Abstract
The immunopathology of respiratory syncytial virus (RSV) infection, the most common cause of lower respiratory tract infections (LRTI) in the pediatric population, with severe disease being the exception. The variability of the clinical presentation is incompletely explained by host, viral and environmental factors but, in infants and young children, disease severity is certainly linked to the physiological immune immaturity. There is evidence that the maturation of the host immune response is, at least in part, promoted by the composition of the nasopharyngeal microbiome that, modulating excessive inflammation, can counteract the predisposition to develop viral respiratory infections and lower the risk of disease severity. However, interaction between the nasopharyngeal microbiota and respiratory viruses can be bidirectional. Microbial dysbiosis can drive disease pathogenesis but may also represents a reflection of the disease-induced alterations of the local milieu. Moreover, viruses like RSV, can also increase the virulence of potential pathogens in nasopharynx, which is a main reservoir of bacteria, and therefore promote their spread to the lower airways causing superinfection. Negative changes in microbial community composition in early life may constitute a heightened risk towards severe RSV respiratory infection and bacterial superinfection, whilst specific groups of microorganisms can be associated with protection. A better understanding into the potential negative and positive role of the different nasopharyngeal bacterial species in disease prevention as well as into the possible benefits of microbiome therapeutic manipulation, may improve patient outcomes.
INTRODUCTION
Respiratory syncytial virus (RSV) is the main cause of acute viral respiratory infection leading to hospitalization in infants and young children worldwide [1]. The spectrum of its clinical manifestations ranges from mild upper tract illness to severe LRTI. The latter, requiring hospitalization in approximately 20% of the cases and of these ~15% require intensive care management [2]. RSV virulence and a variety of environmental factors contribute to increased RSV susceptibility, as well, specific host related conditions have been identified which increase the risk for severe disease [1-3], yet these are not present in most infants hospitalized with RSV LRTI. The mechanisms accounting for the wide variability in the clinical presentation of RSV infection are only partly understood, but recent insights suggest that modifications of the nasopharyngeal microbiota composition established or induced by a previous infection might be involved. In early life, the baseline system structure and function of the airway immunity is regulated by the local microbial milieu, and distinct microbiota have been associated with sequelae of specific viral and bacterial LRTI [4-6]. Recent insights suggest that modifications of the nasopharyngeal microbiota composition, established by a preceding infection, might be involved. Whether imbalance in the microbial community can drive disease pathogenesis or, conversely, reflects disease-induced alterations of the local milieu following respiratory infections is still to be elucidated [5,6]. A paradigmatic example is represented by the bidirectional interplay between resident nasopharyngeal bacterial communities and RSV in bronchiolitis. Modifications of the upper airway microbiota composition appears to be associated with RSV disease severity, at the same time there is evidence that RSV can increase the virulence of potential nasopharyngeal pathogens, a large bacterial reservoir, that can consequently spreads to the lower airways causing superinfection [7,8]. The current evidences on the epidemiologic link between RSV and nasopharyngeal microbiome, the mechanisms involved in these interactions, and the possible clinical consequences will be reviewed, summarized and discussed.
