Increased resistance to virus infections of mice inoculated with BCG (Bacillus calmette-guérin).

CD-1 or OF-1 mice were inoculated intravenously with 1 mg per mouse (i.e. about 10(6) live bacilli) of Pasteur Institute BCG and challenged 15 to 31 days later with the following viruses introduced by various routes: encephalomyocarditis, murine hepatitis, type 1 and 2 herpes simplex, foot-and-mouth disease and A0 and A2 influenza viruses. In most cases, BCG-inoculated mice exhibited a significantly higher resistance to these lethal infections than control mice (overall survival in control: 18%; in BCG-inoculated mice: 41%). Enhancement of resistance by BCG was especially marked in infections with encephalomyocarditis, herpes simplex type 1 and influenza A2 viruses. Intercurrent infection of BCG-inoculated mice with non lethal doses of viruses did not abrogate their resistance towards subsequent challenge with lethal doses of an unrelated virus. The possible mechanisms of this enhancing effect of BCG on host's resistance are discussed in the light of the known effects of this immunostimulating agent on the various facets of the immune response and of the respective roles the latter play in the defence against virus infections.