What's the connection between gut and lungs?

Disturbances in gut microbiota composition, influenced by genetic factors, diet, and antibiotic usage, can compromise local and systemic immune responses. This dysbiosis is associated not only with gastrointestinal inflammation but also with airway conditions like asthma and chronic obstructive pulmonary disease. The human body hosts a vast array of microbes, predominantly in the gut, estimated at around 38 trillion bacteria. While the gut microbiota is extensively studied, research on the lung microbiome is still developing. Variations between upper and lower respiratory tract microbiota challenge the relevance of upper airway samples in reflecting lower airway conditions.

The microbiome influences immune homeostasis by providing cues and metabolites for immune system maturation. Perturbations in microbial composition and function, known as dysbiosis, disrupt immune homeostasis and correlate with inflammatory diseases. Recent attention has focused on the role of short-chain fatty acids (SCFAs) in bone marrow hematopoiesis, particularly in myelopoiesis. SCFAs influence the differentiation of myeloid progenitors, contributing to an anti-inflammatory environment in allergic airway diseases and other inflammatory conditions. 

Moreover, hematopoiesis can occur outside the bone marrow, such as in the spleen or liver, during immune responses against pathogens. The gut microbiota, possibly through SCFAs, may promote myeloid progenitor hematopoiesis in the spleen. The gut–bone marrow–lung axis elucidates how gut microbiota and SCFAs regulate hematopoiesis, priming the immune system against infections and allergic responses. While promising for future therapies, further studies are needed to understand the complex effects of microbial metabolites on immune modulation at the level of hematopoiesis.