The sialic acid-Siglec immune checkpoint: an opportunity to enhance immune responses and therapy effectiveness in melanoma.

Modulation of immune responses through immune checkpoint blockade has revolutionized cutaneous melanoma treatment. However, it is still the case that not all patients respond successfully to these therapies, indicating the presence of as yet unknown resistance mechanisms. Hence, it is crucial to find novel targets to improve therapy efficacy. One of the described resistance mechanisms is regulated by immune inhibitory Siglec receptors, which are engaged by the carbohydrates sialic acids expressed on tumour cells, contributing to programmed cell death protein-1 (PD1)-like immune suppression mechanisms. In this review, we provide an overview on the regulation of sialic acid synthesis, its expression in melanoma, and the contribution of the sialic acid-Siglec axis to tumour development and immune suppressive mechanisms in the tumour microenvironment. Finally, we highlight potential sialic acid-Siglec axis-related therapeutics to improve the treatment of melanoma.

Deciphering the Chemical Language of the Immunomodulatory Properties of Veillonella parvula Lipopolysaccharide.

Veillonella parvula, prototypical member of the oral and gut microbiota, is at times commensal yet also potentially pathogenic. The definition of the molecular basis tailoring this contrasting behavior is key for broadening our understanding of the microbiota-driven pathogenic and/or tolerogenic mechanisms that take place within our body. In this study, we focused on the chemistry of the main constituent of the outer membrane of V. parvula, the lipopolysaccharide (LPS). LPS molecules indeed elicit pro-inflammatory and immunomodulatory responses depending on their chemical structures. Herein we report the structural elucidation of the LPS from two strains of V. parvula and show important and unprecedented differences in both the lipid and carbohydrate moieties, including the identification of a novel galactofuranose and mannitol-containing O-antigen repeating unit for one of the two strains. Furthermore, by harnessing computational studies, in vitro human cell models, as well as lectin binding solid-phase assays, we discovered that the two chemically diverse LPS immunologically behave differently and have attempted to identify the molecular determinant(s) governing this phenomenon. Whereas pro-inflammatory potential has been evidenced for the lipid A moiety, by contrast a plausible "immune modulating" action has been proposed for the peculiar O-antigen portion.

Synthetic Zwitterionic Streptococcus pneumoniae Type 1 Oligosaccharides Carrying Labile O-Acetyl Esters.

We herein report the first total synthesis of the Streptococcus pneumoniae serotype 1 (Sp1) oligosaccharide, a unique zwitterionic capsular polysaccharide carrying labile O-acetyl esters. The target oligosaccharides, featuring rare α-2,4-diamino-2,4,6-trideoxy galactose (AAT) and α-galacturonic acids, were assembled up to the 9-mer level, in a highly stereoselective manner using trisaccharide building blocks. The lability of the O-acetyl esters imposed a careful deprotection scheme to prevent migration and hydrolysis. The migration was investigated in detail at various pD values using NMR spectroscopy, to show that migration and hydrolysis of the C-3-O-acetyl esters readily takes place under neutral conditions. Structural investigation showed the oligomers to adopt a right-handed helical structure with the acetyl esters exposed on the periphery of the helix in close proximity of the neighboring AAT residues, thereby imposing conformational restrictions on the AATα1-4GalA(3OAc) glycosidic linkages, supporting the helical shape of the polysaccharide, that has been proposed to be critical for its unique biological activity.

Human Milk Oligosaccharide 2'-Fucosyllactose Inhibits Ligand Binding to C-Type Lectin DC-SIGN but Not to Langerin.

Human milk oligosaccharides (HMOs) and their most abundant component, 2'-Fucosyllactose (2'-FL), are known to be immunomodulatory. Previously, it was shown that HMOs and 2'-FL bind to the C-type lectin receptor DC-SIGN. Here we show, using a ligand-receptor competition assay, that a whole mixture of HMOs from pooled human milk (HMOS) and 2'-FL inhibit the binding of the carbohydrate-binding receptor DC-SIGN to its prototypical ligands, fucose and the oligosaccharide Lewis-B, (Leb) in a dose-dependent way. Interestingly, such inhibition by HMOS and 2'-FL was not detected for another C-type lectin, langerin, which is evolutionarily similar to DC-SIGN. The cell-ligand competition assay using DC-SIGN expressing cells confirmed that 2'-FL inhibits the binding of DC-SIGN to Leb. Molecular dynamic (MD) simulations show that 2'-FL exists in a preorganized bioactive conformation before binding to DC-SIGN and this conformation is retained after binding to DC-SIGN. Leb has more flexible conformations and utilizes two binding modes, which operate one at a time via its two fucoses to bind to DC-SIGN. Our hypothesis is that 2'-FL may have a reduced entropic penalty due to its preorganized state, compared to Leb, and it has a lower binding enthalpy, suggesting a better binding to DC-SIGN. Thus, due to the better binding to DC-SIGN, 2'-FL may replace Leb from its binding pocket in DC-SIGN. The MD simulations also showed that 2'-FL does not bind to langerin. Our studies confirm 2'-FL as a specific ligand for DC-SIGN and suggest that 2'-FL can replace other DC-SIGN ligands from its binding pocket during the ligand-receptor interactions in possible immunomodulatory processes.

