Expression of hirudin fusion proteins in mammalian cells: a strategy for prevention of intravascular thrombosis

Ethics, virtues and xenotransplantation

Early in 2022 the first pig to human cardiac xenotransplant was performed. The graft initially performed well, and rejection was well controlled. However, the graft failed, and the patient died 60 days after the procedure. The ethical issues relating to xenotransplantation include the risk/benefit to the individual, the risk of porcine-derived infectious agents crossing into humans, animal welfare and rights, issues of human and animal identity and concerns relating to fair allocation of organs and appropriate use of resources.These ethical issues are often addressed using emotional arguments, or through consequentialist or deontological lens. An alternative is to use approaches based on virtue ethics to understand the moral purpose (telos) of the research and the virtues (character traits) needed to be a good research clinician. In this review we will consider the virtues of justice, courage, temperance and practical wisdom, as well as the role of clinical curiosity, and their application to xenotransplantation. This provides an alternative approach for the clinical academic and others involved in the research to reflect on their practice.

Cell-free synthesis of the hirudin variant 1 of the blood-sucking leech Hirudo medicinalis

Synthesis and purification of peptide drugs for medical applications is a challenging task. The leech-derived factor hirudin is in clinical use as an alternative to heparin in anticoagulatory therapies. So far, recombinant hirudin is mainly produced in bacterial or yeast expression systems. We describe the successful development and application of an alternative protocol for the synthesis of active hirudin based on a cell-free protein synthesis approach. Three different cell lysates were compared, and the effects of two different signal peptide sequences on the synthesis of mature hirudin were determined. The combination of K562 cell lysates and the endogenous wild-type signal peptide sequence was most effective. Cell-free synthesized hirudin showed a considerably higher anti-thrombin activity compared to recombinant hirudin produced in bacterial cells.

Hirudin variants production by genetic engineered microbial factory

Hirudin was discovered as an active anticoagulant in leech extracts almost 60 years ago. Since their initial discovery, hirudin and its variants have been produced with various anti-thrombotic, cancer cell inhibition, diabetic cataract treatment and anti-fatigue activities. Some hirudin variants have been approved for clinical use and released into the marketplace. Recent progress has seen made in relation to hirudin variants expressed in several well-established microbial hosts, including Escherichia coli, Saccharomyces cerevisiae, Pichia pastoris and others, with high levels of activity and yield. This review summarizes the current progress on hirudin production using microbial producers, and considers the outlook for future development.

Chinese Medicine Protein and Peptide in Gene and Cell Therapy

The success of gene and cell therapy in clinic during the past two decades as well as our expanding ability to manipulate these biomaterials are leading to new therapeutic options for a wide range of inherited and acquired diseases. Combining conventional therapies with this emerging field is a promising strategy to treat those previously-thought untreatable diseases. Traditional Chinese medicine (TCM) has evolved for thousands of years in China and still plays an important role in human health. As part of the active ingredients of TCM, proteins and peptides have attracted long-term enthusiasm of researchers. More recently, they have been utilized in gene and cell therapy, resulting in promising novel strategies to treat both cancer and non-cancer diseases. This manuscript presents a critical review on this field, accompanied with perspectives on the challenges and new directions for future research in this emerging frontier.

Alpha-1 proteinase inhibitor M358R reduces thrombin generation when displayed on the surface of cells expressing tissue factor

