Adapted NOD/SCID model supports development of phenotypically and functionally mature T cells from human umbilical cord blood CD34(+) cells

Advancing Key Gaps in the Knowledge of Plasmodium vivax Cryptic Infections Using Humanized Mouse Models and Organs-on-Chips

Plasmodium vivax is the most widely distributed human malaria parasite representing 36.3% of disease burden in the South-East Asia region and the most predominant species in the region of the Americas. Recent estimates indicate that 3.3 billion of people are under risk of infection with circa 7 million clinical cases reported each year. This burden is certainly underestimated as the vast majority of chronic infections are asymptomatic. For centuries, it has been widely accepted that the only source of cryptic parasites is the liver dormant stages known as hypnozoites. However, recent evidence indicates that niches outside the liver, in particular in the spleen and the bone marrow, can represent a major source of cryptic chronic erythrocytic infections. The origin of such chronic infections is highly controversial as many key knowledge gaps remain unanswered. Yet, as parasites in these niches seem to be sheltered from immune response and antimalarial drugs, research on this area should be reinforced if elimination of malaria is to be achieved. Due to ethical and technical considerations, working with the liver, bone marrow and spleen from natural infections is very difficult. Recent advances in the development of humanized mouse models and organs-on-a-chip models, offer novel technological frontiers to study human diseases, vaccine validation and drug discovery. Here, we review current data of these frontier technologies in malaria, highlighting major challenges ahead to study P. vivax cryptic niches, which perpetuate transmission and burden.

Humanized mouse model of Rasmussen's encephalitis supports the immune-mediated hypothesis

Rasmussen's encephalitis (RE) is a chronic inflammatory brain disorder that causes frequent seizures and unilateral hemispheric atrophy with progressive neurological deficits. Hemispherectomy remains the only treatment that leads to seizure freedom for this refractory epileptic syndrome. The absence of an animal model of disease has been a major obstacle hampering the development of effective therapies. Here, we describe an experimental mouse model that shares several clinical and pathological features with the human disease. Immunodeficient mice injected with peripheral blood mononuclear cells from RE patients and monitored by video electroencephalography developed severe seizures of cortical origin and showed intense astrogliosis and accumulation of human IFN-γ- and granzyme B-expressing T lymphocytes in the brain compared with mice injected with immune cells from control subjects. We also provide evidence for the efficacy of α4 integrin blockade, an approved therapy for the treatment of multiple sclerosis and Crohn's disease, in reducing inflammatory markers associated with RE in the CNS. This model holds promise as a valuable tool for understanding the pathology of RE and for developing patient-tailored experimental therapeutics.

Humanized Mice to Study Human T Cell Development

While in vitro models exist to study human T cell development, they still lack the precise environmental stimuli, such as the exact combination and levels of cytokines and chemokines, that are present in vivo. Moreover, studying the homing of hematopoietic stem (HSC) and progenitor (HPC) cells to the thymus can only be done using in vivo models. Although species-specific differences exist, "humanized" models are generated to circumvent these issues. In this chapter, we focus on the humanized mouse models that can be used to study early T cell development. Models that study solely mature T cells, such as the SCID-PBL (Tary-Lehmann et al., Immunol Today 16:529-533) are therefore not discussed here, but have recently been reviewed (Shultz et al., Nat Rev Immunol 12:786-798).

Pluripotent stem cell based gene therapy for hematological diseases

Standard treatment for severe inherited hematopoietic diseases consists of allogeneic stem cell transplantation. Alternatively, patients can be treated with gene therapy: gene-corrected autologous hematopoietic stem and progenitor cells (HSPC) are transplanted. By using retro- or lentiviral vectors, a copy of the functional gene is randomly inserted in the DNA of the HSPC and becomes constitutively expressed. Gene therapy is currently limited to monogenic diseases for which clinical trials are being actively conducted in highly specialized centers around the world. This approach, although successful, carries with it inherent safety and efficacy issues. Recently, two technologies became available that, when combined, may enable treatment of genetic defects by HSPC that have the non-functional allele replaced by a functional copy. One technology consists of the generation of induced pluripotent stem cells (iPSC) from patient blood samples or skin biopsies, the other concerns nuclease-mediated gene editing. Both technologies have been successfully combined in basic research and appear applicable in the clinic. This paper reviews recent literature, discusses what can be achieved in the clinic using present knowledge and points out further research directions.

Murine genetically engineered and human xenograft models of chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is a genetically complex disease, with multiple factors having an impact on onset, progression, and response to therapy. Genetic differences/abnormalities have been found in hematopoietic stem cells from patients, as well as in B lymphocytes of individuals with monoclonal B-cell lymphocytosis who may develop the disease. Furthermore, after the onset of CLL, additional genetic alterations occur over time, often causing disease worsening and altering patient outcomes. Therefore, being able to genetically engineer mouse models that mimic CLL or at least certain aspects of the disease will help us understand disease mechanisms and improve treatments. This notwithstanding, because neither the genetic aberrations responsible for leukemogenesis and progression nor the promoting factors that support these are likely identical in character or influences for all patients, genetically engineered mouse models will only completely mimic CLL when all of these factors are precisely defined. In addition, multiple genetically engineered models may be required because of the heterogeneity in susceptibility genes among patients that can have an effect on genetic and environmental characteristics influencing disease development and outcome. For these reasons, we review the major murine genetically engineered and human xenograft models in use at the present time, aiming to report the advantages and disadvantages of each.

Increasing hematopoietic stem cell yield to develop mice with human immune systems

Hematopoietic stem cells (HSCs) are unique in their capacity to give rise to all mature cells of the immune system. For years, HSC transplantation has been used for treatment of genetic and neoplastic diseases of the hematopoietic and immune systems. The sourcing of HSCs from human umbilical cord blood has salient advantages over isolation from mobilized peripheral blood. However, poor sample yield has prompted development of methodologies to expand HSCs ex vivo. Cytokines, trophic factors, and small molecules have been variously used to promote survival and proliferation of HSCs in culture, whilst strategies to lower the concentration of inhibitors in the culture media have recently been applied to promote HSC expansion. In this paper, we outline strategies to expand HSCs in vitro, and to improve engraftment and reconstitution of human immune systems in immunocompromised mice. To the extent that these “humanized” mice are representative of the endogenous human immune system, they will be invaluable tools for both basic science and translational medicine.

