Lymphoid progenitors and primary routes to becoming cells of the immune system

The hematopoietic microenvironment: a network of niches for the development of all blood cell lineages

Blood cell formation (hematopoiesis) takes place mainly in the bone marrow, within the hematopoietic microenvironment, composed of a number of different cell types and their molecular products that together shape spatially organized and highly specialized microstructures called hematopoietic niches. From the earliest developmental stages and throughout the myeloid and lymphoid lineage differentiation pathways, hematopoietic niches play a crucial role in the preservation of cellular integrity and the regulation of proliferation and differentiation rates. Current evidence suggests that each blood cell lineage develops under specific, discrete niches that support committed progenitor and precursor cells and potentially cooperate with transcriptional programs determining the gradual lineage commitment and specification. This review aims to discuss recent advances on the cellular identity and structural organization of lymphoid, granulocytic, monocytic, megakaryocytic, and erythroid niches throughout the hematopoietic microenvironment and the mechanisms by which they interconnect and regulate viability, maintenance, maturation, and function of the developing blood cells.

Unboxing dendritic cells: Tales of multi-faceted biology and function

Often referred to as the bridge between innate and adaptive immunity, dendritic cells (DCs) are professional antigen-presenting cells (APCs) that constitute a unique, yet complex cell system. Among other APCs, DCs display the unique property of inducing protective immune responses against invading microbes, or cancer cells, while safeguarding the proper homeostatic equilibrium of the immune system and maintaining self-tolerance. Unsurprisingly, DCs play a role in many diseases such as autoimmunity, allergy, infectious disease and cancer. This makes them attractive but challenging targets for therapeutics. Since their initial discovery, research and understanding of DC biology have flourished. We now recognize the presence of multiple subsets of DCs distributed across tissues. Recent studies of phenotype and gene expression at the single cell level have identified heterogeneity even within the same DC type, supporting the idea that DCs have evolved to greatly expand the flexibility of the immune system to react appropriately to a wide range of threats. This review is meant to serve as a quick and robust guide to understand the basic divisions of DC subsets and their role in the immune system. Between mice and humans, there are some differences in how these subsets are identified and function, and we will point out specific distinctions as necessary. Throughout the text, we are using both fundamental and therapeutic lens to describe overlaps and distinctions and what this could mean for future research and therapies.

Childhood Acute Leukemias in Developing Nations: Successes and Challenges

PURPOSE OF REVIEW

Acute leukemias represent a tremendous threat to public health around the globe and the main cause of death due to disease in scholar age children from developing nations. Here, we review their current status in Mexico, as a paradigm of study, and the major challenges to control systemic diseases like childhood cancer.

RECENT FINDINGS

A unique molecular epidemiology, late/low precision diagnosis, limited access to treatment, toxicity associated with therapy, continuous exposure to environmental risk factors, and the high frequency of early relapses are some of the factors cooperating to low rates of survival in low-to-medium-income countries. Deliberative dialogues and exhaustive programs have emerged as promising means of advancing evidence-informed policy, by providing a structured forum for key stakeholders to integrate scientific and pragmatic knowledge about complex health concerns. A system-wide strategy based on the comprehensive leukemia identity is essential for a meaningful decline in early childhood mortality.

Ubc9 deficiency selectively impairs the functionality of common lymphoid progenitors (CLPs) during bone marrow hematopoiesis

Hematopoietic development occurs in the bone marrow, and this process begins with hematopoietic stem cells (HSCs). Ubc9 is a unique E2-conjugating enzyme required for SUMOylation, an evolutionarily conserved post-translational modification system. We herein show that a conditional Ubc9 deletion in the hematopoietic system caused decreased thymus weight and reduced lymphocyte to myeloid cell ratio. Importantly, Ubc9 deletion in the hematopoietic system only selectively impaired the development of common lymphoid progenitors (CLPs) in the bone marrow and perturbed their potential to differentiate into lymphocytes, thereby decreasing the number of T/B cells in the periphery. Ubc9 was found to be required for CLP viability, and therefore, Ubc9 deficiency rendered CLPs to undergo apoptosis and attenuated their proliferation. Thus, Ubc9 plays a critical role in the regulation of CLP function during hematopoietic development in the bone marrow.

T cell acute lymphoblastic leukemia (T-ALL): New insights into the cellular origins and infiltration mechanisms common and unique among hematologic malignancies

T-cell acute lymphoblastic leukemia (T-ALL) accounts for 15% and 25% of total childhood and adult ALL cases, respectively. During T-ALL, patients are at risk of organ infiltration by leukemic T-cells. Infiltration is a major consequence of disease relapse and correlates with poor prognosis. Transendothelial migration of leukemic cells is required to exit the blood stream into target organs. While mechanisms of normal T-cell transmigration are well known, the mechanisms of leukemic T-cell extravasation remain elusive; but involvement of chemokines, integrins and Notch signaling play critical roles. Here, we summarize current knowledge about molecular mechanisms of leukemic T-cell infiltration with special emphasis on the newly identified subtype early T-cell-progenitor (ETP)-ALL. Furthermore, we compare the extravasation potential of T-ALL cells with that of other hematologic malignancies such as B-ALL and acute myeloid leukemia (AML).

Genetic abnormalities associated with acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) occurs with high frequency in childhood and is associated with high mortality in adults. Recent technical advances in next‐generation sequencing have shed light on genetic abnormalities in hematopoietic stem/progenitor cells as the precursor to ALL pathogenesis. Based on these genetic abnormalities, ALL is now being reclassified into newly identified subtypes. Philadelphia chromosome‐like B‐lineage ALL is one of the new high‐risk subtypes characterized by genetic alterations that activate various signaling pathways, including those involving cytokine receptors, tyrosine kinases, and epigenetic modifiers. Philadelphia chromosome‐like ALL is essentially heterogeneous; however, deletion mutations in the IKZF1 gene encoding the transcription factor IKAROS underlie many cases as a key factor inducing aggressive phenotypes and poor treatment responses. Whole‐genome sequencing studies of ALL patients and ethnically matched controls also identified inherited genetic variations in lymphoid neoplasm‐related genes, which are likely to increase ALL susceptibility. These findings are directly relevant to clinical hematology, and further studies on this aspect could contribute to accurate diagnosis, effective monitoring of residual disease, and patient‐oriented therapies.

