Human alveolar lining fluid from the elderly promotes Mycobacterium tuberculosis growth in alveolar epithelial cells and bacterial translocation into the cytosol

The elderly population is at significant risk of developing respiratory diseases, including tuberculosis (TB) caused by the airborne Mycobacterium tuberculosis (M.tb). Once M.tb reaches the alveolar space, it contacts alveolar lining fluid (ALF) which dictates host cell interactions. We previously determined that age-associated dysfunctionality in human ALF soluble innate components lead to accelerated M.tb growth within human alveolar macrophages. Here we determined the impact of human ALF on M.tb infection of alveolar epithelial cells (ATs), another critical cellular determinant of infection. We observed that E-ALF-exposed M.tb had significantly increased intracellular growth in ATs compared to adult ALF (A-ALF)-exposed bacteria. Despite this, there were no alterations in AT inflammatory mediators or cell activation. However, exposure to E-ALF altered endosomal trafficking of M.tb, driving bacterial translocation to both endosomal and cytosolic compartments in ATs. Our results indicate that exposure of M.tb to E-ALF promotes translocation of bacteria into the AT cytosol as a potential favorable niche for rapid bacterial growth and at the same time dampens ATs immune responses. Thus, our findings highlight the influence of the elderly lung mucosa on M.tb infection of ATs, an unexplored contributing factor to the elderly populations increased susceptibility of developing active TB disease.