The humoral response to human factor VIII in hemophilia A mice

BACKGROUND: Inhibitory antibodies (Abs) to factor VIII (FVIII inhibitors) constitute the most significant complication in the management of hemophilia A. The analysis of FVIII inhibitors is confounded by polyclonality and the size of FVIII. OBJECTIVES: The goal of this study was to dissect the polyclonal response to human FVIII in hemophilia A mice undergoing a dosage schedule that mimics human use. METHODS: Splenic B-cell hybridomas were obtained following serial i.v. injections of submicrogram doses of FVIII. Results of a novel, anti-FVIII domain-specific enzyme-linked immunosorbent assay were compared to Ab isotype and anti-FVIII inhibitory activity. RESULTS: The robust immune response resulted in the production of approximately 300 hybridomas per spleen. We characterized Abs from 506 hybridomas, representing the most comprehensive analysis of a protein antigen to date. Similar to the human response to FVIII, anti-A2 and anti-C2 Abs constituted the majority of inhibitors. A novel epitope was identified in the A2 domain by competition ELISA. Anti-A2 and anti-C2 Abs were significantly associated with IgG(1) and IgG(2a) isotypes, respectively. Because the IgG(2a) isotype is associated with enhanced Fc receptor-mediated effector mechanisms, this result suggests that anti-C2 Abs and inflammation may be linked. Additionally, we identified a novel class of Abs with dual specificity for the A1 and A3 domains. Forty per cent of the Abs had no detectable inhibitory activity, indicating that they are prominent and potentially pathologically significant. CONCLUSION: The expanded delineation of the humoral response to FVIII may lead to improved management of hemophilia A through mutagenesis of FVIII B-cell epitopes.     

Comparative immunogenicity of recombinant B domain‐deleted porcine factor VIII and Hyate:C in hemophilia A mice presensitized to human factor VIII

Summary. Hyate is a commercial plasma‐derived porcine factor (F)VIII concentrate that is used in the treatment of patients with inhibitory antibodies to FVIII. OBI‐1 is a recombinant B domain‐deleted form of porcine FVIII that is in clinical development for the same indication. Hemophilia A mice were presensitized with human FVIII to simulate clinical inhibitory antibody formation and then were randomized to receive OBI‐1 or Hyate:C in a comparative immunogenicity trial. OBI‐1 or Hyate:C were given in a series of four intravenous injections at weekly intervals at doses of 1, 10, or 100 U kg^?1. Inhibitory antibodies to porcine FVIII were not detected by Bethesda assay in most of the mice given OBI‐1 or Hyate:C at doses of 1 or 10 U kg^?1, but were identified in 81% and 94% of mice given 100 U kg^?1 of OBI‐1 or Hyate:C, respectively. There was no significant difference between OBI‐1 and Hyate:C in inhibitory antibody formation at any dose, although there was a trend toward a lower Bethesda titer in OBI‐1‐treated mice at 10 U kg^?1 (P = 0.09). Total anti‐FVIII antibodies to Hyate:C and OBI‐1 were also measured by ELISA using immobilized purified plasma‐derived porcine FVIII and OBI‐1, respectively, as antigens. At the 10 and 100 U kg^?1 doses, the mean anti‐FVIII response was higher in Hyate:C‐treated‐mice than in OBI‐1‐treated mice (P = 0.02 and P = 0.004, respectively). The results using this model suggest that OBI‐1 may be less immunogenic and safer than Hyate:C in FVIII inhibitor patients.     

