Structural Insights into the Neutralization Properties of the Fully Human, Anti-interferon Monoclonal Antibody Sifalimumab

We report the three-dimensional structure of human interferon α-2A (IFN-α2A) bound to the Fab fragment of a therapeutic monoclonal antibody (sifalimumab; IgG1/κ). The structure of the corresponding complex was solved at a resolution of 3.0 Å using molecular replacement and constitutes the first reported structure of a human type I IFN bound to a therapeutic antibody. This study revealed the major contribution made by the first complementarity-determining region in each of sifalimumab light and heavy chains. These data also provided the molecular basis for sifalimumab mechanism of action. We propose that its interferon-neutralizing properties are the result of direct competition for IFN-α2A binding to the IFN receptor subunit 1 (IFNAR1) and do not involve inhibiting IFN-α2A binding to the IFN receptor subunit 2 (IFNAR2).