Co-inhibition of colony stimulating factor-1 receptor and BRAF oncogene in mouse models of BRAFV600E melanoma

The presence of colony stimulating factor-1 (CSF1)/CSF1 receptor (CSF1R)-driven tumor-infiltrating macrophages and myeloid-derived suppressor cells (MDSCs) is shown to promote targeted therapy resistance. In this study, we demonstrate the superior effect of a combination of CSF1R inhibitor, PLX3397 and BRAF inhibitor, PLX4720, in suppressing primary and metastatic mouse BRAF^V600E melanoma. Using flow cytometry to assess SM1WT1 melanoma-infiltrating leukocytes immediately post therapy, we found that PLX3397 reduced the recruitment of CD11b⁺ Gr1^lo and CD11b⁺ Gr1^int M2-like macrophages, but this was accompanied by an accumulation of CD11b⁺ Gr1^hi cells. PDL1 expression on remaining myeloid cells potentially dampened the antitumor efficacy of PLX3397 and PLX4720 in combination, since PD1/PDL1 axis blockade improved outcome. We also reveal a role for PLX3397 in reducing tumor-infiltrating lymphocytes, and interestingly, this feature was rescued by the co-administration of PLX4720. Our findings, from three different mouse models of BRAF-mutated melanoma, support clinical approaches that co-target BRAF oncogene and CSF1R.

The dendritic cell lineage: ontogeny and function of dendritic cells and their subsets in the steady state and the inflamed setting

Dendritic cells (DCs) form a remarkable cellular network that shapes adaptive immune responses according to peripheral cues. After four decades of research, we now know that DCs arise from a hematopoietic lineage distinct from other leukocytes, establishing the DC system as a unique hematopoietic branch. Recent work has also established that tissue DCs consist of developmentally and functionally distinct subsets that differentially regulate T lymphocyte function. This review discusses major advances in our understanding of the regulation of DC lineage commitment, differentiation, diversification, and function in situ.

Lineage switching in acute leukemias: a consequence of stem cell plasticity?

Acute leukemias are the most common cancer in childhood and characterized by the uncontrolled production of hematopoietic precursor cells of the lymphoid or myeloid series within the bone marrow. Even when a relatively high efficiency of therapeutic agents has increased the overall survival rates in the last years, factors such as cell lineage switching and the rise of mixed lineages at relapses often change the prognosis of the illness. During lineage switching, conversions from lymphoblastic leukemia to myeloid leukemia, or vice versa, are recorded. The central mechanisms involved in these phenomena remain undefined, but recent studies suggest that lineage commitment of plastic hematopoietic progenitors may be multidirectional and reversible upon specific signals provided by both intrinsic and environmental cues. In this paper, we focus on the current knowledge about cell heterogeneity and the lineage switch resulting from leukemic cells plasticity. A number of hypothetical mechanisms that may inspire changes in cell fate decisions are highlighted. Understanding the plasticity of leukemia initiating cells might be fundamental to unravel the pathogenesis of lineage switch in acute leukemias and will illuminate the importance of a flexible hematopoietic development.

The HOXB4 homeoprotein promotes the ex vivo enrichment of functional human embryonic stem cell-derived NK cells

Human embryonic stem cells (hESCs) can be induced to differentiate into blood cells using either co-culture with stromal cells or following human embryoid bodies (hEBs) formation. It is now well established that the HOXB4 homeoprotein promotes the expansion of human adult hematopoietic stem cells (HSCs) but also myeloid and lymphoid progenitors. However, the role of HOXB4 in the development of hematopoietic cells from hESCs and particularly in the generation of hESC-derived NK-progenitor cells remains elusive. Based on the ability of HOXB4 to passively enter hematopoietic cells in a system that comprises a co-culture with the MS-5/SP-HOXB4 stromal cells, we provide evidence that HOXB4 delivery promotes the enrichment of hEB-derived precursors that could differentiate into fully mature and functional NK. These hEB-derived NK cells enriched by HOXB4 were characterized according to their CMH class I receptor expression, their cytotoxic arsenal, their expression of IFNγ and CD107a after stimulation and their lytic activity. Furthermore our study provides new insights into the gene expression profile of hEB-derived cells exposed to HOXB4 and shows the emergence of CD34⁺CD45RA⁺ precursors from hEBs indicating the lymphoid specification of hESC-derived hematopoietic precursors. Altogether, our results outline the effects of HOXB4 in combination with stromal cells in the development of NK cells from hESCs and suggest the potential use of HOXB4 protein for NK-cell enrichment from pluripotent stem cells.

Early lymphoid development and microenvironmental cues in B-cell acute lymphoblastic leukemia

B-cell acute lymphoblastic leukemia (B-ALL) is a hematological disorder characterized by malignant and uncontrolled proliferation of B-lymphoid precursor cells in bone marrow. Over the last few years remarkable advances have been made in identifying genetic aberrations, patterns of abnormal transcriptional activity controlling early fate decisions and environmental cues that may influence leukemic development. In this review we focus on the structure of the early lymphoid system and the current knowledge about cell composition and function of the hematopoietic microenvironment that might control progenitor cell activity and lead to differentiation, proliferation and survival of developing B leukemic precursors. Learning the biology of special leukemic niches is central to understanding the pathogenesis of B-ALL and for the development of novel therapies.

