Activation of LFA-1 through a Ca2(+)-dependent epitope stimulates lymphocyte adhesion

ICAM-3 regulates lymphocyte morphology and integrin-mediated T cell interaction with endothelial cell and extracellular matrix ligands

Leukocyte activation is a complex process that involves multiple cross-regulated cell adhesion events. In this report, we investigated the role of intercellular adhesion molecule-3 (ICAM-3), the third identified ligand for the beta 2 integrin leukocyte function-associated antigen-1 (LFA-1), in the regulation of leukocyte adhesion to ICAM-1, vascular cell adhesion molecule-1 (VCAM-1), and the 38- and 80-kD fragments of fibronectin (FN40 and FN80). The activating anti-ICAM-3 HP2/19, but not other anti-ICAM-3 mAb, was able to enhance T lymphoblast adhesion to these proteins when combined with very low doses of anti-CD3 mAb, which were unable by themselves to induce this phenomenon. In contrast, anti-ICAM-1 mAb did not enhance T cell attachment to these substrata. T cell adhesion to ICAM-1, VCAM-1, FN40, and FN80 was specifically blocked by anti-LFA-1, anti-VLA alpha 4, and anti-VLA alpha 5 mAb, respectively. The activating anti-ICAM-3 HP2/19 was also able to specifically enhance the VLA-4- and VLA-5-mediated binding of leukemic T Jurkat cells to VCAM-1, FN40, and FN80, even in the absence of cooccupancy of the CD3-TcR complex. We also studied the localization of ICAM-3, LFA-1, and the VLA beta 1 integrin, by immunofluorescence microscopy, on cells interacting with ICAM-1, VCAM-1 and FN80. We found that the anti-ICAM-3 HP2/19 mAb specifically promoted a dramatic change on the morphology of T lymphoblasts when these cells were allowed to interact with those adhesion ligands. Under these conditions, it was observed that a large cell contact area from which an uropod-like structure (heading uropod) was projected toward the outer milieu. However, when T blasts were stimulated with other adhesion promoting agents as the activating anti-VLA beta 1 TS2/16 mAb or phorbol esters, this structure was not detected. The anti-ICAM-3 TP1/24 mAb was also unable to induce this phenomenon. Notably, a striking cell redistribution of ICAM-3 was induced specifically by the HP2/19 mAb, but not by the other anti-ICAM-3 mAb or the other adhesion promoting agents. Thus, ICAM-3 was almost exclusively concentrated in the most distal portion of the heading uropod whereas either LFA-1 or the VLA beta 1 integrin were uniformly distributed all over the large contact area. Moreover, this phenomenon was also observed when T cells were specifically stimulated with the HP2/19 mAb to interact with TNF alpha-activated endothelial cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of a novel bovine leukocyte protein involved in cell-cell adhesion

Preliminary characterization of an apparently novel bovine leukocyte adhesion protein is described. Two IgG1 monoclonal antibodies, UC-C1 and UC-H5, raised against established cultures of IL-2-dependent bovine peripheral blood lymphocytes (PBL) were found to react with an antigen expressed by the majority of bovine peripheral blood leukocytes. Immunoprecipitation and polyacrylamide gel electrophoresis of the antigen produced a distinct protein band of molecular weight 160,000, and additional diffuse protein bands of approximate molecular weight 180,000, 175,000, and 150,000. Two-color flow cytometric analyses showed that the antigen was expressed at low density on a small proportion of circulating B lymphocytes, but was highly expressed on all circulating T lymphocytes. The majority of monocytes and all granulocytes expressed the antigen at a density lower than that of T lymphocytes. Peripheral blood lymphocytes stimulated with concanavalin A had an approximately 3-fold increased expression of the antigen, which was apparent within 18 h and remained stable in long-term cultures. Expression of the antigen in thymus, analyzed by the immunoperoxidase technique, was predominantly restricted to thymocytes in the immediate subcapsular cortex and medulla; expression in lymph nodes and spleen was predominantly confined to lymphocytes in T-cell areas. Flow-cytometric analysis demonstrated that thymocytes and the majority of peripheral and mesenteric lymph node-derived T cells had relatively low surface density of antigen compared to circulating T cells. Binding of UC-C1 or UC-H5 to the antigen on lymphocytes induced homotypic aggregation. UC-C1 completely blocked binding of FITC-conjugated UC-H5 to blood mononuclear cells, suggesting that the antibodies recognize the same epitope or proximal epitopes.

Distinct binding of T lymphocytes to ICAM-1, -2 or -3 upon activation of LFA-1

LFA-1 (CD11a/CD18) mediates leukocyte adhesion by binding to one of its ligands: ICAM-1, ICAM-2 or ICAM-3. Here, we investigated whether stimuli known to induce adhesion to ICAM-1 were also capable of inducing LFA-1-mediated adhesion of T lymphocytes to ICAM-2 and -3 transfectants. We observed that phorbol 12-myristate 13-acetate, Mn2+, cross-linking of CD3 or activating antibodies against LFA-1 enhanced LFA-1-mediated T cell adhesion to ICAM-2 and -3, although to a lesser extent than to ICAM-1. These results indicate that, similar to what has been reported for adhesion to ICAM-1, activation of LFA-1 is also required for adhesion to ICAM-2 and -3. Furthermore, the results suggest that ICAM-1 is the major ligand for LFA-1 on activated T lymphocytes. Interestingly, we observed that in contrast to activating antibodies against CD18, activating antibodies against CD11a were incapable of inducing adhesion of LFA-1 to all three ligands. The antibody MEM-83 stimulated binding to ICAM-1, while at the same time inhibiting the interaction of LFA-1 with ICA8M-2 and -3. The antibody NKI-L16 selectively induced adhesion to ICAM-1 and -2, but not to ICAM-3. Our results suggest that different conformations of LFA-1 are required to support adhesion to ICAM-1, -2 or -3, and that ligands may bind on different sites of the LFA-1 molecule.

Xenotransplantation: an examination of the adhesive interactions between human lymphocytes and porcine renal epithelial cells

Specific adhesion molecules stabilize the binding between lymphocytes and antigen bearing cells; this intercellular adhesion is vital to both the affector and effector phases of an immune response. It is not known whether adhesion molecules and their counter-receptors can form the cross-species interactions that will facilitate human T cell recognition of xenogeneic porcine target cells. In this report it is demonstrated that a higher proportion of mitogen-activated than of resting human lymphocytes adhere to cultured porcine renal epithelial cells. Furthermore, antibody blocking experiments demonstrated that at least part of this cell-cell binding is stabilized by the human adhesion molecules LFA-1 (lymphocyte function-associated antigen-1) and the alpha 4-containing integrins. It is possible that this capacity for cross-species adhesion will play a role during the cell-mediated rejection of clinical porcine xenografts.

