Molecular identification of the cross-reacting epitope on alphaM beta2 integrin I domain recognized by anti-alphaIIb beta3 monoclonal antibody 7E3 and its involvement in leukocyte adherence

Immunogenicity evaluation of viral peptides via nonspecific interactions between anti-peptide IgYs and non-cognate peptides

Immunogenicity can be evaluated by detecting antibodies (Abs) induced by an antigen. Presently deployed assays, however, do not consider the negative impacts of Ab poly-specificity, which is well established at the monoclonal antibody level. Here, we studied antibody poly-specificity at the serum level (i.e. nonspecific Ab-probe interactions, NSIs), and ended up establishing a new platform for viral peptide immunogenicity evaluation. We first selected three peptides of high, medium and low immunogenicity, using a 'vaccine serum response rate'-based approach (i.e. the gold standard). These three peptides (Pi) in the bovine serum albumin-Pi form were used to immunize chickens, resulting in longitudinal serum samples for screening with a non-cognate peptide library. The signal intensity of Ab-peptide specific binding and 'NSI count' was used to evaluate the viral peptides' immunogenicity. Only the NSI count agreed with the gold standard. The NSI count also provides more informative data on antibody production than the aggregated signal intensity by whole-protein-based indirect enzyme-linked immunosorbent assay.

Intracellular communication and immunothrombosis in sepsis

Inflammation and coagulation are the critical responses to infection that include leukocytes, platelets, and vascular endothelial cells responding in concert to eradicate the invading pathogen. In sepsis, a variety of cell surface receptors, including toll-like receptors, Fcγ-receptors, G-protein-coupled receptors, and adhesion receptors, detect the pathogens and elicit thromboinflammatory responses. Concurrently, the molecular patterns released from host damaged cells accelerate the immune responses through binding to the same pattern recognition receptors. Cytokines, chemokines, and extracellular vesicles are important mediators for amplifying the responses to distant cells as part of the systemic response to infections. At the same time, cells communicate with each other via direct contact, adhesion molecules, paracrine mediators, and tunneling nanotubes, which are important for regulating inflammation and thrombus formation. Despite increasing attention to immunothrombosis in sepsis, these close communication systems are less understood but play a critical role in host defense mechanisms. In this review, cellular activation and direct intercellular communication systems in sepsis with a focus on the coagulation response will be considered.

The Promiscuous Profile of Complement Receptor 3 in Ligand Binding, Immune Modulation, and Pathophysiology

The β2-integrin receptor family has a broad spectrum of physiological functions ranging from leukocyte adhesion, cell migration, activation, and communication to the phagocytic uptake of cells and particles. Among the members of this family, complement receptor 3 (CR3; CD11b/CD18, Mac-1, αMβ2) is particularly promiscuous in its functional profile and ligand selectivity. There are close to 100 reported structurally unrelated ligands for CR3, and while many ligands appear to cluster at the αMI domain, molecular details about binding modes remain largely elusive. The versatility of CR3 is reflected in its functional portfolio, which includes prominent roles in the removal of invaders and cell debris, induction of tolerance and synaptic pruning, and involvement in the pathogenesis of numerous autoimmune and chronic inflammatory pathologies. While CR3 is an interesting therapeutic target for immune modulation due to these known pathophysiological associations, drug development efforts are limited by concerns of potential interference with host defense functions and, most importantly, an insufficient molecular understanding of the interplay between ligand binding and functional impact. Here, we provide a systematic summary of the various interaction partners of CR3 with a focus on binding mechanisms and functional implications. We also discuss the roles of CR3 as an immune receptor in health and disease, as an activation marker in research and diagnostics, and as a therapeutic target.

Novel tools for functional analysis of CD11c: activation-specific, activation-independent, and activating antibodies

Functions and binding properties of four CD11c-specific mAbs are described here. The mAb 496B stimulated, while 496K inhibited ligand binding of CD11c. The stimulatory mAb, 496B, as well as the inhibitory mAbs BU15 and 496 K appear to act allosterically, as they do not bind the CD11c I domain. The mAb 3.9 bound preferentially to activated forms of CD11c and the binding was divalent cation dependent. CD11c binding to 3.9 recapitulates many of the integrin-ligand interactions. Our data suggest that 3.9 is a competitive antagonist, BU15 and 496K are allosteric antagonists, and 496B is an allosteric agonist of CD11c. These mAbs provide a set of tools to study the functions of the dendritic cell marker, CD11c.

