Everyone knows that bacteria help digest our food, especially in the colon. Fewer people know that gut microbes are also important effectors of the immune system. They help stimulate cells in linings of our guts to produce antibodies to pathogens. The immune system recognizes and fights harmful bacteria, but leaves the helpful species alone. Intestinal bacteria have also been linked to asthma. It turns out that lack of exposure to intestinal bacteria in early childhood can lead to an increase chance of developing asthma. At least in mice.

From Science:

The tricky question is how microorganisms provide this protection. Mice lacking their normal microbial partners and pathogens have now given mucosal immunologist Richard Blumberg of Brigham and Women’s Hospital in Boston and colleagues an insight. Throughout their lives, these so-called germ-free mice dwell in sterile cages and nibble sterile food, so they don’t acquire the intestinal denizens of normal rodents. Compared with their microbe-populated relatives, such mice are more susceptible to colitis, a type of intestinal inflammation, and to asthma, the researchers have now found. In their lungs and intestines, the germ-free mice also harbor an unusually large number of invariant natural killer T (iNKT) cells. These immune cells trigger inflammation after sensing certain microbes or particular molecules, called antigens, made by the body.

The new study suggests that these cells are crucial for conditions such as colitis and asthma. Blumberg and colleagues discovered that genetically altered mice that lack iNKT cells are not prone to colitis, even if they are raised germ-free in sterile surroundings. Furthermore, Blumberg’s team could largely prevent the development of colitis in germ-free mice that do have iNKT cells if they treated the rodents with an antibody that blocks the cells’ ability to detect antigens.

By shifting germ-free mice to cages containing ordinary rodents that teem with bacteria, the researchers demonstrated the importance of early microbe exposure for the distribution of iNKT cells. Transferred mice quickly pick up intestinal bacteria from their cage mates. Moving adult germ-free animals did not reduce the number of iNKT cells in the colon. However, when the researchers rehoused pregnant germ-free mice, ensuring that their offspring would be immersed in bacteria from birth, the pups had fewer iNKT cells in the colon, even after they grew up, Blumberg and colleagues discovered. “It became clear from the study that [iNKT cells are] sensing the composition of the microbial community in the gut and responding to it,” Blumberg says.

Further Reading:

[Science] Torsten Olszak et al., “Microbial Exposure During Early Life Has Persistent Effects on Natural Killer T Cell Function”. Subscription required.