Interstitial leukocyte migration and immune function.

The trafficking of leukocytes into and within lymphoid and peripheral tissues is central to immune cell development, immunosurveillance and effector function. Interstitial leukocyte trafficking is the result of amoeboid polarization and migration, guided by soluble or tissue-bound chemoattractant signals for positioning and local arrest. In contrast to other migration modes, amoeboid movement is particularly suited for scanning cellular networks and tissues. Here, we review mechanisms of leukocyte migration and sensing involved in diapedesis, tissue-based interstitial migration and egress, immune cell positioning in inflammation, and emerging therapeutic interference strategies.

The P2Y1 receptor-mediated leukocyte adhesion to endothelial cells is inhibited by melatonin.

Extracellular ATP (released by endothelial and immune cells) and its metabolite ADP are important pro-inflammatory mediators via the activation of purinergic P2 receptors (P2Y and P2X), which represent potential new targets for anti-inflammatory therapy. Endothelial P2Y1 receptor (P2Y1R) induces endothelial cell activation triggering leukocyte adhesion. A number of data have implicated melatonin as a modulator of immunity, inflammation, and endothelial cell function, but to date no studies have investigated whether melatonin modulates endothelial P2YR signaling. Here, we evaluated the putative effect of melatonin on P2Y1R-mediated leukocyte adhesion to endothelial cells and TNF-α production, using mesenteric endothelial cells and fresh peripheral blood mononuclear cells isolated from rats. Endothelial cells were treated with the P2Y1R agonist 2MeSATP, alone or in combination with melatonin, and then exposed to mononuclear cells. 2MeSATP increased leukocyte adhesion to endothelial cells and TNF-α production in vitro, and melatonin inhibited both effects without altering P2Y1R protein expression. In addition, assays with the Ca2+ chelator BAPTA-AM indicate that the effect of melatonin on 2MeSATP-stimulated leukocyte adhesion depends on intracellular Ca2+ modulation. P2Y1R is considered a potential target to control chronic inflammation. Therefore, our data unveiled a new endothelial cell modulator of purinergic P2Y1 receptor signaling.

Leukocyte receptor expression in chronic periodontitis.

BACKGROUND AND OBJECTIVE: Microbial recognition in the periodontium through specific leukocyte receptors gives rise to the response which in susceptible individuals can lead to periodontal diseases. The aim of this study was to explore the expression of leukocyte receptors in the gingival tissues of chronic periodontitis patients and to analyse differences between diseased and control sites (sites with probing pocket depth <4 mm). MATERIAL AND METHODS: Thirty-seven chronic periodontitis patients were included in the study. Gingival biopsies were harvested from diseased and control sites and processed by flow cytometry for the determination of the expression of 16 leukocyte receptors (CD4, CD8, CD11b, CD14, CD16, CD19, CD25, CD28, CD49d, CD49e, CD62, CD71, CD80, CCR7, Ly6G and HLA-DR). RESULTS: Expression of all studied receptors was higher in test compared with control sites (p < 0.005). Sampled sites with less bleeding on probing exhibited higher expression of CD16 and CD14 receptors (p = 0.020 and 0.011, respectively). CONCLUSIONS: This study points towards considerable differences in the expression of leukocyte receptors between diseased and control sites in the same periodontal patients.

Human neutrophil adherence to laminin in vitro. Evidence for a distinct neutrophil integrin receptor for laminin.

We used mAbs against polymorphonuclear leukocyte (PMN) surface proteins to investigate the mechanisms by which stimulated human neutrophils (PMNs) adhere in vitro to laminin, the major glycoprotein of mammalian basement membrane. mAb IB4, which is directed against the common beta 2 chain of the CD11/CD18, only partially inhibited the adherence of PMA-stimulated PMNs to both laminin and to subendothelial matrices. In contrast, IB4 completely inhibited PMA-stimulated PMN adherence to gelatin, fibronectin, collagen IV, and endothelial cell monolayers. PMA-stimulated PMNs from a patient with severe congenital CD11/CD18 deficiency also adhered to laminin, but not to gelatin or endothelial cell monolayers. Therefore, PMA-stimulated PMNs adhere to laminin by both CD11/CD18-dependent and CD11/CD18-independent mechanisms. Expression of CD11/CD18-independent adherence to laminin was agonist dependent, occurring after stimulation with the calcium ionophore A23187 and recombinant TNF-alpha, but not with the chemotactic factors FMLP, platelet activating factor, or recombinant human C5a. Expression of CD11/CD18-independent adherence was also divalent cation dependent, occurring in the presence of Mg2+ but not Ca2+ as the sole added divalent cation. The mAbs AIIB2 and 13, which are directed against the beta 1 subunit of the VLA integrins, significantly inhibited the CD11/CD18-independent adherence of normal PMNs to laminin, and completely abolished the adherence of CD11/CD18-deficient PMNs to laminin. Both anti-beta 1 mAbs bound to PMNs, as demonstrated by flow cytometry, and immunoprecipitated a membrane molecule of Mr 130,000 daltons from 125I-labeled, detergent-solubilized PMNs. These data suggest that human PMNs possess beta 1 and beta 2 classes of integrins, and that both mediate PMN adherence.

