolated in 1972 and later, the N2 neuraminidase has approximately equal activity towards both types of linkages. The NeuAc alpha 2,6Gal linkage cleaved by the later N2 neuraminidases is the preferred receptor determinant of human H2 and H3 hemagglutinins. Thus, the acquired neuraminidase specificity of the later isolates allows elution of bound virus from erythrocytes derivatized to contain the NeuAc alpha 2,6Gal linkage, while earlier isolates, which cleave only the NeuAc alpha 2,3Gal sequence, fail to elute from these erythrocytes. These results suggest that the observed drift in N2 neuraminidase specificity in the direction of the preferred H2 and H3 receptor determinant may facilitate release of progeny virus from host cells.

Lymphadenopathic Kaposi's sarcoma in a pediatric patient with acquired immune deficiency syndrome.

A six-year-old Hispanic female with acquired immune deficiency syndrome (AIDS) and multiple opportunistic infections was found to have disseminated lymphadenopathic Kaposi's sarcoma at autopsy. She was presumed to have been infected via maternal transmission, although both HIV antibody positive parents were asymptomatic. Kaposi's sarcoma is rarely reported in pediatric AIDS, and children may not have the typical aggressive cutaneous lesions found in adults with AIDS-associated Kaposi's sarcoma.

Sialyloligosaccharides of the respiratory epithelium in the selection of human influenza virus receptor specificity.

Human H3 strains of influenza A virus preferentially bind cell-surface oligosaccharides containing the sequence NeuAc alpha 2,6Gal, while avian influenza strains preferentially recognize the sequence NeuAc alpha 2,3Gal. The distribution of these two types of sialic acid linkages on host respiratory epithelium, the target of influenza infection, may be a factor in the selection of the different receptor specificities observed in human and avian influenza strains. To examine the distribution of these two structures on human tracheal epithelial cells, two sialic acid specific lectins were used. The Sambucus nigra lectin (SNA), which recognizes the sequence NeuAc alpha 2,6Gal/GalNac, primarily binds to the surface of the ciliated tracheal epithelial cells, and only weakly binds to mucins in the surface goblet cells. In contrast, the Maackia amurensis lectin (MAL), which is specific for the NeuAc alpha 2,3Gal sequence, binds strongly to mucus droplets in goblet cells, but not to the surface of ciliated cells. Thus, human ciliated tracheal cells appear to contain sialyloligosaccharides preferentially recognized by human influenza strains. These findings suggest that human H3 influenza strains may have evolved a receptor specificity which favors binding to ciliated cells, and minimizes binding inhibition by respiratory mucus.

Isolation and characterization of a galactose-specific carbohydrate binding protein from human lymphoblastic cells.

A carbohydrate binding protein of Mr = 32,000 (CBP 32) has been isolated from detergent extracts of human B and T lymphoblastoid cells. CBP 32 binds specifically to glycoproteins containing asparagine-linked complex oligosaccharides, and can be eluted from a fetuin affinity matrix by beta-lactose. Binding is not thiol dependent, nor are divalent cations necessary for binding. Native CBP 32 appears to exist as a monomer, with a pI of 8.2. Purified CBP 32 can bind detergent, as shown by charge-shift electrophoresis, and thus appears to be an integral membrane protein.

Characterization of the hemagglutinin receptor specificity and neuraminidase substrate specificity of clinical isolates of human influenza A viruses.

Six clinical isolates of influenza A viruses were examined for hemagglutinin receptor specificity and neuraminidase substrate specificity. All of the viral isolates minimally passaged in mammalian cells demonstrated preferential agglutination of human erythrocytes enzymatically modified to contain NeuAc alpha 2,6Gal sequences, with no agglutination of cells bearing NeuAc alpha 2,3Gal sequences. This finding is consistent with the hemagglutination receptor specificity previously demonstrated for laboratory strains of influenza A viruses. The neuraminidase substrate specificities of the clinical isolates examined were also identical to that described for the N2 neuraminidase of recent laboratory strains of human influenza viruses. The H3N2 viruses all displayed the ability to release sialic acid from both alpha 2, 3 and alpha 2, 6 linkages. In addition, two clinical isolates of H1N1 viruses also demonstrated this dual neuraminidase substrate specificity, a characteristic which has not been previously described for the N1 neuraminidase. These results demonstrate that complementary hemagglutinin and neuraminidase specificities are found in recent isolates of both H1N1 and H3N2 influenza viruses.

