Now researchers explain why that early encounter is good: it helps curb inflammation by altering the amount and function of immune cells.
The work also offers evidence for the ‘hygiene hypothesis,’ which proposes that the increase in asthma and other inflammatory diseases is due to our reduced exposure to microbes – thanks to antibiotics and antibacterials.
A team led by Harvard’s Dennis Kasper and Richard Blumberg show that bacterial communities help regulate quantities and functions of certain immune cells – called invariant natural killer T cells (iNKT) – in mice.
- Germ-free mice that were raised in sterile environments had higher amounts of iNKT cells in their colons and lungs.
- They were also more sensitive to asthma and more susceptible to colitis, an inflammation of the colon.
- Recolonizing these germ-free mice with different populations of bacteria lessened the symptoms of colitis and asthma, and also kept iNKT cell counts low.
- BUT this was only the case if mice were exposed to bacteria as newborn pups. Exposing adults to microbes didn’t reverse the inflammation – showing how early exposure to microbes has lasting effects on the immune system’s sensitivity to allergies and asthma.
Turns out, the germ-free mice also had an increase in the expression of CXCL16, a protein linked to inflammation. Nature News explains:
An analysis of the gene encoding CXCL16 showed that five regions of the gene were hyper-expressed in germ-free mice owing to DNA methylation – the tacking on of molecules to the DNA strand which can alter the production of particular proteins.
They concluded that, without exposure to certain microbes, methylation increases CXCL16 expression, which ultimately increases iNKT cell numbers and inflammation.
"These studies show the critical importance of proper immune conditioning by microbes during the earliest periods of life," Blumberg says in a news release. "Also now knowing a potential mechanism will allow scientists to potentially identify the microbial factors important in determining protection from allergic and autoimmune diseases later in life."
The study was published in Science last week.
[Via Nature News]
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