RESEARCH Vaccine study fuels debate over autism

Experts stressed that many questions remain about the preservative's effects.
KNIGHT RIDDER NEWSPAPERS
WALNUT CREEK, Calif. -- A new study links a mercury-containing preservative once prevalent in many children's vaccines to immune-system dysfunction in mice.
The findings are sure to heighten the debate over whether vaccines play a role in causing autism, especially since there is growing evidence that many autistic children have irregular immune systems.
The researchers released their results with an abundance of caution, stressing that numerous questions remain about thimerosal's effect on humans and its relationship, if any, to the recent rapid rise in the numbers of autistic children.
To use for more research
The study is a road map for future investigations of the effect mercury from a variety of sources has on developing immune systems, said senior author Isaac Pessah, a University of California, Davis, toxicologist.
"This is a clue, a lead -- I hope it will stimulate a lot more research," said Cindy Lawler, a program administrator for the National Institute of Environmental Health Sciences, which funded the study with the University of California, Davis, M.I.N.D. Institute.
Although no longer used widely in children's vaccines in the United States, thimerosal remains in some flu vaccines and over-the-counter products.
The Davis researchers removed dendritic cells from mice, then analyzed how thimerosal affected those cells in culture dishes. A dendritic cell acts as a sentry, helping to marshal the body's immune system against invading viruses or bacteria.
One dendritic cell can activate as many as 300 T-cells, the white blood cells that help find and kill attacking agents.
Findings
Among the study's findings:
UThimerosal dramatically altered dendritic cells by garbling their signaling system.
UAt lower levels, it caused dendritic cells to secrete abnormal levels of cytokine, a substance that initiates inflammatory responses.
UAt higher levels, it caused the death of dendritic cells, preventing them from maturing and activating T-cells.
UEven extremely small amounts of thimerosal interfered significantly with dendritic cell function after a few minutes of exposure.
UImmature dendritic cells were particularly sensitive to thimerosal.
Thimerosal has been used as a preservative in some vaccines and other pharmaceutical products since the 1930s.
Manufacturers use it to prevent fungal and bacterial contamination in multidose vials. It isn't needed in individually packaged doses, a more expensive method.
Took it out
Manufacturers began to remove thimerosal from children's vaccines in 1999 when the American Academy of Pediatrics and the U.S. Public Health Service urged them to do so as a precautionary measure.
The recommendation came after the Food and Drug Administration determined that children who received the full set of recommended immunizations could accumulate doses of mercury exceeding Environmental Protection Agency guidelines.
Today, most children's vaccines in the United States contain only trace amounts of the preservative. The one exception is the flu shot, which is available with and without thimerosal, although the thimerosal version is much more prevalent.
The preservative remains in some ear drops, nose drops, skin creams and cosmetics, as well as adult diphtheria and tetanus vaccines.
Anyone who takes a hard look at thimerosal wades into roiling political waters. While federal health officials insist it is safe, several thousand parents of autistic children have sued pharmaceutical companies, blaming them for causing their children's disorder.
Congressional leaders have entered the fray, arguing about whether to grant pharmaceutical companies immunity from such lawsuits.
Pessah made sure Tuesday he avoided aligning with either side in the debate. "We're just trying to do the best science we can," he said.
Many questions remain, said Pessah, who directs the University of California, Davis Children's Center for Environmental Health and Disease Prevention.
"I want to be clear that I'm not saying thimerosal is causing autism," he said.