Photo by Clay Eals
Autoimmune diseases have all the makings of a spy thriller: Loyal defender against disease turns traitor, leading to unpredictable - and potentially fatal - surprise ending.
And these diseases, characterized by an immune-system attack on normal body tissues, mainly affect only one gender: women.
Intrigue surrounding these illnesses grew in 1998, when Hutch investigator Dr. Lee Nelson and collaborators discovered that trace fetal cells persisting in a mother's bloodstream years after pregnancy - a condition called microchimerism - are associated with a sometimes deadly autoimmune disease called systemic sclerosis, or scleroderma.
Now Nelson, research associate Dr. Nathalie Lambert and colleagues in the Clinical Research Division have found that genetic markers may influence occurrence of fetal microchimersim in the mother.
In two studies published last year, Lambert and Nelson discovered that a particular genetic variant, or allele, in one of the major histocompatibility (MHC) genes correlates with persistence of fetal cells in women and with scleroderma risk in men. The MHC is a large cluster of genes on chromosome 6 that control a multitude of immune system functions.
Previous studies by several labs have shown that the MHC genotype correlates with risk for many autoimmune diseases, including lupus, Graves disease and scleroderma.
The Hutch study showed that women with a genetic variation (also known as an allele) called HLA-DQA1*0501 were more than 13 times as likely to have fetal T-cells in their peripheral T-cells. Even more striking was the observation that all women with T-cell microchimerism had given birth to sons who also harbored the HLA-DQA1*0501 variation.
"The HLA genes form the cornerstone of how the body distinguishes self from non-self," Nelson said. "So it made sense for us to look at whether HLA genotype might be involved in microchimerism, a condition that we think has an important contribution toward the pathogenesis of scleroderma."
Autoimmune diseases predominantly affect women, with symptoms often worsening after pregnancy. Four times more women than men suffer from scleroderma, which can cause symptoms ranging from thickening of the skin to severe damage of the lungs and kidneys.
Persistence of fetal cells in a mother's peripheral blood years after pregnancy helps explain how gender affects susceptibility to scleroderma. Recent studies have shown that not only do fetal cells pass from fetus to mother, but cells also can pass from mother to fetus, meaning that men - who harbor trace cells from their mother - can exhibit microchimerism as well, called maternal microchimerism.
Nelson hypothesizes that the rogue T-cells, which are genetically similar to those of the host, disrupt the normal network of immune system communication.
"The trace fetal cells are analogous to a computer virus," Nelson said. "They are similar enough to the mother's own cells to remain in her body without being destroyed by her immune system, but different enough so that they can interfere with cell-to-cell communication that is critical for the immune system to function normally."
Lambert said an important next step in their research is to isolate the trace fetal cells and to analyze their role in the disease process.
"Up until now, we have used techniques that allow us to detect the presence of fetal DNA in the mother, as opposed to isolating whole cells," she said. "While we suspect that the intact fetal cells are present, we need to be able to collect the cells and conduct functional tests to see whether they cause an anti-maternal reaction."
Lambert and Nelson test for fetal cells using polymerase chain reaction, which can detect minute quantities of DNA. In order to identify fetal-specific DNA, blood samples are drawn from women who have given birth to sons, since male DNA can be distinguished easily by the presence of a Y chromosome.
DNA tests are also used to determine the unique variations in HLA genes of mother and son.
The team's analysis revealed a strong association of HLA genotype of both mother and son with microchimerism. Although microchimerism is observed in both healthy women and those with scleroderma, women with the disease have more fetal cells in their blood and other locations in the body where the disease is manifested.
Lambert and her colleagues analyzed the HLA genotype of men and women with scleroderma compared with a group of healthy controls.
"By analyzing the genetic susceptibility according to sex, we found that the contribution of HLA DQA1*0501 to the risk of scleroderma was substantially greater in men than in women," she said.
Lambert suggested that genetic predisposition may be the major scleroderma risk for men but not for women.
"Because of pregnancy, childbearing women can be exposed to both fetal cells from their children, as well as cells from their own mothers," she said. "But men can be exposed only to cells from their mothers, so just women are subject to multigenerational microchimerism - which may be enough to cause disease. For men, the genetic contribution may be the key factor in disease onset."
Lambert said further support from this theory comes from studies on multi-generational microchimerism.
"Using our polymerase chain-reaction tests, we've found women who have both fetal and maternal cells present in their blood," she said.
Maternal cell detection
Dr. Anne Stevens, a research associate in the Nelson lab and a pediatric rheumatologist, has detected maternal T cells in the thymus of babies up to eight months in age. The thymus, a gland below the neck in which T-cells mature, is removed routinely during infant cardiac surgery, and the excised tissue is sometimes used for research purposes.
"We can stain the X and Y chromosomes in T-cells in the thymus to detect whether any maternal cells are present," she said. "In a male thymus, we see about one in a thousand female cells. We're really interested now in finding out how these maternal cells affect the development of an infant's T lymphocytes."
Other studies in progress in the lab are to develop more sensitive methods for quantifying the levels of microchimerism.
"We know that maternal and fetal cells can persist in both healthy women and women with scleroderma," Lambert said."Now we want to understand how the cells cause disease."