Coordinate expression of beta 1 and beta 2 integrin "activation" epitopes during T cell responses in secondary lymphoid tissue

AJM300, a novel oral antagonist of α4-integrin, sustains an increase in circulating lymphocytes: A randomised controlled trial in healthy male subjects

Aims

AJM300 is an oral antagonist of α4‐integrin that reduces inflammation by blocking leucocyte trafficking. This study aimed to investigate safety, tolerability, pharmacokinetics and pharmacodynamics of AJM300 in healthy male subjects.

Methods

A total of 23 subjects were randomised to receive 240 mg (n = 6), 480 mg (n = 5), 960 mg (n = 6) of AJM300 or the corresponding placebo (n = 2 per group). The study drugs were taken orally 3 times daily after each meal on the first day followed by a 4‐day washout period. Thereafter, multiple‐dose administration was conducted for 6 consecutive days. The pharmacokinetic parameters of AJM300 and its active metabolite (HCA2969) were assessed, and total white blood cells and the differential cell count were used to determine the pharmacodynamic effects. Adverse events (AEs) were also monitored.

Results

The plasma AJM300 and HCA2969 concentration–time curves displayed a triphasic pattern on Day 1 (single‐day administration) and Day 10 (last day of multiple dosing), whereas the concentration of HCA2969 was much higher than that of AJM300. A significant but transient increase in lymphocyte count was observed after AJM300 dosing at all dosages tested compared with the placebo. The increase was sustained over a 24‐h period only at the 960‐mg dosage. In particular, a significant increase in the lymphocyte count compared to placebo (mean, 50.58%; 95% confidence intervals, 20.40–80.76) was observed at the first 960‐mg dose on Day 10. Six (26.1%) subjects reported ≥1 AEs, all of which were mild and resolved spontaneously.

Conclusion

The maximal and 24‐h sustained pharmacodynamic effects were demonstrated at the 960‐mg dosage after oral administration of AJM300 3 times daily for 6 days, which was also found to be safe and well tolerated.

LPS differentially regulates adhesion and transendothelial migration of human monocytes under static and flow conditions

One of the key components of the innate immune response is the recognition of microbial products such as LPS by Toll-like receptors on monocytes and neutrophils. We show here that short-term stimulation of primary human monocytes with LPS led to an increase in adhesion of monocytes to endothelial cells and a dramatic decrease in transendothelial migration under static conditions. In contrast, under normal physiological flow, monocyte adhesion and migration across a human umbilical vein endothelial cell monolayer appeared to be unaffected by LPS treatment. LPS stimulation of monocytes activated beta(1) and beta(2) integrins, but did not increase their surface expression levels. During septic shock, reduction in blood flow as a result of vasodilation and vascular permeability leads to adhesion and accumulation of LPS-stimulated circulating monocytes onto the blood vessel walls. The different findings of monocyte migration under static and flow conditions in our study may offer one explanation for this phenomenon. The rapid engagement of LPS-activated monocytes preventing transendothelial migration could represent a novel mechanism of bacterial exclusion from the vasculature. This occurs during the early stages of sepsis, and in turn may modulate the severity of the pathophysiology.

ERK-MAPK signaling coordinately regulates activity of Rac1 and RhoA for tumor cell motility

We describe two signaling events downstream of ERK-MAP kinase contributing to cell motility in colon carcinoma cells. The Fos family member Fra-1 is expressed in an ERK-dependent manner. Silencing of Fra-1 expression with short interfering RNAs leads to losses of cell polarization, motility, and invasiveness in vitro. These effects of ablating Fra-1 are a consequence of activation of a RhoA-ROCK pathway by beta1-integrin, leading to an increase in the amount of stress fibers and stabilization of focal adhesions. We propose that Fra-1 promotes cell motility by inactivating beta1-integrin and keeping RhoA activity low. This depression of RhoA activity is necessary to permit a second ERK-dependent signaling event via uPAR, the receptor for urokinase-type plasminogen activator, to activate Rac and to drive motility through polarized lamellipodia extension.

Blockade of alpha 5 beta 1 integrins reverses the inhibitory effect of tenascin on chemotaxis of human monocytes and polymorphonuclear leukocytes through three-dimensional gels of extracellular matrix proteins

Tenascin is an extracellular matrix protein found in adults in T cell-dependent areas of lymphoid tissues, sites of inflammation, and tumors. We report here that it inhibited chemotaxis of chemoattractant-stimulated human monocytes and chemoattractant-stimulated polymorphonuclear leukocytes (PMN) through three-dimensional gels composed of collagen I or Matrigel, and chemotaxis of leukotriene B4-stimulated PMN through fibrin gels. The inhibitory effect of tenascin on monocyte or PMN chemotaxis through these matrices was reversed by Abs directed against alpha5beta1 integrins or by a peptide (GRGDSP) that binds to beta1 integrins. Tenascin did not affect leukotriene B4- or fMLP-stimulated expression of beta1 or beta2 integrins, but did exert a small inhibitory effect on PMN adhesion and closeness of apposition to fibrin(ogen)-containing surfaces. Thus, alpha5beta1 integrins mediate the inhibitory effect of tenascin on monocyte and PMN chemotaxis, without promoting close apposition between these leukocytes and surfaces coated with tenascin alone or with tenascin bound to other matrix proteins. This contrasts with the role played by alpha5beta1 integrins in promoting close apposition between fMLP-stimulated PMN and fibrin containing surfaces, thereby inhibiting chemotaxis of fMLP-stimulated PMN through fibrin gels. Thus, chemoattractants and matrix proteins regulate chemotaxis of phagocytic leukocytes by at least two different mechanisms: one in which specific chemoattractants promote very tight adhesion of leukocytes to specific matrix proteins and another in which specific matrix proteins signal cessation of migration without markedly affecting strength of leukocyte adhesion.

