B lymphopoiesis is regulated by cytokines, chemokines and cell surface proteins that initiate signal transduction pathways necessary for maturation to proceed. Many of these factors are expressed by cells in the surrounding bone marrow (BM) microenvironment, which also form the niches that support development. Interleukin-7 (IL-7) is an essential cytokine for progenitor B cells and is important in providing survival, proliferation and maturation signals. By growing BM B cells for extended periods of time in vitro with IL-7 it is possible to select for cells that possess the ability to grow indefinitely, and these cultures can be used to generate cell lines. Data presented herein describe the generation and characterization of IL-7-dependent B cell lines as well as their utility in investigating aspects of B cell development. As B cells mature they lose responsiveness to IL-7, yet retain IL-7 receptor expression. I demonstrate that a B cell’s ability to respond to IL-7 is controlled by the expression of suppressor of cytokine signaling (SOCS) proteins, which are regulated by a variety of signaling pathways including those initiated by IL-7. Development of progenitor B cells to mature immunoglobulin secreting B cells is mediated in part by surface proteins present on stromal cells as well as on B cells themselves. Heparan sulfate and CD19 play important roles in regulating this transition and I provide data that demonstrates how their ability to regulate Erk activation downstream of the pre-B cell receptor (pre-BCR) alters the proliferation and maturation of developing B cells.

B lymphopoiesis is regulated by cytokines, chemokines and cell surface proteins that initiate signal transduction pathways necessary for maturation to proceed. Many of these factors are expressed by cells in the surrounding bone marrow (BM) microenvironment, which also form the niches that support development. Interleukin-7 (IL-7) is an essential cytokine for progenitor B cells and is important in providing survival, proliferation and maturation signals. By growing BM B cells for extended periods of time in vitro with IL-7 it is possible to select for cells that possess the ability to grow indefinitely, and these cultures can be used to generate cell lines. Data presented herein describe the generation and characterization of IL-7-dependent B cell lines as well as their utility in investigating aspects of B cell development. As B cells mature they lose responsiveness to IL-7, yet retain IL-7 receptor expression. I demonstrate that a B cell’s ability to respond to IL-7 is controlled by the expression of suppressor of cytokine signaling (SOCS) proteins, which are regulated by a variety of signaling pathways including those initiated by IL-7. Development of progenitor B cells to mature immunoglobulin secreting B cells is mediated in part by surface proteins present on stromal cells as well as on B cells themselves. Heparan sulfate and CD19 play important roles in regulating this transition and I provide data that demonstrates how their ability to regulate Erk activation downstream of the pre-B cell receptor (pre-BCR) alters the proliferation and maturation of developing B cells.

B lymphocytes develop from pluripotent stem cells, and differentiate to plasma cells (PCs) in reaction to signals from the supportive microenvironment. Different sets of signals, which are derived from multiple sources such as soluble cytokines and cell-cell contacts, are required at different stages of development. For instance, murine B cell progenitors require the action of interleukin-7 (IL-7) in the early phase of their development in the bone marrow (BM). The necessity for IL-7 decreases as the cell matures, and this event is correlated with the appearance of CD22. The first two chapters of this thesis focus on the early stages of B cell development that take place in the BM. In chapter 1, I examine the IL-7 response and, although I do not show a specific role for CD22 in the loss of sensitivity to IL-7, my data suggest that cis interactions involving sialic acids might modulate the IL-7 response. This section is followed by the analysis of the effect of IL-21 on B cell progenitors in the BM. IL-21 is known to regulate the terminal stages of B cell differentiation. In collaboration with Dr. Danijela Konforte, I present evidence that B cell progenitors in the BM also express a functional IL-21 receptor and that stimulation of this receptor with IL-21 accelerates the maturation pace of B cells. I further demonstrate that proB cells stimulated with IL-21 and anti-CD40 can differentiate into immunoglobulin (Ig)-secreting cells, and discuss the possibility that IL-21 plays a role during inflammation for the development of B cell progenitors in peripheral lymphatic organs. Finally, in the last chapter, in collaboration with the laboratory of Dr. Gommerman, I investigate how the microenvironment can shape the development of B cells. It has been demonstrated by my collaborators that IgA+ PCs present in the gut produce iNOS and display traits commonly associated to the myeloid lineage, and in this chapter, I describe a co-culture system with BM and gut stroma to study the conditions that sustain the generation of IgA+iNOS+ cells. In particular, I show that the presence of microbial products is one of the key factors required for their development.

