Scientists have long believed that the inner layer of a healthy woman’s vagina is an effective barrier against HIV invasion during sexual intercourse, but new research from Northwestern University Feinberg School of Medicine has found for the first time that HIV can penetrate a woman’s normal, healthy reproductive tissue and enter the body to infect immune cells. The study’s lead researcher, Thomas Hopper, a professor of cellular and molecular biology at the Feinberg School of Medicine, said, “This is a new study. This is an unexpected but important result,” said Hopper.
We have a new understanding of the way HIV enters the female vagina. Until now, the scientific community still did not understand the details of how HIV is transmitted through sex.”
Hopper and colleagues at Northwestern University and collaborators at Tulane University found that the inner layer of vaginal skin is so fragile that it can be invaded by HIV, entering where skin cells are not tightly bound and killing and replacing them. The study was presented Dec. 16 at the 48th American Society for Cell Biology in San Francisco.
Information from the Centers for Disease Control shows that women and adolescent girls account for 26 percent of new HIV infections in the United States. The latest analysis of the CDC’s 2005 data shows that there were 56,300 new HIV infections in 2005, 31 percent of which came from contact between patients and high-risk heterosexuals. More than half of the new HIV infections were in women.
Hopper said he hopes the findings will help in the development of HIV microbicides and vaccines, if his findings can be further demonstrated through future research. He said, “We urgently need new prevention strategies or treatments to stop HIV from entering the body through female reproductive skin.” Although condoms are effective in preventing the virus, he said, “people don’t use condoms very often for cultural and other reasons.”
By labeling the HIV virus with a light-activated fluorescent label, the Northwestern researchers were able to see the virus penetrate the outermost layer of the female reproductive tract, also called the squamous epithelium, in female tissue obtained through hysterectomy and in animal prototypes. The researchers found that HIV penetrates this reproductive skin barrier by moving rapidly between skin cells in just four hours, reaching 50 microns below the skin, a depth similar to the diameter of a human hair. Here, some immune cells become targets of the HIV virus.
HIV penetration of the outermost surface layer of the skin is more common and more likely to occur during the metabolism of normal skin cells. During metabolism, skin cells are no longer tightly bound together, so water and HIV can easily enter. Hopper said, “As the skin cells are shed, the cells become loose and the virus can get in.” Previously, scientists thought that HIV invaded the female immune system through a single layer of skin cells in the cervical canal, which Hope says, “has always been considered a weak point of the female immune system.”
However, a previous study in Africa found that women using a uterine cap condom to isolate the cervix did not reduce infection rates. Women who had hysterectomies also did not reduce the rate of HIV infection from sexual intercourse. Hopper said researchers also believe that the only way HIV can enter the vaginal canal is if a woman has trauma to her skin, such as that caused by the herpes virus. If there is a fissure in the skin, then HIV should easily enter the skin and bind to immune cells to make it infected. However, in studies where women were given anti-herpes drugs to reduce trauma, researchers found that this approach did not reduce the rate of HIV infection. The researchers of this latest finding believe that this may be because HIV can enter vaginal tissue without any fissures in the body and infect immune cells.
The major error in the field is the idea that HIV is transmitted in only one way,” Hopper said. We believe that HIV can infect the body in more than one way. We think HIV can be transmitted directly through the skin.” The next step will be to show that HIV can indeed infect immune cells in the vaginal canal. Hopper says, “An important future test will be to identify the first infected cells within the epithelium, which was previously thought to be unnecessary to check.”