Thymic stromal-derived lymphopoietin distinguishes fetal from adult B cell development

Lymphocyte pathway analysis using naturally lymphocyte-deficient fish

Comparative phylogenetic analyses are of potential value to establish the essential components of genetic networks underlying physiological traits. For species that naturally lack particular lymphocyte lineages, we show here that this strategy readily distinguishes trait-specific actors from pleiotropic components of the genetic network governing lymphocyte differentiation. Previously, three of the four members of the DNA polymerase X family have been implicated in the junctional diversification process during the somatic assembly of antigen receptors. Our phylogenetic analysis indicates that the presence of terminal deoxynucleotidyl transferase is strictly associated with the facility of V(D)J recombination, whereas PolL and PolM genes are retained even in species lacking Rag-mediated somatic diversification of antigen receptor genes.

Exploring the reprogramming potential of B cells and comprehending its clinical and therapeutic perspective

Initiating from multipotent progenitors, the lineages extrapolated from hematopoietic stem cells are determined by transcription factors specific to each of them. The commitment factors assist in the differentiation of progenitor cells into terminally differentiated cells. B lymphocytes constitute a population of cells that expresses clonally diverse cell surface immunoglobulin (Ig) receptors specific to antigenic epitopes. B cells are a significant facet of the adaptive immune system. The secreted antibodies corresponding to the B cell recognize the antigens via the B cell receptor (BCR). Following antigen recognition, the B cell is activated and thereafter undergoes clonal expansion and proliferation to become memory B cells. The essence of 'cellular reprogramming' has aided in reliably altering the cells to desired tissue type. The potential of reprogramming has been harnessed to decipher and find solutions for various genetically inherited diseases and degenerative disorders. B lymphocytes can be reprogrammed to their initial naive state from where they get differentiated into any lineage or cell type similar to a pluripotent stem cell which can be accomplished by the deletion of master regulators of the B cell lineage. B cells can be reprogrammed into pluripotent stem cells and also can undergo transdifferentiation at the midway of cell differentiation to other cell types. Mandated expression of C/EBP in specialized B cells corresponds to their fast and effective reprogramming into macrophages, reversing the cell fate of these lymphocytes and allowing them to differentiate freshly into other types of cells. The co-expression of C/EBPα and OKSM (Oct4, Sox2, Klf4, c-Myc) amplified the reprogramming efficiency of B lymphocytes. Various human somatic cells including the immune cells are compliant to reprogramming which paves a path for opportunities like autologous tissue grafts, blood transfusion, and cancer immunotherapy. The ability to reprogram B cells offers an unprecedented opportunity for developing a therapeutic approach for several human diseases. Here, we will focus on all the proteins and transcription factors responsible for the developmental commitment of B lymphocytes and how it is harnessed in various applications.

IL-7R signaling activates widespread VH and DH gene usage to drive antibody diversity in bone marrow B cells

Generation of the primary antibody repertoire requires V(D)J recombination of hundreds of gene segments in the immunoglobulin heavy chain (Igh) locus. The role of interleukin-7 receptor (IL-7R) signaling in Igh recombination has been difficult to partition from its role in B cell survival and proliferation. With a detailed description of the Igh repertoire in murine IL-7Rα-/- bone marrow B cells, we demonstrate that IL-7R signaling profoundly influences VH gene selection during VH-to-DJH recombination. We find skewing toward 3' VH genes during de novo VH-to-DJH recombination more severe than the fetal liver (FL) repertoire and uncover a role for IL-7R signaling in DH-to-JH recombination. Transcriptome and accessibility analyses suggest reduced expression of B lineage transcription factors (TFs) and targets and loss of DH and VH antisense transcription in IL-7Rα-/- B cells. Thus, in addition to its roles in survival and proliferation, IL-7R signaling shapes the Igh repertoire by activating underpinning mechanisms.

Deregulation of the Interleukin-7 Signaling Pathway in Lymphoid Malignancies

The cytokine interleukin-7 (IL-7) and its receptor are critical for lymphoid cell development. The loss of IL-7 signaling causes severe combined immunodeficiency, whereas gain-of-function alterations in the pathway contribute to malignant transformation of lymphocytes. Binding of IL-7 to the IL-7 receptor results in the activation of the JAK-STAT, PI3K-AKT and Ras-MAPK pathways, each contributing to survival, cell cycle progression, proliferation and differentiation. Here, we discuss the role of deregulated IL-7 signaling in lymphoid malignancies of B- and T-cell origin. Especially in T-cell leukemia, more specifically in T-cell acute lymphoblastic leukemia and T-cell prolymphocytic leukemia, a high frequency of mutations in components of the IL-7 signaling pathway are found, including alterations in IL7R, IL2RG, JAK1, JAK3, STAT5B, PTPN2, PTPRC and DNM2 genes.

Cell circuits and niches controlling B cell development

Studies over the last decade uncovered overlapping niches for hematopoietic stem cells (HSCs), multipotent progenitor cells, common lymphoid progenitors, and early B cell progenitors. HSC and lymphoid niches are predominantly composed by mesenchymal progenitor cells (MPCs) and by a small subset of endothelial cells. Niche cells create specialized microenvironments through the concomitant production of short-range acting cell-fate determining cytokines such as interleukin (IL)-7 and stem cell factor and the potent chemoattractant C-X-C motif chemokine ligand 12. This type of cellular organization allows for the cross-talk between hematopoietic stem and progenitor cells with niche cells, such that niche cell activity can be regulated by the quality and quantity of hematopoietic progenitors being produced. For example, preleukemic B cell progenitors and preB acute lymphoblastic leukemias interact directly with MPCs, and downregulate IL-7 expression and the production of non-leukemic lymphoid cells. In this review, we discuss a novel model of B cell development that is centered on cellular circuits formed between B cell progenitors and lymphopoietic niches.

Interleukin-7 Receptor Alpha in Innate Lymphoid Cells: More Than a Marker

Innate lymphoid cells (ILCs) are a group of immune cells that are important for defense against pathogens, tissue repair, and lymphoid organogenesis. They share similar characteristics with various subsets of helper T cells but lack specific antigen receptors. Interleukin-7 (IL-7) and thymic stromal lymphopoietin (TSLP) are cytokines that engage the IL-7Rα and have major roles in dictating the fate of ILCs. Recent advances in the field have revealed transcriptional programs associated with ILC development and function. In this article, we will review recent studies of the role of IL-7 and TSLP in ILC development and function during infection and inflammation.

