Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response

We have previously shown that a single portal vein infusion of a recombinant adeno-associated viral vector (rAAV) expressing canine Factor IX (F.IX) resulted in long-term expression of therapeutic levels of F.IX in dogs with severe hemophilia B. We carried out a phase 1/2 dose-escalation clinical study to extend this approach to humans with severe hemophilia B. rAAV-2 vector expressing human F.IX was infused through the hepatic artery into seven subjects. The data show that: (i) vector infusion at doses up to 2 x 10(12) vg/kg was not associated with acute or long-lasting toxicity; (ii) therapeutic levels of F.IX were achieved at the highest dose tested; (iii) duration of expression at therapeutic levels was limited to a period of approximately 8 weeks; (iv) a gradual decline in F.IX was accompanied by a transient asymptomatic elevation of liver transaminases that resolved without treatment. Further studies suggested that destruction of transduced hepatocytes by cell-mediated immunity targeting antigens of the AAV capsid caused both the decline in F.IX and the transient transaminitis. We conclude that rAAV-2 vectors can transduce human hepatocytes in vivo to result in therapeutically relevant levels of F.IX, but that future studies in humans may require immunomodulation to achieve long-term expression.

Major role of local immune responses in antibody formation to factor IX in AAV gene transfer

The risk of an immune response to the coagulation factor IX (F.IX) transgene product is a concern in gene therapy for the X-linked bleeding disorder hemophilia B. In order to investigate the mechanism of F.IX-specific lymphocyte activation in the context of adeno-associated viral (AAV) gene transfer to skeletal muscle, we injected AAV-2 vector expressing human F.IX (hF.IX) into outbred immune-competent mice. Systemic hF.IX levels were transiently detected in the circulation, but diminished concomitant with activation of CD4+ T and B cells. ELISPOT assays documented robust responses to hF.IX in the draining lymph nodes of injected muscle by day 14. Formation of inhibitory antibodies to hF.IX was observed over a wide range of vector doses, with increased doses causing stronger immune responses. A prolonged inflammatory reaction in muscle started at 1.5-2 months, but ultimately failed to eliminate transgene expression. By 1.5 months, hF.IX antigen re-emerged in circulation in approximately 70% of animals injected with high vector dose. Hepatic gene transfer elicited only infrequent and weaker immune responses, with higher vector doses causing a reduction in T-cell responses to hF.IX. In summary, the data document substantial influence of target tissue, local antigen presentation, and antigen levels on lymphocyte responses to F.IX.

Genetic analysis of the antibody response to AAV2 and factor IX

We have analyzed the antibody response against either AAV2 or canine F.IX (cF.IX) in parental C57BL/6 (B6) and DBA/2 (D2) and 18 strains of B6 x D2 (BXD) recombinant inbred (RI) strains of mice after iv administration of AAV2-(ApoE)4/hAAT-cF.IX. There was a higher anti-AAV2 capsid response in B6 compared to D2 mice, with IgG2b being the major isotype separating the high and low responders in these two strains. In contrast, the antibody response to cF.IX was lower than the response to the AAV2 capsid and was limited to the IgG1 isotype in both strains. Genetic linkage analysis of the IgG2b anti-AAV2 antibody response in BXD RI strains revealed a significant locus at D4Mit164 (29 cM, LRS=15.3) and two suggestive loci at D6Mit16 (30.5 cM, LRS=11.2) and D17Mit187 (47.4 cM, LRS=13.1). Genetic linkage analysis of the IgG1 anti-cF.IX antibody response revealed a suggestive locus at D1Mit218 (67 cM, LRS=14.1). Significant epistatic interaction was found between two loci (D8Mit45 and D16Mit47; LOD=6.54) for anti-AAV2 and two other loci (D5Mit348 and D15Mit161; LOD=7.66) for anti-cF.IX. These results indicate that multiple genetic loci independently regulate the isotype-specific antibody response to the AAV2 capsid and the cF.IX transgene.

Acquired haemophilia: management of bleeds and immune therapy to eradicate autoantibodies

Acquired haemophilia is a rare, but often severe bleeding disorder caused by autoantibodies against a coagulation factor, usually factor VIII (FVIII). Between 1997 and 2004 we observed 14 patients (mean age of 78 years) with acquired haemophilia. The aim of the present study was to investigate the effect of activated prothrombin complex concentrate (aPCC) for bleeds and the response to corticosteroids and cyclophosphamide to eradicate the offending autoantibodies. The most common clinical presentations were severe profuse bruising (12) and haematuria (5). Ten patients were classified as idiopathic. At the time of diagnosis all patients had a very low FVIII level, and one patient also showed factor IX < 1%. High levels of antibodies to FVIII varying from 10 to 1340 Bethesda units (BU) and prolonged activated partial thromboplastin time were disclosed in all patients. Eight severe bleeds were treated with aPCC (FEIBA) at a dosage of 70 IU kg(-1) every 8 h until haemostasis. Ten patients received corticosteroids and cyclophosphamide as immunomodulatory therapy. Effective haemostasis was achieved in all bleeds after aPCC. Ten of 11 patients responded either completely or partially to the immunomodulatory regime within 6 months. Five patients achieved complete response (CR) whereas partial responses were seen in five patients. The anti-CD20 monoclonal antibody rituximab was given to two patients in conventional doses and a CR was seen in one patient. aPCC is effective in treating acute bleeds in patients with acquired haemophilia with high inhibitor levels. The combination of oral corticosteroids and cyclophosphamide seems to be effective to eradicate the inhibitor.

Immune tolerance in children with factors VIII and IX inhibitors: a single centre experience

Alloimmune FVIII and FIX inhibitors are the most serious complication of haemophilia in the postviral contamination era and their optimal management remains controversial. We present 15 boys with severe haemophilia (14 with haemophilia A and 1 with haemophilia B) who have received immune tolerance at our centre over a 9-year period. Twelve of them (80%) were successfully tolerized with varying dose intensities, but three of them (including the boy with haemophilia B) failed tolerization. The factors, which were associated with successful tolerance in our group, were a low maximum inhibitor titre and a short interval between diagnosis of the inhibitor and the start of immune tolerance. The time taken to achieve immune tolerance varied from 1 to 27 months and none of the inhibitors have recurred. Two of the three boys who failed immune tolerance had had their inhibitor for 72 and 69 months, respectively before tolerance was attempted.