Human hemato-lymphoid system mice: current use and future potential for medicine

To directly study complex human hemato-lymphoid system physiology and respective system-associated diseases in vivo, human-to-mouse xenotransplantation models for human blood and blood-forming cells and organs have been developed over the past three decades. We here review the fundamental requirements and the remarkable progress made over the past few years in improving these systems, the current major achievements reached by use of these models, and the future challenges to more closely model and study human health and disease and to achieve predictive preclinical testing of both prevention measures and potential new therapies.

Wild-type measles virus interferes with short-term engraftment of human CD34+ hematopoietic progenitor cells

Transient lymphopenia is a hallmark of measles virus (MV)-induced immunosuppression. To address to what extent replenishment of the peripheral lymphocyte compartment from bone marrow (BM) progenitor/stem cells might be affected, we analyzed the interaction of wild-type MV with hematopoietic stem and progenitor cells (HS/PCs) and stroma cells in vitro. Infection of human CD34(+) HS/PCs or stroma cells with wild-type MV is highly inefficient yet noncytolytic. It occurs independently of CD150 in stroma cells but also in HS/PCs, where infection is established in CD34(+) CD150(-) and CD34(+) CD150(+) (in humans representing HS/PC oligopotent precursors) subsets. Stroma cells and HS/PCs can mutually transmit MV and may thereby create a possible niche for continuous viral exchange in the BM. Infected lymphocytes homing to this compartment may serve as sources for HS/PC or stroma cell infection, as reflected by highly efficient transmission of MV from both populations in cocultures with MV-infected B or T cells. Though MV exposure does not detectably affect the viability, expansion, and colony-forming activity of either CD150(+) or CD150(-) HS/PCs in vitro, it efficiently interferes with short- but not long-term hematopoietic reconstitution in NOD/SCID mice. Altogether, these findings support the hypothesis that MV accession of the BM compartment by infected lymphocytes may contribute to peripheral blood mononuclear cell lymphopenia at the level of BM suppression.

Leukemia-initiating cells in human T-lymphoblastic leukemia exhibit glucocorticoid resistance

T-cell acute lymphoblastic leukemia (T-ALL) is associated with a significant risk of disease relapse, but the biological basis for relapse is poorly understood. Here, we identify leukemiainitiating cells (L-ICs) on the basis of functional assays and prospective isolation and report a role for L-ICs in T-ALL disease and relapse. Long-term proliferation in response to NOTCH1 activating signals in OP9-DL1 coculture system or capacity to initiate leukemia in xenografts by the CD7(+)CD1a(-) subset of primary T-ALL samples was superior to other subsets, refining the identity of T-ALL L-ICs. T-ALL engraftment was improved in nonobese diabetic/severe combined immunodeficiency (NOD/scid)IL2Rγ(null) (NSG) mice compared with NOD/scid with anti-CD122 treatment (NS122), but both showed changes in leukemia immunophenotype. Clonal analysis of xenografts using the TCRG locus revealed the presence of subclones of T-ALL L-ICs, some of which possess a selective growth advantage and correlated with the capacity of CD7(+)CD1a(+) xenograft cells to engraft secondary NSG mice. Treatment of high-risk T-ALL xenografts eliminated CD1a(+) T-ALL cells, but CD1a(-) cells were resistant and their number was increased. Our results establish that primary CD1a(-) T-ALL cells are functionally distinct from CD1a(+) cells and that the CD7(+)CD1a(-) subset is enriched for L-IC activity that may be involved in mediating disease relapse after therapy.

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.

In vivo modulation of gene expression by lentiviral transduction in "human immune system" Rag2-/- gamma c -/- mice

Over the last two decades, several humanized mouse models have been used to experimentally analyze the function and development of the human immune system. Recent advances have lead to the establishment of new murine-human chimeric models with improved characteristics, both in terms of human engraftment efficiency and in situ multilineage human hematopoietic development. We describe here the use of newborn BALB/c Rag2(-/-)gamma(c) (-/-) mice as recipients of human hematopoietic progenitor cells to produce "human immune system" (HIS) (BALB-Rag/gamma) mice, using human fetal liver progenitors. The two major subsets of the human dendritic cell lineage, namely, BDCA2(+)CD11c(-) plasmacytoid dendritic cells and BDCA2(-)CD11c(+) conventional dendritic cells, can be found in HIS (BALB-Rag/gamma) mice. In order to manipulate the expression of genes of interest, the human hematopoietic progenitor cells can be genetically engineered ex vivo by lentiviral transduction before performing xenograft transplantation. Using this mouse model, the human immune system can be assessed for both fundamental and pre-clinical purposes.

Effect of ex vivo culture of CD34+ bone marrow cells on immune reconstitution of XSCID dogs following allogeneic bone marrow transplantation

Successful genetic treatment of most primary immunodeficiencies or hematological disorders will require the transduction of pluripotent, self-renewing hematopoietic stem cells (HSC) rather than their progeny to achieve enduring production of genetically corrected cells and durable immune reconstitution. Current ex vivo transduction protocols require manipulation of HSC by culture in cytokines for various lengths of time depending upon the retroviral vector that may force HSC to enter pathways of proliferation, and possibly differentiation, which could limit their engraftment potential, pluripotentiality and long-term repopulating capacity. We have compared the ability of normal CD34(+) cells cultured in a standard cytokine cocktail for 18hours or 4.5 days to reconstitute XSCID dogs following bone marrow transplantation in the absence of any pretransplant conditioning with that of freshly isolated CD34(+) cells. CD34(+) cells cultured under standard gamma-retroviral transduction conditions (4.5 days) showed decreased engraftment potential and ability to sustain long-term thymopoiesis. In contrast, XSCID dogs transplanted with CD34(+) cells cultured for 18hours showed a robust T cell immune reconstitution similar to dogs transplanted with freshly isolated CD34(+) cells, however, the ability to sustain long-term thymopoiesis was impaired. These results emphasize the need to determine ex vivo culture conditions that maintain both the engraftment potential and "stem cell" potential of the cultured cells.

