Direct binding of respiratory syncytial virus to pneumococci: A phenomenon that enhances both pneumococcal adherence to human epithelial cells and pneumococcal invasiveness in a murine model

In a previous study we showed that pneumococcal adherence to epithelial cells was enhanced by a preceding respiratory syncytial virus (RSV) infection. RSV-glycoproteins, expressed on the infected cell surface, may play a role in this enhanced pneumococcal binding, by acting as bacterial receptors. In the current study, it was attempted to analyze the capacity of pneumococci to interact directly with RSV virions. By flow-cytometry, a direct interaction between RSV and pneumococci could be detected. Heparin, an inhibitor of RSV infectivity that interacts with RSV protein-G, blocked RSV-pneumococcal binding, indicating that the latter interaction is indeed mediated by protein-G. RSV-pneumococcal complexes showed enhanced adherence to uninfected human epithelial cells, compared with pneumococcal adherence without bound RSV, and this enhancement was also blocked by heparin. In addition, the significance of these findings in vitro was explored in vivo in a murine model. Both mice that were pretreated with RSV at day 4 before pneumococcal challenge and mice infected with both agents simultaneously showed significantly higher levels of bacteraemia than controls. Simultaneous infection with both agents enhanced the development of pneumococcal bacteraemia most strongly. It was hypothesized that direct viral binding is another mechanism by which RSV can induce enhanced pneumococcal binding to epithelial cells, a phenomenon that is translated in vivo by a higher invasiveness of pneumococci when administered simultaneously with RSV to mice. Apparently, RSV acts in this process as a direct coupling particle between bacteria and uninfected epithelial cells, thereby increasing colonization by and enhancing invasiveness of pneumococci.