Regulated expression and function of CD11c/CD18 integrin on human B lymphocytes. Relation between attachment to fibrinogen and triggering of proliferation through CD11c/CD18.

CD11c/CD18 (p150,95) is a beta 2 integrin expressed by myeloid, natural killer and certain lymphoid cells such as some cytotoxic T cell clones and B cell malignancies. We have studied the expression and function of CD11c on resting and activated B lymphocytes. Flow cytometry, immunoprecipitation, and mRNA analyses showed that cell activation with phorbol esters or with a variety of stimuli such as Staphylococcus aureus or anti-mu antibodies in combination with cytokines induced de novo CD11c/CD18 cell surface expression on most B cells while CD11b expression was not affected. Functional analysis of CD11c/CD18 on B cells revealed that it plays a dual role. First, CD11c/CD18 is implicated in B cell proliferation, as demonstrated by the ability of several anti-CD11c monoclonal antibodies to trigger comitogenic signals; and second, the newly expressed CD11c/CD18 mediates B cell binding to fibrinogen. Our data conclusively demonstrate the role of CD11c/CD18 on both B cell activation and adhesion processes.

Involvement of CD56 (NKH-1/Leu-19 antigen) as an adhesion molecule in natural killer-target cell interaction.

The CD56 differentiation antigen, recognized by anti-Leu-19 and NKH-1 mAbs, is a 200-220-kD glycoprotein that is expressed predominantly on human NK cells and a minor subset of T lymphocytes mediating MHC-unrestricted cytotoxicity. The recent finding that CD56 is an isoform of the neural cell adhesion molecule (N-CAM) prompted us to examine the adhesive function of CD56 in the NK-target cell interaction. Synergistic inhibitory effects of anti-CD56 mAbs with anti-LFA-1 and/or anti-LFA-3 mAbs were demonstrated on NK cell-mediated cytotoxicity and on NK cell binding to target cells only when the target cells also express CD56. These findings indicate that CD56 on NK cells can serve as the third pathway of cell adhesion other than those mediated by the CD2/LFA-3 and LFA-1/ICAM-1 interactions, and is involved in NK cell cytotoxicity when interacting with the cells bearing N-CAM.

Leukocyte response integrin and integrin-associated protein act as a signal transduction unit in generation of a phagocyte respiratory burst.

The leukocyte response integrin (LRI) is a phagocyte integrin which recognizes the basement membrane protein entactin and the synthetic peptide Lys-Gly-Ala-Gly-Asp-Val (KGAGDV). The function of LRI is intimately associated with that of a distinct membrane protein, integrin-associated protein (IAP), as antibodies which recognizes IAP can inhibit all known functions of LRI. When adherent to surface, the LRI ligands entactin and KGAGDV activate the respiratory burst in polymorphonuclear leukocytes (PMN) and monocytes, as do monoclonal antibodies (mAb) directed at either LRI or IAP. When added in solution, peptides and antibodies specific for LRI, and some, but not all, anti-IAP antibodies, can inhibit the respiratory burst activated by any of these surface-adherent ligands. Only monoclonal anti-IAP antibodies which can inhibit LRI function when added in solution are competent to activate the respiratory burst when adherent to a surface. KGAGDV peptide and anti-LRI added in solution can inhibit anti-IAP-stimulated respiratory burst. The LRI-IAP-initiated respiratory burst is independent of CD18, as judged by: (a) blockade of inhibition by anti-CD18 mAb with the protein kinase A inhibitor HA1004; (b) enhanced sensitivity of CD18-dependent respiratory burst compared with LRI/IAP-dependent respiratory burst to the tyrosine kinase inhibitors genestein and herbimicin; and (c) generation of a respiratory burst in response to KGAGDV, anti-LRI, and anti-IAP coated surfaces in PMN from a patient with LAD. Despite its apparent CD18 independence, LRI/IAP-initiated respiratory burst requires a solid phase ligand and is sensitive to cytochalasin B. These data suggest a model in which LRI and IAP act together as a single signal transduction unit to activate the phagocyte respiratory burst, in a manner that requires CD18-independent cell adhesion.

