Immunogenicity and efficacy of two types of West Nile virus-like particles different in size and maturation as a second-generation vaccine candidate.

Virus-like particles (VLPs) of flaviviruses generated from the prM and E genes are a promising vaccine candidate. We have established cell clones continuously releasing VLPs of West Nile virus (WNV) in serum-free conditions. Two types of VLPs were distinguished by sedimenting analyses in sucrose density gradients. Fast sedimenting VLPs (F-VLPs) were large (40-50 nm) and composed of the E and processed mature M proteins, whereas slowly sedimenting VLPs (S-VLPs) were small (20-30 nm) particles consisting of the E and immature prM proteins. F-VLPs induced higher neutralizing antibody and anti-WNV IgG titers than S-VLPs. Furthermore, IgG2a was dominant over IgG1 by immunization with F-VLPs as with whole virion-derived antigens. Mice vaccinated with a low dose (3 ng) of F-VLPs showed higher protective efficacy (83% survivals) against WNV infection than S-VLP-immune mice (17% survivals). These results indicate that F-VLPs more closely resemble the virions and take a better immunogenic form than S-VLPs as WNV vaccine candidates.