Localization of the binding regions of a murine monoclonal anti-factor VIII antibody and a human anti-factor VIII alloantibody, both of which inhibit factor VIII procoagulant activity, to amino acid residues threonine351-serine365 of the factor VIII heavy chain

The role of proteolytic cleavage at Arg336 and Arg372 of the A1 domain in factor VIIa/tissue factor-catalyzed reactions of B domain-deleted factor VIII

BACKGROUND

We previously demonstrated that factor (F)VIII was rapidly activated through proteolytic cleavage at Arg372 and Arg740 by activated FVII (FVIIa)/tissue factor (TF) in very early coagulation phase, followed by inactivation by cleavage at Arg336. The influence of the absence of FVIII B domain in this series of FVIIa/TF-catalyzed reaction remains unclear, however.

AIM

To examine the FVIIa/TF-catalyzed reaction of B domain-deleted (BDD-)FVIII.

METHODS AND RESULTS

The FVIII activity (FVIII:C) of commercial full-length (FL-)FVIII and BDD-FVIII increased by ∼1.7-fold within 0.5 min after addition of FVIIa/TF (1 nM/0.1 nM).

FVIII

C decreased to initial levels with inactivation rate constant (k; ∼0.035) within 15 min of FL-FVIII activation, but decreased gradually to initial levels (k; ∼0.017) within 30 min of BDD-FVIII activation. SDS-PAGE analyses demonstrated that the FVIIa/TF-catalyzed cleavage of BDD-FVIII occurred at Arg336 within 0.5 min in parallel with elevation of FVIII:C, but cleavage at Arg372 was not evident. FVIIa/TF-catalyzed activation of both recombinant BDD-FVIII R336A and R372A mutants that were prepared, were similar to that of wild-type (WT) BDD-FVIII. However, FVIII:C returned to initial levels (k; ∼0.046) within 30 min of R336A mutant activation, but little reduction of FVIII:C was observed with R372A mutant (k; ∼0.0046). SDS-PAGE analysis indicated that FVIIa/TF-catalyzed cleavage of WT and R372A mutant was predominant at Arg336, whereas that of R336A mutant was observed at Arg372.

CONCLUSIONS

FVIIa/TF-catalyzed activation of BDD-FVIII was initiated by cleavage at Arg336, and the FVIII B domain appeared to control FVIIa/TF-catalyzed reactions by altering pattern of cleavage at Arg336 and Arg372.

Impaired factor V-related anticoagulant mechanisms and deep vein thrombosis associated with A2086D and W1920R mutations

Factor V (FV) plays pivotal roles in both procoagulant and anticoagulant mechanisms. Genetic mutations, FV-W1920R (FVNara) and FV-A2086D (FVBesançon), in the C1 and C2 domains of FV light chain, respectively, seem to be associated with deep vein thrombosis. However, the detailed mechanism(s) through which these mutations are linked to thrombophilia remains to be fully explored. The aim of this study was to clarify thrombotic mechanism(s) in the presence of these FV abnormalities. Full-length wild-type (WT) and mutated FV were prepared using stable, human cell lines (HEK293T) and the piggyBac transposon system. Susceptibility of FVa-A2086D to activated protein C (APC) was reduced, resulting in significant inhibition of APC-catalyzed inactivation with limited cleavage at Arg306 and delayed cleavage at Arg506. Furthermore, APC cofactor activity of FV-A2086D in APC-catalyzed inactivation of FVIIIa through cleavage at Arg336 was impaired. Surface plasmon resonance-based assays demonstrated that FV-A2086D bound to Glu-Gly-Arg-chloromethylketone active site-blocked APC and protein S (P) with similar affinities to that of FV-WT. However, weakened interaction between FVa-A2086D and phospholipid membranes was evident through the prothrombinase assay. Moreover, addition of FVa-A2086D to plasma failed to inhibit tissue factor (TF)-induced thrombin generation and reduce prothrombin times. This inhibitory effect was independent of PC, PS, and antithrombin. The coagulant and anticoagulant characteristics of FV(a)-W1920R were similar to those of FV(a)-A2086D. FV-A2086D presented defects in the APC mechanisms associated with FVa inactivation and FV cofactor activity, similar to FV-W1920R. Moreover, both FV proteins that were mutated in the light chain impaired inhibition of TF-induced coagulation reactions. These defects were consistent with congenital thrombophilia.

Acidic Region Residues 1680-1684 in the A3 Domain of Factor VIII Contain a Thrombin-Interactive Site Responsible for Proteolytic Cleavage at Arg1689

Factor VIII (FVIII) is activated by thrombin-catalyzed cleavage at Arg³⁷², Arg⁷⁴⁰, and Arg¹⁶⁸⁹. Our previous studies suggested that thrombin interacted with the FVIII C2 domain specific for cleavage at Arg¹⁶⁸⁹. An alternative report demonstrated, however, that a recombinant (r)FVIII mutant lacking the C2 domain retained >50% cofactor activity, indicating the presence of other thrombin-interactive site(s) associated with cleavage at Arg¹⁶⁸⁹. We have focused, therefore, on the A3 acidic region of FVIII, similar to the hirugen sequence specific for thrombin interaction (54-65 residues). Two synthetic peptides, spanning residues 1659-1669 with sulfated Tyr¹⁶⁶⁴ and residues 1675-1685 with sulfated Try¹⁶⁸⁰, inhibited thrombin-catalyzed FVIII activation and cleavage at Arg¹⁶⁸⁹. Treatment with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide to cross-link thrombin with either peptide showed possible contributions of both 1664-1666 and 1683-1684 residues for thrombin interaction. Thrombin-catalyzed activation and cleavage at Arg¹⁶⁸⁹ in the alanine-substituted rFVIII mutants within 1663-1666 residues were similar to those of wild type (WT). Similar studies of 1680-1684 residues, however, demonstrated that activation and cleavage by thrombin of the FVIII mutant with Y1680A or D1683A/E1684A, in particular, were severely or moderately reduced to 20 to 30% or 60 to 70% of WT, respectively. Surface plasmon resonance-based analysis revealed that thrombin interacted with both Y1680A and D1683A/E1684A mutants with approximately sixfold weaker affinities of WT. Cleavage at Arg¹⁶⁸⁹ in the isolated light-chain fragments from both mutants was similarly depressed, independently of the heavy-chain subunit. In conclusion, the 1680-1684 residues containing sulfated Tyr¹⁶⁸⁰ in the A3 acidic region also contribute to a thrombin-interactive site responsible for FVIII activation through cleavage at Arg¹⁶⁸⁹.

Potential role of activated factor VIII (FVIIIa) in FVIIa/tissue factor-dependent FXa generation in initiation phase of blood coagulation

Factor VIIa/tissue factor (FVIIa/TF) initiates blood coagulation by promoting FXa generation (extrinsic-Xa). Subsequent generation of intrinsic FXa (intrinsic-Xa) amplifies thrombin formation. Previous studies suggested that FVIIa/TF activates FVIII rapidly in immediate coagulation reactions, and FVIIa/TF/FXa activates FVIII prior to thrombin-dependent feedback. We investigated FVIII/FVIIa/TF/FXa relationships in early coagulation mechanisms. Total FXa generated by FVIIa/TF and FVIIa/TF-activated FVIII (FVIIIa_VIIa/TF) was 22.6 ± 1.7 nM (1 min); total FXa with FVIIa-inhibitor was 3.4 ± 0.7 nM, whereas FXa generated by FVIIa/TF or FVIII/TF was 10.4 ± 1.1 or 0.74 ± 0.14 nM, respectively. Little Xa was generated by FVIII alone, suggesting that intrinsic-Xa mechanisms were mediated by FVIIIa_VIIa/TF and FVIII/TF in the initiation phase. Intrinsic-Xa was delayed somewhat by von Willebrand factor (VWF). FVIII activation by FXa with FVIIa/TF was comparable to activation with Glu-Gly-Arg-inactivated-FVIIa/TF. TF counteracted the inhibitory effects of VWF on FXa-induced FVIII activation mediated by Arg³⁷² cleavage. The FVIII-C2 domain bound to cytoplasmic domain-deleted TF (TF^1-243), and VWF blocked this binding by > 80%, indicating an overlap between VWF- and TF^1-243-binding site(s) on C2. Overall, these data suggest that FVIII-associated intrinsic-Xa, governed by both FVIIa/TF-induced and FXa-induced FVIII activation mediated by FVIII-TF interactions, together with FVIIa-dependent extrinsic-Xa mechanisms, may be central to the initiation phase of coagulation.