Targeted indocyanine-green-loaded calcium phosphosilicate nanoparticles for in vivo photodynamic therapy of leukemia

Leukemia is one of the most common and aggressive adult cancers, as well as the most prevalent childhood cancer. Leukemia is a cancer of the hematological system and can be divided into a diversity of unique malignancies based on the onset of the disease as well as the specific cell lineages involved. Cancer stem cells, including recently identified leukemia stem cells (LSCs), are hypothesized to be responsible for cancer development, relapse, and resistance to treatment, and new therapeutics targeting these cellular populations are urgently needed. Nontoxic and nonaggregating calcium phosphosilicate nanoparticles (CPSNPs) encapsulating the near-infrared fluoroprobe indocyanine green (ICG) were recently developed for diagnostic imaging and drug delivery as well as for photodynamic therapy (PDT) of solid tumors. Prior studies revealed that specific targeting of CPSNPs allowed for enhanced accumulation within breast cancer tumors, via CD71 targeting, or pancreatic cancer tumors, via gastrin receptor targeting. In the present study, ICG-loaded CPSNPs were evaluated as photosensitizers for PDT of leukemia. Using a novel bioconjugation approach to specifically target CD117 or CD96, surface features enhanced on leukemia stem cells, in vitro ICG-CPSNP PDT of a murine leukemia cell line and human leukemia samples were dramatically improved. Furthermore, the in vivo efficacy of PDT was dramatically enhanced in a murine leukemia model by utilizing CD117-targeted ICG-CPSNPs, resulting in 29% disease-free survival. Altogether, this study demonstrates that leukemia-targeted ICG-loaded CPSNPs offer the promise to effectively treat relapsing and multidrug-resistant leukemia and to improve the life of leukemia patients.

FADD deficiency impairs early hematopoiesis in the bone marrow

Signal transduction mediated by Fas-associated death domain protein (FADD) represents a paradigm of coregulation of apoptosis and cellular proliferation. During apoptotic signaling induced by death receptors including Fas, FADD is required for the recruitment and activation of caspase 8. In addition, a death receptor-independent function of FADD is essential for embryogenesis. In previous studies, FADD deficiency in embryonic stem cells resulted in a complete lack of B cells and dramatically reduced T cell numbers, as shown by Rag1(-/-) blastocyst complementation assays. However, T-specific FADD-deficient mice contained normal numbers of thymocytes and slightly reduced peripheral T cell numbers, whereas B cell-specific deletion of FADD led to increased peripheral B cell numbers. It remains undetermined what impact an FADD deficiency has on hematopoietic stem cells and progenitors. The current study analyzed the effect of simultaneous deletion of FADD in multiple cell types, including bone marrow cells, by using the IFN-inducible Mx1-cre transgene. The resulting FADD mutant mice did not develop lymphoproliferation diseases, unlike Fas-deficient mice. Instead, a time-dependent depletion of peripheral FADD-deficient lymphocytes was observed. In the bone marrow, a lack of FADD led to a dramatic decrease in the hematopoietic stem cells and progenitor-enriched population. Furthermore, FADD-deficient bone marrow cells were defective in their ability to generate lymphoid, myeloid, and erythroid cells. Thus, the results revealed a temporal requirement for FADD. Although dispensable during lymphopoiesis post lineage commitment, FADD plays a critical role in early hematopoietic stages in the bone marrow.

Bone marrow lymphoid and myeloid progenitor cells are suppressed in 7,12-dimethylbenz(a)anthracene (DMBA) treated mice

In this study we used colony forming unit (CFU) assays to demonstrate rapid suppression (within 6h) of lymphoid (CFU-preB) and myeloid (CFU-GM) progenitor cells in DMBA-treated mice. The duration of these changes were consistent with the blood levels of DMBA and its metabolites that were achieved by either IP or oral DMBA administration. CFU-GM and CFU-preB activities returned to control levels by 2 and 7 days after oral DMBA exposure, respectively, but remained suppressed through 7 days after IP DMBA administration. The continued presence of low levels of DMBA in the bloodstream following IP administration was associated with sustained suppression of CFU-preB, total bone marrow lymphoid cells and peripheral blood lymphocytes. The changes noted above were not observed in Cyp1b1 null mice, demonstrating the need for local DMBA metabolism in the bone marrow by Cyp1b1 to impair bone marrow CFU-preB and CFU-GM. Furthermore, these data provide evidence that myeloid-lineage cells are restored more quickly than lymphoid-lineage cells after DMBA exposure.

A revised scheme for developmental pathways of hematopoietic cells: the myeloid-based model

Blood cells comprise very diverse cell types with a wide range of crucial functions; however, they share a common progenitor cell type-the hematopoietic stem cell (HSC). Clarifying how HSCs differentiate into these diverse cell types is important for understanding how they attain their various functions and offers the potential for therapeutic manipulation. Various theories exist about how HSCs diversify; in particular, one model (the 'classical' model) proposes that lymphocytes and myelo-erythroid lineages branch separately at an early stage of hematopoiesis, whereas another model (the 'myeloid-based' model) proposes that the myeloid potential is retained for much longer among cells that can become lymphocytes. This article describes and compares these models and outlines recent evidence supporting the myeloid-based model.

Withdrawal of sex steroids reverses age- and chemotherapy-related defects in bone marrow lymphopoiesis

A significant decline in immune function is characteristic of aging. Along with the involution of the thymus and associated impaired architecture, which contributes to profound loss of naive T cell production, there are also significant declines in B cell development and the progenitors that support lymphopoiesis. These collectively lead to a reduced peripheral immune repertoire, increase in opportunistic infections, and limited recovery following cytoablation through chemo- or radiotherapy. We have previously shown that sex steroid ablation (SSA) causes a major reversal of age-related thymic atrophy and improves recovery from hematopoietic stem cell transplant. This study focused on the impact of SSA on the B cell compartment and their progenitors in middle-aged and cyclophosphamide-treated mice. In both models, SSA enhanced the number of lymphoid progenitors and developing B cells in the bone marrow (BM) as well as reversing age-related defects in the cycling kinetics of these cells. Enhanced BM lymphopoiesis was reflected in the periphery by an increase in recent BM emigrants as well as immature and mature plasma cells, leading to an enhanced humoral response to challenge by hepatitis B vaccine. In conclusion, SSA improves lymphoid progenitor and B cell recovery from age- and chemotherapy-induced immunodepletion, complimenting the effects on T cells. Since SSA has been achieved clinically for over 25 years, this provides a novel, rational basis for approaching the need for immune recovery in many clinical conditions.

