In lymph nodes, fibroblastic reticular cells (FRCs) form a collagen-based reticular network that supports migratory dendritic cells (DCs) and T cells and transports lymph. A hallmark of FRCs is their propensity to contract collagen, yet this function is poorly understood. Here, we demonstrate that podoplanin (PDPN) regulated actomyosin contractility in FRCs. Under resting conditions, when FRCs are unlikely to encounter mature DCs expressing the PDPN receptor, CLEC-2, PDPN endowed FRCs with contractile function and exerted tension within the reticulum. Upon inflammation, CLEC-2 on mature DCs potently attenuated PDPN-mediated contractility, resulting in FRC relaxation and reduced tissue stiffness. Disrupting PDPN function altered the homeostasis and spacing of FRCs and T cells, resulting in an expanded reticular network and enhanced immunity.

Mucin-type O-linked oligosaccharides (O-glycans) are primary components of the intestinal mucins thatform the mucus gel layer overlying the gut epithelium. Impaired expression of intestinal O-glycans has been observed in patients with ulcerative colitis (UC), but its role in the etiology of this disease is unknown. Here, we report that mice with intestinal epithelial cell-specific deficiency of core 1-derived O-glycans, the predominant form of O-glycans, developed spontaneous colitis that resembled human UC, including massive myeloid infiltrates and crypt abscesses. The colitis manifested in these mice was also characterized by TNF-producing myeloid infiltrates in colon mucosa in the absence of lymphocytes, supporting an essential role for myeloid cells in colitis initiation. Furthermore, induced deletion of intestinal core 1-derived O-glycans caused spontaneous colitis in adult mice. These data indicate a causal role for the loss of core 1-derived O-glycans in colitis. Finally, we detected a biosynthetic intermediate typically exposed in the absence of core 1 O-glycan, Tn antigen, in the colon epithelium of a subset of UC patients. Somatic mutations in the X-linked gene that encodes core 1 β1,3-galactosyltransferase-specific chaperone 1 (C1GALT1C1, also known as Cosmc), which is essential for core 1 O-glycosylation, were found in Tn-positive epithelia. These data suggest what we believe to be a new molecular mechanism for the pathogenesis of UC.

Circulating lymphocytes continuously enter lymph nodes (LNs) for immune surveillance through specialised blood vessels named high endothelial venules (HEVs)1–5, a process that increases dramatically during immune responses. How 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)6–8 in maintaining HEV barrier function. Mice with postnatal deletion of PDPN lost HEV integrity and exhibited spontaneous bleeding in mucosal LNs, and bleeding in the draining peripheral LN after immunisation. 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 cells (FRCs)7, which surround HEVs, functions as an activating ligand for platelet C-type lectin-like receptor 2 (CLEC-2)9,10. Mice lacking FRC PDPN or platelet CLEC-2 exhibited significantly reduced levels of VE-cadherin (VE-cad), which is essential for overall vascular integrity11,12, on HEVs. Infusion of wild-type (WT) platelets restored HEV integrity in CLEC-2-deficient mice. Activation of CLEC-2 induced release of sphingosine-1-phosphate (S1P)13,14 from platelets, which promoted expression of VE-cad on HEVs ex vivo. Furthermore, draining peripheral LNs of immunised mice lacking S1P had impaired HEV integrity similar to PDPN- and CLEC-2-deficient mice. These data demonstrate that local S1P release after PDPN-CLEC-2-mediated platelet activation is critical for HEV integrity during immune responses.