Virosomes as a vaccine delivery system

Virosomes retain the cell entry, and membrane fusion characteristics of the native virus. Thus, influenza-derived virosomes are taken up by cells via receptor-mediated endocytosis, which directs the particles to the endosomes. Subsequently, the virosomal membrane fuses with the endosomal membrane from within the lumen of the endosomes. The fusion process establishes continuity between the lumen of the virosome, and the cell cytosol. Therefore, protein encapsulated in virosomes will be delivered into the cytosol of the cell as shown here for the A subunit of diphtheria toxin. Antigen-containing virosomes interacting with APCs are expected to introduce part of the antigen into the cell cytosol and, thus, into the MHC class I presentation pathway. Delivery of antigens in vivo indeed results in efficient priming of a class I MHC-restricted CTL response making virosomes a promising vehicle for in vivo targeting.