Increased neutralization of SARS-CoV-2 Delta variant by nanobody (Nb22) and the structural basis

Wu, Xilin; Wang, Yaxing; Cheng, Lin; Zhu, Linjing; Ma, Sen; Huang, Bilian; Xu, Shijie; Shi, Haixia; Zhang, Doudou; Liu, Linshuo; Nawaz, Waqas; Ye, Sheng; Wu, Zhiwei

Delta variant, also known as B.1.617.2, has become a predominant circulating variant in many countries since it first emerged in India in December 2020. Delta variant is less sensitive to serum neutralization from COVID-19 convalescent individuals or vaccine recipients, relative to Alpha strains. It was also resistant to neutralization by some anti-receptor binding domain (RBD) and anti-N-terminal domain (NTD) antibodies in clinics. Previously, we reported the discovery of nanobodies isolated from an alpaca immunized with spike protein, exhibiting ultrahigh potency against SARS-CoV-2 and its mutated variants, where a novel inhalable bispecific Nb15 protected SARS-CoV-2 infection in hACE2 mice. Here, we found that Nb22-Fc, among our previously reported nanobodies, exhibited 8.4-fold increased neutralization potency against Delta variant with an IC50 value of 0.41 ng/ml (5.13 pM) relative to Alpha variant. Furthermore, our crystal structural analysis reveals that the binding of Nb22 on SARS-CoV-2 RBD effectively blocks the binding of RBD to ACE2 during virus infection. Furthermore, the L452R mutation in RBD of Delta variant forms an additional hydrogen bond with the hydroxy group of T30 of Nb22, leading to the increased neutralization potency of Nb22 against Delta variant. Thus, Nb22 is a potential therapeutic agent against SARS-CoV-2, especially the highly contagious Delta variant.