Analysis of Synthetic Monodisperse Polysaccharides by Wide Mass Range Ultrahigh-Resolution MALDI Mass Spectrometry.

Carbohydrates, such as oligo- and polysaccharides, are highly abundant biopolymers that are involved in numerous processes. The study of their structure and functions is commonly based on a material that is isolated from complex natural sources. However, a more precise analysis requires pure compounds with well-defined structures that can be obtained from chemical or enzymatic syntheses. Novel synthetic strategies have increased the accessibility of larger monodisperse polysaccharides, posing a challenge to the analytical methods used for their molecular characterization. Here, we present wide mass range ultrahigh-resolution matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) as a powerful platform for the analysis of synthetic oligo- and polysaccharides. Synthetic carbohydrates 16-, 64-, 100-, and 151-mers were mass analyzed and characterized by MALDI in-source decay FT-ICR MS. Detection of fragment ions generated from glycosidic bond cleavage (or cross-ring cleavage) provided information of the monosaccharide content and the linkage type, allowing for the corroboration of the carbohydrate compositions and structures.

Synthesis of Asparagine Derivatives Harboring a Lewis X Type DC-SIGN Ligand and Evaluation of their Impact on Immunomodulation in Multiple Sclerosis.

The protein myelin oligodendrocyte glycoprotein (MOG) is a key component of myelin and an autoantigen in the disease multiple sclerosis (MS). Post-translational N-glycosylation of Asn31 of MOG seems to play a key role in modulating the immune response towards myelin. This is mediated by the interaction of Lewis-type glycan structures in the N-glycan of MOG with the DC-SIGN receptor on dendritic cells (DCs). Here, we report the synthesis of an unnatural Lewis X (LeX )-containing Fmoc-SPPS-compatible asparagine building block (SPPS=solid-phase peptide synthesis), as well as asparagine building blocks containing two LeX -derived oligosaccharides: LacNAc and Fucα1-3GlcNAc. These building blocks were used for the glycosylation of the immunodominant portion of MOG (MOG31-55 ) and analyzed with respect to their ability to bind to DC-SIGN in different biological setups, as well as their ability to inhibit the citrullination-induced aggregation of MOG31-55 . Finally, a cytokine secretion assay was carried out on human monocyte-derived DCs, which showed the ability of the neoglycopeptide decorated with a single LeX to alter the balance of pro- and anti-inflammatory cytokines, inducing a tolerogenic response.

Combining cross-coupling reaction and Knoevenagel condensation in the synthesis of glyco-BODIPY probes for DC-SIGN super-resolution bioimaging.

Lectins are involved in a wide range of carbohydrate mediated recognition processes. Therefore, the availability of highly performant fluorescent tools tailored for lectin targeting and able to efficiently track events related to such key targets is in high demand. We report here on the synthesis of the glyco-BODIPYs 1 and 2, based on the efficient combination of a Heck-like cross coupling and a Knoevenagel condensation, which revealed efficient in addressing lectins. In particular, glyco-BODIPY 1 has two glycosidase stable C-mannose residues, which act as DC-SIGN (dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin) targeting modules. By using live-cell fluorescence microscopy, we proved that BODIPY-mannose 1 was efficiently taken up by immune cells expressing DC-SIGN receptors. Super-resolution stimulated emission depletion (STED) microscopy further revealed that the internalized 1 localized in membranes of endosomes, proving that 1 is a reliable tool also in STED applications. Of note, glyco-BODIPY 1 contains an aryl-azido group, which allows further functionalization of the glycoprobe with bioactive molecules, thus paving the way for the use of 1 for tracking lectin-mediated cell internalization in diverse biological settings.

