3.1 Modulation of RSV infection severity
In infants, RSV bronchiolitis severity is linked to a dysregulation of the immune response, related to a relative immaturity of the host defenses [3]. Recent literature suggests that prior or concomitant colonization of nasopharyngeal microbiota by potentially pathogenic bacteria may enhance the severity of the RSV infection [7,22,23]. A 17-site, multicenter, prospective cohort study of 1,016 infants (age <1 year) hospitalized with bronchiolitis, showed that during RSV infection a greater abundance of Haemophilus influenzae andStreptococcus pneumoniae in the nasal microbiota can be associated with more severe disease (figure 2A) [22]. In a prospective observational study, during four consecutive seasons, 106 young children with a first episode of RSV infection and 26 healthy control subjects were enrolled [7]. Three groups of children less than 2 years of age were evaluated: a) with mild RSV disease (RSV infection), b) with severe RSV disease (RSV hospitalization) and c) healthy controls. Five nasopharyngeal microbiota clusters were identified, characterized by enrichment of eitherHaemophilus influenzae , Streptococcus ,Corynebacterium , Moraxella , and Staphylococcus aureus [7]. RSV infection and hospitalization were positively associated with Haemophilus influenzae and Streptococcus and negatively associated withStaphylococcus aureusabundance (figure 2A). Rates of PICU admission and length of stay tended to be lower in children within the Moraxellacluster than in children included in the other clusters and higher abundance of Moraxella was observed most often in outpatients [7]. Independently from microbiota cluster, children with RSV infection showed overexpression of IFN-related genes. However, in those with transcriptome profiles dominated by Haemophilus influenzaeand Streptococcus , an upregulation of genes linked to toll-like receptor and neutrophil-macrophage activation and signaling was also detected. Overexpression of Haemophilus and loss in commensals, like Veillonella , were described in nasopharyngeal aspirates of a cohort of 54 infants, younger than 6 months of age, hospitalized with RSV infection [24]. Transcriptome profiles dominated byHaemophilus was associated with increase IL-6 and CXCL-8 responses, cytokines that in respiratory virus infections are associated with more severe disease (figure 2A). [24-26]. Moreover, in human bronchial epithelial cell culture pretreated with Haemophilus influenzae , the release of IL-6 and CXCL-8 was synergistically increased after RSV, but not after HRV infection, buttressing the notion that this virus-bacterial interplay is specific for RSV [27]. The clinical implication of these observation is reflected in a multicenter prospective cohort study of 1,005 infants hospitalized for bronchiolitis. In this study the odds ratio for PICU admission was higher for subjects with RSV infection and Haemophilus -dominant profiles compared to RSV-infected infants with profiles dominated by other bacteria, such as Moraxella(figure 2A) [28]. Thus, specific nasopharyngeal microbiota clusters, dominated byHaemophilus influenzae and Streptococcus pneumoniae can modulate the host immune response, potentially affecting clinical disease severity. Further evidence for the ability of the nasopharyngeal microbiota to positively modulate the host immune response is the demonstration of the presence of specific metabolic profiles, linked to the microbiome composition, in children with severe bronchiolitis. In a multicenter prospective cohort study on infants <1 year of age, hospitalized with bronchiolitis, Stewart et al. determined microbiome profiles and metabolome in nasopharyngeal airway samples and their association with disease severity [22]. Among 254 metabolites identified, a panel of 25 showed high sensitivity (84%) and specificity (86%) in predicting the use of positive pressure ventilation (PPV) support. Metagenomic sequencing demonstrated that Haemophilus influenzae , Moraxella catarrhalis , and Streptococcus pneumoniae dominated the nasopharyngeal airway. Of these, however, only the relative abundance of Streptococcus pneumoniaecorrelated with the intensity of these selected metabolites to predict increased risk of PPV support (figure 2A). In the pathway analysis, sphingolipid metabolism was the sub-pathway most significantly enriched in infants with PPV use, and in turn, enrichment of sphingolipid metabolites was positively correlated with the relative abundance ofStreptococcus pneumoniae [22]. In contrast, the relative Moraxella abundance was negatively correlated with respective intensity of the same metabolites. Sphingolipids are linked to the inflammatory-mediated pathogenesis of airway diseases and play an important role in multiple aspects of viral replication, including the initial infection of mammalian cells, depression of host immune response, and assembly and budding of newly synthesized viral components [29]. Thus, curiously, Moraxella spp, thought to be associated with a general increase in infant morbidity [11,13], appears to have a “protective effect” on RSV bronchiolitis.