Study shows how some AIDS vaccines may harm

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - Some viruses being used in experimental AIDS vaccines may damage the immune system by exhausting key cells, researchers reported on Thursday in a finding that may further cloud the field of HIV vaccines.

They said vaccines using the viruses should not be tested on people until more studies are done. But other vaccine experts said the findings, while scientifically interesting, were not a cause for immediate alarm.

The usually harmless viruses are used as so-called vectors to carry genetic material from the AIDS virus into the body so that the immune system can recognize and rally against it.

But the viruses, called adeno-associated viruses, may themselves be doing harm, said Dr. Hildegund Ertl, director of the Wistar Institute Vaccine Center in Philadelphia.

In mice, the adeno-associated virus, or AAV vaccines, directly interfered with immune cells called CD8 T-cells, Ertl's team reported in the Journal of Clinical Investigation. These are the "killer" T-cells that a vaccine is supposed to muster to fight HIV.

"The immune cells become exhausted," Ertl said in a telephone interview.

"It is simply a defense mechanism of T-cells -- if there is too much antigen for too long a time they simply turn themselves off."

Antigens are the proteins the immune system uses to recognize enemies such as bacteria and viruses. In the case of HIV, turned-off T-cells could leave a person more vulnerable than usual to infection.