Early growth response genes regulate B cell development, proliferation, and immune response

Egr-1 (early growth response gene-1) is an immediate early gene encoding a zinc finger motif-containing transcription factor. Upon cross-linking of BCR, mature B cells undergo proliferation with an increase in Egr-1 message. Immature B lymphoma cells that express Egr-1 message and protein constitutively are growth inhibited when Egr-1 is down-regulated by negative signals from BCR or by antisense oligonucleotides. To test the hypothesis that Egr-1 is important for B cell development, we examined B cells from primary and secondary lymphoid organs in Egr-1(-/-) mice. Marginal zone B cell development was arrested in these mice, whereas the B cells in all other compartments were increased. To test the hypothesis that Egr-1 function may be partially compensated by other Egr family members, we developed transgenic mice expressing a dominant negative form of Egr-1, which lacks the trans activation domain but retains the DNA-binding domain, in a B cell-specific manner. There was a decrease in B lymphopoiesis in the bone marrow accompanied by a reduction in splenic immature and mature B cells as well as marginal zone B cells in the transgenic mice. Moreover, transgenic mice respond poorly to BCR cross-linking in vitro and T-independent and T-dependent Ags in vivo.

Reductive isolation from bone marrow and blood implicates common lymphoid progenitors as the major source of thymopoiesis

Ongoing thymopoiesis requires continual seeding from progenitors that reside within the bone marrow (BM), but the identity of the most proximate prethymocytes has remained controversial. Here we take a comprehensive approach to prospectively identify the major source of thymocyte progenitors that reside within the BM and blood, and find that all thymocyte progenitor activity resides within a rare Flk2(+)CD27(+) population. The BM Flk2(+)CD27(+) subset is predominantly composed of common lymphoid progenitors (CLPs) and multipotent progenitors. Of these 2 populations, only CLPs reconstitute thymopoiesis rapidly after intravenous injection. In contrast, multipotent progenitor-derived cells reconstitute the thymus with delayed kinetics only after they have reseeded the BM, self-renewed, and generated CLPs. These results identify CLPs as the major source of thymocyte progenitors within the BM.

Flk2+ common lymphoid progenitors possess equivalent differentiation potential for the B and T lineages

Mature blood cells develop from multipotent hematopoietic stem cells through a series of sequential intermediates in which the developmental potential for particular blood lineages is progressively extinguished. We previously reported the identification of one of these developmental intermediates, the common lymphoid progenitor (CLP), which can give rise to T cells, B cells, dendritic cells (DCs), and natural killer cells (NKs), but lacks myeloid and erythroid potential. Recently, several studies have suggested that the T-cell and DC potential of CLP is limited or absent, and/or that CLP contains significant myeloid potential. Here, we show that the originally identified CLP population can be divided into functionally distinct subsets based on the expression of the tyrosine kinase receptor, Flk2. The Flk2(+) subset contains robust in vivo and in vitro T-cell, B-cell, DC, and NK potential, but lacks myeloid potential and, therefore, represents an oligopotent, lymphoid-restricted progenitor. This population of cells does not appear to be B cell-biased and robustly reconstitutes both B and T lineages in vivo, consistent with its being a physiologic progenitor of both of these subsets. Thus, Flk2 expression defines a homogeneous, readily obtainable subset of bone marrow CLP that is completely lymphoid-committed and can differentiate equivalently well into both B and T lineages.

Launching the T-cell-lineage developmental programme

Multipotent blood progenitor cells enter the thymus and begin a protracted differentiation process in which they gradually acquire T-cell characteristics while shedding their legacy of developmental plasticity. Notch signalling and basic helix-loop-helix E-protein transcription factors collaborate repeatedly to trigger and sustain this process throughout the period leading up to T-cell lineage commitment. Nevertheless, the process is discontinuous with separately regulated steps that demand roles for additional collaborating factors. This Review discusses new evidence on the coordination of specification and commitment in the early T-cell pathway; effects of microenvironmental signals; the inheritance of stem-cell regulatory factors; and the ensemble of transcription factors that modulate the effects of Notch and E proteins, to distinguish individual stages and to polarize T-cell-lineage fate determination.

Identification of an IL-7-dependent pre-T committed population in the spleen

Several extrathymic T cell progenitors have been described but their various contributions to the T cell lineage puzzle are unclear. In this study, we provide evidence for a splenic Lin(-)Thy1.2(+) T cell-committed population, rare in B6 mice, abundant in TCRalpha(-/-), CD3epsilon(-/-), and nude mice, and absent in IL-7- and Rag-2-deficient mice. Neither B nor myeloid cells are generated in vivo and in vitro. The incidence of these pre-T cells is under the control of thymus and/or mature T cells, as revealed by graft experiments. Indeed, IL-7 consumption by mature T cells inhibits the growth of these pre-T cells. Moreover, the nude spleen contains an additional Lin(-)Thy1.2(+)CD25(+) subset which is detected in B6 mice only after thymectomy. We establish that the full pre-T cell potential and proliferation capacity are only present in the c-kit(low) fraction of progenitors. We also show that most CCR9(+) progenitors are retained in the spleen of nude mice, but present in the blood of B6 mice. Thus, our data describe a new T cell lineage restricted subset that accumulates in the spleen before migration to the thymus.

