Multicenter Comparison of von Willebrand Factor Multimer Sizing Techniques

A multicenter study of various types of von Willebrand’s disease (vWD) was conducted in order to compare the different electrophoretic techniques used to evaluate von Willebrand factor multimers in plasma. Seven laboratories participated in the blind study of eight plasma samples from two healthy subjects and six vWD types and subtypes (la, lb, IIA, IIB, IIC and IID). From the results of the multimeric analysis of these samples, it appears that the differential diagnosis of vWD types and subtypes should be first approached by using a low-resolution electrophoretic technique, where each vWF multimer appears as a single band. Low-resolution techniques differentiate type I from type II, subtype la from lb and also subtype IIA from other type II subtypes. When type II subtypes other than IIA are identified with these techniques, samples should be rerun using high resolution techniques that resolve each of the fastest migrating multimers in at least three subbands, and permit the differentiation of subtypes IIB, IIC and IID.

In hemophilia A and autoantibody inhibitor patients: the factor VIII A2 domain and light chain are most immunogenic

Factor VIII (fVIII) is a protein cofactor essential for blood coagulation, and it binds in the factor Xase complex to factors IXa, X, and phospholipid. In about 30% of severe hemophilia A patients, treatment with fVIII leads to production of anti-fVIII antibodies. Anti-fVIII autoantibodies also rarely appear in normal individuals. Those antibodies that inactivate fVIII (inhibitors) prevent optimal fVIII therapy. Inhibitor epitopes were previously localized to the fVIII A2, A3, and C2 domains and to an acidic amino acid region between A1 and A2. Such anti-fVIII antibodies interfere with fVIII binding to components of the factor Xase complex and prevent blood coagulation. When total anti-fVIII titers were determined for each fVIII domain in 43 inhibitor plasmas by immunoprecipitation (IP) and inhibitor neutralization assays, the anti-light chain (LCh) antibody titer was highest, anti-A2 was intermediate, and anti-A1 and anti-B were low. The relative immunogenicity of the fVIII domains in hemophilic and autoantibody inhibitor patients was similar.

Mechanism of the immune response to human factor VIII in murine hemophilia A

Mice genetically deficient in factor VII (fVIII) are a model of hemophilia A. As a first step to reproduce in this mouse model what occurs over time in hemophilia A patients treated with human fVIII (hfVIII), we have investigated the time course and the characteristics of their immune response to hfVIII, after multiple intravenous injections. Anti-hfVIII antibodies appeared after four to five injections. They were IgG1 and to a lesser extent IgG2, indicating that they were induced by both Th2 and Th1 cells. Inhibitors appeared after six injections. CD4+ enriched splenocytes from hfVIII-treated mice proliferated in response to fVIII and secreted IL-10: in a few mice they secreted also IFN-gamma and in one mouse IL-4, but never IL-2. A hfVIII-specific T cell line derived from hfVIII-treated mice secreted both IL-4 and IFN-gamma, suggesting that it included both Th1 and Th2 cells. CD4+ enriched splenocytes of hfIII-treated mice recognized all hfVIII domains. Thus, hemophilic mice develop an immune response to hfVIII administered intravenously similar to that of hemophilia A patients. Their anti-hfVIII antibodies can be inhibitors and belong to IgG subclasses homologous to those of inhibitors in hemophilic patients; their anti-hfVIII CD4+ cells recognize a complex repertoire and both Th1 and Th2 cytokines, and especially IL-10, may drive the antibody synthesis.

Characterization of the immune response to factor VIII using hemophilia A* mice

Inhibitor antibody formation is a major complication of factor VIII replacement therapy in patients with hemophilia A. In order to understand the pathogenesis of this immunologic reaction better, we have characterized the immune response to human factor VIII in a murine model of hemophilia A. Mice with severe factor VIII deficiency caused by targeted gene disruptions were injected intravenously with human factor VIII. A human factor VIII-specific T-cell proliferative response was detected with spleen cells obtained three days after a single injection with human factor VIII and anti-factor VIII antibodies were detected after two intravenous injections. Subsequent exposures led to high titer anti-factor VIII antibodies in both ELISA and inhibitor assays. The anti-factor VIII inhibitor antibody response was shown to be T-cell dependent by its absence in hemophilic mice also deficient for the T-cell co-stimulatory ligand B7-2. In separate experiments, injection of murine CTLA4-Ig completely blocked the primary response to factor VIII in hemophilic mice with intact B7 function. This reagent also prevented or diminished further increases in anti-factor VIII when given to hemophilic mice with low anti-factor VIII antibody titers.

Role of CD154 in the secondary immune response: the reduction of pre-existing splenic germinal centers and anti-factor VIII inhibitor titer

Using the murine model of hemophilia A, we have examined the role of CD154 in the secondary immune response to factor VIII (FVIII). We previously reported that repeated i.v. injection of FVIII in hemophilia A mice induces a T cell-dependent anti-FVIII antibody formation. Herein, blocking of CD154 by a monoclonal antibody in FVIII-primed hemophilia A mice resulted in the disappearance of pre-existing spleen germinal centers (GC) in the white pulp within 24 h of treatment. Moreover, further expansion of GC in response to FVIII challenge was completely inhibited. In parallel, anti-FVIII antibody titers were markedly reduced and T cell responses to FVIII were abolished. The rapid disappearance of the GC after anti-CD154 treatment was not accompanied by increased B cell apoptosis; instead B cells accumulated in the peripheral zone of the splenic white pulp. Interestingly, repeated exposure to FVIII with anti-CD154 antibody administration blocked anti-FVIII antibody formation but failed to induce long-lasting unresponsiveness. Our data demonstrate that the CD40-CD154 interaction is critical for B cell homeostasis and the secondary immune response to FVIII. For potential clinical application, the data also suggest that therapies targeting the CD154 molecule may be useful for the treatment of high titer FVIII inhibitors in hemophilia A.

