There have been many reports on the correlation between decreased diversity of the intestinal microbiome in infants and in school-age children and the increased risk of allergic diseases, such as asthma and atopic eczema, ,as well as the therapeutic strategies aimed at modulating the microbiome [18–20]. Recently, Chiu et al. analysed the intestinal microbiome, the value of IgE in faeces, and the serum allergenic source, which were associated with allergic rhinitis and asthma in children. Furthermore, they found that the interaction between specific species of intestinal microbial dysbacteriosis and IgE-mediated allergen response might lead to early susceptibility to allergic rhinitis and asthma in children [21]. The total serum level thus forms the basis for atopic quality.
However, does the respiratory tract microbiome increase the risk of allergic respiratory disease by regulating IgE levels? In previous studies, the relationship between bacteria in the upper respiratory tract and allergic respiratory diseases, especially asthma, has been reported [22]. When the nasopharyngeal microbiome of children was assessed, the virus and bacteria species that caused acute respiratory tract infection were captured. Haemophilus, Morakot, Bacterias, Staphylococcus, Streptococcus, ectopic Clostridium, and Corynebacterium were found in the nasopharyngeal sample of infants during the first year after birth. These bacteria not only cause inflammatory reactions (such as fever) in the respiratory tract but also directly or indirectly promote persistent asthma [12]. Similar results were also reported for pharyngeal colonisation of Haemophilus, Streptococcus, and Moraxella, which increased the risk of acute asthma or aggravated asthma in children [23]. In addition, Streptococcus pneumoniae, Staphylococcus aureus, Moraxella catarrhalis, Pseudomonas aeruginosa, and Haemophilus influenzae were cultured from the sputum of patients with asthma [24]. These bacteria are also cultured during phases of exacerbations and clinical plateaus in asthma patients. Furthermore, the sputum microbiomes of patients with severe asthma were different from those of healthy individuals and patients with mild asthma, and the level of Streptococcus eosinophilus was especially different [25]. The pathophysiological mechanisms by which these organisms cause asthma are not well known [26]. Since the nasopharynx is thought to be a microbial reservoir associated with acute respiratory infections, its microbial composition is primarily similar to that of the upper airway. There are many studies on the microbiome of the upper respiratory tract in children, but few studies have investigated the microbiome characteristics of the LRT due to the difficulty in obtaining BALF samples from children. Comparing nasopharyngeal samples with BALF samples, it was found that species abundance and diversity of the microbiome in BALF were more abundant than those identified using nasopharynx samples. Actinobacteria species are more abundant in the nasopharynx, while Bacteroidetes is more abundant in the BALF. There are differences in the levels of all species, except for Streptococcus [13].
In this study, although the microbiome diversity in 35% of patients (24/68) was disordered, the remaining 65% of patients (44/68) showed the potential effect of the microbiome in the LRT on allergic respiratory diseases in children by analysing the correlation between the pulmonary microbiome and serum IgE in children undergoing bronchoscopy. Analysis results with PCA showed that there was a statistically significant difference (p < 0.01) between the AS group and the NAS group, suggesting that the LRT microbiome was correlated with TIgE level. Compared with the AS group, Streptococcus, Lactobacillus, and Anoxybacillus were more abundant in the LRT microbiome of the NAS group, with statistically significant differences, while Bacteroidetes was significantly higher in the AS group, which demonstrated that enrichment of Streptococcus, Lactobacillus, and Anoxybacillus in the LRT group may suppress allergy and Bacteroidetes-induced allergy. Previous reports have shown that respiratory infection or treatment with Streptococcus pneumoniae attenuates allergic immune responses and suppresses allergic airway disease by inducing regulatory T cells [27–29]. Furthermore, S. pneumoniae vaccination of asthmatic children and elderly patients reduced the number and severity of asthmatic exacerbations [29]. In contrast, the number of Bacteroidetes fragilis and Bacteroidetes intestinalis was reported to be significantly higher in Japanese cedar pollinosis (JCPsis) subjects than in non-JCPsis subjects before the pollen season, and symptom scores and JCPsis-specific IgE were also positively correlated with these bacteria [30]. In addition, diversity and decrease in the community composition of the microbiome were observed in the lower airway, which is associated with inflammatory phenotypes (Fig. 1). Recurrent cycles of infection-related inflammation of the LRT drive the pathogenesis of persistent wheezing in children.
All children were classified into NAS and AS groups according to the value of TIgE. Comparing the clinical indices of the two groups, the percentage eosinophils count (EO%), the absolute eosinophils count (EO#), and the CD16/56 level were significantly different in the two groups, which demonstrated the children in this study was effectively grouped, because EO%, EO#, and CD16/56 were all reported the meaningful indicators for allergy disease [31–33]. Then the microbiome, such as Bacteroidetes and Streptococcus and, showed positively or negatively correlated with TIgE. Therefore, the respiratory microbiome, as intestinal microbiome, may also be a fertile target for the prevention or management of diseases such as allergic asthma, which are characterised by adaptive immune dysfunction [9]. Using a model of respiratory dysbacteriosis in mice, it was found that respiratory tract microbiome imbalance improved the progress of allergic respiratory diseases, and this was by promoting the local production of IL-33. This information will help to further explore the pathophysiological mechanisms of allergic respiratory diseases and provide new ideas for the diagnosis and treatment of diseases [34]. IgE is most common immunoglobulin in atopic disease and plays an important role in mast cell degranulation and in initiating the T helper 2 (Th2) response. Asthma and atopic disease are typically associated with adaptive immunity with the overexpression of Th2. This is still a relatively new field of research. Although microbiome, such as Streptococcus and Bacteroidetes, are corelated with allergic sensitivity, the mechanism of the correlation between the microbiome of the LRT and early respiratory allergic diseases in children is not clear and needs to be studied further.