MEDICINEYour 72 SCIENTIFIC AMERICAN February 2008“I contain multitudes,” says a line in Walt Whitman’s poem “Song of Myself.” Whit-man was not thinking in biological terms, but the line has biological resonance. Recent studies suggest that each of us possesses—in ad-dition to the trillions of cells descended from the fertilized eggs we once were—a cadre of cells we have acquired from other, genetically distinct individuals. In utero we receive an infusion of them from mom. And women who become pregnant also collect a sampling shed by the de-veloping embryo.That cells cross the placenta is not surprising. After all, the tissue that connects mother and child is not an impenetrable barricade. It is more like a selective border crossing, allowing passage, for instance, of materials needed for the fetus’s development. What is remarkable, however, is the extent to which migrant cells can persist in their new host, circulating in the blood and even taking up residence in various tissues. The intermingling of some cells from one person inside the body of another—a phe-nomenon termed microchimerism—is now drawing intense scrutiny from medical research-ers, because recent work suggests it may con-tribute to both health and disease. Better under-standing of the actions of the transferred cells could someday allow clinicians to harness the stowaways’ beneficial effects while limiting their destructive potential.Surprise after SurpriseScientists gleaned early hints that a mother’s cells could pass to her fetus almost 60 years ago, when a report described the transfer of mater-Many, perhaps all, people harbor a small number of cells from genetically different individuals—from their mothers and, for women who have been pregnant, from their children. What in the world do these foreigners do in the body? BY J. LEE NELSON Cells Are My CellsTWO-WAY TRANSPORT: During pregnancy, some cells travel from mother to baby and some go from baby to mother. A fraction may persist in their new host. The condition is termed microchimerism.KEY CONCEPTS ■ Recent research suggests that each of us harbors some cells that originated in other, genetically dis-tinct individuals—a condi-tion called microchime-rism. All of us probably save cells we have acquired from our mother during gestation, and women who have been pregnant retain cells that come from the fetus.■ The acquired cells can persist for decades and may establish residence inside tissues, becoming an integral part of the body’s organs.■ Microchimerism could contribute to an immune attack in some cases but help the body heal in others. These effects make the acquired cells intriguing new targets for therapeutics that could curb auto immunity or promote regeneration of damaged tissues. —The Editorswww.SciAm.com SCIENTIFIC AMERICAN 73BRYAN CHRISTIE DESIGNCHIMERA in mythology com-bines parts of different animals—a lion, a goat and a snake. A person who har-bors the cells of another person is said to be micro-chimeric because relatively few cells are involved. nal skin cancer cells to the placenta and the infant. By the 1960s biologists began recogniz-ing that normal maternal blood cells can also fi nd their way to the fetus.Data suggesting that cells fl ow in the other direction as well—from fetus to mother—date back even further, to 1893, when a German pa-thologist discovered signs of such transfer in lungs of women who had died from a hyperten-sive disorder of pregnancy. Yet the acquisition of fetal cells by healthy mothers was not well documented in humans until 1979, when a landmark paper by Leonard A. Herzenberg of the Stanford Universit y School of Medicine a nd his colleagues reported fi nding male cells (those with a Y chromosome) in blood from women who were pregnant with boys.Despite evidence of two-way cellular traffi c between mother and fetus, biologists were sur-pris ed in t he 1990s when t hey learned that smal l numbers of the foreign cells often survive indef-initely in healthy individuals. Earlier studies of mother-to-child transfer had shown that mater-nal cells could survive in children with severe combined immunodeficiency, a disorder in which affl icted individuals lack critical infec-tion-fi ghting cells. But scientists had assumed that the ongoing microchimerism in these chil-dren stemmed from their disease and that a nor-mal immune system would destroy any mater-nal cells lurking in a child.That thinking changed when my colleagues and I found maternal cells in adults who had a normal immune system, including in one person aged 46. Evidence that fetal cells can likewise persist in mothers came some years earlier, when Diana W. Bianchi of Tufts University found male DNA in women who had given birth to sons decades before. (In many studies, investiga-tors test for the presence of male cells in women and estimate the number of those cells by measuring the amount of male DNA in blood or tissue samples from the women.)How could transferred cells survive for so long? Most cells live for a limited time and then die. An exception is stem cells, which can divide indefi nitely and give rise to a panoply of specialized cell types, such as ones constituting the im-mune system or the tissue of an organ. The discovery of long-term microchimerism implied that some of the original émigrés were stem cells or were related descendants. Experiments later supported this assumption. I sometimes think of the transferred stem cells or stemlike cells as seeds sprinkled through the body that ultimately take root and become part of the landscape.My Mother, MyselfThe presence of a mother’s cells in her off-spring—termed maternal microchimerism—is probably a double-edged sword, harmful in some cases but helpful in others. On the nega-tive side, maternal cells may contribute to dis-eases typically classifi ed as autoimmune, mean-ing that the immune system unleashes its fi re against the body’s own tissues. Cells derived from the mother appear to play a part, for instance, in juvenile dermatomyositis, an auto-immune disorder that affects primarily the skin and muscles. Research reported in 2004 by Ann M. Reed of the Mayo Clinic showed that mater-nal immune cells isolated from the blood of patients reacted to other cells from those same patients. Reed and her co-workers suggest, therefore, that the disease may arise when trans-ferred maternal immune cells take swipes at a child’s tissues.Maternal
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