Prevention and treatment of factor VIII inhibitors in murine hemophilia A

Inhibitory antibody formation is a major complication of factor VIII replacement therapy in patients with hemophilia A. To better understand the pathogenesis of this immunologic reaction, we evaluated the role of T-cell costimulatory signals for antifactor VIII antibody formation in a murine model of hemophilia A. Repeated intravenous injections of factor VIII in these factor VIII-deficient mice induced an antifactor VIII inhibitor antibody response. This response was shown to be T-cell dependent by its absence in hemophilic mice also deficient for the T-cell costimulatory ligand B7-2. In separate experiments, injection of murine CTLA4-Ig completely blocked the primary response to factor VIII in hemophilic mice with intact B7 function. This reagent also prevented or diminished further increases in antifactor VIII when given to hemophilic mice with low antifactor VIII antibody titers. These studies suggest that strategies targeting the B7-CD28 pathway are potential therapies to prevent and treat inhibitory antifactor VIII antibodies. Moreover, because the development of antibodies to replaced proteins may limit the success of many human gene therapy approaches, our results may be broadly applicable. (Blood. 2000;95:1324-1329)

Inhibitor antibody development and T cell response to human factor VIII in murine hemophilia A

In order to understand better the mechanism of inhibitor formation in hemophilia A patients, we have characterized the immune response to human factor VIII in a murine model of hemophilia A. Mice with severe factor VIII deficiency caused by targeted gene disruptions in exons 16 and 17 were injected intravenously with human factor VIII. Anti-factor VIII was absent or was detected at only very low levels in hemophilic mice of both strains after a single injection of 0.2 microg factor VIII, but it was present in most mice after a second exposure. Subsequent exposures led to high titer anti-factor VIII antibodies in both ELISA and inhibitor assays. A human factor VIII-specific T cell proliferative response was detected with spleen cells obtained three days after a single injection with human factor VIII, before mice had detectable anti-factor VIII antibodies. Subsequent exposures to factor VIII were followed by an increased T cell proliferative response. These studies indicate that murine hemophilia A is a good model for the study of the immune response to human factor VIII, especially the role of the T cell in the early steps in inhibitor antibody formation.

Transgenic pigs produce functional human factor VIII in milk

Deficiency or abnormality of coagulation factor VIII (FVIII) causes a bleeding disorder called hemophilia A. Treatment involves FVIII concentrates prepared from pooled human plasma or recombinant FVIII (rFVIII) prepared from mammalian cell culture. The cost of highly purified FVIII or rFVIII is a major factor in hemophilia therapy and restricts prophylaxis. We have sought to generate a new source of rFVIII by targeting expression of the human FVIII cDNA to the mammary gland of transgenic pigs using the regulatory sequences of the mouse whey acidic protein gene. The identity of processed heterodimeric rFVIII was confirmed using specific antibodies, by thrombin digestion and activity assays. The secretion of as much as 2.7 micrograms/ml of rFVIII in milk was over tenfold higher than in normal plasma. Up to 0.62 U/ml of rFVIII was detected in an assay in which rFVIII restored normal clotting activity to FVIII-deficient human plasma.

Partial correction of a severe molecular defect in hemophilia A, because of errors during expression of the factor VIII gene

Although the molecular defect in patients in a Japanese family with mild to moderately severe hemophilia A was a deletion of a single nucleotide T within an A8TA2 sequence of exon 14 of the factor VIII gene, the severity of the clinical phenotype did not correspond to that expected of a frameshift mutation. A small amount of functional factor VIII protein was detected in the patient's plasma. Analysis of DNA and RNA molecules from normal and affected individuals and in vitro transcription/translation suggested a partial correction of the molecular defect, because of the following: (i) DNA replication/RNA transcription errors resulting in restoration of the reading frame and/or (ii) "ribosomal frameshifting" resulting in the production of normal factor VIII polypeptide and, thus, in a milder than expected hemophilia A. All of these mechanisms probably were promoted by the longer run of adenines, A10 instead of A8TA2, after the delT. Errors in the complex steps of gene expression therefore may partially correct a severe frameshift defect and ameliorate an expected severe phenotype.

