Inhibition of the H3K79 histone methyltransferase DOT1L has exhibited encouraging preclinical and early clinical activity in KMT2A (MLL)-rearranged leukemia, supporting the development of combinatorial therapies. Here, we investigated two novel combinations: dual inhibition of the histone methyltransferases DOT1L and EZH2, and the combination with a protein synthesis inhibitor. EZH2 is the catalytic subunit in the polycomb repressive complex 2 (PRC2), and inhibition of EZH2 has been reported to have preclinical activity in KMT2A-r leukemia. When combined with DOT1L inhibition, however, we observed both synergistic and antagonistic effects. Interestingly, antagonistic effects were not due to PRC2-mediated de-repression of HOXA9. HOXA cluster genes are key canonical targets of both KMT2A and the PRC2 complex. The independence of the HOXA cluster from PRC2 repression in KMT2A-r leukemia thus affords important insights into leukemia biology. Further studies revealed that EZH2 inhibition counteracted the effect of DOT1L inhibition on ribosomal gene expression. We thus identified a previously unrecognized role of DOT1L in regulating protein production. Decreased translation was one of the earliest effects measurable after DOT1L inhibition and specific to KMT2A-rearranged cell lines. H3K79me2 chromatin immunoprecipitation sequencing patterns over ribosomal genes were similar to those of the canonical KMT2A-fusion target genes in primary AML patient samples. The effects of DOT1L inhibition on ribosomal gene expression prompted us to evaluate the combination of EPZ5676 with a protein translation inhibitor. EPZ5676 was synergistic with the protein translation inhibitor homoharringtonine (omacetaxine), supporting further preclinical/clinical development of this combination. In summary, we discovered a novel epigenetic regulation of a metabolic process-protein synthesis-that plays a role in leukemogenesis and affords a combinatorial therapeutic opportunity.

Helper Innate lymphoid cells (ILCs) are tissue resident lymphocytes that play a critical role in a number of biological processes. Several transcription factors are required for the differentiation of hematopoietic stem cells (HSCs) into ILCs. Recent studies demonstrate GATA3 as a transcriptional regulator that plays an essential role in ILC development. We aimed to modulate the differentiation of human cord blood-derived CD34 + cells into ILCs by transient and ectopic expression of mRNA encoding transcription factors known to be important for ILC lineage differentiation, including GATA3, TOX, NFIL3, ID2, and RORγt. Using this experimental protocol, only GATA3 significantly modulated HSCs to differentiate into helper ILCs. Transient overexpression of GATA3 drove the emergence of CD34 + α4β7 + early ILC progenitors during the first few days of culture. These ILC progenitors further acquired IL-7Rα and CD117 to give rise to immediate ILC precursors. In support of these findings, analysis of the genes induced by GATA3 in HSCs showed an upregulation of those associated with ILC development. Moreover, we show GATA3 also acts on more committed progenitors and significantly shifts the differentiation of progenitors away from the ILC1/NK lineage to the ILC2 and ILC3 lineage. In summary, transient overexpression of GATA3 mRNA in CD34 + HSCs enhances the differentiation of HSCs into the helper ILC lineages, at the expense of NK cell development.

Innate lymphoid cells (ILCs) develop from common lymphoid progenitors (CLPs), which further differentiate into the common ILC progenitor (CILP) that can give rise to both ILCs and natural killer (NK) cells. Murine ILC intermediates have recently been characterized, but the human counterparts and their developmental trajectories have not yet been identified, largely due to the lack of homologous surface receptors in both organisms. Here, we show that human CILPs (CD34+CD117+α4β7+Lin−) acquire CD48 and CD52, which define NK progenitors (NKPs) and ILC precursors (ILCPs). Two distinct NK cell subsets were generated in vitro from CD34+CD117+α4β7+Lin−CD48−CD52+ and CD34+CD117+α4β7+Lin−CD48+CD52+ NKPs, respectively. Independent of NKPs, ILCPs exist in the CD34+CD117+α4β7+Lin−CD48+CD52+ subset and give rise to ILC1s, ILC2s, and NCR+ ILC3s, whereas CD34+CD117+α4β7+Lin−CD48+CD52− ILCPs give rise to a distinct subset of ILC3s that have lymphoid tissue inducer (LTi)–like properties. In addition, CD48-expressing CD34+CD117+α4β7+Lin− precursors give rise to tissue-associated ILCs in vivo. We also observed that the interaction of 2B4 with CD48 induced differentiation of ILC2s, and together, these findings show that expression of CD48 by human ILCPs modulates ILC differentiation.

Numerous cell types modulate hematopoiesis through soluble and membrane bound molecules. Whether developing hematopoietic progenitors of a particular lineage modulate the differentiation of other hematopoietic lineages is largely unknown. Here we aimed to investigate the influence of myeloid progenitors on CD34 + cell differentiation into CD56 + innate lymphocytes. Sorted CD34 + cells cultured in the presence of stem cell factor (SCF) and FMS-like tyrosine kinase 3 ligand (FLT3L) give rise to numerous cell types, including progenitors that expressed the prolactin receptor (PRLR). These CD34 + pRLR + myeloid-lineage progenitors were derived from granulocyte monocyte precursors (GMPs) and could develop into granulocytes in the presence of granulocyte-macrophage colonystimulating factor (GM-CSF) in vitro. Moreover, CD34 + pRLR + myeloid progenitors lacked lymphoid developmental potential, but when stimulated with prolactin (PRL) they increased the differentiation of other CD34 + cell populations into the NK lineage in a non-contact dependent manner. Both mRNA and protein analyses show that PRL increased mothers against decapentaplegic homolog 7 (SMAD7) in CD34 + pRLR + myeloid cells, which reduced the production of transforming growth factor beta 1 (TGF-β1), a cytokine known to inhibit CD56 + cell development. Thus, we uncover an axis whereby CD34 + pRLR + GMPs inhibit CD56 + lineage development through TGF-β1 production and PRL stimulation leads to SMAD7 activation, repression of TGF-β1, resulting in CD56 + cell development. Hematopoietic differentiation is specified by a multitude of soluble and membrane-bound molecules produced both within and outside of the hematopoietic system that influence cell fate decisions 1-6. In line with this, numerous cytokines including interleukin (IL)-1, IL-2, IL-7, IL-12, IL-15, IL-18, IL-21, IL-23, IL-25 and IL-33 have been shown to modulate the development of NK cells and other innate lymphoid cells (ILCs) 7-14. Moreover, NK cells and other ILCs require different transcription factors such as Tbet, RORc and Gata3 for their development 7,15,16. Multi-lymphoid progenitor (MLP) differentiate into the common ILC progenitor and this cell then gives rise to NK cells and all helper ILCs (i.e., ILC1, 2 and 3) 12-14,17. NK cells and helper ILCs are distinguished by specific transcription factor expression, cytokine production and function 9,13,14,18,19. We have used in vitro differentiation to study these processes and over the last decade have found that both NK cells and helper ILCs (particularly, ILC1s and ILC3s) develop in this system and similarly express CD56 16,20-23. Therefore, throughout this manuscript we use the term CD56 + lymphocytes to describe all CD56 expressing cells. Prolactin (PRL) is a neuroendocrine hormone best known for its role in lactation. However, PRL also regulates hematopoietic cell development and homeostasis 24-28. Specifically, PRL enhances the development of myeloid and