Effect of Activated Recombinant Human Factor 7 (Niastase) on Laboratory Testing of Inhibitors of Factors VIII and IX

Activated recombinant human factor VIIa (rFVIIa) has been used as a hemostatic agent in patients with hemophilia and acquired inhibitors. Other indications for rFVIIa may include liver disease, warfarin sodium (Coumadin) overdose, or trauma. Monitoring patients on this treatment with standard laboratory testing is problematic. Bleeding risk does not correlate well with the prothrombin time (PT) or the activated partial thromboplastin time (aPTT) during therapy with rFVIIa. In addition, there is no identifiable literature on the effect of rFVIIa on assays of inhibitors in this patient group. Monitoring inhibitors may be important during interventions aimed at acutely reducing inhibitor levels, such as during plasma exchange or protein adsorption. We performed factor assays and evaluated inhibitor levels in plasma from 3 patients with deficiencies in FVIII (2 patients) or FIX (1 patient) and inhibitors (titer range, 5.8-17.4 Bethesda units) before and after adding rFVIIa (range, 0.25-8 microg/mL) in vitro. Additionally, we performed assays of factors of both intrinsic and extrinsic systems to determine the impact of rFVIIa on these tests. We found that both factor levels and inhibitor titers from patients with hemophilia A or B could be measured accurately, even in the presence of suprapharmacologic doses of rFVIIa (8 microg/mL). We also obtained accurate measurements for other assays of the intrinsic coagulation system (FXI and FXII) based on the aPTT. Conversely, we found that assays of the extrinsic system based on the PT (FII, FV, and FX) produced results that were unreliable. FVII results were very high but reproducible. These results suggest that assays based on the PT are inaccurate and should be avoided during FVIIa treatment. Conversely, FVIII and FIX levels and inhibitor titers can be accurately monitored in hemophilia patients receiving rFVIIa according to results of aPTT-based coagulation tests.

Crystallization of an anti-factor IX antibody and its complex

Initiation, propagation and regulatory processes of blood coagulation occur at the cell surface. During blood coagulation, many coagulation factors are anchored onto cell-surface membranes through their N-terminal carboxyglutamic acid-rich (Gla) domains found on such vitamin K-dependent blood coagulation factors as factors VII, IX, X, prothrombin, factors C and S. 10C12 is a conformation-specific calcium-dependent anti-factor IX antibody, which is directed at the calcium-stabilized Gla domain and interferes with factor IX-membrane interaction. In a variety of animal models, 10C12 has been demonstrated to be effective anticoagulant in attenuating thrombosis without severe bleeding. The crystallization of the Fab fragment of 10C12 and its complex with the Gla domain of human factor IX was not trivial. Here, the crystallization conditions and unusual aspects of this crystallization process are reported.

Molecular genotyping of the Italian cohort of patients with hemophilia B

BACKGROUND AND OBJECTIVES

The aim of the study, funded by the Italian Ministry of Health, was to identify the causative mutation in all known patients with hemophilia B in Italy.

DESIGN AND METHODS

Overall, 269 patients followed by 25 regional centers were considered in the study; after exclusion of the related individuals, 238 unrelated patients were analyzed (153 with severe, 59 with moderate and 26 with mild hemophilia B). Screening of the factor IX gene was performed using conformation sensitive gel electrophoresis (CSGE) followed by denaturing high performance liquid chromatography (dHPLC) or direct sequencing in negative cases, or by dHPLC/sequencing (36 cases).

RESULTS

A mutation was identified in 236 of the 238 patients: 6 had large gene deletions (4 total and 2 partial), 14 small deletions, 1 combined deletion/insertion and 215 single nucleotide substitutions. A correlation was observed between the type of mutation and severity of hemophilia; however, a number of patients with the same genotype had varying severities of the disease. Eight of the 169 patients with severe hemophilia B (4.7%) developed inhibitors: 2 of these had a complete gene deletion, 1 had a large partial deletion (from exon A to part of exon H) while 5 had 3 different nonsense mutations. One patient with a nonsense mutation developed anaphylaxis. We also studied 65 families with hemophilia B involving 144 females (14 obligatory carriers, 85 carriers and 45 non-carriers) and performed 12 antenatal diagnoses.

INTERPRETATION AND CONCLUSIONS

The data have been used to build the Italian mutation database to provide each family with knowledge of the disease-causing defect for genetic counseling. This Italian study confirms the marked heterogeneity of factor IX mutations in the population and the presence of a degree of genotype/phenotype discordance. The identification of the mutation can also be used to predict risk of inhibitor development.

Sustained correction of disease in naive and AAV2-pretreated hemophilia B dogs: AAV2/8-mediated, liver-directed gene therapy

Adeno-associated virus 8 (AAV8), a new member of the AAV family isolated from nonhuman primates, is an attractive candidate for hepatic gene transfer applications because of 10- to 100-fold improved transduction efficiency in mouse liver models. Additionally, AAV8 has lesser frequency of pre-existing immunity in humans. These properties could solve some of the problems associated with AAV2 vectors. The benefits of AAV8 demonstrated in mouse models, however, have not been confirmed in larger animals. In this study, we evaluate the efficacy and safety of AAV2/8 vector in both naive and AAV2-pretreated hemophilia B dogs. Two naive hemophilia B dogs that received a single intraportal administration of AAV2/8 vector have achieved sustained expression of 10% and 26% of normal levels of canine factor IX (cFIX) for more than a year. In an AAV2-pretreated hemophilia B dog, cFIX expression increased from less than 1% to 16% of normal levels when treated with an AAV2/8 vector, and a high level of expression has lasted for more than 2 years. No significant liver toxicity or cFIX-specific antibodies have been detected in these animals. Studies here have demonstrated the safety and improved efficacy of AAV2/8 vector in large-animal models for liver-directed gene therapy.

Immune implications of gene therapy for hemophilia

Similar to any novel treatment strategy for hemophilia, gene therapy faces the question of the risk of formation of inhibitory antibodies to the therapeutic factor VIII or factor IX protein. Activation of CD4 (+) or CD8 (+) T cells could lead to antibody formation or cytotoxic T lymphocyte responses to transgene-expressing cells. Preclinical studies in animal models of hemophilia A and B with different mutations in the dysfunctional gene shed light on the risk for such immune responses and point toward strategies to avoid immune activation or even promote tolerance induction. The impacts of variables such as choice and design of gene transfer vector, underlying gene mutation, route of vector administration, and transient immune suppression are discussed. Maintenance of immunological hyporesponsiveness to the therapeutic gene product is critical for successful gene therapy. Recent studies provide evidence for tolerance induction to coagulation factor antigens by viral hepatic or neonatal in vivo gene transfer, by in utero gene delivery, and by oral or nasal administration of protein or peptides.

