Defining the Features of Complement-Active IgM

Immunoglobulin M (IgM) is a class of mammalian antibody that is critical for the early stages of adaptive immunity, and is the most potent Ig-activator 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 promote the activation of complement component C1 remain unresolved. Here we examine in vitro complement activation, C1 binding kinetics, and conformational changes within IgM in different antigen-bound states. Binding studies using biolayer interferometry revealed that only in a multivalent complex with a surface-displayed antigen was IgM fully capable of initiating complement activation. Hydrogen/Deuterium exchange with mass spectrometry revealed the predominant structural changes within the Fc domains during transition to the active conformation. Collectively, this work establishes key structural and functional qualities that define the complement-active form of IgM.