HIV Tat as a latency reversing agent: turning the tables on viral persistence

Abstract

The 'shock and kill' approach to an HIV cure involves the use of latency reversing agents (LRAs) to reactivate latent HIV, with the aim to induce death of infected cells through virus induced cytolysis or immune mediated clearance. Most LRAs tested to date have been unable to overcome the blocks to transcription elongation and splicing that persist in resting CD4+ T cells. Furthermore, most LRAs target host factors and therefore have associated toxicities. Therefore, there remains a high need for HIV-specific LRAs that can also potently upregulate expression of multiply-spliced HIV RNA and viral protein. The HIV Transactivator of Transcription (Tat) protein plays an important role in viral replication - amplifying transcription from the viral promoter - but it is present at low to negligible levels in latently infected cells. As such, it has been hypothesized that providing Tat in trans could result in efficient HIV reactivation from latency. Recent studies exploring different types of Tat-based LRAs have used different nanoparticles for Tat delivery and describe potent, HIV-specific induction of multiply-spliced HIV RNA and protein ex vivo. However, there are several potential challenges to using Tat as a therapeutic, including the ability of Tat to cause systemic toxicities in vivo, limited delivery of Tat to the HIV reservoir due to poor uptake of nucleic acid by resting cells, and challenges in activating truly transcriptionally silent viruses. Identifying ways to mitigate these challenges will be critical to developing effective Tat-based LRA approaches towards an HIV cure.

Keywords: HIV; HIV cure; Tat; latency; latency reversal agent.

Figure 1

Tat-mediated enhancement of RNA polymerase II processivity. Following its translation, the HIV transactivator of transcription (Tat) protein forms a complex with positive transcription elongation factor b (p-TEFb) and binds to the transactivating-response (TAR) element within the early HIV transcript. This causes the hyperphosphorylation (P) of RNA polymerase II (RNAP II), enhancing the transcription processivity of the enzyme. This process is essential for the production of full-length unspliced HIV RNA; and multiply-spliced HIV RNA and virion production thereafter.

Figure 2

Impact of mutations on Tat secretion and uptake. Tat secretion occurs through interactions between phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P₂) the Tat basic domain (residues 49-57), and a conserved tryptophan at position 11 (Trp11). Uptake occurs through the binding of Tat to heparan sulfate proteoglycans, followed by uptake into endosomes. Point and combinatorial substitution mutations in both the basic domain and Trp11 reduce secretion and uptake efficiency of Tat, albeit to different degrees.

Figure 3

Considerations for the development of Tat-based LRAs. There are multiple potential challenges to the development of Tat-based therapeutics, including systemic toxicities in vivo, suboptimal delivery of Tat to latently infected cells, and reservoir diversity, rendering a proportion of proviruses unresponsive to Tat treatment (inner circle). These could be mitigated in several ways (outer circle) including preventing Tat secretion and uptake, utilizing a non-toxic Tat variant, targeting Tat delivery vehicles to CD4+ T cells and anatomical sites of the HIV reservoir; and combining Tat with other HIV-specific latency reversing agents (LRAs) or pro-apoptotic drugs to drive selective killing of HIV DNA+ cells.