Over 35m people worldwide are currently infected by HIV. Antiviral therapies can keep the virus from multiplying. However, no drug can cure infection so far, because various cell types continue to carry the virus in a latent, ie, quiescent, state. Scientists of Helmholtz Zentrum München have now established a model for latent HIV infection of brain cells. The researchers used this model to identify various compounds that affect latency of the virus in the brain.
“Chronic infection is caused by long-lived cells with resting viral genomes that are activated by different factors,” explained Professor Dr Ruth Brack-Werner of the Institute of Virology. “These so-called latently infected cells occur in the blood and in the brain, among others. HIV latency in the brain is particularly difficult to investigate,” she added. Her research group is studying HIV persistence in a very important type of brain cell called astrocytes. The human brain contains billions of them. The many functions of astrocytes include protecting the brain from injury and harmful agents and providing essential support for nerve cells. Mature astrocytes can have a very long lifespan and may exist for years.
Recent studies identified HIV genomes in up to 19% of astrocytes in brain tissues from deceased HIV-1 infected individuals. So far, no experimental model has existed to study HIV latency in these cells. “With our model system, we can simulate latent HIV infection in astrocytes,” said Dr Martha Schneider, first author of the study. The researchers showed that various substances, including the cytokine TNF-alpha, can reactivate the inactive virus. Conversely, it was also possible to inhibit the reactivation of the virus by treating the cells with certain compounds. “These results identify drug candidates that may prevent activation of latent viruses in astrocytes”, Schneider concluded.
In the future, the scientists plan to use this system to study the effect of these and other compounds that may prevent the activation of HIV-1 in the brain. As study director Brack-Werner explained: “Several viral proteins are toxic to neurons and may cause immune damage in the brain. Since only limited replacement of astrocytes occurs in the brain, loss of these cells may cause serious damage. Thus silencing the virus in brain cells is an important goal.” In addition, the researchers plan to test the effect of approved drugs and thus to improve the clinical care of HIV-1 patients in the future.
Objective: Macroglial cells like astrocytes are key targets for the formation of HIV-1 reservoirs in the brain. The ‘shock-and-kill’ HIV-1 cure strategy proposes eradication of reservoirs by clinical treatment with latency reversing agents (LRAs). However, virus activation may endanger the brain, due to limited cell turnover, viral neurotoxicity and poor penetration of antiretroviral drugs. Since the brain is not accessible to clinical sampling, we established an experimental model to investigate the LRA effects on HIV-1 latency in macroglial reservoirs.
Design: Human neural stem cells (HNSC.100) were used to generate a system that models HIV-1 transcriptional latency in proliferating progenitor, as well as differentiated macroglial cell populations and latency-modulating effects of LRAs and compounds targeting HIV-1 transcription were analysed.
Methods: HNSCs were infected with pseudotyped Env-defective HIV-1 viruses. HIV-1 DNA and RNA levels were quantified by qPCR. Expression of latent GFP-reporter viruses was analysed by confocal microscopy and flow cytometry. NF-κB signalling was investigated by confocal microscopy and chromatin immunoprecipitation.
Results: Two of the eight well known LRAs (tumour necrosis factor-alpha, suberoylanilide hydroxamic acid) reactivated HIV-1 in latently infected HNSCs. Tumour necrosis factor-alpha reactivated HIV-1 in progenitor and differentiated populations, whereas suberoylanilide hydroxamic acid was more potent in progenitors. Pre-treatment with inhibitors of key HIV-1 transcription factors (NF-κB, Cdk9) suppressed HIV-1 reactivation.
Conclusion: We conclude that latent HIV-1 in macroglial reservoirs can be activated by selected LRAs. Identification of small molecules that suppress HIV-1 reactivation supports functional cure strategies. We propose using the HNSC model to develop novel strategies to enforce provirus quiescence in the brain.