Neutrophil survival is prolonged in the airways of healthy infants and infants with RSV bronchiolitis.
Large numbers of neutrophils in the airway of infants infected by respiratory syncytial virus (RSV) are recruited by chemokines, such as interleukin-8, and specific inflammatory molecules can delay apoptosis increasing their longevity. The aim of this study was to investigate whether airway secretions in RSV bronchiolitis contain factors that influence neutrophil apoptosis. Nasal lavage fluid (NLF) was obtained from 24 infants with RSV bronchiolitis (31 infant controls and 12 adults). Neutrophils isolated from healthy adult volunteers were incubated with the NLF in Dulbecco modified Eagle medium (DMEM) for 24 h, and apoptosis and necrosis were quantified using Hoechst 33342 and propidium iodide viability dyes. The presence of putative factors that delay neutrophil apoptosis was investigated using inhibitors to leukotriene-B4, lipopolysaccharide and the IL-8 receptor CXCR2, and blocking antibodies to granulocyte-monocyte colony-stimulating factor. Characterisation of NLF involved tests of thermal instability, proteolysis, deoxyribonuclease digestion and molecular filtration. NLF from infants with RSV bronchiolitis and controls significantly delayed neutrophil apoptosis, whereas NLF from healthy adults did not. None of these inhibitor molecules blocked this delay in apoptosis but activity was heat liable and >3 kDa. The study showed that nasal lavage fluid from infants significantly delays neutrophil apoptosis. The speculation is that the prolonged survival of neutrophils in the infant airway contributes to the characteristic accumulation of neutrophils in the airways of infants with respiratory infections.