Scientists Suggest That Immune Cells Help to Maintain Cognition and Brain Cell Renewal

A team of scientists at the Weizm ann Institute of Science, led by Prof. Michal Schwartz of the Neurobiology Department, has come up with new findings that may have implications in delaying and slowing down cognitive deterioration in old age.

Until quite recently, it was generally believed that each individual is born with a fixed number of nerve cells in the brain, and that these cells gradually degenerate and die during the person's lifetime and cannot be replaced. This theory was disproved when researchers discovered that certain regions of the adult brain do in fact retain their ability to support and promote cell renewal (neurogenesis) throughout life, especially under conditions of mental stimuli and physical activity. One such brain region is the hippocampus, which subserves certain memory functions. But how the body delivers the message instructing the brain to step up its formation of new cells is yet unknown.

The central nervous system (CNS), comprising the brain and spinal cord, has been considered for a long time as "a forbidden city", in which the immune system is denied entry as its activity is perceived as a possible threat to the complex and dynamic nerve cell networks. Furthermore, immune cells that recognize the brain's own components("autoimmune" cells) are viewed as a real danger as they can induce autoimmune diseases. Thus, although autoimmune cells are often detected in the healthy individual, their presence there was perceived as an outcome of the body's failure to eliminate them. But Schwartz's group showed that these autoimmune cells have the potential ability – if their levels are controlled – to fight off debilitating degenerative conditions that can afflict the CNS, such as Alzheimer's and Parkinson's diseases, glaucoma, amyotrophic lateral sclerosis (ALS), and the nerve degeneration that results from trauma or stroke.

In their earlier research, Schwartz and her team provided evidence to suggest that T cells directed against CNS components do not attack the brain but instead, recruit the help of the brain's own resident immune cells to safely fight off any outflow of toxic substances from damaged nerve tissues.

In the present study, the scientists showed that the same immune cells may also be key players in the body's maintenance of the normal healthy brain. Their findings led them to suspect that the primary role of the immune system's T cells (which recognize brain proteins) is to enable the "neurogenic" brain regions (such as the hippocampus) to form new nerve cells, and maintaining the individual's cognitive capacity.

It was reported before that rats kept in an environment rich with mental stimulations and opportunities for physical activity exhibit increased formation of new nerve cells in the hippocampus.

In the present work, the scientists showed for the first time that formation of these new nerve cells play a role in this process they repeated the experiment using mice with severe combined immune deficiency (scid mice), which lack T cells and other important immune cells. Significantly fewer new cells were formed in those mice. On repeating the same experiment, this time with mice possessing all of the important immune cells except for T cells, they again found impairment of brain-cell renewal, confirming that the missing T cells were an essential requirement for neurogenesis. They observed that the specific T cells that are helping the formation of new neurons are the ones recognizing CNS proteins.

Schwartz points out that the role of the autoimmune T cells is not to affect the level of intelligence or motivation, but rather, to allow the organism to achieve the full potential of its brainpower. "These findings," she says, "give a new meaning to 'a healthy mind in a healthy body'.

Source:

Invention Intelligence, May - June 2006