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.     

CD43 plays both antiadhesive and proadhesive roles in neutrophil rolling in a context-dependent manner

As the first step in the recruitment of neutrophils into tissues, the cells become tethered to and roll on the vessel wall. These processes are mediated by interactions between the P- and E-selectins, expressed on the endothelial cells of the vessel wall, and their ligands, expressed on the neutrophils. Recently, we reported that CD43 on activated T cells functions as an E-selectin ligand and thereby mediates T cell migration to inflamed sites, in collaboration with P-selectin glycoprotein ligand-1 (PSGL-1), a major P- and E-selectin ligand. Here, we examined whether CD43 on neutrophils also functions as an E-selectin ligand. CD43 was precipitated with an E-selectin-IgG chimera from mouse bone marrow neutrophils. A CD43 deficiency diminished the E-selectin-binding activity of neutrophils when PSGL-1 was also deficient. Intravital microscopy showed that the CD43 deficiency significantly increased leukocyte rolling velocities in TNF-alpha-stimulated venules blocked with an anti-P-selectin mAb, where the rolling was mostly E-selectin dependent, when PSGL-1 was also absent. In contrast, in venules with trauma-induced inflammation, where the rolling was largely P-selectin dependent, the CD43 deficiency reduced leukocyte rolling velocities. Collectively, these observations suggest that CD43 generally serves as an antiadhesive molecule to attenuate neutrophil-endothelial interactions, but when E-selectin is expressed on endothelial cells, it also plays a proadhesive role as an E-selectin ligand.     

CD44 is a major E-selectin ligand on human hematopoietic progenitor cells

E-selectin plays a critical role in mediating tissue-specific homing of T cells into skin, and of primitive hematopoietic progenitor cells (HPCs) into bone marrow (BM). Though it is known that a glycoform of PSGL-1 (CLA) functions as the principal E-selectin ligand on human T lymphocytes, the E-selectin ligand(s) of human HPCs has not been identified. We used a shear-based adherence assay to analyze and define the E-selectin ligand activity of membrane proteins from human HPCs. Our data show that PSGL-1 expressed on human HPCs is an E-selectin ligand, and that HPCs also express a previously unrecognized E-selectin ligand, CD44. The E-selectin ligand activity of CD44 is conferred by the elaboration of sialylated, fucosylated binding determinants on N-glycans. This glycoform of CD44 is expressed on primitive CD34+ human HPCs, but not on more mature hematopoietic cells. Under physiologic flow conditions, this molecule mediates E-selectin-dependent rolling interactions over a wider shear range than that of PSGL-1, and promotes human HPC rolling interactions on E-selectin expressed on human BM endothelial cells. These findings offer new insights into the structural biology and physiology of CD44, and into the molecular basis of E-selectin-dependent adhesive interactions that direct homing of human HPC to BM.     

Selectin-dependent rolling and adhesion of leukocytes in nicotine-exposed microvessels of lung allografts

The interaction of circulating leukocytes with lung microvessels is a critical event in the recruitment of effector cells into the interstitial tissue during episodes of inflammation, including smoking-induced chronic airway disease. In the present study, murine lung tissue transplanted into a dorsal skinfold window chamber in nude mice was used as a model system to study nicotine-induced leukocyte trafficking in vivo. The revascularized lung microvessels were determined to be of pulmonary origin based on their ability to constrict in response to hypoxia. We demonstrated that nicotine significantly enhanced rolling and adhesion of leukocytes within lung microvessels comprising arterioles and postcapillary venules in a dose-dependent manner, but failed to induce leukocyte emigration. Nicotine-induced rolling and adhesion was significantly higher in venules than in arterioles. Treatment of mice with monoclonal antibodies (MAbs) against L-, E-, or P-selectin after exposure of lung allografts to nicotine resulted in variable but significant inhibition of nicotine-induced rolling, whereas nicotine-induced subsequent adhesion was inhibited by MAbs against L- and P-selectin but not E-selectin. Exposure of lung allografts to nicotine along with PD-98059, a mitogen-activated protein kinase (MAPK)-specific inhibitor, resulted in significant inhibition of nicotine-induced rolling and adhesion. In vitro, exposure of murine lung endothelial cells to nicotine resulted in increased phosphorylation of mitogen-activated/extracellular signal-regulated protein kinase 1/2, which could be blocked by PD-98059. Overall, these results suggest that nicotine-induced inflammation in the airways could potentially be due to MAPK-mediated, selectin-dependent leukocyte-endothelial cell interactions in the lung microcirculation.     

