Ultrastructural evidence of distinctive behavior of L-selectin and LFA-1 (αLβ2 integrin) on lymphocytes adhering to the endothelial surface of high endothelial venules in peripheral lymph nodes

 Using an immunoelectron microscopic technique, we demonstrated the distinctive localization of L-selectin, α_L and β₂ integrins (LFA-1) on lymphocytes adhering to high endothelial venules (HEVs) of peripheral lymph nodes. Immunogold staining clearly demonstrated the preferential localization of L-selectin on the faintly adherent microvilli to endothelial surfaces. Often, the particles of L-selectin were found around those microvilli with a dispersed distribution. Examination by antibody-coated latex beads showed that the localization of L-selectin was not restricted to the lymphocyte surface but also found on endothelial cells. These data suggest the molecular shedding from lymphocytes and its transfer to the HEV surface as the ’molecular footprints’ of rolling cells. Concomitant with the dispersion of L-selectin, the gold particles of α_L and β₂ integrins showed significant capping and clustering images on the adherent border of lymphocytes. This redistribution of LFA-1 may be important for inducing the transition of the molecule into the active state to facilitate effective binding to its endothelial ligands. These morphological findings revealed the characteristic behavior of L-selectin and LFA-1 on lymphocytes, and they confirm their respective molecular roles in the current adhesion cascade model between lymphocytes and HEVs. Accepted: 9 June 1998     

A SV-40 immortalized murine endothelial cell line from peripheral lymph node high endothelium expresses a new α-L-fucose binding protein

Summry— Endothelial cells from mouse peripheral lymph nodes were immortalized by cationic liposome-mediate transfection using a plasmid construct containing both the gene coding for the large T antigen of simian virus 40 and a geneticin resistance gene suitable for selection. A cell line (HECal10) was isolated on the basis of its capacity to specifically bind fucoside carrying glycoconjugates; these cells present the main characteristics of endothelial cells: production of angiotensin converting enzyme and of factor VII-related antigen. Upon stimulation, they express E-selectin which binds oligosaccharides containing the Lewis^x determinant (Fucα3[Galβ4 GlcNacβ3Galβ) and the MECA 79 addressin which is characteristics for the peripheral lymph node high endothelium and is a L-selectin. HECa10 cells, as well as peripheral lymph node high endothelial cells in primary culture, express a second fucoside binding protein which differs from E-selectin. Indeed, this new fucoside-binding protein is constitutively expressed on unstimulated cells while E-selectin is not. Furthermore, HECa10 cells mediate selective lymphoid cells adhesion in a selectin/addressin-dependent mechanism, mainly inhibited by MECA 79 antibody and, in a fucose-binding lectin-dependent manner, mainly inhibited by the specifc neoglycoprotein.     

Intravital microscopy comparing T lymphocyte trafficking to the spleen and the mesenteric lymph node

Lymphocyte rolling velocity is determined largely by interactions between leukocyte alpha(4)-integrin (CD49d) and L-selectin and mucosal addressin cell adhesion molecule-1 (MAdCAM-1) in mesenteric postcapillary venules and Peyer's patch high endothelial venules (HEVs). The role of these interactions in other tissue sites of lymphocyte emigration is not known. With the use of real-time intravital confocal microscopy, we found that rolling velocities of T lymphocytes in the murine mesenteric lymph node (MLN) HEV also depend on L-selectin and CD49d. However, in the murine spleen, rolling velocities of T lymphocytes are not influenced by the loss of L-selectin and CD49d. With the use of FITC-dextran and TIE2-GFP mice, we further defined the microvascular compartments of the spleen and showed that adherence of T cells is localized to regions in the white pulp that are not lined by endothelial cells and have shear rates similar to bone marrow sinusoids. These results establish that T cell trafficking to the spleen differs from trafficking to other secondary lymphoid organs and suggest that the mechanical properties of the blood-filtering role of the spleen are important in T cell accumulation in the organ.     

