Mechanisms by which blood cells sense shear stress are poorly characterized. In platelets, glycoprotein (GP)Ib–IX receptor complex has been long suggested to be a shear sensor and receptor. Recently, a relatively unstable and mechanosensitive domain in the GPIbα subunit of GPIb–IX was identified. Here we show that binding of its ligand, von Willebrand factor, under physiological shear stress induces unfolding of this mechanosensory domain (MSD) on the platelet surface. The unfolded MSD, particularly the juxtamembrane ‘Trigger' sequence therein, leads to intracellular signalling and rapid platelet clearance. These results illustrate the initial molecular event underlying platelet shear sensing and provide a mechanism linking GPIb–IX to platelet clearance. Our results have implications on the mechanism of platelet activation, and on the pathophysiology of von Willebrand disease and related thrombocytopenic disorders. The mechanosensation via receptor unfolding may be applicable for many other cell adhesion receptors.

Recombinant human factor VIII expression levels, in vitro and in vivo, are significantly lower than levels obtained for other recombinant coagulation proteins. Here we describe the generation, high level expression and characterization of a recombinant B-domain-deleted porcine factor VIII molecule. Recombinant B-domaindeleted porcine factor VIII expression levels are 10-to 14-fold greater than recombinant B-domain-deleted human factor VIII levels by transient and stable expression in multiple cell lines. Peak expression of 140 units⅐10 6 cells ؊1 ⅐24 h ؊1 was observed from a baby hamster kidneyderived cell line stably expressing recombinant porcine factor VIII. Factor VIII expression was performed in serum-free culture medium and in the absence of exogenous von Willebrand factor, thus greatly simplifying protein purification. Real time reverse transcription-PCR analysis demonstrated that the differences in protein production were not caused by differences in steady-state factor VIII mRNA levels. The identification of sequence(s) in porcine factor VIII responsible for high level expression may lead to a better understanding of the mechanisms that limit factor VIII expression.Factor VIII (fVIII) 1 is a large (ϳ 300 kDa) glycoprotein that functions as an integral component of the intrinsic pathway of blood coagulation. Mutations in the fVIII gene that result in decreased or defective fVIII protein give rise to the genetic disease, hemophilia A, which is phenotypically characterized by recurrent bleeding episodes. Treatment of hemophilia A entails intravenous infusion of either human plasma-derived or recombinant fVIII material. Approximately 25% of all hemophilia A patients treated with fVIII products develop antibodies that inhibit fVIII activity and limit treatment efficacy (1). Patients with fVIII-inhibitory antibodies can be treated using porcine plasma-derived fVIII products, which generally display low cross-reactivity with the human fVIII antibodies (2, 3). Currently there is not a recombinant porcine fVIII product available for clinical use.Since the introduction of recombinant fVIII for the treatment of hemophilia A, commercial suppliers have struggled to keep up with high patient demand (4). The shortage of recombinant fVIII material has precluded prophylactic treatment of severely affected patients, limited the implementation of immune-tolerance regimens, and kept treatment costs high. Unfortunately, fVIII is expressed and recovered at low levels in the heterologous mammalian cell culture systems used for commercial manufacture. The importance of this problem has fueled significant research efforts to overcome the low fVIII expression barrier, and several basic mechanisms have been identified that limit fVIII expression (for review, see Kaufman et al. (5)) Despite these findings, fVIII expression levels remain low, and a product shortage persists.The porcine fVIII cDNA sequence has been reported and shown to encode the homology-defined internal protein domain structure, A1-A2-B-ap-A3-C1-C2 (6, 7). Porcin...

