Identification of the ligand-binding domains of CD22, a member of the immunoglobulin superfamily that uniquely binds a sialic acid-dependent ligand

CD22 regulates B lymphocyte function in vivo through both ligand-dependent and ligand-independent mechanisms

The interaction of CD22 with alpha2,6-linked sialic acid ligands has been widely proposed to regulate B lymphocyte function and migration. Here, we generated gene-targeted mice that express mutant CD22 molecules that do not interact with these ligands. CD22 ligand binding regulated the expression of cell surface CD22, immunoglobulin M and major histocompatibility complex class II on mature B cells, maintenance of the marginal zone B cell population, optimal B cell antigen receptor-induced proliferation, and B cell turnover rates. However, CD22 negative regulation of calcium mobilization after B cell antigen receptor ligation, CD22 phosphorylation, recruitment of SHP-1 to CD22 and B cell migration did not require CD22 ligand engagement. These observations resolve longstanding questions regarding the physiological importance of CD22 ligand binding in the regulation of B cell function in vivo.

CD22 as a target of passive immunotherapy

CD22 is a 135-kd B-cell restricted sialoglycoprotein present in the cytoplasm of virtually all B-lineage cells but expressed on the B-cell surface only at mature stages of differentiation. In humans, the vast majority of IgM(+)IgD(+) B cells express cell-surface CD22, while in lymphoid tissues CD22 expression is high in follicular mantle and marginal zone B cells and weak in germinal center B cells. In B-cell malignancies, CD22 expression ranges from 60% to 80% depending on the histological type and on the assays used. The function of the CD22 molecule is uncertain, although recent studies have suggested roles for the molecule both as a component of the B-cell activation complex and as an adhesion molecule. CD22-deficient mice have a reduced number of mature B cells in the bone marrow and circulation; the B cells have a shorter lifespan and enhanced apoptosis, thus indicating a key role of this antigen in B-cell development/survival. After binding with its natural ligand(s) or antibodies, CD22 is rapidly internalized; this provides a potent costimulatory signal in primary B-cell and proapoptotic signals in neoplastic B cells. Preclinically CD22 has been shown to be an effective target for immunotherapy of B-cell malignancies using either "naked" or toxin-labeled or radiolabeled monoclonal antibodies. Clinical trials in patients with non-Hodgkin's lymphoma (NHL) (both indolent and aggressive disease) are now ongoing with a humanized naked anti-CD22 antibody (epratuzumab, Amgen Inc, thousand Oaks, CA and Immunomedics Inc, Morris Plains, NJ) used as single agent or in combination with other monclonal antibodies (ie, rituximab) and/or chemotherapy. Preliminary data from these studies showed these approaches to be effective and well-tolerated.

Human Siglec-5: tissue distribution, novel isoforms and domain specificities for sialic acid-dependent ligand interactions

Human Siglec-5 is a sialic acid binding immunoglobulin (Ig)-like lectin (Siglec), comprising one N-terminal IgV-SET domain followed by three IgC2-SET domains, and a cytoplasmic domain with ITIM and SAP motifs which regulate cell signalling. We report the differential distribution of hSiglec-5 on neutrophil and macrophage subsets in tissues using monoclonal antibodies, 1A5 and 2H8, which require the first IgC2-SET domain for binding. Interestingly, hSiglec-5 was especially prominent on macrophages in reactive lymph nodes. We have identified four isoforms of hSiglec-5 possessing three (hSiglec-5-3L and -3C) or four (hSiglec-5-4L and -4S) extracellular domains linked to long (hSiglec-5-3L and -4L) or short (hSiglec-5-4S) cytoplasmic tails or existing as a soluble isoform (hSiglec-5-3C). hSiglec-5-4L has the broadest tissue distribution, being detected in adult spleen, thymus, lymph node, peripheral blood leucocytes and bone marrow, and in fetal lung and liver. A soluble Fc chimaeric protein containing the hSiglec-5-4L extracellular domain binds in a sialic acid-dependent manner to glycophorin A on human erythrocytes and to alpha2-3- and alpha2-6-sialyllactose moieties. Domain deletion mutants of hSiglec-5(D1-4)-Fc reveal that the first three IgC2-SET domains are required for optimal binding, with adhesion being abolished if the first IgC2-SET domain is deleted. This indicates that each hSiglec-5 isoform will interact with sialic acid ligands and provides the first step towards defining structure-function relationships of hSiglec-5 isoforms.

