Hematopoietic stem cells (HSCs) are tissue-specific stem cells that are critical for homeostasis and regeneration of the hematopoietic system. Multiple mechanisms exist that help maintain the size and integrity of the HSC pool after exposure to various insults to provide all lineages of blood cells throughout life. Clonal hematopoiesis, an age-related clonal mosaicism detected in the hematopoietic system and governed by aberrant HSC clones with somatic mutations, has recently been identified as an important risk factor for hematological malignancy and cardiovascular disease. Cells with a somatic mutation can present neoantigens via the major histocompatibility complex and can be recognized and eliminated by antigen-specific T cells. However, whether this mechanism also acts to maintain HSC pool integrity remains largely unclear. In this review, I have summarized mechanisms known to support the lifelong maintenance of HSC numbers and function, introduced recent findings that indicate active interaction between HSCs and T cells, and discussed potential effects of its dysregulation on hematological diseases.

Hematopoietic stem cells (HSCs) possess multilineage differentiation capability, which sustains the production of blood and immune cells throughout life. However, the precise mechanisms by which HSCs initiate differentiation toward a particular lineage and the factors that attenuate their lymphopoietic potential with aging are yet to be elucidated. Our group has investigated this issue for over two decades. We initially developed a method for segregating early lymphoid progenitors from HSCs and identified two molecules: endothelial cell-selective adhesion molecule (ESAM), highly expressed in HSCs, and special AT-rich sequence binding protein 1 (SATB1), expressed in early lymphoid progenitors. ESAM marks HSCs across species, including humans. In addition to its significance in stress-induced hematopoiesis, ESAM is also useful in identifying features of human acute myeloid leukemia stem cells. Further, we determined the role of SATB1 in the early HSC differentiation processes toward the lymphoid lineage. Remarkably, SATB1 expression in HSCs significantly decreased with aging, whereas its exogenous induction in aged HSCs rejuvenated their lymphopoietic potential. Furthermore, SATB1-expressing HSCs demonstrated robust lymphopoietic and long-term reconstituting capability, whereas HSCs without SATB1 skewed toward the myeloid lineage. Thus, our continuing research has revealed the significance of ESAM and SATB1 in the fundamental biology of HSCs.

Human leukocyte antigen (HLA) class I allele-lacking [HLA (-)] leukocytes provide compelling evidence that cytotoxic T-lymphocytes are involved in the development of aplastic anemia (AA). However, the clinical significance and precise mechanisms underlying clonal hematopoiesis by HLA (-) hematopoietic stem progenitor cells remain unknown. In HLA (-) cells from patients with AA, we discovered a common nonsense mutation at codon19 (c.19C>T, p.R7X) in exon1 (Exon1mut) of different HLA-A and HLA-B alleles. Exon1 mutation detection using droplet digital polymerase chain reaction (ddPCR) is a powerful tool for diagnosing immune pathophysiology in patients with bone marrow failure. We also looked at the prognosis of 633 patients with AA, including 127 with HLA (-) leukocytes who had been followed up for a long time. In Japanese patients with AA, HLA (-) leukocytes and concomitant aberrant clones were not associated with clonal evolution to MDS/AML. In patients with AA and both marker (-) leukocytes, HLA (-) leukocytes may indicate a lower risk of developing secondary paroxysmal nocturnal hemoglobinuria (PNH). Detecting HLA (-) leukocytes is critical for managing patients with AA and assisting physicians in selecting appropriate therapy.