IntroductionThe development of neutralizing anti-factor VIII (FVIII) antibodies is the major complication in the treatment of patients with hemophilia A with FVIII products. 1,2 Long-term application of high doses of FVIII has evolved as an effective therapy to eradicate the antibodies and to induce long-lasting immune tolerance. [3][4][5][6] Despite clinical experience with the therapy, little is known about the immunologic mechanisms that cause the down-modulation of FVIII-specific immune responses and the induction of long-lasting immune tolerance against FVIII. We asked the question whether the restimulation of FVIII-specific memory B cells is affected by high concentrations of FVIII in vitro or high doses of FVIII in vivo. Memory B cells play an essential role in the maintenance of established antibody responses. On re-exposure to the same antigen, they are rapidly restimulated to proliferate and differentiate into antibody-secreting plasma cells (ASCs) that secrete high-affinity antibodies. 7,8 Furthermore, memory B cells have the potential to act as very efficient antigen-presenting cells and stimulators of CD4 ϩ T cells because of the expression of highaffinity antigen receptors, major histocompatibility complex (MHC) class II and costimulatory molecules. 9 It is, therefore, reasonable to believe that memory B cells have to be eradicated or functionally inactivated during a successful immune tolerance induction therapy with FVIII inhibitors in patients with hemophilia A.We used a murine model of hemophilia A that is characterized by complete deficiency of functionally active FVIII because of a targeted disruption of exon 17 of the F8 gene. 10,11 Intravenous injection of human FVIII into these mice results in high titers of anti-FVIII antibodies that have similar characteristics to those of FVIII inhibitors in patients. [12][13][14][15] Using this model, we demonstrated previously that the differentiation of FVIII-specific memory B cells into ASCs depends on the presence of activated T cells and requires CD40-CD40 ligand and CD80/CD86-CD28 costimulatory interactions. 16 Here, we show that concentrations of FVIII below the physiologic plasma concentration of 0.1 g/mL (1 U/mL) restimulate FVIII-specific memory B cells and induce their differentiation into ASCs. Concentrations above 0.1 g/mL (1 U/mL), however, inhibit memory B-cell restimulation and prevent the formation of ASCs. This inhibition is irreversible and involves the activation of caspases. Materials and methods Hemophilic E-17 miceOur colony of fully inbred hemophilic E-17 mice (characterized by a targeted disruption of exon 17 of the F8 gene) was established with a breeding pair from the original colony 10,11 and crossed into the C57BL/6J background as described. 17 All mice were male and aged 8 to 10 weeks at the beginning of the experiments. All studies were carried out in accordance with Austrian federal law (Act BG 501/1989) regulating animal experimentation and approved by the local authority in Vienna, Austria. Immunization of mice with FV...

Insufficient expression of factor VIII (fVIII) is a major hurdle in the development of successful nucleic acid treatments for hemophilia. However, we recently showed that under myeloablative and reducedintensity total body irradiation (TBI) conditioning, transplantation of hematopoietic stem cells (HSCs) transduced with recombinant retroviruses containing B domain-deleted porcine fVIII (BDDpfVIII) sequences provides curative fVIII levels in a hemophilia A mouse model. In the current study, we tested BDDpfVIII activity after nonmyeloablative conditioning with busulfan, cyclophosphamide, or fludarabine and immunosuppressive agents CTLA4-Ig ؉ anti-CD40L or anti-(murine)-thymocyte serum (ATS). ATS is similar in action to anti-(human)thymocyte globulin (ATG), which is used clinically with busulfan in bone marrow transplantations to increase donor cell engraftment. Mice conditioned with busulfan ؉ ATS and that received a transplant of BDDpfVIIItransduced stem-cell antigen 1-positive cells exhibited moderate levels of donor cell chimerism (between 20% and 60%) and achieved sustained fVIII levels more than 1 U/mL. Similar results were observed in mice preimmunized with human fVIII and conditioned with 5 Gy TBI ؉ ATS or busulfan ؉ ATS. These data demonstrate that it is possible to achieve sufficient fVIII expression after transplantation of BDDpfVIII-transduced HSCs following low-toxicity pretransplantation conditioning with targeted immunosuppression, potentially even in the context of preexisting inhibitors. (Blood. 2007; 110: [2855][2856][2857][2858][2859][2860][2861][2862][2863]

