HIV/AIDS eradication

Abstract

Antiretroviral therapy can inhibit HIV replication in patients and prevent progression to AIDS. However, it is not curative. Here we provide an overview of what antiretroviral drugs do and how the virus persists during therapy in rare reservoirs, such as latently infected CD4+ T cells. We also outline several innovative methods that are currently under development to eradicate HIV from infected individuals. These strategies include gene therapy approaches intended to create an HIV-resistant immune system, and activation/elimination approaches directed towards flushing out latent virus. This latter approach could involve the use of novel chemically synthesized analogs of natural activating agents.

Fig. 1. Essential steps in the HIV life cycle and targets of currently available antiretroviral drugs

1) HIV virus particles (virions) bind to CD4 and a coreceptor (generally CCR5 or CXCR4) on target cells. 2) The viral envelope proteins mediate fusion of the viral and host cell membranes, allowing the viral RNA to be released into the host cell cytoplasm. 3) The viral RNA is reverse transcribed into double stranded DNA by the HIV reverse transcriptase enzyme. 4) Double stranded viral DNA is translocated into the nucleus and the HIV integrase enzyme catalyzes integration of this DNA into the host cell’s chromosomes. At this point the HIV genome is referred to as “proviral DNA” or an “integrated provirus”. 5) Transcription of the HIV genome is mediated by host cell polymerases. 6) and 7) HIV RNA is exported to the cytoplasm for translation or incorporation into new virions. For expression of some proteins, the RNA is spliced prior to nuclear export. 8 and 9) New virions assemble and bud from the plasma membrane. 10) As virions bud, the viral protease enzyme cleaves HIV polyproteins into individual subunits, producing infectious, mature virions. Underscored steps represent those that are targeted by clinically-approved antiretroviral drugs.

Fig. 2. Suppression of HIV replication during therapy

The natural course of HIV infection is associated with extremely high levels of virus replication. Treatment with highly active antiretroviral therapy (HAART) inhibits the vast majority of this replication, but HIV persists at low levels in very rare cellular reservoirs in these individuals. If HAART is stopped for any reason, the virus can emerge from these reservoirs, allowing replication to continue at pretherapy levels.

Fig. 3. Generation of HIV latency

A) Resting CD4+ T cells cannot generally support HIV replication. B) However, if the cells are stimulated by cytokines or recognition of their cognate antigen then they become activated and susceptible to HIV infection. Infection will generally result in death of the host cell, but a small subset of these cells will transition back to a resting state before they can be killed by the virus. C) The result is a shut-down of HIV expression and production of a long-lived latently-infected cell that harbors an HIV provirus that is not producing viral proteins. D) Subsequent activation of this latently infected cell (perhaps many years later) results in re-initiation of virus expression and production of new infectious virions.

Fig 4. Gene therapy approaches for eliminating HIV

Therapeutic genes can be introduced into CD34+ hematopoietic stem cells. As these cells differentiate, the introduced genes will also be present and expressed in progeny cells. In this way CD4+ T cells can be produced that are resistant to HIV infection or encode genes for anti-HIV factors. Examples shown here include: genes encoding T cell receptors against HIV, that could be used to produce cytotoxic T lymphocytes that target HIV-infected cells; genes encoding short-hairpin RNAs (shRNAs) and ribozymes, that can cleave viral RNA or the RNA for the CCR5 receptor for HIV, making the cell resistant to infection; genes encoding recombinases or homing endonucleases, that can inactivate integrated HIV DNA, and various RNAs and proteins that can interfere with HIV protein functions.

Fig. 5. Activation/elimination approaches to purge the latent HIV reservoir

A) A variety of different stimuli have been shown to be capable of inducing a latent HIV provirus to express new viral proteins. In some cases combinations of inducers (such as PKC activators and HDAC inhibitors) can act synergistically in this process. B) Once virus expression has been activated in the latently-infected cell, HIV spread to new cells will be inhibited by the continued presence of HAART. The host cell may be killed directly by cytopathic effects associated with virus replication. Alternatively, the cell could be killed by immune effector mechanisms or novel therapeutic approaches targeted towards viral proteins. HDAC (histone deacetylase), HMT (histone methyltransferase), PKC (protein kinase C), IL (interleukin).

Fig. 6. Examples of HIV latency activating compounds currently under investigation

A) Bryostatin analog (PKC activator) B) Vorinostat (HDAC inhibitor) C) (+)-JQ1 (bromodomain inhibitor).