Direct role of FLT3 in regulation of early lymphoid progenitors

Given that FLT3 expression is highly restricted on lymphoid progenitors, it is possible that the established role of FLT3 in the regulation of B and T lymphopoiesis reflects its high expression and role in regulation of lymphoid-primed multipotent progenitors (LMPPs) or common lymphoid progenitors (CLPs). We generated a Flt3 conditional knock-out (Flt3^fl/fl) mouse model to address the direct role of FLT3 in regulation of lymphoid-restricted progenitors, subsequent to turning on Rag1 expression, as well as potentially ontogeny-specific roles in B and T lymphopoiesis. Our studies establish a prominent and direct role of FLT3, independently of the established role of FLT3 in regulation of LMPPs and CLPs, in regulation of fetal as well as adult early B cell progenitors, and the early thymic progenitors (ETPs) in adult mice but not in the fetus. Our findings highlight the potential benefit of targeting poor prognosis acute B-cell progenitor leukaemia and ETP leukaemia with recurrent FLT3 mutations using clinical FLT3 inhibitors.

PU.1 Is Required for the Developmental Progression of Multipotent Progenitors to Common Lymphoid Progenitors

The transcription factor PU.1 is required for the development of mature myeloid and lymphoid cells. Due to this essential role and the importance of PU.1 in regulating several signature markers of lymphoid progenitors, its precise function in early lymphopoiesis has been difficult to define. Here, we demonstrate that PU.1 was required for efficient generation of lymphoid-primed multipotent progenitors (LMPPs) from hematopoietic stem cells and was essential for the subsequent formation of common lymphoid progenitors (CLPs). By contrast, further differentiation into the B-cell lineage was independent of PU.1. Examination of the transcriptional changes in conditional progenitors revealed that PU.1 activates lymphoid genes in LMPPs, while repressing genes normally expressed in neutrophils. These data identify PU.1 as a critical regulator of lymphoid priming and the transition between LMPPs and CLPs.

Asynchronous lineage priming determines commitment to T cell and B cell lineages in fetal liver

The molecular events that initiate lymphoid-lineage specification remain unidentified because the stages of differentiation during which lineage commitment occurs are difficult to characterize. We isolated fetal liver progenitor cells undergoing restriction of their differentiation potential toward the T cell-innate lymphoid cell lineage or the B cell lineage. Transcripts that defined the molecular signatures of these two subsets were sequentially upregulated in lympho-myeloid precursor cells and in common lymphoid progenitor cells, respectively, and this preceded lineage restriction; this indicates that T cell-versus-B cell commitment is not a binary fate 'decision'. The T cell-bias and B cell-bias transcriptional programs were frequently co-expressed in common lymphoid progenitor cells and were segregated in subsets biased toward T cell differentiation or B cell differentiation, after interleukin 7 (IL-7) signaling that controlled the number of progenitor cells engaging in T cell differentiation versus B cell differentiation.

Permissive roles of cytokines interleukin-7 and Flt3 ligand in mouse B-cell lineage commitment

Hematopoietic cells are continuously generated throughout life from hematopoietic stem cells, thus making hematopoiesis a favorable system to study developmental cell lineage commitment. The main factors incorporating environmental signals to developing hematopoietic cells are cytokines, which regulate commitment of hematopoietic progenitors to the different blood lineages by acting either in an instructive or a permissive manner. Fms-like tyrosine kinase-3 (Flt3) ligand (FL) and Interleukin-7 (IL-7) are cytokines pivotal for B-cell development, as manifested by the severely compromised B-cell development in their absence. However, their precise role in regulating B-cell commitment has been the subject of debate. In the present study we assessed the rescue of B-cell commitment in mice lacking IL-7 but simultaneously overexpressing FL. Results obtained demonstrate that FL overexpression in IL-7-deficient mice rescues B-cell commitment, resulting in significant Ebf1 and Pax5 expression in Ly6D⁺CD135⁺CD127⁺CD19^- precursors and subsequent generation of normal numbers of CD19⁺ B-cell progenitors, therefore indicating that IL-7 can be dispensable for commitment to the B-cell lineage. Further analysis of Ly6D⁺CD135⁺CD127⁺CD19^- progenitors in IL-7- or FL-deficient mice overexpressing Bcl2, as well as in IL-7 transgenic mice suggests that both FL and IL-7 regulate B-cell commitment in a permissive manner: FL by inducing proliferation of Ly6D⁺CD135⁺CD127⁺CD19^- progenitors and IL-7 by providing survival signals to these progenitors.

Decreased IL7Rα and TdT expression underlie the skewed immunoglobulin repertoire of human B-cell precursors from fetal origin

Newborns are unable to mount antibody responses towards certain antigens. This has been related to the restricted repertoire of immunoglobulin (Ig) genes of their B cells. The mechanisms underlying the restricted fetal Ig gene repertoire are currently unresolved. We here addressed this with detailed molecular and cellular analysis of human precursor-B cells from fetal liver, fetal bone marrow (BM), and pediatric BM. In the absence of selection processes, fetal B-cell progenitors more frequently used proximal V, D and J genes in complete IGH gene rearrangements, despite normal Ig locus contraction. Fewer N-nucleotides were added in IGH gene rearrangements in the context of low TdT and XRCC4 expression. Moreover, fetal progenitor-B cells expressed lower levels of IL7Rα than their pediatric counterparts. Analysis of progenitor-B cells from IL7Rα-deficient patients revealed that TdT expression and N-nucleotides additions in Dh-Jh junctions were dependent on functional IL7Rα. Thus, IL7Rα affects TdT expression, and decreased expression of this receptor underlies at least in part the skewed Ig repertoire formation in fetal B-cell precursors. These new insights provide a better understanding of the formation of adaptive immunity in the developing fetus.

Distinct Genetic Networks Orchestrate the Emergence of Specific Waves of Fetal and Adult B-1 and B-2 Development

B cell development is often depicted as a linear process initiating in the fetus and continuing postnatally. Using a PU.1 hypomorphic mouse model, we found that B-1 and B-2 lymphopoiesis occurred in distinct fetal and adult waves differentially dependent on the Sfpi1 14 kB upstream regulatory element. The initial wave of fetal B-1 development was absent in PU.1 hypomorphic mice, while subsequent fetal and adult waves emerged. In contrast, B-2 lymphopoiesis occurred in distinct fetal and adult waves. Whole-transcriptome profiling of fetal and adult B cell progenitors supported the existence of three waves of B-1 and two waves of B-2 development and revealed that the network of transcription factors governing B lineage specification and commitment was highly divergent between B-1 and B-2 progenitors. These findings support the view that the B-1 and B-2 lineages are distinct and provide a genetic basis for layering of immune system development.

