Synthesis of a Candida albicans tetrasaccharide spanning the β1,2-mannan phosphodiester α-mannan junction

Synthesis of a unique mannose α-1-phosphate side chain moiety found in Candida auris cell wall mannan

Candida auris is an emerging fungal pathogen that has become a world-wide public health threat. While there have been numerous studies into the nature, composition and structure of the cell wall of Candida albicans and other Candida species, much less is known about the C. auris cell wall. We have shown that C. auris cell wall mannan contains a unique phosphomannan structure which distinguishes C. auris mannan from the mannans found in other fungal species. Specifically, C. auris exhibits two unique acid-labile mannose α-1-phosphate (Manα1PO4) sidechains that are absent in other fungal mannans and fungal pathogens. This unique mannan structural feature presents an opportunity for the development of vaccines, therapeutics, diagnostic tools and/or research reagents that target C. auris. Herein, we describe the successful synthesis and structural characterization of a Manα1PO4-containing disaccharide moiety that mimics the phosphomannan found in C. auris. Additionally, we present evidence that the synthetic Manα1PO4 glycomimetic is specifically recognized and bound by cell surface pattern recognition receptors, i.e. rhDectin-2, rhMannose receptor and rhMincle, that are known to play important roles in the innate immune response to C. auris as well as other fungal pathogens. The synthesis of the Manα1PO4 glycomimetic may represent an important starting point in the development of vaccines, therapeutics, diagnostics and research reagents which target a number of C. auris clinical strains. In addition, these data provide new insights and understanding into the structural biology of this unique fungal pathogen.

Synthesis of Biotin-Tagged Chitosan Oligosaccharides and Assessment of Their Immunomodulatory Activity

Chitin, a polymer of β-(1→4)-linked N-acetyl-d-glucosamine, is one of the main polysaccharide components of the fungal cell wall. Its N-deacetylated form, chitosan, is enzymatically produced in the cell wall by chitin deacetylases. It exerts immunomodulative, anti-inflammatory, anti-cancer, anti-bacterial, and anti-fungal activities with various medical applications. To study the immunobiological properties of chitosan oligosaccharides, we synthesized a series of β-(1→4)-linked N-acetyl-d-glucosamine oligomers comprising 3, 5, and 7 monosaccharide units equipped with biotin tags. The key synthetic intermediate employed for oligosaccharide chain elongation, a disaccharide thioglycoside, was prepared by orthogonal glycosylation of a 4-OH thioglycoside acceptor with a glycosyl trichloroacetimidate bearing the temporary 4-O-tert-butyldimethylsilyl group. The use of silyl protection suppressed aglycon transfer and provided a high yield for the target disaccharide donor. Using synthesized chitosan oligomers, as well as previously obtained chitin counterparts, the immunobiological relationship between these synthetic oligosaccharides and RAW 264.7 cells was studied in vitro. Evaluation of cell proliferation, phagocytosis, respiratory burst, and Th1, Th2, Th17, and Treg polarized cytokine expression demonstrated effective immune responsiveness and immunomodulation in RAW 264.7 cells exposed to chitin- and chitosan-derived oligosaccharides. Macrophage reactivity was accompanied by significant inductive dose- and structure-dependent protective Th1 and Th17 polarization, which was greater with exposure to chitosan- rather than chitin-derived oligosaccharides. Moreover, no antiproliferative or cytotoxic effects were observed, even following prolonged 48 h exposure. The obtained results demonstrate the potent immunobiological activity of these synthetically prepared chito-oligosaccharides.

