Intravital Imaging of Thymopoiesis Reveals Dynamic Lympho-Epithelial Interactions

SummaryT cell development occurs in the thymus. The thymic microenvironment attracts hematopoietic progenitors, specifies them toward the T cell lineage, and orchestrates their differentiation and egress into the periphery. The anatomical location of the thymus and the intrauterine development of mouse embryos have so far precluded a direct visualization of the initial steps of thymopoiesis. Here, we describe transgenic zebrafish lines enabling the in vivo observation of thymopoiesis. The cell-autonomous proliferation of thymic epithelial cells, their morphological transformation into a reticular meshwork upon contact with hematopoietic cells, and the multiple migration routes of thymus-settling cells could be directly visualized. The unexpectedly dynamic thymus homing process is chemokine driven and independent of blood circulation. Thymocyte development appears to be completed in less than 4 days. Our work establishes a versatile model for the in vivo observation and manipulation of thymopoiesis.

► A foxn1:mCherry transgenic line specifically marks the thymic epithelium in zebrafish ► Development and proliferation of thymic epithelial cells is cell autonomous ► Thymus homing is guided by chemokines and occurs after multiple in-out-in cycles ► Thymocyte development in zebrafish embryos is completed in less than 4 days