Taku Naito

Double-positive thymocytes that have passed positive selection migrate from the cortex to the medulla, where negative selection and the development of thymic regulatory T cells (tTregs) take place. Medullary thymic epithelial cells (mTECs) play important roles in these selections, and their differentiation and maintenance depend on interaction with positively selected CD4+ single-positive cells. Therefore, migration and differentiation after positive selection must be coordinated to establish immune tolerance. However, the regulatory mechanisms of these processes are not fully understood. SATB1 is a genome organizer highly expressed in double-positive thymocytes, and SATB1 deletion causes various defects in T-cell development, including impaired positive and negative selection and tTreg differentiation. Here, we show that SATB1 is critical for temporally coordinated thymocyte trafficking after positive selection in mice. Satb1 knockout (ΔSatb1) led to precocious thymic egress caused by augmented S1pr1 upregulation in positively selected thymocytes, accompanied by lower induction of Ccr7, Tnfsf11, and Cd40lg. Altered thymocyte trafficking and functionality affected the differentiation of mTECs and, in turn, tTreg differentiation. Thus, SATB1 is required to establish immune tolerance, at least in part, by ensuring timely thymic egress and mTEC differentiation.