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Introduction: Primary myelofibrosis (PMF) is a clonal stem cell disorder characterized by myeloid dominant hematopoiesis and dysregulated proliferation of fibroblasts in the bone marrow. However, how these aberrant myeloid cells and fibroblasts are produced remains unclear., Aim and Methods: In this study, we examined in vivo engraftment kinetics of PMF patient-derived CD34+ cells in immunecompromised NOD/SCID/IL2rgKO (NSG) mice. Engrafted human cells were analyzed with flow cytometry, and proliferation of fibroblastic cells and bone marrow fibrosis were assessed with the histo-pathological examination., Results: Transplantation of PMF patient-derived circulating CD34+ fractions into NSG newborns recapitulates clinical features of human PMF. Engraftment of human CD45+ leukocytes resulted in anemia and myeloid hyperplasia accompanied by bone marrow fibrosis by six months post-transplantation. Fibrotic bone marrow contained CD45-vimentin+ cells of both human and mouse origin, suggesting that circulating malignant CD34+ subsets contribute to myelofibrotic changes in PMF through direct and indirect mechanisms., Conclusion: A patient-derived xenotransplantation (PDX) model of PMF allows in vivo examination of disease onset and propagation originating from immature CD34+ cells and will support the investigation of pathogenesis and development of therapeutic modalities for the disorder., (© 2021. Japanese Society of Hematology.)

In human-to-mouse xenogeneic transplantation, polymorphisms of signal-regulatory protein α (SIRPA) that decide their binding affinity for human CD47 are critical for engraftment efficiency of human cells. In this study, we generated a new C57BL/6.Rag2nullIl2rgnull (BRG) mouse line with Sirpahuman/human (BRGShuman) mice, in which mouse Sirpa was replaced by human SIRPA encompassing all 8 exons. Macrophages from C57BL/6 mice harboring Sirpahuman/human had a significantly stronger affinity for human CD47 than those harboring SirpaNOD/NOD and did not show detectable phagocytosis against human hematopoietic stem cells. In turn, Sirpahuman/human macrophages had a moderate affinity for mouse CD47, and BRGShuman mice did not exhibit the blood cytopenia that was seen in Sirpa-/- mice. In human to mouse xenograft experiments, BRGShuman mice showed significantly greater engraftment and maintenance of human hematopoiesis with a high level of myeloid reconstitution, as well as improved reconstitution in peripheral tissues, compared with BRG mice harboring SirpaNOD/NOD (BRGSNOD). BRGShuman mice also showed significantly enhanced engraftment and growth of acute myeloid leukemia and subcutaneously transplanted human colon cancer cells compared with BRGSNOD mice. BRGShuman mice should be a useful basic line for establishing a more authentic xenotransplantation model to study normal and malignant human stem cells., (© 2020 by The American Society of Hematology.)

In hematopoiesis, the expression of critical genes is regulated in a stage-specific manner to maintain normal hematopoiesis. Notch1 is an essential gene involved in the commitment and development of the T-cell lineage. However, the regulation of Notch1 in hematopoiesis is controversial, particularly at the level of hematopoietic stem cell (HSC). Here, we found that the expression of Notch1 is controlled at the post-transcriptional level in HSCs. HSCs express a considerable level of Notch1 mRNA, but its protein level is very low, suggesting a post-transcriptional suppression for Notch1. Using a retroviral sensor vector expressing a fusion mRNA of GFP and 3' untranslated region (3'UTR) of a target gene, we demonstrated that the Notch1-3'UTR had a post-translational suppressive effect only at the HSC but not in the downstream progenitor stages. The sequence motif AUnA was required for this post-transcriptional regulation by the Notch1-3'UTR. Interestingly, this Notch1-3'UTR-mediated suppressive effect was relieved when HSCs were placed in the thymus, but not in the bone marrow. Thus, the expression of Notch1 in HSCs is regulated by microenvironment at the post-transcriptional level, which may control T lymphoid lineage commitment from HSCs.