Factor VIII activation by factor VIIa analog (V158D/E296V/M298Q) in tissue factor-independent mechanisms

Factor (F)VIIa with tissue factor (TF) is a primary trigger of blood coagulation. The recombinant (r)FVIIa analog, NN1731 (V158D/E296V/ M298Q) containing a thrombin/FIXa-mimicking catalytic domain, is ~30-fold more effective on activated platelets without TF, but ~1.2-fold with TF, than rFVIIa for FX activation. We have recently demonstrated the FVIIa/TF-dependent FVIII activation in the early coagulation phase. We assessed the action of NN1731 on FVIII activation. NN1731/TF increased FVIII activity ~2.9-fold within 30 seconds, followed by rapid inactivation, and was slightly more active than rFVIIa/TF. NN1731-catalysed activation, however, was enhanced ~6-fold at 5 minutes (min), and its peak level persisted for ~30 min. NN1731/TF proteolysed FVIII at Arg740, Arg372, and Arg336, similar to rFVIIa/TF, but cleavage by NN1731 alone was much slower at Arg336 than at Arg740 and Arg372. The Km and Vmax for NN1731/TF-catalysed activation were ~1.8-fold lower and ~2.3-fold greater than rFVIIa/TF. The Km for NN1731 alone was ~1.3-fold lower than rFVIIa, whilst the Vmax was ~7.9-fold greater, indicating that the efficiency of FVIII activation by NN1731 and NN1731/TF was ~11- and ~4-fold greater, respectively, than equivalent reactions with rFVIIa. In SPR-based assays, NN1731 bound to FVIII and the heavy chain (Kd; 0.62 and 1.9 nM) with ~1.4- and ~3.1-fold higher affinity than rFVIIa, and the A2 domain contributed to this increase. Von Willebrand factor moderated NN1731-catalysed activation more significantly than NN1731/TF. In conclusion, NN1731 was a greater potential than rFVIIa in up-regulating FVIII activity, and the TF-independent FVIII activation might represent a potential extra mode of its enhanced haemostatic effect.

Mechanisms of human neutrophil elastase-catalysed inactivation of factor VIII(a)

Mechanisms of inflammation and coagulation are linked through various pathways. Human neutrophil elastase (HNE), can bind to activated platelets, might be localised on platelet membranes that provide negatively-charged phospholipid essential for the optimum function of tenase complex. In this study, we examined the effect of HNE on factor (F)VIII. FVIII activity was rapidly diminished in the presence of HNE and was undetectable within 10 minutes. The inactivation rate waŝ8-fold greater than that of activated protein C (APC). This time-dependent inactivation was moderately affected by von Willebrand factor. HNE proteolysed the heavy chain (HCh) of FVIII into two terminal products, A11–358 and A2375–708, by limited proteolysis at Val358, Val374, and Val708. Cleavage at Val708 was much slower than that at Val358 in the >90-kDa A1-A2-B compared to the 90-kDa A1-A2. The 80-kDa light chain (LCh) was proteolysed to 75-kDa product by cleavage at Val1670. HNE-cata- lysed FVIIIa inactivation was markedly slower than that of native FVIII (by ~25-fold), due to delayed cleavage at Val708 in FVIIIa. The inactivation rate mediated by HNE was ~8-fold lower than that by APC. Cleavages at Val358 and Val708 were regulated by the presence of LCh and HCh, respectively. In conclusion, HNE-catalysed FVIII inactivation was associated with the limited-proteolysis that led to A11–358, A2375–708, and A3-C1-C21671–2332, and subsequently to critical cleavage at Val708. HNE-related FVIII(a) reaction might play a role in inactivation of HNE-induced coagulation process, and appeared to depend on the amounts of inactivated FVIII and active FVIIIa which is predominantly resistant to HNE inactivation.

Note: An account of this work was presented at the 51st annual meeting of the American Society of Hematology, December 10, 2009, New Orleans, LA, USA.

Effects of anti-factor VIII inhibitor antibodies on factor VIIa/tissue factor-catalysed activation and inactivation of factor VIII

Factor (F)VIIa/tissue factor (TF) rapidly activates FVIII activity by proteolysis at Arg372 and Arg740, and subsequently inactivates FVIIIa activity by proteolysis at Arg336, although this activation is weaker than that by thrombin. The effects of anti-FVIII inhibitor antibodies on these reactions remain unknown, however. In this study, 13 of anti-FVIII inhibitor antibodies recognising the A2 or C2 domain were prepared. None of them, irrespective of epitope specificity, significantly affected FVIIa/TFcatalysed FVIII activation in one-stage clotting assays. Anti-A2 and anti-C2 type 2 antibodies had little effect on the inactivation phase. Anti-C2 type 1 antibodies, however, modulated inactivation by 40–60% of that seen with control IgG, suggesting that the activity of FVIIIa generated by FVIIa/TF persisted in the presence of this specific type of inhibitor. SDS-PAGE analysis demonstrated that all antibodies had little effect on FVIIa/TF-catalyzed proteolysis at Arg372 and Arg740. Anti-C2 type 1, however, significantly delayed cleavage at Arg336 in dose-dependent manners. Neither anti-A2 nor anti-C2 type 2 affected this reaction, and the findings were consistent with the results of the functional assays. In addition, anti-C2 monoclonal antibodies with type 1 and 2 demonstrated similar patterns of reaction as the anti-C2 polyclonal antibodies in FVIIa/TF-mediated FVIII mechanisms. We demonstrated that FVIIa/TF activated FVIII even in the presence of anti-FVIII antibodies, but inactivation patterns appeared to depend on inhibitor type. It could be important to determine the characteristic of these inhibitor antibodies for prediction of their effects on FVIIa-related FVIII reactions, and the results could have significant therapeutic implications.

Note: An account of this work was presented at the 51st annual meeting of the American Society of Hematology, 2009, New Orleans, LA, USA. This work was supported by grants for MEXT KAKENHI 21591370 in Japan and Bayer Hemophilia Award program.

Novel FV mutation (W1920R, FVNara) associated with serious deep vein thrombosis and more potent APC resistance relative to FVLeiden

FVNara (W1920R), associated with serious deep vein thrombosis, is more resistant to APC relative to FVLeiden (R506Q). This mechanism results from significant decreases in FVa susceptibility to APC and FV cofactor activity for APC.

Contribution of factor VIII light-chain residues 2007-2016 to an activated protein C-interactive site

Although factor (F) VIIIa is inactivated by activated protein C (APC) through cleavages in the FVIII heavy chain-derived A1 (Arg(336)) and A2 subunits (Arg(562), the FVIII light chain (LC) contributes to catalysis by binding the enzyme. ELISA-based binding assays showed that FVIII and FVIII LC bound to immobilised active site-modified APC (DEGR-APC) (apparent K(d) ~270 nM and 1.0 μM, respectively). Fluid-phase binding studies using fluorescence indicated an estimated K(d) of ~590 nM for acrylodan-labelled LC binding to DEGR-APC. Furthermore, FVIII LC effectively competed with FVIIIa in blocking APC-catalysed cleavage at Arg(336) (K(i) = 709 nM). A binding site previously identified near the C-terminal end of the A3 domain (residues 2007-2016) of FVIII LC was subjected to Ala-scanning mutagenesis. FXa generation assays and western and dot blotting were employed to assess the contribution of these residues to FVIIIa interactions with APC. Virtually all variants tested showed reductions in the rates of APC-catalysed inactivation of the cofactor and cleavage at the primary inactivation site (Arg(336)), with maximal reductions in inactivation rates (~3-fold relative to WT) and cleavage rates (~3 to ~9-fold relative to WT) observed for the Met2010Ala, Ser2011Ala, and Leu2013Ala variants. Titration of FVIIIa substrate concentration monitoring cleavage by a dot blot assay indicated that these variants also showed ~3-fold increases relative to WT while a double mutant (Met2010Ala/Ser2011Ala) showed a >4-fold increase in K(m). These results show a contribution of a number of residues within the 2007-2016 sequence, and in particular residues Met2010, Ser2011, and Leu2013 to an APC-interactive site.

