From The New York Times
By NICHOLAS WADE
Published: December 15, 2008
If you return from a trip abroad to find you have projectile vomiting, roaring flatulence, sulfurous belching and explosive diarrhea, the bad news is that you won’t die; you just have an attack of giardiasis, a form of purgatory devised by the single-celled parasite known as giardia.
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Giardia infections can linger for months because the parasite plays a cunning defense against the body’s immune system. In its genomic wardrobe, it has 190 coats to choose from. As soon as the immune system has generated antibodies against one coat, giardia switches to another. Because of the parasite’s persistence and infectivity, some 280 million cases of giardiasis occur in the world each year, the World Health Organization estimates, though most of these are in developing countries where people are more inured to the disease.
Giardia’s offensive game could have a fatal weakness, however. Biologists led by Hugo D. Luján at the Catholic University of Córdoba in Argentina have gained a striking insight into its coat-shuffling stratagem.
With this knowledge, they have accomplished a cunning counterploy: they have forced the parasite to make and wear all its coat proteins at the same time. This altered parasite, they hope, should serve as the perfect vaccine, because it immunizes the body to the full repertoire of giardia’s coat proteins all at once. The idea has worked well in animal tests, Dr. Luján said.
He thinks the same general approach — forcing expression of all coat proteins simultaneously — might help produce vaccines against the other protozoan parasites that rely on coat switching to dodge the immune system. These include malaria and the trypanosomes that cause sleeping sickness and Leishmaniasis.
Dr. Luján and his team have identified the mechanism by which giardia controls its coat proteins, they report in the current issue of Nature. Each of the parasite’s 190 coat genes is the recipe for making a different protein, and the parasite switches its coat every 10 generations or so. To produce the coat, giardia does not switch these genes on one at a time, as might be expected. Instead, it seems to leave them all turned on, allowing each to generate a messenger RNA copy of itself. Usually the messenger RNAs would direct the synthesis of proteins, but giardia then destroys all but one of the messengers, and the survivor makes the coat of the day.
To kill its messenger RNAs, giardia has adapted an ancient cellular system known as RNA interference. The system is designed to destroy foreign RNA, like that of invading viruses, so it was surprising to find it regulating a cell’s own RNAs, Dr. Luján said.
He proved this was the case by disrupting giardia’s production of enzymes, like those known as Dicer and Argonaute, that are components of the RNA interference system. With its RNA selection system out of business, giardia produces many — Dr. Luján believes probably all — of the coat proteins in its repertoire and inserts them into its outer covering.
He said he did not yet know how the organism shifted between coats but suspected that the RNA interference system favored whichever messenger RNA happened to be the most abundant at the time, and destroyed all others.
In an experiment that has not yet been published, Dr. Luján has tested gerbils, the laboratory animal often used in giardia work, with a vaccine consisting just of giardia with its RNA interference system blocked. “We saw complete protection,” he said.
Dr. Theodore E. Nash, a leading expert on giardia at the National Institutes of Health, said the new report was “a major advance in the field.” Since 1979, Dr. Nash has developed many of the methods to study giardia and its coat shuffling, several of which were used by Dr. Luján, who worked for five years in Dr. Nash’s lab.
Another giardia expert, Dr. Rodney Adam of the University of Arizona, said Dr. Luján’s work on giardia’s coat gene control was interesting “but not the whole story.” As for making a vaccine, he said that “this is not an organism to which natural infection will confer immunity.” People in developing countries may get one infection after another, although they do get a much less severe form of the disease.
Malaria also evades the immune system by switching its protein coat. Dr. Kirk Deitsch, an expert on malaria coat genes at the Weill Cornell Medical College, said Dr. Luján’s new finding “may be conceptually applicable to malaria,” although the malaria parasite does not use RNA interference and no one yet knows how to make it display all its 60 coat protein genes at once.
A human vaccine for giardia could be of great benefit if the many mild cases in the developing world do in fact undermine health. Some experts believe persistent giardia infection causes malnutrition, but others are less sure of this.
For the much smaller number of Westerners who are not inured to the disease, a vaccine would be a welcome addition to the few available drugs. It would have been a godsend for the Crusaders, who are known from historical accounts to have suffered terribly from a variety of intestinal diseases that had no respect for rank. In 1249 King Louis IX, who led the Seventh Crusade, had such serious diarrhea that part of the monarch’s breeches were cut away to ease his personal hygiene. Giardia may well have been his tormentor. Using a sensitive immunological test, researchers who excavated a medieval latrine in the city of Acre, once part of the Crusader kingdom of Jerusalem, detected the presence of giardia, they reported in the July issue of The Journal of Archaeological Science.
Giardia itself is far more ancient than any Crusader kingdom. Though a single-celled organism, it belongs to the eukaryotes, the domain that includes all plants and animals. In the tree of eukaryotic life, giardia belongs to one of the earliest branches. It lacks mitochondria, the energy-producing organelles that are almost a badge of eukaryote identity. Even stranger, each giardia cell possesses two nuclei; no one knows what benefit offsets the cost of maintaining the second. Before this enigmatic microbe plagued people, it was doubtless the scourge of many earlier species. Dr. Luján’s discovery may be a critical step in curbing giardia’s merciless torment of its fellow eukaryotes.