Effects of mycobacterial fractions and muramyl dipeptide on the resistance of mice to aerogenic influenza virus infection.

The nonspecific protective effect in mice of pre-exposure to mycobacterial components and muramyl dipeptide three weeks before aerosol infection with influenza virus A/PR/8/34 (H1N1) was studied. Muramyl dipeptide, when combined with trehalose dimycolate and emulsified in an oil-in-water emulsion, conferred complete protection comparable to specific immunization with a high dose of formalin inactivated A/PR/8/34 influenza viral vaccine. Animals pre-exposed to muramyl dipeptide plus trehalose dimycolate showed a marked reduction in lung virus titres, an earlier clearance of detectable infectious virus, and an earlier onset of antibody production in comparison to control mice. Resistance to infection was also observed with BCG-cell wall skeleton combined with trehalose dimycolate and trehalose dimycolate alone when given as oil-in-water preparations. The route of administration of nonspecific stimulants was crucial. Only intravenous but not intradermal inoculation produced significant protection.