Identification of a Human Clonogenic Progenitor with Strict Monocyte Differentiation Potential: A Counterpart of Mouse cMoPs.

Monocytes give rise to macrophages and dendritic cells (DCs) under steady-state and inflammatory conditions, thereby contributing to host defense and tissue pathology. A common monocyte progenitor (cMoP) that is strictly committed to the monocyte lineage has been recently identified in mice. Here, we identified human cMoPs as a CLEC12AhiCD64hi subpopulation of conventional granulocyte-monocyte progenitors (cGMPs) in umbilical cord blood and in bone marrow. Human cMoPs gave rise to monocyte subsets without showing any potential for differentiating into myeloid or lymphoid cells. Within the cGMP population, we also identified revised GMPs that completely lacked DC and lymphoid potential. Collectively, our findings expand and revise the current understanding of human myeloid cell differentiation pathways.

Reinforced antimyeloma therapy via dual-lymphoid activation mediated by a panel of antibodies armed with bridging-BiTE.

Immunotherapy using bispecific antibodies including bispecific T-cell engager (BiTE) has the potential to enhance the efficacy of treatment for relapsed/refractory multiple myeloma. However, myeloma may still recur after treatment because of downregulation of a target antigen and/or myeloma cell heterogeneity. To strengthen immunotherapy for myeloma while overcoming its characteristics, we have newly developed a BiTE-based modality, referred to as bridging-BiTE (B-BiTE). B-BiTE was able to bind to both a human immunoglobulin G-Fc domain and the CD3 molecule. Clinically available monoclonal antibodies (mAbs) were bound with B-BiTE before administration, and the mAb/B-BiTE complex induced antitumor T-cell responses successfully while preserving and supporting natural killer cell reactivity, resulting in enhanced antimyeloma effects via dual-lymphoid activation. In contrast, any unwanted off-target immune-cell reactivity mediated by mAb/B-BiTE complexes or B-BiTE itself appeared not to be observed in vitro and in vivo. Importantly, sequential immunotherapy using 2 different mAb/B-BiTE complexes appeared to circumvent myeloma cell antigen escape, and further augmented immune responses to myeloma relative to those induced by mAb/B-BiTE monotherapy or sequential therapy with 2 mAbs in the absence of B-BiTE. Therefore, this modality facilitates easy and prompt generation of a broad panel of bispecific antibodies that can induce deep and durable antitumor responses in the presence of clinically available mAbs, supporting further advancement of reinforced immunotherapy for multiple myeloma and other refractory hematologic malignancies.

Outcomes of allogeneic hematopoietic cell transplantation under letermovir prophylaxis for cytomegalovirus infection.

Cytomegalovirus (CMV) infection is a major infectious complication following allogeneic hematopoietic cell transplantation (allo-HCT). Although letermovir (LMV) prophylaxis dramatically reduces the incidence of early clinically significant CMV (csCMV) infection, it remains unclear whether it has a beneficial effect on nonrelapse mortality (NRM) and overall survival (OS). Herein, we evaluated the impact of LMV prophylaxis on posttransplant outcomes using the registry database of the Japanese Society for Transplantation and Cellular Therapy. Adult patients who underwent allo-HCT between 2017 and 2019 were analyzed (n = 6004). LMV prophylaxis was administered to 1640 patients (LMV group) and it significantly reduced the incidence of csCMV infection compared with those not administered LMV prophylaxis (15.4% vs 54.1%; p < 0.01). However, it did not improve the 1-year NRM (hazard ratio [HR], 0.93; p = 0.40) and OS (HR, 0.96; p = 0.49). In the LMV group, 74 patients had breakthrough csCMV infection and showed inferior NRM (HR, 3.44; p < 0.01) and OS (HR, 1.93; p = 0.02) compared with those without infection. After completing LMV prophylaxis, 252 patients had late csCMV infection and showed inferior NRM (HR, 1.83; p < 0.01) and OS (HR, 1.58; p < 0.01). Our findings suggest that managing breakthrough and late csCMV infections is important for improving long-term outcomes.

Real-world clinical characteristics of post-essential thrombocythemia and post-polycythemia vera myelofibrosis.