Use of affinity-directed liquid chromatography-mass spectrometry to map the epitopes of a factor VIII inhibitor antibody fraction

BACKGROUND

Neutralizing factor (F) VIII antibodies develop in approximately 30% of individuals with hemophilia A and show specificity to multiple sites in the FVIII protein.

METHODS

Reactive epitopes to an immobilized IgG fraction prepared from a high-titer, FVIII inhibitor plasma were determined after immuno-precipitation (IP) of tryptic and chymotryptic peptides derived from digests of the A1 and A2 subunits of FVIIIa and FVIII light chain. Peptides were detected and identified using highly sensitive liquid chromatography-mass spectrometry (LC-MS).

RESULTS

Coverage maps of the A1 subunit, A2 subunit and light chain represented 79%, 69% and 90%, respectively, of the protein sequences. Dot blots indicated that the inhibitor IgG reacted with epitopes contained within each subunit of FVIIIa. IP coupled with LC-MS identified 19 peptides representing epitopes from all FVIII A and C domains. The majority of peptides (10) were derived from the A2 domain. Three peptides mapped to the C2 domain, while two mapped to the A1 and A3 domains, and single peptides mapped to the a1 segment and C1 domain. Epitopes were typically defined by peptide sequences of < 12 residues.

CONCLUSIONS

IP coupled with LC-MS identified extensive antibody reactivity at high resolution over the entire functional FVIII molecule and yielded sequence lengths of < 15 residues. A number of the peptides identified mapped to known sequences involved in functionally important protein-protein and protein-membrane interactions.

Mechanisms of factor VIIa-catalyzed activation of factor VIII

BACKGROUND

Factor (F)VIIa, complexed with tissue factor (TF), is a primary trigger of blood coagulation, and has extremely restricted substrate specificity. The complex catalyzes limited proteolysis of FVIII, but these mechanisms are poorly understood.

OBJECTIVES

In the present study, we investigated the precise mechanisms of FVIIa/TF-catalyzed FVIII activation.

RESULTS

FVIII activity increased ~4-fold within 30 s in the presence of FVIIa/TF, and then decreased to initial levels within 20 min. FVIIa (0.1 nM), at concentrations present physiologically in plasma, activated FVIII in the presence of TF, and this activation was more rapid than that induced by thrombin. The heavy chain (HCh) of FVIII was proteolyzed at Arg(740) and Arg(372) more rapidly by FVIIa/TF than by thrombin, consistent with the enhanced activation of FVIII. Cleavage at Arg(336) was evident at ~1 min, whilst little cleavage of the light chain (LCh) was observed. Cleavage of the HCh by FVIIa/TF was governed by the presence of the LCh. FVIII bound to Glu-Gly-Arg-active-site-modified FVIIa (K(d), ~0.8 nM) with a higher affinity for the HCh than for the LCh (K(d), 5.9 and 18.9 nm). Binding to the A2 domain was particularly evident. Von Willebrand factor (VWF) modestly inhibited FVIIa/TF-catalyzed FVIII activation, in keeping with the concept that VWF could moderate FVIIa/TF-mediated reactions.

CONCLUSIONS

The results demonstrated that this activation mechanism was distinct from those mediated by thrombin, and indicated that FVIIa/TF functions through a 'priming' mechanism for the activation of FVIII in the initiation phase of coagulation.

Varied immune response to FVIII: presence of proteolytic antibodies directed to factor VIII in different human pathologies

The versatility of antibodies is demonstrated by the various functions that they mediate such as neutralization, agglutination, fixation of the complement and its activation, and activation of effector cells. In addition to this plethora of functions, antibodies are capable of expressing enzymatic activity. Antibodies with catalytic function are a result of the productive interplay between the highly evolved machinery of the immune system and the chemical framework used to induce them (antigens). Catalytic antibodies are immunoglobulins with an ability to catalyze the reactions involving the antigen for which they are specific. Catalytic immunoglobulins of the IgM and IgG isotypes have been detected in the serum of healthy donors. In addition, catalytic immunoglobulins of the IgA isotype have been detected in the milk of healthy mothers. Conversely, antigen-specific hydrolytic antibodies have been reported in a number of inflammatory, autoimmune, and neoplastic disorders. The pathophysiological occurrence and relevance of catalytic antibodies remains a debated issue. Through the description of the hydrolysis of coagulation factor VIII as model target antigen, we propose that catalytic antibodies directed to the coagulation factor VIII may play a beneficial or a deleterious role depending on the immuno-inflammatory condition under which they occur.

Factor VIII inhibitors: role of von Willebrand factor on the uptake of factor VIII by dendritic cells

In patients with haemophilia A, factor VIII (FVIII) therapy leads to the development of anti-FVIII alloantibodies that inhibit FVIII pro-coagulant activity, in up to 25% of the cases. At a time when efficient viral screening procedures are at place, development of inhibitors poses the greatest threat to haemophilia A patients. Various risk factors, both patient and product-related, are responsible for the development of inhibitory antibodies. The role of FVIII-specific CD4+ T lymphocytes in the initiation of the humoral immune response to exogenous FVIII has been well. In view of their capacity to stimulate naïve T cells, dendritic cells (DCs) play a central role in the initiation of the primary immune response. Thus, in the context of a primary alloimmunization against FVIII, i.e. when FVIII-specific B lymphocytes are not there to take up FVIII from the circulation and to serve as antigen presenting cells (APCs), DCs are the only cell type that internalize FVIII, leading to activation of FVIII-specific CD4+ T lymphocytes. von Willebrand factor (VWF) present in plasma-derived FVIII therapeutic concentrates, is known to act as a chaperone molecule for procoagulant FVIII. In addition to its role in reducing the 'antigenicity' of FVIII, the role of VWF in the reduction of the 'immunogenicity' of therapeutic FVIII in patients with haemophilia A has also been suggested. We have recently demonstrated that VWF protects FVIII from being endocytosed by human DCs and subsequently being presented to FVIII-specific T cells. We propose that VWF may reduce the immunogenicity of FVIII by preventing, upstream from the activation of immune effectors, the entry of FVIII in professional antigen presenting cells.

Efficient factor VIII affinity purification using a small synthetic ligand

BACKGROUND

Hemophilia A is currently treated by infusions of the coagulation factor (F) VIII, of which production and purification remain a challenging task. Current purification procedures using immunoaffinity chromatography are cumbersome, expensive, and suffer from the instability of the applied antibody ligands, which elute along with the product and contaminate it. Recently, FVIII was purified using octapeptide ligands, but their use is limited due to the low resistance to proteases.

OBJECTIVE

Our goal was to develop and evaluate a novel ligand for FVIII purification, overcoming the drawbacks of current procedures.

METHODS

Peptide ligands were screened for binding of (125)I-plasma-derived-FVIII (pdFVIII) in a microbead assay. A selected ligand-coated Toyopearl resin was then used for pdFVIII purification from cell-conditioned Delbucco's modified Eagle's medium (DMEM) containing fetal bovine serum. The proteolytic stability of ligand was measured by incubating with human serum and proteinase K, and its cytotoxicity towards human OV-MZ-6 cells was assayed.

RESULTS

A high-affinity octapeptidic FVIII ligand was modified into the small, highly stable and non-toxic peptidomimetic ligand L4 by rational and combinatorial design without affecting its affinity for FVIII. Using ligand L4-coated Toyopearl resin, pdFVIII was isolated from cell-conditioned medium with high purity and 89% column retention after elution with a mild buffer containing 0.6 m NaCl at pH 6.8.

CONCLUSIONS

Ligand L4 offers a valuable alternative to antibody-based procedures for laboratory and industrial production. Its synthesis by established solid-phase procedures is straightforward and considerably cheaper than the biotechnological production of antibodies, and safety concerns associated with the use of biological material are overcome.

