FIGURE 1.

Potential mechanisms on aspiration of oral microorganisms and their cell components and products into the lower respiratory tract to modulate immunoreaction and facilitate adherence and colonization of respiratory pathogens. Indigenous oral microorganisms, as well as their cell components and products such as lipopolysaccharide, peptidoglycans, enzymes and toxins in saliva, may be inhaled into the lower respiratory tract including the lung. Here, we classify the reactions of the lower respiratory tract to invaded oral microbes and their cell components and products into allergy and inflammation. In allergy reaction mode, take C. albicans (C. a) for example, the fungus and its lysates are reported to induce IL-5 and IL-13 production by Th2 cells. IL-5 activates eosinophils which is related with airway eosinophilic inflammation. IL-13 induces goblet cell hyperplasia resulting in increased mucus production and smooth muscle cell hyperplasia and hyperactivity which are prominent pathological features of allergic reaction. In inflammatory reaction mode, typical oral microbes such as F. nucleatum (F. n) and P. gingivalis (P. g) could induce IL-8 and IL-6 production, or soluble TNF receptors, TNF-α, IL-1ß and IL-6 production, respectively, both contributing to inflammation because of the subsequent differentiation, activation and recruitment of neutrophils. TNF-α also induce goblet cell hyperplasia and mucus hypersecretion. Moreover, C. albicans (C. a) infection could induce Th17 CD4⁺ occurrence, whose main functions are the differentiation, activation and recruitment of neutrophils, which contribute to the inflammation and destruction of the lung tissue. In addition of effects on immunoreaction and inflammation, inhaled cell components and products of oral microbes (for example, P. gingivalis) may enhance adherence and colonization of respiratory pathogens on respiratory mucosa, mainly via three ways: (A) modification or upregulating expression of the mucosal epithelium receptors by specific enzymes or by other cell components and products. (B) clearance of the mucous layer that covers the receptors resulting in exposure of surface receptors. (C) the salivary film that protects against the colonization of pathogenic bacteria is destroyed by hydrolytic enzymes.