Project Summary/Abstract The goal of this project is to understand the mechanisms that control effector lymphocyte stability and plasticity, and how this regulation influences human health and disease. Recent studies indicate that innate lymphocytes possess substantial plasticity. The underlying mechanisms, and the implications for health and disease, remain unknown. Our preliminary data indicate that exposure to Notch signaling can elicit innate lymphocyte plasticity and train mature group-2 innate lymphoid cells (ILC2) to acquire the ability to co-produce large amounts of both ILC2- and ILC3- characteristic cytokines, thus converting natural ILC2 (nILC2) into plastic inflammatory ILC2 (iILC2). Our new data suggest that such plastic iILC2 are relatively enriched in the airway of patients with severe refractory asthma. In this project, we will use adoptive transfer, chromatin immunoprecipitation, and RNA sequencing experiments to explore the cellular and molecular mechanisms by which Notch signaling elicits ILC2 plasticity. We will also examine the capability of human and mouse iILC2 to mediate airway inflammation and hyperresponsiveness. Finally, we will investigate the association between the development of plastic iILC2 and the susceptibility to severe refractory asthma in human adult patients. Together, these experiments will shed light on the mechanisms that govern lymphocyte lineage stability and plasticity, and will inform strategies of targeted therapy to treat patients with asthma and other auto-immune and inflammatory disorders.