Study Looks at How HIV May Spread Through Oral Sex

Laboratory studies of mouth tissue suggest that unprotected oral sex does have the potential to transmit HIV, but one expert said it is still less risky than other routes of transmission.

Dr. Xuan Liu of Charles R. Drew University of Medicine and Science and colleagues at the University of California-Los Angeles obtained oral tissue samples from over 50 healthy, HIV-negative patients and exposed the tissue to three different types of HIV. They found that two of the types could infect and reproduce within keratinocytes that line the mouth's surface, and then transfer the infection to adjacent white blood cells. However, the level of infection in the mouth cells was much lower than that seen in white blood cells -- approximately one-fourth to one-eighth lower.

"HIV is able to get into [keratinocytes], but it reproduces less than it would in blood cells... because saliva contains an HIV inhibitor," said Liu.

Researchers found that keratinocytes have two receptors that bind to HIV. However, when the team used inhibitors to block HIV from attaching to those receptors, they noticed that they did not completely block transmission, suggesting that the cells may have lower levels of other receptors used by the virus. Further research is necessary to determine if the laboratory results mimic what actually happens in a living patient, Liu said.

Dr. Jeffrey Laurence, senior scientific consultant for programs at the American Foundation for AIDS Research and director of AIDS Virus Research at Cornell's Weill College of Medicine, said that keratinocytes lack two of the most common receptors for HIV transmission -- CD4 and the CCR5 co-receptor. An effective vaccine would likely have to block these two primary receptors, which are found in cells that line the vagina and rectum.

Laurence believes the findings indicate there is "no reason for altering safer sex guidelines that have been talked about for over 15 years." Laurence said, "No exchange of infected bodily fluids is absolutely safe, but kissing has been shown to be of no risk, and oral sex is of much lower risk than the other traditional factors known to spread HIV."

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