Generation of T cells from adult human hematopoietic stem cells and progenitors in a fetal thymic organ culture system: stimulation by tumor necrosis factor-α

Intrathymic dendritic cell-biased precursors promote human T cell lineage specification through IRF8-driven transmembrane TNF

The cross-talk between thymocytes and thymic stromal cells is fundamental for T cell development. In humans, intrathymic development of dendritic cells (DCs) is evident but its physiological significance is unknown. Here we showed that DC-biased precursors depended on the expression of the transcription factor IRF8 to express the membrane-bound precursor form of the cytokine TNF (tmTNF) to promote differentiation of thymus seeding hematopoietic progenitors into T-lineage specified precursors through activation of the TNF receptor (TNFR)-2 instead of TNFR1. In vitro recapitulation of TNFR2 signaling by providing low-density tmTNF or a selective TNFR2 agonist enhanced the generation of human T cell precursors. Our study shows that, in addition to mediating thymocyte selection and maturation, DCs function as hematopoietic stromal support for the early stages of human T cell development and provide proof of concept that selective targeting of TNFR2 can enhance the in vitro generation of T cell precursors for clinical application.

Multi-objective optimization reveals time- and dose-dependent inflammatory cytokine-mediated regulation of human stem cell derived T-cell development

The generation of T-cells from stem cells in vitro could provide an alternative source of cells for immunotherapies. T-cell development from hematopoietic stem and progenitor cells (HSPCs) is tightly regulated through Notch pathway activation by Delta-like (DL) ligands 1 and 4. Other molecules, such as stem cell factor (SCF) and interleukin (IL)-7, play a supportive role in regulating the survival, differentiation, and proliferation of developing T-cells. Numerous other signaling molecules influence T-lineage development in vivo, but little work has been done to understand and optimize their use for T-cell production. Using a defined engineered thymic niche system, we undertook a multi-stage statistical learning-based optimization campaign and identified IL-3 and tumor necrosis factor α (TNFα) as a stage- and dose-specific enhancers of cell proliferation and T-lineage differentiation. We used this information to construct an efficient three-stage process for generating conventional TCRαβ⁺CD8⁺ T-cells expressing a diverse TCR repertoire from blood stem cells. Our work provides new insight into T-cell development and a robust system for generating T-cells to enable clinical therapies for treating cancer and immune disorders.

Modeling of human T cell development in vitro as a read-out for hematopoietic stem cell multipotency

Hematopoietic stem cells (HSCs) reside in distinct sites throughout fetal and adult life and give rise to all cells of the hematopoietic system. Because of their multipotency, HSCs are capable of curing a wide variety of blood disorders through hematopoietic stem cell transplantation (HSCT). However, due to HSC heterogeneity, site-specific ontogeny and current limitations in generating and expanding HSCs in vitro, their broad use in clinical practice remains challenging. To assess HSC multipotency, evaluation of their capacity to generate T lymphocytes has been regarded as a valid read-out. Several in vitro models of T cell development have been established which are able to induce T-lineage differentiation from different hematopoietic precursors, although with variable efficiency. Here, we review the potential of human HSCs from various sources to generate T-lineage cells using these different models in order to address the use of both HSCs and T cell precursors in the clinic.

A DL-4- and TNFα-based culture system to generate high numbers of nonmodified or genetically modified immunotherapeutic human T-lymphoid progenitors

Several obstacles to the production, expansion and genetic modification of immunotherapeutic T cells in vitro have restricted the widespread use of T-cell immunotherapy. In the context of HSCT, delayed naïve T-cell recovery contributes to poor outcomes. A novel approach to overcome the major limitations of both T-cell immunotherapy and HSCT would be to transplant human T-lymphoid progenitors (HTLPs), allowing reconstitution of a fully functional naïve T-cell pool in the patient thymus. However, it is challenging to produce HTLPs in the high numbers required to meet clinical needs. Here, we found that adding tumor necrosis factor alpha (TNFα) to a DL-4-based culture system led to the generation of a large number of nonmodified or genetically modified HTLPs possessing highly efficient in vitro and in vivo T-cell potential from either CB HSPCs or mPB HSPCs through accelerated T-cell differentiation and enhanced HTLP cell cycling and survival. This study provides a clinically suitable cell culture platform to generate high numbers of clinically potent nonmodified or genetically modified HTLPs for accelerating immune recovery after HSCT and for T-cell-based immunotherapy (including CAR T-cell therapy).

