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There is evidence to suggest that elevated plasma levels of lipoprotein (a) [Lp(a)] represent a risk factor for the development of atherosclerotic vascular disease, but the mechanism by which this lipoprotein localizes to involved vessels is only partially understood. In view of studies suggesting a link between inflammation and atherosclerosis and our previous finding that leukocyte defensin modulates the interaction of plasminogen and tissue-type plasminogen activator with cultured human endothelial cells, we examined the effect of this peptide on the binding of Lp(a) to cultured vascular endothelium and vascular smooth muscle cells. Defensin increased the binding of Lp(a) to endothelial cells approximately fourfold and to smooth muscle cells approximately sixfold. Defensin caused a comparable increase in the amount of Lp(a) internalized by each cell type, but Lp(a) internalized as a consequence of defensin being present was not degraded, resulting in a marked increase in the total amount of cell-associated lipoprotein. Abundant defensin was found in endothelium and in intimal smooth muscle cells of atherosclerotic human cerebral arteries, regions also invested with Lp(a). These studies suggest that defensin released from activated or senescent neutrophils may contribute to the localization and persistence of Lp(a) in human vessels and thereby predispose to the development of atherosclerosis.

SUMMARY Heparin-induced thrombocytopenia/thrombosis (HIT) is a severe thrombotic disorder that occurs in ≈1% of patients treated with heparin. Affected patients commonly develop antibodies that recognize PF4/heparin complexes that may form on the surface of activated platelets and on the endothelium. However, it has not been established that anti-PF4/heparin antibodies are responsible for the clinical manifestations of HIT. To address this issue, we employed a recently developed model of active immunity to study the effect of IgG anti-PF4/heparin antibody in vivo. In previous studies we have shown that it is possible to induce autoimmune diseases such as systemic lupus erythematosus (SLE), anti-phospholipid syndrome (APS) or vasculitis in naive mice by active immunization with anti-DNA, anti-cardiolipin and anti-neutrophil cytoplasmic antibodies, respectively. Immunized animals develop anti-idiotypic antibodies (Ab2) and, after 2–4 months, anti-anti-idiotypic antibodies (Ab3). Ab3s generated in this manner often simulate the binding activity of Ab1 and their expression correlates with the development of specific clinical manifestations typical of the respective human disease. Based on this experience, naive BALB/c mice were immunized with IgG anti-PF4/heparin antibodies isolated from two patients with HIT. The actively immunized mice developed mouse anti-PF4/heparin antibody (Ab3). Administration of unfractionated heparin, but not low molecular weight heparin (LMWH), to the actively immunized animals induced thrombocytopenia by day 4 of drug exposure. There was no evidence of thrombosis. The results of this study support the importance of anti-PF4/heparin antibodies in the pathogenesis of HIT. Further, this model may help to elucidate the factors responsible for thrombosis as well as providing means to assess new treatment options for patients with this disorder.

The mechanism by which single-chain urokinase (scuPA) binds to its receptor (uPAR) is incompletely understood. We report that a fragment comprising the first domain of recombinant soluble uPAR (sDI) as well as a fragment comprising the remaining domains (sDII-DIII) competes with the binding of recombinant full-length soluble uPAR (suPAR) to scuPA with an IC50 = 253 nM and an IC50 = 1569, respectively. sDII-III binds directly to scuPA with Kd = 238 nM. Binding of scuPA to each fragment also induces the expression of plasminogen activator activity. sDI and sDII-DIII (200 nM each) induced activity equal to 66 and 36% of the maximum activity induced by full-length suPAR (5 nM), respectively. Each fragment also stimulates the binding of scuPA to cells lacking endogenous uPAR. Although scuPA binds to sDI and to sDII-DIII through its amino-terminal fragment, the fragments act synergistically to inhibit the binding of suPAR and to stimulate plasminogen activator activity. Furthermore, sDII-DIII retards the velocity and alters the pattern of cleavage of sDI by chymotrypsin. These results suggest that binding of scuPA to more than one epitope in suPAR is required for its optimal activation and association with cell membranes.

The explanation why only a subset of patients with heparin-induced thrombocytopenia (HIT) develop clinically apparent thromboses (HITT) remains uncertain. It has been proposed that platelet activation induced by cross-linking of FcγRIIA by anti-heparin/platelet factor 4 (PF4) antibodies is central to the pathogenesis of thrombosis. The observation that a common functional polymorphism of FcγRIIA, involving either an arginine (R) or histidine (H) at amino acid 131, may underlie disease susceptibility prompted us to investigate the prevalence of receptor isoforms in patients with HIT and HITT. Furthermore, because these isoforms reportedly differ in their avidity for immune complexes containing human IgG2 , we also analyzed sera from patients with HIT and HITT for the prevalence of various subclass-specific IgG anti-heparin/PF4 antibodies. No difference in the allele frequency of FcγRIIA-H131 or R131 was identified among 13 patients with HIT or 23 with HITT compared with 102 controls (χ2 = 1.21, P = .8). Furthermore, although most patients had IgG2 antibodies (62%), IgG1 was the predominant subclass in 30 of the 34 patients with IgG anti-heparin/PF4 antibodies and in 12 was the exclusive subclass found. Also, there was no association between the concordance of IgG2 anti-heparin/PF4 antibodies and the expression of FcγRIIA-H131 in patients with HITT compared with patients with thrombocytopenia alone. These results make it unlikely that the FcγRIIA-H131 isoform or IgG2 anti-heparin/PF4 antibodies are required to develop HITT, suggesting that factors in addition to cross-linking of FcγRIIA receptors contribute to the pathogenesis of thrombosis in patients with heparin-dependent antiplatelet antibodies.

Heparin-induced thrombocytopenia (HIT) is a life-threatening, thrombotic disorder associated with development of anti–platelet factor 4 (anti-PF4)/heparin autoantibodies. Little is known about the antigenic and cellular requirements that initiate the immune response to these complexes. To begin to delineate mechanisms of autoantibody formation in HIT, we studied the immunizing effects of murine PF4 (mPF4)/heparin in mice with and without thymic function. Euthymic mice were injected with mPF4/heparin complexes, mPF4, heparin, or buffer. Mice injected with mPF4/heparin, but not mPF4 or heparin alone, developed heparin-dependent autoantibodies that shared serologic and functional characteristics of human HIT antibodies, including preferential binding to mPF4/heparin complexes and causing heparin- and FcRγIIA-dependent platelet activation. In contrast, athymic mice did not develop HIT-like antibodies. Taken together, these studies establish that PF4/heparin complexes are highly immunogenic and elicit self-reacting anti-PF4/heparin antibodies in a T cell–dependent manner.

The antiphospholipid syndrome is characterized by thrombosis and recurrent fetal loss in patients with antiphospholipid antibodies (APLAs). Most pathogenic APLAs are directed against β2-glycoprotein I (β2GPI), a plasma phospholipid binding protein. One mechanism by which circulating antiphospholipid/anti-β2GPI antibodies may promote thrombosis is by inducing the release of procoagulant microparticles from endothelial cells. However, there is no information available concerning the mechanisms by which anti-β2GPI antibodies induce microparticle release. In seeking to identify proteins phosphorylated during anti-β2GPI antibody-induced endothelial activation, we observed phosphorylation of nonmuscle myosin II regulatory light chain (RLC), which regulates cytoskeletal assembly. In parallel, we observed a dramatic increase in the formation of filamentous actin, a two- to fivefold increase in the release of endothelial cell microparticles, and a 10- to 15-fold increase in the expression of E-selectin, intercellular adhesion molecule 1, vascular cell adhesion molecule 1, and tissue factor messenger RNA. Microparticle release, but not endothelial cell surface E-selectin expression, was blocked by inhibiting RLC phosphorylation or nonmuscle myosin II motor activity. These results suggest that distinct pathways, some of which mediate cytoskeletal assembly, regulate the endothelial cell response to anti-β2GPI antibodies. Inhibition of nonmuscle myosin II activation may provide a novel approach for inhibiting microparticle release by endothelial cells in response to anti-β2GPI antibodies.

Babies are frequently exposed to cerebral hypoxia and ischemia (H/I) during the perinatal period as a result of stroke, problems with delivery or post delivery respiratory management. The sole FDA approved treatment for acute stroke is tissue-type plasminogen activator (tPA). Endogenous tPA is upregulated and potentiates impairment of pial artery dilation in response to hypotension after H/I in pigs. Mitogen-activated protein kinase (MAPK), a family of at least 3 kinases, ERK, p38 and JNK, is also upregulated after H/I, with ERK contributing to impaired vasodilation. This study examined the hypothesis that H/I aggravates the vascular response to two important procontractile mediators released during CNS ischemia, endothelin-1 (ET-1) and thromboxane, which is further enhanced by tPA and ERK MAPK.Cerebral hypoxia (pO(2) 35 mmHg for 10 min via inhalation of N(2)) followed immediately by ischemia (global intracranial pressure elevation for 20 min) was produced in chloralose anesthetized piglets equipped with a closed cranial window.H/I aggravated pial artery vasconstriction induced by ET-1 and the thromboxane mimetic U 46619. Potentiated vasoconstrictor responses were blocked by EEIIMD, an inhibitor of tPA's signaling and vascular activities, but unchanged by its inactive analogue EEIIMR. The cerebrospinal fluid concentration of ERK MAPK determined by ELISA was increased by H/I, potentiated by tPA, but blocked by EEIIMD. The ERK MAPK antagonist U 0126 blocked H/I augmented enhancement of ET-1 and U 46619 vasoconstriction.These data indicate that H/I aggravates ET-1 and thromboxane mediated cerebral vasoconstriction by upregulating endogenous tPA and ERK MAPK.

