Hippo kinases Mst1 and Mst2 sense and amplify IL-2R−STAT5 signaling in Treg cells to establish stable regulatory activity

Interleukin-2 (IL-2) and downstream transcription factor STAT5 are important for maintaining regulatory T (Treg) cell homeostasis and function. Tregs can respond to low levels of IL-2, but the mechanisms by which STAT5 is activated during partial IL-2 deficiency remain uncertain. We identified the serine-threonine kinase Mst1 as a signal-dependent amplifier of IL-2−STAT5 activity in Tregs. Tregs had high Mst1 and Mst2 (Mst1−Mst2) activity that was crucial to prevent tumor resistance and autoimmunity. Mechanistically, Mst1−Mst2 sensed IL-2 signals to promote the STAT5 activation necessary for Treg cell homeostasis and lineage stability, and maintain the highly suppressive phophorylated-STAT5+ Treg cell subpopulation. Unbiased quantitative proteomics revealed an association of Mst1 with the cytoskeletal DOCK8−LRCHs module that shaped activity Rho-family GTPase Rac activity, which in turn mediated STAT5 activation in Tregs. Collectively, IL-2−STAT5 signaling critically depends upon Mst1−Mst2 functions to maintain a stable Treg cell pool and immune tolerance. We used microarrays to compare the global transcription profiles of WT and Mst1/Mst2-null Treg cell populations In total 9 samples were analyzed: Treg cells (WT, n = 5; Mst1/Mst2-null, n = 4).