Thymus Perks Up in HIV Patients

AIDS patients are surviving longer these days, but most still succumb to opportunistic pathogens that elude an HIV-weakened immune system. Now, however, scientists have evidence that an immune-cell proving ground that shuts down naturally with age may be able to return to active duty in some people. The finding, reported in the current Journal of Clinical Investigation, suggests that the body might resume production of certain immune cells after drugs batter HIV into submission.

Immune cells called T lymphocytes--frontline troops in the battle against foreign microbes--are produced in the bone marrow and then battle-readied in the thymus, a small gland at the base of the neck. This training is thought to take place from before birth until puberty, when the thymus atrophies and is generally thought to stop functioning. By adulthood, according to most medical textbooks, the thymus has groomed enough T cells to deal with most of the microbial invaders any one person is likely to encounter in a lifetime. But HIV attacks and destroys T cells, leaving infected patients defenseless against many pathogens. Nevertheless, some studies have shown that patients on powerful new antiviral therapies appear able to regenerate some of these cells once viral burdens have been lowered for a sufficient amount of time--leading researchers to wonder about the source of these new T cells.

To see whether the thymus might be the source of these new T cells, immunologist Joseph McCune and his co-workers at the Gladstone Institute of Virology and Immunology in San Francisco measured thymus size in 99 HIV-positive patients. The team used a technique called computed tomography, which creates a three-dimensional x-ray image of internal organs. About half the subjects had considerably more thymus tissue than HIV-negative controls. What's more, the size of a subject's thymus gland correlated closely to the blood concentration of T cells--indirect evidence that the thymuses might be functioning and releasing these immune cells, say the authors.

Experts caution that the rejuvenation of thymus tissue alone does not prove that the gland is working properly. Still, the results are "a good argument" for a rebirth of thymus function, says immunologist Brigitte Autran of the Pitié-Salpêtrière Hospital in Paris, whose team uncovered some of the first evidence that immune recovery might be possible. "Obviously the immune system is trying harder to bring up T cell counts," adds Mario Roederer, an immunologist at Stanford University.

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