Integrin-mediated mechanotransduction requires its dynamic interaction with specific extracellular matrix (ECM) ligands

The aim of this study is to elucidate the role of integrins in transducing fluid shear stress into intracellular signals in vascular endothelial cells, a fundamental process in vascular biology. We demonstrated that shear stress activates specific integrins in endothelial cells plated on substrates containing the cognate extracellular matrix (ECM) ligands. The shear stress-induced mechanotransduction, as manifested by integrin-Shc association, was abolished when new integrin-ECM ligand interactions were prevented by either blocking the integrin-binding sites of ECM ligands or conjugating the integrins to immobilized antibodies. Our results indicate that the dynamic formation of new connections between integrins and their specific ECM ligands is critical in relaying the signals induced by shear stress to intracellular pathways.

Integrin-mediated mechanotransduction requires its dynamic interaction with specific extracellular matrix (ECM) ligands

The aim of this study is to elucidate the role of integrins in transducing fluid shear stress into intracellular signals in vascular endothelial cells, a fundamental process in vascular biology. We demonstrated that shear stress activates specific integrins in endothelial cells plated on substrates containing the cognate extracellular matrix (ECM) ligands. The shear stress-induced mechanotransduction, as manifested by integrin-Shc association, was abolished when new integrin-ECM ligand interactions were prevented by either blocking the integrin-binding sites of ECM ligands or conjugating the integrins to immobilized antibodies. Our results indicate that the dynamic formation of new connections between integrins and their specific ECM ligands is critical in relaying the signals induced by shear stress to intracellular pathways.

Adhesion molecule expression in CD5-negative/CD10-negative chronic B-cell leukemias: comparison with non-Hodgkin's lymphomas and CD5-positive B-cell chronic lymphocytic leukemia

The classification of CD5-negative/CD10-negative chronic B-cell leukemias (CD5-/CD10- CBL) can be problematic. Most of these cases may represent leukemic non-Hodgkin's lymphoma (NHL) other than B-cell chronic lymphocytic leukemia (BCLL); nonetheless, some investigators still advocate the term "CD5-negative BCLL." Because adhesion molecule (AdMol) expression patterns reflect the biology of lymphoid neoplasms, we studied a series of 106 B-cell lymphoproliferative disorders, including CD5+ BCLL (n = 56), NHL other than BCLL (n = 35), and CD5-/CD10- CBL (excluding hairy cell leukemia and prolymphocytic leukemia) with no prior history of NHL (n = 15) for expression of components of the very late antigen-4 complex (alpha4/beta1 integrin (CD49d/CD29)), components of the mucosal addressin-cell adhesion molecule receptor (alpha4(CD49d)/beta7 integrin), and L-selectin (CD62L). CD62L expression was significantly greater in CD5+ BCLL than in NHL (P < .001). Conversely, CD29, CD49d, and beta7-integrin expression were significantly greater in NHL than in CD5+ BCLL (P < .001 for each marker). These differences persisted when only blood and bone marrow samples were analyzed, with the exception of differences in CD62L expression, which approached, but did not reach, statistical significance (P = .08). The group of CD5-/CD10- CBL displayed an AdMol profile similar to NHL and was significantly different than CD5+ BCLL in expression of beta7 integrin, CD29, CD49d, and CD62L (P range < .001-.011). In summary, CD5-/CD10- CBL display an AdMol profile resembling NHL and significantly different from CD5+ BCLL, supporting the growing notion that "CD5-negative BCLL" generally represents leukemic NHL rather than a variant of true CD5+ BCLL.

CCR3-active chemokines promote rapid detachment of eosinophils from VCAM-1 in vitro

Selective eosinophil recruitment is the result of orchestrated events involving cell adhesion molecules, chemokines, and their receptors. The mechanisms by which chemokines regulate eosinophil adhesion and migration via integrins are not fully understood. In our study, we examined the effect of CCR3-active chemokines on eosinophil adhesion to VCAM-1 and BSA under both static and flow conditions. When eotaxin-2 or other CCR3-active chemokines were added to adherent eosinophils, it induced rapid and sustained eosinophil detachment from VCAM-1 in a concentration-dependent manner. Adhesion was detectably reduced within 3 min and was further reduced at 10-60 min. Simultaneously, eotaxin-2 enhanced eosinophil adhesion to BSA. Preincubation of eosinophils with the CCR3-blocking mAb 7B11 completely prevented chemokine-induced changes in adhesion to VCAM-1 and BSA. Using a different protocol, pretreatment of eosinophils with chemokines for 0-30 min before their use in adhesion assays resulted in inhibition of VCAM-1 adhesion and enhancement of BSA adhesion. By flow cytometry, expression of alpha4 integrins and a beta1 integrin activation epitope on eosinophils was decreased by eotaxin-2. In a flow-based adhesion assay, eotaxin-2 reduced eosinophil accumulation and the strength of attachment to VCAM-1. These results show that eotaxin-2 rapidly reduced alpha4 integrin function while increasing beta2 integrin function. These findings suggest that chemokines facilitate migration of eosinophils by shifting usage away from beta1 integrins toward beta2 integrins.

