The microbiome also important for immunity.
Not all pathogens are bad, some are good.
Emerging data over the last several years has unveiled important roles that commensal
bacteria, which are often referred to as good bacteria, play in human health.
The gut microbiome regulates immune cell populations.
When fermentable fiber, which is the fiber that passes undigested into the colon is consumed,
the bacteria break down some of the fiber for their own use.
The byproducts of this bacterial action are several short chain, fatty acids, including
butyrate, lactate, acetate, propionate, as well as other compounds.
Short chain, fatty acids are important for maintaining gut barrier function, but they
also play a special role in regulating cytokine production and immune cell populations.
The gut microbiome directly influences immune cells that reside in the gut and indirectly
influences immune cells outside of the gut via short chain fatty acids, including the
production of T-regulatory cells, which are immune cells that keep auto-immunity at bay.
Many studies have found that short chain fatty acids derived from the gut microbiome promote
the expansion of T-regulatory cells.
Reduced T-regulatory cell numbers have been found in severe COVID-19 patients.
And T-regulatory cells have been shown to help resolve acute respiratory distress syndrome.
So reduced T-regulatory cell numbers might lead to worse lung pathology.
Gut bacteria also use flavonoids found in fruits, teas, etc., to control the immune
response and protect against severe damage from the influenza virus.
The bacteria metabolize flavonoids, and produce a compound called diaminotetrazole, or DAT
for short, which helped produce interferon, a signaling molecule that activates the immune
system.
This helped protect the lungs against damage from the flu virus.
The DAT did not prevent the flu, but it did significantly reduce the severity of it.
Some of the main dietary sources of flavonoids include tea, citrus fruit, berries, apples,
and legumes.
Mice that were fed fermentable fiber specifically inulin had reduced mortality, better lung
function and less tissue destruction after exposure to influenza virus compared to mice
fed non-fermentable fiber, which was cellulose.
Short chain fatty acids derived from the fermentable fiber prevented neutrophils from infiltrating
lung tissue and this prevented lung tissue damage.
The short chain fatty acids also enhance CD8 T cell effect your function and viral clearance.
The microbiome also plays an important role during early life.
Several studies have shown that early exposure to microorganisms in the dirt, particularly
before the first year of life has been shown to significantly lower the risk of allergies,
wheezing, and asthma in children.
Early exposure to bacteria and certain allergens such as pet dander may have a protective effect
by shaping the child immune responses in such a way that prevent allergies and asthma.
This becomes very important in infants that have genetic predisposition to allergies,
eczema, or asthma particularly if a parent or sibling has one or more of those diseases.
One study specifically found that wheezing was three times as common among children who
grew up without exposure to such allergens compared to children who were exposed before
their first year of life.
Interestingly, exposure to allergens after the first year of life did not decrease the
child's risk of developing allergies or asthma.
Even exposure to certain species of bacteria in early life can have differential effects
on the immune system, particularly with respect to auto-immunity.
An imbalance between the so-called healthier, good bacteria and the pathogenic or bad bacteria
referred to as dysbiosis also plays a role in shaping the immune system.
A healthy or dysbiotic microbiota can influence the host innate immune system by our two types
of signals, microbial cell components and metabolites.
In a dysbiotic state, alterations in the signature of microbial molecules sensed by the host
can lead to different activation state of the immune system.
For example, animal studies have shown that certain species of bacteria in the gut promote
antibody responses to the influenza vaccine, TIV.
They do this through generating a toll-like receptor five signal.
In humans, it was shown that the same toll-like receptor five signal was correlated with a
higher antibody titer four weeks after the influenza vaccine.
In a way, the gut microbiome acts like a natural adjuvant, at least in animal studies.
Ablating the gut microbiome and animals has been shown to affect the gut epigenome and
transcriptional genome of lymphoid cells and the transcriptional genome of myeloid cells.
This occurs through tryptofan metabolites and short chain fatty acids generated via
the microbiome.