Viral legacy may make pregnancy possible.
Millions of years ago, a retrovirus infected our primate ancestors, leaving a legacy of its DNA in their genes. That DNA has been passed down to humans. A study now finds that the protein encoded by this DNA abounds in placental cells--suggesting that the protein may play a useful role today by helping prevent a pregnant woman's immune system from rejecting her fetus.Up to 0.5 percent of the DNA in the human genome may derive from ancient retroviruses, says Mark Boyd of Hahneman University in Philadelphia, but most of this retroviral DNA produces no useful proteins.
However, at least 0.1 percent of the protein in the layer of placental cells that separate the fetus from the mother comes from a known retroviral sequence designated ERV-3. Or so report Boyd and his colleagues at the Kennedy Institute of Rheumatology and Chester Beatty Laboratories, both in London, in the Aug. 20 Virology. That's a significant amount, notes Boyd, since elsewhere in the body, this protein "is barely detectable."
A pregnant woman's body regards the fetus as an interloper in some ways, so its natural reaction is to reject it. One reason for this immune response lies in the structure of the placenta, Boyd says.
In primates, the placenta looks "kind of like a tree stuck into the side of the maternal tissue" as opposed to a flat plate in other mammals. The large surface area of its branching network of blood vessels results in an unusually high degree of contact between the mother and this "foreign" tissue. "It really invades," Boyd says.
In order for the mother's body to retain the placenta, something has to suppress her immune response to it.
"There's no way ERV-3 is the only immunosuppressive thing in the placenta," Boyd says. But he adds that the ERV-3 protein's abundance in placental cells suggests it's an important factor.
Maurice Cohen of Abbott Laboratories in North Chicago, Ill., helped determine the DNA sequence of ERV-3 in 1985, while he was at the National Cancer Institute in Frederick, Md. It resembled sequences in other retroviruses known to suppress immunity, leading Cohen and his group to argue that it might perform some immunosuppressive function. But Boyd's group has "gone a step further and identified a protein."
Boyd and his colleagues plan to test for the protein's suppression of immunity in two ways: first, by trying to block any suppression using antibodies to ERV-3, and then by turning on the ERV-3 DNA in cells that do not normally suppress immunity.
"Even if [ERV-3] isn't immunosuppressive, the evidence that it's doing something useful is just impossible to ignore now," Boyd says. "I quite like the idea that the cell has hijacked the virus and not vice versa."
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| Title Annotation: | Science News of the Week; retrovirus sequence ERV-3 may help suppress human immune response to placenta |
|---|---|
| Author: | Wu, Corinna |
| Publication: | Science News |
| Article Type: | Brief Article |
| Date: | Sep 2, 1995 |
| Words: | 456 |
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