Mucosal innate and adaptive immune responses against herpes simplex virus type 2 in a humanized mouse model

Genital herpes, caused by herpes simplex virus type 2 (HSV-2), is one of the most prevalent sexually transmitted diseases worldwide and a risk factor for acquiring human immunodeficiency virus. Although many vaccine candidates have shown promising results in animal models, they have failed to be effective in human trials. In this study, a humanized mouse strain was evaluated as a potential preclinical model for studying human immune responses to HSV-2 infection and vaccination. Immunodeficient mouse strains were examined for their abilities to develop human innate and adaptive immune cells after transplantation of human umbilical cord stem cells. A RAG2(-/-) gammac(-/-) mouse strain with a BALB/c background was chosen as the most appropriate model and was then examined for its ability to mount innate and adaptive immune responses to intravaginal HSV-2 infection and immunization. After primary infection, human cells in the lymph nodes were able to generate a protective innate immune response and produce gamma interferon (IFN-gamma). After intravaginal immunization and infection, human T cells and NK cells were found in the genital tract and iliac lymph nodes. In addition, human T cells in the spleen, lymph nodes, and vaginal tract were able to respond to stimulation with HSV-2 antigens by replicating and producing IFN-gamma. Human B cells were also able to produce HSV-2-specific immunoglobulin G. These adaptive responses were also shown to be protective and reduce local viral replication in the genital tract. This approach provides a means for studying human immune responses in vivo using a small-animal model and may become an important preclinical tool.

Stem cell concepts renew cancer research

Although uncontrolled proliferation is a distinguishing property of a tumor as a whole, the individual cells that make up the tumor exhibit considerable variation in many properties, including morphology, proliferation kinetics, and the ability to initiate tumor growth in transplant assays. Understanding the molecular and cellular basis of this heterogeneity has important implications in the design of therapeutic strategies. The mechanistic basis of tumor heterogeneity has been uncertain; however, there is now strong evidence that cancer is a cellular hierarchy with cancer stem cells at the apex. This review provides a historical overview of the influence of hematology on the development of stem cell concepts and their linkage to cancer.

Human intrathymic development: a selective approach

Human T lymphocytes can be generated from CD34 progenitor cells from different sources. This can be obtained in an in vivo model wherein human thymic tissue and fetal liver is transplanted in an immunodeficient mouse. However, human T cells are also generated in immunodeficient mice without co-transplantation of human thymus or in in vitro hybrid human-mouse fetal thymus organ culture. This shows that xenogeneic mouse thymus tissue supports human T cell differentiation. Finally, human T cells are generated on co-culture with murine stromal cells that express the Delta-like1 ligand for the Notch receptor. How these different environments influence the human T cell repertoire is reviewed and discussed.

Human BLyS facilitates engraftment of human PBL derived B cells in immunodeficient mice

The production of fully immunologically competent humanized mice engrafted with peripheral lymphocyte populations provides a model for in vivo testing of new vaccines, the durability of immunological memory and cancer therapies. This approach is limited, however, by the failure to efficiently engraft human B lymphocytes in immunodeficient mice. We hypothesized that this deficiency was due to the failure of the murine microenvironment to support human B cell survival. We report that while the human B lymphocyte survival factor, B lymphocyte stimulator (BLyS/BAFF) enhances the survival of human B cells ex vivo, murine BLyS has no such protective effect. Although human B cells bound both human and murine BLyS, nuclear accumulation of NF-κB p52, an indication of the induction of a protective anti-apoptotic response, following stimulation with human BLyS was more robust than that induced with murine BLyS suggesting a fundamental disparity in BLyS receptor signaling. Efficient engraftment of both human B and T lymphocytes in NOD rag1^−/− Prf1^−/− immunodeficient mice treated with recombinant human BLyS is observed after adoptive transfer of human PBL relative to PBS treated controls. Human BLyS treated recipients had on average 40-fold higher levels of serum Ig than controls and mounted a de novo antibody response to the thymus-independent antigens in pneumovax vaccine. The data indicate that production of fully immunologically competent humanized mice from PBL can be markedly facilitated by providing human BLyS.

Mouse models with human immunity and their application in biomedical research

Biomedical research in human beings is largely restricted to in vitro studies that lack complexity of a living organism. To overcome this limitation, humanized mouse models are developed based on immunodeficient characteristics of severe combined immunodeficiency (SCID) or recombination activating gene (Rag)(null) mice, which can accept xenografts. Peripheral constitution of human immunity in SCID or Rag(null) mice has been achieved by transplantation of mature human immune cells, foetal human thymus, bone marrow, liver tissues, lymph nodes or a combination of these, although efficiency needs to be improved. These mouse models with constituted human immunity (defined as humanized mice in the present text) have been widely used to investigate the basic principles of human immunobiology as well as complex pathomechanisms and potential therapies of human diseases. Here, elements of an ideal humanized mouse model are highlighted including genetic and non-genetic modification of recipient mice, transplantation strategies and proposals to improve engraftments. The applications of the humanized mice to study the development and response of human immune cells, human autoimmune diseases, virus infections, transplantation biology and tumour biology are reviewed as well.

Experimental model for the study of the human immune system: production and monitoring of "human immune system" Rag2-/-gamma c-/- mice

Since the late 1980s, the study of the function and development of the human immune system has made intensive use of humanized animal models, among which mouse models have been proven extremely efficient and handy. Recent advances have lead to the establishment of new models with improved characteristics, both in terms of engraftment efficiency and in situ multilineage human hematopoietic development. In particular, the use of newborn BALB/c Rag2-/-gamma c-/- mice as recipients for human hematopoietic stem cells has proven particularly efficient. We describe here how to produce and monitor such "human immune system" (HIS) (BALB-Rag/gamma) mice, which offer large prospects for experimental study of the human immune system and as a preclinical screening tool.