Basic aspects of inhibitors to factors VIII and IX and the influence of non-genetic risk factors

The appearance of polyclonal antibodies inhibiting the function of exogenous factors VIII (FVIII) and IX (FIX) continues to be a major challenge in the treatment of patients with congenital haemophilia. Why these inhibitors develop in 10-20% of patients with haemophilia A, and in 1-5% of patients with haemophilia B, remains largely unexplained. The antibodies, however, are characterized by several features that may have implications for the immune process by which they occur. The FVIII antibodies are mainly directed towards the A2, A3 and C2 domains, thereby interfering with the function of the factor Xase complex, the binding of FVIII to von Willebrand factor, and the binding of FVIII to phospholipid membranes. The FIX epitopes are localized to the NH(2)-terminal gamma-carboxyglutamic acid region and the serine protease domain. Genetic risk factors are known to be of importance in the development of inhibitors, whereas the impact of non-genetic factors is less clear. However, based on studies of related subjects, it is obvious that non-genetic factors are of importance as well. Putative factors currently debated include age at the start of treatment, treatment in association with immune challenges, the type of product, and the mode of administration. Most of the findings reported to date, however, derive from small cohorts that have not been sufficiently well characterized with respect to genetic risk profile. Therefore, additional studies are required to quantify the impact of non-genetic factors on the pathophysiologic process of inhibitor development.

Residues Surrounding Arg336 and Arg562 Contribute to the Disparate Rates of Proteolysis of Factor VIIIa Catalyzed by Activated Protein C

Activated Protein C (APC) inactivates factor VIIIa by cleavage at Arg(336) and Arg(562) within the A1 and A2 subunits, respectively, with reaction at the former site occurring at a rate approximately 25-fold faster than the latter. Recombinant factor VIII variants possessing mutations within the P4-P3' sequences were used to determine the contributions of these residues to the disparate cleavage rates at the two P1 sites. Specific activity values for 336(P4-P3')562, 336(P4-P2)562, and 336(P1'-P3')562 mutants, where indicated residues surrounding the Arg(336) site were replaced with those surrounding Arg(562), were similar to wild type (WT) factor VIII; whereas 562(P4-P3')336 and 562(P4-P2)336 mutants showed specific activity values <1% the WT value. Inactivation rates for the 336 site mutants were reduced approximately 6-11-fold compared with WT factor VIIIa, and approached values attributed to cleavage at Arg(562). Cleavage rates at Arg(336) were reduced approximately 100-fold for 336(P4-P3')562, and approximately 9-16-fold for 336(P4-P2)562 and 336(P1'-P3')562 mutants. Inhibition kinetics revealed similar affinities of APC for WT factor VIIIa and 336(P4-P3')562 variant. Alternatively, the 562(P4-P3')336 variant showed a modest increase in cleavage rate ( approximately 4-fold) at Arg(562) compared with WT, whereas these rates were increased by approximately 27- and 6-fold for 562(P4-P3')336 and 562(P4-P2)336, respectively, using the factor VIII procofactor form as substrate. Thus the P4-P3' residues surrounding Arg(336) and Arg(562) make significant contributions to proteolysis rates at each site, apparently independent of binding affinity. Efficient cleavage at Arg(336) by APC is attributed to favorable P4-P3' residues at this site, whereas cleavage at Arg(562) can be accelerated following replacement with more optimal P4-P3' residues.

Mechanisms of Plasmin-catalyzed Inactivation of Factor VIII

Plasmin not only functions as a key enzyme in the fibrinolytic system but also directly inactivates factor VIII and other clotting factors such as factor V. However, the mechanisms of plasmin-catalyzed factor VIII inactivation are poorly understood. In this study, levels of factor VIII activity increased approximately 2-fold within 3 min in the presence of plasmin, and subsequently decreased to undetectable levels within 45 min. This time-dependent reaction was not affected by von Willebrand factor and phospholipid. The rate constant of plasmin-catalyzed factor VIIIa inactivation was approximately 12- and approximately 3.7-fold greater than those mediated by factor Xa and activated protein C, respectively. SDS-PAGE analysis showed that plasmin cleaved the heavy chain of factor VIII into two terminal products, A1(37-336) and A2 subunits, by limited proteolysis at Lys(36), Arg(336), Arg(372), and Arg(740). The 80-kDa light chain was converted into a 67-kDa subunit by cleavage at Arg(1689) and Arg(1721), identical to the pattern induced by factor Xa. Plasmin-catalyzed cleavage at Arg(336) proceeded faster than that at Arg(372), in contrast to proteolysis by factor Xa. Furthermore, breakdown was faster than that in the presence of activated protein C, consistent with rapid inactivation of factor VIII. The cleavages at Arg(336) and Lys(36) occurred rapidly in the presence of A2 and A3-C1-C2 subunits, respectively. These results strongly indicated that cleavage at Arg(336) was a central mechanism of plasmin-catalyzed factor VIII inactivation. Furthermore, the cleavages at Arg(336) and Lys(36) appeared to be selectively regulated by the A2 and A3-C1-C2 domains, respectively, interacting with plasmin.

Peptides derived from a secretory yeast library restore factor VIII activity in the presence of an inhibitory antibody

The development of autoantibodies against factor VIII represents one of the major complications in the treatment of hemophilia A patients. We have employed a novel library system to obtain peptides that specifically neutralize the interaction between factor VIII and these inhibitors. The random peptides are presented as carboxy-terminal extensions of the eukaryotic initiation factor 5a, an intracellular protein with a molecular mass of 18 kDa. These random peptides formed an unique binding site, as demonstrated by molecular simulations using the computer programs InsightII and GROMACS. The library was screened to identify peptides binding to the murine monoclonal anti-factor VIII antibody ESH8 and to inhibitors derived from patients with factor VIII antibodies. Ten peptides binding to ESH8 were identified. Their specificity was confirmed by displacement assays. Two peptides with the sequences STKTLGRPLHGPAGPVEGGALAGVAEDADLVTAVSGR and YHCKREDLTDRDATCALRQPPQAVRGLGPRVTAVSGR showed the ability to restore the factor VIII activity from 33% up to approximately 90% in functional tests performed in vitro. Three candidates for binding to factor VIII antibodies derived from four different patient's sera were achieved. Three fusion proteins with the peptide sequences PQLGSRRSTTPSLTFQNASWFPAGGPCARSNRG, SGSRQVCKLARSLQPF and WERGRRVGAQVRHARHLVARVLDGAGHQARLTAVNGP bound to inhibitors derived from different patients. Furthermore, two of the obtained fusion proteins with the peptide sequences RHWTALGPAPTHTCADLNYPLLS and WERGRRVGAQVRHARHLVARVLDGAGHQARLTAVNGP did also bind to the monoclonal antibody ESH8. This study demonstrates the potential of this system to identify peptides that inhibit the activity of potent inhibitory antibodies and also shows potential as a method for screening of bioactive peptides.

Role of the B domain in proteolytic inactivation of activated coagulation factor VIII by activated protein C and activated factor X

Hereditary deficiency of factor VIII (FVIII), haemophilia A, is treated by plasma-derived FVIII (pd-FVIII) or recombinant FVIII (rFVIII) infusions. B-domain-deleted FVIII (BDD-rFVIII), although generally safe and effective, was less effective than pd-FVIII in prophylaxis -- evidenced by a 2.5-fold higher bleeding incidence. Assessment of BDD-rFVIII activity in chromogenic and one-stage clotting assays gives up to 50% difference in activity values. As earlier studies demonstrated identical activation and cofactor activity of BDD-rFVIII and pd-FVIII, we decided to study susceptibility of thrombin-activated pd-FVIII, full-length rFVIII and BDD-rFVIII to proteolytic inactivation by activated protein C (APC) and activated factor X (FXa) in a purified system. Proteolysis was monitored by Western blot using monoclonal antibodies C5 and R8B12 specific for the A1 and A2 domains, respectively. Inactivation was monitored by measuring the residual cofactor activity of FVIII forms in a one-stage clotting assay. Proteolysis of A1 and A2 domains of activated BDD-rFVIII proceeded 11 or 13 times faster than that of pd-FVIII or full-length rFVIII. Inactivation of activated BDD-rFVIII was two to three times faster by APC and five to six times faster by FXa. We suggest that differences in proteolytic inactivation may contribute to differences between BDD-rFVIII and pd-FVIII in assaying and in clinical use.