Blood coagulation factor VIII has a domain structure designated A1-A2-B-ap-A3-C1-C2. Human factor VIII is present at low concentration in normal plasma and, comparably, is produced at low levels in vitro and in vivo using transgenic expression techniques. Heterologous expression of B domain-deleted porcine factor VIII in mammalian cell culture is significantly greater than B domain-deleted human or murine factor VIII. Novel hybrid human/porcine factor VIII molecules were constructed to identify porcine factor VIII domains that confer high level expression. Hybrid human/porcine factor VIII constructs containing the porcine factor VIII A1 and ap-A3 domains expressed at levels comparable with recombinant porcine factor VIII. A hybrid construct containing only the porcine A1 domain expressed at intermediate levels between human and porcine factor VIII, whereas a hybrid construct containing the porcine ap-A3 domain expressed at levels comparable with human factor VIII. Additionally, hybrid murine/porcine factor VIII constructs containing the porcine factor VIII A1 and ap-A3 domain sequences expressed at levels significantly higher than recombinant murine factor VIII. Therefore, the porcine A1 and ap-A3 domains are necessary and sufficient for the high level expression associated with porcine factor VIII. Metabolic radiolabeling experiments demonstrated that high level expression was attributable to enhanced secretory efficiency. Factor VIII (fVIII)1 is a plasma protein that functions in proteolytically activated form as a cofactor within the intrinsic pathway of blood coagulation to increase the rate of proteolytic activation of factor X by activated factor IX. fVIII contains a domain structure designated A1-A2-B-ap-A3-C1-C2 that is defined based on internal sequence homology (1, 2). The fVIII A domains share homology with the copper-binding protein ceruloplasmin (3, 4), which has an A1-A2-A3 domain structure in which the three A domains are arranged along a pseudo-3-fold axis of symmetry (5). Before cell secretion, fVIII is cleaved at the B/ap-A3 domain junction into A1-A2-B (heavy chain) and ap-A3-C1-C2 (light chain) subunits. fVIII circulates in the plasma as an inactive heavy chain/light chain heterodimeric procofactor that is non-covalently bound to von Willebrand factor. Proteolytic activation of fVIII by thrombin results from cleavages at Arg-372 between the A1 and A2 domains, Arg-740 between the A2 and B domains, and Arg-1689 between the ap and A3 domains. During this process, the covalent linkage between the A1 and A2 domains is lost, and the B domain and 41-residue ap are released, producing a heterotrimeric,

Human coagulation factor VIII (fVIII) is inefficiently biosynthesized in vitro and has proven difficult to express at therapeutic levels using available clinical gene-transfer technologies. Recently, we showed that a porcine and certain hybrid human/porcine fVIII transgenes demonstrate up to 100-fold greater expression than human fVIII. In this study, we extend these results to describe the use of a humanized, high-expression, hybrid human/porcine fVIII transgene that is 89% identical to human fVIII and was delivered by lentiviral vectors (LVs) to hematopoietic stem cells for gene therapy of hemophilia A. Recombinant human immunodeficiency virus-based vectors encoding the fVIII chimera efficiently transduced human embryonic kidney (HEK)-293T cells. Cells transduced with hybrid human/porcine fVIII encoding vectors expressed fVIII at levels 6- to 100-fold greater than cells transduced with vectors encoding human fVIII. Transplantation of transduced hematopoietic stem and progenitor cells into hemophilia A mice resulted in long-term fVIII expression at therapeutic levels despite <5% genetically modified blood mononuclear cells. Furthermore, the simian immunodeficiency virus (SIV) -derived vector effectively transduced the human hematopoietic cell lines K562, EU1, U.937, and Jurkat as well as the nonhematopoietic cell lines, HEK-293T and HeLa. All cell lines expressed hybrid human/porcine fVIII, albeit at varying levels with the K562 cells expressing the highest level of the hematopoietic cell lines. From these studies, we conclude that humanized high-expression hybrid fVIII transgenes can be utilized in gene therapy applications for hemophilia A to significantly increase fVIII expression levels compared to what has been previously achieved.

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Optimization of a protein’s pharmaceutical properties is usually carried out by rational design and/or directed evolution. Here we test an alternative approach based on ancestral sequence reconstruction. Using available genomic sequence data on coagulation factor VIII and predictive models of molecular evolution, we engineer protein variants with improved activity, stability. biosynthesis potential, and reduced inhibition by clinical anti-drug antibodies. In principle, this approach can be applied to any protein drug based on a conserved gene sequence.