Hematopoietic stem cells (HSCs) that lack HLA-class I alleles as a result of copy-number neutral loss of heterozygosity of the short arm of chromosome 6 (6pLOH) or HLA allelic mutations often constitute hematopoiesis in patients with acquired aplastic anemia (AA), but the precise mechanisms underlying clonal hematopoiesis induced by these HLA-lacking (HLA - ) HSCs remain unknown. To address this issue, we generated induced pluripotent stem cells (iPSCs) from an AA patient who possessed HLA - B4002-lacking (B4002 - ) leukocytes. Three different iPSC clones (wild-type [WT], 6pLOH + , and B *40:02 -mutant) were established from the patient's monocytes. Three-week cultures of the iPSCs in the presence of various growth factors produced hematopoietic cells that make up 50% to 70% of the CD34 + cells of each phenotype. When 10 6 iPSC-derived CD34 + (iCD34 + ) cells with the 3 different genotypes were injected into the femoral bone of C57BL/6.Rag2 mice, 2.1% to 7.3% human multilineage CD45 + cells of each HLA phenotype were detected in the bone marrow, spleen, and peripheral blood of the mice at 9 to 12 weeks after the injection, with no significant difference in the human:mouse chimerism ratio among the 3 groups. Stimulation of the patient's CD8 + T cells with the WT iCD34 + cells generated a cytotoxic T lymphocyte (CTL) line capable of killing WT iCD34 + cells but not B4002 - iCD34 + cells. These data suggest that B4002 - iCD34 + cells show a repopulating ability similar to that of WT iCD34 + cells when autologous T cells are absent and CTL precursors capable of selectively killing WT HSCs are present in the patient's peripheral blood., (© 2018 by The American Society of Hematology.)

The developmental pathway for human megakaryocytes remains unclear, and the definition of pure unipotent megakaryocyte progenitor is still controversial. Using single-cell transcriptome analysis, we have identified a cluster of cells within immature hematopoietic stem- and progenitor-cell populations that specifically expresses genes related to the megakaryocyte lineage. We used CD41 as a positive marker to identify these cells within the CD34 + CD38 + IL-3Rα dim CD45RA - common myeloid progenitor (CMP) population. These cells lacked erythroid and granulocyte-macrophage potential but exhibited robust differentiation into the megakaryocyte lineage at a high frequency, both in vivo and in vitro. The efficiency and expansion potential of these cells exceeded those of conventional bipotent megakaryocyte/erythrocyte progenitors. Accordingly, the CD41 + CMP was defined as a unipotent megakaryocyte progenitor (MegP) that is likely to represent the major pathway for human megakaryopoiesis, independent of canonical megakaryocyte-erythroid lineage bifurcation. In the bone marrow of patients with essential thrombocythemia, the MegP population was significantly expanded in the context of a high burden of Janus kinase 2 mutations. Thus, the prospectively isolatable and functionally homogeneous human MegP will be useful for the elucidation of the mechanisms underlying normal and malignant human hematopoiesis., (© 2017 by The American Society of Hematology.)

Previous studies have predicted that reciprocal activation of GATA-1 and PU.1 regulates myelo-erythroid versus myelo-lymphoid lineage commitment in early hematopoiesis. Such PU.1-activating myelo-lymphoid progenitors exist within the lymphoid-primed multipotent progenitor (LMPP) population at the primitive Lineage(-) Sca-1(+) c-Kit(+) (LSK) stage. We here show that the counterpart of GATA-1-activating myelo-erythroid progenitor resides also at the LSK stage, expressing CD41 at a high level. Purified CD41(hi) LSK cells showed exceedingly strong and prolonged myelo-erythroid-restricted reconstitution, and primed myelo-erythroid gene expression with a more primitive molecular signature as compared to the original common myeloid progenitor (CMP). The CD41(hi) LSK cells more strongly contributed to emergent and malignant myelopoiesis than LMPPs, and produced the original CMP by downregulating Sca-1 and CD41, suggesting that they are the earliest CMPs. Thus, the hematopoietic developmental map should be revised by integrating the primary branchpoint comprised of the new, isolatable CD41(hi) CMP and the LMPP populations., (© 2014 AlphaMed Press.)

To understand the mechanism in lymphoid development, it is critical to identify developmental intermediates downstream of hematopoietic stem cells, including cells responsible for seeding the thymus. Although early studies showed that hematopoietic stem cells choose either the myeloid-erythroid or the lymphoid pathway, recent data suggest that myelo-lymphoid precursors lacking erythroid potential exist in early hematopoiesis before cells are fully committed to the lymphoid lineage. We here summarize the phenotype and functional properties of such progenitors and the current developmental map for the lymphoid lineage.