Vascular Notch Signaling in Stress Hematopoiesis

Canonical Notch signaling is one of the most conserved signaling cascades. It regulates cell proliferation, cell differentiation, and cell fate maintenance in a variety of biological systems during development and cancer (Fortini, 2009; Kopan and Ilagan, 2009; Andersson et al., 2011; Ntziachristos et al., 2014). For the hematopoietic system, during embryonic development, Notch1 is essential for the emergence of hematopoietic stem cells (HSCs) at the aorta-gornado-mesonephro regions of the dorsal aorta. At adult stage, Notch receptors and Notch targets are expressed at different levels in diverse hematopoietic cell types and influence lineage choices. For example, Notch specifies T cell lineage over B cells. However, there has been a long-lasting debate on whether Notch signaling is required for the maintenance of adult HSCs, utilizing transgenic animals inactivating different components of the Notch signaling pathway in HSCs or niche cells. The aims of the current mini-review are to summarize the evidence that disapproves or supports such hypothesis and point at imperative questions waiting to be addressed; hence, some of the seemingly contradictory findings could be reconciled. We need to better delineate the Notch signaling events using biochemical assays to identify direct Notch targets within HSCs or niche cells in specific biological context. More importantly, we call for more elaborate studies that pertain to whether niche cell type (vascular endothelial cells or other stromal cell)-specific Notch ligands regulate the differentiation of T cells in solid tumors during the progression of T-lymphoblastic lymphoma (T-ALL) or chronic myelomonocytic leukemia (CMML). We believe that the investigation of vascular endothelial cells' or other stromal cell types' interaction with hematopoietic cells during homeostasis and stress can offer insights toward specific and effective Notch-related therapeutics.

TNF-alpha and Notch signaling regulates the expression of HOXB4 and GATA3 during early T lymphopoiesis

During the early thymus colonization, Notch signaling activation on hematopoietic progenitor cells (HPCs) drives proliferation and T cell commitment. Although these processes are driven by transcription factors such as HOXB4 and GATA3, there is no evidence that Notch directly regulates their transcription. To evaluate the role of NOTCH and TNF signaling in this process, human CD34⁺ HPCs were cocultured with OP9-DL1 cells, in the presence or absence of TNF. The use of a Notch signaling inhibitor and a protein synthesis inhibitor allowed us to distinguish primary effects, mediated by direct signaling downstream Notch and TNF, from secondary effects, mediated by de novo synthesized proteins. A low and physiologically relevant concentration of TNF promoted T lymphopoiesis in OP9-DL1 cocultures. TNF positively modulated the expression of both transcripts in a Notch-dependent manner; however, GATA3 induction was mediated by a direct mechanism, while HOXB4 induction was indirect. Induction of both transcripts was repressed by a GSK3β inhibitor, indicating that activation of canonical Wnt signaling inhibits rather than induces their expression. Our study provides novel evidences of the mechanisms integrating Notch and TNF-alpha signaling in the transcriptional induction of GATA3 and HOXB4. This mechanism has direct implications in the control of self-renewal, proliferation, commitment, and T cell differentiation.

Dynamic roles of angiopoietin-like proteins 1, 2, 3, 4, 6 and 7 in the survival and enhancement of ex vivo expansion of bone-marrow hematopoietic stem cells

