Glucocorticoid Production in Lymphoid Organs: Acute Effects of Lipopolysaccharide in Neonatal and Adult Mice.

Glucocorticoids (GCs) are critical modulators of the immune system. The hypothalamic-pituitary-adrenal (HPA) axis regulates circulating GC levels and is stimulated by endotoxins. Lymphoid organs also produce GCs; however, it is not known how lymphoid GC levels are regulated in response to endotoxins. We assessed whether an acute challenge of lipopolysaccharide (LPS) increases lymphoid levels of progesterone and GCs, and expression of steroidogenic enzymes and key HPA axis components (eg, corticotropin-releasing hormone [CRH], adrenocorticotropic hormone [ACTH]). We administered LPS (50 µg/kg intraperitoneally) or vehicle control to male and female C57BL/6J neonatal (postnatal day [PND] 5) and adult (PND90) mice and collected blood, bone marrow, thymus, and spleen 4 hours later. We measured progesterone, 11-deoxycorticosterone, corticosterone, and 11-dehydrocorticosterone via liquid chromatography-tandem mass spectrometry. We measured gene expression of key steroidogenic enzymes (Cyp11b1, Hsd11b1, and Hsd11b2) and HPA axis components (Crh, Crhr1, Pomc, and Mc2r) via quantitative polymerase chain reaction. At PND5, LPS induced greater increases in steroid levels in lymphoid organs than in blood. In contrast, at PND90, LPS induced greater increases in steroid levels in blood than in lymphoid organs. Steroidogenic enzyme transcripts were present in all lymphoid organs, and LPS altered steroidogenic enzyme expression predominantly in the spleen. Lastly, we detected transcripts of key HPA axis components in all lymphoid organs, and there was an effect of LPS in the spleen. Taken together, these data suggest that LPS regulates GC production by lymphoid organs, similar to its effects on the adrenal glands, and the effects of LPS might be mediated by local expression of CRH and ACTH.

Discovery of a Highly Potent and Selective Degrader Targeting Hematopoietic Prostaglandin D Synthase via In Silico Design.

Targeted protein degradation by proteolysis-targeting chimera (PROTAC) is one of the exciting modalities for drug discovery and biological discovery. It is important to select an appropriate linker, an E3 ligase ligand, and a target protein ligand in the development; however, it is necessary to synthesize a large number of PROTACs through trial and error. Herein, using a docking simulation of the ternary complex of a hematopoietic prostaglandin D synthase (H-PGDS) degrader, H-PGDS, and cereblon, we have succeeded in developing PROTAC(H-PGDS)-7 (6), which showed potent and selective degradation activity (DC50 = 17.3 pM) and potent suppression of prostaglandin D2 production in KU812 cells. Additionally, in a Duchenne muscular dystrophy model using mdx mice with cardiac hypertrophy, compound 6 showed better inhibition of inflammatory cytokines than a potent H-PGDS inhibitor TFC-007. Thus, our results demonstrated that in silico simulation would be useful for the rational development of PROTACs.

Platelet and myeloid cell phenotypes in a rat model of Fabry disease.

Fabry disease results from a deficiency of the lysosomal enzyme ⍺-Galactosidase-A (⍺-Gal A) and is estimated to occur in approximately 1:4100 live births. Characteristic of the disease is the accumulation of α-Gal-A substrates, primarily the glycosphingolipids (GSLs) globotriaosylceramide and globotriaosylsphingosine. Thrombotic events are a significant concern for Fabry patients, with strokes contributing to a significant decrease in overall lifespan. Currently, the mechanisms underlying the increased risk of thrombotic events experienced by Fabry patients are incompletely defined. Using a rat model of Fabry disease, we provide an improved understanding of the mechanisms linking GSL accumulation to thrombotic risk. We found that ⍺-Gal A-deficient rats accumulate myeloid-derived leukocytes at sites of GSL accumulation, including in the bone marrow and circulation, and that myeloid-derived leukocyte and megakaryocyte populations were prominent among cell types that accumulated GSLs. In the circulation, ⍺-Gal A-deficient rats had increases in cytokine-producing cell types and a corresponding elevation of pro-inflammatory cytokines. Lastly, circulating platelets from ⍺-Gal A-deficient rats accumulated a similar set of ⍺-Galactosidase-A substrates as was observed in megakaryocytes in the bone marrow, and exhibited increased platelet binding to fibrinogen in microfluidic and flow cytometric assays.