A 60-year-old woman with myelodysplastic syndrome/myeloproliferative neoplasm-unclassifiable underwent unrelated bone marrow transplantation from a human leukocyte antigen (HLA) 8/8 allele-matched male donor. Neutrophil engraftment was achieved on day 29. Fluorescence in situ hybridization of sex chromosomes demonstrated complete donor chimerism. The red blood cell and platelet transfusion dependence continued, and the neutrophil count decreased gradually. Despite prolonged administration of broad-spectrum antibiotics for febrile neutropenia, blood cultures on days 46 and 58 returned positive for Stenotrophomonas maltophilia (SM). Contrast-enhanced computed tomography revealed multiple nodules of septic emboli in the lungs and kidneys, suggesting a disseminated SM infection. Antibiotic therapy was conducted based on antimicrobial susceptibility testing. However, the blood cell count failed to normalize and a secondary graft failure was diagnosed. A HLA-haploidentical peripheral-blood stem-cell transplantation from the patient's son was performed on day 134 after the initial transplantation. Neutrophil engraftment was achieved on day 11. Red blood cells and platelets were also engrafted. After the resolution of the SM bacteremia, the patient was discharged on day 63. The prognosis of the SM bacteremia with neutropenia is poor. Antibiotic treatment based on antimicrobial susceptibility testing and a second transplant from an HLA-haploidentical donor likely contributed to the successful outcome in this patient.

With the remarkable scientific advances in stem cells technologies and culture systems, it is no longer a dream to construct in vitro cultured tissue/organ models that respond completely as if they were in vivo. The microphysiological system (MPS) is a symbol of the growing worldwide momentum to promote the use of in vitro methods. The development of MPS devices as a culture system itself has been almost completed both in Japan and overseas, thus, the focus from now on is on the construction of prediction systems and evaluation of their applicability in accordance with the specific requirements (context of use: CoU), for example, of the drug discovery process of pharmaceutical companies. A notable trend at this stage is the close communication between developers (companies and researchers), users, and regulatory authorities, and the organization of the European Organ on a Chip Society (EUROoCS) and the International MPS Summit involving these stakeholders has begun. It is strongly expected that academic studies on in vitro systems symbolized by MPS and their results for the evaluation and prediction of individual responses and their application in society will continue to advance, leading to the promotion of alternatives to animal experiments.

A 22-year-old man with a history of mediastinal germ cell tumor, which was diagnosed at age 20 and remained disease-free after chemotherapy, was diagnosed with acute myeloid leukemia (AML) M2 in January 2020. Karyotype analysis of bone marrow (BM) specimen at diagnosis detected 47,XXY, inv (16) in all cells. Following induction treatment, he achieved complete remission with a remarkable decrease in the minimal residual disease marker. Although considered related to therapy, the AML had a prognostically favorable karyotype, and the initial treatment response was very good. He had no human leukocyte antigen-matched sibling donor candidate. Thus, allogeneic hematopoietic stem cell transplantation was not scheduled at the first complete remission. After three cycles of consolidation therapy, he remained disease-free for over one year. Karyotype analysis of BM during remission revealed that all analyzed cells harbored 47,XXY, and Klinefelter syndrome (KS) was diagnosed. Although the patient experienced an adjustment disorder on KS diagnosis, he had overcome the difficulty with the assistance of psycho-oncologists, clinical psychologists, and genetic counselors. Herein, we report this rare case of KS that manifested after AML diagnosis following mediastinal germ cell tumor treatment.

Sideroblastic anemias (SAs) are a group of heterogeneous congenital and acquired disorders characterized by anemia and presence of ring sideroblasts in the bone marrow. Congenital SA is a rare condition caused by mutations of genes involved in heme biosynthesis, iron-sulfur cluster biosynthesis, and mitochondrial protein synthesis. SAs can also occur following exposure to certain drugs or alcohol or caused by copper deficiency (secondary SA). SAs have been found to be associated with myelodysplastic syndrome (idiopathic SA), which strongly correlates with specific somatic mutations in SF3B1 (splicing factor 3b subunit 1), involved in the RNA splicing machinery. The recent widespread use of genome-editing technology and next-generation sequencing has led to a better understanding of the molecular pathophysiology of SAs. This review discusses the current understanding of the pathophysiology of SAs.

