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

Abstract

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.

Molecular determinants of the factor VIII/von Willebrand factor complex revealed by BIVV001 cryo-electron microscopy

“…Recently, Batsuli et al showed that the pathogenic effect of anti-C1 antibodies directed against noncoagulant epitopes of FVIII results from disrupting interaction with VWF and subsequent rapid FVIII degradation. 66 Our structure supports the hypothesis that this class of inhibitors act via competition with VWF as these group A antibodies 67 bind an epitope at the FVIII-C1/VWF-D9 interface distinct from the phospholipid-binding region of FVIII-C1.…”

Comparison between coagulation factor VIII quantified with one‐stage activity assay and with mass spectrometry in haemophilia A patients: Proof of principle

“…Currently, the major complication in haemophilia A treatment is the development of neutralising anti‐FVIII antibodies, rendering endogenous and exogenous FVIII ineffective. Anti‐FVIII antibodies neutralise the FVIII activity by forming FVIII‐antibody complexes that accelerate clearance of FVIII or by sterically hindering the interaction of FVIII with other coagulation factors 22,23 . For the samples with anti‐FVIII antibodies (in the range 0.8‐5.3 BU/mL, n = 5) present, we found a substantial higher FVIII plasma concentration than activity.…”

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

“…To assess the role of VWF on FVIII-IC with enhanced antibody responses in FVIII -/- mice ( Figure 5B ), mice deficient in both endogenous FVIII and VWF (FVIII -/- /VWF -/- mice) were injected with FVIII, FVIII/2-116 (A1 MAb), FVIII/4A4 (A2 MAb), or FVIII/B136 (C1 MAb) using a “high-dose” FVIII regimen ( Figure 7A ). The “high-dose” regimen was employed to account for increased FVIII clearance in the absence of VWF and lower antibody titers observed in this mouse model ( 58 ). MAb B136 inhibits FVIII binding to VWF; thus, mice immunized with FVIII/B136 were included as a control.…”