Selectins are a family of lectins that mediate tethering and rolling of leukocytes on endothelium in vascular shear flow. To test the hypothesis that the kinetics and the strength of rolling interactions can be independently varied for different selectin:ligand pairs, we have directly compared all three selectins with regard to distinct measures of selectin-mediated interactions in shear flow: tethering, rolling velocity, and strength of rolling adhesions. At comparable site densities of E-selectin, P-selectin, and the L-selectin counter-receptor CD34, neutrophils tethered with similar efficiency and developed rolling adhesions of similar strength as measured by resistance to detachment. Under the same conditions, neutrophils rolled 7.5- to 10.5-fold faster on CD34 than on E-selectin and P-selectin. These findings suggest that the kinetics of bond dissociation and bond formation are faster for L-selectin than for E- and P-selectin. We also compared the behavior of neutrophils and lymphocytes on the same selectin. Both cell types showed comparable strength of binding to CD34; however, neutrophils rolled with faster velocities than lymphocytes.
We have used immunoprecipitation with mAbs to probe folding during biosynthesis of the beta2 integrin subunit of lymphocyte function-associated antigen 1 (LFA-1; CD11a/CD18) before and after association with the alphaL subunit. An evolutionarily conserved region is present in the beta2 subunit between amino acid residues 102 and 344. mAbs to one subregion before the conserved region, and two subregions after the conserved domain, immunoprecipitated both the unassociated beta'2 precursor and mature alphaL/beta2 complex, suggesting portions of these subregions are folded before association with alphaL. An activating mAb to the C-terminal cysteine-rich region, KIM127, preferentially bound to the unassociated beta subunit, suggesting that it may bind to an epitope that is in an alphabeta interface in unactivated LFA-1. By contrast, mAbs to five different epitopes in the conserved region did not react with unassociated beta'2 precursor, suggesting that this region folds after alphaL association and is intimately associated with the alphaL subunit in the alphaL/beta2 complex. mAbs to two different epitopes that involve the border between the conserved region and the C-terminal segment, were fully or partially reactive with the beta'2 precursor, suggesting that this region is partially folded before association with alphaL. The findings suggest that the conserved region is a distinct folding and hence structural unit, and is intimately associated with the alpha subunit.
The N-terminal approximately 440 aa of integrin alpha subunits contain seven sequence repeats. These are predicted here to fold into a beta-propeller domain. A homologous domain from the enzyme phosphatidylinositol phospholipase D is predicted to have the same fold. The domains contain seven four-stranded beta-sheets arranged in a torus around a pseudosymmetry axis. The trimeric G-protein beta subunit (G beta) appears to be the most closely related beta-propeller. Integrin ligands and a putative Mg2+ ion are predicted to bind to the upper face of the beta-propeller. This face binds substrates in beta-propeller enzymes and is used by the G protein beta subunit to bind the G protein alpha subunit. The integrin alpha subunit I domain, which is structurally homologous to the G protein alpha subunit, is tethered to the top of the beta-propeller domain by a hinge that may allow movement of the domains relative to one another. The Ca2+-binding motifs in integrin alpha subunits are on the lower face of the beta-propeller.
We have studied the folding during biosynthesis of the lymphocyte function-associated antigen 1 (LFA-1) alphaL subunit using mAb to epitopes that map to seven different regions within the amino acid sequence. The N-terminal portion of alphaL is predicted to contain a beta-propeller domain, consisting of seven beta-sheets, and an I domain that is predicted to be inserted between beta-sheet 2 and beta-sheet 3 of the beta-propeller. The I domain of alphaL folds before association with the beta2 subunit, as shown by immunoprecipitation of the unassociated alphaL subunit by mAbs specific for four different sequence elements within the I domain. By contrast, the beta-propeller domain is not folded in unassociated alphaL after a chase of as long as 12 h after synthesis, but does fold upon association with beta2. This is shown with mAbs to regions of alphaL, that precede and follow the I domain in the primary structure. A mAb that maps near the junction of the C terminus of the I domain with the beta-propeller domain suggests that this region is partially folded before subunit association. The results show that the I domain and beta-propeller domains fold independently of one another, and suggest that the beta-propeller domain bears an interface for association with the beta subunit.
