The intestinal mucosa is the largest area directly exposed to environmental antigens, the majority of which are innocuous, originating from the diet and indigenous microbiota. In addition to playing a crucial role in physiological processes such as digestion of food and detoxification of bile acids, gut microbes also provide a low-grade stimulation of the intestinal immune system, which contains the majority of the lymphocytes and antibodies in the body. In addition, the production of metabolites by cohorts of commensal bacteria has been reported to modulate local colonic anti-inflammatory T cell populations, abrogate enteric infections by enhancing intestinal epithelial cell integrity and influence physiological responses in sites distal from the gut. The current view is that most of the influence exerted by the microbiota on the host immune system is initiated locally, and may the be propagated by migrating immune cells or blood-borne metabolites produced by the host, in response to microbial stimuli. Surprisingly however, little attention has been given to the possibility of direct, systemic uptake mechanisms of microbial antigens and metabolites. In particular, the vast and highly connected intestinal lymphatic vessels (the lacteals) represent a major cell- and antigen transport route that is largely understudied. This route, which delivers these environmental factors to the mLNs, is not only crucial for immune cell trafficking, but is also the main route for absorption of dietary lipids, packaged into chylomicrons by intestinal enterocytes, and other large or hydrophobic molecules, including oral drugs, that cannot be absorbed by the intestinal blood capillaries. In addition to what they transport, lymphatic endothelial cells themselves are also directly exposed and therefore are likely influenced by luminal insults. In turn, gene expression changes in lymphatic endothelial cells may have consequences both in the regulation of nutrient uptake and in leukocyte migration process. We hypothesize that the intestinal lymphatic system represents a unique route to communicate luminal contents with local and systemic immune system, influencing tolerogenic or pro-inflammatory responses. We will test this hypothesis by mapping the influence of gut microbes on the structure and gene expression profile of gut lymphatics using state-of-the-art imaging and ribosome profiling tools. In parallel studies, we will utilize novel bacterial components and known bacteria species for targeted and unbiased metabolomics screening to define whether microbial molecules are physiologically delivered by the lymph, and define general immune-modulatory properties of these metabolites (proof-of-principle experiments). Using cell-specific genetic tools, microbiota manipulation and sophisticated imaging techniques, we expect to provide an initial characterization of the role played by intestinal lymphatic system in the distribution of gut luminal contents, establishing a platform for subsequent, and much needed, studies defining the precise structural and molecular mechanisms of acellular transport of substances from the intestine to systemic tissues.
The proposed research has significant relevance to public health because it directly addresses the role of intestinal lymphatic system in the modulation of immune responses. By pursuing an understanding of the structure and main metabolites transported by the gut lymphatics, this proposal may provide novel avenues for the prevention of allergies and systemic inflammatory processes.
|Faria, Ana M C; Reis, Bernardo S; Mucida, Daniel (2017) Tissue adaptation: Implications for gut immunity and tolerance. J Exp Med 214:1211-1226|