TCRbeta enhancer activation in early and late lymphoid progenitors

An earlier report from our laboratory indicates that the activation of the T cell receptor (TCR) beta enhancer (Ebeta) is not always an indicator of T lineage potential in bone marrow-resident pre-lymphocytes. In order to more precisely investigate the consequences of Ebeta activation in lymphopoiesis, a genetic reporter animal, in which the expression of green fluorescent protein (GFP) is controlled by Ebeta, was used to examine two well-defined lymphopotent populations. Adoptive transfer experiments suggest that primitive lymphoid precursor populations (specifically, hematopoietic stem cells) consist of two discrete-populations discernible by Ebeta-GFP activation, although the two populations display no overt differences in lineage potential. In contrast, subsets of more differentiated pre-lymphocytes (specifically, common lymphoid progenitors), while also discernible by Ebeta-GFP activation, display different capacities for reconstituting lymphoid compartments. Interestingly, late lymphoid progenitors containing inactive Ebeta elements generated both T and B cells in vivo, in accord with the original description of this population; however, progenitors containing active Ebeta elements displayed an unexpected bias toward the B lineage. Our findings suggest that Ebeta activation is an indicator of B lineage specification in late, but not early lymphoid precursors.

Lymphohematopoietic progenitors do not have a synchronized defect with age-related thymic involution

It has been speculated that aging lymphohematopoietic progenitor cells (LPC) including hematopoietic stem cells (HSC) and early T-cell progenitors (ETP) have intrinsic defects that trigger age-related thymic involution. However, using a different approach, we suggest that that is not the case. We provided a young thymic microenvironment to aged mice by transplanting a fetal thymus into the kidney capsule of aged animals, and demonstrated that old mouse-derived LPCs could re-establish normal thymic lymphopoiesis and all thymocyte subpopulations, including ETPs, double negative subsets, double positive, and CD4(+) and CD8(+) single positive T cells. LPCs derived from aged mice could turn over young RAG(-/-) thymic architecture by interactions, as well as elevate percentage of peripheral CD4(+)IL-2(+) T cells in response to costimulator in aged mice. Conversely, intrathymic injection of ETPs sorted from young animals into old mice did not restore normal thymic lymphopoiesis, implying that a shortage and/or defect of ETPs in aged thymus do not account for age-related thymic involution. Together, our findings suggest that the underlying cause of age-related thymic involution results primarily from changes in the thymic microenvironment, causing extrinsic, rather than intrinsic, defects in T-lymphocyte progenitors.

Germ-line and rearranged Tcrd transcription distinguish bona fide NK cells and NK-like gammadelta T cells

NK cells and gammadelta T cells are distinct subsets of lymphocytes that contextually share multiple phenotypic and functional characteristics. However, the acquisition and the extent of these similarities remain poorly understood. Here, using T cell receptor delta locus-histone 2B-enhanced GFP (Tcrd-H2BEGFP) reporter mice, we show that germ-line transcription of Tcrd occurs in all maturing NK cells. We also describe a population of mouse NK-like cells that are indistinguishable from "bona fide" NK cells using standard protocols. Requirements for V(D)J recombination and a functional thymus, along with very low-level expression of surface TCRgammadelta but high intracellular CD3, define these cells as gammadelta T cells. "NK-like gammadelta T cells" are CD127+, have a memory-activated phenotype, express multiple NK cell receptors and readily produce interferon-gamma in response to IL-12/IL-18 stimulation. The close phenotypic resemblance between NK cells and NK-like gammadelta T cells is a source of experimental ambiguity in studies bridging NK and T cell biology, such as those on thymic NK cell development. Instead, it ascribes chronic TCRgammadelta engagement as a means of acquiring NK-like function.

Molecular dissection of prethymic progenitor entry into the T lymphocyte developmental pathway

Notch signaling activates T lineage differentiation from hemopoietic progenitors, but relatively few regulators that initiate this program have been identified, e.g., GATA3 and T cell factor-1 (TCF-1) (gene name Tcf7). To identify additional regulators of T cell specification, a cDNA library from mouse Pro-T cells was screened for genes that are specifically up-regulated in intrathymic T cell precursors as compared with myeloid progenitors. Over 90 genes of interest were identified, and 35 of 44 tested were confirmed to be more highly expressed in T lineage precursors relative to precursors of B and/or myeloid lineage. To a remarkable extent, however, expression of these T lineage-enriched genes, including zinc finger transcription factor, helicase, and signaling adaptor genes, was also shared by stem cells (Lin(-)Sca-1(+)Kit(+)CD27(-)) and multipotent progenitors (Lin(-)Sca-1(+)Kit(+)CD27(+)), although down-regulated in other lineages. Thus, a major fraction of these early T lineage genes are a regulatory legacy from stem cells. The few genes sharply up-regulated between multipotent progenitors and Pro-T cell stages included those encoding transcription factors Bcl11b, TCF-1 (Tcf7), and HEBalt, Notch target Deltex1, Deltex3L, Fkbp5, Eva1, and Tmem131. Like GATA3 and Deltex1, Bcl11b, Fkbp5, and Eva1 were dependent on Notch/Delta signaling for induction in fetal liver precursors, but only Bcl11b and HEBalt were up-regulated between the first two stages of intrathymic T cell development (double negative 1 and double negative 2) corresponding to T lineage specification. Bcl11b was uniquely T lineage restricted and induced by Notch/Delta signaling specifically upon entry into the T lineage differentiation pathway.

Bone marrow-derived hemopoietic precursors commit to the T cell lineage only after arrival in the thymic microenvironment

T lymphocytes develop in the thymus from hemopoietic precursors that commit to the T cell lineage under the influence of Notch signals. In this study, we show by single cell analyses that the most immature hemopoietic precursors in the adult mouse thymus are uncommitted and specify to the T cell lineage only after their arrival in the thymus. These precursors express high levels of surface Notch receptors and rapidly lose B cell potential upon the provision of Notch signals. Using a novel culture system with complexed, soluble Notch ligands that allows the titration of T cell lineage commitment, we find that these precursors are highly sensitive to both Delta and Jagged ligands. In contrast, their phenotypical and functional counterparts in the bone marrow are resistant to Notch signals that efficiently induce T cell lineage commitment in thymic precursors. Mechanistically, this is not due to differences in receptor expression, because early T lineage precursors, bone marrow lineage marker-negative, Sca-1-positive, c-Kit-positive and common lymphoid progenitor cells, express comparable amounts of surface Notch receptors. Our data demonstrate that the sensitivity to Notch-mediated T lineage commitment is stage-dependent and argue against the bone marrow as the site of T cell lineage commitment.

