Inhibition of SARS-CoV-2 by antibodies to the proteolytic cleavage sites of the spike protein
Final Report Abstract
The initial research plan was to characterize sera from mice that were immunized with peptides derived from linear epitopes of the SARS-CoV-2 spike protein (SARS2-S), one next to the proteolytic S1/S2 cleavage site on the S1 subunit and one overlapping with the S2’ cleavage site on the S2 subunit. This was performed in collaboration with Prof. Winnsinger, Geneva, who had discovered the epitopes and provided peptides, which were also chemically branched for enhanced immunogenicity. Unfortunately, only the peptide derived from the epitope on the S1 subunit, close to the S1/S2 site, was immunogenic in mice. The peptides derived from the S2’ proteolytic site did not elicit antibody responses that significantly exceeded background reactivity as measured by ELISA. We tested sera raised to the peptides in entry assays with lentiviruses that were pseudotyped with SARS2-S and did not detect inhibitory activity. In line with these findings, the sera from the immunized mice did not contain antibodies that detectably bound to SARS2-S expressing, transfected 293T cells. Any strategy to elicit antibodies targeting the proteolytic sites at biologically meaningful concentrations, in particular to the epitope overlapping S2’, will likely require more sophisticated immunization strategies than those employed in our study. The remaining resources from this small two-month project were redirected to participate in a study that compared the B.1.1.7, B.1.351, and P.1 spike proteins with regard to neutralization by therapeutic antibodies and by sera from vaccinated individuals as well as with regard to their functional characteristics such as cell entry and cell-cell fusion. Here, we could demonstrate that all spike variants promoted efficient cell-cell fusion, but we indeed found slightly reduced cell-cell fusion activity of the B.1.351 and P.1 spike proteins in the absence of TMPRSS2 activation, but not of B.1.1.7 compared to the ancestral SARS2-S. While these effects were not particularly pronounced and may be specific to our experimental system, our findings are in agreement with the notion that B.1.351 and P.1 spike proteins are primarily immune escape variants and are probably slightly less functional under some conditions, likely as a trade-off for immune escape. This interpretation is compatible with the fact that neither B.1.351 nor P.1 were able to outcompete e.g. B.1.1.7 on the population level in mostly immune-naïve populations.
Publications
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SARS-CoV-2 variants B.1.351 and P.1 escape from neutralizing antibodies. Cell. 2021 Mar 20:S0092-8674(21)00367-6
Hoffmann M, Arora P, Groß R, Seidel A, Hörnich BF, Hahn AS, Krüger N, Graichen L, Hofmann-Winkler H, Kempf A, Winkler MS, Schulz S, Jäck HM, Jahrsdörfer B, Schrezenmeier H, Müller M, Kleger A, Münch J, Pöhlmann S
