OC 37.1 - Transplacental delivery of recombinant Fc-fused factor VIII (rFVIIIFc) in FVIII-deficient mice.

Background: Injection to pregnant FVIII-KO mice of the Fc-fused A2 and C2 domains of FVIII leads to their transplacental delivery and induction of partial FVIII-immune tolerance in the offspring. Hence, the transplacental delivery of the entire FVIII should confer complete FVIII-immune tolerance. However, recombinant Fc-fused FVIII (rFVIIIFc) crosses the placenta in amounts insufficient to foster FVIII-tolerance. Understanding the rFVIIIFc properties associated with its reduced transplacental delivery could optimize rFVIIIFc transcytosis to levels compatible with tolerance induction.

Aims: To decipher the molecular mechanisms that account for the poor transplacental delivery of rFVIIIFc in FVIII-deficient mice.

Methods: Different Fc-fused proteins were generated: rFVIIIFcN297A, Fc-fused A2, A3C1, C1 and C2 domains of FVIII. Pregnant FVIII-KO mice were injected at day 17.5 of gestation with 170-330 picomoles of the molecules. Blood was collected after 5 min and 4h from mothers and 4h from fetuses. The concentration of the molecules was quantified by ELISA.

Results: rFVIIIFcN297A that cannot bind to Fc𝛾 receptors (Fc𝛾Rs), crossed the placenta to similar levels to that of rFVIIIFc (0.06±0.08 and 0.03±0.01nM, respectively), suggesting that binding of rFVIIIFc to mothers’ FcR𝛾s does not account for its poor transplacental delivery. Preliminary data show that the Fc-fused light chain of FVIII is poorly transferred through the placenta (0.04±0.005nM), suggesting that moieties in the light chain might be implicated in the retention of rFVIIIFc in mother’s circulation or placenta. Interestingly, A2Fc, A3C1Fc, C1Fc and C2Fc were all transferred to levels >1 nM in fetus’s blood.

Conclusion(s): The reduced transplacental delivery of rFVIIIFc is not due to its binding to Fc𝛾Rs. As the C1 and C2 domains of FVIII are implicated in the interaction with different molecules of the coagulation cascade or catabolic receptors, we hypothesize that co-engagement of the C1 and C2 domains of rFVIIIFc may explain, at least in part, the reduced transcytosis of rFVIIIFc through the placenta.