Block-And-Lock: New Horizons for a Cure for HIV-1

Abstract

HIV-1/AIDS remains a global public health problem. The world health organization (WHO) reported at the end of 2019 that 38 million people were living with HIV-1 worldwide, of which only 67% were accessing antiretroviral therapy (ART). Despite great success in the clinical management of HIV-1 infection, ART does not eliminate the virus from the host genome. Instead, HIV-1 remains latent as a viral reservoir in any tissue containing resting memory CD4⁺ T cells. The elimination of these residual proviruses that can reseed full-blown infection upon treatment interruption remains the major barrier towards curing HIV-1. Novel approaches have recently been developed to excise or disrupt the virus from the host cells (e.g., gene editing with the CRISPR-Cas system) to permanently shut off transcription of the virus (block-and-lock and RNA interference strategies), or to reactivate the virus from cell reservoirs so that it can be eliminated by the immune system or cytopathic effects (shock-and-kill strategy). Here, we will review each of these approaches, with the major focus placed on the block-and-lock strategy.

Keywords: HIV-1 latency; HIV-1 reservoir; antiretroviral therapy; block-and-lock; functional cure.

Figure 1

Regulation of HIV transcription. (A) Tat-mediated HIV-1 transcription. The transition from transcriptional initiation to elongation implies the replacement of transcriptional repressors by activators, including NF-κB, SP-1, and histone acetyltransferases (HATs), such as p300/CREB-binding protein (CBP). HATs promote open chromatin and recruitment of polybromo-associated factor (PBAF). PBAF repositions Nuc-1 further downstream of the transcription start site (TSS), enabling efficient transcriptional elongation. Furthermore, the secondary structure of the nascent TAR RNA is also accessible for the biding of Tat protein and further transcription activation. P-TEFb-associated CDK9 phosphorylates the CTD of RNAPII at serine 2, enabling the production of full-length HIV-1 transcripts, which are spliced to produced Tat and an active transcription elongation is established. A Tat-dependent positive feedback loop is generated by increasing HIV-1 transcription and, consequently, exponential replication. (B) Establishment of HIV-1 latency. Several proteins are implicated in the establishment of HIV-1 latency. These include transcription factors such as YY-1 and CBF, enabling the recruitment of histone deacetylases (HDACs). HDACs remove acetyl groups from core histones, namely, at Nuc-1, restricting the accessibility of positive transcription factors to the promoter, promoting viral latency. Histone methyltransferases (HMTs) involved in the establishment of HIV-1 latency include SUV39H1 and G9a, which are involved in Lys9 trimethylation of histone H3 (H3K9me3) and H3K9 dimethylation (H3K9me2), respectively. Two factors, DSIF and NELF, co-operate to pause RNAPII. Repressive factors such as BAF favor HIV-1 latency by positioning Nuc-1 in an energetically unfavorable position downstream of the TSS. Finally, P-TEFb is inactive through its association with the 7SK snRNP, preventing the transition from transcriptional initiation to elongation.

Figure 2

Shock-and-kill approach to eradicate HIV-1 from latently infected cells. During HIV-1 infection, the majority of CD4⁺ T cells are eliminated through cytopathic effects; the few surviving cells revert to a resting memory state harboring latent proviruses. The shock-and-kill approach uses latency reversal agents (LRA) to increase HIV-1 transcription/replication and virions production. This reactivation leads to the elimination of infected cells by cell cytolysis or immune clearance, and simultaneously the remaining viruses are blocked from novel infections by ART.

Figure 3

Block-and-lock approach as a functional cure for HIV-1 infection. The block-and-lock approach entails the long-term durable silencing of viral gene expression. Supplementation of ART with a latency promoting agent (LPA), such as the Tat inhibitor didehydro-Cortistatin A (dCA), could suppress ongoing transcriptional events, induce epigenetic silencing over time and promote a state of “deep latency”, blocking or limiting viral rebound upon treatment interruption.

Figure 4

RNA-based strategies against HIV infection. RNA-based strategies include the use of miRNAs, siRNAs, RNA aptamers, and mRNAs. These RNAs can be synthesized to specifically bind to CCR5, the major co-receptor for HIV entry, or bind to crucial regions into HIV proviral genome. siRNAs assemble with cellular host proteins to form an RNA-induced silencing complex (RISC). Only one strand (guide) is maintained and produces an active RISC that triggers the targeted mRNA degradation, culminating in translational silencing.

Figure 5

CRISPR/Cas system to generate CCR5-resistant stem cells. Hematopoietic stem cells are collected from bone marrow of infected individuals. These cells are expanded ex vivo upon genome editing by the CRISPR/Cas system targeting the CCR5 co-receptor. Subsequently, the CCR5-modified stem cells are infused back to the same individual.