The M358R variant of alpha-1-proteinase inhibitor (API) is a potent soluble inhibitor of thrombin. Previously we engineered AR-API M358R, a membrane-bound form of this protein and showed that it inhibited exogenous thrombin when expressed on transfected cells lacking tissue factor (TF). To determine the suitability of AR-API M358R for gene transfer to vascular cells to limit thrombogenicity, we tested the ability of AR-API M358R to inhibit endogenous thrombin generated in plasma via co-expression co-expressing it on the surface of cells expressing TF. Transfected AR-API M358R formed inhibitory complexes with thrombin following exposure of recalcified, defibrinated plasma to TF on T24/83 cells, but discontinuously monitored thrombin generation was unaffected. Similarly, AR-API M358R expression did not reduce continuously monitored thrombin generation by T24/83 cell suspensions exposed to recalcified normal plasma in a Thrombogram-Thrombinoscope-type thrombin generation assay (TGA); in contrast, 1 μM hirudin variant 3 or soluble API M358R abolished thrombin generation. Gene transfer of TF to HEK 293 conferred the ability to support TF-dependent thrombin generation on HEK 293 cells. Co-transfection of HEK 293 cells with a 9:1 excess of DNA encoding AR-API M358R to that encoding TF reduced peak thrombin generation approximately 3-fold compared to controls. These in vitro results suggest that surface display of API M358R inhibits thrombin generation when the tethered serpin is expressed in excess of TF, and suggest its potential to limit thrombosis in appropriate vascular beds in animal models.

Enhanced effect of inhibition of thrombin on endothelium in murine endotoxaemia: specific inhibition of thrombocytopenia

INTRODUCTION

In systemic endotoxaemia, bacterial lipopolysaccharide causes the rapid expression of tissue factor (TF) and disseminated intravascular coagulation and in animal models, anticoagulants limit pathology and promote survival. Recent studies have emphasised the importance of TF expressed by mononuclear cells for initiating thrombin generation during endotoxaemia and suggested that endothelial cell TF is of little relevance. However, the precise importance of endothelium for intravascular thrombin generation has not been established. In this study, we compared the effect of equivalent levels of hirudin tethered to either endothelium or platelets and monocytes.

MATERIALS AND METHODS

CD31-Hir-Tg mice express a vesicle-targeted, membrane-tethered hirudin fusion protein on endothelium, platelets and monocytes. Bone marrow chimeras between these mice and C57BL/6 were generated The level of intravascular hirudin expressed during endotoxaemia was quantified by inhibition studies using an anti-hirudin antibody and reference to the circulating thrombin anti-thrombin complexes generated in control mice given soluble hirudin.

RESULTS AND CONCLUSIONS

Antibody inhibition studies indicated that individual chimeras expressed similar levels of hirudin fusion protein on endothelium alone as on platelets and leukocytes combined and accordingly, the levels of thrombin anti-thrombin complexes and fibrinogen in each chimera were similar, indicating equivalent inhibition of thrombin generation. However, mice with hirudin on endothelium alone developed significantly less thrombocytopenia. These results suggest a hitherto unrecognized role of endothelium in thrombin-dependent platelet sequestration during endotoxaemia. The data have implications for the development of therapeutic strategies based on targeted anticoagulation to limit disseminated intravascular coagulation.

Fibrocytes mediate intimal hyperplasia post-vascular injury and are regulated by two tissue factor-dependent mechanisms

BACKGROUND

CD34(+) α-smooth muscle actin (SMA)(+) cells mediate intimal hyperplasia (IH) after mechanical endoluminal injury. We previously found that IH is tissue factor (TF) dependent. The precise phenotype of the CD34(+) cells mediating IH is unknown and the mechanisms of TF are also unknown.

OBJECTIVE

To define the phenotype of cells mediating IH and compare the effects of inhibiting TF on different subsets of CD34(+) cells.

METHODS

Endoluminal injury was induced in C57BL/6 and two strains of mice expressing a human tissue factor pathway inhibitor (hTFPI) fusion protein on different subsets of CD34(+) cells. Confocal microscopy, immunocytofluorescence and real-time PCR were used to determine phenotype.

RESULTS

Neointimal cells in C57BL/6 mice were defined as a subset of fibrocytes (CD34(+) CD45(+) collagen-1(+) ) expressing SMA, CD31, TIE-2, CXCR4 and CXCL12. Similar cells circulated post-injury and were also found in mice expressing hTFPI on CD34(+) CD31(+) cells, though in these mice, hTFPI inhibited CD31(+) fibrocyte hyperplasia, so no IH developed. Mice with hTFPI on all CD34(+) α-SMA(+) cells repaired arteries back to a pre-injured state. No CD31(+) fibrocytes were found in these mice unless an anti-hTFPI antibody was administered. Similar findings in protease activated receptor (PAR)-1-deficient mice suggested hTFPI prevented thrombin signaling through PAR-1. In vitro, thrombin increased the number of CD31(+) fibrocytes.