Differential requirements for core2 glucosaminyltransferase for endothelial L-selectin ligand function in vivo

L-selectin is a calcium-dependent lectin on leukocytes mediating leukocyte rolling in high endothelial venules and inflamed microvessels. Many selectin ligands require modification of glycoproteins by leukocyte core2 beta1,6-N-acetylglucosaminyltransferase (Core2GlcNAcT-I). To test the role of Core2GlcNAcT-I for L-selectin ligand biosynthesis, we investigated leukocyte rolling in venules of untreated and TNF-alpha-treated cremaster muscles and in Peyer's patch high endothelial venules (HEV) of Core2GlcNAcT-I null (core2(-/-)) mice. In the presence of blocking mAbs against P- and E-selectin, L-selectin-mediated leukocyte rolling was almost completely abolished in cremaster muscle venules of core2(-/-) mice, but not littermate control mice. By contrast, leukocyte rolling in Peyer's patch HEV was not significantly different between core2(-/-) and control mice. To probe L-selectin ligands more directly, we injected L-selectin-coated beads. These beads showed no rolling in cremaster muscle venules of core2(-/-) mice, but significant rolling in controls. In Peyer's patch HEV, beads coated with a low concentration of L-selectin showed reduced rolling in core2(-/-) mice. Beads coated with a 10-fold higher concentration of L-selectin rolled equivalently in core2(-/-) and control mice. Our data show that endothelial L-selectin ligands relevant for rolling in inflamed microvessels of the cremaster muscle are completely Core2GlcNAcT-I dependent. In contrast, L-selectin ligands in Peyer's patch HEV are only marginally affected by the absence of Core2GlcNAcT-I, but are sufficiently functional to support L-selectin-dependent leukocyte rolling in Core2GlcNAcT-I-deficient mice.     

Circulating Tumor Cells from Prostate Cancer Patients Interact with E-Selectin under Physiologic Blood Flow

Hematogenous metastasis accounts for the majority of cancer-related deaths, yet the mechanism remains unclear. Circulating tumor cells (CTCs) in blood may employ different pathways to cross blood endothelial barrier and establish a metastatic niche. Several studies provide evidence that prostate cancer (PCa) cell tethering and rolling on microvascular endothelium via E-selectin/E-selectin ligand interactions under shear flow theoretically promote extravasation and contribute to the development of metastases. However, it is unknown if CTCs from PCa patients interact with E-selectin expressed on endothelium, initiating a route for tumor metastases. Here we report that CTCs derived from PCa patients showed interactions with E-selectin and E-selectin expressing endothelial cells. To examine E-selectin-mediated interactions of PCa cell lines and CTCs derived from metastatic PCa patients, we used fluorescently-labeled anti-prostate specific membrane antigen (PSMA) monoclonal antibody J591-488 which is internalized following cell-surface binding. We employed a microscale flow device consisting of E-selectin-coated microtubes and human umbilical vein endothelial cells (HUVECs) on parallel-plate flow chamber simulating vascular endothelium. We observed that J591-488 did not significantly alter the rolling behavior in PCa cells at shear stresses below 3 dyn/cm². CTCs obtained from 31 PCa patient samples showed that CTCs tether and stably interact with E-selectin and E-selectin expressing HUVECs at physiological shear stress. Interestingly, samples collected during disease progression demonstrated significantly more CTC/E-selectin interactions than samples during times of therapeutic response (p=0.016). Analysis of the expression of sialyl Lewis X (sLe^x) in patient samples showed that a small subset comprising 1.9-18.8% of CTCs possess high sLe^x expression. Furthermore, E-selectin-mediated interactions between prostate CTCs and HUVECs were diminished in the presence of anti-E-selectin neutralizing antibody. CTC-Endothelial interactions provide a novel insight into potential adhesive mechanisms of prostate CTCs as a means to initiate metastasis.     

