Orthogonal solid-phase synthesis of tetramannosylated peptide constructs carrying three independent branched epitopes

The development of peptide-based drugs requires the chemical synthesis of systems as complex as they appear in nature. Most bioactive peptides have to be associated with co-activators and delivery or targeting domains, the synthesis of such complexes is far from trivial. In efforts to develop a prototype for a new generation of peptide vaccines, a peptide construct was prepared, using an alternating lysine and glycine backbone. A 24-mer major antigen corresponding to the M2 protein of influenza virus, and two shorter T-cell epitopes derived from the hemagglutinin were co-synthesized onto the side chains of the first three lysines. To help the delivery of the constructs inside the antigen presenting cells via the multimeric cell surface mannose receptor, three additional lysines were decorated with four mannosylated serine residues. The synthesis difficulty increased upon addition of the glycoamino acids and alternating the peptide and glycoamino acid branches. The successful solid-phase synthesis of the constructs proceeded with the use of a combination of three quasi-orthogonally removable amino protecting groups and a robust activation strategy. These multi-glycosylated constructs represent some of the most complex synthetic peptides to date, and will be used to study the entire process of antigen delivery, presentation and immunogenicity.