Engineered camelid antibody multimers can potently block SARS-CoV-2 viral entry.
Coronaviruses express a spike (S) glycoprotein containing a receptor-binding domain (RBD) and a region facilitating membrane fusion and subsequent viral entry. SARS-CoV-1 and SARS-CoV-2 utilize the angiotensin converting enzyme 2 (ACE2) as a receptor to enter target cells. During the current global pandemic, potent and reliable immunotherapies against SARS-CoV-2 are the need of the hour. RBD-targeted neutralizing antibodies can block SARS-CoV-1 entry, mainly by maintaining the RBD in an unstable conformation. Recently, the conventional antibody CR3022 was shown to bind to both SARS-CoV-1 and SARS-CoV2. However, this antibody only binds the RBD under restricted conformations and is therefore unable to fully prevent ACE2 binding and viral entry. Other limitations of conventional antibodies include increased instability, resulting in shorter half-life, and weak binding due to steric hindrance associated with large antibody size. Dromedaries produce heavy-chain-only antibodies (HCAbs) with a single variable domain (VHH) that are several times smaller and more tissue-penetrant than the heavy- and light-chain variable domain antibodies seen in primates. Moreover, VHHs display higher thermo- and chemostability and can be engineered into multimers. In this study, Wrapp et al. demonstrate the functional advantages of using single-domain camelid antibodies to neutralize SARS-CoV. By immunizing llamas with prefusion-stabilized betacoronavirus spike proteins, the authors identified three SARS VHH clones (SARS VHH-72, -1, and -6) that bind SARS-CoV-1 S protein. SARS VHH-72 binds with high affinity to the SARS-CoV1 RBD, resulting in conformational changes that bury the ACE2 receptor binding interface and thus block viral entry into target cells. The SARS VHH-72 clone also exhibits cross-reactivity against RBDs of closely related coronavirus strains including SARS-COV-2. The authors bioengineered bivalent variants of SARS VHH-72 (VHH-72-Fc and VHH-72-VHH-72) that display enhanced affinity and stability of binding and potent, antibody dose-dependent blockade of SARS-CoV-2 RBD-SD1 binding to the ACE2 receptor. The authors demonstrate that VHH-72 can neutralize not only SARS-CoV-2 and SARS-CoV-1, but also other pathogenic SARS-CoV–like viruses. The favorable size, enhanced stability, and flexibility to build multimers to strengthen binding render VHHs a potential major step forward in immune therapeutics against SARS-CoV-2 and other pathogenic human coronaviruses.
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