Immunoblot cross-reactivity of factor VIII inhibitors with porcine factor VIII

Porcine factor VIII has been used successfully to treat factor VIII inhibitor patients whose plasmas have minimal cross-reactivity to porcine factor VIII. However, some inhibitor plasmas do inhibit porcine factor VIII, and the extent of procoagulant inhibition often increases after treatment with porcine factor VIII. Because there is no information about the porcine factor VIII epitopes with which these antibodies react, we have compared the immunoblot and enzyme-linked immunosorbent assay (ELISA) reactivities with porcine and human factor VIII for 20 inhibitor plasmas (11 from hemophilia A patients and 9 autoantibodies). Immunoblots identified binding to porcine factor VIII for only 2 of the 12 plasmas from patients who had not received porcine factor VIII, but this reactivity could not be predicted from the inhibitor titer to porcine factor VIII. Immunoblot reactivity with porcine factor VIII was detected for 7 of 8 inhibitor plasmas from patients who had been previously treated with porcine factor VIII, and the strength of this reactivity was generally related to the inhibitor titer. Of the 5 plasmas that were immunoblot positive with the porcine factor VIII A2 domain, 4 had inhibitor titers greater than 45 Bethesda units when tested with porcine factor VIII, whereas only 1 of 15 of the other plasmas had this level of inhibitor activity with porcine factor VIII. In contrast, immunoblot reactivity to the porcine factor VIII A1 domain did not correlate with the antiporcine VIII inhibitor titer. We also determined the effect of preincubation with human or porcine factor VIII on immunoblot reactivity. In one case, immunoblot reactivity with porcine factor VIII was absorbed with porcine, but not human, factor VIII, which is consistent with antibody formation after treatment with porcine factor VIII. In no cases did human factor VIII reduce the reactivity of inhibitor plasmas with the porcine A1 domain, suggesting that these antibodies are directed at unique porcine factor VIII determinants. The reactivity to porcine A2 in 2 plasmas probably represented cross-reactivity of similar A2 determinants, because it was absorbed by both human and porcine factor VIII. Although the ELISA assays with porcine factor VIII detected antibodies in some plasmas that could not be identified by inhibitor assay or immunoblot, the level of ELISA reactivity was generally consistent with the titers of the other assays.

Factor VIII inhibitors

The development of a Factor VIII inhibitor, an antibody that blocks its procoagulant function, is one of the most serious complications of hemophilia A treatment. Similar antibodies are also recognized as a rare cause of bleeding in previously healthy individuals who develop autoimmune anti-Factor VIII antibodies. Recent studies have yielded important information about these antibodies in four different areas: better understanding of the incidence of inhibitors following Factor VIII treatment; identification of patients at highest risk of inhibitor formation; characterization of anti-Factor VIII; and the development of better therapies.

Some factor VIII inhibitor antibodies recognize a common epitope corresponding to C2 domain amino acids 2248 through 2312, which overlap a phospholipid-binding site

The finding that human factor VIII (fVIII) inhibitor antibodies with C2 domain epitopes interfere with the binding of fVIII to phosphatidylserine (PS) suggested that this is the mechanism by which they inactivate fVIII. We constructed a recombinant C2 domain polypeptide and demonstrated that it bound to all six human inhibitors with fVIII light chain specificity. Thus, some antibodies within the polyclonal anti-light chain population require only amino acids within C2 for binding. Recombinant C2 also partially or completely neutralized the inhibitor titer of these plasmas, demonstrating that anti-C2 antibodies inhibit fVIII activity. Immunoblotting of a series of C2 deletion polypeptides, expressed in Escherichia coli, with inhibitor plasmas showed that the epitopes for human inhibitors consist of a common core of amino acid residues 2248 through 2312 with differing extensions for individual inhibitors. The epitope of inhibitory monoclonal antibody (MoAb) ESH8 was localized to residues 2248 through 2285. Three human antibodies and anti-C2 MoAb NMC-VIII/5 bound to a synthetic peptide consisting of amino acids 2303 through 2332, a PS-binding site, but MoAb ESH8 did not. These antibodies also inhibited the binding of fVIII to synthetic phospholipid membranes of PS and phosphatidylcholine, confirming that the blocked epitopes contribute to membrane binding as well as binding to PS. In contrast, MoAb ESH8 did not inhibit binding. As the maximal function of activated fVIII in the intrinsic factor Xase complex requires its binding to a phospholipid membrane, we propose that fVIII inhibition by anti-C2 antibodies is related to the overlap of their epitopes with the PS-binding site. MoAb ESH8 did not inhibit fVIII binding to PS-containing membranes, suggesting the existence of a second mechanism of fVIII inhibition by anti-C2 antibodies.

Residues 484-508 contain a major determinant of the inhibitory epitope in the A2 domain of human factor VIII

The A2 domain (residues 373-740) of human blood coagulation factor VIII (fVIII) contains a major epitope for inhibitory alloantibodies and autoantibodies. We took advantage of the differential reactivity of inhibitory antibodies with human and porcine fVIII and mapped a major determinant of the A2 epitope by using a series of active recombinant hybrid human/porcine fVIII molecules. Hybrids containing a substitution of porcine sequence at segment 410-508, 445-508, or 484-508 of the human A2 domain were not inhibited by a murine monoclonal antibody A2 inhibitory, mAb 413, whereas hybrids containing substitutions at 387-403, 387-444, and 387-468 were inhibited by mAb 413. This indicates that the segment bounded by Arg484 and Ile508 contains a major determinant of the A2 epitope. mAb 413 did not inhibit two more hybrids that contained porcine substitutions at residues 484-488 and 489-508, indicating that amino acid side chains on both sides of the Ser488-Arg489 bond within the Arg484-Ile508 segment contribute to the A2 epitope. The 484-508, 484-488, and 489-508 porcine substitution hybrids displayed decreased inhibition by A2 inhibitors from four patient plasmas, suggesting that there is little variation in the structure of the A2 epitope in the inhibitor population.