Catalytic IgG from Patients with Hemophilia A Inactivate Therapeutic Factor VIII

Factor VIII (FVIII) inhibitors are anti-FVIII IgG that arise in up to 50% of the patients with hemophilia A, upon therapeutic administration of exogenous FVIII. Factor VIII inhibitors neutralize the activity of the administered FVIII by sterically hindering its interaction with molecules of the coagulation cascade, or by forming immune complexes with FVIII and accelerating its clearance from the circulation. We have shown previously that a subset of anti-factor VIII IgG hydrolyzes FVIII. FVIII-hydrolyzing IgG are detected in over 50% of inhibitor-positive patients with severe hemophilia A, and are not found in inhibitor-negative patients. Although human proficient catalytic Abs have been described in a number of inflammatory and autoimmune disorders, their pathological relevance remains elusive. We demonstrate here that the kinetics of FVIII degradation by FVIII-hydrolyzing IgG are compatible with a pathogenic role for IgG catalysts. We also report that FVIII-hydrolyzing IgG from each patient exhibit multiple cleavage sites on FVIII and that, while the specificity of cleavage varies from one patient to another, catalytic IgG preferentially hydrolyze peptide bonds containing basic amino acids.

Mapping of FVIII inhibitor epitopes using cellulose-bound synthetic peptide arrays

Epitope mapping using antibodies against factor VIII (FVIII) has been performed using blotting techniques with truncated and/or digested FVIII molecules. Here, we focused on the precise mapping of affinity purified IgG from patients with an immune response against blood clotting FVIII using synthetic peptide arrays on cellulose membranes comprising the entire sequence of FVIII. The aim was to elucidate the epitope profile from different inhibitors and possibly detect new epitopes, which have not been described before. The epitope patterns from five patients showed reactivity with all domains in the FVIII molecule, but were different between various patients. These results included epitopes usually buried within the folded protein. However, in competition assays using FVIII as competitive agent in a mixture with inhibitor IgG, the most immunogenic regions were located in the FVIII light chain. Our results show that the C1 domain was the region with highest immunogenicity in all patients. Here, we demonstrate that the SPOT method is very well suited for the precise location of epitopes in the core of the protein, which usually cannot be detected by other methods.

Exosite-interactive regions in the A1 and A2 domains of factor VIII facilitate thrombin-catalyzed cleavage of heavy chain

Thrombin catalyzes the proteolytic activation of factor VIII, cleaving two sites in the heavy chain and one site in the light chain of the procofactor. Evaluation of thrombin binding the reaction products from heavy chain cleavage by steady state fluorescence energy transfer using a fluorophore-labeled, active site-modified thrombin as well as by solid phase binding assays using a thrombin Ser(205) --> Ala mutant indicated a high affinity site in the A1 subunit (K(d) approximately 5 nm) that was dependent upon the Na(+)-bound form of thrombin, whereas a moderate affinity site in the A2 subunit (K(d) approximately 100 nm) was observed for both Na(+)-bound and -free forms. The solid phase assay also indicated that hirudin blocked thrombin interaction with the A1 subunit and had little, if any, effect on its interaction with the A2 subunit. Conversely, heparin blocked thrombin interaction with the A2 subunit and showed a marginal effect on A1 binding. Evaluation of the A2 sequence revealed two regions rich in acidic residues that are localized close to the N and C termini of this domain. Peptides encompassing these clustered acidic regions, residues 373-395 and 719-740, blocked thrombin cleavage of the isolated heavy chain at Arg(372) and Arg(740) and inhibited A2 binding to thrombin Ser(205) --> Ala, suggesting that both A2 domain regions potentially support interaction with thrombin. A B-domainless, factor VIII double mutant Asp(392) --> Ala/Asp(394) --> Ala was constructed, expressed, and purified and possessed specific activity equivalent to a severe hemophilia phenotype. This mutant was resistant to cleavage at Arg(740), whereas cleavage at Arg(372) was not affected. These data suggest the acidic region comprising residues 389-394 in factor VIII A2 domain interacts with thrombin via its heparin-binding exosite and facilitates cleavage at Arg(740) during procofactor activation.

Thrombin-catalyzed activation of factor VIII with His substituted for Arg372 at the P1 site

Thrombin-catalyzed proteolysis at Arg372 of factor VIII is essential for procofactor activation. However, hemophilia A patients with the missense mutation Arg372 to His possess a mild to moderate phenotype yet show no detectable cleavage at this bond. To evaluate this discrepancy, we prepared and stably expressed a recombinant, B-domainless factor VIII mutant (R372H) that possessed approximately 1% the specific activity of wild type. Cleavage at R372H by thrombin occurred with an approximately 80-fold decreased rate compared with wild type. N-terminal sequence analysis of the derived A2 subunit confirmed that cleavage occurred at the His372-Ser373 bond. Factor VIII R372H was activated slowly, attained lower activity levels, and exhibited an apparent reduced inactivation rate compared with factor VIII wild type. These observations were attributed to a reduced cleavage rate at His372. Factor Xa generation assays showed similar Michaelis-Menten constant (K(m), apparent) values for thrombin-catalyzed activation for either factor VIII form, but suggested an approximately 70-fold reduced maximum velocity (V(max)) for factor VIII R372H. However, prolonged reaction with thrombin yielded similar activity and stability values for the mutant and wild-type factor VIIIa forms. These results indicate a markedly reduced rate of cleavage following substitution at the P(1)Arg, and this property likely reflects the severity of the hemophilia A phenotype.

Mechanisms of Interactions of Factor X and Factor Xa with the Acidic Region in the Factor VIII A1 Domain

The 337-372 sequence of the factor VIIIa A1 subunit contains interactive sites for both zymogen factor X and the active enzyme, factor Xa. Solid phase binding studies indicated that factor Xa possessed a >20-fold higher affinity for the isolated A1 subunit of factor VIIIa compared with factor X. Heparin completely inhibited zero-length cross-linking of the 337-372 peptide to factor Xa but not to factor X. In the presence of calcium, factor Xa showed greater affinity for heparin than factor X. Studies using factor Xa mutants in which heparin-binding exosite residues were individually replaced by Ala showed that the R240A mutant was defective in recognition of the Lys36 cleavage site, generating the A137-372 intermediate with approximately 20% the catalytic efficiency of wild type. This defect likely resulted from an approximately 4-fold increase in Km for the A1 substrate because kcat values for the wild type and mutant were equivalent. Cleavage of the A1-A2 domain junction by factor Xa R240A was not blocked by the 337-372 peptide. Studies using mutant factor VIII where clustered acidic residues in the 337-372 segment were replaced by Ala showed that a factor VIIIa D361A/D362A/D363A mutant possessed a approximately 1.6-fold increase in Km for factor X compared with wild type. However, similar Km values were observed for recombinant factor X and R240A substrates. These results indicate that the binding regions of factor X and factor Xa for A1 domain overlap and that both utilize acidic residues 361-363. Furthermore, factor Xa but not factor X interacts with high affinity at this site via residues contained within the heparin-binding exosite of the proteinase.

Inhibitors in hemophilia A

Factor VIII (FVIII) replacement therapy remains the mainstay in hemophilia A care. The major complication of replacement therapy is formation of antibodies, which inhibit FVIII activity, thus dramatically reducing treatment efficiency. The present review summarizes the accumulated knowledge on epitopes of FVIII inhibitors and mechanisms of their inhibitory effects. FVIII inhibitors most frequently target the A2, C2 and A3 domains of FVIII and interfere with important interactions of FVIII at various stages of its functional pathway; a class of FVIII inhibitors inactivates FVIII by proteolysis. We discuss therapeutic approaches currently used for treatment of hemophilia A patients with inhibitors and analyze the factors that influence the outcome. The choice between options should depend on the level of inhibitors and consideration of efficacy, safety, and availability of particular regimens. Advances of basic science open avenues for alternative targeted, specific and long-lasting treatments, such as the use of peptide decoys for blocking FVIII inhibitors, bypassing them with human/porcine FVIII hybrids, neutralizing FVIII-reactive CD4 T cells with anti-clonotypic antibodies, or inducing immune tolerance to FVIII with the use of universal CD4 epitopes or by genetic approaches.

