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Scientists have discovered an unexpected antiviral defence mechanism in sea anemones, revealing that animals may have evolved more than one successful way to combat viral infections. The findings challenge a long-standing assumption that all animals inherited the same basic antiviral immune system from a common ancestor, opening new perspectives on how immunity has evolved across millions of years.

The study, led by PhD candidate Ton Sharoni and Prof. Yehu Moran of the Hebrew University of Jerusalem, in collaboration with researchers from the University of North Carolina at Charlotte, was published in Nature Ecology & Evolution. The research suggests that evolution may have produced multiple solutions to the problem of viral infections rather than relying on a single universal strategy.

Viruses have posed a constant threat to living organisms throughout evolutionary history. In humans and other vertebrates, one of the most important antiviral defense mechanisms relies on a protein called MAVS. This protein helps activate the body's immune response after viral genetic material is detected, allowing the immune system to launch a coordinated attack against invading pathogens.

To explore how ancient this defence system might be, researchers turned to sea anemones—marine animals that diverged from the evolutionary lineage leading to humans more than 600 million years ago. As close relatives of corals and jellyfish, sea anemones offer valuable insights into the early evolution of animal immune systems.

During their investigation, scientists identified a previously unknown protein, which they named CARDIB (CARD Inhibitor Binding protein). Structurally, CARDIB closely resembles the human antiviral protein MAVS, leading researchers to initially believe it served a similar role in activating immune defences.

However, the experiments revealed a surprising twist. Instead of switching on antiviral responses like MAVS, CARDIB actually suppresses them under normal conditions.

"Everything about CARDIB suggested it should function like MAVS," said Prof. Yehu Moran, head of the Department of Ecology, Evolution and Behavior at the Hebrew University. "Instead, we discovered that it does the exact opposite. Rather than activating antiviral defences, CARDIB normally suppresses them."

The unexpected finding prompted researchers to investigate why an organism would possess a protein that appears to dampen its own immune system. To answer this question, the team used CRISPR gene-editing technology to remove the CARDIB gene from sea anemones before exposing them to viruses.

The results overturned conventional expectations. Sea anemones lacking CARDIB were significantly more vulnerable to viral infection. The viruses multiplied more rapidly, the animals failed to activate their antiviral defenses effectively, and their overall ability to fight infection was greatly reduced.

"The results were completely counterintuitive," said Sharoni. "Although CARDIB acts as a brake on the immune system under normal conditions, that brake turns out to be essential for mounting an effective antiviral response."

The researchers believe that carefully regulating immune activity may be just as important as activating it. Without CARDIB, the immune system appears to lose its ability to respond efficiently when viruses attack, suggesting that controlled suppression may help maintain the balance needed for an effective defence.

The discovery offers fresh insight into the complexity of immune evolution and indicates that antiviral systems across the animal kingdom may be far more diverse than previously recognized. Rather than following a single evolutionary blueprint, different groups of animals may have independently developed distinct mechanisms to protect themselves from viruses.

Beyond its evolutionary significance, the study could eventually contribute to biomedical research. Understanding alternative immune strategies in ancient organisms may inspire new approaches to treating viral infections or regulating immune responses in humans.

By uncovering a protein that protects against viruses through an unexpected mechanism, the research highlights the remarkable adaptability of life over evolutionary time and underscores that nature has evolved multiple ways to achieve the same goal—defending organisms against persistent viral threats.

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