The mRNA for the seven-transmembrane-spanning G protein-coupled receptor fusin/CXCR-4 is expressed in primary mouse astrocyte cultures and the transformed mouse microglial cell line, N9. Cell surface expression of fusin in these cells was confirmed by staining with a polyclonal anti-fusin Ab. The functional capacity of this chemokine receptor was examined by evaluating the calcium responses following stimulation of glial cells with the CXC-chemokine, stromal-derived cell factor-1alpha (SDF-1alpha). Both astrocytes and microglial cells mobilized calcium following stimulation with chemically synthesized SDF-1alpha. SDF-1alpha- and carbachol-mediated calcium responses of astrocytes were partially inhibited by treatment with pertussis toxin (PTx), suggesting receptor coupling to a combination of G alpha(i) and other G proteins. In contrast, the calcium responses of microglial cells to SDF-1alpha were completely PTx sensitive, while responses to carbachol stimulation were PTx resistant. The ability of SDF-1alpha to induce glial cell migration was also examined. Synthetic SDF-1alpha was a potent chemoattractant for mouse microglial cells at ligand concentrations of 10 to 500 ng/ml; peak responses were noted at 100 ng/ml. In contrast, astrocytes did not migrate toward a gradient of SDF-1alpha. The failure of SDF-1alpha to induce astrocyte migration was specific, as another chemokine, macrophage inflammatory protein-1alpha, triggered astrocyte chemotaxis.
The chemokine receptors CXCR4 and CCR5 function as coreceptors for HIV-1 entry into CD4+ cells. During the early stages of HIV infection, viral isolates tend to use CCR5 for viral entry, while later isolates tend to use CXCR4. The pattern of expression of these chemokine receptors on T cell subsets and their regulation has important implications for AIDS pathogenesis and lymphocyte recirculation. A mAb to CXCR4, 12G5, showed partial inhibition of chemotaxis and calcium influx induced by SDF-1, the natural ligand of CXCR4. 12G5 stained predominantly the naive, unactivated CD26(low) CD45RA+ CD45R0- T lymphocyte subset of peripheral blood lymphocytes. In contrast, a mAb specific for CCR5, 5C7, stained CD26(high) CD45RA(low) CD45R0+ T lymphocytes, a subset thought to represent previously activated/memory cells. CXCR4 expression was rapidly up-regulated on peripheral blood mononuclear cells during phytohemagglutinin stimulation and interleukin 2 priming, and responsiveness to SDF-1 increased simultaneously. CCR5 expression, however, showed only a gradual increase over 12 days of culture with interleukin 2, while T cell activation with phytohemagglutinin was ineffective. Taken together, the data suggest distinct functions for the two receptors and their ligands in the migration of lymphocyte subsets through lymphoid and nonlymphoid tissues. Furthermore, the largely reciprocal expression of CXCR4 and CCR5 among peripheral blood T cells implies distinct susceptibility of T cell subsets to viral entry by T cell line-tropic versus macrophage-tropic strains during the course of HIV infection.