Binding of L-selectin to its vascular and extravascular ligands is differentially regulated by pH

Ligands for L-selectin, a leukocyte adhesion molecule, are expressed in high endothelial venules (HEVs) in lymph nodes and extravascular tissues, such as renal tubules. Here, we report that the binding of L-selectin to its vascular and extravascular ligands is differentially regulated by pH. The optimal L-selectin-dependent binding of leukocytes to HEVs was observed at pH 7.4, a physiological pH in the blood. In contrast, the optimal binding of leukocytes to the renal tubules was observed at pH 5.6. Consistently, optimal binding of soluble recombinant L-selectin to a major vascular ligand, 6-sulfo sialyl Lewis X, was observed at pH 7.4. Binding to extravascular ligands, such as chondroitin sulfate (CS) B, CS E and heparan sulfate, occurred at pH 5.6. Under physiological shear stress ranging from 1 to 2 dynes/cm², maximal leukocyte rolling on vascular ligands was observed at pH 6.8 to 7.4, and no rolling was detected at pH conditions below 5.6. These findings suggest that the pH environment is one important factor that determines leukocyte trafficking under physiological and pathological conditions.     

Comparison of L-selectin and E-selectin ligand specificities: the L-selectin can bind the E-selectin ligands sialyl Le(x) and sialyl Le(a).

The L- and E-selectins are leukocyte and endothelial cell surface molecules which mediate leukocyte-endothelial cell adhesion by interacting with carbohydrate ligands. In the present study we find that L-selectin, like E-selectin, can interact with synthetic neoglycoproteins containing Sialyl Le(x) (Neu5Ac alpha 2-3Gal beta 1-4[Fuc alpha 1-3]GlcNAc beta-R), or Sialyl Le(a) (Neu5Ac-alpha 2-3Gal beta 1-3[Fuc alpha 1-4]GlcNAc beta-R). Additionally, both the E-selectin and L-selectin can bind the peripheral lymph node addressin, a high endothelial venule ligand for L-selectin. Despite overlapping interactions, the L- and E-selectins discriminate between their native ligands. The peripheral lymph node addressin is a preferential ligand for L-selectin; and furthermore, L-selectin expressing cells do not interact detectably with the cutaneous lymphocyte antigen, a native glycoprotein ligand for E-selectin found on a subset of lymphocytes associated with the skin.     

In Situ Analysis of Lymphocyte Migration to Lymph Nodes

Blood-borne lymphocytes migrate continuously to peripheral lymph nodes (PLN) and other organized lymphoid tissues where they are most likely to encounter their cognate antigen. Lymphocyte homing to PLN is a highly regulated process that occurs exclusively in specialized high endothelial venules (HEV) in the nodal paracortex. Recently, it has become possible to explore this vital aspect of peripheral immune surveillance by intravital microscopy of the subiliac lymph node microcirculation in anesthetized mice. This paper reviews technical and experimental aspects of the new model and summarizes recent advances in our understanding of the molecular mechanisms of lymphocyte homing to PLN which were derived from its use. Both lymphocytes and granulocytes initiate rolling interactions via L-selectin binding to the peripheral node addressin (PNAd) in PLN HEV. Subsequently, a G protein-coupled chemoattractant stimulus activates LEA-1 on rolling lymphocytes, but not on granulocytes. Thus. granulocytes continue to roll through the PLN, whereas LEA-I activation allows lymphocytes to arrest and emigrate into the extravascular compartment. We have also identified a second homing pathway that allows L-selectin low/(activated/memory) lymphocytes to home to PLN. P-selectin on circulating activated platelets can mediate simultaneous platelet adhesion to PNAd in HEV and to P-selectin glycoprotein ligand (PSGL)-l on lymphocytes. Through this mechanism, platelets can form a cellular bridge which can effectively substitute for the loss of L-selectin on memory cell subsets.     

