IL1B and infection: The release of IL-1β from RSV infected U937 and BMDMs at 12 h–24 h post-infection was not due to cell death (apoptosis, pyroptosis or necrosis) because annexin-V and Propidium Iodide (PI) staining (representative of cell death) was not observed in RSV infected (at 12 h–24 h post-infection) U937 cells and BMDMs (data not shown).