Review Article

From the Role of Microbiota in Gut-Lung Axis to SARS-CoV-2 Pathogenesis

Figure 1

Relationship between the gut microbiota and lung immunity: the interaction between the intestinal commensal bacteria and establishing of lung immunity is mediated by various factors, including PAMPs, PRRs, SCFAs, intestinal integrity, and immune cells of the lamina propria. In a normal state, DCs are continuously sampled from the lumen through M-cell activity, extension of dendrites, and the gut barrier function, which determine bacterial/PAMP translocation. After DC sampling, these cells migrate to GALT and then MLN to regulate differentiation and homing of lymphocytes (T and B cells) depending on the released certain cytokines in respect to gut microbiota composition. The activated T and B cells are distributed in the lungs via circulation. Also, the levels of CCL20 and CCL17, which are produced by the lungs after microbial exposure, contribute to imprinting of T cell subsets, based on the cognate CCRs. Furthermore, SCFAs can penetrate into the bone marrow and influence lung immunity by affecting MDP differentiation to inflammatory or anti-inflammatory immune cells. Inflammatory macrophages and DCs in the lungs are derived from CDPs and Ly6C+ inflammatory monocytes. Alternatively, activated macrophages (AAMs) are anti-inflammatory immune lung cells, derived from Ly6C- patrolling monocytes subtypes.