Reduce and Control: A Combinatorial Strategy for Achieving Sustained HIV Remissions in the Absence of Antiretroviral Therapy

Abstract

Human immunodeficiency virus (HIV-1) indefinitely persists, despite effective antiretroviral therapy (ART), within a small pool of latently infected cells. These cells often display markers of immunologic memory and harbor both replication-competent and -incompetent proviruses at approximately a 1:100 ratio. Although complete HIV eradication is a highly desirable goal, this likely represents a bridge too far for our current and foreseeable technologies. A more tractable goal involves engineering a sustained viral remission in the absence of ART--a "functional cure." In this setting, HIV remains detectable during remission, but the size of the reservoir is small and the residual virus is effectively controlled by an engineered immune response or other intervention. Biological precedence for such an approach is found in the post-treatment controllers (PTCs), a rare group of HIV-infected individuals who, following ART withdrawal, do not experience viral rebound. PTCs are characterized by a small reservoir, greatly reduced inflammation, and the presence of a poorly understood immune response that limits viral rebound. Our goal is to devise a safe and effective means for replicating durable post-treatment control on a global scale. This requires devising methods to reduce the size of the reservoir and to control replication of this residual virus. In the following sections, we will review many of the approaches and tools that likely will be important for implementing such a "reduce and control" strategy and for achieving a PTC-like sustained HIV remission in the absence of ART.

Keywords: HIV; block and lock; cure; genome editing; reduce and control; shock and kill.

Figure 1

Therapeutic approaches being explored aimed at long term neutralization of the latent HIV reservoir. (bNAbs, broadly neutralizing HIV antibodies; ADCC, antibody dependent cellular cytotoxicity).

Figure 2

Schematic representation of the major classes of LRAs and their molecular mechanism of action.

Figure 3

Targeting of host or viral factors to silence latent HIV proviruses. Therapeutics devised to inhibit HIV-1 replication may target different stages of the viral life cycle. The small molecule drug rapamycin blocks viral transcription initiation by inhibiting mTOR and AKT-mediated induction of NF-κB. Didehydro-cortistatin A interferes with Tat-mediated transactivation by blocking Tat binding to TAR, thereby reducing RNA Pol II elongation. RNA interference predominantly acts by degrading viral transcripts. Gene editing technologies directly target proviral DNA by either excising or corrupting the viral genome. CRISPR interference (CRISPRi) does not cleave the proviral DNA. Rather, a catalytically inactive Cas9 (dCas9) protein is fused to a Krüppel associated box (KRAB) domain and directed to the HIV-1 LTR by sgRNAs. This fusion protein recruits transcriptional repressors producing epigenetic changes associated with gene silencing.

Figure 4

Major gene editing platforms operating through transcriptional control or DNA cleavage and excision. Two functions are present in all gene editing platforms. The first component directs sequence-specific DNA binding while the second promotes either frank DNA cleavage or the recruitment of epigenetic modifiers that positively or negatively control target gene transcription. For example, a catalytically inactive or dead version of Cas9 (dCas9) can be fused to a strong transcriptional activation domain from the RelA transcription factor (NF-κB p65), the replication and transcription activator (RTA), or repeats of the HSV VP16 protein (VP64), to induce transcriptional activation. Alternatively, transcriptional inhibition can be induced by fusing dCas9 to Krueppel-associated box (KRAB) proteins, methyl CpG binding protein 2 (MeCP2), or DNA methyl transferase 3A (DNMT3a). These “baits” effectively recruit transcriptional repressors that silence gene activity via epigenetic mechanisms.

Figure 5

Schematic representation of the “reduce and control” approach. First, a combination of effective, non-toxic LRAs are employed in order to render reservoir cells visible to the immune system. The reservoir is then reduced in size likely through the use of multiple agents including bNAbs, various killer cells and therapeutic vaccines. The residual virus can be controlled by the same agents used to reduce the size of the reservoir. However, this shrunken reservoir could form an attractive target for additional transcriptional silencing approaches. We believe it likely that some form of this combinatorial “reduce and control” strategy will emerge as a means of achieving a sustained HIV remission in the absence of ART.