Heparin-induced thrombocytopenia (HIT) is a thrombotic complication of heparin therapy mediated by antibodies to complexes between platelet factor 4 (PF4) and heparin or cellular glycosaminoglycans. However, only a fraction of patients with anti-PF4-heparin antibodies develop HIT, implying that only a subset of these antibodies is pathogenic. The basis for the pathogenic potential of anti-PF4-heparin antibodies remains unclear. To elucidate the intrinsic PF4-binding properties of HIT-like monoclonal antibody (KKO) versus non-pathogenic antibody (RTO) at the single-molecule level, we utilized optical trap-based force spectroscopy to measure the strength and probability of binding of surface-attached antibodies with oligomeric PF4 to simulate interactions on cells. To mimic the effect of heparin in bringing PF4 complexes into proximity, we chemically cross-linked PF4 tetramers using glutaraldehyde. Analysis of the force histograms revealed that KKO-PF4 interactions had ∼10-fold faster on-rates than RTO-PF4, and apparent equilibrium dissociation constants differed ∼10-fold with similar force-free off-rates (k(off) = 0.0031 and 0.0029 s(-1)). Qualitatively similar results were obtained for KKO and RTO interacting with PF4-heparin complexes. In contrast to WT PF4, KKO and RTO showed lower and similar binding probabilities to cross-linked PF4(K50E), which forms few if any oligomers. Thus, formation of stable PF4 polymers results in much stronger interactions with the pathogenic antibody without a significant effect on the binding of the non-pathogenic antibody. These results suggest a fundamental difference in the antigen-binding mechanisms between model pathogenic and non-pathogenic anti-PF4 antibodies that might underlie their distinct pathophysiological behaviors.

Despite continued achievements in antithrombotic pharmacotherapy, difficulties remain in managing patients at high risk for both thrombosis and hemorrhage. Utility of antithrombotic agents (ATAs) in these settings is restricted by inadequate pharmacokinetics and narrow therapeutic indices. Use of advanced drug delivery systems (ADDSs) may help to circumvent these problems. Various nanocarriers, affinity ligands, and polymer coatings provide ADDSs that have the potential to help optimize ATA pharmacokinetics, target drug delivery to sites of thrombosis, and sense pathologic changes in the vascular microenvironment, such as altered hemodynamic forces, expression of inflammatory markers, and structural differences between mature hemostatic and growing pathological clots. Delivery of ATAs using biomimetic synthetic carriers, host blood cells, and recombinant fusion proteins that are activated preferentially at sites of thrombus development has shown promising outcomes in preclinical models. Further development and translation of ADDSs that spare hemostatic fibrin clots hold promise for extending the utility of ATAs in the management of acute thrombotic disorders through rapid, transient, and targeted thromboprophylaxis. If the potential benefit of this technology is to be realized, a systematic and concerted effort is required to develop clinical trials and translate the use of ADDSs to the clinical arena.

A single exposure to protamine and heparin during CPB is highly sensitizing; 29% of patients develop Abs to PRT/H complexes by day 30 after CPB.PRT/H Abs share several features with platelet factor 4/heparin Abs, including high titer formation after CPB, heparin dependence, antigen specificity, and platelet activation.

Whether single chain urokinase (scuPA) expresses intrinsic enzymatic activity continues to be a subject of controversy. We report that the activity of scuPA is enhanced by a small plasmin substrate, H-D-valyl-L-leucyl-L-lysine-p-nitroanilide diacetate (D-VLK-p), but not by a second plasmin substrate, H-D-norleucyl-hexahydrotyrosyl-lysine-p-nitroanilide diacetate (*L*YK-p). D-VLK-p had no effect on the activity of a plasmin insensitive scuPA variant (scuPA-glu158) indicating that native scuPA can be cleaved by plasmin even at saturating concentrations of D-VLK-P. In contrast, D-VLK-P inhibited the activity of the native and scuPA-glu158 complexed with soluble urokinase receptor. Further, D-VLK-p stimulated the enzymatic activity of low molecular weight scuPA (amino acids 144-410) suggesting that D-VLK-P interacts with a second, previously undescribed regulatory site in scuPA.

Binding of urokinase-type plasminogen activator (uPA) to its glycosylphosphatidylinositol-anchored receptor (uPAR) initiates signal transduction, adhesion, and migration in certain cell types. To determine whether some of these activities may be mediated by associations between the uPA/uPAR complex and other cell surface proteins, we studied the binding of complexes composed of recombinant, soluble uPA receptor (suPAR) and single chain uPA (scuPA) to a cell line (LM-TK- fibroblasts) that does not express glycosylphosphatidylinositol (GPI)-anchored proteins to eliminate potential competition by endogenous uPA receptors. scuPA induced the binding of suPAR to LM-TK- cells. Binding of labeled suPAR/scuPA was inhibited by unlabeled complex, but not by scuPA or suPAR added separately, indicating cellular binding sites had been formed that are not present in either component. Binding of the complex was inhibited by low molecular weight uPA (LMW-uPA) indicating exposure of an epitope found normally in the isolated B chain of two chain uPA (tcuPA), but hidden in soluble scuPA. Binding of LMW-uPA was independent of its catalytic site and was associated with retention of its enzymatic activity. Additional cell binding epitopes were generated within suPAR itself by the aminoterminal fragment of scuPA, which itself does not bind to LM-TK- cells. When scuPA bound to suPAR, a binding site for alpha 2-macroglobulin receptor/LDL receptor-related protein (alpha 2 MR/LRP) was lost, while binding sites for cell-associated vitronectin and thrombospondin were induced. In accord with this, the internalization and degradation of cell-associated tcuPA and tcuPA-PAI- 1 complexes proceeded less efficiently in the presence of suPAR. Further, little degradation of suPAR was detected, suggesting that cell- bound complex dissociated during the initial stages of endocytosis. Thus, the interaction of scuPA with its receptor causes multiple functional changes within the complex including the dis-appearance of an epitope in scuPA involved in its clearance from the cell surface and the generation of novel epitopes that promote its binding to proteins involved in cell adhesion and signal transduction.

DIOPATHIC thrombocytopenic purpura (ITP, also I known as primary immune thrombocytopenic purpura) is a hematologic disorder for which appropriate diagnostic and treatment strategies are uncertain. In 1994, the American Society of Hematology (ASH) established a panel to produce explicitly developed practice guidelines for the diagnosis and management of ITP. “Explicitly developed,” evidencebased practice guidelines, which are being issued increasingly by medical specialty societies, combine a critical appraisal of scientific evidence with practice recommendations that state clearly to what extent the guidelines are based either on published scientific evidence or opinion (eg, clinical experience).I4 More details about the clinical practice guideline movement are provided elsewhere.’.’ This report begins with a brief summary of the panel’s recommendations, followed by a more detailed analysis of its methodology, the findings of the comprehensive literature review, and a full presentation of the recommendations. The report concludes with recommendations for future research. As explained later, the recommendations are based on the panel’s opinion, derived from a systematic scoring methodology. (Only recommendations receiving scores of 1 .O to 3.0 or 7.0 to 9.0, as defined later in the text, are cited in this summary.)

To develop a catheter-directed method to enhance urokinase- mediated thrombolysis with use of ultrasound.A prototype catheter was constructed by using a 9-F piezoelectric crystal capable of producing 640-kHz pulsed ultrasound energy. Clots formed in vitro from whole blood were trace-labeled with iodine-125 fibrinogen, and the release of radiolabeled fibrin degradation products was measured in the presence of urokinase, ultrasound, or a combination of urokinase and ultrasound.By 30 minutes, clot lysis was more complete with urokinase plus ultrasound (78.7% +/- 5.3 [mean +/- SD]) than with ultrasound alone (19.3% +/- 10.0) or urokinase alone (47.9% +/- 10.0) (P.001 for ultrasound and urokinase vs either alone). The time to 50% clot lysis was shortened by 46% on average with the application of urokinase and ultrasound compared with urokinase alone (P.03).Catheter-based ultrasound enhances enzymatic thrombolysis in vitro and may be a practical means to reduce the dose of enzyme and the time needed to achieve clot lysis in vivo.

Urokinase-type plasminogen activator (uPA) is synthesized as single- chain protein (scuPA) with little intrinsic activity. scuPA is activated when it is converted to two-chain urokinase (tcuPA) by plasmin or when it binds as a single-chain molecule to its cellular receptor (uPAR). Previous data indicate that complexes between scuPA and its receptor have somewhat higher affinity for plasminogen than does tcuPA. The current study indicates that plasminogen activator activity of scuPA bound to recombinant, soluble uPAR (suPAR) is also fivefold less sensitive to inhibition by plasminogen activator type 1 (PAI-1) than is soluble or receptor-bound tcuPA. Binding of PaI-1 to suPAR/scuPA complexes is totally reversible and can be overcome by increasing the concentration of plasminogen, suggesting a competitive mechanism of inhibition (Ki = 18 nmol/L). Binding of scuPA to suPAR also retards its cleavage by plasmin. These results indicates that binding of single-chain urokinase to its receptor promotes its activity, retards its inhibition, and protects it from conversion to a two-chain form of the enzyme, a step that may precede its inactivation and clearance from cell surfaces. These results are consistent with a physiologic role for receptor-bound single-chain urokinase as a cellular plasminogen activator.

