HIV Infection of the Brain

In this podcast, Jonathan Karn, PhD, talks about his session "Novel Findings of Neuronal Modulation of HIV Expression in Microglia" at the Conference on Retroviruses and Opportunistic Infections 2021, including some controversies among researchers about HIV infection of the brain.

Additional Resource: 

Jonathan Karn, PhD, is the Reinberger Professor of Molecular Biology at Case Western School of Medicine and the director of AIDs research at Case Western in Cleveland, Ohio. 



Jessica Bard: Hello, everyone, and welcome to another installment of "Podcast 360," your go‑to resource for medical news and clinical updates. I'm your moderator, Jessica Bard, with Consultant360 Specialty Network.

Little is understood about the consequences of HIV neuroinvasion, and there are some major disagreements among researchers about HIV infection of the brain. Dr Jonathan Karn is here to speak with us about that today. Dr Karn is the Reinberger Professor of Molecular Biology at Case Western School of Medicine and the director of AIDS research at Case Western in Cleveland, Ohio.

Thank you for joining us today, Dr Karn. You're presenting your research, "Novel Findings of Neuronal Modulation of HIV Expression in Microglia" at CROI 2021. Can you give us an overview of this study?

Dr Jonathan Karn: I was asked by Dennis Kolson and Serena Spudich to both give an overview of controversial aspects of HIV infection of the brain and then talk a little bit about our own specific research programs.

In the talk, I addressed three major controversies about HIV infection of the brain. Some of these controversies are surprisingly simple in scope, but it just shows how little we understand about the consequences of HIV neuroinvasion.

One of the first controversies is, does HIV establish latency in the central nervous system at all? Preponderance of the evidence is that viruses actually can get in, infect microglia, and become transcriptionally silenced.

This has been very difficult to prove with human studies, because all samples have to be post‑mortem samples. Our understanding of that comes from animal models, including the SIV model and humanized mouse models.

Another controversy, again surprisingly simple, is what cells are infected by HIV and become latent? I think everyone agrees that perivascular macrophages are critical entry points into the brain for the virus. Most people believe microglia can be infected, but it's not so clear how latent they are. There's controversy about whether astrocytes are also able to sustain infections.

One of the things my lab did, in collaboration with Paula Cannon, is work with humanized mouse models, where we were able to introduce human microglia into the brains of mice, infect them with HIV, recover the microglial cells from the population, and reactivate them ex vivo.

We were able to show definitively that cells of human origin got infected with HIV, and that they were latently infected. They required ex vivo reactivation in order to show a viral expression.

The third controversy is whether HIV neurodegenerative disorders are due solely to inflammation. The field has been split between camps that, "is the virus stupid," ‑‑ I'm in that camp ‑‑ and, "is inflammation stupid." Some of my colleagues are in that camp. The answer is that it's a little of both. That's usually what happens with these kinds of controversies.

We have an overarching hypothesis that we've been working on, that intermittent activation of HIV from latent states, which is in response to inflammatory signals, is what's driving neurodegeneration. This is a long, slow process that occurs over many years.

Jessica: Can you elaborate more on how neuronal modulation impacts HIV expression in microglia? What did you and your research team find?

Dr Karn: One of the real surprises we had is when we started doing co‑culture experiments between microglial cells and neurons. We found that if you have an isolated microglial cell ‑‑ it's usually a pretty activated cell ‑‑ the virus comes in, and it's expressing.

If you co‑culture it with neurons, healthy neurons, then their signals that are propagated from the neurons that reduce the inflammatory responses to the microglia. The microglia calm down. The virus goes latent.

This was the first evidence we had that latency in the microglia is a function of the cell activation state, and the cell activation state is mediated by the neurons.

The other surprise we had about this was that it was a two‑way street. If neurons were damaged, then they propagated signals that reactivated the virus. This is actually following a pattern of how microglia cells normally work in the brain.

Their job is to survey damaged neurons, chew them up, get them out of the picture. Their job is also, when neurons are healthy, to facilitate synaptic connections.

Microglia normally have these different polarization states. What we found is that transcription control in HIV mirrored that. When the microglia were in their phagocytic, activated state, HIV was going strong. When the microglia were in their anti‑inflammatory surveyance state, then the HIV tended to be latent.