Interferon-producing killer dendritic cells (IKDCs) arise via a unique differentiation pathway from primitive c-kitHiCD62L+ lymphoid progenitors

Interferon-producing killer dendritic cells (IKDCs) have only recently been described and they share some properties with plasmacytoid dendritic cells (pDCs). We now show that they can arise from some of the same progenitors. However, IKDCs expressed little or no RAG-1, Spi-B, or TLR9, but responded to the TLR9 agonist CpG ODN by production of IFNgamma. The RAG-1(-)pDC2 subset was more similar to IKDCs than RAG-1(+) pDC1s with respect to IFNgamma production. The Id-2 transcriptional inhibitor was essential for production of IKDCs and natural killer (NK) cells, but not pDCs. IKDCs developed from lymphoid progenitors in culture but, unlike pDCs, were not affected by Notch receptor ligation. While IKDCs could be made from estrogen-sensitive progenitors, they may have a slow turnover because their numbers did not rapidly decline in hormone-treated mice. Four categories of progenitors were compared for IKDC-producing ability in transplantation assays. Of these, Lin(-)Sca-1(+)c-Kit(Hi)Thy1.1(-)L-selectin(+) lymphoid progenitors (LSPs) were the best source. While NK cells resemble IKDCs in several respects, they develop from different progenitors. These observations suggest that IKDCs may arise from a unique differentiation pathway, and one that diverges early from those responsible for NK cells, pDCs, and T and B cells.

Fate decisions regulating bone marrow and peripheral B lymphocyte development

In adult mammals, bone marrow pluripotent hematopoietic stem cells generate B lymphoid-specified progeny that progress through a series of well-characterized stages before generating B-cell receptor expressing B lymphocytes. These functionally immature B lymphocytes then migrate to the spleen wherein they differentiate through transitional stages into follicular or marginal zone B lymphocytes capable of responding to T-dependent and -independent antigens, respectively. During the terminal stages of B lymphocyte development in the bone marrow, as well as immediately following egress into the peripheral compartments, B lymphocytes are counterselected to eliminate B lymphocytes with potentially dangerous self-reactivity. These developmental and selection events in the bone marrow and periphery are dependent on the integration of intrinsic genetic programs with extrinsic microenvironmental signals that drive progenitors toward increasing B lineage commitment and maturation. This chapter provides a comprehensive overview of the various stages of primary and secondary B lymphocyte development with an emphasis on the selection processes that affect decisions at critical checkpoints. Our intent is to stress the concept that at many steps in the developmental process leading to a mature immunocompetent B lymphocyte, B lineage cells are integrating multiple and different signaling inputs that are translated into specific and appropriate cell fate decisions.

Cell cycle quiescence of early lymphoid progenitors in adult bone marrow

Lymphocyte production in bone marrow (BM) requires substantial cell division, but the relationship between largely quiescent stem cells and dividing lymphoid progenitors is poorly understood. Therefore, the proliferation and cell cycle status of murine hematopoietic progenitors that have just initiated the lymphoid differentiation program represented the focus of this study. Continuous bromo-2'-deoxyuridine (BrdU) incorporation and DNA/RNA analysis by flow cytometry revealed that a surprisingly large fraction of RAG-1(+)c-kit(hi) early lymphoid progenitors (ELPs) and RAG-1(+)c-kit(lo) pro-lymphocytes (Pro-Ls) in adult BM were in cell cycle quiescence. In contrast, their counterparts in 14-day fetal liver actively proliferated. Indeed, the growth fraction (cells in G(1)-S-G(2)-M phases) of fetal ELPs was on average 80% versus only 30% for adult ELPs. After 5-fluorouracil treatment, as many as 60% of the adult ELP-enriched population was in G(1)-S-G(2)-M and 34% incorporated BrdU in 6 hours. Transcripts for Bcl-2, p21Cip1/Waf1, and p27 Kip1 cell cycle regulatory genes correlated inversely well with proliferative activity. Interestingly, adult lymphoid progenitors in rebound had the high potential for B lymphopoiesis in culture typical of their fetal counterparts. Thus, lymphocyte production is sustained during adult life by quiescent primitive progenitors that divide intermittently. Some, but not all, aspects of the fetal differentiation program are reacquired after chemotherapy.

Primitive lymphoid progenitors in bone marrow with T lineage reconstituting potential

Multiple subsets of the bone marrow contain T cell precursors, but it remains unclear which is most likely to replenish the adult thymus. Therefore, RAG-1+ early lymphoid progenitors (RAG-1+ ELP), and CD62L/L-selectin+ progenitors (LSP), as well as common lymphoid progenitors from C57BL6-Thy1.1-RAG-1/GFP mouse bone marrow were directly compared in transplantation assays. The two c-Kit(high) populations vigorously regenerated the thymus and were superior to common lymphoid progenitors in magnitude and frequency of thymic reconstitution. Regeneration was much faster than the 22 days described for transplanted stem cells, and RAG-1+ ELP produced small numbers of lymphocytes within 13 days. As previously reported, LSP were biased to a T cell fate, but this was not the case for RAG-1+ ELP. Although RAG-1+ ELP and LSP had reduced myeloid potential, they were both effective progenitors for T lymphocytes and NK cells. The LSP subset overlapped with and included most RAG-1+ ELP and many RAG-1- TdT+ ELP. LSP and RAG-1+ ELP were both present in the peripheral circulation, but RAG-1+ ELP had no exact counterpart among immature thymocytes. The most primitive of thymocytes were similar to Lin- c-Kit(high) L-selectin+ TdT+ RAG-1- progenitors present in the marrow, suggesting that this population is normally important for sustaining the adult thymus.