Signaling through L-selectin mediates enhanced chemotaxis of lymphocyte subsets to secondary lymphoid tissue chemokine

L-selectin functions as an important adhesion molecule that mediates tethering and rolling of lymphocytes by binding to high endothelial venule (HEV)-expressed ligands during recirculation. Subsequent lymphocyte arrest and transmigration require activation through binding of HEV-decorated homeostatic chemokines such as secondary lymphoid tissue chemokine (SLC; CCL21) to its counterreceptor, CCR7. Importantly, L-selectin also functions as a signaling molecule. In this study, signaling induced by ligation of L-selectin using mAb or endothelial cell-expressed ligand significantly enhanced the chemotaxis of murine T cells and B cells to SLC but not to other homeostatic chemokines. Consistent with the expression levels of L-selectin in different lymphocyte subsets, L-selectin-mediated enhancement of chemotaxis to SLC was observed for all naive lymphocytes and effector/memory CD8(+) T cells, whereas only a subpopulation of effector/memory CD4(+) T cells responded. During in vivo mesenteric lymph node migration assays, the absence of L-selectin on lymphocytes significantly attenuated both their ability to migrate out of the HEV and their chemotaxis away from the vessel wall. Notably, ligation of L-selectin and/or CCR7 did not result in increased CCR7 expression levels, internalization, or re-expression. Pharmacologic inhibitor studies showed that L-selectin-mediated enhanced chemotaxis to SLC required intact intracellular kinase function. Furthermore, treatment of lymphocytes with the spleen tyrosine kinase family inhibitor piceatannol reduced their ability to migrate across the HEV in peripheral lymph nodes. Therefore, these results suggest that "cross-talk" in the signaling pathways initiated by L-selectin and CCR7 provides a novel mechanism for functional synergy between these two molecules during lymphocyte migration.     

Functional distribution and further characterization of human endothelial ligand for cellular L-selectin.

In the present study we examine the functional distribution of the human endothelial L-selectin ligand, which determines the sites of extravasation of L-selectin-positive cells. A murine cell line transfected with human L-selectin adhered preferentially to the high endothelial venules (HEV) of human peripheral lymph nodes compared to the HEV of mucosal lymphoid tissues (mean of 0.83 compared to a mean of 0.07 cells per HEV respectively). In addition, an antibody against L-selectin differentially inhibited the adhesion of human lymphocytes to peripheral lymphoid tissue versus mucosal lymphoid tissue HEV (mean 41 and 5% inhibition respectively). Although both sulfoglucuronyl-containing glycolipids and sialyl-Lewis X have been proposed as endothelial ligands for L-selectin, an antibody against the former did not bind to peripheral lymph node endothelium, and an antibody against the latter did not block adhesion of L-selectin-expressing cells. The enzyme O-sialoglycoprotein endopeptidase caused up to an 84% reduction in L-selectin-dependent binding, indicating that sialylated glycoproteins containing O-linked glycans are essential for a large majority of adhesion via L-selectin.     

Sulfotransferases of two specificities function in the reconstitution of high endothelial cell ligands for L-selectin.

L-selectin, a lectin-like receptor, mediates rolling of lymphocytes on high endothelial venules (HEVs) in secondary lymphoid organs by interacting with HEV ligands. These ligands consist of a complex of sialomucins, candidates for which are glycosylation- dependent cell adhesion molecule 1 (GlyCAM-1), CD34, and podocalyxin. The ligands must be sialylated, fucosylated, and sulfated for optimal recognition by L-selectin. Our previous structural characterization of GlyCAM-1 has demonstrated two sulfation modifications, Gal-6-sulfate and GlcNAc-6-sulfate in the context of sialyl Lewis x. We now report the cloning of a Gal-6-sulfotransferase and a GlcNAc-6-sulfotransferase, which can modify GlyCAM-1 and CD34. The Gal-6-sulfotransferase shows a wide tissue distribution. In contrast, the GlcNAc-6-sulfotransferase is highly restricted to HEVs, as revealed by Northern analysis and in situ hybridization. Expression of either enzyme in Chinese hamster ovary cells, along with CD34 and fucosyltransferase VII, results in ligand activity, as detected by binding of an L-selectin/IgM chimera. When coexpressed, the two sulfotransferases synergize to produce strongly enhanced chimera binding.     