Cell adhesion and polarity during immune interactions

Intercellular interactions are critical for a coordinated function of different cell types involved in the immune response. Here we review the cellular and molecular events occurring during cell-cell immune contacts. Cognate naïve CD4+ T lymphocyte-dendritic cell (DC) and primed T cell-antigen-presenting B lymphocyte interactions are discussed. The engagement of cytotoxic T lymphocytes (CTL) or natural killer cells (NK) with their targets is analyzed and compared to the process of T cell-antigen-presenting cell (APC) conjugate formation. The immunological synapse, a complex cluster of molecules organized at the contact area of cell conjugates, exhibits common features but shows some differences depending on cell types involved. Cellular interactions occur in sequential stages that involve dramatic changes in cell polarity and dynamic redistribution of cell membrane receptors. The role of membrane microdomains, adaptor molecules and the cytoskeleton in the regulation of the molecular reorganization at cell-cell contacts is also discussed.

CD19, CD21, and CD22: multifaceted response regulators of B lymphocyte signal transduction

B lymphocyte development and function depend upon the activity of intrinsic and B cell antigen receptor (BCR)-induced signals. These signals are interpreted, amplified, fine-tuned, or suppressed through the precise actions of specialized cell surface coreceptors, or "response regulators," that inform B cells of their extracellular environment. Important cell surface response regulators include the CD19/CD21 complex, CD22, and CD72. CD19 establishes a novel Src-family protein tyrosine kinase (PTK) amplification loop that regulates basal signaling thresholds and intensifies Src-family PTK activation following BCR ligation. In turn, CD22 limits the intensity of CD19-dependent, BCR-generated signals through the recruitment of potent phosphotyrosine and phosphoinositide phosphatases. Herein we discuss our current understanding of how CD19/CD21 and CD22 govern the emergence and intensity of BCR-mediated signals, and how alterations in these tightly controlled regulatory activities contribute to autoimmunity in mice and humans.

Molecular characterization and expression of a novel human leukocyte cell-surface marker homologous to mouse Ly-9

Ly-9 is a mouse cell-surface glycoprotein that is selectively expressed on thymocytes and on mature T and B lymphocytes. Ly-9 belongs to the CD2 subset of the immunoglobulin superfamily, an emerging family of cell signaling receptors. Recently, a partial human Ly-9 complementary DNA (cDNA) sequence has been described. Full-length cDNA clones were isolated that included the initiation codon, the sequence encoding the full signal peptide, and 14 amino acids more in the cytoplasmic domain than in the previously reported clone. The predicted extracellular domain of human Ly-9 contains 4 immunoglobulinlike domains, similar to those in mouse Ly-9. Northern blot analysis revealed that the human Ly-9 messenger RNA (2.6 kb) is expressed predominantly in lymph node, spleen, thymus, and peripheral blood leukocytes. Four monoclonal antibodies (mAbs) were raised against human Ly-9 by immunizing mice with the pre-B-cell line 300.19 stably transfected with human Ly-9 full-length cDNA. These mAbs strongly stained the surfaces of cells transfected with human Ly-9 cDNA but not of untransfected cells. Human Ly-9 expression was restricted to T and B lymphocytes and thymocytes, with the highest levels of expression on CD4+CD8− and CD4−CD8+ thymocytes. Monocytes, granulocytes, platelets, and red blood cells were uniformly negative for Ly-9. These mAbs immunoprecipitated major polypeptides of 120 kd from the transfected cells and 120 kd and 100 kd from B-cell line Daudi, probably because of the cell-surface–expressed isoforms. These data demonstrate that human Ly-9 is a new marker for the study of normal and malignant leukocytes.