Two mechanisms have been proposed for regulating rolling velocities on selectins. These are (a) the intrinsic kinetics of bond dissociation, and (b) the reactive compliance, i.e., the susceptibility of the bond dissociation reaction to applied force. To determine which of these mechanisms explains the 7.5-11.5-fold faster rolling of leukocytes on L-selectin than on E- and P-selectins, we have compared the three selectins by examining the dissociation of transient tethers. We find that the intrinsic kinetics for tether bond dissociation are 7-10-fold more rapid for L-selectin than for E- and P-selectins, and are proportional to the rolling velocities through these selectins. The durations of pauses during rolling correspond to the duration of transient tethers on low density substrates. Moreover, applied force increases dissociation kinetics less for L-selectin than for E- and P-selectins, demonstrating that reactive compliance is not responsible for the faster rolling through L-selectin. Further measurements provide a biochemical and biophysical framework for understanding the molecular basis of rolling. Displacements of tethered cells during flow reversal, and measurements of the distance between successive pauses during rolling provide estimates of the length of a tether and the length of the adhesive contact zone, and suggest that rolling occurs with as few as two tethers per contact zone. Tether bond lifetime is an exponential function of the force on the bond, and the upper limit for the tether bond spring constant is of the same order of magnitude as the estimated elastic spring constant of the lectin-EGF unit. Shear uniquely enhances the rate of L-selectin transient tether formation, and conversion of tethers to rolling adhesions, providing further understanding of the shear threshold requirement for rolling through L-selectin.
We have studied the pathways that lead to arrest and firm adhesion of rolling PMN on activated, surface-adherent platelets. Stable arrest and adhesion strengthening of PMN on thrombin-stimulated, surface-adherent platelets in flow required distinct Ca2+- and Mg2+-dependent regions of Mac-1 (alphaMbeta2), and involved interactions of Mac-1 with fibrinogen, which was bound to platelets via alphaIIbbeta3. Mac-1 also bound to other unidentified ligands on platelets, which were not intracellular adhesion molecule-2 (ICAM-2), heparin, or heparan-sulfate proteoglycans. This was shown by inhibition with mAbs or peptides, by treatment of platelets with heparitinase, and by using platelets with defective alphaIIbbeta3 from a patient with Glanzmann thrombasthenia. Tethering of PMN on platelet ICAM-2 via LFA-1 (alphaLbeta2) was observed, which may facilitate the transition between rolling on selectins and Mac-1-dependent arrest. Arrest and adhesion strengthening was pertussis toxin sensitive and in flow was mainly induced by platelet-activating factor but not through activation of the chemokine receptor CXCR2. In stasis, spreading occurred and the CXCR2 contributed to firm adhesion.
The adhesive interactions of eosinophils with purified E-, P-, and L-selectins; vascular cell adhesion molecule-1 molecule; and HUVEC were examined in shear flow. Compared with neutrophils, eosinophils showed markedly less binding to E-selectin, but significantly stronger avidity for P-selectin. Both cell types showed a similar level of tethering and rolling on L-selectin. Eosinophils tethered and arrested abruptly on vascular cell adhesion molecule-1. However, some of the tethers were detached within several seconds; this was prevented by stimulation with eotaxin. Eosinophils also showed immediate arrest on HUVEC stimulated with 100 U/ml TNF-alpha for 6 h. Treatment with L-selectin mAb decreased eosinophil accumulation on the HUVEC by abrogating secondary tethers through interactions between flowing and attached eosinophils. mAb to P-selectin but not to E-selectin strongly inhibited primary tethers and accumulation of eosinophils. mAb to the integrin alpha 4 subunit inhibited arrest, induced rolling or detachment of tethered eosinophils, and resulted in partial reduction of eosinophil accumulation. mAb to the integrin beta 2 subunit had only a slight effect, whereas treatment with mAb to the integrin alpha 4 and beta 2 subunits together abolished rolling interactions as well as arrest, and thus almost totally inhibited eosinophil accumulation. Our data indicate that P-selectin, but not E-selectin, is directly involved in eosinophil tethering on inflammatory endothelium while L-selectin mainly mediates intereosinophil interaction. VLA-4 has a crucial role in eosinophil arrest, and arrest is enhanced by exposure to chemoattractants.
