Figure 1.

Potential mechanisms of Lactobacilli to modulate respiratory immunity via the gut-lung axis (1). Migration of activated immune cells and cytokines from mesenteric lymph nodes (MLNs) and intestinal lamina propria to the lung through the circulation (2). Some endocrine cytokines (such as TNF-α, IL-6) may migrate to the lung tissue through the circulation, and then alter immune environment of the lung (3). SCFAs affect bone marrow hematopoiesis and promote the conversion of macrophage and DC progenitors (MDPs) into Ly6C-monocytes, which reaches lung tissue and differentiates into anti-inflammatory alternatively activated macrophages (AAMs); AAMs inhibit chemokine CXCL1 production thus leading to reduced neutrophils recruitment in lung tissue (4). In the intestinal lumen, Lactobacilli or their components and production of metabolites (such as SCFAs) are taken up by intestinal epithelial cells and then enter to the lung via the circulation (5). Lactobacilli or their components from the intestinal lumen reach lung directly via microbreathing or esophageal reflux.