Cryptic sialic acid binding lectins on human blood leukocytes can be unmasked by sialidase treatment or cellular activation

We recently reported that the sialic acid-specific binding sites of CD22 molecules on B cells are masked by endogenous ligands, and can be unmasked by sialidase treatment or cellular activation. Here, we show that many other human blood leukocyte types have endogenous sialic acid binding sites that can be unmasked by sialidase treatment. Truncation of sialic acid side chains on the soluble probes used for detection abolishes all binding, indicating the specificity of the interaction for the details of sialic acid structure. There is limited overlap between alpha2-6- and alpha2-3-sialic acid-specific binding sites, which are unmasked on monocytes, natural killer cells, a minority of mature T cells, neutrophils, and some cultured human leukemic cell lines. Activation with phorbol ester and calcium ionophore causes spontaneous exposure of some of the binding sites, occurring over a period of minutes on neutrophils and several hours on monocytes and U937 leukemia cells. Activation is accompanied by some evidence for desialylation of cell surface molecules. Thus, many human blood cells have specific binding sites for sialic acids, masked by endogenous sialylated ligands. Cellular activation can unmask these sites, possibly by the action of an endogenous sialidase. The nearly universal masking of such sites in unactivated blood cells could explain why many of these sialic acid-binding lectins have not been previously discovered. Similar considerations may apply to sialic acid binding lectins of other cell types and tissues.

Masking and unmasking of the sialic acid-binding lectin activity of CD22 (Siglec-2) on B lymphocytes

CD22 is a B cell-restricted glycoprotein involved in signal transduction and modulation of cellular activation. It is also an I-type lectin (now designated Siglec-2), whose extracellular domain can specifically recognize alpha2-6-linked sialic acid (Sia) residues. This activity is postulated to mediate intercellular adhesion and/or to act as a coreceptor in antigen-induced B cell activation. However, studies with recombinant CD22 indicate that the lectin function can be inactivated by expression of alpha2-6-linked Sia residues on the same cell surface. To explore whether this masking phenomenon affects native CD22 on B cells, we first developed a probe to detect the lectin activity of recombinant CD22 expressed on Chinese hamster ovary cells (which have no endogenous alpha2-6-linked Sia residues). This probe is inactive against CD22-positive B lymphoma cells and Epstein-Barr virus-transformed lymphoblasts which express high levels of alpha2-6-linked Sia residues. Enzymatic desialylation unmasks the CD22 lectin activity, indicating that endogenous Sia residues block the CD22 lectin-binding site. Truncation of the side chains of cell surface Sia residues by mild periodate oxidation (known to abrogate Sia recognition by CD22) also had this unmasking effect, indicating that the effects of desialylation are not due to a loss of negative charge. Normal resting B cells from human peripheral blood gave similar findings. However, the lectin is partially unmasked during in vitro activation of these cells. Thus, the lectin activity of CD22 is restricted by endogenous sialylation in resting B cells and may be transiently unmasked during in vivo activation, perhaps to modulate intercellular or intracellular interactions at this critical stage in the humoral response.

Myelin-associated glycoprotein binding to gangliosides. Structural specificity and functional implications

Myelin-associated glycoprotein (MAG), which mediates certain myelin-neuron cell-cell interactions, is a lectin that binds to sialylated glycoconjugates. Gangliosides, the most abundant sialylated glycoconjugates in the brain, may be the functional neuronal ligands for MAG. Cells engineered to express MAG on their surface adhered specifically to gangliosides bearing an alpha 2,3-linked N-acetylneuraminic acid on a terminal galactose, with the following relative potency: GQ1b alpha >> GD1a, GT1b >> GM3, GM4 (GM1, GD1b, GD3, and GQ1b did not support adhesion). MAG binding was abrogated by modification of the carboxylic acid, any hydroxyl, or the N-acetyl group of the ganglioside's N-acetylneuraminic acid moiety. Related immunoglobulin (Ig) superfamily members either failed to bind gangliosides (CD22) or bound with less stringent specificity (sialoadhesin), whereas a modified form of MAG (bearing three of its five extra-cellular Ig-like domains) bound only GQ1b alpha. Enzymatic removal of sialic acids from the surface of intact nerve cells altered their functional interaction with myelin. These data are consistent with a role for gangliosides in MAG-neuron interactions.

