The T cell ecosystem: How do T cells co-exist and co-regulate each other during an immune response?

Most entities, such as humans, animals or plants, are living in and benefit from being in a group, society or ecosystem. For example, bees inform each other when they find a food source, and keep all the food they individually find into the beehive, which benefits the whole colony. This proposal is based on the hypothesis some white blood cells called CD8 T cells also organise themselves as an ecosystem during healthy responses, and that this is beneficial for the overall outcome. Each CD8 T cell clone is unique, as it expresses a specific T cell receptor that recognises different sets of antigens with different affinities. It has been proposed that this diversity was required for long-term immunity towards viruses. Diversity happens despite the fact that some CD8 T cells have a competitive advantage and are sufficient to control infections, suggesting that the breadth of CD8 T cell clones recruited to a response is actively regulated. How different T cell clones co-exist and communicate to achieve their consensus goal to provide long-term protection, is unknown. We propose that this resembles an ecosystem based on collective behaviour, and aim to establish by which mechanism the diverse CD8 T cell clones coexist at the initiation of an immune response. In particular, we will:
1) Define T cell inter-communication and co-influence. We will address the fate of diverse CD8 T cell clones when they are by themselves, or when they cohabit with each other.
2) Define the environment that fosters CD8 T cell diversity. We will define the cellular composition in space and time of this environment, and whether this environment might triage recruited T cells.
3) Define the relevance of a T cell ecosystem for the outcome of heterosubtypic immunity.
Altogether, we expect the outcomes will improve our understanding of healthy CD8 T cell responses and ultimately bring new concepts to manipulate those responses. Vaccination is the best example of successful manipulation of the immune system, however, the control of some pathogens, like influenza. CD8 T cells are usually not triggered during vaccination, despite their importance for virus eradication. This project will provide a comprehensive characterisation of the mechanisms and environment necessary for the generation of a T cell ecosystem, and the benefits for immune responses.

CD8 T cell responses are composed of multiple T cell clones recognizing their activating peptide with different affinity. This clonally diverse response is always observed despite the fact that activation of high-affinity clones is favoured and sufficient to control infections. This suggests that the breadth of CD8 T cell clones recruited during an immune response is tightly regulated. We previously showed that CD8 T cells can regulate each other's fate through direct communication and sharing of the cytokine IFNg. In addition, our preliminary data suggest that very low affinity T cells are the most potent T cell producers of IFNg at the early stages of an immune response, and that strength of T cell activation might control responsiveness to IFNg. Based on these, we hypothesise that very low-affinity T cells triggered during an healthy immune response have a specific function, distinct from direct pathogen clearance, in increasing the breadth of CD8 T cell responses through IFNg-dependent inhibition of high-affinity T cells. To test this hypothesis, we will:
1. Characterize the outcome of T cell co-regulation. We will use a reductionist system where we will control T cell affinity for its antigen and mix T cells with different affinities. We will address the role of very low-affinity clones on other clones and characterize the molecular mechanism underpinning co-regulation.
2. We hypothesise that T cell co-regulation is restricted in space and time. Using several imaging methods, we will characterise the dynamics of T cell co-regulation and the microenvironment favouring this.
3. We will define the function of T cell co-regulation in increased diversity and its relevance for heterosubtypic immunity. High T cell diversity and low-affinity T cells have been correlated with cross-protection and heterosubtypic immunity. We will test the involvement of T cell co-regulation in endogenous clonal diversity by using an heterosubtypic model of Influenza infection.

This project will characterise the relevance of direct interactions between CD8 T cells on clonal diversity and cross-protection during viral infection. This is likely to have impact on several areas:
1) Main beneficiaries: scientists working in T cell biology and closely related fields. The work will result in the advancement of our knowledge of CD8 T cell responses, how the breadth of those responses is regulated, and its impacts on the generation of immune responses, especially cross-protection towards highly mutating viruses. As such, the outcomes of this research are likely to be wide reaching, including those who work on protective immunity and tumour immunity. The work may also benefit researchers outside these fields, for example those who work in cytokine signalling; the potential role of integrins in cytokine specification could be of great interest for researcher in this field. Finally, researchers in behavioural fields and mathematical modelling of cell behaviour might be interested in using our data in order to model CD8 T cell behaviour. In the long-term, we speculate that the work contained in this application will have important clinical impacts, both in the design of vaccination strategies that will improve vaccination against high-mutating viruses such as Influenza, but also in other immune-related therapies such as cancer immunotherapies. All of these impacts have the potential to significantly improve the health and quality of life of patients across a spectrum of ages and disease types.
2) Data arising from this project will bring benefit not only academic institutions seeking to develop better vaccines, but also the pharmaceutical industry and the veterinary sector. Successful vaccines have been developed mostly against pathogens that show no or limited antigenic variation and that can be controlled by neutralising serum antibodies. In contrast, the control of pathogens that display more variable antigens and require T cell immunity remains more tenuous. The commercial sector is likely to benefit from this research through the characterisation of the key requirements of CD8 T cell responses in healthy individuals. This could be translated, for example, to new vaccination designs, for humans and animals, that could take into account the function of T cell collectivity for the generation of cross-protection.
3) Health care professionals and patients will benefit from work undertaken in this study. Although this proposal focuses on basic research by characterising interactions between CD8 T cells, this work will also advance of the mechanisms underlying cross-protection, which is essential for successful vaccines towards high-mutating viruses. One of such viruses is Influenza. It induces a highly contagious respiratory illness, which still results in 28,000 hospitalizations and 7,000 deaths in the UK yearly despite current vaccination scheme. This is of particular importance for younger and elderly people, which are more susceptible to Influenza infection. The healthcare costs are substantial and therefore any advances which could be used to design better vaccines will have a potential impact on those costs by reducing the number hospitalisations. This can create economic benefits by saving health care costs, to allow healthy ageing but also to boost the UK-based pharmaceutical industry.