Introduction
Respiratory syncytial virus (RSV) causes lower respiratory tract infections responsible for approximately 200,000 deaths per annum amongst infants worldwide [1]. While the impact of RSV bronchiolitis is greatest in early infancy1,2 it is also a major respiratory pathogen amongst those with chronic obstructive pulmonary disease3,4 and the elderly3. Despite the existence of only two major RSV sub-types and the lack of significant antigenic drift the annual epidemics of RSV infections are associated with frequent reinfections throughout life while there is still no effective vaccine despite this being listed as a research priority by the World Health Organization5,6. Following a natural infection antibody levels wane rapidly7-9, thus making individuals susceptable to re-infection throughout life. It appears likely that this poor long term memory response contributes to poor herd immunity and thus contributes to the annual epidemics of RSV infection10,11. Low levels of passively acquired maternal neutralising antibodies will predispose very young infants to developing significant lower respiratory tract infections (LRTIs) thus explaining the typical preponderance of the virus amongst those admitted with LRTIs in those under 6 months of age. Understanding how RSV can interfere with host immune responses will be key to addressing the pressing need for effective prevention and treatment5-12.
Dendritic cells (DCs) are key antigen presenting cells within the lungs and are critical for the induction of memory immune responses13-15. DC numbers increase significantly in the airways of both infants with RSV bronchiolitis16,17 and in mouse models of the condition18,19. Hence the impaired memory response is unlikely to be attributable to inhibition of DC recruitment. While RSV’s principal site of replication is the respiratory epithelium2,6,7 it has also been shown to infect monocytes, both in vitro 20-22 and in samples from infants with acute bronchiolitis15,16,23. The virus productively infects a proportion of cultured myeloid derived differentiated human DC21. Of note is the virus’s ability to replicate in these cells for several months before adopting a latent state from which replication can be reactivated some months later22. Given that the virus appears to target DCs and other monocytes it seems likely that this ability contributes to the impairment of long-term immune memory and hence to the success of the virus. Despite the potential importance of these findings little is known about the evolution of infection and persistence of the virus in monocytes, including DC, during a natural infection while there is currently no information regarding the type of DCs in which RSV might persist following a natural infection.
In order to better understand the interplay of RSV, monocytes (including DCs) and other immune cells through the course of an acute RSV infection and beyond, young mice were infected with a genetically modified strain of RSV designed to fluoresce when actively replicating22,24. The pattern of inflammatory response and identification of infected cells was determined at various time points from infection through the acute infection and following complete clinical recovery. Multi-parameter flow cytometry was used to track fluorescing RSV and identify co-localisation of the virus with any particular monocyte cell populations, including DCs, from digested lung tissue preparations. RSV viral load from infected lung tissues and lung inflammation measured in bronchoalveolar lavage were also measured in order to correlate this information with pulmonary monocyte infiltration and clearance patterns observed over the course of infection.