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Generation of effective immune responses requires expansion of rare antigen-specific CD4+ T cells. The magnitude of the responding population is ultimately determined by proliferation and survival. Both processes are tightly controlled to limit responses to innocuous antigens. Sustained expansion occurs only when innate immune sensors are activated by microbial stimuli or by adjuvants, which has important implications for vaccination. The molecular identity of the signals controlling sustained T-cell responses is not fully clear. Here, we describe a prominent role for the Notch pathway in this process. Coactivation of Notch allows accumulation of far greater numbers of activated CD4+ T cells than stimulation via T-cell receptor and classic costimulation alone. Notch does not overtly affect cell cycle entry or progression of CD4+ T cells. Instead, Notch protects activated CD4+ T cells against apoptosis after an initial phase of clonal expansion. Notch induces a broad antiapoptotic gene expression program that protects against intrinsic, as well as extrinsic, apoptosis pathways. Both Notch 1 and Notch2 receptors and the canonical effector RBPJ (recombination signal binding protein for immunoglobulin kappa J region) are involved in this process. Correspondingly, CD4+ T-cell responses to immunization with protein antigen are strongly reduced in mice lacking these components of the Notch pathway. Our findings, therefore, show that Notch controls the magnitude of CD4+ T-cell responses by promoting cellular longevity. [ABSTRACT FROM AUTHOR]

Notch has been linked to β-catenin-dependent tumorigenesis; however, the mechanisms leading to Notch activation and the contribution of the Notch pathway to colorectal cancer is not yet understood. By microarray analysis, we have identified a group of genes downstream of Wnt/β-catenin (down-regulated when blocking Wnt/βcatenin) that are directly regulated by Notch (repressed by γ-secretase inhibitors and up-regulated by active Notchi in the absence of β-catenin signaling). We demonstrate that Notch is downstream of Wnt in colorectal cancer cells through β-catenin-mediated transcriptional activation of the Notch-ligand Jaggedi. Consistently, expression of activated Notchi partially reverts the effects of blocking Wnt/β-catenin pathway in tumors implanted s.c. in nude mice. Crossing APC[supMinl]with Jagged1[sup+/Δ] miceis sufficient to significantly reduce the size of the polyps arising in the APC mutant background indicating that Notch is an essential modulator of tumorigenesis induced by nuclear β-catenin. We show that this mechanism is operating in human tumors from Familial Adenomatous Polyposis patients. We conclude that Notch activation, accomplished by β-catenin-mediated up-regulation of Jagged1, is required for tumorigenesis in the intestine. The Notch-specific genetic signature is sufficient to block differentiation and promote vasculogenesis in tumors whereas proliferation depends on both pathways. [ABSTRACT FROM AUTHOR]