The role of commensal microbiota in Natural Killer T cell and liver immune regulation
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Abstract
Invariant Natural Killer T (iNKT) cells are a subset of T cells that bridge innate and adaptive immunity. iNKT cells recognize glycolipids as antigens and are characterized by the ability to rapidly produce cytokines that influence immune responses, and they have been shown to play an important role in various disease models. iNKT cells use conserved T cell receptors to recognize lipids presented by the major class 1b molecule CD1d expressed on the surface of antigen-presenting cells. Accumulating data suggests that α-galactosylceramide (α-GalCer) produced by the human symbiont B. fragilis is an iNKT cell lipid antigen. The goals of this project are to identify and structurally describe symbiont lipid antigens recognized by iNKT cells in the human gut, as well as to determine how these lipid antigens alter iNKT cell populations. We created and characterized a novel B. fragilis ceramide galactosyltransferase (CGT) mutant that selectively lacks α-GalCer to explore the effects of this lipid on the symbiotic relationship between the human immune system and microbiome. To determine the biological consequences of α-GalCer production by B. fragilis, the mutant was analyzed using in vitro activity assays and by high-performance liquid chromatography coupled with mass spectrometry (HPLC-MS). We found that B. fragilis lipids activate iNKT cells, while the CGT mutant lacking production of α-GalCer was unable to activate iNKT cells. This mutant retained both normal growth behavior and normal morphology by electron microscopy. To determine the consequences of α-GalCer production in vivo, we used germ-free (GF) mice that were colonized with an engineered B. fragilis with inducible α-GalCer production when anhydrotetracycline is added to the drinking water. α-GalCer production by B. fragilis in vivo demonstrated that B. fragilis-induced α-GalCer synthesis led to a significant increase in liver iNKT cells, with no impact on iNKT cells in the spleen, lungs, colon, small intestine, mesenteric lymph nodes, or thymus.The relationship between iNKT cell colonization of the liver and a gut symbiont shows that immune development is organ and cell type specific, emphasizing the importance of understanding microbe-host interactions. In this study, we primarily focus on the possible implications of host-microbe interactions and the linkage between gut commensals, lipids, and immune regulation. Continued studies of lipid transportation from the gut to the liver will allow us to understand the transport mechanism and function of iNKT cells in the liver.
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2024