We report here that in chronic lymphocytic leukemia (CLL), the propensity to generate clonal B cells has been acquired already at the hematopoietic stem cell (HSC) stage. HSCs purified from patients ...with CLL displayed lymphoid-lineage gene priming and produced a high number of polyclonal B cell progenitors. Strikingly, their maturation into B cells was restricted always to mono- or oligo-clones with CLL-like phenotype in xenogeneic recipients. These B cell clones were independent of the original CLL clones because they had their own immunoglobulin VDJ genes. Furthermore, they used preferentially VH genes frequently used in human CLL, presumably reflecting the role of B cell receptor signaling in clonal selection. These data suggest that HSCs can be involved in leukemogenesis even in mature lymphoid tumors.
► HSCs in CLL have cell-intrinsic propensity to generate clonal CLL-like B cells ► Abnormal karyotypes are not required for appearance of such B cell clones ► The majority of single HSCs in patients with CLL display lymphoid-lineage priming ► HSCs can be involved in oncogenesis even in mature lymphoid tumors
Adult T‐cell leukemia/lymphoma (ATL) is an aggressive lymphoproliferative disease caused by human T‐cell leukemia virus type 1 (HTLV‐1). Multi‐agent chemotherapy can reduce ATL cells but frequently ...allows relapses within a short period of time. Allogeneic hematopoietic stem cell transplantation (allo‐HSCT) following chemotherapy is now a standard therapy for ATL in Japan as it can achieve long‐term remission in approximately one‐third of recipient ATL patients; however, it also has a risk of treatment‐related mortality. Allo‐HSCT often induces HTLV‐1 Tax‐specific cytotoxic T cells (CTL) as well as graft‐versus‐host (GVH) response in ATL patients. This observation led to development of a new therapeutic vaccine to activate Tax‐specific CTL, anticipating anti‐ATL effects without GVH response. The newly developed Tax‐DC vaccine consists of autologous dendritic cells pulsed with Tax peptides corresponding to CTL epitopes that have been identified in post‐allo‐HSCT ATL patients. In a pilot study of Tax‐DC therapy in three ATL patients after various initial therapies, two patients survived for more than 4 years after vaccination without severe adverse effects (UMIN000011423). The Tax‐DC vaccine is currently under phase I trial, showing a promising clinical outcome so far. These findings indicate the importance of patients’ own HTLV‐1‐specific T‐cell responses in maintaining remission and provide a new approach to anti‐ATL immunotherapy targeting Tax. Although Tax‐targeted vaccination is ineffective against Tax‐negative ATL cells, it can be a safe alternative maintenance therapy for Tax‐positive ATL and may be further applicable for treatment of indolent ATL or even prophylaxis of ATL development among HTLV‐1‐carriers.
Although adult T‐cell leukemia (ATL) is known to be resistant to chemotherapy, recent therapeutic approaches including hematopoietic stem cell transplantation, mogamulizumab and lenalidomide improved prognosis of ATL, all of which act through host immunity. More recently, a clinical study of novel anti‐ATL therapeutic vaccine targeting human T‐cell leukemia virus type 1 Tax was carried out in Japan for the first time with promising clinical outcomes. This review proposes a new concept of immunotherapy in ATL, by dissecting underlying mechanisms.
Thrombopoietin receptor agonist (TPO-RA) is effective for aplastic anemia (AA) and idiopathic thrombocytopenic purpura (ITP). However, the risk of thrombosis during ITP treatment with TPO-RA is ...higher than without TPO-RA. It is unclear whether TPO-RA increases the risk of thrombosis in patients with AA. We report a case of a 66-year-old female with severe AA having paroxysmal nocturnal hemoglobinuria (PNH) clones in the peripheral blood who developed ischemic colitis after three days of starting eltrombopag. Contrast-enhanced computed tomography showed ischemic colitis and contrast enhancement defect in the left atrial appendage, which indicated a thrombus in the heart. Stopping eltrombopag and providing supportive care improved her symptoms, and her blood cell counts gradually increased. Thrombosis should be considered when TPO-RA is administered during the immunosuppressive treatment of AA.
