Bacterial membrane vesicles mediate the secretion of immunomodulatory factors during Mycobacterium tuberculosis infection (MPF3P.804)

Abstract

Abstract Mycobacterium tuberculosis (Mtb) is an intracellular pathogen that infects lung macrophages and releases microbial factors to promote immune evasion and survival. These factors include the lipoglycan LAM which blocks phagosome maturation and Mtb lipoproteins which modulate MHC-II antigen presentation and cytokine production via TLR2 signaling. It is unknown how lipoproteins and lipoglycans are shed from the Mtb cell wall, but they are dispersed throughout Mtb-infected macrophages and released within extracellular vesicles. It was initially presumed that Mtb lipoproteins and LAM trafficked into exosomes, small vesicles (50-150 nm) of endocytic origin, but studies have shown that bacteria can secrete membrane vesicles of similar size (50-250 nm). Using imaging and biochemical approaches, we show that Mtb produces bacterial vesicles within infected macrophages that are subsequently released into the extracellular environment. These Mtb vesicles are enriched for cell wall components (LAM, TLR2 agonists) and are distinct from exosomes labeled with host cell markers (CD9, CD63). Our data suggest that bacterial vesicles are the mechanism employed by Mtb to transfer cell wall components and impair effector functions within the infected macrophage. Furthermore, we propose that released Mtb vesicles, not exosomes, circulate bacterial components beyond infected cells to further modulate immune responses. These mechanisms may promote the survival of Mtb and pathogenesis of infection.