B lymphocytes develop from pluripotent stem cells, and differentiate to plasma cells (PCs) in reaction to signals from the supportive microenvironment. Different sets of signals, which are derived from multiple sources such as soluble cytokines and cell-cell contacts, are required at different stages of development. For instance, murine B cell progenitors require the action of interleukin-7 (IL-7) in the early phase of their development in the bone marrow (BM). The necessity for IL-7 decreases as the cell matures, and this event is correlated with the appearance of CD22. The first two chapters of this thesis focus on the early stages of B cell development that take place in the BM. In chapter 1, I examine the IL-7 response and, although I do not show a specific role for CD22 in the loss of sensitivity to IL-7, my data suggest that cis interactions involving sialic acids might modulate the IL-7 response. This section is followed by the analysis of the effect of IL-21 on B cell progenitors in the BM. IL-21 is known to regulate the terminal stages of B cell differentiation. In collaboration with Dr. Danijela Konforte, I present evidence that B cell progenitors in the BM also express a functional IL-21 receptor and that stimulation of this receptor with IL-21 accelerates the maturation pace of B cells. I further demonstrate that proB cells stimulated with IL-21 and anti-CD40 can differentiate into immunoglobulin (Ig)-secreting cells, and discuss the possibility that IL-21 plays a role during inflammation for the development of B cell progenitors in peripheral lymphatic organs. Finally, in the last chapter, in collaboration with the laboratory of Dr. Gommerman, I investigate how the microenvironment can shape the development of B cells. It has been demonstrated by my collaborators that IgA+ PCs present in the gut produce iNOS and display traits commonly associated to the myeloid lineage, and in this chapter, I describe a co-culture system with BM and gut stroma to study the conditions that sustain the generation of IgA+iNOS+ cells. In particular, I show that the presence of microbial products is one of the key factors required for their development.

BLNK is an adaptor protein that functions in B-cell receptor signalling, and is vitally necessary at signalling checkpoints of mammalian B-cell development. However, its importance to avian B-cell development remains unclear. To explore the function of BLNK in chickens, shRNA-mediated RNA interference was delivered to a chicken B-cell line in vitro by replication-competent avian retrovirus (RCAS), and effective shRNA were determined. To observe an shRNA phenotype on chicken B-cell development, we simultaneously explored whether RCAS penetrance was correlated between red blood cells (RBC) and bursal B-cells by infecting chicken embryos with RCAS expressing a fluorescent tag. We found that RCAS penetrance was correlated between RBC and B-cells, which provides a system to observe any in vivo effects of BLNK shRNA on B-cell development. Furthermore, this system for observing BLNK function may be complemented by genetically-modified BLNK, particularly variants resistant to RNA interference.

BLNK is an adaptor protein that functions in B-cell receptor signalling, and is vitally necessary at signalling checkpoints of mammalian B-cell development. However, its importance to avian B-cell development remains unclear. To explore the function of BLNK in chickens, shRNA-mediated RNA interference was delivered to a chicken B-cell line in vitro by replication-competent avian retrovirus (RCAS), and effective shRNA were determined. To observe an shRNA phenotype on chicken B-cell development, we simultaneously explored whether RCAS penetrance was correlated between red blood cells (RBC) and bursal B-cells by infecting chicken embryos with RCAS expressing a fluorescent tag. We found that RCAS penetrance was correlated between RBC and B-cells, which provides a system to observe any in vivo effects of BLNK shRNA on B-cell development. Furthermore, this system for observing BLNK function may be complemented by genetically-modified BLNK, particularly variants resistant to RNA interference.

