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Bloom labMarch 12, 2025 at 6:31 PM EDT

In study led by @csimonich.bsky.social & Teagan McMahon, we quantify antigenic evolution of RSV F Important because: 1️⃣ RSV top cause of infant hospitalization in USA 2️⃣ New antibodies & vax can prevent hospitalizations 3️⃣ Will virus evolution erode their efficacy? biorxiv.org

RSV has high burden in infants: top cause of infant hospitalization in USA, 2nd-leading cause of infant mortality globally A monoclonal antibody (nirsevimab) recently recommended for infants born in USA in RSV season. It effectively prevents hospitalizations pubmed.ncbi.nlm.nih.gov

RSV vaccines also now approved to protect infants (via maternal vaccination) & elderly But some viruses evolve to erode antibodies and vaccines. Will RSV do same? Worryingly, a Regeneron antibody failed phase 3 trials due to resistance in some RSV strains. pubmed.ncbi.nlm.nih.gov

To enable study of RSV antigenic evolution, we first developed a pseudovirus system to effectively measure neutralization targeting the F protein from recent human strains (prior assays mostly use lab-adapted strains).

With help from Alex Greninger, we validated that this pseudovirus assay measures neutralization titers that correlate well with live-virus assays (see below).

To quantify antigenic evolution, we chose F proteins from older and recent human strains of both RSV A and RSV B subtypes (shown on trees below), and generated pseudoviruses encoding these F proteins.

We tested neutralization of these strains by older human sera to quantify impact of F evolution on serum neutralization. As shown below, F evolution at most modestly affects serum neutralization---in contrast to evolution of influenza HA, which strongly erodes neutralization

However, RSV F evolution has escaped some monoclonal antibodies, and we validated that rare sporadic strains have mutations that escape the currently recommended nirsevimab antibody.

Interestingly, we found that the rare sporadic RSV strains with nirsevimab escape mutations also have reduced neutralization by human sera. Therefore, escape mutations selected by antibodies could have a moderate impact on polyclonal antibody immunity, and so merit monitoring.

Overall, RSV F antigenic evolution slower than for influenza hemagglutinin or SARS2 spike. However, natural mutations can escape antibodies & modestly affect sera. As antibodies become widely used, important to monitor for antigenic changes. Assay we describe here will help enable that monitoring New assays described here use single-cycle pseudoviruses, so enable study of F mutations w/o generating actual mutant viruses. Thanks to @csimonich.bsky.social & Teagan McMahon, Xiaohui Ju, Tim Yu, Natalie Brunette, Terry Stevens-Ayers, Michael Boeckh, @kinglabipd.bsky.social, Alex Greninger Final version of this paper is published in Journal of Virology: journals.asm.org If you are interested in implementing RSV pseudovirus neutralization assays, the plasmids are now available in AddGene (addgene.org) and we are happy to share the 293T-TIM1 cells.

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