The incidence of factor VIII inhibitors in patients with severe hemophilia A

1. Many factors must be considered when retrospective studies are compared, for the intensity of F.VIII treatment and the frequency of inhibitor evaluation have a marked effect on inhibitor incidence. 2. The incidence of F.VIII inhibitors in patients treated with cryoprecipitate and/or intermediate purity concentrates varies greatly in different studies, with cumulative risks of 7-32% after 10-15 years of follow-up. 3. The rate of inhibitor development appears to be higher for patients treated with recombinant F.VIII, but the cumulative incidence is not greater. This may change, however, when there has been an equivalent follow-up period for the patients receiving recombinant F.VIII. 4. Cumulative risk figures may inappropriately overstate the magnitude of the problem since some inhibitors disappear after short periods of time. 5. Only prospective, long-term studies will provide more satisfactory incidence and prevalence data.

Haemophilia A: database of nucleotide substitutions, deletions, insertions and rearrangements of the factor VIII gene, second edition

A large number of different mutations in the factor VIII (F8) gene have been identified as a cause of haemophilia A. This compilation lists known single base-pair substitutions, deletions and insertions in the F8 gene and reviews the status of the inversional events which account for a substantial proportion of mutations causing severe haemophilia A.

Haemophilia A: database of nucleotide substitutions, deletions, insertions and rearrangements of the factor VIII gene, second edition

A large number of different mutations in the factor VIII (F8) gene have been identified as a cause of haemophilia A. This compilation lists known single base-pair substitutions, deletions and insertions in the F8 gene and reviews the status of the inversional events which account for a substantial proportion of mutations causing severe haemophilia A.

Inhibition of human factor VIIIa by anti-A2 subunit antibodies

Human inhibitory alloantibodies and autoantibodies to Factor VIII (FVIII) are usually directed toward the A2 and/or C2 domains of the FVIII molecule. Anti-C2 antibodies block the binding of FVIII to phospholipid, but the mechanism of action of anti-A2 antibodies is not known. We investigated the properties of a patient autoantibody, RC, and a monoclonal antibody, 413, that bind to the region which contains the epitopes of all anti-A2 alloantibodies or autoantibodies studied to date. mAb 413 and RC were noncompetitive inhibitors of a model intrinsic Factor X activation complex (intrinsic FXase) consisting of Factor IXa, activated FVIII (FVIIIa), and synthetic phospholipid vesicles, since they decreased the Vmax of intrinsic FXase by > 95% at saturating concentrations without altering the Km. This indicates that RC and mAb 413 either block the binding of FVIIIa to FIXa or phospholipid or interfere with the catalytic function of fully assembled intrinsic FXase, but they do not inhibit the binding of the substrate Factor X. mAb 413 did not inhibit the increase in fluorescence anisotropy that results from the binding of Factor VIIIa to fluorescein-5-maleimidyl-D-phenylalanyl-prolyl-arginyl-FIXa (Fl-M-FPR-FIXa) on phospholipid vesicles in the absence of Factor X, indicating it does not inhibit assembly of intrinsic FXase. Addition of Factor X to Fl-M-FPR-FIXa, FVIIIa, and phospholipid vesicles produced a further increase in fluorescence anisotropy and a decrease in fluorescence intensity. This effect was blocked completely by mAb 413. We conclude that anti-A2 antibodies inhibit FVIIIa function by blocking the conversion of intrinsic FXase/FX complex to the transition state, rather than by interfering with formation of the ground state Michaelis complex.

Spectrum of mutations in CRM-positive and CRM-reduced hemophilia A

Hemophilia A is due to the functional deficiency of factor VII (FVIII, gene locus F8C). Although half the patients have no detectable FVIII protein in their plasma, the more rare patients (approximately 5%) have normal levels of a dysfunctional FVIII and are termed cross-reacting material (CRM)-positive. More commonly (approximately 45%), patients have plasma FVIII protein reduced to an extent roughly comparable to the level of FVIII activity and are designated CRM-reduced. We used denaturing gradient gel electrophoresis to screen for mutations within the F8C gene of 11 patients (6 CRM-positive, 5 CRM-reduced) and identified 9 different mutations in 9 patients after analyses of all 26 exons, the promoter region, and the polyadenylation site. Six mutations have not been described previously. Five were missense (Ser289Leu, Ser558Phe, Val634Ala, Val634-Met, Asn1441Lys), and the sixth was a 3-bp deletion (delta Phe652). A review of the literature and the assay of FVIII antigen in 5 hemophilia A patients with previously identified missense mutations from this laboratory yielded a total of 20 other unique CRM-reduced and CRM-positive mutations. Almost all CRM-positive/reduced mutations (24/26) were missense, and many (12/26) occurred at CpG dinucleotides. We examined 19 missense mutations for evolutionary conservation using the portions of the porcine and murine F8C sequences that are known, and 18/19 amino acid residues altered by mutation in these patients were conserved. Almost 50% of mutations (11/26) clustered in the A2 domain, suggesting that this region is critical for the function of FVIII.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of dysfunctional factor VIII molecules

Immunopurification and characterization of dysfunctional factor VIII-like molecules in CRM-positive and CRM-reduced hemophilia A permit correlation of structural changes with molecular defects. The technique described here is sufficiently sensitive to characterize the molecular mass and enzymatic fragments of the factor VIII chains in patients with as little VIII: Ag as 0.05 units/ml. Specific abnormalities have been identified in 5 of the first 24 samples tested. In each case, the mutation responsible for factor VIII dysfunction has been determined by sequencing a part of the abnormal gene. Mutations have been identified that abolish critical thrombin cleavage sites or which generate new N-glycosylation sites. The technique provides a useful approach to the study factor VIII structure-function relationships, and it has the potential to clarify further the molecular basis of factor VIII procoagulant activity.

