Fibroblastic reticular cells provide a supportive niche for lymph node-resident macrophages

Area of Science:

• Immunology
• Cell Biology
• Microanatomy

Background:

• Lymph nodes (LNs) host resident macrophage populations vital for immune function and homeostasis.
• The specific factors governing the LN macrophage niche remain largely undefined.
• Understanding these niche-defining factors is critical for immune system research.

Purpose of the Study:

• To identify key cellular components that maintain the macrophage niche within lymph nodes.
• To investigate the role of fibroblastic reticular cells (FRCs) in supporting resident macrophages.
• To elucidate the molecular mechanisms by which FRCs influence macrophage populations.

Main Methods:

• Genetic ablation of FRCs in vivo using two distinct mouse models.
• Co-localization analysis of macrophages and FRCs in human and murine lymph nodes.
• Single-cell RNA-sequencing of murine lymph node cells to identify gene expression patterns.
• In vitro functional assays using purified FRCs and monocytes.

Main Results:

• Genetic ablation of FRCs resulted in a rapid depletion of macrophages and monocytes from LNs.
• Macrophages were observed to co-localize with FRCs in both human and mouse LNs.
• Murine FRC subsets expressed the master macrophage regulator, colony-stimulating factor 1 (CSF1).
• CSF1R signaling, supported by FRCs, was sufficient for macrophage development in functional assays.
• These findings were conserved across mouse and human systems.

Conclusions:

• Fibroblastic reticular cells (FRCs) are essential components of the lymph node macrophage niche.
• FRCs play a critical role in maintaining parenchymal macrophage populations within LNs.
• The CSF1-CSF1R axis, mediated by FRCs, is a key mechanism for supporting LN macrophages.