Lymph node stromal cells support the maturation of pre-DCs into cDCs via colony stimulating factor 1

Conventional dendritic cells (cDCs) arise from committed precursor dendritic cells (pre DCs) in the bone marrow that continuously seed the periphery. Pre-DCs and other upstream progenitors proliferate and mature in response to Fms-related receptor tyrosine kinase 3 ligand (FLT3L), which is considered the most important cytokine for cDC development. However, other cytokines such as stem cell factor (SCF) and colony stimulating factor 1 (CSF1) were also shown to induce pre-DC maturation into DC-like cells. It is not completely understood which cells contribute to cDC development once pre-DCs arrive in peripheral tissues. Here, we analyzed the impact of lymph node (LN) fibroblastic stromal cells (FSCs) on the maturation of pre-DCs into cDCs. We could demonstrate that ex vivo isolated LN FSCs co-cultured with pre-DCs induce precursor maturation into DC-like cells, which were capable of efficiently promoting the proliferation of naïve CD4+ T cells. Interestingly, FSCs isolated from mesenteric lymph nodes (mLNs) or peripheral LNs (pLNs) induced DC-like cells with highly comparable transcriptomes, showing similarity to both ex vivo isolated DCs and macrophages based on characteristic signature genes. Finally, by performing supplementation and receptor blocking studies we could demonstrate that CSF1 is a driving factor for LN FSC-mediated pre-DC maturation into DC-like cells. In summary, we could identify CSF1 as a novel stromal cell-derived factor that supports the maturation of pre-DCs into cDCs within secondary lymphoid organs.