Project Summary CD8 T cells play a critical role in the destruction of insulin-producing pancreatic ? cells leading to type 1 diabetes mellitus (T1DM). Specifically eliminating ? cell-reactive CD8 T cells would be of great therapeutic value; it would prevent diabetes development in susceptible individuals, while leaving the rest of their immune systems largely intact. However, we do not yet fully understand the molecular details underlying the development and maintenance of these pathogenic autoreactive CD8 T cells. The long-term goal of this project is to elucidate the mechanisms regulating ? cell-reactive CD8 T cell differentiation and function. CD4 T cells and CD8 T cells are both required for the development of T1DM. Recent work uncovered the cytokine IL-21 as a critical signal produced by CD4 T cells to help CD8 T cells promote diabetes progression. However, the direct effects of IL-21 on ? cell-reactive CD8 T cell function and the signaling events causing this are currently unknown. Our preliminary data suggest that IL-21 induces expression of the transcription factor BATF, and that this might cooperate with T cell receptor stimulation-induced IRF4 to regulate a transcriptional program in CD8 T cells. This leads us to hypothesize that the interaction between BATF and IRF4 is required for the maintenance of ? cell-reactive CD8 T cell function. In Aim 1, we will determine the mechanism by which CD4 T cell-derived IL-21 helps autoreactive CD8 T cells in T1DM. The effect of CD4 T cell-derived IL-21 on CD8 T cell function and BATF expression will be determined in vivo. Then, the ability of BATF to rescue CD8 T cell function in the absence of IL-21 will be tested, as well as the requirement of the physical interaction between BATF and IRF4 in this process. In Aim 2, we will elucidate the molecular mechanism of BATF-IRF4-mediated CD8 T cell diabetogenic activity. These experiments will determine if BATF and IRF4 are both required for expression of genes involved in CD8 T cell differentiation and effector function. Furthermore, they will test if BATF and IRF4 both bind to cis-regulatory elements of these genes. This proposal will help us understand how ? cell-reactive CD8 T cells differentiate and function during the pathogenesis of T1DM. This is in line with the mission of NIDDK, as the results of this project could lead to identification of the BATF-IRF4 interaction as a novel therapeutic target for the treatment or prevention of T1DM.