Understanding affinity variation in the CD28/ CTLA-4 pathway and its impact on immune function.

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The CD28-CTLA-4 system is central to the control of T cell immune responses. While CD28 is an activating receptor, CTLA-4 opposes this function, inhibiting responses. Indeed, genetic deficiency in CTLA-4 function results in autoimmunity and antibody blockade is used to stimulate immunity to cancer therapeutically.Despite their opposing functions, CD28 and CTLA-4 share two ligands, CD80 and CD86, which differ in both affinity and avidity characteristics. How these distinct ligand characteristics affect immune function is not understood. In this project we propose to explore how changes in ligand characteristics impact immune function. We will generate and characterize different ligand variants in order to study their impact in a range of immune settings.Antigen presenting cells (B cell lines) will be engineered to express different ligands and functional outcomes will be compared at different levels of ligand expression. We anticipate studying human T cell responses to different ligand combinations in vitro, measuring their effect on T cell activation, expansion, survival, cellular phenotype, cytokine production and differentiation. Experiments will be carried out both in the presence and absence of CTLA-4 expression. In addition, we will perform studies to measure how different ligands are controlled by CTLA-4 transendocytosis and how they affect regulatory T cell (Treg) homeostasis. The project will provide training and experience in molecular biology, immunology and cell biology via the cloning and expressing of ligand variants and the study of their immune function. Overall, the project will generate a detailed understanding of the relationship between the biophysical characteristics CD28 and CTLA-4 ligands and their regulation of immune cell function.