BRPF1 is essential for development of fetal hematopoietic stem cells

Hematopoietic stem cells (HSCs) serve as a life-long reservoir for all blood cell types and are clinically useful for a variety of HSC transplantation-based therapies. Understanding the role of chromatin organization and regulation in HSC homeostasis may provide important insights into HSC development. Bromodomain- and PHD finger-containing protein 1 (BRPF1) is a multivalent chromatin regulator that possesses 4 nucleosome-binding domains and activates 3 lysine acetyltransferases (KAT6A, KAT6B, and KAT7), suggesting that this protein has the potential to stimulate crosstalk between different chromatin modifications. Here, we investigated the function of BRPF1 in hematopoiesis by selectively deleting its gene in murine blood cells. Brpf1-deficient pups experienced early lethality due to acute bone marrow failure and aplastic anemia. The mutant bone marrow and fetal liver exhibited severe deficiency in HSCs and hematopoietic progenitors, along with elevated reactive oxygen species, senescence, and apoptosis. BRPF1 deficiency also reduced the expression of multipotency genes, including Slamf1, Mecom, Hoxa9, Hlf, Gfi1, Egr, and Gata3. Furthermore, BRPF1 was required for acetylation of histone H3 at lysine 23, a highly abundant but not well-characterized epigenetic mark. These results identify an essential role of the multivalent chromatin regulator BRPF1 in definitive hematopoiesis and illuminate a potentially new avenue for studying epigenetic networks that govern HSC ontogeny.

Inhibition of B Lymphopoiesis by Adipocytes and IL-1-Producing Myeloid-Derived Suppressor Cells

B lymphopoiesis declines with age, and this decline correlates with increased adipose tissue in the bone marrow (BM). Also, adipocyte-derived factors are known to inhibit B lymphopoiesis. Using cocultures of mouse BM cells with OP9 stromal cells, we found that adipocyte-conditioned medium induces the generation of CD11b(+)Gr1(+) myeloid cells, which inhibit B cell development in vitro. Adipocyte-conditioned medium-induced CD11b(+)Gr1(+) cells express Arg1 (arginase) and Nos2 (inducible NO synthase) and suppress CD4(+) T cell proliferation, indicating that these cells are myeloid-derived suppressor cells (MDSCs). Blocking arginase and inducible NO synthase did not restore B lymphopoiesis, indicating that inhibition is not mediated by these molecules. Transwell and conditioned-medium experiments showed that MDSCs inhibit B lymphopoiesis via soluble factors, and by cytokine array we identified IL-1 as an important factor. Addition of anti-IL-1 Abs restored B lymphopoiesis in BM cultures containing MDSCs, showing that MDSC inhibition of B lymphopoiesis is mediated by IL-1. By treating hematopoietic precursors with IL-1, we found that multipotent progenitors are targets of IL-1. This study uncovers a novel function for MDSCs to inhibit B lymphopoiesis through IL-1. We suggest that inflammaging contributes to a decline of B lymphopoiesis in aged individuals, and furthermore, that MDSCs and IL-1 provide therapeutic targets for restoration of B lymphopoiesis in aged and obese individuals.

Thymic stromal lymphopoietin: a central regulator of allergic asthma

INTRODUCTION

Epithelial cell-derived mediators have emerged as key players for instigating local remodeling and the associated cellular inflammation in asthmatic airways. In particular, the epithelial-derived cytokine, thymic stromal lymphopoietin (TSLP), has been identified as a master switch for allergic inflammation.

AREAS COVERED

TSLP is expressed by structural and immune cells at the site of allergen entry in the airways. Stimuli for release of TSLP include common triggers of asthma symptoms, and TSLP levels correlate with disease severity. TSLP regulates helper T cell 2 (Th2) humoral immunity through upregulating OX40L on dendritic cells (DCs), which drives Th2 lymphocytes; however, activation of several other cells by TSLP also supports the development of Th2 inflammation. Animal models of asthma demonstrate that increased levels of TSLP can induce many of the characteristics of asthma.

EXPERT OPINION

The work conducted to date supports a critical role of TSLP in the pathogenesis of allergic asthma. The first clinical trial to block the downstream effects of OX40L has shown reduced levels of circulating IgE and airway eosinophils, confirming the importance of TSLP-induced OX40L levels on DCs. Clinical trials with TSLP blockade are underway and will unequivocally confirm whether TSLP is indeed a key driver of allergic inflammation in asthma.

B-cell progenitors and precursors change their microenvironment in fetal liver during early development

The microenvironments, in which B lymphocytes develop in fetal liver, are largely still unknown. Among the nonhematopoietic cells, we have identified and FACS-separated two subpopulations, CD45(-) TER119(-) VCAM-1(+) cells that are either CD105(high) LYVE-1(high) or CD105(low) ALCAM(high) . Immunohistochemical analyses find three of four c-Kit(+) IL-7Rα(+) B220(low) CD19(-) SLC(-) B progenitors in contact with vascular endothelial-type LYVE-1(high) cells on embryonic day 13.5. One day later c-Kit(+) IL-7Rα(+) cells develop to CD19(- and +) , SLC-expressing, DHJH-rearranged pre/pro and pro/preB-I cells. Less than 10% are still in contact with LYVE-1(high) cells, but half of them are now in contact with mesenchymally derived ALCAM(high) liver cells. All of these ALCAM(high) cells, but not the LYVE-1(high) cells produce IL-7 and CXCL12, while both produce CXCL10. Progenitors and pro/preB-I cells are chemoattracted in vitro toward CXCL10 and 12, suggesting that lymphoid progenitors with Ig gene loci in germline configuration enter the developing fetal liver at E13.5 from vascular endothelium, attracted by CXCL10, and then migrate within a day to an ALCAM(high) liver cell microenvironment, differentiating to DHJH-rearranging, surrogate light chain-expressing pre/proB and pro/preB-I cells, attracted by CXCL10 and 12. Between E15.5 and E16.5 preB-I cells expand 10-fold in continued contact with ALCAM(high) cells and begin VH- to DHJH-rearrangements in further differentiated c-Kit(-) IL-7Rα(-) preBII cells. STEM Cells 2013;31:2800-2812.

