Studies on Acquired Immunity in Rabbits to Intranasal Infection with Type I Pneumococcus

Abstract

Summary and Conclusions The susceptibility and resistance of rabbits to intranasal infection with a virulent type I pneumococcus were determined in a large series of animals, both in normal and in those immunized locally by specific and non-specific methods. The results may be summarized as follows: The intranasal instillation in normal rabbits of relatively small volumes of pneumococcal cultures causes death by septicemia in more than 90 per cent of instances.Intranasal vaccination of rabbits for five or more days with a formalin-killed culture of pneumococcus type I or with a pneumococcus autolysate, confers practically complete protection against later intranasal instillation of large quantities of living pneumococci of the same strain.Intranasal stimulation by the repeated instillation of non-specific irritants, such as solutions of formalin, tannic acid or alum, or of a bacterial vaccine of killed paratyphoid bacilli, confers no protection against later intranasal infection with living pneumococci. The protective effect of specific immunization, therefore, is not due merely to the local mobilization of cells of inflammation.Intranasal immunization with a pneumococcal vaccine is superior to intragastric immunization, proving that the protective effect is not due to ingestion of the vaccine after intranasal instillation.Intranasally vaccinated rabbits may resist intranasal infection at a time when no protective antibodies can be demonstrated in their blood serums by the mouse-protection test. Such animals, however, may possess general immunity against intravenous injection of living pneumococci. The immunity against pneumococcal infection is, therefore, not a strictly local immunity of the upper respiratory tract in the sense suggested by Besredka and by Bull and McKee, although the antibody level in the blood serum may be extremely low in animals highly protected against intranasal infection. In the case of infection of a highly susceptible animal with a virulent microörganism, the absolute separation of local and general processes of immunity is exceedingly difficult.