Fibroblastic Stromal Cells Expressing Delta-like Notch Ligands Control Extrathymic T Cell Development in Mesenteric Lymph Nodes

Early T cell development is supported by signals mediated by the Notch1 receptor in T lineage progenitors and Delta-like4 (Dll4) ligand in thymic epithelial cells. Although T cell development is normally restricted to the thymus, extrathymic T cell development has been reported in athymic Foxn1^nu/nu nude mice as well as during times of thymopoietic stress such as post-bone marrow transplantation (BMT). A population of CD4+CD8a double positive T cell progenitors can be found in the mesenteric lymph nodes (MLN) in both athymic mice and early post-BMT, suggesting the presence of an extrathymic niche conducive to T cell development. However, whether Notch signaling is required, as well as the cellular source(s) of Notch ligand throughout the process in the MLN, remains unknown. We hypothesize that MLNs harbor a unique environment in which Notch ligands are available to circulating progenitors and critical to sustain extrathymic T cell development in these contexts. To test this, we utilized systemic neutralizing antibodies as well as loss-of-function genetic models to assess whether the Notch ligands Dll4 or Dll1 are important for extrathymic T cell development. We found that, like homeostatic thymocyte development, Dll4 appears to play an essential role in generation of early T cell progenitors found in the MLN. Furthermore, the source of Dll4 appears to reside within subsets of non-hematopoietic fibroblastic stromal cells lineage traced by a Ccl19-Cre transgene. In sum, these findings shed new light on the cellular and molecular cues regulating T cell development outside of the thymus.