Inflammation as a mechanism and therapeutic target for in-stent restenosis

Restenosis following coronary stenting has long been attributed to neointimal proliferation, thrombosis, and negative remodeling. More recently, the important role of inflammation in vascular healing has also been increasingly well understood. From animal models and from clinical experience, we know that endothelial injury, platelet and leukocyte interactions, and subcellular chemoattractant and inflammatory mediators are pivotal in the development of the inflammatory response following stent implantation. By examining the specific mechanisms governing the inflammatory response to percutaneous coronary intervention, we may gain insight into potential therapeutic targets and strategies to prevent restenosis in clinical practice.

Material-induced tissue factor expression but not CD11b upregulation depends on the presence of platelets

Biomaterials activate leukocytes as well as platelets when exposed to blood. One feature of leukocyte activation at least at times beyond a few hours is tissue factor expression, contributing to a procoagulant state. We show here that platelet activation and specifically platelet-monocyte aggregate formation appears to be a precondition for tissue factor expression. Material-induced Tissue Factor (TF) expression by isolated leukocytes (6 x 10(6) cells/mL) resuspended in increasing concentrations of platelets in plasma was elevated when the platelet concentration was 50 x 10(6) platelets/mL or more; at lower platelet concentrations (1-25 x 10(6). cells/mL) the TF expression remained at background levels. On the other hand, significant CD11b upregulation was observed on leukocytes, in bulk and adherent to beads, at all platelet concentrations. This platelet effect on material-induced TF expression appeared to be mediated by the formation of platelet-monocyte aggregates. Anti-P-selectin, which blocked the association between platelets and leukocytes, reduced monocyte adhesion and material-induced TF expression for bulk monocytes. Anti-GPIIb/IIIa, a GPIIb/IIIa platelet antagonist, also reduced monocyte adhesion and material-induced TF expression in the bulk, most likely due to its inhibiting effect on the formation of platelet-monocyte aggregates, secondary to platelet activation. However, the antibody-associated reductions for bulk leukocytes (mainly neutrophils) were small and incomplete. Similar levels of TF expression, in the bulk, were observed with both polystyrene (PS), a strong platelet activator, and polyethylene glycol-modified PEG (PS-PEG), a mild platelet activator. The role of platelets in material-induced TF expression appears to be mediated in part via the formation of platelet-monocyte aggregates, although other mechanisms are likely also involved.

Platelet–monocyte complex formation: effect of blocking PSGL-1 alone, and in combination with αIIbβ3 and αMβ2, in coronary stenting

BACKGROUND

Binding of platelet P-selectin to P-selectin glycoprotein ligand 1 (PSGL-1) is an initial event in the interactions between platelets and monocytes. Platelet-monocyte complexes (PMCs) have been implicated in several vascular disease processes, including acute coronary syndromes (ACS) and complications after percutaneous coronary intervention (PCI). We investigated the effect of ex vivo blockade of PSGL-1, alone and in combination with blockade of the alphaMbeta(2) (Mac-1) and alpha(IIb)beta(3) (GP IIb/IIIa) integrins, on PMC formation.

METHODS AND RESULTS

Dual-label flow cytometry was used to detect PMCs in the blood of 10 volunteers and 10 patients undergoing PCI who received intravenous GP IIb/IIIa antagonists. PSGL-1 blockade, both prior to and after platelet stimulation, markedly reduced the formation of PMCs. Concomitant ex vivo blockade of the alphaMbeta(2) and alpha(IIb)beta(3) integrins did not result in further decreases of PMCs compared to PSGL-1 blockade alone. Antagonism of PSGL-1 also led to near elimination of leukocyte-platelet interactions under flowing conditions.

CONCLUSION

Blockade of PSGL-1 alone is sufficient to inhibit and reverse the formation of PMCs following platelet stimulation. Concurrent antagonism of PSGL-1 and the alpha(IIb)beta(3) and alphaMbeta(2) integrins was not more effective than inhibition of PSGL-1 alone. These results suggest that platelet-monocyte complex formation is mostly dependent on PSGL-1.

