Desired Turbulence? Gut-Lung Axis, Immunity, and Lung Cancer

Figure 2

Proposed pathways of the gut-lung interaction. Microbiota and its products that enter intestinal mucosa (blue arrows) are phagocytosed and transferred to mesenteric lymph nodes (MLN) by antigen presenting cells (APC), where they stimulate priming of the T and B cells. Once activated, with the expression of proper homing receptors, these cells can migrate back to the original site (intestinal mucosa) (black dashed arrow) or to distal locations such as the lung epithelium and lung nodes through lymphatic and blood circulation. There, they can directly act on their target or continue to stimulate other immune cells. On the other hand, bacterial products from the intestinal mucosa or surviving bacteria can also reach the lung by blood or lymphatics to stimulate the immune system in the same way as they would have done in the intestinal tract. Depending on the tissue prestimulation, type of stimulus, and local and general immunological status, the result can be positive, as effective bacterial clearance or antitumour activity, or overinflammatory response, promoting further tissue damage, pathogen colonization, and tumour progression. The same schema is proposed in the other sense, beginning with the lung mucosa and finishing with distal effect on the gut. Although not known for the moment, there is also a possibility that bacterial products of the lung microbiota can exert their effect in the intestinal mucosa, being delivered in the same way as explained above. APC: antigen presenting cell; DC: dendritic cell; GALT: gut-associated lymphatic tissue; IEC: intestinal epithelial cell; LLN: lung lymph node; MLN: mesenteric lymph node; SCFA: short-chain fatty acid. Colour legend (borders/arrows): blue: influence of gut on lung; orange: influence of lung on gut; black: mutual influence.