To elucidate the molecular mechanisms underlying normal and malignant hematopoietic development, it is critical to identify developmental intermediates for each lineage downstream of hematopoietic stem cells. Recent advances in prospective isolation of hematopoietic stem and progenitor cells, and efficient xenogeneic transplantation systems have provided a detailed developmental map in both mouse and human hematopoiesis, demonstrating that surface phenotypes of mouse stem-progenitor cells and their human counterparts are considerably different. Here, we summarize the phenotype and functional properties and their differences of hematopoietic stem and progenitor cell populations between mouse and human.

Prospective isolation of hematopoietic stem and progenitor cells has identified the lineal relationships among all blood-cell types and has allowed their developmental mechanisms to be assayed at the single-cell level. These isolated cell populations are used to elucidate the molecular mechanism of lineage fate decision and of its plasticity directly by stage-specific enforcement or repression of lineage-instructive signaling in purified cells. With an emphasis on the myeloid lineage, this review summarizes current concepts and controversies regarding adult murine hematopoietic development and discusses the potential mechanisms, operated by single or by multiple transcription factors, of myeloid lineage fate decision.

OTT1(RBM15) was originally described as a 5' translocation partner of the MAL(MKL1) gene in t(1,22)(p13;q13) infant acute mega karyocytic leukemia. OTT1 has no established physiological function, but it shares homology with the spen/Mint/SHARP family of proteins defined by three amino-terminal RNA recognition motifs and a carboxyl-terminal SPOC (Spen paralog and ortholog carboxyl-terminal) domain believed to act as a transcriptional repressor. To define the role of OTT1 in hematopoiesis and help elucidate the mechanism of t(1,22) acute megakaryocytic leukemia pathogenesis, a conditional allele of Ott1 was generated in mice. Deletion of Ott1 in adult mice caused a loss of peripheral B cells due to a block in pro/pre-B differentiation. There is myeloid and megakaryocytic expansion in spleen and bone marrow, an increase in the Lin(-)Sca-1(+)c-Kit(+) compartment that includes hematopoietic stem cells, and a shift in progenitor fate toward granulocyte differentiation. These data show a requirement for Ott1 in B lymphopoiesis, and inhibitory roles in the myeloid, megakaryocytic, and progenitor compartments. The ability of Ott1 to affect hematopoietic cell fate and expansion in multiple lineages is a novel attribute for a spen family member and delineates Ott1 from other known effectors of hematopoietic development. It is plausible that dysregulation of Ott1-dependent hematopoietic developmental pathways, in particular those affecting the megakaryocyte lineage, may contribute to OTT1-MAL-mediated leukemogenesis.

The mechanism of lineage specification in multipotent stem cells has not been fully understood. We recently isolated progenitors with the eosinophil, basophil, or mast cell lineage potential, all of which originate from granulocyte/monocyte progenitors (GMPs). By using these prospectively purified progenitors, we show here that the expression timing of GATA-2 and CCAAT enhancer-binding protein alpha (C/EBPalpha) can differentially control their lineage commitment. The expression of GATA-2 instructed C/EBPalpha-expressing GMPs to commit exclusively into the eosinophil lineage, while it induced basophil and/or mast cell lineage commitment if C/EBPalpha was suppressed at the GMP stage. Furthermore, simply by switching the order of C/EBPalpha and GATA-2 transduction, even lymphoid-committed progenitors recaptured these developmental processes to be reprogrammed into each of these lineages. We propose that the order of expression of key transcription factors is critical for their interplay to selectively drive lineage specification programs, by which stem cells could generate multiple lineage cells in a hierarchical manner.

During 'emergency' situations such as infections, host defense requires rapid mobilization of bone marrow granulocyte progenitors. 'Steady-state' granulopoiesis is absolutely dependent on the C/EBPalpha transcription factor, but the transcriptional mechanisms underlying emergency granulopoiesis remain unclear. Here we show that large numbers of granulocytes were generated from C/EBPalpha-deficient progenitors after cytokine stimulation in vivo. Cytokine treatment or fungal infection induced upregulation of C/EBPbeta but not C/EBPalpha or C/EBPepsilon transcripts in granulocyte progenitors, and C/EBPbeta-deficient progenitors showed decreased emergency-induced granulopoiesis in vitro and in vivo. C/EBPbeta inhibited proliferation less severely than did C/EBPalpha. These data suggest a critical function for C/EBPbeta in emergency granulopoiesis, which demands both differentiation and proliferation of granulocyte precursors.