Rapidly induced, T-cell–independent xenoantibody production is mediated by marginal zone B cells and requires help from NK cells

Xenoantibody production directed at a wide variety of T lymphocyte–dependent and T lymphocyte–independent xenoantigens remains the major immunologic obstacle for successful xenotransplantation. The B lymphocyte subpopulations and their helper factors, involved in T-cell–independent xenoantibody production are only partially understood, and their identification will contribute to the clinical applicability of xenotransplantation. Here we show, using models involving T-cell–deficient athymic recipient mice, that rapidly induced, T-cell–independent xenoantibody production is mediated by marginal zone B lymphocytes and requires help from natural killer (NK) cells. This collaboration neither required NK-cell–mediated IFN-γ production, nor NK-cell–mediated cytolytic killing of xenogeneic target cells. The T-cell–independent IgM xenoantibody response could be partially suppressed by CD40L blockade.

Restoration of human B-cell differentiation into NOD-SCID mice engrafted with gene-corrected CD34+ cells isolated from Artemis or RAG1-deficient patients

Severe combined immunodeficiency (SCID) caused by mutation of the recombination-activating gene 1 (RAG1) or Artemis gene lead to the absence of B- and T-cell differentiation. The only curative treatment is allogeneic bone marrow (BM) transplantation, which displays a high survival rate when an HLA compatible donor is available but has a poorer prognosis when the donor is partially compatible. Consequently, gene therapy may be a promising alternative strategy for these diseases. Here, we report that lentiviral gene-corrected BM CD34(+) cells (isolated from Artemis- or RAG1-deficient patients) sustain human B-cell differentiation following injection into non-obese diabetic/SCID (NOD-SCID) mice previously infused with anti-interleukin-2 receptor beta chain monoclonal antibody. In most of the mice BM, engrafted with Artemis-transduced cells, human B-cell differentiation occurred until the mature stage. The B cells were functional as human immunoglobulin M (IgM) was present in the serum. Following injection with RAG1-transduced cells, human engraftment occurred in vivo but B-cell differentiation until the mature stage was less frequent. However, when it occurred, it was always associated with human IgM production. This overall approach represents a useful tool for evaluating gene transfer efficiency in human SCID forms affecting B-cell development (such as Artemis deficiency) and for testing new vectors for improving in vivo RAG1 complementation.

A Humanized Mouse Model of Idiopathic Nephrotic Syndrome Suggests a Pathogenic Role for Immature Cells

Idiopathic nephrotic syndrome is characterized by glomerular proteinuria in the absence of infiltrating cells or immunoglobulin deposits. Although it is suspected that T cells secrete a circulating factor that leads to proteinuria by altering the permeability of the glomerular filtration barrier, the precise etiology of this syndrome is unknown. Because an animal model that mimics human idiopathic nephrotic syndrome does not exist, we developed a humanized mouse model of the disease by injecting CD34(+) stem cells or CD34(-) peripheral blood mononuclear cells from afflicted patients into immunocompromised mice. Even though both CD34(+) and CD34(-) cells induced the engraftment of human CD45(+) leukocytes in mice, only the injection of CD34(+) stem cells induced albuminuria. Ultrastructural analysis of glomeruli from the resulting proteinuric mice revealed effacement of podocyte foot processes, similar to the pathology observed in the human disease. Therefore, our data suggest that the cells responsible for the pathogenesis of idiopathic nephrotic syndrome are more likely to be immature differentiating cells rather than mature peripheral T cells.

Factors affecting human T cell engraftment, trafficking, and associated xenogeneic graft-vs-host disease in NOD/SCID beta2mnull mice

OBJECTIVE

Graft-vs-host disease (GVHD) is the major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation. Models of immunodeficient mice that consistently and efficiently reconstitute with xenoreactive human T cells would be a valuable tool for the in vivo study of GVHD, as well as other human immune responses.

MATERIALS AND METHODS

We developed a consistent and sensitive model of human GVHD by retro-orbitally injecting purified human T cells into sublethally irradiated nonobese diabetic/severe combined immunodeficient (NOD/SCID)-beta2m(null) recipients. In addition, we characterized for the first time the trafficking patterns and expansion profiles of xenoreactive human T cells in NOD/SCID-beta2m(null) recipients using in vivo bioluminescence imaging.

RESULTS

All NOD/SCID-beta2m(null) mice conditioned with 300 cGy total body irradiation and injected with 1 x 10(7) human T cells exhibited human T-cell engraftment, activation, and expansion, with infiltration of multiple target tissues and a subsequent >20% loss of pretransplantation body weight. Importantly, histological examination of the GVHD target tissues revealed changes consistent with human GVHD. Furthermore, we also showed by in vivo bioluminescence imaging that development of lethal GVHD in the NOD/SCID-beta2m(null) recipients was dependent upon the initial retention and early expansion of human T cells in the retro-orbital sinus cavity.

CONCLUSION

Our NOD/SCID-beta2m(null) mouse model provides a system to study the pathophysiology of acute GVHD induced by human T cells and aids in development of more effective therapies for human GVHD.

Human-hemato-lymphoid-system mice: opportunities and challenges

With the recent advances in human-hemato-lymphoid-system mice, this commentary discusses the utility of these mice and further improvements required to generate an accessible system that allows predictive in vivo human hematology and immunology research.