Homotypic aggregation of CD103 (alpha E beta 7)+ lymphocytes by an anti-CD103 antibody, HML-4

One monoclonal antibody, HML-4, directed against the alpha E beta 7 integrin (CD103), an integrin preferentially expressed on human intestinal intraepithelial lymphocytes (IEL), induced the homotypic aggregation of IEL and of a CD103+ MOLT16 cell line. Aggregation was an active adhesion event dependent on an intact cytoskeleton, on tyrosine phosphorylation but not on activation of protein kinase C. It was blocked by four other anti-CD103 antibodies but by none of the antibodies blocking known adhesion lymphocyte pathways. It was associated with a redistribution of the CD103 integrin in the areas of cell-cell contacts. These results indicated that HML-4-induced homotypic adhesion was mediated via CD103 and resulted from the binding of the integrin to an as yet undefined ligand expressed by CD103+ cells. This ligand was distinct from the epithelial ligand of CD103: in contrast with homotypic adhesion, heterotypic adhesion of CD103+ MOLT16 cells on two epithelial intestinal cell lines (DLD1 and HT29) was dependent on the presence of divalent cations, was not enhanced by HML-4, was inhibited by HML-1 but not by the three other antibodies with an inhibitory effect on homotypic adhesion. Finally, the study of conjugates between CD103+ and CD103- sublines derived from the MOLT16 cell line suggested that HML-4-induced homotypic aggregation resulted from homophilic CD103-CD103 interactions.

Effect of lymphocyte activation status on binding to endothelium. The role of CD11a, CD18 and CD49d adhesion molecules

When endothelial cells (EC) were stimulated with IL-1 and/or lymphocytes with rIL-2 or PHA, the binding of lymphocytes to EC was increased. PMA treatment of lymphocytes alone did not increase their binding to EC, but when EC were additionally induced with IL-1 the binding was increased. The expression of LFA-1 was constant, whilst the expression of CD49d was increased after rIL-2 and PHA stimulation. The PMA- and rIL-2-induced lymphocyte binding to IL-1-induced EC was inhibited by anti-CD11a, CD18 and CD49d mAbs; on the other hand, the enhanced binding of PHA-stimulated lymphocytes to EC could not be blocked by these mAbs. These results show that activation of lymphocytes by various stimuli leads to different usage of adhesion pathways in their binding to inflammatory EC.

Two independent sets of monoclonal antibodies define neoepitopes linked to soluble ligand binding and leukocyte adhesion functions of activated alpha M beta 2

The integrin alpha M beta 2 mediates a variety of events, adhesive, phagocytic, and inflammatory. Evidence has suggested that the functional events may be mediated by the "activated" conformational forms of alpha M beta 2 produced by appropriate stimulation of myeloid and monocytic lineage. The activation of alpha M beta 2 may be associated with new epitopes on alpha M beta 2, sites that may be related to the acquired receptor functions. Monoclonal antibodies were produced that preferentially bind neoepitopes expressed by activated alpha M beta 2. These anti-neo antibodies each inhibited three activation-associated specific receptor alpha M beta 2 functions, though to different extents. One set of anti-neo antibodies inhibited in a concordant manner the binding of factor X and of fibrinogen by > 90%, abolished the alpha M beta 2-initiated cellular coagulant response, and inhibited monocyte adhesion to unstimulated endothelial monolayers. A second set of anti-neo antibodies only diminished factor X and fibrinogen binding by approximately 40% to 50% but markedly suppressed Xa generation and only partially inhibited monocyte adherence to unstimulated endothelium. Concordance was observed between binding of factor X or fibrinogen and competence for leukocyte adhesion to unstimulated endothelium. Antibody competition assays segregated the anti-neo antibodies into the same two distinct sets, consistent with recognition of separate neoepitopes that are linked to alpha M beta 2 function. These data support the conclusion that the activated conformer of alpha M beta 2 that binds fibrinogen and factor X also mediates monocyte-endothelial interactions as well as the alternative cellular coagulation pathway.

Regulation of cellular interactions with laminin by integrin cytoplasmic domains: the A and B structural variants of the alpha 6 beta 1 integrin differentially modulate the adhesive strength, morphology, and migration of macrophages

Several integrin alpha subunits have structural variants that are identical in their extracellular and transmembrane domains but that differ in their cytoplasmic domains. The functional significance of these variants, however, is unknown. In the present study, we examined the possibility that the A and B variants of the alpha 6 beta 1 integrin laminin receptor differ in function. For this purpose, we expressed the alpha 6A and alpha 6B cDNAs, as well as a truncated alpha 6 cDNA (alpha 6-delta CYT) in which the cytoplasmic domain sequence was deleted after the GFFKR pentapeptide, in P388D1 cells, an alpha 6 deficient macrophage cell line. Populations of stable alpha 6A, alpha 6B, and alpha 6-delta CYT transfectants that expressed equivalent levels of cell surface alpha 6 were obtained by fluorescence-activated cell sorter and shown to form heterodimers with endogenous beta 1 subunits. Upon attachment to laminin, the alpha 6A transfectants extended numerous pseudopodia. In contrast, the alpha 6B transfectants remained rounded and extended few processes. The transfectants were also examined for their ability to migrate toward a laminin substratum using Transwell chambers. The alpha 6A transfectants were three- to fourfold more migratory than the alpha 6B transfectants. The alpha 6-delta CYT transfectants did not attach to laminin in normal culture medium, but they did attach in the presence of Mn2+. The alpha 6-delta CYT transfectants migrated to a lesser extent than either the alpha 6A or alpha 6B transfectants in the presence of Mn2+. The alpha 6 transfectants differed significantly in the concentration of substratum bound laminin required for half-maximal adhesion in the presence of Mn2+:alpha 6A (2.1 micrograms/ml), alpha 6B (6.3 micrograms/ml), and alpha 6-delta CYT (8.8 micrograms/ml). Divalent cation titration studies revealed that these transfectants also differed significantly in both the [Ca2+] and [Mn2+] required to obtain half-maximal adhesion to laminin. These data demonstrate that the A and B variants of the alpha 6 cytoplasmic domain can differentially modulate the function of the alpha 6 beta 1 extracellular domain.

The dynamic regulation of integrin adhesiveness

The integrins are a family of transmembrane heterodimeric adhesion molecules that play important roles in wound healing, immune system function and organ development. Recent studies indicate that adhesion of integrins to their ligands is not constitutive but is dynamically regulated by intracellular signal transduction pathways.

