BCG-induced T cells shape Mycobacterium tuberculosis infection before reducing the bacterial burden
Growing evidence suggests the outcome of Mycobacterium tuberculosis (Mtb) infection is established rapidly after exposure, but how the current tuberculosis vaccine, BCG, impacts early immunity is poorly understood. Here we found that murine BCG immunization promotes a dramatic shift in infected cell types. While alveolar macrophages (AM) are the major infected cell for the first two 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 antigen presented by infected AM. Mtb-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 co-localization of T cells and infected cells. Understanding how BCG alters early immune responses to Mtb provides new avenues to improve upon the immunity it confers.