HIV-1 latency: an update of molecular mechanisms and therapeutic strategies

Abstract

The major obstacle towards HIV-1 eradication is the life-long persistence of the virus in reservoirs of latently infected cells. In these cells the proviral DNA is integrated in the host's genome but it does not actively replicate, becoming invisible to the host immune system and unaffected by existing antiviral drugs. Rebound of viremia and recovery of systemic infection that follows interruption of therapy, necessitates life-long treatments with problems of compliance, toxicity, and untenable costs, especially in developing countries where the infection hits worst. Extensive research efforts have led to the proposal and preliminary testing of several anti-latency compounds, however, overall, eradication strategies have had, so far, limited clinical success while posing several risks for patients. This review will briefly summarize the more recent advances in the elucidation of mechanisms that regulates the establishment/maintenance of latency and therapeutic strategies currently under evaluation in order to eradicate HIV persistence.

Figure 1

Establishment of post-integration latency and therapeutic approaches. Infection of activated CD4⁺ T cells by HIV-1 mostly results in their rapid death by the cytopathic effect of the virus. A minority of activated CD4⁺ T cells, however, becomes infected just as they are undergoing the transition from an activated to a resting-memory state where the provirus becomes silent. As an alternative, latent infection may arise from direct infection of resting CD4⁺ T cells. The established latent reservoir in the T CD4⁺ resting memory compartment then survives through homeostatic proliferation. Two major approaches to cure HIV-1 infection are the “functional cure” and the “sterilizing cure”. The “functional cure” approach includes different strategies as (I) initiation of ART during the very early primary stage of acute HIV-1 infection, leading to a long lasting control of viremia following c-ART discontinuation; (II) selective depletion of discrete T cell subsets carrying the integrated HIV-1 DNA without viral reactivation; and (III) the so called “shock/kick and kill” strategy consisting in inducing, through drugs, transcription of quiescent, replication-competent HIV-1 provirus (the “shock/kick” phase), in the presence of ART (to block viral spread), making virus reactivating cells susceptible to immune clearance, cytopathic effects and/or the effects of ad hoc therapeutics (the “kill” phase). The “sterilizing cure” approach, manly consists in the engineering of patient’s own T CD4⁺ cells or hematopoietic stem cells through the generation of a CCR5 deletion followed by an autologous infusion/transplant with these cells that are resistant to infection and may take over the original infected cell population.

Figure 2

HIV-1 gene transcription is silenced in latently infected cells: epigenetic mechanisms of silencing affected by epigenetic modulators. Transcription initiation at the HIV-1 LTR is inhibited in latently infected CD4⁺ T cells due to different epigenetic silencing mechanisms. These include: recruitment of histone deacetylases (HDACs) by the NF-κB p50/p50 homodimer resulting in deacetylation of histones at the Nuc0 and Nuc1 nucleosomes, recruitment of histone methyltransferases (HMTs) as Suv39h1, EZH2 and G9a, resulting in methylation of histones and DNA methyltransferases responsible for DNA methylation at CpG islands. Crucial transcription factors responsible for initiating transcription at the LTR, such as NF-κB, NFAT and cJun (a sub unit of AP1) are then sequestered in the cytoplasm in an inactive state, contributing in the establishment/maintenance of latency. The P-TEFb factor, crucial for HIV-1 transcriptional elongation, is part of an inactive complex and together with the low amounts of the P-TEFb subunit cyclin T1 in latently infected CD4⁺ T cells, represent a further mechanism of transcriptional restriction. HDAC, HMT and DNMT inhibitors are all being explored to promote escape from latency in the context of the “shock/kick and kill” strategy.

Figure 3

Transcriptional reactivation of HIV-1 LTR by stimulation/activation of initiation and elongation factors. Several compounds including T cell activators and differentiating agents have been identified that induce transcription of latent HIV-1 integrated genome. These act as inducers of protein PKC and NF-κB/NF-AT pathways and P-TEFb stimulators. Briostatin-1 and Prostratin are activators of PKC that once activated, phosphorylates the NF-κB-inhibitor IκB-α with its subsequent degradation and accumulation in the nucleus of the active NF-κB (RelA/p50) heterodimer. Activated PKC also triggers the MAP kinase pathway activating the AP1 dimeric factor (Fos/Jun) also recruited to the LTR enhancer upon binding to NF-κB. AV6 requires activation of NFAT to stimulate reactivation. HMBA activates the calcium pathway and calcineurin, contributing to NFAT activation, and stimulates the AKT kinase, which in turn phosphorylates HEXIM1, with the release of the P-TEFb (CycT1 and CDK-9) elongation factor from the inhibitory complex (HEXIM1/7SKsnRNP). Disulfiram stimulates the degradation of PTEN, similarly determining the activation of AKT and the release of HEXIM1. JQ1, by targeting BRD4, may release P-TEFb, promoting elongation.