British and American scientists have made quite a forward leap in their understanding of the human immunodeficiency virus (HIV), which could lead to clues in developing a vaccine against AIDS.

Scientists found that if cholesterol is removed from the HIV, then it becomes unable to attack its host's immune system.

"It's like an army that has lost its weapons but still has flags, so another army can recognise it and attack it," said Adriano Boasso of Imperial College London, who led the study, which was published Monday in the journal Blood.

The team now plans to find out how to use this way of inactivating HIV and possibly develop it into a vaccine.

Usually when a person becomes infected with HIV, the body's immune response naturally puts up an immediate defence. But some researchers believe HIV causes the innate immune system to overreact, weakening the next line of defence, known as the adaptive immune response.

"HIV is very sneaky," Boasso said in a statement. "It evades the host's defences by triggering overblown responses that damage the immune system... It's like revving your car in first gear for too long -- eventually the engine blows out," he explained.

For this study -- published yesterday in the journal Blood -- Boasso's team removed cholesterol from the membrane around HIV and found that this stopped HIV from triggering the innate immune response. Without the overreacting if the innate immune system, a stronger adaptive response is developed, orchestrated by a type of immune cells called T cells.

Boasso said this may be why developing a vaccine has proven so tricky.

"Most vaccines prime the adaptive response to recognise the invader, but it's hard for this to work if the virus triggers other mechanisms that weaken the adaptive response."

HIV takes its membrane from the cell that it infects, the researchers explained in their study. This membrane contains cholesterol, which helps keep it fluid and enables it to interact with particular types of cell.

Normally, a subset of immune cells called plasmacytoid dendritic cells (pDCs) recognize HIV quickly and react by producing signaling molecules called interferons. These signals activate various processes which are initially helpful, but which damage the immune system if switched on for too long.

Working with scientists Johns Hopkins University, the University of Milan and Innsbruck University, Boasso's team found that if cholesterol is removed from HIV's envelope, it can no longer activate pDCs.

As a result, T cells, which orchestrate the adaptive response, can fight the virus more effectively.

UK's Mail Online reports that AIDS kills around 1.8 million people a year worldwide. An estimated 2.6 million people caught HIV in 2009, and 33.3 million people are living with the virus.

Major producers of current HIV drugs include Gilead Bristol Myers Squibb, Merck, Pfizer and GlaxoSmithKline.