Jessica: You briefly did touch on this. How did the novel findings of neuronal modulation of HIV expression differ in healthy neurons versus damaged neurons?

Dr Karn: As I was saying, the damaged neurons are releasing inflammatory signals that activate the microglia. This whole process is exacerbated by inflammatory cytokines. There's actually a good correlation between the progression of hand and circulating inflammatory cytokines.

Those are signals that activate microglia. The microglia start chewing up the neurons. The neurons start crying out for help. That initiates a feedback mechanism that leads to neuronal damage.

In the case of healthy neurons, it's the opposite. They are issuing signals that calm the neurons, calm the microglial down. We've been trying to investigate exactly what those signals are.

One of our very recent findings is that the ligand for fractalkine and also CD200 ‑‑ these are two major neuronal signals for interacting with microglia ‑‑ they seem to be at least part of the picture of what gets HIV into latency.

There may be additional signals involving glutamate regulation. That might be a role for astrocytes in the system as well.

There's still a rich area of research to figure out what's the basis for this crosstalk, but the general principle is once there's neuronal damage, perhaps due to inflammation, then the microglia get activated. HIV pops out. You have local damage. When that gets resolved, then the microglia shut back down again.

Jessica: My next question for you, patients with HIV are living longer due to improved antiretroviral therapy. Why is all of this that we've been talking about so important to study now?

Dr Karn: One of the little-known aspects of HIV is that 30 to 50 percent of patients have measurable neurocognitive problems. These aren't overt. They don't compromise people's day‑to‑day activities, but it's significant enough that with careful neurocognitive evaluations and testing, it shows up.

This is a big worry. Despite very effective therapy, there is still viral blipping in the central nervous system and evidence of a long, slow neurological problem. Of course, for patients where therapy is failing, the virus can emerge in the central nervous system. You can see severe damage.

In completely untreated HIV infections, if there's a collapse of the therapy, it can lead to overt dementia. This is a significant, long‑term problem that's affecting up to half of HIV patients even if they're well‑treated.

Jessica: What would you say are the overall take‑home messages of your study and your session at CROI 2021?

Dr Karn: The first message is don't forget the brain. We tend to focus on the periphery, T‑cell. A lot of the cure efforts are focused on eliminating peripheral reservoirs. Clearly, they are where most of the virus is, but there is this other compartment which has very distinct mechanisms and issues.

The second take‑home message is that a lot of long‑term neurological problems are due to dysregulation of microglial cells. That's a theme that's been showing up in Alzheimer's disease, Parkinson's, other neurodegenerative conditions.

In the case of HIV, it's very striking how the virus is fitting into microglial dysbiosis. That may be, ultimately, the source of the neurological complications in HIV infections.

Jessica:  What's next for research on this topic?

Dr Karn:  We've got a long way to go. We still don't know all of the signals. The good news for research is some of the models are now increasingly refined.

We've been working successfully with transgenic mouse models that lets us recover cells, and using single‑cell sequencing methods, dive under the hood and define what is the activation state of the microglial cells when they're infected with HIV, how do they respond locally to a neuronal signal. We're optimistic about that, but we're just scratching the surface there.

The other big technology that's come in has been human organoids. We, and a number of other labs, have been developing methods to introduce microglial cells into these organoids. That gives us another tool to look at close interactions between microglia, astrocytes, and neurons, and get to the bottom of what are the signals.

Finally, of course, we want to do some good for the world. We know that the signals are activating particular nuclear receptors. We think there are opportunities for drug interventions by blocking these receptors, things in the retinoid receptor family. There are existing drugs that are brain‑penetrant. We're hopeful that if we understand the signals, we can find ways of keeping HIV down.

Jessica: Thank you for speaking with us, Dr Karn. We'll give you the last word. Is there anything else that you'd like to add?

Dr Karn: Great, and pleasure to talk to you. This has been a relatively neglected area in HIV research. I would encourage all the clinical folks to be attentive to neurocognitive issues in their patients, even if they're otherwise doing well. I look forward to seeing drug discovery efforts in this area, which will be important for the future.

Finally, all of this kind of issue becomes even worse in drug‑using populations. We don't know what's the impact of opioids or methamphetamine on this, but we certainly know it isn't helping.

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