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.

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.

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.

CD45 modulates galectin-1-induced T cell death: regulation by expression of core 2 O-glycans.

Galectin-1 induces death of immature thymocytes and activated T cells. Galectin-1 binds to T cell-surface glycoproteins CD45, CD43, and CD7, although the precise roles of each receptor in cell death are unknown. We have determined that CD45 can positively and negatively regulate galectin-1-induced T cell death, depending on the glycosylation status of the cells. CD45(+) BW5147 T cells lacking the core 2 beta-1,6-N-acetylglucosaminyltransferase (C2GnT) were resistant to galectin-1 death. The inhibitory effect of CD45 in C2GnT(-) cells appeared to require the CD45 cytoplasmic domain, because Rev1.1 cells expressing only CD45 transmembrane and extracellular domains were susceptible to galectin-1 death. Moreover, treatment with the phosphotyrosine-phosphatase inhibitor potassium bisperoxo(1,10-phenanthroline)oxovanadate(V) enhanced galectin-1 susceptibility of CD45(+) T cell lines, but had no effect on the death of CD45(-) T cells, indicating that the CD45 inhibitory effect involved the phosphatase domain. Expression of the C2GnT in CD45(+) T cell lines rendered the cells susceptible to galectin-1, while expression of the C2GnT in CD45(-) cells had no effect on galectin-1 susceptibility. When CD45(+) T cells bound to galectin-1 on murine thymic stromal cells, only C2GnT(+) T cells underwent death. On C2GnT(+) cells, CD45 and galectin-1 co-localized in patches on membrane blebs while no segregation of CD45 was seen on C2GnT(-) T cells, suggesting that oligosaccharide-mediated clustering of CD45 facilitated galectin-1-induced cell death.

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.

Human homologs of the Xenopus oocyte cortical granule lectin XL35.

The cDNAs encoding two human homologs of the Xenopus oocyte lectin, XL35, were isolated from a small intestine cDNA library and termed HL-1 and HL-2. The deduced amino acid sequence of each homolog is about 60% identical and 80% similar to that of XL35, and none of these sequences contains the C-type lectin motif, although it is known that XL35 requires calcium for ligand binding. By Northern analysis, HL-1 transcripts are present at relatively high levels in heart, small intestine, colon, thymus, ovary, and testis. HL-2 transcripts, by contrast, are expressed only in small intestine. Immunocytochemistry using a polyclonal antibody produced against XL35 shows HL-1 protein to be localized exclusively in endothelial cells in colon, thymus, liver, and other tissues. Primary cultures of human aortic endothelial cells are positive for HL-1 expression by immunoblotting and by PCR analysis, but several other human cell types are not. HL-1 and -2 are both encoded at chromosome 1q23, the same locus that encodes the selectins. XL35, HL-1 and -2, and another mouse homolog are members of a new family of proteins whose members most likely perform diverse functions.

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.

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.

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.

Restricted receptor segregation into membrane microdomains occurs on human T cells during apoptosis induced by galectin-1.

Galectin-1 induces apoptosis of human thymocytes and activated T cells by an unknown mechanism. Apoptosis is a novel function for a mammalian lectin; moreover, given the ubiquitous distribution of the oligosaccharide ligand recognized by galectin-1, it is not clear how susceptibility to and signaling by galectin-1 is regulated. We have determined that galectin-1 binds to a restricted set of T cell surface glycoproteins, and that only CD45, CD43, and CD7 appear to directly participate in galectin-1-induced apoptosis. To determine whether these specific glycoproteins interact cooperatively or independently to deliver the galectin-1 death signal, we examined the cell surface localization of CD45, CD43, CD7, and CD3 after galectin-1 binding to human T cell lines and human thymocytes. We found that galectin-1 binding resulted in a dramatic redistribution of these glycoproteins into segregated membrane microdomains on the cell surface. CD45 and CD3 colocalized on large islands on apoptotic blebs protruding from the cell surface. These islands also included externalized phosphatidylserine. In addition, the exposure of phosphatidylserine on the surface of galectin-1-treated cells occurred very rapidly. CD7 and CD43 colocalized in small patches away from the membrane blebs, which excluded externalized phosphatidylserine. Receptor segregation was not seen on cells that did not die in response to galectin-1, including mature thymocytes, suggesting that spatial redistribution of receptors into specific microdomains is required for triggering apoptosis.

