Tissue-specific T cell migration in the intestine

The intestine contains more immune cells than any other organ in the body. The small intestine and the large intestine (the colon), though connected, perform different functions. The small intestine digests and absorbs food, while the large intestine absorbs water. Both parts of the intestine are constantly exposed to harmless or beneficial microorganisms, and also to other microbes that can cause infections that can be unpleasant at best, and life threatening at worst. The immune cells in our intestine therefore face a formidable challenge. They have to recognise, attack and destroy potentially harmful infectious agents without damaging the tissue, but not mount similar attacks against food or the innocuous microorganisms that live in our intestine. These harmless materials are not just ignored. Deliberate and active processes are triggered to make sure that immune cells cannot respond to them.

Most people have a healthy intestine but ineffective immune responses mean intestinal infections remain major causes of illness and death worldwide. On the other hand, inappropriate attacks on harmless foods or microorganisms can cause inflammatory bowel disease (IBD). Crohn's disease and ulcerative colitis, are the commonest types of IBD. They are life-changing conditions and currently affect over a quarter of a million people in the UK. IBD can also substantially increase the likelihood of developing bowel cancer. Currently there is no cure, and existing medicines are often ineffective at treating symptoms. Our recent work has generated results that will enable us to identify the molecules that direct the immune cells to the intestine. Identifying these molecules will be a major step towards developing new therapies to treat IBD and new ways of boosting responses to immunisation against intestinal infections.

Immune responses in the intestine start within specialised tissues called the mesenteric lymph nodes (MLN). This is a chain of lymph nodes linked end-to-end and connected to the intestine by specialised vessels. These vessels convey information from the intestine that is used by the MLN to instruct immune cells, which can then travel to the intestine to perform specific tasks. We have shown that each individual lymph node in the MLN chain performs distinct functions and appears to be responsible for a specific region of the intestine - The immune cells in a particular lymph node travel to a particular part of the intestine. This is important, because it means that immune cells can be directed to the place where they are most likely to be useful, and can therefore protect us efficiently.

While in the MLN, immune cells make particular molecules that allow them travel to the correct part of the intestine. Two molecules have already been identified that contribute to directing immune cells specifically to the small intestine. Drugs have been developed for the treatment of IBD that interfere with the function of one or other of these two molecules. Other as yet unidentified molecules are also involved in directing immune cells to the small intestine, and very little is known about the molecules that direct immune cells to the large intestine. Our new discoveries about the specialisation of lymph nodes in the MLN chain provide us with an opportunity to compare immune cells in each of the different nodes, determine where they travel to in the intestine, and identify novel molecules required to get them there. We will also be able to determine how immune cells in different lymph nodes acquire the unique sets of molecules they need to get back to the intestine. We will investigate this in healthy animals, and in animals with inflammation in different parts of their intestines. We anticipate that this work will identify new ways to control how immune cells travel in the body, and therefore new strategies to prevent them causing harm in patients with IBD or novel ways of boosting immunisations against intestinal infections.

The small intestine (SI) and colon are physiologically and functionally distinct. Adaptive immune responses in the intestine are initiated in the mesenteric lymph node (MLN) chain. We have found that the individual MLN are immunologically distinct. Dendritic cells (DCs) and lymph from the SI drains to specific small intestinal nodes (sMLN), while DCs and fluid from the colon enter colonic nodes (cMLN). T cells activated in the sMLN can express CCR9 and alpha4beta7 and preferentially home to the SI, and we have shown that T cells in the cMLN selectively migrate to the colon. This separation of sMLN and cMLN provides a mechanism by which immune responses in the SI and colon can be independently controlled. Even greater regional specialisation may exist because CCR9-dependence varies along the SI, and we find that DCs from particular segments of the SI migrate to specific sMLNs. Drugs targeting CCR9 and alpha4beta7 have been developed to treat inflammatory bowel disease (IBD). Further insights into the molecular mechanisms underpinning tissue-specific homing will likely provide future targets for drug development.

We hypothesise that specific nodes in the MLN chain imprint distinct molecular signatures on activated T cells to allow their preferential homing to precise regions of the intestine. We aim to define, with unprecedented precision, how trafficking of T cells activated in different MLNs is controlled; the molecular signals directing T cell migration to parts the intestine; and the cells and molecules that participate in imprinting these migratory preferences. To fully understand the physiological relevance of these processes, we will perform our experiments both in the steady state, and in animals with small intestinal or colonic inflammation. This study will enhance understanding of critically important immunological processes, and identify novel targets whose therapeutic manipulation could enhance mucosal vaccination or provide new therapies for IBD.

This proposal is a basic science discovery project that aims to dissect fundamental processes in intestinal immunity. In the short-term, therefore, the impact of the study will be most keenly felt in the academic community, and the principal beneficiaries will be immunology researchers in the UK and across the world. In addition, by communicating the research through our continuing teaching and outreach activities, we will educate and inform, with impact, other beneficiaries in the University, schools, and the local community.

There may be possibilities for some commercialisation over the tenure of the grant, and we have experience of working with the private sector. For example, SM's expertise in the purification and analysis of intestinal immune cell populations led to a contract (~$550K over 4 years) with a partner company in the USA to provide samples of cells from patients undergoing active immune responses with a view to identifying potential novel therapeutic targets and biomarkers in inflammatory bowel disease (IBD). RN, as co-PI, has also previously received funding from Novartis (£137K) and Almac Sciences (£200K) to explore translational aspects of his chemokine research. We also have a track record of translating findings in experimental animals into studies on humans, and have close links with clinical colleagues in Glasgow from a variety of different specialties. For instance, studies on the basic biology of migrating DCs in rodents have been used to design studies of immune function in patients with IBD, and we have recently published details of mechanisms by which intestinal inflammation contributes to systemic inflammation in patients (Wright et al (2015) Rheumatology). These examples highlight our ambitions to engage effectively with companies and clinicians to enhance the impact of our work, and, if funded, we will continue to actively seek similar outputs from the proposed study over the course of the funding period.

We anticipate that our study will be of broad interest to pharmaceutical companies, particularly those seeking to design new drugs for the treatment of IBD. Insights into leukocyte migration in the intestine have already led to the development of new therapeutics in this area that target CCR9 or the alpha4beta7 integrin, including Traficet-EN (Chemocentryx) and Vedolizumab (Takeda). Interfering with cell migration therefore clearly has therapeutic potential. The economic, physical and psychological impact of IBD is enormous, and patients can struggle to lead normal lives, placing a heavy burden on health services and families alike. Any new developments in the treatment of these diseases would be most welcome and have a significant impact on the health, well being and economic contribution of patients. In addition, the economic impact could be substantial for any company developing new drugs in this area. It is estimated that annual sales of Vedolizumab will hit $500m by 2018, and it has been reported that, under the terms of a licence agreement with GSK, Chemocentryx could receive up to $1.5 billion if Traficet-EN, and five other chemokine receptor blockers, are successfully developed and commercialised. Leukocyte migration is also a key component of responses to vaccines, and there is the possibility that a greater understanding of T cell homing to the intestine could lead to novel ways to augment mucosal vaccines against intestinal infections which remain major causes of morbidity and mortality worldwide. Clearly these potential impacts are some way off, and will require many years of development, but one important starting point is to develop a comprehensive understanding of leukocyte migration in the intestinal immune system. The successful completion of the proposed study will represent a major step towards achieving this goal.