Effects of I domain deletion on the function of the beta2 integrin lymphocyte function-associated antigen-1

A subset of integrin alpha subunits contain an I domain, which is important for ligand binding. We have deleted the I domain from the beta2 integrin lymphocyte function-asssociated antigen-1 (LFA-1) and expressed the resulting non-I domain-containing integrin (DeltaI-LFA-1) in an LFA-1-deficient T cell line. DeltaI-LFA-1 showed no recognition of LFA-1 ligands, confirming the essential role of the I domain in ligand binding. Except for I domain monoclonal antibodies (mAbs), DeltaI-LFA-1 was recognized by a panel of anti-LFA-1 mAbs similarly to wild-type LFA-1. However, DeltaI-LFA-1 had enhanced expression of seven mAb epitopes that are associated with beta2 integrin activation, suggesting that it exhibited an "active" conformation. In keeping with this characteristic, DeltaI-LFA-1 induced constitutive activation of alpha4beta1 and alpha5beta1, suggesting intracellular signaling to these integrins. This "cross-talk" was not due to an effect on beta1 integrin affinity. However, the enhanced activity was susceptible to inhibition by cytochalasin D, indicating a role for the cytoskeleton, and also correlated with clustering of beta1 integrins. Thus, removal of the I domain from LFA-1 created an integrin with the hallmarks of a constitutively active receptor mediating signals into the cell. These findings suggest a key role for the I domain in controlling integrin activity.

The Nonintegrin Laminin Binding Protein (p67 LBP) Is Expressed on a Subset of Activated Human T Lymphocytes and, Together with the Integrin Very Late Activation Antigen-6, Mediates Avid Cellular Adherence to Laminin

A search for genes expressed in activated T cells revealed that the nonintegrin, 67-kDa laminin binding protein (p67 LBP) is expressed on the surface of a subset (10–15%) of activated peripheral blood T cells. Surface p67 LBP expression is detectable by FACS using the anti-p67 LBP mAb, MLuC5, within 6 h of T cell activation with phorbol dibutyrate and ionomycin, peaks 18–36 h postactivation, and persists for 7–10 days. The subset of T cells expressing p67 LBP is composed of mature, single-positive cells (85% CD4+8−, 15% CD4−8+) of memory cell phenotype (100% CD45 RO+/CD45 RA−). The p67 LBP+ T cells also express the integrin α6 chain (CD49f), which is known to associate with p67 LBP on tumor cells. In addition, the p67 LBP+ T cells express the integrin β1, which associates with α6 in the laminin-specific integrin receptor very late activation Ag (VLA)-6 (α6β1). Expression of an exogenous cDNA encoding the 37-kDa LBP precursor (p37 LBPP) confers p67 LBP surface expression on a p67 LBP-negative Jurkat T cell line (B2.7). Expression of p67 LBP induces B2.7 transfectants to adhere to laminin, but avid laminin binding depends on coexpression of VLA-6. Taken together, these data indicate that p67 LBP is an activation-induced surface structure on memory T cells that, together with VLA-6, mediates cellular adherence to laminin.

Analysis of cultured keratinocytes from a transgenic mouse model of psoriasis: effects of suprabasal integrin expression on keratinocyte adhesion, proliferation and terminal differentiation

Many important transgenic mouse models of benign and neoplastic skin diseases have been generated through the use of promoters that target transgene expression to the different epidermal layers. However, more mechanistic studies of the specific effects of the transgenes on keratinocytes have been hampered by difficulties in culturing keratinocytes from adult mouse epidermis and by the low differentiation potential of many established mouse keratinocyte lines. We have used the Rheinwald & Green technique to cultivate primary adult keratinocytes and to generate keratinocyte lines from transgenic mice which have a sporadic psoriatic phenotype due to expression of human integrin subunits under the control of the involucrin promoter. We show that the transgenes are induced when keratinocytes are placed in suspension and that the transgenic integrins are capable of clustering in focal adhesions and mediating cell adhesion and spreading. We also show that suprabasal integrin expression has no direct effect on proliferation of cells in the underlying basal layer, ruling this out as a possible explanation for the epidermal hyperproliferation observed in the transgenic mice.

Sequence Motifs in the Integrin α4 Cytoplasmic Tail Required for Regulation of In Vivo Expansion of Murine Lymphoma Cells