The interaction of single chain urokinase with its receptor accelerates plasminogen activator activity on cell surfaces and induces intracellular signalling in several cell types. To date, no physiologic inhibitor of this binding has been identified. We report that the binding of scuPA to its cellular receptor is inhibited by long chain fatty acids such as oleic acid (C18, delta 9) at physiological plasma concentrations. Inhibition of single chain urokinase binding to human trophoblastic cells by long chain fatty acids was dose-dependent and saturable. Fifty percent of the binding was inhibited at an oleic acid concentration of 27 microM, while inhibition was maximal (75%) at 150 microM oleic acid. The inhibitory potency of oleic acid was unaffected by fatty acid free albumin or human plasma. Inhibition of single chain urokinase binding by free fatty acid analogues was critically dependent on chain length (C14 required for inhibition) and was proportional to the extent of unsaturation. Only the fraction of specific scuPA binding to trophoblasts that was dependent on uPAR was susceptible to inhibition by oleic acid, while binding of scuPA to vitronectin, thombospondin, and the alpha 2-macroglobulin receptor/low-density lipoprotein-related receptor was not. [3H]Oleic acid bound specifically to recombinant soluble uPAR in a 1:1 molar ratio in the presence or absence of plasma and totally blocked its specific binding to a cell line expressing glycosyl phosphatidylinositol-linked single chain urokinase. These results indicate that oleic acid and other unsaturated long chain free fatty acids may serve as physiologic regulators of proteolytic events and intracellular signalling that depend upon the interaction of urokinase with its receptor.

Single-chain urokinase (scuPA), the unique form of urokinase secreted by cells, is converted to an active two-chain molecule through the cleavage of a single peptide bond by plasmin and other specific proteinases. Although scuPA may express limited enzymatic activity, its contribution to plasminogen activation on cell surfaces remains uncertain. Further, although it is well known that scuPA binds to a specific extracellular urokinase-type plasminogen activator receptor, the effect of this interaction on the enzymatic activity of scuPA has not been described. In the present paper we report that the binding of scuPA to cellular an to recombinant soluble urokinase-type plasminogen activator receptors (suPAR) increases its catalytic activity as measured by the cleavage of a urokinase-specific chromogenic substrate. suPAR increased the Vmax of scuPA 5-fold with little change in its Km. suPAR also stimulated the plasminogen activator activity of scuPA by decreasing its Km for Glu-plasminogen from 1.15 microM to 0.022 microM and by increasing the kcat of this reaction from 0.0015 to 0.022 s-1. Preincubation of scuPA with suPAR also enhances its susceptibility to inhibition by plasminogen activator inhibitor type 1, consistent with exposure of its catalytic site. The activity of scuPA bound to suPAR is not accompanied by cleavage of scuPA, which continues to migrate as a single band in SDS-polyacrylamide gel electrophoresis under reducing conditions. Moreover, suPAR increases the plasminogen activator activity of a plasmin-insensitive variant, scuPA (scuPA-Glu158), as well. Enhancement of scuPA activity by suPAR is both prevented and reversed by its aminoterminal fragment (amino acids 1-135), which competes for receptor binding, suggesting that continued binding to the receptor is required for expression of scuPA's enzymatic activity. Thus, our data suggest that scuPA may undergo a reversible transformation between a latent and an active state. The urokinase receptor may induce or stabilize scuPA in its active conformation, thereby contributing to the initiation of plasminogen activation on cell surfaces.

An important biologic function of high-molecular-weight kininogen (HK) is to deliver bradykinin (BK) to its cellular receptors. Internalization and degradation of HK may provide a mechanism by which endothelial cells modulate the production of BK and control its activities. Therefore, we investigated the binding and subsequent distribution of biotinylated-HK (biotin-HK) associated with human umbilical vein endothelial cells (HUVEC). HUVEC bound 3 to 4 times more HK and with greater avidity at 1 to 3 hours at 37 degrees C than at 4 degrees C (Bmax = 1.0 +/- 0.02 x 10(7) molecules/cell, kd = 7 +/- 3 nmol/L v Bmax = 2.6 +/- 0.2 x 10(6) molecules/cell, kd = 46 +/- 8 nmol/L). However, there was no evidence that the difference was caused by internalization of HK at the higher temperature. First, the same amount of biotin-HK was associated with nonpermeabilized and permeabilized HUVEC using buffers containing 20 to 50 mumol/L zinc ion in the absence or presence of 2 mmol/L calcium ion. Second, binding of biotin-HK to HUVEC was approximately 92% reversible at 1 hour when the cells were maintained at both 37 degrees C and 4 degrees C. Third, neither chloroquine nor primaquine altered the amount of biotin-HK bound to HUVEC. Fourth, biotin-HK bound to HUVEC was almost completely removed by pronase. Fifth, the nonpermeable dye, crystal violet, almost completely quenched the fluorescence signal emitted by HUVEC-associated fluorescein isothiocyanate (FITC) HK. Finally, the localization of HUVEC-bound FITC-HK was restricted to the membrane as shown by laser scanning confocal microscopy. The expression of HK binding sites had an absolute requirement for metabolic energy, but was not dependent on new protein synthesis. Membrane-bound HK contributed to the anticoagulant nature of endothelial cells by blocking human alpha-thrombin binding and its resultant induction of prostacyclin formation. These studies indicate that HK is not internalized by HUVEC, but remains primarily on cell surfaces to be accessible for BK liberation and to modulate the binding and actions of alpha-thrombin.

Tissue-type plasminogen activator (tPA) remains the sole thrombolytic approved by the FDA for the treatment of pulmonary embolism (PE). tPA has not been replaced by third generation plasminogen activators, e.g. Reteplase (Ret) and Tenecteplase (TNK) that circulate with longer life-spans and in theory should have more extended potency in vivo. One reason for this paradox is the inability to assign units of activity to plasminogen activators based on specific biologically relevant standards, which impairs objective comparison. Here, we compare clot permeation, retention and fibrinolytic activities of tPA, TNK and Ret in vitro and clot composition over time with outcome in a mouse model of disseminated pulmonary microembolism (ME). When clots were incubated in the continuous presence of drug, tPA, TNK and Ret lysed fibrin clots identically in the absence of PA inhibitor-1 (e.g. PAI-1). Ret, which has lower fibrin affinity and greater susceptibility to inhibition by PAI-1 than tPA, was less effective in lysing plasma clots, while TNK was less effective when the fibrin content of the clots was enhanced. However, when clots were afforded only brief exposure to drug, as occurs in vivo, Ret showed more extensive clot permeation, greater retention and lysis than tPA or TNK. These results were reproduced in vivo in a mouse model of ME. These studies indicate the need for more relevant tests of plasminogen activator activity in vitro and in vivo and they show that clot permeation and retention are important potential predictors of clinical utility.

The paradigm for managing primary immune thrombocytopenia (ITP) in adults has changed with the advent of rituximab and thrombopoietin receptor agonists (TPO-RAs) as options for second-line therapy. Splenectomy continues to provide the highest cure rate (60%-70% at 5+ years). Nonetheless, splenectomy is invasive, irreversible, associated with postoperative complications, and its outcome is currently unpredictable, leading some physicians and patients toward postponement and use of alternative approaches. An important predicament is the lack of studies comparing second-line options to splenectomy and to each other. Furthermore, some adults will improve spontaneously within 1-2 years. Rituximab has been given to more than 1 million patients worldwide, is generally well tolerated, and its short-term toxicity is acceptable. In adults with ITP, 40% of patients are complete responders at one year and 20% remain responders at 3-5 years. Newer approaches to using rituximab are under study. TPO-RAs induce platelet counts > 50 000/μL in 60%-90% of adults with ITP, are well-tolerated, and show relatively little short-term toxicity. The fraction of TPO-RA–treated patients who will be treatment-free after 12-24 months of therapy is unknown but likely to be low. As each approach has advantages and disadvantages, treatment needs to be individualized, and patient participation in decision-making is paramount.

Rapid laboratory assessment of heparin-induced thrombocytopenia (HIT) is important for disease recognition and management. The utility of contemporary immunoassays to detect antiplatelet factor 4 (PF4)/heparin antibodies is hindered by detection of antibodies unassociated with disease. To begin to distinguish properties of pathogenic anti-PF4/heparin antibodies, we compared isotype-matched monoclonal antibodies that bind to different epitopes: KKO causes thrombocytopenia in an in vivo model of HIT, whereas RTO does not. KKO binding to PF4 and heparin is specifically inhibited by human HIT antibodies that activate platelets, whereas inhibition of RTO binding is not differentially affected. Heparin increased the avidity of KKO binding to PF4 without affecting RTO, but it did not increase total binding or binding to nontetrameric PF4K50E. Single-molecule forced unbinding demonstrated KKO was 8-fold more reactive toward PF4 tetramers and formed stronger complexes than RTO, but not to PF4K50E dimers. KKO, but not RTO, promoted oligomerization of PF4 but not PF4K50E. This study reveals differences in the properties of anti-PF4 antibodies that cause thrombocytopenia not revealed by ELISA that correlate with oligomerization of PF4 and sustained high-avidity interactions that may simulate transient antibody-antigen interactions in vivo. These differences suggest the potential importance of epitope specificity in the pathogenesis of HIT.