Hemophilia gene transfer: comparison with conventional protein replacement therapy

One of the unanswered questions in hemophilia is whether gene transfer, if successful, will be a safe and effective alternative to standard clotting factor treatment for hemophilia. Despite life-threatening complications of protein-based clotting factor treatment during the last three decades, including acquired immunodeficiency syndrome and hepatitis C, factor infusion is now considered safe, effective, and compatible with a normal lifespan. Thus, protein-based therapy will be the standard against which the safety and efficacy of gene transfer will be judged. Will the potential risks of gene transfer be sufficiently low to justify its use? Should all individuals with hemophilia consider gene transfer? To answer these questions, the known risks and benefits of current protein-based therapy must be compared with the potential risks and benefits of gene transfer. It is anticipated that risks of gene transfer may include the known risks of protein-based therapies, including allergic reactions, inflammatory responses, inhibitor formation, chronic hepatitis, as well as gene-transfer-specific risks, including germline transmission, insertional mutagenesis, thrombosis, and potential ethical and psychological issues. This article reviews and compares the risks and benefits of standard protein-based therapy with those of gene transfer, and considers how gene transfer might fit into state-of-the-art management of hemophilia.

Novel hemophilia B mouse models exhibiting a range of mutations in the Factor IX gene

Animal models have been critical to the development of novel therapeutics in hemophilia. A deficiency of current murine models of hemophilia B is that they are all due to gene deletions, a type of mutation that is relatively rare in the human hemophilia population. We generated mice with a range of mutations in the Factor IX (F.IX) gene; these more faithfully reflect the types of mutations that cause disease in the human population. Transgenic mice expressing either wild-type human F.IX (hF.IX), or F.IX variants with premature translation termination codons, or missense mutations, under the control of the murine transthyretin promoter, were generated and crossed with mice carrying a large deletion of the murine F.IX gene. Gene copy number, F.IX transcript levels in the liver, intrahepatocyte protein expression, and circulating levels of F.IX protein in the mice were determined and compared with data generated by transient transfection assays using the same F.IX variants. Mice were injected with a viral vector expressing hF.IX and displayed a range of immune responses to the transgene product, depending on the underlying mutation. These new mouse models faithfully mimic the mutations causing human disease, and will prove useful for testing novel therapies for hemophilia. (Blood. 2004;104:2767-2774)

Inhibitors in congenital coagulation disorders

The development of inhibitory 'allo' antibodies to a deficient coagulation factor is arguably now the most severe and important complication of clotting factor concentrate exposure in haemophilia and other congenital coagulation disorders. Furthermore, development of an inhibitor to the factor VIII or factor IX transgene product remains a significant concern in gene therapy protocols for haemophilia. Although the development of an inhibitor does not usually change the rate, initial severity or pattern of bleeding, it does compromise the ability to manage haemorrhage in affected individuals, resulting in a greater rate of complications, cost and disability. The purpose of this review is to summarize current understanding of the epidemiology, immunobiology, laboratory evaluation and management of inhibitors arising in patients with congenital coagulation disorders. An attempt has been made to focus on recent advances in the immunology of inhibitors, and to speculate on their potential clinical application.

Mapping of genes that control the antibody response to human factor IX in mice

We tested the hypothesis that the antibody response to human factor IX in mice is controlled by genetic factors, especially histocompatibility antigens. Seven inbred mouse strains were immunized against human factor IX by adenoviral gene transfer or serial injections of human factor IX protein. A/J mice had the highest antibody response and 2 C57 mouse strains had the lowest response. We used the adenovirus vector to immunize 26 recombinant inbred mouse strains (AXB and BXA) derived from A/J and C57BL/6J mice and observed highly significant linkage (logarithmic odds [LOD] scores approximately 4.8) for the polymorphic D17Mit62 marker that is 1 centimorgan ( approximately 300 000 base pair [bp]) from the mouse major histocompatibility complex (MHC) locus (H-2). Experiments in mice with chimeric MHC genes indicated that class IaK or class II H-2 (or both) genes were critical, but other genes contributed to the antibody response. Polymorphic markers from chromosomes 1 and 10 that are near important immunoregulatory genes such as interleukin 10 and the interferon-gamma gene show suggestive linkage (LOD scores of approximately 2.3-2.6) to the factor IX antibody response. This study confirms the hypothesis that H-2 (and other) genes control factor IX antibody development in mice and suggests their potential importance for factor IX antibody development in humans with hemophilia B.

FEIBAR safety profile in multiple modes of clinical and home-therapy application

The development of neutralizing antibodies to factor VIII or IX therapeutic concentrates remains the most serious and challenging complication in the management of patients with haemophilia A and B. FEIBA, Anti-Inhibitor Coagulant Complex, is an activated prothrombin complex concentrate that has been used to treat patients with such complications for almost 30 years. The mechanism of action of FEIBA has been proposed to involve simultaneous FVIII/FIX inhibitor bypassing action in the common, intrinsic and extrinsic coagulation pathways. FEIBA is derived from human plasma that undergoes stringent viral screening followed by significant viral inactivation and removal. To date, there have been no confirmed reports of transmission of hepatitis A, B or C, or of human immunodeficiency viruses associated with the use of the current, vapour-heat-treated FEIBA concentrate. The incidence of thrombotic adverse events recorded in the Baxter pharmacovigilance database for the 10-year postmarket period (1990-99) was approximately 4 : 100,000 infusions of FEIBA. Almost all documented thrombotic events with FEIBA occurred with doses that exceeded dosing recommendations, and known risk factors for cardiovascular disease were evident in more than 80% of the patients involved. Overall, clinical data have shown FEIBA to be safe and well-tolerated for use in a wide variety of clinical settings, including treatment of bleeding episodes, management of surgical procedures, home therapy, long-term prophylaxis, and prophylaxis during immune tolerance induction, when used according to dosing guidelines.

Continuous antigenic stimulation system (CASS) as a new immunization strategy

Protection against diseases is mediated by a sustained immune response. Here, we describe a new immunization strategy. Mice implanted with encapsulated C2C12 myoblasts secreting human factor IX (hFIX) elicited a strong humoral response against the transgene, as compared to mice immunized with complete Freund's adjuvant (FA). Mice also had increasing IgG2a antibody titer, indicating a switch to a Th1 profile immune response. Mice developed strong hFIX-specific cytotoxic T lymphocytes (CTL) that was detectable 213 days after implantation, demonstrating the sustained immunity elicited by encapsulated cells. Here, we propose continuous antigenic stimulation system (CASS) as a novel immunization strategy with potential application in the design of novel vaccines.

