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. 2021 Sep 7;12:724033. doi: 10.3389/fphar.2021.724033

FIGURE 2.

FIGURE 2

Effects and mainly mechanisms of probiotics and prebiotics in the gut-lung axis and context of respiratory infections. Probiotics and prebiotics administered orally can improve dysbiosis and induce eubiosis in the host, leading to an increase in SCFAs directly (produced by probiotics) or indirectly (produced by commensal microbiota). Furthermore, probiotics can also reduce the burden and epithelial damage induced by intestinal parasites. The uptake of probiotics by DCs in the intestinal submucosa, and their migration to lymph nodes, induces the activation and proliferation of Th1, Th2, Th17, Treg, and B cells. Activated T cells and B cells produce cytokines and antibodies, enter the circulatory and lymphatic systems, and reach the lungs, where they will increase resistance to infections caused by viruses, bacteria, and fungi. The fermentation of prebiotics and production of SCFAs increases the number of DCs precursors in the bone marrow and increases CD8+ T cells activity, that confer protection against infections in the lung. The immunomodulation demonstrated after the administration of probiotics and prebiotics may be linked to the reduced viral titer, bacterial colonization, parasite load, and migration in the lungs. Probiotic-induced immunomodulation can increase the frequency of dendritic cells and CD4+ and CD8+ T cells in the lungs against infections by viruses and bacteria and can increase specific IgG and IgM antibodies to these pathogens. Also, the increase in Treg cells may be related to the reduction of inflammation-induced lung damage. In parasitic infections, probiotics have been linked with increased frequency of Th1 and concentration of IL-12 and IFN-γ, which may justify the reduction in the parasite load and larvae migration in the lung. Because there are no scientific studies that demonstrate the reduction of lung colonization by fungi after oral administration of probiotics, is still unknow if the antimycotic potential from probiotics metabolites, as shown in vitro, could be applied in an in vivo system.