Mechanisms of pre-B-cell receptor checkpoint control and its oncogenic subversion in acute lymphoblastic leukemia

Pre-B cells within the bone marrow represent the normal counterpart for most acute lymphoblastic leukemia (ALL). During normal early B-cell development, survival and proliferation signals are dominated by cytokines, particularly interleukin-7 (IL-7) for murine developing B cells. With expression of a functional pre-B-cell receptor (BCR), cytokine signaling is attenuated and the tonic/autonomous pre-BCR signaling pathway provides proliferation as well as differentiation signals. In this review, we first describe checkpoint mechanisms during normal B-cell development and then discuss how genetic lesions in these pathways function as oncogenic mimicries and allow transformed pre-B cells to bypass checkpoint control. We focus on cytokine receptor signaling that is mimicked by activating lesions in receptor subunits or downstream mediators as well as aberrant activation of non-B lymphoid cytokine receptors. Furthermore, we describe the molecular switch from cytokine receptor to pre-BCR signaling, how this pathway is of particular importance for certain ALL subtypes, and how pre-BCR signaling is engaged by genetic lesions, such as BCR-ABL1. We discuss the transcriptional control mechanisms downstream of both cytokine- and pre-BCR signaling and how normal checkpoint control mechanisms are circumvented in pre-B ALL. Finally, we highlight new therapeutic concepts for targeted inhibition of oncogenic cytokine or pre-BCR signaling pathways.

Human lymphoid development in the absence of common γ-chain receptor signaling

Despite the power of model systems to reveal basic immunologic mechanisms, critical differences exist between species that necessitate the direct study of human cells. Illustrating this point is the difference in phenotype between patients with SCID caused by mutations affecting the common γ-chain (γc) cytokine signaling pathway and mice with similar mutations. Although in both species, null mutations in either IL-2RG (which encodes γc), or its direct downstream signaling partner JAK3, result in T and NK cell deficiency, an associated B cell deficiency is seen in mice but not in humans with these genetic defects. In this study, we applied recent data that have revised our understanding of the earliest stages of lymphoid commitment in human bone marrow (BM) to determine the requirement for signaling through IL-2RG and JAK3 in normal development of human lymphoid progenitors. BM samples from SCID patients with IL-2RG (n = 3) or JAK3 deficiency (n = 2), which produce similar "T-NK-B+" clinical phenotypes, were compared with normal BM and umbilical cord blood as well as BM from children on enzyme treatment for adenosine deaminase-deficient SCID (n = 2). In both IL-2RG- and JAK3-SCID patients, the early stages of lymphoid commitment from hematopoietic stem cells were present with development of lymphoid-primed multipotent progenitors, common lymphoid progenitors and B cell progenitors, normal expression patterns of IL-7RA and TLSPR, and the DNA recombination genes DNTT and RAG1. Thus, in humans, signaling through the γc pathway is not required for prethymic lymphoid commitment or for DNA rearrangement.

The development and survival but not function of follicular B cells is dependent on IL-7Rα Tyr449 signaling

IL-7 is a critical cytokine for lymphocyte development. Recent work has highlighted critical roles for IL-7 signaling in mature T cell homeostasis and function, but its role in B cells is less well characterized. Using a knock-in mouse possessing a Tyr to Phe mutation at position 449 (IL-7Rα(449F/449F) mice) within the cytoplasmic SH2-binding motif of IL-7Rα, we evaluated the role of IL-7Rα Y449 motif in spleen B cells. IL-7Rα(449F/449F) mice had reduced numbers and increased death of follicular B cells compared to WT, but had significantly more follicular cells than IL-7Rα(-/-). The death of IL-7Rα(449F/449F) follicular cells was not due to a failure to respond to BAFF or lower levels of BAFF, a critical B cell survival factor. Marginal zone B cells were unaffected by the IL-7Rα(449F/449F) mutation. Any role for TSLP was ruled out, as TSLPR(-/-) mice had an identical B cell phenotype to wild-type mice. Bone marrow chimeras and the absence of IL-7Rα on B cells suggested that IL-7 did not directly regulate mature B cells, but that an IL-7-responsive cell was influencing B cells. IL-7 was also critical at the checkpoint between the T1 and T2 stages in the spleen. IL-7Rα(-/-) mice fail to develop T2 cells, but IL-7Rα(449F/449F) show a reduction compared to WT but not complete absence of T2 cells. We also tested the functional responses of IL-7Rα(449F/449F) to antigens and infection and found no difference in antibody responses to T-dependent or T-independent antigens, or to Influenza/A. IL-7 was important for generation of antibody responses to the intestinal worm H. polygyrus and for naive levels of IgA. Taken together, this suggests that IL-7 regulates follicular B cell numbers and survival in a cell-extrinsic manner, via a bone-marrow derived cell, but is not critical for antibody production outside the gut.

Lack of functional TSLP receptors mitigates Th2 polarization and the establishment and growth of 4T1 primary breast tumours but has different effects on tumour quantities in the lung and brain

The 4T1 mammary carcinoma cell line produces TSLP. We had hypothesized that TSLP promotes the development of a permissive environment for the growth and metastasis of primary tumour and that this is associated with a Th2-polarized antitumour immune response. We found that, in Tslpr(-/-) mice, the mean tumour diameters were smaller from days 27 to 40, and relatively fewer tumour cells were present in the lung, compared with wild-type mice. Polarization of the Th2 cytokine profile was also diminished in Tslpr(-/-) mice. These findings confirmed those reported previously by others. Here, we further show that primary tumours are established less often in Tslpr(-/-) mice and that, unexpectedly, the relative number of tumour cells in the brain is greater in Tslpr(-/-) mice compared with wild-type mice. Findings from our cytotoxicity assays show that 4T1-directed lysis is undetectable in both WT and Tslpr(-/-) mice, ruling out the possibility that altered cytotoxic responses in Tslpr(-/-) mice are responsible for the differences we observed. In a human tissue microarray, positive staining for TSLP was seen in tumour cells from breast cancer tissue, but it was also seen in normal glandular epithelial cells from normal breast tissue, which has not been shown before. Thus, our findings provide new insight into the effects of TSLP in metastatic breast cancer.

Transcriptional control of pre-B cell development and leukemia prevention

The differentiation of early B cell progenitors is controlled by multiple transcriptional regulators and growth-factor receptors. The triad of DNA-binding proteins, E2A, EBF1, and PAX5 is critical for both the early specification and commitment of B cell progenitors, while a larger number of secondary determinants, such as members of the Ikaros, ETS, Runx, and IRF families have more direct roles in promoting stage-specific pre-B gene-expression program. Importantly, it is now apparent that mutations in many of these transcription factors are associated with the progression to acute lymphoblastic leukemia. In this review, we focus on recent studies that have shed light on the transcriptional hierarchy that controls efficient B cell commitment and differentiation as well as focus on the oncogenic consequences of the loss of many of the same factors.

