Despite the urgency of the COVID-19 pandemic, many questions related to the immune response to SARS viruses remain unanswered. Of particular interest is the complement system, which is responsible for immune recognition of foreign particles and pathogens. Opsonization of pathogens with the third complement protein (C3) is one of the main triggers of immune clearance. Neutralizing high-affinity antibodies against the receptor binding domain (RBD) of SARS-CoV-2 are known to emerge in convalescent and vaccinated individuals, but their role in complement activation is unknown. Studies of complement opsonization using purified viral particles present a technical and logistical challenge. Here we study complement opsonization of engineered nano-sized pseudovirus-like iron oxide particles bearing ~70 copies of functional surface-bound recombinant SARS-CoV-2 RBD protein. In non-diluted human sera, nanoparticles bind significantly more IgG, but not IgM, in vaccinated and convalescent donors than in naïve donors. The level of nanoparticle-bound IgG positive correlates with the serum level of anti-RBD IgG. The results also show a significant C3 deposition in vaccinated (8/10), convalescent (8/10) and in some naïve donors (6/11) in the order of vaccinated&gt;convalescent&gt;naïve sera. The complement deposition level positively correlates with nanoparticle-bound IgG level. Anti-RBD IgG promotes the opsonization exclusively <i>via</i> the alternative pathway without the involvement of calcium-sensitive classical/lectin pathways. On average, each bound anti-RBD IgG promotes deposition of less than one C3 molecule, and almost all deposited C3 is covalently bound to other proteins rather than IgG. These findings demonstrate that anti-RBD antibodies incite complement via the alternative pathway and suggest that engineered pseudovirus-like nanoparticles could be valuable tools for dissecting innate immune responses towards viral surface proteins.

Nanoparticles functionalized with the receptor binding domain of SARS-CoV-2 spike protein for probing serum complement responses in vaccinated, COVID-19 convalescent and naive subjects

ACS Spring 2022

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