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-expressing FRCs co-localize with DCs and express FLT3L/Flt3l in human and mouse lymph nodes. Using a new genetic model, we provide evidence that FLT3L produced by GREM1+ FRCs maintains lymph node DC precursors (preDCs), and both conventional (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 preDCs and cDCs within the interfollicular zone (IFZ). Functionally, loss of GREM1+ FRC-derived FLT3L impairs cDC activation of antigen-specific T cell responses to both immunization and infection. These findings provide key mechanistic insights underlying stromal cell support of DC homeostasis and function.Dendritic cells were isolated by FACS from lymph nodes of 4x wild-type (Grem1 wt/ki Flt3l wt/wt) and 4x FLT3L floxed (Grem1 wt/ki Flt3l fl/fl) mice for single-cell profiling (total 8). Samples were indexed using TotalSeq-A hashtag antibodies (B0101-B0108). Stromal, myeloid and lymphoid cell compartments were isolated by FACS from lymph nodes of 4x wild-type (Grem1 wt/ki Flt3l wt/wt) and 4x FLT3L floxed (Grem1 wt/ki Flt3l fl/fl) for single-cell profiling (total 8). For spatial profiling, lymph nodes were isolated from 4x wild-type (Grem1 wt/ki Flt3l wt/wt) and 4x FLT3L floxed (Grem1 wt/ki Flt3l fl/fl) mice for spatial transcriptomics profiling using the Visium assay (10X Genomics).