Recent advances in hematopoietic stem cells (HSCs) expansion by growth factors including angiopoietin-like proteins (Angptls) have opened up the possibility to use HSCs in regenerative medicine. However, the unavailability of true in vitro HSCs expansion by these growth factors has limited the understanding of the cellular and molecular mechanism of HSCs expansion. Here, we report the functional role of mouse Angptls 1, 2, 3, 4, 6 and 7 and growth factors SCF, TPO, IGF-2 and FGF-1 on purified mouse bone-marrow (BM) Lineage(-)Sca-1(+)(Lin-Sca-1(+)) HSCs. The recombinant retroviral transduced-CHO-S cells that secrete Angptls in serum-free medium were used alone or in combination with growth factors (SCF, TPO, IGF-2 and FGF-1). None of the Angptls stimulated HSC proliferation, enhanced or inhibited HSCs colony formation, but they did support the survival of HSCs. By contrast, any of the six Angptls together with saturating levels of growth factors dramatically stimulated a 3- to 4.5-fold net expansion of HSCs compared to stimulation with a combination of those growth factors alone. These findings lead to an understanding of the basic function of Angptls on signaling pathways for the survival as well as expansion of HSCs in the bone marrow niche.

The mouse is out of the bag: insights and perspectives on HIV-1-infected humanized mouse models

Human immunodeficiency virus type 1 (HIV-1), which is the causative agent of acquired immunodeficiency syndrome, is a human-specific virus. Because HIV-1 cannot infect and cause disorders in other animals, it has been an arduous struggle to study the dynamics of HIV-1 infection in vivo. To understand and elucidate HIV-1 pathogenesis in vivo, several small animal models for HIV-1 infection have been established and improved over the last 20 years. Recently, a novel murine model, 'humanized mouse', has been generated. A humanized mouse has the potential to maintain human hematopoiesis including human CD4(+) leukocytes and, therefore, is able to support persistent HIV-1 infection in vivo. We herein describe the current state-of-the-art in HIV-1-infected humanized mice and introduce insights and perspectives of their use for HIV-1 studies in vivo.

Dynamics of memory and naïve CD8+ T lymphocytes in humanized NOD/SCID/IL-2Rgammanull mice infected with CCR5-tropic HIV-1

Creating a novel small animal model of HIV-1 infection that can support long-term systemic HIV-1 infection and produce HIV-1-specific immune response has a great benefit for studying HIV-1 pathogenesis in vivo. In the present study, we have generated a humanized mouse, NOG-hCD34 mouse, by transplanting newborn NOD/SCID/IL-2Rgamma(null) mice with human hematopoietic stem cells through hepatic injection. These mice were infected with a CCR5-tropic HIV-1 and were analyzed for plasma viral load, changes in peripheral blood T lymphocytes, and HIV-1-specific antibody production. High level of viral replication, increase in effector/memory CD8(+) T lymphocytes, class-switching to IgG, and production of HIV-1-specific IgGs were observed. Our findings suggest that NOG-hCD34 mice may have a wide variety of application in HIV-1 research.

Modulating T-cell homeostasis with IL-7: preclinical and clinical studies

Interleukin-7 (IL-7) is required for the development and survival of T cells and plays a critical role in modulating T-cell homeostasis. This review will address current understanding of IL-7 biology, review recent clinical experiences and discuss potential future clinical applications of IL-7, or IL-7 blockade, in the setting of disease.

Cord blood stem cell transplantation for haemoglobinopathies

Despite improvements in supportive care, patients with beta-thalassaemia major or sickle cell disease (SCD) may benefit from haematopoietic stem cell transplantation at some point during their lives. Human leucocyte antigen (HLA)-matched sibling bone marrow donors are not always available and alternative sources of stem cells have been sought, including related and unrelated donor cord blood transplants (CBT). The outcome of CBT from related donors for the treatment of both thalassaemia major and SCD is now approaching that for bone marrow transplantation, with around 90% of patients surviving disease-free. The main complication is graft rejection, which may be reduced by increasing pretransplant immune suppression. Transplant-related mortality following HLA-identical matched related donor CBT is extremely low but is significant in the small series of unrelated and/or mis-matched donor CBT. The principal limitation to extending the use of CB stem cells for the cure of haemoglobinopathies is the need to better understand the mechanisms of action and optimal conditioning regimens used to secure long-term engraftment while minimizing morbidity and mortality. Further biological studies and clinical trials are needed to address this aim.

