ICAT expression disrupts β-catenin-TCF interactions and impairs survival of thymocytes and activated mature T cells

T cell factor (TCF) family of transcription factors and β-catenin critically regulate T cell development as demonstrated by the deletion of the tcf gene, which results in a block early in development that becomes complete in mice bearing tcf/lef double deletion. However, the role of β-catenin, a major TCF cofactor, remains controversial. To directly address this, we have generated transgenic mice expressing Inhibitor of β-catenin and TCF (ICAT), a naturally occurring inhibitor that specifically disrupts TCF and β-catenin interactions. In this report, we demonstrate that disrupting the interaction of β-catenin with TCF renders adult thymocytes and activated T cells highly susceptible to apoptosis. In contrast to previously reported observations during fetal thymocyte development, these data show that in adult mice, survival and not differentiation of thymocytes, depends on transcription by TCF and β-catenin. Indeed, we demonstrate that expression of ICAT impedes thymocyte survival by reducing the expression of Bcl_xL in thymocytes below a critical threshold. Survival of activated mature T cells was also impaired due to diminished expression of activation-induced Bcl_xL. Accordingly, expression of transgenic Bcl-2 rescued activated ICAT-Tg CD4 T cells from apoptosis. Thus, disruption of TCF-β-catenin interactions specifically impairs the survival of thymocytes and activated T cells.