The Angiotensin-(1-12)/Chymase axis as an alternate component of the tissue renin angiotensin system.

The identification of an alternate extended form of angiotensin I composed of the first twelve amino acids at the N-terminal of angiotensinogen has generated new knowledge of the importance of noncanonical mechanisms for renin independent generation of angiotensins. The human sequence of the dodecapeptide angiotensin-(1-12) [N-Asp1-Arg2-Val3-Tyr4-Ile5-His6-Pro7-Phe8-His9-Leu10-Val1-Ile12-COOH] is an endogenous substrate that in the rat has been documented to be present in multiple organs including the heart, brain, kidney, gut, adrenal gland, and the bone marrow. Newer studies have confirmed the existence of Ang-(1-12) as an Ang II-forming substrate in the blood and heart of normal and diseased patients. Studies to-date document that angiotensin II generation from angiotensin-(1-12) does not require renin participation while chymase rather than angiotensin converting enzyme shows high catalytic activity in converting this tissue substrate into angiotensin II directly.

Fatty acid synthase, a novel poor prognostic factor for acute lymphoblastic leukemia which can be targeted by ginger extract.

Altered metabolism of fatty acid synthesis is considered a hallmark characteristic of several malignancies, including acute lymphoblastic leukemia (ALL). To evaluate the impact of fatty acid synthase (FASN) on drug resistant ALL, bone marrow samples were collected from 65 pediatric ALLs, including 40 de novo and 25 relapsed patients. 22 non-cancer individuals were chosen as controls. Quantitative RT-PCR showed increased expression levels of FASN in drug resistant patients compared with the therapy responders. Single and combined treatment of malignant cells were analyzed using Annexin-V/PI double staining and MTT assays. Incubation of resistant primary cells with ginger showed simultaneous increased apoptosis rates and reduced FASN expression levels. Furthermore, docking studies demonstrated high affinity bindings between ginger derivatives and FASN thioesterase and ketosynthase domains, compared with their known inhibitors, fenofibrate and morin, respectively. Finally, combined treatment of in-house multidrug resistant T-ALL subline with ginger and dexamethasone induced drug sensitivity and down regulation of FASN expression, accordingly. To the best of our knowledge, this is the first study that introduces FASN upregulation as a poor prognostic factor for drug resistant childhood ALL. Moreover, it was revealed that FASN inhibition may be applied by ginger phytochemicals and overcome dexamethasone resistance, subsequently.

Inhibiting Glycogen Synthase Kinase-3 Mitigates the Hematopoietic Acute Radiation Syndrome in a Sex- and Strain-dependent Manner in Mice.

The Radiation and Nuclear Countermeasures Program at the National Institute of Allergy and Infectious Diseases (NIAID) mandated that medical countermeasures for treating Acute Radiation Syndrome (ARS) must have efficacy when administered at least 24 h after radiation exposure. At this time point, many cells within key target tissues, such as the hematopoietic system and the gastrointestinal (GI) tract, will already be dead. Therefore, drugs that promote the regeneration of surviving cells may improve outcomes. The serine/threonine kinase glycogen synthase kinase-3 (GSK-3) regulates stem and progenitor cell self-renewal and regeneration in the hematopoietic and GI compartments. We tested inhibition of GSK-3ß by SB216763 24 h after total body irradiation (TBI) and sub-total body irradiation (SBI). Here, we show that subcutaneous administration of SB216763 promotes the regeneration of surviving hematopoietic stem/progenitor cells (HSPCs), including myeloid progenitor cells, and improves survival of C57Bl/6 male mice when administered 24 h after TBI. However, these results were not recapitulated in female C57Bl/6 animals, suggesting a sex difference in GSK-3ß signaling in HSPCs. Subcutaneous administration of SB216763 in male mice stimulated activation of Sox2 transcription but failed to induce Sox2 transcription in female C57Bl/6 mice. Using TCF/lef-GFP reporter mice, we examined Wnt signaling in HSPCs of irradiated male and female mice treated with SB216763. GSK-3 inhibition elevated Wnt reporter activity in HSPCs isolated from male but not female mice. SB216763 did not mitigate hematopoietic ARS in males or females of a second strain of wild-type mice, C3H. In addition, administration of SB216763 did not mitigate hematopoietic ARS beyond the currently available standard approved therapy of ciprofloxacin and granulocyte-colony stimulating factor (G-CSF) in male C57Bl/6 mice. Further, SB216763 did not mitigate GI-ARS after SBI in C57Bl/6 male mice. The lack of efficacy in both sexes and multiple strains of mice indicate that SB216763 is not suitable for further drug development as a mitigator of ARS. Our studies demonstrate that activation of Wnt signaling in HSPCs promotes hematopoietic regeneration following radiation exposure, and targeting this pathway downstream of GSK-3ß may mitigate ARS in a sex- and strain-independent manner.