Thirty years of hemophilia treatment in the Netherlands, 1972-2001

Since the introduction of replacement therapy in the early 1960s by the infusion of plasma-derived factor VIII and IX preparations, important changes have occurred for hemophilia patients. We studied the medical and social developments over 30 years of hemophilia treatment. Since 1972, 5 cross-sectional national postal surveys among all hemophilia patients in the Netherlands were performed, the latest in 2001. The prestructured questionnaires included items on treatment, the presence of inhibitory antibodies against factor VIII or IX, the annual number of bleeding episodes, use of inpatient hospital care, and hepatitis C and HIV infections. Response rate in 2001 was 70%. Young patients (<16 years) with severe hemophilia showed the largest increase in use of prophylaxis, from 34% in 1972 to 86% in 2001. The occurrence of hemorrhages has gradually decreased. Hospital admissions decreased from 47% of all patients in 1972 to 18% in 2001. Our study shows that the treatment of patients with severe hemophilia in the Netherlands has focused on the use of prophylactic treatment, especially in children. This has resulted in a decrease in bleeding frequency and an improvement of the medical and social circumstances of patients.

An antibody specific for coagulation factor IX enhances the activity of the intrinsic factor X-activating complex

During hemostasis the zymogen factor X (FX) is converted into its enzymatically active form factor Xa by the intrinsic FX-activating complex. This complex consists of the protease factor IXa (FIXa) that assembles, together with its cofactor, factor VIIIa, on a phospholipid surface. We have studied the functional properties of a FIXa-specific monoclonal antibody, 224AE3, which has the potential to enhance intrinsic FX activation. Binding of the antibody to FIXa improved the catalytic properties of the intrinsic FX-activating complex in two ways: (i) factor VIIIa bound to the FIXa-antibody complex with a more than 18-fold higher affinity than to FIXa, and (ii) the turnover number (kcat) of the enzyme complex increased 2- to 3-fold whereas the Km for FX remained unaffected. The ability of 224AE3 to increase the FXa-generation potential (called the "booster effect") was confirmed in factor VIII (FVIII)-depleted plasma, which was supplemented with different amounts of recombinant FVIII. In the presence of antibody 224AE3 the coagulant activity was increased 2-fold at physiological FVIII concentration and up to 15-fold at low FVIII concentrations. The booster effect that we describe demonstrates the ability of antibodies to function as an additional cofactor in an enzymatic reaction and might open up a new principle for improving the treatment of hemophilia.

Permanent phenotypic correction of hemophilia B in immunocompetent mice by prenatal gene therapy

Hemophilia B, also known as Christmas disease, arises from mutations in the factor IX (F9) gene. Its treatment in humans, by recombinant protein substitution, is expensive, thus limiting its application to intermittent treatment in bleeding episodes and prophylaxis during surgery; development of inhibitory antibodies is an associated hazard. This study demonstrates permanent therapeutic correction of his disease without development of immune reactions by introduction of an HIV-based lentiviral vector encoding the human factor IX protein into the fetal circulation of immunocompetent hemophiliac and normal outbred mice. Plasma factor IX antigen remained at around 9%, 13%, and 16% of normal in the 3 hemophilia B mice, respectively, until the last measurement at 14 months. Substantial improvement in blood coagulability as measured by coagulation assay was seen in all 3 mice and they rapidly stopped bleeding after venipuncture. No humoral or cellular immunity against the protein, elevation of serum liver enzymes, or vector spread to the germline or maternal circulation were detected.

The incidence of factor VIII and factor IX inhibitors in the hemophilia population of the UK and their effect on subsequent mortality, 1977-99

BACKGROUND

Previous studies of the development of inhibitors and their impact on mortality have been small.

OBJECTIVES

To examine the development of inhibitors in people with hemophilia in the UK and their effect on subsequent mortality.

PATIENTS

6078 males with hemophilia A and 1172 males with hemophilia B registered in the UK Haemophilia Centre Doctors' Organisation database, 1977-98.

RESULTS

In severe hemophilia A inhibitors developed at rates of 34.4, 5.2 and 3.8 per 1000 years at ages <5, 5-14 and 15+years; cumulative risks at ages 5 and 75 were 16% and 36%. In hemophilia A the rate of inhibitor development decreased during 1977-90, but increased during the 1990s. In severe hemophilia B inhibitors developed at rates of 13.3 and 0.2 per 1000 years at ages <5 and 5+ and cumulative risks at ages 5 and 75 were 6% and 8%. With HIV, inhibitor development did not increase mortality. In severe hemophilia without HIV, inhibitor development doubled mortality during 1977-92, but during 1993-99 mortality was identical with and without inhibitors. In severe hemophilia without HIV but with inhibitors, mortality from causes involving bleeding decreased during 1977-99 (P = 0.001) as did mortality involving intracranial hemorrhage (P = 0.007).

CONCLUSIONS

These data provide estimates of the rate of inhibitor development in hemophilia A and hemophilia B, and they show that the rate of inhibitor development has varied over time, although the reasons for this remain unclear. They also show that in severe hemophilia the substantial increase in mortality previously associated with inhibitors is no longer present.

Rituximab in the treatment of alloimmune factor VIII and IX antibodies in two children with severe haemophilia

We report the use of rituximab (Genentech, San Francisco, CA, USA) in two children with severe haemophilia with inhibitors to factors VIII and IX, which failed to respond to conventional immune tolerance therapy. The treatment was well tolerated by both children. The child with haemophilia B had no clinical improvement or fall in CD19 and he is currently being treated with recombinant activated factor VII (NovoSeven, Novo Nordisk, Denmark) for bleeding episodes. The child with haemophilia A had a good clinical response with a negative inhibitor assay at 11 months follow-up.

Recombinant factor VIIa: review of efficacy, dosing regimens and safety in patients with congenital and acquired factor VIII or IX inhibitors

Recombinant factor (rF)VIIa has been available to clinicians since 1996 and has an excellent safety record after almost three-quarters of a million doses have been administered. This paper will review the current clinical experience with rFVIIa dosing in acquired and congenital hemophilia with inhibitors and chronicle all spontaneous and clinical trial reports of thrombotic adverse events as of April 2003. Standard dosing of rFVIIa (90 micro g kg(-1)) allows binding of FVIIa to the surface of an activated platelet and can directly activate factor X in the absence of tissue factor. Experience with bolus dosing suggests that higher dosing (>200 micro g kg(-1)) may be more efficacious in treating hemophilia patients. Clinical trials are ongoing to validate this observation. Continuous infusion dosing may be efficacious for major surgery but high infusion rates (50 micro g kg(-1) h(-1)) might be needed. The relationship between dose of rFVIIa, amount of thrombin generated and measurable FVIIa level is still not known and perhaps newer testing which measures thrombin generation might be more advantageous. Relatively few thrombotic events have been associated with rFVIIa. Known factors predisposing to thrombosis were present in 20 of the 25 (80%) hemophilia patients who were reported spontaneously or who developed a thrombosis during a clinical trial. Additionally, thrombotic events have not increased despite a growing experience with higher dosing of rFVIIa.