Myeloid cell leukemia-1 (MCL-1) is an essential survival factor for hematopoiesis. In humans, hematopoietic stem cells (HSCs) express MCL-1 at the highest level in response to FMS-like tyrosine ...kinase-3 (FLT3) signaling. We here show that this FLT3-dependent stem cell maintenance system also plays a critical role in survival of leukemic stem cells (LSCs) in acute myeloid leukemia (AML). The CD34+CD38− LSC fraction expresses high levels of FLT3 as well as MCL-1, even compared with normal HSCs. Treatment with FLT3 ligand induced further MCL-1 up-regulation in LSCs in all AML cases tested. Interestingly, the group of samples expressing the highest levels of MCL-1 constituted AML with FLT3–internal tandem duplications (ITD). In FLT3-ITD AML cell lines, cells expressed a high level of MCL-1, and an inhibition of MCL-1 induced their apoptotic cell death. A tyrosine kinase inhibitor suppressed MCL-1 expression, and induced apoptosis that was reversed by the enforced MCL-1 expression. Finally, transduction of FLT3-ITD into HSCs strongly activated MCL-1 expression through its signal transducer and activator of transcription 5 (STAT5)–docking domains. This effect was completely abrogated when STAT5 activation was blocked. Thus, the acquisition of FLT3-ITD ensures LSC survival by up-regulating MCL-1 via constitutive STAT5 activation that is independent of wild-type FLT3 signaling.
The developmental pathway for human megakaryocytes remains unclear, and the definition of pure unipotent megakaryocyte progenitor is still controversial. Using single-cell transcriptome analysis, we ...have identified a cluster of cells within immature hematopoietic stem- and progenitor-cell populations that specifically expresses genes related to the megakaryocyte lineage. We used CD41 as a positive marker to identify these cells within the CD34+CD38+IL-3RαdimCD45RA− common myeloid progenitor (CMP) population. These cells lacked erythroid and granulocyte-macrophage potential but exhibited robust differentiation into the megakaryocyte lineage at a high frequency, both in vivo and in vitro. The efficiency and expansion potential of these cells exceeded those of conventional bipotent megakaryocyte/erythrocyte progenitors. Accordingly, the CD41+ CMP was defined as a unipotent megakaryocyte progenitor (MegP) that is likely to represent the major pathway for human megakaryopoiesis, independent of canonical megakaryocyte-erythroid lineage bifurcation. In the bone marrow of patients with essential thrombocythemia, the MegP population was significantly expanded in the context of a high burden of Janus kinase 2 mutations. Thus, the prospectively isolatable and functionally homogeneous human MegP will be useful for the elucidation of the mechanisms underlying normal and malignant human hematopoiesis.
•A prospectively isolatable and functionally homogeneous human megakaryocyte progenitor population resides in CMP fraction of adult bone marrow.•This newly identified unipotent megakaryocyte progenitor significantly contributes to normal and malignant megakaryopoiesis.
Cyclins are regulatory subunits of cyclin-dependent kinases. Cyclin A, the first cyclin ever cloned, is thought to be an essential component of the cell-cycle engine. Mammalian cells encode two ...A-type cyclins, testis-specific cyclin A1 and ubiquitously expressed cyclin A2. Here, we tested the requirement for cyclin A function using conditional knockout mice lacking both A-type cyclins. We found that acute ablation of cyclin A in fibroblasts did not affect cell proliferation, but led to prolonged expression of another cyclin, cyclin E, across the cell cycle. However, combined ablation of all A- and E-type cyclins extinguished cell division. In contrast, cyclin A function was essential for cell-cycle progression of hematopoietic and embryonic stem cells. Expression of cyclin A is particularly high in these compartments, which might render stem cells dependent on cyclin A, whereas in fibroblasts cyclins A and E play redundant roles in cell proliferation.
In hematopoiesis, the expression of critical genes is regulated in a stage-specific manner to maintain normal hematopoiesis.
Notch1
is an essential gene involved in the commitment and development of ...the T-cell lineage. However, the regulation of
Notch1
in hematopoiesis is controversial, particularly at the level of hematopoietic stem cell (HSC). Here, we found that the expression of
Notch1
is controlled at the post-transcriptional level in HSCs. HSCs express a considerable level of
Notch1
mRNA, but its protein level is very low, suggesting a post-transcriptional suppression for
Notch1
. Using a retroviral sensor vector expressing a fusion mRNA of GFP and 3′ untranslated region (3′UTR) of a target gene, we demonstrated that the
Notch1
-3′UTR had a post-translational suppressive effect only at the HSC but not in the downstream progenitor stages. The sequence motif AUnA was required for this post-transcriptional regulation by the
Notch1
-3′UTR. Interestingly, this
Notch1
-3′UTR-mediated suppressive effect was relieved when HSCs were placed in the thymus, but not in the bone marrow. Thus, the expression of
Notch1
in HSCs is regulated by microenvironment at the post-transcriptional level, which may control T lymphoid lineage commitment from HSCs.