Fcε Receptor I-Associated lyn-Dependent Phosphorylation of Fcγ Receptor IIB During Negative Regulation of Mast Cell Activation

FcγRIIB are low-affinity receptors for IgG whose intracytoplasmic domain contains an immunoreceptor tyrosine-based inhibition motif (ITIM). FcγRIIB inhibit cell activation triggered by receptors that signal via immunoreceptor tyrosine-based activation motifs. This inhibition requires ITIM tyrosyl phosphorylation and is correlated with the binding of SH2 domain-containing phosphatases that may mediate the inhibitory signal. In the present work, we investigated the mechanism of FcγRIIB phosphorylation and its consequences in mast cells. We demonstrate that the phosphorylation of FcγRIIB requires coaggregation with FcεRI and that, once phosphorylated, FcγRIIB selectively recruit the inositol polyphosphate 5 phosphatase SHIP, in vivo. In vitro, however, the phosphorylated FcγRIIB ITIM binds not only SHIP, but also the two protein tyrosine phosphatases, SHP-1 and SHP-2. We show that the coaggregation of FcγRIIB with FcεRI does not prevent FcεRI-mediated activation of lyn and syk. Both kinases can phosphorylate FcγRIIB in vitro. However, when coaggregated with FcεRI, FcγRIIB was in vivo phosphorylated in syk-deficient mast cells, but not in lyn-deficient mast cells. When FcεRI are coaggregated with FcγRIIB by immune complexes, FcεRI-associated lyn may thus phosphorylate FcγRIIB. By this mechanism, FcεRI initiate ITIM-dependent inhibition of intracellular propagation of their own signals.

CD84 Leukocyte Antigen Is a New Member of the Ig Superfamily

cDNA isolated from a human B-cell line Raji library was analyzed and shown to encode the full-length cDNA sequence of a novel cell-surface glycoprotein, initially termed HLy9-β. The predicted mature 307-amino acid protein was composed of two extracellular Ig-like domains, a hydrophobic transmembrane region, and an 83-amino acid cytoplasmic domain. The extracellular Ig-like domains presented structural and sequence homology with a group of members of the Ig superfamily that included CD2, CD48, CD58, and Ly9. Northern blot analysis showed that the expression of HLy9-β was predominantly restricted to hematopoietic tissues. Chromosome localization studies mapped the HLy9-β gene to chromosome 1q24, where other members of this Ig superfamily (CD48 and HumLy9) have been mapped. CD84 monoclonal antibodies (MoAbs) were shown to react with cells transfected with the cloned cDNA. These MoAbs were further used to show that CD84 is expressed as a single chain cell-surface glycoprotein of Mr 64,000 to 82,000, which was highly glycosylated. CD84 had a unique pattern of expression, being found predominantly on lymphocytes and monocytes. Thus, the glycoprotein HLy9-β is recognized by MoAbs previously clustered as CD84 and represents a newly identified member of the Ig superfamily that may play a significant role in leukocyte activation.

CD84 Leukocyte Antigen Is a New Member of the Ig Superfamily

cDNA isolated from a human B-cell line Raji library was analyzed and shown to encode the full-length cDNA sequence of a novel cell-surface glycoprotein, initially termed HLy9-β. The predicted mature 307-amino acid protein was composed of two extracellular Ig-like domains, a hydrophobic transmembrane region, and an 83-amino acid cytoplasmic domain. The extracellular Ig-like domains presented structural and sequence homology with a group of members of the Ig superfamily that included CD2, CD48, CD58, and Ly9. Northern blot analysis showed that the expression of HLy9-β was predominantly restricted to hematopoietic tissues. Chromosome localization studies mapped the HLy9-β gene to chromosome 1q24, where other members of this Ig superfamily (CD48 and HumLy9) have been mapped. CD84 monoclonal antibodies (MoAbs) were shown to react with cells transfected with the cloned cDNA. These MoAbs were further used to show that CD84 is expressed as a single chain cell-surface glycoprotein of Mr 64,000 to 82,000, which was highly glycosylated. CD84 had a unique pattern of expression, being found predominantly on lymphocytes and monocytes. Thus, the glycoprotein HLy9-β is recognized by MoAbs previously clustered as CD84 and represents a newly identified member of the Ig superfamily that may play a significant role in leukocyte activation.