Signaling through CD50 (ICAM-3) stimulates T lymphocyte binding to human umbilical vein endothelial cells and extracellular matrix proteins via an increase in beta 1 and beta 2 integrin function

Regulated adhesion of T lymphocytes to antigen-presenting cells, endothelial cells and extracellular matrix proteins is crucial in T lymphocyte activation and migration to the sites of injury. In this study, we show that three monoclonal antibodies (mAb) recognizing different epitopes on the CD50 (ICAM-3) molecule increase T lymphocyte adhesion to tumor necrosis factor (TNF)-stimulated human umbilical vein endothelial cells and extracellular matrix proteins. These phenomena are mediated by an increase in beta 1 and beta 2 integrin avidity since (a) CD50-induced adhesion to endothelial cells was abrogated by simultaneous blocking of beta 1- and beta 2-mediated adhesion pathways but not by interfering with either one individually, (b) CD50 mAb increased beta 1 integrin-mediated adhesion to extracellular matrix proteins and to fibronectin-derived synthetic peptides, (c) CD50 mAb enhanced T lymphocyte binding to ICAM-1 transfectants, and (d) CD50 mAb did not modify surface expression patterns of beta 1 or beta 2 integrins on T lymphocytes. Our data suggest that constitutively expressed CD50 (ICAM-3) can play a pivotal role in initiating a cascade of adhesion events which may be crucial in immune activation and in the development of inflammatory lesions.

Molecular analysis of antigen-independent adhesion forces between T and B lymphocytes

The low-affinity interactions underlying antigen recognition by T-cell receptors (TCRs) are thought to involve antigen-independent adhesion mechanisms. Using a hydrodynamic approach, we found that antigen-independent adhesion occurred between human B cells and resting T cells in a transient and temperature-dependent fashion. The mean cell-cell adhesion force was 0.32 x 10(-9) N and was generated by similar contributions (0.16 x 10(-9) N) of the LFA-1- and CD2-dependent adhesion pathways. After T-cell stimulation with a phorbol ester, the force contributed by LFA-1 was drastically increased, while that of CD2 was unaffected. We propose that weak receptor-mediated adhesion initiates antigen-independent intercellular contacts required for antigen recognition by the TCR and is upregulated following TCR engagement. The method used permits adhesion forces between living cells to be resolved at the molecular level and should prove valuable for the rapid assessment of interaction forces between various types of cells and cell-sized particles.

Stimulation of integrin-mediated adhesion of T lymphocytes and monocytes: two mechanisms with divergent biological consequences

We show that the adhesion of T lymphoid cells to immobilized fibronectin can be increased by two distinct mechanisms. The first is by increasing the affinity of the fibronectin receptor/ligand interaction using the anti-beta 1 integrin monoclonal antibody 8A2. The second is by treating the cells with phorbol 12-myristate 13-acetate (PMA), which alters events that occur after receptor occupancy (e.g., cell spreading) without affecting receptor affinity. The effects of these two mechanisms on adhesion in the presence of physiological concentrations of soluble fibronectin suggest that they have different biological consequences. Under these conditions, the net effect of increasing the affinity of the fibronectin receptors is to decrease cell adhesion, whereas the increase in adhesion induced by PMA is unaffected. This suggests that the high affinity receptors are not primarily available for cell adhesion under these circumstances, and that they have an alternative function. We further show that high affinity binding of soluble fibronectin can be induced by either differentiation of the monocytic cell line THP-1 or by cross-linking the T cell receptor complexes on the T lymphoid cell line HUT-78. The differentiated monocytic cells express two populations of fibronectin receptors: a minority in a high affinity state, and the majority in a low affinity state. Thus they will both continue to adhere in the presence of physiological concentrations of soluble fibronectin and bind significant amounts of soluble fibronectin at the cell surface.

Extracellular Ca2+ modulates leukocyte function-associated antigen-1 cell surface distribution on T lymphocytes and consequently affects cell adhesion

Transition of leukocyte function-associated antigen-1 (LFA-1), from an inactive into an activate state depends on the presence of extracellular Mg2+ and/or Ca2+ ions. Although Mg2+ is directly involved in ligand binding, the role of Ca2+ in LFA-1 mediated adhesion remained obscure. We now demonstrate that binding of Ca2+, but not Mg2+, directly correlates with clustering of LFA-1 molecules at the cell surface of T cells, thereby facilitating LFA-1-ligand interaction. Using a reporter antibody (NKI-L16) that recognizes a Ca(2+)-dependent epitope on LFA-1, we found that Ca2+ can be bound by LFA-1 with different strength. We noticed that weak binding of Ca2+ is associated with a dispersed LFA-1 surface distribution on T cells and with non-responsiveness of these cells to stimuli known to activate LFA-1. In contrast, stable binding of Ca2+ by LFA-1 correlates with a patch-like surface distribution and vivid ligand binding after activation of LFA-1. Mg(2+)-dependent ligand binding does not affect binding of Ca2+ by LFA-1 as measured by NKI-L16 expression, suggesting that Mg2+ binds to a distinct site, and that both cations are important to mediate adhesion. Only Sr2+ ions can replace Ca2+ to express the L16 epitope, and to induce clustering of LFA-1 at the cell surface. We conclude that Ca2+ is involved in avidity regulation of LFA-1 by clustering of LFA-1 molecules at the cell surface, whereas Mg2+ is important in regulation of the affinity of LFA-1 for its ligands.

Pertussis toxin inhibition of T-cell hybridoma invasion is reversed by manganese-induced activation of LFA-1

Pertussis toxin (PT) inhibits invasiveness of T-cell hybridomas in vitro and metastasis formation in vivo. We present evidence for the hypothesis that PT interferes with functional activation of LFA-1. Invasion by TAM2D2 T-cell hybridoma cells of fibroblast monolayers was completely blocked by PT pretreatment, but the cells regained invasiveness in the presence of Mn2+, which activates LFA-1. This invasion was blocked by anti-LFA-1 mAb, and Mn2+ did not stimulate invasiveness of LFA-1-deficient TAM2D2 mutants. TAM2D2 cells did not adhere to surfaces coated with the LFA-1 counterstructure ICAM-1, but Mn2+ induced adhesion. Hence, LFA-1 on TAM2D2 cells requires activation before it can participate in the invasion process. The hypothesis is further supported by the slightly different results obtained with the TAM8C4 T-cell hybridoma. PT inhibited invasion strongly but not completely. This reduced invasion was increased by Mn2+. TAM8C4 cells did adhere to ICAM-1, but Mn2+ enhanced adhesion. Thus, part of LFA-1 on TAM8C4 cells is constitutively active, allowing for some PT-insensitive invasion, but further activation is required for optimal adhesion and invasion. PT blocks G-protein-mediated signals, suggesting that an extracellular factor is involved. This is not a serum component or an autocrine motility factor, since the PT effect was serum-independent, and PT did not inhibit motility. Therefore, it is probably produced by the fibroblasts, and either secreted or associated with the cell surface. These results are in line with the hypothesis that a fibroblast constituent activates LFA-1 via a PT-sensitive G-protein and thus stimulates invasion of T-cell hybridomas into the fibroblast monolayer.

