Listeria pneumonitis: influence of route of immunization on resistance to airborne infection.

Mice that are immunized with an airborne inoculum of BCG are more highly resistant to airborne challenge with Mycobacterium tuberculosis than are mice that are immunized by the subcutaneous or intravenous route. To discover whether this phenomenon is peculiar to tuberculosis, we studied the influence of the route of immunization upon pulmonary resistance in Listeria monocytogenes infection. Mice were immunized by the airborne, intravenous, or footpad route and were subsequently challenged by the same route at 1 to 4 weeks after immunization. Mice were highly and uniformly resistant to intravenous challenge, regardless of the route of immunization. The route of immunization bore no influence upon resistance to footpad infection, but resistance was appreciably better in mice challenged within 2 weeks of immunization than it was at later time points. In mice immunized by the footpad and intravenous routes, the pattern of resistance to airborne and footpad challenges was similar, in that there was substantially less immunity at 4 weeks than at 2 weeks after immunization. However, mice immunized by the airborne route were highly resistant to airborne challenge, regardless of the interval between immunization and reinfection. In this last respect, resistance of the lungs to reinfection was similar after Listeria and tuberculosis pneumonitis. It is suggested that a similar pattern of resistance may prevail in pneumonitis caused by other facultative intracellular parasites.