Immunogenicity of FVIII: a Role for Thrombin.

Abstract 3175

Poster Board III-115

Administration of human Factor VIII to hemophilia A (FVIII^-/-) mice is a useful small animal model to study the immune response in patients given therapeutic FVIII. These mice manifest a robust, T-cell dependent, antibody response to exogenous FVIII treatment even when encountered through traditionally tolerogenic routes, whereas a different foreign protein like ovalbumin (OVA) is much less immunogenic by these routes. Interestingly, administration of FVIII with OVA led to an immune response to both proteins, suggesting that the function of FVIII in the clotting cascade led to additional “danger” signals to co-administered proteins. Thus, we propose that FVIII is particularly immunogenic because of its function in the coagulation cascade that leads to thrombin formation. We showed that native FVIII is poorly immunogenic when it cannot activate downstream coagulation factors. Thus, heat-inactivated FVIII is poorly immunogenic despite containing normal T-cell epitopes. Importantly, native FVIII is less immunogenic in mice treated with warfarin (which blocks vitamin K-dependent enzymes) or with the direct thrombin-inhibitor, hirudin. Based on the hypothesis that thrombin generation is necessary for the immunogenicity and formation of inhibitors to FVIII, we wished to uncouple this immune response from the disease. When hemostatically normal BALB/c mice were injected directly with thrombin and OVA, they formed increased immune responses to OVA compared to mice given OVA alone. Thus, a likely mechanism is that FVIII treatment initiates a thrombin burst, which we propose is an immunogenic “danger” signal, leading indirectly to dendritic cell activation and promotion of an immune response. (Supported by NIH RO1 HL061883, NIH T32 HL007698, and AHA Fellowship 0815219E)