Bipotential B-macrophage progenitors are present in adult bone marrow

Mis-expression of GATA6 re-programs cell fate during early hematopoiesis

The traditional view of hematopoiesis is that myeloid cells derive from a common myeloid progenitor (CMP), whereas all lymphoid cell populations, including B, T, and natural killer (NK) cells and possibly plasmacytoid dendritic cells (pDCs), arise from a common lymphoid progenitor (CLP). In Max41 transgenic mice, nearly all B cells seem to be diverted into the granulocyte lineage. Here, we show that these mice have an excess of myeloid progenitors, but their CLP compartment is ablated, and they have few pDCs. Nevertheless, T cell and NK cell development proceeds relatively normally. These hematopoietic abnormalities result from aberrant expression of Gata6 due to serendipitous insertion of the transgene enhancer (Eμ) in its proximity. Gata6 mis-expression in Max41 transgenic progenitors promoted the gene-regulatory networks that drive myelopoiesis through increasing expression of key transcription factors, including PU.1 and C/EBPa. Thus, mis-expression of a single key regulator like GATA6 can dramatically re-program multiple aspects of hematopoiesis.

A novel phenotype of B cells associated with enhanced phagocytic capability and chemotactic function after ischemic stroke

Accumulating evidence has demonstrated the involvement of B cells in neuroinflammation and neuroregeneration. However, the role of B cells in ischemic stroke remains unclear. In this study, we identified a novel phenotype of macrophage-like B cells in brain-infiltrating immune cells expressing a high level of CD45. Macrophage-like B cells characterized by co-expression of B-cell and macrophage markers, showed stronger phagocytic and chemotactic functions compared with other B cells and showed upregulated expression of phagocytosis-related genes. Gene Ontology analysis found that the expression of genes associated with phagocytosis, including phagosome- and lysosome-related genes, was upregulated in macrophage-like B cells. The phagocytic activity of macrophage-like B cells was verified by immunostaining and three-dimensional reconstruction, in which TREM2-labeled macrophage-like B cells enwrapped and internalized myelin debris after cerebral ischemia. Cell-cell interaction analysis revealed that macrophage-like B cells released multiple chemokines to recruit peripheral immune cells mainly via CCL pathways. Single-cell RNA sequencing showed that the transdifferentiation to macrophage-like B cells may be induced by specific upregulation of the transcription factor CEBP family to the myeloid lineage and/or by downregulation of the transcription factor Pax5 to the lymphoid lineage. Furthermore, this distinct B cell phenotype was detected in brain tissues from mice or patients with traumatic brain injury, Alzheimer's disease, and glioblastoma. Overall, these results provide a new perspective on the phagocytic capability and chemotactic function of B cells in the ischemic brain. These cells may serve as an immunotherapeutic target for regulating the immune response of ischemic stroke.

Cutaneous Histiocytic Sarcoma With Cellular Cannibalism

Cutaneous histiocytic sarcoma (HS) is a rare malignant tumor. An 82-year-old woman presented with a 4 × 2-cm irregular-shaped red nodule on the left posterior scalp. A biopsy specimen revealed sheets of pleomorphic atypical cells in the dermis and subcutis. A diagnosis of HS was made based on the results of a panel of immunohistochemical stains that revealed positivity of leukocyte common antigen, CD4, CD163, and HLA-DR. At the time of resection, the tumor grew rapidly to 12 × 6.5 × 5 cm in size in 2 months. The resected tumor comprised round, oval, plasmacytoid, and spindled cells. Signet-ring cell type tumor cells were also observed. The histiocytic nature of HS was confirmed owing to the presence of cellular cannibalism, emperipolesis, Langhans giant cell-like cells, Touton giant cell-like cells, foreign-body giant cell-like cells, and hemosiderin laden cells. In some foci, a storiform pattern and fascicular pattern were occasionally observed. Local recurrence occurred shortly after resection. Subsequent radiation therapy showed insufficient effectiveness. It is challenging to make a diagnosis of HS without performing immunohistochemical studies; however, a variety of histiocytic features confirmed in hematoxylin and eosin-stained sections may suggest HS.

Anti-CD20-Mediated B Cell Depletion Is Associated With Bone Preservation in Lymphoma Patients and Bone Mass Increase in Mice

Immunotherapy with anti-CD20-specific antibodies (rituximab), has become the standard of care for B cell lymphoproliferative disorders and many autoimmune diseases. In rheumatological patients the effect of rituximab on bone mass yielded conflicting results, while in lymphoma patients it has not yet been described. Here, we used cross-sectional X-ray imaging (CT/PET-CT) to serially assess bone density in patients with follicular lymphoma receiving rituximab maintenance therapy. Remarkably, this treatment prevented the decline in bone mass observed in the control group of patients who did not receive active maintenance therapy. In accordance with these data, anti-CD20-mediated B cell depletion in normal C57BL/6J female mice led to a significant increase in bone mass, as reflected by a 7.7% increase in bone mineral density (whole femur), and a ~5% increase in cortical as well as trabecular tissue mineral density. Administration of anti-CD20 antibodies resulted in a significant decrease in osteoclastogenic signals, including RANKL, which correlated with a reduction in osteoclastogenic potential of bone marrow cells derived from B-cell-depleted animals. Taken together, our data suggest that in addition to its anti-tumor activity, anti-CD20 treatment has a favorable effect on bone mass. Our murine studies indicate that B cell depletion has a direct effect on bone remodeling.

Erythropoietin receptor in B cells plays a role in bone remodeling in mice

Erythropoietin (EPO) is a key regulator of erythropoiesis. However, EPO receptors (EPO-Rs) are also expressed on non-erythroid cell types, including myeloid and bone cells. Immune cells also participate in bone homeostasis. B cells produce receptor activator of nuclear factor kappa-Β ligand (RANKL) and osteoprotegerin (OPG), two pivotal regulators of bone metabolism. Here we explored the ability of B cells to transdifferentiate into functional osteoclasts and examined the role of EPO in this process in a murine model.

Methods: We have combined specifically-designed experimental mouse models and in vitro based osteoclastogenesis assays, as well as PCR analysis of gene expression.

