HIV is a remarkable escape artist. In the past 15 years the world's scientific community has poured millions into the search for a vaccine, drug or treatment that could kill it off, and failed.
Our understanding of the workings of the virus and how our immune systems respond to it has advanced tremendously, giving us insights into key life processes, but a final cure for the illness remains beyond our grasp. Being elusive is part of what this virus is all about, says Prof William Hall, head of the virus reference laboratory at University College Dublin. It escapes the immune system by mutation, that is by changing into different forms that evade the initial immune cell attack. "It replicates very, very rapidly, which enhances its ability to mutate," he explained.
Its second ploy is to conceal its own genetic code in that of its host. Once bound into the host's own DNA it represents a "reservoir of latent viral material", Prof Hall explained, and when installed, "it is there forever".
HIV is a retrovirus, which means it doesn't follow the conventional pattern of DNA-based life. The genetic code is made of DNA, which in effect is a recipe for making proteins inside the cell. Like any recipe it is only a list of instructions. These must be put into action by RNA. The RNA is like one single step in the recipe which tells the cell how to make the protein.
"The normal flow of life is DNA and that goes to RNA to code proteins," says Prof Hall. "Retroviruses and HIV are different in that they begin as RNA and go back to DNA." HIV enters the cell and then uses the cell's own processing system to install its own genetic code into that of its host. The effectiveness of drug treatments to control HIV is described on this page by Dr Muiris Houston; undoubtedly they have had tremendous success. New drugs have changed HIV from a death sentence to a chronic illness with which patients can live.
Work is proceeding internationally on finding a cure or vaccine. Here, too, researchers are enjoying some tentative successes. A team at Oxford University, for example, began a new vaccine trial in September, one of 70 such HIV trials under way around the world. Eighteen volunteers, including Dr Evan Harris, MP for Oxford West and Abingdon, have been vaccinated to assess safety and also to see whether it produces a useful immune response, according to a report in the British Medical Journal.
Their vaccine approach focuses on a promising finding that, far from being overcome by the virus, the body's immune system mounts a powerful and partially effective initial defence, with its killer Tcells quickly destroying HIV infected cells. But the virus then unleashes its ability to replicate and mutate. Some viral mutants escape and take refuge in the host's DNA where they are safe from the T-cells.
Important research on this response was published in Nature in September by a group from the University of Wisconsin Madison. They studied the immune response of rhesus macaques infected with the monkey equivalent of HIV.
The original viral form was killed off by the T-cells, but mutants survived over time as expected. The researchers found the viral mutants changed a protein called Tat, essential for viral replication but also unfortunately the protein originally recognised and attacked by the T-cells.
The team now believes a vaccine which targets HIV's Tat protein might allow the body to clear out the virus on first exposure.