Stem cells and cloning
The promise of stem cells to generate tissue for cell-based therapies is an exciting one, however several hurdles need to be overcome before this can be realised.
A major problem with the use of ES cells to generate tissue for transplant is that the immune system of the patient would detect them as foreign and attack. Immune rejection is a major problem in all transplant therapies.
Cells in our body display a set of markers, known as MHC (major histocompatibility complex) which the immune system recognises – it helps work out what is meant to be in the body (self), and what is foreign (non-self). Invading pathogens and transplant tissue display different patterns of MHC, which the body recognises as foreign and thus the immune system attacks them.
Australian Stem Cell Centre
One strategy for overcoming this could involve the use of somatic cell nuclear transfer (this used to be called therapeutic cloning, but scientists no longer use the term as it is misleading). This would involve replacing the nucleus of an egg cell with that from a cell from the patient’s body and allowing it to develop to form a blastocyst. ES inner mass cells would then be harvested and used to establish an ES cell line that has the genetic makeup of the patient. These cells would then be directed to develop into the tissue needed for transplant. This tissue would display the same antigen markers as the patient’s, and therefore would not be rejected. However, this technology is not legal in Australia.
This potential application of cloning technology would not result in the formation or development of a new foetus or adult human. However, there has been considerable opposition to this research as it involves the generation of embryos specifically for research which is considered unethical by many. Additionally, the embryos are genetic clones of the patient, and thus could, in theory, be used to generate a new human. Reproductive cloning is widely regarded as unethical by the medical and scientific community.
Alternative strategies for overcoming immune rejection
The current strategy for overcoming tissue rejection is the use of drugs that suppress the immune system. However, this means that the patient may then be more susceptible to infections and researchers are investigating new drugs that have fewer side effects.
An alternative strategy for overcoming immune rejection is to use adult stem cells from patients themselves to derive the transplant tissue. These cells would display the MHC markers that the patient recognises as self, so that the immune system would not reject it.
Researchers are also working to establish stem cell banks that contain tissue of many different types. Cells from those that most closely match the patient would then be used for transplant.
The thymus gland is the organ most responsible for programming our immune system to recognise its own cells very early in life. It ceases this function shortly after birth so that any cells entering the body after that will be rejected. Scientists are researching ways of possibly reactivating the thymus gland so that it will recognise the transplanted tissue.
This work is still in its early stages and is very complex – treatments such as this are likely not to be available for at least ten years.