Thrombomodulin-bound thrombin cleaves protein C (PC) zymogen in blood plasma producing activated protein C (APC), which exerts anti-coagulant, anti-inflammatory, anti-apoptotic and CNS-protective effects. Recombinant APC and thrombomodulin (TM) are both in clinical studies for management of acute conditions including sepsis. Methods that permit accurate measurement of APC in plasma are needed for clinical monitoring and mechanistic studies in animal models. However, the two existing methods require either long incubation periods with substrate, resulting in high background or they also recognize protein C inhibitor (PCI) complexed with APC (APC:PCI), which convolutes analysis of the amount of APC generated. Here we describe a robust quantitative in vivo assay that measures APC generation at both low levels of human protein C seen in chronic inflammatory disease and at physiological levels that shows a >99% fit with in vitro data.

uPA (urokinase-type plasminogen activator) stimulates cell migration through multiple pathways, including formation of plasmin and extracellular metalloproteinases, and binding to the uPAR (uPA receptor; also known as CD87), integrins and LRP1 (low-density lipoprotein receptor-related protein 1) which activate intracellular signalling pathways. In the present paper we report that uPA-mediated cell migration requires an interaction with fibulin-5. uPA stimulates migration of wild-type MEFs (mouse embryonic fibroblasts) (Fbln5+/+ MEFs), but has no effect on fibulin-5-deficient (Fbln5−/−) MEFs. Migration of MEFs in response to uPA requires an interaction of fibulin-5 with integrins, as MEFs expressing a mutant fibulin-5 incapable of binding integrins (FblnRGE/RGE MEFs) do not migrate in response to uPA. Moreover, a blocking anti-(human β1-integrin) antibody inhibited the migration of PASMCs (pulmonary arterial smooth muscle cells) in response to uPA. Binding of uPA to fibulin-5 generates plasmin, which excises the integrin-binding N-terminal cbEGF (Ca2+-binding epidermal growth factor)-like domain, leading to loss of β1-integrin binding. We suggest that uPA promotes cell migration by binding to fibulin-5, initiating its cleavage by plasmin, which leads to its dissociation from β1-integrin and thereby unblocks the capacity of integrin to facilitate cell motility.

Heparin-induced thrombocytopenia is a prothrombotic adverse drug effect induced by platelet-activating antibodies against multimolecular complexes of platelet factor 4 and heparin. Diagnosis rests on a clinical assessment of disease probability and laboratory testing. Management involves immediate discontinuation of heparin and initiation of an alternative anticoagulant. Because of the frequency of thrombocytopenia among heparinized patients, the limited specificity of widely available immunoassays, the limited availability of more specific functional assays, and clinicians' fears of missing a case of true disease, overtesting, overdiagnosis, and overtreatment have become common. As a result, a substantial number of thrombocytopenic patients are unnecessarily exposed to costly alternative anticoagulants and their attendant risk of bleeding. In this review, we describe not only our approach to the evaluation and management of patients with heparin-induced thrombocytopenia, but also the measures we use to minimize misdiagnosis and unnecessary treatment of patients without the disease. In addition, we propose areas of investigation for improvement of the diagnosis and management of this potentially fatal disorder.

Bradykinin (BK), a potent vasoactive nonapeptide formed by proteolytic cleavage of kininogen, mediates its activity by binding to specific cell surface receptors. Delivery of BK to these receptors is limited by cell-bound and plasma kininases that degrade BK to inactive fragments. Binding of its parent compound, kininogen, to specific endothelial cell receptors may provide an environment in which the degradation of BK by kininases is restricted. The determinants that mediate kininogen binding to endothelial cells have not been fully elucidated. The present studies suggest that part of BK and the amino-terminal amino acids of kininogens' common light chain constitute part of this recognition sequence. Human umbilical vein endothelial cells (HUVEC) in culture at 37 degrees C express 2-3-fold more binding sites for biotinylated high molecular weight kininogen (biotin-HK) containing BK than for biotin-HK from which BK has been liberated by plasma kallikrein. Binding of BK-free biotin-HK was not restored by preincubating HUVEC with BK, arguing against the possibility that BK released from biotin-HK stimulated expression of additional HK receptors. The extent of biotin-HK binding to HUVEC at 37 degrees C directly correlated with the amount of BK retained within the protein. Four lines of evidence suggest that part of BK and the amino terminus of the light chain of kininogens are part of the sequence recognized by the endothelial cell kininogen receptor. First, monoclonal antibodies to the carboxyl terminus of BK (MBK3) and the common light chains of the kininogens (HKL6, HKL8) inhibited biotin-HK binding to HUVEC. Second, a synthetic, dimeric bradykinin receptor antagonist blocked biotin-HK (IC50 = 9 microM) binding to HUVEC as did two synthetic tissue kallikrein inhibitors modeled after the carboxyl-terminal sequence of BK. Third, synthetic peptides containing the carboxyl-terminal portion of BK and the amino terminus of kininogens' common light chain, MKRPPGFSPFRSSRIG and GFSPFRSSRIG, blocked binding of biotin-HK (IC50 = 2-3 mM), whereas an overlapping peptide, SPFRSSRIGEIKEETT, at 5 mM and a scrambled peptide, FSGPKRSPIMGRPSFR, did not. Fourth, biotinylated GMKRPPGFSPFRSSRIG specifically bound to HUVEC, and its binding was blocked by HK. Since the presence of the nonapeptide BK in HK contributes to maximal binding of HK to HUVEC, there is a novel type of BK receptor in which part of the nonapeptide is recognized within the context of its parent molecule.(ABSTRACT TRUNCATED AT 400 WORDS)

Heparin induced thrombocytopenia (HIT) is an iatrogenic antibody-mediated disorder with a paradoxically high propensity for thrombosis. We have shown previously that human HIT IgGs and the HIT-like monoclonal antibody (MAb) KKO bind to platelet factor 4 (PF4) complexed with glycosaminoglycans (GAGs) on the surface of platelets and monocytes, initiating cell activation in vitro, thrombocytopenia in a transgenic mouse model, and thrombus formation in a laser microvascular injury model in vivo even in the absence of exogenous heparin. Monocytes bind PF4 and HIT Ab more readily than platelets because they express higher affinity GAGs, heparan sulfate and dermatan sulfate, in addition to chondroitin sulfate found on both cell types. To study changes in the structure of the monocytes that accompany HIT, we used scanning electron microscopy, confocal microscopy and flow cytometry to characterize the morphology and function of isolated human monocytes and mouse transgenic Fcg receptor IIA positive (FcγRIIA+) or wt (FcγRIIA-) monocytes in the absence or presence of platelets. We show by scanning electron microscopy that upon binding of pathogenic HIT Abs to PF4/GAG complexes on FcgRIIA expressing monocytes, they initiate profound remodeling of the cell membrane. Addition of 100 μg/ml recombinant human PF4 in the absence of HIT Abs initiates the activation process with the appearance of 177 ± 53 nm "knobs" on the surface of 70% of monocytes. Subsequent addition of the HIT-like monoclonal antibody KKO at 50 μg/ml dramatically alters the cellular surface with the appearance of large 701 ± 208 nm membrane "blebs" that were not seen on FcγRIIA-mouse monocytes. These large, membrane-associated structures likely engage FcγRIIA, clustering them in proximity to cell-bound immune complexes, which promotes cell activation that leads to thrombosis. These blebs increase in size over time and are then shed from the cells as monocyte-derived microparticles, which self-aggregate. As a result of shedding of these blebs, the monocytes lose much of their typical ruffled surface (only 67% of monocytes maintain ruffles in the presence of PF4 plus KKO, compared to 97% of control monocytes) and appear smoother, sometimes with pores indicating degranulation. In the presence of platelets, monocytes exposed to PF4 and KKO formed heterocellular aggregates in addition to these subcellular changes. In contrast to KKO, addition of the non-pathogenic MAb RTO not only did not induce blebbing, but largely inhibited PF4-induced changes in the monocyte surface. This suggests that RTO might prevent monocyte activation by interfering with PF4 tetramerization. Structural analysis of the shed microparticles by microscopy revealed that they had an average diameter of 356 ± 307 nm, with many larger particles and aggregates. Flow cytometry confirmed that the shed particles contain cell membrane lipids and receptors. Confocal microscopy showed uniform binding of labeled PF4 to the monocyte cell membrane followed by rapid clustering into large complexes after the addition of KKO, but not RTO. These studies affirm the centrality of cell surface PF4/GAG complexes in the pathogenesis of HIT and provide quantitative morphometric characteristics of the changes in the monocyte membrane structure. We propose that PF4 released from activated platelets binds to the surface of GAG-expressing monocytes in vivo, forming clusters of PF4/GAG complexes that likely promote antibody binding and cause monocyte activation through FcγRIIA along with large-scale remodeling of the cell membrane and shedding of procoagulant microparticles. Disclosures No relevant conflicts of interest to declare.

