The origins of immunity?

When the going gets tough, social amoebas get together. Most of the time, these unusual amoebas live in the soil as single-celled organisms, but when food runs short, tens of thousands of them band together to form a sluglike multicellular cluster, which then slithers away in search of a more bountiful patch of dirt.

New research shows that, within this slug, specialized cells rove around vacuuming up invading bacteria and toxins, thus forming a kind of rudimentary immune system. The discovery could provide a molecular link between the bacteria-eating behavior of single-celled amoebas and similar behavior by cells of animals’ immune systems.

Adam Kuspa of the Baylor College of Medicine in Houston and his colleagues showed that, to identify foreign bacteria, the slug’s roving cells require a protein called toll/interleukin-1 receptor A (TirA). When the team disabled this protein in the amoebas, the specialized cells lost their ability to hunt down bacteria, the researchers report in the Aug. 3 Science. In addition, when the impaired slug was teased apart, its individual cells could no longer identify and eat soil bacteria, which are the amoebas’ natural food.

Because TirA is closely related to bacteria-identification proteins that are active in animals’ immune systems, Kuspa says that the work may indicate an ancient evolutionary connection between microbes and immune system cells. “People wondered whether it might be the same conserved molecular mechanism, and here we see it’s a related protein in both cases,” he says.