Basophils and mast cells, which are selectively endowed with the high-affinity IgE receptor and mediate a range of adaptive and innate immune responses, have an unknown developmental relationship. Here, by evaluating the expression of the beta7 integrin, a molecule that is required for selective homing of mast cell progenitors (MCPs) to the periphery, we identified bipotent progenitors that are capable of differentiating into either cell type in the mouse spleen. These basophil/mast cell progenitors (BMCPs) gave rise to basophils and mast cells at the single-cell level and reconstituted both mucosal and connective tissue mast cells. We also identified the basophil progenitor (BaP) and the MCP in the bone marrow and the gastrointestinal mucosa, respectively. We further show that the granulocyte-related transcription factor CCAAT/enhancer-binding protein alpha (C/EBPalpha) plays a primary role in the fate decision of BMCPs, being expressed in BaPs but not in MCPs. Thus, circulating basophils and tissue mast cells share a common developmental stage at which their fate decision might be controlled principally by C/EBPalpha.

Homozygous loss of function of Runx1 (Runt-related transcription factor 1 gene) during murine development results in an embryonic lethal phenotype characterized by a complete lack of definitive hematopoiesis. In light of recent reports of disparate requirements for hematopoietic transcription factors during development as opposed to adult hematopoiesis, we used a conditional gene-targeting strategy to effect the loss of Runx1 function in adult mice. In contrast with the critical role of Runx1 during development, Runx1 was not essential for hematopoiesis in the adult hematopoietic compartment, though a number of significant hematopoietic abnormalities were observed. Runx1 excision had lineage-specific effects on B- and T-cell maturation and pronounced inhibition of common lymphocyte progenitor production. Runx1 excision also resulted in inefficient platelet production. Of note, Runx1-deficient mice developed a mild myeloproliferative phenotype characterized by an increase in peripheral blood neutrophils, an increase in myeloid progenitor populations, and extramedullary hematopoiesis composed of maturing myeloid and erythroid elements. These findings indicate that Runx1 deficiency has markedly different consequences during development compared with adult hematopoiesis, and they provide insight into the phenotypic manifestations of Runx1 deficiency in hematopoietic malignancies.

Hematopoiesis has provided a valuable model for examining how genetic programs are established and executed in terms of cell fate decision. Identification of common myeloid and lymphoid progenitors allows us to directly assess the regulatory mechanisms of lineage commitment. Multiple markers of hematopoietic lineages are coexpressed in hematopoietic stem cells and progenitors, a phenomenon referred to as lineage priming. Promiscuous expression of several lineage-affiliated genes precedes lineage commitment but does not alter the biological potential of hematopoietic stem cells and multipotent progenitors. Promiscuous accessibility of multiple programs allows flexibility in cell fate commitment at the multipotent stages, indicating that transcriptional promiscuity can operate in stem cells and progenitors to control their transition from multipotency to single-lineage commitment.

Hematopoiesis has provided a valuable model for understanding how genetic programs are established to decide cell fates in multipotent stem or progenitor cells. The identification of common myeloid and lymphoid progenitors has allowed us to directly assess the regulatory mechanisms of lineage commitment. Multiple genes of hematopoietic lineages, including transcription factors, are coexpressed in hematopoietic stem cells and progenitors, a phenomenon referred to as "lineage priming." The accessibility for multiple transcription factors promiscuously allows flexibility in cell fate commitments at the multipotent stages. The changes in the expression levels and timing of transcription factors can induce lineage conversion of committed cells, indicating that the regulation of transcription factors might be primarily critical for maintaining hierarchical hematopoietic development.

