Gun blasts naked-DNA vaccine into cell.

After a decade of ever more elaborate genetic engineering procedures -- snipping, stitching, and packaging DNA -- some vaccine researchers are finding that the simplest method works best. Just administer plain DNA to mice, they recommended, and the animals will muster an immune response strong enough to protect them from an otherwise lethal virus.

A research team led by Harriet Robinson of the University of Massachusetts Medical School in Worcester inoculated mice with purified DNA, which encodes a protein of the influenza virus. Between 67 and 95 percent of these test animals developed flu symptoms and then recovered, whereas 87 percent of the control animals, which had not been vaccinated, died.

Five different routes of inoculation conferred immunity, but by far the most efficient was to shoot DNA into the mice's skin with a gene gun. The scientists describe their study in the Dec. 15 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

"I think this is a powerful new technique," Robinson says. "It will allow us to make vaccine for diseases that we did not previously have vaccines for."

"It is a revolutionary approach in the sense of using DNA as a vaccine," comments Dominich Iacuzio, who directs the influenza program at the National Institute for Allergy and Infectious Diseases in Bethesda, Md.

At this time, though, the technique remains an experimental concept, both researchers caution. "It's promising but too early to tell how these animals results will related to a possible vaccine for humans," Iacuzio says.

A vaccine provokes the body's immune system to prepare for a later attack of a pathogen. Traditional vaccines consist of live, weakened viruses, dead viruses, or purified proteins. Live viruses carry the risk of reverting to a more active form, and dead viruses and proteins do not always achieve lasting immunity. With pure DNA, however, "you get the advantages of a [weakened] virus without the risk of [that] virus actually growing out," Robinson says.

That was not the mainstream opinion until now. "The whole idea of injecting DNA as a vaccine is contrary to anything that was previously envisioned or even allowed by the Food and Drug Administration," Iacuzio explains.

Besides safety concerns, researchers doubted whether DNA could enter cells in amounts sufficient to trigger a strong immune reaction.

Host cells take up very little DNA, Robinson acknowledges, but she suggested that immunity depends on quality, not quantity. Cells read the DNA and produce proteins that touch off a cascade of cellular interactions leading to stable immunity. The techique is so effective because the immnune systgem recognizes a viral protein manufactured by the body much better than one made outside, she adds.

Moreover, the skin and the linings of the nose and trachea possess unique patrol systems of lumphoid cells that detect foreign proteins when they enter the body and promptly inform the immune system of the invasion.

All of the five routes of inoculation tested by Robinson's team conferrred immunity: nose drops, injections -- into muscle, veins, and under the skin -- and a "shotgun" application to the skin. "We show for the first time that all these multiple routes work," Robinson says.

Tissues differ in DNA uptake and efficacy, the researchers noticed. The skin proved by far the most effective route. So rather than inject DNA and hope that enough of it would sneak into cells, Robinson's co-workers used a gene gun to blast DNA-covered gold particles into outer skin cells.

Thanks to the gun and the nearby lymphoid cells, the amount of DNA needed to immunize mice through the skin dwindled to 0.4 microgram, roughly one-thousandth the amount required by the other routes. "As the method gets more refined, we will need to apply even less DNA," Robinson predicts.

"The technique has tremendous implications, most immediately for the generation of new vaccines. As it outgrows its infancy, however, you will see it made into drug-delivery regimens," says David B. Weiner, an immunologist at the University of Pennsylvania School of Medicine in Philadelphia.

Using a similar approach, Weiner and his colleagues elicited an immune response against a type of human immunodeficiency virus (HIV) in macaques, they report in the Nov. 9 DNA AND CELL BIOLOGY.

The researchers inoculated monkeys with DNA of gp 160, a coat protein of HIV, the virus that causes AIDS. In response, to monkeys' immune system made antibodies that blocked HIV infection in laboratory-cultured cells. After four inoculations, 95 to 100 percent of of the macaques exhibited that response. The team has yet to enact these monkeys with HIV to test whether they are truly immune.

"Our study is the first to demonstrate a successful vaccination of nonhuman primates using a DNA vaccine, "Weiner says. Others may quickly follow suit. Several research teams are devising similar tests in monkeys.