There are few prospective studies on patients with post-essential thrombocythemia myelofibrosis (PET-MF) and post-polycythemia vera myelofibrosis (PPV-MF). Therefore, we conducted a nationwide longitudinal prospective survey to clarify the clinical characteristics of these diseases. A total of 197 PET-MF and 117 PPV-MF patients diagnosed between 2012 and 2021 were analyzed. The median age at diagnosis was 70.0 years for both diseases. The time from diagnosis of ET or PV to that of MF was 9.6 and 10.4 years, respectively, with no significant difference. Patients with PPV-MF had higher hemoglobin levels and white blood cell counts than those with PET-MF, whereas those with PET-MF had higher platelet counts than those with PPV-MF. Although splenomegaly was more frequent in patients with PPV-MF at diagnosis, there was no difference in the frequency of constitutional symptoms. Ruxolitinib was the most common treatment administered to 74.6% and 83.8% of patients with PET-MF and PPV-MF, respectively. Patients with PET-MF and PPV-MF had similar prognoses, with 3-year overall survival (OS) of 0.742 in PET-MF and 0.768 in PPV-MF patients. In both diseases, leukemic transformation was the leading cause of death, followed by infection. The 3-year OS for patients with PET/PPV-MF and primary MF diagnosed during the same period was 0.754 and 0.626, respectively, with no significant difference. This survey provides real-world clinical features and prognostic data on secondary myelofibrosis in the ruxolitinib era.

[Development of novel bispecific antibody therapy for multiple myeloma].

Over the past decade, new therapeutic modalities have markedly improved clinical outcomes for patients with multiple myeloma. Recently, immunotherapy using both bispecific antibodies (BsAb) and chimeric antigen receptor T cells (CAR-T cells) has induced further anti-myeloma responses. Different agents must be combined to overcome the heterogeneity of myeloma cell clones, and new modalities for the treatment of refractory myeloma must also be developed to strengthen therapeutic effects. We have developed a novel BiTE (bispecific T-cell engager)-based modality, referred to as bridging-BiTE (B-BiTE). B-BiTE is able to bind to both an Fc domain of a human immunoglobulin G monoclonal antibody (mAb) and the human CD3 molecule. This enables rapid generation of a mAb/B-BiTE complex and safely induces dual-lymphoid activation of both human T cells and NK cells against myeloma cells. Importantly, sequential immunotherapy using two different mAb/B-BiTE complexes can produce deep and durable anti-myeloma responses. To further advance treatment of multiple myeloma, it is important to determine how to combine and sequence immunotherapy with other agents while considering management of unique adverse events caused by activated immune cells.

Long-term transition of antibody titers in healthcare workers following the first to fourth doses of mRNA COVID-19 vaccine: Comparison of two automated SARS-CoV-2 immunoassays.

Anti-spike receptor binding domain (S-RBD) antibody against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) which best correlates with virus-neutralizing antibody is useful for estimating the period of protection and identifying the timing of additional booster doses. Long-term transition of the S-RBD antibody titer and the antibody responses among healthy individuals remain unclear. In the present study, therefore, we monitored the S-RBD antibody titers of 16 healthcare workers every 4 weeks for 76 weeks after vaccination with a fourth dose of mRNA-1273 (Moderna) following three doses of BNT162b2 (Pfizer/BioNTech) using two commercial automated immunoassays (Roche and Abbott). Two antibody responses to the vaccine were similar with an up-down change before and after the second (weeks 3), third (weeks 40) and fourth (week 72) vaccinations, but the titer did not fall below the assay's positivity threshold in any individual. The peak level of the geometric mean titer (GMT) in the Roche assay was highest after the third vaccination, and that in Abbott assay was highest after the fourth vaccination but almost equal to that after the third vaccination. Both the geometric mean fold rise (GMFR) demonstrated by the Roche and Abbott assays were highest after the third vaccination. Antibody titers determined by the Roche and Abbott assays showed a positive strong correlation (correlation coefficient: 0.70 to 0.99), but the ratio (Roche/Abbott) of antibodies demonstrated by both assays increased 0.46- to 8.26-fold between weeks 3 and 76. These findings will be helpful for clinicians when interpreting results for SARS-CoV-2 antibody levels and considering future vaccination strategies.

[Allogeneic hematopoietic cell transplantation for myeloproliferative neoplasms].

