Left behind: social amoebae
This week's paper, published in Science (317:679) is "Immune-like phagocyte activity in the social amoeba" by Guokai Chen, Olga Zhuchenko, and Adam Kuspa of the Baylor College of Medicine.
Cells of the social amoeba, Dictyostyleium discoideum forage individually, but eventually group together into a "slug", which crawls through the soil for days before eventually forming a spore-tipped stalk. Previous work with this species has looked at conflicts of interest over which cells have to sacrifice future reproduction (as spores) and become part of the stalk. This week's paper uncovers another example of apparent altruism in Dictyostelium, which may shed light on the evolution of a key part of our immune system.
As a Dictyostelium slug crawls through the soil, some cells are left behind. Are these just random sluggards? Or do they function like human phagocytes, the immune system cells that gobble up bacteria?
About 1% of cells in the slug were found to accumulate various fluorescent dyes at up to 10 times the rate of other cells. The authors suggest that these "S cells" may accumulate a variety of toxins, thereby protecting other cells from poisoning. No calculations were presented to show that the rates of toxin accumulation are high enough to provide useful protection, however.
In any case, accumulation of fluorescent molecules makes it easy to identify S cells, and even to separate them from other Dictyostelium cells. The cells can be sorted using a flow cytometer, in which thousands of tiny droplets per second, some containing cells, pass individually through a laser beam. Droplets can be deflected into different tubes, depending on their fluorescence.
Apparently the cells left behind by the slug are mostly these S cells. Ideally, the fluorescent microscopy photo showing fluorescent cells left behind would also include some way of detecting non-S cells, such as a dye taken up by all cell types.
When slugs were injected with bacteria, at least half of the cells that took them up were S cells, even though S cells were only 1% of the total. The other bacteria-ingesting cells may have been S cells that could not be identified as such. The slugs eventually ejected almost all of the bacteria, apparently by leaving them behind inside S cells. If so, the S cells apparently died in the process, as the ejected bacteria "appeared to be surrounded by cell debris."
A gene similar to an immune defense gene in animals, tirA, was found to be expressed at eight times the rate in S cells. So was a plant-related defense gene. When the animal-like gene was knocked out, the S cells were killed by bacteria to which they are normally resistant.
So Dictyostelium S cells act like our phagocytes and use a similar gene. Could our immune system really have an evolutionary origin that predates the evolution of true multicellularity?
Humans did not evolve from modern Dictyostelium , but the common ancestor of humans and Dictyostelium was probably more like them than like us. The authors note that, because amoebae live in the soil and interact with bacteria all the time, "they might have retained key characteristics of plant and animal innate immunity if those functions existed in their common ancestor."
If so, then S cells might be similar to phagocytes in other ways. For example, do human phagocytes have the same enhanced uptake of fluorescent dyes as S cells?
If the S cells are genetically identical to the other cells in the slug, sacrificing themselves to protect their clone-mates from bacteria is not much more surprising than similar self-sacrifice by our phagocytes. But it would still be interesting to know how the "volunteers" are chosen.