Cutting Edge: Bacillus Calmette–Guérin–Induced T Cells Shape Mycobacterium tuberculosis Infection before Reducing the Bacterial Burden

: Growing evidence suggests the outcome of Mycobacterium tuberculosis infection is established rapidly after exposure, but how the current tuberculosis vaccine, bacillus Calmette-Guerin (BCG), impacts early immunity is poorly understood. In this study, we found that murine BCG immunization promotes a dramatic shift in infected cell types. Although alveolar macrophages are the major infected cell for the first 2 weeks in unimmunized animals, BCG promotes the accelerated recruitment and infection of lung-infiltrating phagocytes. Interestingly, this shift is dependent on CD4 T cells, yet does not require intrinsic recognition of Ag presented by infected alveolar macrophages. M. tuberculosis-specific T cells are first activated in lung regions devoid of infected cells, and these events precede vaccine-induced reduction of the bacterial burden, which occurs only after the colocalization of T cells and infected cells. Understanding how BCG alters early immune responses to M. tuberculosis provides new avenues to improve upon the immunity it confers.