The biology of thymic stromal lymphopoietin (TSLP)

Originally shown to promote the growth and activation of B cells, thymic stromal lymphopoietin (TSLP) is now known to have wide-ranging impacts on both hematopoietic and nonhematopoietic cell lineages, including dendritic cells, basophils, eosinophils, mast cells, CD4⁺, CD8⁺ and natural killer T cells, B cells and epithelial cells. While TSLP's role in the promotion of TH2 responses has been extensively studied in the context of lung- and skin-specific allergic disorders, it is becoming increasingly clear that TSLP may impact multiple disease states within multiple organ systems, including the blockade of TH1/TH17 responses and the promotion of cancer and autoimmunity. This chapter will highlight recent advances in the understanding of TSLP signal transduction, as well as the role of TSLP in allergy, autoimmunity and cancer. Importantly, these insights into TSLP's multifaceted roles could potentially allow for novel therapeutic manipulations of these disorders.

Regulation of murine natural killer cell commitment

Natural killer (NK) cells can derive from the same precursors as B and T cells, however, to achieve lineage specificity, several transcription factors need to be activated or annulled. While a few important transcription factors have been identified for NK genesis the mechanisms of how this is achieved is far from resolved. Adding to the complexity of this, NK cells are found and potentially develop in diverse locations in vivo and it remains to be addressed if a common NK cell precursor seeds diverse niches and how transcription factors may differentially regulate NK cell commitment in distinct microenvironments. Here we will summarize some recent findings in NK cell commitment and discuss how a NK cell transcriptional network might be organized, while addressing some misconceptions and anomalies along the way.

Functions of thymic stromal lymphopoietin in immunity and disease

Thymic stromal lymphopoietin (TSLP) is an interleukin 7-like cytokine expressed mainly by epithelial cells. Current studies provide compelling evidence that TSLP is capable of activating dendritic cells to promote T helper (Th) 2 immune responses. TSLP has also been shown to directly promote Th2 differentiation of naïve CD4(+) T cell and activate natural killer T cells, basophils and other innate immune cells at the initial stage of inflammation. In addition, TSLP affects B cell maturation and activation and can also influence regulatory T (Treg) cell differentiation and development. TSLP-induced Th2 responses are associated with the pathogenesis of allergic inflammatory diseases, including atopic dermatitis, asthma, and rhinitis. Based on recent findings in humans and mouse models, TSLP might also be involved in the pathogenesis of inflammatory bowel disease and progression of cancer. In this review, we will summarize our current understanding of the biology of TSLP and highlight the important issues for future investigations.

The multiple facets of thymic stromal lymphopoietin (TSLP) during allergic inflammation and beyond

Originally shown to promote the growth and activation of B cells, TSLP is now known to have wide-ranging impacts on hematopoietic and nonhematopoietic cell lineages, including DCs, basophils, eosinophils, mast cells, CD4(+), CD8(+), and NK T cells, B cells, and epithelial cells. Whereas the role of TSLP in the promotion of TH2 responses has been studied extensively in the context of lung- and skin-specific allergic disorders, it is becoming increasingly clear that TSLP may impact multiple disease states within multiple organ systems, including the blockade of TH1/TH17 responses and the promotion of cancer and autoimmunity. This review will highlight recent advances in the understanding of TSLP signal transduction, as well as the role of TSLP in allergy, autoimmunity, and cancer. Importantly, these insights into the multifaceted roles of TSLP could potentially allow for novel, therapeutic manipulations of these disorders.

The many roles of IL-7 in B cell development; mediator of survival, proliferation and differentiation

Interleukin-7 (IL-7) plays several important roles during B cell development including aiding in; the specification and commitment of cells to the B lineage, the proliferation and survival of B cell progenitors; and maturation during the pro-B to pre-B cell transition. Regulation and modulation of IL-7 receptor (IL-7R) signaling is critical during B lymphopoiesis, because excessive or deficient IL-7R signaling leads to abnormal or inhibited B cell development. IL-7 works together with E2A, EBF, Pax-5 and other transcription factors to regulate B cell commitment, while also functions to regulate Ig rearrangement by modulating FoxO protein activation and Rag enhancer activity. Suppressor of cytokine signaling (SOCS) proteins are inhibitors of cytokine activation and, in B cells, function to fine tune IL-7R signaling; ensuring that appropriate IL-7 signals are transmitted to allow for efficient B cell commitment and development.

The P4-type ATPase ATP11C is essential for B lymphopoiesis in adult bone marrow

B lymphopoiesis begins in fetal liver, switching to bone marrow after birth where it persists for life. The unique developmental outcomes of each phase are well documented, yet their molecular requirements are not. Here we describe two allelic X-linked mutations in mice that caused a cell-intrinsic arrest of adult B lymphopoiesis. Mutant fetal liver progenitors generated B cells in situ, but not in irradiated adult bone marrow, highlighting a necessity for the affected pathway only in the context of adult bone marrow. The causative mutation was ascribed to Atp11c, which encodes a P4-type ATPase with no previously described function. Our data establish an essential, cell-autonomous and context-sensitive function for ATP11C, a putative aminophospholipid flippase, in B cell development.

IL-33 activates B1 cells and exacerbates contact sensitivity

B1 B cells produce natural IgM and play a critical role in the early defense against bacterial and viral infection. The polyreactive IgM also contributes to the clearance of apoptotic products and plays an important role in autoimmune pathogenesis. However, the mechanism of activation and proliferation of B1 cells remains obscure. In this study, we report that IL-33, a new member of IL-1 family, activates B1 cells, which express the IL-33 receptor α, ST2. IL-33 markedly activated B1 cell proliferation and enhanced IgM, IL-5, and IL-13 production in vitro and in vivo in a ST2-dependent manner. The IL-33-activated B1 cell functions could be largely abolished by IL-5 neutralization and partially reduced by T cell or mast cell deficiency in vivo. ST2-deficient mice developed less severe oxazolone-induced contact sensitivity (CS) than did wild-type (WT) mice. Furthermore, IL-33 treatment significantly exacerbated CS in WT mice with enhanced B1 cell proliferation and IgM and IL-5 production. Moreover, IL-33-activated B1 cells from WT mice could adoptively transfer enhanced CS in ST2(-/-) mice challenged with IL-33. Thus, we demonstrate, to the best of our knowledge, a hitherto unrecognized mechanism of B1 cell activation and IL-33 function, and suggest that IL-33 may play an important role in delayed-type hypersensitivity.

