Tadpoles' tips for immunity

Unexpected links between the innate immune system and metamorphosis in invertebrates have provided scientists with glimpses of some of evolution's most significant milestones in the fight against infection.

A team from the University of Washington, Seattle, in the United States, has identified genes and unusual cell behaviour in sea squirts, one of the most primitive members of the phylum chordata, which humankind shares with other mammals, fish and reptiles.

Billie Swalla, associate professor of zoology, outlined the findings at the American Association for the Advancement of Science's annual meeting. He and colleague Brad Davidson identified a complex of 14-16 genes among the many that "switch on" prior to the sea squirt tadpole metamorphosing into an adult. This group of genes was already known to be behind innate immunity in invertebrates, the defence system that enables killer cells to recognise invading bacteria.

The scientists then used time-lapse photography and found that a group of body cells moved around the tadpole's body as metamorphosis began, just like human white blood cells. Some even left the organism.

Professor Swalla believes these cells may be activated by the genes she has found, so that they can clear space within the tadpole's body as it is completely restructured during metamorphosis. The defensive capability may simply be a useful secondary function.

While humankind possesses an adaptive immune system that makes antibodies to fight off infection, it has recently emerged that we also carry duplicates of the innate immune system genes in our genome.

If the scientists can discover how the sea squirt activates its primitive version of this system during metamorphosis, it raises the prospect of artificially boosting natural defences in immuno-compromised human patients such as those with Aids or leukaemia.