Notch proteins are cell surface receptors that mediate developmental cell specification events. To explore the function of murine Notch1, an essential portion of the gene was flanked with loxP sites and inactivation induced via interferon-regulated Cre recombinase. Mice with a neonatally induced loss of Notch1 function were transiently growth retarded and had a severe deficiency in thymocyte development. Competitive repopulation of lethally irradiated wild-type hosts with wild-type- and Notch1-deficient bone marrow revealed a cell autonomous blockage in T cell development at an early stage, before expression of T cell lineage markers. Notch1-deficient bone marrow did, however, contribute normally to all other hematopoietic lineages. These findings suggest that Notch1 plays an obligatory and selective role in T cell lineage induction.

No abstract

The transcription factor interferon regulatory factor 1 (IRF‐1) binds tightly to the interferon (IFN)‐beta promoter and has been implicated in the induction of type I IFNs. We generated mice devoid of functional IRF‐1 by targeted gene disruption. As reported by others, IRF‐1‐deficient mice showed a discrete phenotype: the CD4/CD8 ratio was increased and IFN‐gamma‐induced levels of macrophage iNO synthase mRNA were strongly diminished. However, type I IFN induction in vivo by virus or double‐stranded RNA was unimpaired, as evidenced by serum IFN titers and IFN mRNA levels in spleen, liver and lung. There was also no impairment in the response of type I IFN‐inducible genes. Therefore, IRF‐1 is not essential for these processes in vivo.

Gene disruptions and deletions of up to 20kb have been generated by homologous recombination with appropriate targeting vectors in murine embryonic stem (ES) cells. Because we could not obtain a deletion of about 200 kb in the mouse amyloid precursor protein gene by the classical technique, we employed strategies involving the insertion of loxP sites upstream and downstream of the region to be deleted by homologous recombination and elicited excision of the loxP-flanked region by introduction of a Cre expression vector into the ES cells. In the first approach, the loxP sequences were inserted in two successive steps and after each step, ES cell clones were isolated and characterized. Deletion of the loxP-flanked sequence was accomplished by introducing the cre gene in a third step. In the second approach, ES cells containing the upstream loxP cassette were electroporated simultaneously with the downstream loxP targeting vector and the Cre expression plasmid. ES cells were obtained that gave rise to chimeric mice capable of germ-line transmission of the deleted amyloid precursor protein allele.

The IL-1 receptor-related protein T1 is expressed on the surface of Th2, but not Th1 cells. Studies with anti-T1 monoclonal antibodies have suggested that T1 is critical for development of normal Th2-type responses. To elucidate the role of T1 in vivo, we generated T1deficient mice and a T1-transgenic strain which secretes soluble T1-Fc fusion protein into the serum. These were analyzed for the Th2 immune response induced by infection with the parasitic nematode Nippostrongylus brasiliensis. Although Th2 cytokine production by lymph node cells was similar in all groups of N. brasiliensis-infected mice, a decrease in IL-5 production by lung lymphocytes was detected in both T1-deficient and T1-Fc-transgenic mice compared to control littermates. This difference in IL-5 production did not influence blood eosinophilia, but recruitment of eosinophils into lung tissue, especially in T1-Fctransgenic mice was slightly decreased. However, induction of all other immune parameters was normal and both T1-deficient and T1-Fc-transgenic mice were able to clear the parasite infection within 12 days with kinetics similar to those in control mice. Therefore, in contrast to previous suggestions, we conclude that the T1 protein is not obligatory for normal development of Th2 immune responses.