n e w s m a k e r s : A Dreaded Bull's Eye

by Jill Aitoro

fter the discovery of Lyme disease in the mid-1970s, people dutifully checked for ticks and the dreaded bull’s eye pattern on the skin indicating infection. A successful vaccine developed two years ago was good news, but recently a look at the composition of the Lyme bacteria Borrelia burgdorferi showed that the vaccine could stand an upgrade.

"The biology of what goes on inside the tick has been ignored," says Aravinda M. de Silva, assistant professor of microbiology and immunology. Past research had revealed that the bacteria change their surface proteins when transmitted from the tick to the mammal, so de Silva and Jun Ohnishi, a postdoctoral fellow from Japan, wanted to examine the process of such changes during transmission. To do that Ohnishi infected a colony of ticks and put the ticks on mice.

"Inside the tick the bacteria all had one major protein on the surface," de Silva says. But when the tick started to feed and the bacteria made its way from the gut to the salivary glands, the surface proteins changed unpredictably.

The study revealed that the protein used to develop the current vaccine was found on the surface of the bacteria still inside the gut of the tick but was not on the majority of the bacteria after the change took place during transmission. The fact that the vaccine was effective was a stroke of luck, de Silva says. So, if the vaccine doesn’t contain the same proteins as the bacteria entering the mammal, why does it work?

"The transmission from the tick to the mammal takes 48 hours," de Silva says. During the first 36 hours, the bacteria are still in the gut of the tick. In the meantime, the tick is sucking blood into the gut, where the unchanged protein is still present. The mammal is never infected because the mammal’s vaccinated blood kills the bacteria while still in the tick.

he problem is that such transmission-blocking vaccines are less effective after about a year. Antibody levels go down, as does protection from the disease, unless booster shots are injected. "Conventional vaccines work so that when exposed to the bacteria, the immune cells remember that you were vaccinated," de Silva says. In the current vaccine, there is nothing for the immune cells to remember because the bacteria were blocked inside the tick—there was never any exposure.

There are over 1,000 genes in each bacteria, of which 150 have the potential to produce proteins that localize to the surface of the bacteria. Of the 150, the researchers looked at three. They hope that looking at a greater number will reveal a surface protein that is constant among all bacteria entering the mammal.

If a constant surface protein could be added to the current vaccine, a person would have two lines of defense. Should the blood fail to kill the bacteria in the gut of the tick, de Silva says, the memory response of the antibodies would be triggered when they came in contact with the other surface protein as it entered the host.

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