The mechanisms controlling the exit of lymphocytes from tissues via lymphatics are practically unknown. We have now identified a 270-300-kDa molecule designated common lymphatic endothelial and vascular endothelial receptor-1 (CLEVER-1) on human lymphatic endothelium and high endothelial venules. We show that it mediates binding of lymphocytes both to high endothelial venules and to lymphatic vessels. Moreover, blocking of the function of CLEVER-1 results in significant reduction of lymphocyte traffic in vivo. Notably, CLEVER-1 is also an inducible vascular adhesion molecule for other classes of leukocytes at sites of inflammation in peripheral tissues. These findings suggest that CLEVER-1 is involved in regulation of lymphocyte recirculation and migration of leukocytes to sites of inflammation and is a potential new target to control inflammation.

Continuous lymphocyte recirculation between blood and lymphoid tissues forms a basis for the function of the immune system. Lymphocyte entrance from the blood into the tissues has been thoroughly characterized, but mechanisms controlling lymphocyte exit from the lymphoid tissues via efferent lymphatics have remained virtually unknown. In this work we have identified mannose receptor (MR) on human lymphatic endothelium and demonstrate its involvement in binding of lymphocytes to lymphatic vessels. We also show that the binding requires L-selectin, and L-selectin and MR form a receptor–ligand pair. On the other hand, L-selectin binds to peripheral lymph node addressins (PNAds) on high endothelial venules (HEVs) that are sites where lymphocytes enter the lymphatic organs. Interestingly, MR is absent from HEVs and PNAds from lymphatic endothelium. Thus, lymphocyte L-selectin uses distinct ligand molecules to mediate binding at sites of lymphocyte entrance and exit within lymph nodes. Taken together, interaction between L-selectin and MR is the first molecularly defined mechanism mediating lymphocyte binding to lymphatic endothelium.

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SUMMARYKnowledge of the histological distribution of leucocytes and adhesion molecules in the human genital tract is scarce although local immunity in this region is important. Using immunohistochemical methods, we here describe the organization of CD3+, CD8+ and CD4+ T cells, CD19+ B cells, CD38+ plasma cells, major histocompatibility complex (MHC ) class II+ antigen-presenting cells and CD14+ monocytes, as well as the expression of endothelial addressins in normal human ectocervical and vaginal mucosa. T cells were clustered in a distinct band beneath the epithelium and were also dispersed in the epithelium and the lamina propria, whereas CD38+ plasma cells were present only in the lamina propria. MHC class II+ cells were numerous in the lamina propria and in the epithelium, where they morphologically resembled dendritic cells. Lymphoid aggregates containing CD19+ and CD20+ B cells as well as CD3+, CD4+ and CD8+ cells were also found in the cervix. The mucosal addressin cell adhesion molecule-1 (MAdCAM-1) was not expressed on the vascular endothelium in the cervical or vaginal mucosa. In contrast, intercellular adhesion molecule-1 (ICAM-1), vascular adhesion protein-1 ( VAP-1) and P-selectin were expressed in all tissue samples, and vascular cell adhesion molecule-1 ( VCAM-1) and E-selectin were found in four of seven samples. We conclude that the distribution of leucocytes and adhesion molecules is very similar in the ecto-cervical and the vaginal mucosa and that the regulation of lymphocyte homing to the genital tract is different from that seen in the intestine. Our results also clearly suggest that the leucocytes are not randomly scattered in the tissue but organized in a distinct pattern.