WASHINGTON: In what is being seen as a breakthrough in finding a "vaccine" to combat AIDS, scientists have discovered a "critical" new way a man can transmit HIV to a woman during sex.
Researchers had long believed that the normal lining of the female vaginal tract was an effective barrier to the invasion of HIV during sexual intercourse. They thought the virus could not penetrate the tissue. But, now a team at Northwestern University has shown for the first time that HIV does penetrate a woman's normal, healthy genital tissue to a depth where it can gain access to its immune cell targets.
"This is an unexpected and important result. We have a new understanding of how HIV can invade female vaginal tract. Until now, science had really had no idea about the details of how sexual transmission of HIV actually works. The mechanism was all very murky," team leader Thomas Hope said.
According to him, if confirmed by future studies, the findings will provide information to help develop microbicides and vaccines to protect against HIV.
"We urgently need new prevention strategies or therapeutics to block the entry of HIV through a woman's genital skin. While condoms are 100 per cent effective in blocking the virus, people don't always use them for cultural and other reasons," he said.
In fact, in their study the scientists have discovered that the interior vaginal skin is vulnerable to HIV invasion at the level where it naturally sheds and replaces skin cells, a point where the cells are not as tightly bound together.
By labelling the HIV viruses with photo-activated fluorescent tags, the team was able to view the virus as it penetrated the outermost lining of the female genital tract, called the squamous epithelium, in female human tissue obtained from a hysterectomy and in animal models.
They found that HIV penetrated the genital skin barrier primarily by moving quickly -- in just four hours -- between skin cells to reach 50 microns beneath the skin, a depth similar to the width of a human hair. This is the depth at which some of the immune cells targeted by HIV are located.
HIV penetration is common in the outermost superficial layers of skin and occurs during the shedding of skin cells. In the process, the skin cells are no longer as tightly bound together so water -- and HIV -- can easily enter. "As pieces of the skin flake off, that's the loose point in the system where the virus can get in," Hope said.
The findings of the study were presented at the annual meeting of the American Society for Cell Biology