Sialylated, fucosylated ligands for L-selectin expressed on leukocytes mediate tethering and rolling adhesions in physiologic flow conditions

Interaction of leukocytes in flow with adherent leukocytes may contribute to their accumulation at sites of inflammation. Using L- selectin immobilized in a flow chamber, a model system that mimics presentation of L-selectin by adherent leukocytes, we characterize ligands for L-selectin on leukocytes and show that they mediate tethering and rolling in shear flow. We demonstrate the presence of L- selectin ligands on granulocytes, monocytes, and myeloid and lymphoid cell lines, and not on peripheral blood T lymphocytes. These ligands are calcium dependent, sensitive to protease and neuraminidase, and structurally distinct from previously described ligands for L-selectin on high endothelial venules (HEV). Differential sensitivity to O-sialo- glycoprotease provides evidence for ligand activity on both mucin-like and nonmucin-like structures. Transfection with fucosyltransferase induces expression of functional L-selectin ligands on both a lymphoid cell line and a nonhematopoietic cell line. L-selectin presented on adherent cells is also capable of supporting tethering and rolling interactions in physiologic shear flow. L-selectin ligands on leukocytes may be important in promoting leukocyte-leukocyte and subsequent leukocyte endothelial interactions in vivo, thereby enhancing leukocyte localization at sites of inflammation.     

Leukocyte rolling velocities and migration are optimized by cooperative L-selectin and intercellular adhesion molecule-1 functions

Selectin family members largely mediate initial tethering and rolling of leukocytes on vascular endothelium, whereas integrin and Ig family members are essential for leukocyte firm adhesion. To quantify functional synergy between L-selectin and Ig family members during leukocyte rolling, the EA.hy926 human vascular endothelial line was transfected with either fucosyltransferase VII (926-FtVII) cDNA to generate L-selectin ligands alone or together with ICAM-1 cDNA (926-FtVII/ICAM-1). The ability of transfected 926 cells to support human leukocyte interactions was assessed in vitro using parallel plate flow chamber assays. Lymphocyte rolling on 926-FtVII cells was increased by approximately 70% when ICAM-1 was expressed at physiological levels. Although initial tether formation was similar for both cell types, lymphocyte rolling was 26% slower on 926-FtVII/ICAM-1 cells. Pretreatment of lymphocytes with an anti-CD18 mAb eliminated the increase in rolling, and all rolling was blocked by anti-L-selectin mAb. In addition, rolling velocities of lymphocytes from CD18-hypomorphic mice were 48% faster on 926-FtVII/ICAM-1 cells, with a similar reduction in rolling frequency relative to wild-type lymphocytes. CD18-hypomorphic lymphocytes also showed an approximately 40% decrease in migration to peripheral and mesenteric lymph nodes during in vivo migration assays compared with wild-type lymphocytes. Likewise, wild-type lymphocyte migration to peripheral lymph nodes was reduced by approximately 50% in ICAM-1(-/-) recipient mice. Similar to human lymphocytes, human neutrophils showed enhanced rolling interactions on 926-FtVII/ICAM-1 cells, but also firmly adhered. Thus, in addition to mediating leukocyte firm adhesion, CD18 integrin/ICAM-1 interactions regulate leukocyte rolling velocities and thereby optimize L-selectin-mediated leukocyte rolling.     

N-acetylglucosamine 6-O-sulfotransferase-1 regulates expression of L-selectin ligands and lymphocyte homing

Lymphocyte homing is initiated by the binding of L-selectin on lymphocytes to its ligands on high endothelial venules (HEV). Sialyl 6-sulfo Lewis X is a major capping group of L-selectin ligands. N-Acetylglucosamine (GlcNAc) 6-sulfation is essential for the ligand activity, and is catalyzed by GlcNAc 6-O-sulfotransferases (GlcNAc6STs) of which GlcNAc6ST-1 and GlcNAc6ST-2 are expressed in HEV. Here, we report that mice deficient in GlcNAc6ST-1 were impaired in the elaboration of sialyl 6-sulfo Lewis X in HEV and that an epitope of L-selectin ligands recognized by the MECA-79 anti-body was greatly reduced or abolished in the abluminal aspect of HEV. Lymphocyte homing to peripheral lymph nodes, mesenteric lymph nodes, and Peyer's patches was significantly reduced in GlcNAc6ST-1 null mice. These results demonstrate that GlcNAc6ST-1 is involved in lymphocyte homing in vivo, and indicate that GlcNAc6ST-1 and -2 play complementary roles. The importance of GlcNAc6ST-1 is particularly high-lighted by its involvement in lymphocyte homing to Peyer's patches where GlcNAc6ST-2 expression is undetectable.     

