DESCRIPTION (provided by applicant): This R21 is very simple. It addresses our previous demonstrations that newly emerging clinical isolates of tuberculosis, now the most successful bacterial pathogen on the planet, can induce regulatory T cells which seem to directly interfere with the expression of protective immunity elicited by vaccination with BCG. Here, we will determine if "prime boosting" BCG with two very potent "live attenuated mutants" improve immunity and survival beyond that achieved by BCG alone, and whether this still happens in guinea pigs infected with a Beijing strain that potently induces regulatory T cells. The research design will consist of BCG vaccination followed by boosting and then challenge with one of two highly virulent clinical isolates, one of which induces regulatory T cells and one that does not. We will use bacteriology, immunopathology, flow cytometry, RT-PCR, and Kaplan-Meier analysis to monitor study outcomes.