Tadafumi Iino

Reciprocal Activation of GATA-1 and PU.1 Marks Initial Specification of Hematopoietic Stem Cells into Myeloerythroid and Myelolymphoid Lineages

A hierarchical hematopoietic development with myeloid versus lymphoid bifurcation has been proposed downstream of the multipotent progenitor (MPP) stage, based on prospective isolation of progenitors capable of generating only myeloerythroid cells (common myeloid progenitor, CMP) or only lymphocytes (common lymphoid progenitor, CLP). By utilizing GATA-1 and PU.1 transcription factor reporters, here we identified progenitor populations that are precursors for either CMPs or CLPs. Two independent populations expressing either GATA-1 or PU.1 resided within the CD34(+)Sca-1(+)c-Kit(+) MPP fraction. The GATA-1(+) MPP displayed potent myeloerythroid potential without giving rise to lymphocytes, whereas the PU.1(+) MPP showed granulocyte/monocyte/lymphoid-restricted progenitor activity without megakaryocyte/erythroid differentiation. Furthermore, GATA-1(+) and PU.1(+) MPPs possessed huge expansion potential and differentiated into the original CMPs and CLPs, respectively. Thus, the reciprocal activation of GATA-1 and PU.1 primarily organizes the hematopoietic lineage fate decision to form the earliest hematopoietic branchpoint that comprises isolatable myeloerythroid and myelolymphoid progenitor populations.

Self-Renewing Hematopoietic Stem Cell Is the Primary Target in Pathogenesis of Human Chronic Lymphocytic Leukemia

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.

Cyclin A Is Redundant in Fibroblasts but Essential in Hematopoietic and Embryonic Stem Cells

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.

FLT3-ITD up-regulates MCL-1 to promote survival of stem cells in acute myeloid leukemia via FLT3-ITD–specific STAT5 activation

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.

Human Flt3 Is Expressed at the Hematopoietic Stem Cell and the Granulocyte/Macrophage Progenitor Stages to Maintain Cell Survival

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.

Detection of human herpesvirus-8 in peripheral blood mononuclear cells from adult Japanese patients with multicentric Castleman's disease.

Summary. Human herpesvirus‐8 (HHV‐8) encodes viral homologues of cellular genes, including viral interleukin 6 (vIL‐6), which induces endogenous human IL‐6 (hIL‐6) secretion. Unregulated overproduction of hIL‐6 in lymph nodes (LN) is thought to be responsible for the systemic manifestations of multicentric Castlemans disease (MCD). In the present study, we assessed the presence of HHV‐8 and HHV‐8‐encoded viral homologues in LN and peripheral blood mononuclear cells (PBMC) from adult Japanese patients with MCD. HHV‐8 DNA was amplified by nested polymerase chain reaction (PCR) and was detected in LN from 13 out of 16 MCD patients (81%). HHV‐8 DNA was also detected in PBMC from six out of seven patients (86%) whose LN were positive for HHV‐8 DNA. Because mRNA could not be successfully extracted from LN sections that were either formalin‐fixed or embedded in paraffin, we examined the expression of mRNA for HHV‐8‐encoded viral homologues, such as vIL‐6, vBCL‐2, vCyclin‐D and viral G‐protein‐coupled receptor (vGPCR) by nested reverse transcription (RT)‐PCR in PBMC from 10 MCD patients. However, mRNA of these HHV‐8‐encoded viral homologues was not detected in any patients tested. Although our results do not indicate a role for HHV‐8‐encoded viral homologues in the pathogenesis of MCD, they do suggest that HHV‐8 infection may be associated with MCD in adult Japanese patients.

T Cell Leukemia/Lymphoma 1 and Galectin-1 Regulate Survival/Cell Death Pathways in Human Naive and IgM+ Memory B Cells through Altering Balances in Bcl-2 Family Proteins

