Unexpected Importance of Thymus Revealed in Adults

The thymus gland, which creates immunological T cells before birth and during childhood, is frequently thought to be non-functional in adults and is occasionally removed during cardiac surgery to allow for easier access to the heart and major blood veins.

Evidence from recent studies headed by Massachusetts General Hospital (MGH) researchers and reported in the New England Journal of Medicine suggests that the thymus is crucial for adult health in general, for preventing cancer, and possibly for autoimmune disease prevention.

The team compared the mortality, cancer, and autoimmune disease risks between 1,146 adults who had their thymus surgically removed and 1,146 demographically matched patients who underwent comparable cardiothoracic surgery without thymectomy to determine whether the thymus has any health benefits for adults. In a subgroup of patients, the researchers also assessed T cell production and blood levels of immune-related chemicals.

A thymectomy increased the chance of death by 8.1% five years after surgery compared to 2.8% for patients who did not have their thymus removed, or a 2.9-fold increase. Additionally, during that time, a 2.0-times increased risk of cancer emerged in 7.4% of patients who underwent thymectomy surgery compared to 3.7% of patients in the control group.

“By studying people who had their thymus removed, we discovered that the thymus is absolutely required for health. If it isn’t there, people’s risk of dying and risk of cancer is at least double,” says senior author David T. Scadden, MD, director of the Center for Regenerative Medicine at MGH and co-director of the Harvard Stem Cell Institute. “This indicates that the consequences of thymus removal should be carefully considered when contemplating thymectomy.”

The overall mortality rate was greater in the thymectomy group than in the general U.S. population in a separate analysis that included all patients in the thymectomy group with more than five years of follow-up (9.0% vs. 5.2%), as was the mortality owing to cancer (2.3% vs. 1.5%).

When patients who had infections, cancer, or autoimmune diseases prior to surgery were taken out of the analysis, Scadden and his colleagues noticed a difference even though they found that overall there was no significant difference in the risk of autoimmune disease between the thymectomy and control groups in their study.

After eliminating these patients, 12.3% of patients in the thymectomy group and 7.9% in the control group experienced the development of autoimmune illness, resulting in a 1.5-times greater risk.

With an average follow-up of 14.2 postoperative years, the subgroup of patients in which T cell production and immune-related molecules were assessed (22 in the thymectomy group and 19 in the control group) consistently produced fewer new T cells than controls and had higher blood levels of pro-inflammatory molecules.

Now, Scadden and his team want to examine how varied thymus function levels in adulthood impact people’s health. According to him, “We can assess the relative vigor of the thymus and determine whether the level of thymus activity, rather than just its presence, is associated with better health.”

Kameron A. Kooshesh, MD, Brody H. Foy, DPhil, David B. Sykes, MD, Ph.D., and Karin Gustafsson, Ph.D. are additional co-authors.

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