Fusion of Fc domain to Factor IX (FIX) modulates the Th2 biased response to FIX and triggers Th1 immunity in hemophilia B treated mice

Fusion of the IgG1 constant region domain (Fc) to therapeutic proteins increases the plasma half-life of drugs via binding to the neonatal Fc receptor (FcRn). However, potential interactions with Fc gamma-receptors (FcγR) expressed by immune cells may effect the immunogenicity of these drugs. The coagulation factors VIII (FVIII) and IX (FIX) for the treatment of hemophilia A & B (HA & HB) are the only drugs available with and without Fc fusion and offer an opportunity to experimentally study the effect of Fc fusion. Anti FVIII & FIX antibodies (ADAs) and rare cases of anaphylaxis in HB patients, are serious safety concerns. Using a mouse model for HB, we compared the immune responses to infusions of recombinant human FIX (hFIX) and hFIX fused to mouse IgG2a-Fc (hFIX-mFc). The mFc allowed species-specific Fc-FcγR interactions and prevented immune response to the foreign hFc. Treatment with hFIX-mFc caused earlier onset of anti-FIX IgG and elicited higher FIX-neutralizing antibody levels. Treatment with hFIX triggered Th2 biased immunity i.e., elevated plasma IgE titers, and Th2 cytokine profile of FIX-specific CD4 T cells and a few anaphylactic events. The fusion of Fc altered the response to a Th1 type with increased plasma IFN-γ levels, and IFN-γ secretion by effector CD4 T cells. hFIX-mFc binds to soluble FcγRs and FcγRs on natural killer cells (which also secrete IFNγ) and antigen presenting cells (dendritic cells, and macrophages source of IL-12). We show that Fc fusion enhances the presentation of hFIX to CD4 T cells. We propose that high plasma availability and targeting of APC provide plausible mechanisms by which the Fc fusion may modulate the CD4 T cell response to hFIX towards a Th1 like response, thus avoiding allergic responses.