October 8, 2001
The "master" hormone that commands the body's response to stress is also directly involved in the process that prevents a mother's immune system from destroying an embryo that has implanted in her uterus, according to the results of a study by researchers at the the National Institute of Child Health and Human Development, The National Institute of Diabetes and Digestive and Kidney Diseases, and several other Institutions.
"This finding opens up promising new ground in the quest to treat recurrent miscarriage, " explained Duane Alexander, M.D., director of the NICHD. "Similarly, the finding may also lead to new insights for preventing and treating preeclampsia, a life-threatening complication of pregnancy, as well as certain forms of cancer."
The study appears in the October 9 issue of Nature Immunology.
Briefly, the researchers have found that the stress hormone corticotropin releasing hormone (CRH) is also produced by trophoblasts--early cells that later give rise to the placenta, explained the study's senior author, George P. Chrousos, M.D., Chief of NICHD's Pediatric and Reproductive Endocrinology Branch. In turn, CRH causes these same cells to secrete a protein known as Fas ligand-FasL, for short. Like a key fits into a lock, FasL fits into the Fas molecule that sits on the surface of immune cells known as T-cells. Once FasL binds to the Fas molecule, the T-cell enters into a stage known as apoptosis, a programmed cycle that brings on its own death. The entire process destroys the T cells before they can attack the developing embryo. The Fas molecule is technically known as a receptor. The "lock and key" mechanism of a molecule and its receptor occurs with a vast number of cell and hormone combinations, triggering a variety of courses of action for the cell.
In an earlier study, these researchers found that the cells lining the uterus also produce CRH. In this study, the researchers found that the cells lining the uterus also manufacture FasL.
"The embryo is foreign to its mother, and, in theory, should be rejected by her immune system," Dr. Chrousos said. "When the embryo first implants, the surrounding tissue even looks inflamed, as if it were responding to an infection."
Dr. Chrousos and his colleagues reached their conclusions after performing an elaborate series of experiments on cells in culture and in animals. First, the researchers tested both laboratory cultures of human trophoblast cells and human cancer cells that resemble trophoblast cells. Both types of cells were found to produce large amounts of CRH. Also, both cell types had receptors for CRH on their cell surfaces, which suggests that the cells react to the CRH that they themselves produce.
In the next series of experiments, the researchers added varying amounts of CRH to the cell cultures. The more CRH the cells were exposed to, the greater the amounts of FasL they produced. Similarly, cultures of cells from the endometrium also produced FasL when they were exposed to CRH.
Next, Chrousos and his colleagues added activated T cells to cultures of the trophoblast cells. Activated T-cells have the Fas molecule-the receptor for FasL-on their surfaces and are primed to attack disease-causing organisms. Inactive T cells do not have the receptor and typically do not attack foreign organisms. Roughly 15 percent of the activated T cells died after exposure to the trophoblast cells. However, when the trophoblasts were first treated with CRH, about 68 percent of the T cells died, presumbably because the cells began manufacturing greater amounts of FasL. In contrast, inactive T cells-which do not have the FasL receptor-survived when exposed to the trophoblasts cultures, regardless of whether the trophoblasts had been treated with CRH.
In another series of experiments, the researchers treated female rats with varying doses of antalarmin, within 6 days after the animals had mated. Antalarmin binds to a receptor for CRH. However, antalarmin does not fit into the receptor, but instead blocks it so that no other molecule can fit into it. Antalarmin prevents CRH from locking into its receptor, and in so doing, indirectly prevents the cell from making FasL. Without FasL to stop them, the T cells can attack the embryo.
Within the first five days after mating, the more antalarmin the rats received, the less likely the embryos were to implant in their mother's uteruses. Similarly, rats treated with greater amounts of antalarmin produced smaller amounts of FasL at the site of embryo implantation. However, if rats were treated with antalarmin between 6 and 20 days after mating, their pregnancies were not affected.
"This finding suggests that the CRH/FasL relationship only plays an anti-rejection role in very early pregnancy-not in later pregnancy," Dr. Chrousos said. "Quite possibly, another hormonal system takes over to suppress the immune reaction later in pregnancy."
Strains of rats that are genetically incapable of producing T cells can conceive an embryo and carry the pregnancy to term, although they do so at much lower rates than do rats with normal T cell function.
Dr. Chrousos added that a likely candidate for a "back up" system that could suppress immune function in the absence of FasL, early in pregnancy as well as suppress the immune system later in pregnancy, is the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). TRAIL also brings on the pre-programmed death of cells. However, in tests done by the researchers, CRH did not increase the amounts of TRAIL found in the cell cultures.
Dr. Chrousos explained that the key role the CRH/FasL relationship plays makes it a prime candidate for studies to determine the causes of unexplained infertility. The relationship may also play a part in preeclampsia. This life-threatening complication of pregnancy results in high blood pressure in pregnancy; without warning, preeclampsia can progress to eclampsia, a combination of dangerously high blood pressure and seizures that can often result in the death of the mother. Both conditions, he said, are characterized by high levels of CRH in blood and inflammation around the placenta and uterine wall, which may result from a failure of the CRH/FasL system to halt the mother's immune response.
Also, because certain types of cancer invade the body's tissues in a way similar to the way the trophoblast invades the uterine wall, study of the CRH/FasL relationship might lead to clues that could help in the design of new cancer treatments. Although the CRH/FasL relationship has never been implicated in any cancer, other researchers are now studying how FasL relates to cancer cases.
Finally, Dr. Chrousos said, the findings mean that researchers who are testing a class of drugs for treating depression and anxiety disorders by blocking the CRH receptor should be especially careful when testing the drug in women who might be in the very early stages of pregnancy. He added, however, that the most popular class of drugs currently prescribed to treat depression is unlikely to hinder implantation of the embryo. These drugs, known as selective serotonin reuptake inhibitors, act directly on the brain and do not influence the CRH/FasL relationship.
Other authors of the study were A. Magrigiannis, of the, University of Crete, Heraklion, Greece, Hammersmith Hospital, London, and the Onassis Foundation, Athens, Greece; E. Zoumakis, University of Crete, and NICHD; S. Kalantaridou, the Onassis Foundation and NICHD; A.N. Margioris, University of Crete; C. Coutifaris, University of Pennsylvania Medical Center, Philadelphia; K.C. Rice, National Institute of Diabetes and Digestive and Kidney Diseases, NIH; and A. Gravanis, University of Crete.
The NICHD is part of the National Institutes of Health, the biomedical research arm of the federal government. The Institute sponsors research on development before and after birth; maternal, child, and family health; reproductive biology and population issues; and medical rehabilitation. NICHD publications, as well as information about the Institute, are available from the NICHD website, http://www.nichd.nih.gov, or from the NICHD Information Resource Center, 1-800-370-2943; E-mail NICHDInformationResourceCenter@mail.nih.gov.