An attenuated variant of Eastern encephalitis virus: biological properties and protection induced in mice.

Wild type Eastern equine encephalitis virus (E) was compared with a mutant (Em) derived from it. The latter was tested as an attenuated vaccine in mice. They differed in the following properties: Em formed smaller plaques on chick embryo (CE) cell monolayers and, unlike E, did not plaque on mouse embryo (ME) monolayers. Futher, Em had a longer latent period and attained a lower peak titer than E after infection of CE cells, was more senssitive than E to chick interferon, and was less virulent for mice (SC and IP routes) and hamsters (IP route) than E. Both viruses were similar in several other properties tested. The mutant was found to induce a gradient in the specificity of protection in mice against challenge by selected viruses after a single subcutaneous injection of living virus. The protection was best against autologous (Em) challenge, was next best against challenge by the virulent parent (E) virus, but was not demonstrable against cross challenge by Venezuelan encephalitis (V) virus. Conventional hemagglutination-inhibiting (HI), complement-fixing (F), and neutralizing (N) antibodies could not be detected in Em-immunized mice even when fresh monkey or guinea pig serum was included in Ntests to provide complement and/or accessory factor(s). However, N antibodies were detected in protected mice by an indirect antiglobulin test. Passive protection by serum or ascites fluids (a.f.) was characterized by a lower but otherwise similar protection gradient like that found after active immunization with virus as described above. Interferon was not detected in the a.f. used for passive protection, nor was heterologous interference evident in Em immunized mice challenged 18 days later with vaccinia or vesicular stomatitis virus. Immunized mice that survived autologous (Em) challenge showed broadened protection against a second challenge by parent E virus, and cross protection against V virus. This typical protection was associated with the presence of HI and conventional N antibodies, except for V which showed no detectable neutralizing antibodies by either a standard or antiglobulin technique.