Galectin-1 specifically modulates TCR signals to enhance TCR apoptosis but inhibit IL-2 production and proliferation.

Galectin-1 is an endogenous lectin expressed by thymic and lymph node stromal cells at sites of Ag presentation and T cell death during normal development. It is known to have immunomodulatory activity in vivo and can induce apoptosis in thymocytes and activated T cells (1-3). Here we demonstrate that galectin-1 stimulation cooperates with TCR engagement to induce apoptosis, but antagonizes TCR-induced IL-2 production and proliferation in a murine T cell hybridoma and freshly isolated mouse thymocytes, respectively. Although CD4+ CD8+ double positive cells are the primary thymic subpopulation susceptible to galectin-1 treatment alone, concomitant CD3 engagement and galectin-1 stimulation broaden susceptible thymocyte subpopulations to include a subset of each CD4- CD8-, CD4+ CD8+, CD4- CD8+, and CD4+ CD8- subpopulations. Furthermore, CD3 engagement cooperates with suboptimal galectin-1 stimulation to enhance cell death in the CD4+ CD8+ subpopulation. Galectin-1 stimulation is shown to synergize with TCR engagement to dramatically and specifically enhance extracellular signal-regulated kinase-2 (ERK-2) activation, though it does not uniformly enhance TCR-induced tyrosine phosphorylation. Unlike TCR-induced IL-2 production, TCR/galectin-1-induced apoptosis is not modulated by the expression of kinase inactive or constitutively activated Lck. These data support a role for galectin-1 as a potent modulator of TCR signals and functions and indicate that individual TCR-induced signals can be independently modulated to specifically affect distinct TCR functions.

Molecular basis for the generation in pigs of influenza A viruses with pandemic potential.

Genetic and biologic observations suggest that pigs may serve as "mixing vessels" for the generation of human-avian influenza A virus reassortants, similar to those responsible for the 1957 and 1968 pandemics. Here we demonstrate a structural basis for this hypothesis. Cell surface receptors for both human and avian influenza viruses were identified in the pig trachea, providing a milieu conducive to viral replication and genetic reassortment. Surprisingly, with continued replication, some avian-like swine viruses acquired the ability to recognize human virus receptors, raising the possibility of their direct transmission to human populations. These findings help to explain the emergence of pandemic influenza viruses and support the need for continued surveillance of swine for viruses carrying avian virus genes.

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.

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.

Galectin-1 is expressed by thymic epithelial cells in myasthenia gravis.

Galectin-1, a member of a family of carbohydrate binding proteins, is synthesized by thymic epithelial cells in normal juvenile thymus, and mediates adhesion of immature T cells to thymic epithelium. Because cell adhesion molecules are proposed to play a role in the thymic hyperplasia and neoplasia seen in the autoimmune disease myasthenia gravis, we examined the expression of galectin-1 in myasthenic thymi. We detected abundant galectin-1 expression in thymic epithelial cells in 27 hyperplastic and neoplastic thymi from patients with myasthenia gravis. Primary cultures of neoplastic epithelial cells from a thymoma continued to express galectin-1, and bound immature T cells; T cell binding was inhibited by the addition of the beta-galactosides lactose and thiodigalactoside, suggesting that galectin-1 on the thymoma cells and a saccharide ligand on the T cells participated in cell-cell adhesion. Expression of galectin-1 by thymic epithelial cells may play a role in the thymic pathology seen in myasthenia gravis.

Characterization of terminal sialic acid linkages on human thymocytes. Correlation between lectin-binding phenotype and sialyltransferase expression.