The binding of integrins to cognate ligands is tightly controlled by intracellular signals. Conversely, integrin occupancy generates biochemical signals inside the cell. The present study examined whether concepts of integrin function established by in vitro analysis apply to regulation of receptor function in complex biologic settings in vivo using a mouse model of tumor metastasis. Integrin α4 subunits were truncated at amino acid Gln1014 (A4-1014), preserving the conserved GFFKR motif, and at position Glu1021 (A4-1021). In vitro adhesion assays revealed that cytoplasmic tail truncations did not affect constitutive ligand binding of α4 integrins, while agonist-induced adhesion was abolished by the A4-1014, but not by the A4-1021, mutation. Inducible ligand binding of α4 integrins was dependent on cytoskeletal function, whereas constitutive adhesion was not. In vivo metastasis formation assays demonstrated that expansion of murine T lymphoma cells in spleen is strongly inhibited by the wild-type α4 subunit and the A4-1021 mutant. In contrast, the in vivo phenotype of α4 integrin expression in lymphoma cells was completely abrogated by the A4-1014 mutation. Cross-linking of α4 integrins in vitro inhibited proliferation and induced apoptosis of LB cells expressing wild-type α4 subunits or the A4-1021 mutant, but not of LB-A4-1014 cells. In summary, these results demonstrate that sequence motifs regulating cytoskeleton-dependent α4 integrin activation in vitro are essential for the control of LB lymphoma cell expansion both in vitro and in vivo.

Lack of intrinsic polarity in the ligand-binding ability of keratinocyte beta1 integrins

Within the basal layer of the epidermis the beta1 integrins have a pericellular distribution. Two monoclonal antibodies, 15/7 and 12G10, that detect a conformation of the beta1 integrin subunit that is induced following cation or ligand occupancy selectively recognized beta1 integrins at the basement membrane zone in vivo and in focal adhesions of cultured keratinocytes; they did not recognize integrins on the apical and upper lateral membranes of basal keratinocytes nor integrins on the suprabasal keratinocytes of hyperproliferative epidermis. Inhibition of intercellular adhesion did not induce the 15/7 epitope on the lateral and apical membrane domains. The surface distribution of the epitopes was consistent with the antibodies acting as reporters of ligand-binding; in addition, the 15/7 epitope was exposed on unglycosylated, immature beta1 integrins. Although the apical membrane of basal keratinocytes is not normally in contact with extracellular matrix proteins, we found that it was capable of binding fibronectin-coated beads and that the 15/7 epitope was exposed on plasma membrane in contact with the beads. When a chimeric molecule consisting of the extracellular domain of CD8 and the cytoplasmic domain of the beta1 integrin subunit, used to mimic a constitutively active beta1 heterodimer, was introduced into keratinocytes it localized to the basal, lateral and apical membrane domains. We conclude that although the conformation of the keratinocyte beta1 integrins differs between the basal and the lateral/apical membrane domains there is no intrinsic polarity in the ligand binding potential of the receptors.

CD4+ T lymphocytes migrating in three-dimensional collagen lattices lack focal adhesions and utilize beta1 integrin-independent strategies for polarization, interaction with collagen fibers and locomotion

Cell migration may depend on integrin-mediated adhesion to and deadhesion from extracellular matrix ligands. This concept, however, has not yet been confirmed for T lymphocytes migrating in three-dimensional extracellular matrices. We investigated receptor involvement in T cell migration combining a three-dimensional collagen matrix model with time-lapse videomicroscopy, computer-assisted cell tracking and confocal microscopy. In collagen lattices, the migration of CD4+ T cells (1) involved interactions with collagen fibers at the leading edge and uropod likewise, (2) occurred independently of the co-clustering of beta1, beta2, or beta3 integrins with F-actin, focal adhesion kinase, and phosphotyrosine at interactions with collagen fibers, (3) was counteracted by high-affinity beta1 integrin binding induced by antibody TS2/16; however, (4) the migration could not be blocked by a combination of adhesion-perturbing anti-beta1, -beta2, -beta3, and alpha v integrin antibodies. Integrin blocking neither affected cell polarization, interaction with fibers, beta1 integrin distribution, migration velocity, path structure, nor the number of locomoting cells in spontaneously migrating or concanavalin A-activated cells. Hence, T lymphocytes migrating in three-dimensional collagen matrices may utilize highly transient interactions with collagen fibers of low adhesivity, thereby differing from focal adhesion-dependent migration strategies employed by other cells.

Pentoxifylline Inhibits Adhesion and Activation of Human T Lymphocytes

We have herein studied the effect of pentoxifylline (PTX) on the adhesion and activation of human T lymphocytes. We found that PTX inhibited the adhesion of T cells to the β1 and β2 integrin ligands VCAM-1 and ICAM-1; this inhibitory activity was dose dependent, with a maximal effect from 12 to 24 h. We also found that PTX was able to interfere with the activation of β1 integrins induced by intracellular signals; however, the conformational change of β1 integrins induced by extracellular stimuli (e.g., activating mAbs, or Mn2+) was not significantly affected by this drug. In addition, the homotypic aggregation of T cells induced by anti-β1 and -β2 integrin chain mAbs was also inhibited by PTX. PTX had a significant inhibitory effect on the T lymphocyte expression of the activation Ags CD25 (IL-2Rα-chain), CD69 (activation-inducer molecule), and CD98 (4F2) induced by PHA. Accordingly, PTX also interfered with early cell activation events such as the rise in intracellular Ca2+ and the activation of the Na+/H+ antiporter induced by PHA and phorbol esters, respectively. Furthermore, this drug inhibited both the cell cycle progression and cell proliferation of T cells induced through the CD3/TCR complex. However, this drug did not show any effect on the cell activation/proliferation induced by PMA plus ionomycin. Our results indicate that PTX interferes efficiently with the activation and cell adhesion of human T lymphocytes. These effects may be of relevance for the clinical uses of this drug.