Progression of regulatory gene expression states in fetal and adult pro-T-cell development

Precursors entering the T-cell developmental pathway traverse a progression of states characterized by distinctive patterns of gene expression. Of particular interest are regulatory genes, which ultimately control the dwell time of cells in each state and establish the mechanisms that propel them forward to subsequent states. Under particular genetic and developmental circumstances, the transitions between these states occur with different timing, and environmental feedbacks may shift the steady-state accumulations of cells in each state. The fetal transit through pro-T-cell stages is faster than in the adult and subject to somewhat different genetic requirements. To explore causes of such variation, this review presents previously unpublished data on differentiation gene activation in pro-T cells of pre-T-cell receptor-deficient mutant mice and a quantitative comparison of the profiles of transcription factor gene expression in pro-T-cell subsets of fetal and adult wildtype mice. Against a background of consistent gene expression, several regulatory genes show marked differences between fetal and adult expression profiles, including those encoding two basic helix-loop-helix antagonist Id factors, the Ets family factor SpiB and the Notch target gene Deltex1. The results also reveal global differences in regulatory alterations triggered by the first T-cell receptor-dependent selection events in fetal and adult thymopoiesis.

Coordinated acquisition of inhibitory and activating receptors and functional properties by developing human natural killer cells

The stages of human natural killer (NK) cell differentiation are not well established. Culturing CD34(+) progenitors with interleukin 7 (IL-7), IL-15, stem cell factor (SCF), FLT-3L, and murine fetal liver cell line (EL08.1D2), we identified 2 nonoverlapping subsets of differentiating CD56(+) cells based on CD117 and CD94 (CD117(high)CD94(-) and CD117(low/-)CD94(+) cells). Both populations expressed CD161 and NKp44, but differed with respect to NKp30, NKp46, NKG2A, NKG2C, NKG2D, CD8, CD16, and KIR. Only the CD117(low/-) CD94(+) population displayed cytotoxicity and interferon-gamma production. Both populations arose from a single CD34(+)CD38(-) Lin(-) cell and their percentages changed over time in a reciprocal fashion, with CD117(high)CD94(-) cells predominating early and decreasing due to an increase of the CD117(low/-)CD94(+) population. These 2 subsets represent distinct stages of NKcell differentiation, since purified CD117(high) CD94(-) cells give rise to CD117(low/-)CD94(+) cells. The stromal cell line (EL08.1D2) facilitated the transition from CD117(high)CD94(-) to CD117(low/-)CD94(+) via an intermediate phenotype (CD117(low)CD94(low/-)). EL08.1D2 also maintained the mature phenotype, preventing the reversion of CD117(low/-)CD94(+) cells to the intermediate (CD117(low)CD94(low/-)) phenotype. An analogous population of CD56(+)CD117(high)CD94(-) cells was found in cord blood. The identified stages of NK-cell differentiation provide evidence for coordinated acquisition of HLA-specific inhibitory receptors (ie, CD94/NKG2A) and function in developing human NK cells.

The continuum model of marrow stem cell regulation

PURPOSE OF REVIEW

We review the continuum model of stem cell regulation. A series of studies on purified lineage negative rhodamine low Hoechst low murine stem cells driven through cell cycle by cytokine exposure have shown that many phenotypic features show reversible changes with cycle progression.

RECENT FINDINGS

We and others have shown that purified murine marrow stem cells are a cycling population. Features that are labile with cycle progression are in-vivo engraftment, progenitor numbers, expression of adhesion proteins and cytokine receptors, global gene expression and differentiation into granulocytes and megakaryocytes. These observations have led to a theory that regulation of hematopoietic stem cells is on a continuum and not in a hierarchy. Out-of-tissue plasticity in which marrow cells show a capacity to produce nonhematopoietic cells in non-marrow tissues also exists. We have shown 'robust' production of lung and skeletal muscle cells by marrow cells in the presence of appropriate tissue injury and demonstrated that the capacity of marrow cells to produce nonhematopoietic cells in the lung also varies reversibly with cell cycle status.

SUMMARY

Thus, stem cells show a plastic plasticity and the continuum appears to hold for both nonhematopoietic and hematopoietic lineages.

Xenobiotic-induced alterations in thymocyte development

The thymus is a very sensitive target for environmental pollutants, which can affect this organ as well as thymocyte differentiation. A failure in thymocyte development can be due to the exacerbation of apoptosis, arrest of thymocyte maturation, generation of autoreactive T cells, and inhibition or stimulation of the output of recent thymic emigrants to the periphery. Recent data demonstrate that the immune system has the potential to maintain homeostasis under conditions of elevated risk, and the thymus plays a crucial role in this process. Environmental xenobiotics can exert their effects through receptor-mediated interactions or independently on receptor involvement. Under natural conditions organisms are exposed to a variety of xenobiotics. The final effect of such exposure is not related to the action of a single chemical, but to the action of a mixture of chemicals. The toxic effect of environmental xenobiotics on the generation and functions of immune cells may result in suppression or stimulation of the immune response. The most intensive studies have been done on halogenated aromatic hydrocarbons, heavy metals and various chemicals acting as endocrine disrupters. Recently, special interest has focused on the action of air particulate matter.

E proteins and Notch signaling cooperate to promote T cell lineage specification and commitment

The helix-loop-helix protein, E47, is essential for both B- and T-lineage development. Here we demonstrate that in vitro E47 and Notch signaling act in concert to promote T cell development from fetal hematopoietic progenitors and to restrain development into the natural killer and myeloid cell lineages. The expression of an ensemble of genes associated with Notch signaling is activated by E47, and additionally, Notch signaling and E47 act in parallel pathways to induce a T lineage-specific program of gene expression. Enforced expression of the intracellular domain of Notch rescues the developmental arrest at the T cell commitment stage in E2A-deficient fetal thymocytes. Finally, we demonstrate that regulation of Hes1 expression by Notch signaling and E47 is strikingly similar to that observed during Drosophila melanogaster sensory development. Based on these observations, we propose that in developing fetal thymocytes E47 acts to induce the expression of an ensemble of genes involved in Notch signaling, and that subsequently E47 acts in parallel with Notch signaling to promote T-lineage maturation.

