When brain injuries stimulate the infiltration of immune cells, it may cause inflammation and tissue damage in the brain and impair patient recovery. Thus, the brain’s recruiting of immune system cells to the brain may cause more harm than good. Johns Hopkins researchers report that they have identified how brain cells trigger a response in immune cells when the brain is injured.

As described in Science Signaling, in a new study using mice, researchers described how after a brain injury, immune cells in the brain called astrocytes released vesicles (fat-like virus-size molecules) carrying proteins, RNAs, and microRNAs into the circulation. When these vesicles reached the liver, they stimulated the secretion of cytokines that mobilized peripheral immune cells to infiltrate the brain. Researchers examined what the signal was, and how, exactly, the signal got all the way to the liver from the brain, particularly considering the blood-brain barrier.

Identifying the pathway helped pinpoint ways to reduce brain damage brought on by the body’s own immune response. Inhibiting this communication between the brain and the liver could accelerate and improve recovery from brain injuries.

According to Norman Haughey, Ph.D., professor of neurology at the Johns Hopkins University School of Medicine, “exosomes” — miniature vesicles released from cells — form inside cell compartments and release outside the cell when these compartments fuse with the cell’s surrounding membrane. Exosomes are surrounded by bits of cell membrane and filled with proteins and different types of the genetic material RNA. Researchers found that white blood cells in healthy mice who received exosomes from the blood of mice with brain damage traveled to the site of brain injury, which demonstrates that exosomes released from the brain in response to damage alert the immune system to send the immune cell sentinels to the brain. When researchers stripped the vesicles of protein and their genetic cargo and injected them back into mice, the blood cells no longer went to the site of brain injury.