WASHINGTON - Researchers have found another handle that the AIDS virus uses to attack cells, and said this one may explain how it gets into the gut, where it hides out and multiplies for a full assault on the body.

The handle is a cell receptor, and its discovery could open new ways to fight the fatal and so far incurable virus, the team at the National Institute of Allergy and Infectious Diseases reported on Sunday.

The human immunodeficiency virus, or HIV, attacks immune system cells, or lymphocytes, known as CD4 T-cells. It was known to use three main receptors, or molecular doorways, to infect cells: CD4, CCR5 and CXCR4.

Writing in the journal Nature Immunology, the NIAID scientists said they found a similar receptor, more a handle than a doorway, called integrin alpha 4 beta 7.

"It is a homing receptor for lymphocytes to get to the gut. It is the very molecule that steers lymphocytes to the gut and keeps them there," NIAID director Dr. Anthony Fauci said in a telephone interview.

"It is not only important in that it is a homing receptor to the gut. But it also can play a role in enhancing the ability of HIV to spread in the body."

Much of the body's immune response takes place in the gut, in gut-associated lymphoid tissue, or GALT.

Soon after HIV has infected someone it quickly moves to the gut tissue and starts replicating. This kills off the CD4 T-cells, leaving patients with a poorly functioning immune system and vulnerable to the infections that mark AIDS.

"The gut is where the virus really takes hold," Elena Martinelli, who worked on the study, said in a statement.

"We found that integrin alpha 4 beta 7, whose natural function is to direct T cells to the GALT, is also a receptor for HIV. It is very unlikely that this is a coincidence."

This could help explain why drug cocktails called highly active antiretroviral therapy, or HAART, cocktails often fail to completely suppress the virus. They can keep patients healthy for years, but never eradicate the virus.

"It has been suggested that the massive depletion of memory CD4 T-cells from the gut, soon after transmission, is central to HIV disease," the researchers wrote.

They said an important next step will be developing drugs that stop HIV from attaching to this receptor.

More than 33 million people are infected with HIV globally and 25 million have died of AIDS. There is no vaccine.