An observed increase in the prevalence of certain autoimmune diseases has been linked to the lack of childhood exposure to microbes. A study by Dennis Kasper and Richard Blumberg at Harvard Medical School in Boston, Massachusetts, and their colleagues reveals a possible cellular mechanism for this 'hygiene hypothesis'.

The authors found that when they induced asthma or colitis in juvenile mice raised in a sterile environment, the animals had higher-than-normal levels of a type of immune cell called invariant natural killer T cells in their lungs or colon, respectively. These cells trigger inflammation and have been linked to ulcerative colitis and asthma. Moreover, expression of CXCL16, an inflammatory signalling molecule linked to the T cells, was also elevated in the lungs and colon, and seemed to be regulated by microbes.

Mice exposed to microbes as neonates, but not as adults, showed a decreased accumulation of the T cells, emphasizing the importance of early exposure.

Science http://dx.doi.org/10.1126/science.1219328 (2012)