Previous studies have predicted that reciprocal activation of GATA-1 and PU.1 regulates myelo-erythroid versus myelo-lymphoid lineage commitment in early hematopoiesis. Such PU.1-activating ...myelo-lymphoid progenitors exist within the lymphoid-primed multipotent progenitor (LMPP) population at the primitive Lineage(-) Sca-1(+) c-Kit(+) (LSK) stage. We here show that the counterpart of GATA-1-activating myelo-erythroid progenitor resides also at the LSK stage, expressing CD41 at a high level. Purified CD41(hi) LSK cells showed exceedingly strong and prolonged myelo-erythroid-restricted reconstitution, and primed myelo-erythroid gene expression with a more primitive molecular signature as compared to the original common myeloid progenitor (CMP). The CD41(hi) LSK cells more strongly contributed to emergent and malignant myelopoiesis than LMPPs, and produced the original CMP by downregulating Sca-1 and CD41, suggesting that they are the earliest CMPs. Thus, the hematopoietic developmental map should be revised by integrating the primary branchpoint comprised of the new, isolatable CD41(hi) CMP and the LMPP populations.
FLT3/FLK2, a member of the receptor tyrosine kinase family, plays a critical role in maintenance of hematopoietic homeostasis, and the constitutively active form of the FLT3 mutation is one of the ...most common genetic abnormalities in acute myelogenous leukemia. In murine hematopoiesis, Flt3 is not expressed in self-renewing hematopoietic stem cells, but its expression is restricted to the multipotent and the lymphoid progenitor stages at which cells are incapable of self-renewal. We extensively analyzed the expression of Flt3 in human (h) hematopoiesis. Strikingly, in both the bone marrow and the cord blood, the human hematopoietic stem cell population capable of long-term reconstitution in xenogeneic hosts uniformly expressed Flt3. Furthermore, human Flt3 is expressed not only in early lymphoid progenitors, but also in progenitors continuously along the granulocyte/macrophage pathway, including the common myeloid progenitor and the granulocyte/macrophage progenitor. We further found that human Flt3 signaling prevents stem and progenitors from spontaneous apoptotic cell death at least through up-regulating Mcl-1, an indispensable survival factor for hematopoiesis. Thus, the distribution of Flt3 expression is considerably different in human and mouse hematopoiesis, and human FLT3 signaling might play an important role in cell survival, especially at stem and progenitor cells that are critical cellular targets for acute myelogenous leukemia transformation.
Two distinct dendritic cell (DC) subsets, conventional DCs (cDCs) and plasmacytoid DCs (pDCs), have been shown to develop via either the myeloid or the lymphoid pathway in murine hematopoiesis. ...Lineage-specific phenotypes or functions of “myeloid” and “lymphoid” DCs, however, still remain elusive. Furthermore, such analysis has been particularly difficult in humans, due to lack of an assay system appropriate for the analysis of human stem and progenitor cell differentiation. Here, using a highly efficient xenotransplantation model, we extensively analyze the origin and the molecular signature of human DCs. Purified human common myeloid progenitors (CMPs) and common lymphoid progenitors (CLPs) were intravenously transplanted into nonobese diabetic–severe combined immunodeficiency (NOD-scid)/IL2rγnull newborn mice. CMPs and CLPs displayed significant expansion in the xenogeneic host, and human cDC and pDC progeny were isolatable. Strikingly, each human DC subset possessed indistinguishable expression patterns of surface phenotype and gene transcripts regardless of their CMP or CLP origin, even at the genome-wide level. Thus, cDC and pDC normally develop after cells have committed to the myeloid or the lymphoid lineage in human hematopoiesis, while their transcriptional signatures are well preserved irrespective of their lineage origin. We propose that human DCs use unique and flexible developmental programs that cannot be categorized into the conventional myeloid or lymphoid pathway.
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