Molecular mapping of functional domains of the leukocyte receptor for endothelium, LAM-1

Heterotropic modulation of selectin affinity by allosteric antibodies affects leukocyte rolling

Selectins are a family of adhesion receptors designed for efficient leukocyte tethering to the endothelium under shear. As a key property to resist premature bond disruption, selectin adhesiveness is enhanced by tensile forces that promote the conversion of a bent into an extended conformation of the N-terminal lectin and epidermal growth factor-like domains. Conformation-specific Abs have been invaluable in deciphering the activation mechanism of integrins, but similar reagents are not available for selectins. In this study, we show that the anti-human L-selectin mAbs DREG-55 and LAM1-5 but not DREG-56, DREG-200, or LAM1-1 heterotropically modulate adhesion presumably by stabilizing the extended receptor conformation. Force-free affinity assays, flow chamber, and microkinetic studies reveal a ligand-specific modulation of L-selectin affinity by DREG-55 mAb, resulting in a dramatic decrease of rolling velocity under flow. Furthermore, secondary tethering of polymorphonuclear cells was blocked by DREG-200 but significantly boosted by DREG-55 mAb. The results emphasize the need for a new classification for selectin Abs and introduce the new concept of heterotropic modulation of receptor function.

Structural basis for selectin mechanochemistry

Selectins are adhesion molecules that resist large tensile forces applied by hydrodynamic forces to leukocytes binding to vessel walls. In crystals, the liganded (high-affinity) and unliganded (low-affinity) conformations differ in orientation between their tandem lectin and EGF domains. I examine how tensile force exerted on a selectin-ligand complex in vivo could favor the more extended, high-affinity conformation. Allostery is transmitted from the EGF-lectin domain interface to the ligand-binding interface on the lectin domain, 30 A away. Trp-1 of the lectin domain and the long axis of the EGF domain form an L-shaped prybar that is welded together by hydrogen bonds to the Trp-1 alpha-amino group. Pivoting of the prybar induced by force demolishes an interface between the Trp-1 side chain and the lectin domain at a switch1 region. These changes are transmitted by rigid body movement of the switch2 region to rearrangements in the switch3 region at the ligand binding site. Another switch region corresponds to a single residue in the EGF domain with large effects on ligand binding and rolling adhesion. Allostery in selectins, and the alignment of tensile force on a selectin-ligand complex with the transition pathway for conformational change, explain much of the structural basis for selectin mechanochemistry.

Transmission of allostery through the lectin domain in selectin-mediated cell adhesion

The selectins are cell adhesion proteins that must resist applied forces to mediate leukocyte tethering and rolling along the endothelium and have 2 conformational states. Selectin-ligand bond dissociation increases only modestly with applied force, and exhibits catch bond behavior in a low-force regime where bond lifetimes counterintuitively increase with increasing force. Both allosteric and sliding-rebinding models have emerged to explain catch bonds. Here, we introduce a large residue into a cleft that opens within the lectin domain to stabilize the more extended, high-affinity selectin conformation. This mutation stabilizes the high-affinity state, but surprisingly makes rolling less stable. The position of the mutation in the lectin domain provides evidence for an allosteric pathway through the lectin domain, connecting changes at the lectin-EGF interface to the distal binding interface.

Membrane markers of the immune cells in swine: an update

Besides their breeding value, swine are increasingly used as biomedical models. As reported in three international swine clusters of differentiation (CD) workshops and in the animal homologue section of the last workshop for the determination of human leukocyte differentiation antigens (HLDA 8), characterisation of leukocyte surface antigens by monoclonal antibodies and other molecular studies have determined the cell lineages and blood leukocyte subsets implicated in the immune response, including cell adhesion molecules involved in cell trafficking. This review focusses on the current state of knowledge of porcine leukocyte differentiation and major histocompatibility complex (SLA) molecules. Examples of porcine particularities such as the double-positive T lymphocytes with the phenotype CD(4+)CD8(low) and CD(4-)CD8(low) alphabeta T cell subsets and the persistence of SLA class II after T-lymphocyte activation are illustrated, as well as the shared characteristics of the Artiodactyla group, such as the high proportion of gammadelta TcR (T cell receptor) T cells in blood and other lymphoid tissues. Furthermore, discrepancies between swine and humans, such as CD16 expression on dendritic cells and CD11b (wCD11R1) tissue distribution are outlined. The rapidly growing information should facilitate manipulation of the swine immune system towards improving disease control, and open new avenues for biomedical research using the pig as a model.

Remodeling of the lectin-EGF-like domain interface in P- and L-selectin increases adhesiveness and shear resistance under hydrodynamic force

Crystal structures of the lectin and epidermal growth factor (EGF)-like domains of P-selectin show 'bent' and 'extended' conformations. An extended conformation would be 'favored' by forces exerted on a selectin bound at one end to a ligand and at the other end to a cell experiencing hydrodynamic drag forces. To determine whether the extended conformation has higher affinity for ligand, we introduced an N-glycosylation site to 'wedge open' the interface between the lectin and EGF-like domains of P-selectin. This alteration increased the affinity of P-selectin for its ligand P-selectin glycoprotein 1 (PSGL-1) and thereby the strength of P-selectin-mediated rolling adhesion. Similarly, an asparagine-to-glycine substitution in the lectin-EGF-like domain interface of L-selectin enhanced rolling adhesion under shear flow. Our results demonstrate that force, by 'favoring' an extended selectin conformation, can strengthen selectin-ligand bonds.

L-selectin: adhesion, signalling and its importance in pathologic posttraumatic endotoxemia and non-septic inflammation

The leucocyte expressed surface-bound L-selectin belongs to the selectin family of adhesion molecules. It exhibits adhesive as well as signalling functions. Mainly, it is of importance in lymphocyte homing and in the extravasation of leucocytes into the surrounding tissue during inflammation. Acting in the initial step of the cell adhesion cascade, L-selectin is responsible for the rolling of leucocytes on endothelial layers. Therefore, L-selectin is thought to be an adequate target for pharmacological interventions. Beneath the discussion of the molecules' general features like molecule structure and its regulation, the review focuses firstly on L-selectin in the context of posttraumatic inflammatory disorders, and secondly on the importance of L-selectin specific signalling events.

