Ripk1 is critical for preserving effector regulatory T cells and the suppressive transcriptional program in regulatory T cells.

Ripk1 plays an important role as a regulator of programmed cell death processes such as apoptosis and necroptosis and is involved in initiating pro-inflammatory NF-κB signaling. Immune tolerance depends on the proper function and homeostasis of regulatory T (Treg) cells. Here, we show that specific ablation of Ripk1 in Treg cells leads to systemically reduced Treg cell numbers resulting in spontaneous whole-body pathology. Using chimeric mice that allowed us to study Treg cells in the absence of inflammatory conditions, we observed a competitive disadvantage in vivo of Ripk1-deficient compared to Ripk1-proficient Treg cells. Furthermore, single-cell RNA sequencing revealed that Ripk1 is required for the maintenance of the effector Treg cell transcriptional signature, which is essential to prevent immune dysregulation. To overcome the limitation of low cell numbers in the chimeric mice, we isolated Treg cells from mice, in which Ripk1 could be deleted in a tamoxifen-inducible manner. Despite the strong reduction detected in Ripk1-deficient Treg cells of the chimeric mice, we did not observe impaired viability by the sole absence of Ripk1 in Treg cells from the inducible system. Of note, we observed reduced viability of activated Ripk1-deficient Treg cells in the presence of TNF. Together, these findings highlight the fundamental role of Ripk1 in maintaining immune homeostasis by preserving the highly suppressive effector Treg cells.