The Anglerfish Deleted Its Immune System to Fuse With Its MateFeedzy

There are few animals more bizarre than the anglerfish, a species that has so much trouble finding a mate that when the male and female do connect underwater, males actually fuse their tissue with the females for life. After the merger, the two share a single respiratory and digestive system.

Now scientists have discovered that the anglerfish accomplishes this sexual parasitism because it has lost a key part of its immune system, which then allows two bodies to become one without tissue rejection. (Remember symbiont Jadzia Dax of Deep Space Nine?)

All vertebrates, including humans, have two kinds of immune systems. The first is the innate system, which responds quickly to attacks by microscopic invaders with a variety of chemicals like mucous physical barriers like hair and skin, and disease-munching cells called macrophages. The second line of defense is an adaptive system that produces both “killer” T cells to attack the pathogen and antibodies custom-made to fight specific bacteria or viruses. The two systems work together to fight infections and prevent disease.

But in a study published Thursday in the journal Science, researchers from Germany’s Max Planck Institute and the University of Washington found that many anglerfish species (there are more than 300) have evolved over time to lose the genes that control their adaptive immune systems, meaning that they can’t create antibodies and lack those T cells.

“Anglerfish have traded in their immune faculties, which we believe to be essential, for this reproductive behavior,” says Thomas Boehm, a professor at the institute’s Department of Immunobiology and Epigenetics in Freiburg, Germany, and lead author on the paper.

To come to this conclusion, Boehm and his colleagues spent the past six years conducting genetic tests on tissue samples of anglerfish taken from around the world. They tried catching them by using deep-sea trawls that collect specimens 1,000 feet below the surface, but because the anglerfish is both rare and elusive, they were unable to collect any live specimens. So to get enough tissue for their genetic analysis, the researchers instead scoured museum collections and other laboratories that had anglerfish pickled in preservatives, some of them decades old.

Within the anglerfish family, there are several reproductive methods. The females of some species fuse with one male; others fuse with multiple males; and still another group have only a temporary fusion. After grinding up 31 tissue samples of 10 species, the team conducted genetic tests and found that species that temporarily fuse to their mates lack the genes responsible for the maturation of antibodies. Species that create a permanent attachment to their mates had also lost a set of additional genes that are responsible for the assembly of T cell receptors and antibody genes that are the foundation of the innate immune system in all vertebrates.

“It was intuitive to think that there is some genetic proclivity to allow this to happen,” Boehm says about the anglerfish species’ unusual immune systems. “This is the first bit of evidence that these animals do have this inability to reject a part of themselves and allow for these couplings to take place.”

In all other vertebrate species, including humans, a fusion of tissues would provoke an immune response, because the host body would treat the new tissue as an invader. In fact, successful organ transplants require doctors to carefully match donor and recipient tissue, as well as to prescribe drugs that temporarily suppress the recipient’s immune system so their body doesn’t mount a potentially deadly immune response. The anglerfish has done the same thing through evolution, Boehm says.

“They have gotten rid of everything that is essential for a proper immune response,” says Boehm, whose research focuses on immune systems from several different animal species. “They have no receptors to recognize foreign invaders. They are basically defenseless. A patient like this would never survive and would be dead in no time.”

Boehm worked with Ted Pietsch, professor emeritus at the University of Washington, who has studied the anglerfish and its strange physiology ever since he was a first-year graduate student in 1970. Piesch says he’s been thinking about the mystery of how the female and male can join tissue for the past 50 years. “It’s been an enigma, and it’s wonderful to think that we’ve finally got an answer,” he says.

Other biologists have been trying to solve the puzzle of the anglerfish as well. Arseny Dubin, a graduate student at the Nord University in Bodø, Norway, published a study last year in the journal Biology Letters detailing the loss of an immune system pathway in one local species of anglerfish. Dubin, who wrote his recent doctoral thesis on the anglerfish immune system, praised the new study for broadening the understanding of this strange animal. “It’s sad that it wasn’t us to publish it,” Dubin says. “I was planning to apply for a grant to do the same thing. It’s very interesting.”

Boehm says he hopes that the finding will perhaps lead to a new understanding of immunosuppression in humans, and perhaps better treatments for organ transplant recipients in the future. “From an evolutionary perspective, any immunologist would say it’s impossible to disentangle the innate and adaptive arms of the immune system,” Boehm says. “They’ve been together for more than 500 million years. If we fiddle with one or the other arm, it’s a catastrophic event. This is the first big surprise—that there is hope and that there is life without one of these two arms.”

Boehm believes that the anglerfish somehow boosts its own innate immune system to make up for the loss of the adaptive system. Understanding how—and if—that happens might open up new kinds of treatments for human transplant patients and will be the subject of future research by his group.

“I am a strong believer in looking at nature just to get an idea of the variability that is tolerated in the system,” Boehm says. “If we find the right things, we might be able to learn what is possible in the system. Then we might be able to manipulate it.”

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