A new study from Sweden suggests that repeated exposure to HIV via oral sex may trigger neutralizing antibodies that protect against the virus.
Some HIV-negative men in long term relationships with HIV-positive men have an antibody response in saliva which may inhibit HIV infection, report Swedish researchers in an article published online ahead of print in AIDS. This is the first time that such a response has been described in saliva, and may help explain why infection through oral sex is somewhat infrequently reported even in serodiscordant couples.
While it is well established that while HIV infection during fellatio and other types of oral sex can and does happen, the number of infections that can be attributed to oral sex is relatively small in comparison with the number of times that unprotected oral sex is practiced. One reason is that saliva contains enzymes which partially inhibit HIV infection.
Klara Hasselrot and colleagues from the Karolinska Institutet in Stockholm wished to investigate whether in long term relationships where one partner has HIV, the HIV-negative partner develops IgA1 antibodies in saliva that would help inhibit HIV infection during oral sex.
They recruited 25 HIV-negative men who were in a relationship of at least six months duration with an HIV-positive man. In addition, 22 HIV-negative men who were not in a serodiscordant relationship were recruited at a blood donor clinic to act as controls.
Analysis of the medical records of the HIV-positive partners showed that whilst most were on treatment at the time of the study, only two had been on antiretroviral treatment with undetectable viral loads for the entire length of their relationship. The researchers judge that this means that, with two exceptions, all HIV-negative partners have probably been exposed to HIV at some point.
 Hasselrot K et al. Oral HIV-exposure elicits mucosal HIV-neutralizing antibodies in uninfected men who have sex with men. AIDS (online edition), 2009.