MEDICINE Your 72 SCIENTIFIC AMERICAN February 2008 Cells Are My Cells 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 contain multitudes says a line in Walt Whitman s poem Song of Myself Whitman was not thinking in biological terms but the line has biological resonance Recent studies suggest that each of us possesses in addition 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 developing 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 BRYAN CHRISTIE DESIGN I 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 phenomenon termed microchimerism is now drawing intense scrutiny from medical researchers because recent work suggests it may contribute to both health and disease Better understanding of the actions of the transferred cells could someday allow clinicians to harness the stowaways beneficial effects while limiting their destructive potential Surprise after Surprise Scientists 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 TWO WAY TRANSPORT During pregnancy some cells travel from mother to baby and some go from baby to KEY CONCEPTS Recent research suggests that each of us harbors some cells that originated in other genetically distinct individuals a condition called microchimerism 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 autoimmunity or promote regeneration of damaged tissues The Editors mother A fraction may persist in their new host The condition is termed microchimerism w w w S c i A m c o m SCIENTIFIC AMERICAN 73 74 SCIENTIFIC AMERICAN Microchimerism is now drawing intense scrutiny from medical researchers CHIMERA in mythology com bines parts of different animals a lion a goat and a snake A person who harbors the cells of another person is said to be microchimeric because relatively few cells are involved 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 Myself The presence of a mother s cells in her offspring termed maternal microchimerism is probably a double edged sword harmful in some cases but helpful in others On the negative side maternal cells may contribute to diseases typically classified as autoimmune meaning 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 autoimmune disorder that affects primarily the skin and muscles Research reported in 2004 by Ann M Reed of the Mayo Clinic showed that maternal 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 transferred maternal immune cells take swipes at a child s tissues Maternal microchimerism also seems to contribute albeit in a different way to neonatal lupus syndrome believed to arise in part from the destructive activity of certain antibodies that travel from the mother s circulation into her developing baby s These antibodies apparently home in on fetal tissue and thereby place the newborn at risk for a variety of problems the most serious of which is a life threatening inflammation in the heart Even though the mothers of affected infants have the disease causing antibody in their circulation they themselves are often healthy and infants born later on to the same woman generally are not affected That pattern led my coworkers and me to suspect that although the antibodies are important in the disease they are not the whole story Indeed when Anne M Stevens in my group examined cardiac tissue from boys with neonatal lupus who had died from heart failure she discovered that it contained female cells which we presume came from the mother Such cells were absent or rare in fetuses that died from other causes More than 80 percent of these maternal cells produced February 2008 BRIDGEMAN ART LIBRARY GETTY IMAGES nal skin cancer cells to the placenta and the infant By the 1960s biologists began recognizing that normal maternal blood cells can also fi nd their way to the fetus Data suggesting that cells flow in the other direction as well from fetus to mother date back even further to 1893 when a German pathologist discovered signs of such transfer in lungs of women who had died from a hypertensive 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 University School of Medicine and his colleagues reported finding male cells those with a Y chromosome in blood from women who were pregnant with boys Despite evidence of two way cellular traffic between mother and fetus biologists were surprised in the 1990s when they learned that small numbers of the foreign cells often survive indefinitely in healthy individuals Earlier studies of mother to child transfer had shown that maternal cells could survive in children with severe combined immunodeficiency a disorder in which affl icted individuals lack critical infection fighting cells But scientists had assumed
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