Embryonic stem cell transplantation in patients is not yet practical, but Israeli researchers have identified the stage in the fetus's life when its cells are most suitable for transplantation
The idea of using embryonic stem cells for tissue transplantation does not fall from the scientific agenda. The reason for this is the severe shortage of organs for transplantation. But even though in recent years more and more researchers have managed to make the embryonic stem cells differentiate into specific cells, the transplantation of the cells in patients who need organs and tissues is still not in sight.
Embryonic stem cell transplantation has two main dangers. One is that the stem cells will cause a rejection reaction after being transplanted. To prevent rejection, young cells should be transplanted as much as possible. But stem cells that are in early stages of development may cause another problem. Some of these cells are still undifferentiated multipurpose cells, and after being transplanted they may form tumors called teratomas, which contain a large array of tissues. Therefore, in order to perform a successful transplant, one must find the window of time in which the stem cells have already "committed" to a certain differentiation direction (and therefore will not develop into teratomas), but they still do not provoke an immune response towards them (and therefore no rejection occurs).
Prof. Yair Reisner, from the Department of Immunology at the Weizmann Institute, decided to look for this window of time. Reisner chose to conduct experiments with embryonic kidney cells, due to the high demand for kidneys for transplantation. The experiments were conducted in human embryos and pig embryos. "Various pig organs are similar in function to human organs, and therefore they may be an important substitute due to the ethical difficulty of using aborted human embryos," explains Reisner.
In both types of embryos, the researchers located the areas from which the kidney is supposed to develop a few weeks after the beginning of pregnancy. "We removed a tiny piece of tissue from these areas, transplanted it into mice and tested whether the tissue cells succeed in developing into a normal, functioning kidney." The team of researchers, which also included Dr. Benny Dekel and Dr. Tanya Burkova from Reisner's laboratory, and Prof. Justen Passwell from the Sheba Medical Center, transplanted the embryonic cells into a small space under the kidney of mice lacking an immune system (in such mice, any tissue without a rejection reaction occurring). After implantation, the embryonic tissue was absorbed into the mouse's body and continued to develop; The embryonic cells turned into kidney cells and produced urine.
"In mice in which cells from pig embryos were transplanted, we saw that when the cells were taken from a three-week-old embryo - teratomas developed. But when the cells were taken from four-week-old embryos - a kidney developed from the cells. In other words, stem cells capable of causing teratomas disappear in pig embryos at the age of four weeks, and from that moment all the cells will turn into kidney tissue." In an identical experiment in which mice were transplanted with stem cells from human embryos, it became clear to the researchers that only when the cells were taken from embryos seven weeks old and older did no teratomas develop.
The next question was when the transplant's immune system starts attacking the fetal tissue. "This happens when the transplanted fetal cells begin to signal the mouse's immune system of their presence," says Reisner. "Our mice did not have an immune system, but when we added immune system cells to them from the outside. These cells settled on the embryonic cells and destroyed them."
The researchers discovered that in mice in which pig embryo cells were transplanted, the immune response arose when the embryo was six weeks old or older. In cells of human origin, tissue rejection occurred from the tenth week of fetal life onwards. The window of opportunity for transplanting human fetal cells is therefore when the fetus is 8-7 weeks old (nine-week-old cells are too close to the point of rejection), and the window of opportunity for transplanting pig fetal cells is when the fetus is 5-4 weeks old. "In these periods of time, we believe, if we take stem cells for transplantation, we will not get teratomas and the rejection reaction will be negligible, if at all." This study by Reisner and his partners was published in the journal "Nature Medicine".
Later Reisner turned to check the sharp transition from a situation where there is no rejection to a situation where it occurs. "This issue has occupied researchers for decades," he says. "One of the great mysteries in nature is why the fetus is not attacked by the mother's immune system, because it is just like a foreign implant. It is known that the fetus is wrapped in a special membrane that physically separates it from the mother's immune system and does not allow the passage of cells that may cause rejection. The placenta also filters substances that pass from the mother to the fetus. But these mechanisms do not work perfectly, and in newborns they find the cells of the mother's immune system. There must therefore be additional mechanisms that prevent rejection.
"In the first step", Reisner continues, "we decided to check what happens to the genes responsible for the immune response during the same time window in which we found relative protection against rejection of the developing fetal kidney." With the assistance of Prof. Gidi Ravavi from the Sheba Medical Center and his research group, the researchers found that during this period, changes occur in 57 genes of the human embryo that are involved in the immune response. "The large number of genes shows that it is not a single mechanism, and the examination of these genes shows several possible mechanisms," says Reisner.
So far, researchers have made progress in understanding two main mechanisms that contribute to the ability of the young fetal cells to escape the immune system of the mother (or the future transplant). The first mechanism is related to the cells that stimulate the immune response. With the help of sensitive methods developed in recent years, which make it possible to show the presence of these cells in a developing human embryo, the researchers found that the cells that stimulate the immune system appear only in the tenth week - at the upper limit of the time window that the researchers identified earlier.
The second mechanism is related to the formation of blood vessels around the transplanted tissue. "For a tissue to function, it must create a connection to blood vessels," explains Reisner. "In transplantation, the main attack of the immune system is against the blood vessels of the transplanted organ. When the blood vessels are destroyed, the implant does not receive a supply of oxygen and food, and it is destroyed." The team members also found that the formation of blood vessels in the kidney tissue, like the appearance of the cells that trigger the immune response, begins in the human embryo only in the tenth week.