INFANT NASOPHARYNGEAL MICROBIOME IN HEALTH AND DISEASE
The airways represent a large surface area that interacts with the surrounding environment. They exhibit a gradient of bacterial load that is relatively high in the nasopharynx and substantially diminished in the lower respiratory tract [1]. A growing body of literature has demonstrated that the nasopharyngeal microbiome, with its mixed microbial communities, plays an important role in maturation and homeostasis of the host immune response in the upper and lower airways [1,4,5]. The balanced competitive/synergistic interplay between commensal and potentially pathogenic taxa, that is the hallmark of a healthy status, can be altered by acquisition of new pathogenic bacteria, increased virulence or prevalence of potential pathogens, and/or decreased efficiency of host defenses, the latter being the cause but also the result of dysbiosis [1,4,6]. In newborns and infants, obligatory nasal breathers, nasopharyngeal airways are both the first line of defense but also the main port of entry for bacteria and viruses. Overgrowth of virulent taxa will lead to symptomatic infection, with surrounding tissue invasion and inflammation. [9]. Therefore, in early life, microbiota composition of nasopharyngeal airways reflects exposures to the surrounding atmospheric environment which, shaping the host immune responses, will favor or inhibit the predisposition to respiratory disorders [1,4,10-12]. Nasopharyngeal colonization by environmental potential respiratory pathogens is established early in childhood [2]. In a study on 234 infants, it was shown that nasopharyngeal microbiome has a relatively simple structure, dominated by six clusters: Moraxella ,Haemophilus (Gram negative bacteria), Staphylococcus ,Corynebacterium , Streptococcus and Alloiococcus(Gram positive bacteria) (figure 1) [13]. Healthy nasopharyngeal samples collected around 2 months of age were dominated byStaphylococcus and Corynebacterium, but the frequency of these microbiome profile groups declined with age and, at 12 months of age, an increased prevalence of Alloiococcus and Moraxellaspecies (spp) was detected (figure 1A) [13]. Transient incursions ofStreptococcus , Moraxella and/or Haemophilusoccurred during acute viral respiratory tract infections,Streptococcus and Moraxella colonizing, respectively, 54% and 72% of the children by 1 year of age [14] and a progressive decline in Staphylococcus andCorynebacterium spp (figure 1B) [13]. Antibiotic treatments were associated with higher abundances of the potential respiratory pathogens Haemophilus , Streptococcus andMoraxella , with lower abundances of Alloiococcus andCorynebacterium (figure 1C) [13]. This microbial dysbiosis may predict subsequent respiratory infections [2,15-17]. Colonization with Streptococcus , Moraxella orHaemophilus spp was linked to an increased risk for LRTI, whilst nasopharyngeal microbiota dominated by Corynebacterium orAlloiococcus clusters was associated with less respiratory morbidity (figure 1D) [11-13]. When related to early exposure to noxious environmental factors negative changes in microbial community composition may constitute a heightened risk towards severe viral respiratory infections and bacterial superinfections [11,17]. This can be observed in severely premature infants admitted to neonatal intensive care units (NICU). These infants face the combined effects of deprivation of the normal intrauterine physical environment, supplemental oxygen with relative hyperoxia, barotrauma from mechanical respiratory support, exposure to nosocomial pathogens, and broad-spectrum antimicrobials [16]. They show abnormal nasal airway immune responses against respiratory viruses, persisting beyond the PICU period and, in comparison with full term infants, a higher nasopharyngeal microbial heterogeneity and within-group dissimilarity [18]. Emergence of a mixed flora, including Moraxella and other Gram-negative bacteria (Burkholderia , Neisseria and Janthinobacterium ), was detected. At phylum level, an increase in Proteobacteria (a major group of Gram-negative bacteria) was associated with a decrease inFirmicute (Gram-positive bacteria such as Lactobacillus ) (figure 1E) [14]. This airway microbiome signature has been recently described also in preterm newborns at increased risk of bronchopulmonary dysplasia [16,19]. The presence of within-group dissimilarity, identified also in the nasopharyngeal microbiota of otherwise healthy premature infants, suggests that this group does not develop a stable microbiome in early life. Longitudinal data showed that these prematurity-related microbiota features persisted during viral infection and played important negative role in modulating airway inflammatory and immune responses in this vulnerable group [16,19]. Upper airways colonization by potential pathogens in neonates, also in term infants, may be associated with increased susceptibility to LRTI during the first 3 years of life. A prospective birth cohort study of 411 children born to mothers with asthma, showed that neonatal hypopharyngeal colonization with Streptococcus pneumoniae , Haemophilus influenzae , orMoraxella catarrhalis , at age 4 weeks, was associated with increased risk of bronchiolitis, recurrent wheeze, asthma and pneumonia, during the first 3 years of life [20,21]. The viral etiology of bronchiolitis, i.e. whether it was RSV or human rhinovirus (HRV) related, was not reported in this study.
3. NASOPHARYNGEAL MICROBIOTA COMMUNITY COMPOSITION AND RSV INFECTION