Inhibitor of apoptosis protein survivin regulates vascular injury

Survivin (also termed Birc5) belongs to the family of genes known as inhibitors of apoptosis, and it has been implicated in both prevention of cell death and control of mitosis. The survivin pathway is exploited in cancer, but its potential role in vascular injury is unknown. Here, we show that balloon-mediated arterial injury in rabbits resulted in expression of survivin in vascular cells. Serum or PDGF-AB stimulated survivin expression in cultured smooth-muscle cells (SMCs), which suppressed apoptosis and prevented caspase activation. Adenoviral delivery of a phosphorylation-defective survivin mutant reversed the cytoprotective effect of PDGF in SMCs without affecting mitotic progression, suppressed neointimal formation in wire-injured mouse femoral arteries, and induced vascular cell apoptosis in vivo. These data identify survivin as a critical regulator of SMC apoptosis after acute vascular injury. Disrupting the survivin pathway may provide a novel therapy to limit pathological vessel-wall remodeling.

The GP IIb/IIIa inhibitor abciximab (c7E3) inhibits the binding of various ligands to the leukocyte integrin Mac-1 (CD11b/CD18, alphaMbeta2)

Cross-reactivity with integrins other than glycoprotein IIb/IIIa (GP IIb/IIIa) is discussed as a potential reason for the overall clinical benefits of the GP IIb/IIIa-blocking antibody-fragment abciximab. We evaluated whether abciximab binds to the leukocyte integrin Mac-1, whether it inhibits binding of the distinct ligands and thereby may modulate inflammation, cell proliferation and coagulation. Binding of fluorescence-labelled abciximab to phorbolmyristate acetate-stimulated monocytes and to a monocytic cell line (THP-1) could be detected in flow cytometry. The binding of fibrinogen, the inactivated complement factor 3b (iC3b), and the coagulation factor X to Mac-1 could be inhibited by abciximab (10 microg/ml) in vitro. As a functional consequence, the conversion of factor X to factor Xa mediated by Mac-1, as detected by the chromogenic substrate SZ-2222, was impaired by abciximab. Adhesion of THP-1 cells to immobilized intercellular adhesion molecule 1 (ICAM-1) and to fibrinogen was reduced significantly by abciximab. Fibrinogen-mediated cell aggregation was also impaired. In conclusion, we describe binding of abciximab to Mac-1 on stimulated monocytes. Thereby, abciximab inhibits binding of the ligands fibrinogen, ICAM-1, iC3b and factor X. Furthermore, we demonstrated that Mac-1-dependent conversion from factor X to factor Xa is impaired by abciximab, arguing for the direct modulation of the coagulation cascade by abciximab. Overall, the inhibition of Mac-1 could provide additional clinical benefits of abciximab beyond the well-described blockade of GP IIb/IIIa.

Activated monocytes induce smooth muscle cell death: role of macrophage colony-stimulating factor and cell contact

BACKGROUND

Plaque disruption is the inciting event for coronary thrombosis and acute coronary syndromes. Multiple factors influence plaque rupture, including the loss of vascular smooth muscle cells (VSMCs). We hypothesized that monocytes/macrophages (MMs) activated by macrophage colony-stimulating factor (M-CSF) are responsible for VSMC death.

METHODS AND RESULTS

VSMC apoptosis was markedly increased in the presence of both M-CSF and MMs (58.8+/-3.3%) compared with VSMCs plus M-CSF without MMs (15.7+/-1.5%, P< or =0.00005), VSMCs plus MMs without M-CSF (22.7+/-3.7%, P< or =0.0001), or control VSMCs alone (13.2+/-2.1%, P< or =0.0001). MM cell contact was required for M-CSF-stimulated killing of VSMCs, and MMs displayed an M-CSF concentration-dependent killing effect. Abciximab binds Mac-1 (CD11b/CD18) on MMs. When added to VSMCs exposed to MMs and M-CSF, abciximab (7 microg/mL) significantly reduced VSMC apoptosis (19.1+/-2.2%, P< or =0.0003). Therapeutic doses of tirofiban (0.35 microg/mL) and eptifibatide (5 microg/mL), which inhibit platelet glycoprotein (GP) IIb/IIIa but not Mac-1, did not block activated MM-induced VSMC apoptosis (65.0+/-3.4% and 51.3+/-2.5%, respectively). A recombinant anti-CD-18 antibody had an effect similar to that of abciximab (16.5+/-0.4%).

