Pathogenesis of Venezuelan equine encephalitis virus infection in mice and hamsters

Comparative neurovirulence and tissue tropism of wild-type and attenuated strains of Venezuelan equine encephalitis virus administered by aerosol in C3H/HeN and BALB/c mice

To assess the potential for aerosol administration of vaccines for Venezuelan equine encephalitis virus (VEE), we compared the neurovirulence and tissue tropism of the wild-type Trinidad donkey (TrD) strain to those of the attenuated TC83 and V3526 strains of VEE in mice. Six to 8-week-old female C3H/HeN and BALB/c mice were aerosol exposed to one of the three VEE strains. Three mice of each strain were euthanatized at different times and their tissues were processed and stained using hematoxylin and eosin, immunohistochemistry, and in situ hybridization. All three viral strains infected the brains of mice and induced encephalitis. TrD spread caudally from the olfactory bulbs to all regions of the brain, caused widespread necrotizing panencephalitis by day 5, and resulted in 100% mortality (geometric mean = 7 days) in both mouse strains. By comparison, TC83 relatively spared the caudal regions of the brain but still caused 100% mortality in the C3H/HeN mice (geometric mean = 12 days), yet it did not kill any BALB/c mice. V3526 infectivity of the brain was the most limited, mainly affecting the neocortex and diencephalon. This virus was not lethal in either mouse strain. The TrD strain also infected the olfactory neuroepithelium, local lymphoid tissues, teeth, and vomeronasal organs, whereas the affinity of TC83 and V3526 outside the brain was essentially limited to the olfactory neuroepithelium. Attenuated VEE strains administered to mice by aerosol have restricted tissue tropism as compared with wild-type virus; however, even attenuated strains can infect the brain and induce encephalitis.

Replicon-helper systems from attenuated Venezuelan equine encephalitis virus: expression of heterologous genes in vitro and immunization against heterologous pathogens in vivo

A replicon vaccine vector system was developed from an attenuated strain of Venezuelan equine encephalitis virus (VEE). The replicon RNA consists of the cis-acting 5' and 3' ends of the VEE genome, the complete nonstructural protein gene region, and the subgenomic 26S promoter. The genes encoding the VEE structural proteins were replaced with the influenza virus hemagglutinin (HA) or the Lassa virus nucleocapsid (N) gene, and upon transfection into eukaryotic cells by electroporation, these replicon RNAs directed the efficient, high-level synthesis of the HA or N proteins. For packaging of replicon RNAs into VEE replicon particles (VRP), the VEE capsid and glycoproteins were supplied in trans by expression from helper RNA(s) coelectroporated with the replicon. A number of different helper constructs, expressing the VEE structural proteins from a single or two separate helper RNAs, were derived from attenuated VEE strains Regeneration of infectious virus was not detected when replicons were packaged using a bipartite helper system encoding the VEE capsid protein and glycoproteins on two separate RNAs. Subcutaneous immunization of BALB/c mice with VRP expressing the influenza HA or Lassa virus N gene (HA-VRP or N-VRP, respectively) induced antibody responses to the expressed protein. After two inoculations of HA-VRP, complete protection against intranasal challenge with influenza was observed. Furthermore, sequential immunization of mice with two inoculations of N-VRP prior to two inoculations of HA-VRP induced an immune response to both HA and N equivalent to immunization with either VRP construct alone. Protection against influenza challenge was unaffected by previous N-VRP immunization. Therefore, the VEE replicon system was characterized by high-level expression of heterologous genes in cultured cells, little or no regeneration of plaque-forming virus particles, the capability for sequential immunization to multiple pathogens in the same host, and induction of protective immunity against a mucosal pathogen.

Venezuelan equine encephalitis virus vaccines induce mucosal IgA responses and protection from airborne infection in BALB/c, but not C3H/HeN mice

Immunization with either a live-attenuated (TC-83) or formalin-inactivated (C-84) vaccine for Venezuelan equine encephalitis (VEE) virus protected BALB/c mice from lethal VEE infection acquired subcutaneously or by aerosol. While vaccinated C3H/HeN mice were also protected from parenteral infection, neither vaccine protected these mice from an aerosol infection. The apparent vaccine failures in C3H/HeN mice could not be attributed to deficiencies in virus-neutralizing antibodies in serum, as these responses were typically of equal or higher titer than those observed in protected BALB/c mice before challenge. IgG subclass analysis offered no facile explanation: profiles of IgG2 alpha dominance were observed in C3H/HeN mice given either vaccine and in BALB/c mice given the live-attenuated vaccine, whereas BALB/c antibody responses shifted toward IgGl dominance after immunization with the killed C-84 vaccine. Data from immunized congenic mice showed that the H-2 genes from the C3H/He mice were not singularly responsible for the inability of these mice to resist aerosol infection with VEE virus. VEE virus-specific IgA responses were detected more frequently in respiratory and vaginal secretions obtained from the protected BALB/c mice.

A viral vaccine vector that expresses foreign genes in lymph nodes and protects against mucosal challenge

A candidate live-virus vaccine strain of Venezuelan equine encephalitis virus (VEE) was configured as a replication-competent vector for in vivo expression of heterologous immunogens. Three features of VEE recommend it for use as a vaccine vector. (i) Most human and animal populations are not already immune to VEE, so preexisting immunity to the vector would not limit expression of the heterologous antigen. (ii) VEE replicates first in local lymphoid tissue, a site favoring the induction of an effective immune response. (iii) Parenteral immunization of rodents and humans with live, attenuated VEE vaccines protects against mucosal challenge, suggesting that VEE vaccine vectors might be used successfully to protect against mucosal pathogens. Upon subcutaneous (s.c.) inoculation into the footpad of mice, a VEE vector containing the complete influenza virus hemagglutinin (HA) gene expressed HA in the draining lymph node and induced anti-HA immunoglobulin G (IgG) and IgA serum antibodies, the levels of which could be increased by s.c. booster inoculation. When immunized mice were challenged intranasally with a virulent strain of influenza virus, replication of challenge virus in their lungs was restricted, and they were completely protected from signs of disease. Significant reduction of influenza virus replication in the nasal epithelia of HA vector-immunized mice suggested an effective immunity at the mucosal surface. VEE vaccine vectors represent an alternative vaccination strategy when killed or subunit vaccines are ineffective or when the use of a live attenuated vaccine might be unsafe.

Spread of Venezuelan equine encephalitis virus in mice olfactory tract

Spread of Venezuelan equine encephalitis (VEE) virus and damage of the central nervous system (CNS) in mice infected by respiratory route was studied. Virus concentration in organs and blood, "dose-effect" relationships, and ultrastructural lesions in various tissues were examined in immune and normal mice. We showed, via three independent methods--characteristic curve investigations, tissue virus concentration dynamics, and ultrastructural methods--the spread of VEE virus through the olfactory tract into the brain of immune mice. From these experiments it was concluded that in case of respiratory challenge VEE virus can enter the CNS of normal mice by both vascular and olfactory pathways, while in immune mice the main route is olfactory.