Translational inhibition of E-selectin expression stimulates P-selectin-dependent neutrophil recruitment

Although known for its role in hemostasis, there is a growing body of evidence that thrombin can induce leukocyte recruitment and contribute to the inflammatory response. An in vitro parallel-plate flow chamber was used to systematically examine thrombin-induced neutrophil interactions with human endothelium. Stimulation of endothelial cells with thrombin (1 U/ml) resulted in an immediate, P-selectin-dependent increase in neutrophil rolling and adhesion that was comparable in magnitude to optimal levels of histamine (the classical inducer of P-selectin). However, thrombin, but not histamine, induced a delayed (4 h) E-selectin-dependent rolling similar to that of tumor necrosis factor-alpha, suggesting that thrombin has the unique ability to recruit neutrophils by an early P-selectin and a delayed E-selectin pathway. Surprisingly, inhibition of E-selectin expression with the general protein synthesis inhibitor cycloheximide induced P-selectin expression 4 h after thrombin stimulation. Cycloheximide and thrombin (4 h) induced sufficient P-selectin-dependent rolling to recruit as many neutrophils as were recruited with 4 h of stimulation with thrombin alone. Histamine in the presence of cycloheximide or cycloheximide alone did not evoke the P-selectin response at 4 h, suggesting that this was not due to direct cycloheximide induction of P-selectin. Treatment of endothelium with tumor necrosis factor-alpha (an E-selectin inducer) and cycloheximide also eliminated E-selectin expression but, much like thrombin, induced P-selectin expression and neutrophil recruitment. In conclusion, inhibition of E-selectin via protein synthesis inhibition activates the protein synthesis-independent pathway of P-selectin expression to support adequate leukocyte recruitment.     

Interplay between shear stress and adhesion on neutrophil locomotion

Leukocyte locomotion over the lumen of inflamed endothelial cells is a critical step, following firm adhesion, in the inflammatory response. Once firmly adherent, the cell will spread and will either undergo diapedesis through individual vascular endothelial cells or will migrate to tight junctions before extravasating to the site of injury or infection. Little is known about the mechanisms of neutrophil spreading or locomotion, or how motility is affected by the physical environment. We performed a systematic study to investigate the effect of the type of adhesive ligand and shear stress on neutrophil motility by employing a parallel-plate flow chamber with reconstituted protein surfaces of E-selectin, E-selectin/PECAM-1, and E-selectin/ICAM-1. We find that the level and type of adhesive ligand and the shear rate are intertwined in affecting several metrics of migration, such as the migration velocity, random motility, index of migration, and the percentage of cells moving in the direction of flow. On surfaces with high levels of PECAM-1, there is a near doubling in random motility at a shear rate of 180 s(-1) compared to the motility in the absence of flow. On surfaces with ICAM-1, neutrophil random motility exhibits a weaker response to shear rate, decreasing slightly when shear rate is increased from static conditions to 180 s(-1), and is only slightly higher at 1000 s(-1) than in the absence of flow. The random motility increases with increasing surface concentrations of E-selectin and PECAM-1 under static and flow conditions. Our findings illustrate that the endothelium may regulate neutrophil migration in postcapillary venules through the presentation of various adhesion ligands at sites of inflammation.     

