The greatest breakthrough in understanding how HIV infects the human body was made by a Dutch research team: mucosa cells covering the human genitalia synthesize a protein that destroys HIV, keeping its infectious ability much lower than it would otherwise be.

Researchers even speculate that boosting the activity of this protein, named Langerin, could be a important means of cutting off HIV transmission.

The cells producing the new discovered protein are the Langerhans cells, disposed in a web-like network in skin and mucosa, a barrier HIV must surpass on its way to infecting the host.

"We observed that Langerin is able to scavenge viruses from the surrounding environment, thereby preventing infection," said lead researcher Teunis Geijtenbeek, an immunologist researcher at Vrije University Medical Center in Amsterdam.

"And since generally all tissues on the outside of our bodies have Langerhans cells, we think that the human body is equipped with an antiviral defense mechanism, destroying incoming viruses," he added.

The discovery "is very interesting and unexpected," said Dr. Jeffrey Laurence, director of the Laboratory for AIDS Virus Research at the Weill Cornell Medical College, in New York City.

"It may explain part of the relative inefficiency of HIV in being transmitted."

Even if HIV has produced 22 million dead victims since first discovered 25 years ago, it is in fact relatively weak at infecting humans.

"For example, the human papillomavirus (HPV), which causes cervical cancer, is nearly 100 % infectious," Laurence noted.

In other words, each sexual contact with an infected individual leads to a new infection.

"On the other hand, during one episode of penile-vaginal intercourse with an HIV-infected partner, the chance that you are going to get HIV is somewhere between one in 100 and one in 200," Laurence said.

This has been puzzling the scientists.

The Dutch team investigated Langerhans cells from 13 human donors.

HIV infects the immune system T-cells, but these ones are relatively far away (in lymph tissues), and the virus must pass first through the nearby Langerhans cells.

It was long thought that HIV easily infects Langerhans cells, but the new research revealed that the Langerhans cells "do not become infected by HIV-1, because the cells have the protein Langerin on their cell surface," Geijtenbeek said.

"Langerin captures HIV-1 very efficiently, and this Langerin-bound HIV-1 is taken up (a bit like eating) by the Langerhans cells and destroyed. Langerhans cells act more like a virus vacuum cleaner," said Geijtenbeek.

In some circumstances, like very high levels of HIV exposure or weak Langerin activity, the Langerhans cells get infected.

"The finding is exciting for many reasons, not the least of which is its potential for HIV prevention. We are currently investigating whether we can enhance Langerin function by increasing the amount of Langerin on the cell surface of Langerhans cells. This might be a real possibility, but it will take time. I am also confident that other researchers will now also start exploring this possibility," said Geijtenbeek.

This protein explains variance in HIV resistance amongst different individuals.

"It is known that the Langerin gene is different in some individuals. These differences could affect the function of Langerin. Thus, Langerhans cells with a less functional Langerin might be more susceptible to HIV-1, and these individuals are more prone to infection. We are currently investigating this," said Geijtenbeek.

The discovery could lead to the development of topical protective microbicides for the vagina against HIV infection.

"Choosing compounds that allow Langerin to continue to work its magic will enhance any candidate microbicide's effectiveness," added Geijtenbeek.

Still, Laurence warns.

"In the test tube, this is a very important finding. But there are many things in the test tube that don't occur when you get into an animal or a human. Having said that, though, this is a very intriguing finding."