Tumor necrosis factor promotes T-cell at the expense of B-cell lymphoid development from cultured human CD34+ cord blood cells

OBJECTIVE

Human CD34+ cord blood (CB) cells are hematopoietic progenitors useful for stem cell transplantation, even after ex vivo expansion. We investigated the effect of tumor necrosis factor (TNF) on lymphoid development from cultured CD34+ CB cells.

MATERIALS AND METHODS

Human CD34+ CB cells were cultured in cytokine mixes with or without TNF. Preculture during 60 hours was followed by in vitro differentiation assays, including fetal thymus organ culture and coculture on murine stromal MS-5 cells. In a next step, experiments were extended to CD34+CD38- and CD34+CD38+ CB cells and prolonged preculture.

RESULTS

Preculture in the presence of TNF improved differentiation into T cells and diminished the ability to generate B cells, while NK potential and myeloid development were unaffected. Sorted CD34+CD38- CB cells were more potent T-cell precursors after preculture in TNF, compared to CD34+CD38+ CB cells. In precultured CD34+CD38- CB cells, TNF increased GATA3 but decreased EBF1 expression, in line with the skewed lymphoid differentiation induced by TNF. However, when preculture in the presence of TNF was extended to 1 week, T-cell precursors were lost.

CONCLUSION

After short-term culture of CD34+ CB cells in the presence of TNF, T-cell generation is stimulated at the expense of B-cell generation. T-cell progenitors are enriched in the CD34+CD38- fraction. These results have implications on the culture conditions to be used for CB CD34+ cells prior to transplantation.

Effect of hepatocyte growth factor on long term hematopoiesis of human progenitor cells in transgenic-sever combined immunodeficiency mice

Hepatocyte growth factor (HGF), which was originally isolated as a liver generating factor, enhances hematopoiesis. To study the effect of HGF on hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs), we generated severe combined immunodeficiency (SCID) mice producing human (h) HGF and/or stem cell factor (SCF) by transferring the relevant genes to fertilized eggs, and then transplanted hematopoietic progenitors from human cord blood into the transgenic (Tg) SCID mice. Six months after transplantation, a significantly larger number of human cells were found in the Tg SCID mice than in non-Tg controls. Characteristically, the recipient SCID mice producing h HGF (HGF-SCID) had a significantly increased number of h CD41+ cells, whereas the SCF-SCID recipients had more CD11b+ cells. Significantly large numbers of CD34+ progenitors were found in the SCID mice transferred with both h HGF and h SCF genes (HGF/SCF-SCID) when compared with HGF-SCID or SCF-SCID mice. These results imply that HGF supports the differentiation of progenitors in megakaryocyte lineage, whereas SCF supports that in myeloid lineage. The results also imply that HGF acts on HSCs/HPCs as a synergistic proliferative factor combined with SCF. We have demonstrated the advantage of the human cytokine-producing animal in the maintenance of human HSCs.

Simultaneous isolation of human BM hematopoietic, endothelial and mesenchymal progenitor cells by flow sorting based on aldehyde dehydrogenase activity: implications for cell therapy

BACKGROUND

ALDH(br) cells express high aldehyde dehydrogenase (ALDH) activity and have progenitor cell activity in several contexts. We characterized human BM ALDH(br) cells to determine whether cell sorting based on ALDH activity isolates potentially useful populations for cell therapy.

METHOD

We measured the expression of ALDH and cell-surface Ag by flow cytometry and compared the ability of sorted ALDH(br), and BM populations remaining after ALDH(br) cells were removed (ALDH(dim) populations), to develop into several cell lineages in culture.

RESULTS

The ALDH(br) population comprised 1.2+/-0.8% (mean+/-SD, n=30) nucleated cells and was enriched in cells expressing CD34, CD117, CD105, CD127, CD133 and CD166, and in primitive CD34(+) CD38(-) and CD34(+) CD133(+) progenitors. Most of the CD34(+) and CD133(+) cells were ALDH(dim). ALDH(br) populations had 144-fold more hematopoietic colony-forming activity than ALDH(dim) cells and included all megakaryocyte progenitors. ALDH(br) populations readily established endothelial cell monolayers in cultures. Cells generating endothelial colonies in 7 days were 435-fold more frequent in ALDH(br) than ALDH(dim) populations. CFU-F were 9.5-fold more frequent in ALDH(br) than ALDH(dim) cells, and ALDH(br) cells gave rise to multipotential mesenchymal cell cultures that could be driven to develop into adipocytes, osteoblasts and chondrocytes.

