The thymus is the gateway for immune cell creation, but it is only highly active in the first two decades of life. After that it atrophies. This effectively places a cap on the number of immune cells supported by the body, which is a limit we'd like to work around in order to better maintain immune function in the old. One approach is to rebuild the thymus, such as via tissue engineering or some form of treatment that alters cell signaling to convince existing cells in the body that they are young and thus should be recreating a large and active thymus.
It is as big as an apple in children, but shrinks to peanut size in later life. A quirk of evolution had transformed a "raging torrent", which pumps out [a large number of] T-cells a day in the young, into a "dripping tap" in adults. "The human body was programmed to live for two quick generations in the jungle. Nowadays we all live much longer. We're dealing with an ageing population sitting there without a thymus."
Now [scientists] say they have created the "seeds" to regrow the organ which produces the immune system's soldier cells. [The researchers] have found a way of fashioning stem cells that can develop into the thymus, a pyramid-shaped organ near the heart. The breakthrough [is] the first step in rewiring humans' immune systems to keep pace with their longer life expectancy. The study, which took three years, converted human embryonic stem cells into "thymic" stem cells capable of sprouting a full-sized thymus. The cells could be injected into adult thymuses, triggering renewed growth, or used to grow another thymus from scratch elsewhere in the body.
The next step is to convert the cells into a three-dimensional structure, by cultivating a cluster of the cells around a framework of microscopic fibres in a test tube, and transplanting it under a fold of skin or a membrane around the kidney. The budding thymus is then "logged on" to blood vessels so that it can attract the blood stem cells it converts into T-cells. Last year [scientists] succeeded in doing this in mice. While clinical trials on humans are about five years away, [the] approach could be a major boost for AIDS sufferers or people with depleted immunity. They include cancer patients on chemotherapy or radiation therapy, transplant recipients on anti-immunity drugs, and elderly people who have become so run down that ordinary viruses pose a significant threat.