Results: (i) EPO treatment in vivo increased RANKL expression in bone marrow (BM) B cells, suggesting a paracrine effect on osteoclastogenesis; (ii) B cell-derived osteoclastogenesis occured in vivo and in vitro, as demonstrated by B cell lineage tracing in murine models; (iii) B-cell-derived osteoclastogenesis in vitro was restricted to Pro-B cells expressing CD115/CSF1-R and is enhanced by EPO; (iv) EPO treatment increased the number of B-cell-derived preosteoclasts (β3⁺CD115⁺), suggesting a physiological rationale for B cell derived osteoclastogenesis; (v) finally, mice with conditional EPO-R knockdown in the B cell lineage (cKD) displayed a higher cortical and trabecular bone mass. Moreover, cKD displayed attenuated EPO-driven trabecular bone loss, an effect that was observed despite the fact that cKD mice attained higher hemoglobin levels following EPO treatment.

Conclusions: Our work highlights B cells as an important extra-erythropoietic target of EPO-EPO-R signaling and suggests their involvement in the regulation of bone homeostasis and possibly in EPO-stimulated erythropoietic response. Importantly, we present here for the first time, histological evidence for B cell-derived osteoclastogenesis in vivo.

Differentiation of c-Kit+ CD24+ natural killer cells into myeloid cells in a GATA-2-dependent manner

Myeloid progenitor cells have generally been considered the predominant source of myeloid cells under steady-state conditions. Here we show that NK cells contributed to a myeloid cell lineage pool in naïve and tumor-bearing mice. Using fate tracing of NKp46⁺ cells, we found that myeloid cells could be derived from NK cells. Notably, among mature CD11b⁺ CD27⁺ NK cells, c-Kit⁺ CD24⁺ NK cells were capable of differentiating into a range of myeloid lineages in vitro and produced neutrophils and monocytes in vivo. The differentiation was completely inhibited by NK-stimulating cytokines. In addition to the potential for differentiation into myeloid cells, c-Kit⁺ CD24⁺ NK cells retained NK cell phenotypes and effector functions. Mechanistically, GATA-2 was necessary for the differentiation of c-Kit⁺ CD24⁺ NK cells. Therefore, we discovered that GATA-2-dependent differentiation of c-Kit⁺ CD24⁺ NK cells contributes to myeloid cell development and identified a novel pathway for myeloid lineage commitment under physiological conditions.

Human NK Cell Development: One Road or Many?

CD3⁻CD56⁺ NK cells develop from CD34⁺ hematopoietic progenitors (HPCs) in vivo, and this process can be recapitulated in vitro. The prevailing model is that human NK cell development occurs along a continuum whereby common lymphocyte progenitors (CLPs) gradually downregulate CD34 and upregulate CD56. Acquisition of CD94 marks commitment to the CD56^bright stage, and CD56^bright NK cells subsequently differentiate into CD56^dim NK cells that upregulate CD16 and killer immunoglobulin-like receptors (KIR). Support for this linear model comes from analyses of cell populations in secondary lymphoid tissues and in vitro studies of NK cell development from HPCs. However, several lines of evidence challenge this linear model and suggest a more branched model whereby different precursor populations may independently develop into distinct subsets of mature NK cells. A more definitive understanding of human NK cell development is needed to inform in vitro differentiation strategies designed to generate NK cells for immunotherapy. In this review, we summarize current evidence supporting the linear and branched models of human NK cell development and the challenges associated with reaching definitive conclusions.

From Discrete to Continuous Modeling of Lymphocyte Development and Plasticity in Chronic Diseases

The molecular events leading to differentiation, development, and plasticity of lymphoid cells have been subject of intense research due to their key roles in multiple pathologies, such as lymphoproliferative disorders, tumor growth maintenance and chronic diseases. The emergent roles of lymphoid cells and the use of high-throughput technologies have led to an extensive accumulation of experimental data allowing the reconstruction of gene regulatory networks (GRN) by integrating biochemical signals provided by the microenvironment with transcriptional modules of lineage-specific genes. Computational modeling of GRN has been useful for the identification of molecular switches involved in lymphoid specification, prediction of microenvironment-dependent cell plasticity, and analyses of signaling events occurring downstream the activation of antigen recognition receptors. Among most common modeling strategies to analyze the dynamical behavior of GRN, discrete dynamic models are widely used for their capacity to capture molecular interactions when a limited knowledge of kinetic parameters is present. However, they are less powerful when modeling complex systems sensitive to biochemical gradients. To compensate it, discrete models may be transformed into regulatory networks that includes state variables and parameters varying within a continuous range. This approach is based on a system of differential equations dynamics with regulatory interactions described by fuzzy logic propositions. Here, we discuss the applicability of this method on modeling of development and plasticity processes of adaptive lymphocytes, and its potential implications in the study of pathological landscapes associated to chronic diseases.

Role of mir-15a/16-1 in early B cell development in a mouse model of chronic lymphocytic leukemia

In both human chronic lymphocytic leukemia (CLL) and the New Zealand Black (NZB) murine model of CLL, decreased levels of microRNAs miR-15a/16 play an important role in the disease. Here we investigate the effects of this microRNA on early steps of B cell development and the capacity of miR-15a-deficient hematopoietic stem cells (HSC) and B1 progenitor cells (B1P) to reproduce CLL-like phenotype both in vitro and in vivo. Our results demonstrate that both miR-15a deficient HSC and B1P cells are capable of repopulating irradiated recipients and produce higher numbers of B1 cells than sources with normal miR-15a/16 levels. Furthermore, induced pluripotent stem (iPS) cells derived for the first time from NZB mice, provided insights into the B cell differentiation roadblock inherent in this strain. In addition, exogenously delivered miR-15a into the NZB derived B cell line provided valuable clues into novel targets such as Mmp10 and Mt2. Our data supports the hypothesis that miR-15a/16 deficient stem cells and B1Ps experience a maturation blockage, which contributes to B1 cells bias in development. This work will help understand the role of miR-15a in early events of CLL and points to B1P cells as potential cells of origin for this incurable disease.

Transcriptional and Microenvironmental Regulation of Lineage Ambiguity in Leukemia

Leukemia is characterized by the uncontrolled production of leukemic cells and impaired normal hematopoiesis. Although the combination of chemotherapies and hematopoietic stem cell transplantation has significantly improved the outcome of leukemia patients, a proportion of patients still suffer from relapse after treatment. Upon relapse, a phenomenon termed “lineage switch” is observed in a subset of leukemia patients, in which conversion of lymphoblastic leukemia to myeloid leukemia or vice versa is observed. A rare entity of leukemia called mixed-phenotype acute leukemia exhibits co-expression of markers representing two or three lineages. These two phenotypes regarding the lineage ambiguity suggest that the fate of some leukemia retain or acquire a certain degree of plasticity. Studies using animal models provide insight into how lineage specifying transcription factors can enforce or convert a fate in hematopoietic cells. Modeling lineage conversion in normal hematopoietic progenitor cells may improve our current understanding of how lineage switch occurs in leukemia. In this review, we will summarize the role of transcription factors and microenvironmental signals that confer fate plasticity to normal hematopoietic progenitor cells, and their potential to regulate lineage switching in leukemias. Future efforts to uncover the mechanisms contributing to lineage conversion in both normal hematopoiesis and leukemia may pave the way to improve current therapeutic strategies.