Identification of a factor Xa-interactive site within residues 337-372 of the factor VIII heavy chain

We recently demonstrated that the residues 337-372, comprising the acidic C-terminal region in A1 subunit, interact with factor Xa during the proteolytic inactivation of factor VIIIa (Nogami, K., Wakabayashi, H., and Fay, P. J. (2003) J. Biol. Chem. 278, 16502-16509). We now show this sequence is important for factor Xa-catalyzed activation of factor VIII. Peptide 337-372 markedly inhibited cofactor activation, consistent with a delay in the rate of cleavage at the A1-A2 junction. Studies using the isolated factor VIII heavy chain indicated that the peptide completely blocked cleavage at the A1-A2 junction (IC50 = 11 microm) and partially blocked cleavage at the A2-B junction (IC50 = 100 microm). Covalent cross-linking was observed between the 337-372 peptide and factor Xa following reaction with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide, and the peptide quenched the fluorescence of dansyl-Glu-Gly-Arg active site-modified factor Xa, suggesting that residues 337-372 directly interact with factor Xa. Studies using a monoclonal antibody recognizing residues 351-365 as well as the peptide to this sequence further restricted the interactive region. Mutant factor VIII molecules in which clustered acidic residues in the 337-372 segment were converted to alanine were evaluated for activation by factor Xa. Of the mutants tested, only factor Xa-catalyzed activation of the D361A/D362A/D363A mutant was inhibited with peak activity of approximately 50% and an activation rate constant of approximately 30% of the wild type values. These results indicate that the 337-372 acidic region separating A1 and A2 domains and, in particular, a cluster of acidic residues at position 361-363 contribute to a unique factor Xa-interactive site within the factor VIII heavy chain that promotes factor Xa docking during cofactor activation.

Mechanisms of factor Xa-catalyzed cleavage of the factor VIIIa A1 subunit resulting in cofactor inactivation

Activation of factor VIII by factor Xa is followed by proteolytic inactivation resulting from cleavage within the A1 subunit (residues 1-372) of factor VIIIa. Factor Xa attacks two sites in A1, Arg(336), which precedes the highly acidic C-terminal region, and a recently identified site at Lys(36). By using isolated A1 subunit as substrate for proteolysis, production of the terminal fragment, A1(37-336), was shown to proceed via two pathways identified by the intermediates A1(1-336) and A1(37-372) and generated by initial cleavage at Arg(336) and Lys(36), respectively. Appearance of the terminal product by the former pathway was 7-8-fold slower than the product obtained by the latter pathway. The isolated A1 subunit was cleaved slowly, independent of the presence of phospholipid. The A1/A3-C1-C2 dimer demonstrated an approximately 3-fold increased cleavage rate constant, and inclusion of phospholipid further enhanced this value by approximately 2-fold. Although association of A1 or A1(37-372) with A3-C1-C2 enhanced the rate of cleavage at Arg(336), inclusion of A3-C1-C2 did not affect the cleavage at Lys(36) in A1(1-336). A synthetic peptide 337-372 blocked the cleavage at Lys(36) (IC(50) = 230 microm) while showing little if any effect on cleavage at Arg(336). Proteolysis at Lys(36), and to a lesser extent Arg(336), was inhibited in a dose-dependent manner by heparin. These results suggest that inactivating cleavages catalyzed by factor Xa at Lys(36) and Arg(336) are regulated in part by the A3-C1-C2 subunit. Furthermore, cleavage at Lys(36) appears to be selectively modulated by the C-terminal acidic region of A1, a region that may interact with factor Xa via its heparin-binding exosite.

Altered interactions between the A1 and A2 subunits of factor VIIIa following cleavage of A1 subunit by factor Xa

Factor VIIIa consists of subunits designated A1, A2, and A3-C1-C2. The limited cofactor activity observed with the isolated A2 subunit is markedly enhanced by the A1 subunit. A truncated A1 (A1(336)) was previously shown to possess similar affinity for A2 and retain approximately 60% of its A2 stimulatory activity. We now identify a second site in A1 at Lys(36) that is cleaved by factor Xa. A1 truncated at both cleavage sites (A1(37-336)) showed little if any affinity for A2 (K(d)>2 microm), whereas factor VIIIa reconstituted with A2 plus A1(37-336)/A3-C1-C2 dimer demonstrated significant cofactor activity ( approximately 30% that of factor VIIIa reconstituted with native A1) in a factor Xa generation assay. These affinity values were consistent with values obtained by fluorescence energy transfer using acrylodan-labeled A2 and fluorescein-labeled A1. In contrast, factor VIIIa reconstituted with A1(37-336) showed little activity in a one-stage clotting assay. This resulted in part from a 5-fold increase in K(m) for factor X when A1 was cleaved at Arg(336). These findings suggest that both A1 termini are necessary for functional interaction of A1 with A2. Furthermore, the C terminus of A1 contributes to the K(m) for factor X binding to factor Xase, and this parameter is critical for activity assessed in plasma-based assays.

Autoantibodies to factor VIII with catalytic activity

Hemophilia A is an X-linked, recessive, bleeding disorder caused by defective or deficient factor VIII (FVIII) molecules. Infusion of purified FVIII to patients with severe hemophilia A results in approximately 25% of the cases, in the emergence of anti-FVIII antibodies (inhibitors) that are known to neutralize the pro-coagulant activity of FVIII by steric hindrance. We recently reported on the proteolysis of FVIII by allo-antibodies in the plasma of high responder patients with severe hemophilia A, demonstrating a new mechanism by which FVIII inhibitors may prevent the pro-coagulant function of FVIII. Hemophilia is the first model where a direct link between the hydrolysis of the target molecule and the occurrence of the clinical manifestations may be established. It also represents the first example in humans, of the induction of catalytic antibodies following the exogenous administration of an antigen. The characterization of FVIII inhibitors as site-specific proteases may provide new approaches to the treatment of inhibitors.

Antibodies with hydrolytic activity towards factor VIII in patients with hemophilia A

Antibodies endowed with hydrolytic properties have been described in humans for over a decade in a variety of pathological conditions such as systemic lupus erythematosus (SLE), autoimmune thyroiditis, asthma, and Bence Jones disease. Although the identified target substrate molecules have always been autoantigens (i.e., DNA, thyroglobulin, vasoactive intestinal peptide), a direct role of hydrolysis of the autoantigen in pathology of the disease has not been clearly documented. We have described in multitransfused patients with hemophilia A the presence of anti-factor VIII (FVIII) IgG antibodies that hydrolyze FVIII. The estimated kinetic parameters derived for FVIII cleavage by anti-FVIII antibodies are in line with the previously described catalytic antibodies. The identified cleavage sites are evenly spread throughout the FVIII molecule and are located after an arginine or a lysine in most cases. We have recently shown that the catalytic antibodies are highly prevalent among hemophilia A patients with FVIII inhibitors. Catalytic antibodies to FVIII are the first example where the hydrolysis of the target molecule by hydrolytic antibodies may be directly relevant to the etiology of the disease. The characterization of FVIII inhibitors as site-specific proteases may provide novel strategies in the design of therapy against FVIII inhibitors in patients with hemophilia A.

A novel mechanism of factor VIII protection by von Willebrand factor from activated protein C-catalyzed inactivation

The protective effect of von Willebrand factor (VWF) toward activated protein C (APC)-catalyzed inactivation of factor VIII (FVIII) has been attributed mainly to inhibition of FVIII binding to phospholipid. In the present study, we demonstrated that VWF-mediated FVIII protection from APC also results from direct inhibition of FVIII binding to APC. Inhibition of FVIII binding to anhydro-APC by VWF would be consistent with partial or complete overlap of the FVIII binding sites for APC and VWF. We examined, therefore, the inhibitory effects of 6 synthetic peptides spanning residues 1996 to 2028 around the previously localized APC binding region (FVIII residues 2009-2018). Peptide 2009 to 2018 inhibited FVIII binding to anhydro-APC by 83% (50% inhibition, 55 microM). Similarly, peptide 2013 to 2022 inhibited FVIII binding to VWF by 84% (50% inhibition, 25 microM). It was also found that peptides 2009 to 2018 and 2013 to 2022 optimally bound to anhydro-APC and VWF, respectively. A rabbit antipeptide IgG, raised against peptide 2009 to 2022, blocked the binding of both anhydro-APC and VWF to FVIII. This immunoglobulin G inhibited proteolytic cleavage of FVIII by APC. Our results indicate that the essential regions for the binding of APC and VWF to FVIII overlap and that the protective effect of VWF on APC-catalyzed FVIII inactivation includes competitive inhibition of APC binding to FVIII by VWF.

