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Like subsurface ninjas, the cells of a particular fungus are slipping into the skins of amphibians worldwide, killing them, and now a new study hints at why this particular fungus has been so successful.
In 1998, a new species of chytrid fungus called Batrachochytrium dendrobatidis was identified. In recent decades, it has contributed to rendering dozens of frog species extinct, researchers think. They know the fungus inserts itself into the skin of frogs, drying out a layer they require to be hydrated, but just how the fungus renders its final fatal jab hasn't been clear.
"We know that amphibians have great immune responses," said researcher Louise Rollins-Smith, of Vanderbilt University. "So it's always been a puzzle why many species haven't been able to clear this fungus and survive."
Rollins-Smith and her colleagues proposed that B. dendrobatidis follows in the footsteps of other pathogenic fungi, blocking aspects of its host's immune system. To figure out which of the fungus's molecules could perform this trick, the researchers mixed fungal cells with amphibian immune cells, including those involved in amphibians' first and second lines of defense (its innate and adaptive immune responses, respectively).
They observed the fungus to have no effect on cells involved in the innate immune response, but to greatly inhibit the more tailored immune response carried out by B- and T-lymphocytes. This adaptive immune response is necessary to clear fungal infection.
"It was a surprise that this fungus didn't appear to interfere with the first line of phagocytic defenses," said Rollins-Smith.
The scientists conducted further experiments in mammalian lymphocytes (mice and human), finding the same result. Only the second line, the lymphocyte-mediated defense, was impaired.
"The immunosuppressive factors released by the amphibian chytrid fungus seem to target a conserved vulnerability shared by amphibian and mammalian lymphocytes," Rollins-Smith said. "This mechanism of cellular suppression may be unique to this fungus or shared by other fungi. That is a question that awaits further research."
Because lymphocyte inhibition only happened when the researchers used mature fungal cells, and not young zoospores (which lack a cell wall), Rollins-Smith and colleagues propose that whatever fungal molecule is causing deadly damage to frogs is in the fungal cell wall.
This study, which shows how B. dendrobatidis has evolved to evade the amphibian immune system, explains why this fungus has been so devastating to amphibian populations; even frogs with a strong innate immune response can be susceptible.
This project was supported by the National Science Foundation and the National Institutes of Health.
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