The thymus becomes active again
Today, AIDS patients survive longer, but most eventually succumb to pathogens that evade the HIV-compromised immune system. However, scientists have now found evidence that the thymus - the school for immune cells - can be reactivated in some people. The body may be able to start producing certain immune cells again after the HI viruses have been suppressed by medication. Immune cells, or more precisely the T-lymphocytes, are produced in the bone marrow and then prepared for use against pathogens in the thymus, a small gland at the base of the neck. This training begins before birth and continues until puberty, at which point the thymus atrophies and ceases to function-at least, that's what was thought. By the time humans reach adulthood, the thymus has generated enough T cells to deal with most of the microbial invaders that a human being is likely to encounter in his or her lifetime. However, HIV attacks and destroys T-cells of all things, so that infected patients no longer have any defenses against many other pathogens. Despite this, some studies have found that patients undergoing powerful new antiviral therapies are able to regenerate some of these cells once the viral load has been reduced for a sufficiently long period of time. Where the new T cells came from was initially a mystery.
To determine whether the thymus might be the source of these new T cells, immunologist Joseph McCune and his collaborators at the Gladstone Institute of Virology and Immunology in San Francisco measured the thymus size of 99 HIV-positive patients (Journal of Clinical Investigation from 1. June 1998 abstract). The team used computer tomography, which creates a three-dimensional X-ray image of the internal organs. About half of the subjects had significantly more thymus tissue than HIV-negative controls. More importantly, the size of a subject's thymus was proportional to the concentration of T cells in their blood. The authors see this as indirect evidence that the thymus might be functioning and releasing these immune cells.
But experts warn that resuscitation of the thymus tissue alone does not prove the gland is functioning properly. Nevertheless, they see the results as "a good argument" for revitalizing the thymus function. "Apparently the immune system is trying very hard to increase the number of T cells," concludes Mario Roederer, an immunologist at Stanford University.
