May 3-9 2021
Could antibodies from convalescents be used?
SARS-CoV-2 is the third β-coronavirus to appear in humans. After the SARS-CoV and MERS-CoV epidemics, very few antibodies had been isolated for therapeutic use. Currently there are numerous attempts underway to purify and characterise those antibodies that are effective against SARS-CoV-2 so as to guide the choices of treatment strategies and to assess their responsiveness to current variants.
American researchers (at the Massachusetts Institute of Technology, California Institute of Technology, Harvard and Caltech) studied the production of antibodies in 14 COVID-19 patients who had been convalescing for at least a month. In 11 patients, neutralization activity was observed in vitro in blood samples, with varying degrees of effectiveness. The B cells, which produce antibodies able to recognise the spike protein or its RBD (Receptor Binding Domain), were purified (flow cytometry) so as to examine their genetic profiles (VDJ sequencing) and their transcription (scRNA-seq). It was found that these cells are enriched with VH3-53/VH3-66 genes, characteristic of class 1 neutralizing antibodies having a common mode of action. Six distinct transcription groups were identified (TC, grouped according to overexpressed genes), of which some were typical of activated B cells (TC4) and memory cells (TC3). The cells producing the strongest neutralizing antibodies corresponded to activated B and memory cells (TC3 and TC4).
This can be schematically represented as follows:
Then 92 antibodies that interacted strongly with the spike, were purified from the 4 patients with the most neutralizing antibodies. Amongst these antibodies, 56 bind effectively to Spike, but only 27 were found to have a neutralizing activity: they target the RBD (ELISA testing) and originate from B cells expressing IgG. The 4 most effective antibodies were also the most effective against the UK variant (B.1.1.7), while only 2 (BG10-19 and BG7-15) neutralize the South African variant (B.1.351).
Of the 92 antibodies, only the BG10-19 was capable of neutralizing SARS-CoV, and did so more effectively than the S309 (the standard neutralizing antibody against the 2 strains). Structural analysis showed that the BG10-19 antibody targets a conserved area of the RBD, different from that which liaises with the ACE2 (cryo-electron microscopy, SPR). By bridging adjacent RBD from the spike trimer, this antibody keeps the RBD in a closed conformation and prevents it from interacting with the ACE2.
Finally, the researchers effected several mutations on spike’s RBD (including current variants in circulation) and evaluated the ability of the 6 most powerful antibodies to bind to the mutated protein or neutralize in vitro infection through the corresponding pseudovirus. These capacities are diminished in the majority of the antibodies, except for the BG7-15, which remains active against the N493K mutation, found in the Brazilian and South African variants. The BG4-25 and BG10-19 antibodies remain effective only against certain mutations. When combined, only the BG4-25 and BG10-19 antibodies demonstrate synergistic neutralisation activity. In vitro experiments on immune evasion show that the BG10-19 antibody induces two immune escape mutants that have never been detected in humans.
This integrated approach means that the laws governing neutralisation can be better understood. The potent neutralizing antibodies against SARS-CoV-2 come from active B and memory cells, with common transcription profiles, although originating from different convalescing individuals. The ability of the serum to neutralize the virus does not correlate with the antibody potency of one individual, but rather with the size of the B-cell populations with a high affinity for the spike.
It is however important to verify whether this correlation exists in vaccinated individuals. The unique properties of the BG10-19 antibody make it a potentially useful treatment that could be effective against current and future variants. This study should help with the development of immunogens capable of delivering immune responses that protect against SARS-CoV-2 and its variants, as well as other coronavirus strains