The golden star tunicate may look like a flower, but this marine invertebrate is a brawler, attacking its tunicate neighbors in melees that feature ferocious cell-to-cell combat. Now, scientists have discovered that the immune system of this pugnacious animal shows some unexpected similarities to our own. The finding could help uncover new approaches for preventing rejection of transplanted organs or treating cancer.
“It’s pretty exciting,” says comparative immunologist Larry Dishaw of the University of South Florida College of Medicine in St. Petersburg, who wasn’t connected to the research. “They’ve laid out a nice, convincing story here.”
Tunicates are the closest living relatives of vertebrates—the group that includes humans, sharks, mice, and turtles—but the two evolutionary lines separated about 500 million years ago. The 3-millimeter-long, tube-shaped animals cluster in colonies on rocks and other hard underwater surfaces, fanning out like petals. When one growing colony contacts another, they have to decide “are they going to fight or are they going to fuse,” says study co-author Benyamin Rosental, a cellular immunologist now at Ben-Gurion University of the Negev in Beersheba, Israel.
Unless both colonies carry the same version of a particular protein, they fight. Cells from the two colonies attack and destroy one another in battles akin to what happens when the human immune system rejects a transplanted organ.
To probe how the golden star tunicate’s immune system works, a team led by Rosental and bioinformatician Mark Kowarsky of Stanford University in Palo Alto, California, isolated 34 types of cells from the animal. They found some cells switched on the same genes that are active in our hematopoietic stem cells, the blood-forming cells that spawn all the cells of our immune system. Like vertebrate hematopoietic stem cells, the tunicate versions can divide and specialize into different cell types, the scientists determined.
The researchers identified other parallels between the tunicate and vertebrate immune systems. Cells such as macrophages that devour invaders are a key part of vertebrate defenses. The animals harbored three kinds of these protectors. One type had never been detected before in the tunicates, and it shared a similar gene activity pattern with macrophages.
Another way in which the tunicates’ immune system mirrors the vertebrate version involves cells that are specialized to kill other cells. In our bodies, these assassins include natural killer cells, which target tumor cells or cells infected by viruses. As the scientists report online today in Nature, tunicates also deploy such cell executioners. When the researchers staged fights in the lab dish between cells from different tunicates, they found that the bodies piled up. Analyzing the genes that are active in these killer cells may help researchers pin down the crucial genes that spur organ rejection, Rosental says, and could suggest new ways to eliminate cancer cells.
The body of a tunicate seems simple, Dishaw says, but the new study shows “this simple system has incredible complexity” in its immune system. The overlap with humans indicates some features of the vertebrate immune system originated in our invertebrate ancestors. Rosental and colleagues are now studying other invertebrates such as sea urchins to determine how much further back in evolutionary history these features extend.