The leukocyte integrin alpha L beta 2 (LFA-1) is important in transendothelial migration. Since it is not fully understood how LFA-1 mediates transmigration, we studied the effects of alpha L and beta 2 cytoplasmic domain mutants that alter LFA-1 adhesiveness for intercellular adhesion molecule-1. Monocyte chemotactic protein-1 (MCP-1) induced LFA-1-dependent transendothelial migration of Jurkat and J-beta 2.7 transfectants coexpressing the MCP-1 receptor CCR2B and wild-type alpha L. No transendothelial chemotaxis was observed with truncation mutants of the alpha L cytoplasmic tail, which rendered LFA-1 constitutively active or locked LFA-1 in a low avidity state unresponsive to cellular activation. Moreover, transendothelial chemotaxis of lymphoblastoid SLA transfectants was abolished by truncation of the beta 2 cytoplasmic domain, but not by mutation of its TTT motif, which is important in phorbol ester-induced adhesion. These data indicate that transmigration may require both alpha L and beta 2 cytoplasmic domains. We further show that MCP-1-induced transendothelial chemotaxis of PBMC was inhibited by sustained activation of LFA-1 with Mn2+ or a stimulatory mAb to beta 2. Dimeric soluble intercellular adhesion molecule-1 also reduced transendothelial chemotaxis of PBMC. Taken together, our data suggest that transendothelial chemotaxis of mononuclear cells may involve dynamic changes in LFA-1 avidity.
Antibodies immobilized on the wall of a flow chamber can support leukocyte rolling in shear flow. IgM mAb to Lewis(x) (CD15) and sialyl Lewis(x) (CD15s), which are carbohydrate antigens related to selectin ligands, plus mAb to CD48 and CD59, could mediate rolling. IgM and IgG mAb to L-selectin (CD62L), lymphocyte function-associated antigen 1 (CD11a), CD43, intercellular adhesion molecule 3 (CD50), and CD45 mediated only firm adhesion. In contrast to selectins, antibodies supported rolling only within a restricted range of site densities and wall shear stresses, outside of which firm adhesion or detachment occurred. When wall shear stress was increased, rolling velocity increased rapidly for antibodies but not for selectins. The kinetics of dissociation from the substrate of transiently tethered cells also increased more rapidly as a function of shear stress for antibodies than for selectins. These comparisons emphasize a number of remarkable features of selectins, including the lack of development of firm adhesion, and suggest that specialized molecular or cellular mechanisms must be required to explain their ability to support rolling over a wide range of environmental variables.
Tenascin is a large extracellular matrix molecule expressed at specific sites in the adult, including immune system tissues such as the bone marrow, thymus, spleen, and T cell areas of lymph nodes. Tenascin has been reported to have both adhesive and anti-adhesive effects in static assays. We report here that tenascin supports the tethering and rolling of lymphocytes and lymphoblastic cell lines under flow conditions. Binding was calcium dependent and was not inhibited by treatment of lymphocytes with O-glycoprotease or a panel of glycosidases including neuraminidase and heparitinase but was inhibited by treatment of cells with proteinase K. Binding was to the fibrinogen-like terminal domain of tenascin as determined by antibody blocking studies and binding to recombinant tenascin proteins. When compared to rolling of the same cell type on E-selectin, rolling on tenascin was found to be smoother at all shear stresses tested, suggesting that cells formed a larger number of bonds on the tenascin substrate than on the E-selectin substrate. When protein plating densities were adjusted to give similar profiles of cell detachment under increasing shears, the density of tenascin was 8.5-fold greater than that of E-selectin. Binding to tenascin was not dependent on any molecules previously identified as tenascin receptors and is likely to involve a novel tenascin receptor on lymphocytes. We postulate that the ability of tenascin to support lymphocyte rolling may reflect its ability to support cell migration and that this interaction may be used by lymphocytes migrating through secondary lymphoid organs.
