Editors' ChoiceLymphatic System

Tricellular Cooperation

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Sci. Signal.  08 Oct 2013:
Vol. 6, Issue 296, pp. ec241
DOI: 10.1126/scisignal.2004788

Lymphocytes enter the lymph nodes through special blood vessels called high endothelial venules (HEVs), and infection or immunization increases the rate of this trafficking. This movement of cells across the vessels must occur without compromising vascular integrity. Platelets are critical to preventing excessive bleeding and aid in blood vessel repair. Podoplanin (PDPN) is a ligand for the platelet-activating receptor CLEC-2 and is abundant in lymph nodes. Herzog et al. created mice with postnatal-inducible knockout of PDPN or selective knockout in fibroblastic reticular cells that surround HEVs. Both types of mice exhibited spontaneous bleeding into the mucosal lymph nodes, a characteristic that also occurred in mice with platelet-specific deficiency of CLEC-2. Mice deficient in CLEC-2 (in platelets) or PDPN (global inducible) also exhibited bleeding into peripheral lymph nodes after immune challenge, which was also associated with increased permeability of the HEVs in peripheral lymph nodes. Blocking lymphocyte trafficking into lymph nodes by treating lymphocytes with an antibody to the adhesion molecule L-selectin prevented the bleeding from peripheral lymph nodes associated with immune challenge. Platelets were detected on the abluminal side of HEVs in peripheral lymph nodes in immunized wild-type mice, confirming that platelets could migrate across HEVs to bind to PDPN on fibroblastic reticular cells. The cellular adhesion molecule VE-cadherin and other proteins associated with adherens junctions were reduced in the lymph nodes of mice deficient in PDPN or CLEC-2. Blocking platelet aggregation did not prevent an increase in permeability of peripheral lymph nodes of immunized wild-type mice. Instead, isolated wild-type platelets, but not CLEC-2–deficient platelets, released sphingosine 1-phosphate (S1P), and PDPN-positive—but not PDPN-negative—melanoma cells stimulated S1P release from wild-type platelets. VE-cadherin abundance decreased in mucosal lymph node slices cultured in medium containing lipid-depleted fetal bovine serum. Addition of wild-type platelets, but not those from S1P-deficient mice, to the lipid-depleted medium prevented this reduction in VE-cadherin in wild-type slices but not in slices from PDPN-deficient mice. Thus, as platelets migrate with lymphocytes across HEVs, they are activated by PDPN on the surrounding fibroblastic reticular cells and secrete S1P, which stimulates the HEVs to maintain adhesion.

B. H. Herzog, J. Fu, S. J. Wilson, P. R. Hess, A. Sen, J. M. McDaniel, Y. Pan, M. Sheng, T. Yago, R. Silasi-Mansat, S. McGee, F. May, B. Nieswandt, A. J. Morris, F. Lupu, S. R. Coughlin, R. P. McEver, H. Chen, M. L. Kahn, L. Xia, Podoplanin maintains high endothelial venule integrity by interacting with platelet CLEC-2. Nature 502, 105–109 (2013). [PubMed]