Divalent cations and ligands induce conformational changes that are highly divergent among beta1 integrins

Here we show striking differences in conformational regulation among beta1 integrins. Upon manganese stimulation, a beta1 epitope defined by monoclonal antibody (mAb) 9EG7 was induced strongly (on alpha4beta1), moderately (on alpha5beta1), weakly (on alpha2beta1), or was scarcely detectable (on alpha6beta1 and alpha3beta1). Comparable results were seen for the beta1 epitope defined by mAb 15/7. Likewise, soluble ligands caused strong (alpha4beta1), moderate (alpha5beta1), weak (alpha2beta1, alpha6beta1), or minimal (alpha3beta1) induction of the 9EG7 epitope. Exchange or deletion of alpha chain cytoplasmic tails did not alter Mn2+-induced 9EG7 epitope levels. Upon removal of calcium by EGTA or EDTA, the hierarchy of 9EG7 epitope induction was similar (alpha5beta1 > alpha2beta1 > alpha6beta1 > alpha3beta1), except that EGTA reduced rather than induced 9EG7 expression on alpha4beta1. Thus in contrast to other beta1 integrins, calcium uniquely supports constitutive expression of the 9EG7 epitope on alpha4beta1. Likewise, calcium supported vascular cell adhesion molecule-stimulated 9EG7 appearance on alpha4beta1, whereas calcium inhibited ligand-induced 9EG7 epitope on other integrins. Constitutive expression of 9EG7 on alpha4beta1 was eliminated by a D698E mutation in alpha4, suggesting that Asp-698 may play a key role in maintaining atypical alpha4beta1 response to calcium. In conclusion, our results (i) demonstrate that mAb such as 9EG7 and 15/7 have limited diagnostic utility as reporters of ligand or Mn2+ occupancy for beta1 integrins, (ii) indicate pronounced differences in conformational flexibilities (alpha4beta1 > alpha5beta1 > alpha2beta1 > alpha6beta1 > alpha3beta1), (iii) allow us to hypothesize that beta1 integrins may differ markedly in conformation-dependent inside-out signaling, and (iv) have uncovered an atypical alpha4beta1 response to calcium that requires alpha4 Asp-698.

In vitro transendothelial migration of blood T lymphocytes from HIV-infected individuals

OBJECTIVE

We hypothesized that differential extravasation of circulating CD4+ or CD8+ T lymphocytes contributes to HIV-associated CD8+ lymphocytic alveolitis. Differences in T-cell transendothelial migration may be intrinsic or emerge at sites where vascular endothelium is activated by overexpression of tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma.

DESIGN

We used an in vitro model of lymphocyte extravasation to assess transendothelial migration of peripheral blood mononuclear cells (PBMC) from HIV-positive individuals. We assayed bronchoalveolar lavage (BAL) fluid from HIV-positive and normal individuals to determine if increased levels of TNF-alpha and IFN-gamma were present in the lungs of HIV-infected individuals.

METHODS

Transendothelial migration was assessed by determining the number and flow cytometric phenotype of PBMC adherent to or migrating across unstimulated or TNF-alpha and IFN-gamma-activated endothelial cell monolayers. We measured BAL fluid cytokine concentrations using standard antigen-capture enzyme-linked immunosorbent assays for TNF-alpha and IFN-gamma.

RESULTS

T cells migrating across unactivated endothelial cells were significantly enriched for CD4+ T cells. Cytokine activation of endothelial cells allowed significantly greater transendothelial migration of CD8+ T cells compared to unactivated endothelial cells. TNF-alpha was increased in BAL fluid from HIV-positive individuals relative to controls.

CONCLUSIONS

These data suggest that, in HIV-positive individuals, CD4+ T cells are migration competent and blood CD8+ T cells do not have enhanced migration competence relative to CD4+ T cells. CD8+ T cell extravasation is aided by TNF-alpha and IFN-gamma-induced endothelial cells activation.

The laminin-binding activity of the alpha 7 integrin receptor is defined by developmentally regulated splicing in the extracellular domain

The expression pattern of the laminin-binding alpha 7 beta 1 integrin is developmentally regulated in skeletal, cardiac, and smooth muscle. The X1/X2 alternative splicing in the extracellular domain of alpha 7 is found in the variable region between conserved alpha-chain homology repeat domains III and IV, a site implicated in ligand binding. To assess differences in X1/X2 isoform activity, we generated MCF-7 cell lines transfected with alpha 7-X1/X2 cDNAs. Transfectants expressing the alpha 7-X2 variant adhered rapidly to laminin 1, whereas those expressing alpha 7-X1 failed to attach. That alpha 7-X1 exists in an inactive state was established in assays using an activating beta 1 antibody that induced X1-dependent cell adhesion and spreading. Furthermore, the activation of alpha 7-X1 was cell type specific, and when expressed in HT1080 cells, the integrin was converted into a fully functional receptor capable of promoting adhesion. Thus, the expression of the alpha 7-X1/X2 integrin is a novel mechanism that regulates receptor affinity states in a cell-specific context and may modulate integrin-dependent events during muscle development and repair.

Regulation of alpha 4 integrin-mediated adhesion of human eosinophils to fibronectin and vascular cell adhesion molecule-1

BACKGROUND

Eosinophils selectively accumulate at sites of allergic inflammation. Their recruitment is dependent on both the expression and functional activity of cell adhesion molecules. How the functional activity of cell adhesion molecules on eosinophils is regulated is poorly understood.

