Respiratory syncytial virus (RSV) infection is a major cause of bronchiolitis in infants while influenza A infection usually manifests as upper respiratory tract infection. We hypothesised that the immunological responses of infants to RSV infection and influenza A infection are different. This prospective study was undertaken to compare the cytokine responses during RSV and influenza A infection. Sera and nasopharyngeal aspirates (NPA) were collected from infants with a coryzal illness with or without wheeze who were admitted to the paediatric wards during 1998. Cytokines, adhesion molecules, RANTES, IgE and eosinophil cationic protein (ECP) were measured by enzyme linked immunosorbent assay or fluorescence enzyme immunoassay. The diagnosis of RSV and influenza infections was based on direct immunofluorescence and viral culture. Of the 39 infants studied, RSV infection was confirmed in 11 patients and Influenza A in 10 patients. All RSV patients and one influenza A patient had wheeze during the infection. The serum concentrations of interleukin (IL)-4 and IL-5, regulated upon activation normal T cell expressed and secreted (RANTES) and soluble intercellular adhesion molecule 1 (sICAM-1) in infants with RSV infection were significantly higher than those with influenza A infection (all P < 0.02). The concentration of tumour necrosis factor-alpha (TNF-alpha) in NPA was significantly lower in infants with RSV infection (P < 0.01). Conclusion: A predominant T helper cell type 2 cytokine and related immunological response was observed in infants with respiratory syncytial virus infection whereas a predominant pro-inflammatory cytokine response was observed in infants with influenza A infection. This may explain the different clinical manifestations of the two viral infections in infants.
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