CD22 is a sialic acid-binding immunoglobulin-like lectin (Siglec) that is highly expressed on B-cells and B cell lymphomas, and is a validated target for antibody and nanoparticle based therapeutics. However, cell targeted therapeutics are limited by their complexity, heterogeneity, and difficulties in production. We describe here a chemically defined natural N-linked glycan scaffold that displays high affinity CD22 glycan ligands and outcompetes the natural ligand for the receptor, resulting in single molecule binding to CD22 and endocytosis into cells. Binding affinity is increased by up to 1500-fold compared to the monovalent ligand, while maintaining the selectivity for hCD22 over other Siglecs. Conjugates of these multivalent ligands with auristatin and saporin toxins are efficiently internalized via hCD22 resulting in killing of B-cell lymphoma cells. This single molecule ligand targeting strategy represents an alternative to antibody- and nanoparticle-mediated approaches for delivery of agents to cells expressing CD22 and other Siglecs.

•All H3N2 influenza viruses recognize human-type receptors with extended glycan chains•Recent H3 and pandemic H1 hemagglutinins prefer extended, branched N-glycan receptors•Lipid-linked glycan receptors restore infectivity to receptor-deficient MDCK cells•Molecular dynamics simulation shows bidentate binding of N-glycans to one HA trimerHuman and avian influenza viruses recognize different sialic acid-containing receptors, referred to as human-type (NeuAcα2-6Gal) and avian-type (NeuAcα2-3Gal), respectively. This presents a species barrier for aerosol droplet transmission of avian viruses in humans and ferrets. Recent reports have suggested that current human H3N2 viruses no longer have strict specificity toward human-type receptors. Using an influenza receptor glycan microarray with extended airway glycans, we find that H3N2 viruses have in fact maintained human-type specificity, but they have evolved preference for a subset of receptors comprising branched glycans with extended poly-N-acetyl-lactosamine (poly-LacNAc) chains, a specificity shared with the 2009 pandemic H1N1 (Cal/04) hemagglutinin. Lipid-linked versions of extended sialoside receptors can restore susceptibility of sialidase-treated MDCK cells to infection by both recent (A/Victoria/361/11) and historical (A/Hong Kong/8/1968) H3N2 viruses. Remarkably, these human-type receptors with elongated branches have the potential to increase avidity by simultaneously binding to two subunits of a single hemagglutinin trimer.Download high-res image (191KB)Download full-size image

The siglec family of sialic acid-binding proteins are endocytic immune cell receptors that are recognized as potential targets for cell directed therapies. CD33 and CD22 are prototypical members and are validated candidates for targeting acute myeloid leukaemia and non-Hodgkin's lymphomas due to their restricted expression on myeloid cells and B-cells, respectively. While nanoparticles decorated with high affinity siglec ligands represent an attractive platform for delivery of therapeutic agents to these cells, a lack of ligands with suitable affinity and/or selectivity has hampered progress. Herein we describe selective ligands for both of these siglecs, which when displayed on liposomal nanoparticles, can efficiently target the cells expressing them in peripheral human blood. Key to their identification was the development of a facile method for chemo-enzymatic synthesis of disubstituted sialic acid analogues, combined with iterative rounds of synthesis and rapid functional analysis using glycan microarrays.

Sialosides on N- and O-linked glycoproteins play a fundamental role in many biological processes, and synthetic glycan probes have proven to be valuable tools for elucidating these functions. Though sialic acids are typically found α2-3- or α2-6-linked to a terminal nonreducing end galactose, poly-LacNAc extended core-3 O-linked glycans isolated from rat salivary glands and human colonic mucins have been reported to contain multiple internal Neu5Acα2-6Gal epitopes. Here, we have developed an efficient approach for the synthesis of a library of N- and O-linked glycans with multisialylated poly-LacNAc extensions, including naturally occurring multisialylated core-3 O-linked glycans. We have found that a recombinant α2-6 sialyltransferase from Photobacterium damsela (Pd2,6ST) exhibits unique regioselectivity and is able to sialylate internal galactose residues in poly-LacNAc extended glycans which was confirmed by MS/MS analysis. Using a glycan microarray displaying this library, we found that Neu5Acα2-6Gal specific influenza virus hemagglutinins, siglecs, and plant lectins are largely unaffected by adjacent internal sialylation, and in several cases the internal sialic acids are recognized as ligands. Polyclonal IgY antibodies specific for internal sialoside epitopes were elicited in inoculated chickens.