The potential role of sialoadhesin as a macrophage recognition molecule in health and disease

Sialoadhesin is a macrophage-restricted transmembrane glycoprotein of 185 kDa that mediates cell-cell interactions through recognition of Neu5Ac alpha2,3Gal in glycoconjugates. The extracellular region of sialoadhesin is composed of seventeen immunoglobulin-like domains, of which the amino-terminal two are highly-related structurally and functionally to the amino-terminal domains of CD22, myelin associated glycoprotein and CD33. These proteins, collectively known as the sialoadhesin family, are able to mediate sialic acid-dependent binding with distinct specificities for both the type of sialic acid and its linkage to subterminal sugars. In this review we discuss our recent studies on sialoadhesin and suggest how this molecule may contribute to a range of macrophage functions, both under normal conditions as well as during inflammatory reactions.

Binding specificities of the sialoadhesin family of I-type lectins. Sialic acid linkage and substructure requirements for binding of myelin-associated glycoprotein, Schwann cell myelin protein, and sialoadhesin

The carbohydrate binding specificities of three sialoadhesins, a subgroup of I-type lectins (immunoglobulin superfamily lectins), were compared by measuring lectin-transfected COS cell adhesion to natural and synthetic gangliosides. The neural sialoadhesins, myelin-associated glycoprotein (MAG) and Schwann cell myelin protein (SMP), had similar and stringent binding specificities. Each required an alpha2,3-linked sialic acid on the terminal galactose of a neutral saccharide core, and they shared the following rank-order potency of binding: GQ1balpha >> GD1a = GT1b >> GM3 = GM4 >> GM1, GD1b, GD3, GQ1b (nonbinders). In contrast, sialoadhesin had less exacting specificity, binding to gangliosides that bear either terminal alpha2,3- or alpha2,8-linked sialic acids with the following rank-order potency of binding: GQ1balpha > GD1a = GD1b = GT1b = GM3 = GM4 > GD3 = GQ1b >> GM1 (nonbinder). CD22 did not bind to any ganglioside tested. Binding of MAG, SMP, and sialoadhesin was abrogated by chemical modification of either the sialic acid carboxylic acid group or glycerol side chain on a target ganglioside. Synthetic ganglioside GM3 derivatives further distinguished lectin binding specificities. Deoxy and/or methoxy derivatives of the 4-, 7-, 8-, or 9-position of sialic acid attenuated or eliminated binding of MAG, as did replacement of the sialic acid acetamido group with a hydroxyl. In contrast, the 4- and 7-deoxysialic acid derivatives supported sialoadhesin binding at near control levels (the other derivatives did not support binding). These data are consistent with sialoadhesin binding to one face of the sialic acid moiety, whereas MAG (and SMP) may have more complex binding sites or may bind sialic acids only in the context of more restricted oligosaccharide conformations.

Co-receptors of B lymphocytes

The past year has seen advances in our understanding of accessory membrane proteins that modulate the B cell response to antigen-receptor stimulation. The generation of complement receptor deficient mice has reinforced our appreciation of the importance of complement receptors in the B cell response to antigen. The association of inositol polyphosphate 5-phosphatase with FcgammaRIIB suggests another mechanism, in addition to recruitment of the phosphotyrosine phosphatase SHP-1, by which secreted immunoglobulin can limit further response to antigen. The in vivo function of CD22 in regulating the threshold of antigen-receptor signalling has been shown using CD22-deficient mice. Lastly, B cell receptor signalling in the B-1 subset of B lymphocytes has been demonstrated to be negatively regulated by CD5.

