There are several types of stem cell therapy that are in development. They can be broadly broken down into cells that come from the same patient, called autologous, and cells that derived from another source, called allogeneic. Autologous stem cells have the advantage not causing worries regarding immune rejection. Unfortunately, in many conditions that require stem cell therapy, such as peripheral artery disease, or coronary heart disease, the original stem cell pool in the patient is depleted. This is because on the one hand the body is constantly using the stem cells to try to heal itself, and on the other hand, there is an underlying inflammation in the conditions mentioned that suppress stem cell activity. Therefore, in some situations it is better to use "fresh" stem cells from another donor.
Mesenchymal stem cells can be extracted from bone marrow, fat, and several other sources. In contrast to the current dogma (which appears to be scientifically incorrect) that stem cells that make blood can not be transplanted without immune suppression, the current thinking for mesenchymal stem cells is that immune suppression is not needed when they are transplanted. Part of the reason for this is that mesenchymal stem cells have been published in many papers to actually "immune modulate". In other words, mesenchymal stem cells appear to have the ability to reprogram the immune system so as to not destroy them, but at the same time they allow the immune system to continue performing its usual function of destroying pathogens.
In a recent paper (Chin et al. Cryopreserved mesenchymal stromal cell treatment is safe and feasible for severe dilated ischemic cardiomyopathy. Cytotherapy. 2009 Nov 2) the use of mesenchymal stem cells was evaluated after the cells have been stored frozen. The importance of this is that an ideal stem cell treatment would be a "universal donor" stem cell "drug" in that cells could be shipped to the point of care frozen and used in the convenience of a doctor’s office, without the need for expensive equipment that is currently a requirement in the medical practice of stem cell therapy.
In the publication the scientists treated three patients with dilated cardiomyopathy with mesenchymal stem cells that were previously frozen. Stem cells were injected directly into the heart muscle as the patients were undergoing coronary artery bypass surgery. All three patients responded better than what would have been expected had they undergone surgery alone in terms of cardiac function, ejection volume, and reduction of scarring. Although the study was uncontrolled and therefore efficacy data is not solid, the fact that the procedure was performed safely, without adverse effects at 1-year follow-up suggests that more studies need to be performed to evaluate efficacy of this approach.