Control of leucocyte function-associated antigen-1-dependent cellular conjugation by divalent cations

The control of integrin activation is fundamental to an understanding of the integrin-dependent cellular adhesion thought to be important for a plethora of basic cellular functions. Using a cell-cell conjugation assay the role of divalent cations in leucocyte function-associated antigen-1 (LFA-1)-dependent cellular adhesion was further investigated. The conjugation of interleukin-2 (IL-2)-activated lymphocytes to tumour cells was found to be energy dependent and required the presence of various divalent cations, removal of which decreased the level of conjugation. Increased concentrations of calcium, magnesium and manganese ions resulted in a corresponding increase in levels of conjugation. This increase in conjugation was LFA-1 dependent. Interestingly, when calcium ions were first removed from LFA-1, treatment of lymphocytes with magnesium and manganese ions gave significantly higher levels of conjugation than in the presence of calcium. Using a simple displacement study, calcium ions were shown to displace magnesium ions resulting in decreased conjugation. However, calcium ions were unable to displace manganese ions for binding to LFA-1. That manganese was exerting its effect via an LFA-1-dependent mechanism was confirmed using monoclonal antibodies to CD11a which negated the increased conjugation frequency due to manganese.

Role of LFA-1/ICAM-1 in interleukin-2-stimulated lymphocyte proliferation

Major adhesion routes between lymphoid cells involve the receptor/ligand pairs LFA-1/ICAM-1 and CD2/LFA-3, in addition to VLA or CD44 molecules. In this study we evaluated the role of these adhesion receptors in the proliferative response of lymphoid cells to interleukin-2 (IL-2). Blocking studies were performed with a panel of monoclonal antibodies (mAb) directed against these adhesion molecules. Selective inhibition of recombinant (r)IL-2-induced cell proliferation was observed with mAb directed against the alpha or beta subunit of LFA-1 or to its ligand ICAM-1. Interestingly, rIL-2-induced proliferation was also inhibited by NKI-L16, and anti-1 alpha antibody known to enhance cell-cell interaction. Resting lymphocytes were preferentially susceptible to the inhibition, particularly in an early phase of culture and when stimulated with a relatively low dose of rIL-2. By using mAb that specifically could block distinct rIL-2 activation pathways, LFA-1/ICAM-1 interaction was found to be required for p55 IL-2 receptor (IL-2R)-mediated interaction of rIL-2 with its high-affinity receptor, but not for p75 IL-2R-mediated responses. Furthermore, it was shown that the rIL-2 response of T lymphocytes, but not of natural killer cells, was dependent on LFA-1/ICAM-1 interaction. This suggests that LFA-1/ICAM-1 interaction is required for an optimal rIL-2 response of cells capable of IL-2 secretion. Our data provide evidence for the hypothesis that adhesion receptor-directed release of IL-2 may result in a locally high concentration of IL-2 that triggers high-affinity IL-2R signaling and up-regulates p55 IL-2R to enhance cytokine responsiveness.

Human naive and memory T-helper cells display distinct adhesion properties to ICAM-1, LFA-3 and B7 molecules

In this paper the contribution of different accessory molecules to the adhesion of resting, naive and memory CD4+ T cells was examined utilizing a panel of CHO cell transfectants as model antigen-presenting cells (APCs). CD4+ T lymphocytes demonstrated strong adhesion to HLA-DR4 transfected CHO cells co-expressing B7, ICAM-1 or LFA-3 molecules, suggesting that all three adhesion pathways is utilized by resting CD4+ cells. Monoclonal antibodies (MoAbs) against the corresponding receptors on T cells, e.g. anti-CD28, anti-LFA-1 beta and anti-CD2, inhibited completely T-cell adhesion to natural ligands expressed on transfected CHO cells. Pretreatment of CD4+ T cells with NKI-L16 MoAb, which interact with an activation epitope on LFA-1 alpha chain, enhanced adhesion to ICAM-1 but not B7 or LFA-3-expressing CHO cells. Analysis of T helper-cell subsets revealed that memory T cells bound several fold stronger to ICAM-1 expressing transfectants compared to the CD4+ 45RA+ naive T cells, whereas adhesion to B7, LFA-3- and B7/LFA-3-expressing CHO cells was similar in both T-cell subsets. The kinetics of adhesion of naive and memory CD4+ T cells to ICAM-1 was rapid and similar in both subsets. The NKI-L16 MoAb multiplied several times ICAM-1-dependent adhesion in naive compared to memory cells, which enabled the naive cells to reach a similar adhesion level as memory cells. The results suggest that resting naive CD4+ T cells utilize preferentially the CD2/LFA-3 or CD28/B7 adhesion pathways upon adhesion to APCs, while memory CD4+ T cells utilize the CD2/LFA-3, CD28/B7 and LFA-1/ICAM-1 adhesion pathways. The NKI-L16 MoAb-induced upregulation of adhesion involves an increased affinity of LFA-1 for its ligand and not a change in the number of LFA-1 molecules. This is compatible with a view that naive cells express a large number of inactive LFA-1 molecules, whereas memory cells express preferentially activated LFA-1 molecules. The inherent low number of active LFA-1 molecules on naive CD4+ T cells may be important in keeping these cells in a resting state.

Control of leukocyte integrin activation

The integrin receptors on leukocytes are transiently activated by "triggering" molecules that may be other leukocyte membrane structures such as the T-cell receptor complex or small molecules such as PAF, which bind to their own specific receptors. This "inside out" signaling is essential for high affinity integrin/ligand pairing. In the example of LFA-1/ICAM-1, binding is positively supported by Mg2+ but negatively supported by Ca2+. How specific divalent cations affect receptor activation and subsequent ligand binding has still to be fully understood. However, the fact that activation can be mimicked from outside the cell via special anti-LFA-1 monoclonal antibodies such as MEM-83 suggests that activated integrins undergo conformational changes. Further alteration occurs as a result of the interaction of integrin with ligand, and the resulting novel epitopes are named "ligand-induced binding sites." For a brief period of time the integrin/ligand complex is able to transmit signals from "outside in." The transient activation of leukocyte integrins determines that cell-cell adhesion will be short lived and serves the purpose of permitting recycling of effector cells with their targets.

