Ellen A. Robey

Somatic rearrangement of the genes encoding antigen receptors allows the mammalian immune system to produce receptors that can recognize virtually any foreign protein. This rearrangement process also generates nonfunctional antigen receptors as well as receptors that can recognize self-proteins. During thymic development, T cells go through a testing process which ensures that cells expressing useless or harmful antigen receptors do not mature. The selection of T cells in the thymus consists of several components driven by distinct recognition events that have distinct consequences for the cell. T cell selection appears to begin before the rearrangement process is complete. Evidence is emerging that T cells have a developmental checkpoint to make sure that the antigen receptor beta gene is successfully rearranged before development can proceed. This checkpoint also appears to be linked to the regulation of rearrangement and mechanisms to ensure that each cell expresses only one antigen receptor (allelic exclusion). After a T cell has successfully rearranged and expressed both its alpha and beta antigen receptor genes, a second phase of selection occurs. During this phase, T cells with self-reactive antigen receptors are eliminated by negative selection, and T cells that can recognize foreign peptides bound to polymorphic self-MHC molecules are selected to mature (positive selection. How these seemingly incompatible forms of selection both occur is a subject of considerable interest. Positive selection is linked also to the choice between the CD4 helper lineage and the CD8 cytotoxic T cell lineage; the mechanisms by which these two events are linked is an ongoing area of investigation.

The Notch/Lin-12/Glp-1 receptor family participates in cell-cell signaling events that influence cell fate decisions. Although several Notch homologs and receptor ligands have been identified, the nuclear events involved in this pathway remain incompletely understood. A truncated form of Notch, consisting only of the intracellular domain (NotchIC), localizes to the nucleus and functions as an activated receptor. Using both an in vitro binding assay and a cotransfection assay based on the two-hybrid principle, we show that mammalian NotchIC interacts with the transcriptional repressor CBF1, which is the human homolog of Drosophila Suppressor of Hairless. Cotransfection assays using segments of mouse NotchIC and CBF1 demonstrated that the N-terminal 114-amino-acid region of mouse NotchIC contains the CBF1 interactive domain and that the cdc10/ankyrin repeats are not essential for this interaction. This result was confirmed in immunoprecipation assays in which the N-terminal 114-amino-acid segment of NotchIC, but not the ankyrin repeat region, coprecipitated with CBF1. Mouse NotchIC itself is targeted to the transcriptional repression domain (aa179 to 361) of CBF1. Furthermore, transfection assays in which mouse NotchIC was targeted through Gal4-CBF1 or through endogenous cellular CBF1 indicated that NotchIC transactivates gene expression via CBF1 tethering to DNA. Transactivation by NotchIC occurs partially through abolition of CBF1-mediated repession. This same mechanism is used by Epstein-Barr virus EBNA2. Thus, mimicry of Notch signal transduction is involved in Epstein-Barr virus-driven immortalization.