The integrin LFA-1 mediates activation-dependent leukocyte adhesion. The beta subunit cytoplasmic domain has been demonstrated previously to modulate the adhesiveness of LFA-1. To investigate whether the alpha subunit cytoplasmic domain is also involved in the regulation of LFA-1-adhesive function, we stably expressed cytoplasmic domain truncated forms of the alpha subunit in a Jurkat mutant (Jurkat-beta2.7) deficient in the endogenous LFA-1 alpha subunit and in K562 cells. Clones expressing similar levels of cell surface LFA-1 were tested for their ability to bind to immobilized ICAM-1. Truncation of the alpha subunit cytoplasmic domain before, but not after, the conserved GFFKR sequence motif resulted in constitutive ICAM-1 binding of both Jurkat-beta2.7 and K562 transfectants. However, truncation after the GFFKR motif reduced sensitivity to stimulation by PMA or stimulatory Abs. Internal deletion of the GFFKR motif, or point mutations of the Gly (G), the two Phe (F), or the Arg (R) in the GFFKR motif to Ala (A) rendered LFA-1 constitutively active. Mutation of the Lys (K) did not affect LFA-1 adhesion to ICAM-1. These findings indicate that the GFFKR motif maintains the low adhesive state of LFA-1, possibly by restraining the receptor conformation. We further demonstrate that the alpha subunit cytoplasmic domain and the conserved GFFKR motif are also required for efficient formation of LFA-1 alphabeta heterodimers.
Selectins are cell adhesion molecules that bind carbohydrate ligands and promote interaction between leukocytes and the vessel wall in vascular shear flow. Selectin-ligand bonds have high mechanical strength, allowing initial tethering to the vessel wall through one or few bonds, and have fast on and off rates, permitting rolling in response to hydrodynamic drag. The L-selectin molecule on leukocytes binds to peripheral node addressin on high endothelial venules of lymph nodes to mediate leukocyte rolling and binds to a ligand on neutrophils to mediate rolling of leukocytes over one another. Here we describe a surprising mechanism for regulation of these interactions, both in vitro and in vivo. Shear above a critical threshold is required to promote and maintain rolling interactions through L-selectin, but not through E-selectin, P-selectin or VCAM-1. The shear threshold requirement for L-selectin may be physiologically important in low shear to prevent inappropriate aggregation of leukocytes and interaction with the vessel wall.
The ability of chemokines, particularly MCP-1, to induce integrin-dependent binding of T lymphocytes to endothelial adhesion molecules or extracellular matrix (ECM) components was examined. MCP-1 induced significant adhesion to fibronectin (FN) and to endothelial-secreted ECM but not to purified ICAM-1 or VCAM-1, or to activated endothelium. The MCP-1-induced binding of T lymphocytes to FN was rapid, dose dependent, and resulted from activation of both VLA-4 and VLA-5. Like MCP-1, the chemokines RANTES and MIP-1 beta induced T lymphocyte binding to FN, but not to ICAM-1. We suggest therefore, that these T lymphocyte chemokines may be most important, not in initiating integrin-dependent firm adhesion of T lymphocytes to the vascular wall, but rather, in subsequent adhesive interactions during migration into tissue.
Little is known about how lymphocytes migrate within secondary lymphoid organs. Stromal cells and their associated reticular fibers form a network of fibers that radiate from high endothelial venules to all areas of the lymph node and may provide a scaffold for lymphocyte migration. We studied interactions of lymphocytes with cultured human tonsillar stromal cells and their extracellular matrix using shear stress to distinguish transient interactions from firm adhesion. Tonsillar lymphocytes and SKW3 T lymphoma cells tethered and rolled on monolayers of cultured tonsillar stromal cells and their matrix. A significant proportion of these rolling interactions were independent of divalent cations and were mediated by CD44 binding to hyaluronan, as shown by inhibition with mAb to CD44, soluble hyaluronan, as hyaluronidase treatment of the substrate, and O-glycoprotease treatment of the rolling cells. O-glycoprotease treatment of the substrate also blocked binding completely to stromal matrix and partially to stromal monolayers. SKW3 cells tethered and rolled on plastic-immobilized hyaluronan, confirming the specificity of this interaction. By contrast, monolayers of resting or stimulated human umbilical vein endothelial cells failed to support CD44- and hyaluronan-dependent rolling. SKW3 cells added under flow conditions to frozen sections of human tonsil bound and rolled along reticular fibers in the presence of EDTA. Rolling was blocked by either CD44 mAb or hyaluronan. We propose that lymphocytes migrating through secondary lymphoid organs may use CD44 to bind to hyaluronan immobilized on stromal cells and reticular fibers.