CONCLUSIONS

Inhibition of TF on CD31(+) fibrocytes inhibits IH whereas inhibition on all CD34(+) α-SMA(+) cells (or PAR-1 deficiency) inhibits the appearance of CD31(+) fibrocytes and promotes repair. These data enhance our understanding of IH and suggest novel ways to promote regenerative repair.

Retention of thrombin inhibitory activity by recombinant serpins expressed as integral membrane proteins tethered to the surface of mammalian cells

BACKGROUND

Serpins form a widely distributed protein superfamily, but no integral membrane serpins have been described.

OBJECTIVES

To anchor three serpins -α(1) -proteinase inhibitor (α(1) PI) (M358R), antithrombin (AT), and heparin cofactor II (HCII) - in the plasma membranes of transfected mammalian cells and assess their ability to inhibit thrombin.

METHODS

Serpin cDNAs were altered to include N-terminal, non-cleavable plasma membrane-targeting sequences from the human transferrin receptor (TR) (TR-serpin) or the human asialoglycoprotein receptor (AR) (AR-serpin), and used to transfect COS-1 or HEK 293 cells. Cells were analyzed for serpin expression by immunoblotting of subcellular fractions, by immunofluorescence microscopy, or by flow cytometry, with or without exposure to exogenous thrombin; AR-serpins and TR-serpins were also compared with their soluble recombinant counterparts.

RESULTS

Both TR-α(1) PI (M358R) and AR-α(1) PI (M358R) were enriched in the integral membrane fraction of transfected COS-1 or HEK 293 cells, and formed inhibitory complexes with thrombin, although less rapidly than soluble α(1) PI (M358R). Thrombin inhibition was abrogated by an additional T345R mutation in AR-α(1) PI (M358R). Surface-displayed AR-AT also formed serpin-enzyme complexes with thrombin, but to a lesser extent than AR-α(1) PI (M358R); AR-HCII inhibitory function was not detected. Immunofluorescence detection and flow cytometric quantification of bound thrombin also supported the status of AR-α(1) PI (M358R) and AR-AT as thrombin inhibitors.

CONCLUSIONS

Two of three thrombin-inhibitory serpins retained functionality when expressed as integral membrane proteins. Our findings could be applied to create and screen hypervariable serpin libraries expressed in mammalian cells, or to confer protease resistance on engineered cells in vivo.

Critical roles for thrombin in acute and chronic inflammation

Thrombin can amplify inflammation induced by other stimuli, either through ischemia (consequent upon thrombosis), indirectly through generation of downstream mediators such as activated protein C, or directly via signals through protease activated receptors (PAR). This paper will summarize recent data from our laboratory indicating that thrombin is required to initiate CCR2-dependent leukocyte recruitment and that it is the principal determinant of the outcome after vascular injury, via PAR-1 activation of a distinct subset of smooth muscle cell progenitors. In both, tissue factor (TF) initiates thrombin generation and the thrombin acts locally, exemplifying that the initiation phase can generate autocrine or paracrine signalling molecules. Thrombin is an important constituent of innate immunity, able to amplify and modify responses to invading pathogens or tissue damage. With novel anti-thrombin therapeutics and agents to target PAR, a new understanding of the importance of thrombin may allow the development of innovative anti-inflammatory strategies.

Postinjury vascular intimal hyperplasia in mice is completely inhibited by CD34+ bone marrow-derived progenitor cells expressing membrane-tethered anticoagulant fusion proteins

BACKGROUND

Coagulation proteins promote neointimal hyperplasia and vascular remodelling after vessel injury, but the precise mechanisms by which they act in vivo remain undetermined.

OBJECTIVES

This study, using an injury model in which the neointima is derived from bone marrow (BM)-derived cells, compared inhibition of tissue factor or thrombin on either BM-derived or existing vascular smooth muscle cells.