T and IgA B lymphocytes of the pharyngeal and palatine tonsils: differential expression of adhesion molecules and chemokines

The pharyngeal (Ph) and palatine (Pa) tonsils, although located in different regions of the upper aero-digestive tract (UADT), are thought to protect the respiratory tract similarly against infections by inducing and disseminating T and surface IgA(+) (sIgA(+)) B cells. We investigated the factors controlling the migratory properties of T and sIgA(+) B lymphocytes in the UADT of pigs by comparing the expression of vascular addressins, homing receptors and chemokine transcripts in Ph/Pa tonsils, Peyer's patches (PP) and their draining lymph nodes (LN). The vascular addressin PNAd was detected on high endothelial venules in both tonsils, whereas mucosal addressin cell adhesion molecule-1, otherwise present in PP and mesenteric LN, was not detected. More importantly, the vascular cell adhesion molecule-1 (VCAM-1) addressin was present in Ph tonsil and LN but neither in Pa tonsil nor in PP vascular cells, whereas both T and sIgA(+) B lymphocytes displayed similar levels of alpha4beta1(high) integrin, the ligand of VCAM-1. Analysis of transcript levels for several lymphoid (CCL19, CXCL12 and CCL21) and epithelial chemokines also demonstrated opposite chemokine mRNA ratios for Ph tonsil (CCL28 > CCL25) and PP, with Pa tonsil expressing very low levels of CCL28. Collectively, these data indicate that the differential compartmentalization of sIgA(+) lymphocytes between Pa and Ph tonsils may partly result from the differential expression of VCAM-1 and CCL28. They also suggest that tonsillar addressins and epithelial chemokines, rather than the cells intravasating it, control the regionalization of sIgA(+) lymphocytes in the UADT.

Adhesion molecules, catecholamines and leucocyte redistribution during and following exercise

The circulating blood normally contains no more than 1-2% of the body's population of leucocytes. The numbers and phenotypes of circulating leucocyte subsets can change dramatically during and immediately following exercise. The surface expression of adhesion molecules makes an important contribution to such responses by changing patterns of cell trafficking. Alterations in the surface expression of adhesion molecules could reflect a shedding of molecules, selective apoptosis or differential trafficking of cells with a particular phenotype, effects from mechanical deformation of the cytoplasm, active biochemical processes involving cytokines, catecholamines, glucocorticoids or other hormones, or changes in the induction of adhesion molecules. The expression of adhesion molecules changes with maturation and activation of leucocytes. Typically, mature cells express lower densities of L-selectin (CD62L), the homing receptor for secondary lymphoid organs, and higher densities of LFA-1 (CD11a), the molecule associated with trafficking to non-lymphoid reservoir sites. The neutrophils and natural killer cells that are mobilised during exercise also express high levels of Mac-1 (CD11b), a marker associated with cellular activation. Possibly, exercise demarginates older cells that are awaiting destruction in the spleen. Plasma concentrations of catecholamines rise dramatically with exercise, and there is growing evidence that catecholamines, acting through a cyclic adenosine monophosphate second messenger system, play an important role in modifying the surface expression of adhesion molecules. Analogous changes can be induced by other forms of stress that release catecholamines or by catecholamine infusion, and responses are blocked by beta(2)-blocking agents. Catecholamines also modify adherence and expression of adhesion molecules in vitro. Cell trafficking is modified by genetic deficiencies in the expression of adhesion molecules, but leucocyte responses to exercise and catecholamines are generally unaffected by splenectomy. A number of clinical conditions including atherogenesis and metaplasia are marked by an altered expression of adhesion molecules. The effects of exercise on these molecules could thus have important health implications.

CD19 can regulate B lymphocyte signal transduction independent of complement activation

B lymphocytes are critically regulated by signals transduced through the CD19-CD21 cell surface receptor complex, where complement C3d binding to CD21 supplies an already characterized ligand. To determine the extent that CD19 function is controlled by complement activation, CD19-deficient mice (that are hyporesponsive to transmembrane signals) and mice overexpressing CD19 (that are hyperresponsive) were crossed with CD21- and C3-deficient mice. Cell surface CD19 and CD21 expression were significantly affected by the loss of CD21 and C3 expression, respectively. Mature B cells from CD21-deficient littermates had approximately 36% higher cell surface CD19 expression, whereas CD21/35 expression was increased by approximately 45% on B cells from C3-deficient mice. Negative regulation of CD19 and CD21 expression by CD21 and C3, respectively, may be functionally significant because small increases in cell surface CD19 overexpression can predispose to autoimmunity. Otherwise, B cell development and function in CD19-deficient and -overexpressing mice were not significantly affected by a simultaneous loss of CD21 expression. Although CD21-deficient mice were found to express a hypomorphic cell surface CD21 protein at low levels that associated with mouse CD19, C3 deficiency did not significantly affect B cell development and function in CD19-deficient or -overexpressing mice. These results, and the severe phenotype exhibited by CD19-deficient mice compared with CD21- or C3-deficient mice, collectively demonstrate that CD19 can regulate B cell signaling thresholds independent of CD21 engagement and complement activation.

Soluble P-selectin moderates complement-dependent reperfusion injury of ischemic skeletal muscle

P-selectin is an adhesion molecule expressed on activated endothelial and platelet surfaces. The function of the short consensus repeats (SCRs) of P-selectin, homologous with the SCRs of complement regulatory proteins is largely unknown. In a model of murine hindlimb ischemia where local reperfusion injury is partly mediated by IgM natural antibody and classical complement pathway activation, we hypothesized that human soluble P-selectin (sP-sel) would moderate the complement component of the inflammatory response. Infusion of sP-sel supernatant or purified (p) sP-sel prepared from activated human platelets, reduced ischemic muscle vascular permeability by 48% and 43%, respectively, following reperfusion. Hindlimb immunohistochemistry demonstrated negligible C3 staining colocalized with IgM in these groups compared with intense staining in the untreated injured mice. In vitro studies of mouse serum complement hemolytic activity showed that psP-sel inhibited the classical but not alternative complement pathway. Flow cytometry demonstrated that psP-sel inhibited C1q adherence to sensitized red blood cells. From these data we conclude that sP-sel moderates skeletal muscle reperfusion injury by inhibition of the classical complement pathway.

An activated L-selectin mutant with conserved equilibrium binding properties but enhanced ligand recognition under shear flow

Selectins mediate the initial tethering and rolling of leukocytes on vessel walls. Adhesion by selectins is a function of both ligand recognition at equilibrium and mechanical properties of the selectin-ligand bond under applied force. We describe an EGF domain mutant of L-selectin with profoundly augmented adhesiveness over that of native L-selectin but conserved ligand specificity. This mutant, termed LPL, was derived by a substitution of the EGF-like domain of L-selectin with the homologous domain from P-selectin. The mutant bound soluble carbohydrate L-selectin ligand with affinity comparable with that of native L-selectin but interacted with all surface-bound ligands much more readily than native L-selectin, in particular under elevated shear flow. Tethers mediated by both native and mutant L-selectin exhibited similar lifetimes under a range of shear stresses, but the rate of bond formation by the mutant was at least 10-fold higher than that of native L-selectin toward distinct L-selectin ligands. Enhanced rate of bond formation by the mutant was associated with profoundly stronger rolling interactions and reduced dependence of rolling on a threshold of shear stress. This is the first demonstration that the EGF domain can modulate the binding of the lectin domain of a selectin to surface-immobilized ligands under shear flow without affecting the equilibrium properties of the selectin toward soluble ligands.

