US lead scientist Dr Howard Petrie, from the Scripps Research Institute in San Diego, California, said: "The thymus ages more rapidly than any other tissue in the body, diminishing the ability of older individuals to respond to new immunologic challenges, including evolving pathogens and the vaccines that may otherwise offer protection from them.
"We provide, for the first time, a mechanistic link between antioxidants and normal immune function, opening new avenues for potential treatment strategies that could improve immune defences in the ageing population."
The thymus is a small gland that lies behind the breastbone between the lungs. Its function is to manufacture T-cells, essential immune system cells in the front-line of the body's defences against harmful foreign invaders and cancer.
T-cells are continuously lost and replaced throughout life. But from about the time of puberty onwards, the thymus rapidly shrinks and its T-cell generating ability diminishes.
Why this happens has been unclear. A leading theory is that sex hormones such as testosterone play a role, but this fails to explain why the thymus seems to age so much faster than other organs and tissues.
The new research highlights the unique damaging effect hydrogen peroxide - best known as a bleaching agent but also a natural by-product of metabolism - has on the thymus.
Hydrogen peroxide can be compared with dangerous waste from a nuclear reactor. It is produced by all cells as a result of the process that converts food into energy using oxygen, but is highly destructive. Oxidative damage inflicted by hydrogen peroxide and other "reactive oxygen species" (ROS) chemicals can tear apart cell membranes and scramble DNA.
Antioxidants, some of which are produced naturally in the body, help to block the devastating effects of chemicals such as hydrogen peroxide.
The Scripps scientists found that one natural antioxidant, an enzyme called catalase, is especially important in the thymus.
As the organ aged, it became deficient in catalase, allowing the damage caused by hydrogen peroxide to go unchecked.
In genetically engineered mice with raised levels of the enzyme, the thymus was preserved as they aged. And the addition of two other antioxidants - vitamin C and the chemical n-acetylcysteine - to the drinking water of normal mice also protected the thymus.
N-acetylcysteine is used in medicine to treat the effects of paracetamol overdose and certain lung conditions.
After 10 weeks, the thymuses of mice given the antioxidant supplements were "significantly larger" than those of untreated animals, the research published in the journal Cell Reports showed.
The scientists concluded: "We propose that irreversible thymic atrophy represents a conventional ageing process that is accelerated by catalase deficiency."
The findings suggest that oxidation rather than hormones is chiefly to blame for age-related thymus damage.
Dr Petrie said: "Our study shows that the fundamental mechanism of ageing in the thymus, namely accumulated metabolic damage, is the same as in other body tissues.
"However, the process is accelerated in the thymus by a deficiency in the essential protective effects of catalase, which is found at higher levels in almost all other body tissues."
