A study published in Science identified four neutralising small, single-chain antibodies that target the receptor-binding domain (RBD) of SARS-CoV-2 spike and demonstrates their inhibitory activity in different in vitro infection models.
These small variable domains of heavy-chain-only antibodies, also known as nanobodies, were obtained from an alpaca and a llama immunised with the RBD and inactivated virus. Then, the researchers used electron cryomicroscopy (cryo-EM) to study in detail how the various nanobodies interact with the spike protein of the virus.
They found that the premature activation of the fusion machinery on virions was an unexpected mechanism of neutralisation. Activation of the spike in the absence of target membranes likely induces irreversible conformational changes to assume the energetically favourable post-fusion conformation, but without catalysing fusion, the authors hypothesise.
Based on their observations, they were able to design bi- and tri-valent nanobodies with improved neutralising properties. Furthermore, the simultaneous targeting of two independent epitopes reduced the virus’s probability of becoming resistant through escape mutations.
These multivalent nanobody constructs are small, extremely stable, easy to engineer, and economic to produce in simple expression systems, therefore representing a versatile alternative to conventional antibodies for passive immunisation against SARS-CoV-2, the authors conclude.