TGF-<sup>2</sup>signaling in thymic epithelial cells regulates thymic involution and postirradiation reconstitution
Hauri-Hohl MM., Zuklys S., Keller MP., Jeker LT., Barthlott T., Moon AM., Roes J., Holländer GA.
The thymus constitutes the primary lymphoid organ responsible for the generation of naive T cells. Its stromal compartment is largely composed of a scaffold of different subsets of epithelial cells that provide soluble and membrane-bound molecules essential for thymocyte maturation and selection. With senescence, a steady decline in the thymic output of T cells has been observed. Numeric and qualitative changes in the stromal compartment of the thymus resulting in re-duced thymopoietic capacity have been suggested to account for this physiologic process. The precise cellular and molecular mechanisms underlying thymic senescence are, however, only incompletely understood. Here, we demonstrate that TGF-β signaling in thymic epithelial cells exerts a direct influence on the cell's capacity to support thymopoiesis in the aged mouse as the physiologic process of thymic senescence is mitigated in mice deficient for the expression of TGF-ßRII on thymic epithelial cells. Moreover, TGF-ß signaling in these stromal cells transiently hinders the early phase of thymic reconstitution after myeloablative conditioning and hematopoietic stem cell transplantation. Hence, inhibition of TGF-ß signaling decelerates the process of age-related thymic involution and may hasten the reconstitution of regular thymopoiesis after hematopoietic stem cell transplantation. © 2008 by The American Society of Hematology.