Introduction: We previously demonstrated prolonged circulation and improved efficacy of erythrocyte (red blood cell, RBC) coupled thrombomodulin (TM) fusion proteins as thromboprophylactic and anti-inflammatory agents. To further this therapeutic platform, we produced humanized analogues and investigated their efficacy in models where RBC-coupling may provide not only a pharmacokinetic advantage, but also a pharmacodynamic advantage by local delivery to RBC membranes. Previous fusions were constructed with mouse TM fused to single-chain variable fragments (scFv) of a rat-derived anti-glycophorin A antibody. Therefore, to produce clinically translatable therapeutic fusion proteins, modification of both cargo and targeting moiety are necessary for use in humans to render them non-immunogenic and capable of binding to human RBCs. We aimed to use human-like antibodies against high-prevalence antigens, fuse these antibodies to the extracellular domain of TM, confirm their binding and enzymatic activity, and demonstrate their efficacy in whole-blood models of human vascular pathology. Methods: An IgG Fab phage display library was prepared from a Rhesus macaque immunized with human RBCs. By panning on intact RBCs, Fab/phage specific for human RBCs were identified. We selected two clones from this library, one against a high-prevalence Band 3 antigen (Wright B (Wrb), Diego blood group, >106 copies/RBC) and one against a high-prevalence RhCE antigen (Rh17, Rhesus blood group, ~105 copies/RBC). The variable chain sequences of these antibodies were cloned into a scFv construct and fused to the extracellular domain of human TM. Fusion proteins were produced in S2 cells using a metallothionein promoter expression system. The soluble extracellular domain of human TM was similarly cloned and produced as a control. Binding of the fusions to RBCs was measured by indirect agglutination and by ELISA with immobilize erythrocyte ghosts. Activity of the TM fusions was measured by colorimetric cleavage of an APC substrate. Adverse effects of fusions on RBCs were investigated in a model of osmotic stress in hypotonic saline as well as mechanical integrity against agitation with glass beads. Assays of endothelial protection were performed on human umbilical vein endothelial cells (HUVEC) activated lipopolysaccharide (LPS), TNF-α, heme, and HMGB1. Culture supernatants were analyzed by ELISA for IL-6, IL-8, and vWF. An endothelialized, whole-blood microfluidic platform was developed using the Bio-Flux (Fluxion) microfluidic system. Models of vascular pathology included TNF-α activation of the endothelialized channels as well as localized, light-induced injury with hematoporphyrin. Results: We successfully produced fusion proteins of human TM and human-like scFv antibody derivatives capable of specifically binding to human RBCs (Figure 1, a). The fusions demonstrated affinities suitable for translation, and enabled comparison of different levels of surface loading. The scFv/TM fusions did not induce direct RBC agglutination, and did not have adverse effects on RBC integrity under osmotic and mechanical stress. The fusions maintained similar enzymatic activity to their soluble TM counterparts (Figure 1, b) and remained active when bound to RBCs (Figure 1, c). The TM fusions also demonstrated efficacy in protection of HUVECs against activation by inflammatory mediators such as LPS and thrombin (Figure 1, d-f), both as soluble proteins and when bound to RBCs. In a whole-blood endothelialized microfluidic system, the fusions reduced fibrin deposition and channel occlusion in activated and injured endothelium. Figure 1 - (a) hTM-Wrb binds specifically to human RBC with detectable binding at Conclusion: Thrombomodulin was successfully coupled to human RBCs by fusion to human-like antibodies against high-prevalence epitopes. The RBC-coupled TM maintained its anti-inflammatory and anti-thrombotic activity without adversely affecting RBC integrity, and had therapeutic activity in a microfluidic model of vascular pathology. Figure 1. Figure 1. Disclosures No relevant conflicts of interest to declare.

Human neutrophil peptides (HNPs) or alpha (α)-defensins are a family of small antimicrobial peptides which play important roles in innate immunity against invading bacteria, fungi, and viruses. HNPs contain 29-33 amino acid residues which share a distinct pattern of disulfide bonding. HNPs can be further subdivided into myeloid (HNPs 1-4) and enteric (HD5-6) forms. HNPs-1, -2, and -3 are structurally identical except for the first amino acid residue and are predominantly expressed in human neutrophils from which they are released at sites of infection or inflammation. Previous studies have demonstrated that HNP1 promotes thrombus formation. However, the mechanisms underlying its prothrombotic effects are not fully understood. In the present study, we demonstrate that HNP-1, -2, and -3 all inhibit plasma-derived and recombinant ADAMTS13 activity in a concentration-dependent manner as determined by the cleavage of FRETS-VWF73 (Fig. 1A & 1B) and multimeric VWF under denaturing conditions. At final concentrations of ~10-15 µM, purified and synthetic HNP-1, -2, and -3 completely abolish ADAMTS13Õs ability to cleave FRETS-VWF73 (Fig. 1A & 1B). The concentrations required for complete inhibition of proteolytic cleavage of pre-denatured VWF by ADAMTS13 using the urea dialysis method is higher, likely resulting from the removal of peptides from the reaction. HNP-1 binds to ultra large VWF released from endothelial cells, soluble multimeric VWF, GST-VWF73 peptide, and ADAMTS13 as determined by cultured endoethelial cells in a microfluidic channels and by surface plasmon resonance. The affinity (the dissociation constant, KD) for HNP-1 to bind VWF, GST-VWF73, and ADAMTS13 is 8.0 micro mol/L, 1.0 micro mol/L, and 3.2 micro mol/L, respectively. Sequence analysis reveals that the amino acid residues of HNP-1, -2, and -3 all contain a RRY motif that is also found in the spacer domain (i.e. 659RRYGEEY665) of ADAMTS13. We hypothesize that competition by HNPs with ADAMTS13 for binding to VWF-A2 domain mediates their inhibition. As shown, a deletion or alanine substitution of RRY within HNP1 nearly abolishes its ability to inhibit ADAMTS13 activity determined by the cleavage of FRETS-VWF73 (in Fig. 1C) and multimeric VWF under denaturing conditions. Similarly, HNP-beta and aliphatic (with no aromatic rings directly on the nitrogen atom) HNP-1 exhibit no inhibitory activity on ADAMTS13 (Fig. 1C). To further demonstrate the inhibitory activity of HNP1 towards ADAMTS13 under more physiological conditions, a BioFlux microfluidic system is employed. Addition of purified (native) HNP-1 (6-15 micro mol/L) to D-phenylalanyl-prolyl-arginyl chloromethyl ketone (PPACK) anticoagulated whole blood dramatically augments the rate of platelet adhesion and aggregation to the fibrillar collagen-coated surface under arterial shear stress (approximately 100 dyne per square centimeter). These results indicate that HNP-1 plays a role in the inhibition of VWF proteolysis by ADAMTS13 under flow. We conclude that HNP-1, -2, and -3 released from activated neutrophils at sites of infection or inflammation could significantly augment thrombus formation by inhibiting the local or residual plasma ADAMTS13 activity, when the circulating ADAMTS13 activity is already at critically low levels as in cases of hereditary or acquired autoimmune TTP and HUS, thereby triggering the onset of thrombotic complications. (*authors contribute equally to this work). Figure 1. Figure 1. HNPs are potent inhibitors of ADAMTS13 metalloprotease. A. Purified HNPs inhibit ADAMTS13-mediated cleavage of FRETS-VWF73; B. HNP-1, -2, and -3 all inhibit the cleavage of FRETS-VWF73 by ADAMTS13; C. No inhibition of the ADAMTS13 dependent cleavage of FRETS-VWF73 by HNP1 mutants, HNP-beta, and aliphatic HNP1 compared with WT control. Disclosures No relevant conflicts of interest to declare.

Replicating the complexities of the human blood vessel include the establishment of a 3-D confluency of viable endothelial cells (ECs) on an appropriate matrix, use of human whole blood or specific components of blood, varied shear stresses, and the induction of a localized and controlled injury within the observable field to understand and intervene in hemostatic events. This array of complexities have made vascular modeling an important unmet challenge. Such a model would enhance our understanding of the pathogenesis of diverse coagulation disorders, such as the prothrombotic disorder HIT. In HIT, the platelet-rich clots lead to its other designation as the "white clot syndrome". To provide an improved injury model, we adapted a hematoporphyrin-photochemical injury using a Fluxion Bioflux microfluidic system. When illuminated with 405 nm light, hematoporphyrin releases reactive oxygen species, inducing localized EC injury within the exposed field, but without denuding ECs. Unlike rose bengal, hematoporphyrin does not cause fluorescent interference with quantitative analysis of the developing thrombus. We propose that the model permits refined analysis of ECs transitioning from areas of quiescence to injury to quiescence, allowing us to localize and quantify the contribution of various components to the growing thrombus. In HIT, patients form antibodies to complexes of the platelet-specific chemokine, platelet factor 4 (PF4), and negatively-charged molecules such as infused heparin and heparans found on the surface of platelets and monocytes. ECs also bind PF4 immune complexes due to their highly negatively charged glycocalyx-rich surface. Prior murine cremaster laser injury studies showed that HIT antibodies bind predominantly to the EC surface rather than platelet within the thrombus itself. Was this observation related to the nature of the cremaster injury? Would antibody binding to the EC lining also be important in a wholly human HIT detection system? Using the described photochemical microfluidic system, we created a localized injury in human umbilical vein EC-lined channels through which we flowed whole human blood. Activated platelets established growing aggregates at the site of EC injury, releasing more PF4 that then bound to non-activated ECs downstream of the injury. Whole blood containing a HIT-like antibody to simulate the prothrombotic state of HIT was then flowed over this injured vasculature. HIT antibodies and then platelets bound sequentially to this new site of HIT antigen, allowing the thrombus to propagate downstream. We have named this phenomenon "rolling barrage" and suggest that a key part of the prothrombotic nature of HIT lies in antibody-mediated activation of downstream EC with subsequent thrombus propagation. HIT often occurs in the setting in surgery, which might prime EC dysfunction. We therefore treated the ECs (TNF-α) prior to injury and introducing the HIT-like antibody. TNF-α activated the EC lining, leading to loss of the anticoagulant EC surface, which enhanced clot formation downstream of the site of photochemical injury. Therefore, in a prothrombotic state such as HIT, we propose that a local injury acts as a nidus for thrombus initiation. The procoagulant process spreads distally in part because of released PF4 adhering to the downstream EC glycocalyx, which is exacerbated by mediators of inflammation. We anticipate that the described model can be used to study novel interventions to block this cycle in a wholly human system with control over the contribution of individual cellular elements, and will further understanding of the importance of this mechanism in other prothrombotic disorders. Disclosures No relevant conflicts of interest to declare.

