Testing the binding and functional consequences of FcγR engagement by humanized anti-FcRn antibodies.

The neonatal Fc receptor (FcRn), expressed on phagocytic leukocytes as well as on non-hematopoietic cells, prolongs the circulating half-life of IgG by facilitating its recycling from endosomes. This process can preserve the half-life of pathogenic autoantibodies and therefore targeting FcRn is of interest as a therapeutic approach in autoimmune diseases that are driven by IgG autoantibodies, such as myasthenia gravis. Rozanolixizumab is a humanized anti-neonatal Fc receptor (FcRn) monoclonal antibody (mAb) of the immunoglobulin G4 (IgG4) sub-class, currently in clinical development for the treatment of IgG autoantibody-driven diseases. The current study explored the engagement of FcγRs and potential functional consequences with rozanolixizumab and FcRn.

Binding of fluorophore labelled anti-FcRn antibodies was investigated by flow cytometry using cells stably expressing the high affinity FcγR, FcγRI. Surface expression of FcγRs following incubation with anti-FcRn antibodies was investigated by high-content imaging using monocyte-derived macrophages. Cytokine release following incubation with anti-FcRn antibodies was measured from whole blood, HUVEC and macrophage cultures.

Rozanolixizumab was able to bind to FcγRs in cell-based assays, and mediate antibody bipolar bridging (ABB), leading to a down-regulation of FcγRI from the cell surface. An IgG1 format anti-FcRn more potently reduced surface labelling of FcγRI in an Fc-dependent manner. Both binding and ABB were prevented by low concentrations of exogenous human IgG and occurred in the absence of measurable cytokine release.

These data indicate that in the absence of exogenous IgG, IgG4 format therapeutic antibodies may down-regulate FcγRI but, in the presence of physiological concentrations of IgG, the engagement of FcγRs by IgG4 format antibodies is expected to be rare. Thus the IgG4 format, frequently used for therapeutic mAbs, is likely to be relatively inert with respect to unwanted engagement of Fc receptors.