Seiki Kamisue

Role of factor VIII C2 domain in factor VIII binding to factor Xa

Factor VIII (FVIII) is activated by proteolytic cleavages with thrombin and factor Xa (FXa) in the intrinsic blood coagulation pathway. The anti-C2 monoclonal antibody ESH8, which recognizes residues 2248–2285 and does not inhibit FVIII binding to von Willebrand factor or phospholipid, inhibited FVIII activation by FXa in a clotting assay. Furthermore, analysis by SDS-polyacrylamide gel electrophoresis showed that ESH8 inhibited FXa cleavage in the presence or absence of phospholipid. The light chain (LCh) fragments (both 80 and 72 kDa) and the recombinant C2 domain dose-dependently bound to immobilized anhydro-FXa, a catalytically inactive derivative of FXa in which dehydroalanine replaces the active-site serine. The affinity (K d ) values for the 80- and 72-kDa LCh fragments and the C2 domain were 55, 51, and 560 nm, respectively. The heavy chain of FVIII did not bind to anhydro-FXa. Similarly, competitive assays using overlapping synthetic peptides corresponding to ESH8 epitopes (residues 2248–2285) demonstrated that a peptide designated EP-2 (residues 2253–2270; TSMYVKEFLISSSQDGHQ) inhibited the binding of the C2 domain or the 72-kDa LCh to anhydro-FXa by more than 95 and 84%, respectively. Our results provide the first evidence for a direct role of the C2 domain in the association between FVIII and FXa.

Common inhibitory effects of human anti‐C2 domain inhibitor alloantibodies on factor VIII binding to von Willebrand factor

Summary. Factor VIII (FVIII) Inhibitor alloantibodies obtained from seven severe haemophilia A patients were examined for their binding regions and their effects on FVHI binding to von Willebrand factor (vWF). Immunoblotting analysis with a panel of recombinant fragments demonstrated that the binding regions of antibodies in cases 1‐5 were contained in the C2 domain of the light chain. Antibodies from cases 1 and 2, which recognized an epitope within residues 2248‐2312, completely inhibited FVIII/ vWF binding in an FXISA (IC50: 5‐0 and 9‐0μg/ml, respectively). Antibodies from case 3 recognizing 2170‐2312 and case 5 recognizing 2170‐2327 also inhibited FVIII/vWF binding (IC50:110 and 400μg/mI, respectively). Case 4 antibodies recognizing 2218‐2307 showed barely detectable inhibition and cases 6 and 7 antibodies recognizing the 44 kD heavy chain, did not inhibit. Our results demonstrate that all anti‐C2 alloantibodies with epitopes that extend to the residue 2312 inhibit vWF binding and that an overlap of the inhibitor epitope with residues 2308‐2312 is critical for maximal inhibition of vWF binding. Prevention of FVIII/vWF binding appears to be a common property of anti‐C2 domain inhibitor alloantibodies.

Mapping of the minimal domain encoding a conformational epitope by λ phage surface display : factor VIII inhibitor antibodies from haemophilia A patients

Haemophilia A patients who receive repeated transfusion of fVIII concentrates often develop inhibitor alloantibodies, resulting in reduced efficacy of the therapy. Determination of fVIII epitopes for the alloantibodies is essential for an understanding of their inhibitory effect on blood coagulation. Random fragments of fVIII displayed on lambda phage particles were selected using two patient plasmas immobilized onto the surface of a microtiter plate. A set of clones defined the minimal domain that consisted of 157 amino acid residues including cysteine at both boundaries. The minimal domain absorbed most of the binding activities of the plasmas to fVIII, suggesting that the domain contains a major determinant for the plasmas. Site-directed mutagenesis and chemical denaturation of the domain confirmed that a tertiary structure formed by the disulfide bridge was recognized by the antibodies. The epitope domain defined overlaps with fVIII binding sites to vWf and phospholipid, and may play an important role in blood coagulation. Thus, the bacteriophage lambda surface display may be useful for mapping the minimal folding domain of various protein antigens that contain a conformational epitope.