Hematopoietic stem cells' (HSCs) metabolic dynamics regulate cell fate and differentiation. To maintain the stemness of HSCs, it is necessary to maintain an appropriate balance of adenosine triphosphate (ATP) production by the glycolytic system and mitochondria. In conventional intracellular ATP measurement, snapshot analysis is performed after the lysis of a large number of cells, resulting in the loss of single-cell level and spatiotemporal information. To overcome this technical limitation, we recently developed a methodology for measuring ATP concentration in living HSCs using knock-in mice of an intracellular ATP concentration biosensor called GO-ATeam2, which employs the principle of Förster resonance energy transfer. The current understanding of metabolism in HSCs, the metabolic dependence of HSCs revealed by the GO-ATeam2 system, and the prospects for novel metabolic measurement techniques in the future are reviewed in this paper.

A 66-year-old man with fever was diagnosed with B-cell acute lymphoblastic leukemia. He failed to achieve complete remission after initial hyperfractionated cyclophosphamide, vincristine, doxorubicin, and dexamethasone (Hyper-CVAD) therapy and was referred to our hospital to undergo allogeneic stem cell transplantation. Bone marrow aspiration showed 97.5% lymphoblasts positive for CD19. Blood tests revealed the presence of broad antihuman leukocyte antigen (HLA) antibodies. After blinatumomab therapy, bone marrow aspiration showed 19.6% blasts. Furthermore, after additional mini-mitoxantrone, etoposide, and cytarabine (mini MEC) therapy, the patient achieved complete remission. Interestingly, after blinatumomab therapy, the blood tests revealed that the titers of anti-HLA antibodies had decreased, and cord blood transplantation was performed in complete remission. This case report revealed that chemotherapy including blinatumomab, which targets CD19-positive cells, has the potential to decrease antibody-producing cells, thus leading to a dramatic reduction of anti-HLA antibodies.

Chronic graft-versus-host disease (GVHD) affects various organs and causes significant morbidity and mortality after allogeneic hematopoietic cell transplantation. The 2005 National Institutes of Health consensus criteria for chronic GVHD have set international standards for endpoints and designing and reporting of clinical trials; these criteria were revised in 2014 to incorporate accumulated evidence and controversies. In addition, preclinical studies of chronic GVHD have identified treatment targets such as regulatory T cells, B-cell signaling, Th17 cells, Tc17 cells, follicular helper T cells, follicular regulatory T cells, and fibrosis-promoting factors. These efforts led to the approval of ibrutinib, belumosudil, and ruxolitinib by the U.S. Food and Drug Administration for treating chronic GVHD after failure of one or more lines of systemic therapy, and an increasing number of investigational agents that target different biological pathways of chronic GVHD are under development in clinical trials. To address challenges in a rapidly evolving field, a third National Institutes of Health consensus project was held in 2020, in which investigators, patient advocacy organizations, and pharmaceutical companies aimed to define basic and clinical research roadmaps that may lead to significant change in chronic GVHD management over the next 5 years.

Graft-versus-host disease (GVHD), which is characterized by T-cell activation following allogeneic hematopoietic cell transplantation (allo-HCT), remains one of the most significant hurdles directly or indirectly affecting transplant outcomes. With improved knowledge, experimental and clinical models have been established over the last two decades to predict the clinical utility of novel therapeutic agents, and clinical care has progressed. In addition, recent advances have revealed mechanisms underlying GVHD that focus on the breakdown of tissue tolerance following allo-HCT, complementing established concepts of immunological intolerance that allo-reactive T cells and antigen-presenting cells contribute to the development and aggravation of GVHD. The pathophysiology of GVHD as influenced by various cells and tissues and therapeutic strategies based on mechanistic findings to advance the understanding of GVHD and individualize allo-HCT have been discussed in this review.