BCR signaling plays a critical role in purging the self-reactive repertoire, or in rendering it anergic to establish self-tolerance in the periphery. Differences in self-reactivity between human naive and IgM+ memory B cells may reflect distinct mechanisms by which BCR signaling dictates their survival and death. Here we demonstrate that BCR stimulation protected naive B cells from apoptosis with induction of prosurvival Bcl-2 family proteins, Bcl-xL and Mcl-1, whereas it rather accelerated apoptosis of IgM+ memory B cells by inducing proapoptotic BH3-only protein Bim. We found that BCR-mediated PI3K activation induced the expression of Mcl-1, whereas it inhibited Bim expression in B cells. Phosphorylation of Akt, a downstream molecule of PI3K, was more sustained in naive than IgM+ memory B cells. Abundant expression of T cell leukemia/lymphoma 1 (Tcl1), an Akt coactivator, was found in naive B cells, and enforced expression of Tcl1 induced a high level of Mcl-1 expression, resulting in prolonged B cell survival. In contrast, Galectin-1 (Gal-1) was abundantly expressed in IgM+ memory B cells, and inhibited Akt phosphorylation, leading to Bim up-regulation. Enforced expression of Gal-1 induced accelerated apoptosis in B cells. These results suggest that a unique set of molecules, Tcl1 and Gal-1, defines distinct BCR signaling cascades, dictating survival and death of human naive and IgM+ memory B cells.

Influence of transplanted dose of CD56+ cells on development of graft-versus-host disease in patients receiving G-CSF-mobilized peripheral blood progenitor cells from HLA-identical sibling donors.

Summary:We investigated effects of variations in the cellular composition of G-CSF-mobilized peripheral blood progenitor cell (G-PBPC) allografts on clinical outcomes of allogeneic PBPC transplantation. We retrospectively analyzed transplanted doses of various immunocompetent cells from 27 HLA-identical sibling donors in relation to engraftment, incidence of graft-versus-host disease (GVHD), and survival. Significant variability was documented in both absolute numbers and relative proportions of CD34+, CD2+, CD3+, CD4high+, CD4+25+, CD8high+, CD19+, CD56+, and CD56+16+ cells contained in these allografts. Stepwise Cox regression analysis revealed that the CD56+ cell dose was significantly inversely correlated with the incidence of GVHD. Thus, there was a significantly higher incidence of grade II acute GVHD in patients receiving a lower CD56+16+ cell dose (hazard ratio (HR) 0.0090; 95% confidence interval (CI), <0.00001–3.38; P=0.031), a higher incidence of chronic GVHD in those receiving allografts with a lower CD56+16+ to CD34+ ratio (HR <0.00001; 95% CI <0.00001–0.0007; P=0.0035), and a higher incidence of extensive chronic GVHD in those receiving allografts with a lower CD56+ to CD34+ ratio (HR <0.00001; 95% CI <0.00001–0.053; P=0.0083). These results suggest that CD56+ cells in G-PBPC allografts from HLA-identical sibling donors may play an important role in preventing the development of GVHD.

Clinical outcomes of a novel therapeutic vaccine with Tax peptide-pulsed dendritic cells for adult T cell leukaemia/lymphoma in a pilot study

Adult T cell leukaemia/lymphoma (ATL) is a human T cell leukaemia virus type‐I (HTLV‐I)‐infected T cell malignancy with poor prognosis. We herein developed a novel therapeutic vaccine designed to augment an HTLV‐I Tax‐specific cytotoxic T lymphocyte (CTL) response that has been implicated in anti‐ATL effects, and conducted a pilot study to investigate its safety and efficacy. Three previously treated ATL patients, classified as intermediate‐ to high‐risk, were subcutaneously administered with the vaccine, consisting of autologous dendritic cells (DCs) pulsed with Tax peptides corresponding to the CTL epitopes. In all patients, the performance status improved after vaccination without severe adverse events, and Tax‐specific CTL responses were observed with peaks at 16–20 weeks. Two patients achieved partial remission in the first 8 weeks, one of whom later achieved complete remission, maintaining their remission status without any additional chemotherapy 24 and 19 months after vaccination, respectively. The third patient, whose tumour cells lacked the ability to express Tax at biopsy, obtained stable disease in the first 8 weeks and later developed slowly progressive disease although additional therapy was not required for 14 months. The clinical outcomes of this pilot study indicate that the Tax peptide‐pulsed DC vaccine is a safe and promising immunotherapy for ATL.

CD41 Marks the Initial Myelo‐Erythroid Lineage Specification in Adult Mouse Hematopoiesis: Redefinition of Murine Common Myeloid Progenitor

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 CD41hi 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 CD41hi 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 CD41hi CMP and the LMPP populations. Stem Cells 2015;33:976–987