Discovery of nanobodies against SARS-CoV-2 and an uncommon neutralizing mechanism (preprint)

SARS-CoV-2 and its variants continue to threaten public health. The virus recognizes the host cell by attaching its Spike receptor-binding domain (RBD) to the host receptor ACE2. Therefore, RBD is a primary target for neutralizing antibodies and vaccines. Here we report the isolation, and biological and structural characterization of two single-chain antibodies (nanobodies, DL4 and DL28) from RBD-immunized alpaca. Both nanobodies bind Spike with affinities that exceeded the detection limit (picomolar) of the biolayer interferometry assay and neutralize the original SARS-CoV-2 strain with IC50 of 86 ng mL-1 (DL4) and 385 ng mL-1 (DL28). DL4 and a more potent, rationally designed mutant, neutralizes the Alpha variant as potently as the original strain but only displays marginal activity against the Beta variant. By contrast, the neutralizing activity of DL28, when in the Fc-fused divalent form, was less affected by the mutations in the Beta variant (IC50 of 414 ng mL-1 for Alpha, 1060 ng mL-1 for Beta). Crystal structure studies reveal that DL4 blocks ACE2-binding by direct competition, while DL28 neutralizes SARS-CoV-2 by an uncommon mechanism through which DL28 distorts the receptor-binding motif in RBD and hence prevents ACE2-binding. Our work provides two neutralizing nanobodies for potential therapeutic development and reveals an uncommon mechanism to design and screen novel neutralizing antibodies against SARS-CoV-2.

Sequence Analysis for SNP Detection and Phylogenetic Reconstruction of SARS-CoV-2 Isolated from Nigerian COVID-19 Cases (preprint)

Background: Coronaviruses are a group of viruses that belong to the Family Coronaviridae, Genus Betacoronavirus. In December 2019, a new coronavirus disease (COVID-19) characterized by severe respiratory symptoms was discovered. The causative pathogen was a novel coronavirus known as 2019-nCoV and later as SARS-CoV-2. Within two months of its discovery, COVID-19 became a pandemic causing widespread morbidity and mortality. Methodology: Whole genome sequence data of SARS-CoV-2 isolated from Nigerian COVID-19 cases were retrieved by downloading from GISAID database. A total of 18 sequences that satisfied quality assurance (length > 29700 nts and number of unknown bases denoted as N < 5%) were used for the study. Multiple sequence alignment (MSA) was done in MAFFT (Version 7.471) while SNP calling was implemented in DnaSP (Version 6.12.03) respectively and then visualized in Jalview (Version 2.11.1.0). Phylogenetic analysis was with MEGA X software. Results: Nigerian SARS-CoV-2 had 99.9% genomic similarity with four large conserved genomic regions. A total of 66 SNPs were identified out of which 31 were informative. Nucleotide diversity assessment gave Pi = 0.00048 and average SNP frequency of 2.22 SNPs per 1000 nts. Non-coding genomic regions particularly 5 UTR and 3 UTR had a SNP density of 3.77 and 35.4 respectively. The region with the highest SNP density was ORF10 with a frequency of 8.55 SNPs/1000 nts). Majority (72.2%) of viruses in Nigeria are of L lineage with preponderance of D614G mutation which accounted for 11 (61.1%) out of the 18 viral sequences. Nigeria SARS-CoV-2 revealed 3 major clades namely Oyo, Ekiti and Osun on a maximum likelihood phylogenetic tree. Conclusion and Recommendation: Nigerian SARS-CoV-2 reveals high mutation rate together with preponderance of L lineage and D614G mutants. Implication of these mutations for SARS-CoV-2 virulence and the need for more aggressive testing and treatment of COVID-19 in Nigeria is discussed. Additionally, attempt to produce testing kits for COVID-19 in Nigeria should consider the conserved regions identified in this study. Strict adherence to COVID-19 preventive measure is recommended in view of Nigerian SARS-CoV-2 phylogenetic clustering pattern, which suggests intensive community transmission possibly rooted in communal culture characteristic of many ethnicities in Nigeria.

