Regulation of Intestinal Immune Responses by the Atypical Chemokine Receptor CCRL1.

The immune system in our intestine faces a formidable challenge. As in all other tissues, it has to recognise, attack and destroy potentially harmful infectious agents, without irreversibly damaging tissue function. However, it also has to ensure that it does not mount similar attacks against food or the innocuous bacteria that live in our intestine. These harmless materials are not just ignored. Deliberate and active processes are triggered to make sure that the entire immune system cannot respond to them. This is called 'oral tolerance'. Most people maintain a healthy balance between immune attack and oral tolerance. However, ineffective immune responses mean intestinal infections remain major causes of morbidity and mortality worldwide. Conversely, inappropriate attacks on harmless foods or bacteria in the gut can cause chronic inflammation, such as coeliac disease or Crohn's disease. By understanding how decisions are made and regulated in the intestinal immune system, we will be able to find new ways of boosting responses to immunisation against intestinal infections, and develop new medicines for treating inflammatory bowel diseases.

Cells called dendritic cells are of central importance in immune responses in the intestine. Their function is critically dependent on their ability to move into, within, and between the organs of the intestinal immune system. Large numbers of dendritic cells are present in the wall of the intestine where they pick up information about the health of the tissue and its contents, and about whether these contents are potentially harmful or not. Having done this, the dendritic cells enter specialised tubes called lymphatic vessels which channel dendritic cells away from the intestine to organs called mesenteric lymph nodes. Here, dendritic cells leave the lymphatic vessels and penetrate deep into the mesenteric lymph nodes where they interact with other cells called T cells. It is during these interactions that key decisions are made about whether oral tolerance or immune attack should be initiated, and it is the T cells that are ultimately responsible for executing these decisions. To do this, T cells, just like dendritic cells, have to be able to move into, within, and between the organs of the intestinal immune system and spread around the entire body.

In health and disease, the movement of dendritic cells and T cells around the intestine is controlled by 'attractants' released by other cells that reside in the intestine, the walls of lymphatic channels, and in the mesenteric lymph node. These attractants tell dendritic cells and T cells where to go, and when. An exciting new drug interfering with one of these attractants has been developed, and may be useful in the treatment of Crohn's disease. We have been studying a molecule called CCRL1 that controls the quantity and distribution of attractants at work in the intestinal immune system. As a result, it indirectly plays an important role in controlling the movement of cells. Strikingly, we have found that CCRL1 is absolutely required for oral tolerance in mice. Furthermore, mice lacking CCRL1 show enhanced responses to oral immunisation. The work we are proposing aims to precisely define the role and importance of CCRL1 in the intestinal immune system, and reveal how it exert its functions. Importantly, we believe that a deeper understanding of CCRL1 will pave the way for future studies exploring if disrupting the regulation of attractants in the intestine has medical benefit in inflammatory bowel diseases, or whether it can enhance the effectiveness of oral vaccination against intestinal infections.

Leukocyte migration directed by CCR7 and CCR9 plays a central role in regulating intestinal immunity and tolerance, and drugs targeting CCR9 are being developed to treat inflammatory bowel diseases. The atypical chemokine receptor CCRL1 (CCX-CKR) binds the same chemokines as CCR7 and CCR9, but seems to act as a chemokine scavenger. We have found that CCRL1 deficient mice are completely refractory to the induction of oral tolerance to protein antigen, and show enhanced responses to mucosal immunisation. This is associated with a striking reduction in plasmacytoid dendritic cells (pDC) in mesenteric lymph nodes (MLN) arising from a stromal defect in these mice. Lymphatic endothelial cells (LEC) appear to be the main CCRL1-expressing cells in the intestine and MLN, raising unexplored questions about LEC regulation of pDC trafficking and function. Moreover, CCRL1 might also control conventional DC (cDC) in the intestine, as these cells have to traverse LEC barriers using CCR7 during their migration from the lamina propria into lymph, and from lymph into MLN. Therefore, we hypothesise that, by governing homeostatic chemokine abundance and/or distribution at LEC surfaces, CCRL1 controls DC in trans to regulate the intestinal immune system. We will test this hypothesis by answering the following questions: (i) Does CCRL1 loss affect cDC in the intestinal immune system? (ii) How and why are pDC in MLN altered by CCRL1 deficiency? (iii) How does CCRL1 influence antigen presentation and T cell differentiation in the MLN? (iv) What role do pDC play in CCRL1-dependent intestinal immune regulation? (v) Are mucosal defects in CCRL1 deficient mice due to loss of chemokine control by stromal cells? By providing new perspectives on pDC function, chemokine control of intestinal DC, the regulation of chemokine networks, and immunoregulation by LEC, this work could identify CCRL1 as a novel target for treating inflammatory bowel disease or enhancing responses to mucosal vaccination.

This proposal is a basic science research project that aims to understand fundamental processes in intestinal immunity. Consequently, in the short-term, the impact of the results 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. This is discussed in detail in the Academic Beneficiaries section. In addition, by communicating the research through our continuing teaching and outreach activities (see Communications Plan), we also hope to inform and educate, with impact, other beneficiaries in the University and local community.

We anticipate that the findings of the proposed research will also be of interest to those in pharmaceutical companies seeking to design new drugs. Insights into leukocyte migration in intestinal immunity have already led to the development of new therapeutics targeting the alpha4beta7 integrin, MadCAM-1 and, most notably, the chemokine receptor CCR9. An exclusive license to develop and commercialise the CCR9 antagonist Traficet-EN as a therapy for inflammatory bowel disease (IBD) was recently purchased from Chemocentryx by GSK, and is currently in phase III trials for Crohn's disease, phase II trials for coeliac disease, with trials in ulcerative colitis under consideration. It is not known if CCRL1 inhibition could have therapeutic utility in IBD, but it is an exciting possibility that merits close examination in the future. It is foreseeable, for example, that chemokine dysregulation stemming from CCRL1 blockade could disorientate the leukocytes expressing CCR7 or CCR9 that contribute to the pathogenesis of these diseases, and thereby alleviate symptoms. Moreover, preliminary data presented in the Case for Support suggest that CCRL1 antagonists might enhance responses to oral vaccines, so could be used to improve protection against intestinal infections that are major causes of morbidity and mortality worldwide. Should any of these potential outcomes be achievable, the economic impact could be substantial. It is reported that Chemocentryx could receive up to $1.5 billion if Traficet-EN, and five other chemokine receptor blockers, are successfully developed and commercialised. Importantly, individuals affected by IBD or intestinal infections would then emerge as another group of beneficiaries, through improved health, well-being and economic contribution. Clearly, these potential impacts are some way off and require many years of development, but the starting point has to be an in-depth understanding of CCRL1 function in vivo, and the successful completion of the proposed study will represent a major step towards achieving this goal.