The ongoing worldwide COVID-19 pandemic is caused by the SARS-CoV-2 novel coronavirus, which obtained a mutation (D614G) early on that increased its transmissibility. The D614G strain is now the dominant strain worldwide, however, most vaccines in development were designed against the original (D614) strain. To predict if currently developed vaccines will be effective against the G614 strain, we must better understand its susceptibility to neutralization by antibodies.
A recent paper in Cell Host and Microbe compared D614 and G614 strains of SARS-CoV-2 and their respective susceptibilities to neutralizing antibodies.
This paper began by immunizing mice, rhesus macaques, and humans with mRNA-LNP vaccines. To generate and optimize their own vaccine for rodent and primate studies, the researchers relied on two TriLink BioTechnologies products. They utilized TriLink’s N1-methylpseudouridine-5’-triphosphate (N-1081) to modify their mRNA sequence, and TriLink’s CleanCap Reagent AG 3’Ome (N-7413) to co-transcriptionally cap the in vitro transcribed mRNAs. The latter product has been optimized for mammalian systems and produces the ideal Cap 1 structure. The capped, modified mRNA was then encapsulated in a lipid nanoparticle for immunization.
After immunization with the mRNA-LNP vaccines, serum was collected for pseudovirus neutralization assays. Through these assays, researchers demonstrated that the predominant G614 strain is consistently but modestly more susceptible to neutralization by a variety of antibodies compared to the D614 strain. This result was maintained across mice, macaques, and humans, as well as across the four different versions of mRNA that were used. Importantly, this means that the D614G strain should still be neutralized by the antibodies produced by currently developed vaccines.
The paper also offered a possible mechanistic explanation for the change in neutralization characteristics of the G614 strain. The D614G mutation is located in the spike protein S1 subunit, but outside of the receptor-binding domain. Using negative stain electron microscopy, the G614 mutation was found to have a conformational change in the spike protein. This conformational change was speculated to expose additional neutralizing epitopes on the receptor-binding domain for antibodies to bind to.
Overall, this study showed that the G614 strain of SARS-CoV-2 can be bound by the antibodies produced from immunization against the D614 strain. Additionally, the G614 strain has a conformational change in its spike protein that likely explains its modestly increased susceptibility to neutralizing antibodies in rodent, non-human primate, and human studies.
There were reasonable limitations to this work, as this paper only examined effects after mRNA-LNP vaccine administration, which is the vaccine platform utilized in both the Pfizer/BioNTech and Moderna vaccines that are currently being distributed worldwide. Rather than including live virus neutralization or animal challenge experiments, the researchers only conducted pseudovirus neutralization assays. However, a recent paper described a very high correlation between pseudovirus and live virus neutralization experiments, suggesting the pseudoviral experiments are sufficient to address neutralization. Finally, the structural microscopy did not use a fully native spike protein, and differences could potentially be elucidated if the native protein were to be studied.
As the pandemic persists, newer strains of SARS-CoV-2 have been identified and more will likely be discovered. Additional studies like this one will be needed to evaluate evolving strains for changes in transmissibility and neutralization susceptibility. These works will inform vaccine development and public health strategies as the world continues to combat COVID-19.