The cytoplasmic tail of L-selectin interacts with members of the Ezrin-Radixin-Moesin (ERM) family of proteins: cell activation-dependent binding of Moesin but not Ezrin.

L-selectin regulates the recruitment of naive lymphocytes from the bloodstream to secondary lymphoid organs, mediating their initial capture and subsequent rolling along high endothelial cell surface-expressed ligands in peripheral lymph nodes. In vivo, distribution of L-selectin and cell surface levels determine the tethering efficiency and rolling velocity of leukocytes, respectively. Treatment of naive lymphocytes with phorbol myristate acetate (PMA) induces rapid ectodomain proteolytic down-regulation (shedding) of surface L-selectin via a protein kinase C (PKC)-dependent pathway. In an attempt to isolate proteins that are involved in regulating L-selectin expression, an affinity column was constructed using the 17-amino acid cytoplasmic tail of L-selectin. Affinity purification of extracts from lymphocytes, pre-treated with or without PMA, allowed identification of proteins that interact with the affinity column under one condition but not the other. By using this approach, members of the Ezrin-Radixin-Moesin family of proteins were found to interact specifically with the cytoplasmic tail of L-selectin. Moesin from PMA-stimulated lymphocytes, but not from unstimulated lymphocytes, bound to L-selectin tail. In contrast, ezrin from unstimulated or PMA-stimulated lymphocytes associated with L-selectin tail with equal affinity. Furthermore, the PKC inhibitor Ro 31-8220 significantly reduced the interaction of moesin, but not ezrin, with L-selectin. Alanine mutations of membrane-proximal basic amino acid residues in the cytoplasmic domain of L-selectin identified arginine 357 as a critical residue for both ezrin and moesin interaction. Finally, BIAcore affinity analysis confirmed that N-terminal moesin interacts specifically with L-selectin cytoplasmic tail, with relatively high affinity (K(d) approximately 40 nm). Based on these findings, although moesin and ezrin bind to a similar region of the cytoplasmic tail of L-selectin, moesin binding is dependent on PKC activation, which suggests that ezrin and moesin are regulated differently in lymphocytes.     

Expression of L-selectin and efficient binding to high endothelial venules do not modulate the dissemination potential of murine B-cell lymphoma

The homing receptor L-selectin is essential for the migration of naive lymphocytes into peripheral lymph nodes. In contrast to naive lymphocytes, activated and memory cells down-regulate L-selectin and enter peripheral lymph nodes by an L-selectin-independent mechanism. In view of the concept that lymphomas present the malignant counterparts of normal lymphocytes at a defined stage of differentiation, it has been suggested that in contrast to lymphomas with a memory/activated cell phenotype, L-selectin is essential for dissemination of lymphomas that represent naive cells. 38C-13 is a murine B-cell lymphoma with an immature naive cell phenotype. 38C-13 cells express high levels of L-selectin and bind to lymph node high endothelial venules in an L-selectin-dependent manner. In this study we demonstrate that treatment of 38C-13 tumor-bearing mice with anti-L-selectin antibodies did not inhibit tumor dissemination to peripheral lymph nodes. Moreover, L-selectin-negative 38C-13 variant cells disseminated as efficiently as wild-type cells. Thus, in spite of its expression, L-selectin is not required and does not affect the metastatic potential of the tumor. L-selectin of the malignant cells and of normal lymphocytes appears to be functionally different. Thus, whereas antibody cross-linking of L-selectin resulted in down-modulation of the receptor in normal lymphocytes, cross-linking had no effect on L-selectin expression in 38C-13 cells, suggesting that, in spite of comparable levels of surface expression in normal and malignant cells, L-selectin may be functionally impaired in some malignant cells. Received: 9 November 2000 / Accepted: 11 January 2001     

