Podoplanin maintains high endothelial venule integrity by interacting with platelet CLEC-2

A transmembrane O-glycoprotein podoplanin (PDPN) expressed on fibroblastic reticular cells is the activating ligand for platelet receptor CLEC-2; this interaction leads to perivenular release of sphingosine-1-phosphate and expression of VE-cadherin on high endothelial venules, a key process for the maintenance of vascular integrity in lymph nodes. The lymph nodes, essential sites for immune responses, are supplied with circulating lymphocytes that enter via specialized post-capillary blood vessels known as high endothelial venules (HEVs). How the integrity of this vascular barrier is maintained in the face of the increase in lymphocyte homing during immune responses is not known. Lijun Xia and colleagues now show that the mechanism involves cross-talk between HEVs, platelets and the fibroblastic reticular cells (FRCs) that surround the HEVs. The transmembrane protein podoplanin, expressed on FRCs, interacts with the platelet activation receptor CLEC-2, prompting the release of sphingosine-1-phosphate that in turn preserves the adhesion molecule VE-cadherin on the HEVs. This finding may suggest new approaches to the regulation of lymph node homeostasis in conditions of increased HEV proliferation, such as chronic inflammation. Circulating lymphocytes continuously enter lymph nodes for immune surveillance through specialized blood vessels named high endothelial venules1,2,3,4,5, a process that increases markedly during immune responses. How high endothelial venules (HEVs) permit lymphocyte transmigration while maintaining vascular integrity is unknown. Here we report a role for the transmembrane O-glycoprotein podoplanin (PDPN, also known as gp38 and T1α)6,7,8 in maintaining HEV barrier function. Mice with postnatal deletion of Pdpn lost HEV integrity and exhibited spontaneous bleeding in mucosal lymph nodes, and bleeding in the draining peripheral lymph nodes after immunization. Blocking lymphocyte homing rescued bleeding, indicating that PDPN is required to protect the barrier function of HEVs during lymphocyte trafficking. Further analyses demonstrated that PDPN expressed on fibroblastic reticular cells7, which surround HEVs, functions as an activating ligand for platelet C-type lectin-like receptor 2 (CLEC-2, also known as CLEC1B)9,10. Mice lacking fibroblastic reticular cell PDPN or platelet CLEC-2 exhibited significantly reduced levels of VE-cadherin (also known as CDH5), which is essential for overall vascular integrity11,12, on HEVs. Infusion of wild-type platelets restored HEV integrity in Clec-2-deficient mice. Activation of CLEC-2 induced release of sphingosine-1-phosphate13,14 from platelets, which promoted expression of VE-cadherin on HEVs ex vivo. Furthermore, draining peripheral lymph nodes of immunized mice lacking sphingosine-1-phosphate had impaired HEV integrity similar to Pdpn- and Clec-2-deficient mice. These data demonstrate that local sphingosine-1-phosphate release after PDPN–CLEC-2-mediated platelet activation is critical for HEV integrity during immune responses.