The Siglec family of sialic acid-binding proteins are differentially expressed on white blood cells of the immune system and represent an attractive class of targets for cell-directed therapy. Nanoparticles decorated with high-affinity Siglec ligands show promise for delivering cargo to Siglec-bearing cells, but this approach has been limited by a lack of ligands with suitable affinity and selectivity. Building on previous work employing solution-phase sialoside library synthesis and subsequent microarray screening, we herein report a more streamlined ‘on-chip’ synthetic approach. By printing a small library of alkyne sialosides and subjecting these to ‘on-chip’ click reactions, the largest sialoside analogue library to date was generated. Siglec-screening identified a selective Siglec-7 ligand, which when displayed on liposomal nanoparticles, allows for targeting of Siglec-7+ cells in peripheral human blood. In silico docking to the crystal structure of Siglec-7 provides a rationale for the affinity gains observed for this novel sialic acid analogue.

Invariant natural killer T (iNKT) cells induce a protective immune response triggered by foreign glycolipid antigens bound to CD1d on antigen-presenting cells (APCs). A limitation of using glycolipid antigens to stimulate immune responses in human patients has been the inability to target them to the most effective APCs. Recent studies have implicated phagocytic CD169+ macrophages as major APCs in lymph nodes for priming iNKT cells in mice immunized with glycolipid antigen in particulate form. CD169 is known as sialoadhesin (Sn), a macrophage-specific adhesion and endocytic receptor of the siglec family that recognizes sialic acid containing glycans as ligands. We have recently developed liposomes decorated with glycan ligands for CD169/Sn suitable for targeted delivery to macrophages via CD169/Sn-mediated endocytosis. Here we show that targeted delivery of a lipid antigen to CD169+ macrophages in vivo results in robust iNKT cell activation in liver and spleen using nanogram amounts of antigen. Activation of iNKT cells is abrogated in Cd169−/− mice and is macrophage-dependent, demonstrating that targeting CD169+ macrophages is sufficient for systemic activation of iNKT cells. When pulsed with targeted liposomes, human monocyte–derived dendritic cells expressing CD169/Sn activated human iNKT cells, demonstrating the conservation of the CD169/Sn endocytic pathway capable of presenting lipid antigens to iNKT cells.

Cell-specific delivery of therapeutic agents using ligand targeting is gaining interest because of its potential for increased efficacy and reduced side effects. The challenge is to develop a suitable ligand for a cell-surface receptor that is selectively expressed on the desired cell. Sialoadhesin (Sn, Siglec-1, CD169), a sialic acid-binding immunoglobulin-like lectin (Siglec) expressed on subsets of resident and inflammatory macrophages, is an attractive target for the development of a ligand-targeted delivery system. Here we report the development of a high-affinity and selective ligand for Sn that is an analogue of the natural ligand and is capable of targeting liposomal nanoparticles to Sn-expressing cells in vivo. An efficient in silico screen of a library of ∼8400 carboxylic acids was the key to identifying novel 9-N-acyl-substituted N-acetylneuramic acid (Neu5Ac) substituents as potential lead compounds. A small panel of targets were selected from the screen and synthesized to evaluate their affinities and selectivities. The most potent of these Sn ligands, 9-N-(4H-thieno[3,2-c]chromene-2-carbamoyl)-Neu5Acα2–3Galβ1–4GlcNAc (TCCNeu5Ac), was conjugated to lipids for display on a liposomal nanoparticle for evaluation of targeted delivery to cells. The TCCNeu5Ac liposomes were found to target liposomes selectively to cells expressing either murine or human Sn in vitro, and when administered to mice, they exhibited in vivo targeting to Sn-positive macrophages.

Myelin associated glycoprotein (Siglec-4) is a myelin adhesion receptor, that is, well established for its role as an inhibitor of axonal outgrowth in nerve injury, mediated by binding to sialic acid containing ligands on the axonal membrane. Because disruption of myelin–ligand interactions promotes axon outgrowth, we have sought to develop potent ligand based inhibitors using natural ligands as scaffolds. Although natural ligands of MAG are glycolipids terminating in the sequence NeuAcα2–3Galβ1–3(±NeuAcα2−6)GalNAcβ-R, we previously established that synthetic O-linked glycoprotein glycans with the same sequence α-linked to Thr exhibited ∼1000-fold increased affinity (∼1 μM). Attempts to increase potency by introducing a benzoylamide substituent at C-9 of the α2–3 sialic acid afforded only a two-fold increase, instead of increases of >100-fold observed for other sialoside ligands of MAG. Surprisingly, however, introduction of a 9-N-fluoro-benzoyl substituent on the α2–6 sialic acid increased affinity 80-fold, resulting in a potent inhibitor with a Kd of 15 nM. Docking this ligand to a model of MAG based on known crystal structures of other siglecs suggests that the Thr positions the glycan such that aryl substitution of the α2–3 sialic acid produces a steric clash with the GalNAc, while attaching an aryl substituent to the other sialic acid positions the substituent near a hydrophobic pocket that accounts to the increase in affinity.