OBJECTIVE

Our objective was to examine the functional activity of alpha 4 integrins on human eosinophils and its regulation by various agents.

METHODS

Function of alpha 4 integrins on human eosinophils was examined by testing adhesion to immobilized fibronection and vascular cell adhesion molecule-1 (VCAM-1) in the presence or absence of a monoclonal antibody (mAb) (8A2) that activates beta 1 integrin function.

RESULTS

Spontaneous eosinophil adhesion to VCAM-1 was enhanced by 8A2, but adhesion to fibronectin could only be detected in the presence of 8A2. Concentrations of 8A2 that were approximately 100-fold less than saturating induced maximal eosinophil adhesion. Adhesion to VCAM-1 in the presence of 8A2 was effectively inhibited by alpha 4 and beta 1 integrin mAbs: beta 7 mAb had partial inhibitory activity. Connecting segment-1 peptide and alpha 4 mAb blocked 8A2-dependent fibronectin binding: beta 1, beta 2, and beta 7 integrin mAbs had partial inhibitory activity. Eosinophils obtained from bronchoalveolar lavage fluids and blood eosinophils stimulated with IL-5, platelet-activating factor, or RANTES displayed increased beta 2 integrin-dependent, not alpha 4 integrin-dependent, attachment. Spontaneous adhesion of eosinophils to VCAM-1 was significantly reduced by the tyrosine kinase inhibitor tyrphostin B46 (inhibitory concentration of 50% approximately equal to 20 mumol/L); this effect was reversed by 8A2.

CONCLUSIONS

The functional activity of integrins on eosinophils can be positively and negatively regulated. Altered integrin avidity may influence eosinophil recruitment in vivo.

Prevention of cytokine-induced changes in leukocyte adhesion receptors by nonsteroidal antiinflammatory drugs from the oxicam family

OBJECTIVE

To explore the effect of the nonsteroidal antiinflammatory drugs (NSAIDs) piroxicam and meloxicam on quantitative and qualitative changes in leukocyte adhesion receptors induced by cytokines and other activation stimuli.

METHODS

The expression of CD11b and L-selectin during neutrophil activation with tumor necrosis factor alpha (TNF alpha), granulocyte-macrophage colony-stimulating factor (GM-CSF), FMLP, phorbol myristate acetate (PMA), and calcium ionophore A23187 was assessed by flow cytometry. Enzyme-linked immunosorbent assays were used to quantitate soluble L-selectin shed after neutrophil stimulation. Enzyme release was measured to determine neutrophil degranulation by proinflammatory stimuli. Changes in affinity state of beta 1 and beta 2 integrins after neutrophil and T lymphocyte stimulation were assessed, by flow cytometry, using the monoclonal antibodies (MAb) HUTS-21 (anti-beta 1) and CBRM1/5 (anti-CD11b), which recognize activation-dependent epitopes on these two integrins.

RESULTS

Pretreatment of neutrophils with either NSAID prevented the changes in L-selectin and CD11b expression induced by TNF alpha, GM-CSF, and FMLP, but not those induced by PMA or A23187. Furthermore, piroxicam significantly decreased the amount of L-selectin shed by cytokine-treated neutrophils, whereas it did not exert this effect on PMA- or A23187-treated neutrophils. Piroxicam also decreased the release of gelatinase and lysozyme induced by TNF alpha, but not by PMA. Interestingly, piroxicam prevented the conformational changes that beta 2 integrins underwent upon activation of neutrophils: the appearance of the activation epitope of CD11b, detected by the CBRM1/5 MAb, was blocked by piroxicam in TNF alpha-treated neutrophils. Moreover, in chemokine-treated T lymphocytes, the expression of activation epitopes on beta 1 integrins was also diminished by piroxicam. In contrast, this NSAID did not affect the beta 1 integrin conformational changes induced by PMA or Mn++.

CONCLUSION

Our results indicate that members of the oxicam family are able to interfere with events of neutrophil function, such as their degranulation and cytokine-mediated activation changes in adhesion molecules, both in neutrophils and in lymphocytes. Such effects may significantly contribute to the antiinflammatory activity of these drugs.

Activated conformations of very late activation integrins detected by a group of antibodies (HUTS) specific for a novel regulatory region (355-425) of the common beta 1 chain

The very late activation antigens (VLA) or beta 1 integrins mediate cell attachment to different extracellular matrix proteins and intercellular adhesions. The ligand binding activity of these adhesion receptors is not constitutive and can be regulated by temperature, presence of extracellular divalent cations, stimulatory monoclonal antibodies (mAbs), and cellular activation. We have generated three novel mAbs, HUTS-4, HUTS-7, and HUTS-21, recognizing specific epitopes on the common beta 1 subunit (CD29) of VLA integrins whose expression correlates with the ligand binding activity of these heterodimeric glycoproteins. This correlation has been demonstrated for several integrin heterodimers in different cell systems using a variety of extracellular and intracellular stimuli for integrin activation. Thus, the presence of micromolar concentrations of extracellular Mn2+, preincubation with the activating anti-beta 1 mAb TS2/16, and cell treatment with phorbol esters or calcium ionophores, induced the expression of the HUTS beta 1 epitopes on T lymphoblasts. Using a panel of human-mouse beta 1 chimeric molecules, we have mapped these epitopes to the 355-425 sequence of the beta 1 polypeptide. This segment represents therefore a novel regulatory region of beta 1 that is exposed upon integrin activation. Interestingly, binding of HUTS mAbs to partially activated VLA integrins results in maximal activation of these adhesion receptors and enhancement of cell adhesion to beta 1 integrin ligands collagen, laminin, and fibronectin.

