The gastrointestinal tract is made up of trillions of bacteria that are largely ignored by the body’s immune system. Why is it that the body’s immune system knows to ignore these beneficial bacteria that are so important for our ability to live a healthy life? The answer to this question could play an important role in understanding how to maintain a healthy gut and how to treat diseases. Scientists led by Gregory Sonnenberg at Weill Cornell Medical College may have answered this question in a study published last week in Science.
The researchers studied T cells, cells that are made in the thymus and are trained there to kill-off foreign microbes and other intruders that make their way into the human body. But why don’t these T cells attack helpful bacteria in the GI tract? They found that the T cells are again educated in the gut to not attack beneficial bacteria but when this education is disrupted, it can lead to disease. For example, inflammatory bowel diseases like Crohn’s disease and ulcerative colitis occur when the immune system attacks the GI tract and bacteria in the GI tract.
In the thymus, T cells that could attack the body are destroyed before they are released into circulation. In the gut, a type of cell called innate lymphoid cells (ILCs) educate the T cells to not attack beneficial bacteria. These ILCs had previously been found to make a physical barrier between the bacteria in the gut and the immune system.
In mice, they found that ILCs attacked T cells that were destroying beneficial bacteria and when they prevented this attack by ILCs on the T cells, severe intestinal inflammation resulted. They also looked at intestinal biopsies of young patients with Crohn’s disease. In the biopsies they found that the ILCs lacked specific molecules that are important for educating the T cells not to attack the bacteria in the gut. They found that a decrease in this molecule correlated with an increase in pro-inflammatory cells in children with Crohn’s disease.
The authors state that it may be possible to get rid of these T cells that are causing the inflammation and by doing so you may be able to help treat the disease. By restoring this molecule (Major Histocompatibility Complex class II) that is preventing the education of the T cells, pro-inflammatory T cells may be reduced resulting in reduced intestinal inflammation.