Detection of In Vivo Activated Platelets in Experimental Cerebral Thrombosis: Studies Using a New Monoclonal Antibody 2T60, Specific for Activated Human and Rabbit Platelets

Development of liposomal nanoconstructs targeting P-selectin (CD62P)-expressing cells by using a sulfated derivative of sialic acid

PURPOSE

NMSO3, a sulfated derivative of sialic acid, is a specific inhibitor for P-selectin (CD62P)-mediated cell adhesion. We attempted to apply liposomes modified with NMSO3 for selective targeting of activated platelets.

METHODS

The binding of fluorescently labeled NMSO3-containing liposomes (NMSO3-liposomes) to CHO cells expressing P-selectin (CHO-P cells) and activated platelets were examined. The distribution of NMSO3-liposomes incorporated into the cells was observed by fluorescence microscopy.

RESULTS

The binding assay revealed that NMSO3-liposomes specifically bound to immobilized P-selectin and CHO-P cells in a dose-dependent manner. The binding of NMSO3-liposomes to CHO-P cells was much stronger than that to the parental CHO-K1 cells. Fluorescence microscopic observation showed that NMSO3-liposomes were incorporated into CHO-P cells after the binding and distributed throughout the cytoplasm of the cell. NMSO3-liposomes bound more strongly to thrombin-activated platelets than to resting platelets, as assessed by flow cytometry.

CONCLUSIONS

These results suggest that NMSO3-liposomes can be applied for selective drug delivery to activated platelets.

Cytokine secretion from human monocytes potentiated by P-selectin-mediated cell adhesion

BACKGROUND/AIM

P-selectin is a carbohydrate-recognizing cell adhesion molecule expressed on activated platelets and endothelial cells. It plays a crucial role in the recruitment of leukocytes to inflammatory and hemorrhagic sites. Cell adhesion mediated by P-selectin induces leukocyte activation, such as the generation of reactive oxygen species and the expression of blood coagulation factors. We assessed how P-selectin-mediated cell adhesion affects cytokine secretion from monocytes.

METHODS

Human peripheral blood monocytes were cultured in a plate that had been coated with P-selectin purified from human platelets, and cytokines released in the culture supernatant from monocytes were determined by ELISA.

RESULTS

The secretion of tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, IL-8, IL-12 and macrophage inflammatory protein-1β increased 3- to 10-fold in response to P-selectin compared with unstimulated monocytes. We next examined the effects of cytokine treatment of monocytes on their susceptibility to P-selectin. The secretion of TNF-α from monocytes in response to P-selectin was increased when monocytes were preincubated with granulocyte/macrophage colony-stimulating factor, monocyte chemotactic protein-1 or interferon-γ (IFN-γ); IFN-γ was the most effective in potentiating TNF-α secretion from monocytes.

CONCLUSION

These results suggest that the interaction of monocytes with P-selectin plays an important role not only in their trafficking but also in the regulation of cytokine production by these cells.

Circulating platelet and neutrophil activation correlates with the clinical course of unstable angina

Recent studies have suggested important roles of inflammation in the pathophysiology of unstable angina (UA). We investigated whether activation of the circulating platelets and neutrophils were implicated in inflammatory reactions associated with unstable angina Expressions of platelet P-selectin and neutrophil CD11b, and neutrophil-platelet aggregates were evaluated by flow cytometry in anticoagulated peripheral venous blood from 71 patients with UA and 22 patients with stable angina (SA). Expressions of platelet P-selectin and neutrophil CD11b, and neutrophil-platelet aggregates on the admission day were all significantly higher in 71 patients with UA than 22 with SA (median, mean fluorescence intensity [MFI]: 7.00 vs 4.51, P < 0.01, 64.68 vs 47.75, P = 0.0007; and % of 10 000 neutrophils: 7.84 vs 3.40, P = 0.0001, respectively). These three parameters in 43 patients with UA were significantly decreased (MFI: 4.23, P = 0.003, 50.82, P = 0.0003; and % of 10 000 neutrophils: 5.04, P = 0.0001, respectively) 7 days after the first measurement. These results indicate that circulating activated platelets and neutrophils are more strongly implicated in the acute phase of UA. These findings also suggest that thrombus formation after rupture of atherosclerotic plaques as well as plaque formation involves inflammatory reactions.

