Many aspects of innate immune responses to SARS viruses remain unclear. Of particular
interest is the role of emerging neutralizing antibodies against the receptor-binding
domain (RBD) of SARS-CoV-2 in complement activation and opsonization. To overcome
challenges with purified virions, here we introduce “pseudovirus-like”
nanoparticles with ∼70 copies of functional recombinant RBD to map complement
responses. Nanoparticles fix complement in an RBD-dependent manner in sera of all
vaccinated, convalescent, and naïve donors, but vaccinated and convalescent
donors with the highest levels of anti-RBD antibodies show significantly higher IgG
binding and higher deposition of the third complement protein (C3). The opsonization
via
anti-RBD antibodies is not an efficient process: on average, each
bound antibody promotes binding of less than one C3 molecule. C3 deposition is
exclusively through the alternative pathway. C3 molecules bind to protein deposits, but
not IgG, on the nanoparticle surface. Lastly, “pseudovirus-like”
nanoparticles promote complement-dependent uptake by granulocytes and monocytes in the
blood of vaccinated donors with high anti-RBD titers. Using nanoparticles displaying
SARS-CoV-2 proteins, we demonstrate subject-dependent differences in complement
opsonization and immune recognition.