Influenza virus binding to host cells and neutrophil trafficking to sites of inflammation are diverse aspects of biology mediated by receptor recognition of sialic acids that terminate glycans on cell surface glycoproteins and glycolipids. The first crystal structure of a mammalian sialyltransferase provides insights into the biosynthesis of the rich spectrum of sialic acid–containing glycans in the mammalian glycome.Sialic acids occupy the terminal position of glycan chains of glycoproteins and glycolipids and contribute to the huge range of glycan structures that mediate cell surface biology. They are recognized as ligands by mammalian glycan-binding proteins and as receptors for pathogens such as influenza and polyoma virus, bacteria and many bacterial toxins as they attach to the host cell1.

CD22, a negative regulator of B cell signaling, is a member of the siglec family that binds to α2-6-linked sialic acids on glycoproteins. Previous reports demonstrated that binding of multivalent sialoside probes to CD22 is blocked, or “masked,” by endogenous (cis) ligands, unless they are first destroyed by sialidase treatment. These results suggest that cis ligands on B cells make CD22 functionally unavailable for binding to ligands in trans. Through immunofluorescence microscopy, however, we observed that CD22 on resting B cells redistributes to the site of contact with other B or T lymphocytes. Redistribution is mediated by interaction with trans ligands on the opposing cell because it does not occur with ligand-deficient lymphocytes from ST6GalI-null mice. Surprisingly, CD45, proposed as both a cis and trans ligand of CD22, was not required for redistribution to sites of cell contact, given that redistribution of CD22 was independent of CD45 and was observed with lymphocytes from CD45-deficient mice. Furthermore, CD45 is not required for CD22 masking as similar levels of masking were observed in the WT and null mice. Comparison of the widely used sialoside–polyacrylamide probe with a sialoside–streptavidin probe revealed that the latter bound a subset of B cells without sialidase treatment, suggesting that cis ligands differentially impacted the binding of these two probes in trans. The combined results suggest that equilibrium binding to cis ligands does not preclude binding of CD22 to ligands in trans, and allows for its redistribution to sites of contact between lymphocytes.

The sialic-acid-binding immunoglobulin-like lectins (siglecs) comprise a family of receptors that are differentially expressed on leukocytes and other immune cells. The restricted expression of several siglecs to one or a few cell types makes them attractive targets for cell-directed therapies. The anti-CD33 (also known as Siglec-3) antibody gemtuzumab (Mylotarg™) is approved for the treatment of acute myeloid leukemia, and antibodies targeting CD22 (Siglec-2) are currently in clinical trials for treatment of B cell non-Hodgkins lymphomas and autoimmune diseases. Because siglecs are endocytic receptors, they are well suited for a ‘Trojan horse’ strategy, whereby therapeutic agents conjugated to an antibody, or multimeric glycan ligand, bind to the siglec and are efficiently carried into the cell. Although the rapid internalization of unmodified siglec antibodies reduces their utility for induction of antibody-dependent cellular cytotoxicity or complement-mediated cytotoxicity, antibody binding of Siglec-8, Siglec-9 and CD22 has been demonstrated to induce apoptosis of eosinophils, neutrophils and depletion of B cells, respectively. Here, we review the properties of siglecs that make them attractive for cell-targeted therapies.

•Natural mutations in H5N1 virus exhibit increased binding to human type receptors.•H5N1 variants with human type receptor specificity are transmissible in a ferret model.•Receptor specificity of H5N1 should be monitored for assessment of pandemic risk.The high pathogenicity of H5N1 viruses in sporadic infections of humans has raised concerns for its potential to acquire the ability to transmit between humans and emerge as a highly pathogenic pandemic virus. Because avian and human influenza viruses differ in their specificity for recognition of their host cell receptors, receptor specificity represents one barrier for efficient transmission of avian viruses in human hosts. Over the last century, each influenza virus pandemic has coincided with the emergence of virus with an immunologically distinct hemagglutinin exhibiting a ‘human-type’ receptor specificity, distinct from that of viruses with the same hemagglutinin circulating in zoonotic species. Recent studies suggest that it is possible for H5N1 to acquire human type receptor specificity, but this has not occurred in nature. This review covers what is known about the molecular basis for the switch between avian and human-type receptor specificity for influenza viruses that have successfully adapted to man, the potential for H5N1 to evolve to human-type receptor specificity and its relevance to pandemic risk.

The siglec family of sialic acid-binding proteins are endocytic immune cell receptors that are recognized as potential targets for cell directed therapies. CD33 and CD22 are prototypical members and are validated candidates for targeting acute myeloid leukaemia and non-Hodgkin's lymphomas due to their restricted expression on myeloid cells and B-cells, respectively. While nanoparticles decorated with high affinity siglec ligands represent an attractive platform for delivery of therapeutic agents to these cells, a lack of ligands with suitable affinity and/or selectivity has hampered progress. Herein we describe selective ligands for both of these siglecs, which when displayed on liposomal nanoparticles, can efficiently target the cells expressing them in peripheral human blood. Key to their identification was the development of a facile method for chemo-enzymatic synthesis of disubstituted sialic acid analogues, combined with iterative rounds of synthesis and rapid functional analysis using glycan microarrays.