Molecules and structures involved in the adhesion of natural killer cells to vascular endothelium.

The present study was designed to define molecules and structures involved in the interaction of natural killer (NK) cells with the vascular endothelium in vitro. Resting and interleukin 2 (IL-2)-activated NK cells were studied for their capacity to adhere to resting and IL-1-treated human umbilical vein endothelial cells (EC). In the absence of stimuli, NK cells showed appreciable adhesion to EC, with levels of binding intermediate between polymorphs and monocytes. The binding ability was increased by pretreatment of NK cells with IL-2. Using the appropriate monoclonal antibody, the beta 2 leukocyte integrin CD18/CD11a was identified as the major adhesion pathway of NK cells to unstimulated EC. Activation of EC with IL-1 increased the binding of NK cells. In addition to the CD18-CD11a/intercellular adhesion molecule pathway, the interaction of resting or IL-2-activated NK cells to IL-1-activated EC involved the VLA-4 (alpha 4 beta 1)-vascular cell adhesion molecule 1 receptor/counter-receptor pair. No evidence for appreciable involvement of endothelial-leukocyte adhesion molecule was obtained. Often, NK cells interacted either with the culture substrate or with the EC surface via dot-shaped adhesion structures (podosomes) protruding from the ventral surface and consisting of a core of F-actin surrounded by a ring of vinculin and talin. The identification of molecules and microanatomical structures involved in the interaction of NK cells with EC may provide a better understanding of the regulation of NK cell recruitment from blood, their extravasation, and their migration to tissues.

Crosslinking of the T cell-specific accessory molecules CD7 and CD28 modulates T cell adhesion.

Regulated adhesion enables T cells to migrate through tissue and transiently interact with an endless succession of cells. Monoclonal antibody (mAb) engagement of the CD3/T cell receptor (TCR) complex results in a rapid and transient augmentation of the adhesion function of LFA-1 and VLA integrin molecules on human T cells. We show in this study that mAb crosslinking of the T cell-specific accessory molecules CD7 and CD28, or treatment with the Ca2+ ionophore A23187, results in the rapid induction of integrin-mediated adhesion to three distinct ligands: the extracellular matrix protein fibronectin, and the cell surface molecules ICAM-1 and VCAM-1. Like CD3 crosslinking, increased adhesion via CD7 and CD28 crosslinking appears to involve both protein kinase C (PKC) and cAMP-dependent protein kinases. In contrast, A23187 induction of adhesion is unaffected by PKC inhibitors. CD7 is preferentially expressed on naive T cells and is unique in being a potent inducer of naive T cell adhesion. Enhanced expression/function of adhesion-inducing molecules thus overcomes relative deficits in adhesion receptor expression.

Neutrophil-activating protein 1/interleukin 8 stimulates the binding activity of the leukocyte adhesion receptor CD11b/CD18 on human neutrophils.

The cytokine NAP-1/IL-8 is produced by a variety of different cells in response to inflammatory stimuli and elicits several biological responses from PMN. Experiments presented here demonstrate that PMN exposed to NAP-1/IL-8 expressed increased amounts of CD11b/CD18, as well as CD11c/CD18 and CR1, on their cell surface, while expression of Fc gamma RIII and HLA-A,B,C remained essentially unchanged. Increased CD11b/CD18 and CD11c/CD18 appears to correspond with the release of specific granules by NAP-1/IL-8. NAP-1/IL-8 was also a potent stimulator of several of the binding activities of CD11b/CD18. Ligation of EC3bi by CD11b/CD18 was rapidly enhanced by NAP-1/IL-8, but phagocytosis of the ligated particles was not induced by the agonist. In addition, enhanced binding of EC3bi was observed in the absence of an increase in receptor expression as shown with PMN cytoplasts. NAP-1/IL-8 promoted additional adhesive interactions between CD11b/CD18 and the biosynthetic precursor of LPS, lipid IVa, fibrinogen, and endothelial cells, suggesting that NAP-1/IL-8 may promote leukocyte adhesion in vivo that could lead to recruitment of PMN to sites of tissue inflammation.