CONCLUSIONS

These data suggest that monocytes and physiological concentrations of M-CSF trigger VSMC apoptosis. Abciximab and specific inhibitors of the Mac-1 receptor can antagonize this process.

Therapeutic potential of monoclonal antibodies in myocardial reperfusion injury

While reperfusion therapy in myocardial infarction is associated with better short- and long-term outcomes, it paradoxically results in reperfusion injury mediated by interactions between leukocytes, endothelial cells, platelets, and the myocardium. Several surface receptors, adhesion molecules, and ligands have been shown to be important in the pathogenesis of myocardial reperfusion injury, and therapeutic strategies employing the use of monoclonal antibodies have been attempted against many of them. These have included monoclonal antibodies against activated complement 5 (C5a) to inhibit leukotaxis, monoclonal antibodies against P-selectin, P-selectin glycoprotein ligand (PSGL)-1, L-selectin and E-selectin to inhibit leukocyte rolling, and monoclonal antibodies against the Mac-1 (CD11b/CD18) receptor and intercellular adhesion molecule (ICAM)-1 to block firm adhesion of leukocytes to endothelial cells. In addition, although initially developed as an antiplatelet agent, the glycoprotein IIb/IIIa receptor antagonist abciximab shows significant ability to diminish or prevent reperfusion injury, presumably through its ability to block the Mac-1 receptor on leukocytes. Finally, monoclonal antibodies have also been tested against several cytokines and adhesion molecules implicated in so-called subacute endothelial activation, including interleukin-8 and vascular cell adhesion molecule (VCAM)-1. Studies in animals evaluating the use of monoclonal antibodies in reperfusion injury against various potential targets have largely been successful; however, studies in humans have been disappointing, underscoring the pitfalls of using animal models for the study of complex diseases. Based upon current knowledge, it is becoming clear that a successful strategy against reperfusion injury will require targeting several pathways at once, rather than attempting to block one final common pathway. In addition, inhibition of subacute endothelial activation through inhibition of transcription factors, namely nuclear factor (NF)-kappa B, may be a prerequisite to significantly reducing the extent of myocardial damage in this condition. The future of monoclonal antibodies in the overall strategy remains unclear. Newer small molecule inhibitors are also under development, and the eventual role of gene therapy remains to be elucidated.

Monocyte adhesion to human vein grafts: a marker for occult intraoperative injury?

OBJECTIVE

Monocyte adhesion to the vessel wall is believed to be an important initiating event in atherosclerosis and intimal hyperplasia. We hypothesized that occult intraoperative vein injury induces an immediate increase in monocyte adhesion that may be critical to the development of vein graft disease.

METHODS

Vein segments were obtained from patients (n = 23) undergoing lower extremity bypass. The initial segment (V1, n = 17) was excised immediately at the time of conduit harvest. A second segment (V2, n = 23) was obtained from the distal conduit just before performing the distal anastomosis. Segments were incubated with radiolabeled THP-1 cells (monocytoid cell line) for 1 hour at 37 degrees C, then rinsed and solubilized for determination of bound radioactivity. In a subset of grafts (n = 4), THP-1 cells were preincubated with monoclonal antibody (mAB) 7E3 (which binds to the monocyte integrin Mac-1 at its fibrinogen [Fg]-binding site) or control (mAB 14E11). Fg deposition and endothelial coverage were evaluated by immunohistochemistry (n = 10). Statistical analysis was performed using the paired t test and analysis of variance. Follow-up graft patency data were obtained and correlated with adhesion values using an exact test (StatXact, Cytel Software, Cambridge, Mass).

RESULTS

Monocyte adhesion was significantly increased after surgical manipulation (V1, 2400 +/- 770 versus V2, 7343 +/- 1555 cells/cm(2); P <.02). Fg deposition was abundant in V2 sections and not seen in V1. Monocyte adhesion to V2 segments was significantly reduced (58% of control, P <.01) by 7E3 treatment. Graft follow-up was complete with a mean interval of 11 months. Higher V2 adhesion values were associated with occluded grafts (P =.07). The median value for the six occluded grafts was 6234 cells/cm(2) versus 3892 cells/cm(2) for the 17 patent grafts.

