Wenlan Chen

Discovery of Glycosyltransferases Using Carbohydrate Arrays and Mass Spectrometry

Glycosyltransferases (GTs) catalyze the reaction between an activated sugar donor and an acceptor to form a new glycosidic linkage. GTs are responsible for the assembly of oligosaccharides in vivo and are also important for the in vitro synthesis of these biomolecules. However, the functional identification and characterization of new GTs are both difficult and tedious. This paper describes an approach that combines arrays of reactions on an immobilized array of acceptors with analysis by mass spectrometry to screen putative GTs. A total of 14,280 combinations of GT, acceptor and donor in four buffer conditions were screened and led to the identification and characterization of four new GTs. This work is significant because it provides a label-free method for the rapid functional annotation of putative enzymes.

Adaptability of the semi-invariant natural killer T-cell receptor towards structurally diverse CD1d-restricted ligands

The semi‐invariant natural killer (NK) T‐cell receptor (NKTcr) recognises structurally diverse glycolipid antigens presented by the monomorphic CD1d molecule. While the α‐chain of the NKTcr is invariant, the β‐chain is more diverse, but how this diversity enables the NKTcr to recognise diverse antigens, such as an α‐linked monosaccharide (α‐galactosylceramide and α‐galactosyldiacylglycerol) and the β‐linked trisaccharide (isoglobotriaosylceramide), is unclear. We demonstrate here that NKTcrs, which varied in their β‐chain usage, recognised diverse glycolipid antigens with a similar binding mode on CD1d. Nevertheless, the NKTcrs recognised distinct epitopic sites within these antigens, including α‐galactosylceramide, the structurally similar α‐galactosyldiacylglycerol and the very distinct isoglobotriaosylceramide. We also show that the relative roles of the CDR loops within the NKTcr β‐chain varied as a function of the antigen. Thus, while NKTcrs characteristically use a conserved docking mode, the NKTcr β‐chain allows these cells to recognise unique aspects of structurally diverse CD1d‐restricted ligands.

l-Rhamnose Antigen: A Promising Alternative to α-Gal for Cancer Immunotherapies

The targeting of autologous vaccines toward antigen presenting cells (APCs) via the in vivo complexation between anti α-Gal (anti-Gal) antibodies and α-Gal antigens presents a promising cancer immunotherapy with enhanced immunogenicity. This strategy takes advantage of the ubiquitous anti-Gal antibody in human serum. In contrast to the α-Gal epitope, the recent identification of high titers of anti-l-rhamnose (anti-Rha) antibodies in humans reveals a new approach toward immunotherapy employing l-rhamnose (Rha) monosaccharides. In order to evaluate this simple antigen in preclinical applications, we have synthesized Rha-conjugated immunogens and successfully induced high titers of anti-Rha antibodies in wildtype mice. Moreover, our studies demonstrate for the first time that wildtype mice could replace α1,3galactosyltransferase knockout (α1,3GT KO) mice in such antigen/antibody-mediated vaccine design when developing cancer immunotherapies.

Combining carbochips and mass spectrometry to study the donor specificity for the Neisseria meningitidis β1,3-N-acetylglucosaminyltransferase LgtA.

A library of 11 UDP-N-acetylglucosamine analogs were rapidly screened for their activities as donors for the Neisseria meningitidis β1,3-N-acetylglucosaminyltransferase (LgtA) by direct on-chip reaction and detection with SAMDI-TOF mass spectrometry. Six of the analogs were active in this assay and were analyzed by SAMDI to characterize the kinetics toward LgtA. The analysis revealed that substitutions on C-2, C-4, and C-6 affect the activity of the donors, with bulky groups at these positions decreasing affinity of the donors for the enzyme, and also revealed that activity is strongly affected by the stereochemistry at C-3, but not C-4, of the donor. The study is also significant because it demonstrates that SAMDI can be used to both profile glycosyltransferase activities and to provide a quantitative assessment of enzyme activity.

Efficient synthesis of galactosylceramide analogues for iNKT cell stimulation

Glycolipids are potential antigens for iNKT cells recognition and demonstrate important roles in both innate and adaptive immunity. However, the difficulties in the preparation of pure configuration defined glycolipids limit the exploration of their different profiles in activating iNKT cells. We report here a concise and stereospecific preparation of novel galactosylceramide analogues by oxime ligation. This strategy would provide an efficient way to generate varied glycolipid analogues with either synthetic or natural carbohydrates for biological evaluations.

Introduction of aromatic group on 4′-OH of α-GalCer manipulated NKT cell cytokine production

The glycosphingolipid α-GalCer has been found to influence mammalian immune system significantly through the natural killer T cells. Unfortunately, the pre-clinical and clinical studies revealed several critical disadvantages that prevented the therapeutic application of α-GalCer in treating cancer and other diseases. Recently, the detailed illustration of the CD1d/α-GalCer/NKT TCR complex crystal structural, together with other latest structural and biological understanding on glycolipid ligands and NKT cells, provided a new platform for developing novel glycolipid ligands with optimized therapeutic effects. Here, we designed a series of novel aromatic group substituted α-GalCer analogues. The biological activity of these analogues was characterized and the results showed the unique substitution group manipulated the immune responses of NKT cells. Computer modeling and simulation study indicated the analogues had unique binding mode when forming CD1d/glycolipid/NKT TCR complex, comparing to original α-GalCer.

