Injuries to the head and spine can be devastating because nerves in the brain and spinal cord cannot regenerate. According to Israeli researchers, this lack of a repair mechanism may be a product of evolution.
"There seems to have been an evolutionary trade-off," says Prof. Michal Schwartz of the Weizmann Institute of Science. "Higher animals protected their central nervous system from invasion by the immune system, but paid the price by forfeiting their ability to regenerate injured nerves. Thus, an evolutionary advantage that protects the healthy brain turns into a disadvantage in the case of injury."
When tissue damage occurs, immune cells called macrophages are attracted to the injured site to fight infection. They remove damaged cells and release substances that promote healing. But the central nervous system in mammals is an exception: when damaged, it is not effectively assisted by the immune system.
According to findings by Schwartz and her associates, the mammalian central nervous system contains an active component that suppresses the macrophages. This results in relatively few macrophages being recruited to come to the site of central nervous system injuries. And the few that make it are often ineffective.
"We have shown that the immune system's assistance is just as vital for the repair of the mammalian central nervous system as it is for any other tissue," said Schwartz. "However, because of a suppressive mechanism which seems to have developed in the course of evolution, this assistance does not operate."
The researchers at the Weizmann Institute learned that they could "educate" immune cells by conducting experiments on rats. They removed rat macrophages and activated them by exposing the cells to injured sciatic nerves, which, as part of the peripheral nervous system, are capable of regeneration. The scientists then applied the activated macrophages to injured optic nerves, which, as part of the central nervous system, normally do not regenerate. But in the presence of the pre-activated macrophages in place, the optic nerves began to re-grow.
"The observation of the effects of activated macrophages on the injured nervous system is an interesting part of a very big picture," says Mary Ellen Cheung, program manager for Central Nervous System Trauma and Regeneration at the National Institute of Neurologic Disorders and Stroke and National Institute of Health.
"I think [the] observation that macrophages activated by sciatic nerve have a particular effect on the optic nerve, is very interesting and can be used as a part of the puzzle on the central nervous system injury," said Cheung. "But I think it is a very long way from using this type of technique directly in a clinical application."
Schwartz also admits that a treatment development may take many years. "Macrophages may be the missing link in the process of wound healing in the central nervous system," said Schwartz. "Transplanting activated macrophages into injured nerves may help the central nervous system overcome its failure to heal after injury."
SOURCE: FASEB Journal, September, 1996
