Bone marrow mesenchymal stem/stromal cells (BM-MSCs) are key components of the hematopoietic niche thought to have a direct role in leukemia pathogenesis. BM-MSCs from patients with acute myeloid leukemia (AML) have been poorly characterized due to disease heterogeneity. We report a functional, genetic, and immunological characterization of BM-MSC cultures from 46 AML patients, stratified by molecular/cytogenetics into low-risk (LR), intermediate-risk (IR), and high-risk (HR) subgroups. Stable MSC cultures were successfully established and characterized from 40 of 46 AML patients irrespective of the risk subgroup. AML-derived BM-MSCs never harbored tumor-specific cytogenetic/molecular alterations present in blasts, but displayed higher clonogenic potential than healthy donor (HD)-derived BM-MSCs. Although HD- and AML-derived BM-MSCs equally provided chemoprotection to AML cells in vitro, AML-derived BM-MSCs were more immunosuppressive/anti-inflammatory, enhanced suppression of lymphocyte proliferation, and diminished secretion of pro-inflammatory cytokines. Multivariate analysis revealed that the level of interleukin-10 produced by AML-derived BM-MSCs as an independent prognostic factor negatively affected overall survival. Collectively our data show that AML-derived BM-MSCs are not tumor related, but display functional differences contributing to therapy resistance and disease evolution.

The immunosuppressive and anti-inflammatory properties of mesenchymal stromal cells (MSC) have prompted their therapeutic application in several autoimmune diseases, including rheumatoid arthritis. Adult MSC are finite and their clinical use is restricted by the need for long-term expansion protocols that can lead to genomic instability. Inhibition of Smad2/3 signaling in human pluripotent stem cells (hPSC) provides an infinite source of MSC that match the phenotype and functional properties of adult MSC. Here, we test the therapeutic potential of hPSC-MSC of embryonic origin (embryonic stem cell-derived mesenchymal stromal cells, hESC-MSC) in the experimental model of collagen-induced arthritis (CIA). CIA was induced in DBA/1 mice by immunization with type II collagen (CII) in Complete Freund’s Adjuvant (CFA). Mice were treated with either a single dose (106 cells/mouse) of hESC-MSC on the day of immunization (prophylaxis) or with three doses of hESC-MSC every other day starting on the day of arthritis onset (therapy). Arthritis severity was evaluated daily for six weeks and ten days, respectively. Frequency of Treg (FoxP3+), Th1 (IFNγ+) and Th17 (IL17+) CD4+ T cells in inguinal lymph nodes (ILN) was quantified by flow cytometry. Serum levels of anti-CII antibodies were determined by ELISA. Detection of hESC-MSC and quantification of murine and human indoleamine 2,3 dioxygenase (IDO1) expression was performed by quantitative real-time PCR. Statistical differences were analyzed by ANOVA and the Mann-Whitney U test. Administration of hESC-MSC to mice with established arthritis reduced disease severity compared to control-treated mice. Analysis of CD4 T cell populations in treated mice showed an increase in FoxP3+ Treg and IFNγ+ Th1 cells but not in Th17 cells in the ILN. Anti-CII antibody levels were not affected by treatment. Migration of hESC-MSC to the ILN in treated mice was associated with the induction of murine IDO1. Treatment with hESC-MSC ameliorates CIA by inducing IFNγ+ Th1 cells and IDO1 in the host. Thus, hESC-MSC can provide an infinite cellular source for treatment of rheumatoid arthritis.