The median survival duration of myelofibrosis is about 5 years and at present, allogeneic hematopoietic cell transplantation is the only curative treatment. Although myelofibrosis's clinical course and prognosis vary from patient to patient, the time and indication of transplantation should be determined in light of the likelihood of transplant-related death or long-term prognosis, as well as any information on each patient's gene mutation risk. Previous reports have demonstrated that allogeneic hematopoietic cell transplantation can be a curative treatment for myelofibrosis. However, the transplant-related mortality rate is as high as 30-50%, and the overall survival rate is only around 40%. Future research should clarify how to decide between JAK2 inhibitors and allogeneic hematopoietic cell transplantation, how to lower high transplant-related mortality, how to choose a stem cell source, how to create the best pretransplant treatment, and how to incorporate JAK2 inhibitors before transplantation.

A human SIRPA knock-in xenograft mouse model to study human hematopoietic and cancer stem cells.

In human-to-mouse xenogeneic transplantation, polymorphisms of signal-regulatory protein α (SIRPA) that decide their binding affinity for human CD47 are critical for engraftment efficiency of human cells. In this study, we generated a new C57BL/6.Rag2nullIl2rgnull (BRG) mouse line with Sirpahuman/human (BRGShuman) mice, in which mouse Sirpa was replaced by human SIRPA encompassing all 8 exons. Macrophages from C57BL/6 mice harboring Sirpahuman/human had a significantly stronger affinity for human CD47 than those harboring SirpaNOD/NOD and did not show detectable phagocytosis against human hematopoietic stem cells. In turn, Sirpahuman/human macrophages had a moderate affinity for mouse CD47, and BRGShuman mice did not exhibit the blood cytopenia that was seen in Sirpa-/- mice. In human to mouse xenograft experiments, BRGShuman mice showed significantly greater engraftment and maintenance of human hematopoiesis with a high level of myeloid reconstitution, as well as improved reconstitution in peripheral tissues, compared with BRG mice harboring SirpaNOD/NOD (BRGSNOD). BRGShuman mice also showed significantly enhanced engraftment and growth of acute myeloid leukemia and subcutaneously transplanted human colon cancer cells compared with BRGSNOD mice. BRGShuman mice should be a useful basic line for establishing a more authentic xenotransplantation model to study normal and malignant human stem cells.

Prognostic impact of complex karyotype on post-transplant outcomes of myelofibrosis.

Chromosomal abnormalities in the role of prognostic factor for transplant patients with myelofibrosis (MF) are not fully investigated. Regarding complex karyotype (CK), we retrospectively analyzed 241 patients with primary and secondary MF who received a first allogeneic hematopoietic cell transplantation (HCT). Based on an unfavorable karyotype in the Dynamic International Prognostic Scoring System, we compared the outcomes in 3 groups: favorable karyotype, unfavorable karyotype including CK (unfavorable-CK(+)), and unfavorable karyotype not including CK (unfavorable-CK(-)). Overall survival was significantly shorter in the unfavorable-CK(+) group (hazard ratio (HR) 2.49, 95% CI: 1.46-4.24, P < 0.001), whereas there was no difference between the unfavorable-CK(-) group and the favorable group (HR 0.57, 95% CI: 0.20-1.59, P = 0.28). In addition, a significantly higher proportion of patients in the unfavorable-CK(+) group did not achieve complete remission after HCT (P = 0.007). The cumulative incidence of disease progression was significantly higher in the unfavorable-CK(+) group (HR 2.5, 95% CI 1.6-3.92, P < 0.001), whereas that in the unfavorable-CK(-) group was comparable to that in the favorable group (HR 0.49, 95% CI 0.12-1.94, P = 0.31). Further investigations will be needed to clarify the impact of CK on transplant outcomes in MF.

Cyclosporine/methotrexate versus tacrolimus/methotrexate with or without anti-thymocyte globulin as GVHD prophylaxis in adult patients with aplastic anemia.

