Defining the Structural Features of Complement-Active IgM: 3-state comparison

Immunoglobulin mu (IgM) is a class of mammalian antibodies that are critical for the early stages of adaptive immunity and are the most potent activators of the classical complement cascade. While the relationship between IgM and complement has been appreciated for decades, the structural transitions within IgM upon antigen binding that lead to the activation of the C1 complex remain unresolved. Here we examine the complement activation profiles, binding kinetics, and conformational changes in IgM in different antigen-bound states. Only in the complex with a surface-displayed antigen with multiple Fab arms engaged was the IgM fully capable of driving complement activation. The structural changes within IgM upon antigen binding were monitored by Hydrogen/Deuterium Exchange reveal the predominant structural changes within the Fc core domains upon activation. Collectively, this work establishes key structural and functional qualities that define the complement-active form of IgM.