Neutralization of antifactor VIII inhibitors by recombinant porcine factor VIII

BACKGROUND: Inhibitory antibodies to factor (F) VIII (FVIII inhibitors) present a major clinical challenge as a complication of hemophilia A and as acquired autoantibodies in non-hemophiliacs. Porcine FVIII is a potentially useful therapeutic agent because of its low crossreactivity with many inhibitors. Recombinant porcine FVIII (rpFVIII) is undergoing clinical trials in inhibitor patients. OBJECTIVES: The goals of this study were to neutralize human FVIII inhibitors in vitro with rpFVIII and to characterize the relationship between recovery of FVIII activity and the antiporcine FVIII inhibitor titer. METHODS: rpFVIII was added to 28 antihuman FVIII inhibitor plasmas and assayed either immediately or after a 2 h preincubation at 37 degrees C. The concentration of rpFVIII required for recovery of FVIII activity to 50% of normal (EC(50)) was determined by polynomial regression analysis and compared with the antiporcine FVIII inhibitor titer measured by Bethesda assay. RESULTS: Six of nine plasmas classified as non-crossreactive by Bethesda assay had detectable inhibitory activity in the FVIII recovery assay. Recovery was decreased following a 2 h preincubation compared with immediate assay for the 19 plasmas with detectable antiporcine FVIII inhibitors. There was a linear relationship between EC(50) and inhibitor titer for plasmas with antiporcine FVIII titers ranging from 0.6 to 10 BU per milliliter in both the immediate and the 2 h preincubation assays. CONCLUSION: A quantitative method for analysis of inhibitor neutralization in vitro has been developed, which may be useful for comparison with pharmacodynamic studies of rpFVIII in FVIII inhibitor patients and subsequent rational dosing in this patient population.     

Successful preoperative apheresis of factor VIII antibody using factor VIII concentrate as a replacement fluid

Plasma exchange was performed on a patient with hemophilia A and inhibitor to factor VIIIC prior to bilateral cataract removal. Koate was used as replacement fluid with effective reduction of the inhibitor level. From the technical standpoint we found out that factor VIII is best reconstituted in water and directly infused through the venous line and not the centrifuge bowl.     

Immune tolerance therapy for factor VIII inhibitors: moving from empiricism to an evidence-based approach

Summary.  Currently, the only proven strategy for achieving antigen-specific tolerance to factor VIII (FVIII) is immune tolerance induction (ITI) therapy. This paper discusses our current knowledge of the host and treatment factors, as well as supportive care initiatives, known or suspected to influence the outcome of ITI in the treatment of inhibitors arising in patients with severe hemophilia A. Among these, questions surrounding the choice of therapeutic product and/or dosing regimen generate the most controversy, given the lack of a definitive evidence-based approach to either. Furthermore, the potential for central venous access device (CVAD) and intercurrent bleeding complications to impact the ultimate success of ITI remains unclear. The ongoing clinical trials designed to further clarify several of these polarizing issues are reviewed. This paper also explores the current and future role of immune modulation in possible salvage, ancillary or primary alternative tolerance induction strategies. The special cases of low titer/ responding inhibitors and inhibitors developing in mild hemophilia A patients are considered. Finally, this paper summarizes the currently recommended approach to ITI and makes the case for a move from empiric therapeutics to a risk-stratified evidence-based approach to FVIII inhibitor eradication.     

Induction of immune tolerance by neonatal intravenous injection of human factor VIII in murine hemophilia A.

Inhibitory antibody formation is the most serious complication of factor (F)VIII replacement therapy in hemophilia A patients. FVIII-deficient mice were used to study new approaches for induction of immune tolerance. Neither antiFVIII inhibitory antibodies nor antiFVIII IgGs were observed in 13 of 14 adult mice that received 0.05 U g(-1) body weight of human FVIII intravenously within 24 h after birth and repeated injections as adults. In contrast, high FVIII antibody titers (>50 Bethesda Units mL(-1)) developed in seven of 13 mice injected on day 3 postpartum and in all adult mice not treated neonatally. One of nine mice and three of 17 mice developed high-titer antiFVIII inhibitory antibody when they were treated initially with 2-fold (0.1 U g(-1) body weight) and 10-fold higher doses (0.5 U g(-1) body weight) FVIII on day 0, respectively. A human FVIII-specific T-cell proliferative response was absent in splenocytes from neonatally treated mice. The tolerance was FVIII specific because antitoxoid antibodies developed after immunization with tetanus toxoid. Splenocytes failed to proliferate or produce interferon (IFN)-gamma in response to FVIII stimulation, yet still secreted interleukin-2. A proliferative response was restored with exogenous IFN-gamma or interleukin-12, suggesting that lack of inhibitor to FVIII was due to IFN-gamma-dependent anergy. Thus, exposure on day 0 to physiological levels of FVIII antigen might be important for induction of immune tolerance. This immune tolerance model may provide a basis for new approaches to prevention of FVIII inhibitors during replacement therapy.     

