Lung microbiome in healthy and diseased individuals

Figure 1 Ecological determinants of the lung microbiome. The respiratory microbiome is determined by three factors: Microbial immigration, microbial elimination (mainly in heathy individuals) and regional growth conditions (mainly in advanced lung disease) (adapted from Dickson RP 2015).

Figure 2 Lung microbiome disturbances following respiratory diseases exacerbations (adapted from Dickson RP 2014). The triggers like virus, allergens, pollutants initiate airway inflammation with activation of alveolar macrophages, neutrophils, eosinophils, dendritic cells, lymphocytes, which alters growth conditions of airway microbiota. Altered growth conditions result in a disturbed microbiome, which promotes further airway inflammation via pathogen-associated molecular patterns and pattern recognition receptor interactions.

Figure 3 Model of intestinal microbiome effects on lung immunology (adapted from Samuelson DR 2015). Microbes in the intestine is sampled by dendritic cells (DCs) either directly from the lumen or following translocation through M-cells to the gut-associated lymphoid tissue. A combination of signals from the microbes results in phenotypic changes in the DCs. DCs promote activation of various T-cell subsets within the mesenteric lymph nodes (MLN) and production of regulatory cytokines. Following the immune challenge in the airways T-cells activated in the gastrointestinal associated lymphoid tissue (GALT) and MLN move to the respiratory mucosa where they promote protective and anti-inflammatory responses. Production of various bacterial metabolites (e.g., SCFAs) also affects the gut-lung axis, as these products get to the lung, where they can alter the levels of inflammation. SCFA: Short chain fatty acid; IL: Interleukin; TNF: Tumor necrosis factor.