Cutting Edge: A Thymocyte-Thymic Epithelial Cell Cross-Talk Dynamically Regulates Intrathymic IL-7 Expression In Vivo

Abstract

Thymic epithelial cells (TECs) are the predominant intrathymic source of the essential thymopoietin IL-7. Whether thymocyte-TEC interactions have a role in the regulation of IL-7 expression is not known. By exploiting IL-7 reporter mice in which yellow fluorescent protein expression identifies TECs expressing high levels of IL-7 (Il7+ TECs), we show that Il7+ TECs segregate from emerging medullary TECs during thymic organogenesis. Although Il7+ TECs normally diminish with age, we found that Il7+ TECs are markedly retained in alymphoid Rag2−/−Il2rg−/− IL-7 reporter mice that manifest a profound thymopoietic arrest. Transfer of Tcra−/− or wild-type (but not Rag2−/−) hematopoietic progenitors to alymphoid IL-7 reporter recipients normalizes the frequency of Il7+ TECs and re-establishes cortical TEC/medullary TEC segregation. Although thymocyte-derived signals are often considered stimulatory for TEC maturation, our findings identify a negative feedback mechanism in which signals derived from TCRβ-selected thymocytes modulate TEC-dependent IL-7 expression.