Fibroblastic FLT3L supports lymph node dendritic cells in the interfollicular niche

Dendritic cell (DC) homeostasis is maintained in secondary lymphoid organs (SLOs) by Fms-like tyrosine kinase 3 ligand (FLT3L). The specific niche providing this DC growth factor within human and mouse SLOs is unclear. Here, we show that Gremlin1 (Grem1)-expressing lymph node fibroblastic reticular cells (FRCs) support DC homeostasis via provision of FLT3L. Grem1 FRCs co-localize with DCs and express FLT3L in human and mouse lymph nodes. Using a new genetic model, we provide evidence that FLT3L produced by GREM1 FRCs maintains lymph node preDCs, cDCs, and plasmacytoid DCs (pDCs). Spatial transcriptomics and cytofluorometry reveal that Grem1 FRC-derived FLT3L supports not only proliferation, but also survival of lymph node cDCs within the interfollicular zone (IFZ). Functionally, loss of Grem1 FRC-derived FLT3L impairs cDC priming of antigen-specific T cell responses. These findings provide key mechanistic insights underlying stromal cell support of DC homeostasis and function.