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Editorial: HDAC inhibition begets more MDSCs Pavan Reddy1 Department of Internal Medicine, University of Michigan Comprehensive Cancer Center, Ann Arbor, Michigan, USA RECEIVED NOVEMBER 3, 2011; REVISED NOVEMBER 22, 2011; ACCEPTED DECEMBER 6, 2011. DOI: 10.1189/jlb.1111541 ‹ SEE CORRESPONDING ARTICLE ON PAGE 701

merging data demonstrate that cytotoxic doses [1, 2]. The mecha- HDACs remove acetyl groups from HDACi, initially developed as nisms and promise of HDACi-medi- ␧-N-acetyl lysine amino acids on his- E anticancer agents, are also po- ated immune modulation are tone tails and regulate chromatin tent anti-inflammatory agents at non- increasingly understood [1]. In this structure and dynamics [5–7]. Emerg- issue, Rosborough and colleagues [3] ing data also show that in addition to report a novel role for HDACi in regu- regulating acetylation of lysines within

Abbreviations: BMϭbone marrow, GVHDϭgraft- lating immune responses (Fig. 1). versus-host disease, HDACiϭhistone deacety- They demonstrate that HDACi en- 1. Correspondence: Department of Internal lase inhibitors, HSCϭhematopoietic stem cell, hance the generation and expansion Medicine, University of Michigan Comprehen- MDSCϭmyeloid-derived suppressor cell, of MDSCs, a key subset of regulatory sive Cancer Center, 3312 CCC, 1500 East Med- SAHAϭsuberoylinalide hydroxamic acid, ical Center Dr., Ann Arbor, MI 48109-0942, Tregϭregulatory T cell, TSAϭtrichostatin-A APCs [4]. USA. E-mail: [email protected]

www.jleukbio.org Volume 91, May 2012 Journal of Leukocyte Biology 679 SUMMARY: mechanisms of suppression of T cell re- • HDAC inhibition regulates innate responses and DC functions sponses by MDSCs include a high level of • Inhibition of HDAC enzymes in GMCSF treated BM progenitors impairs DC but promotes arginase activity, NO, ROS production, MDSC differentiation induction of Tregs, secretion of TGF-␤, • The MDSCs generated by HDAC inhibition demonstrate the expected in vitro suppressive depletion of cysteine, and up-regulation functions that are dependent on iNOS and HO-1 of PGE2. The distinct subsets of MDSCs • HDAC inhibition augments MDSCs numbers in vivo use differential pathways for regulating immune responses [4, 22, 23]. For exam- IMPLICATIONS: ple, granulocytic MDSCs are reported to • Insight into HDAC inhibition mediated immune regulation be dependent on ROS, whereas mono-

