Injecting a mixture of immune cells from chronically stressed mice into other mice positively affects their mood and behavior, a new study has found. Rather than exhibiting signs of depression, which was anticipated, the recipient mice actually became more social and even experienced a boost in cell growth in one area of the brain. According to the researchers, these surprising yet intriguing results could possibly help scientists identify novel ways for treating depression. The study has been published in The Journal of Neuroscience.
It’s been known for some time that both acute (short-term) and chronic stress have an impact on the immune system. While acute stress can actually stimulate the immune system, chronic stress may have dampening effects and can sometimes lead to depression.
While this much is known, the majority of studies investigating the relationship between the immune system and mood have focused on one particular branch of the immune system, the innate branch, and have neglected the adaptive immune system. The former refers to nonspecific mechanisms that constitute our first line of defense against infectious organisms, such as chemical and physical barriers like the skin. The latter refers to a more specific, targeted response that is called into play when pathogens evade the innate immune system, such as the production of neutralizing antibodies.
Since cells of the adaptive immune system retain memories of previous infections which make future responses towards a particular pathogen more efficient, scientists wondered whether these cells may also keep memories of stress and thereby affect the mood of the host. To test this hypothesis, scientists from the National Institute of Health in Maryland added an aggressive “bully” mouse into the cages of male mice. Two weeks later, the stressed-out, bullied mice displayed depression-like symptoms: They became withdrawn, unsociable and disinterested in the scent of female mice.
Next, they isolated adaptive immune cells from both the stressed mice and unstressed control mice and injected them into a strain of mice lacking these cells, essentially providing them with the adaptive immunity of the donor. They predicted that the mice receiving cells from the stressed mice would exhibit depression-like symptoms, but in fact they observed the opposite. Compared with controls, these mice became less anxious, more sociable and even displayed cell growth in a region of the brain involved in learning and memory.
Taking this one step further, they repeated the experiment but injected the cells into a particular strain of mice known for being shy and unresponsive. Remarkably, these mice showed a complete personality change, becoming active and curious.
At the moment, the researchers aren’t too sure why this is happening, although they postulate that adaptive immune cells may perhaps react to stress by developing mood-boosting capabilities, but how they do this is hazy. Furthermore, what is particularly puzzling is why the adaptive cells in the bullied mice don’t eventually enhance their mood, allowing them to cope with the stressful situation. It’s possible that it has something to do with the innate immune system, which could be interfering with the adaptive branch.
[Via New Scientist and The Journal of Neuroscience]