RNA bacteriophage capsid-mediated drug delivery and epitope presentation

Objective: To use our knowledge of the three-dimensional structure and self-assembly mechanism of RNA bacteriophage capsids to develop novel virus-like particles (VLPs) for drug delivery and epitope presentation. Methods: Site-directed mutagenesis of a recombinant MS2 coat protein expression construct has been used to generate translational fusions encompassing short epitope sequences. These chimeric proteins still self-assemble in vivo into T =3 shells with the foreign epitope in an accessible location. Covalent conjugation has also been used to generate RNA stem-loops attached to the toxin, ricin A chain, or to nucleotide-based drugs, that are still capable of stimulating self-assembly of the capsid in vitro. These packaged drugs can then be directed to specific cells in culture by further covalent decoration of the capsids with targeting molecules. Results: Chimeric VLPs are strongly immunogenic when carrying either B or T cell epitopes, the latter generating cytokine profiles consistent with memory responses. Immune responses to the underlying phage epitopes appear to be proportional to the area of the phage surface accessible. Phage shells effectively protect nucleic acid-based drugs and, for the toxin construct, make cell-specific delivery systems with LD₅₀ values in culture sub-nanomolar. Conclusion: VLP technology has potential for therapeutic and prophylactic intervention in disease.