Neutrophil survival is prolonged in the airways of healthy infants and infants with RSV bronchiolitis

Abstract

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-B 4 , 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 w3 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. Respiratory syncytial virus (RSV) is responsible for annual epidemics of respiratory disease affecting the whole population [1-3] and particularly infants of whom the majority are infected during their first winter [1,4,5]. Of these, 20-30% develop lower respiratory tract symptoms and 0.5-2% are hospitalised with RSV bronchiolitis [1,2,4,5]. It is increasingly recognised that RSV is a major cause of respiratory morbidity in the elderly [6,7]. Several prospective studies have also indicated that lower respiratory tract RSV infections in hospitalised [8] and nonhospitalised [9] infants, result in an excess of recurrent lower respiratory tract symptoms in subsequent years. These symptoms decline progressively over the first decade of life but the cause for this excess morbidity remains unclear [10].There has been much speculation regarding the nature of the host/virus interaction [3,11] and more than 30 yrs ago it was proposed that RSV bronchiolitis is a consequence of a specific immunopathology.Despite much research to demonstrate this, no distinct immunopathological process has been identified. The current authors have shown that neutrophils are the predominant inflammatory cell in the upper and lower airway in RSV infections in infants, accounting for y80% of cells recovered from the tracts [12]. These finding have been subsequently replicated by others [13]. Increased protease activity in airway secre...

Ciliary dyskinesia is an early feature of respiratory syncytial virus infection

“…This is consistent with the observation that neutrophils are the dominant cell type recovered from bronchoalveolar lavage in infants with RSV bronchiolitis [23]. Furthermore, neutrophils are thought to augment RSV-induced epithelial damage [24,25], and the absence of these cells in our system may explain the lack of observed cytopathology. Ciliated cell cultures also show an elevated production of the IFN-cinduced protein CXCL10 following RSV infection, which is consistent with previous studies [26].…”

In vitro modeling of respiratory syncytial virus infection of pediatric bronchial epithelium, the primary target of infection in vivo

“…Massive lung infiltration of neutrophils, monocytes, lymphocytes, and, to a much lesser extent, eosinophils is characteristic of severe RSV disease in infants (2,7,40,41). Chemokines released by RSV-infected epithelial cells are undoubtedly important in this infiltration.…”

Respiratory Syncytial Virus Activates Innate Immunity through Toll-Like Receptor 2

“…Neutrophils are innate leukocytes that traffic to sites of infection via chemokine gradients and greatly aid the clearance of virus-infected cells. The presence of neutrophils correlates with RSV disease severity and, further, these cells secrete inflammatory mediators, including reactive oxygen species and cytokines (i.e., , that further drive inflammation and indiscriminately kill both infected and uninfected tissues (4,24,52,53). Therefore, heightened TLR2 signaling may mediate neutrophil-associated airway inflammation through CCL2 production.…”