Total Synthesis of Phospholipomannan of Candida albicans

First, total synthesis of the cell surface phospholipomannan anchor [β-Manp-(1 → 2)-β-Manp]_n-(1 → 2)-β-Manp-(1 → 2)-α-Manp-1 → P-(O → 6)-α-Manp-(1 → 2)-Inositol-1-P-(O → 1)-phytoceramide of Candida albicans is reported. The target phospholipomannan (PLM) anchor poses synthetic challenges such as the unusual kinetically controlled (1 → 2)-β-oligomannan domain, anomeric phosphodiester, and unique phytoceramide lipid tail linked to the glycan through a phosphate group. The synthesis of PLM anchor was accomplished using a convergent block synthetic approach using three main appropriately protected building blocks: (1 → 2)-β-tetramannan repeats, pseudodisaccharide, and phytoceramide-1-H-phosphonate. The most challenging (1 → 2)-β-tetramannan domain was synthesized in one pot using the preactivation method. The phytoceramide-1-H-phosphonate was synthesized through an enantioselective A³ three-component coupling reaction. Finally, the phytoceramide-1-H-phosphonate moiety was coupled with pseudodisaccharide followed by deacetylation to produce the acceptor, which on subsequent coupling with tetramannosyl-H-phosphonate provided the fully protected PLM anchor. Final deprotection was successfully achieved by Pearlman's hydrogenation.

Synthetic Phosphodiester-Linked 4-Amino-4-deoxy-l-arabinose Derivatives Demonstrate that ArnT is an Inverting Aminoarabinosyl Transferase

4‐Amino‐4‐deoxy‐l‐arabinopyranose (Ara4N) residues have been linked to antibiotic resistance due to reduction of the negative charge in the lipid A and core regions of the bacterial lipopolysaccharide (LPS). To study the enzymatic transfer of Ara4N onto lipid A, which is catalysed by the ArnT transferase, we chemically synthesised a series of anomeric phosphodiester‐linked lipid Ara4N derivatives containing linear aliphatic chains as well as E‐ and Z‐configured monoterpene units. Coupling reactions were based on sugar‐derived H‐phosphonates, followed by oxidation and global deprotection. The enzymatic Ara4N transfer was performed in vitro with crude membranes from a deep‐rough mutant from Escherichia coli as acceptor. Product formation was detected by TLC and LC‐ESI‐QTOF mass spectrometry. Out of seven analogues tested, only the α‐neryl derivative was accepted by the Burkholderia cenocepacia ArnT protein, leading to substitution of the Kdo₂‐lipid A acceptor and thus affording evidence that ArnT is an inverting glycosyl transferase that requires the Z‐configured double bond next to the anomeric phosphate moiety. This approach provides an easily accessible donor substrate for biochemical studies relating to modifications of bacterial LPS that modulate antibiotic resistance and immune recognition.

Importance of Candida Antigenic Factors: Structure-Driven Immunomodulation Properties of Synthetically Prepared Mannooligosaccharides in RAW264.7 Macrophages

The incidence and prevalence of serious fungal infections is rising, especially in immunosuppressed individuals. Moreover, co-administration of antibiotics and immunosuppressants has driven the emergence of new multidrug-resistant pathogens. The significant increase of multidrug-resistant pathogens, together with their ability to form biofilms, is associated with morbidity and mortality. Research on novel synthetically prepared immunomodulators as potential antifungal immunotherapeutics is of serious interest. Our study demonstrated the immunobiological activity of synthetically prepared biotinylated mannooligosaccharides mimicking Candida antigenic factors using RAW264.7 macrophages. Macrophage exposure to the set of eight structurally different mannooligosaccharides induced a release of Th1, Th2, Th17, and Treg cytokine signature patterns. The observed immune responses were tightly associated with structure, dose, exposure time, and selected signature cytokines. The viability/cytotoxicity of the mannooligosaccharide formulas was assessed based on cell proliferation. The structure-based immunomodulatory activity of the formulas was evaluated with respect to the length, branching and conformation of the various formulas. Glycoconjugate formulas with terminal β-mannosyl-units tended to be more potent in terms of Candida relevant cytokines IL-12 p70, IL-17, GM-CSF, IL-6, and TNFα induction and cell proliferation, and this tendency was associated with structural differences between the studied glycoconjugate formulas. The eight tested mannooligosaccharide conjugates can be considered potential in vitro immunomodulative agents suitable for in vitro Candida diagnostics or prospectively for subcellular anti-Candida vaccine design.