The urokinase receptor (uPAR) is linked to plasma membranes through a glycosylphosphatidylinositol (GPI) anchor. It has been posited that the GPI anchor facilitates clearance of uPAR-bound complexes between two chain urokinase (tcuPA) and plasminogen activator inhibitor type 1 (PAI-1) by the alpha 2-macroglobulin receptor (alpha 2MR) which permits re-expression of unoccupied uPA receptors on the cell surface. To test this hypothesis we compared internalization and degradation of 125I-labeled tcuPA-PAI-1 by COS cells expressing either transfected wild-type, GPI-linked uPAR (uPAR/GPI), or a chimeric receptor composed of the extracellular domains of uPAR linked to the transmembrane and cytosolic domains of the alpha chain (p55 subunit) of the interleukin-2 receptor (uPAR/IL-2R alpha). The kinetics of binding, internalization and degradation of tcuPA-PAI-1 by COS cells expressing each form of uPAR were virtually identical. However, internalization of complexes by uPAR/IL-2R alpha was more susceptible to inhibition by recombinant soluble 39-kDa alpha 2MR-associated protein (RAP) which competes for binding of tcuPA-PAI-1 complexes to alpha 2MR (p < 0.001), and the internalization was accompanied by a greater reduction in the number of surface uPAR/IL-2R alpha, than uPAR/GPI (p < 0.05). These studies indicate that the rate of internalization of tcuPA-PAI-1 is governed primarily by the extracellular domains of uPAR, whereas the GPI anchor may facilitate internalization of complexes and re-expression of uPAR when binding sites on alpha 2MR are limiting.

N-methyl-D-aspartate (NMDA)-induced pial artery dilation (PAD) is reversed to vasoconstriction after fluid percussion brain injury (FPI). Tissue type plasminogen activator (tPA) is up-regulated and the tPA antagonist, EEIIMD, prevents impaired NMDA PAD after FPI. Mitogen-activated protein kinase (MAPK), a family of at least three kinases, ERK, p38, and JNK, is also up-regulated after traumatic brain injury (TBI). We hypothesize that tPA impairs NMDA-induced cerebrovasodilation after FPI in a MAPK isoform-dependent mechanism.Lateral FPI was induced in newborn pigs. The closed cranial window technique was used to measure pial artery diameter and to collect cerebrospinal fluid (CSF). ERK, p38, and JNK MAPK concentrations in CSF were quantified by ELISA.CSF JNK MAPK was increased by FPI, increased further by tPA, but blocked by JNK antagonists SP600125 and D-JNKI1. FPI modestly increased p38 and ERK isoforms of MAPK. NMDA-induced PAD was reversed to vasoconstriction after FPI, whereas dilator responses to papaverine were unchanged. tPA, in post-FPI CSF concentration, potentiated NMDA-induced vasoconstriction while papaverine dilation was unchanged. SP 600125 and D-JNKI1, blocked NMDA-induced vasoconstriction and fully restored PAD. The ERK antagonist U 0126 partially restored NMDA-induced PAD, while the p38 inhibitor SB203580 aggravated NMDA-induced vasoconstriction observed in the presence of tPA after FPI.These data indicate that tPA contributes to impairment of NMDA-mediated cerebrovasodilation after FPI through JNK, while p38 may be protective. These data suggest that inhibition of the endogenous plasminogen activator system and JNK may improve cerebral hemodynamic outcome post-TBI.

The concentration of urokinase plasminogen activator (uPA) is elevated in pathological settings such as acute lung injury, where pulmonary arterial contractility and permeability are disrupted. uPA limits the accretion of fibrin after injury. Here we investigated whether uPA also regulates pulmonary arterial contractility and permeability. Contractility was measured using isolated pulmonary arterial rings. Pulmonary blood flow was measured in vivo by Doppler and pulmonary vascular permeability, according to the extravasation of Evans blue. Our data show that uPA regulates the in vitro pulmonary arterial contractility induced by phenylephrine in a dose-dependent manner through two receptor-dependent pathways, and regulates vascular contractility and permeability in vivo. Physiological concentrations of uPA (≤1 nM) stimulate the contractility of pulmonary arterial rings induced by phenylephrine through the low-density lipoprotein receptor–related protein receptor. The procontractile effect of uPA is independent of its catalytic activity. At pathophysiological concentrations, uPA (20 nM) inhibits contractility and increases vascular permeability. The inhibition of vascular contractility and increase of vascular permeability is mediated through a two-step process that involves docking to N-methyl-d-aspartate receptor–1 (NMDA-R1) on pulmonary vascular smooth muscle cells, and requires catalytic activity. Peptides that specifically inhibit the docking of uPA to NMDA-R, or the uPA variant with a mutated receptor docking site, abolished both the effects of uPA on vascular contractility and permeability, without affecting its catalytic activity. These data show that uPA, at concentrations found under pathological conditions, reduces pulmonary arterial contractility and increases permeability though the activation of NMDA-R1. The selective inhibition of NMDAR-1 activation by uPA can be accomplished without a loss of fibrinolytic activity.

Tissue-type plasminogen activator (tPA) is a potent fibrinolytic enzyme used to treat acute coronary artery obstruction. However, tPA has shown limited utility in other disorders caused by thrombotic vascular occlusion, such as pulmonary embolism. We found that tPA caused dose-dependent effects on the contractility of pulmonary arterial rings that may affect its effectiveness as a thrombolytic agent. At low concentrations (1 nM), tPA stimulated pulmonary vascular contraction in response to phenylephrine, whereas at higher concentrations (20 nM) tPA inhibited pulmonary arterial contractility and promoted pulmonary vascular permeability through an interaction between its “docking site” and N-methyl d-aspartate receptor type 1 (NMDA-R1) expressed by pulmonary arteries. A hexapeptide derived from plasminogen activator inhibitor type 1 that blocked the docking site of tPA, but not its catalytic activity, inhibited its interaction with NMDA-R1, abolished inhibition of pulmonary artery contractility, attenuated vascular permeability, and facilitated fibrinolysis in a murine model of pulmonary embolism. Similar outcomes were seen using a tPA variant that lacks the docking site but retains catalytic activity. These data suggest that it is feasible to attenuate the deleterious extrafibrinolytic effects of tPA and improve its benefit:risk profile in the management of pulmonary embolism.

Urokinase plasminogen activator (uPA) and PA inhibitor type 1 (PAI-1) are elevated in acute lung injury, which is characterized by a loss of endothelial barrier function and the development of pulmonary edema. Two-chain uPA and uPA-PAI-1 complexes (1-20 nM) increased the permeability of monolayers of human pulmonary microvascular endothelial cells (PMVECs) in vitro and lung permeability in vivo. The effects of uPA-PAI-1 were abrogated by the nitric-oxide synthase (NOS) inhibitor L-NAME (N(D)-nitro-L-arginine methyl ester). Two-chain uPA (1-20 nM) and uPA-PAI-1 induced phosphorylation of endothelial NOS-Ser(1177) in PMVECs, which was followed by generation of NO and the nitrosylation and dissociation of β-catenin from VE-cadherin. uPA-induced phosphorylation of eNOS was decreased by anti-low density lipoprotein receptor-related protein-1 (LRP) antibody and an LRP antagonist, receptor-associated protein (RAP), and when binding to the uPA receptor was blocked by the isolated growth factor-like domain of uPA. uPA-induced phosphorylation of eNOS was also inhibited by the protein kinase A (PKA) inhibitor, myristoylated PKI, but was not dependent on PI3K-Akt signaling. LRP blockade and inhibition of PKA prevented uPA- and uPA-PAI-1-induced permeability of PMVEC monolayers in vitro and uPA-induced lung permeability in vivo. These studies identify a novel pathway involved in regulating PMVEC permeability and suggest the utility of uPA-based approaches that attenuate untoward permeability following acute lung injury while preserving its salutary effects on fibrinolysis and airway remodeling.

Outcome of traumatic brain injury (TBI) is impaired by hyperglycemia, hypotension, and glutamate, and improved by insulin. Insulin reduces glutamate concentration, making it uncertain whether its beneficial effect accrues from euglycemia. Glucagon decreases CNS glutamate, lessens neuronal cell injury, and improves neurological scores in mice after TBI. In vitro, glucagon limits NMDA-mediated excitotoxicity by increasing cAMP and protein kinase A (PKA). NMDA receptor activation couples cerebral blood flow (CBF) to metabolism. Dilation induced by NMDA is impaired after fluid percussion brain injury (FPI) due to upregulation of endogenous tPA, which further disturbs cerebral autoregulation during hypotension after fluid percussion injury (FPI). We hypothesized that glucagon prevents impaired NMDA receptor-mediated dilation after FPI by upregulating cAMP, which decreases release of tPA. NMDA-induced pial artery dilation (PAD) was reversed to vasoconstriction after FPI. Glucagon 30 min before or 30 min after FPI blocked NMDA-mediated vasoconstriction and restored the response to vasodilation. PAD during hypotension was blunted after FPI, but protected by glucagon. Glucagon prevented FPI-induced reductions in CSF cAMP, yielding a net increase in cAMP, and blocked FPI-induced elevation of CSF tPA. Co-administration of the PKA antagonist Rp 8Br cAMPs prevented glucagon-mediated preservation of NMDA-mediated dilation after FPI. The pKA agonist Sp 8Br cAMPs prevented impairment of NMDA-induced dilation. These data indicate that glucagon protects against impaired cerebrovasodilation by upregulating cAMP, which decreases release of tPA, suggesting that it may provide neuroprotection when given after TBI, or prior to certain neurosurgical or cardiac interventions in which the incidence of perioperative ischemia is high.

