A group of Swedish researchers has found that, in a group of gay men in serodiscordant relationships (that is, one guy is positive, the other not), the HIV-negative partner’s saliva produced IgA1 antibodies that help inhibit HIV infection during oral sex. While it’s been long-proven that it’s possible to contract HIV through oral sex, it’s relatively uncommon compared to transmission via anal intercourse. The scientists believe they’ve found the reason.
“[An] 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.
Analysis of whole saliva samples showed that saliva from 15 of the men in serodiscordant relationships had HIV neutralising capacity. This was also the case for six of the control group, which confirms saliva’s usual HIV inhibiting activity.
Further tests were performed on samples of the IgA1 antibodies only. In these tests, antibodies from 13 of the serodiscordant partners, but none of the control samples, neutralised HIV.
The researchers believe that repeated exposure to HIV during oral sex produces this specific immune response in saliva. Moreover they argue that the inhibitory effect of IgA1 is likely to be a significant contributor to neutralisation in the whole-saliva samples. Looking at the 13 men whose IgA1 was able to neutralise HIV, they re-tested saliva samples from which IgA1 had been removed. Only five of these samples had neutralising activity.
Two years after enrollment into the study, new samples were taken and tested. The situation was unchanged for almost all serodiscordant partners (although one man’s saliva showed neutralising capacity for the first time, and another man lost this ability). Moreover, they all remained HIV-negative.
The researchers also found that men who had neutralising capacity in their saliva tended to have partners with a higher viral load than men who did not have this capacity. This would suggest that neutralising capacity is determined by the amount of exposure to virus.”