Characterization of an apparently conserved epitope in E- and P-selectin identified by dual-specific monoclonal antibodies

E- and P-selectin recognize a wide and overlapping range of oligosaccharide ligands including sialyl-Lewis X (sLeX) through their highly homologous C-type lectin domains. We report that an epitope apparently conserved between E- and P-selectin is functionally involved in ligand recognition although distantly located from the conventional carbohydrate binding site. We found that a previously established anti-E-selectin monoclonal antibody (mAb), 1.2B6, is cross-reactive with P-selectin, and that the 1.2B6 epitope is in the C-type lectin domain and identical to or overlapping with an epitope recognized by other independently established anti-E- and P-selectin dual-specific mAb. The epitope has been mapped by others to a region distant from the previously identified carbohydrate binding site of E-selectin in its three-dimensional structure. Nevertheless, it is of note that all dual-specific mAb, including 1.2B6, inhibited E- or P-selectin-mediated cell adhesion and also binding to sLeX. Engagement of the apparently conserved epitope by the dual-specific mAb may lead to inhibition of the ligand binding ability of E- and P-selectin by a previously uncharacterized mechanism(s) rather than by direct inhibition of sLeX binding to the hitherto identified ligand binding site.

Reconstitution of functional L-selectin ligands on a cultured human endothelial cell line by cotransfection of alpha1-->3 fucosyltransferase VII and newly cloned GlcNAcbeta:6-sulfotransferase cDNA

Recently, we proposed sialyl 6-sulfo Lewis X as a major carbohydrate-capping group of the L-selectin ligands on high endothelial venules in human lymph nodes. In this study we succeeded in reconstituting functional L-selectin ligands on a cultured human endothelial cell line, ECV304, by transfecting the alpha1-->3fucosyltranseferase VII (Fuc-T VII) and newly cloned GlcNAcbeta:6-sulfotransferase (6-Sul-T) cDNAs. The ECV304 cells transfected with Fuc-T VII cDNA expressed conventional sialyl Lewis X detected with specific antibodies including 2H5, whereas the cells transfected with 6-Sul-T cDNA expressed sialyl 6-sulfo lactosamine as well as MECA-79-defined carbohydrate determinants, but these singly transfected cells failed to express sialyl 6-sulfo Lewis X, as detected with the antisialyl 6-sulfo Lewis X mAb G152. Sialyl 6-sulfo Lewis X appeared only on the cells that were cotransfected with both 6-Sul-T and Fuc-T VII cDNAs. Significant adhesion of L-selectin-expressing cells was seen only to the doubly transfected ECV304 cells and was inhibited by G152. No adhesion was observed to the cells transfected either with 6-Sul-T or with Fuc-T VII cDNA alone. The mRNAs of the two enzymes were expressed or were inducible upon interleukin 1 stimulation in human endothelial cells. These results indicate that a set of carbohydrate determinants synthesized by the concerted action of the two enzymes, as typically represented by the sialyl 6-sulfo Lewis X-capping group, serves as an essential component of the ligand for L-selectin and that the reagents 2H5 and MECA-79, utilized in earlier studies to detect L-selectin ligand on high endothelial venules, recognize two different aspects of the same set of synthetic products.

Two Candidate Downstream Target Genes for E2A-HLF

The E2A-HLF fusion gene, formed by the t(17;19)(q22;p13) chromosomal translocation, is thought to drive the leukemic transformation of early B-cell precursors by repressing an evolutionarily conserved apoptotic pathway. To test this hypothesis, we sought to identify downstream targets of E2A-HLF in t(17;19)+ pro-B leukemia cells (UOC-B1) that had been transfected with a zinc-inducible vector encoding a dominant-negative suppressor (E2A-HLF[dn]) of the oncoprotein. Representational difference analysis of mRNAs from E2A-HLF(dn)+ UOC-B1 cells grown with (E2A-HLF inactive) or without (E2A-HLF active) the addition of zinc yielded several differentially expressed cDNA fragments that were individually subcloned. Two of the clones, designated F-5 and G-4, hybridized with mRNAs that were upregulated by E2A-HLF. Levels of both transcripts declined sharply within 8 to 12 hours after suppression of E2A-HLF DNA-binding activity, becoming undetectable after 96 hours. The F-5 cDNA was identified as a portion of ANNEXIN VIII, whose product was expressed in promyelocytic leukemia cells and UOC-B1 cells, but not in other leukemic cell lines. A novel full-length cDNA cloned with the G-4 fragment encoded a protein that we have named SRPUL (sushi-repeat protein upregulated in leukemia). It is normally expressed in heart, ovary, and placenta, but could not be detected in leukemic cell lines other than UOC-B1. Neither protein prevented apoptosis in interleukin-3–dependent murine pro-B cells, suggesting that they have paraneoplastic roles in leukemias that express E2A-HLF, perhaps in the disseminated intravascular coagulopathy and hypercalcemia that characterize these cases.

Two Candidate Downstream Target Genes for E2A-HLF

The E2A-HLF fusion gene, formed by the t(17;19)(q22;p13) chromosomal translocation, is thought to drive the leukemic transformation of early B-cell precursors by repressing an evolutionarily conserved apoptotic pathway. To test this hypothesis, we sought to identify downstream targets of E2A-HLF in t(17;19)+ pro-B leukemia cells (UOC-B1) that had been transfected with a zinc-inducible vector encoding a dominant-negative suppressor (E2A-HLF[dn]) of the oncoprotein. Representational difference analysis of mRNAs from E2A-HLF(dn)+ UOC-B1 cells grown with (E2A-HLF inactive) or without (E2A-HLF active) the addition of zinc yielded several differentially expressed cDNA fragments that were individually subcloned. Two of the clones, designated F-5 and G-4, hybridized with mRNAs that were upregulated by E2A-HLF. Levels of both transcripts declined sharply within 8 to 12 hours after suppression of E2A-HLF DNA-binding activity, becoming undetectable after 96 hours. The F-5 cDNA was identified as a portion of ANNEXIN VIII, whose product was expressed in promyelocytic leukemia cells and UOC-B1 cells, but not in other leukemic cell lines. A novel full-length cDNA cloned with the G-4 fragment encoded a protein that we have named SRPUL (sushi-repeat protein upregulated in leukemia). It is normally expressed in heart, ovary, and placenta, but could not be detected in leukemic cell lines other than UOC-B1. Neither protein prevented apoptosis in interleukin-3–dependent murine pro-B cells, suggesting that they have paraneoplastic roles in leukemias that express E2A-HLF, perhaps in the disseminated intravascular coagulopathy and hypercalcemia that characterize these cases.

L-selectin-mediated lymphocyte aggregation: role of carbohydrates, activation and effects on cellular interactions

L-selectin on lymphocytes reacts with glycosylated ligands on high endothelial venule walls in lymphoid organs. Through this carbohydrate-dependent interaction, rolling and initial attachment of lymphocytes to endothelium is mediated. Here we have studied an earlier described L-selectin-induced homotypic aggregation, to further elucidate the events that occur after engagement of L-selectin. It was found that the interaction of L-selectin with fucoidan, but not with other carbohydrates, or with monoclonal antibodies directed against the carbohydrate recognition domain of L-selectin, resulted in homotypic aggregation among both B- or T lymphocytes. Importantly, this aggregation was shown to be both lymphocyte function-associated antigen-1 (LFA-1) and calcium-independent. Furthermore, for aggregation metabolic energy was required, and signalling via protein tyrosine kinase appeared to be involved. Neither de novo protein synthesis, protein kinase C mediated signalling, Gi-protein mediated signal transduction, nor calcium mobilization were required for aggregation. During aggregation, L-selectin was not shed from the lymphocyte's cell surface. Finally, it was found that the lymphocyte binding capacity to high endothelial venules on cryostat sections was not altered upon triggering these lymphocytes via L-selectin. Interestingly, L-selectin-triggered cells showed increased binding to paracortical areas in peripheral lymph nodes. Our data suggest that signals via L-selectin, might lead to altered expression of cell surface molecules, important in interactions other than the first stage of lymphocyte rolling.

