Prospects for treatment of latent HIV

Abstract

Recent advances in antiretroviral therapy (ART) have drastically improved the quality of life for people with HIV infection. However, owing to the persistence of latent HIV in the presence of therapy, patients must remain on therapy indefinitely. Currently, the solution to the HIV pandemic rests on the prevention of new infections and many decades of ART for the steadily expanding number of people infected worldwide. ART is costly, requires ongoing medical care, and can have side effects, thereby preventing its universal availability. Therefore, to escape the ironic burdens of therapy, efforts have begun to develop treatments for latent HIV infection. Current approaches propose either complete eradication of infection or induction of a state of stringent control over viral replication without ART. This review will discuss these strategies in detail and their potential for clinical development.

Figure 1

Induction and clearance strategies. (a) Induction. Several small molecules have been identified that are able to induce transcription of quiescent HIV proviruses. Prostratin, tumor necrosis factor-α, and bryostatin have been demonstrated to act on the protein kinase C/nuclear factor κB (PKC/NF-κB) pathway. Activated PKC phosphorylates IκBα, causing release of NF-κB. NF-κB then translocates to the nucleus, where it binds to the promoter of HIV and promotes transcription. Histone deacetylase (HDAC) inhibitors directly inhibit deacetylation of histones at the HIV long terminal repeat (LTR), which is associated with induction of transcription. Similarly, histone methyltransferase inhibitors (HMTi) inhibit the methylation of histone tails, which are associated with transcription from the HIV promoter in some model systems. Hexamethylene bisacetamide (HMBA) causes activation of the Akt pathway, which may result in phosphorylation of HMBA inducible 1 (HEXIM). This may result in the release of positive transcription elongation factor b (p-TEFb) from the inhibitory HEXIM complex, allowing p-TEFb access to the HIV LTR, where it phosphorylates RNA pol II and primes the LTR for transcription. Disulfiram has been shown to act upstream of HMBA, perhaps by causing degradation of phosphatase and tensin homolog, an inhibitor of AKT. Activated AKT then phosphorylates HEXIM, and LTR induction ensues. Vaccination may activate some T cells, inducing transcription from quiescent HIV LTRs. (b) Clearance. Once viral transcription has been induced, cells containing HIV must be cleared. There are several mechanisms that may contribute to clearance. If HIV transcription is sufficient to produce HIV protein, HIV antigens may be displayed on the cell surface, which may allow the immune system to target and destroy the infected cells. Alternatively, the cell may undergo apoptosis in response to HIV production. Antiretroviral therapy (ART) will be used to prevent de novo infection.

Figure 2

Gene therapy approaches to eradication: (a) HIV-1-specific recombinase. (i) The HIV provirus contains a sequence within each long terminal repeat (LTR) that is recognized by Tre recombinase. (ii) Tre is introduced into the cell and binds the TRS. (iii) Tre dimerizes and loops out the HIV proviral DNA. (iv) The DNA is recombined, resulting in excision of the intervening sequence containing the provirus and religation of the provirus-free host DNA. (b) Suicide genes. (i) High-level induction of HIV proviral expression can lead to a flood of infectious viral particles. In the presence of antiretroviral therapy (ART), most de novo infections will be prevented, whereas in the absence of sufficient ART, infection can occur. Alternatively, low-level induction may fail to produce sufficient viral proteins for the immune system to be able to identify the infected cell, allowing these cells to persist and possibly expand by homeostatic proliferation. (ii) HIV-dependent suicide gene constructs encoding toxic or proapoptotic genes driven by the HIV LTR and regulated by inhibitory sequences from HIV gag (I) and a Rev response element (R) result in cell death on expression of HIV Tat and Rev, while sparing uninfected cells. TRS, Tre recognition sequence.

Figure 3

Strategies to control persistent infection: (a) Zinc-finger nucleases (ZFNs). (i) CCR5-specific ZFNs bind sequences within the CCR5 gene, forming a ZFN heterodimer around a central target sequence. (ii) The ZFN nuclease domains (Nuc Dom) cleave the CCR5 gene, creating a DNA double-strand break (DSB). (iii) The DSB can be repaired by homologous recombination (not shown) or nonhomologous end joining, which often leads to deletions at the break site and a reading frame shift in the encoded messenger RNA (mRNA) that gives rise to nonfunctional CCR5 protein. (b) Triple-R expression. The Triple-R vector encodes 1a (an anti-CCR5 ribozyme), 2a (short hairpin RNAs targeting Tat and Rev), and 3 (a transactivation response element (TAR) decoy containing a nucleolar-localization tag). 1b, The ribozyme binds CCR5 mRNA, cleaves it, and then RNA is degraded. 2b, The short hairpin RNAs are processed into small interfering RNAs (siRNAs) and the guide strand is loaded into RNA-induced silencing complex (RISC). Upon binding of the siRNA/RISC to Tat or Rev mRNA, the mRNA is cleaved and degraded. 3, Tat protein that re-enters the nucleus is bound by the TAR decoy and sequestered at the nucleolus, thus titrating it away from HIV TAR. (c) Reversal of immune exhaustion using antibodies against inhibitory receptors. Costimulatory and inhibitory molecules are expressed on both T-cell and antigen-presenting-cell surfaces. T cells exert their specific effector functions by engagement of costimulatory molecules, and the immune response is terminated due to expression of inhibitory receptors. During chronic HIV infection, in which the immune system is continuously exposed to the antigen, inhibitory receptors are overexpressed, and the cells become anergic. The use of monoclonal antibodies directed against inhibitory receptors, or their ligands, may restore T cells’ capacity to develop their specific immune functions.