Induction of protective immune responses against Venezuelan equine encephalitis (VEE) virus aerosol challenge with microencapsulated VEE virus vaccine.

Venezuelan equine encephalomyelitis (VEE) virus, a member of the family Togaviridae, genus Alphavirus, causes disease in humans and equids. The virus is normally transmitted by the bite of an infected mosquito however, it can also be highly infectious by aerosol. The purpose of the present study was to determine the effectiveness of formalin-fixed, 60Co-irradiated VEE virus microencapsulated in poly DL-lactide-co-glycolide in inducing immune responses protective against aerosol challenge with virulent VEE virus. Balb/c mice were primed by subcutaneous injection of microencapsulated VEE virus vaccine, followed 30 days later by a single immunization with the same vaccine given via the oral, intratracheal (i.t.) or subcutaneous (s.c.) route. Mice boosted by the i.t. or s.c. route had higher plasma IgG anti-VEE virus levels than orally immunized animals. The responses in the former groups were similar in magnitude to those seen in mice primed and boosted by the i.t. route. Antibody activity was detected in bronchial-alveolar and intestinal washes, fecal extracts and saliva from immunized animals. The levels of IgG and IgA antibody activity in bronchial-alveolar wash fluids from mice boosted by the i.t. route were higher than those seen in animals immunized by the oral or s.c. route with the microsphere vaccine. Mice immunized with the microencapsulated VEE virus vaccine were protected from lethal VEE virus infection following aerosol challenge at approximately three months after the initial immunization. Mucosal immunization via the i.t. route appeared to be the most effective regimen, since 100% of the mice resisted aerosol challenge.

Enhancement of protective immune responses to Venezuelan equine encephalitis (VEE) virus with microencapsulated vaccine.

Venezuelan equine encephalomyelitis (VEE) virus is a mosquito-borne arbovirus of major human health significance in the New World. Currently two forms of VEE virus are used for immunization of humans and horses, i.e. a live attenuated and a formalin-inactivated vaccine. Clinical evidence suggests that these vaccines are not fully efficacious and may produce certain undesirable side-effects. In the present study, microspheres composed of biocompatible and biodegradable poly (DL-lactide-co-glycolide) (DL-PLG) were evaluated for their effectiveness as a delivery system of whole, inactivated VEE virus vaccine for the induction of protective immune responses. Mice receiving 50 micrograms VEE virus in microspheres composed of an equimolar ratio of DL-lactide and glycolide (50:50 DL-PLG) exhibited a primary circulating IgG antibody response which was approximately 32-times higher than the response induced with the same dose of unencapsulated (free) virus. A similar difference in responses was seen with antigen doses ranging from 3.1 to 50 micrograms. A rapid increase in antibody activity was seen after the secondary immunization (day 50). Formalin fixation of inactivated VEE virus was important for immunogenicity since the circulating anti-VEE virus antibody response induced with microencapsulated nonformalin-fixed virus vaccine was lower than that induced with microencapsulated formalin-fixed virus vaccine. Furthermore, at low antigen concentrations, DL-PLG microsphere vaccines prepared with the solvent methylene chloride induced higher antibody responses than those prepared using ethyl acetate as the solvent. Microencapsulated vaccine also induced higher VEE virus neutralization titers than did free virus vaccine. Finally, the microencapsulated virus was more effective than the free virus in inducing immune responses protective against systemic challenge with virulent VEE virus. These results demonstrate that DL-PLG microspheres containing formalin-fixed, inactivated VEE virus were effective in augmenting circulating IgG antibody levels and neutralization titers to the VEE virus following systemic immunization and in affording enhanced protection against systemic challenge with virulent VEE virus. The effects of antigen form and the microsphere processing solvent on the immunogenicity of the vaccine are discussed.