Human adaptive immune system Rag2-/-gamma(c)-/- mice

Although many biologic principles are conserved in mice and humans, species-specific differences exist, for example, in susceptibility and response to pathogens, that often do not allow direct implementation of findings in experimental mice to humans. Research in humans, however, for ethical and practical reasons, is largely restricted to in vitro assays that lack components and the complexity of a living organism. To nevertheless study the human hematopoietic and immune system in vivo, xenotransplantation assays have been developed that substitute human components to small animals. Here, we summarize our recent findings that transplantation of human cord blood CD34(+) cells to newborn Rag2(-/-)gamma(c)(-/-) mice leads to de novo development of major functional components of the human adaptive immune system. These human adaptive immune system Rag2(-/-)gamma(c)(-/-) (huAIS-RG) mice can now be used as a technically straightforward preclinical model to evaluate in vivo human adaptive immune system development as well as immune responses, for example, to vaccines or live infectious pathogens.

In vitro expanded cells contributing to rapid severe combined immunodeficient repopulation activity are CD34+38-33+90+45RA-

Expansion of hematopoietic stem cells could be used clinically to shorten the prolonged aplastic phase after umbilical cord blood (UCB) transplantation. In this report, we investigated rapid severe combined immunodeficient (SCID) repopulating activity (rSRA) 2 weeks after transplantation of CD34(+) UCB cells cultured with serum on MS5 stromal cells and in serum- and stroma-free cultures. Various subpopulations obtained after culture were studied for rSRA. CD34(+) expansion cultures resulted in vast expansion of CD45(+) and CD34(+) cells. Independent of the culture method, only the CD34(+)33(+)38(-) fraction of the cultured cells contained rSRA. Subsequently, we subfractionated the CD34(+)38(-) fraction using stem cell markers CD45RA and CD90. In vitro differentiation cultures showed CD34(+) expansion in both CD45RA(-) and CD90(+) cultures, whereas little increase in CD34(+) cells was observed in both CD45RA(+) and CD90(-) cultures. By four-color flow cytometry, we could demonstrate that CD34(+)38(-)45RA(-) and CD34(+)38(-)90(+) cell populations were largely overlapping. Both populations were able to reconstitute SCID/nonobese diabetic mice at 2 weeks, indicating that these cells contained rSRA activity. In contrast, CD34(+)38(-)45RA(+) or CD34(+)38(-)90(-) cells contributed only marginally to rSRA. Similar results were obtained when cells were injected intrafemorally, suggesting that the lack of reconstitution was not due to homing defects. In conclusion, we show that after in vitro expansion, rSRA is mediated by CD34(+)38(-)90(+)45RA(-) cells. All other cell fractions have limited reconstitutive potential, mainly because the cells have lost stem cell activity rather than because of homing defects. These findings can be used clinically to assess the rSRA of cultured stem cells.

Ex Vivo Expansion Does Not Alter the Capacity of Umbilical Cord Blood CD34+Cells to Generate Functional T Lymphocytes and Dendritic Cells

We examined whether ex vivo expansion of umbilical cord blood progenitor cells affected their capacity to generate immune cells such as T lymphocytes (TLs) and dendritic cells (DCs). The capacity to generate TLs from cord blood CD34(+) cells expanded for 14 days (d14) was compared with that of nonexpanded CD34(+) cells (d0) using fetal thymus organ cultures or transfer into nonobese diabetic/severe combined immunodeficient mice. The cell preparations yielded comparable percentages of immature (CD4(+)CD8(-), CD4(+)CD8(+)) TLs and functional mature (CD3(+)CD4(+), CD3(+)CD8(+)) TLs with an analogous TCR (T-cell receptor)-Vbeta repertoire pattern. As regards DCs, d0 and d14 CD34(+) cells also yielded similar percentages of CD1a(+) DCs with the same expression levels of HLA-DR, costimulatory and adhesion molecules, and chemokine receptors. DCs derived from either d14 or d0 CD34(+) stimulated allogeneic TLs to the same extent, and the cytokine pattern production of these allogeneic TLs was similar with no shift toward a predominant Th1 or Th2 response. Even though the intrinsic capacity of d14 CD34(+) cells to generate DCs was 13-fold lower than that of d0 CD34(+) cells, this reduction was offset by the prior amplification of the CD34(+) cells, resulting in the overall production of 15-fold more DCs. These data indicate that ex vivo expansion of CD34(+) cells does not impair T lymphopoiesis nor DC differentiation capacity.

CD52 is a novel costimulatory molecule for induction of CD4+ regulatory T cells

We previously reported that 4C8 monoclonal antibody (mAb) provides a costimulatory signal to human CD4+ T cells and consequently induces regulatory T (Treg) cells, which are hypo-responsive and suppress the polyclonal response of bystander CD4+ cells in a contact-dependent manner. In this study, we identified the antigen of 4C8 mAb as CD52. Costimulation with Campath-1H, a humanized anti-CD52 mAb, also induced Treg cells. Anti-CD52-induced Treg cells suppressed the proliferation of both CD4+ and CD8+ T cells provided with polyclonal or allogeneic stimulation. When Treg cells were induced from Staphylococcal enterotoxin B (SEB) treated cells, they suppressed the response to SEB more efficiently than that to another superantigen, SEA. Furthermore, anti-CD52-induced Treg cells could be expanded by culture with IL-2 followed by CD52-costimulation, and co-injection of expanded Treg cells suppressed lethal xenogeneic graft versus host disease (GvHD) reactions in SCID mice caused by human peripheral blood mononuclear cells (PBMCs).

Experimental Models to Study Development and Function of the Human Immune System In Vivo

The study of development and function of the immune system in vivo has made intensive use of animal models, but performing such work in humans is difficult for experimental, practical, and ethical reasons. Confronted with this scientific challenge, several pioneering groups have developed in the late 1980s mouse models of human immune system development. Although these experimental approaches were proven successful and useful, they were suffering from limitations due to xenograft transplantation barriers. By reviewing the characteristics of the successive models over the last 20 years, it becomes apparent that screening of potentially interesting mouse strains and usage of combinations of genetic deficiencies has led to major advances. This is particularly true for human T cell development in the murine thymus. This review will focus on these advances and the potential future improvements that remain to be accomplished.