Langerhans cell histiocytosis (LCH) is characterized by immature dendritic cell proliferation, which is currently classified as an inflammatory myeloid neoplasm. Clinical features and outcomes vary from spontaneously regressing isolated bone disease to fatal liver, spleen, or hematopoietic system (risk organ) involvement-positive multisystem disease. LCH cells have the only mutation in the mitogen-activated protein kinase (MAPK) signaling pathway gene, represented by the BRAF V600E mutation, which is the driver mutation. The type of disease depends on the stage of hematopoietic cell differentiation at which the mutation occurs. LCH cells acquire anti-apoptosis and senescence-associated secretory phenotype by oncogene-induced senescence, with migration failure to lymph nodes. These cause LCH cell accumulation and various inflammatory cell recruitment in the lesion, resulting in severe inflammation. Tissue damage in LCH is due to this inflammation, not the LCH cell proliferation. Patients with a risk of organ involvement without the initial treatment response may be rescued by allogeneic hematopoietic stem cell transplantation after reducing the disease activity with MAPK inhibitors. Intravenous zoledronic acid and intrathecal cytarabine injections have been introduced into the ongoing clinical trial in Japan to reduce bone recurrence and prevent neurodegeneration as sequelae.

Recent studies have demonstrated that cancer-specific metabolism plays a crucial role in a variety of malignancies, including acute myeloid leukemia (AML). To identify a novel therapeutic target for AML, we conducted a metabolite screen on AML cells and normal hematopoietic stem/progenitor cells (HSPCs) and detected that the metabolism of glycerol-3-phosphate (G3P) is reprogrammed in AML. Glycerol-3-phosphate acyltransferases (GPATs), the first and rate-limiting enzymes in the lipid biosynthesis pathway, convert G3P into lysophosphatidic acid (LPA). Among various GPAT isozymes, GPAT1 was highly expressed in AML cells and silencing it inhibited the cell growth of AML. GPAT1 is located on the outer membrane of the mitochondria and regulates mitochondrial fusion and oxidative phosphorylation (OXPHOS). Silencing GPAT1 promoted mitochondrial fission and reduced OXPHOS. In AML, the GPAT1 inhibitor also suppressed cell proliferation and mitochondrial metabolism. However, this inhibitor had no effect on normal hematopoiesis in vivo. In conclusion, these findings indicate that targeting GPAT1 may be a promising therapeutic strategy for AML, since it suppresses leukemia-specific metabolism without impairing normal HSPCs.

Using post-transplant cyclophosphamide (PTCy-haplo), haploidentical allogeneic hematopoietic stem cell transplantation has shown a surge in popularity in recent years. There are, however, only a few reports of PTCy-haplo being used to treat myelodysplastic syndromes (MDS) that have been complicated by myeloid sarcoma (MS). An immuno-suppressive therapy was given to a 25-year-old man who was diagnosed with low-risk MDS in September 2007. After an ileocecal ulcer biopsy that revealed MS in July 2019, a chromosomal analysis of the bone marrow cells in August 2019 revealed loss of chromosome 7, which is associated with poor prognosis. Because the patient lacked an HLA-matched sibling donor, he underwent PTCy-haplo in December 2019. On day 33, complete remission and donor chimerism was achieved. Ileocecal ulcer scarring was discovered by a colonoscopy on day 54. Grade I cutaneous acute graft-versus-host disease was discovered approximately on day 30 and treated with topical steroids. PTCy-haplo may be an effective treatment for MS.

In 21 of 47 mice, tumor development was initiated by intraperitoneal injection of one side population(SP)cell derived from a mouse ascites tumor and the host mice died of the ascites tumor. The SP cells were thought to be cancer stem-like cells. Examination by differential interference contrast microscopy revealed that the SP cells were smaller in size(ϕ 27.5µm), and had a higher N/C ratio, sparser and shorter microvilli on the cell surface, and significantly fewer mitochondria, as compared to the non-SP cells(ϕ 30.0µm). These findings could be related to the stemness of the SP cells.

Gene panel test, which has been used in the field of solid tumors, is expected to be introduced in the field of hematopoietic tumors in the near future. Unintended germline variants may be detected due to the comprehensive nature of the gene panel test, and these are called "secondary findings." Additionally, it sometimes reveals that a person has germline variants that predispose the carriers to develop the disease. There are several impeding issues to be addressed regarding these germline variants, including detection methods, obtaining informed consent, scope of variants to be disclosed, and genetic counseling methods. In the field of hematopoietic tumors, the problem is further complicated by the possibility of detecting germline variants in donors because allogeneic hematopoietic stem cell transplantation constitutes an important treatment modality in this field. Thus, this paper aims to outline the issues related to germline variants in gene panel tests, which were discussed at the symposium "NGS-based multi-gene panel testing in hematology/oncology" at the annual meeting of the Japanese Society of Hematology in 2021.

