Following immunogenic challenge, infiltrating and dividing lymphocytes significantly increase lymph node (LN) cellularity leading to organ expansion,. Here we report that the physical elasticity of LNs is maintained in part by podoplanin (PDPN) signalling in stromal fibroblastic reticular cells (FRCs) and its modulation by CLEC-2 expressed on dendritic cells (DCs). We show that PDPN induces actomyosin contractility in FRCs via activation of RhoA/C and downstream Rho-kinase. Engagement by CLEC-2 causes PDPN clustering and rapidly uncouples PDPN from RhoA/C activation, relaxing the actomyosin cytoskeleton and permitting FRC stretching. Notably, administration of CLEC-2 protein to immunised mice augments LN expansion. In contrast, the latter is significantly constrained in mice selectively lacking CLEC-2 expression in DCs. Thus, the same DCs that initiate immunity by presenting antigens to T lymphocytes also initiate remodeling of LNs by delivering CLEC-2 to FRCs. CLEC-2 modulation of PDPN signalling permits FRC network stretching and allows for the rapid LN expansion driven by lymphocyte influx and proliferation that is the critical hallmark of adaptive immunity.
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