Mechanism of Transcriptional Regulation of Th2 Cell Development

PROJECT SUMMARY CD4+ T helper (Th) cells of different Th1, Th2, Th17 and Treg subtypes, developed in thymus, have important functions in adaptive immunity. Their dysregulation has also been implicated in diseases such as Th2- associated inflammatory and allergic diseases including asthma, Crohn’s disease and atopic dermatitis. Th2 lineage-specific differentiation from naïve CD4+ T cells is tightly controlled by cytokines (IL-4, 5, 13) through highly ordered subtype-specific gene transcription directed by a set of transcription factors and regulators with opposing activities to promote (YY1, Gata3, Stat6) or inhibit (Foxp3) Th2 cell differentiation. However, the underlying molecular mechanism remains elusive. BRD4, a key member of the bromodomain and extra- terminal (BET) protein family, is well known for its role in gene transcriptional activation, but whether and how it functions in gene transcriptional repression in a cell-type specific manner has remained elusive. Recently, we discovered that Brd4 works with Polycomb repressive complex 2 (PRC2) to repress transcriptional expression of Th2-negative regulators Foxp3 and E3-ubiqutin ligase Fbxw7 in lineage-specific differentiation of Th2 cells from mouse primary naïve CD4+ T cells. Brd4 through its second bromodomain (BD2) binds to lysine- acetylated-EED subunit of the PRC2 complex to control target gene repression through histone H3 lysine 27 trimethylation (H3K27me3). We found that Foxp3 represses transcription of Th2-specific transcription factor Gata3, while Fbxw7 controls Gata3 protein stability via ubiquitination-mediated protein degradation, which in turn ensures Gata3-mediated transcriptional activation of Th2 cytokines including Il4, Il5 and Il13. Chemical inhibition of BRD4 BD2 induces transcriptional de-repression of Foxp3 and Fbxw7, and transcriptional down- regulation of Il4, Il5 and Il13, resulting in inhibition of Th2 cell differentiation. Our study uncovers previously unappreciated BRD4 functions in directing Th2-specific gene transcriptional repression to safeguard Th2 cell lineage differentiation. Building on our promising findings, in this study, we seek to define the mechanistic details of BRD4’s transcriptional repression function in the temporal regulation of Th2 cells, develop and use new BRD4-BD2 selective inhibitors to study the transcriptional regulation of Th2 cell differentiation, and investigate the therapeutic potential of BRD4-BD2 inhibition as new Th2 immunomodulation to block Th2- associated allergic inflammation in mouse models of asthma.