Synthesis of bacterial mimetics of HIV-1 gp120 glycan epitopes

Combating the worldwide pandemic caused by the human immunodeficiency virus (HIV-1) is still a formidable challenge despite the progress made in antiviral drug design. The development of broadly cross-protective and neutralizing antibodies is counteracted by various immune evasion strategies of the virus. Still, the tight glycan clusters at the viral cell envelope spikes being present in the oligomannosidic glycoprotein gp120 may be exploited as „Achilles heel“ target for antibody recognition. A limited number of gp120 directed monoclonal antibodies has been elaborated with highly unique epitope specificities but due to the close similarity of these epitopes with mammalian glycoprotein glycans the binding of gp120-specific antibodies is usually characterized by a low immunogenicity and low affinity. Recently, a gp120-related oligomannosyl glycan has been found connected to the core unit of the Gram-negative lipopolysaccharide of the soil bacterium Rhizobium radiobacter. In preliminary experiments these glycan structures cross-reacted with HIV-specific antibodies with modest affinity. Structural data of a gp120 ligand and the bacterial glycan in the binding sites of two monoclonal anti HIV antibodies indicated that the bacterial scaffold might provide additional binding interactions with mannosyl residues. Moreover, since the bacterial sugars are inherently recognized as foreign by the immune system, an enhanced immune response against these antigens and reduced autoreactivity may be expected. Two major hypotheses may thus be put forward: Do novel, modified HIV-related bacterial glycans mimic the viral gp120 epitopes? Second, will these glycans be more efficiently recognized by anti-HIV monoclonal antibodies and could elicit neutralizing antibodies? The project will focus on the chemical synthesis of oligosaccharides up to the undecasaccharide level as modified and chain-extended mimics of gp120 glycan epitopes by combining the structurally conserved bacterial lipopolysaccharide component with two oligomannosyl chains. As controls, the mannosyl units will also be provided without the bacterial scaffold. The ligands will be prepared as spacer derivatives to be conjugated biotin and to protein carriers for testing of monoclonal anti-HIV antibodies. In addition large glycan clusters will be assembled, which should lead to a substantial increase in binding affinities due to the multivalent presentation of ligands. NMR spectroscopic studies will be performed to get insight into the detailed recognitions motifs of HIV-specific antibodies with the ligands. Immunochemical studies will be jointly performed with Dr. Kunert at the Department of Biotechnology (BOKU) and with Dr. Pantophlet at Simon Fraser University in Canada. Provided the binding studies are successful, future studies will include immunization with the neoglycoconjugates in order to elicit cross-reactive and neutralizing antibodies. The outcome of the project should thus substantially contribute to novel approaches in anti-HIV vaccine development and/or supplement combination therapies and will have significant impact on anti-AIDS immunogen design strategies.