Essentials Fc-fusion increases a therapeutic's half-life, but FcγR interactions may impact immunogenicity. Species-specific Fc-FcγR interactions allow for mechanistic in vivo studies using mouse models. Fc fusion modulates the immune response to factor IX in hemophilia B mice by eliciting Th1 bias. This model could inform future studies of IgE-associated anaphylaxis in hemophilia B patients.

Summary: Background Fc fusion is a platform technology used to increase the circulating half-life of protein and peptide therapeutics. However, there are potential immunological consequences with this approach, such as changes in the molecule's immunogenicity as well as possible interactions with a repertoire of Fc receptors (FcR) that can modulate immune responses. Objectives/Methods Using a mouse hemophilia B (HB) model, we compared the immune responses to infusions of recombinant human factor IX (hFIX) and hFIX fused to mouse IgG2a-Fc (hFIX-mFc). The mFc was employed to allow species-specific Fc-FcγR interactions. Results Although treatment with hFIX-mFc altered the early development of anti-FIX IgG, no significant differences in anti-FIX antibody titers were observed at the end of the treatment regimen (5 weeks) or upon anamnestic response (5 months). However, treatment with hFIX-mFc elicited higher FIX-neutralizing antibody levels and resulted in reduced IgE titers compared with the hFIX-treated group. Additionally, differences in plasma cytokine levels and in vitro CD4⁺ T-cell responses suggest that whereas hFIX treatment triggered a Th2-biased immune response, hFIX-mFc treatment induced Th1-biased CD4⁺ T cells. We also show that hFIX-mFc bound to soluble FcγRs and engaged with FcγRs on different cell types, which may impact antigen presentation. Conclusions These studies provide a model system to study how Fc-fusion proteins may affect immune mechanisms. We used this model to demonstrate a plausible mechanism by which Fc fusion may modulate the IgE response to hFIX. This model may be appropriate for investigating the rare but severe IgE-mediated anaphylaxis reaction to hFIX infusions in HB patients.