Synthesis and Antibody Binding Studies of Schistosome-Derived Oligo-α-(1-2)-l-Fucosides.

Schistosomiasis is caused by blood-dwelling parasitic trematodes of the genus Schistosoma and is classified by the WHO as the second most socioeconomically devastating parasitic disease, second only to malaria. Schistosoma expresses a complex array of glycans as part of glycoproteins and glycolipids that can be targeted by both the adaptive and the innate part of the immune system. Some of these glycans can be used for diagnostic purposes. A subgroup of schistosome glycans is decorated with unique α-(1-2)-fucosides and it has been shown that these often multi-fucosylated fragments are prime targets for antibodies generated during infection. Since these α-(1-2)-fucosides cannot be obtained in sufficient purity from biological sources, we set out to develop an effective route of synthesis towards α-(1-2)-oligofucosides of varying length. Here we describe the exploration of two different approaches, starting from either end of the fucose chains. The oligosaccharides have been attached to gold nanoparticles and used in an enzyme-linked immunosorbent assay ELISA and a microarray format to probe antibody binding. We show that binding to the oligofucosides of antibodies in sera of infected people depends on the length of the oligofucose chains, with the largest glycans showing most binding.

Carbohydrate-based adjuvants.

Carbohydrate adjuvants are safe and biocompatible compounds usable as sustained delivery systems and stimulants of ongoing humoral and cellular immune responses, being especially suitable for the development of vaccines against intracellular pathogens where alum is useless. The development of new adjuvants is difficult and expensive, however, in the last two years, seven new carbohydrate-based adjuvants have been patented, also there are twelve ongoing clinical trials of vaccines that contain carbohydrate-based adjuvants, as well as numerous publications on their mechanism of action and safety. More research is necessary to improve the existent adjuvants and develop innovative ones.

The Core Fucose on an IgG Antibody is an Endogenous Ligand of Dectin-1.

The core fucose, a major modification of N-glycans, is implicated in immune regulation, such as the attenuation of the antibody-dependent cell-mediated cytotoxicity of antibody drugs and the inhibition of anti-tumor responses via the promotion of PD-1 expression on T cells. Although the core fucose regulates many biological processes, no core fucose recognition molecule has been identified in mammals. Herein, we report that Dectin-1, a known anti-ß-glucan lectin, recognizes the core fucose on IgG antibodies. A combination of biophysical experiments further suggested that Dectin-1 recognizes aromatic amino acids adjacent to the N-terminal asparagine at the glycosylation site as well as the core fucose. Thus, Dectin-1 appears to be the first lectin-like molecule involved in the heterovalent and specific recognition of characteristic N-glycans on antibodies.

Loading dendritic cells with gold nanoparticles (GNPs) bearing HIV-peptides and mannosides enhance HIV-specific T cell responses.

Gold nanoparticles (GNPs) decorated with glycans ameliorate dendritic cells (DC) uptake, antigen-presentation and T-cells cross-talk, which are important aspects in vaccine design. GNPs allow for high antigen loading, DC targeting, lack of toxicity and are straightforward prepared and easy to handle. The present study aimed to assess the capacity of DC to process and present HIV-1-peptides loaded onto GNPs bearing high-mannoside-type oligosaccharides (P1@HM) to autologous T-cells from HIV-1 patients. The results showed that P1@HM increased HIV-specific CD4+ and CD8+ T-cell proliferation and induced highly functional cytokine secretion compared with HIV-peptides alone. P1@HM elicits a highly efficient secretion of pro-TH1 cytokines and chemokines, a moderate production of pro-TH2 and significant higher secretion of pro-inflammatory cytokines such as TNF-α and IL-1ß. Thus, co-delivery of HIV-1 antigens and HM by GNPs is an excellent vaccine delivery system inducing HIV-specific cellular immune responses in HIV+ patients, being a promising approach to improve anti-HIV-1 vaccines.

Synthesis and evaluation of fluorescent Pam3Cys peptide conjugates.

