Synthesis of an Undecasaccharide Featuring an Oligomannosidic Heptasaccharide and a Bacterial Kdo-lipid A Backbone for Eliciting Neutralizing Antibodies to Mammalian Oligomannose on the HIV-1 Envelope Spike.

Nino Trattnig,Paul Kosma,Ralph Pantophlet,Jean-François Bruxelle,Markus Blaukopf

doi:10.1021/jacs.9b02872

Abstract

Lipooligosaccharides (LOS) from the bacterium Rhizobium radiobacter Rv3 are structurally related to antigenic mammalian oligomannoses on the HIV-1 envelope glycoprotein spike that are targets for broadly neutralizing antibodies. Here, we prepared a hybrid structure of viral and bacterial epitopes as part of a vaccine design strategy to elicit oligomannose-specific HIV-neutralizing antibodies using glycoconjugates based on the Rv3 LOS structure. Starting from a Kdo2GlcNAc2 tetrasaccharide precursor, a central orthogonally protected mannose trichloroacetimidate donor was coupled to OH-5 of the innermost Kdo residue. To assemble larger glycans, the N-acetylamino groups of the glucosamine units were converted to imides to prevent formation of unwanted imidate byproducts. Blockwise coupling of the pentasaccharide acceptor with an α-(1→2)-linked mannotriosyl trichloroacetimidate donor introduced the D1-arm fragment. Glycosylation of O-6 of the central branching mannose with an α-(1→2)-α-(1→6)-linked mannotriosyl trichloroacetimidate donor unit then furnished the undecasaccharide harboring a D3-arm extension. Global deprotection yielded the 3-aminopropyl ligand, which was activated as an isothiocyanate or adipic acid succinimidoyl ester and conjugated to CRM197. However, representative oligomannose-specific HIV-neutralizing antibodies bound the undecasaccharide conjugates poorly. Possible reasons for this outcome are discussed herein along with paths for improvement.

Highlights

HIV-1 remains a major threat to human health in many countries, and it is generally agreed that only a prophylactic vaccine is likely to curb infection rates globally.[1,2] To be effective, an HIV vaccine may need to elicit both humoral and cellular immune responses to blunt infection and protect against disease
The antigenic similarity to mammalian oligomannose was shown by binding to 2G12, which is specific for the D1 arm oligomannose, and later confirmed by the crystal structure of the bacterial mannotetraose fragment in complex with 2G12.12 The crystal structure of the bacterial ligand was used to model and construct derivatives that would more closely resemble
The structure of the imidate 4 was derived from proton and carbon NMR data, which revealed the loss of one NH signal of the GlcNAc-backbone and showed a Journal of the American Chemical Society Scheme 2

Summary

Introduction

HIV-1 remains a major threat to human health in many countries, and it is generally agreed that only a prophylactic vaccine is likely to curb infection rates globally.[1,2] To be effective, an HIV vaccine may need to elicit both humoral and cellular immune responses to blunt infection and protect against disease. The glycans on Env derive from the biosynthetic machinery of the mammalian host cell and are meant to allow the virus to evade the host antibody response.[4] As such, it was long thought that these glycans are immunologically silent.[5] over the last several years, it has become evident that some HIV-infected individuals develop neutralizing antibodies to this “glycan shield”.6 2G12 was the first human monoclonal antibody described that targets the glycans on HIV Env.[7] Later studies showed that 2G12 binds an array of oligomannoses on Env.[8] Following an extended period without any description of 2G12-like antibodies, the past decade has seen a sharp increase in the discovery of additional neutralizing antibodies, some with substantial breadth and potency, targeting what is defined as the “high-mannose patch” on HIV gp120.9 the high-mannose patch has gained substantial attraction as a target for vaccine development.[10] The lipooligosaccharide (LOS) of the phytopathogenic Rhizobium radiobacter strain Rv3 was shown previously to consist of a mannotetraose subunit α-Man-(1→2)-α-Man(1→2)-α-Man-(1→3)-α-Man that resembles the so-called D1 arm of mammalian oligomannose (Figure 1).[11] The antigenic similarity to mammalian oligomannose was shown by binding to 2G12, which is specific for the D1 arm oligomannose, and later confirmed by the crystal structure of the bacterial mannotetraose fragment in complex with 2G12.12 The crystal structure of the bacterial ligand was used to model and construct derivatives that would more closely resemble

Objectives

Results

Conclusion