Membrane repair balances host cell killing by Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is the pathogen responsible for tuberculosis and the major cause of death by infection worldwide. Only 10% of those infected with Mtb develop active disease whereas 90% become latently infected. The outcome is determined both by the virulence of the bacterium and by host defences, but it is not clear what eventually flips the balance. Our main hypothesis is disruption of host cell membranes, which can trigger inflammatory cell death and favor Mtb replication and spread. To cause persistent infection (latency), Mtb manipulates the immune system and establishes a compartment inside host macrophages where it avoids degradation. However, in order to spread, Mtb has to leave the compartment and the macrophage and get into the alveolar space. It is shown that Mtb can disrupt cell membranes and kill the host cell. The key to contain Mtb infection is thus to preserve membrane integrity, and host cells have different means to repair damages. We propose that host cells die when the damage inflicted by Mtb to the cellular membranes exceeds membrane repair. We use live-cell imaging and high-resolution 3D microscopy of Mtb-infected macrophages to reveal mechanisms of membrane damage and repair, and how this relates to host cell killing, Mtb survival, and spread. We previously demonstrated that Mtb damages the plasma membrane of host macrophages, which resulted in inflammatory signaling and cell death. Membrane repair and sequestration of the bacteria in a process called autophagy seemed to be protective. Currently, we are investigating various mechanisms of cell death, how this impacts bacterial spread, and the regulation of these processes by the epithelium. For this, we have established a new infection model using human pluripotent stem cells to make macrophages and alveolar epithelial cells from the same origin. The success of this project will provide new insight into cellular processes that are relevant for tuberculosis and other infectious and sterile diseases where membrane damage can cause inflammatory cell death (e.g. atherosclerosis, silicosis, gout). It will also pave the way for the development of adjunct host-directed therapeutic strategies to improve tuberculosis treatment.

Mycobacterium tuberculosis (Mtb) is the intracellular pathogen responsible for tuberculosis and is the major cause of death by infection worldwide. Only 10% of those infected with Mtb develop active disease whereas 90% become latently infected. The outcome is determined both by the virulence of the bacterium and by host defenses, but it is not clear what eventually flips the balance. To cause persistent infection (latency), Mtb must establish and maintain a compartment inside macrophages that does not fuse with lysosomes. However, Mtb also needs to translocate to the cytosol and kill the host cell for efficient replication and spread. This requires membrane disruptive activity, and we hypothesize that preservation of membrane integrity is key to contain Mtb infections. We further propose that the Endosomal Sorting Complexes Required for Transport (ESCRT) machinery repairs smaller damages caused by Mtb to phagosome- and plasma membranes whereas autophagy sequesters ruptured Mtb phagosomes. If the damage is too extensive, cells die from necrosis, of which inflammasome activation and pyroptosis is one major pathway. We will use time-lapse live-cell imaging and correlative high-resolution 3D microscopy of Mtb-infected macrophages to reveal mechanisms of membrane damage and repair, and how this relates to autophagy, inflammasome activation and pyroptosis. The success of this project will provide substantial new mechanistic insight into cellular processes of high relevance for tuberculosis and other infectious and sterile diseases where membrane damage can cause inflammasome activation and programmed necrosis (ref atherosclerosis, silicosis, Alzheimer). It will also pave the way for development of adjunct host-directed therapeutic strategies targeting host cell killing by Mtb to improve tuberculosis treatment.