Even if both the safety and the viral variability issues could be overcome, HIV presents one final challenge to vaccine developers. The immune system can fight only enemies that it can see, but HIV creates many unseen enemies. The virus has a latency period, where the host cell is infected, harbors viral genes and has the potential to reproduce new viral particles at any moment but does not. In this quiescent state the infected cell displays no viral peptides on its surface and has no way of signaling its distress. Developing a vaccine that will distinguish infected but quiescent cells from uninfected cells is an intellectually imposing task.

In one particularly innovative approach, peptides are administered consistently over time, so that the immune system remains at a high state of readiness against HIV. The hope here is that a highly alert immune system might be able to kill off the virus swiftly, either to avert initial infection or to quell reproduction of new viral particles that emerge as latent cells become activated. The strategy taken in this approach is to package the genes for the immunologically stimulatory peptides inside another organism that will continuously produce them and "remind" the immune system what the HIV perpetrator looks like. The way this vaccine works is that genes for some HIV proteins are placed inside a single-celled organism called BCG, a harmless derivative of the bacillus that causes tuberculosis. As a vaccine against tuberculosis, BCG has already been widely used and proven safe in humans. What makes it especially useful for vaccines where it is desirable to maintain a high level of immunological readiness is that the organism is not killed. Rather, the immune system isolates the bacillus in a ball of cells called a granuloma. Even though BCG is held prisoner inside the granuloma, it can still produce substances and secrete them into the blood. Potentially, BCG can be engineered to secrete HIV peptides for an individual's entire life. Thus, if the strategy works, it carries the potential for imparting lifetime immunity. Like all of the other vaccine strategies, however, this one is still in the early experimental stages and is not yet ready for human use.

Therapeutics

As creative as vaccine research has been, scientists who work on AIDS are generally more optimistic about new advances in treating people who already have AIDS with drugs than in preventing infection with vaccines, at least for the immediate future. Broadly speaking, drugs aimed at combating HIV can do one of two things: they can augment the



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