Controlled WASp activity regulates the proliferative response for Treg cell differentiation in the thymus

The Wiskott-Aldrich syndrome protein (WASp) regulates actin cytoskeletal dynamics and function of hematopoietic cells. Mutations in the WAS gene lead to two different syndromes; Wiskott-Aldrich syndrome (WAS) caused by loss-of-function mutations, and X-linked neutropenia (XLN) caused by gain-of-function mutations. We previously showed that WASp-deficient mice have a decreased number of regulatory T (Treg) cells in the thymus and the periphery. We here evaluated the impact of WASp mutations on Treg cells in the thymus of WAS and XLN mouse models. Using in vitro Treg differentiation assays, WAS CD4 single-positive thymocytes have decreased differentiation to Treg cells, despite normal early signaling upon IL-2 and TGF-β stimulation. They failed to proliferate and express CD25 at high levels, leading to poor survival and a lower number of Foxp3⁺ Treg cells. Conversely, XLN CD4 single-positive thymocytes efficiently differentiate into Foxp3⁺ Treg cells following a high proliferative response to IL-2 and TGF-β, associated with high CD25 expression when compared with WT cells. Altogether, these results show that specific mutations of WASp affect Treg cell development differently, demonstrating a critical role of WASp activity in supporting Treg cell development and expansion.