Epigenetic features that regulate IgH locus recombination and expression

Precisely regulated rearrangements that yield imprecise recombination junctions are hallmarks of antigen receptor gene assembly. At the immunoglobulin heavy chain (IgH) gene locus this is initiated by rearrangement of a D (H) gene segment to a J (H) gene segment to generate DJ(H) junctions, followed by rearrangement of a V (H) gene segment to the DJ(H) junction to generate fully recombined VDJ alleles. In this review we discuss the regulatory features of each step of IgH gene assembly and the role of epigenetic mechanisms in achieving regulatory precision.

Stromal cell-free conditions favorable for human B lymphopoiesis in culture

Progress has been slow in defining molecular requirements for human B lymphopoiesis in part because of differences from experimental animals and also because of the lack of culture conditions that efficiently support the process. We recently found that human CD10+ lymphocytes were produced when CD34+ hematopoietic stem and progenitor cells were cultured in contact with human mesenchymal stem cells (hMSC). Further investigation revealed that it occurred even when progenitors were separated from hMSC by membrane filters. Experiments with neutralizing antibodies suggested that important heat labile factors produced by hMSC are unlikely to be IL-7, TSLP, CXCL12 or hemokinin-1. Further manipulation of culture conditions revealed that optimal lymphopoiesis required careful selection of fetal calf serum lots, maintenance of high cell densities, as well as recombinant cytokines (SCF, FL and G-CSF). G-CSF was particularly important when adult bone marrow rather than umbilical cord blood derived CD34+ cells were used to initiate the cultures. These improved methods should facilitate identification of molecules that can be used to speed regeneration of the humoral immune system following chemotherapy and might suggest ways to inhibit growth of B lineage malignancies.

CpG inhibits pro-B cell expansion through a cathepsin B-dependent mechanism

TLR9 is expressed in cells of the innate immune system, as well as in B lymphocytes and their progenitors. We investigated the effect of the TLR9 ligand CpG DNA on the proliferation of pro-B cells. CpG DNA inhibits the proliferation of pro-B, but not pre-B, cells by inducing caspase-independent cell death through a pathway that requires the expression of cathepsin B. This pathway is operative in Rag-deficient mice carrying an SP6 transgene, in which B lymphopoiesis is compromised, to reduce the size of the B lymphocyte precursor compartments in the bone marrow. Thus, TLR9 signals can regulate B lymphopoiesis in vivo.

TSLP directly impairs pulmonary Treg function: association with aberrant tolerogenic immunity in asthmatic airway

Background

Even though thymic stromal lymphopoietin (TSLP) has been implicated in the development of allergic inflammation, its influence on immune tolerance mediated by regulatory T cells (Treg) have not been explored. We aimed to dissect the influence of TSLP on immunosuppressive activities of Treg and its potential consequences in human allergic asthma.

Methods

In vitro culture system was utilized to study the effects of TSLP on human Treg. The functional competency of pulmonary Treg from a cohort of 15 allergic asthmatic, 15 healthy control, and 15 non-allergic asthmatic subjects was also evaluated by suppression assays and flow cytometric analysis.

Results

Activated pulmonary Treg expressed TSLP-R and responded to TSLP-mediated activation of STAT5. TSLP directly and selectively impaired IL-10 production of Treg and inhibited their suppressive activity. In human allergic asthma, pulmonary Treg exhibited a significant decrease in suppressive activity and IL-10 production compared to healthy control and non-allergic asthmatic counterparts. These functional alterations were associated with elevated TSLP expression in bronchoaveolar lavage fluid (BAL) of allergic asthmatic subjects. Furthermore, allergic asthmatic BAL could suppress IL-10 production by healthy control pulmonary Treg in a TSLP-dependent manner.

Conclusions

These results provide the first evidences for a direct role of TSLP in the regulation of suppressive activities of Treg. TSLP mediated inhibition of Treg function might present a novel pathologic mechanism to dampen tolerogenic immune responses in inflamed asthmatic airway.

Thymic stromal lymphopoietin

Thymic stromal lymphopoietin (TSLP) is an epithelial cell-derived cytokine expressed in skin, gut, lungs, and thymus. TSLP signals via a TSLP receptor (TSLPR), a heterodimer of the IL-7 receptor alpha chain and the TSLPR chain. The TSLPR chain is closely related to the common receptor gamma chain that is expressed on a wide range of cell types in the adaptive and innate immune system. TSLP exerts a profound influence on the polarization of dendritic cells to drive T helper (Th) 2 cytokine production. TSLP also directly promotes T-cell proliferation in response to T-cell receptor activation and Th2 cytokine production and supports B-cell expansion and differentiation. TSLP further amplifies Th2 cytokine production by mast cells and natural killer T cells. These properties confer on TSLP a critical role in driving Th2-mediated inflammation. This role is supported by the finding that TSLP expression is upregulated in keratinocytes of atopic dermatitis skin lesions and in bronchial epithelial cells in asthma.

Atopic dermatitis-like disease and associated lethal myeloproliferative disorder arise from loss of Notch signaling in the murine skin

Background

The Notch pathway is essential for proper epidermal differentiation during embryonic skin development. Moreover, skin specific loss of Notch signaling in the embryo results in skin barrier defects accompanied by a B-lymphoproliferative disease. However, much less is known about the consequences of loss of Notch signaling after birth.

Methodology and Principal Findings

To study the function of Notch signaling in the skin of adult mice, we made use of a series of conditional gene targeted mice that allow inactivation of several components of the Notch signaling pathway specifically in the skin. We demonstrate that skin-specific inactivation of Notch1 and Notch2 simultaneously, or RBP-J, induces the development of a severe form of atopic dermatitis (AD), characterized by acanthosis, spongiosis and hyperkeratosis, as well as a massive dermal infiltration of eosinophils and mast cells. Likewise, patients suffering from AD, but not psoriasis or lichen planus, have a marked reduction of Notch receptor expression in the skin. Loss of Notch in keratinocytes induces the production of thymic stromal lymphopoietin (TSLP), a cytokine deeply implicated in the pathogenesis of AD. The AD-like associated inflammation is accompanied by a myeloproliferative disorder (MPD) characterized by an increase in immature myeloid populations in the bone marrow and spleen. Transplantation studies revealed that the MPD is cell non-autonomous and caused by dramatic microenvironmental alterations. Genetic studies demontrated that G-CSF mediates the MPD as well as changes in the bone marrow microenvironment leading to osteopenia.

Significance

Our data demonstrate a critical role for Notch in repressing TSLP production in keratinocytes, thereby maintaining integrity of the skin and the hematopoietic system.

