FLORENCE, Italy -- The AIDS virus, long known to be transmitted through the bloodstream, can also enter the body through cells in the mucous membranes, researchers reported yesterday.
Their findings appear to shed light on one of the puzzles of the epidemic: how heterosexual transmission occurs in the apparent absence of sores, tears or other openings that would allow the virus to enter the bloodstream directly.
In two separate studies reported at the Seventh International Conference on AIDS, U.S. and Italian researchers report finding the human immunodeficiency virus in dendritic and mucosal cells. Dendritic cells are usually near lymph nodes; mucosal cells are found in the lining of the genitals, anus and mouth, as well as in semen and vaginal fluids.
"It helps explain heterosexual transmission," said the lead U.S. researcher, Dr. William Haseltine of Harvard University's Dana-Farber Cancer Institute.
"Just as we know the exact routes of transmission for gonorrhea, syphilis and candida, we now have the route for this virus."
It has long been known that the virus can be transmitted when infected blood or semen enters a person's bloodstream. In the United States, HIV is spread mostly by sharing of needles among drug users and by homosexual intercourse. In most of the world, however, it is chiefly spread by heterosexual sex, which usually does not involve bleeding. To explain this, researchers have theorized that genital sores or rough sexual practices may provide a route in to the bloodstream. The new research explains how sexual transmission can occur without such means -- and also suggests that it could occur through oral sex.
The finding, Dr. Haseltine said, means that acquired immune deficiency syndrome should be regarded like other sexually transmitted diseases.
"Look," Dr. Haseltine said, "let's be candid. AIDS is a venereal disease. . . . It's time people stopped thinking there was something special about somebody else that put them at risk for HIV. Mucosa is mucosa; VD is VD."
Neither dendritic nor mucosal cells have CD4 receptors, special proteins long thought to be the doorknobs that HIV uses to gain entry into a cell. For the last 10 years, AIDS researchers have assumed that the primary target for HIV was CD4-bearing T cells of the immune system.
But, Dr. Haseltine said, it is beginning to look as if the dendritic cells are the first target of the virus and that T cells are secondary.
