Amino acid availability acts as a critical environmental rheostat of mucosal ILC2 responses

Our intestines are continually exposed to a wide range of stimuli from the environment in the form of bacteria - both beneficial and harmful - and metabolically active molecules released during the digestion of food. The immune system continually acts to keep the intestine healthy and functioning normally, and to prevent any damage caused by infections or chemicals entering the body during feeding. Recent advances have shown that the balance of bacterial and dietary-derived signals in the intestine dramatically alters the way the immune system responds, and changes in this balance can result in reduced immunity to infection, inflammation or even the progression of obesity or cancer. However, the precise nature of these environmental signals and the way immune cells respond to them remains unclear, blocking the development of new treatments aimed at modifying environmental signals in the gut, or targeting the immune cell sensors that detect them.

In this project we will build upon new and exciting early work in our lab that suggest that a population of tissue-resident innate immune cells continually sense the intestinal environment for changes in a family of so-called "essential" amino acids. These amino acids are critical to keep us healthy but cannot be made by human cells and must be ingested from digestion of food in the diet.

This particular population of gut-resident immune cells constantly surveys the intestine for potential danger, and responds quickly in response to danger or infections to launch protective immunity and to repair the tissue. The speed of this response is highly reliant on the ability to sense changes in the gut environment, as well as the cells ability to import basic building blocks of proteins - in the form of amino acids - that act to "fuel" immune function. We show these cells have a much higher ability to sense changes in intestinal amino acid levels compared to other immune cells, which allows them to generate an appropriate fast and powerful immune response. In this proposal we suggest that the ability of these immune cells to sense essential amino acids is critical for the intestinal immune system to sense infections or potential danger.

We propose to further explore this hypothesis using exciting, new experimental tools and approaches which will allow us to determine exactly how these critical immune cells respond to changes in amino acids in the intestinal environment. In particular, we have identified two key genes that encode for amino acid "transporters" that detect and take up amino acids into immune cells. Using models in which these genes have been deleted within immune cells we have generated early findings which suggest the levels of amino acids both outside and inside an immune cell determine the degree to which that cell can perform it's tissue protective functions and respond to intestinal infection.

Our central objectives are to utilize new technologies and experimental tools in the lab to better understand how immune cells sense their environment - particularly how they respond to changes in important nutrients and metabolites to ensure appropriate responses that subsequently keep our intestines healthy. These findings could have important consequences for a wide range of intestinal diseases by helping us to understand how environment risk factors such as diet and infections alter the function of the immune system and determine intestinal health.

Tissue health at mucosal barrier sites, such as the gastrointestinal tract, is determined in part through a network of immune mechanisms that act to repair damaged tissue, maintain tolerance to dietary proteins and commensal microbes and prevent infection. It is increasingly appreciated that intestinal-resident immune cells are acutely sensitive to environmental cues in the form of microbial and dietary metabolites, which act to regulate immune function, signal potential threats, and align metabolically costly responses with the availability of key substrates required to fuel cellular function.

Innate lymphoid cells (ILC) are tissue-resident sentinels that respond rapidly upon tissue damage or infection to restore health. While ILC are activated by host-derived signals the extent to which the intestinal environmental regulates ILC responses is poorly understood. We demonstrate that group 2 ILC (ILC2) are preferentially poised to sense availability of essential amino acids, key dietary-derived metabolic substrates. Through elevated expression of amino acid transporters ILC2 were found to constitutively import essential amino acids, and in striking preliminary data we demonstrate amino acid sensing and import is critical to license optimal ILC2 responses.

Despite these advances, why ILC2 preferentially exhibit this capacity and how this regulates effector function remains unclear. In this project we will test the hypothesis that sensing of environmental amino acid levels is advantageous for ILC2 function, particularly in the context of infection with large multicellular helminths which compete with host immune cells for metabolic resources. Using novel transgenic tools and established ex vivo assays will dissect the mechanisms through which amino acid import regulates ILC2 cell biology and metabolism. Together these studies will uncover unappreciated mechanisms through which the environmental metabolic cues act as a checkpoint of innate immunity.

Impact for Training and Career Development
This project will directly support the training and career development of a postdoctoral researcher, exposing them to training in a wide range of scientific disciplines as well as key bioinformatics and soft skills, which are increasingly essential for a career in basic biomedical sciences. In addition the postdoctoral researcher will further benefit from our ongoing collaborations and will have the opportunity to visit those laboratories and environments to learn new skill sets and further their depth of understanding from experts in their respective research areas (e.g. signalling, metabolism, proteomics).
Timescale: These interactions will occur continually throughout the life course of the funding, and beyond.

Impact for Industry and Biotech
ILC2 are increasingly appreciated to be key immune players in the maintenance of normal healthy tissue functions, as well as across a range of allergic and inflammatory diseases. Indeed dysregulated ILC2 activation and cytokine production have been reported in patient cohorts, while the biotech industry is currently heavily investing in a range of biologics that target central aspects of the ILC2 axis (e.g. anti IL-5, anti IL-33, anti-TSLP mAbs indicated for allergic diseases). Our work could have particular relevance for industry as cellular metabolic pathways, including amino acid uptake, are highly druggable with small molecule inhibitors and may prove an attractive target for therapeutic intervention.
Timescale: We will engage throughout the course of this project with potential beneficiaries within the biotech industry, utilizing pre-existing interactions with drug companies (GlaxoSmithKline, Astra Zeneca) and will engage appropriately with the Universities dedicated business engagement and IP teams where possible.

Impact for Patients, Charitable Organisations and the General Public
While this project will not directly address the clinical implications of environmental and metabolic regulation of ILC2, it nonetheless has the potential to impact upon multiple patient cohorts, who suffer from diseases in which ILC2 have been implicated in disease onset and/or progression. For example, current therapeutics for allergic diseases remain limited and thus new intervention strategies are required. As such the work has the potential to impact upon patients suffering from diseases associated with this underlying etiology, their family members and charitable organisations campaigning for an increased understanding of these diseases. We further aim to communicate our work with patients, where possible, through bespoke events organised via our clinical colleagues in respiratory medicine, through engagement with charitable organisations or through public outreach events (see below).

The General Public As highlighted above, and addition to these activities, the UoM engages in multiple outreach activites with the general public. Furthermore, research findings and implications will be disseminated through social media (i.e. Twitter) as well as laboratory, departmental and university webpages. Research findings, published through open access journals, will also be disseminated to the general public via press releases. The principal investigator and lab members also endeavour to engage the general public regarding our work to raise awareness of science careers, biomedical research and the importance of the immune system in determining general health.
Timescale: Interactions with general public beneficiaries will be fostered throughout the funding period and actively sought via pre-exiting UoM and departmental public outreach activities (e.g. International Night of Science, Pint of Science, University public engagement events, UoM Immunology-led activities and talks at music festivals e.g Green Man festival, Blue Dot festival)