The impact of calcineurin inhibitor types and anti-thymocyte globulin (ATG) in conditioning on overall survival (OS) and GVHD-free, relapse-free survival (GRFS) has not yet been analyzed in detail for aplastic anemia. We herein examined 517 adult patients with aplastic anemia who underwent BMT from HLA-matched sibling donors (MSD, n = 255) and unrelated donors (UD, n = 262) and were treated with cyclosporine A (CSA) + methotrexate (MTX) (n = 258) and tacrolimus (TAC) + MTX (n = 259). In total, 330 patients received ATG in conditioning. CSA + MTX versus TAC + MTX did not have a significant impact on acute and chronic GVHD, OS, or GRFS in each donor type. The use of ATG in conditioning reduced the risk of grade II-IV acute GVHD in the MSD and UD cohorts (HR 0.42, P = 0.014, and HR 0.3, P < 0.001, respectively); however, a differential impact on GRFS was identified, namely, better GRFS in MSD recipients (HR 0.56, P = 0.016), but not in UD recipients (HR 1.1, P = 0.657). In conclusion, CSA + MTX and TAC + MTX were similar as GVHD prophylaxis regardless of the donor type, and ATG in conditioning increased GRFS in MSD transplants, but not in UD transplants.

Histone H3K27 Demethylase Negatively Controls the Memory Formation of Antigen-Stimulated CD8+ T Cells.

Although the methylation status of histone H3K27 plays a critical role in CD4+ T cell differentiation and its function, the role of Utx histone H3K27 demethylase in the CD8+ T cell-dependent immune response remains unclear. We therefore generated T cell-specific Utx flox/flox Cd4-Cre Tg (Utx KO) mice to determine the role of Utx in CD8+ T cells. Wild-type (WT) and Utx KO mice were infected with Listeria monocytogenes expressing OVA to analyze the immune response of Ag-specific CD8+ T cells. There was no significant difference in the number of Ag-specific CD8+ T cells upon primary infection between WT and Utx KO mice. However, Utx deficiency resulted in more Ag-specific CD8+ T cells upon secondary infection. Adoptive transfer of Utx KO CD8+ T cells resulted in a larger number of memory cells in the primary response than in WT. We observed a decreased gene expression of effector-associated transcription factors, including Prdm1 encoding Blimp1, in Utx KO CD8+ T cells. We confirmed that the trimethylation level of histone H3K27 in the Prdm1 gene loci in the Utx KO cells was higher than in the WT cells. The treatment of CD8+ T cells with Utx-cofactor α-ketoglutarate hampered the memory formation, whereas Utx inhibitor GSK-J4 enhanced the memory formation in WT CD8+ T cells. These data suggest that Utx negatively controls the memory formation of Ag-stimulated CD8+ T cells by epigenetically regulating the gene expression. Based on these findings, we identified a critical link between Utx and the differentiation of Ag-stimulated CD8+ T cells.

Tyrosine kinase inhibitors induce alternative spliced BCR-ABLIns35bp variant via inhibition of RNA polymerase II on genomic BCR-ABL.

To elucidate dynamic changes in native BCR-ABL and alternatively spliced tyrosine kinase inhibitor (TKI)-resistant but function-dead BCR-ABLIns35bp variant, following commencement or discontinuation of TKI therapy, each transcript was serially quantified in patients with chronic myeloid leukemia (CML) by deep sequencing. Because both transcripts were amplified together using conventional PCR system for measuring International Scale (IS), deep sequencing method was used for quantifying such BCR-ABL variants. At the initial diagnosis, 7 of 9 patients presented a small fraction of cells possessing BCR-ABLIns35bp , accounting for 0.8% of the total IS BCR-ABL, corresponding to actual BCR-ABLIns35bp value of 1.1539% IS. TKI rapidly decreased native BCR-ABL but not BCR-ABLIns35bp , leading to the initial increase in the proportion of BCR-ABLIns35bp . Thereafter, both native BCR-ABL and BCR-ABLIns35bp gradually decreased in the course of TKI treatment, whereas small populations positive for TKI-resistant BCR-ABLIns35bp continued fluctuating at low levels, possibly underestimating the molecular response (MR). Following TKI discontinuation, sequencing analysis of 54 patients revealed a rapid relapse, apparently derived from native BCR-ABL+ clones. However, IS fluctuating at low levels around MR4.0 marked a predominant persistence of cells expressing function-dead BCR-ABLIns35bp , suggesting that TKI resumption was unnecessary. We clarified the possible mechanism underlying mis-splicing BCR-ABLIns35bp , occurring at the particular pseudo-splice site within intron8, which can be augmented by TKI treatment through inhibition of RNA polymerase II phosphorylation. No mutations were found in spliceosomal genes. Therefore, monitoring IS functional BCR-ABL extracting BCR-ABLIns35bp would lead us to a correct evaluation of MR status, thus determining the adequate therapeutic intervention.