Clinical scenario: An otherwise healthy 25-year-old woman returns to your office for management of chronic primary immune thrombocytopenia. She was diagnosed 6 months earlier and continues to require prednisone 15 mg daily and periodic infusions of intravenous immunoglobulin to maintain a hemostatic platelet count. You discuss second-line treatment options, including splenectomy. The patient asks if there are any means by which to predict likelihood of response to splenectomy. You have heard about the use of indium-labeled autologous platelet scanning for this purpose and wonder what the evidence shows. Immune thrombocytopenia (ITP) comprises a heterogeneous group of disorders characterized by autoimmune-mediated platelet destruction by tissue macrophages in the spleen and elsewhere, variable impairment of thrombopoiesis, and an elevated risk of bleeding. Splenectomy remains a highly effective and well-tolerated treatment for patients refractory to or intolerant of first-line therapy with long-term complete response rates of 66%. 1‐3 Nevertheless, splenectomy is not without its drawbacks, including complications related to surgery and anesthesia in the short-term and lifelong risks of serious infection and possibly thromboembolism. 2 Indium-labeled autologous platelet scanning (ILAPS) is a nuclear imaging modality in which autologous platelets are reinfused into the patient after ex vivo labeling with 111 In. Subsequent scintigraphy demonstrates the site(s) of platelet sequestration and clearance. It has been proposed that patients with purely or predominantly splenic sequestration determined by ILAPS may be more likely to respond to splenectomy than those exhibiting hepatic, mixed, or diffuse patterns. 4 To evaluate the potential of ILAPS to predict response to splenectomy, we performed a comprehensive literature review of all studies in which ILAPS was performed prior to splenectomy in patients with ITP.

Women with anticardiolipin antibodies (ACLA) are at increased risk for fetal loss. One potential explanation for this outcome is that sera from these individuals contain antibodies reactive with trophoblast cells, which are involved in the establishment of the uteroplacental vasculature and maintenance of placental blood fluidity. To examine this hypothesis, we compared the incidence of trophoblast-reactive antibodies in 27 patients with ACLA and a history of fetal loss with that in 29 normal pregnant women. Sera from 20 patients, but only one control, contained trophoblast-reactive antibodies (P < .001). These antibodies were not directed against major histocompatibility class I antigens, and reacted with both term and first-trimester trophoblast cells. In most cases, sera from which ACLA were adsorbed by cardiolipin- containing liposomes maintained reactivity against cells. In addition, patient Ig fractions immunoprecipitated an approximately 62-kD protein from the trophoblast cell surface, stimulated the release of arachidonic acid and thromboxane A2 by trophoblasts, and inhibited the binding of prourokinase to trophoblast urokinase receptors. These observations show that sera from women with ACLA and a history of fetal loss contain antitrophoblast antibodies. These antibodies may be serologically distinct from ACLA, and may contribute to the pathogenesis of fetal demise.

The vasoactive compound bradykinin (BK) is liberated by proteolytic cleavage from high molecular weight kininogen (HK) and low molecular weight kininogen (LK). Expression of kininogens on cell surface receptors may affect the delivery of BK at sites of inflammation. Therefore, we investigated whether BK itself alters the expression of binding sites for its parent molecules, HK and LK, on the surface of cultured human umbilical vein endothelial cells (HUVEC). 125I-LK and 125I-HK each bind to a single class of sites on HUVEC in reactions that are saturable, reversible, and zinc-dependent (Bmax = 9.7 +/- 0.2 x 10(5) sites/cell; kd = 43.3 +/- 8 nmol/L; n = 5 and Bmax = 10.3 +/- 0.4 x 10(5) sites/cell; kd = 40.3 +/- 0.9 nmol/L; n = 3 for LK and HK, respectively). HK and LK compete for the same binding site (Ki = 19.4 +/- 5 nmol/L HK v 125I-LK; Ki = 24.5 +/- 4 nmol/L LK v 125I-HK, n = 3). Moreover, 50-fold molar excess light chain of HK inhibits 125I-LK binding 51% and 50-fold molar excess LK and the heavy chain of HK inhibit 125I-light chain of HK binding 92% and 76%, respectively. Preincubation of HUVEC with BK produces a transient, concentration- dependent increase in the binding of HK and LK, reaching a maximum 3 to 4 hours after addition of BK (46% increase over control for HK; 57% increase over control for LK; P < .005 for each ligand). Des-Arg9- bradykinin, a B1 receptor agonist, increases kininogen binding to the same extent as BK; the upregulation of kininogen binding sites by BK is partially blocked by a B1 but not by a B2 receptor antagonist. The protein kinase C inhibitors (PKC), sphingosine and H7, completely block the induction of HK receptors by BK. Phorbol 12-myristate 13-acetate (PMA), which also activates PKC, stimulates the binding of HK and the purified light chain of HK to HUVEC as well. However, unlike HK and its light chain, binding of LK and the heavy chain of HK are increased by PMA only in the presence of added calcium ion. These studies show that BK upregulates a common binding site for HK, LK, and each chain of HK on HUVEC. Induction of kininogen receptors on endothelial cells by BK may modulate the generation of this vasoactive compound at sites of vascular injury.

The purpose of this study was to determine if the plasma concentration of endothelin-1 is elevated in pregnant women abusing cocaine and to determine how these levels differ from those in patients with preeclampsia and in women with uncomplicated pregnancies.Plasma endothelin-1 levels were measured in 30 women with acute cocaine intoxication, 32 women with preeclampsia, 14 pregnant women with chronic hypertension, 26 women with uncomplicated pregnancies, and 16 nonpregnant individuals. Serial samples after delivery were obtained in 12 women with preeclampsia, 10 with cocaine abuse, 4 with chronic hypertension, and 7 with uncomplicated pregnancies.The mean endothelin-1 concentration in those with cocaine abuse was 18.2 +/- 8.1 pg/ml (95% confidence interval 15.2 to 21.2). This was similar to that in women with preeclampsia (21.1 +/- 5.9 pg/ml, 95% confidence interval 19 to 23.3) (p = 0.2) but significantly different from that in women with chronic hypertension (11.5 +/- 3.6 pg/ml, 95% confidence interval 9.4 to 13.6) (p0.001) and women with uncomplicated pregnancies (6.7 +/- 3.9 pg/ml, 95% confidence interval 5.1 to 8.2) (p0.001).Endothelin-1 levels in women abusing cocaine are comparable to those in women with preeclampsia and are significantly higher than those in gravid women with chronic hypertension and women with uncomplicated pregnancies. Elevated levels of endothelin-1 may contribute to some of the pregnancy-related complications in women abusing cocaine.

Binding of urokinase-type plasminogen activator (u-PA) to specific receptors (u-PAR) on the surface of endothelial cells contributes to the regulation of plasmin-dependent processes such as fibrinolysis and angiogenesis. We studied the effect of raising intracellular levels of cyclic AMP (cAMP) and/or activating protein kinase C on the expression of u-PAR in cultured human umbilical vein endothelial cells (HUVEC). Incubation of HUVEC with forskolin stimulated a time- and concentration-dependent increase in the expression of u-PAR, measured both by an increase in the specific binding of radiolabeled single-chain u-PA (scu-PA) and by increased binding of anti-u-PAR antibodies. Maximal increase in u-PAR expression (81 +/- 11% above control, n = 11) was not associated with a change in receptor affinity for scu-PA when HUVEC were incubated for 20 hours at 37 degrees C with 50 microM forskolin. Receptor induction by forskolin was inhibited when HUVEC were preincubated with deoxyadenosine monophosphate (DAM), an inhibitor of adenylyl cyclase. A similar increase in receptor expression (128 +/- 27% above control, n = 3) was induced by the cAMP analogue 8-bromoadenosine 3':5'-cyclic monophosphate (50 mM). Forskolin induced an approximately twofold increase in the expression of a single approximately 1.4-kb u-PAR messenger RNA (mRNA) transcript within 2 hours. Phorbol myristate acetate (PMA) also stimulated a time- and concentration-dependent increase in specific scu-PA binding. The maximal increase in u-PAR expression (254 +/- 27% above control, n = 11) was observed when HUVEC were preincubated with 10 nM PMA for 20 hours. Induction of u-PAR by PMA was inhibited when HUVEC were preincubated with either cycloheximide or H7 but was unaffected by DAM. u-PAR induced by PMA showed a reduced affinity for scu-PA (Kd, 14 +/- 2 nM versus 3.6 +/- 0.6 nM, p < 0.001; n = 8). PMA stimulation for 20 hours resulted in a sixfold increase in a single approximately 1.4-kb u-PAR mRNA transcript, with increased levels detectable within 30 minutes. Coincubation of HUVEC with optimal concentrations of forskolin and PMA for 20 hours produced a fully additive increase in u-PAR expression at both the mRNA and protein levels. These data suggest that both cAMP-dependent and protein kinase C-dependent protein kinase pathways may independently regulate u-PAR expression in human endothelial cells.