Interactions through L-selectin between leukocytes and adherent leukocytes nucleate rolling adhesions on selectins and VCAM-1 in shear flow

We demonstrate an additional step and a positive feedback loop in leukocyte accumulation on inflamed endothelium. Leukocytes in shear flow bind to adherent leukocytes through L-selectin/ligand interactions and subsequently bind downstream and roll on inflamed endothelium, purified E-selectin, P-selectin, L-selectin, VCAM-1, or peripheral node addressin. Thus adherent leukocytes nucleate formation of strings of rolling cells and synergistically enhance leukocyte accumulation. Neutrophils, monocytes, and activated T cell lines, but not peripheral blood T lymphocytes, tether to each other through L-selectin. L- selectin is not involved in direct binding to either E- or P-selectin and is not a major counterreceptor of endothelial selectins. Leukocyte- leukocyte tethers are more tolerant to high shear than direct tethers to endothelial selectins and, like other L-selectin-mediated interactions, require a shear threshold. Synergism between leukocyte- leukocyte and leukocyte-endothelial interactions introduces novel regulatory mechanisms in recruitment of leukocytes in inflammation.     

Endothelial chemokines destabilize L-selectin-mediated lymphocyte rolling without inducing selectin shedding

Chemokines presented on specialized endothelial surfaces rapidly up-regulate leukocyte integrin avidity and firm arrest through G(i)-protein signaling. Here we describe a novel, G-protein-independent, down-regulatory activity of apical endothelial chemokines in destabilizing L-selectin-mediated leukocyte rolling. Unexpectedly, this anti-adhesive chemokine suppression of rolling does not involve L-selectin shedding. Destabilization of rolling is induced only by immobilized chemokines juxtaposed to L-selectin ligands and is an energy-dependent process. Chemokines are found to interfere with a subsecond stabilization of selectin tethers necessary for persistent rolling. This is a first indication that endothelial chemokines can attenuate in situ L-selectin adhesion to endothelial ligands at subsecond contacts. This negative feedback mechanism may underlie the jerky nature of rolling mediated by L-selectin in vivo.     

Tissue-specific expression of LeY antigen in high endothelial venules of human lymphoid tissues

In this study, we demonstrated that the anti-Le^Yantibody (BM-1) especially reacted with high endothelial venules (HEVs) in peripheral lymph nodes of blood group O individuals. The Le^Yexpression on HEVs showed a unique tissue-specific pattern, i.e., a large amount of the Le^Yexpression in peripheral lymph nodes and no or small amounts in mesenteric lymph node. Statistical analysis showed that there was the significant difference between the percentage of Le^Y-positive HEVs in peripheral lymph nodes and mesenteric lymph nodes. No expression of Le^Ywas observed in vessels of Payer's patch, thymus, spleen and other non-lymphoid organs. In blood group A or B individuals, the reactivity between HEVs and anti-Le^Yantibody increased after enzyme digestion with -N-acetylgalactosaminidase or -galactosidase. These findings show that the expression of difucosylated blood group ABH antigens are especially expressed on HEVs in peripheral lymph nodes. Furthermore, the tissue-specific pattern suggests that these antigens may be related to intercellular adhesion between lymphocytes and HEVs.     

Isolation and characterization of high endothelial cell lines derived from mouse lymph nodes

Summary Two long-term cultured cell lines were established from BALB/c mouse axillary and cervical lymph nodes that exhibited a combination of functional, morphological, and phenotypic characteristics consistent only with high endothelial venule cells. Spleen lymphocytes selectively bound and migrated across the cell lines. On Matrigel, these cell lines formed tubules with lumens, a characteristic unique to endothelial cells. Morphologically the cells were 20–30 μm in diameter and exhibited contact inhibition. The cells were not myeloid in origin because they lacked sodium fluoride-inhibitable nonspecific esterase activity, myeloperoxidase activity, and F4/80 antigen. The cell line phenotypes were compared to high endothelial venule (HEV) cells in tissue sections. HEV cells in lymph node tissue sections expressed endoglin, PECAM-1, ICAM-1, VCAM-1, laminin, fibronectin, collagen IV, H2K^d, MECA 79, MECA 325, and vWF. The cell lines expressed endoglin, VCAM-1, fibronectin, and H2K^d. The cell line derived from cervical lymph nodes also expressed laminin and H2D^d. Neither cell line expressed collagen IV, IA^d, ICAM-1, ICAM-2, dendritic cell antigen, or PECAM-1. They also did not express MECA antigens or intracellular vWF, consistent with reports of many cultured endothelial cells. To further substantiate cell line identification, antiserum generated against the cell lines bound specifically to HEV cells in frozen lymph node tissue sections and to both of the lymph node-derived cell lines but not control cell lines. Thus, the lymph node derived-cell lines expressed molecules found on HEV cellsin vivo and most importantly retained the functions of tubule formation, lymphocyte adhesion, and promotion of lymphocyte migration.     