Favorable Effect of Cytomegalovirus Reactivation on Outcomes in Cord Blood Transplant and Its Differences Among Disease Risk or Type.

The effects of cytomegalovirus (CMV) reactivation on cord blood transplant (CBT) are unclear. We assessed the effect of CMV reactivation in adult single-unit CBT without in vivo T cell depletion. Of 3147 eligible cases, 2052 were acute myeloid leukemia (AML), 643 acute lymphoblastic leukemia (ALL), and 452 myelodysplastic syndrome (MDS). CMV reactivation up to 100 days after CBT was associated with better overall survival (OS) compared with no reactivation cases (57.3% versus 52.6% at 3 years after CBT), whereas nonrelapse mortality (NRM) was increased in ALL (16.2% versus 8.9%) and standard disease risk (17.1% versus 10.6%, P = .014) by CMV reactivation. On multivariate analysis, CMV reactivation had favorable effects on relapse in MDS (hazard ratio [HR], .55; P = .044) and high disease risk (HR, .77; P = .047). In NRM, only standard-risk cases showed adverse effects of CMV reactivation (HR, 1.56; P = .026). OS was significantly improved with CMV reactivation in a subgroup of patients with AML (HR, .84; P = .044), MDS (HR, .68; P = .048), and high disease risk (HR, .81; P = .013). This favorable effect of CMV reactivation on OS in AML and high disease risk cases was maintained even after considering the effect of grades II to IV acute graft-versus-host disease. Thus, CMV reactivation might have beneficial or adverse effects on relapse, NRM, and OS, depending on the disease type or disease risk.

The Iowa Gambling Task on HIV-infected subjects.

HIV-associated neurocognitive disorders (HAND) are characterized by cognitive, behavioral, and motor dysfunctions, which impact daily functioning and are predictive of poor survival among patients. The diagnosis of HAND is marked by clinically significant declines in multiple domains of neurocognitive functioning. Some patients diagnosed with HAND have social problem; however, higher brain dysfunction is not detected in general neuropsychological assessments and the intelligence quotient may remain unchanged. Impaired decision-making may reduce social and occupational qualities of life. The Iowa Gambling Task (IGT) has been developed as a task to evaluate risk predictions at the time of decision-making. In the present study, 38 HIV-infected patients enrolled in our hospital performed IGT and we investigated whether the results obtained are associated with HAND. The median net IGT score of all HIV-infected subjects was significantly lower than that of healthy controls. Patients diagnosed with HAND accounted for 43.8% of the negative net score group. We elucidated the relationship between the net IGT score and HAND for the first time. We think that IGT is a good tool to detect decision-making impairment for ANI and MND. Careful follow-ups of the progression of HAND and increased awareness among HIV-infected patients and medical care workers of the risk of social behavioral disorders, which negatively impact daily life before they are detected, are needed in order to prevent deteriorations in the quality of life of these patients.

Successful allogeneic stem cell transplantation in a case with acute myeloid leukemia and invasive Schizophyllum commune rhinosinusitis.

Schizophyllum commune, a basidiomycete fungus, is a quite rare cause of invasive sinusitis for which no standard treatment has yet been established. We report herein a 59-year-old woman who developed S. commune rhinosinusitis after remission induction chemotherapy for her acute myeloid leukemia. No causative microorganisms were identified in the sinus lavage fluid culture, whereas nucleotide sequencing of the internal transcribed spacer region using endoscopic sinus biopsy specimen could confirm the pathogen as S. commune. Liposomal amphotericin B and voriconazole (VRCZ) treatment ameliorated both her clinical symptoms and laboratory findings. The patient was successfully treated with allogeneic stem cell transplantation, under continuous VRCZ administration, without aggravation of S. commune sinusitis. Molecular diagnosis and prompt intervention with suitable antifungal drugs may be crucial to manage this rare infectious complication.

[Recent advances in the treatment of myelofibrosis].

