Innate Immune Responses to Respiratory Syncytial Virus: Age-associated Changes

Respiratory syncytial virus (RSV) infection causes ~64 million cases of respiratory disease and 200,000 deaths annually worldwide, yet there is no broadly effective prophylactic or treatment regimen. RSV can produce acute respiratory illness in patients of all ages but strikes the age extremes, infants and the elderly, with highest frequency presumably due to innate immune deficiencies. A higher morbidity and mortality has been reported for the elderly above 65 years of age, which has been attributed to immune senescence. Efforts to generate an effective vaccine have thus far been unsuccessful. The innate immune system provides the first line of defense against viral pathogens with a repertoire of anatomical barriers, phagocytic immune cells, pattern recognition receptors (PRRs) and antiviral cytokines like interferons (IFNs). The precise mechanism of subversion of innate immunity in young and aged is poorly understood. A better understanding of innate immune pathways is expected to aid in the development of appropriate vaccines or prophylactics for these high-risk groups. Previously, the RSV nonstructural protein 1 (NS1) was shown to antagonize IFN responses by disrupting components of the innate immune system, although the mechanism is not well defined. We hypothesized that NS1 targets constituents of the PRR pathways to evade innate immunity and thus ensure viral survival. Using microscopy and co-immunoprecipitation assays, we found that NS1 localizes to the mitochondria and binds to the mitochondrially associated adaptor protein MAVS, thus preventing MAVS interaction with the RNA helicase, RIG-I. Expression of NS1 was also correlated with upstream IFN-response regulator, LGP2, and its expression was inducible in the absence of a viral infection. Tetracycline-inducible expression of recombinant NS1 in a cell model also promoted viral replication and emphasizes the key contribution of NS1 to RSV survival. Through this study, we demonstrated a mechanism for RSV NS1 in the disruption of early innate responses through mitochondrial localization and alteration of the RLH signaling. Whereas the above studies showed the importance RSV-induced innate immune pathways, whether the expression and signaling of innate immune pathways were adversely affected upon RSV infection in the high-risk groups remains unknown. Since elderly individuals are at an increased risk for severe bronchiolitis and RSV-induced pneumonia, often resulting in hospitalization and medical intervention and adaptive immune cell functionality and responsiveness reportedly decline with age, we hypothesized a similar age-related deterioration of the innate antiviral system. In this investigation, we used an aged mouse model to correlate age-associated changes in innate immune gene expression with RSV pathology. Of 84 antiviral genes examined, five genes including RIG-I, IFNAR1, TLR8, IL-1Β, and osteopontin (OPN) were associated with both age and infection. In response to RSV infection, aged mice had delayed induction of antiviral genes and diminished ability to secrete IL-6 in response to TLR7/8 agonist in primary alveolar macrophages. Lungs from aged, RSV-infected mice had increased cellular infiltration and prolonged infection as compared to young mice. In summary, age-related decline in expression and functionality of antiviral defenses were correlated with enhance RSV-induced lung disease in aged mice. In the absence of infection, aged mice chronically overproduced IL-1Β and OPN relative to young mice. Upon infection, aged mice had impaired ability to secrete higher levels of IL-1Β and mucus. In contrast, OPN secretion remained high and prolonged in aged mice throughout infection. The age-related decline in host antiviral gene induction and delayed cytokine production correlated with enhanced disease pathology. Using a transgenic strain of mice deficient in OPN (OPN-KO), we observed greater resistance to RSV and enhanced secretion of mucus, but unaltered cellular infiltration into the lungs. Therefore, OPN overproduction and defective mucus production likely contribute to pathology in aged mice. These findings demonstrate that RSV targets the innate virus recognition and antiviral cytokine activation pathways but also that the antiviral defense system is significantly affected by age. Consequently, efforts to generate vaccines or develop therapies that stimulate IFN induction may prove unsuccessful in the elderly given that RSV virulence factors and age weaken these responses. This study contributes to our understanding of how aging relates to the RSV subversion of the host antiviral response and should help with the development of better antiviral therapies suited to the growing elderly population