A high-affinity RBD-targeting nanobody improves fusion partner's potency against SARS-CoV-2 (preprint)

A key step to the SARS-CoV-2 infection is the attachment of its Spike receptor-binding domain (S RBD) to the host receptor ACE2. Considerable research have been devoted to the development of neutralizing antibodies, including llama-derived single-chain nanobodies, to target the receptor-binding motif (RBM) and to block ACE2-RBD binding. Simple and effective strategies to increase potency are desirable for such studies when antibodies are only modestly effective. Here, we identify and characterize a high-affinity synthetic nanobody (sybody, SR31) as a fusion partner to improve the potency of RBM-antibodies. Crystallographic studies reveal that SR31 binds to RBD at a conserved and greasy site distal to RBM. Although SR31 distorts RBD at the interface, it does not perturb the RBM conformation, hence displaying no neutralizing activities itself. However, fusing SR31 to two modestly neutralizing sybodies dramatically increases their affinity for RBD and neutralization activity against SARS-CoV-2 pseudovirus. Our work presents a tool protein and an efficient strategy to improve nanobody potency.

A potent synthetic nanobody targets RBD and protects mice from SARS-CoV-2 infection (preprint)

SARS-CoV-2, the causative agent of COVID-191, recognizes host cells by attaching its receptor-binding domain (RBD) to the host receptor ACE22-7. Neutralizing antibodies that block RBD-ACE2 interaction have been a major focus for therapeutic development8-18. Llama-derived single-domain antibodies (nanobodies, ~15 kDa) offer advantages including ease of production and possibility for direct delivery to the lungs by nebulization19, which are attractive features for bio-drugs against the global respiratory disease. Here, we generated 99 synthetic nanobodies (sybodies) by in vitro selection using three libraries. The best sybody, MR3 bound to RBD with high affinity (KD = 1.0 nM) and showed high neutralization activity against SARS-CoV-2 pseudoviruses (IC50 = 0.40 μg mL-1). Structural, biochemical, and biological characterization of sybodies suggest a common neutralizing mechanism, in which the RBD-ACE2 interaction is competitively inhibited by sybodies. Various forms of sybodies with improved potency were generated by structure-based design, biparatopic construction, and divalent engineering. Among these, a divalent MR3 conjugated with the albumin-binding domain for prolonged half-life displayed highest potency (IC50 = 12 ng mL-1) and protected mice from live SARS-CoV-2 challenge. Our results pave the way to the development of therapeutic nanobodies against COVID-19 and present a strategy for rapid responses for future outbreaks.

Potent synthetic nanobodies against SARS-CoV-2 and molecular basis for neutralization (preprint)

SARS-CoV-2, the Covid-19 causative virus, adheres to human cells through binding of its envelope Spike protein to the receptor ACE2. The Spike receptor-binding domain (S-RBD) mediates this key event and thus is a primary target for therapeutic neutralizing antibodies to mask the ACE2-interacting interface. Here, we generated 99 synthetic nanobodies (sybodies) using ribosome and phage display. The best sybody MR3 binds the RBD with KD of 1.0 nM and neutralizes SARS-CoV-2 pseudovirus with IC50 of 0.40 g mL-1. Crystal structures of two sybody-RBD complexes reveal a common neutralizing mechanism through which the RBD-ACE2 interaction is competitively inhibited by sybodies. The structures allowed the rational design of a mutant with higher affinity and improved neutralization efficiency by [~]24-folds, lowering the IC50 from 12.32 to 0.50 g mL-1. Further, the structures explain the selectivity of sybodies between SARS-CoV strains. Our work presents an alternative approach to generate neutralizers against newly emerged viruses. One sentence summaryStructural and biochemical studies revealed the molecular basis for the neutralization mechanism of in vitro-selected and rationally designed nanobody neutralizers for SARS-CoV-2 pseudovirus.