The effect of oral Mycobacterium vaccae on subsequent responses of mice to BCG sensitization

It has been postulated that previous exposure to mycobacteria in the environment may be a contributing factor to the variable efficacy of BCG vaccination in protecting against human tuberculosis or leprosy, in different geographical regions. To test this hypothesis mice were given Mycobacterium vaccae in their drinking water for 3 weeks immediately, 27 days, or 54 days before they were injected subcutaneously with BCG. Spleen cells were examined, 50 days after injection of the mice, for ability to proliferate in vitro in response to killed mycobacteria added to the cultures. The results show that, depending on the timing of the exposure of the mice in vivo to M. vaccae before BCG injection and the dose of the mycobacterial challenge in vitro, oral administration of this species of environmental mycobacteria can either enhance, mask or interfere with the expression of sensitization by BCG. Thus, our data from mice support the hypothesis that the results of BCG vaccination for a particular human population are influenced by exposure to indigenous mycobacteria.