A new study of mice supports the idea that exposure to germs in childhood helps develop the immune system and thereby prevent allergies and other immune-related diseases such as asthma and colitis later on in life. Researchers at the Brigham and Women’s Hospital in Boston, US, led the study, a report of which is in the 22 March online issue of Science.

The “hygiene hypothesis” proposes that a lack of early childhood exposure to microbes increases susceptibility to certain diseases by suppressing development of the immune system. The new study not only supports this idea, but may also explain the whys and hows.

However, the researchers caution that they investigated mice, and this does not necessarily mean the same results would occur in humans.

The study was led by two senior authors, both from Brigham and Women’s Hospital (BWH). Dr Richard Blumberg is chief for the BWH Division of Gastroenterology, Hepatology and Endoscopy, and Dr Dennis Kasper is director of BWH’s Channing Laboratory.

Blumberg, Kasper and colleagues studied “germ-free” (GF) mice, that are bred in a sterile environment, without exposure to microbes, and specific-pathogen-free (SPF) mice raised in a normal laboratory environment.

They bred both mice to develop forms of asthma and inflammatory bowel disease (IBD) and compared their immune systems.

They found that the GF mice had more invariant natural killer T (iNKT) cells in their lungs and bowel, and developed more severe disease symptoms:

” … we show that, in germ-free (GF) mice, invariant natural killer T (iNKT) cells accumulate in the colonic lamina propria and lung, resulting in increased morbidity in models of IBD and allergic asthma compared to specific pathogen-free (SPF) mice,” they write.

iNKT cells help fight infection, but they can also fight the body’s own tissue, making it more susceptible to inflammatory diseases.

The researchers also found, when they exposed GF mice to germs in their first few weeks of life, they did not develop high levels of iNKT cells, and they did not develop the more severe symptoms seen in those kept germ-free.

However, if they left this until the GF mice were adults, it had no effect. So they concluded, for benefits to occur, the exposure to germs had to happen before the mice reached adulthood.

They also found the disease-protection the GF mice with early-life exposure to microbes received proved to be long-lasting.

Blumberg told the press:

“These studies show the critical importance of proper immune conditioning by microbes during the earliest periods of life.”

“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,” he added.

The researchers said their findings are a first step to understanding the global increase in allergic and autoimmunie diseases in urban settings.

However, only by doing studies on humans will we know if the same or similar mechanism is at work, with the same effect.

And, should that be the case, then we may see germ exposure in a new way: and find, perhaps, that contact with germs at the right time may actually be good for us.

Written by Catharine Paddock PhD