For years, the roots of depression have long been thought to be from outward sources like substance abuse, medications, loss of a loved one, or a major life-changing event. But studies are now showing that some people may be susceptible to depression due to an over-reactive immune system.
Scientists have found that when exposed to stress, the immune systems of some mice reacted by overproducing a substance called Interleukin 6. IL-6 is a cytokin, a molecule used for cell-to-cell communication that is important in immune response. Mice with spiked amounts of IL-6 tended to show more symptoms of depression than mice with non-reactive immune systems.
These findings could suggest hope for the nearly 121 million people who suffer from depression world-wide because scientists were able to successfully treat these mice.
Researcher Georgia Hodes says that stress can be thought of like an allergen, like tree pollen. Unlike other allergens, stress can trigger the immune system to cause symptoms of depression rather than itchy eyes and sneezing.
"In some ways, it is an analogy to an allergy," Hodes said. "You have something that is not really dangerous, but your body thinks it is, so you have this massive immune response. In this case, the stressor is what they're having this massive immune response to."
In 2000 and 2003, Finnish studies showed a link between allergies and depression and there has been research showing that people with severe allergies are twice as likely to develop depression. It was thought that allergy symptoms like fatigue, headaches and sleeplessness, could cause a slump in patient’s moods and could lead to strong holds of depression.
People with depression did show an increase in production of IL-6 but it was unclear whether it was the source or a symptom of the depression. Hodes and her team tried to answer that question by exposing young mice to old mice, a scenario certain to induce a great amount of stress because the older mice are aggressive and will pin down and even bite the younger mice.
After repeated exposure to high levels of stress, the scientists found that some mice had a normal amount of IL-6 in their systems while others had over-produced it. The mice that had an elevation of IL-6 showed social withdrawal as well as a reduction in the enjoyment of things they previously liked. They showed symptoms associated with depression in mice.
They then injected these mice with a drug that stops cytokins like IL-6 from traveling to the brain and found that the previously depressed mice began to act normally. The scientists finished by testing and verifying that the increase in IL-6 came from the immune system alone in order to isolate the cause of the elevation.
Researchers plan to present their findings at the annual meeting of the society for Neuroscience in New Orleans, on October 16th. They are currently working to find whether mice that are genetically modified not to produce IL-6 can be used as bone marrow donors to cure mice who are stress-susceptible.
The drugs used to block the cytokins in mice are currently available and are used to treat rheumatoid arthritis in humans. This research suggests they may be able to be used to successfully treat depression as well. This could very well be an incredible breakthrough in treatment for those suffering from depression.