IL-7 derived from lymph node fibroblastic reticular cells is dispensable for naive T cell homeostasis but crucial for central memory T cell survival.

The survival of peripheral T cells is dependent on their access to peripheral LNs (pLNs) and stimulation by IL-7. In pLNs fibroblastic reticular cells (FRCs) and lymphatic endothelial cells (LECs) produce IL-7 suggesting their contribution to the IL-7-dependent survival of T cells. However, IL-7 production is detectable in multiple organs and is not restricted to pLNs. This raises the question whether pLN-derived IL-7 is required for the maintenance of peripheral T cell homeostasis. Here, we show that numbers of naive T cells (T ) remain unaffected in pLNs and spleen of mice lacking Il7 gene activity in pLN FRCs, LECs, or both. In contrast, frequencies of central memory T cells (T ) are reduced in FRC-specific IL-7 KO mice. Thus, steady state IL-7 production by pLN FRCs is critical for the maintenance of T , but not T , indicating that both T cell subsets colonize different ecological niches in vivo.