High endothelial venules of the lymph nodes express Fas ligand.

Fas (CD95, APO-1) is widely expressed on lymphatic cells, and by interacting with its natural ligand (Fas-L), Fas induces apoptosis through a complex caspase cascade. In this study we sought to survey Fas-L expression in vascular and sinusoidal structures of human reactive lymph nodes. Immunohistochemical Fas-L expression was present in all paracortical high endothelial venules (HEVs), in cells lining the marginal sinus wall, and in a few lymphocytes, but only occasionally in non-HEV vascular endothelium. In the paracortical zone over 60% of all vessels and all paracortical HEVs showed Fas-L expression, whereas in the medullary zone less than 10% of the blood vessels were stained with Fas-L. Normal vessels outside lymph nodes mostly showed no Fas-L expression. We show that in human reactive lymph nodes Fas-L expression is predominantly present in HEVs. Because the circulating lymphocytes gain entry to nodal parenchyma by transendothelial migration through HEVs, the suggested physiological importance of Fas-L expression in these vessels lies in the regulation of lymphocyte access to lymph node parenchyma by possibly inducing Fas/Fas-L mediated apoptosis of activated Fas-expressing lymphoid cells. The Fas-L expressing cells in the marginal sinus might have a similar function for cells accessing the node in afferent lymph.     

L-Selectin Ligands That Are O-glycoprotease Resistant and Distinct from MECA-79 Antigen are Sufficient for Tethering and Rolling of Lymphocytes on Human High Endothelial Venules

During the process of lymphocyte recirculation, lymphocytes bind via L-selectin to sulfated sialyl-Lewis^x (sLe^x)–containing carbohydrate ligands expressed on the surface of high endothelial venules (HEV). We have examined the expression of sLe^x on HEV using a panel of mAbs specific for sLe^x and sLe^x-related structures, and have examined the function of different sLe^x-bearing structures using an in vitro assay of lymphocyte rolling on HEV. We report that three sLe^x mAbs, 2F3, 2H5, and CSLEX-1, previously noted to bind with high affinity to glycolipid-linked sLe^x, vary in their ability to stain HEV in different lymphoid tissues and bind differentially to O-linked versus N-linked sLe^x on glycoproteins. Treatment of tissue sections with neuraminidase abolished staining with all three mAbs but slightly increased staining with MECA-79, a mAb to a sulfation-dependent HEV-associated carbohydrate determinant. Treatment of tissue sections with O-sialoglycoprotease under conditions that removed the vast majority of MECA-79 staining, only partially reduced staining with the 2F3 and 2H5 mAbs. Using a novel rolling assay in which cells bind under flow to HEV of frozen tissue sections, we demonstrate that a pool of O-sialoglycoprotease–resistant molecules is present on HEV that is sufficient for attachment and rolling of lymphocytes via L-selectin. This interaction is not inhibited by the mAb MECA-79. Furthermore, MECA-79 mAb blocks binding to untreated sections by only 30%, whereas the sLe^x mAb 2H5 blocks binding by ~60% and a combination of MECA-79 and 2H5 mAb blocks binding by 75%. We conclude that a pool of O-glycoprotease-resistant sLe^x-like L-selectin ligands exist on human HEV that is distinct from the mucin-associated moieties recognized by MECA-79 mAb. We postulate that these ligands may participate in lymphocyte binding to HEV.