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.