• Provide a platform for ex vivo expansion of MDSCs that may be exploited therapeutically cytic MDSCs are more dependent on argi- nase and NO for regulating T cell re- • The stability, phenotype and functional relevance of the in vivo expansion of MDSCs remain to be analyzed sponses [4]. The powerful immune-suppressive • The specific HDACs and HATs involved in MDSC differentiation need to be determined features of MDSC make them attrac- • The critical epigenetic and the non-histone proteins that must be acetylated need to be understood tive candidates for use in cell therapy to reduce unwanted and exuberant Figure 1. Summary and implications. immune responses, such as in autoim- munity, graft rejection, and GVHD [23, 24]. To facilitate such studies, it histones, HDAC enzymes regulate the activity [7]. They have been shown to would be essential to generate, ex acetylation of several nonhistone pro- regulate the function of various im- vivo, relatively large and stable im- teins [8–11]. They are classified into mune cells and modulate in vivo dis- mune-suppressive MDSCs. Murine four main classes: class I HDACs in- ease states in experimental models [1, studies have demonstrated that G-CSF clude HDAC1, -2, -3, and -8; class II 17]. Specifically, their impact on Tregs expands MDSCs in vitro and in vivo HDACs are HDAC4, -5, -7, and -9 and T cell responses is being appreci- [23]. Rosborough et al. [3] analyzed (class IIa) and HDAC6 and -10 (class ated increasingly [13, 18]. Recent data the impact of HDAC inhibition on the IIb); class III HDACs are homologs of have demonstrated the ability of these generation and function of MDSCs. yeast silent information regulator 2 agents to inhibit the production of They demonstrate that HDAC inhibi- proteins; and class IV HDAC is multiple proinflammatory cytokines tion of BM GM-CSF-treated cultures HDAC11 [11]. Class I HDAC enzymes from various APCs [1]. HDAC inhibi- with TSA or SAHA expanded the HSC are expressed in most cells and are tion has been shown to enhance IDO and progenitor compartments, skewed largely, but not exclusively, restricted expression in DCs, promote conver- myeloid differentiation, impaired the to the nucleus. By contrast, class II sion of inflammatory macrophages development of DCs, and enhanced HDAC enzymes demonstrate a more into a tolerogenic phenotype, decrease the generation of MDSCs. Although restricted, tissue-specific expression TLR signaling, reduce costimulatory addition of rIL-4 to the cultures ex- and shuttle between the nucleus and molecule expression, and increase panded the progenitor cells and dem- cytoplasm [7, 12]. Most HDACi inhibit IL-10 expression [19–22]. But, the onstrated skewed myeloid differentia- class I and II enzymes with varying ef- mechanisms of action of HDACi on tion, it did not enhance the genera- ficiency [11]. The catalytic activities of different APC subsets and their gener- tion of MDSCs. Upon further the class I and II HDACs differ [7]. ation and function are not understood characterization, they found that these ϩ Class I HDAC enzymes exhibit strong completely. MDSCs expressed CD11b F4/80int deacetylase activity, whereas most class MDSCs are now being appreciated in- and Ly-6Chigh, suggesting generation II HDACs are enzymatically less active creasingly as key APC subsets that are re- of a monocytic MDSC phenotype. and act primarily as scaffolding pro- sponsible for regulating immune re- These cells demonstrated equivalent teins within large multimolecular com- sponses. They potently suppress T effector suppression of allogeneic T cell re- plexes. cell responses while enhancing Tregs sponses in vitro. However, in contrast HDACi belong to different classes of [23]. They are a heterogeneous popula- to control MDSCs, those derived fol- drugs with distinct chemical structures tion of immature myeloid cells that con- lowing HDAC inhibition showed re- and abilities to inhibit HDAC enzymes sists of myeloid progenitors and precur- duced expression of arginase, iNOS, [11, 13–15]. The two HDACi, used by sors [4]. MDSCs are identified in murine and HO-1. Furthermore, arginase was Rosborough et al. [3]—TSA and studies as cells that are positive for CD11b not required, whereas iNOS and HO-1 SAHA—are nonselective, pan-HDACi and Gr-1. Based on expression of Ly-6C activity was required for the suppres- that inhibit class I and II [11, 16]. or Ly-6G, they can be characterized fur- sive effects on allogeneic T cells. Im- HDACi can also modulate the acetyla- ther as monocytic MDSCs (CD11bϩ Ly- portantly, HDAC inhibition also aug- tion of HDAC proteins themselves, 6GϪ Ly-6Chigh) or granulocytic MDSCs mented in vivo expansion of MDSCs causing alterations in their stability or (CD11bϩ Ly-6Gϩ Ly-6Clow) [23]. The in BM and spleen by GM-CSF, al-

680 Journal of Leukocyte Biology Volume 91, May 2012 www.jleukbio.org EDITORIAL Reddy HDAC inhibition begets more MDSCs though the functional impact of this ACKNOWLEDGMENTS deacetylase homologue bound to the TSA and SAHA inhibitors. Nature 401, 188–193. in vivo expansion was not addressed. 17. Reddy, P., Maeda, Y., Hotary, K., Liu, C., This study, like all interesting and P.R. was supported by NIH grants AI- Reznikov, L. L., Dinarello, C. A., Ferrara, J. L. 075284, HL-090775, and CA-143379. (2004) Histone deacetylase inhibitor suberoylani- seminal observations, while illuminat- lide hydroxamic acid reduces acute graft-versus- ing a role for HDAC inhibition in the host disease and preserves graft-versus-leukemia generation of MDSCs, also raises sev- effect. Proc. Natl. Acad. Sci. USA 101, 3921–3926. 18. Tao, R., de Zoeten, E. F., Ozkaynak, E., Chen, C., eral additional questions. Why did the REFERENCES Wang, L., Porrett, P. M., Li, B., Turka, L. 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