Research on monkeys has found that as few as seven cells productively infected with HIV may be able to establish an infection via sex (Kaizu).
The unexpected ease with which the researchers infected animals that, in some cases, were intended to be control animals prompts them to speculate that we may have underestimated the contribution infected T-cells make to HIV transmission, compared with free virus in body fluids.
It has been known since HIV was discovered that an infection can be established either with free-floating viral particles or by cells infected with, and producing, virus. It has been found that as few as two productively-infected cells can establish an infection if they are directly injected into the bloodstream, and in cases of urethritis semen may contain as many as a million T- and B-cell lymphocytes per millilitre of semen.
However it was thought that for cell-mediated infection to occur via sex, there had to be a significant breach of the integrity of the genital or rectal mucosa.
Researchers at the US national Primate Research Center in Wisconsin, wishing to establish a model for cell-mediated infection for future prevention experiments, vaginally inoculated ten cynomolgus (pig-tailed) macaques with peripheral blood mononuclear cells (PBMCs – white blood cells of which 35-45% were CD4 cells) which had been infected with a variety of simian immunodeficiency virus called SIVmac239.
They had induced temporary genital ulcers in five of the monkeys by inoculating them with 5ml of a lubricant containing the detergent benzalkonium chloride.
Three monkeys, two with induced ulcers and one so-called ‘control’ with no genital inflammation, were inoculated three times in three days with 500,000 cells. Four monkeys, two with induced ulcers, were inoculated three times (or in one case twice) with 5,000 cells. And three monkeys, one with induced ulcers, were inoculated just once with 5,000 cells.
About 4-10% of the cells in the inoculum (viral dose) expressed the SIVmac239 antigen on their surface and were therefore infected with proviral DNA. However, the vast majority were not productively infected, that is, they were not expressing viral particles on the cell surface nor were they displaying receptors indicative of activation that might indicate productive infection.
Because of this, the number of productively infected cells as measured by PCR testing of the fluid surrounding cells varied from 384 to 8,192 in the 500,000 cell inoculum and from 4 to 14 cells in the 5,000 cell inoculum.
All the animals with induced ulcers that received three inocula were infected, though the animal that only received one dose was not.
Surprisingly, however, two of the control animals with no inflammation were also infected. One was the other animal given 500,000 cells for three days. The other was the one that was given 5,000 cells just twice: the third inoculum was withheld because the monkey began menstruating on the third day. Neither of them had clinical evidence of inflammation before infection though there was mild inflammation at two or three days (the presumed time of infection).
In contrast, a monkey given three 5,000-cell doses that did have vaginal inflammation and discharge which had not been deliberately induced but was due to a bacterial infection was not infected with SIV.
“Unexpectedly, some of the animals that had no clinical evidence of inflammation became infected,” say the researchers.
“These results suggest that though major inflammation may increase the probability of infection, it is not an absolute requisite for this route of transmission.”
Why is this important? Firstly, productively-infected cells shedding virus may be present on mucosal surfaces like the lining of the foreskin, vagina and rectum at any time during sex and do not have to be contained in a ‘body fluid’.
Secondly, as the researchers say, “infected cells can migrate through epithelial abrasion, or they may adhere to the mucosal epithelium, serving as a continued source of budding virus.”
And thirdly, because the cells already contain proviral DNA within their genome, they are “particularly difficult to neutralise” – imported cells may smuggle HIV into another body in the guise of a genetic sequence that cannot be directly targeted by antiretroviral drugs.
This reinforces the message that the period of risk for transmission during sex occurs throughout penetration, and not just via release of body fluid. It adds to the evidence for circumcision as a preventative measure as this reduces the mucosal area that might also be shedding productively-infected cells.
Kaizu M et al. Repeated intravaginal inoculation with cell-associated simian immunodeficiency virus results in persistent infection of nonhuman primates. J Infect Dis 194: 912-916, 2006.