Mary Carmichael in Newsweek:
At first glance, Paul's HIV vaccine looks familiar; it uses the “neutralizing antibody” strategy, which calls on the body's B cells to make proteins that fight the virus. This approach is how all existing vaccines for other diseases work, but it hasn't succeeded against HIV. The virus is too smart to fall victim to the human immune system. It hides many of the identifying proteins on its outer coat, cloaking them from the prying eyes of B cells, and thus no antibodies are made.
A few proteins on the outside of the HIV virus remain naked and exposed. They have to, in order to bind to human cells and kill them. Paul has his eye on one of these proteins, called gp120. According to his theory, it is a superantigen, a protein related to a fragment of a retrovirus that wormed its way into the human genome hundreds of thousands of years ago and stayed there.
Paul says that because gp120 is a superantigen, it's similar to something the body has seen before. That means the immune system can make antibodies against it—just not enough of them, because after infection, the viral protein sabotages the B cells' assembly line. This is where Paul's vaccine comes in. By chemically manipulating gp120 and administering it as a vaccine, he says, he can cause the B cells to ramp up their production of unusually powerful antibodies, thwarting the virus's attempts at sabotage, arming the immune system, and protecting the body against HIV.
More here.