Hemophilia A due to mutations that create new N-glycosylation sites

In studying the molecular defects responsible for cross-reacting material-positive hemophilia A, we have identified two patients in whom the nonfunctional factor VIII-like protein has abnormal, slower-moving heavy or light chains on SDS/PAGE. Both patients have severe hemophilia A (less than 1% of normal factor VIII activity) with a normal plasma level of factor VIII antigen. The molecular defects were identified by denaturing gradient gel electrophoresis screening of PCR-amplified products of the factor VIII-coding DNA sequence followed by nucleotide sequencing of the abnormal PCR products. In patient ARC-21, a methionine-to-threonine substitution at position 1772 in the factor VIII light chain creates a potential new N-glycosylation site at asparagine-1770. In patient ARC-22, an isoleucine-to-threonine substitution at position 566 creates a potential new N-glycosylation site at asparagine-564 in the A2 domain of the factor VIII heavy chain. The mobility of these chains on SDS/PAGE was normal after N-Glycanase digestion and procoagulant activity was generated--to a maximum of 23% and 45% of control normal plasma. Abnormal N-glycosylation, blocking factor VIII procoagulant activity, represents a newly recognized mechanism for the pathogenesis of severe hemophilia A.

A soluble recombinant factor VIII fragment containing the A2 domain binds to some human anti-factor VIII antibodies that are not detected by immunoblotting

Human factor VIII (fVIII) inhibitors are pathologic antibodies that inactivate fVIII. A cDNA clone was modified to encode fVIII amino acid residues 373-740 for expression in a baculovirus vector in insect cells. The encoded protein fragment H2 was produced as a soluble, secreted protein, and it was used to test inhibitor plasmas for the presence of antibodies that were not detected by immunoblotting. Seven of 13 inhibitors that bound only to the fVIII light chain by immunoblotting also bound to fragment H2 in an immunoprecipitation assay. Thus multi-chain inhibitor reactivity of inhibitors is more frequent than previously reported. One of these inhibitors was shown to share the epitope for other inhibitors that bind to H2 within amino acid residues 373-541 in immunoblotting assays. The sensitive immunoprecipitation assay described allows determination of relative H2 binding capacity of the total IgG and epitope localization of inhibitors that cannot be similarly characterized by immunoblotting.

Cysteamine enhances the procoagulant activity of Factor VIII-East Hartford, a dysfunctional protein due to a light chain thrombin cleavage site mutation (arginine-1689 to cysteine)

We have recently identified the molecular defect responsible for cross-reacting material-positive hemophilia A in two unrelated patients in which the substitution of cysteine for arginine-1689 (Factor VIII-East Hartford[FVIII-EH]) abolishes a critical Factor VIII light chain thrombin cleavage site. As other mutant proteins with a cysteine for arginine substitution have been modified in the presence of cysteamine, we have determined the effect of this and other reducing agents on FVIII-EH function. Cysteamine concentrations between 0.1 and 10 mM caused dose- and time-dependent increases in FVIII-EH VIII:C activity, as much as 14-fold (to 35 and 62 U/dl for the two patients tested). Comparable data were obtained in a standard one-stage VIII:C coagulation assay and in a chromogenic substrate assay measuring Factor Xa generation. Thrombin cleavage of the FVIII-EH light chain in the presence of cysteamine was documented by immunoadsorption and analysis. Cystamine and cysteamine-S-phosphate, similar compounds that do not possess a free thiol group, had no effect. Cysteamine augmentation of FVIII-EH VIII:C was abolished by the simultaneous addition of N-ethyl maleimide or iodoacetamide, but these sulfhydryl blocking agents did not prevent the VIII:C increase and light chain cleavage by thrombin if the plasma samples were dialyzed to remove the inhibitors before adding the cysteamine. However, incubation with DTT before iodoacetamide prevented the cysteamine effect after dialysis. These data suggest that when isolated from patient plasma, FVIII-EH cysteine-1689 is present in a disulfide bond. This bond is cleaved by cysteamine to form a new mixed disulfide, a pseudolysine that restores a thrombin cleavage site that is essential for procoagulant function.