METHODS

Two transgenic (Tg) mouse strains expressing membrane-tethered tissue factor pathway inhibitor (TFPI) or hirudin (Hir) fusion proteins driven by an alpha smooth muscle actin (SMA) promoter were generated (alpha-TFPI-Tg and alpha-Hir-Tg) and the phenotype after wire-induced endovascular injury was compared with that in wild-type (WT) controls.

RESULTS

WT mice developed progressive neointimal expansion, whereas injury in either Tg was followed by repair back to a preinjured state. This was also seen when WT mice were reconstituted with BM from Tg mice but not when Tgs were reconstituted with WT BM, in which injury was followed by slowly progressive neointimal expansion. Injection of CD34+ cells from Tg mice into injured WT mice resulted in the accumulation of fusion protein-expressing cells from day 3 onwards and an absence of neointimal hyperplasia in those areas.

CONCLUSIONS

Neointimal development after wire-induced endovascular injury in mice was completely inhibited when BM-derived cells infiltrating the damaged artery expressed membrane tethered anticoagulant fusion proteins under an alpha-SMA promoter. These findings enhance our understanding of the pathological role that coagulation proteins play in vascular inflammation.

Gene therapy of the ischemic lower limb--Therapeutic angiogenesis

The limitations of surgical revascularisation and pharmacological treatment in peripheral arterial occlusive disease (PAOD) are well recognized. Therapeutic options for critical leg ischemia are consequently limited to percutaneous transluminal angioplasty (PTA) or surgical revascularisation. Unfortunately, many patients with critical leg ischemia are poor candidates for either procedure. Therapeutic angiogenesis is a novel promising tool to treat these patients. Experimental and clinical and trials of gene transfer for therapeutic angiogenesis have already shown some clinical efficacy. This review is focused on gene transfer techniques in preclinical and clinical therapeutic angiogenesis, angiogenic growth factors, vectors, delivery methods and routes. The results of clinical and experimental studies, safety and side effects of gene therapy, and the perspectives of future research are also discussed.

Endothelial cell protection in xenotransplantation: looking after a key player in rejection

The endothelium, as an organ at the interface between the intra- and extravascular space, actively participates in maintaining an anti-inflammatory and anti-coagulant environment under physiological conditions. Severe humoral as well as cellular rejection responses, which accompany cross-species transplantation of vascularized organs as well as ischemia/reperfusion injury, primarily target the endothelium and disrupt this delicate balance. Activation of pro-inflammatory and pro-coagulant pathways often lead to irreversible injury not only of the endothelial layer but also of the entire graft, with ensuing rejection. This review focuses on strategies targeted at protecting the endothelium from such damaging effects, ranging from genetic manipulation of the donor organ to soluble, as well as membrane-targeted, protective strategies.

A new recombinant thrombolytic and antithrombotic agent with higher fibrin affinity--a staphylokinase variant. I. In vitro study

We attempted to construct a new recombinant protein characterized by fibrin-specific properties of plasminogen activation combined with antithrombin and antiplatelet activities. To the C-terminal part of recombinant staphylokinase (r-SAK), which is a promising profibrinolytic agent, we assembled: (i) the Kringle 2 domain (K2) of tissue-type plasminogen activator (t-PA), containing a fibrin-specific binding site, (ii) the RGD sequence (Arg-Gly-Asp) for the prevention of platelet aggregation and (iii) the antithrombotic agent - hirudin. The cDNA for hybrid protein SAK-RGD-K2-Hir was cloned into pESP-3 yeast protein expression vector. The introduction of K2 t-PA, RGD sequence and hirudin into r-SAK molecule did not alter the SAK activity. The plasminogen activation rate (determined by K(M) and K(cat)) of SAK-RGD-K2-Hir was not significantly different from that of r-SAK. Affinity and binding strength of the recombinant protein to fibrin immobilized on the biosensor were higher than to r-SAK. We observed a higher clot lysis potency of SAK-RGD-K2-Hir as evidenced by a faster and more profound lysis of 125I-labeled human fibrin clots. The potency of thrombin inhibition by the hirudin part of the recombinant fusion protein SAK-RGD-K2-Hir was the same as that of r-Hir alone. In conclusion, the results of the in vitro study suggest that the SAK-RGD-K2-Hir construct can be a more potent and faster-acting thrombolytic agent with antithrombin and antiplatelet properties compared with standard r-SAK.

