Notch3 and canonical NF-kappaB signaling pathways cooperatively regulate Foxp3 transcription.

Notch3 overexpression has been previously shown to positively regulate the generation and function of naturally occurring regulatory T cells and the expression of Foxp3, in cooperation with the pTα/pre-TCR pathway. In this study, we show that Notch3 triggers the trans activation of Foxp3 promoter depending on the T cell developmental stage. Moreover, we discovered a novel CSL/NF-κB overlapping binding site within the Foxp3 promoter, and we demonstrate that the activation of NF-κB, mainly represented by p65-dependent canonical pathway, plays a positive role in Notch3-dependent regulation of Foxp3 transcription. Accordingly, the deletion of protein kinase C, which mediates canonical NF-κB activation, markedly reduces regulatory T cell number and per cell Foxp3 expression in transgenic mice with a constitutive activation of Notch3 signaling. Collectively, our data indicate that the cooperation among Notch3, protein kinase C, and p65/NF-κB subunit modulates Foxp3 expression, adding new insights in the understanding of the molecular mechanisms involved in regulatory T cell homeostasis and function.