Factor VIII (FVIII) gene mutations in 120 patients with hemophilia A: detection of 26 novel mutations and correlation with FVIII inhibitor development

BACKGROUND: As the publication of the sequence of the factor VIII gene (FVIII) in 1984, a large number of mutations that cause hemophilia A (HA) have been identified. Thanks to the advances in the detection of mutations, it is now possible to identify a putative FVIII sequence alteration in the vast majority of patients with HA.OBJECTIVES: Our main objective was to report on the spectrum of FVIII mutations and their distribution throughout the gene in 120 patients with HA. METHODS: Screening of FVIII mutations was performed using direct sequencing. Newly described missense mutations were further studied by molecular modeling. RESULTS: A total of 47 different HA causative FVIII mutations have been identified, 26 of which are described for the first time. These novel mutations include 14 missense and six nonsense mutations, two small deletions, one large deletion and three splice-site mutations. We further investigated the development of FVIII-specific inhibitors in all patients with HA. We found that four novel mutations (Ser882X, Tyr1786Ser, Ala2218Thr and a splice-site defect in intron 22) were associated with inhibitor development. CONCLUSION: These data extend our insight into the mechanisms by which novel amino acid substitutions may lead to HA, and how HA patient genotypes influence the risk of FVIII inhibitor development.     

慢病毒介导的B区缺失的人凝血因子Ⅷ在NOD/SCID小鼠中的持续表达(英文)

近年来,基因治疗作为一种新的治疗方法给血友病A的治疗提供了新的思路。本研究旨在探讨在体外和NOD/SCID小鼠中应用慢病毒载体介导的血友病A基因疗法的可能性。构建含有B区缺失的人凝血因子Ⅷ(BDDhFⅧ)基因和IRES-eGFP编码序列的慢病毒表达载体pXZ9/BDDFⅧ。通过3质粒共转染293FT包装细胞,包装后感染293FT,HLF,Chang-liver和人骨髓间充质干细胞。在感染后分别通过酶联免疫吸附试验(ELISA),一期法,逆转录-聚合酶链反应(RT-PCR)和聚合酶链反应(PCR)法检测凝血因子Ⅷ(FⅧ)活性,FⅧ抗原,FⅧ的mRNA转录和基因整合情况。超速离心收集病毒颗粒,并通过门静脉注射感染NOD/SCID小鼠。ELISA分析小鼠血浆FⅧ抗原,荧光显微镜观察绿色荧光蛋白的表达,转导后1个月RT-PCR分析小鼠肝脏人FⅧ的转录情况。结果表明:成功制备高浓度的重组慢病毒,并能在体外高效转导靶细胞。感染后72 h能检测到高水平的FⅧ活性和FⅧ抗原。RT-PCR和PCR法能敏感检测到人FⅧ基因转录和整合至感染后的细胞中。在所有接受重组慢病毒颗粒注射后的NOD/SCID小鼠肝脏中均能检测到人FⅧ基因的转录,同时重组慢病毒也能在体内高效转导小鼠肝细胞。在感染后72 h小鼠血浆中人FⅧ水平为(49±6)mU,1周后为(54±8)mU,1个月后为(23±4)mU。结论:携带BDDhFⅧ基因的慢病毒颗粒在体内外能高效转导靶细胞,且所有被转导的靶细胞都能有效的分泌人FⅧ。经过门静脉注射慢病毒颗粒的NOD/SCID小鼠可以持续表达人FⅧ。     

