Notch3 and pT{alpha}/pre-TCR sustain the in vivo function of naturally occurring regulatory T cells.

Dysregulated generation and/or function of naturally occurring ‘CD41CD251 regulatory T cells’ (Tregs) play key role in the development of autoimmune diseases, including type 1 diabetes. Recent findings suggest that Notch3 signaling activation promotes thymic generation and peripheral expansion and in vivo function of naturally occurring Tregs, thus preventing autoimmune diabetes progression in mouse models. However, the mechanisms underlying these effects have remained elusive, thus far. Here, we show that the expression of pTa gene is up-regulated in naturally occurring Tregs, at both mRNA and protein levels. Moreover, by using double mutant mice, with T cell-targeted constitutive activation of Notch3 in a pTa2/2 background, we demonstrate that pTa deletion significantly counteracts the Notch3-dependent expansion, the increased FoxP3 expression and the enhanced in vitro activity of naturally occurring Tregs. Notably, the absence of pTa also impairs the Notch3- dependent protection against experimentally induced autoimmune diabetes. Finally, by adoptive cell transfer experiments, we demonstrated that this failure is directly related to the impaired in vivo function of naturally occurring Tregs bearing pTa deletion. Collectively, our data suggest that pTa expression is required for the in vivo function of naturally occurring Tregs and that the activation of Notch3 signaling may positively regulate the function of this population, through the pTa/pre-T cell receptor pathway.