Immunization of mice with live attenuated encephalomyocarditis virus: local immunity and survival

Mice were vaccinated with an attenuated encephalomyocarditis (EMC) virus strain by the intraperitoneal (i.p.) route and by various ways of respiratory administration: aerosol exposure and intratracheal (i.t.) and intranasal (i.n.) instillation. A linear relationship was found between vaccine dose and the resulting serum antibody titer. The effectiveness of the vaccine was determined by measuring the 50% protective doses (ED(50) values) after a lethal challenge with live virulent virus given by the i.p. route. For all three methods of respiratory immunization essentially the same ED(50) value was found, about 200 plaqueforming units (PFU), but i.p. immunization was less effective, the ED(50) value being about 600 PFU. To investigate the protective effect of local immunity, mice were vaccinated i.p. or i.n. and challenged by the i.n. route. The same ED(50) values were found as after i.p. challenge, indicating that the degree of protection afforded by the vaccine depends only on the route of vaccination and not on the route of challenge. This means that protection depends largely on systemic immunity and that local immunity plays only a minor role in this system. The results are discussed in relation to the feasibility of respiratory immunization against animal viruses.