Intracellular heterogeneity in adhesiveness of endothelium affects early steps in leukocyte adhesion

Endothelial cell junctions are thought to be preferential sites for transmigration. However, the factors that determine the site of transmigration are not well defined. Our data show that the preferential role of endothelial cell junctions is not limited to transmigration but extends to earlier steps of leukocyte recruitment, such as rolling and arrest. We used primary mouse neutrophils and mouse aortic endothelium in a flow chamber system to compare adhesive interactions near endothelial cell junctions to interactions over endothelial cell centers. We found differences in both rolling velocity and arrest frequency for neutrophils at endothelial cell junctions vs. more central areas of endothelial cells. Differences were governed by adhesion molecule interactions, not local topography. Interestingly, the role of particular adhesion molecules depended on their location on the endothelial cell surface. Although ICAM-1 stabilized and slowed rolling over central areas of the cell, it did not influence rolling velocity over endothelial cell junctions. P-selectin and VCAM-1 were more important for rolling near endothelial cell junctions than E-selectin. This demonstrates that adhesive properties of endothelial cell junctions influence early events in the adhesion cascade, which may help explain how leukocytes are localized to sites of eventual transmigration. endothelial cells; rolling; selectins; integrins     

Direct real-time observation of E- and P-selectin-mediated rolling on cutaneous lymphocyte-associated antigen immobilized on Western blots

Human memory T cells associated with cutaneous inflammatory responses are characterized by their expression of cutaneous lymphocyte-associated Ag (CLA), a carbohydrate determinant differentially expressed on P-selectin glycoprotein ligand-1 (PSGL-1). Although expression of the CLA epitope on PSGL-1 (CLA(+) PSGL-1) by memory T cells is associated with acquisition of E-selectin ligand activity, it is not known whether CLA(+) PSGL-1, itself, is a ligand for E-selectin on human T cells or whether other glycoproteins, with or without CLA modification, support E-selectin-dependent rolling in shear flow. To address this issue, we developed a method for real-time analysis of functional adhesive interactions between selectin-bearing cells in shear flow with leukocyte ligands resolved by SDS-PAGE and immobilized on standard Western blots. The results of these studies provide direct evidence that CLA(+) PSGL-1 is a functional ligand for both E- and P-selectin, confirm that the P-selectin ligand activity of PSGL-1 is independent of CLA modification, and identify a distinct, non-PSGL-1 E-selectin ligand on CLA-positive human memory T cells.     

E-selectin-dependent signaling via the mitogen-activated protein kinase pathway in vascular endothelial cells

E-selectin, a cytokine-inducible adhesion molecule, supports rolling and stable arrest of leukocytes on activated vascular endothelium. Previous studies have suggested that this transmembrane protein can also transduce signals into the endothelial cell. We now demonstrate activation of the mitogen-activated protein kinase (MAPK) signaling cascade in cultured HUVEC in response to E-selectin-dependent leukocyte adhesion and Ab-mediated cross-linking of cell surface E-selectin. Adhesion of increasing numbers of HL60 cells to IL-1beta-activated HUVEC stimulated robust increases in MAPK activity that were abrogated by an E-selectin blocking Ab. Cross-linking of cell surface E-selectin with Abs, as a mimic of multivalent ligand engagement, strongly stimulated MAPK/extracellular signal-related kinase (ERK) kinase (MEK)-dependent MAPK activation and concomitant up-regulation of mRNA for c-fos, an immediate early response gene, whereas Ab cross-linking of HLA class I molecules (present at comparable density) failed to do so. Coimmunoprecipitation documented Ras, Raf-1 and, phospho-MEK complex formation. Unactivated HUVEC transduced with a full-length adenoviral E-selectin construct also exhibited cross-link-induced MAPK activation, macromolecular complex formation, and c-fos up-regulation, whereas HUVEC transduced with a cytoplasmic domain deletion mutant failed to respond. These observations indicate that E-selectin can transduce an activating stimulus via the MAPK cascade into the endothelial cell during leukocyte adhesion.     

