Sensitization and recall of anti-Brucella immunity in Guinea pig macrophages by attenuated and virulent Brucella.

Peritoneal macrophages from guinea pigs vaccinated with strain Rev I of Brucella melitensis were only moderately activated thereby to limit, in an in vitro system, the intracellular growth of Rev I bacilli. Nevertheless, the appropriate memory cells had been primed, as demonstrated by the observation that reinfection of animals with virulent B. melitensis followed by intraperitoneal inoculation of mineral oil called forth macrophages in immunized guinea pigs which inhibited strongly the intracellular growth of brucellae. These macrophages slowed the growth of brucellae in the absence of immune serum. The intensity of the recall response was related to the challenge route and to the virulence of the challenge strain. After equal doses of attenuated or virulent brucellae, resistance was highest in macrophages recalled by the virulent strain. An important basis for the attenuation of the Rev I strain may lie in its initially low degree of macrophage activation during primary infection, although still retaining the capacity to prime stem cells. This property is associated with a protein found in fraction I, because 600 mug/ml in Freund's adjuvant primed guinea pigs so that challenge by strain 6015 evoked activated macrophages. This was seen microscopically as a reduced spread of infection in and amongst the macrophage population. Immune serum further reduced this spread and limited the number of viable intracellular brucellae.