High P-selectin expression and low CD36 occupancy on circulating platelets are strong predictors of restenosis after coronary stenting in patients with coronary artery disease

Recent studies have shown that circulating platelets play an important role in the development of restenosis early after coronary stent implantation. We investigated P-selectin expression and CD36 blockade on platelets by flow cytometry in 48 consecutive patients who underwent coronary stenting. P-selectin expression was significantly higher 1 day after stenting in patients who had restenosis (n = 15) than in those who had no restenosis (n = 28), and the odds ratio for restenosis in patients with high P-selectin levels (MFI > 6.5) was 11.67 (P < 0.001) as compared with patients who had intermediate and low P-selectin levels. CD36 blockade was assessed with the use of two anti-CD36 antibodies, OKM5 and GS95 (our new anti-CD36 antibody), the binding of which indicates total CD36 amount and free CD36 unoccupied by lipid-related ligands, respectively. Binding of OKM5 to platelets was similar before and after stenting in both groups. CD36 blockade on platelets was seen 1 day after stenting in the non-restenosis group, and the odds ratio for restenosis in patients without CD36 blockade [GS95 binding ratio >0.8 as compared with binding before stenting] on day 1 was 28.60 (P < 0.001). P-selectin expression and unoccupied CD36 on platelets shortly after stenting may be strong predictors of post-stent restenosis.

Inhibition of P-selectin-mediated cell adhesion by a sulfated derivative of sialic acid

P-selectin, a carbohydrate-binding cell adhesion molecule expressed on activated endothelial cells and platelets, plays a key role in the recruitment of leukocytes to inflammatory and hemorrhagic sites. It simultaneously recognizes a sialic acid-containing carbohydrate chain and the sulfated tyrosine residues of a specific counter-receptor expressed on the leukocyte surface. We examined the inhibitory effects of a synthetic sulfated derivative of sialic acid (NMSO3) on P-selectin-mediated cell adhesion and found the following: (1) P-selectin/IgG chimera bound to immobilized NMSO3. (2) The binding of P-selectin/IgG chimera to purified P-selectin glycoprotein ligand-1 was inhibited by soluble NMSO3. (3) The adhesion of HL60 cells to P-selectin-expressing CHO cells was inhibited by NMSO3. (4) NMSO3 inhibited P-selectin-induced tumor necrosis factor-alpha production in monocytes and activated platelet-induced generation of reactive oxygen species in neutrophils. In conclusion, NMSO3 acts as a specific inhibitor for P-selectin-mediated cell adhesion and for adhesion-dependent leukocyte activation.

Reversibility versus Persistence of GPIIb/IIIa Blocker-Induced Conformational Change of GPIIb/IIIa (αIIbβ3, CD41/CD61)

Clinically used GPIIb/IIIa blockers are ligand mimetics, and thereby their binding can induce conformational changes of the platelet integrin GPIIb/IIIa. Since the reversibility of these conformational changes may be an important determinant of potential adverse effects of GPIIb/IIIa blockers, we produced a new monoclonal antibody (anti-LIBS-mAb), and by using its binding properties, we investigated the conformational changes of GPIIb/IIIa during the binding and especially the dissociation of GPIIb/IIIa blockers. Production of monoclonal antibody (mAb) clones was performed using purified GPIIb/IIIa in a high affinity conformation and using activated platelets. Clone anti-LIBS-145-mAb was chosen, since it allowed the sensitive probing of eptifibatide-induced conformational changes of GPIIb/IIIa. On resting and activated platelets and on GPIIb/IIIa-expressing Chinese hamster ovary cells, anti-LIBS-145-mAb binding returned to background binding after dissociation of eptifibatide, indicating a complete reversibility of the eptifibatide-induced conformational change. Furthermore, with the mixing of eptifibatide-preincubated and nonincubated cells, a fast reversibility could be demonstrated. However, when fibrinogen was present in a physiological concentration, the GPIIb/IIIa blocker-induced conformation was partially retained after the dissociation of eptifibatide and to the same extent binding of fibrinogen and the activation-specific mAb Pac-1 was induced. In conclusion, a fast reversibility of the conformational change of GPIIb/IIIa after dissociation of GPIIb/IIIa blockers could be demonstrated as an intrinsic property of the GPIIb/IIIa receptor. This mechanism prevents general platelet aggregation after dissociation of ligand mimetic GPIIb/IIIa blockers. Nevertheless, in the presence of fibrinogen this reversibility is not complete, which may explain some of the side effects of GPIIb/IIIa blockers, especially those of the oral GPIIb/IIIa blockers.