Fine mapping of the epitopes of humanized anti-L-selectin monoclonal antibodies HuDREG-55 and HuDREG-200

Blocking the function of L-selectin with a monoclonal antibody (mAb) is a promising way to prevent neutrophils from causing tissue damage during inflammation. HuDREG-55 and HuDREG-200 are humanized mAb which bind to human L-selectin and block its function as an adhesion molecule. To understand the mechanism of the action of HuDREG-55 and HuDREG-200, we determined their epitopes on L-selectin at the amino acid level. The analysis of human E- and L-selectin chimeric proteins demonstrated that the lectin domain of L-selectin is necessary for the binding of HuDREG-55 and HuDREG-200. Mutational analysis of Escherichia coli-expressed L-selectin showed that HuDREG-55 binding is sensitive to amino acid changes at positions 11, 56, 87, 89, 105, 107 and 111 (counting from the amino-terminus of mature L-selectin) while HuDREG-200 binding is sensitive to amino acid changes at 45, 46 and 47. Both epitopes are located close to the predicted carbohydrate binding site, indicating that HuDREG-55 and HuDREG-200 block the function of L-selectin by directly inhibiting the binding to carbohydrate ligands.

Heparin inhibits leukocyte rolling in pial vessels and attenuates inflammatory changes in a rat model of experimental bacterial meningitis

Heparin is a natural proteoglycan that was first described in 1916. In addition to its well characterized effect on blood coagulation, it is becoming clear that heparin also modulates inflammatory processes on several levels, including the interference with leukocyte-endothelium interaction. Anecdotal observations suggest a better clinical outcome of heparin-treated patients with bacterial meningitis. The authors demonstrate that heparin, a glycosaminoglycan, inhibits significantly in the early phase of experimental pneumococcal meningitis the increase of 1) regional cerebral blood flow (125 +/- 18 versus 247 +/- 42%), 2) intracranial pressure (4.5 +/- 2.0 versus 12.1 +/- 2.2 mm Hg), 3) brain edema (brain water content: 78.23 +/- 0.33 versus 79.49 +/- 0.46%), and 4) influx of leukocytes (571 +/- 397 versus 2400 +/- 875 cells/microL) to the cerebrospinal fluid compared with untreated rats. To elucidate the possible mechanism of this observation, the authors investigated for the first time leukocyte rolling in an inflammatory model in brain venules by confocal laser scanning microscopy in vivo. Heparin significantly attenuates leukocyte rolling at 2, 3, and 4 hours (2.8 +/- 1.3 versus 7.9 +/- 3.2/100 microm/min), as well as leukocyte sticking at 4 hours (2.1 +/- 0.4 versus 3.5 +/- 1.0/100 microm/min) after meningitis induction compared with untreated animals. The authors conclude that heparin can modulate acute central nervous system inflammation and, in particular, leukocyte-endothelium interaction, a key process in the cascade of injury in bacterial meningitis. They propose to evaluate further the potential of heparin in central nervous system inflammation in basic and clinical studies.

Fucoidin, a potent inhibitor of L-selectin function, reduces contact hypersensitivity reaction in mice

In order to investigate the role of L-selectin (CD62L) in delayed-type hypersensitivity (DTH) reaction, effect of fucoidin, a potent inhibitor of CD62L function, was examined in a model of mouse contact hypersensitivity reaction. Intravenous injection of fucoidin to sensitized mice just before hapten challenge resulted in a significant and dose-dependent reduction of the ear swelling in contact hypersensitivity. The ear swelling caused by the hapten challenge was also inhibited when fucoidin was administered at the sensitization phase. Histological analyses of the ear sections revealed that the fucoidin-induced suppression of contact hypersensitivity reflected a marked inhibition of the ear edema and the leukocyte infiltration. The activity of fucoidin was specific in that its related saccharides exerted little effect on the reaction. These results suggest that CD62L may play an important role in both afferent and efferent phases of cutaneous DTH reaction. Since DTH response is one of the most significant features of several chronic inflammatory diseases, our data also show that blocking of CD62L function may be beneficial for the treatment of these diseases in humans.

Soluble Monomeric P-Selectin Containing Only the Lectin and Epidermal Growth Factor Domains Binds to P-Selectin Glycoprotein Ligand-1 on Leukocytes

Under shear stress, leukocytes use P-selectin glycoprotein ligand-1 (PSGL-1) to tether to and roll on P-selectin expressed on activated platelets or endothelial cells. P-selectin has an NH2-terminal lectin domain, an epidermal growth factor (EGF)-like motif, nine consensus repeats (CRs), a transmembrane domain, and a cytoplasmic tail. To determine whether the CRs are required for P-selectin to bind PSGL-1, we expressed a soluble protein (Lec-EGF) that contained only the lectin and EGF domains, plus a short C-terminal epitope tag. Electron microscopy and hydrodynamic analysis confirmed that Lec-EGF was monomeric, as previously shown for soluble P-selectin (sPS) that contained the lectin and EGF domains plus all nine CRs. Fluid-phase Lec-EGF or sPS inhibited binding of oligomeric125I-labeled membrane-derived P-selectin (mPS) to PSGL-1 on neutrophils and binding of 125I-PSGL-1 to immobilized mPS. The IC50 for inhibiting binding of mPS to neutrophils was fivefold greater for Lec-EGF than for sPS, whereas the IC50 for inhibiting binding of mPS to purified PSGL-1 was indistinguishable for Lec-EGF and sPS. Under static or shear conditions, neutrophils used PSGL-1 to tether to or roll on Lec-EGF that was captured by an immobilized monoclonal antibody to the C-terminal epitope. These data show that P-selectin requires only the lectin and EGF domains to bind to PSGL-1.