Lymphocyte homing and homeostasis

The integration and control of systemic immune responses depends on the regulated trafficking of lymphocytes. This lymphocyte "homing" process disperses the immunologic repertoire, directs lymphocyte subsets to the specialized microenvironments that control their differentiation and regulate their survival, and targets immune effector cells to sites of antigenic or microbial invasion. Recent advances reveal that the exquisite specificity of lymphocyte homing is determined by combinatorial "decision processes" involving multistep sequential engagement of adhesion and signaling receptors. These homing-related interactions are seamlessly integrated into the overall interaction of the lymphocyte with its environment and participate directly in the control of lymphocyte function, life-span, and population dynamics. In this article a review of the molecular basis of lymphocyte homing is presented, and mechanisms by which homing physiology regulated the homeostasis of immunologic resources are proposed.

Regulation of beta1-integrin function in cultured human vascular smooth muscle cells

Avidity modulation and function of beta1-integrin receptors in cultured human vascular smooth muscle cells (SMCs) were investigated using monoclonal antibody (mAb) 8A2, which binds to the beta1 subunit of integrin heterodimers and induces a high avidity state. The adhesion of SMCs to extracellular matrix proteins, but not to poly-L-lysine, was enhanced by pretreatment with mAb 8A2. A qualitative alteration of beta1 integrin was assessed with mAb 15/7, which binds to an activation-dependent epitope on the beta1 subunit. Binding of mAb 15/7 was enhanced by mAb 8A2 in a dose-dependent manner. Arg-Gly-Asp peptide and soluble fibronectin also enhanced expression of the 15/7 epitope, suggesting that the 15/7 epitope is closely related to the ligand-occupied state of beta1 integrin. Platelet-derived growth factor (PDGF)-AA and -BB increased SMC adhesion to type I collagen but did not augment mAb 15/7 binding, suggesting that PDGFs increase binding avidity by a postreceptor mechanism. In addition, mAb 8A2 inhibited PDGF-BB-induced SMC migration through Matrigel-coated filters. These results suggest that avidity modulation of beta1 integrin may play an important role in the function of SMCs.

Monoclonal antibody 9EG7 defines a novel beta 1 integrin epitope induced by soluble ligand and manganese, but inhibited by calcium

The monoclonal antibody 9EG7 has been previously found to recognize an epitope induced by manganese on the integrin beta 1 chain (Lenter, M., Uhlig, H., Hamann, A., Jeno, P., Imhof, B., and Vestweber, D. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 9051-9055). Here we show that treatment of beta 1 integrins with manganese or soluble integrin ligands (e.g. fibronectin and RGD peptide) induced the 9EG7 epitope. This epitope was also induced upon EGTA treatment to remove calcium, and the addition of calcium inhibited 9EG7 epitope induction by manganese or by ligand. Further emphasizing the importance of the 9EG7 epitope, the 9EG7 antibody itself stimulated adhesion mediated by multiple beta 1 integrins, and conversely, ligands for alpha 2 beta 1, alpha 3 beta 1, alpha 4 beta 1, and alpha 5 beta 1 all stimulated 9EG7 expression. Together these results support a model whereby (i) calcium inhibits beta 1 integrin function because it prevents the appearance of a conformation favorable to ligand binding and (ii) manganese enhances beta 1 integrin function because it induces the same favorable conformation that is induced by adding ligand, or removing calcium. Notably, other beta 1-stimulating agents (magnesium and mAb TS2/16) did not induce 9EG7 expression unless ligand was also present. Thus, although 9EG7 may reliable detect the ligand-bound conformation of beta 1 integrins, its expression does not always correlate with integrin "activation". Finally, mouse/chicken beta 1 chimeric molecules were used to map the 9EG7 epitope to beta 1 residues 495-602 within the cysteine-rich region, and antibody cross-blocking studies showed that the 9EG7 epitope is distinct from all previously defined human beta 1 epitopes.

Expression and functional significance of an activation-dependent epitope of the beta 1 integrins in chronic inflammatory diseases