Regulation of alpha 6 beta 1 integrin laminin receptor function by the cytoplasmic domain of the alpha 6 subunit

The alpha 6 beta 1 integrin is expressed on the macrophage surface in an inactive state and requires cellular activation with PMA or cytokines to function as a laminin receptor (Shaw, L. M., J. M. Messier, and A. M. Mercurio. 1990. J. Cell Biol. 110:2167-2174). In the present study, the role of the alpha 6 subunit cytoplasmic domain in alpha 6 beta 1 integrin activation was examined. The use of P388D1 cells, an alpha 6-integrin deficient macrophage cell line, facilitated this analysis because expression of either the alpha 6A or alpha 6B subunit cDNAs restores their activation responsive laminin adhesion (Shaw, L. S., M. Lotz, and A. M. Mercurio. 1993. J. Biol. Chem. 268:11401-11408). A truncated alpha 6 cDNA, alpha 6-delta CYT, was constructed in which the human cytoplasmic domain sequence was deleted after the GFFKR pentapeptide. Expression of this cDNA in P388D1 cells resulted in the surface expression of a chimeric alpha 6-delta CYT beta 1 integrin that was unable to mediate laminin adhesion or increase this adhesion in response to PMA under normal conditions, i.e., in medium that contained physiological concentrations of Ca++ and Mg++. The alpha 6A-delta CYT transfectants adhered to laminin, however, when Ca++/Mg++ was replaced with 150 microM Mn++. We also assessed the role of serine phosphorylation in the regulation of alpha 6A beta 1 integrin function by site-directed mutagenesis of the two serine residues present in the alpha 6A cytoplasmic domain because this domain is phosphorylated on serine residues in response to stimuli that activate the laminin receptor function of alpha 6 A beta 1. Point mutations were introduced in the alpha 6A cDNA that changed either serine residue #1064 (M1) or serine residue #1071 (M2) to alanine residues. In addition, a double mutant (M3) was constructed in which both serine residues were changed to alanine residues. P388D1 transfectants which expressed these serine mutations adhered to laminin in response to PMA to the same extent as cells transfected with wild-type alpha 6A cDNA. These findings provide evidence for a novel mode of integrin regulation that is distinct from that reported for other regulated integrins (O'Toole, T. E., D. Mandelman, J. Forsyth, S. J. Shattil, E. F. Plow, and M. H. Ginsberg. 1991. Science (Wash. DC). 254:845-847. Hibbs, M. L., H. Xu, S. A. Stacker, and T. A. Springer. 1991. Science (Wash. DC). 251:1611-1613), and they demonstrate that serine phosphorylation of the alpha 6A cytoplasmic domain is not involved in this regulation.

ICAM-3 interacts with LFA-1 and regulates the LFA-1/ICAM-1 cell adhesion pathway

The interaction of lymphocyte function-associated antigen-1 (LFA-1) with its ligands mediates multiple cell adhesion processes of capital importance during immune responses. We have obtained three anti-ICAM-3 mAbs which recognize two different epitopes (A and B) on the intercellular adhesion molecule-3 (ICAM-3) as demonstrated by sequential immunoprecipitation and cross-competitive mAb-binding experiments. Immunoaffinity purified ICAM-3-coated surfaces were able to support T lymphoblast attachment upon cell stimulation with both phorbol esters and cross-linked CD3, as well as by mAb engagement of the LFA-1 molecule with the activating anti-LFA-1 NKI-L16 mAb. T cell adhesion to purified ICAM-3 was completely inhibited by cell pretreatment with mAbs to the LFA-1 alpha (CD11a) or the LFA-beta (CD18) integrin chains. Anti-ICAM-3 mAbs specific for epitope A, but not those specific for epitope B, were able to trigger T lymphoblast homotypic aggregation. ICAM-3-mediated cell aggregation was dependent on the LFA-1/ICAM-1 pathway as demonstrated by blocking experiments with mAbs specific for the LFA-1 and ICAM-1 molecules. Furthermore, immunofluorescence studies on ICAM-3-induced cell aggregates revealed that both LFA-1 and ICAM-1 were mainly located at intercellular boundaries. ICAM-3 was located at cellular uropods, which in small aggregates appeared to be implicated in cell-cell contacts, whereas in large aggregates it appeared to be excluded from cell-cell contact areas. Experiments of T cell adhesion to a chimeric ICAM-1-Fc molecule revealed that the proaggregatory anti-ICAM-3 HP2/19 mAb was able to increase T lymphoblast attachment to ICAM-1, suggesting that T cell aggregation induced by this mAb could be mediated by increasing the avidity of LFA-1 for ICAM-1. Moreover, the HP2/19 mAb was costimulatory with anti-CD3 mAb for T lymphocyte proliferation, indicating that enhancement of T cell activation could be involved in ICAM-3-mediated adhesive phenomena. Altogether, our results indicate that ICAM-3 has a regulatory role on the LFA-1/ICAM-1 pathway of intercellular adhesion.

Mutation of putative divalent cation sites in the alpha 4 subunit of the integrin VLA-4: distinct effects on adhesion to CS1/fibronectin, VCAM-1, and invasin

To investigate the functional significance of putative integrin divalent cation binding sites, several mutated alpha 4 subunit cDNAs were constructed. Mutants contained the conservative substitution of Glu for Asp or Asn at the third position in each of three putative divalent cation sites. Transfection of wild-type or mutated alpha 4 into K562 cells yielded comparable expression levels and immunoprecipitation profiles. However, for all three alpha 4 mutants, adhesion to CS1/fibronectin was greatly diminished in either the presence or absence of the stimulatory anti-beta 1 mAb TS2/16. Constitutive adhesion to vascular cell adhesion molecule (VCAM) 1 was also diminished but, unlike CS1 adhesion, was restored upon TS2/16 stimulation. In contrast, adhesion to the bacterial protein invasin was minimally affected by any of the three mutations. For each of the mutants, the order of preference for divalent cations was unchanged compared to wild-type alpha 4, on CS1/fibronectin (Mn2+ > Mg2+ > Ca2+), on VCAM-1 (Mn2+ > Mg2+ = Ca2+) and on invasin (Mg2+ = Ca2+). However for the three mutants, the efficiency of divalent cation utilization was decreased. On VCAM-1, 68-108 microM Mn2+ was required to support half-maximal adhesion for the mutants compared with 14-18 microM for wild-type alpha 4. These results indicate (a) that three different ligands for VLA-4 show widely differing sensitivities to mutations within putative divalent cation sites, and (b) each of the three putative divalent cation sites in alpha 4 have comparable functional importance with respect to both divalent cation usage and cell adhesion.

KIM185, a monoclonal antibody to CD18 which induces a change in the conformation of CD18 and promotes both LFA-1- and CR3-dependent adhesion

Unactivated peripheral blood leukocytes show little tendency to bind to other cells or matrix components, whilst, in the presence of inflammatory mediators, adhesive interactions can rapidly increase. The Leu-CAM (beta 2 integrin) family of adhesion molecules have been shown to mediate a variety of these induced adhesion events. Here we describe a monoclonal antibody against CD18, KIM185, which stimulates JY cell homotypic aggregation by a CD11 a pathway as well as inducing the adherence of neutrophils to protein-coated plastic by a CD11b-dependent mechanism. The antibody recognizes an epitope distinct from the previously described KIM127 antibody and evidence is presented that the binding of KIM185 can cause a change in the conformation of the CD18 molecule.

