NEW DELHI: Researchers from the University of Washington in Seattle have, for the first time, found exactly how the human immunodeficiency virus (HIV) enters the female body during sex and which are its very first-target cells in the vagina.

The discovery provides hope to a devastated scientific community working on developing an effective microbicide, to help women protect themselves against HIV, after clinical trials of the most promising anti-HIV vaginal gel were recently halted in India and Africa. The gel was found to heighten the risk of contracting HIV rather than providing protection.

Now, the knowledge of how the vagina allows the virus to enter the body will provide a crucial boost for these scientists to discover the most effective microbicide. Microbicides are creams or gels that aim to block HIV from invading a woman's body.

Using a unique model system they developed, the scientists found that HIV-1 simultaneously enters two different types of cells in the vaginal epithelium (outer lining of vaginal cells) associated with the immune system � Langerhans cells (LC) and CD4+ T-cells � but through different mechanisms. The scientists reported in the journal Immunity, that both LC and CD4+ cells move out of the vaginal epithelium after the infection.

Researcher Julie McElrath reported, "Understanding the initial events in the establishment of vaginal HIV type-1 entry and infection has been hampered by the lack of appropriate experimental models. We have now shown that upon contact, HIV-1 rapidly penetrated both LC and CD4+ T-cells. HIV-1 entered CD4+ T-cells almost exclusively by CD4 and CCR5 receptor without requiring passage from LC. By contrast, HIV-1 entered LC primarily by means of multiple receptors. Our findings shed light on the very earliest steps of HIV infection and may guide the design of effective strategies to block local transmission and prevent HIV-1 spread."

Scientists have for long known that HIV-1 enters CD4+ cells by interacting with a protein called CCR5. But the researchers wanted to know what happens when women are exposed to HIV through vaginal sex. The team then used a chemical treatment to separate the outermost layer of vaginal skin from the underlying tissue. They then exposed this layer to HIV viruses that had been tagged with a glowing dye. The team could then see which cells the viruses had infected. Within two hours, HIV had attacked the CD4+ T-cells, and infected almost half of them. "Now we know that any effective microbicide must prevent HIV infection from entering both T-cells and LC," a NACO official said.

Women are physically more susceptible to HIV infection than men. Married women in developing countries are among those at highest risk due to their multi-partner husbands. Most new infections, about 14,000 every day, are in women.

According to WHO, 17.5 million women are living with HIV/AIDS, the worst affected being those aged 15-24 years, who are three to four times more likely to be infected with HIV than young men of the same age. UNAIDS says over 38% of those living with HIV in India are women.