From stem cell to T cell: one route or many?

T cells developing in the adult thymus ultimately derive from haematopoietic stem cells in the bone marrow. Here, we summarize research into the identity of the haematopoietic progenitors that leave the bone marrow, migrate through the blood and settle in the thymus to generate T cells. Accumulating data indicate that various different bone-marrow progenitors are T-cell-lineage competent and might contribute to intrathymic T-cell development. Such developmental flexibility implies a mechanism of T-cell-lineage commitment that can operate on a range of T-cell-lineage-competent progenitors, and further indicates that only those T-cell-lineage-competent progenitors able to migrate to, and settle in, the thymus should be considered physiological T-cell progenitors.

Trafficking from the bone marrow to the thymus: a prerequisite for thymopoiesis

T-cell development in the thymus requires periodic importation of hematopoietic progenitors from the bone marrow. Such thymus settling progenitors arise from hematopoietic stem cells (HSCs) that are retained in a specific bone marrow microenvironmental niche. Vacation of this niche is required for HSC proliferation and differentiation into downstream progenitors. In order to reach the thymus, progenitors must then be mobilized from bone marrow to blood. Finally, progenitors in blood must settle in the thymus. Here we review signals and molecular interactions that are likely to play a role in trafficking from the bone marrow to the thymus, focusing on how these interactions may regulate which progenitors physiologically contribute to thymopoiesis.

The κ-like pre-B receptor: Surplus biology or a missing link?

After the demonstration that surrogate JCkappa polypeptides could covalently bind mu heavy chain and upon the characterization of the Vkappa-like component of the kappa-like pre-B cell receptor, it became evident that germline transcription is not sterile. The present review discusses the concept of the alternative usage of kappa-like pre-B cell receptors and classical pre-B cell receptors utilizing the lambda-like surrogate light chain composed of lambda5 and VpreB. We propose that both kappa-like and lambda-like pre-B cell receptors work in concert in a fail-safe mechanism to promote light chain rearrangement, heavy chain allelic exclusion and B-lymphocyte maturation.

Identification of a B-1 B cell-specified progenitor

The B-1 subpopulation of B lymphocytes differs phenotypically and functionally from conventional B-2 B cells. B-1 B cells are proposed to derive from a distinct progenitor, but such a population has not been isolated. Here we identify and characterize a B-1 B cell progenitor whose numbers peaked in fetal bone marrow but were less abundant in postnatal bone marrow. These Lin(-)CD45R(lo-neg)CD19(+) cells responded to thymic stromal lymphopoietin and 'preferentially' reconstituted functional sIgM(hi)CD11b(+)CD5(lo-neg) B-1 B cells, but not sIgM(+)CD11b(-) B-2 B cells, in vivo. These data indicate that the CD45R(lo-neg)CD19(+) population includes B-1 B cell-specified progenitors and support models proposing distinct developmental pathways for B-1 B cells.

Human embryonic stem cell-derived NK cells acquire functional receptors and cytolytic activity

Human embryonic stem cells (hESCs) provide a unique resource to analyze early stages of human hematopoiesis. However, little is known about the ability to use hESCs to evaluate lymphocyte development. In the present study, we use a two-step culture method to demonstrate efficient generation of functional NK cells from hESCs. The CD56(+)CD45(+) hESC-derived lymphocytes express inhibitory and activating receptors typical of mature NK cells, including killer cell Ig-like receptors, natural cytotoxicity receptors, and CD16. Limiting dilution analysis suggests that these cells can be produced from hESC-derived hemopoietic progenitors at a clonal frequency similar to CD34(+) cells isolated from cord blood. The hESC-derived NK cells acquire the ability to lyse human tumor cells by both direct cell-mediated cytotoxicity and Ab-dependent cellular cytotoxicity. Additionally, activated hESC-derived NK cells up-regulate cytokine production. hESC-derived lymphoid progenitors provide a novel means to characterize specific cellular and molecular mechanisms that lead to development of specific human lymphocyte populations. These cells may also provide a source for innovative cellular immune therapies.

The thymus exports long-lived fully committed T cell precursors that can colonize primary lymphoid organs

Thymic export of cells is believed to be restricted to mature T cells. Here we show that the thymus also exports fully committed T cell precursors that colonize primary lymphoid organs. These precursor cells exited the thymus before T cell receptor rearrangements and colonized lymphoid organs such as the thymus and the gut. Migration of the thymic T cell-committed precursors led to permanent colonization of the gut precursor compartment, improved the capacity of gut precursors to further differentiate into T cells and was sufficient for the generation of 'euthymic like' CD8alphaalpha(+) intraepithelial lymphocytes. These data demonstrate a new function for the thymus in peripheral seeding with T cell precursors that become long lived after thymus export.

Life before the pre-B cell receptor checkpoint: specification and commitment of primitive lymphoid progenitors in adult bone marrow

The production of B cells is a complex process determined by well-timed combinations of intrinsic factors and environmental cues that guide the differentiation of primitive progenitors in the bone marrow. Expression of several key transcription factors and receptor-stromal cell ligand interactions are landmarks of the earliest events in B lymphopoiesis in adult bone marrow. We describe this as a gradual loss of options for other blood cell lineages coincident with gain of essential properties. Experimental, stress or infection-related deregulation may change B cell fate specification, commitment or population dynamics, and consequently the production rate of mature populations.

Many roads, one destination for T cell progenitors

The thymus manufactures new T cells throughout life but contains no self-renewing potential. Instead, replenishment depends on recruitment of bone marrow-derived progenitors that circulate in the blood. Attempts to identify thymic-homing progenitors, and to assess the degree to which they are precommitted to the T cell lineage, have led to complex and sometimes conflicting results. As described here, this probably reflects the existence of multiple distinct types of T cell lineage progenitors as well as differences in individual experimental approaches.