IL-17 and TNF-α sustain neutrophil recruitment during inflammation through synergistic effects on endothelial activation

IL-17A (IL-17) is the signature cytokine produced by Th17 cells and has been implicated in host defense against infection and the pathophysiology of autoimmunity and cardiovascular disease. Little is known, however, about the influence of IL-17 on endothelial activation and leukocyte influx to sites of inflammation. We hypothesized that IL-17 would induce a distinct pattern of endothelial activation and leukocyte recruitment when compared with the Th1 cytokine IFN-γ. We found that IL-17 alone had minimal activating effects on cultured endothelium, whereas the combination of TNF-α and IL-17 produced a synergistic increase in the expression of both P-selectin and E-selectin. Using intravital microscopy of the mouse cremaster muscle, we found that TNF-α and IL-17 also led to a synergistic increase in E-selectin-dependent leukocyte rolling on microvascular endothelium in vivo. In addition, TNF-α and IL-17 enhanced endothelial expression of the neutrophilic chemokines CXCL1, CXCL2, and CXCL5 and led to a functional increase in leukocyte transmigration in vivo and CXCR2-dependent neutrophil but not T cell transmigration in a parallel-plate flow chamber system. By contrast, endothelial activation with TNF-α and IFN-γ preferentially induced the expression of the integrin ligands ICAM-1 and VCAM-1, as well as the T cell chemokines CXCL9, CXCL10, and CCL5. These effects were further associated with a functional increase in T cell but not neutrophil transmigration under laminar shear flow. Overall, these data show that IL-17 and TNF-α act in a synergistic manner to induce a distinct pattern of endothelial activation that sustains and enhances neutrophil influx to sites of inflammation.     

Threshold Levels of Fluid Shear Promote Leukocyte Adhesion through Selectins (CD62L,P,E)

Leukocyte adhesion through L-selectin to peripheral node addressin (PNAd, also known as MECA-79 antigen), an L-selectin ligand expressed on high endothelial venules, has been shown to require a minimum level of fluid shear stress to sustain rolling interactions (Finger, E.B., K.D. Puri, R. Alon, M.B. Lawrence, V.H. von Andrian, and T.A. Springer. 1996. Nature (Lond.). 379:266–269). Here, we show that fluid shear above a threshold of 0.5 dyn/cm² wall shear stress significantly enhances HL-60 myelocyte rolling on P- and E-selectin at site densities of 200/μm² and below. In addition, gravitational force is sufficient to detach HL60 cells from P- and E-selectin substrates in the absence, but not in the presence, of flow. It appears that fluid shear–induced torque is critical for the maintenance of leukocyte rolling. K562 cells transfected with P-selectin glycoprotein ligand-1, a ligand for P-selectin, showed a similar reduction in rolling on P-selectin as the wall shear stress was lowered below 0.5 dyn/cm². Similarly, 300.19 cells transfected with L-selectin failed to roll on PNAd below this level of wall shear stress, indicating that the requirement for minimum levels of shear force is not cell type specific. Rolling of leukocytes mediated by the selectins could be reinitiated within seconds by increasing the level of wall shear stress, suggesting that fluid shear did not modulate receptor avidity. Intravital microscopy of cremaster muscle venules indicated that the leukocyte rolling flux fraction was reduced at blood centerline velocities less than 1 mm/s in a model in which rolling is mediated by L- and P-selectin. Similar observations were made in L-selectin–deficient mice in which leukocyte rolling is entirely P-selectin dependent. Leukocyte adhesion through all three selectins appears to be significantly enhanced by a threshold level of fluid shear stress.     