Non-classical anti-factor VIII C2 domain antibodies are pathogenic in a murine in vivo bleeding model

Summary.  Objective: The pathogenicity of anti-human factor (F) VIII monoclonal antibodies (MAbs) was tested in a murine bleeding model. Methods: MAbs were injected into the tail veins of hemophilia A mice to a peak plasma concentration of 60 n m , followed by injection of human B domain-deleted FVIII at 180 U kg⁻¹, producing peak plasma concentrations of ∼2 n m . At 2 h, blood loss following a 4-mm tail snip was measured. The following MAbs were tested: (i) 4A4, a type I anti-A2 FVIII inhibitor, (ii) I54 and 1B5, classical type I anti-C2 inhibitors, (iii) 2–77 and B45, non-classical type II anti-C2 inhibitors, and (iv) 2–117, a non-classical anti-C2 MAb with inhibitory activity less than 0.4 Bethesda Units per mg IgG. Results: All MAbs except 2–117 produced similar amounts of blood loss that were significantly greater than control mice injected with FVIII alone. Increasing the dose of FVIII to 360 U kg⁻¹ overcame the bleeding diathesis produced by the type II MAbs 2–77 and B45, but not the type I antibodies, 4A4, I54, and 1B5. These results were consistent with the in vitro Bethesda assay in which 4A4 completely inhibited both 1 U mL⁻¹ and 3 U mL⁻¹ FVIII, while there was 40% residual activity at saturating concentrations of 2–77 at either concentration of FVIII. Conclusions: For patients with an inhibitor response dominated by non-classical anti-C2 antibodies both the in vivo and in vitro results suggest that treatment with high-dose FVIII rather than bypassing agents may be warranted.     

Severe hemophilia A due to a 1.3 kb factor VIII gene deletion including exon 24: homologous recombination between 41 bp within an Alu repeat sequence in introns 23 and 24.

Partial or complete factor (F)VIII gene deletions are found in about 5% of families with severe hemophilia A. Relatively few deletions have been well characterized and, of these, recombination occurred between either common repeat elements or non-homologous sequences. In evaluating a family with severe hemophilia A, an exon 24 deletion was suspected when no fragment was obtained on attempted PCR amplifications. A combination of the 5' primer of exon 23 and the 3' primer of exon 25 fragments was used with prolonged extension times to amplify a normal 2.9 kb fragment that included exons 23 through 25; the patient's amplified product was 1.6 kb indicating a 1.3 kb deletion. A mixture of normal and patient DNA showed both sized fragments as did that from an obligate carrier. Carrier detection was applied to two women at risk; one was and one was not a carrier. Sequencing the proband's 1.6 kb fragment revealed that a 1328 bp deletion occurred between homologous sequences of 287 and 285 bp in introns 23 and 24, respectively; these share 85% identity. Blast nucleotide search revealed that these represent Alu elements. Comparison with an alignment of each of the two homologous sequences further localized recombination to a 41-bp segment. However, a simple recombination event would not account for the proband's sequence. The most likely explanation is that the homologous recombination was accompanied by incomplete mismatch repair.     

A human monoclonal antibody inhibiting partially factor VIII activity reduces thrombus growth in baboons