Distinct mutations in two patients with leukocyte adhesion deficiency and their functional correlates.

Two patients with leukocyte adhesion deficiency (LAD), one with a moderate phenotype (patient 14) and one with a severe phenotype (patient 2) who had been shown to have a normal sized beta subunit protein precursor, were analyzed in an attempt to determine the molecular basis for their disease. RNase mapping located possible mutations to two distinct but adjacent regions of the beta subunit cDNA. Sequencing of patient-derived cDNA clones in this region revealed a C for T difference at amino acid 149 in patient 14 which resulted in the substitution of a leucine for a proline, and an A for G substitution at amino acid 169 in patient 2 which mutated a glycine to an arginine. The mutated amino acids are in a region of the cDNA that is highly conserved between the beta subunits of the integrin family and are identical in all known integrin beta subunits. Co-transfection of the beta subunit cDNA containing the patient 2 mutation with the wild-type alpha subunit of LFA-1 in a mammalian expression system resulted in no expression of LFA-1. In the case of the mutation in patient 14 there was markedly diminished expression of LFA-1 with loss of function and loss of the epitope for a number of anti-beta mAbs. Normal half-life of the mutant beta subunits, and previous demonstration of a lack of alpha/beta complex formation during biosynthesis in patient cells, suggest a defect in association with the alpha subunit. Association with beta is required for expression of the alpha subunit of LFA-1. Loss of functional expression with both of these beta subunit mutations suggests that they lie in a site critical for association with the alpha subunit.

Inducible cell adhesion molecule 110 (INCAM-110) is an endothelial receptor for lymphocytes. A CD11/CD18-independent adhesion mechanism.

Inducible cell adhesion molecule 110 (INCAM-110) is a 110-kD glycoprotein expressed on cytokine-activated human vascular endothelial cells. mAb blocking studies indicate that INCAM-110 and intercellular adhesion molecule 1 (ICAM-1) independently support the adhesion of lymphocytes to activated human umbilical vein endothelial cell monolayers. Anti-CD11a/CD18 antibodies with anti-INCAM-110 mAb E1/6 produce greater inhibition of lymphocyte adhesion than either reagent alone, suggesting that INCAM-110 and LFA-1 are not an obligate receptor-ligand pair. Blood monocytes, but not polymorphonuclear leukocytes, also appear to bind endothelial INCAM-110. Endothelial expression of INCAM-110 is upregulated at sites of inflammation, suggesting a role in the recruitment of mononuclear leukocytes.

The specific interaction of helper T cells and antigen-presenting B cells. IV. Membrane and cytoskeletal reorganizations in the bound T cell as a function of antigen dose.

We have used double-immunofluorescence labeling to determine the surface distributions of LFA-1 and CD4, and the intracellular distributions of the cytoskeletal protein talin and of the microtubule organizing center (MTOC) of cloned Th cells in 1:1 cell couples with antigen (Ag)-specific APC of the B cell type (B-APC). The Th cell was directed to a peptide fragment of the Ag OVA in the context of IAd. The B-APC was the transfected A20 B hybridoma cell A20-HL, bearing on its surface a surface Ig specific for the hapten TNP, and pulsed with different concentrations of DNP-OVA. At sufficiently high doses of DNP-OVA (greater than 100 ng/ml), in essentially all couples, LFA-1, CD4, and talin were each concentrated at the Th cell membrane where it was in contact with the B-APC, and the MTOC inside the Th cell was reoriented to face the contact region. At lower doses of DNP-OVA (between 50 and 10 ng/ml), in all couples, LFA-1 and talin were concentrated at the Th/B-APC contact region, but the extent of CD4 clustering, MTOC reorientation, and Th cell proliferation all decreased with decreasing Ag dose. With no Ag, none of these effects was observed. These and other data indicate that two distinct signals are received by the Th cell that is specifically bound to its B-APC. The first signal, at low Ag doses, stimulates a linkage of LFA-1 and talin in the Th cell, and a specific LFA-1-mediated intercellular adhesion; the second signal, at higher Ag doses, is required to induce Th cell proliferation, with which the Th-MTOC reorientation and CD4 clustering are correlated.

