"Previous research has suggested that it's not individual HIV viruses that get into the brain but rather HIV-infected immune cells known as monocytes," says Dr. Harris Goldstein, director of the Einstein-Montefiore Medical Center for AIDS Research, and senior author of the study published in the Journal of Virology.
"Using an animal model, we wanted to find out first of all whether being infected with HIV enables monocytes to do what they don't usually do - escape from blood vessels and enter brain tissue," the researcher added.
For their study, the researcher first created transgenic mice whose monocytes coded for green fluorescent protein (GFP), so that they would become easy to detect.
The group later injected HIV-infected GFP monocytes and the normal ones into control mice.
When the brains of the subjects were examines four days later, the researchers observed that there was no sign of monocytes in the brains of any of the mice injected with uninfected GFP monocytes.
However, monocytes were found to be present at very low levels in the brains of nearly one third of the mice injected with the HIV-producing monocytes.
"These results demonstrated very clearly that being infected with HIV somehow gives monocytes the capacity to cross an intact blood brain barrier (BBB). But we also suspected that something else was making it easier for HIV-infected monocytes to breach the defences protecting the brain from infection," says Dr. Goldstein.
In 2006, scientists at the National Institutes of Health had reported that HIV infection breaks down barriers in the intestine that normally prevent intestinal bacteria from entering the bloodstream.
Scientists associated with that study noticed that the blood of HIV-infected people contained markedly elevated levels of lipopolysaccharide (LPS), a component of certain bacteria that are normally confined to the intestine but leak out due to HIV infection.
"So we hypothesized that the combination of HIV-infected monocytes and elevated LPS levels would amplify the ability of HIV to cross the BBB and get into the brain," says Dr. Goldstein.
With a view to testing their hypothesis, the researchers injected control mice with very low doses of LPS that were comparable to the levels in the bloodstream of HIV-infected individuals, and would only minimally weaken their BBB's.
Half the mice were injected with HIV-infected GFP monocytes three hours later, while the remaining were injected with normal GFP monocytes.
Four days later, monocytes could not be detected in the brains of any of the 15 mice that were pre-treated with LPS, and then injected with normal monocytes producing GFP alone.
However, monocytes were readily detected in the brains of about 25 per cent of mice pre-treated with LPS, and then injected with HIV-and-GFP-producing monocytes.
"Clearly, HIV-infected monocytes uniquely benefit from the LPS that is present in high amounts in the blood of HIV-infected people. So when HIV-infected monocytes are 'knocking on the door' of the BBB and starting to crack it open, the LPS facilitates their entry by making the BBB more permeable, apparently by weakening blood vessel structure," says Dr. Goldstein.
In another experiment, the researchers studied a mouse strain they named HIV-TG, in which HIV could replicate inside brain cells associated with the BBB.
These mice, along with control mice, were injected with LPS and, three hours later, intravenously injected with HIV-and-GFP-producing monocytes from the HIV/GFP-TG mouse strain.
Four days later, HIV-producing monocytes could be detected in the brains of about 25 percent of the control mice, as in the preceding experiment.
By contrast, more than twice as many (70 percent) of the brains of HIV-TG mice that support systemic HIV infection contained HIV-producing monocytes.
When present, HIV-producing monocytes were three times more numerous in the brains of HIV-TG mice than in the brains of control mice.
Dr. Goldstein said: "These results demonstrate very dramatically that HIV infection of cells associated with the BBB, in conjunction with LPS exposure, contributes to BBB breakdown. So when HIV infection occurs, we seem to have a 1-2-3 combination of punches working in concert to facilitate entry of HIV-infected monocytes into the BBB-protected brain: HIV-infection of monocytes increases their capacity to cross even an intact BBB; HIV infection in the gut releases LPS into the bloodstream allowing it to erode the BBB; and HIV infection of the cells of the BBB makes them more sensitive to the deleterious effects of LPS."
The researchers believe that their findings may lead to preventive or therapeutic strategies.
Dr. Goldstein says that one approach to help maintain the integrity of the BBB in HIV-infected people might be to monitor the LPS level in their bloodstream, and then reduce elevated levels.
"We may be able to use antibiotics that kill intestinal bacteria that make LPS, and drugs are already available that can bind to LPS and clear it from the bloodstream. Ideally, we would promptly start newly diagnosed HIV-infected patients on a treatment to reinforce their BBB's so that HIV can't penetrate it - and perhaps we could even strengthen the BBB's of people who've been infected for quite a while. But before we can prevent the tragedy of HIV-associated dementia, we need to better understand the mechanism by which these molecular and cellular 'punches' interact to undermine the BBB," said the researchers.
The study has been published in the Journal of Virology.