RESPIRATORY SYNCYTIAL VIRUS-INFECTION OF HUMAN PRIMARY NASAL AND BRONCHIAL EPITHELIAL-CELL CULTURES AND BRONCHOALVEOLAR MACROPHAGES

In adults, clinical symptoms caused by respiratory syncytial virus (RSV) are usually confined to the upper respiratory tract, whereas RSV infection in infants frequently causes bronchiolitis and pneumonia. The preferential localization of RSV infection to the upper airways may partially be due to protective immunity, but may also depend on a difference in susceptibility of epithelial cells from upper and lower airways, or on antiviral activities of bronchoalveolar macrophages (AM). In this study, we have compared the susceptibility of primary adult human nasal epithelium, primary adult human bronchial epithelium, a human bronchial epithelial cell line (BEAS-2B), and adult human AM to infection with RSV. The cell cultures were infected with multiplicities of infection (moi) of 1 and 0.1. Virus release into the supernatants was assayed at days 1, 2, 4, and 7, and the percentage of virus-positive cells determined by immunofluorescence at the same time points. Similar proportions of nasal epithelial cells (NE) and bronchial epithelial cells (BE) were infected with RSV. Approximately 50 to 75% (with moi 1) and 2 to 10% (with moi 0.1) of the cells were infected by 24 h; almost all the cells were RSV positive by day 4. However, BE released less infectious RSV than do NE. With moi 0.1, 10-fold less virus was released over 4 days of culture. By days 4 to 7, cytopathic effects (CPE) were maximal in all epithelial cell cultures, but CPE developed latest in BE infected with moi 0.1. AM were also productively infected with RSV, with peak virus production at day 2. AM released 13-fold less virus than BE and 46-fold less virus than NE at this time point. When the amounts of cell-associated virus in AM and BEAS were compared, the epithelial cells contained 100-fold more virus than AM. In contrast to all the epithelial cells, infection of AM appeared to be restricted to the initially infected cells as determined by the constant proportion of virus-positive cells throughout the time course of in vitro culture. Based on these observations, we conclude that at least under the conditions of primary culture both adult human NE and BE are highly susceptible to RSV infection. However, the NE release more infectious virus than do BE. Although AM are infected by RSV, virus replication and spread by reinfection are restricted in these cells, suggesting that AM are probably not important in spread of virus and may play a protective role in RSV infection of the bronchoalveolar region of the lung.