The advantage of residualizing radiolabels for targeting B-cell lymphomas with a radiolabeled anti-CD22 monoclonal antibody

CD22 antibodies (Abs) bound to B-cell lymphomas are known to be internalized and catabolized rapidly. Therefore, it would be expected that use of CD22 as a target for radioimmunotherapy should be enhanced by the use of "residualizing" radiolabels, which are trapped within the cell after catabolism of the Ab to which they had been conjugated. Our study was intended to evaluate this hypothesis using Ab LL2. In initial experiments, we found that LL2 binding was strongly temperature dependent, with approximately 15-fold greater binding at 37 degrees C than at 0 degrees C. A series of experiments suggested that this difference is due to a conformational change in the antigen at low temperature, so that the LL2 epitope is partially blocked. In vitro, residualizing labels-including 125I-dilactitol tyramine and 111In-DTPA-were retained by cells much longer than a conventional iodine label. In vivo, residualizing labels also showed a marked advantage in terms of uptake by Ramos B-cell lymphoma xenografts in nude mice. However, the absolute Ab uptake by xenografts was quite low, in comparison with results obtained with many carcinoma xenografts, which appears to be due in part to vascular properties of the B-cell lymphoma xenografts.

Animal lectins

Protein and lipid glycosylation is no longer considered as a topic whose appeal is restricted to a limited number of analytical experts perseveringly pursuing the comprehensive cataloguing of structural variants. It is in fact arousing curiosity in various areas of basic and applied bioscience. Well founded by the conspicuous coding potential of the sugar part of cellular glycoconjugates which surpasses the storage capacity of oligonucleotide- or oligopeptide-based code systems, recognition of distinct oligosaccharide ligands by endogenous receptors, i.e. lectins and sugar-binding enzymes or antibodies, is increasingly being discovered to play salient roles in animal physiology. Having inevitably started with a descriptive stage, research on animal lectins has now undubitably reached maturity. Besides listing the current categories for lectin classification and providing presentations of the individual families and their presently delineated physiological significance, this review places special emphasis on tracing common structural and functional themes which appear to reverberate in nominally separated lectin and animal categories as well as lines of research which may come to fruition for medical sciences.

Sialic acids in molecular and cellular interactions

Sialic acids (Sias) are terminal components of many glycoproteins and glycolipids especially of higher animals. In this exposed position they contribute significantly to the structural properties of these molecules, both in solution and on cell surfaces. Therefore, it is not surprising that Sias are important regulators of cellular and molecular interactions, in which they play a dual role. They can either mask recognition sites or serve as recognition determinants. Whereas the role of Sias in masking and in binding of pathogens to host cells has been documented over many years, their role in nonpathological cellular interaction has only been shown recently. The aim of this chapter is to summarize our knowledge about Sias in masking, for example, galactose residues, and to review the progress made during the past few years with respect to Sias as recognition determinants in the adhesion of pathogenic viruses, bacteria, and protozoa, and particularly as binding sites for endogenous cellular interaction molecules. Finally, perspectives for future research on these topics are discussed.

CD22, a B lymphocyte-specific adhesion molecule that regulates antigen receptor signaling

The development of B lymphocytes is a highly regulated process that depends in part on lineage-specific cell surface molecules. In addition, transmembrane signals generated through the B cell antigen receptor and other surface molecules regulate B cell responses to foreign antigens. Recent studies reveal CD22 to be a functionally significant receptor during these processes. CD22 is first expressed in the cytoplasm of pro-B and pre-B cells, and on the surface as B cells mature to become IgD+. CD22 is a member of the Ig superfamily that serves as an adhesion receptor for sialic acid-bearing ligands expressed on erythrocytes and all leukocyte classes. In addition to its potential role as a mediator of intercellular interactions, signal transduction through CD22 can activate B cells and modulate antigen receptor signaling in vitro. CD22 signaling is mediated via interactions with a number of kinases and phosphatases that bind the cytoplasmic domain through phosphorylated tyrosine residues located within consensus TAM and TIM motifs. The phenotype of CD22-deficient mice suggests that CD22 is primarily involved in the generation of mature B cells within the bone marrow, blood, and marginal zones of lymphoid tissues. Most notable in CD22-deficient mice is a significant diminution of surface Ig levels in these B cell subpopulations, which suggests that CD22 functions in vivo to adjust the signaling threshold of cell surface antigen receptors. A further understanding of CD22 function is required and may reveal roles for CD22 in disease susceptibility or the development of autoimmunity.

