Latent infection of resting CD4+
T cells is established early during HIV-1 infection, making eradication of HIV unachievable with current ART [1
]. Following integration of viral DNA into the cellular genome, the HIV long terminal repeat (LTR) promoter can revert to transcriptional silence in the absence of stimulation [4
]. One of the mechanisms through which HIV latency is maintained is by the action of histone deacetylases (HDACs) at the HIV-1 LTR (reviewed in [7
Histone deacetylases are lysine deacetylases that modify histones and induce transcriptional repression, but can also exert influences on cellular activities that are independent of transcriptional repression (reviewed in [8
]). HDACs are generally divided into three classes. The class I HDACs comprise HDAC1, 2, 3 and 8, whereas class II HDACs include 4, 5, 7 and 9 (subclass IIa) and 6 and 10 (subclass IIb). The catalytic domain of HDAC11 shares homology with both class I and II and this enzyme is sometimes classified as a class IV HDAC. The class III HDACs, the sirtuins, differ from the other classes in that they require nicotinamide adenine dinucleotide (NAD) to function and are not affected by HDAC inhibitors active against class I, II and IV.
Histone deacetylases repress transcription mainly through their ability to covalently modify the lysine tail of core histones of nucleosomes through deacetylation. Deacetylation of lysine residues on histone tails decreases the access of transcription factors to the DNA, and recruits other histone-modifying complexes that result in further transcriptional repression.
In tissue culture models of latent HIV infection, HDAC1 is recruited to the LTR by multiple DNA-binding complexes. HDAC1 recruitment by the transcription factor SV40 late specific factor (LSF) in concert with YY1 was the first mechanism reported [9
]. Later studies suggested that AP-4 [10
], heterodimers of the activation domain-deficient NFκB p50 subunit [11
], C-myc through interaction with Sp1 [12
], and CBF-1 by binding near the NFκB/NFAT enhancer element [13
] could also recruit HDAC1. CTIP2 was reported to recruit HDAC1 or HDAC2 to the Sp1-binding site of the LTR [14
]. Finally, it has been suggested that HDAC3 associates with the LTR [15
Disruption of HDAC1 recruitment to the LTR by specific DNA-binding molecules, or inhibition of HDAC activity by global HDAC inhibitors leads to LTR activation and the escape of HIV from latency in both cell line models and primary cells obtained from patients [9
]. Furthermore, the HDAC inhibitors (HDACis) valproic acid (VPA) and suberoylanilide hydroxamic acid (SAHA; vorinostat) induce viral outgrowth from resting CD4+
T cells of aviremic HIV-infected individuals on ART [18
]. HDAC2 and 3 can also occupy a site at the HIV LTR, and may play a role in the repression of LTR expression [14
]. These observations have led to the investigation of HDAC inhibition as a putative therapeutic strategy to induce HIV from latency.
As global HDAC inhibition may have adverse effect on host cells, we studied the ability of selective HDAC inhibitors to de-repress the HIV LTR in both a cell line model of latency and in CD4+ resting T cells isolated from aviremic patients on ART. We found that inhibitors that target class I HDACs 1, 2, 3 alone or in tandem with the class II HDAC6 were efficient inducers of HIV expression, yielding the outgrowth of replication-competent HIV from the resting CD4+ cells of patients. However, inhibition of HDAC6 alone, or of other class II HDACs resulted in marginal LTR activation in cell line systems, and did not result in significant recovery of virus from patient’s cells. Of note, a selective inhibitor targeting HDAC1 and 2 was not very effective at inducing LTR activation in cell lines and virus outgrowth from resting CD4+ T cells of HIV-infected individuals. These findings suggest the design of inhibitors selective for a limited array of HDACs as therapies to target persistent HIV infection.