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Microbes & Immunity Understanding lung development, health, and diseases

influenza result in the loss of epithelial cells. This infection
triggers the activation of many progenitor cells related to a
particular compartment of the lung. In particular, alveolar
type 2 cells respond to Wnt signaling and begin to grow
rapidly during the first 2 – 3 weeks following infection,
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contributing to the restoration of alveolar function.
4.2. Non-infectious injury
Lung injury can also be caused by chemicals such as
naphthalene and polidocanol, as well as gases like sulfur
dioxide. Special pollution-resistant Clara-like epithelial
(CE) stem cells have been demonstrated to survive in
the terminal bronchioles following naphthalene-induced
damage that impairs the normal progenitor (Clara)
cells. The study concluded that due to their resilience to Figure 4. Placenta-gut-lung triad. This illustration represents the
hypothetical interplay among the placenta, gut, and lung. Placental
pollutants and their role in regenerating and renewing signaling promotes early lung development and initiates microbial
the terminal bronchiolar epithelium, CE cells constitute a priming. Postnatally, the gut–lung axis facilitates bidirectional immune
crucial stem cell population for lung repair. 52 and microbial communication. Together, this triad supports the
coordinated maturation of mucosal immunity. Image created with NIH
5. Placenta-gut-lung triad BioArt.
The placenta-gut-lung triad highlights the overlapping
developmental and functional relationships among these According to a 16S rRNA sequencing study, the
organs. Lung morphogenesis is directly impacted by the healthy lung microbiota at the family level is dominated
placenta through its regulation of immunological and by Firmicutes, Bacteroidetes, Proteobacteria, Fusobacteria,
hormonal signals, as well as its provision of oxygen and and Actinobacteria, while at the Operational Taxonomic
nutrients. Unit level, Prevotella, Veillonella, and Streptococcus species
are most prominent. 56
The foregut is the common embryonic organ that gives
rise to both the lung and the gut. To generate the two initial Changes in lung microbiota have been implicated in
lung buds, the foregut endoderm specifically evaginates chronic lung diseases, such as asthma and COPD. In fact,
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and extends into the surrounding mesenchyme. As the the beneficial lung bacteria may also enhance the efficacy
interface between mother and fetus, the placenta facilitates of radiotherapy by reducing radiation-induced damage.
the exchange of nutrients and is necessary to preserve a The organs in the respiratory system are in charge of
healthy intrauterine environment. Through the lung- absorbing oxygen and releasing carbon dioxide. The upper
gut axis, the gut, which houses diverse microbiota, plays and lower respiratory tracts make up the two functioning
a modulatory role in lung immunity. Figure 4 provides a sections of the respiratory system. The lungs facilitate the
schematic description of the placenta-gut-lung triad. transport of oxygen from inhaled air into the bloodstream
and the removal of carbon dioxide from the blood into the
6. Lung microbiota in health and diseases air. Given the lung’s extensive surface area and continuous
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The term microbiota first appeared in the early 1900s, when exposure to the external environment through breathing,
it was discovered that a vast number of microorganisms, they are constantly exposed to foreign substances.
such as bacteria, yeasts, and viruses, cohabit in diverse parts However, it is surprising that despite this exposure and the
of the human body. In addition, the human microbiota, volume of air inhaled daily, inflammatory responses within
popularly known as “the hidden organ,” contributes about the lungs are relatively rare.
150 times more genetic information than the entire human 6.1. Lung microbiota in maintaining homeostasis
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genome. The United States National Institutes of Health
launched the Human Microbiome Project (HMP) in A state of immunological tolerance is established and
2007, which catalyzed a surge in research on the human maintained by the lung microbiota, which stops superfluous
microbiome. The primary purpose of the HMP was to and damages inflammatory responses to minor stimuli.
discover and characterize microbiomes at critical locations Microbial compounds derived from the lung
across the human body. This review focuses specifically microbiota are detected by pattern recognition receptors
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on the lungs. in the lung, which promote the differentiation of naive T

Volume 2 Issue 3 (2025) 52 doi: 10.36922/mi.7719