Are Some People Immune to AIDS?

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So why aren't they sick? Clinicians and researchers have poked, prodded, questioned and bled their healthy human guinea pigs four to six times a year every year in search of any relevant information. Last month the NIH held its first scientific conference to evaluate the mounting evidence. So far, success in fighting HIV does not appear to be closely linked to good diets, the lack of drug use or stress or the absence of other sexually transmitted diseases. Finding no simple patterns, researchers are zeroing in on the men's individual immune responses, even searching through their genetic makeups for the reasons behind their special status. Thanks to advances that have been made in molecular biology and immunology since the start of the AIDS epidemic, scientists have found some tantalizing clues.

Their investigation begins with a white blood cell called CD4. It is the linchpin of the immune system and the main target of HIV. As a general rule, people who become infected by HIV suffer a drop in their CD4 count from a normal level of about 1,200 cells per 1/1000th of a mL of blood to 500 cells or less. The risk of developing one or more of the illnesses associated with AIDS rises dramatically if the CD4 count drops below 200.

One of the most striking things about the healthy survivors is that after the initial drop, their CD4 count stabilizes -- usually above 500. Assaulted but not overwhelmed, they no longer lose any ground against HIV. One possible explanation is that these men were exposed to a strain of the virus that is naturally weaker than most. The immune system subdues the less malevolent virus, allowing the body to fend off any new attacks by more dangerous strains. In the same way, English milkmaids who suffered from cowpox in the 18th century developed an immunity to the disease that also protected them against its more lethal cousin, smallpox. After studying these women in 1796, Edward Jenner developed his smallpox vaccine.

Investigators are also excited by the possibility that some of the HIV survivors' immune systems are cannier than a chess master. They apparently do not allow their opponent much freedom of movement and prevent the virus from mutating very often. This makes the infection easier to control because the body does not have to recognize and subdue new variations every few months. If researchers could figure out how a survivor can keep such tight control of the chessboard, then perhaps they can find a way to give other patients the same ability. Clinicians might even be able to boost the defenses of people whose immune systems have already suffered serious damage.

The healthy survivors may lead a genetically charmed life. Each of the body's cells possesses an identical inherited molecular trait, dubbed its HLA type, that allows an individual to distinguish friend from microscopic foe. Some people's HLA types are more common than others. Heredity specialists have already identified a few genetic types that appear to increase a person's chance of developing AIDS after infection. Now they are trying to determine if long-term survivors hold any inherited molecular configurations in common that could be responsible for their ability to resist HIV.