Innate immune responses to Mycobacterium tuberculosis infection : How extracellular traps and trained immunity can restrict bacterial growth
Abstract: Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis, and the cause of 1.5 million deaths in 2018. During a pulmonary TB infection, the bacterium reaches the lungs and is phagocytosed by cells of the innate immune system, primarily macrophages. The macrophages are either able to eradicate the bacteria or the bacteria start to replicate, and the following immune response leads to the formation of a large cluster of different cell types called a granuloma. In the granuloma the mycobacteria are contained in a latent infection, or they can start to replicate causing rupture of the granuloma and spread of the disease. Neutrophils are also innate immune cells that are rapidly recruited to the site of infection. They are phagocytes, but they also exert extracellular effector mechanisms by their release of microbicidal granule proteins, reactive oxygen species and neutrophil extracellular traps. M. tuberculosis has co-evolved and adapted to the human host making it ingenious at exploiting the human immune response, promoting its survival and replication in human host cells. The human immune system has also evolved mechanisms to limit M. tuberculosisreplication and spread. This thesis covers work on the innate immune response to TB and how neutrophils and macrophages respond to a mycobacterial infection and can control M. tuberculosis-replication.Neutrophils and macrophages can respond to M. tuberculosis by releasing extracellular traps. We demonstrated that neutrophil extracellular traps contain the danger signal heat-shock protein 72 when induced by mycobacteria, which subsequently mediate a proinflammatory activation of adjacent macrophages. Macrophages can also release extracellular traps, and we observed the release of macrophage extracellular traps in response to M. tuberculosis that grow in cord-structures. We further demonstrated that the induction of extracellular traps also required the mycobacterial virulence factor ESAT-6.Trained immunity is an epigenetically regulated memory of the innate immune system that results in a heightened response to a later encounter of the same or different pathogen. β-glucans are structural components of microbial cell walls and known inducers of trained immunity. We studied the effects of β-glucan from a bacterial source (curdlan from Alcaligenes faecalis), from yeast (WGP dispersible from Saccharomyces cerevisiae) and from the supernatant of a multicellular fungi (Alternaria) in search of functional changes in human macrophages which enhanced their anti-mycobacterial capacity. M. tuberculosis growth reduction was observed in WGP dispersible-trained macrophages when co-cultured with neutrophils. We also discovered that the interferon-gamma (IFNγ) signaling pathway, which is important for mycobacterial control, is hypomethylated in the WGP dispersible-trained macrophages. Since hypomethylation of genes typically is associated with gene activation, this suggests a more active IFNγ signaling in response to β-glucan innate immune training.Most of our studies were performed using in vitro culturing of primary human macrophages and neutrophils. However, an in vitro 3D tissue model is a valuable tool when studying complex events that occur during a TB infection that involves both multiple cell types and requires knowledge of the spatial movement of cells. In this thesis we also describe an in vitro lung tissue model, which we could use to observe the clustering of monocytes around mycobacteria and quantify the size and number of macrophage clusters.In conclusion, this thesis comprises work on innate immune functions during tuberculosis infection. We describe extracellular trap formation in macrophages and neutrophils in response to M. tuberculosis. We also explore trained immunity and how β-glucan training can enhance mycobacterial growth restriction.
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