Soluble Monomeric P-Selectin Containing Only the Lectin and Epidermal Growth Factor Domains Binds to P-Selectin Glycoprotein Ligand-1 on Leukocytes

Under shear stress, leukocytes use P-selectin glycoprotein ligand-1 (PSGL-1) to tether to and roll on P-selectin expressed on activated platelets or endothelial cells. P-selectin has an NH2-terminal lectin domain, an epidermal growth factor (EGF)-like motif, nine consensus repeats (CRs), a transmembrane domain, and a cytoplasmic tail. To determine whether the CRs are required for P-selectin to bind PSGL-1, we expressed a soluble protein (Lec-EGF) that contained only the lectin and EGF domains, plus a short C-terminal epitope tag. Electron microscopy and hydrodynamic analysis confirmed that Lec-EGF was monomeric, as previously shown for soluble P-selectin (sPS) that contained the lectin and EGF domains plus all nine CRs. Fluid-phase Lec-EGF or sPS inhibited binding of oligomeric125I-labeled membrane-derived P-selectin (mPS) to PSGL-1 on neutrophils and binding of 125I-PSGL-1 to immobilized mPS. The IC50 for inhibiting binding of mPS to neutrophils was fivefold greater for Lec-EGF than for sPS, whereas the IC50 for inhibiting binding of mPS to purified PSGL-1 was indistinguishable for Lec-EGF and sPS. Under static or shear conditions, neutrophils used PSGL-1 to tether to or roll on Lec-EGF that was captured by an immobilized monoclonal antibody to the C-terminal epitope. These data show that P-selectin requires only the lectin and EGF domains to bind to PSGL-1.

Adhesion molecules in haemopoiesis

In the adult mammal, haemopoiesis is restricted to the extravascular compartment of the bone marrow (BM) where primitive haemopoietic stem cells (HSC) and their clonogenic progeny develop in intimate contiguity with a heterogeneous population of stromal cells that comprise the haemopoietic micro-environment (HM). Although the importance of cellular interactions between primitive haemopoietic progenitor cells (HPC) and marrow stromal cells is well established, precise definition of the nature of many of these interactions at the molecular level is lacking and remains an objective of fundamental importance to understanding of haemopoietic regulation. Current data suggest that a wide variety of cell surface molecules representing several adhesion molecule superfamilies, including integrins, selectins, sialomucins and the immunoglobulin gene superfamily, are involved in supporting cell-cell and cell-extracellular matrix (ECM) interactions. These diverse CAM-ligand interactions, rather than simply serving to initiate and maintain contact between HPC and stromal cells and ECM components, also have an additional, more direct role in controlling the growth and development of primitive haemopoietic cells.

Histamine (H1) receptor antagonist inhibits leukocyte rolling in pial vessels in the early phase of bacterial meningitis in rats

The present study tested the hypothesis whether a histamine dependent pathway is involved in leukocyte-endothel interaction in the early phase of bacterial meningitis. Using confocal laser scanning microscopy we investigated leukocyte rolling in brain venules in vivo during 4 h in experimental pneumococcal meningitis in the rat. Leukocyte rolling, but not firm adhesion induced by intracisternally (i.c.) injected pneumococcal cell wall components, was temporarily inhibited (2 h, 5.6 +/- 1.9 vs. 2.3 +/- 0.9; 3 h, 7.4 +/- 2.7 vs. 3.1 +/- 1.3/100 microm/min) by diphenhydramine, a histamine H1 receptor antagonist. Histamine, possibly released by activated mast cells, is known to initiate P-selectin upregulation and subsequent leukocyte rolling. This data suggest that histamine is a mediator of leukocyte rolling in the early phase of bacterial meningitis.

L-selectin from human, but not from mouse neutrophils binds directly to E-selectin

L-Selectin on neutrophils as well as inducible E- and P-selectin on endothelium are involved in the recruitment of neutrophils into inflamed tissue. Based on cell attachment assays, L-selectin was suggested to function as a carbohydrate presenting ligand for E- and P-selectin. However, previous affinity isolation experiments with an E-selectin-Ig fusion protein had failed to detect L-selectin among the isolated E-selectin ligands from mouse neutrophils. We show here that L-selectin from human neutrophils, in contrast to mouse neutrophils, can be affinity-isolated as a major ligand from total cell extracts using E-selectin-Ig as affinity probe. Binding of human L-selectin to E-selectin was direct, since purified L-selectin could be reprecipitated with E-selectin-Ig. Recognition of L-selectin was abolished by sialidase-treatment, required Ca2+, and was resistant to treatment with endoglycosidase F. Binding of L-selectin to a P-selectin-Ig fusion protein was not observed. In agreement with the biochemical data, the anti-L-selectin mAb DREG56 inhibited rolling of human neutrophils on immobilized E-selectin-Ig but not on P-selectin-Ig. No such inhibitory effect was seen with the anti-mouse L-selectin mAb MEL14 on mouse neutrophils. Rolling of E-selectin transfectants on purified and immobilized human L-selectin was inhibited by mAb DREG56. We conclude that L-selectin on human neutrophils is a major glycoprotein ligand among very few glycoproteins that can be isolated by an E-selectin affinity matrix. The clear difference between human and mouse L-selectin suggests that E-selectin-binding carbohydrate moieties are attached to different protein scaffolds in different species.

The Ly-49 family: genes, proteins and recognition of class I MHC

The Ly-49 family consists of at least nine members, of which Ly-49A and C have been found to be NK-cell inhibitory receptors specific for class I MHC. The functions of other Ly-49 molecules are still unclear. Further analysis of Ly-49 is complicated by the cross-reactivities of some anti-Ly-49 antibodies initially thought to be specific for individual Ly-49 molecules. Studies on the role of Ly-49 in hybrid resistance as well as on allelic exclusion are also complicated by our recent finding that a novel Ly-49CB6 gene is the likely allelic form of Ly-49CBALB as opposed to a previously reported highly related but distinct gene in B6 mice. In cell-cell binding assays, only Ly-49A and C show significant binding to class I MHC. Ly-49A and C also bind some polysaccharides, and carbohydrates on class I MHC seem to be important for its binding to Ly-49. However, this interaction involves not only the carbohydrate recognition domain of Ly-49 but also a part of the stalk region, suggesting that both carbohydrates and peptide backbone of class I MHC may be recognized by Ly-49. It is likely that additional Ly-49 molecules yet to be identified function as NK-inhibitory receptors specific for class I MHC.

L-selectin activates the Ras pathway via the tyrosine kinase p56lck

Selectins mediate rolling, the initial step of leukocyte adhesion to endothelial cells [Springer, T. A. (1995) Annu. Rev. Physiol. 57, 827-872 and Butcher, E. C. (1991) Cell 67, 1033-1036]. In this study we show that L-selectin triggering of Jurkat cells using different antibodies or glycomimetics resulted in activation of the src-tyrosine kinase p56lck; tyrosine phosphorylation of intracellular proteins, in particular mitogen-activating protein kinase and L-selectin; and association of Grb2/Sos with L-selectin. This association correlated with an activation of p21Ras, mitogen-activating protein kinase, Rac2, and a transient increase of 2-O synthesis. Stimulation of the Ras pathway by L-selectin requires functional p56lck, since p56lck-deficient Jurkat cells (JCaM1.6) do not show tyrosine phosphorylation, association of L-selectin with Grb2/Sos, and activation of Ras upon L-selectin triggering. Transfection of JCaM1.6 cells with p56lck reconstitutes the observed signaling events. Genetic inhibition of Ras or Rac2 prevented Rac2 stimulation and 2-O synthesis, respectively. The specificity and the physiological significance of the observed signaling cascade is indicated by stimulation of L-selectin-transfected P815, L-selectin-positive CEM or peripheral blood lymphocytes resulting in the same activation events as in Jurkat cells. Our results point to a signaling cascade from L-selectin via p56lck, Grb2/Sos, Ras, and Rac2 to 2-O.