CONCLUSIONS

Monocyte adhesion to the vein wall is immediately increased after surgical manipulation and is inhibited by mAB 7E3. Early monocyte adhesion to vein grafts is likely to involve interactions between Mac-1 and Fg. Heightened levels of monocyte adhesion at implantation may be a marker for subsequent vein graft failure.

Monocyte adhesion to balloon-injured arteries: the influence of endothelial cell seeding

OBJECTIVE

Deendothelialization of injuries of the artery disrupts normal vascular homeostasis, affecting both the structural integrity of the blood vessel wall, as well as the interaction of the arterial surface with blood components such as platelets, leukocytes, and circulating proteins. Leukocyte and, in particular, monocyte recruitment to damaged vessels has been implicated in the pathogenesis of intimal hyperplasia. We hypothesize that reendothelialization is an important modulator of monocyte adhesion to healing arterial surfaces.

METHODS

New Zealand white rabbits (n = 20) were subjected to bilateral iliofemoral artery balloon injury. Cultured, autologous venous endothelial cells (ECs) were immediately seeded onto one vessel, whereas the contralateral artery received medium alone, to accelerate endothelial relining. Vessels were harvested (5-9 days after injury) for analysis of permeability (Evans Blue dye exclusion), endothelial coverage (anti-CD31 immunohistochemistry), monocyte adhesion (ex vivo binding of 51Na2CrO4-labeled monocytic THP-1 cells), and monocyte recruitment (RAM-11 immunohistochemistry).

RESULTS

Improved EC coverage was evidenced by positive staining for CD31 in the seeded vessels. Vessel wall permeability was markedly reduced in EC-seeded arteries (29% +/- 10% vs 99% +/- 0% surface Evans blue staining, P <.005), consistent with restoration of a functional endothelial barrier. EC seeding significantly reduced ex vivo THP-1 binding to vessels explanted at a mean of 8 days after injury (45,170 +/- 8939 vs 85,994 +/- 16,500 cells/cm2, P <.05). However, RAM-11 staining revealed no significant difference in overall macrophage accumulation between seeded and control vessels 1 week after injury (111 +/- 22 vs 95 +/- 14 cells/section, P =.36).

CONCLUSIONS

Immediate seeding of a balloon-injured rabbit artery with cultured ECs results in accelerated restoration of the endothelial lining. At 1 week, barrier function is improved, and the seeded vessel surface is less adhesive to activated monocytes ex vivo, as compared with injured controls. Nonetheless, EC-seeded and nonseeded arteries demonstrate similar total macrophage accumulation over 1 week. These data suggest that after mechanical arterial injury, endothelial coverage may be one important variable influencing leukocyte adhesion.

Integrin alpha(M)beta(2)-mediated cell migration to fibrinogen and its recognition peptides

Leukocyte migration is the hallmark of inflammation, and integrin alpha(M)beta(2) and its ligand fibrinogen (Fg) are key participants in this cellular response. Cells expressing wild-type or mutant alpha(M)beta(2) and Fg or its derivatives have been used to dissect the molecular requirements for this receptor-ligand pair to mediate cell migration. The major conclusions are that (a) Fg, its D fragment, and its P1 and P2 alpha(M)beta(2) recognition peptides support a chemotactic response; (b) when the I domain of alpha(L) was replaced with the I domain of alpha(M), the chimeric receptor supported cell migration to Fg; however, the alpha(M) subunit, containing the I domain but lacking the beta(2) subunit, supported migration poorly, thus, the alpha(M)I domain is necessary but not sufficient to support chemotaxis, and efficient migration requires the beta(2) subunit and alpha(M)I domain; and (c) in addition to supporting cell migration, P2 enhanced alpha(M)beta(2)-mediated chemotaxis to Fg and the P1 peptide. This activation was associated with exposure of the activation-dependent epitope recognized by monoclonal antibody 7E3 and was observed also with human neutrophils. Taken together, these data define specific molecular requirements for alpha(M)beta(2) to mediate cell migration to Fg derivatives and assign a novel proinflammatory activity to the P2 peptide.