Genomics based analysis of interactions between developing B-lymphocytes and stromal cells reveal complex interactions and two-way communication

Background

The use of functional genomics has largely increased our understanding of cell biology and promises to help the development of systems biology needed to understand the complex order of events that regulates cellular differentiation in vivo. One model system clearly dependent on the integration of extra and intra cellular signals is the development of B-lymphocytes from hematopoietic stem cells in the bone marrow. This developmental pathway involves several defined differentiation stages associated with specific expression of genes including surface markers that can be used for the prospective isolation of the progenitor cells directly from the bone marrow to allow for ex vivo gene expression analysis. The developmental process can be simulated in vitro making it possible to dissect information about cell/cell communication as well as to address the relevance of communication pathways in a rather direct manner. Thus we believe that B-lymphocyte development represents a useful model system to take the first steps towards systems biology investigations in the bone marrow.

Results

In order to identify extra cellular signals that promote B lymphocyte development we created a database with approximately 400 receptor ligand pairs and software matching gene expression data from two cell populations to obtain information about possible communication pathways. Using this database and gene expression data from NIH3T3 cells (unable to support B cell development), OP-9 cells (strongly supportive of B cell development), pro-B and pre-B cells as well as mature peripheral B-lineage cells, we were able to identify a set of potential stage and stromal cell restricted communication pathways. Functional analysis of some of these potential ways of communication allowed us to identify BMP-4 as a potent stimulator of B-cell development in vitro. Further, the analysis suggested that there existed possibilities for progenitor B cells to send signals to the stroma. The functional consequences of this were investigated by co-culture experiments revealing that the co-incubation of stromal cells with B cell progenitors altered both the morphology and the gene expression pattern in the stromal cells.

Conclusions

We believe that this gene expression data analysis method allows for the identification of functionally relevant interactions and therefore could be applied to other data sets to unravel novel communication pathways.

Critical roles for c-Myb in lymphoid priming and early B-cell development

c-Myb is a transcription factor with functions in many hematopoietic lineages. c-Myb-deficient mice display reduced numbers of B cells; however, it is unknown what role c-Myb plays in B lymphopoiesis because no critical target genes have been identified in the B-cell lineage. We demonstrate that conditional deletion of c-Myb in B-cell progenitors completely abolishes B-cell development. c-Myb is required for lymphoid progenitors to respond to the cytokines interleukin-7 and thymic stromal lymphopoietin; in the absence of sufficient c-Myb activity, mice display a B lymphopenia that closely resembles that observed in interleukin-7 receptor alpha-deficient animals. Analysis of the multipotent progenitor compartment indicates that c-Myb is also required for up-regulation of multiple lymphoid-associated genes, including Il7r, and for the subsequent development of the common lymphoid progenitor population. These data show that c-Myb plays a critical role in the regulatory pathways governing lymphoid specification and early B-cell differentiation.

A Flt3- and Ras-dependent pathway primes B cell development by inducing a state of IL-7 responsiveness

Ras plays an important role in B cell development. However, the stage at which Ras governs B cell development remains unclear. Moreover, the upstream receptors and downstream effectors of Ras that govern B cell differentiation remain undefined. Using mice that express a dominant-negative form of Ras, we demonstrate that Ras-mediated signaling plays a critical role in the development of common lymphoid progenitors. This developmental block parallels that found in flt3(-/-) mice, suggesting that Flt3 is an important upstream activator of Ras in early B cell progenitors. Ras inhibition impaired proliferation of common lymphoid progenitors and pre-pro-B cells but not pro-B cells. Rather, Ras promotes STAT5-dependent pro-B cell differentiation by enhancing IL-7Ralpha levels and suppressing socs2 and socs3 expression. Our results suggest a model in which Flt3/Ras-dependent signals play a critical role in B cell development by priming early B cell progenitors for subsequent STAT5-dependent B cell differentiation.

Loss of the pro-apoptotic BH3-only Bcl-2 family member Bim sustains B lymphopoiesis in the absence of IL-7

IL-7 is pivotal for B cell development. Proteins of the Bcl-2 family are essential regulators of lymphocyte survival. Particularly, the pro-apoptotic BH3-only members Bim and Puma mediate lymphocyte apoptosis provoked by cytokine deprivation. Herein, we addressed whether the absence of Bim or Puma within the hematopoietic compartment could bypass the requirement for IL-7-driven B cell development in adult mice. We found that deficiency of Bim, but not Puma, partially rescued B cell development in the absence of IL-7. The numbers of both sIgM(-) and sIgM(+) B cells were markedly increased in the bone marrow of recipients lacking IL-7 upon reconstitution with Bim-deficient hematopoietic progenitors, compared with their control or Puma-deficient counterparts. The augmentation of B cell lymphopoiesis in the absence of Bim was reflected in the mature peripheral compartment by an increase in both the number of immature and mature B cells in the spleen and in the circulating IgM levels. Bim-deficient B cells were also increased in IL-7-sufficient recipients suggesting that peripheral B cells homeostasis is governed by a Bim-dependent and IL-7-independent mechanism. Our data highlight the role of Bim as a key regulator of cell survival during B lymphocyte development in vivo.

IL-7 Dependence in human B lymphopoiesis increases during progression of ontogeny from cord blood to bone marrow

IL-7 is critical for B cell production in adult mice; however, its role in human B lymphopoiesis is controversial. One challenge was the inability to differentiate human cord blood (CB) or adult bone marrow (BM) hematopoietic stem cells (HSCs) without murine stroma. Here, we examine the role of IL-7 in human B cell development using a novel, human-only model based on coculturing human HSCs on primary human BM stroma. In this model, IL-7 increases human B cell production by >60-fold from both CB and adult BM HSCs. IL-7-induced increases are dose-dependent and specific to CD19(+) cells. STAT5 phosphorylation and expression of the Ki-67 proliferation Ag indicate that IL-7 acts directly on CD19(+) cells to increase proliferation at the CD34(+) and CD34(-) pro-B cell stages. Without IL-7, HSCs in CB, but not BM, give rise to a small but consistent population of CD19(lo) B lineage cells that express EBF (early B cell factor) and PAX-5 and respond to subsequent IL-7 stimulation. Flt3 ligand, but not thymic stromal-derived lymhopoietin (TSLP), was required for the IL-7-independent production of human B lineage cells. As compared with CB, adult BM shows a reduction of in vitro generative capacity that is progressively more profound in developmentally sequential populations, resulting in an approximately 50-fold reduction in IL-7-dependent B lineage generative capacity. These data provide evidence that IL-7 is essential for human B cell production from adult BM and that IL-7-induced expansion of the pro-B compartment is increasingly critical for human B cell production during the progression of ontogeny.

