Depression and the immune system: recent research

Behavioral scientists have been studying depression and depressive symptoms for a long time, yet the etiology is still not fully known. There are many myths in public about what depression is, and what causes it. And it doesn’t help that the scientific community can’t come to an agreement either. But that’s what makes research so fun… so many problems to solve! Often depression is thought of as a purely psychological phenomena.  Research has shown however, that there is a mind-body connection in depression. Interestingly, the immune system might be the “body” part depression is related to.

When your hurt yourself, trip on a rock and scratch your knee, or get a paper cut, your immune system works to rebuild the damaged tissues. The immune system’s primary response to an invader or to tissue damage is to swarm it with immune cells and particles. This is called inflammation.  Your immune cells call in all the needed infantry, clear out the damage, and start rebuilding. So inflammation sounds like a good thing, right?

Dr. Lampkin presented this question on September 28th in the UCLA Health Psychology & Behavioral Medicine lecture series. His talked posed the question: Is inflammation always a good thing?

In 1991, Roger Smith presented the “macrophage theory of depression,” suggesting that in fact, inflammation might not always be a good thing because increased inflammation has a strong relationship with being depressed. The relationship between inflammation and depression has been studied extensively since then. In a 2009 meta-analysis published by Howren et al. in Psychosomatic Medicine, it was reported that there was a positive and reliable correlation between depression and inflammation, in all markers of inflammation (Il-6, IL-1, etc). Not only that, the relation was “dose-dependent” – meaning that the more severe the depression is, the more severe the inflammation levels were.

Lampkin’s research group was intrigued by this theory and used a ovarian cancer and mouse model to test it. Cancerous tumors are like wounds that never heal. Although they never heal, the immune system (and inflammatory actors) are always there to trying to figure out how to fix the tissue.  Ironically, those inflammatory pieces end up helping the tumor grow even more.

What Lampkin and his research group did, was surgically place an ovarian cancer tumor in mice. Their hypothesis was that since the tumor will increase levels of inflammation (measured by drawing blood), depressive symptoms in the mice should increase too. Now you might be thinking… how do you measure depression in mice? Well, by measuring the amount of interest they have in pleasurable activities, like drinking sugar water. If the mice before the tumor are happily drinking their sugar water, and then after the tumor, stop drinking they yummy sugar water, this indicates a loss of interest in pleasurable things. In humans, it’s a critical symptom of a depressive disorder. And in fact, Lampkin and his colleagues found that the mice with tumors did increase their depressive-like behavior more than the mice who did not have the tumor implanted. Hypothesis supported.

So could this mean that the 30% of cancer patients that are diagnosed with depression are not diagnosed just because they are distraught about their cancer, but actually because of the tumor itself? The results by this group suggest that as a possible answer. Stayed tuned for more work that will surely be done to test this hypothesis.

This post was based on a talk by professor Dr. Donald M. Lamkin titled “Cancer, Inflammation, and Major Depressive Disorder: Correlations and Experimental Evidence of a Connection.” Read more about this specific study here.