Chemical Released by Leaky Gut May Allow HIV to Infect the Brain

Scientists at the Albert Einstein College of Medicine of Yeshiva University in New York City have shed new light on a possible mechanism by which HIV enters the brain and contributes to neurocognitive problems, potentially leading to strategies for preventing HIV infection of the brain and its complications.

The new findings were published in the August 2008 issue of the Journal of Virology and summarized in a press release issued by the university.

In up to 20% of people with HIV, the virus manages to escape from the bloodstream and cross into the brain, resulting in HIV-associated dementia and other cognitive disorders.

The investigators found strong evidence that lipopolysaccharide -- a component of the cell walls of bacteria -- helps the virus penetrate the usually impregnable blood brain barrier (BBB).

"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," said senior study author Dr. Harris Goldstein of the Einstein-Montefiore Medical Center for AIDS Research. "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."

Monocyte (photo from Wikipedia)

Monocytes are a type of immune system white blood cell, or leukocyte, that can harbor HIV. Since HIV does not infect mouse cells directly, the researchers used a strain of mice they developed that carry viral genes that produce HIV in monocytes and T-cells. These were bred with another strain of mice with genes for a green fluorescent protein (GFP), so that the resulting mice (dubbed HIV/GFP-TG) had HIV-infected monocytes that could be detected either by looking for the glowing green marker protein under a microscope or using a PCR assay to detect GFP DNA.

The researchers isolated millions of monocytes from HIV/GFP-TG mice and HIV-free GFP-TG mice, which carried genes for the marker protein but not HIV, and injected them into control mice. After 4 days, the researchers examined the brains of the injected mice to see whether monocytes had crossed the BBB.

While there were no monocytes detected in the brains of any of the control mice injected with HIV-free GFP-containing monocytes, ultrasensitive DNA analysis showed that HIV-infected monocytes were present at very low levels in the brains of nearly one third of the mice injected with HIV-producing monocytes.

"These results demonstrated very clearly that being infected with HIV somehow gives monocytes the capacity to cross an intact BBB," said Dr. Goldstein. "But we also suspected that something else was making it easier for HIV-infected monocytes to breach the defenses protecting the brain from infection."

In 2006, scientists at the National Institutes of Health reported that HIV infection disrupts barriers in the intestine that normally prevent intestinal bacteria from spilling into the bloodstream. They found that the blood of HIV-infected people conatined markedly elevated levels of lipopolysaccharide, a component of certain bacteria that are normally confined to the intestine but can leak out due to HIV infection. In addition, previous animal studies showed hat exposure to elevated lipopolysaccharide levels compromised the integrity of the BBB.

"So we hypothesized that the combination of HIV-infected monocytes and elevated lipopolysaccharide levels would amplify the ability of HIV to cross the BBB and get into the brain," Dr. Goldstein continued.

To test this hypothesis, his team injected control mice with very small amounts of lipopolysaccharide -- comparable to levels in the blood of people with HIV -- that would only minimally weaken the BBB. After 3 hours, half the mice were injected with HIV-producing GFP monocytes and the rest were injected with HIV-free GFP monocytes.

Four days later, HIV-producing monocytes could not be detected in the brains of any of the 15 control mice that were pre-treated with lipopolysaccharide and then injected with HIV-free GFP monocytes. These monocytes were readily detected, however, in the brains of about one quarter of the mice pre-treated with lipopolysaccharide and then injected with HIV-producing GFP monocytes.

"Clearly, HIV-infected monocytes uniquely benefit from the lipopolysaccharide that is present in high amounts in the blood of HIV-infected people," Dr. Goldstein said. "So when HIV-infected monocytes are 'knocking on the door' of the BBB and starting to crack it open, the lipopolysaccharide facilitates their entry by making the BBB more permeable, apparently by weakening blood vessel structure."

If HIV-infected monocytes and lipopolysaccharide in the bloodstream can be considered a "one-two punch" for entry into the brain, a third punch -- simply having a systemic HIV infection -- also seems to help "soften up" the BBB, according to the press release.

In making this discovery, Dr. Goldstein's team used an HIV transgenic (HIV-TG) mouse strain they developed in which HIV replicates inside brain cells associated with the BBB. These mice, along with normal control mice, were injected with lipopolysaccharide and after 3 hours injected with HIV-producing GFP monocytes from the HIV/GFP-TG mouse strain.

Four days later, HIV-producing monocytes could be detected in the brains of about 25% of the control mice, as before. But the brains of 70% of the HIV-TG mice (which support systemic HIV infection) contained HIV-producing monocytes. Furthermore, when present, the number of HIV-producing monocytes was 3 times higher in the brains of the HIV-TG mice compared with the brains of control mice.

"These results demonstrate very dramatically that HIV infection of cells associated with the BBB, in conjunction with However lipopolysaccharide exposure, contributes to BBB breakdown," said Dr. Goldstein. "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 lipopolysaccharide 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 lipopolysaccharide."

These findings could potentially lead to preventive or therapeutic strategies, the researchers suggested. To help maintain the integrity of the BBB in people with HIV Dr. Goldstein said one approach might be to monitor the lipopolysaccharide level in their bloodstream and reduce elevated levels.

"We may be able to use antibiotics that kill intestinal bacteria that make lipopolysaccharide, and drugs are already available that can bind to lipopolysaccharide and clear it from the bloodstream," he stated. "Ideally, we would promptly start newly diagnosed HIV-infected patients on a treatment to reinforce their BBBs so that HIV can't penetrate it -- and perhaps we could even strengthen the BBBs of people who've been infected for quite a while."

But, he added, "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."

Blood-Brain Barrier. Graphic from Society of Neuroscience

9/05/08

Reference
H Wang, J Sun, and H Goldstein. Human immunodeficiency virus type 1 infection increases the in vivo capacity of peripheral monocytes to cross the blood-brain barrier into the brain and the in vivo sensitivity of the blood-brain barrier to disruption by lipopolysaccharide. Journal of Virology 82(15): 7591-7600. August 2008. (Abstract).

Other Source
Albert Einstein College of Medicine. Chemical liberated by leaky gut may allow HIV to infect the brain, Einstein scientists find. Press Release. August 18, 2008.