Infertility is one of the late side effects of cancer treatment. Expansion of anti-cancer treatment allow patients to have more life time, however infertility is becoming a matter damaging QOL during the young cancer survivors. The passive strategy such as avoiding the gonad-toxic drug or decreasing the total volume of them and shielding the gonads against cancer therapy has been conducted. To preserve the fertility of young female, ovary tissue cryopreservation is becoming a standard over the world after the success of offspring from cryopreserved ovary tissue autograft was reported. Sperm preservation method is established for the male fertility preservation method, however this is only applicable for sexually matured male patients. For the sake of preserving fertility of sexually immature male patients, many trials using cryopreserved testis tissues or testicular cells have been undergone. Recently, in vitro gametogenesis from stem cell of the human and the mouse to primordial germ cell like cell has been achieved. Here the previous challenges and the latest reports for obtaining functional sperm from immature testis and the reconstruction of spermatogonial niche as a potential approach for preserving fertility procedure are described.

The patient is a 34-year-old HIV antibody-negative female with normal immunocompetence. The patient was referred to the hospital of the current study due to diarrhea and abdominal pain, which developed in May 2014. On conducting computed tomography (CT), remarkable wall thickening was noted in the terminal ilium over the ascending colon, suggesting a malignant tumor. However, making a definite diagnosis by lower gastrointestinal endoscopic biopsy and left hemicolectomy was not possible. The dense proliferation of plasma cell-like cells and plasmablasts was noted; CD20, CD19, CD79a, CD3, CD4, and Epstein-Barr virus-encoded miRNAs (EBER) were negative and CD138 was positive on immunostaining. Based on the aforementioned data, the patient was diagnosed with plasmablastic lymphoma (PBL). High-dose chemotherapy combined with autologous peripheral blood stem cell transplantation (PBSCT) was performed in the first remission period after the completion of four cycles of hyper CVAD/MTX-AraC alternating therapy. Remission was confirmed by FDG-PET/CT 3 months after autologous PBSCT. No signs of recurrence have been observed in 6 years after the transplantation. Although no standard treatment for PBL has been established, autologous peripheral blood stem cell transplantation combined with high-dose chemotherapy during the first remission period may be a beneficial treatment option.

This paper reports a case of a 56-year-old male with IgG lambda plasmablastic myeloma exhibiting multiple chromosomal abnormalities. The patient initially presented with plasmablastic ascites and underwent early auto stem cell transplantation and achieved minimal residual disease-negative status but relapsed after 1.5 months and became refractory to novel drugs, such as proteasome inhibitor and daratumuab. Performing differential diagnosis of plasmablastic myeloma with extramedullary masses or fluid retention observed at the initial presentation in comparison to plasmablastic lymphoma and pleural effusion lymphoma is difficult, and patients often have a poor prognosis even with novel drugs. Hence, finding a treatment strategy for such patients is difficult. Thus, further novel drugs are expected to emerge in the future.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19), which results in a rapid increase in the number of patients and deaths. In addition, various mutant strains have emerged and to be considered to accelerate the number of infected persons. To overcome this situation, effective strategies against COVID-19 include the development of vaccines to prevent SARS-CoV-2 infection and therapeutic agents that suppress the severity after infection. The drug repositioning approach, which search existing drugs that are effective against COVID-19, are expected to develop anti-COVID-19 drugs. In addition, various methods using human iPSC-derived differentiated cells has been developed to evaluate the efficacy and safety of drugs, and are also used for searching for therapeutic drugs for COVID-19. Here, we would like to describe the recent research and future perspectives for COVID-19 therapeutic drugs from the viewpoint of human iPS cell technology.