Comb jelly immune system can spot old enemies

Comb jellies may look like little more than filmy ghosts twinkling in the sea. Yet their immune systems can tell whether the jellies have encountered certain bacteria before.

Biologists once thought only vertebrates could recognize a familiar foe and tailor an immune response to it, says Sören Bolte of the University of Kiel in Germany. But insects and crustaceans turn out to have their own ways of accomplishing the same memorylike task.

Now, plunging further down to the ancient branches at the base of animals’ genealogical tree, Bolte and his colleagues report similar immune-system abilities in the comb jelly Mnemiopsis leidyi. Four probable immune-reaction genes behaved differently the second time they encountered harmful bacteria, Bolte and his colleagues say November 19 in Biology Letters. The first bacterial assault “primed” the jelly immune system to react differently during the second attack, he says.

The animal’s immune accomplishment shouldn’t be a big surprise, says Werner E.G. Müller of the University of Mainz in Germany. Sponges, another anicent goup of animals, seem to have genetic capacity for a sophisticated immune response.

Whether sponges or comb jellies rank as the most ancient animal lineage has been a boiling debate for several years (SN: 5/18/13, p. 20). However the ranking turns out, Bolte calls for including diverse animals from ancient branches on the tree of life in the effort to understand the evolution of immune systems.

Comb jellies bob through a soup of microbes, some of them vicious, so the jelly immune system needs to fight pathogens. To understand how the jellies battle microbes, Bolte, then at the GEOMAR Helmholtz Center for Ocean Research Kiel, turned to M. leidyi, a comb jelly commonly studied in labs. It’s an invasive species that moved from the Gulf of Mexico and the waters off New England into Europe.

In earlier studies, researchers had injected comb jellies with the common marine pathogenic bacterium Listonella anguillarum and identified genes that appeared to respond to bacterial attack. In the new research, Bolte and his colleagues used L. anguillarum plus an unrelated marine pathogen, Planococcus citreus. The jellies received paired injections 3.5 days apart, with the second either of the same bacterium as the first or of the different one.  

A gene for metabolic function burst into more intense activity the second time jellies encountered the same pathogen. The first brush might have alerted the body to be ready for trouble and react vigorously the second time. But three other genes previously connected with the immune reactions became somewhat less active the second time around.  Perhaps the immune system didn’t recognize the bacteria, which the researchers had killed, as a threat, Bolte speculates. The milder reaction the second time would have conserved resources. Or perhaps those three genes weren’t involved in any process needed for a second response.