Health

The tonsils might serve as a passageway through which HIV enters the body, suggests an analysis of cells from the mouth and throat.

Researchers stress that the risk of acquiring HIV through oral sex is far too low to warrant preventive tonsillectomies. But they say the finding might have implications for preventing transmission of the virus from HIV-infected mothers to their children through breastfeeding.

Niki Moutsopoulos at the National Institute of Dental and Craniofacial Research in Bethesda, Maryland, US, and her colleagues examined gum tissues taken from dental patients to make room for dental crowns, and tonsils taken from eight individuals as part of a treatment for sleep apnoea, a breathing disorder. "They were pretty much going to throw them out if I didn't use them for my research," Moutsopoulos says of the tissue samples.

With the help of specially designed antibody molecules, the team was able to determine the level of various proteins within these tissues.

About 12% of the tonsil cells tested positive for the protein CD4, which sits on the surface of immune cells that are particularly vulnerable to infection by the virus. By comparison, less than 4% of the gum cells contained this protein.

Moutsopolous and her teammates also conducted a genetic analysis of the tissue samples, using special probes that bind to specific DNA sequences. This experiment revealed that the CXCR4 gene was 11 times more active in the tonsils than in the gum samples. The gene codes for a protein, also called CXCR4, that also helps HIV infect immune cells.

Condoms protect

The scientists found further evidence that the tonsils are perhaps more susceptible to HIV infection that other oral areas, including the cheek lining and floor of the mouth.

The new findings provide biological data to complement clinical and epidemiological evidence suggesting that HIV can - in exceptional cases - spread through oral sex, according to John Greenspan, director of the AIDS Research Institute at the University of California in San Francisco, US. "It's a further piece of the jigsaw puzzle," he told New Scientist.

But he and others note that while HIV transmission can occur through oral sex, it appears to happen very rarely. Experts have estimated that at most 4 out of every 10,000 oral sex exposures result in infection, compared with 1 out of every 200 to 1000 episodes of genital contact.

For this reason, researchers suggest people should use condoms to protect themselves from acquiring HIV and other sexually transmitted diseases through oral sex. Undergoing a tonsillectomy to remove the tonsils would be too drastic a measure based on current evidence, they add. "I think that would be a stretch," says Moutsopoulos. "We can't go taking out every tissue that HIV could pass through."

Probiotic treatments

She adds, though, that if future studies bear out a link between tonsils and HIV transmission, there could be implications for blocking the spread of the virus from mother to child in parts of the world where infant formula and antiretroviral drugs are in short supply. Scientists have estimated that a child has a 15% chance of acquiring the virus through breastfeeding within the first year of life.

Greenspan notes that some scientists have started investigating whether probiotic treatments - which promote the growth of healthful bacteria - might reduce the transmission of HIV to very young children through the oral cavity and gastrointestinal tract.

Moutsopoulos stresses that the tonsils - which were removed routinely up until the late twentieth century - are now seen as a first line of defence in the immune system, alerting the body to potentially dangerous foreign particles. For this reason, she does not believe there is enough evidence to warrant their removal as part of a prophylactic measure against HIV transmission.

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Journal reference: American Journal of Pathology (DOI: 10.2353/ajpath.2007.061006)

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