Signaling through the LFA-1 leucocyte integrin actively regulates intercellular adhesion and tumor necrosis factor-alpha production in natural killer cells

The LFA-1 leucocyte integrin is known to participate in natural killer (NK) cytolytic activity, mediating effector target interactions. The possibility that LFA-1 may also play an active regulatory role in NK cells has been explored. To this end, we have employed a monoclonal antibody (HP1N) raised against recombinant interleukin-2 (rIL-2)-activated NK cells, which recognizes the alpha chain of the LFA-1 heterodimer (CD11a). In contrast to other anti-CD11a mAb the HP1N and its F(ab')2 fragment did not affect NK cell-mediated cytotoxicity and triggered a strong homotypic adhesion of NK cells and other LFA-1+ cells. Cellular aggregation was inhibited by anti-CD18 mAb, anti-ICAM-1 mAb, and other anti-CD11a mAb. Remarkably, the HP1N mAb was also shown to induce tumor necrosis factor-alpha (TNF-alpha) production from NK cells upon costimulation with anti-CD16 mAb. Such an effect appeared to be independent from homotypic adhesion since it took place in Mg(2+)-free medium, where NK cell aggregation was inhibited. Moreover, incubation with the HP1N mAb triggered a Ca2+ influx into the cytosol; this effect was clearly observed upon cross-linking of cell bound HP1N and was also substantiated with other anti LFA-1 (CD11a and CD18) mAb. Taken together these results indicate that the LFA-1 molecule is capable of transducing signals in NK cells, which regulate the intercellular interaction with its ligand, and enhance the activation via Fc gamma receptor type III.

Inflammatory roles of P-selectin

Polymorphonuclear leukocytes (PMNs) bind rapidly and reversibly to endothelial cells induced to express P-selectin, a glycoprotein that mediates adhesive intercellular interactions. In addition, PMNs adherent to endothelium expressing P-selectin demonstrate an intracellular Ca2+ transient, functionally up-regulate beta-2-integrins (CD11/CD18 glycoproteins), become polarized in shape, and are primed for enhanced degranulation when subsequently stimulated with chemotactic factors. However, P-selectin induces none of these responses directly when used alone, when incorporated into model membranes, or when expressed by transfected cells. The absence of direct activation of the PMNs is not due to competing antiinflammatory effects of P-selectin; instead, purified P-selectin and P-selectin in membranes support agonist-stimulated PMN responses. Furthermore, tethering of PMNs to endothelial surfaces by P-selectin is required for priming to occur efficiently, as shown by experiments with blocking monoclonal antibodies. The priming event is directly mediated by the signaling molecule, platelet-activating factor (PAF), and is inhibited by blocking the PAF receptor on PMNs. Thus, P-selectin and PAF are components of an adhesion and activation cascade, but have distinct roles: P-selectin tethers and captures the PMN, whereas PAF mediates juxtacrine activation. In vivo, selectins may facilitate interaction of target cells with membrane-bound molecules that send intercellular signals, in addition to mediating rolling of leukocytes and other adhesive functions.

Lymphocyte-endothelial interactions in inflamed synovia: involvement of several adhesion molecules and integrin epitopes

The role of several adhesion molecules for lymphocyte-endothelial interactions in the synovia of rheumatoid arthritis patients was studied using the frozen section assay. Partial inhibition of lymphocyte binding to endothelium of synovial sections could be observed with antibodies against CD44, L-selectin, and beta 1- and beta 2-integrins, pointing to the participation of several adhesion molecules in the regulation of lymphocyte immigration into inflamed synovia rather than the presence of a unique homing receptor. Different degrees of inhibition were found within a series of antibodies against alpha 4- and beta 1-integrins known to have functional effects in other interaction systems. In addition, increased binding to endothelial cells was induced when lymphocytes were pretreated with TS2/16 anti-beta 1 IgG, whereas binding to non-endothelial components of synovia was increased after treatment with HP 2/4 (anti-alpha 4) Fab. The data suggest a multifunctional role of alpha 4/beta 1-integrins in directly mediating adhesion as well as regulating adhesive interactions in the rheumatoid synovia.

Induction of aggregation of Raji human B-lymphoblastic cells by vasoactive intestinal peptide

Subsets of neurons in the thymic cortex, Peyer's patches and lymphoid tissues of the respiratory system deliver vasoactive intestinal peptide (VIP) at nanomolar concentrations. The possible effects of VIP on B-cell adhesiveness in these tissues were examined in studies of the homotypic aggregation (HA) of human B-lymphoblastoid cells of the Raji line, which express a mean of 27,950 VIP receptors/cell with a mean Kd of 0.8 nM. Mean HA, assessed microscopically, attained a maximum of 54% after 8 hr with 0.1 microgram/ml of phorbol 12-myristate 13-acetate (PMA) (P < 0.01) and 31% after 24 hr with 10(-8) M VIP (P < 0.05), as contrasted with 13% and 20% at the respective times in medium alone, and both stimuli also increased the mean size of aggregates. The presence of the phosphodiesterase inhibitor Ro 20-1724 permitted 10(-9) M VIP, which had no effect alone, to raise the mean cyclic AMP content of Raji cells by more than 10-fold and concurrently to elevate mean HA from 55% in medium alone at 48 hr to 70% and from 55% at 72 hr to 68% (P < 0.05 for both). Monoclonal antibodies to lymphocyte function-associated (LFA-1) adhesive protein and to intercellular adherence molecule-1 (ICAM-1) suppressed significantly the HA of Raji cells induced by VIP and PMA. The effects of VIP on compartmental immunity in the lungs and intestines thus may be mediated in part by increases in lymphocyte adhesiveness, which could contribute to the regional accumulation of specifically immunocompetent cells.

Plasma membrane lipid packing and leukocyte function-associated antigen-1-dependent aggregation of lymphocytes

A simple model system for study of adhesion mediated by leukocyte function-associated antigen-1 (LFA-1) is aggregation of lymphocytes stimulated in vitro. Although aggregation is blocked by monoclonal antibodies to LFA-1, not all lymphocytes expressing LFA-1 aggregate, indicating that LFA-1 is necessary but not sufficient for aggregation. To investigate whether the lipid bilayer plays a role in the functional activation of LFA-1, human peripheral blood lymphocytes and murine splenic lymphocytes were stimulated in culture, and measurements made of aggregation vs. packing of plasma membrane lipids. Progression of cells into aggregates was paralleled by a decrease in lipid packing of the population as a whole, as monitored by increased staining with the fluorescent probe merocyanine 540. Cells from aggregates stained more intensely than nonaggregated cells from the same population, indicating that aggregates are preferentially formed from cells in the population with the loosest packed membrane. In contrast, aggregated cells were found to express equivalent or even lower amounts of LFA-1 than nonaggregated cells. Looser lipid packing is therefore associated with the development of LFA-1-dependent aggregation, and might be involved in the functional activation of this cell adhesion molecule.

