NeuroAIDS — the syndrome of neurologic damage caused by HIV — has emerged one of the most frustrating complications of HIV infection. While antiretroviral therapy has tamed the acute ravages of AIDS for many patients, the neurological damage done by the virus is often progressive and largely immune to antiviral treatment.
The problem is complicated by a lack of fundamental knowledge of the mechanisms by which the virus damages the CNS. In the words of Christina Marra, MD, a neurologist and infectious disease expert: “Despite twenty-plus years of investigation, we still don't know how HIV causes brain injury.”
A new study indicates a possible role, and potentially a significant one, for astrocytes. The finding spotlights a group of cells that, until recently, were thought to be “uninteresting” for understanding neuroAIDS, since so few astrocytes are infected with the virus. But the new research shows that those few cells may cause widespread toxicity to their neighbors, disrupting the blood-brain barrier and triggering apoptosis in many cells that are not themselves infected.
Only 5 percent of astrocytes become infected with HIV, according to Eliseo Eugenin, PhD, assistant professor of pathology at the Albert Einstein College of Medicine of Yeshiva University in New York, who was the lead researcher on the study published June 29 in The Journal of Neuroscience.
Even in these, he said, viral production is extremely low, explaining the lack of interest in them among many scientists. The majority of virus in the brain resides instead in microglia and macrophages. The importance of the CSF as a reservoir for HIV that is relatively resistant to antiretroviral treatment has recently become an important topic in the field, as researchers contemplate whether complete eradication of the virus is a practical therapeutic strategy.
But the new study suggests that astrocytes may play a more important role than their numbers indicate. “We have identified an amplification system,” he said.
The system involves gap junctions between adjacent astrocytes. Gap junctions are large channels that link the cytoplasm of neighboring cells, allowing ions, small molecules, and even proteins to pass freely between them. Astrocytic gap junctions allow glutamate, for instance, to be absorbed in an area of high neuronal activity, and then to be diffused to other astrocytes to prevent a potentially toxic build-up.
Astrocytes also play a key role in maintaining the blood-brain barrier. Astrocytic “end feet” surround the blood vessels, helping to regulate both the passage of materials into the brain, and the flow of blood to areas of high metabolic demand.
In cell culture, Dr. Eugenin showed, infection of only a small percentage of astrocytes caused a disruption of the blood-brain barrier, increasing its permeability to molecules normally excluded, including large proteins, and leading to apoptosis of endothelial cells. These effects could be blocked by blocking the gap junctions linking the astrocytes. The effects of infection were especially noticeable on the astrocyte end feet, which displayed misshapen termini and aberrant interactions with the endothelium. These effects, too, could be prevented by blocking the gap junctions, indicating the flow of signals from infected to uninfected cells through these channels.
In a model of neuroAIDS, pigtail macaques infected with simian immunodeficiency virus (SIV), about 4 percent of the astrocytes were infected, mimicking the human condition. As in cell culture, cells in the macaque brain that were in contact with infected astrocytes underwent apoptosis. These cells included endothelial cells and uninfected astrocytes. Remarkably, the infected astrocytes themselves did not succumb to infection.
“The cells are immortalized,” Dr. Eugenin said. Previous work from the same lab has indicated that astrocytic infection increases inflammation. “This brings in macrophages, and the astrocytes can then transfer cells to the macrophages.” This reactivates the virus, spreading the infection, he explained.
“I think these results are really exciting, for two reasons,” Dr. Eugenin said. “We are demonstrating that this viral reservoir is important, because the HIV is hijacking this gap-junction communication system to spread inflammation, apoptosis, and new infection. This means we have to target not only those immortalized cells, but also the toxic signals that arise in these few cells. It also may explain why many people receiving antiretroviral therapy who have undetectable viral load still develop cognitive disorders. It's not the virus. It's the virus amplifying these toxic signals.”
Much is still unknown about the role of astrocytes in neuroAIDS, and about the mechanism through which they exert their destruction. Dr. Eugenin is moving on to characterize the signals that pass from cell to cell through the gap junctions, hoping their identity may reveal potential therapeutic targets.
The study was supported by grants from the NIH, including the National Institute of Mental Health and the Centers for AIDS Research.
The uncertainty about the role of astrocytes was echoed by Avindra Nath, MD, clinical director of the NINDS, and chief of the Section of Infections of the Nervous System. “We don't know the relative contributions” of different cell types in the central nervous system, he said. “I think it is fair to say there are multiple mechanisms involved, and this may be one of them.”
Dr. Nath termed the current study “an interesting preliminary step,” but indicated much more work needs to be done. He pointed out that in humans with neuroAIDS, the damage follows a long period of infection, rather than occurring in the acute phase, as in the cell culture and SIV models. “The more important phase to study is the later stages.”
“The most important questions related to this paper and to neuroAIDS in general,” he said, “is whether the brain is a reservoir, and if so, where does the virus reside, and what can we do to control or eliminate the reservoir? In this paper, they show that infected astrocytes can have profound effects on other cell types, even though there are few of them. I think that is an interesting concept, and an important one for us to consider.”
Dr. Marra, professor of medicine at the University of Washington School of Medicine in Seattle, said that the role of the astrocytic population as a potential viral reservoir is only now getting the attention it deserves. “The mantra has been that they are not productively infected,” she said. The current study indicates they may be playing a previously unanticipated role in neuroAIDS nonetheless.
The role suggested for astrocytes is “a reasonable hypothesis,” she said, although the limitations of the models limit the conclusions about the importance of that role. No single mechanism, whether dysregulation of blood flow or increased apoptosis or some other consequence of infection, may be central. “The problem is probably so much more complicated than we can fathom,” she said. “But I think this research does draw attention to a phenomenon that wasn't getting attention before.”