Notch3 and pTα/pre-TCR sustain the in vivo function of naturally occurring regulatory T cells

Dysregulated generation and/or function of naturally occurring 'CD4 ⁺CD25 ⁺ regulatory T cells' (T _regs) 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 T _regs, 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 pTα gene is up-regulated in naturally occurring T _regs, at both mRNA and protein levels. Moreover, by using double mutant mice, with T cell-targeted constitutive activation of Notch3 in a pTα ^-/- background, we demonstrate that pTα deletion significantly counteracts the Notch3-dependent expansion, the increased FoxP3 expression and the enhanced in vitro activity of naturally occurring T _regs. Notably, the absence of pTα 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 T _regs bearing pTα deletion. Collectively, our data suggest that pTα expression is required for the in vivo function of naturally occurring T _regs and that the activation of Notch3 signaling may positively regulate the function of this population, through the pTα/pre-T cell receptor pathway.