Pathophysiology of inhibitors to factor VIII in patients with haemophilia A

The occurrence of factor VIII (FVIII) inhibitors is one of the major complications of the treatment of haemophilia A. We present this review as a description of the major players of the antiFVIII immune response, with particular emphasis on the nature and properties of the different antiFVIII antibodies, their mechanisms of action in inhibiting FVIII activity, their potential neutralization by anti-idiotypic antibodies, and the importance of the T cell in participating in the induction of FVIII inhibitors. We briefly conclude on the avenues that remain to be explored in order to establish efficient therapeutic approaches aimed at eliminating FVIII inhibitors in patients with haemophilia A.

The concept of idiotypic vaccination against factor VIII inhibitors in haemophilia A

Idiotypic vaccination has proven successful in several animal models and human trials. Here we suggest that the expression of cross-reactive idiotypes on factor VIII (FVIII) inhibitors of patients with haemophilia A, patients with anti-FVIII autoimmune disease and natural anti-FVIII antibodies of healthy individuals, together with the ability of anti-idiotypic reagents to neutralize anti-FVIII antibodies, provides a rationale for designing a vaccine strategy aimed at preventing the occurrence of or suppressing inhibitors, based on the induction of protective anti-idiotypes. Here we discuss the rationale supporting the concept of using idiotypic vaccination to prevent the occurrence of FVIII inhibitors in patients with haemophilia A.

The prevalence of proteolytic antibodies against factor VIII in hemophilia A

BACKGROUND

Factor VIII inhibitors are IgG alloantibodies that arise during replacement therapy in 25 to 50 percent of patients with severe hemophilia A. The hydrolysis of factor VIII by anti--factor VIII antibodies has been proposed as a mechanism of inactivation of factor VIII.

METHODS

We purified IgG from patients with severe hemophilia A. The proteolytic activity of the antibodies was assessed by incubating the IgG with biotinylated human factor VIII and analyzing patterns of factor VIII cleavage by sodium dodecyl sulfate--polyacrylamide-gel electrophoresis and immunoblotting. The controls were normal human IgG and IgG purified from plasma of patients with hemophilia who did not have inhibitory antibodies.

RESULTS

Significant proteolytic activity was detected in IgG from 13 of 24 inhibitor-positive patients. No hydrolytic activity was detected in control antibodies of IgG from patients without inhibitors. The rate of hydrolysis of factor VIII by purified IgG correlated positively with the factor VIII--neutralizing activity of IgG in plasma (r2=0.67, P=0.029). Principal-component analysis of migration profiles of digestion fragments demonstrated the heterogeneity of the catalytic potential of factor VIII inhibitors among patients.

CONCLUSIONS

Proteolysis is a mechanism by which IgG antibodies against factor VIII can inactivate factor VIII.

Haemophilia A: effects of inhibitory antibodies on factor VIII functional interactions and approaches to prevent their action

Factor VIII (FVIII) is an essential component of the intrinsic pathway of blood coagulation. Normal functioning of FVIII requires its interactions with other components of the coagulation cascade. In the circulation, it exists as a complex with von Willebrand factor (vWF). Upon activation by thrombin or activated factor X (FXa), activated FVIII (FVIIIa) functions as a cofactor for the serine protease factor IXa. Their complex assembled on the phospholipid surface activates FX to FXa, which consequently participates in formation of thrombin, the key protease of the coagulation cascade. Genetic deficiency in FVIII results in a coagulation disorder haemophilia A, which is treated by infusions of FVIII products. Approximately 25-30% of patients develop antibodies inhibiting FVIII activity (FVIII inhibitors). The major epitopes of inhibitors are located within the A2, C2 and A3 domains of the FVIII molecule. The inhibitory effects of antibodies are manifested at various stages of the FVIII functional pathway, including FVIII binding to vWF, activation of FVIII by thrombin, and FVIIIa incorporation into the Xase complex. We summarize the current knowledge of the FVIII sites involved in interaction with its physiological ligands and different classes of inhibitory antibodies and describe their inhibitory mechanisms. We outline the strategies aimed to overcome the effects of inhibitory antibodies such as development of human/porcine FVIII molecules, resistant to inhibitors. We also discuss approaches to modulate the antibody response, as well as efforts to develop a long-term immunotolerance to FVIII protein.

Long-term induction of immune tolerance after blockade of CD40-CD40L interaction in a mouse model of hemophilia A

A factor VIII-deficient knockout mouse was used as a model for severe hemophilia A to characterize the immune response to recombinant human factor VIII (fVIII) and to study new approaches for induction of immune tolerance to fVIII. Mice initially received periodic injections of fVIII in doses similar to those used for the treatment of human hemophilia A. To induce immune tolerance, a hamster monoclonal antibody specific for murine CD40 ligand (CD40L or CD154) was injected with fVIII. Control mice received fVIII alone or fVIII and hamster immunoglobulin G. After treatment, humoral and cellular immune responses were evaluated. Ninety-five percent of anti-CD40L-treated mice had lower titers of anti-fVIII antibody (less than 1 microg/mL) compared with fVIII-injected control mice (mean, 18 microg/mL). To determine whether anti-CD40L treatment induces long-term immune tolerance, mice were rechallenged 3 times with fVIII alone. At 150 days after treatment, 12 of 22 anti-CD40L-treated mice remained tolerant to fVIII (anti-fVIII antibody titers less than 1 microg/mL). However, tolerant mice immunized with tetanus toxoid (TT) developed high anti-TT antibody, demonstrating that tolerance is fVIII specific. T cells from tolerant mice showed impaired proliferative responses after stimulation with fVIII in vitro and lack of production of the cytokines interleukin-2 (IL-2), IL-4, interferon gamma, and IL-10. These results demonstrate that long-term immune tolerance to fVIII was effectively induced after early blockade of CD40-CD40L interaction. In addition, the lack of tolerance in this model was associated with the expression of a Th2 phenotype.

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.

Circulating factor VIII immune complexes in patients with type 2 acquired hemophilia A and protection from activated protein C-mediated proteolysis

Factor VIII (FVIII) inhibitor antibodies are classified into 2 groups according to the kinetic pattern of FVIII inactivation. Type 2 antibodies are more commonly observed in patients with acquired hemophilia A and do not completely inhibit FVIII activity; in most cases, substantial levels of circulating FVIII are detected. Three type 2 autoantibodies from patients who had normal levels of FVIII antigen despite having low levels of FVIII activity were studied. The antibodies reacted exclusively with the light chain of FVIII but not with the C2 domain, and their epitopes were therefore ascribed to the regions in the A3-C1 domains. Heavy and light chains of FVIII were detected in plasma-derived immune complexes extracted by using protein G Sepharose. Direct binding assays using anhydro-activated protein C (anhydro-APC), a catalytically inactive derivative of activated protein C (APC) in which the active-site serine is converted to dehydroalanine, were used to examine the relation between immune complexes and APC. The intact FVIII, 80-kd light chain, and 72-kd light chain bound in a dose-dependent manner to anhydro-APC, with K(d) values of 580, 540, and 310 nM, respectively, whereas no appreciable binding was detected for the heavy chain. The 3 autoantibodies blocked FVIII binding to anhydro-APC by approximately 80% and consequently inhibited APC-induced FVIII proteolytic inactivation. These antibodies also bound to a synthetic peptide, His2009-Val2018, which contains the APC binding site. The findings suggest that binding of type 2 autoantibodies, recognizing residues His2009 to Val2018, protects FVIII from APC-mediated proteolysis and might contribute to the presence of FVIII immune complexes in the circulation.

Factor VIII C2 domain contains the thrombin-binding site responsible for thrombin-catalyzed cleavage at Arg1689

Thrombin-catalyzed factor VIII activation is an essential positive feedback mechanism regulating intrinsic blood coagulation. A factor VIII human antibody, A-FF, with C2 epitope, exclusively inhibited factor VIII activation and cleavage at Arg(1689) by thrombin. The results suggested that A-FF prevented the interaction of thrombin with factor VIII and that the C2 domain was involved in the interaction with thrombin. We performed direct binding assays using anhydro-thrombin, a catalytically inactive derivative of thrombin in which the active-site serine is converted to dehydroalanine. Intact factor VIII, 80-kDa light chain, 72-kDa light chain, and heavy chain fragments bound dose-dependently to anhydro-thrombin, and the K(d) values were 48, 150, 106, and 180 nm, respectively. The C2 and A2 domains also dose-dependently bound to anhydro-thrombin, and the K(d) values were 440 and 488 nm, respectively. The A1 domain did not bind to anhydro-thrombin. A-FF completely inhibited C2 domain binding to anhydro-thrombin (IC(50), 18 nm), whereas it did not inhibit A2 domain binding. Furthermore, C2-specific affinity purified F(ab)'(2) of A-FF, and the recombinant C2 domain inhibited thrombin cleavage at Arg(1689). Our results indicate that the C2 domain contains the thrombin-binding site responsible for the cleavage at Arg(1689).