Thymic stromal lymphopoietin, OX40-ligand, and interleukin-25 in allergic responses

Allergic diseases are often triggered by environmental allergens that induce dominant type 2 immune responses, characterized by the infiltrated T-helper type 2 (TH2) lymphocytes, eosinophils, and elevated TH2 cytokines. In addition to TH2 type immune responses, epithelial stress and injury linked to tissue remodelling are often observed, suggesting that epithelial cells may play important role in regulating allergic responses. Dendritic cells (DCs), the professional antigen-presenting cells with the capabilities of sampling allergens, are considered as the key player on instructing TH2 immune responses. Whether inflamed epithelium can regulate innate immunity, such as macrophages and DCs, which in turn instructs adaptive immunity has long been hypothesized. Studies of thymic stromal lymphopoietin (TSLP), an epithelial cells-derived cytokine, that can strongly activate DCs, provide important evidences that the epithelial barrier can trigger allergic diseases by regulating immune responses. The finding that OX40/OX40Ligand (OX40L) interactions are the molecular trigger responsible for the induction and maintenance of TH2 responses by TSLP-activated DCs provides a plausible molecular explanation for TSLP-mediated allergy. Recent progresses in characterizing the pro-inflammatory IL-17 cytokine family have added an additional layer of complexity on the regulation of allergic inflammation. TSLP-DCs can induce a robust expansion of TH2 memory cells and strengthen functional attributes by up-regulating their surface expression of IL-17RB (IL-25R), the receptor for cytokine IL-17E (IL-25), a distinct member of IL-17 cytokine family. IL-17E (also known as IL-25) produced by epithelial cells, and other innate cells, such as eosinphils, basophils, and mast cells, are shown to regulate adaptive immunity by enhancing TH2 cytokine productions. These exciting findings expand our knowledge of the complex immunological cascades that result in allergic inflammation and may provide novel therapeutic approaches for the treatment of allergic diseases.

Novel molecular and cellular therapeutic targets in acute lymphoblastic leukemia and lymphoproliferative disease

While the outcome for pediatric patients with lymphoproliferative disorders (LPD) or lymphoid malignancies, such as acute lymphoblastic leukemia (ALL), has improved dramatically, patients often suffer from therapeutic sequelae. Additionally, despite intensified treatment, the prognosis remains dismal for patients with refractory or relapsed disease. Thus, novel biologically targeted treatment approaches are needed. These targets can be identified by understanding how a loss of lymphocyte homeostasis can result in LPD or ALL. Herein, we review potential molecular and cellular therapeutic strategies that (i) target key signaling networks (e.g., PI3K/AKT/mTOR, JAK/STAT, Notch1, and SRC kinase family-containing pathways) which regulate lymphocyte growth, survival, and function; (ii) block the interaction of ALL cells with stromal cells or lymphoid growth factors secreted by the bone marrow microenvironment; or (iii) stimulate innate and adaptive immune responses.

Welcome to the neighborhood: epithelial cell-derived cytokines license innate and adaptive immune responses at mucosal sites

There is compelling evidence that epithelial cells (ECs) at mucosal surfaces, beyond their role in creating a physical barrier, are integral components of innate and adaptive immunity. The capacity of these cells to license the functions of specific immune cell populations in the airway and gastrointestinal tract offers the prospect of novel therapeutic strategies to target multiple inflammatory diseases in which barrier immunity is dysregulated. In this review, we discuss the critical functions of EC-derived thymic stromal lymphopoietin (TSLP), interleukin-25 (IL-25), and IL-33 in the development and regulation of T-helper 2 (Th2) cytokine-dependent immune responses. We first highlight recent data that have provided new insights into the factors that control expression of this triad of cytokines and their receptors. In addition, we review their proinflammatory and immunoregulatory functions in models of mucosal infection and inflammation. Lastly, we discuss new findings indicating that despite their diverse structural features and differential expression of their receptors, TSLP, IL-25, and IL-33 cross-regulate one another and share overlapping properties that influence Th2 cytokine-dependent responses at mucosal sites.

TSLP in epithelial cell and dendritic cell cross talk

Dendritic cells (DCs) are professional antigen-presenting cells that have the ability to sense infection and tissue stress, sample and present antigen to T lymphocytes, and instruct the initiation of different forms of immunity and tolerance. The functional versatility of DCs depends on their remarkable ability to translate collectively the information from the invading microbes, as well as their resident tissue microenvironments. Recent progress in understanding Toll-like receptor (TLR) biology has illuminated the mechanisms by which DCs link innate and adaptive antimicrobial immune responses. However, how tissue microenvironments shape the function of DCs has remained elusive. Recent studies of TSLP (thymic stromal lymphopoietin), an epithelial cell-derived cytokine that strongly activates DCs, provide strong evidence at a molecular level that epithelial cells/tissue microenvironments directly communicate with DCs, the professional antigen-presenting cells of the immune system. We review recent progress on how TSLP expressed within thymus and peripheral lymphoid and nonlymphoid tissues regulates DC-mediated central tolerance, peripheral T cell homeostasis, and inflammatory Th2 responses.

FLT3 ligand and not TSLP is the key regulator of IL-7-independent B-1 and B-2 B lymphopoiesis

Phenotypically and functionally distinct progenitors and developmental pathways have been proposed to exist for fetally derived B-1 and conventional B-2 cells. Although IL-7 appears to be the primary cytokine regulator of fetal and adult B lymphopoiesis in mice, considerable fetal B lymphopoiesis and postnatal B cells are sustained in the absence of IL-7; in humans, B-cell generation is suggested to be largely IL-7-independent, as severe combined immune-deficient patients with IL-7 deficiency appear to have normal B-cell numbers. However, the role of other cytokines in IL-7-independent B lymphopoiesis remains to be established. Although thymic stromal lymphopoietin (TSLP) has been proposed to be the main factor driving IL-7-independent B lymphopoiesis and to distinguish fetal from adult B-cell progenitor development in mice, recent studies failed to support a primary role of TSLP in IL-7-independent fetal B-cell development. However, the role of TSLP in IL-7-independent adult B lymphopoiesis and in particular in regulation of B-1 cells remains to be established. Here we demonstrate that, rather than TSLP, IL-7 and FLT3 ligand are combined responsible for all B-cell generation in mice, including recently identified B-1-specified cell progenitors. Thus, the same IL-7- and FLT3 ligand-mediated signal-ing regulates alternative pathways of fetal and adult B-1 and B-2 lymphopoiesis.