Crystal Structure of the Calcium-stabilized Human Factor IX Gla Domain Bound to a Conformation-specific Anti-factor IX Antibody

The binding of Factor IX to membranes during blood coagulation is mediated by the N-terminal gamma-carboxyglutamic acid-rich (Gla) domain, a membrane-anchoring domain found on vitamin K-dependent blood coagulation and regulatory proteins. Conformation-specific anti-Factor IX antibodies are directed at the calcium-stabilized Gla domain and interfere with Factor IX-membrane interaction. One such antibody, 10C12, recognizes the calcium-stabilized form of the Gla domain of Factor IX. We prepared the fully carboxylated Gla domain of Factor IX by solid phase peptide synthesis and crystallized Factor IX-(1-47) in complex with Fab fragments of the 10C12 antibody. The overall structure of the Gla domain in the Factor IX-(1-47)-antibody complex at 2.2 A is similar to the structure of the Factor IX Gla domain in the presence of calcium ions as determined by NMR spectroscopy (Freedman, S. J., Furie, B. C., Furie, B., and Baleja, J. D. (1995) Biochemistry 34, 12126-12137) and by x-ray crystallography (Shikamoto, Y., Morita, T., Fujimoto, Z., and Mizuno, H. (2003) J. Biol. Chem. 278, 24090-24094). The complex structure shows that the complementarity determining region loops of the 10C12 antibody form a hydrophobic pocket to accommodate the hydrophobic patch of the Gla domain consisting of Leu-6, Phe-9, and Val-10. Polar interactions also play an important role in the antibody-antigen recognition. Furthermore, the calcium coordination network of the Factor IX Gla domain is different than in Gla domain structures of other vitamin K-dependent proteins. We conclude that this antibody is directed at the membrane binding site in the omega loop of Factor IX and blocks Factor IX function by inhibiting its interaction with membranes.

Use of implantable venous access devices in children with severe hemophilia: benefits and burden

BACKGROUND AND OBJECTIVES

Since venous access in small children can be difficult to obtain, implantable venous access devices (IVAD) are used to administer clotting factor in such patients with severe hemophilia. The aim of our study was to evaluate how many children in our center needed an IVAD in order to be able to start early prophylaxis, what the differences were between children who needed an IVAD and those who did not and what the complications of the IVAD were.

DESIGN AND METHODS

All 70 patients with severe hemophilia born between January 1987 and October 2000 treated at our center before they were 6 years old were studied.

RESULTS

An IVAD was placed in 23 children (33%). Children with an IVAD started prophylactic treatment at a mean age of 2 years (SD 1.3), those without at a mean age of 3.6 years (SD 1.6) (p< 0.001). Home treatment was feasible at a mean age of 2.8 years (SD 1.3) in children with an IVAD and at 4.5 years in those without an IVAD (SD 1.8) (p = 0.001). Infection was the most frequent complication; the mean number of infections per IVAD was 0.61. Thrombosis was more common than initially thought (15%). The infection rate in children with inhibitory antibodies was 3.1 per 1000 patient-days; in children without an inhibitor it was 0.72 per 1000 patient-days.

INTERPRETATION AND CONCLUSIONS

In 33% of the children in our cohort an IVAD was needed in order to start early prophylaxis. IVAD are needed more frequently when prophylaxis is started at an early age, but have the advantage that home treatment is feasible earlier. Infection is the most common complication, particularly in children with inhibitors.

Safety and efficacy of factor IX gene transfer to skeletal muscle in murine and canine hemophilia B models by adeno-associated viral vector serotype 1

Adeno-associated viral (AAV) vectors (serotype 2) efficiently transduce skeletal muscle, and have been used as gene delivery vehicles for hemophilia B and for muscular dystrophies in experimental animals and humans. Recent reports suggest that AAV vectors based on serotypes 1, 5, and 7 transduce murine skeletal muscle much more efficiently than AAV-2, with reported increases in expression ranging from 2-fold to 1000-fold. We sought to determine whether this increased efficacy could be observed in species other than mice. In immunodeficient mice we saw 10- to 20-fold higher levels of human factor IX (hF.IX) expression at a range of doses, and in hemophilic dogs we observed approximately 50-fold higher levels of expression. The increase in transgene expression was due partly to higher gene copy number and a larger number of cells transduced at each injection site. In all immunocompetent animals injected with AAV-1, inhibitory antibodies to F.IX developed, but in immunocompetent mice treated with high doses of vector, inhibitory antibodies eventually disappeared. These studies emphasize that the increased efficacy of AAV-1 vectors carries a risk of inhibitor formation, and that further studies will be required to define doses and treatment regimens that result in tolerance rather than immunity to F.IX.

Environmental and genetic factors influencing inhibitor development

Inhibitor formation occurs at a frequency of 20% to 30% in severe hemophilia A, and 3% in hemophilia B. Today, it represents the major complication in patient care and renders classical substitution therapy ineffective. Genetic factors, such as factor VIII (FVIII) gene mutations and immune response genes, particularly the major histocompatibility complex, have been shown to constitute decisive risk factors for the development of inhibitors. In severe hemophilia A and B, those mutations that result in the absence or severe truncation of the FVIII/factor IX (FIX) proteins are associated with the highest risk for inhibitor formation, indicating that a major driving force in inhibitor development is the presentation of a novel antigen to the patient's immune system. An alternative pathomechanism may underlie inhibitor development in patients with mild hemophilia A. Missense mutations, especially those in the C1/C2 domains, may alter the immunogenicity of the FVIII protein, eliciting an inhibitor response against the mutated epitope. In some patients with hemophilia B, especially those with large deletions to the FIX gene, a severe allergic reaction occurs simultaneously with inhibitor onset. Despite the obviously strong genetic predisposition, discordant inhibitor status in monozygotic hemophilia A twins demonstrates that environmental factors also play a role in the development of inhibitors.