Bone marrow niche-derived extracellular matrix-degrading enzymes influence the progression of B-cell acute lymphoblastic leukemia.

Specific and reciprocal interactions with the bone marrow microenvironment (BMM) govern the course of hematological malignancies. Matrix metalloproteinase-9 (MMP-9), secreted by leukemia cells, facilitates tumor progression via remodeling of the extracellular matrix (ECM) of the BMM. Hypothesizing that leukemias may instruct the BMM to degrade the ECM, we show, that MMP-9-deficiency in the BMM prolongs survival of mice with BCR-ABL1-induced B-cell acute lymphoblastic leukemia (B-ALL) compared with controls and reduces leukemia-initiating cells. MMP-9-deficiency in the BMM leads to reduced degradation of proteins of the ECM and reduced invasion of B-ALL. Using various in vivo and in vitro assays, as well as recipient mice deficient for the receptor for tumor necrosis factor (TNF) α (TNFR1) we demonstrate that B-ALL cells induce MMP-9-expression in mesenchymal stem cells (MSC) and possibly other cells of the BMM via a release of TNFα. MMP-9-expression in MSC is mediated by activation of nuclear factor kappa B (NF-κB) downstream of TNFR1. Consistently, knockdown of TNF-α in B-ALL-initiating cells or pharmacological inhibition of MMP-9 led to significant prolongation of survival in mice with B-ALL. In summary, leukemia cell-derived Tnfα induced MMP-9-expression by the BMM promoting B-ALL progression. Inhibition of MMP-9 may act as an adjunct to existing therapies.

Identification of DOT1L inhibitors by structure-based virtual screening adapted from a nucleoside-focused library.

Disruptor of Telomeric Silencing 1-Like (DOT1L), the sole histone H3 lysine 79 (H3K79) methyltransferase, is required for leukemogenic transformation in a subset of leukemias bearing chromosomal translocations of the Mixed Lineage Leukemia (MLL) gene, as well as other cancers. Thus, DOT1L is an attractive therapeutic target and discovery of small molecule inhibitors remain of high interest. Herein, we are presenting screening results for a unique focused library of 1200 nucleoside analogs originally produced under the aegis of the NIH Pilot Scale Library Program. The complete nucleoside set was screened virtually against DOT1L, resulting in 210 putative hits. In vitro screening of the virtual hits resulted in validation of 11 compounds as DOT1L inhibitors clustered into two distinct chemical classes, adenosine-based inhibitors and a new chemotype that lacks adenosine. Based on the developed DOT1L ligand binding model, a structure-based design strategy was applied and a second-generation of non-nucleoside DOT1L inhibitors was developed. Newly synthesized compound 25 was the most potent DOT1L inhibitor in the new series with an IC50 of 1.0 µM, showing 40-fold improvement in comparison with hit 9 and exhibiting reasonable on target effects in a DOT1L dependent murine cell line. These compounds represent novel chemical probes with a unique non-nucleoside scaffold that bind and compete with the SAM binding site of DOT1L, thus providing foundation for further medicinal chemistry efforts to develop more potent compounds.

Human Bone Marrow Is Comprised of Adipocytes with Specific Lipid Metabolism.

Under caloric restriction, bone marrow adipocytes (BM-Ads) do not decrease in size compared to white adipocytes, suggesting they harbor unique metabolic properties. We compare human primary BM-Ads with paired subcutaneous adipocytes (SC-Ads) using proteomic and lipidomic approaches. We find that, although SC-Ads and BM-Ads share similar morphological features, they possess distinct lipid metabolism. Although BM-Ad shows enrichment in proteins involved in cholesterol metabolism, correlating with increased free cholesterol content, proteins involved in lipolysis were downregulated. In particular, monoacylglycerol lipase expression is strongly reduced in BM-Ads, leading to monoacylglycerol accumulation. Consequently, basal and induced lipolytic responses are absent in BM-Ads, affirming their differences in metabolic fitness upon caloric restriction. These specific metabolic features are not recapitulated in vitro using common protocols to differentiate bone marrow mesenchymal stem cells. Thus, contrary to classical SC-Ads, BM-Ads display a specific lipid metabolism, as they are devoid of lipolytic activity and exhibit a cholesterol-orientated metabolism.