Pre/pro-B cells generate macrophage populations during homeostasis and inflammation

Most tissue-resident macrophages (Mφs) are believed to be derived prenatally and are assumed to maintain themselves throughout life by self-proliferation. However, in adult mice we identified a progenitor within bone marrow, early pro-B cell/fraction B, that differentiates into tissue Mφs. These Mφ precursors have non-rearranged B-cell receptor genes and coexpress myeloid (GR1, CD11b, and CD16/32) and lymphoid (B220 and CD19) lineage markers. During steady state, these precursors exit bone marrow, losing Gr1, and enter the systemic circulation, seeding the gastrointestinal system as well as pleural and peritoneal cavities but not the brain. While in these tissues, they acquire a transcriptome identical to embryonically derived tissue-resident Mφs. Similarly, these Mφ precursors also enter sites of inflammation, gaining CD115, F4/80, and CD16/32, and become indistinguishable from blood monocyte-derived Mφs. Thus, we have identified a population of cells within the bone marrow early pro-B cell compartment that possess functional plasticity to differentiate into either tissue-resident or inflammatory Mφs, depending on microenvironmental signals. We propose that these precursors represent an additional source of Mφ populations in adult mice during steady state and inflammation.

B Cell Functions Can Be Modulated by Antimicrobial Peptides in Rainbow Trout Oncorhynchus mykiss: Novel Insights into the Innate Nature of B Cells in Fish

B cells in fish were recently proven to have potent innate immune activities like macrophages. This inspired us to further explore the innate nature of B cells in fish. Moreover, antimicrobial peptides (AMPs) are representative molecules of innate immunity, and they can modulate the functions of macrophages. These make fish an appropriate model to study the interactions between B cells and AMPs. Interestingly, the results in this study revealed that the IgM⁺ and IgT⁺ B cells of rainbow trout could express multiple AMP genes, including four cathelicidin genes and one β-defensin gene. The expression levels of the cathelicidin genes were obviously higher than that of the β-defensin gene. Further studies revealed that intracellular, extracellular, in vitro, and in vivo stimulations could significantly increase the expression of the cathelicidin genes in trout IgM⁺ and IgT⁺ B cells but not the expression of the β-defensin gene, indicating that cathelicidin peptides are the main innate immune effectors of trout B cells. More interestingly, we found that cathelicidin peptides could significantly enhance the phagocytic, intracellular bactericidal, and reactive oxygen species activities of trout IgM⁺ and IgT⁺ B cells, a phenomenon previously reported only in macrophages, and these activities might also be mediated by the P2X₇ receptor. These results collectively suggest that B cells play multiple roles in the innate immunity of fish, and they provide new evidence for understanding the close relationship between B cells and macrophages in vertebrates.

The effect of environmental temperature on reptilian peripheral blood B cell functions

Recent studies have identified phagocytic B cells in a variety of species, yet little is understood about their function and how it is influenced by natural environmental variation, such as temperature. Phagocytic B-cells are present in red-eared slider turtles, Trachemys scripta, and the wide range of temperatures experienced by these ectotherms may have an effect on immunity, including B cell antibody secretion and phagocytosis. We examined the impact of environmental temperature on B cell function in vitro using phagocytic and ELISpot assays conducted at biologically relevant temperatures. We found a significant effect of temperature on antibody secretion, with maximal antibody secretion occurring at intermediate temperatures (estimated maximum of 28.8°C). There was no effect of temperature on phagocytosis. We also noted a difference in the efficiency of phagocytosis in this assay between B cells and non-B cells. Interestingly, in our in vitro assay, phagocytic B cells engulfed more foreign fluorescent beads per cell than phagocytes lacking surface immunoglobulin. This work sheds light on our understanding of phagocytic B cells and the importance of environmental temperature on the behavior of reptilian immune cells, which may have relevance for organismal fitness.

An Atypical Splenic B Cell Progenitor Population Supports Antibody Production during Plasmodium Infection in Mice

Hematopoietic stem and progenitor cells (HSPCs) function to replenish the immune cell repertoire under steady-state conditions and in response to inflammation due to infection or stress. Whereas the bone marrow serves as the primary niche for hematopoiesis, extramedullary mobilization and differentiation of HSPCs occur in the spleen during acute Plasmodium infection, a critical step in the host immune response. In this study, we identified an atypical HSPC population in the spleen of C57BL/6 mice, with a lineage(-)Sca-1(+)c-Kit(-) (LSK(-)) phenotype that proliferates in response to infection with nonlethal Plasmodium yoelii 17X. Infection-derived LSK(-) cells upon transfer into naive congenic mice were found to differentiate predominantly into mature follicular B cells. However, when transferred into infection-matched hosts, infection-derived LSK(-) cells gave rise to B cells capable of entering into a germinal center reaction, and they developed into memory B cells and Ab-secreting cells that were capable of producing parasite-specific Abs. Differentiation of LSK(-) cells into B cells in vitro was enhanced in the presence of parasitized RBC lysate, suggesting that LSK(-) cells expand and differentiate in direct response to the parasite. However, the ability of LSK(-) cells to differentiate into B cells was not dependent on MyD88, as myd88(-/-) LSK(-) cell expansion and differentiation remained unaffected after Plasmodium infection. Collectively, these data identify a population of atypical lymphoid progenitors that differentiate into B lymphocytes in the spleen and are capable of contributing to the ongoing humoral immune response against Plasmodium infection.

