Respiratory syncytial virus (RSV) is the leading cause of hospitalization in infants and young children worldwide. In addition to the acute morbidity, the association between RSV lower respiratory infection (LRTI) and the development of recurrent wheezing has been established in several well-controlled prospective studies conducted in different parts of the world.1–3 A recent case-cohort multicenter study conducted in Europe and Canada demonstrated that preterm infants who received anti-RSV prophylaxis with palivizumab had a significant lower risk of subsequent recurrent wheezing at 24-month follow-up compared with untreated matched-controls.4 Among children admitted to the hospital for severe RSV bronchiolitis, more than 30% will develop persistent wheezing up to 13 years of age which may extend into early adulthood.5
Three theories have been proposed to explain the link between early RSV infection and subsequent wheezing: (1) a causal relationship in which changes induced by the virus early in life will alter the normal development of the infant's lung; (2) an “effect” relationship in which RSV infection early in life in infants with pre-existing abnormal immune response and/or airway function serves as a triggering event that leads to subsequent episodes of wheezing; or (3) a multifactorial relationship: the response to RSV will depend on both the host predisposition (genetic makeup,6,7 airway, and immune system maturation at the time of the infection) coupled with external or environmental factors.
Recent evidence derived from in vitro experiments,8 animal models,9–13 and even from studies in humans14–16 suggest that RSV may persist “latently” and/or at low level replication in immunologically privileged sites within the lung. On the basis of these observations, investigators began to examine whether the persistence of the virus could contribute to the long-term airway disease observed in children after RSV LRTI.
This article briefly summarizes some recent studies focusing on novel perspectives and strategies to understand the pathogenesis and consequences of long-term RSV infection. The following topics will be reviewed: chronic manifestations of RSV infection, evidence and significance of RSV RNA persistence, and the differences on gene expression profiles elicited by RSV in the mouse model.
RSV Persistence: Evidence in the Mouse Model
Our laboratory has established a mouse model of RSV lower respiratory tract infection that allows assessment of viral replication, disease severity, and characterization of the immune response during both the acute and chronic phases of RSV disease.9,10,17,18
RSV Induces Chronic Pulmonary Morbidity.
After clearing the virus and recovering from the acute disease, RSV-infected mice progressed into a chronic phase characterized by airway hyperreactivity (AHR) and persistent airway inflammation for up to 154 days after the infection. Methacholine challenge elicited a modest increase in AHR in sham-inoculated controls. However, the magnitude of the response was significantly elevated in RSV-infected animals for up to 154 days after infection.9,17 The persistent AHR observed was associated with the presence of a chronic inflammatory mononuclear infiltrate in the lungs of RSV-infected mice, which was quite different from the infiltrate observed during the acute phase of the disease. On day 154, the inflammatory infiltrates were localized mainly around the larger central vessels and airways and consisted of lymphocytes, macrophages, and scattered plasma cells. Sham-inoculated mice showed rare, small lymphocytic infiltrates unchanged over time.17 These findings provide a histologic correlation to the abnormal pulmonary responses documented by plethysmography.9,17
In addition, the administration of an anti-RSV neutralizing monoclonal antibody significantly reduced RSV replication, which was associated with parallel reductions in lung inflammation and concentrations of proinflammatory cytokines, as well as significant modulation of pulmonary function abnormalities, including long-term AHR, compared with untreated controls.10,18
RSV RNA Persistence: An Unexpected Observation.
To determine whether the virus could be detected when it is no longer identified by plaque assay cultures, bronchoalveolar lavage (BAL) and lung specimens from mice inoculated with live and inactivated RSV were obtained during the acute and chronic phases of the disease and assayed for the presence of RSV RNA using a real-time reverse transcription polymerase chain reaction assay (RT-PCR) targeting the N gene.9–11 In contrast to viral cultures, which were consistently negative on day 7 postinoculation, RT-PCR detected RSV RNA in infected mice up to day 77 postinoculation. RSV RNA persistence was independent of the mouse strain evaluated (BALB/c versus C57Bl/6). In addition, RSV RNA copy number and AHR significantly correlated on day 14 postinoculation.9
We have recently confirmed these findings in a new set of experiments in which mice inoculated with live RSV or RSV inactivated by heat or ultraviolet light were followed over time to measure RSV load by culture and RT-PCR, as well as pulmonary function. Live RSV induced acute disease and long-term AHR, and was associated with persistence of RSV RNA. However, there were no RSV RNA detected nor development of AHR in mice inoculated with inactivated virus.11 Whether this correlation only reflects a similar time course of these 2 variables, or indeed suggests a possible association between the persistence of RSV RNA and long-term airway disease, is an intriguing possibility that will require further studies.
Gene Expression Profiles Correlate With Disease Severity in Experimental RSV-Induced Chronic Airway Disease.
We have recently applied microarray analysis to further understand and characterize the impact of RSV infection at the gene expression level. Whole lung RNA was extracted from RSV-infected mice and sham-inoculated controls and analyzed during the acute and chronic phases of the disease.19 RSV induced significant changes in lung gene expression compared with sham-inoculated controls. We observed that the gene expression patterns identified changed over time. Indeed, on day 42, which represents the chronic phase of the disease, statistical analysis identified 13 genes significantly overexpressed in RSV-infected mice compared with controls that strongly correlated with the AHR observed.19 This initial analysis provides additional evidence of the ability of RSV to induce chronic changes in the lower respiratory tract. It also shows the value of a genomics approach to identify markers that correlate with disease severity and at the same time to potentially provide novel insights into the pathogenesis of this infection.19
Among the many complex factors that contribute to the development of asthma in children, viral respiratory infections, in particular infections with RSV, have been clearly associated with an increased risk for recurrent wheezing.2,3
Despite the significant efforts aimed to facilitate the understanding of the pathogenesis of RSV-induced chronic airway disease, many questions remain unresolved. Studies in humans and in animal models have shown that interventions directed at reducing viral replication with anti-RSV neutralizing antibodies resulted in improvement of acute disease severity and long-term pulmonary abnormalities. Recent observations in the mouse model have demonstrated the persistence of RSV RNA in the lower respiratory tract, which coincides in time with chronic pulmonary findings. It is possible that the recently recognized RSV persistence could also occur in children. If that is the case, it may contribute to the long-term pulmonary abnormalities observed in children after RSV LRTI and open the possibility of new preventive and/or therapeutic interventions for those patients with viral-associated recurrent wheezing.
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