DISCUSSION

Hematopoietic, endothelial and mesenchymal progenitor cells can be isolated simultaneously from human BM by cell sorting based on ALDH activity. BM ALDH(br) populations may be useful in several cell therapy applications.

Higher expression of transcription targets and components of the nuclear factor-kappaB pathway is a distinctive feature of umbilical cord blood CD34+ precursors

Delayed engraftment, better reconstitution of progenitors, higher thymic function, and a lower incidence of the graft-versus-host disease are characteristics associated with umbilical cord blood (UCB) transplants, compared with bone marrow (BM). To understand the molecular mechanisms causing these intrinsic differences, we analyzed the differentially expressed genes between BM and UCB hematopoietic stem and progenitor cells (HSPCs). The expressions of approximately 10,000 genes were compared by serial analysis of gene expression of magnetically sorted CD34(+) cells from BM and UCB. Differential expression of selected genes was evaluated by real-time polymerase chain reaction on additional CD34(+) samples from BM (n = 22), UCB (n = 9), and granulocyte colony stimulating factor-mobilized peripheral blood (n = 6). The overrepresentation of nuclear factor-kappaB (NF-kappaB) pathway components and targets was found to be a major characteristic of UCB HSPCs. Additional promoter analysis of 41 UCB-overrepresented genes revealed a significantly higher number of NF-kappaB cis-regulatory elements (present in 22 genes) than would be expected by chance. Our results point to an important role of the NF-kappaB pathway on the molecular and functional differences observed between BM and UCB HSPCs. Our study forms the basis for future studies and potentially for new strategies to stem cell graft manipulation, by specific NF-kappaB pathway modulation on stem cells, prior to transplant.

Different thresholds of Notch signaling bias human precursor cells toward B-, NK-, monocytic/dendritic-, or T-cell lineage in thymus microenvironment

Notch receptors are involved in lineage decisions in multiple developmental scenarios, including hematopoiesis. Here, we treated hybrid human-mouse fetal thymus organ culture with the gamma-secretase inhibitor 7 (N-[N-(3,5-difluorophenyl)-l-alanyl]-S-phenyl-glycine t-butyl ester) (DAPT) to establish the role of Notch signaling in human hematopoietic lineage decisions. The effect of inhibition of Notch signaling was studied starting from cord blood CD34(+) or thymic CD34(+)CD1(-), CD34(+)CD1(+), or CD4ISP progenitors. Treatment of cord blood CD34(+) cells with low DAPT concentrations results in aberrant CD4ISP and CD4/CD8 double-positive (DP) thymocytes, which are negative for intracellular T-cell receptor beta (TCRbeta). On culture with intermediate and high DAPT concentrations, thymic CD34(+)CD1(-) cells still generate aberrant intracellular TCRbeta(-) DP cells that have undergone DJ but not VDJ recombination. Inhibition of Notch signaling shifts differentiation into non-T cells in a thymic microenvironment, depending on the starting progenitor cells: thymic CD34(+)CD1(+) cells do not generate non-T cells, thymic CD34(+)CD1(-) cells generate NK cells and monocytic/dendritic cells, and cord blood CD34(+)Lin(-) cells generate B, NK, and monocytic/dendritic cells in the presence of DAPT. Our data indicate that Notch signaling is crucial to direct human progenitor cells into the T-cell lineage, whereas it has a negative impact on B, NK, and monocytic/dendritic cell generation in a dose-dependent fashion.

