HIV Persistence, Reservoirs and Eradication Strategies Workshop: Day 1
The workshop began on December 6, 2011 with a symposium organized by the Division of AIDS Research at the US National Institute of Mental Health on "Unique Challenges and Opportunities for Eradication of CNS HIV Reservoirs". We report here the main discussions that occured during this session.
Joseph Jeymohan introduced the symposium by saying that eradication strategies must eliminate HIV from all reservoirs including the brain. As T cells do not reside in the brain for long, he argued that we have to identify the exact other cells that are involved in this persistence.
Tae-Wook Chun has analyzed patients on long-term HAART and found persistence of HIV DNA in activated cells in the blood compartment. This could be explained by their continuous infection in tissues. There are 2 potential explanations for continued cell tissue infection during HAART:
-ARV drugs do not reach tissue cells wel, in particular in the gut, and at sufficient levels,
-cell to cell virus propagation is not stopped by ARVs.
Steve Deeks pointed out that we first have to stop HIV replication in each compartment before speaking of a possible cure. He mentioned the work from Joe Wong group on raltegravir intensification and their results on gut biopsies, in particular at the terminal ileum level. But the final frontier will be the brain, according to him. Some authors think that despite effective plasma viral suppression, something continues to happen in the brain, but it is hard to prove.
He also addressed the role of persistent inflammation in HIV persistence, and the potential interest of using drugs that tackle this mechanism.
However, a lot of current ongoing eradication trials will fail to reach the CNS, in his view, and even future ones like anti-PD1 antibodies will not reach the brain.
Melissa Churchill recalled that HIV enters the CNS very early. Can astrocytes be a reservoir for HIV? They actually contain integrated proviral HIV-1 DNA and up to 20% of them might be infected according to in vitro analysis. Particular modes of HIV latency regulation might also exist in the CNS that are different from those in CD4+ T cells.
It is not known whether astrocytes can produce virions but they could however contribute to CNS pathogenesis by "simply" transfering HIV from cell to cell.
Mario Stevenson then stressed that discussions have always been T cell centric and that we now are at the stage that we need to understand that other cells are, at least qualitatively, as important as T cells. His lab has demonstrated that macrophages are a chronic reservoir in vivo. For example, in a SHIV model, there is a huge depletion of CD4+ T cells but plasma viremia remains high, which is maintained by a population of infected macrophages. Macrophages are also resistant to the cytopathic effects of HIV and their life-span is unaffected by HIV infection. Cytopathicity of macrophages is dependant of MCSF. HIV induces the production of this survival cytokine and in vitro the administration of Gleevec can reduce this cytopathicity. It is urgent to determine what agents can be used to purge the macrophage reservoir and how HIV latency is regulated in these cells.
Sarah Palmer addressed the issue of measuring HIV in CSF during suppressive therapy. Using the single-copy assay (SCA) at the CSF level she analyzed 18 patients with plasma viremia who were intensified by raltegravir for 12 weeks. No effect was found on CSF in this pilot study, even by measuring surrogate markers like neopterin. An other study involving 50 patients is ongoing. These studies are funded by the amfAR and the NIH.
Jonathan Karn addressed the mechanisms of HIV latency regulation in glial cells. TNF-alpha reactivates latent HIV in these cells and this is mediated by NF-kappa B. SAHA is also a strong activator. Cocaine also activates HIV in glial cells via NF-kappa B. Microglial cells are probably restricted by the CoREST system whereas T-cells are restricted by polycomb. Consequently, some spectrum of differences might exist concerning HDAC inhibitors.
Janice Clements emphasized the challenges of macrophages and astrocytes for HIV eradication strategies. She raised the question whether a bone marrow transplant can replace microglia. She also raised the issue of blood brain barrier access of eradication approaches. Finally, as she already asked in an editorial in AIDS last year, can reactivation approaches cause brain injury?
In a SIV macaque model of HAART, we see complete suppression of CSF viral RNA but we do see residual viral replication in the brain. Not only there is persistent viral RNA there, but also persistent expression of inflammatory proteins. CSF/plasma CCL2 ratio is reduced by HAART in this model but then maintains a plateau.
Avi Nath began by a joke saying that scientists studying glial cells are less respected than those studying neurons. He argued that eradication approaches fell into 3 categories that each have the CNS as a limit:
-Enhancement of cytotoxic T cell responses
The approaches to activate latent HIV also are in this situation as we hope that the immune system will get rid of the cells from which the virus has been activated.
But activating HIV may cause important brain damage, as already raised by Janice Clements.
-Create cells resistant to HIV. This is the gene therapy approach. But astrocytes can still transfer HIV to T cells, and even in the context of bone marrow transplantation, these infected astrocytes are not replaced.
-Making a therapeutic vaccine. But it would have to work against the different strains found in the brain.
Finally, Avi Nath said that analyzing the CSF is interesting but that we have to find new ways to "sample" the brain tissue itself or to develop imaging techniques that are able to replace that in a non invasive way.
The symposium was followed by an hour of intense discussions that raised more question marks than answers.
The brain HIV reservoir is definitely the main compartment that will attract the focus of scientists in the next few months. The NIH will devote funding for several studies trying to answer the following questions:
-what are the main cells involved?
-what are the mechanisms of HIV persistence in these cells?
-what kind of ablative strategies can be tested?
-what tools can be used to measure more precisely what is ongoing there?
-what animal models can help resolve these question? Because even the most humanized mouse will keep mouse, not human, neurons!
Key words: HIV CNS, HIV brain, HIV cure, HIV reservoir