The multiplicity of Notch receptors raises the question of the contribution of specific isoforms to T-cell development. Notch3 is expressed in CD4(-)8(-) thymocytes and is down-regulated across the CD4(-)8(-) to CD4(+)8(+) transition, controlled by pre-T-cell receptor signaling. To determine the effects of Notch3 on thymocyte development, transgenic mice were generated, expressing lck promoter-driven intracellular Notch3. Thymuses of young transgenics showed an increased number of thymocytes, particularly late CD4(-)8(-) cells, a failure to down-regulate CD25 in post-CD4(-)8(-) subsets and sustained activity of NF-kappaB. Subsequently, aggressive multicentric T-cell lymphomas developed with high penetrance. Tumors sustained characteristics of immature thymocytes, including expression of CD25, pTalpha and activated NF-kappaB via IKKalpha-dependent degradation of IkappaBalpha and enhancement of NF-kappaB-dependent anti-apoptotic and proliferative pathways. Together, these data identify activated Notch3 as a link between signals leading to NF-kappaB activation and T-cell tumorigenesis. The phenotypes of pre-malignant thymocytes and of lymphomas indicate a novel and particular role for Notch3 in co-ordinating growth and differentiation of thymocytes, across the pre-T/T cell transition, consistent with the normal expression pattern of Notch3.

C/EBP beta is considered a key element of interleukin‐6 (IL‐6) signalling as well as an important transcriptional regulator of the IL‐6 gene itself. We describe here how mice lacking C/EBP beta develop a pathology similar to mice overexpressing IL‐6 and nearly identical to multicentric Castleman's disease in human patients, with marked splenomegaly, peripheral lymphadenopathy and enhanced haemopoiesis. Humoral, innate and cellular immunity are also profoundly distorted, as shown by the defective activation of splenic macrophages, the strong impairement of IL‐12 production, the increased susceptibility to Candida albicans infection and the altered T‐helper function. Our data show that C/EBP beta is crucial for the correct functional regulation and homeostatic control of haemopoietic and lymphoid compartments.

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