A novel anti-CD18 mAb recognizes an activation-related epitope and induces a high-affinity conformation in leukocyte integrins

Monoclonal antibody MEM-148 was previously shown to recognize CD18 chains in a free form unassociated within leukocyte integrin heterodimers, but yet it is paradoxically able to induce a high-affinity conformation in the native, cell surface expressed LFA-1 molecules. Our results based on kinetics of binding, immunoprecipitation and cell-aggregation experiments demonstrate that the mAb does bind to and stabilizes a specific conformation of LFA-1 heterodimers apparently distinguished by an increased affinity to its cellular ligand(s). A similar high-affinity conformation of LFA-1, in which the MEM-148 epitope becomes exposed, is induced also by a Mg2+/EDTA or low pH (5.5-6.5) treatments which may mimic physiologically relevant situations in normal or inflamed tissues. Thus, mAb MEM-148 is a novel valuable tool for detection and induction of specific conformations of human leukocyte integrins.

c7E3Fab reduces postischemic leukocyte-thrombocyte interaction mediated by fibrinogen. Implications for myocardial reperfusion injury

Reperfusion injury after coronary occlusion is in part mediated by leukocyte activation and adhesion. Platelets may interact with polymorphonuclear granulocytes (PMNs), causing aggravated reperfusion injury. We studied whether c7E3Fab, a chimeric Fab fragment blocking platelet glycoprotein (GP) IIb/IIIa, decreases PMN-platelet-dependent myocardial dysfunction after ischemia. Isolated guinea pig hearts (n=5 per group) perfused at a constant flow of 5 mL/min were subjected to ischemia (15 minutes, 37 degrees C) and reperfusion. Human PMNs (10x10(6) cells, 3 mL), platelets (400x10(6), 3 mL), and fibrinogen (1 mg/mL) were infused for 3 minutes after 2 minutes of reperfusion, with or without c7E3Fab. Flow cytometry detected GPIIb/IIIa (platelets) and MAC-1 (aMbeta2, PMNs) as well as coaggregates of both in the effluent, whereas double-fluorescence microscopy visualized intracoronary PMN-platelet coaggregates. Postischemic recovery of pressure-volume work (12-cm H(2)O preload and 60-mm Hg afterload) was defined as the ratio of postischemic to preischemic external heart work (mean+/-SEM). c7E3Fab reduced platelet GPIIb/IIIa detection to 10% of controls, blocked a transcoronary MAC-1 increase (+25% without versus -23% with c7E3Fab), and inhibited PMN-platelet coaggregation in the effluent (49+/-12% without versus 17+/-2% with c7E3Fab) as well as in the hearts themselves (5.0+/-0.7/cm(2) without versus 1.2+/-0.3/cm(2) surface area with c7E3Fab). Postischemic recovery of external heart work (83+/-5% in cell-free hearts) declined to 46+/-4% after postischemic PMN-platelet infusion, but not in the presence of c7E3Fab (74+/-11%) or LPM19c (71+/-6%). We conclude that c7E3Fab inhibits formation of PMN-platelet aggregates during myocardial reperfusion, an effect that protects against PMN-platelet-dependent stunning.

Divergent effects of platelet-endothelial cell adhesion molecule-1 and beta 3 integrin blockade on leukocyte transmigration in vivo

The final stage in the migration of leukocytes to sites of inflammation involves movement of leukocytes through the endothelial cell layer and the perivascular basement membrane. Both platelet-endothelial cell adhesion molecule-1 (PECAM-1/CD31) and the integrin alphavbeta3 have been implicated in this process, and in vitro studies have identified alphavbeta3 as a heterotypic ligand for PECAM-1. In the present study we have addressed the roles of these molecules by investigating and comparing the effects of PECAM-1 and alphavbeta3 blockade on leukocyte migration in vivo. For this purpose we have examined the effects of neutralizing Abs directed against PECAM-1 (domain 1-specific, mAb 37) and beta3 integrins (mAbs 7E3 and F11) on leukocyte responses in the mesenteric microcirculation of anesthetized rats using intravital microscopy. The anti-PECAM-1 mAb suppressed leukocyte extravasation, but not leukocyte rolling or firm adhesion, elicited by IL-1beta in a dose-dependent manner (e.g., 67% inhibition at 10 mg/kg 37 Fab), but had no effect on FMLP-induced leukocyte responses. Analysis by electron microscopy suggested that this suppression was due to an inhibition of neutrophil migration through the endothelial cell barrier. By contrast, both anti-beta3 integrin mAbs, 7E3 F(ab')2 (5 mg/kg) and F11 F(ab')2 (5 mg/kg), selectively reduced leukocyte extravasation induced by FMLP (38 and 46%, respectively), but neither mAb had an effect on IL-1beta-induced leukocyte responses. These findings indicate roles for both PECAM-1 and beta3 integrins in leukocyte extravasation, but do not support the concept that these molecules act as counter-receptors in mediating leukocyte transmigration.

