The Pros and Cons of Trained Immunity Induced by Vaccines for Tuberculosis Prevention

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ABSTRACT The 2015 WHO Global Tuberculosis Report states that 9.6 million people, among them 1 million children, fell ill with TB in 2014. The Bacille Calmette-Guérin (BCG) vaccine to prevent tuberculosis (TB) has been introduced 95 years ago and is still administered to >80% of newborns worldwide. BCG is highly effective in preventing severe complications associated with TB disease in infants, and the introduction of the BCG vaccine caused a drop in overall childhood mortality. Thus, the beneficial effects of BCG extend far beyond the protection against TB infection in infants. Recent studies in adults demonstrated that BCG induces epigenetic changes in monocyte populations that result in improved functional capacity that can persist for months and extends to BCG-unrelated unrelated pathogens, a phenomenon referred to as “trained immunity”. Prolonged and improved innate responses also shaped the T cell response with a shift towards Th1 and Th17 responses. Despite these advantages, a new TB vaccine is urgently needed to stop the global spread of TB. BCG-induced immunity wanes over time, and the vaccine is not effective in adults. Furthermore, HIV-infected infants have an increased risk to develop local or disseminated BCG disease. Considering the high overlap of TB and HIV infections, we developed a pediatric combination HIV-TB vaccine based on auxotroph human-adapted Mtb mutants. We could demonstrate that our Mtb-SIV vaccine was safe in healthy and SIV-infected newborn macaques and could induce TB and SIV-specific immune responses. We present preliminary data that our Mtb vaccine strain enhanced monocyte function for up to 5 months, consistent with vaccine-induced trained immunity by BCG. At the same time, CD4+ T cells, showed signs of persistent immune activation that could prove detrimental in areas with high HIV prevalence, but their potential to promote Th1 responses that would be advantageous against intracellular pathogens. We will use our attenuated Mtb (AMtb) vaccine as a model for TB vaccine candidates to answer key questions related to the beneficial (or detrimental) effects of BCG vaccination that are mediated by trained and heterologous immunity. First, we will confirm that epigenetic modifications induced by BCG vaccination also occur in Malawian infants. Using archived infant rhesus macaque samples, we will test the hypothesis that epigenetic changes induced by BCG or novel auxotroph Mtb vaccine candidates are not restricted to monocytes, but also occur in dendritic cells, and that vaccine-induced trained immunity is maintained by tissue macrophages and dendritic cells. Finally, we will test whether innate imprinting shapes the CD4+T cell response and is associated with epigenetic changes in CD4+ T cells. The data are expected to inform the design of preventative pediatric TB vaccines.