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A. Li et al.
Table 1. Preclinical studies of acute graft-versus-host disease investigating epigenetic mechanisms.
Enzyme
EZH2
DNMT
HDAC (Pan)
HDAC6 HDAC11
SIRT3
HDAC (Pan)
HDAC (Pan)
Cells
CD8+/CD4+ T cells CD8+/CD4+ T cells
CD8+ T cells
CD8+/CD4+ T cells
Treg CD8+/CD4+ T cells
CD8+ T cells CD8+/CD4+ T cells
CD8+/CD4+ T cells
APC
IEC
Key findings
Ezh2 KO impairs proliferation, differentiation and expansion; it reduces aGvHD but preserves GvL.28 Ezh2 inhibition with DZNep inhibits ongoing GvHD but preserves the GvL effect.30
Administration of the EZH2 inhibitor GSK126, which specifically reduces H3K27me3 without affecting the protein, failed to prevent aGvHD in mice. In contrast, targeting T-cell EZH2 protein by inhibiting HSP90 reduced aGvHD in mice undergoing allo-HSCT.33
EZH2 controls CD8+ T memory precursor formation and antitumor activity.32
Inhibition impairs activation, expansion, and secretion of cytokines.43
Inhibition by Aza increased Treg frequency through hypomethylation of Foxp3.42, 43, 67
Pan-inhibition using SAHA results in reduced proliferative and cytotoxic activity of
anti-CD3 activated T cells.34
Inhibition of HDAC6 impairs CD8+ T-cell proliferation and function in a GvHD-like model.38
KO of Hdac11 increased T-cell proliferation rates and effector function resulting in more rapid and potent aGvHD.36
Loss of Sirt3 results in decreased aGvHD severity due to decreased activation and production of ROS while maintaining GvT.40
Pan-inhibition of HDAC with SAHA reduced aGvHD, resulting in a drastic decrease in pro-inflammatory cytokine expression and induced high level expression of IDO to suppress alloreactive T cells.20, 23
Butyrate treatment reduced GvHD severity, improved IEC junction integrity and reduced IEC apoptosis. In addition, the decrease in H4 acetylation, butyrate transporter and receptor levels due to allo-HSCT inflammation were reversed.90
EZH2: enhancer of zeste homolog 2; KO: knockout; aGvHD: acute graft-versus-host-disease; GvL:graft-versus-leukemia; H3K27me3: histone 3 lysine 27 trimethylation; HSP90: heat shock protein 90;allo-HSCT:allogeneic hematopoietic stem cell transplantation;DNMT:DNA methyltransferases;Treg:regulatoryT cell;Aza:5-azacytidine;HDAC:histone deacety- lase; SAHA: suberoylanilide hydroxamic acid; ROS: reactive oxygen species; GvT: graft-versus-tumor; IDO: indoleanime-2,3-deoxygenase; APC: antigen-presenting cell; IEC: intes- tinal epithelial cell; H4: histone 4
shaping T-cell responses.5,12-15 Immunosuppressive mole- cules such as IL-10, indoleamine-2,3-dioxygenase (IDO) and programmed death ligand 1 (PD-L1) may be upregu- lated to repress alloreactive T-cell responses, shifting them to become tolerogenic.18-21 Epigenetic regulators convert these signals into the aforementioned markers and mole- cules.
Histone deacetylases’ multiple functions in the sensitization of hematopoietic antigen-presenting cells Two classes of enzyme regulate histone acetylation sta-
tus: histone acetyltransferases (HAT) and histone deacetylases (HDAC). HAT acetylate histone lysine sub- strates and open compacted chromatin, allowing tran- scription factors to access DNA.22 HDAC decrease histone lysine tail acetylation, repressing gene transcription.16 Epigenetic studies of hematopoietic APC sensitization have primarily focused on the impact of HDAC (Figure 1).
One of the first studies anchoring epigenetics to aGvHD, helmed by Reddy and colleagues, brought to light the role of histone acetylation in aGvHD.23 HDAC are important for APC production of pro-inflammatory cytokines and immunosuppressive molecules.20,23 Preclinical studies have shown that in vivo administration of the pan-HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) reduced aGvHD.23 SAHA treatment did not impair T-cell responses to host antigens, but significantly decreased the production of inflammatory cytokines, TNF-a, IL-1 and IFN-γ, by APC. Subsequent studies con- firmed that treatment with SAHA resulted in a marked decrease in pro-inflammatory cytokines (e.g., TNF-a, IL-
12, IL-6) in APC, which are important in promoting allore- active T-cell responses.20 SAHA inhibited IL-6 production in dendritic cells stimulated by variable toll-like receptor (TLR) agonists (e.g., TLR2, TLR3, TLR4 and TLR9). In lipopolysaccharide-stimulated dendritic cells, SAHA treat- ment induced high-level expression of IDO to suppress alloreactive T-cell responses.20
Villagra et al. highlighted the importance of HDAC11 in repressing the negative regulation that murine APC exert- ed on T-cell responses.18 Using chromatin immunoprecipi- tation, researchers determined that upon overexpression of HDAC11 in APC, there was decreased acetylation of histone 4 (H4) at the distal Il10 promoter. This was asso- ciated with decreased IL-10 transcription upon lipopolysaccharide stimulation. In contrast, HDAC11 knockdown using shRNA had the opposite effect, result- ing in the induction of IL-10 expression. Accordingly, silencing HDAC11 expression in APC impaired antigen- specific T-cell responses, whereas overexpression of HDAC11 in APC caused tolerant CD4+ T cells to transi- tion to an immunogenic phenotype.18
HDAC6 is a positive regulator of tolerogenic APC. Normally, HDAC6 forms a complex with signal transduc- er and activator of transcription (STAT) 3 that is recruited to the Il10 promoter. Silencing HDAC6 resulted in decreased STAT3 phosphorylation and reduced IL-10 pro- duction.19 The opposing effects of HDAC11 and HDAC6 in regulating IL-10, a cytokine that can tip the balance between reactivity and tolerance in dendritic cells,18,19 underline the importance of understanding how individ- ual HDAC regulate APC function. More specific HDAC
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haematologica | 2020; 105(11)