A quantitative method for detection of circulating fms related tyrosine kinase 3 (FLT-3) in acute myeloid leukemia (AML) patients.

FMS related tyrosine kinase 3 (FLT-3) is a tyrosine kinase expressed in early hematopoietic precursor cells and has roles in survival, proliferation, and differentiation. Bone marrow expression and mutagenic analysis of FLT-3 in Acute Myeloid Leukemia (AML) patients is well-characterized. However, the levels of circulating FLT-3 in serum have not been previously described. In this study we describe a quantitative electrochemiluminescent immunoassay that detects FLT-3 in human serum. Using this method we find that AML patients have elevated levels of circulating FLT-3 and these levels correlated to the percent blast counts in the bone marrow (BM).

Comparison of methodologies for the detection of BRAF mutations in bone marrow trephine specimens.

AIMS: BRAF V600E detection assists in the diagnosis of hairy cell leukaemia (HCL); however, testing practices vary. We evaluated the clinical utility of 5 BRAF mutation testing strategies for use on bone marrow trephines (BMT). METHODS: 11 HCL, 5 HCL 'mimic', 2 treated HCL and 10 normal BMT specimens were tested for mutant BRAF, comparing Sanger sequencing, pyrosequencing, amplicon-based next generation sequencing (NGS), automated (Idylla) PCR and immunohistochemistry (IHC). RESULTS: PCR and IHC were cheaper and identified V600E in 100 % of HCL cases. Pyrosequencing detected the mutation in 91%, NGS in 55% of cases and Sanger sequencing in 27%. All assays gave wild-type BRAF results in HCL mimics and normal BMT samples. CONCLUSIONS: PCR and IHC were most sensitive and cost-effective, but these have limited scope for multiplexing and are likely to be replaced by NGS gene panels or whole genome sequencing in the medium to long term.

Mitochondrial protein OPA mediates osteoporosis induced by radiation through the P38 signaling pathway.

OBJECTIVE: Bone marrow is full of mitochondria. However, the role of bone marrow mitochondrial protein in bone marrow damage and related signal transduction mechanism remains to be further studied. OPA is a newly discovered mitochondrial transmembrane protein. Its expression pattern and function in the physiological and pathological conditions of bone marrow are still elusive. The purpose of this study is to investigate the potential role of OPA in osteoporosis. PATIENTS AND METHODS: A mouse osteoporosis model was established by radiation. The OPA expression was tested by Western blot and qRT-PCR. The P38 signaling activity was evaluated by enzymatic activity kit. The mitochondrial ATP production was determined by flow cytometry. The bone marrow cell apoptosis was detected by flow cytometry. U0126 was used to pretreat mouse before modeling. Bone marrow tissue was collected from patients who received osteoporosis surgery to test the OPA expression, P38 activation and cell apoptosis. The OPA and P38 levels were analyzed by correlation. RESULTS: The mouse osteoporosis model was successfully established by radiation induction. In this osteoporosis model, the expression of OPA was increased. The P38 signaling was activated while the mitochondrial ATP production was reduced, with the increase of apoptosis of bone marrow cells. By contrast, U0126 pretreatment markedly inhibited the OPA expression, restrained the P38 signaling pathway, enhanced mitochondrial ATP production and suppressed the bone marrow cell apoptosis in mouse osteoporosis model. A significantly positive correlation was found between OPA and P38. CONCLUSIONS: The down-regulation of OPA inhibits cell apoptosis and improves osteoporosis via inducing mitochondrial ATP production and suppressing the P38 signaling pathway.

Attenuation of PKCδ enhances metabolic activity and promotes expansion of blood progenitors.