Summary.  Background: The inhibitory activity of an anti-factor VIII (FVIII) antibody can be modulated through glycosylation of the antigen binding site, as has recently been described. This offers the opportunity to develop an optimized anticoagulant agent targeting partial FVIII inhibition. Objectives: We investigated in non-human primates the antithrombotic activity, pharmacokinetics,and pharmacodynamics of a human monoclonal antibody, Mab-LE2E9Q, inhibiting FVIII activity partially. Methods: The ability of Mab-LE2E9Q to prevent thrombosis was evaluated in baboons after administration of 1.25 and 5 mg kg⁻¹ antibody or saline as a single intravenous (i.v.) bolus. Thrombus development was recorded in expansion ('venous') and in Dacron^® ('arterial') thrombosis chambers incorporated in an extracorporeal arteriovenous shunt implanted between the femoral vessels 1 h, 24 h and 7 days after the administration of Mab-LE2E9Q. Results: Mab-LE2E9Q reduced thrombus growth to a similar extend 1 h, 1 day and 1 week after administration of the antibody. Ex vivo pharmacodynamic analysis indicated that the evaluation of the residual FVIII activity was strongly dependent on the type of FVIII assay and on the phospholipid concentration in the assay. No significant difference in bleedings was observed between animals treated with Mab-LE2E9Q or with saline. Conclusions: Understanding the role of glycosylation in FVIII inhibition by a human monoclonal antibody allowed selection of an antibody inhibiting only moderately FVIII activity while significantly reducing thrombus development in a baboon extracorporeal model. As that antibody did not increase the bleeding tendency, it may represent a novel type of a long-acting antithrombotic agent with an optimal safety/efficacy profile.     

T‐cell responses in two unrelated hemophilia A inhibitor subjects include an epitope at the factor VIII R593C missense site

Summary. Background: Development of neutralizing anti‐factor (F)VIII antibodies (‘inhibitors’) is a serious clinical problem in hemophilia A. Increased inhibitor risk has been associated with certain FVIII missense substitutions, including R593C in the A2 domain. Objectives: The aim of the present study was to identify T‐cell epitopes in FVIII and characterize T‐cell responses in two unrelated hemophilia A subjects sharing F8‐R593C and HLA‐DRB1*1101 genotypes. We hypothesized that the hemophilic substitution site coincides with an important T‐cell epitope. Patients/methods: The binding affinities of peptides for recombinant HLA‐DR proteins were measured and compared with epitope prediction results. CD4+ T cells were stimulated using peptides and stained with fluorescent, peptide‐loaded tetramers. Results: The inhibitor subjects, but not HLA‐matched controls, had high‐avidity HLA‐DRB1*1101‐restricted T‐cell responses against FVIII_589–608, which contains the hemophilic missense site. Antigen‐specific T cells secreted Th1 and Th2 cytokines and proliferated in response to FVIII and FVIII_592–603. FVIII_589–608 bound with physiologically relevant (micromolar) IC₅₀ values to recombinant DR0101, DR1101 and DR1501 proteins. Conclusions: Hemophilia A patients with R593C missense substitutions and these HLA haplotypes had an increased incidence of inhibitors in our cohorts, supporting a paradigm in which presentation of FVIII epitopes containing the wild‐type R593 influences inhibitor risk in this hemophilia A sub‐population.     

A single adeno-associated virus (AAV)-murine factor VIII vector partially corrects the hemophilia A phenotype

Summary.  A major obstacle for delivery of factor (F)VIII using adeno-associated virus (AAV) vectors is the large size of FVIII cDNA, which is well above the 5 kb packaging limit for AAV. Here we construct a < 5 kb FVIII-AAV vector using murine FVIII cDNA and a strong liver-specific albumin promoter. We assessed the efficacy of this vector using three different routes of administration, intraportal, intrasplenic and tail vein injection, in FVIII knockout (FVIII KO) mice. The peak level of FVIII observed was about 8% of normal mouse FVIII activity. Even at 9 months, post vector injection, 14 of 19 mice receiving FVIII-AAV demonstrated phenotypic correction and roughly 2% FVIII activity. The transgene copy number ranged from 0.001 to 0.1 copies per cell, depending upon the somatic tissue. The potential for germline transmission of AAV was assayed in 34 pups obtained from five pairs of treated, phenotypically corrected adult hemophilic mice. Although the parents harbored the transgene in liver, spleen, and gonads, none of the 34 offspring was positive for the transgene, suggesting that the risk of inadvertent germline transmission is low.