The avidity of VLA integrins for their ligands can be increased by their transition to an active conformational state. This conformational change can be detected with a novel monoclonal antibody (mAb), termed 15/7, that recognizes an activation-dependent conformational epitope on the common beta 1 polypeptide of different VLA alpha beta 1 integrins. In an attempt to understand the possible role of the active conformational state of beta 1 integrins in vivo, we first investigated the expression of 15/7 epitope on T lymphocytes from patients with chronic inflammatory joint diseases. An enhanced expression of the 15/7 epitope was found in the synovial fluid (SF) T lymphocytes from these patients as compared to their peripheral blood (PB) T cells. The effect of different cytokines on the appearance of the 15/7 activation epitope in PB T lymphocytes was subsequently analyzed; interferon-gamma, interleukin-2 and, to a lower extent, tumor necrosis factor-alpha were able to induce an increased expression of the 15/7 epitope. This enhanced 15/7 expression correlated with a higher binding ability to fibronectin of cytokine-activated T cells. The presence of this activation epitope was detected in a small proportion of T lymphocytes scattered within inflammatory foci of synovial membrane from rheumatoid arthritis and thyroid glands from Hashimoto's chronic thyroiditis. We then analyzed the possible role of 15/7 epitope expression on cell adhesion in vitro. Immunofluorescence studies showed that the 15/7 epitope displayed a spot-like distribution, selectively decorating adhesive contacts of U-937 myelomonocytic cells attached to the 80 kDa proteolytic fragment of fibronectin (FN80). Furthermore, the anti-beta 1 15/7 mAb was able to induce both T lymphocyte, Jurkat and U-937 cellular binding and spreading on FN80. Altogether these results indicate that an activated conformation of beta 1 integrins is detected in vivo in lymphocyte infiltrates from chronic inflammatory conditions. The active conformations of beta 1 integrins are regulated by physiologic mediators such as cytokines, play an important role in cellular attachment and spreading, and appear to be involved in the development of inflammatory processes.

An enzyme-linked immunosorbent assay to identify inhibitors of activation of platelet integrin alpha IIb beta 3

The affinity of integrin alpha IIb beta 3 for adhesive ligands is tightly regulated by the platelet such that fibrinogen binding is observed only after platelet activation. Ligand binding is necessary for platelet aggregation, which contributes to vascular occlusion in pathological states. Therefore, we have developed an ELISA assay to screen for compounds that inhibit alpha IIb beta 3 activation. Washed platelets were incubated in microtitre wells with potential inhibitory compounds and stimulated with an agonist to activate alpha IIb beta 3. After the addition of biotin-PAC1, a fibrinogen-mimetic monoclonal antibody, the activation state of alpha IIb beta 3 was measured by sedimenting the platelets and quantitating the residual biotin-PAC1 in the cell-free supernatant in a streptavidin-based ELISA. This assay detected (1) specific PAC1 binding to activated platelets in response to a variety of agonists, and (2) dose-dependent inhibition of PAC1 binding by function-blocking anti-alpha IIb beta 3 monoclonal antibodies, by the tetrapeptide, RGDS, and by an alpha IIb beta 3-selective RGD peptidomimetic. Furthermore, the assay detected inhibition of PAC1 binding by intracellular inhibitors of platelet activation, including bisindolylmaleimide, a selective protein kinase C antagonist, and wortmannin, an inhibitor of phosphatidylinositol 3-kinase. These studies demonstrate that this integrin activation ELISA can detect pharmacological blockade of platelet alpha IIb beta 3 by extracellular and intracellular inhibitors. Its use may facilitate the search for clinically useful anti-platelet drugs.

Recirculation, phenotype and functions of lymphocytes in mice treated with monoclonal antibody MEL-14

The effect of a single injection of an antibody against the peripheral lymph node (PLN) homing receptor or L-selectin (gp90MEL-14) was studied in vivo in C57BL/6 mice. L-selectin is known to be rapidly shed from leukocytes in humans and in mice following activation or cross-linking in vitro. Here we demonstrate that in vivo a single injection of MEL-14 antibody induces a rapid, almost complete and reversible down-regulation of L-selectin expression on both T and B cells. This modulation is dose dependent, specific for L-selectin and lasts for 10 days. On neutrophils, L-selectin expression was moderately decreased, and the injected antibody was detectable on the cell surface for several days. Thus, L-selectin expression after antibody binding in vivo was affected differently on neutrophils and lymphocytes. MEL-14 treatment induces profound alterations of cell traffic. Loss of L-selectin on lymphocytes leads to drastic PLN depletion and increased spleen cellularity. Depleted PLN were highly enriched in MEL-14-/lo, CD44hi and CD11ahi cells, which may represent transiently sessile memory/activated cells. The unresponsiveness in mixed lymphocyte reaction of PLN cells from treated animals and of purified L-selectin- PLN T cells from normal mice supports this view. However, PLN and spleen cells from treated animals responded normally to non-antigen-specific stimuli.

Adhesion receptors in lymphocyte activation

The past several years have seen significant progress in understanding the role of T lymphocyte coreceptors in adhesion and activation. New insights have been gained in several areas: the avidity regulation of beta 1 and beta 2 integrins and their role in signal transduction; the regulation of CD8 avidity; the role of Lck in CD4 coreceptor activity; and the novel role for CD2 adhesion in the T cell antigen response.

Control of lymphocyte homing

The expression of immunity, both protective and pathologic, is critically dependent on the appropriate distribution of 'lymphoid resources' among the tissues of the body. The 'homing' mechanisms mediating this distribution have proven to have an astounding plasticity--directing, under strict microenvironmental control, the selective recruitment of specific lymphocyte subsets to the various secondary lymphoid tissues and extralymphoid immune effector sites. The past year has seen significant progress in our understanding in three areas: the molecular basis of lymphocyte interactions with endothelium, providing new insight into the complex multistep process of lymphocyte extravasation; the role of extravascular matrix and cells in retaining lymphocytes within tissues; and the mechanisms by which local microenvironments differentially regulate adhesion molecule expression and function so as to provide for site-selective lymphocyte homing.