Because the human body rejects most introduced tissue, locating an organ that will be accepted by a patient's immune system remains a significant challenge for transplantation medicine. Creating cells that are genetically identical to those of the patient offers one solution to this problem. Now the results of a study published online today by the journal Nature Biotechnology illustrates that, though it is not yet feasible in humans, this approach can work in cows. Using nuclear transplantation (known as therapeutic cloning), scientists found that cloned cells can function in the body of a bovine transplant recipient without being rejected.

To clone an animal, scientists extract the DNA-containing nucleus from a donor cell and insert it into a so-called denucleated egg—one that has had its DNA removed but is otherwise intact. Cloned cells thus possess genetic material identical to that of the donor and, in theory, should not be rejected if they are transplanted back in to the donor's body. But a small amount of foreign DNA from the egg's mitochondria (the energy powerhouse of the cell) remains in cloned cells. To investigate whether this small bit of unfamiliar genetic material is enough to instigate an immune response, Robert P. Lanza of Advanced Cell Technology in Worcester, Mass., and his colleagues experimented on cows, animals with fairly sophisticated immune systems. The researchers sucked the nucleus from a cow's egg and inserted a second cow's DNA in its place. They next inserted the developing ball of cells into the uterus of a third cow and waited for it to develop. After several weeks of gestation, the group extracted three types of tissue—skeletal muscle, cardiac cells and kidney cells—from the developing fetus. After culturing these cells, the team transplanted them into the cow that had originally donated its DNA. Weeks later, no immune response had materialized, and the researchers report that the cells demonstrated normal development.

Millions of people suffer from ailments such as lung, heart and kidney disease that could be alleviated through the introduction of healthy tissue. Some scientists propose that therapeutic cloning in humans may one day be possible using lab-grown stem cells to generate genetically matched tissues to replace those that are missing or damaged. This particular experiment took place on a developing cloned bovine fetus; analogous work could not be done on people because of ethical considerations. The findings, however, represent one step toward understanding the bodys response to introduced cloned tissue. Says Lanza, "This study furnishes the first scientific evidence that cloned tissues can be transplanted back into animals without being destroyed by the bodys immune system."