Les xénogreffes finiront-elles par être acceptées ?

Transplantation represents a major advance in modern medicine with a major impact on the interactions between individuals and society. The numbers of patients undergoing organ transplantation increased steadily over the years and around 250,000 individuals are living nowadays in Europe with a transplanted organ. On the other hand, the numbers of cadaveric (brain-dead) donors used for organ transplantation remains stable, at around 5,000 each year, and the numbers of transplantation from living donors only slowly increase in Europe. Therefore, a gap is growing between the numbers of patients in need of a transplant and the numbers of organs available for transplantation. About 45,000 patients are currently on renal transplant waiting lists in Europe and, depending on the countries considered, 15 to 30 % of candidates for liver or heart transplantation die before a life-saving transplant becomes available to them. There is therefore an urgent need to implement innovative research and to take full advantage of recent biotechnological advances to explore new avenues in xenotransplantation, and to simultaneously address the ethical, societal and public health issues related to organ replacement. Much progresses have been accomplished in the understanding of xenograft rejection processes that include hyperacute, acute vascular and cellular rejection mechanisms. Strategies to promote xenograft survival that are currently under evaluation include genetic engineering of donor pigs, adapted immunosuppressive treatments and tolerance induction. Also, the psychological acceptance has been evaluated.

Complete inhibition of acute humoral rejection using regulated expression of membrane-tethered anticoagulants on xenograft endothelium

Xenotransplantation promises an unlimited supply of organs for clinical transplantation. However, an aggressive humoral immune response continues to limit the survival of pig organs after transplantation into primates. Because intravascular thrombosis and systemic coagulopathy are prominent features of acute humoral xenograft rejection, we hypothesized that expression of anticoagulants on xenogeneic vascular endothelium might inhibit the process. Hearts from novel transgenic mice, expressing membrane-tethered fusion proteins based on human tissue factor pathway inhibitor and hirudin, respectively, were transplanted into rats. In contrast to control non-transgenic mouse hearts, which were all rejected within 3 days, 100% of the organs from both strains of transgenic mice were completely resistant to humoral rejection and survived for more than 100 days when T-cell-mediated rejection was inhibited by administration of ciclosporin A. These results demonstrate the critical role of coagulation in the pathophysiology of acute humoral rejection and the potential for inhibiting rejection by targeting the expression of anticoagulants to graft endothelial cells. This genetic strategy could be applied in a clinically relevant species such as the pig.

Inhibition of intravascular thrombosis in murine endotoxemia by targeted expression of hirudin and tissue factor pathway inhibitor analogs to activated endothelium

We have generated transgenic mice expressing the leech anticoagulant hirudin and human tissue factor pathway inhibitor tethered to the cell surface by fusion with fragments of human CD4 and P-selectin. Expression of the transgenes is under the control of the CD31 (platelet endothelial cell adhesion molecule [PECAM]) promoter, limiting expression to endothelial cells, monocytes, and platelets. In addition, the P-selectin sequence directs expression to secretory granules. Functional cell surface expression only occurs when the cells are activated. In a mouse model of systemic lipopolysaccharide (LPS)–induced endotoxemia, we show that expression of either anticoagulant on activated endothelium inhibits the widespread intravascular thrombosis, thrombocytopenia, and consumptive coagulopathy associated with endotoxemia. Importantly, non– LPS-treated transgenic mice had normal baseline bleeding times. We speculate that targeted delivery of anticoagulants to the endothelium may be a strategy worth pursuing in clinical sepsis to improve efficacy of systemic anticoagulation while minimizing potential hemorrhagic side effects.

