Overview of flavivirus molecular biology and future vaccine development via recombinant DNA

Studies in many laboratories over the last several years have elucidated the structures of several different flavivirus genomes. Conserved features include the production of at least 10 different virus encoded proteins from a single long open reading frame by a combination of host and virus-encoded proteases. The established gene order is 5'-C- prM(M)-E-NS1-NS2A-NS2B-NS3-NS4A-NS4B-NS 5-3' and these proteins exhibit varying degrees of homology in comparisons among flaviviruses. Conserved RNA sequences and structures have also been identified for the mosquito-borne flaviviruses but are absent in sequenced tick-bone viruses. Relevant to the development of efficacious flavivirus vaccines, studies aimed at defining the antigenic determinants necessary for eliciting protective immunity have focused primarily on the structural proteins, in particular the E protein, as well as the nonstructural secreted glycoprotein, NS1. Other work, which has led to the derivation of live-attenuated flavivirus strains, should eventually allow the genetic determinants of flavivirus attenuation and pathogenesis to be understood at the molecular level. The successful recovery infectious flaviviruses from cloned cDNA raises the possibility of manipulating these viral genomes as cDNA to construct or propagate candidate live-attenuated vaccine strains. Several applications of this technology are discussed.