Factor VIII-East Hartford (arginine 1689 to cysteine) has procoagulant activity when separated from von Willebrand factor

Factor VIII East Hartford (FVIII-EH) procoagulant activity is reduced because the substitution of cysteine for arginine 1689 abolishes an essential Factor VIII light chain thrombin cleavage site. Incubation of FVIII-EH plasma with penicillamine or DTT causes a five- to sixfold increase in FVIII-EH VIII:C, at 80 and 1 mM, respectively. While there is no FVIII-EH light chain cleavage when thrombin is added in the presence of penicillamine or DTT, these reducing agents disrupt the FVIII-vWf complex. For example, the addition of 5 mM DTT to normal or FVIII-EH plasma causes a 50% reduction in Factor VIII binding to vWf. These observations suggested that DTT increases FVIII-EH VIII:C by partial dissociation of FVIII-EH from vWf. This was verified by showing that vWf-free FVIII-EH had VIII:C activity of 21 U/dl, while the starting plasma level was 2.5 U/dl. Removal of other FVIII-EH plasma proteins by agarose gel filtration had no effect on VIII:C activity. The demonstration that this mutant Factor VIII has cofactor function when separated from vWf indicates that the dissociation of Factor VIII from vWf is an essential effect of Factor VIII light chain cleavage at arginine-1689.

Future approaches to factor VIII inhibitor therapy

Future progress in our ability to treat acquired factor VIII (FVIII) inhibitors must be based on advances in knowledge of both the FVIII molecule and the nature of the human immune response. New therapeutic approaches to patients with acquired FVIII inhibition likely will emphasize modifications of the immune response. This concept holds considerable promise, because studies have characterized the critical steps leading to tolerance of self-antigens. Development of FVIII inhibitors represents a loss of self-tolerance, which any successful therapy must restore. Conceivably, restoration of self-tolerance can be accomplished in many ways: prevention of antigen binding to helper T lymphocytes, deletion of self-antigen-reactive T cells, inhibition of major histocompatibility complex (MHC) recognition, or enhancement of the antigen-specific suppressor T lymphocyte population. Recent data have demonstrated that highly specific methods can suppress ongoing immune responses against defined autoantigens. Antibodies that inhibit T-cell activation, peptides that block self-antigen binding, and antibodies that inhibit MHC recognition all have been successful in modifying experimentally induced autoimmune diseases. Whether any of these immunotherapeutic approaches will be effective in the treatment of acquired FVIII inhibition remains to be determined. Until data from animal model systems establish the feasibility of immune intervention, scrutiny of other new therapeutic approaches to patients with spontaneous inhibitors will continue to be important. Administration of FVIII-bypassing procoagulant proteins shows promise, as does removal of inhibitors by affinity reagents, such as FVIII peptides containing relevant epitopes (antigenic sites). Farther on the horizon is development of recombinant FVIII molecules so modified as to remove antigenic determinants while preserving procoagulant function. Articles in this supplement summarize several avenues for treatment of patients with acquired FVIII inhibitors. Alternatives include treatment with sufficient human or porcine FVIII to offset inhibitors, use of materials that reestablish hemostasis even though FVIII levels are not increased (the so-called FVIII-bypassing agents), manipulation of immune responses through physical depletion of inhibitor by plasmapheresis or affinity chromatography, and administration of intravenous immunoglobulin or immunosuppressive cytotoxic drugs. Thus, the heterogeneous clinical presentation is paralleled by the wide range of available therapeutic approaches.

Haemophilia A: database of nucleotide substitutions, deletions, insertions and rearrangements of the factor VIII gene

Mutations at the factor VIII gene locus causing Haemophilia A have now been identified in many patients from many ethnic groups. Earlier studies used biased methods which detected repetitive mutations at a few CG dinucleotides. More recently rapid gene scanning methods have uncovered an extreme diversity of mutations. Over 80 different point mutations, 6 insertions, 7 small deletions, and 60 large deletions have been characterised. Repetitive mutation has been proved for at least 16 CpG sites. All nonsense mutations cause severe disease. Most missense mutations appear to cause instability of the protein, but some are associated with production of dysfunctional factor VIII molecules, thereby localising functionally critical regions of the cofactor. Variable phenotype has been observed in association with three of the latter class of genotype. This catalogue of gene lesions in Haemophilia A will be updated annually.

Histocompatibility antigen patterns in haemophilic patients with factor VIII antibodies

A number of studies suggest that there is a genetic basis for the formation by some haemophilia A patients of antibodies that inactivate factor VIII. In our study, human leucocyte antigen (HLA)-A, B, C, DR and DQ typing was carried out for 44 haemophilia A patients, including 16 who had developed an antibody to factor VIII. In contrast to previous reports, we found no association between HLA-DR antigens and haemophilia A per se or the formation of a factor VIII inhibitor. However, there was an absence of HLA-Cw5 in the 16 haemophilic patients who had formed an antibody to factor VIII. This finding, consistent with a previous report, identified a statistically significant difference in HLA-Cw5 frequency when the inhibitor patient group was compared to multi-transfused haemophilic patients who had no inhibitor (11/28).

Characterization of a thrombin cleavage site mutation (Arg 1689 to Cys) in the factor VIII gene of two unrelated patients with cross-reacting material-positive hemophilia A

The molecular defect responsible for moderate and severe hemophilia A has been identified for two unrelated patients with the CRM-positive form of this disorder (factor VIII activity of 0.02 and 0.05 U/mL with factor VIII antigen of 0.87 and 2.20 U/mL). In both cases, the immunopurified dysfunctional factor VIII protein is abnormal, in that the 80 Kd light chain is not cleaved by thrombin at arginine-1689. The basis for this failure was identified by polymerase chain reaction amplification of exon 14 of the variant factor VIII genes and direct sequencing of the amplified products. In both cases, a single base substitution (C to T) was identified that produces an arginine to cysteine substitution at amino acid residue 1689. These data identify the molecular defects of the two identical factor VIII variant proteins. The dysfunctional factor VIII has been designated "Factor VIII-East Hartford," the residence of the patient in whom the defect was first identified.