DEVELOPING A PORCINE TRANSPLANTATION MODEL: EFFICIENT GENE TRANSFER INTO PORCINE VASCULAR CELLS

BACKGROUND

The pig is generally regarded as likely to be the preferred donor animal in xenotransplantation. Although many hurdles remain to be cleared, it would be useful to be able to manipulate porcine endothelium genetically, among other reasons, to test approaches in the modulation of inflammation. However, as a nondividing cell, it is less easy to manipulate.

METHODS

The authors performed in vivo and in vitro gene transfection experiments using as an adjunct the DNA-binding agent 4',6'-diamidino-2-phenylindole (DAPI), which protects DNA from degradation.

RESULTS

The introduction of DAPI into a liposomal transfection system was able to increase in vitro transfection efficiency of both endothelial and vascular smooth muscle cells from the pig, even in the presence of small amounts of serum. This last observation encouraged the authors to use this system in vivo in porcine carotid arteries. In this model, the authors were also able to demonstrate a high degree of transfection efficiency using DAPI, which seemed to work by protecting DNA from degradation.

CONCLUSIONS

The authors believe this technique may allow them to address many biological questions relating to intervening in vascular disease, inflammation, and immune responses in the context of transplantation and beyond.

Human thrombin and FXa mediate porcine endothelial cell activation; modulation by expression of TFPI-CD4 and hirudin-CD4 fusion proteins

Aside from their critical role in thrombosis, activated coagulation factors also have inflammatory properties and these may be important during delayed xenograft rejection (DXR). This study assessed whether porcine EC could be activated by factor Xa (FXa) and thrombin (FIIa) and whether expression of tissue factor pathway inhibitor (TFPI)-CD4 and hirudin-CD4 fusion proteins could prevent such activation. Incubation of porcine EC with human FXa and FIIa induced cell surface expression of E-selectin, VCAM and tissue factor (TF) in a time-dependent and concentration-dependent manner. In contrast, porcine EC transfected with a human TFPI-CD4 fusion protein were selectively resistant to these pro-inflammatory effects of FXa but not FIIa. Likewise, the transfectants expressing the hirudin-CD4 fusion protein were selectively resistant to the pro-inflammatory effects of FIIa but not those of FXa. When combined, the FXa and FIIa had an additive effect on the activation of control EC. In contrast, coexpression of both hirudin-CD4 and TFPI-CD4 fusion proteins completely inhibited the upregulation of VCAM with the FXa/FIIa mix. These results indicate that expression of novel anticoagulant fusion proteins on the surface of porcine EC can protect against EC activation induced by human coagulation factors FXa and FIIa. In vivo, we anticipate that expression of these fusion proteins on the endothelium of transplanted xenografts, besides preventing intravascular thrombosis, will also protect against EC activation induced by trace amounts of FIIa and FXa, thereby further protecting the grafts from DXR.

Prolonged in vivo anticoagulant activity of a hirudin-albumin fusion protein secreted from Pichia pastoris

Hirudin is a small, proteinaceous thrombin inhibitor that clears rapidly from the circulation. A hexahistidine-tagged hirudin-rabbit serum albumin (RSA) fusion protein, HLAH6, was characterized following secretion from Pichia pastoris. HLAH6 bound to immobilized nickel, anti-RSA, and anti-hexahistidine antibodies, and contained the expected (ITYTD) N-terminus. Its spectrometric mass was 74,490 (versus the theoretical mass of 74,410 and sodium dodecyl sulfate-polyacrylamide gel electrophoresis mobility of 84 kDa). The terminal catabolic half-life in rabbits of HLAH6, recombinant Pichia-derived His-tagged RSA, or plasma-derived RSA did not differ. Injection of 2 mg/kg HLAH6 into rabbits raised the activated partial thromboplastin time (aPTT) above initial values for 4-24 h, while the equimolar dose of unfused hirudin was without significant effect. A higher dose of HLAH6 (3 mg/kg functional HLAH6, equivalent to 37.6 thrombin-inhibitory units/g) raised the aPTT by 2.0- to 2.5-fold; the elevation persisted for > 48 h. Importantly, both HLAH6 and unfused hirudin inhibited clot-bound thrombin. Our results suggest that HLAH6 exhibits not only delayed clearance, but also prolonged biological activity in vivo compared with unfused hirudin.