Differentiation and characterization of T cells

This paper outlines current standard methods of inducing T lymphopoiesis in vitro and in vivo. Reference is made to both murine and human systems. In addition to differentiation assays, methods to optimally characterize output cells are discussed. In bringing together a number of existing protocols, many techniques important in investigating T cell development can be reviewed in one place.

SCL/TAL1 expression level regulates human hematopoietic stem cell self-renewal and engraftment

The fate of hematopoietic stem cells (HSCs) is regulated through a combinatorial action of proteins that determine their self-renewal and/or their commitment to differentiation. Stem cell leukemia/T-cell acute lymphoblastic leukemia 1 (SCL/TAL1), a basic helix-loop-helix (bHLH) transcription factor, plays key roles in controlling the development of primitive and definitive hematopoiesis during mouse development but its function in adult HSCs is still a matter of debate. We report here that the lentiviral-mediated enforced expression of TAL1 in human CD34+ cells marginally affects in vitro the differentiation of committed progenitors, whereas in vivo the repopulation capacity of the long-term SCID (severe combined immunodeficient) mouse–repopulating cells (LT-SRCs) is enhanced. As a consequence, the production of SRC-derived multipotent progenitors as well as erythroid- and myeloid-differentiated cells is increased. Looking at the lymphoid compartment, constitutive TAL1-enforced expression impairs B- but not T-cell differentiation. Expression of a mutant TAL1 protein that cannot bind DNA specifically impairs human LT-SRC amplification, indicating a DNA-binding dependent effect of TAL1 on primitive cell populations. These results indicate that TAL1 expression level regulates immature human hematopoietic cell self-renewal and that this regulation requires TAL1 DNA-binding activity.

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.

Development and Activation of Human Dendritic Cells In Vivo in a Xenograft Model of Human Hematopoiesis

Dendritic cells (DCs) are derived from CD34+ progenitors and play a central role in the development of immune responses and in tolerance. Their therapeutic potential underscores the need for in vivo models that accurately recapitulate human DC development and function to provide a better understanding of DC biology in health and disease. Using nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice transplanted with human CD34+ cells as a model of human hematopoiesis, we examined DC ontogeny. Progenitors of both myeloid (m) and plasmacytoid (p) DCs were identified in the bone marrow of mice up to 24 weeks after transplant, indicating ongoing and sustained production of DCs after initial engraftment. To determine whether human DCs derived from transplanted stem cells were functional, their response to acute inflammation using lipopolysaccharide (LPS) was examined. Eighteen hours after LPS administration, a dramatic increase in the plasma levels of the human inflammatory cytokines interleukin (IL)-8, IL-10, tumor necrosis factor-alpha, and IL-12p70 was observed. Only mDCs and not pDCs responded in vivo to LPS by upregulating CD86 and CD83. In vivo activation of human mDCs resulted in a substantial increase in the ability of mDCs to induce the proliferation of naive human T cells. Taken together, these data indicate that human CD34+ cells seem to have differentiated appropriately within the NOD/SCID microenvironment into DCs that are developmentally, phenotypically, and functionally similar to the DC subsets found in humans.

Restriction of transgene expression to the B-lymphoid progeny of human lentivirally transduced CD34+ cells

Development of gene transfer strategies will necessitate improved efficiency and control of transduction and transgene expression. We here provide evidence that targeting expression of the GFP reporter gene to the B-lymphoid progeny of genetically modified human hematopoietic progenitor cells can be achieved through the insertion of regulatory sequences from the human CD19 gene promoter into a lentiviral vector. Based on a bioinformatics approach, three human CD19-derived sequences were designed and inserted into a self-inactivated lentiviral vector backbone upstream of the GFP gene: S.CD19 (230 bp), M.CD19 (464 bp), and L.CD19 (1274 bp). These new lentiviral vectors efficiently transduced cord blood CD34(+) cells. The M.CD19 and especially L.CD19 sequences preferentially targeted GFP expression to in vitro and in vivo differentiated CD19(+) progeny; moreover, transgene expression was detected from the CD34(+) pro/pre-B cell to the mature peripheral IgM(+) B cell stage. In contrast, GFP expression was weak or absent in primary T-lymphoid and uncommitted progenitor cells or in erythroid, natural killer, or myeloid differentiated cells. Such B-lineage-specific lentiviral vectors may be useful for correcting inherited disorders that affect B-lymphoid cells or for deciphering the transcriptional program that controls B cell commitment and differentiation.

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.

In vivo selection for human and murine hematopoietic cells transduced with a therapeutic MGMT lentiviral vector that inhibits HIV replication

We have developed an HIV-based lentiviral vector, VRX496, which efficiently transduces human CD34+ progenitors and CD4+ T lymphocytes. VRX496 contains an antisense sequence against the HIV envelope and is currently being evaluated for safety in a clinical trial for treatment of HIV. Selective outgrowth of transduced hematopoietic cells in vivo is anticipated to increase the therapeutic efficacy of this treatment by maximizing the persistence of virus-resistant cells in the body. Although HIV resistance is selective, additional selection may aid in treatment efficacy due to the vast quantity of target cells. Therefore, we engineered VRX496 to express the P140K MGMT gene to drive potent drug-mediated in vivo selection for transduced hematopoietic long-term repopulating cells. Suboptimally transduced T cell cultures treated with O6-benzylguanine and BCNU were selected from 3 to 100%, and after selection cultures did not support HIV replication. Primary CD34+ progenitors derived from G-CSF-mobilized peripheral blood were transduced at 27 to 35% efficiency. Approximate sixfold selection was observed for transduced CD34+ progenitors, colony-forming units, and long-term culture-initiating cells. Multilineage in vivo selection was demonstrated for transduced murine hematopoietic cells in human CD34(+)-derived hematopoietic cells in NOD-SCID mice. These results establish efficient ex vivo and in vivo selection for hematopoietic cells transduced with lentiviral vectors and support the potential therapeutic benefit of this strategy in human gene therapy.