Dynamic aspects of adhesion receptor function--integrins both twist and shout

The recognition of extracellular molecules by cell surface receptors is the principal mechanism used by cells to sense their environment. Consequently, signals transduced as a result of these interactions make a major contribution to the regulation of cellular phenotype. Historically, particular emphasis has been placed on elucidating the intracellular consequences of growth factor and cytokine binding to cells. In addition to these interactions, however, cells are usually in intimate contact with a further source of complex structural and functional information, namely immobilised extracellular matrix and/or cell surface adhesion proteins. A key question in recent years has been whether cells use the myriad of adhesion protein-receptor interactions purely for structural and migratory function, or whether these interactions also make a more varied contribution to cell phenotype. Here we review dynamic aspects of the function of one major class of adhesion receptor, the integrins. In particular, we focus on the evidence for shape changes in integrin molecules, the mechanisms responsible for regulating ligand binding, and the signals transduced following integrin occupancy.

Surface phenotype and adhesion activity of B-cell chronic lymphoid leukemias

Surface phenotypes and adhesion activity to human umbilical vein endothelial cells (HUVECs) were studied using leukemic cells from 12 Japanese patients with B-cell chronic lymphoid leukemias including 7 with chronic lymphocytic leukemia (CLL), 1 with prolymphocytic leukemia (PLL), 2 with hairy cell leukemia (HCL) and 2 with HCL variant (HCL-V). CD13 and CD23 were found to be characteristically positive in CLL, whereas they were not expressed in non-CLL cases except for positivity of CD23 in two such cases. Except for CD11b, all other leukocyte integrins examined (CD11a, CD11c and CD18) and their ligand (CD54) were highly expressed in non-CLL cases. Adhesion activity of leukemic cells to HUVECs after co-culture with HUVECs was well correlated with the expression of CD11b, CD18 and CD54, but showed no predilection for any leukemia subtype. Positivity for CD5, CD21, CD23 and CD13 changed after the co-culture with HUVEC. These results suggest that adhesion activity after co-culture. does not correlate with the leukemia subtype and that endothelial cells activate or differentiate leukemic cells.

Role of intracellular Ca2+ levels in the regulation of CD11a/CD18 mediated cell adhesion

CD2, CD3, and MHC class II have been demonstrated to stimulate lymphocyte function-associated antigen (LFA)-1 (CD11a/CD18) mediated adhesion (Van Kooyk et al., 1989, Dustin and Springer, 1989; Mourad et al., 1990). Activation of LFA-1 may be mediated by different intracellular signals generated from these stimuli, since previous findings suggest that triggering of LFA-1 through CD2 or CD3 leads to sustained and transient cell adhesion respectively (Van Kooyk et al., 1989). We investigated the role of intracellular signalling pathways in more detail. The results demonstrate that, in addition to protein tyrosine kinase (PTK) and protein kinase C (PKC) mediated signalling, increase in cytosolic-free calcium ([Ca2+]i) levels play a major role in the activation of LFA-1. The calcium ionophore Ionomycin, which increases [Ca2+]i is capable of directly activating LFA-1. Furthermore, activation of LFA-1 by triggering through CD2, CD3 or MHC class II is associated with an increase in [Ca2+]i levels, with kinetics that directly correlate with cell adhesiveness. Moreover, entry of extracellular Ca2+ via Ca-channels is involved in both the CD3- and MHC class II, as well as part of the CD2 induced LFA-1 activation. Depletion of intracellular calcium results in unresponsiveness of LFA-1 to these stimuli, further demonstrating a regulatory role for [Ca2+]i in LFA-1 mediated adhesion.

Homotypic leukocyte aggregation triggered by a monoclonal antibody specific for a novel epitope expressed by the integrin beta 1 subunit: conversion of nonresponsive cells by transfecting human integrin alpha 4 subunit cDNA

The monoclonal antibody 33B6 was found to be specific for the beta 1 integrin subunit. Treatment of leukocytes with this antibody induced a vigorous homotypic aggregation that had similar physiologic conditions as aggregation induced by a monoclonal antibody specific for the alpha 4 subunit. Expression of a beta 1 subunit on the cell surface was not sufficient for mAb 33B6-mediated aggregation to occur, since cells of the K562 erythroleukemia line failed to respond even though they expressed the beta 1 subunit and the 33B6 epitope. However, after transfection with cDNA encoding the alpha 4 subunit, K562 cells acquired the ability to aggregate in response to mAb 33B6 binding. By contrast, mAb 33B6 blocked cell binding to the endothelial surface protein vascular cell adhesion molecule-1 and the extracellular matrix protein fibronectin. These results suggest that the beta 1 epitope defined by mAb 33B6 may play a novel role in regulating leukocyte adhesive interactions.

A novel LFA-1 activation epitope maps to the I domain

A panel of 21 alpha-subunit (CD11a) and 10 beta-subunit (CD18) anti-LFA-1 mAbs was screened for ability to activate LFA-1. A single anti-CD11a mAb, MEM-83, was identified which was able to directly induce the binding of T cells to purified ICAM-1 immobilized on plastic. This ICAM-1 binding could be achieved by monovalent Fab fragments of mAb MEM-83 at concentrations equivalent to whole antibody, was associated with appearance of the "activation reporter" epitope detected by mAb 24, and was completely inhibited by anti-ICAM-1 and LFA-1 blocking mAbs. The epitope recognized by mAb MEM-83 was distinct from that recognized by mAb NKI-L16, an anti-CD11a mAb previously reported to induce LFA-1 activation, in that it was constitutively present on freshly isolated peripheral blood mononuclear cells and was not divalent cation dependent for expression. The ICAM-1 binding activity induced by mAb MEM-83 was, however, dependent on the presence of Mg2+ divalent cations. Using an in vitro-translated CD11a cDNA deletion series, we have mapped the MEM-83 activation epitope to the "I" domain of the LFA-1 alpha subunit. These studies have therefore identified a novel LFA-1 activation epitope mapping to the I domain of LFA-1, thereby implicating this domain in the regulation of LFA-1 binding to ICAM-1.