Neonatal gene transfer with a retroviral vector results in tolerance to human factor IX in mice and dogs

The effect of neonatal gene transfer on antibody formation was determined using a retroviral vector (RV) expressing human factor IX (hFIX). Normal mice from different strains injected intravenously with RV as newborns achieved therapeutic levels of hFIX without antibody production and were tolerant as adults to challenge with hFIX. Neonatal hemophilia B mice that received different amounts of RV achieved stable and dose-related expression of hFIX without anti-hFIX antibody formation. After protein challenge, antibody formation was markedly reduced for animals that expressed hFIX at levels higher than 14 ng/mL (0.3% of normal). However, antibodies developed for animals that received the lowest dose of RV and expressed hFIX at approximately 2 ng/mL before protein challenge. In dogs, neonatal injection of a high dose of RV resulted in 500 ng/mL hFIX in plasma without antibody formation. We conclude that neonatal gene transfer with RV does not induce antibody responses to hFIX in mice or dogs and that mice achieving levels greater than 3 x 10-10 M hFIX are usually tolerant to protein injection as adults. Low-dose gene therapy or frequent protein injections in the neonatal period might induce tolerance to subsequent injections of protein with a low risk for adverse effects.

Targeting lentiviral vector expression to hepatocytes limits transgene-specific immune response and establishes long-term expression of human antihemophilic factor IX in mice

Stable gene replacement by in vivo administration of lentiviral vectors (LVs) has therapeutic potential for metabolic disorders and other systemic diseases. We studied the expression of intracellular and secreted proteins by LVs in immunocompetent mice. Liver, spleen, and bone marrow cells were efficiently transduced. However, transgene expression, driven by a ubiquitous promoter, was limited by transgene-specific cellular and humoral immune responses, leading to the clearance of transduced cells. After green fluorescent protein (GFP) gene transfer, the liver showed infiltration of CD8(+) cytotoxic T cells, and GFP-specific CD8(+) T cells were isolated from the spleen. After human factor IX (hF.IX) gene transfer, anti-hF.IX antibodies were induced. These immune responses were not detected in mice injected with heat-inactivated or genome-lacking LVs or in GFP-transgenic mice, indicating that they were specifically triggered by transgene expression in vivo. Intriguingly, selective targeting of LV expression to hepatocytes limited the immune responses to the transgenes. By this approach, high levels of hF.IX, potentially in the therapeutic range, were reached and maintained long term in immunocompetent mice, without inducing antibody formation. These results prompt further studies in relevant animal models to explore the potential of in vivo LV administration for the gene therapy of hemophilias and other liver-based diseases.

Reduced bleeding events with subcutaneous administration of recombinant human factor IX in immune-tolerant hemophilia B dogs

Intravenous administration of recombinant human factor IX (rhFIX) acutely corrects the coagulopathy in hemophilia B dogs. To date, 20 of 20 dogs developed inhibitory antibodies to the xenoprotein, making it impossible to determine if new human FIX products, formulations, or methods of chronic administration can reduce bleeding frequency. Our goal was to determine whether hemophilia B dogs rendered tolerant to rhFIX would have reduced bleeding episodes while on sustained prophylactic rhFIX administered subcutaneously. Reproducible methods were developed for inducing tolerance to rhFIX in this strain of hemophilia B dogs, resulting in a significant reduction in the development of inhibitors relative to historical controls (5 of 12 versus 20 or 20, P <.001). The 7 of 12 tolerized hemophilia B dogs exhibited shortened whole blood clotting times (WBCTs), sustained detectable FIX antigen, undetectable Bethesda inhibitors, transient or no detectable antihuman FIX antibody titers by enzyme-linked immunosorbent assay (ELISA), and normal clearance of infused rhFIX. Tolerized hemophilia B dogs had 69% reduction in bleeding frequency in year 1 compared with nontolerized hemophilia B dogs (P =.0007). If proven safe in human clinical trials, subcutaneous rhFIX may provide an alternate approach to prophylactic therapy in selected patients with hemophilia B.

A novel therapeutic approach combining human plasma-derived Factors VIIa and X for haemophiliacs with inhibitors: evidence of a higher thrombin generation rate in vitro and more sustained haemostatic activity in vivo than obtained with Factor VIIa alone

BACKGROUND AND OBJECTIVES

Therapy with recombinant Factor VIIa (rFVIIa) for haemophiliacs with inhibitors still has some unresolved problems, such as the requirement for frequent infusions of rFVIIa every 2-3 h to sustain haemostatic activity for an extended time-period and that the therapeutic dose of rFVIIa is not always predictable. In the present study, we searched for an effective combination of plasma-derived FVIIa with other blood coagulation factors, and demonstrated that a therapeutic approach combining plasma-derived FVIIa and Factor X (FX) was more useful for treating haemophiliacs with inhibitors than FVIIa alone.

MATERIALS AND METHODS

The haemostatic effects of FVIIa and FX were evaluated in vitro and in vivo. In in vitro experiments we assessed the following: the ability to enhance the thrombin generation rate in a reconstituted blood coagulation model without Factor VIII (FVIII) or Factor IX (FIX); the ability to correct the activated partial prothrombin time (APTT) of FVIII-depleted plasma or FIX-depleted plasma; and the ability to correct the clotting time of haemophilia-like whole blood using thromboelastography (TEG). In in vivo experiments, the haemostatic activity of the combination treatment of FVIIa and FX was determined by measuring the bleeding time and TEG using a monkey haemophilia B model produced by the injection of anti-human FIX polyclonal antibodies. The degree of thrombogenicity of the combination was evaluated using the rabbit stasis model.

RESULTS

The addition of FX to FVIIa dramatically enhanced the thrombin generation rate in the reconstituted blood coagulation model and corrected the prolonged APTTs of FVIII- and FIX-depleted plasmas to levels achieved by the replacement therapies. In contrast, the addition of prothrombin to FVIIa did not show such enhancing activity. Furthermore, FVIIa-induced whole blood clotting times in the FVIII- and FIX-inhibited states were also shortened by the addition of FX in a concentration-dependent manner. Finally, the co-administration of FVIIa (80 microg/kg) and FX (800 microg/kg) in a monkey haemophilia B model resulted in a more robust and persistent haemostatic effect on the secondary bleeding time and whole-blood clotting time of TEG than that of FVIIa alone. The results of rabbit stasis tests for evaluating the risk of thrombogenicity showed that the combination of FVIIa and FX was less thrombogenic than FEIBA.

CONCLUSIONS

The present study demonstrated that the combination of FVIIa and FX appeared to have a higher and more sustainable haemostatic potential than FVIIa alone, and less thrombogenicity than FEIBA. A therapeutic approach combining FVIIa and FX could be a promising and novel approach to compensate for the disadvantages of rFVIIa and FEIBA for haemophiliacs with inhibitors.

AAV-mediated gene therapy for hemophilia

Gene therapy for hemophilia has been contemplated since the coagulation Factor genes responsible for the disease were cloned 20 years ago. Multiple approaches towards the delivery of Factors VIII or IX, the defective genes in the most common forms of hemophilia, have resulted in positive results in animals, and largely equivocal results in human clinical testing. Use of vectors based on adeno-associated virus has led to robust and sustained cures in hemophilic mice and dogs, and intriguing preliminary results in small or ongoing clinical trials. As more clinical experience is gained, solving delivery issues will be of paramount importance and will lead to more clinical success. This success will permit hemophilia to be cured following a single injection of the normal gene.

