Study suggests they trigger an immune response.

The controversial use of stem cells to help patients recover from a heart attack may work, but not because it grows new heart muscle.

Research in mice has found that injecting stem cells into the heart triggers an immune response that makes the scar stronger and the heart beat more forcefully.

The study, published in the journal Nature, suggests the current practice of injecting stem cells into a patient’s blood may not be optimal: direct injection into the heart could be more effective.

The team, led by Jeffery D Molkentin from the Cincinnati Children’s Hospital Medical Centre, US, wanted to resolve the thorny debate about whether stem cell therapy works after a heart attack and, if so, how.

Heart attacks happen when a coronary artery gets blocked and starves oxygen to a zone of heart muscle. The damaged area heals up with a scar, which means the heart doesn’t pump so well.

The result can be fluid build-up on the lungs and shortness of breath – your classic heart failure.

For decades, the mainstay of treatment has been drugs, which restore normal heart function in up to 40% of people.

But more recently stem cells – taken from the person’s heart or bone marrow – have been injected into the bloodstream in the hope they will make their way to the heart and form new heart muscle.

Despite numerous trials, however, evidence of benefit is sparse. The European Society of Cardiology recently concluded “the early promise of cell therapy has not yet been fulfilled”.

Nonetheless, 61 US businesses reportedly offer the treatment, which has not been approved by the US Food and Drug Administration for heart failure. Hence the urgent need for clarity on what the stem cells actually do. To find out, Molkentin’s team induced heart attacks in a group of eight-week-old mice.