Molecular analysis of human anti-factor VIII antibodies by V gene phage display identifies a new epitope in the acidic region following the A2 domain

One of the major binding sites for factor VIII inhibitors is located within the A2 domain. In this study, phage display technology was used to isolate 2 human monoclonal antibodies, termed VK34 and VK41, directed toward the heavy chain of factor VIII. The VHdomain of a single-chain variable domain antibody fragment (scFv) VK34 is encoded by germline gene segment DP-10. Epitope-mapping studies revealed that scFv VK34 is directed against amino acid residues Arg484–Ile508 , a previously identified binding site for factor VIII inhibitors in the A2 domain. ScFv VK34 inhibited factor VIII activity with a titer of 280 BU/mg. The VH domain of VK41 was encoded by germline gene segment DP-47. A phage corresponding to VK41 competed with a monoclonal antibody for binding to amino acid residues Asp712–Ala736 in the acidic region adjacent to the A2 domain. Reactivity of VK41 with a factor VIII variant in which we replaced amino acid residues Asp712–Ala736for the corresponding region of heparin cofactor II was strongly reduced. In addition, substitution of Tyr718719723 for Phe abrogated binding of VK41 to factor VIII. ScFv VK41 did not inhibit factor VIII activity. This study not only defines the primary structure of human anti-factor VIII antibodies reactive with the A2 domain, it also describes an antibody with an epitope not previously identified in the antibody repertoire of hemophilia patients with an inhibitor.

Molecular analysis of human anti-factor VIII antibodies by V gene phage display identifies a new epitope in the acidic region following the A2 domain

One of the major binding sites for factor VIII inhibitors is located within the A2 domain. In this study, phage display technology was used to isolate 2 human monoclonal antibodies, termed VK34 and VK41, directed toward the heavy chain of factor VIII. The VHdomain of a single-chain variable domain antibody fragment (scFv) VK34 is encoded by germline gene segment DP-10. Epitope-mapping studies revealed that scFv VK34 is directed against amino acid residues Arg484–Ile508 , a previously identified binding site for factor VIII inhibitors in the A2 domain. ScFv VK34 inhibited factor VIII activity with a titer of 280 BU/mg. The VH domain of VK41 was encoded by germline gene segment DP-47. A phage corresponding to VK41 competed with a monoclonal antibody for binding to amino acid residues Asp712–Ala736 in the acidic region adjacent to the A2 domain. Reactivity of VK41 with a factor VIII variant in which we replaced amino acid residues Asp712–Ala736for the corresponding region of heparin cofactor II was strongly reduced. In addition, substitution of Tyr718719723 for Phe abrogated binding of VK41 to factor VIII. ScFv VK41 did not inhibit factor VIII activity. This study not only defines the primary structure of human anti-factor VIII antibodies reactive with the A2 domain, it also describes an antibody with an epitope not previously identified in the antibody repertoire of hemophilia patients with an inhibitor.

Partial correction of murine hemophilia A with neo-antigenic murine factor VIII

We have previously reported a factor VIII knockout (FVIII KO) mouse model for hemophilia A. Here we demonstrate the presence of nonfunctional heavy chain factor VIII protein in the mouse, making it an excellent model for cross-reacting material (CRM)-positive hemophilia A patients, who express normal levels of a dysfunctional FVIII protein. We attempted to correct these mice phenotypically by transduction of wild-type mouse factor VIII cDNA delivered in an E1/E3-deleted adenoviral vector by tail vein injection. All treated mice displayed initial high-level FVIII expression that diminished after 1 month. Ten of 12 mice administered between 6 x 10(9) and 1 x 10(11) particles/mouse along with anti-CD4 antibody showed long-term FVIII activity (0.03-0.05 IU/ml, equivalent to 3-5% of normal FVIII) that corrected the phenotype. Wild-type murine FVIII was a neo-antigen to the KO mice, generating both cytotoxic and humoral immune responses. Immune suppression with anti-CD4 antibody abrogated these immune responses. These data demonstrate that despite the presence of endogenous FVIII protein the immune system still recognizes a species-specific transgene protein as a neo-antigen, eliciting a cytotoxic T cell response. This phenomenon may exist in the treatment of other genetic disorders by gene therapy.

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.

Interacting Regions in the A1 and A2 Subunits of Factor VIIIa Identified by Zero-Length Cross-Linking

Factor VIIIa is a heterotrimer of A1, A2, and A3-C1-C2 subunits, the activity of which is labile due to a weak affinity interaction of the A2 subunit with the A1/A3-C1-C2 dimer. We have used the zero-length cross-linking reagent, 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC), to localize regions of interaction within the A1 and A2 subunits. Reaction of factor VIIIa with EDC resulted in the formation of a cross-linked product of approximately 90 kD consisting of the A1 and A2 subunits as judged by Western blotting. Alkaline resistance of this product indicated an amide rather than ester linkage. Factor VIIIa activity decreased as the concentration of cross-linked product increased, suggesting that flexibility in the inter-subunit interaction may be required for proper cofactor function. This product was not formed in the contiguous A1-A2 domains of factor VIII, suggesting that, upon cofactor activation, a conformational change occurs that leads to the formation of a new interdomainal salt bridge(s). Reaction of the EDC-treated factor VIIIa with activated protein C (APC), which cleaves the A1 subunit at Arg336 and bisects the A2 subunit at Arg562, resulted in the formation of an approximately 30 kD product that contains the C-terminus region of A1 covalently linked to the N-terminal half of the A2. The approximately 90 kD cross-linked product was generated after reaction of A2 subunit with A1/A3-C1-C2 dimer but not with A1336/A3-C1-C2, a form of the dimer produced by APC cleavage and lacking the C-terminal acidic region of A1. A synthetic peptide corresponding to this acidic region (Met337-Arg372) was found to covalently cross-link to the isolated A2 subunit in 1:1 stoichiometry, suggesting that this region is both necessary and sufficient for the interaction of the A1 and A2 subunits. Sequence analysis of this product suggested that Glu344 in the A1 peptide may contribute to the cross-linkage. These results indicate that activation of factor VIII results in formation of a new ionic linkage(s) localized to the acidic C-terminal region of A1 and the N-terminal half of A2.

The Inhibitor Antibody Response Is More Complex in Hemophilia A Patients Than in Most Nonhemophiliacs With Factor VIII Autoantibodies

Approximately 25% of hemophilia A patients infused with factor VIII (fVIII) mount an immune response, which leads to its inactivation. Anti-fVIII autoantibodies are also seen rarely in individuals with normal fVIII. We have previously demonstrated that some anti-A2 and anti-C2 domain antibodies are fVIII inhibitors and that many patients have additional inhibitors with a fVIII light chain (LCh) epitope outside C2. Because the contribution of the different antibodies to the plasma inhibitor titer had been examined in a limited number of patients (14), we report in this study a more extensive analysis of 55 plasmas. The dominant inhibitors in 62% (13 of 21) of autoantibody plasmas were directed only against C2 or A2, but not both, whereas this pattern was found in only 15% (5 of 34) of hemophilic plasmas. In addition, anti-A2 inhibitors were present in 71% (24 of 34) of hemophilic plasmas, but only 33% (7 of 21) of autoantibody plasmas. These results demonstrated that the inhibitor response in hemophiliacs was more complex and the epitope specificity was somewhat different. A comparison of hemophiliacs treated only with plasma fVIII or recombinant fVIII showed no significant differences in the complexity of the inhibitor response, as ≥ 2 different inhibitor antibodies were present in 78% (18 of 23) of the former and 82% (9 of 11) of the latter. In contrast, the major inhibitors in 35% (8 of 23) of hemophiliacs treated with plasma fVIII were directed against C2 and another LCh epitope within residues 1649-2137, but not A2, while none (0 of 11) treated with recombinant fVIII had this pattern.