Intracellular localization of glycoprotein VI in human platelets and its surface expression upon activation

Glycoprotein (GP) VI is a major receptor for collagen and belongs to the immunoglobulin super family. Here, we examined the localization of GPVI in resting and activated human platelets by immunogold scanning and transmission electron microscopy and flow cytometry. Ultrastructural observation detected immunolabelling for GPVI that was distributed uniformly over the entire surface of resting platelets, and revealed that GPVI was also localized on both the membranes of the surface-connected open canalicular system (OCS) and alpha-granules. The OCS- and alpha-granule-associated GPVI pools were an estimated 35.4 +/- 3.2% (mean +/- standard deviation) of the total. Little GPVI labelling was observed in any part of GPVI-deficient platelets. A remarkable time-dependent increase in GPVI surface expression was found by flow cytometry when platelets were activated by collagen-related peptide (CRP) and convulxin. The GPVI-mediated activation of platelets by CRP or convulxin resulted in similar ultrastructural changes and an increased GPVI labelling density on the activated platelet surface, which was accompanied by a decreased interior expression. GPVI was also expressed on microparticles generated from activated platelets. Thus, our study demonstrates that platelets have internal pools of GPVI, and that GPVI is increasingly redistributed to the surface membrane and to microparticles during platelet activation.

Density-dependent induction of TNF-alpha release from human monocytes by immobilized P-selectin

P-selectin purified from human platelets, when immobilized on a solid surface, induced monocytes to release tumor necrosis factor-alpha (TNF-alpha). The induction of TNF-alpha release was dependent on the concentration of P-selectin used for the immobilization, and the maximal stimulation was observed when the plate was coated with 0.3 microgram/ml of P-selectin. Use of either a higher or a lower concentration of P-selectin for the plate-coating was found to elicit less TNF-alpha release, although the higher concentration of P-selectin caused a stronger adhesion of HL-60 leukemic cells. The expression of mRNA for TNF-alpha roughly paralleled the TNF-alpha secretion, as assessed by RT-PCR. These results indicate that monocytes are activated by immobilized P-selectin in a density-dependent manner.

Redistribution of selectin counter-ligands induced by cytokines

Soluble recombinant (r) P-selectin and rP-selectin immobilized on plastic surfaces were tested for their capacity to activate neutrophils to produce superoxide anion. Soluble rP-selectin was incapable of activating leukocytes, whereas immobilized rP-selectin was able to induce leukocyte activation. When neutrophils were pretreated with a low dose of IL-8, granulocyte colony stimulating factor or granulocyte macrophage colony stimulating factor, soluble rP-selectin was no longer inert. These cytokine-primed leukocytes produced superoxide anion in the presence of soluble rP-selectin. During this priming period, sialyl Lewis X (sLe(X)) epitopes redistributed to one end of the leukocytes. Similar polarization of sLe(X) epitopes was observed at the attachment site of cells that adhered to immobilized rP-selectin. Cap formation and superoxide anion production induced by solid-phase P-selectin or by IL-8 and soluble rP-selectin treatment were inhibited by treatment of the leukocytes with cytochalasin B. These observations suggest that the redistribution of the carbohydrate ligands and the polarization of the leukocyte surface through an active process is a prerequisite but not sufficient to leukocyte superoxide production through P-selectin.

P-selectin and vascular cell adhesion molecule-1 are focally expressed in aortas of hypercholesterolemic rabbits before intimal accumulation of macrophages and T lymphocytes

The accumulation of monocyte/macrophages and T lymphocytes in arterial intima is a hallmark of early atherogenesis. To investigate the temporal relationships between endothelial expression of adhesion molecules (eg, P-selectin and vascular cell adhesion molecule-1 [VCAM-1]) and intimal accumulation of macrophages and T lymphocytes, immunostaining was performed by using serial frozen sections from intercostal branch points of thoracic aortas of New Zealand White rabbits that had been fed a 0.3% cholesterol diet. After 1 week of cholesterol feeding, neither macrophages nor T lymphocytes were detected, although endothelial expression of P-selectin and VCAM-1 was observed. After 3 weeks, macrophages were detectable in 75% and T lymphocytes were present in 25% of the rabbits. Expression of P-selectin and VCAM-1 was sustained until 10 weeks. Infiltration of T lymphocytes was restricted in areas in which macrophages were accumulated and did not appear to precede macrophage infiltration. E-selectin expression was not detectable before accumulation of mononuclear leukocytes; however, very few endothelial cells covering foam cell lesions expressed E-selectin after 6 weeks. Similar results were obtained in Watanabe heritable hyperlipidemic rabbits aged 1, 2, and 3 months. Taken together, localized expression of P-selectin and VCAM-1 may play a key role in the initial recruitment of macrophages and T lymphocytes in early atherogenesis.