CD43 functions as a ligand for E-Selectin on activated T cells

E-selectin, an inducible cell adhesion molecule expressed on endothelial cells, mediates the rolling on endothelium of leukocytes expressing E-selectin ligands, such as neutrophils and activated T cells. Although previous studies using mice lacking P-selectin glycoprotein ligand-1 (PSGL-1) have indicated that PSGL-1 on Th1 cells functions as an E-selectin ligand, the molecular nature of E-selectin ligands other than PSGL-1 remains unknown. In this study, we show that a 130-kDa glycoprotein was precipitated by an E-selectin-IgG chimera from mouse Th1 cells. This protein was cleaved by O-sialoglycoprotein endopeptidase and required sialic acid for E-selectin binding. The mAb 1B11, which recognizes the 130-kDa glycoform of CD43, recognized the 130-kDa band in the E-selectin-IgG precipitate. In addition, immunoprecipitation of the E-selectin-IgG precipitate with 1B11 depleted the 130-kDa protein, further confirming its identity as CD43. CD43 was also precipitated with E-selectin-IgG from cultured human T cells. E-selectin-dependent cell rolling on CD43 was observed under flow conditions using a CD43-IgG chimera generated in Chinese hamster ovary cells expressing alpha-1,3-fucosyltransferase VII and a core 2 beta-1,6-N-acetylglucosaminyltransferase. These results suggest that CD43, when modified by a specific set of glycosyltranferases, can function as an E-selectin ligand and therefore potentially mediate activated T cell migration into inflamed sites.     

differential L-selectin binding activities of human hematopoietic cell L-selectin ligands, HCELL and PSGL-1

Expression of L-selectin on human hematopoietic cells (HC) is associated with a higher proliferative activity and a more rapid engraftment after hematopoietic stem cell transplantation. Two L-selectin ligands are expressed on human HCs, P-selectin glycoprotein ligand-1 (PSGL-1) and a specialized glycoform of CD44 (hematopoietic cell E- and L-selectin ligand, HCELL). Although the structural biochemistry of HCELL and PSGL-1 is well characterized, the relative capacity of these molecules to mediate L-selectin-dependent adhesion has not been explored. In this study, we examined under shear stress conditions L-selectin-dependent leukocyte adhesive interactions mediated by HCELL and PSGL-1, both as naturally expressed on human HC membranes and as purified molecules. By utilizing both Stamper-Woodruff and parallel-plate flow chamber assays, we found that HCELL displayed a 5-fold greater capacity to support L-selectin-dependent leukocyte adherence across a broad range of shear stresses compared with that of PSGL-1. Moreover, L-selectin-mediated leukocyte binding to immunopurified HCELL was consistently >5-fold higher than leukocyte binding to equivalent amounts of PSGL-1. Taken together, these data indicate that HCELL is a more avid L-selectin ligand than PSGL-1 and may be the preferential mediator of L-selectin-dependent adhesive interactions among human HCs in the bone marrow.     

Mac-2 binding protein is a novel e-selectin ligand expressed by breast cancer cells

Hematogenous metastasis involves the adhesion of circulating tumor cells to vascular endothelium of the secondary site. We hypothesized that breast cancer cell adhesion is mediated by interaction of endothelial E-selectin with its glycoprotein counter-receptor(s) expressed on breast cancer cells. At a hematogenous wall shear rate, ZR-75-1 breast cancer cells specifically adhered to E-selectin expressing human umbilical vein endothelial cells when tested in parallel plate flow chamber adhesion assays. Consistent with their E-selectin ligand activity, ZR-75-1 cells expressed flow cytometrically detectable epitopes of HECA-452 mAb, which recognizes high efficiency E-selectin ligands typified by sialofucosylated moieties. Multiple E-selectin reactive proteins expressed by ZR-75-1 cells were revealed by immunoprecipitation with E-selectin chimera (E-Ig chimera) followed by Western blotting. Mass spectrometry analysis of the 72 kDa protein, which exhibited the most prominent E-selectin ligand activity, corresponded to Mac-2 binding protein (Mac-2BP), a heretofore unidentified E-selectin ligand. Immunoprecipitated Mac-2BP expressed sialofucosylated epitopes and possessed E-selectin ligand activity when tested by Western blot analysis using HECA-452 mAb and E-Ig chimera, respectively, demonstrating that Mac-2BP is a novel high efficiency E-selectin ligand. Furthermore, silencing the expression of Mac-2BP from ZR-75-1 cells by shRNA markedly reduced their adhesion to E-selectin expressing cells under physiological flow conditions, confirming the functional E-selectin ligand activity of Mac-2BP on intact cells. In addition to ZR-75-1 cells, several other E-selectin ligand positive breast cancer cell lines expressed Mac-2BP as detected by Western blot and flow cytometry, suggesting that Mac-2BP may be an E-selectin ligand in a variety of breast cancer types. Further, invasive breast carcinoma tissue showed co-localized expression of Mac-2BP and HECA-452 antigens by fluorescence microscopy, underscoring the possible role of Mac-2BP as an E-selectin ligand. In summary, breast cancer cells express Mac-2BP as a novel E-selectin ligand, potentially revealing a new prognostic and therapeutic target for breast cancer.     

