Challenge of Hamsters with Japanese B, St. Louis and Murray Valley Encephalitis Viruses after Immunization by West Nile Infection Plus Specific Vaccine

Abstract

Groups of hamsters were basically immunized with active WN virus following administration of WN inactivated vaccine. Most of these animals were given 1 or 2 supplementary injections of formalin inactivated vaccines of JBE, SLE or MVE viruses. Others were given 1 to 3 injections of vaccine without the preceding WN immunization. Groups of animals with these various immunization schedules were then challenged subcutaneously in a quantitative manner with JBE, SLE or MVE virus, depending on the virus vaccine used. For JBE and SLE virus challenge the animals were treated with cortisone to render them susceptible to peripheral inoculation. Quantitative neutrahzation and complement fixation tests against WN and one or more other viruses were made on sera drawn from animals of the various groups just prior to virus challenge. In general, the character of challenge and serological responses was similar with all viruses. WN infection plus 1 injection of vaccine always afforded better protection than 1 injection of vaccine alone and usually gave protection equal to or better than that obtained after 3 injections of vaccine. Neutralizing antibody titers showed an association with protection as afforded by inactivated virus vaccine of homologous type when used alone. Occasionally, however, those animals with WN virus plus vaccine, the group which was most resistant, did not have the highest neutralization index. Complement-fixing antibodies were never detected after vaccine alone, but appeared only in animals which had had a preceding infection with active WN virus. Detectable complement-fixing antibody response to the virus of the type represented by the vaccine appeared to correlate best with maximum protection. Among the 3 viruses used, the most striking difference observed in protection against challenge occurred between the groups tested with JBE or SLE viruses and those with MVE. The latter resisted challenge to a much greater extent after all types of immunizing procedures. This was attributed in large part to the fact that cortisone was not used during challenge with MVE virus. It was suggested that cortisone interfered with the booster response ordinarily associated with infection in a partially immunized animal. Immunity in a naturally susceptible host, such as man, it was predicted, would probably be of a higher order for JBE and SLE than that observed in cortisone treated hamsters. Results should parallel those of hamsters challenged with MVE—a situation in which cortisone was not needed. Parallels have been pointed out between the serological responses occurring in hamsters and man after a variety of immunological experiences with hve and inactivated agents of Group B, singly and in combination. It is concluded that when and if a suitably benign or attenuated strain of WN virus becomes available for safe administration to man as a basic immunizing procedure to these Group B viruses, one or more subsequent injections of inactivated vaccine of the virus type against which protection is desired should provide better immunity than vaccine alone.