Summary There are at least 49 members of the BTB-ZF gene family, which includes PLZF, BCL6, ThPOK, LRF and others. Only a few have been studied in the context of immune system development or function. Expression of the known BTB-ZF genes correlates with specific functions of the cells – in many ways, the BTB-ZF gene actually defines the subset. Therefore, our lab has hypothesized that analysis of BTB-ZF gene expression at the level of the single cell will result in the identification of new T cell subsets. What is clear is that understanding the diversity of lymphocyte subsets defined by this gene family will have a profound impact on our understanding of the overall complexity and function of the immune response. Indeed, the identification of novel immune subsets, such as Th17 T cells, Tregs, Tfh cells, has proven over the last few years to have major implications for vaccine development, cancer treatment and the management of autoimmune diseases. In this application, we show that that a specific BTB-ZF family member defines a unique subset of CD4+ T cells and a subset of Tregs. Through the use of genetically engineered mouse models we show that these cells rapidly produce effector cytokines following stimulation. We also show that, analogous to NKT cells, these cells have “pre-formed” cytokine message. The significance of this application lies in the definition of two novel T cell subsets that we hypothesize play unique roles in immune homeostasis. In this application, using genetically modified mice, adoptive cell transfers and disease models, we will study the functions of these cells particularly when they are resident in specific tissues. We also will explore the interrelationships between the cells in spleen, intestine and visceral fat.