Distinct myeloid progenitor-differentiation pathways identified through single-cell RNA sequencing

According to current models of hematopoiesis, lymphoid-primed multi-potent progenitors (LMPPs) (Lin(-)Sca-1(+)c-Kit(+)CD34(+)Flt3(hi)) and common myeloid progenitors (CMPs) (Lin(-)Sca-1(+)c-Kit(+)CD34(+)CD41(hi)) establish an early branch point for separate lineage-commitment pathways from hematopoietic stem cells, with the notable exception that both pathways are proposed to generate all myeloid innate immune cell types through the same myeloid-restricted pre-granulocyte-macrophage progenitor (pre-GM) (Lin(-)Sca-1(-)c-Kit(+)CD41(-)FcγRII/III(-)CD150(-)CD105(-)). By single-cell transcriptome profiling of pre-GMs, we identified distinct myeloid differentiation pathways: a pathway expressing the gene encoding the transcription factor GATA-1 generated mast cells, eosinophils, megakaryocytes and erythroid cells, and a pathway lacking expression of that gene generated monocytes, neutrophils and lymphocytes. These results identify an early hematopoietic-lineage bifurcation that separates the myeloid lineages before their segregation from other hematopoietic-lineage potential.

Macrophage colony-stimulating factor receptor marks and regulates a fetal myeloid-primed B-cell progenitor in mice

Although it is well established that unique B-cell lineages develop through distinct regulatory mechanisms during embryonic development, much less is understood about the differences between embryonic and adult B-cell progenitor cells, likely to underpin the genetics and biology of infant and childhood PreB acute lymphoblastic leukemia (PreB-ALL), initiated by distinct leukemia-initiating translocations during embryonic development. Herein, we establish that a distinct subset of the earliest CD19(+) B-cell progenitors emerging in the E13.5 mouse fetal liver express the colony-stimulating factor-1 receptor (CSF1R), previously thought to be expressed, and play a lineage-restricted role in development of myeloid lineages, and macrophages in particular. These early embryonic CSF1R(+)CD19(+) ProB cells also express multiple other myeloid genes and, in line with this, possess residual myeloid as well as B-cell, but not T-cell lineage potential. Notably, these CSF1R(+) myeloid-primed ProB cells are uniquely present in a narrow window of embryonic fetal liver hematopoiesis and do not persist in adult bone marrow. Moreover, analysis of CSF1R-deficient mice establishes a distinct role of CSF1R in fetal B-lymphopoiesis. CSF1R(+) myeloid-primed embryonic ProB cells are relevant for infant and childhood PreB-ALLs, which frequently have a bi-phenotypic B-myeloid phenotype, and in which CSF1R-rearrangements have recently been reported.

Goldfish (Carassius auratus L.) as a model system to study the growth factors, receptors and transcription factors that govern myelopoiesis in fish

The process of myeloid cell development (myelopoiesis) in fish has mainly been studied in three cyprinid species: zebrafish (Danio rerio), ginbuna carp (Carassius auratus langsdorfii) and goldfish (C. auratus, L.). Our studies on goldfish myelopoiesis have utilized in vitro generated primary kidney macrophage (PKM) cultures and isolated primary kidney neutrophils (PKNs) cultured overnight to study the process of macrophage (monopoiesis) and neutrophil (granulopoiesis) development and the key growth factors, receptors, and transcription factors that govern this process in vitro. The PKM culture system is unique in that all three subpopulations of macrophage development, namely progenitor cells, monocytes, and mature macrophages, are simultaneously present in culture unlike mammalian systems, allowing for the elucidation of the complex mixture of cytokines that regulate progressive and selective macrophage development from progenitor cells to fully functional mature macrophages in vitro. Furthermore, we have been able to extend our investigations to include the development of erythrocytes (erythropoiesis) and thrombocytes (thrombopoiesis) through studies focusing on the progenitor cell population isolated from the goldfish kidney. Herein, we review the in vitro goldfish model systems focusing on the characteristics of cell sub-populations, growth factors and their receptors, and transcription factors that regulate goldfish myelopoiesis.

In Vivo Deletion of the Cebpa +37 kb Enhancer Markedly Reduces Cebpa mRNA in Myeloid Progenitors but Not in Non-Hematopoietic Tissues to Impair Granulopoiesis

The murine Cebpa gene contains a +37 kb, evolutionarily conserved 440 bp enhancer that directs high-level expression to myeloid progenitors in transgenic mice. The enhancer is bound and activated by Runx1, Scl, GATA2, C/EBPα, c-Myb, Pu.1, and additional Ets factors in myeloid cells. CRISPR/Cas9-mediated replacement of the wild-type enhancer with a variant mutant in its seven Ets sites leads to 20-fold reduction of Cebpa mRNA in the 32Dcl3 myeloid cell line. To determine the effect of deleting the enhancer in vivo, we now characterize C57BL/6 mice in which loxP sites flank a 688 bp DNA segment containing the enhancer. CMV-Cre mediated germline deletion resulted in diminution of the expected number of viable Enh(f/f);CMV-Cre offspring, with 28-fold reduction in marrow Cebpa mRNA but normal levels in liver, lung, adipose, intestine, muscle, and kidney. Cre-transduction of lineage-negative marrow cells in vitro reduced Cebpa mRNA 12-fold, with impairment of granulocytic maturation, morphologic blast accumulation, and IL-3 dependent myeloid colony replating for >12 generations. Exposure of Enh(f/f);Mx1-Cre mice to pIpC led to 14-fold reduction of Cebpa mRNA in GMP or CMP, 30-fold reduction in LSK, and <2-fold reduction in the LSK/SLAM subset. FACS analysis of marrow from these mice revealed 10-fold reduced neutrophils, 3-fold decreased GMP, and 3-fold increased LSK cells. Progenitor cell cycle progression was mildly impaired. Granulocyte and B lymphoid colony forming units were reduced while monocytic and erythroid colonies were increased, with reduced Pu.1 and Gfi1 and increased Egr1 and Klf4 in GMP. Finally, competitive transplantation indicated preservation of functional long-term hematopoietic stem cells upon enhancer deletion and confirmed marrow-intrinsic impairment of granulopoiesis and B cell generation with LSK and monocyte lineage expansion. These findings demonstrate a critical role for the +37 kb Cebpa enhancer for hematopoietic-specific Cebpa expression, with enhancer deletion leading to impaired myelopoiesis and potentially preleukemic progenitor expansion.

