Final Report Summary - NKT CELLS IN MUCOSA (NKT cells, CD1 expression and lipid presentation in intestinal immunity) The mammalian intestine is colonized but a vast community of commensal microorganism that establish a mutual relationship with their host. Immune cells control the number and composition of bacteria that colonize the intestine whereas the presence of commensals is essential for immune system development/maturation. A deregulation in the microbial composition (dysbiosis) is associated with infection and inflammatory diseases including inflammatory bowel disease (IBD) and asthma. Understanding the crosstalk between the microbiota and the immune system will help to prevent or alleviate these conditions.One of the major immune cell populations present in the intestinal mucosa is the NKT cell subset. NKT cells respond to foreign and self-lipids presented in CD1 (CD1d in mice), including lipids from commensal bacterial species such as Bacteroides. NKT cells are implicated in the maintenance of intestinal homeostasis by promoting IL10 production by intestinal epithelial cells (IEC), are essential players in IBD, and are involved in the immune response to intestinal pathogens including Salmonella thiphymurium and Toxoplasma gondii. Despite the role of NKT cells in intestinal homeostasis and disease, and the high content of microbial lipids in the intestine, the mechanism by which intestinal NKT cells respond to commensal lipids remained unexplored. The main aim of this project was to understand how CD1d-mediated lipid presentation and NKT cells modulate intestinal immune responses in homeostatic conditions. To address this question we have analysed the features of NKT cells in intestine-associated immune tissues, explored the cellular mechanism involved in Iipid presentation within the intestinal mucosa and studied the role of NKT cells in IgA production and diversification. First we investigated the role of NKT cells in the regulation of intestinal homeostasis by analysing the immune cell compartment of CD1d deficient mice. One of the main features of intestinal immunity is the secretion of Immunoglobulin A (IgA) to the gut lumen. We have analysed IgA production in a mouse model that is deficient both in CD1d and in NKT cells (CD1dKO) and found that IgA diversification is skewed in CD1dKO mice, despite the fact that B cell, and T cell responses are normal and the ability to produce IgA is not affected. In vivo NKT cell activation has also altered IgA repertoire, implying that NKT cells/CD1d are important in IgA diversification. Moreover, antibiotic treatment and microbiota sequencing suggest that the skew IgA repertoire is due to a change in the small intestine microbial components of CD1dKO mice. Moreover, CD1dKO microbiota has shown to be less efficient in protecting from entero-pathogen infection.Preliminary data from our lab and others suggests that intestinal NKT cells respond to lipids from the commensal bacteria, which modulate their phenotype and function. While exploring the mechanisms that underlie the presentation of commensal lipids to NKT cells, we unexpectedly identified a role for innate lymphoid cells (ILCs) in lipid immunity. ILCs constitute a complex family of immune cells that have been appreciated as orchestrators of immune defences at mucosal surfaces, playing a central role in the anatomical containment of commensals and in the maintenance of epithelial integrity. Our data demonstrated that ILCs from different intestinal compartments express CD1d with the higher levels of expression corresponding to group 3 ILCs (ILC3s). Moreover, we showed that ILC3s have the capacity to internalise and present lipids on CD1d leading to the modulation NKT cell activation. Our data suggests a possible role for an NKT-ILC axis in the regulation of mucosal immunityOur findings highlight the importance of NKT cells and CD1d in maintaining intestinal homeostasis by shaping the commensal microbial communities; and reveal how commensal-derived lipids can modulate intestinal NKT cell function. Different components of the microbiota are implicated in intestinal and non-intestinal inflammatory diseases, like IBD and asthma. New treatments, like microbiota transplant, are now been developing to try to prevent and control these diseases. However, understanding how the immune system and the commensal communicate with each other will be essential to obtain optimal treatments. This study goes a step further to comprehend this crosstalk, and to establish the bases to prevent and to develop new treatments for microbiota related diseases.