Primary myelofibrosis (PMF) is classified as a clonal myeloproliferative neoplasm (MPN) characterized by bone marrow fibrosis and subsequent extramedullary hematopoiesis that causes progressive anemia, symptomatic splenomegaly, and various constitutional symptoms and eventual transformation into acute leukemia. The main MPN pathophysiology is the constitutive activation of JAK2/STAT signaling. JAK2, MPL, and CALR mutations, known as phenotypic driver mutations, are directly implicated in the disease pathogenesis by the activation of JAK2/STAT signaling. Moreover, other gene mutations, including methylation-related regulators, histone modification-related factors, and RNA splicing molecules, also contribute to the pathogenesis of MPN development. Patients with PMF, unlike other MPNs, experience a significantly worse prognosis. Thus, the risk of disease should be evaluated individually, and a tailored treatment plan should be developed based on each patient's disease risk. Gene mutation information is becoming more important in evaluating the risk of disease and determining treatment options. Allogeneic hematopoietic stem cell transplantation is the only curative treatment, but its indication is limited because of the age of onset. A JAK2 inhibitor, ruxolitinib, improves splenomegaly and disease-related constitutive symptoms. To date, new JAK2 inhibitors and drugs that delay the progression of fibrosis and leukemic transformation are under development and are expected to improve the prognosis for PMF.

Comparison of Outcomes of Allogeneic Transplantation for Primary Myelofibrosis among Hematopoietic Stem Cell Source Groups.

The choice of alternative donor is a major issue in allogeneic hematopoietic stem cell transplantation (HSCT) for patients with primary myelofibrosis (PMF) without an HLA-matched related donor. We conducted this retrospective study using the Japanese national registry data for 224 PMF patients to compare the outcomes of first allogeneic HSCT from HLA-matched related donor bone marrow (Rtd-BM), HLA-matched related donor peripheral blood stem cells (Rtd-PB), HLA-matched unrelated donor bone marrow (UR-BM), unrelated umbilical cord blood (UR-UCB), and other hematopoietic stem cell grafts. Nonrelapse mortality (NRM) rates at 1 year after Rtd-BM, Rtd-PB, UR-BM, UR-UCB, and other transplantations were 16%, 36%, 30%, 41%, and 48%, respectively. Multivariate analysis identified UR-UCB transplantation, other transplantation, frequent RBC transfusion before transplantation, and frequent platelet (PLT) transfusion before transplantation as predictive of higher NRM. Relapse rates at 1 year after Rtd-BM, Rtd-PB, UR-BM, UR-UCB, and other transplantation were 14%, 17%, 11%, 14%, and 15%, respectively. No specific factor was associated with the incidence of relapse. Overall survival (OS) at 1 and 4 years after Rtd-BM, Rtd-PB, UR-BM, UR-UCB, and other transplantation were 81% and 71%, 58% and 52%, 61% and 46%, 48% and 27%, and 48% and 41%, respectively. Multivariate analysis identified older patient age, frequent RBC transfusion before transplantation, and frequent PLT transfusion before transplantation as predictive of lower OS. In conclusion, UR-UCB transplantation, as well as UR-BM transplantation, can be selected for PMF patients without an HLA-identical related donor. However, careful management is required for patients after UR-UCB transplantation because of the high NRM. Further studies including more patients after HLA-haploidentical related donor and HLA-mismatched unrelated donor transplantation would provide more valuable information for patients with PMF when making decisions regarding the choice of alternative donor.

Polymorphism in Sirpa modulates engraftment of human hematopoietic stem cells.

Graft failure in the transplantation of hematopoietic stem cells occurs despite donor-host genetic identity of human leukocyte antigens, suggesting that additional factors modulate engraftment. With the nobese diabetic (NOD)-severe combined immunodeficiency (SCID) xenotransplantation model, we found that the NOD background allowed better hematopoietic engraftment than did other strains with equivalent immunodeficiency-related mutations. We used positional genetics to characterize the molecular basis for this strain specificity and found that the NOD Sirpa allele conferred support for human hematopoiesis. NOD SIRP-alpha showed enhanced binding to the human CD47 ligand, and its expression on mouse macrophages was required for support of human hematopoiesis. Thus, we have identified Sirpa polymorphism as a potent genetic determinant of the engraftment of human hematopoietic stem cells.

Identification of unipotent megakaryocyte progenitors in human hematopoiesis.

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.