Chemokine polyreactivity of IL7Rα+CSF-1R+ lympho-myeloid progenitors in the developing fetal liver

In murine ontogeny, fetal liver is the major hemato- and B-lymphopoietic site until birth. Hematopoiesis develops in largely non-hematopoietic niches, which provide contacts, chemokines and cytokines that induce migration, residence, proliferation and differentiation of progenitors. Within early multipotent progenitors an IL7Rα⁺CSF-1R⁺ subset expressed a mixture of lymphoid- and myeloid-specific genes and differentiated to lymphoid and myeloid lineages in vitro. By contrast, IL7Rα⁺ cells were lymphoid-committed, and CSF-1R⁺ cells were erythro-myeloid-restricted. To respond to a multitude of chemokines single biphenotypic cells expressed CXCR4 and as many as five other chemokine receptors. The monopotent IL7Rα⁺ and CSF-1R⁺progenitors all expressed CXCR4, and mutually exclusive, more restricted sets of the analysed five chemokine receptors. This study proposes that chemokine polyreactive, cytokine-bipotent and monopotent progenitors transmigrate through LYVE-1^high endothelium, attracted by selected chemokines, and reach the IL7- and CSF-1-producing ALCAM^high mesenchymal niche, attracted by other sets of chemokines, to differentiate to B-lymphoid respectively myeloid cells.

The first wave of B lymphopoiesis develops independently of stem cells in the murine embryo

In the developing mouse embryo, there are several waves of hematopoiesis. Primitive and definitive erythromyeloid lineages appear prior to hematopoietic stem cell (HSC) emergence, and these waves are considered to be transient and support embryonic homeostasis until HSC-derived hematopoiesis is established. However, recent evidence strongly suggests that HSC-independent immune cells, such as tissue macrophages and some innate lymphoid cells, develop in the mouse embryo and persist into postnatal life. Innate type B-1 cells have also been reported to emerge from hemogenic endothelial cells in the extraembryonic yolk sac and para-aortic splanchnopleura, and continue to develop in the fetal liver, even in HSC-deficient mouse embryos. Here, this review discusses B-1 cell development in the context of the layered immune system hypothesis of B lymphopoiesis and the emergence of B-1 cells independent of HSCs.

B cell signatures of BCWD-resistant and susceptible lines of rainbow trout: a shift towards more EBF-expressing progenitors and fewer mature B cells in resistant animals

Bacterial cold water disease (BCWD) is a chronic disease of rainbow trout, and is caused by the Gram-negative bacterium Flavobacterium psychrophilum (Fp), a common aquaculture pathogen. The National Center for Cool and Cold Water Aquaculture has bred two genetic lines of rainbow trout: a line of Fp-resistant trout (ARS-Fp-R or R-line trout) and a line of susceptible trout (ARS-Fp-S, or S-line). Little is known about how phenotypic selection alters immune response parameters or how such changes relate to genetic disease resistance. Herein, we quantify interindividual variation in the distribution and abundance of B cell populations (B cell signatures) and examine differences between genetic lines of naive animals. There are limited trout-specific cell surface markers currently available to resolve B cell subpopulations and thus we developed an alternative approach based on detection of differentially expressed transcription factors and intracellular cytokines. B cell signatures were compared between R-line and S-line trout by flow cytometry using antibodies against transcription factors early B cell factor-1 (EBF1) and paired domain box protein Pax5, the pro-inflammatory cytokine IL-1β, and the immunoglobulin heavy chain mu. R-line trout had higher percentages of EBF(+) B myeloid/ progenitor and pre-B cells in PBL, anterior and posterior kidney tissues compared to S-line trout. The opposite pattern was detected in more mature B cell populations: R-line trout had lower percentages of both IgM(+) mature B cells and IgM-secreting cells in anterior kidney and PBL compared to S-line trout. In vitro LPS-activation studies of PBL and spleen cell cultures revealed no significant induction differences between R-line and S-line trout. Together, our findings suggest that selective resistance to BCWD may be associated with shifts in naive animal developmental lineage commitment that result in decreased B lymphopoiesis and increased myelopoiesis in BCWD resistant trout relative to susceptible trout.

Re-thinking the functions of IgA(+) plasma cells

The intestinal mucosa harbors the largest population of antibody (Ab)-secreting plasma cells (PC) in the human body, producing daily several grams of immunoglobulin A (IgA). IgA has many functions, serving as a first-line barrier that protects the mucosal epithelium from pathogens, toxins and food antigens (Ag), shaping the intestinal microbiota, and regulating host-commensal homeostasis. Signals induced by commensal colonization are central for regulating IgA induction, maintenance, positioning and function and the number of IgA(+) PC is dramatically reduced in neonates and germ-free (GF) animals. Recent evidence demonstrates that the innate immune effector molecules tumor necrosis factor α (TNFα) and inducible nitric oxide synthase (iNOS) are required for IgA(+) PC homeostasis during the steady state and infection. Moreover, new functions ascribed to PC independent of Ab secretion continue to emerge, suggesting that PC, including IgA(+) PC, should be re-examined in the context of inflammation and infection. Here, we outline mechanisms of IgA(+) PC generation and survival, reviewing their functions in health and disease.

Functional B-1 progenitor cells are present in the hematopoietic stem cell-deficient embryo and depend on Cbfβ for their development

The fetal liver is a major hematopoietic site containing progenitor cells that give rise to nearly all blood cells, including B-1 cells. Because the fetal liver is not a de novo site of hematopoietic stem cell (HSC) or progenitor-cell emergence, it must be seeded by yolk sac (YS)-derived erythromyeloid progenitors at embryonic day (E) 8.5-E10 and aorta-gonado-mesonephros (AGM)-derived HSCs at E10.5-E11.5. Although the B-1 progenitor cell pool in the fetal liver is considered to be of HSC origin, we have previously proposed that YS-derived B-1 progenitors may also contribute to this pool. Until now, it has been impossible to determine whether HSC-independent B-1 progenitor cells exist in the fetal liver. Here, we demonstrate the presence of transplantable fetal-liver B-1 and marginal zone B progenitor cells in genetically engineered HSC-deficient embryos. HSC-deficient YS and AGM tissues produce B-1 progenitors in vitro and thus may serve as sites of origin for the B-1 progenitors that seed the fetal liver. Furthermore, we have found that core-binding factor beta (Cbfβ) expression is required for fetal-liver B-1 progenitor cell maturation and expansion. Our data provide, to our knowledge, the first evidence for the presence of B-1 progenitor cells in the fetal liver that arise independently of HSCs and implicate Cbfβ as a critical molecule in the development of this lineage.

Murine B-1 B cell progenitors initiate B-acute lymphoblastic leukemia with features of high-risk disease

B-1 and B-2 B cells derive from distinct progenitors that emerge in overlapping waves of development. The number of murine B-1 progenitors peaks during fetal development whereas B-2 B cell production predominates in adult bone marrow. Many genetic mutations that underlie B-acute lymphoblastic leukemia (B-ALL) occur in the fetus, at which time B-1 progenitor numbers are high. However, whether B-ALL can initiate in B-1 progenitors is unknown. In the present study, we report that BCR-ABL-transformed murine B-1 progenitors can be B-ALL cells of origin and demonstrate that they initiate disease more rapidly than do oncogene-expressing B-2 progenitors. We further demonstrate that B-1 progenitors exhibit relative resistance to apoptosis and undergo significant growth following oncogene expression, and we propose that these properties underlie the accelerated kinetics with which they initiate leukemia. These results provide a developmental perspective on the origin of B-ALL and indicate B cell lineage as a factor influencing disease progression.