Granulocyte colony-stimulating factor (G-CSF) is the principal cytokine regulating granulopoiesis. G-CSF receptor-deficient mice (G-CSFR-/-) are neutropenic but have only a modest reduction of committed myeloid progenitors. Since it is likely that compensatory mechanisms are induced by the severe neutropenia present in G-CSFR-/- mice, a competitive repopulation assay was performed. These data show that under basal conditions, G-CSF drives nearly all of granulopoiesis through multiple mechanisms. Most importantly, G-CSFR signals regulate the production and/or maintenance of committed-myeloid progenitors. Surprisingly, G-CSFR signals also play a significant role in the regulation of primitive multipotential progenitors in vivo. The contribution of G-CSFR-/- cells to the hematopoietic stem cell compartment is modestly reduced. Moreover, a marked decrease in the contribution of G-CSFR-/- cells to other progenitors in the myeloid pathway, including erythroid and megakaryocytic progenitors, is observed. In contrast, relative to the hematopoietic stem cell compartment, the contribution of G-CSFR-/- cells to the lymphoid lineages is increased. These data suggest that G-CSFR signals may play a role in directing the commitment of primitive hematopoietic progenitors to the common myeloid lineage. Thus, regulation of G-CSF levels may provide a mechanism for directing primitive hematopoietic progenitors into the common myeloid lineage in response to environmental stresses.

Hematopoietic stem cells (HSCs) maintain hematopoiesis by giving rise to all types of blood cells. Recent reports suggest that HSCs also possess the potential to generate nonhematopoietic tissues. To evaluate the underlying mechanisms in the commitment of HSCs into multitissue and multihematopoietic lineages, we performed oligonucleotide array analyses targeting for prospectively purified HSCs, multipotent progenitors (MPPs), common lymphoid progenitors (CLPs), and common myeloid progenitors (CMPs). Here we show that HSCs coexpress multiple nonhematopoietic genes as well as hematopoietic genes; MPPs coexpress myeloid and lymphoid genes; CMPs coexpress myeloerythroid, but not lymphoid genes, whereas CLPs coexpress T-, B-, and natural killer-lymphoid, but not myeloid, genes. Thus, the stepwise decrease in transcriptional accessibility for multilineage-affiliated genes may represent progressive restriction of developmental potentials in early hematopoiesis. These data support the hypothesis that stem cells possess a wide-open chromatin structure to maintain their multipotentiality, which is progressively quenched as they go down a particular pathway of differentiation.

We demonstrate here that "promiscuous" expression of myeloid or lymphoid genes precedes lineage commitment in hematopoiesis. Prospectively purified single common myeloid progenitors (CMPs) coexpress myelo-erythroid but not lymphoid genes, whereas single common lymphoid progenitors (CLPs) coexpress T and B lymphoid but not myeloid genes. Genes unrelated to the adopted lineage are downregulated in bipotent and monopotent descendants of CMPs and CLPs. Promiscuous gene expression does not alter the biological potential of multipotent progenitors: CMPs with an activated endogenous M lysozyme locus yield normal proportions of myelo-erythroid colonies, and CLPs expressing the pre-T cell receptor alpha gene differentiate into normal numbers of B cells. Thus, the accessibility for multiple myeloid or lymphoid programs promiscuously may allow flexibility in fate commitments at these multipotent stages.

The CCAAT enhancer binding protein alpha (C/EBP alpha) transcription factor plays a critical role in granulocytopoiesis. Mice with a disruption of the C/EBP alpha gene demonstrate an early block in granulocytic differentiation, and disruption of C/EBP alpha function is a common theme in many types of human acute myelogenous leukemia, which is characterized by a block in myeloid development. To characterize further the nature of this block, we derived cell lines from the fetal liver of C/EBP alpha-deficient animals. These lines resembled morphologically the immature myeloid blasts observed in C/EBP alpha(-/-) fetal livers and did not express messenger RNA encoding early myeloid genes such as myeloperoxidase. Similarly, granulocytic markers such as Mac-1 and Gr-1 were not expressed; nor were erythroid and lymphoid surface antigens. Introduction of an inducible C/EBP alpha gene into the line revealed that conditional expression of C/EBP alpha induced the C/EBP family members C/EBP beta and C/EBP epsilon and subsequent granulocyte differentiation. Similar results were obtained when C/EBP alpha(-/-) cells were stimulated with the cytokines interleukin-3 and granulocyte-macrophage colony-stimulating factor, but not with all-trans retinoic acid, supporting a model of at least 2 pathways leading to the differentiation of myeloid progenitors to granulocytes and implicating induction of other C/EBP family members in granulopoiesis.