Structure and function of the epidermal growth factor domain of P-selectin

P-selectin is a multidomain adhesion protein on the surface of activated platelets and endothelial cells that functions in the recruitment of leukocytes to the site of inflammation. The amino-terminal lectin and EGF domains constitute the ligand recognition unit. We have produced a synthetic 40-residue P-selectin EGF domain (P-sel:EGF) to examine the structure and function of this domain independent of P-selectin. The peptide was folded in vitro and exhibited the same disulfide bonding pattern as other EGF-like domains. P-sel:EGF did not inhibit P-selectin-mediated cellular adhesion assays, indicating that the lectin domain is also required. We undertook the study of the P-selectin EGF by 1H NMR to determine its structure independent of the lectin domain and to compare its structure to that of E-selectin determined crystallographically [Graves et al. (1994) Nature 367, 532]. Although the binding of P-selectin to its carbohydrate ligand is calcium dependent, and some EGF domains have calcium binding sites, addition of calcium had no effect on the NMR spectrum or on the pH-induced changes. Nearly complete resonance assignments were made from 2D 1H NMR spectra at pH 6.0. Two sections of antiparallel beta-sheet were identified on the basis of the pattern of long-range NOEs, 3JHN alpha coupling constants, and slowly exchanging amides. The solution structure of the peptide backbone was determined using distance geometry and simulated annealing calculations. The backbone RMSD to the geometric average for 19 final structures is 0.64 +/- 0.17 A. The resulting fold closely resembles that of other EGF-like peptides, including the E-selectin EGF domain (RMSD approximately 1.08 A). However, compared to the E-selectin EGF structure which also contains the lectin domain, some residues from 1-11 are less ordered, and novel contacts occur between the amino terminus and the core beta-sheet. Despite marked structural homology of the selectin polypeptide backbones, the selectin EGF surfaces show unique distributions of charged residues, a feature that likely correlates to the functional differences.

Differential regulation of tissue-specific lymph node high endothelial venule cell adhesion molecules by tumour necrosis factor and transforming growth factor-beta 1

Lymphocytes migrate from blood into lymph nodes (LN) of rats specifically at segments of venules lined by high endothelium (HEV). We have previously shown that pretreatment of LN HEV cells with pro-inflammatory cytokines such as tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma), augments their adhesiveness for thoracic duct lymphocytes (TDL). Here we report that a mouse monoclonal antibody, 3C10, recognized tissue-specific endothelial determinants on rat LN HEV cells and blocked their adhesiveness for TDL and EL-4J cells transfected with rat L-selectin. In contrast, 3C10 antibody did not inhibit lymphocyte attachment to Peyer's patch (PP) frozen sections or cultured PP HEV cells. The antibody immunoprecipitated from LN HEV cells two proteins with apparent molecular weights of 90,000 and 50,000. The expression of 3C10 antigen on LN HEV cells was increased by incubation with TNF-alpha or IFN-gamma. Furthermore, pretreatment of cytokine-stimulated LN HEV cells with 3C10 antibody blocked TDL binding in a dose-dependent manner. In contrast, 3C10 antigen expression on LN HEV cells was significantly decreased following incubation of cells with transforming growth factor-beta 1 (TGF-beta 1). In addition, TGF-beta 1 also abrogated the adhesiveness of LN HEV cells stimulated with TNF-alpha, IFN-gamma or both cytokines. Together, these data suggest that endothelial determinants recognized by the 3C10 antibody are tissue-specific ligands for lymphocyte adhesion and cytokines such as TNF-alpha and TGF-beta differentially regulate their expression and function.

Anionic phospholipids bind to L-selectin (but not E-selectin) at a site distinct from the carbohydrate-binding site

It is known that L-selectin binds to glycoconjugates containing the tetrasaccharide sialyl Lewis X in a Ca2+-dependent manner. In addition, a number of other acidic oligosaccharides (for example heparin or chondroitin sulphate) or glycolipids (for example sulphatides) bind to L-selectin independent of cations. In this paper we have established that L-selectin binds to charged phospholipids, such as cardiolipin and phosphatidylserine, but not to neutral phospholipids such as phosphatidylcholine. No interaction between E-selectin and any phospholipid was observed. The interaction between L-selectin cardiolipin was inhibited by dextran sulphate, fucoidan, mannose 6-phosphate and monoclonal antibodies previously reported to block the interaction between L-selectin and its natural ligands. Analysis of the amino acid sequence of the selectins indicated that L-selectin, but no E-selectin, contains a sequence homologous to the putative cardiolipin-binding epitope found in plasma glycoprotein beta2I. Glycoprotein beta2I and a peptide corresponding to the putative cardiolipin-binding epitope in beta2I inhibited the binding of L-selectin to cardiolipin or fucoidin. Based on the binding characteristics, sequence analysis and structural modelling of L-selectin, we suggest that the amino acid sequence KKNKED (residues 84-89) is a novel site for the binding of acidic species to L-selectin. This motif is localized close to the putative carbohydrate-binding domain of L-selectin and may be a second site within the lectin domain for the interaction of leucocyte L-selectin with its natural endothelial ligands.

Calcium-dependent lectin activity with novel distribution on thymocyte subsets

In this study we use carbohydrate probes to search for novel cell surface lectins in the immune system. Many carbohydrate binding proteins are members of the C-type lectin superfamily, whose ligand binding is dependent on calcium. To identify potential new members of this superfamily, fluorescein-conjugated carbohydrate polymers were used to probe for calcium-dependent cell surface binding. This approach offers advantages over the use of monoclonal antibody probes since only carbohydrate binding proteins would be identified. We have identified a cell surface lectin, referred to as thy-lec, detected by the probe fucoidan-fuorescein isothiocyanate. This calcium-dependent lectin has a novel distribution on thymocyte subsets. It is present on the surface of immature CD4+8+ and on large, cycling CD4-8- cells and CD8+4- cells, but not on small, mature phenotype CD8+4- or CD4+8- thymocytes. This lectin is not found on mature T cells or other leukocytes in lymph nodes, spleen or bone marrow. It is proposed that this novel cell surface has a function in the maturation of T cells in the thymus.