Cost of care and quality of life for patients with hemophilia complicated by inhibitors: the COCIS Study Group

Inhibitors in patients with hemophilia are a rare complication of a rare disease causing pain and disability in patients and impairment to the quality of their lives. Recent advances in treatment have brought improvements, but they have done so by absorbing larger amounts of financial resources. This study involved 52 Italian patients with hemophilia with high-responding inhibitors who were longitudinally observed for 18 months to evaluate concomitantly cost of care and quality of life. Overall, 0.6 bleeding episodes per patient per month were recorded. This frequency of events was lower than that reported in other cohorts of patients with hemophilia who were not taking inhibitors. The average monthly cost of care was, in euros, 18,000 (18,000 US dollars) per patient, mainly because of treatment products. Recombinant activated factor VII, mostly used for orthopedic surgery, represented 50% of the expenses. Quality of life, measured through validated questionnaires, was similar to that of patients with severe hemophilia without inhibitors. In particular, physical quality of life was similar to that in patients with diabetes and on dialysis, whereas mental quality of life was comparable to that in the general population. This study shows that hemophilia complicated by inhibitors, a prototype of rare disease, requires high amounts of resources for management that provides a satisfactory quality of life.

Immunogenicity and immune tolerance coagulation Factors VIII and IX

Some of the major issues related to the development and control of antibodies that occur during treatment of haemophilia with replacement factors (Factor VIII and Factor IX) are reviewed. Information on analytical issues, immunogenicity, and immune tolerance may be applicable to the study of other therapeutic proteins. Conversely, new information obtained from evaluation of other therapeutic protein products may address issues that remain unresolved for Factor VIII and FIX replacement therapy.

Immune tolerance therapy dose as an outcome predictor

The question of dose as a successful ITT outcome predictor in haemophilia A and B is an important one on many levels. Increased morbidity associated with inhibitor development has been documented. Currently, immune tolerance is the only proven method for inhibitor eradication. Furthermore, given the morbidity and the high cost of less effective bypass therapy, a study by Harvard health economists using Markov decision analysis predicted an increased life expectancy of 4.5 years at a lifetime cost savings of $1.6 million with the use of ITT rather than lifelong APCC therapy. However, the immediate short-term cost of this expensive therapeutic intervention has the capacity to strain the financial resources of many countries in the developed world, regardless of the system of medical reimbursement. It also assures the inaccessibility of this treatment modality to most of the world's inhibitor patients living in the developing world. Cost aside, recent experience has taught us that clotting factor supply is precarious, not inexhaustible as previously assumed. Finally, in the paediatric patient who is most likely to require and undergo ITT, venous access for daily infusions is problematic with or without a central venous access device. Indeed, for these reasons alone, finding the answer to the question of optimal cost-effective dosing immune tolerance is a matter of medical urgency and moral obligation to the global haemophilia community.

Factor XI deficiency - from molecular genetics to clinical management

Factor XI (FXI) deficiency is a rare bleeding disorder, but is known to occur more frequently in a number of well-defined populations. FXI deficiency is most notable for its variable clinical phenotype. The FXI gene is located at the distal end of the long arm of chromosome 4 and encodes a 607 amino acid mature protein, which is a zymogen for a serine protease. Although the serine protease domain is similar to that of many other coagulation factors, the heavy chain differs in that it contains four tandem Apple domains. FXI is also unique in that it exists as a homodimer, with this dimerization appearing essential for normal function. A total of 39 different FXI mutations have been identified to date, affecting both the catalytic and Apple domains. This article will review the molecular genetics of FXI deficiency with particular focus on the implications of these findings for the clinical management of this condition. The potential utility of alternatives to plasma-derived FXI concentrate, such as recombinant factor VIIa (rFVIIa, NovoSeven) will also be explored.

Induction of immune tolerance to coagulation factor IX antigen by in vivo hepatic gene transfer

Gene replacement therapy is an attractive approach for treatment of genetic disease, but may be complicated by the risk of a neutralizing immune response to the therapeutic gene product. There are examples of humoral and cellular immune responses against the transgene product as well as absence of such responses, depending on vector design and the underlying mutation in the dysfunctional gene. It has been unclear, however, whether transgene expression can induce tolerance to the therapeutic antigen. Here, we demonstrate induction of immune tolerance to a secreted human coagulation factor IX (hF.IX) antigen by adeno-associated viral gene transfer to the liver. Tolerized mice showed absence of anti-hF.IX and substantially reduced in vitro T cell responses after immunization with hF.IX in adjuvant. Tolerance induction was antigen specific, affected a broad range of Th cell subsets, and was favored by higher levels of transgene expression as determined by promoter strength, vector dose, and mouse strain. Hepatocyte-derived hF.IX expression induced regulatory CD4(+) T cells that can suppress anti-hF.IX formation after adoptive transfer. With a strain-dependent rate of success, tolerance to murine F.IX was induced in mice with a large F.IX gene deletion, supporting the relevance of these data for treatment of hemophilia B and other genetic diseases.

In utero gene transfer of human factor IX to fetal mice can induce postnatal tolerance of the exogenous clotting factor

The fundamental hypotheses behind fetal gene therapy are that it may be possible (1) to achieve immune tolerance of transgene product and, perhaps, vector; (2) to target cells and tissues that are inaccessible in adult life; (3) to transduce a high percentage of rapidly proliferating cells, and in particular stem cells, with relatively low absolute virus doses leading to clonal transgene amplification by integrating vectors; and (4) to prevent early disease manifestation of genetic diseases. This study provides evidence vindicating the first hypothesis; namely, that intravascular prenatal administration of an adenoviral vector carrying the human factor IX (hFIX) transgene can induce immune tolerance of the transgenic protein. Following repeated hFIX protein injection into adult mice, after prenatal vector injection, we found persistence of blood hFIX and absence of hFIX antibodies in 5 of 9 mice. Furthermore, there was substantial hFIX expression after each of 2 reinjections of vector without detection of hFIX antibodies. In contrast, all adult mice that had not been treated prenatally showed a rapid loss of the injected hFIX and the development of high hFIX antibody levels, both clear manifestations of a strong immune reaction.

