Indeed, anti-apoptotic Bcl-2 upregulation heavily depends on IL-15 transpresentation in the intestinal epithelium (30). to re-express coreceptors after OP9-DL1 culture, they eventually mature and accumulate in the spleen where TCR and IL-15/STAT5 signaling promotes their conversion to CD8 cells and their expression of gut-homing receptors. Adoptive transfer of splenic DN cells gives rise to CD8 cells in the gut, establishing their precursor relationship in vivo. Interestingly, Bim does not restrict the IL-15Cdriven maturation of CD8 cells that is critical for intestinal homeostasis. Thus, we found a temporal and tissue-specific role for Bim in limiting thymic agonist selection of CD8 precursors and their TCR repertoire, but not in the maintenance of CD8 intraepithelial lymphocytes in the intestine. The intestinal epithelium continuously contacts food Ags and intestinal flora and relies on a complex network of intestinal immune cells that control immune homeostasis in the gut. Besides CD4+ regulatory T cells (Tregs) and TCRg+ cells, a high level of TCR+CD8+ and TCR+CD4?CD8? double-negative (DN) T cells reside in the intestinal intraepithelial lymphocyte (iIEL) compartment. Both DN and CD8 iIELs play an immune regulatory role in the intestine, secreting immune-suppressive cytokines to prevent inflammatory bowel diseases (1C4). With regard to TCR CD8 iIELs, there has been some controversy surrounding their development. Although earlier work suggested that this population is extrathymically derived (5, 6), more recent studies suggest that most TCR CD8 iIELs in euthymic mice are derived from thymic precursor cells, which are selected by the agonist peptide/MHC complex stimulating TCRs with a strong affinity (7C12). This agonist selection developmental model of the CD8 iIELs and DN T cells was further strengthened by recent experiments using TCR-transgenic mice in which the TCRs were derived from CD8 iIELs, in that the mice bearing these iIEL-derived self-reactive TCRs had an abundance of CD8 iIELs and DN T cells (13C15). These studies showed that the TCRs favoring DN and CD8 T cell differentiation may promiscuously interact with multiple ligands, including Byakangelicol MHC Byakangelicol class I, MHC class II (MHC II), or nonclassic MHC ligands (14, 15). During thymic development, a considerable portion of thymocytes with these TCRs undergo apoptosis. How the self-reactive thymic precursors of CD8 T and DN T cells survive negative selection-associated apoptosis and develop into mature iIELs are not well understood mechanistically. T cells with strong avidity for self-antigens are limited in the thymus, whereas a broad T cell repertoire restricted to self-MHC molecules is maintained by the mechanisms Byakangelicol of central tolerance. After TCR V(D)J rearrangement, positive selection takes place in thymic cortex and promotes the survival of CD4+CD8+ double-positive (DP) thymocytes whose TCR has at least a basal affinity for self-antigen peptide/MHC complexes. Positively selected thymocytes then undergo two waves of thymic negative selection. One wave occurs in response to ubiquitous self-antigen (UbA) and endogenous Isl1 viral superantigen (SAg) in the thymic cortex or corticomedullary junction (16). A second wave occurs when CCR7 signals direct thymocytes into thymic medulla and responses to tissue-restricted self-antigen (TRA) driven by medullary thymic epithelial cells and dendritic cells in an Aire-dependent manner (16). Those having too strong affinity are eliminated during negative selection (17). The negative selection process has been reported to be mediated Byakangelicol by the BH3-only Bcl-2 family member Bim, as Bim?/? mice are resistant to thymic negative selection in five independent models (18). Paradoxically, Bim?/? mice do not have an increase in DP (CD4+CD8+) thymocytes as would be expected if thymic negative selection were impaired (19). Additionally, other groups have failed to find a substantial role for Bim in other models of thymic negative selection, including endogenous SAg- or UbA-induced negative selection, which are thought to better represent physiologic negative selection (20, 21). One possibility is that other proapoptotic factors, such as Puma, may serve redundant roles with Bim to fully promote negative selection (22). Another nonCmutually exclusive possibility is that Bim plays a more important role in a certain type of thymocytes undergoing negative selection, such as those responding to TRA (23, 24). However, many of these studies only focused on the negative selection of conventional T cells. The spatial and temporal role of Bim Byakangelicol in agonist selection.