Modulating immunogenicity of factor IX by fusion to an immunoglobulin Fc domain: a study using a hemophilia B mouse model
Fusion of therapeutic molecules to the Fc domain of immunoglobulin has been used to increase plasma half-life through the neonatal Fc receptor recycling mechanism. Recently this has been used in hemophilia for Factor VIII and Factor IX (F.IX) replacement. Fc-fusion was noted to alter the development of anti-F.IX and F.IX inhibitory antibodies, although the mechanism was not clear from clinical studies.
The authors developed a F.IX fusion product with mouse Fc, to allow the evaluation of immunological response in a mouse model of hemophilia B. These mice were administered either recombinant human F.IX (hFIX), hFIX with a mouse Fc fusion (hFIX-mFC), and hFIX with a human Fc fusion (hFIX-hFC). Anti-F.IX IgG was measured, and IgE levels were also monitored. Inhibitory antibodies to F.IX were determined by a modified Bethesda assay. Cytokine and immunological (B, memory B, and T cell) repertoires were also measured from cultured splenic cells.
Mice showed a strong immunological response to hFIX-hFC, highlighting the importance of using the murine Fc in the study. Mice treated with the hFIX-mFC had higher levels of anti-F.IX IgG and inhibitory antibodies compared with only hFIX treated mice, but lower IgE titres. The authors postulated that Fc fusion modulates the T-cell response to hFIX, switching from a Th2 to a Th1 response, and further evaluated this with cytokine profiles. Further evaluation of the interaction with cell surface receptors reveal that hFIX-mFC, but not hFIX, bound specifically to Fc receptors on professional antigen presenting cells (APCs). Targeting of the F.IX antigen to these APCs allows enhanced presentation and priming of immunity, and this strong signalling may also encourage the Th1 phenotype.
Because these studies were conducted in a mouse model, the extrapolation of results to humans is always uncertain. However, there is good validity and plausibility in the results.