Smoking cigarettes is a habit well-known for its association with an increased risk for lung cancer, heart disease, and countless other conditions. Today, you can add one more to the list: inflammatory bowel disease. It may seem farfetched that disease in the lungs can lead to disease in the gut, but a new study from scientists at South Korea’s Kyung Hee University shows that these two regions are more connected than you might realize.
Previous findings have shown that smoking increases a person’s risk of Crohn’s disease, a type of inflammatory bowel disease, but the present study is the first to confirm a direct causative relationship between smoking and intestinal inflammation.
“Crohn's disease is more likely to occur in people with airway diseases, suggesting that inflammation in the lungs is linked with inflammation in the gut,” explained study researcher Hyunsu Bae.
Researchers began their study by exposing groups of mice to cigarette smoke, comparing any observations they made with a control group that was not exposed to smoke. As expected, mice exposed to smoke developed inflammation in the lungs. However, researchers also saw evidence of inflammation in the colon, along with blood in the feces and abnormally high levels of an immune cell called CD4+ T cells, which were also producing the pro-inflammatory protein interferon-gamma.
With the relationship between smoking and intestinal inflammation narrowed down to the involvement of CD4+ T cells and interferon gamma, researchers then asked: How do the effects from smoking reach the gut all the way from the lungs?
They repeated the same experiment except with one group of mice with few CD4+ T cells and one group that was unable to produce interferon-gamma. Neither group developed gut inflammation after smoke exposure, confirming the role of CD4+ T cells in causing gut inflammation after smoking.
Researchers performed one last experiment, again exposing mice to smoke. This time, though, they took from the lungs of the mice their population of CD4+ T cells and injected them into a different group of mice that had never been exposed to smoke. However, these mice developed colitis anyway, confirming that CD4+ T cells are directly responsible for colitis from cigarette smoking.
"Our results suggest that cigarette smoking activates specific white blood cells in the lung, which might later move to the colon, triggering bowel inflammation," explained study researcher Jinju Kim. "Smokers, especially those who also have bowel disease, should reduce their smoking."
Bae, Kim, and others are hoping that elucidating the direct relationship between smoking and inflammatory bowel disease could lead to new treatments and increased awareness among smokers concerning their risk of developing the disease.
The present study was published in the journal Frontiers in Immunology.