Finding an antibody less affected by variants

Administering messenger RNA (mRNA) encoding the broadly neutralizing antibody Ab401 before infection reduced weight loss and deaths in mice infected with SARS-CoV-2 or a bat-derived sarbecovirus. Published in PNAS, this preclinical study tested the potential of delivering genetic instructions instead of antibody protein, following antibody discovery and structural analysis.

Antigenic changes in SARS-CoV-2 can alter sites recognized by existing antibodies and reduce their neutralizing activity. The researchers targeted a spike region relatively conserved across several sarbecoviruses. Their goal was to identify candidates for preventive approaches for immunocompromised people who may not mount an adequate vaccine response, and to prepare for future human infections caused by animal viruses.

The team isolated 20 lineages of broadly neutralizing antibodies targeting the receptor-binding domain (RBD) from samples collected over time from one participant vaccinated after infection. The five antibodies with the broadest neutralization each neutralized all 18 pseudoviruses in the panel, which included SARS-CoV-2 variants such as XBB.1.5 and JN.1, as well as other sarbecoviruses. These results apply to the viruses tested in the panel and do not mean the antibodies work against every sarbecovirus.

Protective effect of Ab401 administered two days before infection

A 2.6 Å crystal structure of the Ab401–RBD complex showed that the antibody recognizes a conserved class 1/4 site. The antibody’s heavy-chain complementarity-determining region 3 (CDRH3) loop forms hydrogen bonds with the RBD. This binding pattern provides a structural basis for recognition of multiple sarbecoviruses.

The animal experiments used K18-hACE2 mice expressing human angiotensin-converting enzyme 2 (ACE2). The researchers administered either Ab401 protein at 20 mg/kg or lipid nanoparticle (LNP)-encapsulated mRNA encoding the antibody’s heavy and light chains at 30 μg per mouse, both by the retro-orbital route. Controls received empty LNPs; each treatment–virus combination included six mice. Two days later, the mice were infected intranasally with 1 million plaque-forming units (PFU) of either SARS-CoV-2_WA1 or the bat-derived BANAL-236 virus.

Immediately before infection, median serum Ab401 concentrations were 92 μg/mL in the protein group and 18 μg/mL in the mRNA group. Plasma neutralizing titers were also lower in the mRNA group. The evidence therefore does not support interpreting mRNA delivery as more potent than protein administration. Through day 10 after WA1 infection, all six mice in the protein group and five in the mRNA group survived. Both antibody groups maintained their pre-infection body weight, while all control mice died within nine days.

After BANAL-236 infection, both antibody groups maintained their weight and survived. This virus was less pathogenic than WA1 in this model. Control mice had substantial weight loss, but only one died. The two infection conditions therefore cannot be considered equally lethal.

Expression and durability remain to be tested before human use

The results show that mRNA-LNPs could be an alternative way to deliver a broadly neutralizing antibody. The ability to update candidates by changing the antibody sequence may also offer a development advantage. However, this experiment did not quantify reductions in manufacturing costs or benefits for storage and distribution.

Protection was demonstrated with pre-infection dosing and a 10-day post-infection observation period. Therapeutic efficacy after infection, long-term protection, and safety and preventive efficacy in people require separate evaluation. The researchers noted that low expression had been an obstacle when moving mRNA antibody delivery to nonhuman primates and people. Antibody discovery also began with one participant, so further studies must determine whether the same response can be elicited in a diverse range of people.

The study design and results are reported in the PNAS paper and the provided open-access full text.