B lymphocytes produce a wide array of antibodies to recognize a countless number of antigens. This highly diverse repertoire is produced during B cell development in the bone marrow from the immunoglobulin heavy chain (Igh) and light chain (Igk and Igl) loci. The mouse Igh is a large (~3Mb) multigene locus that contains 195 variable (V), 10 diversity (D) and 4 joining (J) genes that undergo developmentally regulated V(D)J recombination to produce the variable region of the antibody. Gene expression depends on spatial organisation of chromatin. To ensure that all V genes have a chance to recombine, they are brought into physical proximity to the D-J region by locus contraction and DNA looping. Not all V genes recombine with equal frequencies and we aim to investigate how dynamic changes in 3D structure of the Igh locus facilitate V(D)J recombination. Chromosome conformation capture techniques have revolutionised studies of genome conformation. I have applied a novel form of enriched Hi-C to study both intra-locus (cis) and genome-wide (trans) interactions of the immunoglobulin loci in pro-B and pre-B cells. This method provides a higher resolution than Hi-C and is less biased than 4C and 5C. I have mapped all cis interactions within the Igh locus to produce a comprehensive view of the structure of the locus prior to recombination. This approach has shown that the 3’ superanchor (3’CBEs) and the Intergenic Control Region 1 (IGCR1) containing CTCF sites are the two most interacting regions in the locus making long-range contacts with all V genes. A second major conformational feature is that the distal V genes form a large tightly looped domain forming the centre of mass of the locus to which the 3’CBEs and IGCR1 loop. Thanks to a collaboration on polymer modelling, 5000 single conformations were simulated based on the ensemble Hi-C data. This showed that every structure is different, supporting a model of dynamic and flexible organisation of the locus rather than hierarchical subdomains therein. Moreover, there is only a slight trend for V genes interacting more often with the D-J region to have higher recombination scores, supporting an ‘equal opportunity for all’ model in which participation of V genes in V(D)J recombination is not constrained by linear genomic distance from the DJ region. Nevertheless, CTCF binding level does contribute to V gene recombination frequency. I have also discovered that Igh and Igk loci participate in a highly specialised network of genome-wide (trans) interactions involving genes encoding B cell-specific factors essential for activation and maintenance of B cell identity, including Pax5, Foxo1, Ebf1, and Runx1. I have validated these by 3D DNA FISH and found that at the pro-B cell stage the Igh is involved in many trans interactions, whereas Igk does not make any contacts. In contrast, Igk gains numerous trans interactions at the pre-B cell stage, many of which overlap with the interactions Igh participates in at both developmental stages. Together, these findings reveal a complex developmentally regulated orchestration of genome conformation changes that underpins B cell development.

Proteolytic processing of transmembrane receptors and ligands can have dramatic effects on cell signaling and subsequent cellular responses. Previous studies demonstrated that a disintegrin and metalloproteinase 10 (ADAM10) may cleave numerous B cell-expressed receptors, including the low affinity IgE receptor (CD23). However, lethality of ADAM10-deficient embryos has limited examination of these cleavage events in lymphocytes. To investigate their role in B cell development and function, we generated B cell-specific ADAM10 knockout mice. Intriguingly, deletion prevented development of the entire marginal zone B cell (MZB) lineage. Further analysis revealed that ADAM10 is required for S2 cleavage of the Notch2 receptor and initiation of Notch2 signaling, which is required for MZB development. Additionally, cleavage of CD23 was dramatically impaired in ADAM10-deficient B cells. This finding and results of ex vivo cleavage assays demonstrated that ADAM10 is the principal in vivo sheddase of CD23. Previous studies have demonstrated that Notch signaling and CD23 cleavage regulate antibody production. Accordingly, deletion of ADAM10 profoundly inhibited germinal center formation, and T-dependent and T-independent antibody responses to immunization, implicating ADAM10 as a novel regulator of adaptive immunity. Additionally, to determine the role of ADAM10 activity in hematopoiesis, we generated transgenic mice (A10Tg) that overexpress the protease on lymphoid and myeloid progenitors. Surprisingly, this markedly suppressed B2 cell development and promoted dramatic expansion of myeloid-derived suppressor cells (MDSCs) via a cell intrinsic mechanism. A10Tg MDSCs inhibited T cell proliferation and adoptive immunotherapy of B16 melanoma, resulting in exacerbated metastatic progression that was prevented by MDSC depletion. Thus, A10Tg mice represent a novel model for the examination of MDSC development and MDSC-mediated immune suppression in a tumor-free environment. Finally, hematopoietic stem cell cultures revealed that ADAM10 overexpression directs myeloid development by dysregulating Notch signaling via uncoupling the highly regulated proteolysis of Notch receptors. Collectively, these findings demonstrate that ADAM10 is a critical regulator of Notch signaling, B cell development, and MDSC expansion. Moreover, they have important implications for the treatment of numerous CD23 and Notch mediated pathologies, ranging from allergy to cancer.