Aberrant hypermethylation of tumor suppressor genes plays an important role in the development of many tumors. Recently identified new DNA methyltransferase (DNMT) genes, DNMT3A and DNMT3B, code for de novo methyltransferases. To determine the roles of DNMT3A, DNMT3B, as well as DNMT1, in the development of leukemia, competitive polymerase chain reaction (PCR) assays were performed and the expression levels of DNMTs were measured in normal hematopoiesis, 33 cases of acute myelogenous leukemia (AML), and 17 cases of chronic myelogenous leukemia (CML). All genes were constitutively expressed, although at different levels, in T lymphocytes, monocytes, neutrophils, and normal bone marrow cells. Interestingly, DNMT3B was expressed at high levels in CD34(+) bone marrow cells but down-regulated in differentiated cells. In AML, 5.3-, 4.4-, and 11.7-fold mean increases were seen in the levels of DNMT1, 3A, and 3B, respectively, compared with the control bone marrow cells. Although CML cells in the chronic phase did not show significant changes, cells in the acute phase showed 3.2-, 4.5-, and 3.4-fold mean increases in the levels of DNMT1, 3A, and 3B, respectively. Using methylation-specific PCR, it was observed that the p15(INAK4B) gene, a cell cycle regulator, was methylated in 24 of 33 (72%) cases of AML. Furthermore, AML cells with methylated p15(INAK4B) tended to express higher levels of DNMT1 and 3B. In conclusion, DNMTs were substantially overexpressed in leukemia cells in a leukemia type- and stage-specific manner. Up-regulated DNMTs may contribute to the pathogenesis of leukemia by inducing aberrant regional hypermethylation. (Blood. 2001;97:1172-1179)

The thymus has been regarded as the major site of T cell differentiation. We find that in addition to alphabeta and gammadelta T cells, a significant number (approximately 3 x 104 per day) of B220+IgM+ mature B cells are exported from the thymus of C57BL/6 mice. Of these emigrating B cells, we estimate that at least approximately 2 x 104 per day are cells which developed intrathymically, whereas a maximum of approximately 0.8 x 104 per day are cells which circulated through the thymus from the periphery. The thymus possesses a significant number of pro-B and pre-B cells that express CD19, VpreB, lambda5, and pax-5. These B cell progenitors were found in the thymic cortex, whereas increasingly mature B cells were found in the corticomedullar and medullary regions. Other lymphoid cells, including NK cells and lymphoid dendritic cells, are not exported from the thymus at detectable levels. Thus, the thymus contributes to the formation of peripheral pools of B cells as well as of alphabeta and gammadelta T cells.

Lymphopoiesis of mature and diverse populations of T, B and NK (natural killer) cells from multipotent hematopoietic stem cells is an ideal model of tissue generation and regeneration. Identification and isolation of hematolymphoid stem and progenitor cells in several laboratories over the past several years have provided populations that can be studied biologically for lineage commitment and biochemically for receptor function, signal transduction and selective gene expression. These studies may ultimately provide candidate genes involved in lineage commitment, cell death or survival, self-renewal and migratory capacities of progenitors.

Haematopoietic stem cells give rise to progeny that progressively lose self-renewal capacity and become restricted to one lineage. The points at which haematopoietic stem cell-derived progenitors commit to each of the various lineages remain mostly unknown. We have identified a clonogenic common lymphoid progenitor that can differentiate into T, B and natural killer cells but not myeloid cells. Here we report the prospective identification, purification and characterization, using cell-surface markers and flow cytometry, of a complementary clonogenic common myeloid progenitor that gives rise to all myeloid lineages. Common myeloid progenitors give rise to either megakaryocyte/erythrocyte or granulocyte/macrophage progenitors. Purified progenitors were used to provide a first-pass expression profile of various haematopoiesis-related genes. We propose that the common lymphoid progenitor and common myeloid progenitor populations reflect the earliest branch points between the lymphoid and myeloid lineages, and that the commitment of common myeloid progenitors to either the megakaryocyte/erythrocyte or the granulocyte/macrophage lineages are mutually exclusive events.

Mice lacking functional IL-7 or IL-7R alpha genes are severely deficient in developing thymocytes, T cells, and B cells. IL-7 and IL-7 receptor functions are believed to result in lymphoid cell proliferation and cell maturation, implying signal transduction pathways directly involved in mitogenesis and elaboration of developmentally specific new gene programs. Here, we show that enforced expression of the bcl-2 gene in T-lymphoid cells (by crossing in the Emu-bcl-2 transgene) in IL-7R alpha-deficient mice results in a significant restoration of thymic positive selection and T cell numbers and function. We propose cell survival signals to be the principal function of IL-7R engagement in thymic and T cell development.