A subset of high-titer anti-factor VIII A2 domain antibodies is responsive to treatment with factor VIII

Factor VIII Antibodies Demonstrate Type I or Type II Kinetics in Acquired Haemophilia A

BACKGROUND

Acquired haemophilia A (AHA) is an acquired bleeding disorder resulting from autoantibodies against Factor VIII (FVIII). Previous studies have reported differences in FVIII inhibitor kinetics (type I or type II) in AHA compared to severe haemophilia A.

AIM

To characterise inhibitor kinetics in AHA and evaluate the proportions displaying type I, II or indeterminate kinetics.

METHODS

Single-centre retrospective study of inhibitor kinetics in adults with AHA. Type I kinetics were defined as linear FVIII inhibition with ≥ 97% FVIII inactivation. Type II kinetics were defined as non-linear kinetics and inability to completely neutralise FVIII. Inhibitor titres were calculated using two methods outlined by the International Council for Standardisation in Haematology.

RESULTS

Baseline samples from 34 patients were included. Fifteen samples (44.1%) exhibited type I kinetics, 16 samples (47.1%) exhibited type II kinetics and 3 (8.8%) were indeterminate. Plateau mean residual FVIII:C was higher for inhibitors displaying type II compared to type I kinetics (18.6 vs. 2.9 IU/dL, p < 0.0001). Non-linear regression using a dose-response curve without categorisation for kinetics type yielded a poor fit (R2 = 38%), which improved with refitting using categories of type I or II kinetics that explained 87% and 85% of the variability. The median difference in inhibitor titre between the two reporting methods was 5% and 15% in the type I and II kinetics groups, respectively.

CONCLUSION

FVIII autoantibodies demonstrate either type I or type II kinetics. Greater discrepancy in reported inhibitor titres depending on the method used is seen for inhibitors with type II kinetics.

Factor VIII antibody immune complexes modulate the humoral response to factor VIII in an epitope-dependent manner

Introduction

Soluble antigens complexed with immunoglobulin G (IgG) antibodies can induce robust adaptive immune responses in vitro and in animal models of disease. Factor VIII immune complexes (FVIII-ICs) have been detected in individuals with hemophilia A and severe von Willebrand disease following FVIII infusions. Yet, it is unclear if and how FVIII-ICs affect antibody development over time.

Methods

In this study, we analyzed internalization of FVIII complexed with epitope-mapped FVIII-specific IgG monoclonal antibodies (MAbs) by murine bone marrow-derived dendritic cells (BMDCs) in vitro and antibody development in hemophilia A (FVIII-/-) mice injected with FVIII-IC over time.

Results

FVIII complexed with 2-116 (A1 domain MAb), 2-113 (A3 domain MAb), and I55 (C2 domain MAb) significantly increased FVIII uptake by BMDC but only FVIII/2-116 enhanced antibody titers in FVIII-/- mice compared to FVIII alone. FVIII/4A4 (A2 domain MAb) showed similar FVIII uptake by BMDC to that of isolated FVIII yet significantly increased antibody titers when injected in FVIII-/- mice. Enhanced antibody responses observed with FVIII/2-116 and FVIII/4A4 complexes in vivo were abrogated in the absence of the FVIII carrier protein von Willebrand factor.

Conclusion

These findings suggest that a subset of FVIII-IC modulates the humoral response to FVIII in an epitope-dependent manner, which may provide insight into the antibody response observed in some patients with hemophilia A.

Impact of different factor VIII inhibitor kinetic profiles on the inhibitor titer quantification using the modified Nijmegen-Bethesda assay

Background

Coagulation factor VIII (FVIII) inhibitor titer quantification is vital for optimizing care in people with hemophilia A.

Objectives

This study analyzed the impact of the different kinetic profiles of four FVIII monoclonal antibodies on inhibitor titer quantification using the modified Nijmegen-Bethesda assay.

Methods

Concentration-related and time-related profiles of FVIII antibodies (4A4, BO2C11, 2-54, ESH-8) were evaluated in vitro. FVIII residual activity was measured using a one-stage clotting assay and chromogenic substrate assay. Profiles of the FVIII antibodies were compared with the theoretical kinetic model: the ideal log (residual activity)-linear (inhibitor concentration) relationship. Different theoretical kinetic model-dependent and -independent criteria to calculate FVIII inhibitor titer were compared.