A finely tuned balance of self-renewal, differentiation, proliferation, and survival governs the pool size and regenerative capacity of blood-forming hematopoietic stem and progenitor cells (HSPCs). Here, we report that protein kinase C delta (PKCδ) is a critical regulator of adult HSPC number and function that couples the proliferative and metabolic activities of HSPCs. PKCδ-deficient mice showed a pronounced increase in HSPC numbers, increased competence in reconstituting lethally irradiated recipients, enhanced long-term competitive advantage in serial transplantation studies, and an augmented HSPC recovery during stress. PKCδ-deficient HSPCs also showed accelerated proliferation and reduced apoptosis, but did not exhaust in serial transplant assays or induce leukemia. Using inducible knockout and transplantation models, we further found that PKCδ acts in a hematopoietic cell-intrinsic manner to restrict HSPC number and bone marrow regenerative function. Mechanistically, PKCδ regulates HSPC energy metabolism and coordinately governs multiple regulators within signaling pathways implicated in HSPC homeostasis. Together, these data identify PKCδ as a critical regulator of HSPC signaling and metabolism that acts to limit HSPC expansion in response to physiological and regenerative demands.

Proteinase 3 Limits the Number of Hematopoietic Stem and Progenitor Cells in Murine Bone Marrow.

Hematopoietic stem and progenitor cells (HSPCs) undergo self-renewal and differentiation to guarantee a constant supply of short-lived blood cells. Both intrinsic and extrinsic factors determine HSPC fate, but the underlying mechanisms remain elusive. Here, we report that Proteinase 3 (PR3), a serine protease mainly confined to granulocytes, is also expressed in HSPCs. PR3 deficiency intrinsically suppressed cleavage and activation of caspase-3, leading to expansion of the bone marrow (BM) HSPC population due to decreased apoptosis. PR3-deficient HSPCs outcompete the long-term reconstitution potential of wild-type counterparts. Collectively, our results establish PR3 as a physiological regulator of HSPC numbers. PR3 inhibition is a potential therapeutic target to accelerate and increase the efficiency of BM reconstitution during transplantation.

Inflammasome Activation Aggravates Cutaneous Xanthomatosis and Atherosclerosis in ACAT1 (Acyl-CoA Cholesterol Acyltransferase 1) Deficiency in Bone Marrow.

Objective- ACAT1 (Acyl-CoA cholesterol acyltransferase 1) esterifies cellular free cholesterol, thereby converting macrophages to cholesteryl ester-laden foam cells in atherosclerotic lesions and cutaneous xanthoma. Paradoxically, however, loss of ACAT1 in bone marrow causes the aggravation of atherosclerosis and the development of severe cutaneous xanthoma in hyperlipidemic mice. Recently, it has been reported that cholesterol crystals activate NLRP3 (NACHT, LRR [leucine-rich repeats], and PYD [pyrin domain] domain-containing protein 3) inflammasomes, thereby contributing to the development of atherosclerosis. The present study aimed to clarify the role of NLRP3 inflammasomes in the worsening of atherosclerosis and cutaneous xanthoma induced by ACAT1 deficiency. Approach and Results- Ldlr-null mice were transplanted with bone marrow from WT (wild type) mice and mice lacking ACAT1, NLRP3, or both. After the 4 types of mice were fed high-cholesterol diets, we compared their atherosclerosis and skin lesions. The mice transplanted with Acat1-null bone marrow developed severe cutaneous xanthoma, which was filled with numerous macrophages and cholesterol clefts and had markedly increased expression of inflammatory cytokines, and increased atherosclerosis. Loss of NLRP3 completely reversed the cutaneous xanthoma, whereas it improved the atherosclerosis only partially. Acat1-null peritoneal macrophages showed enhanced expression of CHOP (C/EBP [CCAAT/enhancer binding protein] homologous protein) and TNF-α (tumor necrosis factor-α) but no evidence of inflammasome activation, after treatment with acetylated LDL (low-density lipoprotein). Conclusions- Elimination of ACAT1 in bone marrow-derived cells aggravates cutaneous xanthoma and atherosclerosis. The development of cutaneous xanthoma is induced mainly via the NLRP3 inflammasome activation.

Effects of JAK1/2 inhibition on bone marrow stromal cells of myeloproliferative neoplasm (MPN) patients and healthy individuals.