Chirally pure R- and S-epimers of TLR2 ligand Pam3CysSK4 were prepared and separately conjugated to an OVA model epitope, in which lysine was replaced by azidonorleucine. The azide function in the conjugate permitted labelling with different fluorophores by use of strain-promoted 3+2 cycloaddition. The R-epimer of the labelled conjugates induced TLR2-dependent DC maturation, while S-epimer proved to be inactive. Combining the lipophilicity of Pam3CysSK4 ligand with fluorophores influenced the solubility of the resulting conjugates in an unpredictable way and only the conjugates labelled with Cy-5 were suitable for confocal fluorescence microscopy experiments. It was shown that both epimers of the Cy-5 labelled lipopeptides were internalized equally well, indicating TLR2-independent cellular uptake. The presented results demonstrate the usefulness of strain-promoted azide-alkyne cycloaddition in the labelling of highly lipophilic lipopeptides without disturbing the in vitro activity of these conjugates with respect to activation of TLR-2.

A quantitative study of the intracellular dynamics of fluorescently labelled glyco-gold nanoparticles via fluorescence correlation spectroscopy.

The dynamic behaviour of gold nanoparticles functionalised with glucose (Glc-Au NPs) has been studied here by means of fluorescence correlation spectroscopy (FCS). Meaningful data on the state of aggregation and dynamics of Glc-Au NPs fluorescently-labelled with HiLyte Fluor647 (Glc-Au-Hi NPs) in the intracellular environment were obtained. Moreover, the work presented here shows that FCS can be used to visualise the presence of single NPs or NP aggregates following uptake and to estimate, locally, NP concentrations within the cell. FCS measurements become possible after applying a "prebleaching" methodology, when the immobile NP fraction has been effectively removed and thus significant FCS data has been recorded. In this study, Glc-Au-Hi NPs have been incubated with HepG2 cells and their diffusion time in the intracellular environment has been measured and compared with their diffusion value in water and cell media.

Effective targeting of DC-SIGN by α-fucosylamide functionalized gold nanoparticles.

Dendritic Cells (DCs), the most potent antigen-presenting cells, play a critical role in the detection of invading pathogens, which are recognized also by multiple carbohydrate-specific receptors. Among them, DC-SIGN is one of the best characterized, with high-mannose and Lewis-type glycan specificity. In this study, we present a potent DC-SIGN targeting device developed using gold nanoparticles functionalized with α-fucosyl-ß-alanyl amide. The nanoparticles bound to cellular DC-SIGN and induced internalization as effectively as similar particles coated with comparable amounts of Lewis(X) oligosaccharide. They were found to be neutral toward dendritic cell maturation and IL-10 production, thus envisaging a possible use as targeted imaging tools and antigen delivery devices.

Glyconanoparticles as multifunctional and multimodal carbohydrate systems.

The quest for the construction of multivalent carbohydrate systems, with precise geometries that are highly efficient in interacting with carbohydrate binding proteins, has been a goal of synthetic chemists since the discovery of the multivalent nature of carbohydrate-mediated interactions. However, the control of the spatial and topological requirements for these systems is still a challenge. Glyconanoparticles (GNPs) are sugar-coated gold, iron oxide or semiconductor nanoparticles with defined thiol-ending glycosides that combine the multivalent presentation of carbohydrates (glycoclusters) with the special chemico-physical properties of the nano-sized metallic core. The possibility of attaching different types of carbohydrates and other molecules (such as luminescent probes, peptides, and magnetic chelates) onto the same gold nanoparticle in a controlled way (multifunctional GNPs), as well as modifying the core in order to obtain glyconanoparticles with magnetic or fluorescence properties (multimodal GNPs) makes this multivalent glyco-scaffold suitable for carrying out studies on carbohydrate-mediated interactions and applications in molecular imaging. In this review, we focus mainly on the rational design of glyconanoparticles as scaffolds for combining different ligands and survey the most recent examples of glyconanoparticles as both multivalent carbohydrate systems and probes for molecular imaging.

Glycosystems in nanotechnology: Gold glyconanoparticles as carrier for anti-HIV prodrugs.

The therapeutic approach for the treatment of HIV infection is based on the highly active antiretroviral therapy (HAART), a cocktail of antiretroviral drugs. Notwithstanding HAART has shown different drawbacks like toxic side effects and the emergence of viral multidrug resistance. Nanotechnology offers new tools to improve HIV drug treatment and prevention. In this scenario, gold nanoparticles are an interesting chemical tool to design and prepare smart and efficient drug-delivery systems. Here we describe the preparation and antiviral activity of carbohydrate-coated gold nanoparticles loaded with anti-HIV prodrug candidates. The nucleoside reverse transcriptase inhibitors abacavir and lamivudine have been converted to the corresponding thiol-ending ester derivatives and then conjugated to ~3 nm glucose-coated gold nanoparticles by means of "thiol-for-thiol" ligand place exchange reactions. The drugs-containing glyconanoparticles were characterized and the pH-mediated release of the drug from the nanoparticle has been determined. The antiviral activity was tested by evaluating the replication of NL4-3 HIV in TZM-bl infected cells. The proof-of-principle presented in this work aims to introduce gold glyconanoparticles as a new multifunctional drug-delivery system in the therapy against HIV.

