B‐lymphocyte biology

Abstract

B cells and antibodies are critical elements of humoral immune responses against a wide range of pathogens. B-cell immune responses can be thought of as a series of converging processes, finalizing in the generation of memory B cells or antibody-forming plasma cells with the added ingredients of somatic mutation and class-switch recombination. In this volume, a range of contemporary viewpoints are integrated related to the broad topic of ‘B-lymphocyte biology’. Thus, some readers may feel that each topic has been randomly selected, thereby appearing like a pot-au-feu. However, the warp and woof of this volume is how B lymphocytes are differentiated from progenitor B cells to antibody-forming plasma cells (warp), and during each differentiation process, how supra- and intracellular communications take place (woof). In the process of hematopoiesis, it has been accepted that changes in cell surface markers and patterns of gene expression are sequential events. However, Ichii et al. (1) now suggest that the order may not be as rigid as once thought. Using recombination-activating gene-1 (RAG-1)-based reporter mice (RAG-1 expression is permanently marked by fluorescence) to trace early lymphoid progenitors (ELPs), they found that essential events in B lymphopoiesis are not tightly synchronized and that ELPs are found to be non-homogeneous with respect to cell cycle and expression of lymphoid-related genes. This lack of homogeneity is seen at the level of hematopoietic stem cells (HSCs) (1). Antibody diversity is generated through the rearrangement of the V, (D), and J segments of immunoglobulin (Ig) heavy and light chain genes. However, only one allele in each B cell successfully rearranges, and, once this occurs, it prevents the rearrangement of the other allele. This well-known phenomenon, allelic exclusion, is an important event in the B-cell developmental process, guaranteeing the ‘one-B cell-one specificity’ concept. However, the underlying mechanism is still enigmatic. Vettermann and Schilissel (2) discuss various mechanistic models, including the asynchronous recombination model initially proposed by his group and the feedback inhibition model, to explain the establishment of allelic exclusion. They also speculate why monoallelic Ig expression is critical for B-cell function. Skok and colleagues (3) show their spatiotemporal studies of allelic exclusion. They describe our current understanding about the dynamic changes in nuclear location of the V, (D), J segments of Ig genes during the V(D)J recombination processes. These data have been obtained using an elegant FISH (fluorescence in situ hybridization) imaging technique. The excluded Igh allele moves to repressive DNA domains near the pericentromeric heterochromatin (PCH), while the productive allele remains within accessible domains. Furthermore, upon successful Igh recombination, the excluded allele de-condenses at the PCH, which physically separates the distal V(h) genes from the proximal V(h) domain, thus preventing further rearrangements (3). In addition, they also review current ideas about how key transcription factors such as Pax5 affect the nuclear dynamics of Ig genes. These new data provide a deep insight into the mechanism of allelic exclusion. Pre-B-cell receptor (pre-BCR) and interleukin-7 receptor (IL-7R) signaling are critical for expansion of pro/pre-B cells and their subsequent differentiation to immature B cells. Werner et al. (4) review what is taking place inside the cells at these stages of differentiation. They particularly focus on the phosphoinositide 3-kinase (PI3K)-AKT-FOXO signaling axis in the pre-B-cell stage, where the signal through the pre-BCR regulates the proliferation and survival of the cells in conjunction with the IL-7R. This signaling axis controls two important events in the B-lineage differentiation: VDJ recombination and Ig light chain recombination. They also review the roles of the PI3K-AKT-FOXO signaling in mature and activated B cells. After encountering antigen, B cells become activated and differentiate into memory B cells or plasma cells. These dynamic processes occur in the germinal center (GC) and extrafollicular areas. Several contributors review this area: the mechanism(s) of GC formation by Vinuesa (5) and Brink (6), heterogeneity of memory B cells by Kurosaki (7) and Yoshida (8), and the differentiation into plasma cells and their subsets by Yoshida (8) and Tarlinton (9). It has long been recognized that the GC is critically important to B-cell immune responses, in particular in T-dependent responses to protein antigens. In the GC, B cells proliferate rapidly and can undergo isotype switching and somatic hypermutation (SHM). SHM is thought to be critical for selecting B cells with increasing BCR (or antibody) affinity and specificity for the immunizing antigen, and these B cells can be differentiated into memory B cells or plasma cells. The formation of the GC results from cellular interactions involving the antigen-responsive B cells, the T follicular helper (Tfh) cells, and the follicular dendritic cells (FDCs). Vinuesa et al. (5) compare the molecular mechanism(s) of T-cell-independent GC formation with that of the more ‘classical’ T-cell-dependent GC