Direct characterization of factor VIII in plasma: detection of a mutation altering a thrombin cleavage site (arginine-372----histidine)

An immunoadsorbent method has been developed for the direct analysis of normal and variant plasma factor VIII. Using this method, the molecular defect responsible for mild hemophilia A has been identified for a patient whose plasma factor VIII activity is 0.05 unit/ml, even though the factor VIII antigen content is 3.25 units/ml. Although the variant factor VIII has an apparently normal molecular mass and chain composition, the 92-kDa heavy chain accumulates when the variant protein is incubated with thrombin and the 44-kDa heavy chain fragment cannot be detected. In contrast, thrombin cleavage of the 80-kDa light chain to the 72-kDa fragment is normal. As these data indicate a loss of factor VIII cleavage by thrombin at arginine-372, the genetic defect was determined by polymerase-chain-reaction amplification of exon 8 of the factor VIII gene and direct sequencing of the amplified product. A single-base substitution (guanine----adenine) was identified that produces an arginine to histidine substitution at amino acid residue 372. These data identify the molecular basis of an abnormal factor VIII, "factor VIII-Kumamoto," that lacks procoagulant function because of impaired thrombin activation.

Molecular basis of factor VIII inhibition by human antibodies. Antibodies that bind to the factor VIII light chain prevent the interaction of factor VIII with phospholipid

Most antibodies to factor VIII have recently been shown to react with discrete regions of the factor VIII light chain (within the C2 domain) and/or the factor VIII heavy chain (within the amino-terminal segment of the A2 domain). The mechanism by which these antibodies, usually designated "factor VIII inhibitors," interfere with factor VIII function has been examined by determining their effect on factor VIII binding to a phospholipid. Factor VIII-phosphatidylserine binding was prevented by all seven factor VIII inhibitors that had strong factor VIII light chain reactivity and reduced by two inhibitors with weak anti-light chain reactivity. None of four inhibitors with heavy chain reactivity prevented factor VIII-phosphatidylserine interaction, though a partial reduction (less than 50%) was noted for the intact IgG preparations. However, when Fab' fragments were substituted, no detectable reduction in factor VIII-phosphatidylserine binding was noted for the anti-heavy chain inhibitors and complete inhibition was retained by the anti-light chain inhibitors. These data suggest that a subset of factor VIII inhibitors, those that bind to light chain determinants, inactivate factor VIII by preventing its effective interaction with phospholipid.

The natural history of factor VIII:C inhibitors in patients with hemophilia A: a national cooperative study. II. Observations on the initial development of factor VIII:C inhibitors

During a 4-year multicenter cooperative study of acquired factor VIII inhibitors in persons with hemophilia A, new inhibitors were detected in 31 of 1,306 patients who entered the study without an inhibitor or the history of an inhibitor. The incidence of new inhibitors was eight per 1,000 patient-years of observation. The factor VIII:C level before inhibitor development was less than or equal to 0.03 U/mL in 29 individuals and 0.06 U/mL and 0.07 U/mL in the remaining two. Factor VIII:Ag levels were measured in 27 individuals and were less than 0.03 U/mL in 23 and 0.05 to 0.11 U/mL in the remaining four. Maximum inhibitor levels ranged from 1.0 to 9,044 Bethesda U/mL. In seven patients under the age of 20, relatively weak inhibitors (none higher than 4.3 Bethesda U/mL) were detected on only a single occasion despite continued factor VIII challenge. In the other 24 patients with inhibitors detected on multiple occasions, 50% had appeared by age 20 and 71% by age 30. Seventeen of the 31 inhibitors, including 12 of 15 with maximum values greater than 10 Bethesda U/mL, developed within 75 exposure days to factor VIII.

Mild hemophilia A associated with a cryptic donor splice site mutation in intron 4 of the factor VIII gene

Hemophilia A, an X-linked disease caused by deficiency of factor VIII, is characterized by variation in clinical severity and coagulation activity. This variation is though to reflect heterogeneity of mutations in the factor VIII gene. Here we describe a CG-to-CA mutation within a potential cryptic donor splice site in intron 4 of the factor VIII gene from a patient with mild disease. This mutation makes the cryptic sequence resemble more closely the consensus sequence for donor splice sites. We infer that the mutation activates the cryptic donor splice site, which in turn causes a defect in RNA processing.

