It has long been known that contracting HIV through oral sex is rare. Klara Hasselrot of Stockholm’s Karolinska Institutet recently wrapped up a study–detailed in a forthcoming paper in the international AIDS journal _AIDS_–that might shed some light on why this is. It provides the first-ever evidence that humans can develop resistance to HIV in their saliva.
Hasselrot and her colleagues recruited 25 healthy men, all in long-term homosexual relationships with an HIV-positive partner, and tested their spit for IgA1 antibodies—the same ones that apparently protect some Kenyan sex workers from infection. Thirteen of the men who were regularly exposed to HIV through oral sex with their infected partners had significant amounts of salivary IgA1. The researchers isolated the antibodies and tested their ability to neutralize a lab strain of HIV. The conclusion? “Exposure to HIV, as a consequence of oral intercourse, is sufficient to produce an IgA1-mediated HIV-neutralizing response in the oral mucosa of uninfected men with a male partner.” In other words, being exposed to HIV orally can induce an immune response in saliva that neutralizes the virus. This immunity is also fairly durable–the men in the study were retested after two years, and all but one retained their ability to neutralize the virus. Even more interesting is the fact that men whose partners carried a higher viral load of HIV had a higher capacity to neutralize, suggesting that their antibody production is sensitive to fluctuations in the amount of virus they are exposed to.
Obviously it’s a bad idea to go around willy-nilly having unprotected oral sex with HIV-infected men, but the participants in Hasselrot’s study reported relying on oral sex as a “safe” substitute for anal sex, and it turns out that they were on to something.