Search

Over the last few years we have utilized a system to genetically engineer foot-and-mouth disease virus (FMDV) to produce live-attenuated vaccine candidates. These candidates have been generated in the genetic background of a tissue culture-adapted strain of serotype A12 virus. Based on this A12 system, we created a virus lacking the sequence encoding the leader (L) proteinase (Piccone et al., 1995), and demonstrated that this leaderless virus, A12-LLV2 was avirulent in bovine and swine, and could be used as an attenuated vaccine (Mason et al., 1997; Chinsangaram et al., 1998). The current study shows that a similar leader-deleted chimeric virus containing the genome of the type A12 virus with a substituted type O1 capsid coding region from a bovine-virulent virus can be constructed, and that the virus has low, but detectable virulence in swine. A second chimera specifying a tissue culture-adapted type O1 capsid lacking the RGD cell binding site, was avirulent in swine, but was not sufficiently immunogenic to provide protection from challenge. These results are described with respect to mechanisms of attenuation and antigen formation in live-attenuated virus-inoculated animals.

In 1997, a devastating outbreak of foot-and-mouth disease (FMD) in Taiwan was caused by a serotype O virus (referred to here as OTai) with atypical virulence. It produced high morbidity and mortality in swine but did not affect cattle. We have defined the genetic basis of the species specificity of OTai by evaluating the properties of genetically engineered chimeric viruses created from OTai and a bovine-virulent FMD virus. These studies have shown that an altered nonstructural protein, 3A, is a primary determinant of restricted growth on bovine cells in vitro and significantly contributes to bovine attenuation of OTai in vivo.

We have developed naked DNA vaccine candidates for foot-and-mouth disease (FMD), an important disease of domestic animals. The virus that causes this disease, FMDV, is a member of the picornavirus family, which includes many important human pathogens, such as poliovirus, hepatitis A virus, and rhinovirus. Picornaviruses are characterized by a small (7-9000 nucleotide) RNA genome that encodes capsid proteins, processing proteinases, and enzymes required for RNA replication. We have developed two different types of DNA vaccines for FMD. The first DNA vaccine, pP12X3C, encodes the viral capsid gene (P1) and the processing proteinase (3C). Cells transfected with this DNA produce processed viral antigen, and animals inoculated with this DNA using a gene gun produced detectable antiviral immune responses. Mouse inoculations with this plasmid, and with a derivative containing a mutation in the 3C proteinase, indicated that capsid assembly was essential for induction of neutralizing antibody responses. The second DNA vaccine candidate, pWRMHX, encodes the entire FMDV genome, including the RNA-dependent RNA polymerase, permitting the plasmid-encoded viral genomes to undergo amplification in susceptible cells. pWRMHX encodes a mutation at the cell binding site, preventing the replicated genomes from causing disease. Swine inoculated with this vaccine candidate produce viral particles lacking the cell binding site, and neutralizing antibodies that recognize the virus. Comparison of the immune responses elicited by pP12X3C and pWRMHX in swine indicate that the plasmid encoding the replicating genome stimulated a stronger immune response, and swine inoculated with pWRMHX by the intramuscular, intradermal, or gene gun routes were partially protected from a highly virulent FMD challenge.