The thymic medulla is required for Foxp3+ regulatory but not conventional CD4+ thymocyte development

Abstract

A key role of the thymic medulla is to negatively select autoreactive CD4⁺ and CD8⁺ thymocytes, a process important for T cell tolerance induction. However, the involvement of the thymic medulla in other aspects of αβ T cell development, including the generation of Foxp3⁺ natural regulatory T cells (nT_reg cells) and the continued maturation of positively selected conventional αβ T cells, is unclear. We show that newly generated conventional CD69⁺Qa2⁻ CD4 single-positive thymocytes mature to the late CD69⁻Qa2⁺ stage in the absence of RelB-dependent medullary thymic epithelial cells (mTECs). Furthermore, an increasing ability to continue maturation extrathymically is observed within the CD69⁺CCR7^−/loCCR9⁺ subset of conventional SP4 thymocytes, providing evidence for an independence from medullary support by the earliest stages after positive selection. In contrast, Foxp3⁺ nT_reg cell development is medullary dependent, with mTECs fostering the generation of Foxp3⁻CD25⁺ nT_reg cell precursors at the CD69⁺CCR7⁺CCR9⁻ stage. Our results demonstrate a differential requirement for the thymic medulla in relation to CD4 conventional and Foxp3⁺ thymocyte lineages, in which an intact mTEC compartment is a prerequisite for Foxp3⁺ nT_reg cell development through the generation of Foxp3⁻CD25⁺ nT_reg cell precursors.