CD2-positive B-cell precursor acute lymphoblastic leukemia with an early switch to the monocytic lineage

Switches from the lymphoid to myeloid lineage during B-cell precursor acute lymphoblastic leukemia (BCP-ALL) treatment are considered rare and thus far have been detected in MLL-rearranged leukemia. Here, we describe a novel BCP-ALL subset, switching BCP-ALL or swALL, which demonstrated monocytosis early during treatment. Despite their monocytic phenotype, 'monocytoids' share immunoreceptor gene rearrangements with leukemic B lymphoblasts. All swALLs demonstrated BCP-ALL with CD2 positivity and no MLL alterations, and the proportion of swALLs cases among BCP-ALLs was unexpectedly high (4%). The upregulation of CEBPα and demethylation of the CEBPA gene were significant in blasts at diagnosis, prior to the time when most of the switching occurs. Intermediate stages between CD14(neg)CD19(pos)CD34(pos) B lymphoblasts and CD14(pos)CD19(neg)CD34(neg) 'monocytoids' were detected, and changes in the expression of PAX5, PU1, M-CSFR, GM-CSFR and other genes accompanied the switch. Alterations in the Ikaros and ERG genes were more frequent in swALL patients; however, both were altered in only a minority of swALLs. Moreover, switching could be recapitulated in vitro and in mouse xenografts. Although children with swALL respond slowly to initial therapy, risk-based ALL therapy appears the treatment of choice for swALL. SwALL shows that transdifferentiating into monocytic lineage is specifically associated with CEBPα changes and CD2 expression.

Clonally related follicular lymphomas and Langerhans cell neoplasms: expanding the spectrum of transdifferentiation

The traditional model of hematopoiesis is based on unidirectional maturation of hematopoietic precursors into lineage-committed cells. However, recent studies indicate that mature B lymphocytes may demonstrate significant lineage plasticity. We and others have reported transdifferentiation of follicular lymphomas (FLs) into clonally related histiocytic/dendritic cell neoplasms. Here, we describe 2 patients with FL who developed clonally related Langerhans cell neoplasms. The first was a 52-year-old man diagnosed with FL, grade 1. He received immunochemotherapy and had stable disease for 8 years. He then developed increasing lymphadenopathy, and lymph node biopsy showed Langerhans cell sarcoma with no evidence of FL. The second patient was a 77-year-old woman who presented with lymphadenopathy, an abdominal mass, and pulmonary nodules. Lymph node biopsy showed both Langerhans cell histiocytosis and minimal involvement by FL, grade 1. In each case, a combination of immunoglobulin gene rearrangement and fluorescence in situ hybridization studies provided evidence to support a clonal relationship between the FL and Langerhans cell neoplasm. These cases provide striking examples of neoplastic transdifferentiation and expand the spectrum of lesions clonally identical to otherwise typical FL. Awareness of this phenomenon may aid in diagnosis when histologically dissimilar tumors arise synchronously or metachronously in patients with lymphoma.

Human B-1 cells take the stage

B-1 cells play critical roles in defending against microbial invasion and in housekeeping removal of cellular debris. B-1 cells secrete natural antibody and manifest functions that influence T cell expansion and differentiation and in these and other ways differ from conventional B-2 cells. B-1 cells were originally studied in mice where they are easily distinguished from B-2 cells, but their identity in the human system remained poorly defined for many years. Recently, functional criteria for human B-1 cells were established on the basis of murine findings, and reverse engineering resulted in identification of the phenotypic profile, CD20(+)CD27(+)CD43(+)CD70(-), for B-1 cells found in both umbilical cord blood and adult peripheral blood. Human B-1 cells may contribute to multiple disease states through production of autoantibody and stimulation/modulation of T cell activity. Human B-1 cells could be a rich source of antibodies useful in treating diseases present in elderly populations where natural antibody protection may have eroded. Manipulation of human B-1 cell numbers and/or activity may be a new avenue for altering T cell function and treating immune dyscrasias.

Lineage switch in childhood acute leukemia: an unusual event with poor outcome

Although rarely, switches between lymphoid and myeloid lineages may occur during treatment of acute leukemias (AL). Correct diagnosis relies upon confirmation by immunophenotyping of the lineage conversion and certification that the same cytogenetic/molecular alterations remain despite the phenotypic changes. From a total of 1,482 AL pediatric patients, we report nine cases of lineage conversion (0.6%), seven from lymphoid (four Pro-B, two Pre-B, one Common) to myelo-monocytic, and two from myeloid (bilineal, with myeloid predominance) to Pro-B. Eight patients were infants. Switches were suggested by morphology and confirmed with a median of 15 days (range: 8 days-6 months) from initiation of therapy. Of note, in five cases switches occurred before day 15. Stability of the clonal abnormalities was assessed by cytogenetic, RT-PCR/Ig-TCR rearrangement studies in all patients. Abnormalities in 11q23/MLL gene were detected in seven cases. Treatment schedules were ALL (two pts), Interfant-99 (five pts) and AML (two pts) protocols. Despite changing chemotherapy according to the new lineage, all patients died. Our findings support the association of lineage switches with MLL gene alterations and the involvement of a common lymphoid B-myeloid precursor. New therapies should be designed to address these rare cases. Possible mechanisms implicated are discussed.

Novel functions of murine B1 cells: active phagocytic and microbicidal abilities

B1 cells are evolutionarily conserved innate-like cells that share many features with macrophages. It has also been established that B1 cells have a close developmental relationship with macrophages. However, whether B1 cells are able to act as professional phagocytic cells is not clear. In this study, we report that mouse peritoneal cavity (PerC) B cells demonstrate in vivo and in vitro phagocytic activities for Staphylococcus aureus, Escherichia coli, and polystyrene fluorescent microspheres. Approximately 5% of PerC B cells, mainly B1b cells, showed phagocytic activity. Ingested microbes were killed efficiently in the phagolysosome. The antigen-specific B-cell antigen receptor promoted B-cell phagocytosis, resulting in antigen presentation to T cells after uptake of bacteria. Our results reveal for the first time that mouse B1 cells have active phagocytic capabilities and thereby act as a bridge linking innate and adaptive immunity.

