For researchers trying to untangle the roots of the current epidemic of asthma, one observation is especially intriguing: Children who grow up on dairy farms are much less likely than the average child to develop the respiratory disease. Now, a European team studying mice has homed in on a possible explanation: Bits of bacteria found in farm dust trigger an inflammatory response in the animals’ lungs that later protects them from asthma. An enzyme involved in this defense is sometimes disabled in people with asthma, suggesting that treatments inspired by this molecule could ward off the condition in people.

For researchers trying to untangle the roots of the current epidemic of asthma, one observation is especially intriguing: Children who grow up on dairy farms are much less likely than the average child to develop the respiratory disease. Now, a European team studying mice has homed in on a possible explanation: Bits of bacteria found in farm dust trigger an inflammatory response in the animals’ lungs that later protects them from asthma. An enzyme involved in this defense is sometimes disabled in people with asthma, suggesting that treatments inspired by this molecule could ward off the condition in people.

The study, published on page 1106, offers new support for the so-called hygiene hypothesis, a 26-year-old idea that posits that our modern zeal for cleanliness and widespread use of antibiotics have purged the environment of microorganisms that once taught a child’s developing immune system not to overreact to foreign substances. “This gives us a tantalizing molecular mechanism for understanding the epidemiological evidence,” says pediatric immunologist Stuart Turvey of the University of British Columbia in Vancouver, Canada, who was not involved with the new work. But others caution that the finding is probably far from the only explanation for why early exposure to microbes can make kids less allergy-prone.

About 20 studies in Europe and elsewhere have found that children raised on farms have relatively low rates of allergies and asthma. Some researchers suspect a key reason is that the kids breathe in air full of molecules from the cell wall of certain bacteria, called lipopolysaccharides for their fat-sugar structure. Also known as endotoxins, these fragments—from dying bacteria in cow manure and fodder—cause a temporary low state of inflammation in the lungs that somehow dampens the immune system’s response to allergens, the thinking goes.

Pulmonologist Bart Lambrecht and â€¨immunologist Hamida Hammad of Ghent University and Flanders Institute for Biotechnology in Belgium and their collaborators wanted to probe the mechanism for endotoxins’ protective effect. They first showed that injecting the molecules into the noses of 6- to 12-week-old mice every other day for 2 weeks protects the rodents from developing asthma later, in response to dust mites. In the endotoxin-exposed mice, the epithelial cells lining the lungs made lower levels of proinflammatory molecules called cytokines when they encountered the dust mites; the mice also had fewer dendritic cells, the immune sentinels activated by cytokines.

Continue reading at Science/AAAS.

Girls on farm image via Shutterstock.