Tumor necrosis factor promotes human T-cell development in nonobese diabetic/severe combined immunodeficient mice

A major problem after clinical hematopoietic stem cell transplantations is poor T-cell reconstitution. Studying the mechanisms underlying this concern is hampered, because experimental transplantation of human stem and progenitor cells into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice usually results in low T-lymphocyte reconstitution. Because tumor necrosis factor alpha (TNFalpha) has been proposed to play a role in T-lineage commitment and differentiation in vitro, we investigated its potential to augment human T-cell development in vivo. Administration of TNF to irradiated NOD/SCID mice before transplantation of human mononuclear cells from either cord blood or adult G-CSF-mobilized peripheral blood (MPBL) led 2-3 weeks after transplantation to the emergence of human immature CD4(+)CD8(+) double-positive T-cells in the bone marrow (BM), spleen, and thymus, and in this organ, the human cells also express CD1a marker. One to 2 weeks later, single-positive CD4(+) and CD8(+) cells expressing heterogenous T-cell receptor alpha beta were detected in all three organs. These cells were also capable of migrating through the blood circulation. Interestingly, human T-cell development in these mice was associated with a significant reduction in immature lymphoid human CD19(+) B cells and natural killer progenitors in the murine BM. The human T cells were mostly derived from the transplanted immature CD34(+) cells. This study demonstrates the potential of TNF to rapidly augment human T lymphopoiesis in vivo and also provides clinically relevant evidence for this process with adult MPBL progenitors.

Identification of hematopoietic stem/progenitor cells: strength and drawbacks of functional assays

A major challenge in hematopoiesis is to conceive assays that could bring useful insights into experimental and clinical hematology. This means identifying separately the various classes of hematopoietic progenitors that are produced sequentially during the progression from stem cells to differentiated functional cells. Standardized short-term colony assays easily quantify lineage-committed myeloid precursors, but identification of primitive cells, which have both the ability to repopulate durably myeloid and lymphoid lineages and perhaps to self-renew, still depends on in vivo assays. Whatever the assay, two important requisites have to be solved: one is the definition of appropriate read-outs that will depend solely on the function of these cells, and the second is to evaluate precisely their numbers and proliferative potential in quantitative assays. When evaluating hematopoiesis, three parameters have to be taken into account: (1) the lack of reliable correlation between the phenotype of a given cell and its function. This is especially problematic in post-transplantation situations where cells from transplanted animals are analysed; (2) functionally heterogeneous cells are identified in a single assay; and (3) ontogeny-related changes in hematopoietic cell proliferation and self-renewal that, in human beings, hampers the exploration of adult stem cells. Nevertheless, years of progress in the manipulation of hematopoietic stem cells have recently resulted in the purification of a cell subset that repopulates irradiated recipients with absolute efficiency.

Complete reconstitution of human lymphocytes from cord blood CD34+ cells using the NOD/SCID/gammacnull mice model

Establishment of an assay capable of generating all classes of human lymphocytes from hematopoietic stem cells (HSCs) will provide new insight into the mechanism of human lymphopoiesis. We report ontogenic, functional, and histologic examination results of reconstituted human lymphocytes in NOD/SCID/ gammacnull mice after the transplantation of human cord blood (CB) CD34+ cells. After transplantation, human B, natural killer (NK), and T cells were invariably identified in these mice, even though no human tissues were cotransplanted. Immature B cells resided mainly in bone marrow (BM), whereas mature B cells with surface immunoglobulins were preferentially found in spleen. NK cells were identified in BM and spleen. T cells were observed in various lymphoid organs, but serial examinations after transplantation confirmed human T lymphopoiesis occurring in the thymus. These human lymphocytes were also functionally competent. Human immunoglobulin M (IgM), IgA, and IgG were detected in the sera of these mice. T cells showed a diverse repertoire of T-cell-receptor Vbeta (TCR Vbeta) chains, proliferated in response to phytohemagglutinin, and were cytotoxic against cell lines. NK activity was demonstrated using the K562 cell line. Immunohistochemical analysis revealed that human lymphocytes formed organized structures in spleen and thymus that were analogous to those seen in humans. In the thymus, CD4 and CD8 double-positive T cells were predominant and coexpressed CD1a and Ki-67, thereby supporting the notion that T lymphopoiesis was taking place. NOD/SCID/ gammacnull mice provide a unique model to investigate human lymphopoiesis without the cotransplantation of human tissues.