IL-7 effect on immunological reconstitution after HSCT depends on MHC incompatibility

Considerable progress has been recently accomplished in the management of patients who have undergone haplo-incompatible haematopoietic stem cell transplantation (HSCT) in terms of intake and prevention of graft-versus-host disease. Nevertheless haplo-incompatible HSCT is a procedure limited to a small number of patients because of the long-lasting immunodeficiency that is responsible for more than 50% of deaths within the first 3 months. Interleukin (IL)-7, which plays a unique and key role in T-cell development both in the mouse and in the human, is particularly attractive for attempting to speed up T-cell reconstitution. However, controversial results have been obtained after bone marrow graft in murine and primate models. To elucidate the impact of IL-7 treatment, we have performed HSCT in irradiated murine recombination activating gene (RAG) immunodeficient recipients, using donors that exhibited increased major histocompatibilty complex (MHC) incompatibility. Although irradiation performed prior to HSCT lead to a profound defect in the thymic stromal cells responsible for IL-7 production, IL-7 treatment had no significant effect on immune reconstitution in the MHC compatible and partially compatible settings. Interestingly, in the MHC fully incompatible setting in which only one-third of the recipients demonstrated active thymopoiesis, probably because of the rejection of donor cells by host natural killer cells, IL-7 treatment had a beneficial effect on T-cell development.

ABC transporter inhibitors that are substrates enhance lentiviral vector transduction into primitive hematopoietic progenitor cells

High gene transfer efficiencies have been difficult to achieve in hematopoietic progenitor cells (HPCs) but are important to therapeutic success of HPC gene therapy. Efficient gene transfer is especially challenging with use of column-purified vector for clinical application, as opposed to centrifuged vector commonly used for research. We investigated novel approaches to increase transduction by using a clinically applicable protocol and quantities of column-purified lentiviral vector. Recognizing the association of adenosine 5′-triphosphate (ATP)-binding cassette (ABC) transporters with HPC biology, we investigated the effect of transporter inhibitors on transduction. We found the ABC transporter inhibitor verapamil improved transduction efficiency 2- to 6-fold into CD34+ cells isolated from mobilized peripheral blood, bone marrow, and cord blood. Verapamil also improved transduction in human SCID (severe combined immunodeficient) repopulating cell (SRC) transduction 3- to 4-fold, resulting in 80% to 90% transduction levels in mice receiving primary and secondary transplants without alterations in multilineage reconstitution. Additional ABC transporter substrate inhibitors like quinidine, diltiazem, and ritonavir also enhanced transduction 2- to 3-fold, although ABC transporter inhibitors that are not substrates did not. Enhanced transduction was not observed in mature hematopoietic cells, neurospheres, mesenchymal stem cells, or hepatocytes. Enhancement of transduction in HPCs was observed with vesicular stomatitis virus-G (VSV-G)-pseudotyped lentiviral vector but not with vector pseudotyped with RD114. Thus, we present a new approach for efficient delivery to primitive HPCs by VSV-G-pseudotyped lentiviral vectors. (Blood. 2004;104:364-373)

Development of a human adaptive immune system in cord blood cell-transplanted mice

Because ethical restrictions limit in vivo studies of the human hemato-lymphoid system, substitute human to small animal xenotransplantation models have been employed. Existing models, however, sustain only limited development and maintenance of human lymphoid cells and rarely produce immune responses. Here we show that intrahepatic injection of CD34+ human cord blood cells into conditioned newborn Rag2-/-gammac-/- mice leads to de novo development of B, T, and dendritic cells; formation of structured primary and secondary lymphoid organs; and production of functional immune responses. This provides a valuable model to study development and function of the human adaptive immune system in vivo.

Cotransplantation of third-party mesenchymal stromal cells can alleviate single-donor predominance and increase engraftment from double cord transplantation

Although the infusion of umbilical cord blood (UCB) from multiple donors can be a strategy to overcome the cell dose limitation frequently encountered in UCB transplantation, clinical trials have revealed that cells from one donor dominate engraftment. To investigate the origin of and the factors influencing this inequality, we performed mixed transplantation of 2 UCB units with varying degrees of HLA disparities into NOD/SCID mice and determined donor origins by polymerase chain reaction-sequence-specific oligonucleotide probe (PCR-SSOP) or real-time quantitative (RQ)-PCR for human short tandem repeats (STRs). When total mononuclear cells from 2 units were transplanted as a mixture, cells from one donor predominated (ratio, 81:19), despite comparable overall engraftment when infused as single units, and no augmentation in overall engraftment was observed when compared with the single-unit controls. However, lineage depletion or cotransplantation of mesenchymal stromal cells (MSCs) expanded from third-party bone marrow resulted in more balanced coengraftment. Direct comparison of double UCB transplantation in the presence or absence of MSCs showed that the reduced deviation in the donor ratio (1.8:1 vs. 2.8:1) correlated with a higher overall level of engraftment with MSC cotransplantation. These results indicate that third-party MSCs can be used to alleviate donor deviation and to facilitate engraftment of multidonor UCB.

A new xenograft model for graft-versus-host disease by intravenous transfer of human peripheral blood mononuclear cells in RAG2-/- gammac-/- double-mutant mice

The safe application of new strategies for the treatment of graft-versus-host disease (GVHD) is hampered by the lack of a clinically relevant model for preclinical testing. Current models are based on intraperitoneal transfer of human peripheral blood mononuclear cells (huPBMCs) into NOD-SCID (nonobese diabetic-severe combined immunodeficient)/SCID mice. Intravenous transfer would be preferred but this has always been ineffective. We developed a new model for xenogeneic GVHD (X-GVHD) by intravenous transfer of huPBMCs into RAG2-/- gammac-/-mice. Our results show a high human T-cell chimerism of more than 20% (up to 98%) in more than 90% of mice, associated with a consistent development of XGVHD within 14 to 28 days and a total mortality rate of 85% shorter than 2 months. After murine macrophage depletion, engraftment was earlier and equally high with lower doses of huPBMCs. Human macrophages were also absent in these mice. Purified huCD3+ cells showed a similar X-GVH effect with contribution of both CD4 and CD8 phenotypes. Human immunoglobulins and cytokines were produced in diseased mice. One of 30 mice developed chronic X-GVHD with skin histology similar to human GVHD. In conclusion, we present a new model for X-GVHD by intravenous transfer of huPBMCs in RAG2-/- gammac-/- mice. Murine and human macrophages do not seem to be necessary for acute X-GVHD in this model.

