Peptide Nanocapsules As Novel Immunogens:Design And Biophysical Analysis Of A Prototype SARS Vaccine

Abstract

Severe Acute Respiratory Syndrome (SARS) is an infectious disease caused by a novel coronavirus that cost nearly 800 lives. While there have been no recent outbreaks of the disease, the threat remains as SARS coronavirus (SARS-CoV) like strains are still existing in animal reservoirs. Therefore, the development of a vaccine is in grave need. We have designed and produced a prototype SARS vaccine: a self-assembling polypeptide nanocapsule that repetitively displays a SARS B-cell epitope from the C-terminal heptad repeat of the virus' spike protein. The peptide forming the nanocapsule consists of the pentameric coiled-coil domain of COMP at the N-terminus joined by a short linker segment to a de novo designed trimeric coiled-coil domain at the C-terminus. The SARS epitope is ideally suited to extend this trimeric coiled-coil as it is itself a trimeric coiled-coil. Circular dichroism of the refolded nanocapsules revealed a highly α-helical structure. Proper self-assembly of the peptide into nanocapsules was verified by TEM and DLS, both showing nanocapsules in the 25nm to 30nm size range. The number of peptide chains per nanocapsule was then determined by analytical ultracentrifugation and the average was 110 peptide chains per nanocapsule. Immunization experiments with these SARS-nanocapsules were performed with Balb/c mice. An investigation of the binding properties of the elicited antibodies showed that they were highly conformation specific for the coiled-coil epitope since they specifically recognized the native trimeric conformation of C-terminal heptad repeat region. The antisera also exhibited neutralization activity in an in vitro infection inhibition assay. We conclude that these peptide nanocapsules represent a promising platform for vaccine design, in particular for diseases that are characterized by neutralizing epitopes with coiled-coil conformation such as SARS-CoV or other enveloped viruses.