ABSTRACT Asthma is a chronic inflammatory disease of the airways that affects 26 million Americans and results in approximately 3600 deaths per year in the US alone (1a). Asthma is a heterogeneous disease and both Th2 predominant and Th17 predominant forms have been described with different etiologies. Interestingly, it was recently determined that a subset of patients with particularly severe asthma contain T cells that make both Th2 and Th17 cytokines. However, mechanisms that prevent allergen specific T cells from becoming Th2/17 dual producers have not been described. We have identified a novel inhibitory pathway that limits the differentiation and pathogenicity of both Th2 and Th2/17 dual-producing T cells and protects against airway remodeling in mice. This pathway is controlled by the E3 ubiquitin ligase Cul5. Supporting this, we recently generated mice in which Cul5 was deleted only in T cells, and determined that Cul5 limits Th2 and Th2/17 differentiation and airway remodeling after asthma induction. Specifically, we found that mice lacking Cul5 in T cells showed increased lung inflammation, eosinophilia, goblet cell hyperplasia and fibrosis following house dust mite exposure. T cells from Cul5fl/flCD4-Cre mice were more likely to produce Th2 cytokines and were much more likely to become Th2/17 dual cytokine producing T cells. Using a screen to reveal Cul5 binding partners, we identified two substrate receptors that cooperate with Cul5 in T cells. Based on these preliminary data we hypothesize that Cul5 associates with substrate receptors and other factors to ubiquitylate substrates and thus limits the differentiation of Th2 and Th2/17 dual cytokine producing cells and prevents asthma. Based on our preliminary data, our long term goal is to develop novel therapeutic strategies that activate the Cul5 pathway to turn Th2 and Th2/17 cells off in patients with asthma. However, to do this effectively we must first determine 1) how Cul5 regulates T cell biology, 2) determine how interacting partners aid Cul5 function, and 3) delineate how, on a mechanistic level, Cul5 complexes limit T cell differentiation and pathogenicity (i.e identify substrates). In this proposal we will determine key aspects of how Cul5 restricts T cell differentiation and function, thus revealing the signaling pathways that allow Th2/17 cells to develop and drive asthma. Additionally, we will identify regulatory mechanisms that promote the activation and function of Cul5. This information will provide crucial information needed as we begin to develop therapies to target Cul5 to reduce the differentiation of Th2 and Th2/17 cells in allergic asthma.