Since moving to Finland, I’ve become accustomed to asking guests whether they have any allergies before I prepare dinner. I grew up in the developing world where allergies and asthma seem to be much less common than they are here; in fact, various studies have found higher rates of allergy and autoimmune conditions in developed than developing countries. One explanation for this is the “hygiene hypothesis“, which proposes that excessive hygiene early in life can affect the development of the immune system and result in allergic conditions and autoimmune diseases in later life. In a recent study appearing in Science, a team of scientists in Germany and the United States present evidence supporting the hygiene hypothesis and the importance of an early challenge to the immune system.
The researchers tested this idea in mice, which are commonly used as a model system to study the human immune response; they compared the immune systems and responses of germ-free (GF) mice, which were completely free of any microorganisms, and specific pathogen-free (SPF) mice, which had normal gut microbiota but were free of pathogens. The researchers measured the level of invariant Natural Killer T (iNKT) cells, which are an important part of the immune system, and found that the germ-free mice had more iNKT cells in their colon and lungs than their SPF counterparts. In addition to playing a vital role in our immune response, iNKT cells have also been implicated in several autoimmune conditions; the GF mice were more susceptible to induction of asthma and colitis (an autoimmune bowel inflammation), perhaps due to the increased quantity of iNKT cells. Although allowing the adult germ-free mice to be recolonized by microbes didn’t reduce their iNKT levels or their susceptibility to asthma or colitis, recolonization of pregnant GF mice just before delivery did lead to normal iNKT levels and reduced susceptibility in their offspring. Simply having microbiota wasn’t enough; the microbes had to be present at the right developmental stage in order to properly regulate the immune response.
To confirm that the increased sensitivity was linked to iNKT cells, the team treated the GF mice with an antibody which blocked the molecule that normally interacts with iNKT cells in the immune response. Young GF mice treated with this antibody didn’t accumulate excessive iNKT cells in their colon or lungs; these mice grew up to be no more sensitive to colitis and asthma than the SPF mice.
The researchers were even able to show that the increase in iNKT cells was due to increased expression of a specific gene, Cxcl16. While Cxcl16 levels were similar in GF and SPF mice at birth, they increased specifically in the colon and lungs of GF mice later in life, implying that microbial exposure somehow provided a signal which determined the level of Cxcl16 in these tissues. Treatment of newborn GF mice with a CXCL16-neutralizing antibody led to normal levels of iNKT cells in the colon and liver as well as normal sensitivity to colitis or asthma induction. The overexpression of Cxcl16 wasn’t due to a genetic difference but rather to something called “methylation”; methylation is an epigenetic factor, or a method of affecting gene regulation without any change in the underlying DNA sequence. Cxcl16 was more highly methylated in GF mice, leading to higher expression levels; when methylations levels were reduced either chemically or by the introduction of conventional microbiota, Cxcl16 expression was reduced to normal levels.
Altogether, these findings tell quite a remarkable story. Early in life, the microbiota of a mouse are somehow able reduce the methylation of the mouse gene Cxcl16 in the colon and lungs, though the precise mechanism by which this happens isn’t identified in this study. In the absence of this regulation, higher methylation levels lead to an increased level of Cxcl16 expression. Increased amounts of CXCL16 result in the accumulation of iNKT cells in those tissues, making them more susceptible to autoimmune inflammations like asthma and colitis. Although there are differences between the immune systems of humans and mice, the iNKT cell system is very similar in the two organisms and the researchers predict that their findings will also be applicable to humans.
It’s important to realize that the challenge to the immune system of the SPF mice came from their own microbiota and not from a pathogen. I mentioned in an earlier post that each of us is really a vast ecosystem; results like these demonstrate the mutual importance of that ecostystem and the interactions which comprise it. This is likely to be a very active research area in the coming years, so hopefully we’ll learn a great deal more about the microorganisms that inhabit our body and the interactions that shape our biology. I also have to admit to a certain sense of satisfaction in seeing the hygiene hypothesis get support from such an excellent study, if only because it reinforces the notion that we are and are part of a complex, dynamic system in which well-intentioned changes such as dramatically improved hygiene may have unintended consequences.
Olszak, T., An, D., Zeissig, S., Vera, M., Richter, J., Franke, A., Glickman, J., Siebert, R., Baron, R., Kasper, D., & Blumberg, R. (2012). Microbial Exposure During Early Life Has Persistent Effects on Natural Killer T Cell Function Science (advance online publication)