P21-activated kinase 2 is essential in maintenance of peripheral Foxp3+ regulatory T cells.

The p21-activated kinase 2 (Pak2), an effector molecule of the Rho family GTPases Rac and Cdc42, regulates diverse functions of T cells. Previously, we showed that Pak2 is required for development and maturation of T cells in the thymus, including thymus-derived regulatory T (Treg) cells. However, whether Pak2 is required for the functions of various subsets of peripheral T cells, such as naive CD4 and helper T-cell subsets including Foxp3+ Treg cells, is unknown. To determine the role of Pak2 in CD4 T cells in the periphery, we generated inducible Pak2 knockout (KO) mice, in which Pak2 was deleted in CD4 T cells acutely by administration of tamoxifen. Temporal deletion of Pak2 greatly reduced the number of Foxp3+ Treg cells, while minimally affecting the homeostasis of naive CD4 T cells. Pak2 was required for proliferation and Foxp3 expression of Foxp3+ Treg cells upon T-cell receptor and interleukin-2 stimulation, differentiation of in vitro induced Treg cells, and activation of naive CD4 T cells. Together, Pak2 is essential in maintaining the peripheral Treg cell pool by providing proliferation and maintenance signals to Foxp3+ Treg cells.