Acute neutrophil (PMN) recruitment to postischemic cardiac or pulmonary tissue has deleterious effects in the early reperfusion period, but the mechanisms and effects of neutrophil influx in the pathogenesis of evolving stroke remain controversial. To investigate whether PMNs contribute to adverse neurologic sequelae and mortality after stroke, and to study the potential role of the leukocyte adhesion molecule intercellular adhesion molecule-1 (ICAM-1) in the pathogenesis of stroke, we used a murine model of transient focal cerebral ischemia consisting of intraluminal middle cerebral artery occlusion for 45 min followed by 22 h of reperfusion. PMN accumulation, monitored by deposition of 111In-labeled PMNs in postischemic cerebral tissue, was increased 2.5-fold in the ipsilateral (infarcted) hemisphere compared with the contralateral (noninfarcted) hemisphere (P < 0.01). Mice immunodepleted of neutrophils before surgery demonstrated a 3.0-fold reduction in infarct volumes (P < 0.001), based on triphenyltetrazolium chloride staining of serial cerebral sections, improved ipsilateral cortical cerebral blood flow (measured by laser Doppler), and reduced neurological deficit compared with controls. In wild-type mice subjected to 45 min of ischemia followed by 22 h of reperfusion, ICAM-1 mRNA was increased in the ipsilateral hemisphere, with immunohistochemistry localizing increased ICAM-1 expression on cerebral microvascular endothelium. The role of ICAM-1 expression in stroke was investigated in homozygous null ICAM-1 mice (ICAM-1 -/-) in comparison with wild-type controls (ICAM-1 +/+). ICAM-1 -/- mice demonstrated a 3.7-fold reduction in infarct volume (P < 0.005), a 35% increase in survival (P < 0.05), and reduced neurologic deficit compared with ICAM-1 +/+ controls. Cerebral blood flow to the infarcted hemisphere was 3.1-fold greater in ICAM-1 -/- mice compared with ICAM-1 +/+ controls (P < 0.01), suggesting an important role for ICAM-1 in the genesis of postischemic cerebral no-reflow. Because PMN-depleted and ICAM-1-deficient mice are relatively resistant to cerebral ischemia-reperfusion injury, these studies suggest an important role for ICAM-1-mediated PMN adhesion in the pathophysiology of evolving stroke.
Rat mAbs were raised against murine intercellular adhesion molecule-2 (ICAM-2). Immune precipitation and purification reveal that the murine ICAM-2 glycoprotein is 55 kDa and is similar in size to human ICAM-2. ICAM-2 is expressed on a variety of leukocyte cell lines, including T and B lymphoma, mastocytoma, and macrophage lines. ICAM-2 is well expressed on endothelioma cell lines, and in contrast to ICAM-1, expression is not increased by inflammatory cytokines. One of the mAb to ICAM-2 partially or completely inhibits binding of cells expressing LFA-1 to purified ICAM-2, and binding of cells expressing ICAM-2 to purified LFA-1. The findings in the mouse are congruent with those in the human, suggesting functional conservation of ICAM-2 across species.
We isolated a partial genomic clone encoding ITGAD, a novel beta 2-integrin alpha subunit. The ITGAD gene is highly homologous to the three previously known alpha subunit-encoding genes, that compose the beta 2 integrin family, in deduced amino acid sequence, intron/exon structure and mapping location (chromosome 16p11).