The in vivo development of human T cells from CD34(+) cells in the murine thymic environment

There is increasing evidence that human hematopoietic stem cells can develop into lymphocytes expressing T cell surface markers in the organ culture of murine embryonic thymic lobes. If human T cells with functional maturity are inducible from human stem cells in the mouse, it may be a useful model to investigate human T cell development and the human immune response in vivo. To approach this, we produced a hybrid cluster of murine fetal thymic epithelial cells and human cord blood-derived CD34(+) cells (hu/m cluster) using reaggregate thymic organ culture, and subsequently implanted it under the kidney capsule of NOD/SCID mice. The implanted hu/m cluster grew in volume under the kidney capsule and contained increased numbers of CD4(+)CD8(+)cells as well as CD4 or CD8 single-positive cells with low CD1a expression. These lymphocytes were also shown to possess activity for producing IL-2 and IL-4. Characteristics similar to human T cells also developed in the thymus of newly established mice lacking NK activity from NOD/SCID mice. These results indicate that functionally mature T cells can develop in vivo from human hematopoietic progenitors in the murine environment composed of thymic epithelial cells.

Differential long-term and multilineage engraftment potential from subfractions of human CD34+ cord blood cells transplanted into NOD/SCID mice

Over the past decade xenotransplantation systems have been used with increasing success to gain a better understanding of human cells that are able to initiate and maintain the hematopoietic system in vivo. The nonobese diabetic/severe combined immunodeficiency (SCID) mouse has been a particularly useful model. Human cells capable of hematopoietic repopulation in this mouse, termed SCID-repopulating cells, have been assumed to represent the most primitive elements of the hematopoietic system, responsible for long-term maintenance of hematopoiesis. However, we demonstrate that SCID-repopulating cells present in the CD34(+) cell fraction of cord blood can be segregated into subpopulations with distinct repopulation characteristics. CD34(+)/CD38(+) progenitors can repopulate recipients rapidly, but can only maintain the graft for 12 weeks or less and have no secondary repopulation potential. Conversely, the more primitive CD34(+)/CD38(-) subpopulation repopulates recipients more gradually, can maintain the graft for at least 20 weeks, and contains cells with serial repopulation potential throughout the engraftment period. Additionally, a much higher frequency of T cell precursors are found among SCID-repopulating cells in the CD34(+)/CD38(-) subpopulation. These findings demonstrate that cells with variable repopulation potential comprise the human CD34(+) population and that short- and long-term potential of human precursors can be evaluated in the mouse model.

Development of human lymphohematopoietic stem and progenitor cells defined by expression of CD34 and CD81

In this study, cord blood CD34(+) cells expressed CD81, a member of the transmembrane 4 superfamily, and were classified into 3 subpopulations on the basis of their expression levels: CD34(+)CD81(+), CD34(low)CD81(+), and CD34(+)CD81(high). The lymphohematopoietic activity of each subpopulation was then examined by using suspension and clonogenic cultures for hematopoietic potential, coculture with MS-5 cells for B-cell potential, organ cultures of thymus lobes from nonobese diabetic/severe combined immunodeficiency disease (NOD/SCID) fetal mice, coculture with stromal cells derived from NOD/SCID fetal-mouse liver tissue for natural killer (NK) cell and mast cell potentials, and xenotransplantation into NOD/SCID mice for long-term repopulating (LTR) ability. CD34(+)CD81(+) cells represented a heterogeneous population that had all the lymphohematopoietic activities, including NOD/SCID mouse-repopulating ability. CD34(low)CD81(+) cells were enriched in erythroid, megakaryocytic, and NK lineage potentials but had lost T-cell and B-cell potentials and LTR ability. The CD34(+)CD81(high) fraction was depleted of most lymphohematopoietic potentials except NK cell and mast cell potentials. Thus, along the differentiation cascade from CD34(+)CD81(+) lymphohematopoietic stem cells, an up-regulation of CD81 or a down-regulation of CD34 results in a change in lymphohematopoietic properties. CD81 may serve as a marker for defining developmental stages of lymphohematopoietic stem cells. (Blood. 2001;97:3755-3762)