DNA Nanostructure-Templated Antibody Complexes Provide Insights into the Geometric Requirements of Human Complement Cascade Activation

The classical complement
pathway is activated by antigen-bound
IgG antibodies. Monomeric IgG must oligomerize to activate complement
via the hexameric C1q complex, and hexamerizing mutants of IgG appear
as promising therapeutic candidates. However, structural data have
shown that it is not necessary to bind all six C1q arms to initiate
complement, revealing a symmetry mismatch between C1 and the hexameric
IgG complex that has not been adequately explained. Here, we use DNA
nanotechnology to produce specific nanostructures to template antigens
and thereby spatially control IgG valency. These DNA-nanotemplated
IgG complexes can activate complement on cell-mimetic lipid membranes,
which enabled us to determine the effect of IgG valency on complement
activation without the requirement to mutate antibodies. We investigated
this using biophysical assays together with 3D cryo-electron tomography.
Our data revealed the importance of interantigen distance on antibody-mediated
complement activation, and that the cleavage of complement component
C4 by the C1 complex is proportional to the number of ideally spaced
antigens. Increased IgG valency also translated to better terminal
pathway activation and membrane attack complex formation. Together,
these data provide insights into how nanopatterning antigen–antibody
complexes influence the activation of the C1 complex and suggest routes
to modulate complement activation by antibody engineering. Furthermore,
to our knowledge, this is the first time DNA nanotechnology has been
used to study the activation of the complement system.