Results

Factor VIII monoclonal antibodies had different concentration-related and time-related profiles, ideal for comparative analysis using the modified Nijmegen-Bethesda assay. The kinetic profile of 4A4 was similar to the theoretical kinetic model, while BO2C11 showed a steeper curve, and 2-54 and ESH-8 a flatter curve, than the model. In the modified Nijmegen-Bethesda assay, conversion of measured FVIII residual activities for different inhibitor dilutions into FVIII inhibitor titer is based on the theoretical kinetic model. Therefore, titer calculations for FVIII inhibitors that deviate from the model are prone to underestimation or overestimation. Calculating a theoretical dilution at 50% FVIII residual activity by sigmoidal regression reflecting different kinetic inhibition profiles can provide a more accurate titer result.

Conclusion

Kinetic profiles of FVIII antibodies can deviate from the theoretical kinetic model in the modified Nijmegen-Bethesda assay, leading to differences in FVIII inhibitor titer quantification.

Structural insights into blood coagulation factor VIII: Procoagulant complexes, membrane binding, and antibody inhibition

Advances in structural studies of blood coagulation factor VIII (FVIII) have provided unique insight into FVIII biochemistry. Atomic detail models of the B domain-deleted FVIII structure alone and in complex with its circulatory partner, von Willebrand factor (VWF), provide a structure-based rationale for hemophilia A-associated mutations which impair FVIII stability and increase FVIII clearance rates. In this review, we discuss the findings from these studies and their implications toward the design of a recombinant FVIII with improved circulatory half-life. Additionally, we highlight recent structural studies of FVIII bound to inhibitory antibodies that have refined our understanding of FVIII binding to activated platelet membranes and formation of the intrinsic tenase complex. The combination of bioengineering and structural efforts to understand FVIII biochemistry will improve therapeutics for treating hemophilia A, either through FVIII replacement therapeutics, immune tolerance induction, or gene therapy approaches.

Inhibitors: Diagnostic challenges, unknowns of inhibitor development, treatment of bleeding and surgery, and insights into diagnosis and treatment in China

Factor (F) VIII inhibitors develop in around 30% of previously untreated patients (PUPs) with severe haemophilia, to a lesser extend in moderate and mild haemophilia A and in up to 10% in severe haemophilia B. Diagnostic challenges and questions remain including access to high quality testing, the role for functional inhibitor testing and binding antibody testing, and the adaptations needed in the presence of non-factor replacement therapy. Despite significant gains in knowledge there are still many unanswered questions underlying the immunologic mechanisms of inhibitor development and tolerance. Therapeutic options include eradication of inhibitors using immune tolerance induction therapy (ITI), prophylaxis with bypassing agents (i.e., recombinant activated factor VII /rFVIIa or activated prothrombin complex concentrate/aPCC) or non-factor replacement therapies (e.g., emicizumab) and treatment of bleeds or coverage of surgeries/invasive procedure. Recently a haemophilia centre capacity building program was launched in China to further develop the infrastructure and support needed to improve the diagnosis of haemophilia, detection of inhibitors, and continue to improve the care of patients with haemophilia and inhibitors.

Removal of single-site N-linked glycans on factor VIII alters binding of domain-specific monoclonal antibodies

BACKGROUND

A portion of individuals with hemophilia A develop neutralizing antibodies called inhibitors to glycoprotein factor VIII (FVIII). There are multiple risk factors that contribute to the risk of inhibitor formation. However, knowledge of the role of FVIII asparagine (N)-linked glycosylation in FVIII immunity is limited.

OBJECTIVE

To evaluate the effect of site-specific N-linked glycan removal on FVIII biochemical properties, endocytosis by murine bone marrow-derived dendritic cells (BMDCs), and antibody responses.

METHODS

Four recombinant B domain-deleted (BDD) FVIII variants with single-site amino acid substitutions to remove N-linked glycans were produced for experimental assays.

RESULTS

BDD FVIII-N41G, FVIII-N239A, FVIII-N1810A, and FVIII-N2118A with confirmed removal of N-linked glycans and similar glycosylation profiles to BDD FVIII were produced. There were no differences in thrombin activation or von Willebrand factor binding of FVIII variants compared with BDD FVIII; however, reduced FVIII expression, activity, and specific activity was observed with all variants. BDD FVIII-N41G and FVIII-N1810A had reduced uptake by BMDCs, but there were no differences in antibody development in immunized hemophilia A mice compared with BDD FVIII. Half of a repertoire of 12 domain-specific FVIII MAbs had significantly reduced binding to ≥1 FVIII variant with a 50% decrease in A1 domain MAb 2-116 binding to FVIII-N239A.