Toll-like receptor 4-related immunostimulatory polysaccharides: Primary structure, activity relationships, and possible interaction models

Toll-like receptor (TLR) 4 is an important polysaccharide receptor; however, the relationships between the structures and biological activities of TLR4 and polysaccharides remain unknown. Many recent findings have revealed the primary structure of TLR4/MD-2-related polysaccharides, and several three-dimensional structure models of polysaccharide-binding proteins have been reported; and these models provide insights into the mechanisms through which polysaccharides interact with TLR4. In this review, we first discuss the origins of polysaccharides related to TLR4, including polysaccharides from higher plants, fungi, bacteria, algae, and animals. We then briefly describe the glucosidic bond types of TLR4-related heteroglycans and homoglycans and describe the typical molecular weights of TLR4-related polysaccharides. The primary structures and activity relationships of polysaccharides with TLR4/MD-2 are also discussed. Finally, based on the existing interaction models of LPS with TLR4/MD-2 and linear polysaccharides with proteins, we provide insights into the possible interaction models of polysaccharide ligands with TLR4/MD-2. To our knowledge, this review is the first to summarize the primary structures and activity relationships of TLR4-related polysaccharides and the possible mechanisms of interaction for TLR4 and TLR4-related polysaccharides.

Oligosaccharides and peptide displayed on an amphiphilic polymer enable solid phase assay of hapten specific antibodies

Copovidone, a copolymer of vinyl acetate and N-vinyl-2-pyrrolidone, was synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization, and after deacetylation the polymer was functionalized by introduction of amino, azide, and alkyne pendant groups to allow attachment of glycans and peptide. Candida albicans β-mannan trisaccharides 1 and 2 and M. tuberculosis arabinan hexasaccharide 3 with appropriate tethers were conjugated to the polymers by squarate or click chemistry. C. albicans T-cell peptide 4 bearing a C-terminal ε-azidolysine was also conjugated to copovidone by click chemistry. The resulting conjugates provide convenient non-protein-based antigens that are readily adsorbed on ELISA plates, and display excellent characteristics for assay of antibody binding to the haptenic group of interest. Copovidone and BSA glycoconjugates exhibited similar adsorption characteristics when used to coat ELISA plates, and both conjugates were optimal when used as coating solutions at low nanogram/mL concentrations. Provided that the copovidone conjugated glycan is stable to acid, assay plates can be easily processed for reuse at least three times without detectable variation or degradation in ELISA readout.

Synthetically prepared glycooligosaccharides mimicking Candida albicans cell wall glycan antigens--novel tools to study host-pathogen interactions

The immunobiological efficacy of synthetically prepared mannooligosaccharides and a glucooligosaccharide mimicking the structure of Candida albicans cell wall glycans was assessed in vivo and in vitro to exploit immune responses. The exposure of mice splenocytes to BSA-based conjugates of synthetic oligomannosides and oligoglucoside revealed intense influence on T-cell subset polarization. The conjugates biased the immune responses towards Th1 and Th17 with respect to the prevalence of interferon-gamma (IFN-γ) and interleukin (IL)-17 (IL-17) over IL-4 and IL-10 levels. The inflammatory activity of the conjugates has been evaluated based on the induction of pro-inflammatory cytokines. Postvaccination, antimannooligosaccharide and antiglucooligosaccharide antisera were subjected to an evaluation of the structure-immunomodulation activity relationship. Clinical isolates of C. albicans CCY 29-3-32 and C. albicans CCY 29-3-164 were applied to study interactions between Candida cells and anti-oligosaccharide antibodies. In situ recognition of parietal oligomannosyl and oligoglucosyl sequences in C. albicans cell wall by the antisera raised against BSA-based conjugates of synthetic oligomannosides and oligoglucoside revealed the effective recognition of specific distribution of natural oligosaccharide sequences in the cell wall of C. albicans serotype A. With respect to these results, it can be concluded that new, synthetically prepared oligosaccharides mimicking Candida cell wall structures represent prospective immunobiologically effective components for further immunopharmacologically relevant Candida vaccine design.