Treatment of the bleeding inhibitor patient

The development of inhibitory antibodies to factor (F) VIII and FIX continues to be a major challenge in the treatment of patients with hemophilia. In patients with low-responding inhibitors, it is usually possible to saturate the inhibitor with the deficient factor and to achieve hemostasis, but in patients with high-responding inhibitors, two major tasks have to be considered. One is how to treat the acute bleedings and the other is how to permanently eliminate the immune response, in other words, to induce tolerance. There are several hemostatic agents available for bleeding patients with high-responding inhibitors. Nonactivated and activated prothrombin complex concentrates (PCCs) have been used for almost 30 years, and since the beginning of the 1980s, porcine FVIII has also been used. In more recent years, recombinant FVIIa has been added to the therapeutic armamentarium and has been shown to control hemostasis in most patients. Immunoadsorption may temporarily reduce the inhibitor, enabling replacement therapy for several days. Available data on these alternative regimens will be discussed with a focus on the mechanisms of action, pharmacokinetics, safety, monitoring, and clinical experience.

Discontinuous residues of factor IX constitute a surface for binding the anti-factor IX monoclonal antibody A-5

Anti-human factor IX monoclonal antibody, A-5 (Mab A-5), has been widely used in structure-function studies for factor IX. Mab A-5 recognizes the catalytic domain of human factor IX (FIX). Regions important for Mab A-5 binding have previously been localized to the amino terminus of the heavy chain of factor IX, encompassing amino acid residues 181-310 [Blood (74) 971]. We have selected 20 positions in this region for alanine-scanning mutagenesis. We found that Mab A-5 failed to react with the recombinant factor IX mutants with substitutions at positions 228 and 252. Mab A-5 also reacted to a lesser extent to FIXD276A (factor IX with alanine substitution for aspartic acid at residue 276) and FIXK201A/D203A (double alanine substitutions at residues 201 and 203). The apparent dissociation rate constants (K(D)) in binding Mab A-5 were 6.0 x 10(-9), 1.4 x 10(-8) and 2.0 x 10(-8) M, for wild-type FIX, FIXK201A/D203A and FIXD276A, respectively. The increased K(D) values of the two FIX mutants are mainly owing to the increased dissociation rates. These affected residues constitute a surface opposite from the factor VIIIa binding surface. We conclude that the epitope for Mab A-5 is on a surface composed of residues 228, 252, 276, and 201 or 203. This surface, which may not be important for factor VIII binding, contains a strong antigenic region on factor IX.

Identification of T-cell epitopes in clotting factor IX and lack of tolerance in inbred mice

Immune responses to the factor IX protein pose problems for hemophilia B patients who develop antibodies against factor IX and for potential future treatment with gene therapy. To better define the response to human factor IX, we analyzed T-cell responses to human factor IX in factor IX knockout mice on BALB/c and C57BL/6 (B6) backgrounds, both strains having CD4+ T cells that proliferate in response to human factor IX. Surprisingly, wild-type mice have similar factor IX-recognizing CD4+ T cells. We defined a dominant CD4+ epitope for each strain (CVETGVKITVVAGEH for BALB/c and LLELDEPLVLNSYVTPIC for B6) that was recognized by both factor IX knockout and wild-type mice. While human factor IX did not cross-react with the mouse homologs of these epitopes, immunization with peptides from murine factor IX stimulated proliferation in factor IX knockout mice and wild-type mice, demonstrating a failure to delete murine factor IX-specific T cells in normal mice.

The N-terminal epidermal growth factor-like domain of coagulation factor IX. Probing its functions in the activation of factor IX and factor X with a monoclonal antibody

The absence or reduced activity of coagulation factor IX (FIX) causes the severe bleeding disorder hemophilia B. FIX contains an N-terminal Gla domain followed by two epidermal growth factor-like (EGF) domains and a serine protease domain. In this study, the epitope of monoclonal antibody AW, which is directed against the C-terminal part of the first EGF domain in human FIX, was defined, and the antibody was used to study interactions between the EGF domain of FIX and other coagulation proteins. Antibody AW completely blocks activation of FIX by activated factor XI, but activation by activated factor FVII-tissue factor is inhibited only slightly. The antibody also causes a marginal reduction in the apparent k(cat) for factor X both in the presence and absence of activated factor VIII. Based on these results, we produced a preliminary model of the structure of the activated factor IX-activated factor VIII-AW complex on the surface of phospholipid. The model suggests that in the Xase complex, EGF1 of activated factor IX is not involved in direct binding to activated factor VIII. Studies of the interaction of antibody AW with a mutated FIX molecule (R94D) also suggest that the Glu(78)-Arg(94) salt bridge is not important for maintaining the structure of FIX.

Use of a non-depleting anti-CD4 antibody to modulate the immune response to coagulation factors VIII and IX

The generation of antibodies to therapeutic factors VIII or IX is a major problem in the management of haemophilia and places potential limitations on the application of gene therapy. We have investigated the administration of a non-depleting anti-CD4 antibody for modulation of the immune response to human recombinant coagulation factors VIII and IX. In mice given these clotting factors, co-administration of anti-CD4 antibody significantly reduced the appearance of factor-specific antibodies. These data provide evidence that the neutralizing antibody response to exogenous coagulation factors may be controllable if non-depleting anti-CD4 antibody is co-administered at the time of initial replacement therapy.

Influence of vector dose on factor IX-specific T and B cell responses in muscle-directed gene therapy

Intramuscular injection of an adeno-associated virus (AAV) vector has resulted in vector dose-dependent, stable expression of canine factor IX (cF.IX) in hemophilia B dogs with an F.IX missense mutation (Herzog et al., Nat. Med. 1999;5:56-63). The use of a species-specific transgene allowed us to study risks and characteristics of antibody formation against the therapeutic transgene product. We analyzed seven dogs that had been injected at a single time point at multiple intramuscular sites with varying vector doses (dose per kilogram, dose per animal, dose per site). Comparison of individual animals suggests an increased likelihood of inhibitory anti-cF.IX (inhibitor) development with increased vector doses, with dose per site showing the strongest correlation with the risk of inhibitor formation. In six of seven animals, such immune responses were either absent or transient, and therefore did not prevent sustained systemic expression of cF.IX. Transient inhibitory/neutralizing anti-cF.IX responses occurred at vector doses of 2 x 10(12)/site, whereas a 6-fold higher dose resulted in a longer lasting, higher titer inhibitor. Anti-cF.IX was efficiently blocked in an eighth animal that was injected with a high vector dose per site, but in addition received transient immune suppression. Inhibitor formation was characterized by synthesis of two IgG subclasses and in vitro proliferation of lymphocytes to cF.IX antigen, indicating a helper T cell-dependent mechanism. Anti-cF.IX formation is likely influenced by the extent of local antigen presentation and may be avoided by limited vector doses or by transient immune modulation.