Human NK cell development in NOD/SCID mice receiving grafts of cord blood CD34+ cells

Definition of the cytokine environment, which regulates the maturation of human natural killer (NK) cells, has been largely based on in vitro assays because of the lack of suitable animal models. Here we describe conditions leading to the development of human NK cells in NOD/SCID mice receiving grafts of hematopoietic CD34+ precursor cells from cord blood. After 1-week-long in vivo treatment with various combinations of interleukin (IL)-15, flt3 ligand, stem cell factor, IL-2, IL-12, and megakaryocyte growth and differentiation factor, CD56+CD3- cells were detected in bone marrow (BM), spleen, and peripheral blood (PB), comprising 5% to 15% of human CD45+ cells. Human NK cells of NOD/SCID mouse origin closely resembled NK cells from human PB with respect to phenotypic characteristics, interferon (IFN)-gamma production, and cytotoxicity against HLA class 1-deficient K562 targets in vitro and antitumor activity against K562 erythroleukemia in vivo. In the absence of growth factor treatment, CD56+ cells were present only at background levels, but CD34+CD7+ and CD34-CD7+ lymphoid precursors with NK cell differentiation potential were detected in BM and spleen of chimeric NOD/SCID mice for up to 5 months after transplantation. Our results demonstrate that limitations in human NK cell development in the murine microenvironment can be overcome by treatment with NK cell growth-promoting human cytokines, resulting in the maturation of IFN-gamma-producing cytotoxic NK cells. These studies establish conditions to explore human NK cell development and function in vivo in the NOD/SCID mouse model.

Albumin-expressing hepatocyte-like cells develop in the livers of immune-deficient mice that received transplants of highly purified human hematopoietic stem cells

Rodent bone marrow cells can contribute to liver. If these findings are applicable to humans, marrow stem cells could theoretically be harvested from a patient and used to repair his/her damaged liver. To explore this potential, CD34(+) or highly purified CD34(+)CD38(-)CD7(-) human hematopoietic stem cells from umbilical cord blood and bone marrow were transplanted into immunodeficient mice. One month after transplantation, carbon tetrachloride (CCl(4)) was administered into the mice to induce liver damage and hepatocyte proliferation. Mice were analyzed in comparison with CCl(4)-injured mice that did not receive transplants and noninjured controls that received transplants with the same stem cell populations, one month after liver damage. Human-specific albumin mRNA and protein were expressed in the mouse liver and human albumin was detected in the serum of mice that had received CCl(4) injury. Human alpha-fetoprotein was never expressed, but in some mice, human cytokeratin 19 was expressed, which may indicate bile duct development in addition to the albumin-secreting hepatocyte-like cells. Human albumin was not expressed in the starting stem cell populations in injured mice that did not receive transplants nor in noninjured mice that had received transplants of human stem cells. Human albumin expression was detected only in CCl(4)-treated mice that received transplants of human stem cells, and recovery was increased by administration of human hepatocyte growth factor 48 hours after the CCl(4)-mediated liver injury. Our studies provide evidence that human "hematopoietic" stem/progenitor cell populations have the capacity to respond to the injured liver microenvironment by inducing albumin expression.

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.

The essential roles of the chemokine SDF-1 and its receptor CXCR4 in human stem cell homing and repopulation of transplanted immune-deficient NOD/SCID and NOD/SCID/B2m(null) mice

Hematopoietic stem cells are identified based on their functional ability to migrate via the blood circulation of transplanted recipients, to home to the host bone marrow and to durably repopulate this organ with high levels of maturing myeloid and lymphoid cells. While a small pool of undifferentiated stem cells with the potential to repeat the entire process in serially transplanted recipients is maintained within the bone marrow, maturing cells are continuously released into the circulation. In recent years pre-clinical, functional in vivo models for human stem cells have been developed, using immune-deficient mice or pre-immune, fetal sheep as recipients. The mechanism of human stem cell migration, homing and repopulation in transplanted immune-deficient NOD/SCID and NOD/SCID/B2m(null) mice as well as the accessory mediators that facilitate these processes, will be reviewed. In particular, the essential roles of the chemokine SDF-1 and its receptor CXCR4 which mediate and regulate stem cell homing and repopulation will be discussed.

Active form of Notch imposes T cell fate in human progenitor cells

The crucial role of Notch signaling in cell fate decisions in hematopoietic lineage and T lymphocyte development has been well established in mice. Overexpression of the intracellular domain of Notch mediates signal transduction of the protein. By retroviral transduction of this constitutively active truncated intracellular domain in human CD34+ umbilical cord blood progenitor cells, we were able to show that, in coculture with the stromal MS-5 cell line, depending on the cytokines added, the differentiation toward CD19+ B lymphocytes was blocked, the differentiation toward CD14+ monocytes was inhibited, and the differentiation toward CD56+ NK cells was favored. The number of CD7+cyCD3+ cells, a phenotype similar to T/NK progenitor cells, was also markedly increased. In fetal thymus organ culture, transduced CD34+ progenitor cells from umbilical cord blood cells or from thymus consistently generated more TCR-gammadelta T cells, whereas the other T cell subpopulations were largely unaffected. Interestingly, when injected in vivo in SCID-nonobese diabetic mice, the transduced cells generated ectopically human CD4+CD8+ TCR-alphabeta cells in the bone marrow, cells that are normally only present in the thymus, and lacked B cell differentiation potential. Our results show unequivocally that, in human, Notch signaling inhibits the monocyte and B cell fate, promotes the T cell fate, and alters the normal T cell differentiation pathway compatible with a pretumoral state.