Acute myeloid leukemia (AML) is the most common acute leukemia in adults. Disease heterogeneity is well documented, and patient stratification determines treatment decisions. Patient-derived xenografts (PDXs) from risk-stratified AML are crucial for studying AML biology and testing novel therapeutics. Despite recent advances in PDX modeling of AML, reproducible engraftment of human AML is primarily limited to high-risk (HR) cases, with inconsistent or very protracted engraftment observed for favorable-risk (FR) and intermediate-risk (IR) patients. We used NSGS mice to characterize the engraftment robustness/kinetics of 28 AML patient samples grouped according to molecular/cytogenetic classification and assessed whether the orthotopic coadministration of patient-matched bone marrow mesenchymal stromal cells (BM MSCs) improves AML engraftment. PDX event-free survival correlated well with the predictable prognosis of risk-stratified AML patients. The majority (85-94%) of the mice were engrafted in bone marrow (BM) independently of the risk group, although HR AML patients showed engraftment levels that were significantly superior to those of FR or IR AML patients. Importantly, the engraftment levels observed in NSGS mice by week 6 remained stable over time. Serial transplantation and long-term culture-initiating cell (LTC-IC) assays revealed long-term engraftment limited to HR AML patients, fitter leukemia-initiating cells (LICs) in HR AML samples, and the presence of AML LICs in the CD34− leukemic fraction, regardless of the risk group. Finally, orthotopic coadministration of patient-matched BM MSCs and AML cells was dispensable for BM engraftment levels but favored peripheralization of engrafted AML cells. This comprehensive characterization of human AML engraftment in NSGS mice offers a valuable platform for in vivo testing of targeted therapies in risk-stratified AML patient samples.

Acute myeloid leukemia (AML) is the most common acute leukemia in adults. Patients with AML harboring a constitutively active internal tandem duplication mutation (ITD^MUT) in the FMS-like kinase tyrosine kinase (FLT3) receptor generally have a poor prognosis. Several tyrosine kinase/FLT3 inhibitors have been developed and tested clinically, but very few (midostaurin and gilteritinib) have thus far been FDA/EMA-approved for patients with newly diagnosed or relapse/refractory FLT3-ITD^MUT AML. Disappointingly, clinical responses are commonly partial or not durable, highlighting the need for new molecules targeting FLT3-ITD^MUT AML. Here, we tested EC-70124, a hybrid indolocarbazole analog from the same chemical space as midostaurin with a potent and selective inhibitory effect on FLT3. In vitro, EC-70124 exerted a robust and specific antileukemia activity against FLT3-ITD^MUT AML primary cells and cell lines with respect to cytotoxicity, CFU capacity, apoptosis and cell cycle while sparing healthy hematopoietic (stem/progenitor) cells. We also analyzed its efficacy in vivo as monotherapy using two different xenograft models: an aggressive and systemic model based on MOLM-13 cells and a patient-derived xenograft model. Orally disposable EC-70124 exerted a potent inhibitory effect on the growth of FLT3-ITD^MUT AML cells, delaying disease progression and debulking the leukemia. Collectively, our findings show that EC-70124 is a promising and safe agent for the treatment of AML with FLT3-ITD^MUT.

The cell-of-origin of NPM1- and FLT3-mutated acute myeloid leukemia (AML) is still a matter of debate. Here, we combined in vitro clonogenic assays with targeted sequencing to gain further insights into the cell-of-origin of NPM1 and FLT3-ITD-mutated AML in diagnostic bone marrow (BM) from nine NPM1+/FLT3-ITD (+/−) AMLs. We reasoned that individually plucked colony forming units (CFUs) are clonal and reflect the progeny of a single stem/progenitor cell. NPM1 and FLT3-ITD mutations seen in the diagnostic blasts were found in only 2/95 and 1/57 individually plucked CFUs, suggesting that BM clonogenic myeloid progenitors in NPM1-mutated and NPM1/FLT3-ITD-mutated AML patients do not harbor such molecular lesions. This supports previous studies on NPM1 mutations as secondary mutations in AML, likely acquired in an expanded pool of committed myeloid progenitors, perhaps CD34−, in line with the CD34^−/low phenotype of NPM1-mutated AMLs. This study has important implications on the cell-of-origin of NPM1+ AML, and reinforces that therapeutic targeting of either NPM1 or FLT3-ITD mutations might only have a transient clinical benefit in debulking the leukemia, but is unlikely to be curative since will not target the AML-initiating/preleukemic cells. The absence of NPM1 and FLT3-ITD mutations in normal clonogenic myeloid progenitors is in line with their absence in clonal hematopoiesis of indeterminate potential.