OBJECTIVE: Philadelphia-negative myeloproliferative neoplasms (MPNs) commonly share hyperactive JAK-STAT signaling affecting hematopoietic stem cells (HSC) and their progeny. The JAK1/2 inhibitor Ruxolitinib has remarkable clinical efficacy, including spleen reduction, improvement of constitutional symptoms, and bone marrow (BM) fibrosis reversal. Whether this is due to inhibition of JAK2-mutated HSC only, or whether Ruxolitinib also affects BM stroma is not known. METHODS: This study investigated potential effects of Ruxolitinib on BM mesenchymal stromal cells (MSC), which are not only major regulators of hematopoiesis but also contribute to fibrosis, from 10 healthy donors and 7 JAK2V617F -positive MPN patients. RESULTS: Ruxolitinib moderately inhibited the growth of healthy donor MSC (HD-MSC) and MSC from JAK2V617F+ MPN patients (P-MSC) in short- and long-term assays. The clonogenic potential of HD-MSC was not affected by Ruxolitinib. JAK-STAT signaling, however, was markedly inhibited in both HD-MSC and P-MSC, the latter of which showed higher expression of fibrosis-associated and hematopoiesis-maintenance genes. Moreover, Ruxolitinib reduced MSC secretion of MCP-1 and IL-6. CONCLUSION: Ruxolitinib affected JAK2 signaling in MSC at clinically relevant doses, which is likely to contribute to the normalization of the inflammatory milieu in MPNs. Thus, combined HSC and stroma-directed interventions have the potential to improve constitutional symptoms and reduce stromal proliferation in MPNs.

Role of the angiotensin-converting enzyme in the G-CSF-induced mobilization of progenitor cells.

In addition to being a peptidase, the angiotensin-converting enzyme (ACE) can be phosphorylated and involved in signal transduction. We evaluated the role of ACE in granulocyte-colony-stimulating factor (G-CSF)-induced hematopoietic progenitor cell (HPC) mobilization and detected a significant increase in mice-lacking ACE. Transplantation experiments revealed that the loss of ACE in the HPC microenvironment rather than in the HPCs increased mobilization. Indeed, although ACE was expressed by a small population of bone-marrow cells, it was more strongly expressed by endosteal bone. Interestingly, there was a physical association of ACE with the G-CSF receptor (CD114), and G-CSF elicited ACE phosphorylation on Ser1270 in vivo and in vitro. A transgenic mouse expressing a non-phosphorylatable ACE (ACES/A) mutant demonstrated increased G-CSF-induced HPC mobilization and decreased G-CSF-induced phosphorylation of STAT3 and STAT5. These results indicate that ACE expression/phosphorylation in the bone-marrow niche interface negatively regulates G-CSF-induced signaling and HPC mobilization.

Quantifying Caspase-1 Activity in Murine Macrophages.

The caspase-1 protease is a core component of multiprotein inflammasome complexes, which play a critical role in regulating the secretion of mature, bioactive pro-inflammatory cytokines interleukin (IL)-1ß and IL-18. The activity of caspase-1 is often measured indirectly, by monitoring cleavage of cellular caspase-1 substrates, processing of caspase-1 itself, or by quantifying cell death. Here we describe methods for eliciting caspase-1 activity in murine macrophages, via activation of the NLRP3, NAIP/NLRC4 or AIM2 inflammasomes. We then describe a simple fluorogenic assay for directly quantifying cellular caspase-1 activity.

Overexpression of Arginase 1 is linked to DNMT3A and TET2 mutations in lower-grade myelodysplastic syndromes and chronic myelomonocytic leukemia.

Immune dysregulation is a common feature of myelodysplastic syndromes (MDS) and chronic myelomonocytic leukemia (CMML), particularly in early stages. However, the genetic basis remains poorly understood. We recently reported that macrophages from mice deficient in tet methylcytosine dioxygenase 2 (Tet2), a model of MDS/CMML, are hyperinflammatory and have increased expression of arginase 1 (Arg1). In macrophages and myeloid derived suppressor cells (MDSCs) expression of Arg1 contributes to T-cell suppression and immune evasion by L-arginine depletion, in the setting of chronic inflammation and cancer. Since human MDS and CMML are driven by TET2 mutations and associated with chronic inflammation, we hypothesized that arginase enzymatic activity and ARG1 expression would be increased in human MDS/CMML bone marrow. Elevated arginase activity was observed in bone marrow mononuclear cells of MDS and CMML patients with lower-grade features. Immunohistochemical studies confirmed that myelomonocytic cells overexpress ARG1. Additionally, mutations in the epigenetic regulators TET2 and DNMT3A corresponded to high ARG1 expression and activity. These findings suggest ARG1 is a biomarker of immune dysregulation in early MDS and CMML. Recent murine findings have implicated Tet2 and Dnmt3a in regulation of innate immunity. Our study suggests similar changes may be driven by human TET2 and DNMT3A mutations.