B-1 B cell development in the fetus and adult

Models of hematopoiesis often depict lymphocyte production as a uniform process in which a homogenous population of hematopoietic stem cells (HSCs) generates progenitors from which all types of lymphocytes are derived. However, it is increasingly evident that these schemes are too simplistic and that the lymphoid potential of HSCs and precursors arising in the embryo, fetus, neonate, and adult is remarkably distinct. We review recent findings regarding the development of B lymphocytes, and the B-1 B cell lineage in particular, as a case in point. These studies show that B-1 and B-2 B cells involved in innate and adaptive immune responses, respectively, arise in staggered waves of development from distinct progenitors. We discuss the implications of this layered model of B cell development for understanding normal and dysregulated B lymphopoiesis.

Acquisition of a multifunctional IgA+ plasma cell phenotype in the gut

The largest mucosal surface in the body is in the gastrointestinal tract, a location that is heavily colonized by microbes that are normally harmless. A key mechanism required for maintaining a homeostatic balance between this microbial burden and the lymphocytes that densely populate the gastrointestinal tract is the production and transepithelial transport of poly-reactive IgA (ref. 1). Within the mucosal tissues, B cells respond to cytokines, sometimes in the absence of T-cell help, undergo class switch recombination of their immunoglobulin receptor to IgA, and differentiate to become plasma cells. However, IgA-secreting plasma cells probably have additional attributes that are needed for coping with the tremendous bacterial load in the gastrointestinal tract. Here we report that mouse IgA(+) plasma cells also produce the antimicrobial mediators tumour-necrosis factor-α (TNF-α) and inducible nitric oxide synthase (iNOS), and express many molecules that are commonly associated with monocyte/granulocytic cell types. The development of iNOS-producing IgA(+) plasma cells can be recapitulated in vitro in the presence of gut stroma, and the acquisition of this multifunctional phenotype in vivo and in vitro relies on microbial co-stimulation. Deletion of TNF-α and iNOS in B-lineage cells resulted in a reduction in IgA production, altered diversification of the gut microbiota and poor clearance of a gut-tropic pathogen. These findings reveal a novel adaptation to maintaining homeostasis in the gut, and extend the repertoire of protective responses exhibited by some B-lineage cells.

Pivotal advance: peritoneal cavity B-1 B cells have phagocytic and microbicidal capacities and present phagocytosed antigen to CD4+ T cells

Breaking the long-held paradigm that primary B cells are not phagocytic, several studies have demonstrated recently that B cells from fish, amphibians, and reptilians have a significant phagocytic capacity. Whether such capacity has remained conserved in certain mammalian B cell subsets is presently an enigma. Here, we report a previously unrecognized ability of PerC B-1a and B-1b lymphocytes to phagocytose latex beads and bacteria. In contrast, B-2 lymphocytes had an almost negligible ability to internalize these particles. Upon phagocytosis, B-1a and B-1b cells were able to mature their phagosomes into phagolysosomes and displayed the ability to kill internalized bacteria. Importantly, B-1a and B-1b cells effectively present antigen recovered from phagocytosed particles to CD4(+) T cells. However, these cells showed a much lower competence to present soluble antigen or antigen from large, noninternalized particles. B-1 B cells presented particulate and soluble antigen to CD4(+) T cells more efficiently than macrophages, whereas DCs were the most potent APCs. The novel phagocytic and microbicidal abilities identified in B-1 B lymphocytes strengthen the innate nature that has long been attributed to these cells. In the context of adaptive immunity, we show that these innate immune processes are relevant, as they enable B-1 B cells to present phagocytosable particulate antigen. These capacities position these cells at the crossroads that link innate with adaptive immune processes. In a broader context, these newly identified capacities of B-1 B cells further support the previously recognized functional, developmental, and evolutionary relationships between these cells and macrophages.

Human dendritic cell deficiency: the missing ID?

Animal models and human in vitro systems indicate that dendritic cells (DCs) have a crucial role in priming naive T cells, but just how important are they in the intact human? Recent descriptions of human DC deficiency have begun to shed light on this question and to illuminate other puzzles of human DC biology, including their haematopoietic origin, developmental regulation and homeostatic equilibrium with other leukocytes. In this Review, we explore the recently described DC deficiency syndromes, discussing what these have taught us with regard to DC function in humans and the important issues that remain unsolved.

A map for lineage restriction of progenitors during hematopoiesis: the essence of the myeloid-based model

Most hematology and immunology textbooks describe that the first branch point from the hematopoietic stem cells (HSCs) produces two progenitors, one for myelo-erythroid cells and the other for lymphoid cells including T and B cells. This model is based on the concept that the blood cell family can be subdivided into two major lineages, a myelo-erythroid lineage and a lymphoid lineage. Several alternative models have been proposed during the last three decades. We proposed the myeloid-based model in 2001, in which myeloid potential is retained in an early stage of branches toward erythroid, T-, and B-cell lineages. In this review, we focus on the point that cell differentiation models have two different facets: as a map of developmental potential and a cell fate map. These two are expressed in other words as a map for lineage restriction and a map for physiological production routes. We argue that a map of potential is first and foremost essential for the study of molecular mechanisms of lineage commitment, which is the least clarified aspect of cell differentiation. The validity of the myeloid-based model of hematopoiesis will be discussed in reference to these two issues, developmental potential and cell fate.

ADAM10 overexpression shifts lympho- and myelopoiesis by dysregulating site 2/site 3 cleavage products of Notch

Although the physiological consequences of Notch signaling in hematopoiesis have been extensively studied, the differential effects of individual notch cleavage products remain to be elucidated. Given that ADAM10 is a critical regulator of Notch and that its deletion is embryonically lethal, we generated mice that overexpress ADAM10 (ADAM10 transgenic [A10Tg]) at early stages of lympho- and myeloid development. Transgene expression resulted in abrogated B cell development, delayed T cell development in the thymus, and unexpected systemic expansion of CD11b(+)Gr-1(+) cells, also known as myeloid-derived suppressor cells. Mixed bone marrow reconstitution assays demonstrated that transgene expression altered hematopoiesis via a cell-intrinsic mechanism. Consistent with previously reported observations, we hypothesized that ADAM10 overexpression dysregulated Notch by uncoupling the highly regulated proteolysis of Notch receptors. This was confirmed using an in vitro model of hematopoiesis via culturing A10Tg hematopoietic Lineage(-)Sca-1(+)c-Kit(+) cells with OP-9 stromal cells in the presence or absence of Delta-like 1, a primary ligand for Notch. Blockade of the site 2 (S2) and site 3 (S3) cleavage of the Notch receptor demonstrated differential effects on hematopoiesis. OP9-DL1 cultures containing the ADAM10 inhibitor (S2 cleavage site) enhanced and rescued B cell development from wild-type and A10Tg Lineage(-)Sca-1(+)c-Kit(+) cells, respectively. In contrast, blockade of γ-secretase at the S3 cleavage site induced accumulation of the S2 product and consequently prevented B cell development and resulted in myeloid cell accumulation. Collectively, these findings indicate that the differential cleavage of Notch into S2 and S3 products regulated by ADAM10 is critical to hematopoietic cell-fate determination.

