Perplexing new studies from Rush University Medical Center and Wallenberg Neuroscience researchers suggest cells transplanted into the brains of people with Parkinson’s disease (to produce dopamine) “catch” the disorder from the surrounding tissue.

They studied brains of three people who’d received grafts between 11 and 16 years before death. Some cells in the grafts contained structures called Lewy bodies, a hallmark sign of the disease. Most grafted neurons were free from Lewy bodies, suggesting that grafts still offer benefits even if they start to become affected.

Scientists don’t know why the grafts became affected. One theory is that the disease crossed from the patient’s own brain tissue in a process similar to that caused by “prion” proteins. Another theory is that inflammatory reaction is to blame, and so grafts cloned from the patients themselves would reduce the risk.