Emergent physical properties of tissues are not readily understood by reductionist studies of their constituent cells. Here, we show molecular signals control cellular physical properties to collectively determine tissue mechanics of lymph nodes (LNs). LNs paradoxically maintain robust tissue architecture in homeostasis yet are continually poised for extensive tissue expansion upon immune challenge.
The most populous stromal cell component of the LN are fibroblastic reticular cells (FRCs) which span the whole tissue generating an interconnected cellular network surrounding bundles of extracellular matrix (ECM) fibres, termed the conduit. During immunogenic challenge, fluid flow and lymphocyte packing dramatically increase, driving expansion of the LN. Failure to respond to changing mechanical pressures can lead to tissue damage and dysfunction. We questioned how FRCs, and the associated ECM conduit structure mechanically responds to this challenge to preserve the integrity of the FRC network, required for optimal immune responses. By using a novel FRC-targeting mouse model alongside ex vivo and in vitro biophysical assays we test the hypothesis that the FRC network generates mechanical tension to buffer and respond to physical forces in the expanding LN.
Laser ablation of the FRC network demonstrated that the cytoskeletal mechanics of FRCs determine tissue tension in the steady state and throughout inflammation, independently of extracellular matrix scaffolds. Analysis of FRC cell surface mechanics using optical tweezers and osmotic swelling revealed that CLEC-2, expressed on migratory dendritic cells, binding to Podoplanin on the surface of FRCs decreases membrane tension, permitting cell elongation and interdigitation through expedited access to plasma membrane reservoirs. Importantly, we found that increased mechanical tension in the FRC network gates the initiation of fibroblast proliferation, restoring homeostatic cellular ratios with lymphocytes through expansion. This study demonstrates that there is a mechanical balancing act orchestrated by the FRC network to control LN expansion and maintain tissue architecture.