formation, and the role of Tfh cells in the initiation of GC formation is also described. Bcl6-expressing T cells (Tfh cells) capable of signaling through signaling lymphocyte activation molecule (SLAM)-associated protein (SAP) are essential for T-dependent GC formation. Furthermore, they emphasize the importance of CD40 ligands on T cells and on FDCs in the T-dependent and T-independent GC formation, respectively. The role of Tfh-derived CD40 ligands, IL-21, and IL-4 in B-cell proliferation, survival, and affinity maturation in the GC are also discussed along with the essential role of FDC-derived integrin signaling through vascular cell adhesion molecule-1 (VCAM-1) and/or intercellular adhesion molecule-1 (ICAM-1). GCs are thought to be a special microenvironment where activated B cells are mutated to undergo affinity maturation; however, because of lack of appropriate model systems, the underlying mechanisms are still unclear. To overcome this problem, Brink and his colleagues (6) established an anti-hen egg lysosome (HEL) BCR knockin system. They review the data using this powerful model system, which can give us a significant insight into how affinity maturation takes place at molecular and cellular levels. They also describe the molecular basis of the movement of activated B cells towards extrafollicular sites or the GC. The expression level of the orphan G-protein-coupled receptor Epstein–Barr virus (EBV)-induced gene 2 (EBI2) by activated B cells has turned out to be one of the critical determinants for such movement. As for the memory B-cell subsets, my colleagues and I (7) review the heterogeneity of memory B cells. Through consideration of the identification, developmental pathway, localization, and maintenance of memory B cells, we highlight and compare the IgM and IgG type memory B cells. Particularly, we place considerable emphasis on the biological significance of these two types of memory B cells, where the IgM+ memory B cells contribute to replenishment of the memory pool and the IgG+ memory B cells differentiate into isotype-switched plasma cells. Furthermore, we note that the IgM+ memory B cells consist of two types of cells with unmutated and somatically mutated V regions. The mechanism(s) to generate these two types of memory cells is another interesting unresolved question that we discuss (7). Memory B cells do not constitutively secrete antibodies, requiring restimulation before they can contribute to the humoral memory responses. Because long-lived plasma cells secrete abundant antibodies for a long time, they are also important contributors to the memory responses. Thus, to understand the humoral memory responses as a whole, Hoyer and colleagues (8) review two types of cells: memory B cells and long-lived plasma cells, the latter referred to as memory plasma cells. Long-lived plasma cells are generated in the context of memory immune reactions and migrate to the bone marrow, where they persist for years and even decades. Furthermore, they review signals controlling the formation of memory B cells and long-lived plasma cells in the GC, in particular the importance of CD40L (CD154), inducible costimulator (ICOS), and SAP. In the review of regulation of plasma cell differentiation, Tarlinton and colleagues (9) describe two pathways: one occurring in the extrafollicular region and the other in the GC. The short-lived plasma cells secreting low affinity antibodies are generated in the former pathway, while the long-lived plasma cells producing high affinity antibodies are differentiated by the latter one (9). The fate decision of B cells might thus be influenced in several different ways, including types of antigens, plasma cell subsets, etc. Tarlinton and colleagues (9) also review the current concepts about transcription networks regulating plasma cell differentiation and about plasma cell subsets. Finally, they discuss the ‘niches’ that are formed by stromal elements and that might be essential for the survival and maintenance of plasma cells. IgM and IgG are well characterized antibody isotypes. Recently, unique mechanisms of IgD and IgA generation have been established, which are reviewed by Cerutti (10) and Fagarasan (11). IgD is an enigmatic antibody that mature B cells express together with IgM through alternative RNA splicing. However, it has been reported that some B cells class-switch from IgM to IgD, and Chen and Cerutti (10) recently clarified the T-cell-dependent and T-cell-independent IgM to IgD class-switch mechanisms in B cells in the human upper respiratory mucosa. Here, Chen and Cerutti (10) describe the regulation and function of IgD in addition to providing us with some historical and ontogenical insights. Furthermore, the section on ‘IgD in diseases’ is an excellent overview that summarizes the relationship between dysregulation of IgD synthesis and immune-associated diseases including infection, malignancies, allergy, and autoimmunity. IgA is the most abundant antibody in mucosal secretions, and IgA+ B cells are present in the gut-associated lymphoid tissues (GALT). In this issue, Fagarasan and colleagues (11) discuss the role of B-1 and B-2 cells in IgA antibody responses. It has been widely accepted that IgA B cells are efficiently derived from the B2 