The Sialoadhesins--a family of sialic acid-dependent cellular recognition molecules within the immunoglobulin superfamily

For many years evidence has accumulated that sialic acids function in cellular interactions either by masking or as a recognition site. However, receptors or adhesion molecules mediating such functions between eukaryotic cells were unknown until about 5 years ago, when it was found that the members of the Selectin family mediate adhesion of leukocytes to specific endothelia through binding to sialylated glycans like sialyl Lewis. More recently, the Sialoadhesin family of sialic acid-dependent adhesion molecules was defined within the superfamily of immunoglobulin-like molecules. So far, it has been shown that sialoadhesin (Sn), CD22, CD33, the myelin-associated glycoprotein (MAG) and the Schwann cell myelin protein (SMP) belong to this family. In contrast to the Selectins, these proteins are associated with diverse biological processes, i.e. hemopoiesis, neuronal development and immunity. In this review their properties, carbohydrate specificities and potential biological functions are discussed. Finally, we provide perspectives with respect to the nature of ligands, implications of sialic acid modifications and future research.

Carbohydrate recognition systems: functional triads in cell-cell interactions

Considerable progress is being made in our understanding of the molecular basis for mammalian carbohydrate recognition systems. Selectins, related proteins and sialoadhesins are carbohydrate-binding proteins which serve as receptors in the orchestration of innate and acquired immune responses, inflammation and other forms of cell-cell communication. Protein structural studies and gain-of-function and loss-of-function mutations are providing clues to ways in which the receptors interact with monosaccharide elements of the oligosaccharide ligands. Binding experiments using oligosaccharides on lipid or protein carriers indicate that modes of presentation such as the clustered state and the manner of display on proteins are crucial factors determining whether a functional triad of receptor and ligand + carrier (counter-receptor) is formed.

A single N-linked glycosylation site is implicated in the regulation of ligand recognition by the I-type lectins CD22 and CD33

CD22 is an immunoglobulin superfamily B lymphocyte-specific adhesion receptor and a member of the recently identified I-type class of lectins. Recent work has shown that CD22 specifically recognizes sialic acid linked alpha2,6 to terminal N-linked oligosaccharides on selected cell surface glycoproteins. CD22-ligand interaction is regulated by the activity of a beta-galactoside alpha2, 6-sialyltransferase that can inactivate CD22-mediated binding by sialylating the CD22 receptor itself. These observations suggest that N-linked glycosylation sites on the CD22 molecule may play a role in the regulation of CD22-mediated adhesion. In this work we have performed site-specific mutagenesis of potential N-linked glycosylation sites on CD22 in an effort to determine whether they might be involved in ligand recognition. We show that mutation of a single potential N-linked glycosylation site in the first immunoglobulin domain of CD22 completely abrogates ligand recognition. Interestingly, this site is characterized by the sequence NCT, where the cysteine is thought to be involved in an intrachain disulfide bond. Site-directed mutagenesis of similar NC(T/S) motifs in the first or second Ig domains of the I-type lectins myelin-associated glycoprotein, and sialoadhesin did not disrupt their ability to mediate sialic acid binding. In contrast, mutation of a NCS motif in the first Ig domain of the I-type lectin CD33 unmasked its sialic acid binding activity. These observations suggest that a single N-linked glycosylation site located at a similar position in the CD22 and CD33 glycoproteins is critical for regulating ligand recognition by both receptors.