The Roles of 3 ' and 4 ' Hydroxy Groups in alpha-Galactosylceramide Stimulation of Invariant Natural Killer T Cells

The marine-derived α-galactosylceramide is an exogenous ligand for natural killer T cells and leads to the secretion of both T help 1 (Th1) and Th2 cytokines. The relationship between the sugar moiety structure and invariant natural killer T (iNKT) cell stimulation ability has not been fully understood. With the series α-galactosylceramide analogues varied on C3′ and C4′ position, subjected to a murine system, we discovered that the 3′ hydroxyl is very crucial in maintaining the molecule’s immunogenicity. Any modification on this position will lead to the losing of activity. We also found that the C4′ position is not so sensitive and can tolerate some small modifications on it. Moreover, the C4′ substituted analogues induced biased Th2 cytokines release was observed.

Chemoenzymatic Synthesis of Uridine 5′-Diphospho-2-acetonyl-2-deoxy-α-d-glucose as C2-Carbon Isostere of UDP-GlcNAc

2-ketoGlc, which is the C(2)-carbon isostere of GlcNAc, is a novel GlcNAc analogue with a ketone group. The corresponding glycosyltransferase donor substrate, UDP-2-ketoGlc, is necessary for synthesizing 2-ketoGlc-containing molecules and is thus highly important for metabolic polysaccharide remodeling and engineering. We report here the first chemoenzymatic synthesis of UDP-2-ketoGlc using our two-enzyme (NahK and GlmU) system in vitro.

Erratum: Adaptability of the semi-invariant natural killer T-cell receptor towards structurally diverse CD1d-restricted ligands (The EMBO Journal (2009) 28 (3579-3590) DOI: 10.1038/emboj.2009.286

Department of Microbiology and Immunology, Vanderbilt University, School of Medicine, Nashville, TN, USA, Department of Chemistry and Biochemistry, Ohio State University, Columbus, OH, USA, National Jewish Centre for Allergy and Immunology Research, Denver, CO, USA, Division of Developmental Immunology, La Jolla Institute for Allergy and Immunology, La Jolla, CA, USA, Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, NY, USA, Department of Chemistry, University of Connecticut, Storrs, CT, USA, Department of Microbiology and Immunology, University of Melbourne, Melbourne, Victoria, Australia, Department of Biochemistry and Molecular Biology, Monash University, Melbourne, Victoria, Australia, Fox Chase Cancer Centre, Philadelphia, PA, USA and Division of Cell Biology, La Jolla Institute for Allergy and Immunology, La Jolla, CA, USA

Enhanced Antitumor Effects by Chemical Modified IGb3 Analogues

Certain glycolipid antigens for natural killer T (NKT) cells can direct the overall cytokine balance of the immune response. However, the molecular mechanism of Th1- or Th2-biased cytokine secretion by NKT cells is still unknown. Previously, we synthesized isoglobotrihexosylceramide (iGb3) analogues by introducing a hydroxyl group at C4 on the ceramide portion of iGb3 to produce 4-HO-iGb3 or to further deoxylation on the terminal galactose to produce 4‴-dh-iGb3. Both modified iGb3, especially 4‴-dh-iGb3, stimulated more IFN-γ production by hepatic NKT cells, and thus elicited preferential Th1 responses. Here, we found that 4‴-dh-iGb3–loaded bone marrow–derived dendritic cells (DC) could significantly inhibit growth of subcutaneous melanoma and suppress lung metastasis in C57BL/6 mice compared with unmodified iGb3-loaded DCs. In investigating the mechanisms of this improved activity, we found that 4‴-dh-iGb3 stimulation increased STAT1 signaling by NKT cells, whereas the phosphorylation of Th2 type cytokine–associated transcription factor STAT6 signaling was not affected. Analysis of the structures of iGb3 and 4‴-dh-iGb3 revealed that 4‴-dh-iGb3 provides greater stability and affinity between glycolipid and CD1d or NKT TCR complex than iGb3. Thus, 4‴-dh-iGb3 can improve the antitumor effects of a DC-based vaccine possibly by stabilizing the CD1d/glycolipid/TCR complex and stimulating IFN-γ signaling of NKT cells. Furthermore, chemical modification of iGb3 can elicit Th1-biased responses by NKT cells, and 4‴-dh-iGb3 combined with a DC vaccine may serve as a potent new NKT-based therapy against tumors and infectious diseases. Mol Cancer Ther; 10(8); 1375–84. ©2011 AACR.