

Posted: Sunday, May 11, 2003


But not in the way you might think.

What's more, you probably still carry a part of them.

It turns out that even decades after a woman gives birth, she can still have cells in her blood and tissues that came from her children during pregnancy.

And by the same token, many adults appear to harbor such cells they picked up from Mom during their time in the womb.

As a recent editorial in a pediatrics journal put it, So you think your mother is always looking over your shoulder? She may be IN your shoulder!''

The big question for scientists now is whether harboring these foreign cells is good, bad or irrelevant for a person's health. Some evidence suggests they may set the stage for several kinds of autoimmune disease, in which the body mistakenly attacks its own tissues. But some scientists suspect they may also be helpful.

Actually, these foreign cells aren't the very ones that slipped from fetus to Mom, or vice versa, during pregnancy. Rather, they are descendants of stem cells that transplanted themselves, took root, and started pumping out the progeny found decades later.

The result is a vanishingly small dose of foreign cells in adults. One study, for example, found up to 61 fetal blood cells per tablespoon of blood from women. That's less than one in a million, notes Dr. J. Lee Nelson, a key figure in the young field of studying the phenomenon.

Harboring a small dose of fetal cells from a long-ago pregnancy is definitely very common in healthy individuals,'' said Nelson, an autoimmune disease specialist at the Fred Hutchinson Cancer Research Center and the University of Washington in Seattle. The great majority of women who've had pregnancies even those ending in miscarriage or abortion probably carry detectable fetal cells, she said.

As for adults carrying cells from Mom, Nelson said a recent study found them in the bloodstreams of almost a quarter of women tested. She said she suspects the proportion will be higher when more extensive studies are done. In fact, she thinks it's at least not at all uncommon, and the probable answer is ... that most of us have a low level.''

The foreign-cell phenomenon is called microchimerism (MY-croh-KY-mer-ism). Scientists using genetic tools to identify such cells find them repeatedly in both sick and healthy people.

One research team found male cells in women's bloodstreams up to 27 years after they'd given birth to a boy. On the other side of the coin, Nelson and colleagues found that adults in their 40s still carried blood cells from Mom.

Of course, women who've been pregnant could have cells from their own Moms as well as their children. And sure enough, last year scientists documented that a deceased woman in her 40s harbored cells from her children, born six and eight years before, and her own mother. Both kinds were found in the lung, heart, spleen, gut and liver. Only fetal cells showed up in the right kidney and the ovary, while only cells from Mom appeared in the bone marrow.

That woman had scleroderma, a chronic autoimmune disease with a wide array of symptoms that can include muscle and joint pain, thickened skin, digestive problems, abnormal sensitivity to cold and other problems. The disease is getting the most scrutiny for a possible connection to the foreign cells. The Scleroderma Foundation says that about 300,000 Americans, mostly women, are thought to have the disease, which typically strikes between ages 25 and 55.

The trigger for the disease isn't known, but its immune system attack that can reach the skin, lungs, gut, kidneys, heart and other internal organs, as well as blood vessels, muscles and joints. It can be fatal.

Foreign cells have been found at the attack sites within scleroderma patients, and they're apparently there early in the disease process, said Carol Artlett of Thomas Jefferson University in Philadelphia.

The fact that they're there early is provocative,'' she said, but we don't know what was there first.''

Maybe the foreign cells, which include T cells of the immune system, were just drawn to the site after something else maybe a virus, maybe some toxin initiated the attack, she said.

While there's no conclusive evidence tying the foreign cells to scleroderma, Artlett said research provides enough hints to make her suspect some kind of connection.

Dr. John Varga, who heads the medical advisory board of the scleroderma foundation, said he hasn't yet seen convincing evidence that the foreign cells cause the disease. But research into a possible link is a pretty hot area,'' he said.

If the cells do contribute to autoimmune disease, of course, such research might lead to new treatments. But how could a relative handful of foreign cells cause so much trouble?

Nelson's hypothesis is that just harboring those cells probably isn't the problem, because they're seen too commonly in healthy people.

The real hazard, she suggests, might arise when there's a slight genetic mismatch between the foreign cells and the body they live in. The problem appears to be in the so-called HLA genes, the same ones that doctors seek to match when they transplant organs.

With a very slight mismatch, the body's immune system accepts them as normal much like a computer unwittingly accepts viruses into its inner workings, Nelson reasons.

The problem would arise if the person then encounters some sort of trigger for scleroderma none has been identified yet, but perhaps it's a biological virus or a toxin. At that point, the foreign cells might disrupt the delicate communications between immune system cells that allows for fierce attacks on germs without damage to the body. And that disruption could lead to the misguided attacks on normal tissue that characterize scleroderma.

But might foreign cells provide a benefit too? Nelson notes that women with rheumatoid arthritis often go into remission during pregnancy, especially if their fetus has a much different lineup of HLA genes. And just having given birth reduces a woman's risk for rheumatoid arthritis. Nelson wonders if cells escaping from the fetus might be responsible.

One thing about foreign cells is already clear to Nelson: they are not any reason to blame anybody for disease. It's not an issue of blame. These are rogue cells deriving from the pregnancy,'' she said.

Quite the contrary. Dr. Judith Hall of the University of British Columbia recalls lecturing at a medical school about adults carrying cells from their mothers. Afterwards, she said, a young woman came up to her with an extraordinary reaction.

My Mom died about five months ago, and I've had the hardest time adjusting to her not being around. Now she's here,'' Hall recalls the young woman as saying.

It really was very touching,'' Hall said. It just meant a huge amount to her.''

On the Net:

Scleroderma Foundation: www.scleroderma.org

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