Background: CD4+ T lymphocyte subsets orchestrate immune responses in health and disease. Little is known on control of T cell differentiation exerted by microRNA that affect mRNA translation. The identification of microRNA and their targets that regulate differentiation of T cell subsets may provide new therapeutic targets for immune-mediated diseases. Since microRNA are released in exosomes and circulate in blood, activities of tissue-derived lymphocytes could be assessed by microRNA signatures in the serum. We have defined microRNAs present in resting lymphocyte subsets from peripheral blood and measured lymphocyte-derived microRNAs in the serum. We have also solved important challenges for the identification of microRNA targets, the definition of signatures of activated T cells and their monitoring in the serum, which form the key topics of this application. Advancing State-of-the-Art and objectives: We will identify microRNA of CD4+ T cell subsets purified from inflamed organs and investigate microRNA target network that regulates T cell differentiation. We will exploit this knowledge to profile signatures of in vivo activated T cells and to map genes that could improve understanding of T cell commitment. We will also develop quantitative assays to monitor microRNA signatures in the serum and provide functional evidence of key genes targeted by microRNA which could be targets of immunomodulatory drugs. Significance: This application addresses important challenges at the frontiers of immunology and could lead to significant advances in immunotherapies and diagnostic tools for patients with immune mediated diseases. New ways of identifying microRNA targets and techniques to quantify microRNA signatures in the serum, could be widely applicable in biomedical research.