Monocyte attachment to activated human vascular endothelium in vitro is mediated by leukocyte adhesion molecule-1 (L-selectin) under nonstatic conditions.

The receptors that mediate monocyte adhesion to cytokine-stimulated endothelial monolayers were assessed using a nonstatic (rotating) cell-attachment assay. In this system, leukocyte adhesion molecule-1 (LAM-1) (L-selectin) mediated a major portion (87 +/- 15% at 37 degrees C) of monocyte attachment to activated endothelium. mAb blocking of endothelial leukocyte adhesion molecule-1 (41% inhibition), CD18 (36%), and vascular cell adhesion molecule-1 (25%) function had lesser effects on attachment. These results suggest that LAM-1 may serve an important role in monocyte attachment to endothelium at sites of inflammation.

Differential roles of Mac-1+ cells, and CD4+ and CD8+ T lymphocytes in primary nonfunction and classic rejection of islet allografts.

The high rate of persistent hyperglycemia, termed primary nonfunction, after islet allotransplantation in C57BL/6 mice recipients of B10.BR strain islets, as compared with B10.BR recipients of C57BL/6 islets, led to a series of experiments to determine whether islet allograft primary nonfunction was attributable to technical aspects of the transplant procedure or whether it was a consequence of alloimmunity. Primary nonfunction was prevented by systemic pharmacologic immunosuppression of the host with cyclosporine. Selective immunodepletion of host CD4+ and CD8+ T lymphocytes significantly extended the time of classic rejection but did not significantly affect the rate of primary nonfunction. However, modulation of macrophages by administration to the host of silica completely abolished primary nonfunction. These observations, in conjunction with the immunohistological findings of intense macrophage infiltration in islet allografts from recipients exhibiting persistent post-transplant hyperglycemia, support the hypothesis that primary nonfunction results from a cell-mediated host-immune response of rapid onset that is dependent on macrophages or macrophage byproducts as the main effectors.

Constitutive and stimulus-induced phosphorylation of CD11/CD18 leukocyte adhesion molecules.

The leukocyte CD11/CD18 adhesion molecules (beta 2 integrins) are a family of three heterodimeric glycoproteins each with a distinct alpha subunit (CD11a, b, or c) and a common beta subunit (CD18). CD11/CD18 mediate crucial leukocyte adhesion functions such as chemotaxis, phagocytosis, adhesion to endothelium, aggregation, and cell-mediated cytotoxicity. The enhanced cell adhesion observed upon activation of leukocytes is associated with increased surface membrane expression of CD11/CD18, as well as a qualitative upregulation of CD11/CD18 functions. To elucidate the nature of the qualitative modifications that occur, we examined the phosphorylation status of these molecules in resting human leukocytes and upon activation with PMA or with the chemotactic peptide F-met-leu-phe (FMLP). In unstimulated cells, all three CD11 subunits were found to be constitutively phosphorylated. In contrast, phosphorylation of the common CD18 subunit was minimal. PMA induced rapid and sustained phosphorylation of CD18 that occurred at high stoichiometry, but had only minimal effects on phosphorylation of the associated CD11 subunits. FMLP also induced rapid phosphorylation of CD18, but the effect was of short duration. FMLP-induced phosphorylation of CD18 was not related to its Ca++-mobilizing effect, as CD18 phosphorylation was not observed upon treatment of leukocytes with the Ca++ ionophore, ionomycin. Phosphoamino acid analysis of CD11/CD18 in PMA- or FMLP-treated monocytes revealed a predominance of phosphoserine residues in all CD11/CD18 subunits. A small component of phosphothreonine was present in CD11c and CD18 and a minor component of phosphotyrosine was also detected in CD18 upon leukocyte activation may regulate the adhesion functions mediated by the CD11/CD18 family of molecules.

