Visualizing the early immune response to Mycobacterium tuberculosis infection in macaques immunized with intravenous BCG

Abstract Tuberculosis (TB) causes 1.6 million deaths annually and the standard human vaccine, intradermal (ID) Bacille-Calmette Guérin (BCG), does not prevent pulmonary TB. Previously, we showed that intravenous (IV) BCG immunization elicited robust TB-specific lung-resident T cell responses compared to ID BCG in rhesus macaques (RMs). Following Mycobacterium tuberculosis (Mtb) challenge, 6 out of 10 RMs had no detectable disease as demonstrated by bacterial burden, granuloma formation, and primary Mtb-specific immune responses. Together, these data suggest that Mtb is cleared rapidly from the lungs of IV BCG-vaccinated RMs. To investigate the vaccine-induced lung environment and anamnestic response that might mediate protection against Mtb early after challenge, we developed a high-dose Mtb infection model in RMs that allowed us to locate and live-image areas of Mtb infection in the lungs using confocal microscopy. RMs immunized for 5 months with either IV or ID BCG were challenged with Mtb Erdman mCherry and euthanized 4 or 11 days later to characterize the lung microenvironment. We observed reductions in bacterial burdens in the lungs of RMs that received IV BCG compared to ID BCG-vaccinated and unvaccinated controls by CFU plating, flow cytometry, and confocal microscopy despite using a high challenge dose. Increased T cells were quantified in the lungs of both IV and ID BCG-vaccinated RMs compared to unvaccinated controls by flow cytometry and confocal microscopy. This research provides an opportunity to mine immune correlates of protection in a preclinical model, at the site of infection, and at a post-challenge time point that has been minimally characterized. NIH Intramural Program