MEASUREMENT OF ANTI-FACTOR IX IGG SUBCLASSES IN HAEMOPHILIA B PATIENTS WHO DEVELOPED INHIBITORS WITH EPISODES OF ALLERGIC REACTIONS TO FACTOR IX CONCENTRATES

We have established a simple enzyme-linked immunosorbent assay (ELISA) for the detection of anti-factor IX IgG subclasses in haemophilia B patients with inhibitors. The assay was performed using immobilized purified factor IX. Specific IgG subclasses were detected by peroxidase-conjugated anti-human IgG1,2,3 and 4. Ten plasma samples from 6 haemophilia B patients with inhibitors ranging from 1.0 to 253 Bethesda Units/ml were analyzed. All samples were positive for IgG4. Six out of 10 samples were positive only for IgG4. Three samples were positive for IgG2. Five of the 6 patients had previously had allergic reactions to factor IX concentrates. Three patients had allergic episodes within the past month. Three samples from these latter patients taken on the day when the allergy had occurred showed positive also for IgG1. In later samples, however, taken at 4 days and 4 weeks respectively from two of these same patients. IgG1 was not detected. In two of the five patients in whom allergic reactions had occurred more than one month previously IgG1 was not detected. The results suggested that allergic reactions in patients with haemophilia B treated with factor IX concentrates were associated with the development of the specific IgG1 subclass of antibody to factor IX.

Abnormal factor VIII Hiroshima: defect in crucial proteolytic cleavage by thrombin at Arg1689 detected by a novel ELISA.

We have established an ELISA for detecting thrombin cleavage of the FVIII light chain at Arg1689. The method used a coating alloantibody which recognized amino acid residues 2248–2312 in the C2 domain, together with a second monoclonal antibody, NMC‐VIII/10, which recognized residues 1675–1684 in the amino‐terminal region of the light chain. FVIII antigen (FVIII:Ag) was measured after treatment of plasma with various concentrations of thrombin. The FVIII:Ag of normal plasma was reduced in a dose‐dependent manner by the thrombin, falling to 28% in the presence of 100 U/ml enzyme. The concentration of thrombin that achieved 50% reduction (IC50) was approximately 1·0 U/ml. The plasma of four haemophilia A positive (A+) and two haemophilia A reduced (AR) patients were analysed. The IC50 of all patients was more than 1·0 U/ml, indicating that thrombin cleavage of the FVIII light chain was defective. One haemophilia A+ plasma did not respond to thrombin in this ELISA system. The patient (TI) was a haemophiliac with FVIII coagulant activity of 0·04 U/ml and FVIII:Ag of 1·78 U/ml. In addition, immunoblotting of the purified FVIII from TI showed that thrombin cleavage of the 80 kilodalton (kD) light chain was impaired. The patients DNA was amplified using the polymerase chain reaction with a set of synthetic oligonucleotide primers spanning amino acid residues 1646–1714. Sequence analysis of the amplified DNA fragments revealed a cytosine to thymine transition, converting an arginine 1689 to cysteine. This abnormal FVIII was designated as FVIII Hiroshima. Our ELISA system is a simple and useful method of evaluating the proteolytic cleavage by thrombin at Arg1689.

PRESCHOOL SARCOIDOSIS MIMICKING JUVENILE RHEUMATOID ARTHRITIS : THE SIGNIFICANCE OF GALLIUM SCINTIGRAPHY AND SKIN BIOPSY IN THE DIFFERENTIAL DIAGNOSIS

Preschool sarcoidosis occurring in children less than 6 years old is rare and characterized by the triad of skin, joint and eye manifestations without any pulmonary lesion. Because of similar clinical manifestations, the diagnosis of preschool sarcoidosis and juvenile rheumatoid arthritis (JRA) is confusing. A girl with preschool sarcoidosis, initially diagnosed and treated as having JRA, is reported here. Ophthalmologic examinations revealed posterior involvement of the eye. A gallium scintigram of the head showed panda appearance. Biopsy of the cutaneous lesion demonstrated non‐caseating granuloma. Gallium scanning may be an important clue to correct diagnosis.