Multivalent protein-carbohydrate interactions. A new paradigm for supermolecular assembly and signal transduction.

Many biological recognition processes involve the binding and clustering of ligand-receptor complexes and concomitant signal transduction events. Such interactions have recently been observed in human T cells in which binding and cross-linking of specific glycoprotein counter-receptors on the surface of the cells by an endogenous bivalent carbohydrate binding protein (galectin-1) leads to apoptosis [Pace, K. E., et al. (1999) J. Immunol. 163, 3801-3811]. Importantly, different counter-receptors associated with specific phosphatase or kinase activities were shown to form separate clusters on the surface of the cells as a result of galectin-1 binding to the carbohydrate moieties of the respective glycoproteins. This suggests that the unique separation and organization of signaling molecules that results from galectin-1 binding is involved in delivering the signal to die. The ability of galectin-1 to induce the separation of specific glycoprotein receptors was modeled on the basis of molecular and structural studies of the binding of multivalent carbohydrates to lectins that result in the formation of specific two- and three-dimensional cross-linked lattices. These latter studies have been recently highlighted by X-ray crystallographic results showing that a single tetravalent lectin forms distinct cross-linked complexes with four different bivalent oligosaccharides [Olsen, L. R., et al. (1997) Biochemistry 36, 15073-15080]. In this report, binding and cross-linking of multivalent carbohydrates with multivalent lectins is shown to be a new paradigm for supermolecular assembly and signal transduction in biological systems.

Expression of a specific glycosyltransferase enzyme regulates T cell death mediated by galectin-1.

Galectin-1 induces apoptosis of immature thymocytes and activated T cells, suggesting that galectin-1 regulates cell death in the thymus during selection and in the periphery following an immune response. Although it is known that galectin-1 recognizes lactosamine (Gal-GlcNAc) as a minimal ligand, this disaccharide is ubiquitously expressed on a variety of cell surface glycoproteins. Thus, susceptibility to galectin-1 may be regulated by the presentation of lactosamine on specific oligosaccharide structures created by specific glycosyltransferase enzymes. The core 2 beta-1, 6-N-acetylglucosaminyltransferase (core 2 GnT) creates a branched structure on O-glycans that can be elongated to present multiple lactosamine sequences. In the thymus, the core 2 GnT is expressed in galectin-1-sensitive thymocyte subsets. In the periphery, an oligosaccharide epitope created by the core 2 GnT is expressed on galectin-1-sensitive activated T-cells. In this report, we demonstrate that expression of the core 2 GnT was necessary and sufficient for galectin-1-induced death of murine T cell lines. In addition, overexpression of the core 2 GnT in mice increased the susceptibility of double positive thymocytes to galectin-1. These data demonstrate that expression of a specific glycosyltransferase can control susceptibility to galectin-1, suggesting that developmentally regulated glycosyltransferase expression may be a mechanism to modulate cell death during T cell development and function.

Transtentorial herniation caused by an intracranial mass lesion following high-dose methotrexate.

A patient with intracranial osteosarcoma that arose 16 years after radiation therapy for hereditary retinoblastoma developed fatal cerebral edema and brainstem herniation after she received a single dose of intravenous methotrexate. Autopsy demonstrated extensive necrosis of the tumor mass, as well as necrotizing vascular damage within the neoplasm. Although high-dose methotrexate has been shown to be useful in the treatment of primary osteogenic sarcoma, the tumoricidal effects of therapy appear to have caused a fatal rise in intracranial pressure.

CD7 delivers a pro-apoptotic signal during galectin-1-induced T cell death.

Galectin-1, an endogenous lectin expressed in lymphoid organs and immune-privileged sites, induces death of human and murine thymocytes and T cells. Galectin-1 binds to several glycoproteins on the T cell surface, including CD7. However, the T cell surface glycoprotein receptors responsible for delivering the galectin-1 death signal have not been identified. We show that CD7 is required for galectin-1-mediated death. This demonstrates a novel function for CD7 as a death trigger and identifies galectin-1/CD7 as a new biologic death signaling pair.