Stem cells have been used to repair damaged heart tissues by indirectly boosting the immune system. The study from the University of Cincinnati injected adult stem cells into injured mice and found their hearts were rejuvenated despite no increase in cardiomyocytes.

The rationale behind injecting adult stem cells into damaged tissues is that the cells will migrate to the injured site and proliferate into the desired type, resulting in a natural plaster. This would also avoid the ethical implications and debate of obtaining embryonic stem cells. However, previous studies have shown no long-term benefit to these therapies, or that the cells underwent any differentiation at all.

The hearts of mice had been temporarily deprived of oxygen to mimic a heart attack. This was followed by an injection of either saline, stem cells, or zymosan. Although the adult stem cells improved heart condition, there was an equal benefit to injecting zymosan – an immune boosting drug. The zymosan benefits lasted longer than the stem cells, suggesting that the trigger of the immune response is caused by the injection of the cells, not the fact they are stem cells.

This was confirmed by injecting cellular debris of dead stem cells into the mice hearts and an increased macrophage response was detected along with heart rejuvenation.

The boost to the immune system helped to explain some of the underlying mechanisms of stem cell therapies. Clinical trials using adult stem cells are already happening to help repair hearts after heart attacks are already underway, despite questions over their efficacy and biological function.

Mammals have two distinct groups of stem cells; embryonic and adult. Embryonic stem cells are found in the blastocyst (a day 5/6 embryo consisting of ~200 cells) and can differentiate into any cell found in the body – also termed pluripotent. Adult stem cells are termed ‘multipotent’ as they differentiate into cell types that are closely related to where they are found in the body e.g. hematopoietic stem cells into platelets or red and white blood cells.

The ability of cells to differentiate into other cell types is due to the fact that all share the same genetic sequence and epigenetics. By determining the epigenetic difference between stem cells and differentiated cells, the process can be reversed, restoring cells pluripotent ability. As certain cell types are incredibly rare or difficult to obtain, creating stem cells from more common types or from patients directly is a viable alternative.

This study highlights how the biological mechanisms of stem-cell therapies are still poorly understood. Although there are benefits to using stem-cells, it is important to know why these benefits arise to avoid unforeseen side-effects and allow the production of more targeted therapeutics.