Factor VIII gene analysis in Japanese CRM-positive and CRM-reduced haemophilia A patients by single-strand conformation polymorphism

Haemophilia A is the most common X‐linked blood coagulation disorder; it is caused by deficiency of factor VIII activity (FVIII:C). Half of the affected patients do not have detectable levels of FVIII protein in their plasma, whereas about 5% have normal levels of the FVIII antigen (FVIII:Ag) (> 50 u/dl), and are called cross‐reacting material (CRM) positive (CRM+ or A+). About 45% of patients have reduced levels of the FVIII:Ag (1–50 u/dl), classified as CRM reduced (CRMR or AR). We screened the FVIII gene of 13 Japanese patients (five CRM+ and eight CRMR) by single‐strand conformation polymorphism, and identified 11 different mutations in 13 patients by analysing all 26 exons (Trp255Cys, Tyr473Cys, Gly479Arg, Arg531His, Thr667Arg, Arg1689Cys, Arg1941Gln, Arg2150His, Arg2159Cys, Thr2245Ala and Gly2285Val). Seven mutations were identified in the A domains (four in the A2 domain). All the mutations are point mutations resulting in missense codons. Four mutations (Trp255Cys, Thr667Arg, Thr2245Ala and Gly2285Val) have not been described previously.

The Role of Platelet Von Willebrand Factor in the Binding of Factor VIII to Activated Platelets

Factor VIII binds to activated platelets and contributes to the tenase complex assembled on the platelet membrane surface. We have examined the role of platelet von Willebrand factor in the binding of factor VIII to platelets using a platelet captured enzyme-linked immunosorbent assay. Purified factor VIII bound to activated normal platelets in a dose dependent manner. Factor VIII also bound to platelets obtained from a patient with Type 2N von Willebrand disease, although in this case the binding was reduced to approximately 50% of that seen with control platelets. Furthermore, factor VIII bound to Type 3 von Willebrand disease platelets in the absence of detectable von Willebrand factor. In this instance the binding reaction appeared to be approximately 30% of that seen with the same number of normal platelets. An anti-A3 domain monoclonal antibody, NMC-VIII/10, which recognizes the amino-terminal acidic region of the factor VIII light chain, and an anti-C2 domain monoclonal antibody, NMC-VIII/5, which also moderates the binding of factor VIII to phosphatidylserine, inhibited the association between factor VIII and platelets. Inhibition was more remarkable with NMC-VIII/5 than with NMC-VIII/ 10 but not complete. The findings suggest that the binding of factor VIII to activated platelets is not based on a single ligand-receptor relationship, although a predominant role exists for the platelet von Willebrand factor. Furthermore, both the amino-terminal acidic region of the A3 domain and the C2 domain participate in the binding of factor VIII to activated platelets.

Factor IX inhibition and epitope localization of factor IX inhibitor antibodies in haemophilia B patients with anaphylactoid reactions

Summary. We describe the results of immunological studies, reaction kinetics and epitope localization of six inhibitor antibodies to factor IX (FIX) developed in severe haemophilia B patients. Three of the six patients had suffered recent anaphylactoid reactions to FIX concentrates, two others had in the past and one had none. All six inhibitors rapidly inactivated FIX activity in vitro, and the prominent immunoglobulin (IgG) subclass of the antibody was IgG4 when analysed with ELISA. Interestingly, we found an additional IgG1 component in the antibody samples from the patients who had recently experienced anaphylactoid reactions to FIX. When analysed with Western blotting in these three patients, the IgG4 antibody bound with enhanced affinity to the heavy chain or the light chain of FIX, and in two of the three the IgG1 antibody also bound strongly to the FIX heavy chain. The results suggest that the heavy chain of FIX might play a more significant role than the light chain in the pathogenesis of anaphylactoid reactions in haemophilia B patients with FIX inhibitors.