Structural requirements regulate endoproteolytic release of the L-selectin (CD62L) adhesion receptor from the cell surface of leukocytes

L-selectin mediates leukocyte rolling on vascular endothelium at sites of inflammation and lymphocyte migration to peripheral lymph nodes. L-selectin is rapidly shed from the cell surface after leukocyte activation by a proteolytic mechanism that cleaves the receptor in a membrane proximal extracellular region. This process may allow rapid leukocyte detachment from the endothelial surface before entry into tissues. In this study, the structural requirements for regulation of human L-selectin endoproteolytic release were examined through analysis of chimeric selectin molecules and mutant L-selectin receptors. The use of chimeric selectins and a cytoplasmic tail truncation mutant demonstrated that the extracellular membrane-proximal 15-amino acid region of L-selectin is required for endoproteolytic release. The introduction of alanine-scanning mutations within this membrane-proximal region did not prevent endoproteolytic release, indicating that a specific amino acid motif was not an absolute requirement for cleavage. Furthermore, alterations within the putative primary cleavage site (K283-S284) resulted in either constitutive endoproteolytic release of the receptor or inhibition of cell activation-induced shedding to variable extents. The length of the membrane-proximal region was also critical since truncations of this region completely abolished endoproteolytic release. Thus, release of L-selectin is likely to be regulated by the generation of an appropriate tertiary conformation within the membrane-proximal region of the receptor which allows recognition by a membrane-bound endoprotease with relaxed sequence specificity that cleaves the receptor at a specific distance from the plasma membrane. These observations suggest a generalized protein-processing pathway involved in the endoproteolytic release of specific transmembrane proteins which harbor widely differing primary sequences at or neighboring their cleavage sites.

Reversible stimulation of lymphocyte motility by cultured high endothelial cells: mediation by L-selectin

Lymphocyte emigration from blood into peripheral lymph nodes is mediated by specialized high endothelial cells (HEC) lining the postcapillary venules. A current model for this process postulates that it occurs in three steps: weak, selectin-mediated interactions tether lymphocytes to the blood vessel wall; the lymphocytes are activated to increase the affinity of integrin-dependent adhesion and enhance motility; and finally the lymphocytes migrate actively across the endothelial cell layer. Some features of this model are simulated in vitro by cultured HEC, which support the adhesion and transmigration of lymphocytes. In particular, cultured HEC stimulate lymphocytes to change shape from spherical to polar. This shape change provides a convenient assay of the motility activation of lymphocytes. In this paper it is shown that this occurs without the lymphocytes becoming tightly adherent, but depends on contact with the endothelial cell surface. The shape change is labile: non-adherent polar lymphocytes removed from HEC revert to round with a half-time of less than 8 min. Reagents which block the interaction of L-selectin with its ligands inhibit the HEC-induced shape change; these include mannose-6-phosphate, fucoidan, polyphosphomannan ester, treatment of HEC with sialidases and an anti-L-selectin monoclonal antibody known to block its lectin function. The change in shape is partially inhibited by antisera to the L-selectin ligand GlyCAM-1. Thus it is concluded that in this in vitro system, L-selectin-mediated binding of lymphocytes to HEC is essential for optimal induction of the shape change. Lymphocytes change shape in response to cultured HEC without loss of surface L-selectin, although activation stimuli are known to promote shedding of neutrophil L-selectin as well as motility and increased adhesiveness. However, the lymphocyte change in shape is a reversible process, and this may have implications for the nature and sequence of the signals transmitted from endothelium to lymphocytes during homing to peripheral lymph nodes.

Characterization of E-selectin-deficient mice: demonstration of overlapping function of the endothelial selectins

The initial rolling interaction of leukocytes with the blood vessel wall during leukocyte trafficking has been postulated to rely on members of the selectin family of adhesion molecules. Two selectins, E-selectin and P-selectin, have been identified that are expressed on activated endothelial cells. Mice deficient in E-selectin expression have been produced in order to examine the role of this selectin in leukocyte trafficking. Mice homozygous for an E-selectin null mutation were viable and exhibited no obvious developmental alterations. E-selectin-deficient mice displayed no significant change in the trafficking of neutrophils in several models of inflammation. However, blocking both endothelial selectins by treatment of the E-selectin-deficient animals with an anti-murine P-selectin antibody, 5H1, significantly inhibited neutrophil emigration in two distinct models of inflammation. While neutrophil accumulation at early times during thioglycollate-induced peritonitis was dependent on P-selectin, neutrophil accumulation at later time points was blocked by 5H1 only in E-selectin-deficient mice but not in wild-type mice. Similarly, edema as well as leukocyte accumulation in a model of delayed-type hypersensitivity in the skin was almost completely prevented by blockade of P-selectin function with 5H1 in the E-selectin-deficient mice while the same treatment had no effect in wild-type mice. These data demonstrate that the majority of neutrophil migration in both models requires an endothelial selectin but that E-selectin and P-selectin are functionally redundant. These data have important implications in the use of selectin antagonists in the treatment of inflammatory disease.

Two sites on P-selectin (the lectin and epidermal growth factor-like domains) are involved in the adhesion of monocytes to thrombin-activated endothelial cells

P-selectin, also known as GMP-140, PADGEM or CD62, is expressed on the surface of thrombin-activated platelets and endothelial cells (EC). It is a member of the selectin family of adhesion molecules that regulate leucocyte interactions with the blood vessel wall. In this study we have found that peptides derived from both the lectin (residues 19-34 and 51-61) and epidermal growth factor (EGF)-like (residues 127-139) domains inhibit the adhesion of peripheral blood mononuclear cells (PBMC), elutriated monocytes and a monocytic cell line (U937) to thrombin-activated EC. This inhibition occurred in a concentration-dependent manner and the peptide most active at the lowest concentrations was the one derived from the EGF-like motif (127-139). The scrambled forms of these peptides, identical in amino acid composition to the authentic peptides but with altered sequences, were not inhibitory. Thrombin-activated platelets supported adhesion of U937 cells and this adhesion was dramatically inhibited by the two peptides derived from the lectin-like domain (residues 19-34 and 51-61). All three peptides, when conjugated to BSA and coated on plastic plates, mediated U937 cell adhesion. This study shows, for the first time, that two sites on P-selectin, the lectin and EGF-like domains, are involved in the adhesion of monocytes to thrombin-activated EC.

Lymphocyte homing and leukocyte rolling and migration are impaired in L-selectin-deficient mice

L-selectin, a cell adhesion molecule expressed by leukocytes, mediates the attachment of lymphocytes to high endothelial venules (HEV) of peripheral lymph nodes and mediates the earliest interactions between leukocytes and activated vascular endothelium. Mice possessing a mutant L-selectin gene that results in the complete loss of cell surface receptor expression were generated by gene targeting. Lymphocytes from these mice did not bind to peripheral lymph node HEV and these mice had a severe reduction in the number of lymphocytes localized to peripheral lymph nodes. Short-term homing experiments demonstrated that L-selectin was also involved in lymphocyte migration to mucosal lymph nodes, Peyer's patches, and spleen. Furthermore, significant defects in leukocyte rolling and neutrophil migration into the peritoneum in response to an inflammatory stimulus were observed. Thus, L-selectin plays an essential role in leukocyte homing to lymphoid tissues and sites of inflammation.

