Elucidating mechanisms of protection conferred by a primary Mycobacterium tuberculosis (Mtb) infection to a secondary Mtb infection in non-human primates

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), has lived with man for thousands of years. Yet, TB is still a major global health problem killing more than a million people every year. Bacille Calmette-Guerin was developed almost a hundred years ago and is still the only licensed vaccine for TB. The main hindrance to current vaccine development is the lack of correlates and immune targets of protection. Early published data suggest that individuals with latent TB have a lower risk of developing active TB disease after Mtb re-exposure. A more recent study showed that macaques with an ongoing primary Mtb infection are protected against establishment of granulomas and bacterial growth from a secondary Mtb challenge. The precise immune mechanisms of this protection are largely unknown. The main goal of this dissertation is to investigate the importance of bacterial viability and CD4 T cells in the protection against Mtb reinfection using DNA-barcoded Mtb strains. Eliminating live Mtb bacilli by drug treatment reduced but did not abolish the protection against the establishment of and bacterial growth in granulomas arising from the second infection; although the effect of long term primary Mtb infection against a second infection needs to be further studied. Depletion of CD4 T cells in macaques before reinfection resulted in increased bacterial burden in secondary granulomas, however it only increased the number of secondary granulomas in some macaques showing heterogeneity in these animals. Moreover, we showed that CD4 T cells are important in preventing Mtb dissemination to the lymph nodes. Bacterial burden in secondary granulomas, lungs and lymph nodes in the CD4 T cell-depleted macaques did not reach the level of bacterial burden in the naïve controls suggesting protection is multifactorial. Lastly, we showed that lymph nodes are more than just sites of antigen presentation and immune activation; rather, they are sites of Mtb persistence and growth. Overall, this dissertation provided elements to consider and target in the design and testing of vaccines and therapeutics.