PB0218 - Establishment of inhibitor development system and analysis of inhibitor properties in novel mild/moderate hemophilia A model mice

Background: Hemophilia A is a bleeding disorder due to an inherent deficiency of coagulation factor VIII and its severity is classified by FVIII activity (FVIII:C). The infusion of FVIII concentrates is the basis of hemostatic treatment. However, inhibitors that reduce FVIII activity occur in ~30% of severe cases by administration of FVIII concentrates. On the other hand, ~10% of mild/moderate hemophilia A (MHA) patients also develop inhibitors despite the presence of autologous FVIII protein. We experienced different types of inhibitor properties in two MHA patients with P1809L mutation; autologous FVIII:C remained in case 1 and autologous FVIII:C declined to less than 1% in case 2 after the development of inhibitors (Furukawa, JSTH 2020).

Aims: To elucidate the mechanism of inhibitor development with different properties in MHA.

Methods: We generated knock-in mice (P1777L-KI) by introducing a point mutation of P1777L in the mouse F8 gene, corresponding to P1809L in the human F8 gene, using genome editing techniques and established an inhibitor development and analysis system.

Results: FVIII:C of P1777L-KI was 4.6 % and FVIII antigen was almost the same level as that of wild type mice (WT). After repeated administration of recombinant FVIII concentrate to P1777L-KI, inhibitor development was observed (after 4 times of dose, median 13.3BU/mL). Interestingly, as in the clinical cases, some retained autologous FVIII:C (type R), while others had diminished FVIII:C to less than 1% (type D). When analyzed these inhibitor properties, type D inhibitor suppressed almost equally FVIII:C in plasma of both WT and P1777L-KI, and type R inhibitor had lower inhibitory effect on FVIII:C in the plasma of P1777L-KI than that in the plasma of WT.

Conclusion(s): These results suggested that the P1777L-KI exhibits similar inhibitor properties to those observed in clinical cases, and it could be a valuable tool for clarifying the mechanism behind inhibitor development in MHA.