How B cells influence bone biology in health and disease

It is now well established that important regulatory interactions occur between the cells in the hematopoietic, immune and skeletal systems (osteoimmunology). B lymphocytes (B cells) are responsible for the generation and production of antibodies or immunoglobulins in the body. Together with T cells these lymphocytes comprise the adaptive immune system, which allows an individual to develop specific responses to an infection and retain memory of that infection, allowing for a faster and more robust response if that same infection occurs again. In addition to this immune function, B cells have a close and multifaceted relationship with bone cells. B cells differentiate from hematopoietic stem cells (HSCs) in supportive niches found on endosteal bone surfaces. Cells in the osteoblast lineage support HSC and B cell differentiation in these niches. B cell differentiation is regulated, at least in part, by a series of transcription factors that function in a temporal manner. While these transcription factors are required for B cell differentiation, their loss causes profound changes in the bone phenotype. This is due, in part, to the close relationship between macrophage/osteoclast and B cell differentiation. Cross talk between B cells and bone cells is reciprocal with defects in the RANKL-RANK, OPG signaling axis resulting in altered bone phenotypes. While the role of B cells during normal bone remodeling appears minimal, activated B cells play an important role in many inflammatory diseases with associated bony changes. This review examines the relationship between B cells and bone cells and how that relationship affects the skeleton and hematopoiesis during health and disease.

The phosphatidylserine receptors, T cell immunoglobulin mucin proteins 3 and 4, are markers of histiocytic sarcoma and other histiocytic and dendritic cell neoplasms

The T cell immunoglobulin mucin (TIM) proteins are a family of cell surface phosphatidyserine receptors that are important for the recognition and phagocytosis of apoptotic cells. Because TIM-4 is expressed by macrophages and dendritic cells in human tissue, we examined its expression in a range of histiocytic and dendritic cell neoplasms and found moderate to strong immunohistochemical staining in cases of juvenile xanthogranuloma and histiocytic sarcoma, and lower level staining in interdigitating dendritic cell sarcoma, Langerhans cell histiocytosis, acute monocytic leukemia (leukemia cutis), and blastic plasmacytoid dendritic cell neoplasm (hematodermic tumor). TIM-3 was first described on activated T(H)1 cells but was recently shown to also be a phosphatidylserine receptor and mediate phagocytosis. We found TIM-3 was expressed by peritoneal macrophages, monocytes and splenic dendritic cells. We found that it, like TIM-4, is expressed in a range of histiocytic and dendritic cell neoplasms, typically with strong immunohistochemical staining. Cases of diffuse large B cell lymphoma, anaplastic large cell lymphoma, metastatic malignant melanoma, and metastatic poorly differentiated carcinoma generally exhibited negative to minimal heterogenous staining for TIM-4 and TIM-3. We conclude that histiocytic and dendritic cell neoplasms consistently express TIM-3 and TIM-4 and that these molecules are new markers of neoplasms derived from histiocytic and dendritic cells.

Revised map of the human progenitor hierarchy shows the origin of macrophages and dendritic cells in early lymphoid development

The classical model of hematopoiesis posits the segregation of lymphoid and myeloid lineages as the earliest fate decision. The validity of this model in the mouse has been questioned; however, little is known about the lineage potential of human progenitors. Here we provide a comprehensive analysis of the human hematopoietic hierarchy by clonally mapping the developmental potential of seven progenitor classes from neonatal cord blood and adult bone marrow. Human multilymphoid progenitors, identified as a distinct population of Thy-1(neg-lo)CD45RA(+) cells in the CD34(+)CD38(-) stem cell compartment, gave rise to all lymphoid cell types, as well as monocytes, macrophages and dendritic cells, which indicated that these myeloid lineages arise in early lymphoid lineage specification. Thus, as in the mouse, human hematopoiesis does not follow a rigid model of myeloid-lymphoid segregation.

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.

A new paradigm for hematopoietic cell lineages: revision of the classical concept of the myeloid-lymphoid dichotomy

The concept that blood cells arising from hematopoietic stem cells (HSC) can be subdivided into two major lineages, a myelo-erythroid and a lymphoid lineage, has long persisted. Indeed, it has become almost axiomatic that the first branch point from the HSC produces two progenitors, one for myelo-erythroid cells and the other for lymphoid cells. However, recent studies have provided a battery of findings that cannot be explained by this classical model. We will outline how this classical model arose before describing how we came to propose an alternative 'myeloid-based model', in which myeloid potential is retained in erythroid, T, and B cell branches even after these lineages have segregated from each other.

Resolution of unique Sca-1highc-Kit- lymphoid-biased progenitors in adult bone marrow

We have identified a distinctive lymphoid-restricted progenitor population in adult mouse bone marrow based on a unique c-Kit(-)Sca-1(high)Flt3(+) AA4(+) surface phenotype. These cells are highly lymphoid biased and rapidly generate B and T cells after adoptive transfer. However, whereas previously described lymphoid progenitors such as common lymphoid progenitors express TdT and relatively high levels of RAG2, and are enriched for cells with an active V(D)J recombinase, Flt3(+) AA4(+) cells within the c-Kit(-)Sca-1(high) bone marrow fraction are TdT(-), are RAG2(low), and do not display evidence for ongoing or past recombinase activity. Furthermore, unlike common lymphoid progenitors that readily generate B cells upon stimulation with IL-7, c-Kit(-)Sca-1(high)Flt3(+) precursors do not express abundant levels of the IL-7R, and require costimulation with Flt3 ligand and IL-7 to generate B cells in vitro. Moreover, these findings suggest that hematopoietic stem cells in adults generate an array of lymphoid-biased progenitor populations characterized by distinct gene expression and cytokine response profiles.