The sole FDA approved treatment for acute stroke is tissue type plasminogen activator (tPA). However, tPA potentiates impairment of pial artery dilation in response to hypotension after hypoxia/ischemia (H/I) in pigs. ATP and Ca sensitive K channels (Katp and Kca) are important regulators of cerebrovascular tone and mediate cerebrovasodilation in response to hypotension. Mitogen activated protein kinase (MAPK), a family of at least 3 kinases, ERK, p38 and JNK, is upregulated after H/I, with the ERK isoform contributing to vasodilator impairment. This study examined the effect of H/I on Katp and Kca induced pial artery dilation and the roles of tPA and ERK during/after injury in piglets equipped with a closed cranial window. H/I blunted vasodilation induced by the Katp agonists cromakalim, calcitonin gene related peptide (CGRP) and the Kca agonist NS 1619; the effect of each was exacerbated by tPA. Pre- or post-injury treatment with EEIIMD, a hexapeptide derived from plasminogen activator-1, and ERK antagonist U 0126 prevented Katp and Kca channel agonist induced vasodilator impairment while the inactive analogue EEIIMR had no effect. ERK was upregulated after H/I, which was potentiated by tPA. These data indicate that H/I impairs K channel mediated cerebrovasodilation. tPA augments loss of K channel function after injury by upregulating ERK. These data suggest that thrombolytic therapy for treatment of CNS ischemic disorders can dysregulate cerebrohemodynamics by impairing cation-mediated control of cerebrovascular tone.

SummaryThe physiologic activation of the plasma kallikrein-kinin system requires the assembly of its constituents on a cell membrane. High-molecular-weight kininogen (HK) and cleaved HK (HKa) both interact with at least three endothelial cell binding proteins: urokinase plasminogen activator receptor (uPAR), globular C1q receptor (gC1qR,) and cytokeratin 1 (CK1). The affinity of HK and HKa for endothelial cells are KD=7–52 nM. The contribution of each protein is unknown. We examined the direct binding of HK and HKa to the soluble extracellular form of uPAR (suPAR), gC1qR and CK1 using surface plasmon resonance. Each binding protein linked to a CM-5 chip and the association, dissociation and KD (equilibrium constant) were measured. The interaction of HK and HKa with each binding protein was zinc-dependent. The affinity for HK and HKa was gC1qR>CK1>suPAR, indicating that gC1qR is dominant for binding. The affinity for HKa compared to HK was the same for gC1qR, 2.6-fold tighter for CK1 but 53-fold tighter for suPAR. Complex between binding proteins was only observed between gC1qR and CK1 indicating that a binary CK1-gC1qR complex can form independently of kininogen. Although suPAR has the weakest affinity of the three binding proteins, it is the only one that distinguished between HK and HKa. This finding indicates that uPAR may be a key membrane binding protein for differential binding and signalling between the cleaved and uncleaved forms of kininogen. The role of CK1 and gC1qR may be to initially bind HK to the membrane surface before productive cleavage to HKa.

Heparin-induced thrombocytopenia (HIT) is a life- and limb-threatening thrombotic disorder that develops after exposure to heparin, often in the setting of inflammation. We have shown previously that HIT is associated with antibodies to complexes that form between platelet factor 4 and glycosaminoglycan (GAG) side chains on the surface of platelets. However, thrombosis can occur in the absence of thrombocytopenia. We now show that platelet factor 4 binds to monocytes and forms antigenic complexes with their surface GAG side chains more efficiently than on platelets likely due to differences in GAG composition. Binding to monocytes is enhanced when the cells are activated by endotoxin. Monocyte accumulation within developing arteriolar thrombi was visualized by situ microscopy. Monocyte depletion or inactivation in vivo attenuates thrombus formation induced by photochemical injury of the carotid artery in a modified murine model of HIT while paradoxically exacerbating thrombocytopenia. These studies demonstrate a previously unappreciated role for monocytes in the pathogenesis of arterial thrombosis in HIT and suggest that therapies targeting these cells might provide an alternative approach to help limit thrombosis in this and possibly other thrombotic disorders that occur in the setting of inflammation.

Rationale: The involvement of neutrophil activation in the sentinel, potentially reversible, events in the pathogenesis of acute lung injury (ALI) is only partially understood. α-Defensins are the most abundant proteins secreted by activated human neutrophils, but their contribution to ALI in mouse models is hindered by their absence from murine neutrophils and the inability to study their effects in isolation in other species. Objectives: To study the role of α-defensins in the pathogenesis of ALI in a clinically relevant setting using mice transgenic for polymorphonuclear leukocyte expression of α-defensins. Methods: Transgenic mice expressing polymorphonuclear leukocyte α-defensins were generated. ALI was induced by acid aspiration. Pulmonary vascular permeability was studied in vivo using labeled dextran and fibrin deposition. The role of the low-density lipoprotein–related receptor (LRP) in permeability was examined. Measurements and Main Results: Acid aspiration induced neutrophil migration and release of α-defensins into lung parenchyma and airways. ALI was more severe in α-defensin–expressing mice than in wild-type mice, as determined by inspection, influx of neutrophils into the interstitial space and airways, histological evidence of epithelial injury, interstitial edema, extravascular fibrin deposition, impaired oxygenation, and reduced survival. Within 4 hours of insult, α-defensin–expressing mice showed greater disruption of capillary–epithelial barrier function and ALI that was attenuated by systemic or intratracheal administration of specific inhibitors of the LRP. Conclusions: α-Defensins mediate ALI through LRP-mediated loss of capillary–epithelial barrier function, suggesting a potential new approach to intervention.

Human umbilical vein endothelial cells (HUVEC) in culture express two classes of binding sites for tissue-type plasminogen activator (t-PA). The high-affinity binding site has been identified as PA inhibitor type 1 (PAI-1), which binds to the catalytic portion of the molecule, while the second site binds t-PA through an active-site independent domain. Because recombinant t-PA (rt-PA) is often administered concomitantly with heparin, we investigated the effects of heparin on rt-PA binding to HUVEC. Preincubation of HUVEC with heparin at 4 degrees C increased the binding of radiolabeled rt-PA in a time- and dose-dependent manner. One-half maximal increase in binding was observed within 10 minutes of heparin addition. When HUVEC were preincubated with optimal concentrations (5 U/mL) of heparin for 4 hours at 4 degrees C, a 2.5- +/- 0.2-fold increase in specific binding was observed (mean +/- SEM, n = 12, P less than .01). Other highly sulfated glycosaminoglycans and fucoidan (a sulfated polymer of fucose) stimulated rt-PA binding as well, whereas glycosaminoglycans with lower sulfate content than heparin did not. Several results suggested that heparin increased the binding of rt-PA to “cell-associated” PAI-1. First, only active-site- dependent binding was enhanced by heparin, whereas binding of active- site blocked rt-PA was not affected. Second, extracts from HUVEC preincubated with heparin contained increased amounts of rt-PA-PAI-1 complexes as shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Third, antibodies to PAI-1 blocked the increased binding entirely. HUVEC preincubated with heparin also bound increased amounts of enzymatically active radiolabeled urokinase-type PAs. However, HUVEC preincubated with heparin did not express increased amounts of immunoreactive PAI-1. Therefore, heparin, at therapeutic concentrations, may enhance or stabilize the association of PAs with endothelial cell-associated PAI-1.

It is generally recognized that nascent proteins destined to be processed to a phosphatidylinositol-glycan (PI-G)-anchored membrane form contain a hydrophobic signal peptide at both their NH2 and COOH termini. In previous studies we showed that rough microsomal membranes (RM) prepared from CHO cells can carry out COOH-terminal processing. We have now investigated RM prepared from many additional cell types, including frog oocytes, B cells, and T cells, and found that all are competent with respect to COOH-terminal processing. Exceptions were certain mutant T cells that had been shown to be defective at various steps of PI-G anchor biosynthesis [Sugiyama, E., De Gasperi, R., Urakaze, M., Chang, H.-M., Thomas, L. J., Hyman, R., Warren, C. D. & Yeh, E. T. H. (1991) J. Biol. Chem. 266, 12119-12122]. In one such defective mutant, COOH-terminal processing activity of RM could be restored either by transfecting the intact cells with the gene for the deficient step in PI-G synthesis or by adding PI-G extracts to the RM in vitro. Cleavage of the COOH-terminal signal peptide in the RM is therefore dependent on the presence of intact PI-G incorporated into the mature protein.

The processes of implantation and placentation are both dependent on the invasion and remodeling of the uterine endometrium and vasculature by trophoblasts. Because the secretion and autocrine binding of urokinase (uPA) appears to be a common mechanism used by cells to facilitate plasmin-dependent tissue invasion, we measured the production of uPA and expression of uPA receptors by trophoblasts. Prourokinase bound specifically, reversibly, and with high affinity to cultured trophoblasts, via the uPA epidermal growth factor-like domain. Trophoblasts derived from two first-trimester placentae bound more prourokinase than cells isolated from term placentae. Furthermore, in vitro differentiation of cultured cytotrophoblasts into syncytiotrophoblasts was associated with diminished expression of urokinase receptors and a parallel decrease in the cellular content of uPA receptor mRNA. Trophoblasts also secreted prourokinase and plasminogen activator inhibitors types 1 and 2 (PAI-1 and PAI-2). Although prourokinase was secreted in amounts sufficient to endogenously saturate trophoblast uPA receptors, trophoblasts secreted greater amounts of PAI-1 and PAI-2 than uPA, and no net plasminogen activator activity was detected in trophoblast conditioned medium. In contrast, plasminogen added directly to cultured trophoblasts was readily converted to plasmin. Although the invasion and remodeling of uterine tissues by trophoblasts is a complex process dependent on several proteases of varying specificity, our findings suggest that the expression and modulation of urokinase receptors on the trophoblast cell surface may play an important role in this process.