Notch signaling augments T cell responsiveness by enhancing CD25 expression.

Notch receptors signal through a highly conserved pathway to influence cell fate decisions. Notch1 is required for T lineage commitment; however, a role for Notch signaling has not been clearly defined for the peripheral T cell response. Notch gene expression is induced, and Notch1 is activated in primary CD4(+) T cells following specific peptide-Ag stimulation. Notch activity contributes to the peripheral T cell response, as inhibition of endogenous Notch activation decreases the proliferation of activated T cells in a manner associated with the diminished production of IL-2 and the expression of the high affinity IL-2R (CD25). Conversely, forced expression of a constitutively active Notch1 in primary T cells results in increased surface expression of CD25, and renders these cells more sensitive to both cognate Ag and IL-2, as measured by cell division. These data suggest an important role for Notch signaling during CD4(+) T cell responses, which operates through augmenting a positive feedback loop involving IL-2 and its high affinity receptor.