In times of stress, signals from the brain quell potentially damaging inflammation in other parts of the body. The process involves release of the neurotransmitter acetylcholine in the spleen, but the source of this chemical in the organ has been unclear, as the nerves that link the spleen to the brain cannot make it. So Kevin Tracey at the Feinstein Institute for Medical Research in Manhasset, New York, and his team looked for other sources. They homed in on a specific group of immune cells, called 'memory' T cells, in the spleens of mice that make acetylcholine when the vagus nerve, which connects the brain to many internal organs, is stimulated.

Stimulating the vagus nerve lowered the production of the inflammatory protein called TNF-α in the spleens of normal mice, but had no effect on mice lacking T cells. This was resolved by implanting the acetylcholine-synthesizing T cells into these mice.

Science 10.1126/science.1209985 (2011)