T cell surface sialylation changes during maturation in the thymus. We have previously demonstrated increased expression of mRNA encoding the Gal beta 1, 3GalNAc alpha 2,3-sialyltransferase in mature medullary human thymocytes, compared with immature cortical thymocytes. For this enzyme, increased expression of transferase mRNA correlated with increased sialylation of O-glycans. We have now examined the pattern of expression in the human thymus of two additional sialyltransferases, the Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase (ST6N) and the Gal beta 1,3/4GlcNAc alpha 2,3-sialyltransferase (ST3N). The patterns of mRNA expression were compared with the pattern of binding of two sialic acid-specific plant lectins, Sambucus nigra agglutinin and Maackia amurensis agglutinin, which preferentially recognize alpha 2,6- and alpha 2,3-linked sialic acids, respectively, on N-glycans. By in situ hybridization, mRNA encoding ST3N was detected uniformly throughout the thymus. All thymocytes bound M. amurensis agglutinin, demonstrating a direct correlation between the level of ST3N mRNA expression and cell-surface glycosylation. In contrast, mRNA encoding ST6N was also expressed uniformly throughout the thymus; however, only mature (CD3hi) medullary thymocytes bound S. nigra agglutinin. On mature thymocytes, S. nigra agglutinin appeared to bind primarily to the cell-surface glycoprotein CD45; since only the mature thymocytes expressed the CD45RA isoform, while both mature and immature populations expressed the CD45R0 isoform, CD45RA may be a preferred substrate for ST6N. These results demonstrate that glycoprotein sialylation is tightly regulated during T cell development and that the developmentally regulated expression of specific oligosaccharide structures on the cell surface may be influenced by expression of both the relevant glycosyltransferase and specific acceptor substrates.

Apoptosis of T cells mediated by galectin-1.

Galectin-1, a member of the family of beta-galactoside binding proteins, has growth regulatory and immunomodulatory activities. We report here that galectin-1, expressed by stromal cells in human thymus and lymph nodes, is present at sites of cell death by apoptosis during normal T-cell development and maturation. Galectin-1 induced apoptosis of activated human T cells and human T leukaemia cell lines. Resting T cells also bound galectin-1, but did not undergo apoptosis. Human endothelial cells that expressed galectin-1 induced apoptosis of bound T cells. Galectin-1-induced apoptosis required expression of CD45, and was decreased when N-glycan elongation was blocked by treatment of the cells by swainsonine, whereas inhibition of O-glycan elongation potentiated the apoptotic effect of galectin-1. Induction of apoptosis by an endogenous mammalian lectin represents a new mechanism for regulating the immune response.

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.

Synthesis of an endogeneous lectin, galectin-1, by human endothelial cells is up-regulated by endothelial cell activation.

The pattern of expression of an endogenous lectin, galectin-1, was examined in human lymphoid tissue. Galectin-1 was detected in the endothelial cells lining specialized vessels, termed high endothelial venules, in activated lymphoid tissue, but not in a resting lymph node. Cultured endothelial cells (human aortic and umbilical vein endothelial cells (HAECs and HUVECs)) expressed galectin-1. Activation of the cultured endothelial cells increased the level of galectin-1 expression, as determined by ELISA. Northern blot analysis and high throughput cDNA sequencing. These results suggest that galectin-1 expressed by endothelial cells may bind to and affect the trafficking of cells emigrating from blood into tissues.

Receptor specificity of influenza virus influences severity of illness in ferrets.

Weanling ferrets were inoculated intranasally with either wild-type or receptor-variant clones of influenza A/Memphis/102/72 to determine if changes in receptor specificity influence virulence of influenza virus infection. Over the 5 days after inoculation, receptor-variant inoculated ferrets had a lower mean elevation in body temperature, greater weight gain and less sneezing than the wild-type group. Influenza virus was recovered from the lungs of fewer receptor-variant infected ferrets (5/12 vs 11/12) and in lower titers than in wild-type infected ferrets at 5 days after inoculation. The viruses recovered from lung homogenates retained the same receptor specificity as the inoculum. Serum hemagglutination inhibition titers for the two groups were similar. These findings suggest that the receptor-variant clone is less virulent but elicits a similar immunogenic response compared with the wild-type clone.