Infection by RNA viruses is detected by the host through Toll-like receptors or RIG-I-like receptors. Toll-like receptors and RIG-I-like receptors signal through the adaptors MyD88 and MAVS, respectively, to induce type I IFNs (IFN-I) and other antiviral molecules, which are thought to be essential for activating the adaptive immune system. We investigated the role of these adaptors in innate and adaptive immune responses against respiratory syncytial virus (RSV), a common human pathogen. Deletion of Mavs abolished the induction of IFN-I and other proinflammatory cytokines by RSV. Genome-wide expression profiling in the lung showed that the vast majority of RSV-induced genes depended on MAVS. Although Myd88 deficiency did not affect most RSV-induced genes, mice lacking both adaptors harbored a higher and more prolonged viral load and exhibited more severe pulmonary disease than those lacking either adaptor alone. Surprisingly, Myd88 ؊/؊ Mavs ؊/؊ mice were able to activate a subset of pulmonary dendritic cells that traffic to the draining lymph node in response to RSV. These mice subsequently mounted a normal cytotoxic T-lymphocyte response and demonstrated delayed but effective viral clearance. These results provide an example of a normal and effective adaptive immune response in the absence of innate immunity mediated by MAVS and MyD88.adaptive immunity ͉ RSV ͉ IFN

Inflammatory mediators play a major role in the pathogenesis of respiratory syncytial virus (RSV) infection. The objective of this study was to evaluate the effect of i.v. dexamethasone on cytokine concentrations in tracheal aspirates (TA) of children with severe RSV disease and to correlate them with disease severity. Twenty-five cytokines were measured in TA obtained from children <2 yr old intubated for severe RSV disease, and enrolled in a double-blind study of i.v. dexamethasone (0.5 mg/kg; n = 22) vs. placebo (n = 19). Cytokine concentrations, measured at baseline and days 1 and 5 post-randomization using a multiplex assay, were compared within both treatment groups and correlated with: (i) tracheal white blood cell counts, (ii) tracheal RSV loads by culture and (iii) parameters of disease severity, including number of days of requirement for mechanical ventilation, intensive care unit (ICU), and hospitalization. At baseline interleukin (IL)-13 and IL-15 concentrations were significantly higher in the dexamethasone treatment group. On day 1 post-treatment, only MCP-1, eotaxin and IL-6 concentrations were significantly different but higher in the placebo group. On day 5: IL-13, IL-7, IL-8 and MIP-1alpha concentrations were higher in dexamethasone-treated patients. In both groups MIP-1beta inversely correlated with the days of ventilator support; MIP-1alpha, MIP-1beta and eotaxin inversely correlated with ICU days; and IL-6 inversely correlated with hospitalization regardless of the treatment assigned. Systemic administration of dexamethasone did not have a consistent effect on TA concentrations of pro-inflammatory cytokines. This may help explain, at least in part, the lack of clinical benefit of steroid treatment in children with severe RSV bronchiolitis.

Several studies have described a clear association between respiratory syncytial virus (RSV) lower respiratory tract infection in infancy and the subsequent development of persistent wheezing in children. Using the mouse model we demonstrated that RSV induces long-term airway disease characterized by chronic airway inflammation and airway hyperreactivity (AHR). The RSV murine model offers great advantages to study the immunopathogenesis of RSV-induced long-term airway disease. Mice can be challenged with aerosolized methylcholine to determine the presence of AHR. We can apply the reverse transcription-polymerase chain reaction assay (RT-PCR) to detect RSV RNA in the respiratory tract and we can perform lung gene expression analysis to further characterize the chronic changes induced by RSV infection. Compared with sham-inoculated controls, RSV-infected mice developed chronic airway disease characterized by AHR and persistent airway inflammation. Forty-two days after RSV infection, a time point when RSV could no longer be isolated, RT-PCR demonstrated, quite unexpectedly, the presence of RSV RNA in the lower respiratory tract of mice. The presence of genomic RNA persisted for months after inoculation. Furthermore, preliminary studies also demonstrated that on day 42 there were a number of genes differentially expressed in RSV-infected mice compared with controls. RSV-infected mice with persistent AHR exhibited presence of abnormal chronic inflammatory changes, altered gene expression profiles, and persistence of RSV RNA, which may contribute to long-term airway disease induced by RSV. Future studies are needed to define the significance of persistent RSV RNA in the mouse model, and its potential role in the pathogenesis of RSV-induced persistent wheezing in children.

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