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

The human lymphocyte homing receptor LAM-1, like its murine counterpart MEL-14, functions as a mammalian lectin, and mediates the binding of leukocytes to specialized high endothelial cells in lymphoid organs (HEV). LAM-1 is a member of a new family of cell adhesion molecules, termed selectins or LEC-CAMs, which also includes ELAM-1 and PAD-GEM (GMP-140/CD62). To localize the regions of LAM-1 that are involved in cell adhesion, we developed chimeric selectins, in which various domains of PAD-GEM were substituted into LAM-1, and used these chimeric proteins to define the domain requirements for carbohydrate binding, and to localize the regions recognized by several mAb which inhibit the adhesion of lymphocytes to lymph node HEV. The binding of PPME or fucoidin, soluble complex carbohydrates that specifically define the lectin activity of LAM-1 and MEL-14, required only the lectin domain of LAM-1. The LAM1-1, LAM1-3, and LAM1-6 mAb each strongly inhibit the binding of lymphocytes to HEV in the in vitro frozen section assay, and defined three independent epitopes on LAM-1. Blocking of PPME or fucoidin binding by LAM1-3 indicated that this site is identical, or in close proximity, to the carbohydrate binding site, and analysis of the binding of LAM1-3 to chimeric selectins showed that the epitope detected by LAM1-3 is located within the lectin domain. Although the LAM1-6 epitope is also located in the lectin domain, LAM1-6 did not affect the binding of PPME or fucoidin. The LAM1-1 epitope was located in, or required, the EGF domain, and, importantly, binding of LAM1-1 significantly enhanced the binding of both PPME and fucoidin. These results suggest that adhesion mediated by LAM-1 may involve cooperativity between functionally and spatially distinct sites, and support previous data suggesting a role for the EGF domain of LAM-1 in lymphocyte adhesion to HEV.

An alternative leukocyte homotypic adhesion mechanism, LFA-1/ICAM-1-independent, triggered through the human VLA-4 integrin.

The VLA-4 (CD49d/CD29) integrin is the only member of the VLA family expressed by resting lymphoid cells that has been involved in cell-cell adhesive interactions. We here describe the triggering of homotypic cell aggregation of peripheral blood T lymphocytes and myelomonocytic cells by mAbs specific for certain epitopes of the human VLA alpha 4 subunit. This anti-VLA-4-induced cell adhesion is isotype and Fc independent. Similar to phorbol ester-induced homotypic adhesion, cell aggregation triggered through VLA-4 requires the presence of divalent cations, integrity of cytoskeleton and active metabolism. However, both adhesion phenomena differed at their kinetics and temperature requirements. Moreover, cell adhesion triggered through VLA-4 cannot be inhibited by cell preincubation with anti-LFA-1 alpha (CD11a), LFA-1 beta (CD18), or ICAM-1 (CD54) mAb as opposed to that mediated by phorbol esters, indicating that it is a LFA-1/ICAM-1 independent process. Antibodies specific for CD2 or LFA-3 (CD58) did not affect the VLA-4-mediated cell adhesion. The ability to inhibit this aggregation by other anti-VLA-4-specific antibodies recognizing epitopes on either the VLA alpha 4 (CD49d) or beta (CD29) chains suggests that VLA-4 is directly involved in the adhesion process. Furthermore, the simultaneous binding of a pair of aggregation-inducing mAbs specific for distinct antigenic sites on the alpha 4 chain resulted in the abrogation of cell aggregation. These results indicate that VLA-4-mediated aggregation may constitute a novel leukocyte adhesion pathway.

A peptide derived from the intercellular adhesion molecule-2 regulates the avidity of the leukocyte integrins CD11b/CD18 and CD11c/CD18.

beta 2 integrin (CD11a,b,c/CD18)-mediated cell adhesion is required for many leukocyte functions. Under normal circumstances, the integrins are nonadhesive, and become adhesive for their cell surface ligands, the intercellular adhesion molecules (ICAMs), or soluble ligands such as fibrinogen and iC3b, when leukocytes are activated. Recently, we defined a peptide derived from ICAM-2, which specifically binds to purified CD11a/CD18. Furthermore, this peptide strongly induces T cell aggregation mainly mediated by CD11a/CD18-ICAM-1 interaction, and natural killer cell cytotoxicity. In the present study, we show that the same ICAM-2 peptide also avidly binds to purified CD11b/CD18, but not to CD11c/CD18. This binding can be blocked by the CD11b antibody OKM10. The peptide strongly stimulates CD11b/CD18-ICAM-1-mediated cell aggregations of the monocytic cell lines THP-1 and U937. The aggregations are energy and divalent cation-dependent. The ICAM-2 peptide also induces CD11b/CD18 and CD11c/CD18-mediated binding of THP-1 cells to fibrinogen and iC3b coated on plastic. These findings indicate that in addition to induction of CD11a/CD18-mediated cell adhesion, the ICAM-2 peptide may also serve as a "trigger" for high avidity ligand binding of other beta 2 integrins.