Complement activation by IgG subclasses is governed by their ability to oligomerize upon antigen binding.

Area of Science:

• Immunology
• Biophysics
• Structural Biology

Background:

• Complement activation by antibody-antigen complexes is vital in immunity and antibody-based therapies.
• Previous understanding attributed IgG subclass-mediated complement activation to hinge flexibility and C1 affinity.
• A unified mechanism explaining how IgG subclasses modulate complement activation was lacking.

Purpose of the Study:

• To elucidate the unifying mechanism behind differential complement activation by IgG subclasses.
• To investigate the role of IgG oligomerization on antigenic surfaces in C1 activation.
• To develop a mechanistic model for C1 binding to IgG oligomers.

Main Methods:

• High-speed atomic force microscopy to visualize IgG oligomer structures.
• Quartz crystal microbalance experiments to quantify complement recruitment efficiency.
• Tumor cell lines and vesicle-based assays to assess complement activation and lysis.

Main Results:

• Complement activation efficacy is determined by the ability of IgG subclasses to form C1-activating oligomers on surfaces.
• Direct visualization revealed distinct IgG oligomer structures and distributions.
• A mechanistic model accurately described C1 binding to IgG oligomers, enabling kinetic and equilibrium dissociation constant calculations.

Conclusions:

• IgG subclass-dependent complement activation is governed by their varying capacity to form multivalent IgG oligomers on antigenic surfaces.
• This oligomerization-driven mechanism provides a comprehensive understanding of complement activation by IgG subclasses.
• Findings can inform the rational design of antibody-based immunotherapies.