Activation of CD44 induces ICAM-1/LFA-1-independent, Ca2+, Mg(2+)-independent adhesion pathway in lymphocyte-endothelial cell interaction

We have established an endothelial cell line KOP2.16 from pooled mouse lymph nodes. Resting lymphocytes avidly bound to KOP2.16 and migrated underneath the cytoplasm. The binding was partly mediated by VLA-4 and VCAM-1, but apparently independent of CD44 since anti-CD44 antibody examined failed to inhibit the binding. However, pretreatment of lymphocytes with anti-CD44 resulted in the rapid appearance of Ca(2+)-, Mg(2+)-independent, LFA-1/ICAM-1-, CD2/LFA-3, VLA-4/VCAM-1-independent lymphocyte binding, indicating that a novel adhesion pathway was induced by the anti-CD44 treatment. Interestingly, the elicited adhesion was observed only when anti-CD44 that block hyaluronate recognition of CD44 were used for lymphocyte pretreatment. Neither hyaluronate itself nor non-blocking anti-CD44 up-regulated the adhesion. Fab fragment of the blocking anti-CD44 did not induce the up-regulation unless cross-linked with a second antibody, indicating that cross-linking of surface CD44 is necessary for induction of a novel adhesion pathway. We propose that the agonistic anti-CD44 antibodies induce a novel adhesion pathway by mimicking ligand binding to CD44 on the lymphocyte surface and that non-hyaluronate ligand(s) is involved in regulation of adhesive function of CD44. Potential involvement of such a regulatory mechanism in lymphocyte homing is discussed.

Regulatory effect of interleukin-4 (IL-4) on the expression and function of lymphocyte adhesion receptors involved in IL-2-induced cell aggregation

Human recombinant interleukin-4 (rIL-4) was studied for its capacity to inhibit rIL-2-induced lymphoid cell aggregation. In contrast to rIL-2, rIL-4 was unable to induce cluster formation by itself. However, when added simultaneously with rIL-2 to cultures of freshly isolated peripheral blood lymphocytes (PBL), rIL-4 inhibited cell aggregation in a dose-dependent way. In contrast, PBL, preactivated by a 4-day culture in the presence of 500 U/ml rIL-2, were not inhibited in their adhesive capacity by rIL-4. Inhibition of cell aggregation was most prominent at 24 hr and virtually lost after 72 hr of culture. Phenotypical analysis revealed that rIL-4, with similar kinetics, decreased the rIL-2-mediated up-regulation of the CD2, CD54 and CD49e adhesion molecules. In addition, it was observed that up-regulation of the activation epitope on CD11a recognized by the mAb NKI-L16, was prevented. During 24hr of culture rIL-4 itself did not alter the expression of these antigens. Blocking experiments with mAb directed against adhesion structures did not reveal a direct role for CD49e, but obviously demonstrated involvement of CD11a/CD18-CD54 and CD2-CD58 interactions in the rIL-2-induced adhesion. Therefore, rIL-4 appears to inhibit the early phase of rIL-2-induced aggregation by preventing the up-regulation of CD54 and CD2 antigens and by inhibiting the generation of the activated state of the CD11a/CD18 receptor.

A subpopulation of Mac-1 (CD11b/CD18) molecules mediates neutrophil adhesion to ICAM-1 and fibrinogen

We report that a subpopulation (10%) of the Mac-1 (CD1 1b/CD18) molecules on activated neutrophils mediates adhesion to ICAM-1 and fibrinogen. We describe a novel mAb (CBRM1/5) that binds to an activation-specific neoepitope on a subset of Mac-1 molecules on neutrophils and monocytes after stimulation with chemoattractants or phorobol esters but does not recognize Mac-1 on resting myeloid cells. CBRM1/5 immunoprecipitates a subpopulation of Mac-1 molecules from detergent lysates of neutrophils, binds to immunoaffinity-purified Mac-1, and localizes to the I domain on the alpha chain of Mac-1. Because CBRM1/5 recognizes a fraction of Mac-1 on activated neutrophils, but still blocks Mac-1-dependent adhesion to fibrinogen and ICAM-1, we suggest that only a small subset of Mac-1 molecules is competent to mediate adhesion.

Lymphocyte function-associated antigen 1 dominates very late antigen 4 in binding of activated T cells to endothelium

Lymphocyte function-associated antigen 1/intercellular adhesion molecule 1 (LFA-1/ICAM-1)-and very late antigen 4/vascular cell adhesion molecule 1 (VLA-4/VCAM-1)-mediated adhesion of T lymphocytes to endothelial cells (EC) can be regulated by increased expression of ICAM-1 and VCAM-1 upon cytokine treatment of EC, or by activation of the integrin molecules LFA-1 and VLA-4 on T cells. Here, we provide evidence that preferential usage of LFA-1 over VLA-4 is yet another mechanism to control T cell adhesion. We observed that binding of activated T lymphocytes, as opposed to resting T cells, to EC is essentially mediated through LFA-1 and not through VLA-4. VLA-4-mediated adhesion of T cells to EC is only found when LFA-1 is not expressed or not functional, as observed for several T cell leukemia cell lines. These results suggest that LFA-1-mediated adhesion dominates and may downregulate VLA-4-mediated adhesion through an unidentified mechanism.

Integrins and their ligands

Integrins are expressed on almost every cell type and are responsible for the linkage of the extracellular matrix with the cytoskeleton. In this review I have focused on the intra- and extracellular proteins that bind to integrins. Although many integrins bind to the same extracellular ligand, they mostly recognize different sites on these ligands. Some integrins interact with the same site but then there are requirements for different additional sequences to obtain high affinity. By modulating the expression and activity of integrins in the plasma membrane, cells can adapt their capacity of binding to the matrix. How integrins become activated is as yet not clear, but interaction with other proteins or lipids may be critical. Binding to ligands could also be modulated by alternative splicing of mRNAs for ligand binding sites in the extracellular domain. In Drosophila, the mRNA for the extracellular domain of the PS2 integrin is spliced near a site implicated in ligand binding. In humans, however, there are no indications that alternative splicing contributes to the regulation of function of the extracellular domain of integrins. The only splice variant of the extracellular domain of an integrin identified so far concerns are alpha subunit of the alpha IIb beta 3 complex, but the splicing occurs in a region that has not been implicated in cell adhesion. There is also no evidence as yet that integrin function can be modulated by alternative splicing of mRNA for the cytoplasmic domain of integrin subunits. However, the loss of function seen with some deletion mutants of the cytoplasmic domains of integrin subunits suggests that such a mechanism may well exist. In a different way the binding capacity of a given cell can be influenced by regulating the expression of its ligand or by alternative mRNA splicing of sequences encoding the cell binding domain in their ligands. In the case of fibronectin, the mRNA for one of the integrin binding sites is subject to alternative splicing. The mRNAs for the three chains of laminin appear not to be subject to alternative splicing but, by combining different variant chains of laminin, isoforms can be generated which may have different affinities for integrins. Binding of cells to the matrix therefore does not only depend on the expression and activity of the correct integrin but also of the correct variant of the ligand.(ABSTRACT TRUNCATED AT 400 WORDS)

Neutrophil adhesion: A point for therapeutic intervention?

It has become increasingly clear over the last 20 years that the potential exists to modulate inflammatory responses with compounds that interfere with intercellular adhesion. This review highlights the adhesion interactions that occur during neutrophil extravasation and indicates some of the possible ways of disrupting these interactions.