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

Neutrophils play a dual role in respiratory syncytial virus (RSV) infection, contributing to both viral clearance and lung damage. Analysis of peripheral blood-derived neutrophils from infants with RSV admitted to the Paediatric Intensive Care Unit (PICU) at Great Ormond Street Hospital (GOSH) highlighted myeloperoxidase (MPO), a hallmark of neutrophil degranulation, as a key indicator of disease severity when compared to control infants. The aim of this study is to investigate MPO as a functional readout of therapeutic efficacy. To mechanistically investigate these clinical observations, we developed a physiologically relevant in vitro model of the paediatric airway. Differentiated paediatric airway epithelial cells (AECs) were cultured at air-liquid interface (ALI), with or without an underlying layer of vascular endothelial cells. 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. To evaluate the therapeutic modulation of neutrophil activation, two antiviral compounds-remdesivir (RDV) and RSV604 were evaluated. While both compounds significantly reduced RSV viral load at 24 hours post-infection, only RSV604 attenuated MPO expression. RDV had no measurable effect on MPO levels, suggesting limited efficacy in mitigating neutrophil-driven inflammation. These findings highlight MPO as a biomarker of RSV severity and a functional readout of therapeutic impact. Targeting MPO-driven neutrophil activation may be key to reducing both viral load and inflammation.