From the article:
Activation of the immune system caused mice to learn to run less on wheels in their cages – an activity they normally like. The mice resumed their normal activity when the action of interleukin-6, an immune hormone that carries “sickness” signals to the brain, was blocked.
“Our findings suggest that blocking the action of interleukin-6 might reduce depression symptoms, like fatigue or loss of interest in pleasurable activities, in people who are depressed and who have elevated levels of interleukin-6,” said Simon Sydserff, PhD, a senior research scientist at BrainCells Inc., who conducted the research while with AstraZeneca Pharmaceuticals.
Scientists previously observed that some people became depressed due to an immune response to illness or stress. Elevated levels of immune hormones like interleukin-6 have been found in some depressed patients who are otherwise healthy.