B cell development is a carefully orchestrated process involving many transcription factors acting in concert with cytokine signals, particularly IL-7. The transcription factor c-Myb has long been implicated in B cell development, however surprisingly little is known about the function of c-Myb in B cell progenitors. I have used several mouse models of c-Myb deficiency to investigate the role of c-Myb in the B cell lineage. Conditional deletion of c-Myb in early B cell progenitors using mb-1Cre (c MybΔmb1/Δmb1) leads to a striking lack of B cells from the pre-pro-B cell stage onwards, demonstrating that c-Myb is absolutely required for B cell development. Mice homozygous for a hypomorphic allele of c-Myb (c MybPlt4/Plt4) also display a severe reduction in B cells; in these mice, defects in lymphoid development can be detected within the multipotent progenitor compartment of bone marrow. c-Myb activates transcription via coactivator proteins, particularly CBP and p300. Mice bearing a point mutation in p300 (p300Plt6/Plt6) that inhibits the interaction of p300 with c Myb display a partial block in B cell development, highlighting the importance of the c Myb-p300 complex for B cell development. Together, these mice demonstrate that c-Myb regulates B cell development by functioning both in multipotent progenitor cells and directly in B cell progenitors. In addition, I show that the B-lymphopenia in c-Myb deficient mice is related to a profound defect in IL-7 signalling. IL-7 normally stimulates the proliferation, survival and differentiation of B cell progenitors, however pro-B cells from c-MybPlt4/Plt4 and c MybΔmb1/Δmb1 mice fail to respond to IL 7. Expression of the IL-7Rα chain is reduced on pro-B cells from c MybPlt4/Plt4 and c-MybΔmb1/Δmb1 mice, suggesting that Il7r may be a c-Myb target gene in B cells. Reporter gene assays show that c-Myb can activate the Il7r promoter in synergy with the transcription factor Pu.1. Overall, this work demonstrates that c-Myb is essential for early B cell development and plays a critical role in linking cytokine signals to the transcription factor networks in B cell progenitors.

The chicken bursa of Fabricius is a primary lymphoid tissue important for B-cell development. Our long-term goal is to understand the role of bursal microenvironment in an early B-cell differentiation event initiating repertoire development through immunoglobulin gene-conversion in the chick embryo. We hypothesize that early bursal B-cell differentiation is guided by signals through cytokine receptors. Our theory is based on previous evidence for expression of the receptor tyrosine kinase superfamily members and interleukin receptors in unseparated populations of bursal B-cells and bursal tissue. Knowledge of the expressed genes that are responsible for B-cell differentiation is a prerequisite for understanding the bursal microenvironment’s function. This project uses transcriptomic analysis to examine gene expression across an early B-cell differentiation event. RNA-seq was performed with total RNA isolated from developing B-cells at embryonic day (ED) 16 and ED 19 (n=3). Approximately 90 million high quality clean reads where obtained from the cDNA libraries. The analysis revealed differentially expressed genes involved in Wnt signaling pathway, Jak-STAT pathway, metabolic pathways, tyrosine metabolism, Toll-like receptor signaling pathway, MAPK signaling pathway, and cellhesion molecules. The transcripts for surface receptors, signal transduction and transcription factors identified in this study represent gene candidates for controlling B-cell differentiation in response to bursal microenvironmental factors.