Atomic-Level Dissection of DC-SIGN Recognition of Bacteroides vulgatus LPS Epitopes.

The evaluation of Bacteroides vulgatus mpk (BVMPK) lipopolysaccharide (LPS) recognition by DC-SIGN, a key lectin in mediating immune homeostasis, has been here performed. A fine chemical dissection of BVMPK LPS components, attained by synthetic chemistry combined to spectroscopic, biophysical, and computational techniques, allowed to finely map the LPS epitopes recognized by DC-SIGN. Our findings reveal BVMPK's role in immune modulation via DC-SIGN, targeting both the LPS O-antigen and the core oligosaccharide. Furthermore, when framed within medical chemistry or drug design, our results could lead to the development of tailored molecules to benefit the hosts dealing with inflammatory diseases.

Enhanced glycan nanoprofiling by weak anion exchange preparative chromatography, mild acid desialylation, and nanoliquid chromatography-mass spectrometry with nanofluorescence detection.

The structural characterization and quantification of the glycome of cells and glycoproteins is necessary for the understanding of glycan functions in Biology, the development of diagnostics tests, and the monitoring of glycoprotein pharmaceuticals. Classical N-glycan characterization methods involve enzymatic release followed by derivatization with a fluorochrome and separation by normal-phase HPLC. We have recently developed glycan nanoprofiling, a method for the simultaneous quantification and characterization of the N-glycans without the need of external standardization. Although glycan nanoprofiling allows the characterization of both neutral and sialylated glycans within the same chromatographic run, a significant drawback is the coelution of similar glycans when complex glycan mixtures are analyzed. To overcome this problem, we have developed enhanced glycan nanoprofiling. This new method introduces a weak anion-exchange HPLC separation step to fractionate glycans according to their sialic acid content followed by a mild acid desialylation. Glycans are then resolved by nano-LC-coupled ESI-MS with an intercalated nanofluorescence detector. Neutral glycans have a better analytical separation, better ionization profiles, and provide significantly higher MS signals allowing a detailed characterization of rare glycan species. Enhanced glycan nanoprofiling is a powerful approach that provides a fast and sensitive alternative to available N-glycan profiling methods.

Simple engineering of hybrid cellulose nanocrystal-gold nanoparticles results in a functional glyconanomaterial with biomolecular recognition properties.

Cellulose nanocrystal and gold nanoparticles are assembled, in a unique way, to yield a novel modular glyconanomaterial whose surface is then easily engineered with one or two different headgroups, by exploiting a robust click chemistry route. We demonstrate the potential of this approach by conjugating monosaccharide headgroups to the glyconanomaterial and show that the sugars retain their binding capability to C-type lectin receptors, as also directly visualized by cryo-TEM.

Schistosoma mansoni egg-derived thioredoxin and Sm14 drive the development of IL-10 producing regulatory B cells.

During chronic schistosome infections, a complex regulatory network is induced to regulate the host immune system, in which IL-10-producing regulatory B (Breg) cells play a significant role. Schistosoma mansoni soluble egg antigens (SEA) are bound and internalized by B cells and induce both human and mouse IL-10 producing Breg cells. To identify Breg-inducing proteins in SEA, we fractionated SEA by size exclusion chromatography and found 6 fractions able to induce IL-10 production by B cells (out of 18) in the high, medium and low molecular weight (MW) range. The high MW fractions were rich in heavily glycosylated molecules, including multi-fucosylated proteins. Using SEA glycoproteins purified by affinity chromatography and synthetic glycans coupled to gold nanoparticles, we investigated the role of these glycan structures in inducing IL-10 production by B cells. Then, we performed proteomics analysis on active low MW fractions and identified a number of proteins with putative immunomodulatory properties, notably thioredoxin (SmTrx1) and the fatty acid binding protein Sm14. Subsequent splenic murine B cell stimulations and hock immunizations with recombinant SmTrx1 and Sm14 showed their ability to dose-dependently induce IL-10 production by B cells both in vitro and in vivo. Identification of unique Breg cells-inducing molecules may pave the way to innovative therapeutic strategies for inflammatory and auto-immune diseases.