CD43 collaborates with P-selectin glycoprotein ligand-1 to mediate E-selectin-dependent T cell migration into inflamed skin

Activated T cell migration into nonlymphoid tissues is initiated by the interactions of P- and E-selectin expressed on endothelial cells and their ligands on T cells. P-selectin glycoprotein ligand-1 (PSGL-1) has been the only E-selectin ligand demonstrated to function during the in vivo migration of activated T cells. We show in this study that CD43-deficient Th1 cells, like PSGL-1-deficient cells, exhibited reduced E-selectin-binding activity compared with wild-type cells. Th1 cells with a PSGL-1 and CD43 double deficiency showed even less E-selectin-binding activity. In migration assays in which adoptively transferred cells migrate to inflamed skin P- and E-selectin dependently, CD43 contributed significantly to PSGL-1-independent Th1 cell migration. In addition, in vivo activated T cells from the draining lymph nodes of sensitized mice deficient in PSGL-1 and/or CD43 showed significantly decreased E-selectin-binding activity and migration efficiency, with T cells from double-deficient mice showing the most profound decrease. Collectively, these results demonstrate that the CD43 expressed on activated T cells functions as an E-selectin ligand and thereby mediates T cell migration to inflamed sites, in collaboration with PSGL-1.     

Alpha 4 integrin-dependent leukocyte recruitment does not require VCAM-1 in a chronic model of inflammation

Rats immunized with Mycobacterium butyricum in Freund's adjuvant develop a chronic vasculitis, with large increases in leukocyte rolling and adhesion in mesenteric postcapillary venules that are significantly inhibited with an alpha 4 integrin Ab. Using intravital microscopy to visualize chronically inflamed microvessels, we demonstrated that alpha 4 integrin-dependent leukocyte rolling and adhesion was inhibited with a beta 1 integrin, but not a beta 7 integrin Ab. To date, VCAM-1 has been presumed to be the primary ligand for alpha 4 beta 1 integrin in the vasculature. However, alpha 4 beta 1 integrin-dependent interactions were not reduced by monoclonal or polyclonal VCAM-1 Abs or a VCAM-1 antisense oligonucleotide despite increased VCAM-1 expression in the mesenteric vasculature. To ensure that the VCAM-1 Abs were functional and used at saturating concentrations, blood from Ab-treated rats was perfused over monolayers of CHO cells transfected with rat VCAM-1. Sufficient alpha 4 integrin or VCAM-1 Ab was present to inhibit leukocyte interactions with rat VCAM-1 by 95-100%. Under in vitro flow conditions, only mononuclear leukocytes were recruited from blood of control rats onto purified VCAM-1. However, neutrophils were also recruited onto VCAM-1 from whole blood of adjuvant-immunized animals via alpha 4 integrin. Another ligand for alpha 4 beta 1 integrin is the connecting segment-1 (CS-1) region of fibronectin. An Ab to the CS-1 portion of fibronectin, which did not reduce rolling and adhesion in adjuvant arthritis animals, completely inhibited leukocyte adhesion to CS-1 under static conditions. These findings provide the first evidence that alpha 4 beta 1 integrin-dependent leukocyte rolling and adhesion can occur in vivo via a mechanism other than VCAM-1.     

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.