CONCLUSIONS

Modifications of FVIII N-linked glycans reduced FVIII endocytosis by BMDCs and binding of domain-specific FVIII MAbs, but did not alter de novo antibody production in hemophilia A mice, suggesting that N-glycans do not significantly contribute to inhibitor formation.

Anti-C1 domain antibodies that accelerate factor VIII clearance contribute to antibody pathogenicity in a murine hemophilia A model

Essentials Inhibitor formation remains a challenging complication of hemophilia A care. The Bethesda assay is the primary method used for determining bleeding risk and management. Antibodies that block factor VIII binding to von Willebrand factor can increase FVIII clearance. Antibodies that increase clearance contribute to antibody pathogenicity.

SUMMARY

Background The development of neutralizing anti-factor VIII (FVIII) antibodies remains a challenging complication of modern hemophilia A care. In vitro assays are the primary method used for quantifying inhibitor titers, predicting bleeding risk, and determining bleeding management. However, other mechanisms of inhibition are not accounted for in these assays, which may result in discrepancies between the inhibitor titer and clinical bleeding symptoms. Objectives To evaluate FVIII clearance in vivo as a potential mechanism for antibody pathogenicity and to determine whether increased FVIII dosing regimens correct the associated bleeding phenotype. Methods FVIII-/- or FVIII-/- /von Willebrand factor (VWF)-/- mice were infused with anti-FVIII mAbs directed against the FVIII C1, C2 or A2 domains, followed by infusion of FVIII. Blood loss via the tail snip bleeding model, FVIII activity and FVIII antigen levels were subsequently measured. Results Pathogenic anti-C1 mAbs that compete with VWF for FVIII binding increased the clearance of FVIII-mAb complexes in FVIII-/- mice but not in FVIII-/- /VWF-/- mice. Additionally, pathogenic anti-C2 mAbs that inhibit FVIII binding to VWF increased FVIII clearance in FVIII-/- mice. Anti-C1, anti-C2 and anti-A2 mAbs that do not inhibit VWF binding did not accelerate FVIII clearance. Infusion of increased doses of FVIII in the presence of anti-C1 mAbs partially corrected blood loss in FVIII-/- mice. Conclusions A subset of antibodies that inhibit VWF binding to FVIII increase the clearance of FVIII-mAb complexes, which contributes to antibody pathogenicity. This may explain differences in the bleeding phenotype observed despite factor replacement in some patients with hemophilia A and low-titer inhibitors.

A microengineered vascularized bleeding model that integrates the principal components of hemostasis

Hemostasis encompasses an ensemble of interactions among platelets, coagulation factors, blood cells, endothelium, and hemodynamic forces, but current assays assess only isolated aspects of this complex process. Accordingly, here we develop a comprehensive in vitro mechanical injury bleeding model comprising an "endothelialized" microfluidic system coupled with a microengineered pneumatic valve that induces a vascular "injury". With perfusion of whole blood, hemostatic plug formation is visualized and "in vitro bleeding time" is measured. We investigate the interaction of different components of hemostasis, gaining insight into several unresolved hematologic issues. Specifically, we visualize and quantitatively demonstrate: the effect of anti-platelet agent on clot contraction and hemostatic plug formation, that von Willebrand factor is essential for hemostasis at high shear, that hemophilia A blood confers unstable hemostatic plug formation and altered fibrin architecture, and the importance of endothelial phosphatidylserine in hemostasis. These results establish the versatility and clinical utility of our microfluidic bleeding model.

Combination of hemostatic therapies for treatment of patients with hemophilia A and inhibitors

BACKGROUND

Therapy application and monitoring of patients with hemophilia A (HA) and inhibitors are challenging. In the current study, combined FVIII - bypass therapy was implemented for a cohort of severe HA patients with inhibitors.

METHODS

Plasma of 15 HA patients with inhibitors was spiked ex vivo with FVIII, rFVIIa, FEIBA and their combinations and thrombin generation (TG) was studied. Some patients who experienced hemarthroses or required minor surgeries were treated by a combined concomitant administration of FVIII+FEIBA as IV bolus doses.