Reperfusion injury after focal myocardial ischaemia: polymorphonuclear leukocyte activation and its clinical implications

The only way to rescue ischaemic tissue is to re-instate the oxygen supply to the tissue. However reperfusion of the ischaemic area not only oxygenates the tissue but also initiates a cascade of processes, which may in some cases result in temporary dysfunction of the myocardium. In order to devise protective measures, it is essential to understand the mechanisms and the triggers of this reperfusion phenomenon. In this review we will mainly focus on the inflammatory response caused by reperfusion. We will cover the different steps of polymorphonuclear leukocyte activation and will briefly discuss the molecular biology of the receptors involved. The currently used pharmacological medications in acute cardiology will be reviewed and in particular their actions on polymorphonuclear leukocyte activation, adhesion and degranulation. This review is a compilation of the current knowledge in the field and the therapeutic progress in the prevention of reperfusion injury made today.

Heparin inhibits ligand binding to the leukocyte integrin Mac-1 (CD11b/CD18)

BACKGROUND

The clinical benefits of heparin reach beyond its anticoagulative properties. Recently, it has been described that leukocytes adhere on immobilized heparin mediated by the integrin Mac-1 (CD11b/CD18, alphaMbeta2, or CR3). Because inhibition of this versatile adhesion molecule could explain various aspects of the beneficial clinical effects of heparin, we evaluated whether soluble heparin modulates Mac-1 function in vitro and in vivo.

METHODS AND RESULTS

Binding of unfractionated heparin to Mac-1 on PMA-stimulated monocytes and granulocytes was directly demonstrated in flow cytometry, whereas no binding of heparin was detected on unstimulated leukocytes. Unfractionated heparin inhibited binding of the soluble ligands fibrinogen, factor X, and iC3b to Mac-1. Adhesion of the monocytic cell line THP-1 and of peripheral monocytes and granulocytes to immobilized ICAM-1 was impaired by unfractionated heparin, to the same extent as with inhibition of Mac-1 by monoclonal antibodies such as c7E3. Low-molecular-weight heparin also inhibits binding of fibrinogen to Mac-1. Additionally, flow cytometry of whole blood preparations of patients treated with unfractionated heparin revealed an inhibitory effect of heparin on the binding of fibrinogen to Mac-1 that correlates (n= 48, r=0.63, P<0.001) to the extent of prolongation of the activated partial thromboplastin time.

CONCLUSIONS

We describe a pharmacologically relevant property of heparin that may contribute to its benefits in clinical use. The binding of heparin to Mac-1 and the resulting inhibition in binding of Mac-1 ligands may directly modulate coagulation, inflammation, and cell proliferation.

Potential non-glycoprotein IIb/IIIa effects of abciximab

The antithrombotic effect of abciximab is believed to be primarily due to its blockade of platelet glycoprotein IIb/IIIa receptors, leading to the inhibition of platelet aggregation. Studies have, however, identified that antibody 7E3, the parent molecule of abciximab, and/or abciximab itself, binds to both "activated" alphaMbeta2 receptors and alphaVbeta3 receptors. Because alphaMbeta2 receptors are present on granulocytes and monocytes, cells that have been implicated in contributing to atherosclerosis, intimal hyperplasia after vascular injury, reperfusion injury, and thrombin generation, it is possible that some of abciximab's effects relate to this reactivity. Similarly, because alphaVbeta3 has been implicated in platelet adhesion to osteopontin, intimal hyperplasia after vascular injury, and platelet-mediated thrombin generation, it is possible that some of abciximab's beneficial effects relate to this reactivity. Blockade of alphaVbeta3 receptors may also be beneficial in other disease states because, in animal models, such blockade inhibits tumor angiogenesis and sickle cell adhesion to blood vessel endothelium. Despite these intriguing observations, there are no direct data to support any beneficial roles or any unwanted side effects related to the reactivities of abciximab with "activated" alphaMbeta2 or alphaVbeta3 receptors.