Monocyte rolling, arrest and spreading on IL-4-activated vascular endothelium under flow is mediated via sequential action of L-selectin, beta 1-integrins, and beta 2-integrins

Leukocyte interactions with vascular endothelium at sites of inflammation can be dynamically regulated by activation-dependent adhesion molecules. Current models, primarily based on studies with polymorphonuclear leukocytes, suggest the involvement of multiple members of the selectin, integrin, and immunoglobulin gene families, sequentially, in the process of initial attachment (rolling), stable adhesion (arrest), spreading and ultimate diapedesis. In the current study, IL-4-activated human umbilical vein endothelium, which selectively expresses VCAM-1 and an L-selectin ligand but not E-selectin, and appropriate function blocking monoclonal antibodies, were used to study monocyte-endothelial interactions in an in vitro model that mimics microcirculatory flow conditions. In this system, L-selectin mediates monocyte rolling and also facilitates alpha 4 beta 1-integrin-dependent arrest, whereas beta 2-integrins are required for spreading of firmly attached monocytes on the endothelial cell surface but not their arrest. These findings provide the first in vitro evidence for human monocyte rolling on cytokine-activated endothelium, and suggest a sequential requirement for both beta 1- and beta 2-integrin-dependent adhesive mechanisms in monocyte-endothelial interactions.

Neutrophil tethering to and rolling on E-selectin are separable by requirement for L-selectin

Neutrophil tethering and rolling in shear flow are mediated by selectins and have been thought to be two indistinguishable manifestations of a single molecular interaction between selectin and ligand. However, we report that under physiologic flow conditions, tethering to E-selectin requires a ligand distinct from the one that supports neutrophil rolling. Tethering under shear to E-selectin requires a carbohydrate ligand that is closely associated with the lectin domain of L-selectin on the neutrophil surface, as enzymatic removal of L-selectin, chemotactic factor-induced shedding of L-selectin, and L-selectin MAbs effectively block tethering. In contrast, this ligand is dispensable for the ability to roll on E-selectin, since rolling adhesions formed after static incubations were not affected by the presence or absence of L-selectin. Thus, E-selectin interactions with ligands on neutrophils persist after L-selectin shedding. These findings add an additional step for regulation of leukocyte localization in inflammatory sites.

Non-carbohydrate binding partners/domains of animal lectins

1. Protein-carbohydrate interactions are involved in a large number of biologically important recognition processes. 2. Among the participating classes of proteins lectins are defined as carbohydrate-binding proteins other than an antibody or an enzyme. 3. In addition to the essential carbohydrate-binding domain other functionally and/or structurally important sites, defined by sequence comparison or by experimental demonstration of protein-protein interactions, can be present within the lectin molecule and may be relevant for its physiological significance. 4. Sequence motifs of lectins for protein-protein interactions include amino acid structures designed for cell adhesion, growth regulatory biosignalling, intracellular routing and enzymatic activity. 5. Elucidation of the complete functional role(s) of a lectin requires accurate delineation of its carbohydrate and, if present, of its protein ligands. 6. Presence of more than one carbohydrate-binding domain in a single lectin, potential ligand properties of the glycopart of a lectin, regulatory interplay between different sites and possible interaction of complementarily shaped peptide sequences to the sugar-recognizing site should all be assessed in the quest to comprehensively explain the physiological role(s) of a lectin.

Inhibition of leukocyte rolling with polysaccharide fucoidin prevents pleocytosis in experimental meningitis in the rabbit

Inflammatory recruitment of leukocytes into the cerebrospinal fluid (CSF) during bacterial meningitis has been shown to contribute significantly to the neurological damage commonly associated with this serious disease. In this study we tested whether or not inhibition of leukocyte rolling, a precondition for firm leukocyte adhesion to vascular endothelium in vivo, may reduce CSF leukocyte recruitment and associated inflammatory changes in rabbits with experimental meningitis. As documented by intravital microscopy of small venules in the rabbit mesentery and tenuissimus muscle, leukocyte rolling was rapidly and profoundly reduced by intravenous treatment with the polysaccharide fucoidin, a homopolymer of sulfated L-fucose known to block the function of the leukocytic "rolling receptor" L-selectin. Moreover, fucoidin treatment dramatically reduced the accumulation of both leukocytes and plasma protein in the CSF of rabbits challenged intrathecally with pneumococcal antigen. These main findings thus illustrate that inhibition of leukocyte rolling, an early and obligatory step in the process of leukocyte extravasation, may be an effective therapeutic approach to attenuate leukocyte-dependent central nervous system damage in bacterial meningitis.

A role for the epidermal growth factor-like domain of P-selectin in ligand recognition and cell adhesion

The selectin family of adhesion molecules mediates the initial interactions of leukocytes with endothelium. The extracellular region of each selectin contains an amino-terminal C-type lectin domain, followed by an EGF-like domain and multiple short consensus repeat units (SCR). Previous studies have indirectly suggested a role for each of the extracellular domains of the selectins in cell adhesion. In this study, a panel of chimeric selectins created by exchange of domains between L- and P-selectin was used to directly examine the role of the extracellular domains in cell adhesion. Exchange of only the lectin domains between L- and P-selectin conferred the adhesive and ligand recognition functions of the lectin domain of the parent molecule. However, chimeric selectins which contained both the lectin domain of L-selectin and the EGF-like domain of P-selectin exhibited dual ligand-binding specificity. These chimeric proteins supported adhesion both to myeloid cells and to high endothelial venules (HEV) of lymph nodes and mesenteric venules in vivo. Exchange of the SCR domains had no detectable effect on receptor function or specificity. Thus, the EGF-like domain of P-selectin may play a direct role in ligand recognition and leukocyte adhesion mediated by P-selectin, with the lectin plus EGF-like domains collectively forming a functional ligand recognition unit.

Substance P and adrenocorticotropic hormone do not affect T-lymphocyte adhesion to vascular endothelium or surface expression of adhesion receptors

Substance P (SP) and adrenocorticotropic hormone (ACTH) are peptides that have been shown to have both neurological and immunological effects. Because of the demonstrated effects upon immune function, we examined the effects of these peptides on T-lymphocyte adhesion to vascular endothelium and surface adhesion receptor expression. Neither the adhesion assays nor the expression assays showed any statistically significant effect of SP (10 microM) or ACTH (1 microM) for any incubation period used. We conclude that, while SP and ACTH have a variety of immunomodulatory effects, direct modulation of T-lymphocyte adhesion to vascular endothelium is probably not one of them.

Specific disengagement of cell-bound anti-LAM-1 (anti-L-selectin) antibodies by aurintricarboxylic acid

Brief treatment of human peripheral blood lymphocytes with the potential anti-HIV compound aurintricarboxylic acid (ATA) prompts the selective release of already bound L-selectin-specific anti-Leu8 and anti-LAM1-1 antibodies from the cells. Two other anti-LAM1 antibodies, anti-LAM1-3 and anti-LAM1-5 stay antigen-bound at the same time. Interestingly, the ATA-sensitive anti-Leu8 strongly competes with the ATA-resistant anti-LAM1-3 for binding. Photobleaching fluorescence resonance energy transfer (pFRET) measurements on flow-sorted cells suggests that these two antibodies compete for the same epitope, while anti-LAM1-5-FITC and anti-Leu8-PE bind to distinct sites, although they also compete for binding. Combining the data on competition, pFRET and ATA effect, we suggest that the ATA sensitive anti-Leu8 and resistant anti-LAM1-3 bind to overlapping but non-identical epitopes. This remarkably specific effect may be exploited for designing anti-inflammatory drugs that modulate leukocyte adhesion.