IMPC: Activation and effector functions of novel antiviral T cell populations in vivo

Viral infections represent a major health threat. Thus, understanding the mechanisms involved in defence against viruses is an important goal. In studies of human subjects we have identified a new group of cells which appear to respond to acute and chronic virus infections. The cells studied are described as MAIT cells (Mucosal associated invariant T cells) - this a recently described but abundant cell type in human blood, and the dominant T cell at certain sites such as in the liver. They are known to respond to certain types of bacteria, but we found they could readily respond to viral infections in patients - the diseases studied include Hepatitis C, Influenza and Dengue. We have gone on to show activation by other viruses such as HIV, cytomegalovirus and also novel vaccines.

The research to date has provided a mechanism by which MAIT cells can get activated. This depends on the immune system responding to the presence of the virus by making particular chemical messengers or cytokines. A key switch in this, which appears to be crucial in most cases, is the production of a cytokine Interleukin 18 (IL-18). This (in combination with a second signal) is necessary for MAIT cell activation and MAIT cells possess a very high level of a receptor for this molecule (IL18RAP), making them very sensitive to it. Once activated the MAIT cells can do a range of things to protect the body against viruses, including releasing more chemical messengers such as interferons, which can suppress virus growth, or by killing cells which are infected.

Finally by way of background, we have extended these findings to other related cell types in the human body. These are all characterised by high level expression of IL18RAP and a set of related molecules - in humans notably one named CD161 - which makes them easily identifiable. This includes different types of T cells (CD4+ T cell, CD8+ T cell, gamma-delta T cell), which appear to have the same features - response to viruses via IL18 and homing to tissues. Thus we have a "team" of related cell types which likely provide a first line of defence against viruses, as well as other organisms.

One problem with such human studies is that it is not possible to prove whether the MAIT cell response (or those of the other cell types) is protective against infection, and also how the cells are activated in the body. One way of studying this is to analyse the response in vivo using a mouse model. In this study we will:

1. Analyse which cells in the mouse represent the equivalent IL18RAP expressing subsets to those seen in humans -this will include MAIT cells but also related subsets in tissues. This can be done using a mouse where such cells are labelled fluorescently using a genetic tag.

2. This way we will also be able to track the expression of Il18RAP as they develop and expand in the body, either naturally or in response to virus infections. We will aim to use virus infections which are important for human health and where we already have some data in humans.

3. We can also test to what extent IL18 signalling is needed for the function of these cells in vivo.

We are fortunate that a mouse exists which has been engineered to express such a genetic tag and which we can use to track the cells and test the importance of this pathway. We have experience in use of viral models and have recent unpublished data with collaborators which show that MAIT cells are important in protection against sever Influenza. We aim to establish this model so we can go on to test other pathways which will be importance in function of MAITs and related cell types and then expand this to other models including a new model for viral hepatitis. In the long run I will (with collaborators) apply for project and/or Developmental Pathway Funding Scheme grants to fund experiments to establish if we can harness the power of these cells and their signalling system to protect against challenging infections in humans.

Immunity against infections is classically divided into innate and adaptive responses. However, there are a number of "bridging" cell populations that straddle these two responses, especially at mucosal sites. Amongst these, in humans, MAIT (Mucosa Associated Invariant T) cells represent an abundant subset, have recently emerged as important mediators. These cells are characterised by a restricted TCR usage (Va7.2 and limited beta chains) which allows recognition of bacterial metabolites, but also high level expression of one chain of the IL18 receptor (IL18RAP). This is an important marker but also signalling molecule for the cells as we have recently observed an IL18-dependent TCR-independent activation which allows the MAIT cell pool to respond to viruses. Related T cell types in humans which express high level IL18RAP and co-express the C Type lectin CD161 are also abundant "bridging" populations are mucosal sites and are able to respond similarly to viruses. These 2 observations together massively increase the potential pool of available pro-inflammatory and anti-viral T cells in human disease, but many questions remain about their biology and role in vivo. These questions form the basis for this pump-priming project.

We will take advantage of an ideal mouse strain Il18raptm1a(KOMP)Wtsi, which allows both targeting of the specific receptor and reporter tagging. This will allow us to address the following questions in vivo.

1. What is the distribution of the phenotype of IL18RAP expressing cells (including murine MAIT cells) amongst murine T cell subsets in naïve mice.
2. What is the responsiveness of IL18RAP+ cells to relevant viral infections?
3. What is the role of IL18 receptor signaling in the steady state and on the response to infection?

This work will establish the model of tracking MAIT cell and related cell types in murine viral infections and generate important data underpinning collaborative project grant applications.