Targets for gene therapy of vein grafts

Poor long-term patency and a lack of suitable systemic pharmacologic therapy for the prevention of vein graft failure have prompted the search for effective local gene therapy. Vein grafts are particularly well suited for gene transfer in the clinic because direct access to vein is available during surgical preparation for grafting. In this review, the available animal models are discussed and a new mouse model is highlighted. Recent advances in gene transfer technology are reviewed, including the use of adeno-associated virus and modified adenoviruses that can prolong in vivo transgene expression for months. Gene therapy is intended to reduce early thrombosis, reduce neointima formation, and prevent atherosclerosis in vein grafts. Promising antithrombotic targets include tissue plasminogen activator and thrombomodulin. Nitric oxide synthase, prostacyclin synthase, and tissue inhibitors of metalloproteinases have been used to reduce neointima formation, and vein graft atheroma remains a challenge for the future.

Regulated inhibition of coagulation by porcine endothelial cells expressing P-selectin-tagged hirudin and tissue factor pathway inhibitor fusion proteins

BACKGROUND

Thrombotic vascular occlusion resulting in infarction occurs during hyperacute rejection of allografts transplanted into sensitized patients and remains a major problem in experimental xenotransplantation. A similar process is also found in disorders of diverse etiology including atherosclerosis, vasculitis, and disseminated intravascular coagulation.

METHODS

We have previously constructed two membrane-tethered anticoagulant fusion proteins based on human tissue factor pathway inhibitor and the leech anticoagulant hirudin and demonstrated their functional efficacy in vitro. These constructs have now been modified by the addition of a P-selectin sequence to the cytoplasmic tail to localize them in Weibel-Palade bodies. They have been transfected into Weibel-Palade body-positive endothelial cells isolated from the inferior vena cava of normal pigs.

RESULTS

In resting endothelial cells, fusion protein expression colocalized with P-selectin and was confined to Weibel-Palade bodies. These cells had a procoagulant phenotype in recalcified human plasma. However, after activation with phorbol ester the anticoagulant proteins were rapidly relocated to the cell surface where they specifically inhibited the clotting of human plasma.

CONCLUSIONS

Novel anticoagulant molecules may prove useful therapeutic agents for gene therapy in thrombotic disease and postangioplasty or for transgenic expression in animals whose organs may be used for clinical xenotransplantation. Expression in vascular endothelial cells may be regulated by inclusion of P-selectin cytoplasmic sequence, to restrict cell surface expression to activated endothelium.

Inhibition of tissue factor-dependent and -independent coagulation by cell surface expression of novel anticoagulant fusion proteins

BACKGROUND

Thrombotic vascular occlusion occurs in disorders of diverse etiology, including atherosclerosis, vasculitis, and disseminated intravascular coagulation. The same process results in hyperacute rejection of renal allografts transplanted into sensitized patients and remains a major problem in experimental xenotransplantation.

METHODS

We have previously described the design and expression of several genetic constructs encoding novel fusion proteins with anticoagulant properties. They are based on two naturally occurring soluble anticoagulant proteins, human tissue factor pathway inhibitor (hTFPI) and the leech protein hirudin, which act early and late in the clotting cascade, respectively. We report the expression of human hTFPI-CD4 on the surface of immortalized porcine endothelial cells (IPEC), and show that it functions across the species divide as evidenced by the binding of membrane-expressed porcine tissue factor (pTF)-human factor VIIa complexes.

RESULTS

Using a human plasma recalcification clotting assay, we distinguished between pTF-dependent and pTF-independent fibrin generation, and we have demonstrated that expression of hTFPI-CD4 on IPEC effectively prevented pTF-dependent clotting. Moreover, we show that when hTFPI-CD4 was co-expressed with the hirudin construct, the procoagulant properties of in vitro cultured, activated IPEC were almost completely abolished.

CONCLUSIONS

These results suggest that these novel anticoagulant molecules may prove useful therapeutic agents for gene therapy or for transgenic expression in animals whose organs may be used for cliniCal xenotransplantation.