Neutrophil myeloperoxidase as a functional biomarker for RSV severity: implications for in vitro therapeutic screening

Abstract Respiratory syncytial virus (RSV) is a leading cause of severe lower respiratory tract infections in infants, yet effective therapeutics are lacking. The aim of this study is to develop an in vitro model that recapitulates key clinical outcomes in infants with RSV bronchiolitis to help accelerate the discovery of effective therapeutics. Neutrophil activation and influx into the airways are hallmarks of severe RSV infection, but these responses are difficult to quantify in clinical trials. Here we profile peripheral blood-derived neutrophils from infants with RSV admitted to the Paediatric Intensive Care Unit (PICU) and identify myeloperoxidase (MPO) as a key indicator of disease severity when compared to age matched controls. To mechanistically model this response, we established a paediatric airway epithelial air–liquid interface (ALI) system incorporating an endothelial layer and primary neutrophils to recapitulate the tissue microenvironment at the blood-airway barrier. Following RSV infection, neutrophil migration and activation were assessed using flow cytometry. The inclusion of an endothelial layer enhanced physiological relevance and more accurately replicated in vivo MPO responses. We then evaluated two antiviral candidates (remdesivir (RDV) and RSV604) to assess their ability to modulate neutrophil activation. While both compounds reduced viral load at 24 hours post-infection, only RSV604 attenuated MPO expression. These findings establish MPO as both a biomarker of RSV disease severity and a functional readout of therapeutic efficacy and demonstrate that targeting neutrophil⍰driven inflammatory pathways may be critical for reducing pathology in infant RSV infection. Take home message Antiviral drug discovery should include neutrophil MPO reduction as a readout of therapeutic efficacy. Graphical Abstract