Differential expression of CD22 (Lyb8) on murine B cells

Previous studies have established the distribution, biochemistry and functional attributes of human CD22, a B cell-restricted glycoprotein. Recently, molecular cloning of the murine CD22 equivalent revealed this molecule to be the same as the previously described Lyb8 alloantigen. Using the anti-Lyb8 mAb Cy34.1.2, the present report documents the expression patterns of CD22 within the murine B cell compartment. The results demonstrate that in the bone marrow, murine CD22 is absent on the surface of pro-B cells, pre-B cells and newly emerging IgM+ B cells. CD22 is present at a low density on immature IgMhi B cells and fully expressed on mature recirculating B cells. In the periphery, murine CD22 is expressed at mature levels on all B cell subsets including follicular, marginal zone, B1 and switched B cells. Further studies showed CD22 to be retained on activated murine B cells for extended periods. Finally, in combination with CD23 and heat stable antigen, CD22 can be used to delineate the immature splenic B cells, and distinguish them from follicular and marginal zone cells. Together, the results demonstrate murine CD22 to be a useful pan marker for all mature B cell subsets.

Activation of B lymphocytes: integrating signals from CD19, CD22 and Fc gamma RIIb1

Three accessory membrane proteins, CD19, CD22 and Fc gamma RIIb1, alter signaling through membrane immunoglobulin of B cells by binding cytosolic proteins containing SH2 domains. Recent biochemical and genetic studies have shown that these receptors enable B cells to amplify responses to certain T-cell-dependent antigens (CD19), to restrict their response to T-cell zones of secondary lymphoid organs (CD22), and to dampen their response to antigens for which IgG is already available (Fc gamma RIIb1).

Lec32 is a new mutation in Chinese hamster ovary cells that essentially abrogates CMP-N-acetylneuraminic acid synthetase activity

LEC29.Lec32 is a glycosylation mutant that was isolated from a selection of mutagenized Chinese hamster ovary (CHO) cells for lectin resistance. Compared with LEC29 CHO cells, the double mutant exhibited an unusually high sensitivity to the toxic lectin, ricin, indicating increased exposure of galactose residues on cell surface carbohydrates. Structural analysis of LEC29.Lec32 cellular glycoproteins showed a nearly complete lack of sialic acid residues. Genetic analysis demonstrated that the lec32 mutation is recessive and novel. Biochemical analysis showed that the mutant cells contained less than 5% of the cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-NeuAc) present in parental CHO cells (1.6 nmol/mg of cell protein). A sensitive radiochemical assay used to measure CMP-NeuAc synthetase activity showed that the properties of this enzyme in parental CHO cells were essentially identical to those of CMP-NeuAc synthetase in various mammalian tissues. However, no CMP-NeuAc synthetase activity was detected in LEC29.Lec32 extracts. Mixing experiments provided no evidence for an inhibitor in the mutant CHO cells, and two revertants, which expressed only the LEC29 phenotype, had normal CMP-NeuAc synthetase levels. The combined evidence indicates that the lec32 mutation resides in either the structural gene encoding CMP-NeuAc synthetase or in a gene that regulates the production of active enzyme.

The amino-terminal immunoglobulin-like domain of sialoadhesin contains the sialic acid binding site. Comparison with CD22

Sialoadhesin and CD22 are members of a recently characterized family of sialic acid-dependent adhesion molecules belonging to the immunoglobulin superfamily. Sialoadhesin is a macrophage-restricted receptor containing 17 extracellular Ig-like domains which recognizes oligosaccharides terminating in NeuAc alpha 2-3Gal in N- and O-linked glycans. CD22 is a B cell-restricted receptor with seven Ig-like domains which selectively recognizes oligosaccharides terminating in NeuAc alpha 2-6Gal in N-glycans. Sequence similarity between these proteins is highest within their first four amino-terminal Ig-like domains. Here we identify the domain(s) containing the binding sites of both molecules by generating a series of extracellular domain deletion mutants fused to the Fc portion of human IgG1. Binding activity was analyzed by solid phase cell adhesion assays and also by surface plasmon resonance using purified glycophorin and CD45 as ligands for sialoadhesin and CD22, respectively. For sialoadhesin, the amino-terminal V-set Ig-like domain was both necessary and sufficient to mediate sialic acid-dependent adhesion of the correct specificity. In contrast, for murine CD22, only constructs containing both the V-set domain and the adjacent C2-set domain were able to mediate sialic acid-dependent binding. These results are consistent with the sialic acid binding site for both proteins residing in the membrane distal V-set domain, but for CD22 a direct contribution in binding from the neighboring C2-set domain cannot be excluded.