RESULTS

TG spiking studies showed individual responses not correlated to inhibitor titer. Combinations of agents augmented TG as compared to any single agent, while combined FVIII+FEIBA yielded the highest TG, supporting it as a potential treatment. Following emergent successful surgery of child treated by concomitant FVIII+FEIBA, a total of 396 episodes in 7/15 patients were treated with concomitant FVIII+FEIBA. Five patients were treated for bleeding episodes only, whereas 2 were children undergoing immune tolerance induction (ITI) with FEIBA prophylaxis. Four minor surgeries were performed on FVIII+FEIBA repeated infusions. Neither thrombosis nor any other adverse events were documented.

CONCLUSION

A combination of FVIII+FEIBA may be effective and safe as an alternative treatment option for some high-responding inhibitor patients.

Platelet-Microcapsule Hybrids Leverage Contractile Force for Targeted Delivery of Hemostatic Agents

We report a cell-mediated, targeted drug delivery system utilizing polyelectrolyte multilayer capsules that hybridize with the patient's own platelets upon intravenous administration. The hybridized platelets function as the sensor and actuator for targeted drug delivery and controlled release in our system. These capsules are biochemically and mechanically tuned to enable platelet adhesion and capsule rupture upon platelet activation and contraction, enabling the targeted and controlled "burst" release of an encapsulated biotherapeutic. As platelets are the "first responders" in the blood clot formation process, this platelet-hybridized system is ideal for the targeted delivery of clot-augmenting biotherapeutics wherein immediate therapeutic efficacy is required. As proof-of-concept, we tailored this system to deliver the pro-clotting biotherapeutic factor VIII for hemophilia A patients that have developed inhibitory antifactor VIII antibodies. The polyelectrolyte multilayer capsules physically shield the encapsulated factor VIII from the patient's inhibitors during circulation, preserving its bioactivity until it is delivered at the target site via platelet contractile force. Using an in vitro microfluidic vascular injury model with factor VIII-inhibited blood, we demonstrate a 3.8× increase in induced fibrin formation using capsules loaded with factor VIII at a concentration an order of magnitude lower than that used in systemic delivery. We further demonstrate that clot formation occurs 18 min faster when factor VIII loaded capsules are used compared to systemic delivery at the same concentration. Because platelets are integral in the pathophysiology of thrombotic disorders, cancer, and innate immunity, this paradigm-shifting smart drug delivery system can be similarly applied to these diseases.

High-affinity, noninhibitory pathogenic C1 domain antibodies are present in patients with hemophilia A and inhibitors

Inhibitor formation in hemophilia A is the most feared treatment-related complication of factor VIII (fVIII) therapy. Most inhibitor patients with hemophilia A develop antibodies against the fVIII A2 and C2 domains. Recent evidence demonstrates that the C1 domain contributes to the inhibitor response. Inhibitory anti-C1 monoclonal antibodies (mAbs) have been identified that bind to putative phospholipid and von Willebrand factor (VWF) binding epitopes and block endocytosis of fVIII by antigen presenting cells. We now demonstrate by competitive enzyme-linked immunosorbent assay and hydrogen-deuterium exchange mass spectrometry that 7 of 9 anti-human C1 mAbs tested recognize an epitope distinct from the C1 phospholipid binding site. These mAbs, designated group A, display high binding affinities for fVIII, weakly inhibit fVIII procoagulant activity, poorly inhibit fVIII binding to phospholipid, and exhibit heterogeneity with respect to blocking fVIII binding to VWF. Another mAb, designated group B, inhibits fVIII procoagulant activity, fVIII binding to VWF and phospholipid, fVIIIa incorporation into the intrinsic Xase complex, thrombin generation in plasma, and fVIII uptake by dendritic cells. Group A and B epitopes are distinct from the epitope recognized by the canonical, human-derived inhibitory anti-C1 mAb, KM33, whose epitope overlaps both groups A and B. Antibodies recognizing group A and B epitopes are present in inhibitor plasmas from patients with hemophilia A. Additionally, group A and B mAbs increase fVIII clearance and are pathogenic in a hemophilia A mouse tail snip bleeding model. Group A anti-C1 mAbs represent the first identification of pathogenic, weakly inhibitory antibodies that increase fVIII clearance.