Molecular pathology and immunology of factor VIII (hemophilia A and factor VIII inhibitors)

Factor VIII is a large procoagulant glycoprotein that circulates in plasma in a noncovalent complex with von Willebrand factor. It is essential for the efficient cleavage of coagulation factor X by factor IXa, and its absence causes a severe bleeding disorder. Plasma factor VIII is reduced from the normal range of approximately 100 to 200 ng/ml in patients with the hereditary coagulation defect, hemophilia A, as well as in patients who develop autoantibodies that inactivate factor VIII. The understanding of factor VIII structure has been enhanced by recent studies that have characterized the X chromosome gene responsible for its synthesis, and preliminary information is now available about specific genetic defects. The basis for antibody formation in approximately 15 per cent of repeatedly transfused hemophilic patients is less clear at this time, however, for these individuals appear to have a variety of genetic defects that are not characteristically different from the patients who do not develop inhibitors. Although the antibodies cause a serious problem for affected individuals, they have been very useful in characterizing normal factor VIII and nonfunctional factor VIII-like protein that is found in the plasmas of 10 per cent of patients with mild hemophilia. Moreover, they are very useful reagents that can be used for immunoassay of factor VIII that has been modified in ways that have destroyed its procoagulant function.

von Willebrand factor abnormalities in primary pulmonary hypertension

In primary pulmonary hypertension of recent clinical onset, pulmonary endothelial cells show injury. To characterize this phenomenon, we measured plasma von Willebrand factor (vWF) by immunologic and ristocetin cofactor assays in 6 patients with primary pulmonary hypertension, 17 patients with secondary pulmonary artery hypertension associated with congenital heart disease or cystic fibrosis, and 13 patients with congenital heart disease and normal pulmonary artery pressure. In selected cases, we also determined the vWF multimer pattern. In all 6 cases of primary pulmonary hypertension, the ristocetin cofactor activity was increased relative to the vWF antigen (vWF:Ag) concentration (a ratio of 2.55 +/- 0.36; normal range, 0.8 to 1.4); 4 of the 6 also had a similar and abnormal vWF multimer pattern--an increased proportion of the fastest moving bands. In the other 2, the multimer pattern was normal. Of the other 30 patients, a mild increase in ristocetin cofactor/vWF:Ag was seen in only 2 with secondary pulmonary hypertension and 1 with normal pulmonary artery pressure: these also had an abnormal vWF multimer pattern that was different from that observed in patients with primary pulmonary hypertension. The vWF abnormalities we describe in primary pulmonary hypertension offer a marker of the disease and could be helpful in understanding its pathogenesis.

Localization of binding sites within human von Willebrand factor for monomeric type III collagen

Purified human plasma von Willebrand factor (vWf) binds to pepsin-digested monomeric type III collagen in a saturable (KD = 1 X 10(-8) M), specific, and rapid manner with a stoichiometry of approximately 1:15 [vWf subunit (Mr 270,000):collagen trimer (Mr 300,000)]. Two reduced and alkylated CNBr peptides of vWf, termed M11 residues 542-622 and M20 residues 948-998 [Titani, K., Kumar, S., Takio, K., Ericsson, L. H., Wade, R. D., Ashida, K., Walsh, K. A., Chopek, M. W., Sadler, J. E., & Fujikawa, K. (1986) Biochemistry 25, 3171-3184], inhibited vWf binding to collagen. With 125I-vWf (2 X 10(-9) M) as ligand, M11, M20, fragment III (a dimeric, V8 protease, NH2-terminal fragment, Mr 320,000 referenced above), and unlabeled vWf inhibited binding to collagen with EC50 values of 4.8 X 10(-7), 9.4 X 10(-7), 1.1 X 10(-7), and 0.2 X 10(-7) M, respectively. M11 and M20 bind to collagen directly when 125I-labeled peptides are used as ligands. Other CNBr fragments of vWf were less effective as inhibitors (5-fold or less) and bound less avidly to collagen (5-fold or less) compared to M11 and M20. A murine anti-human vWf monoclonal antibody (MR5), which blocks the binding of vWf to collagen, bound selectively to both M11 and M20 when tested in an enzyme-linked immunoadsorbent assay.(ABSTRACT TRUNCATED AT 250 WORDS)

The dental management of patients with spontaneous acquired factor VIII inhibitors

The histories of two patients without hemophilia but with spontaneous acquired factor VIII inhibitors are reported. A protocol is suggested for the combined hematologic and dental management of such patients.

von Willebrand factor and platelet function

The complex interactions of platelets, von Willebrand factor (VWF), and vascular endothelium have been explored by many investigators during the last decade. With the development of better methods to assess these components has come an increased recognition of diseases associated with defects in the platelet-VWF interaction. Moreover, alternative therapeutic approaches have become available for the clinician who treats patients with disorders of VWF function.

Multicenter comparison of von Willebrand factor multimer sizing techniques. Report of the Factor VIII and von Willebrand Factor Subcommittee

A multicenter study of various types of von Willebrand's disease (vWD) was conducted in order to compare the different electrophoretic techniques used to evaluate von Willebrand factor multimers in plasma. Seven laboratories participated in the blind study of eight plasma samples from two healthy subjects and six vWD types and subtypes (Ia, Ib, IIA, IIB, IIC and IID). From the results of the multimeric analysis of these samples, it appears that the differential diagnosis of vWD types and subtypes should be first approached by using a low-resolution electrophoretic technique, where each vWF multimer appears as a single band. Low-resolution techniques differentiate type I from type II, subtype Ia from Ib and also subtype IIA from other type II subtypes. When type II subtypes other than IIA are identified with these techniques, samples should be rerun using high resolution techniques that resolve each of the fastest migrating multimers in at least three subbands, and permit the differentiation of subtypes IIB, IIC and IID.