Protection against an aerogenic Mycobacterium tuberculosis infection in BCG-immunized and DNA-vaccinated mice is associated with early type I cytokine responses

Although vaccination against tuberculosis (TB) was initiated more than 80 years ago, the correlates of protective immunity against infection by Mycobacterium tuberculosis have still not been well defined. To investigate the vaccine-induced immune responses against TB, we evaluated the early pulmonary cytokine responses elicited by a low dose M. tuberculosis aerogenic challenge in mice that had been immunized with either BCG or a TB DNA vaccine cocktail, two vaccine preparations that induce long-term protection in the mouse model of pulmonary TB. Using three different assays, we showed that specific cytokine responses were elevated in the lungs of vaccinated mice (relative to naïve controls) during the second week post-challenge. By measuring cytokine levels in the bronchoalveolar lavage fluid (BAL) and cytokine mRNA concentrations in pulmonary cells, the levels of IFN-γ, IL-12, and RANTES were shown to be elevated from days 7–14 post-challenge in the lungs. By intracellular cytokine staining (ICS), increased numbers of lung CD4 and CD8 cells expressing IFN-γ were also seen at days 10 and 14 after the infection. Moreover, increased post-challenge IFN-γ levels were detected using the ICS and cytokine mRNA assays in aging BCG-immunized mice that had been effectively boosted with a TB DNA vaccine. Taken together, these data suggest that the post-infection induction of early type 1 cytokine responses correlate with the induction of long-term protective immunity in vaccinated mice.