Researchers at Fred Hutchinson Cancer Research Center, in Seattle, have found experimental data to support a new hypothesis about the genesis of scleroderma, an autoimmune disorder that primarily affects middle-aged women.
In a study funded by the National Institute of Allergy and Infectious Diseases (NIAID), J. Lee Nelson, M.D., and her colleagues report in the Feb. 21 issue of The Lancet, that women with scleroderma have significantly higher levels of non-self fetal cells circulating in their blood decades after pregnancy than healthy women who have previously given birth. Nelson says the persistence of these cells may indirectly dysregulate the mother's immune system interactions, somewhat like a computer virus disrupting the workings of a computer.
"Traditionally, the autoimmune diseases are described as your cells attacking your own normal, healthy tissue," Nelson, associate member of the Hutchinson Center and associate professor of rheumatology at the University of Washington, explains. "Our findings," she continues, "raise the question as to whether some autoimmune diseases are not entirely autoimmune, whether they actually have a component that is non-self. It's really an entirely new paradigm." Nelson says the mix of self and non-self cells, a phenomenon known as chimerism, is somewhat analagous to what happens in a person who receives a non-identical bone marrow transplant and develops graft-versus-host disease (GVHD).
"These findings are intriguing," says Elaine Collier, M.D., chief of NIAID's autoimmunity section, "because they offer another potential explanation, in addition to sex hormones, for the higher incidence of autoimmune diseases in women. While these data are not definitive, they are provocative because of their implications."
Scleroderma, or systemic sclerosis, affects people of all races, with an estimated incidence of 14.1 cases per million population worldwide. Its cause remains unknown. The disease is at least three times more common in women than in men. The peak incidence occurs between ages 35 and 54. The course of the disease varies greatly, with localized or systemic symptoms. The localized form of the disease primarily affects the skin of the face and hands, causing it to become shiny, tight and thick like leather. In contrast, the systemic form, which can lead to premature death, attacks the skin as well as internal organs, including the kidneys, lungs and heart, gastrointestinal tract and joints. Currently there is no way to prevent or cure scleroderma, but treatment can relieve some symptoms.
The study was conducted in two parts. Part one enrolled 40 women, average age mid-40s, who had previously given birth to at least one son. This group included 17 scleroderma patients with onset of disease around age 40, seven healthy sisters of these patients and 16 healthy unrelated women who served as controls.
In collaboration with co-author Diana W. Bianchi, M.D., Nelson's team used a sensitive assay known as polymerase chain reaction (PCR) to look for a specific snippet of the male chromosome in blood samples taken from these women. Women who had had sons rather than daughters were chosen for study simply for technical reasons, because within the milieu of the mother's cells, it is easier to detect evidence of the Y chromosome than to distinguish a daughter's DNA. The researchers tested the samples blinded, meaning they had no information by which they could identify the volunteers with the samples.
PCR revealed that women with scleroderma had an average of 11.1 male cells (range from 1 to 61) per 1 tablespoon of blood compared with an average of 1.3 male cells (range 0 to 5) among their sisters and 0.38 (range 0 to 2) in the control group. Use of immunosuppressant drugs by some of the patients did not affect the number of cells measured.
The average length of time since birth of the youngest son was 18.5 years in the scleroderma patients compared with 15.4 years in the control group. Notably, some scleroderma patients had levels of male DNA greater than the average level found in most women when pregnant with a son. Even so, fetal cells constituted less than 1 percent of the mother's total white blood cells in all cases.
The fact that the cells came from a son is irrelevant, says Nelson. "The important point is that non-self fetal cells occur more often and in greater numbers in scleroderma patients than in healthy women."
However, the development of scleroderma in men and in women without children may also be explained by the new hypothesis. Cells can traffic in both directions during pregnancy, so non-self maternal cells could also engraft and persist in a child.
To explore further whether microchimerism, this intermixing of very low levels of non-self cells with one's own, might be involved in scleroderma, they did a second substudy. Here they investigated whether a women who has given birth to a child with "compatible" immune cell markers known as human leukocyte antigens (HLA) class II has a greater risk of developing scleroderma.
Every individual has a specific pair of these markers on each of his or her immune cells, one marker inherited from the mother and one from the father. These markers act like ID tags to identify these cells as self. If the markers on a child's cells are compatible with the mother's, that is, they differ only slightly, the child's cells may not be recognized by the mother's immune system as foreign and would not be destroyed. For example, if the mother's immune cells carry markers A and B, and the child inherits an A gene from the mother and an A gene from the father, that child's cells can escape detection by the mother's immune system, even though they are mismatched.
This substudy enrolled 21 women with scleroderma and all 47 of their children, and 32 healthy women and all 58 of their children. The researchers were particularly interested in genes that encode the class II markers DR and DQ, since they are associated with susceptibility to scleroderma and other autoimmune diseases. The analysis showed that women with scleroderma had previously given birth to at least one child who was HLA class II-compatible significantly more often than healthy women. The strongest association was found for the gene that encodes for DR, where 62 percent of patients had at least one child compatible in this gene compared with 16 percent of healthy controls. Presumably, HLA compatibility is a risk factor for autoimmune disease because the mother's immune system has difficulty recognizing the fetal cells as foreign and killing them. Yet, because the cells are different, their presence may increase the mother's susceptibility to autoimmune disease.
Although Nelson has been studying pregnancy and autoimmune diseases for 10 years, colleagues at Fred Hutchinson helped the new theory gel in her mind. They frequently perform bone marrow transplants, and much discussion revolves around GVHD, which, by definition, she says, is
chimerism. Moreover, clinically GVHD looks like scleroderma. Then two years ago, Bianchi reported that fetal cells can persist in the mother's circulation up to 27 years after pregnancy. "When the finding came out about fetal cells persisting, it all fell in place for me," Nelson says.
"It will take time before we know how much this approach helps us understand what causes scleroderma," Nelson adds. "But this paradigm certainly is potentially applicable to some other autoimmune diseases as well."
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