cells in organized follicular structures such as Peyer’s patches. However, IgA plasma cells are generated not only from B2 but also from B1 cells. They first describe the characterization, generation, and maintenance, and migration of B1 cells and underscore the importance of gut bacteria, which stimulate Toll-like receptors on B1 cells, causing their relocalization from the peritoneal cavity to the gut followed by their differentiation into plasma cells. Two pathways leading to the generation of IgA+ B cells have now been reported: a T-cell-dependent pathway in the follicular structures, e.g. in the GC of Peyer’s patches, and a T-cell-independent mechanism in the extrafollicular structures. They then summarize the site and cellular interactions in the T-cell-independent pathway of IgA+ B-cell formation, an area in which her group’s studies have made major contributions. BCR, BAFFR [B-cell activating factor belonging to the TNF (tumor necrosis factor) family receptor], and CD40 are key cell surface receptors that govern B-cell survival, proliferation, and differentiation. Their biological functions and how these receptors exert cellular responses are reviewed by Batista (12), Mackay (13), and Bishop (14). The activation of B cells through the BCR by antigenic stimuli is a first gate of B-cell immune responses, and the appropriate activation of B cells is critical in this process. In response to membrane-bound ligand stimulation, antigen aggregation occurs in BCR microclusters containing IgM and IgD, which recruit the kinase Syk and transiently associate with CD19. CD19 is essential for B-cell activation and CD19-deficient B cells are significantly defective in the initiation of BCR-dependent signaling. In this volume, Batista et al. (12) discuss a number of recent studies that have offered insight into the early molecular events in B-cell activation. In particular, they introduce imaging technologies that enable direct visualization of the role for the cytoskeleton (12). Batista et al. (12) also describe the role of Wiskott–Aldrich syndrome protein, which is known to be involved in peripheral B-cell homeostasis. Mackay et al. (13) comprehensively review the functions of BAFF and also its role in autoimmunity and B-cell tolerance. In addition, they incorporate recent data into a new model in which survival signals from BAFFR and BCR are required in a B-cell stage and context-dependent manners (13). Related to this subject, they also describe the role of TACI (transmembrane activator and calcium modulator and cyclophilin ligand interactor) in B-cell biology and note that the dual functions of TACI have been clarified using TACI-deficient mice in which TACI upregulates or downregulates B-cell activation. As for BAFFR signaling, Brink and his colleagues (6) review the importance of TNF receptor-associated factors 2 and 3 (TRAF2 and TRAF3). Binding of BAFF to BAFFR reversed TRAF2-TRAF3-mediated suppression of B-cell survival by triggering the depletion of TRAF3 protein. This finding indicates that the process is TRAF2 dependent and that TRAF2 has dual roles in regulating B-cell homeostasis. Bishop and colleagues (14) review the recent progress about signaling properties of CD40 and LMP1 (latent membrane protein-1), a constitutively active mimic of CD40 made by EBV. They mainly focus on adapter molecules, TRAFs, which bind to the cytoplasmic domain at the C-terminus of LMP1 and CD40. TRAFs are intracellular co-inducers of downstream signaling from CD40, and LMP1 uses specific TRAFs differently from CD40, resulting in amplified and dysregulated CD40-like activation of B cells. They further discuss in the last part of their review the relationship between CD40/LMP1 and cancer and autoimmunity as a problem to be addressed in the near future. Signals through BCR are not only critical for the activation of B cells but also play an essential role in establishing B-cell tolerance to self-antigens. Cambier and colleagues (15) point out that as many as 70% of newly generated B cells are autoreactive and must be silenced to prevent the development of autoimmune diseases. At least three mechanisms, receptor editing, clonal deletion, and anergy, are thought to participate in this silencing, and it appears that most autoreactive cells are silenced by anergy. Thus, anergic autoreactive B cells are continuously present in the periphery in an unresponsive state. They start their review with an historical view of B-cell tolerance, and later they discuss the B-cell receptor signaling and related molecules in anergic B cells which are chronically stimulated by autoantigens (15). The development of therapeutic antibodies has evolved considerably over the past decade, and the use of B-cell-targeted therapies to treat autoimmune diseases is one of the important areas of interest in the field. In this volume, Chan and colleagues (16) first review the mechanisms of B-cell autoimmunity and later describe the use of therapeutic antibodies, especially anti-CD20 antibody (rituximab) in patients with rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, and type 1 diabetes. They further describe the targeting of BAFF [also known as B-lymphocyte stimulator (BLyS)] and APRIL (a proliferation-inducing ligand), introducing belimumab, a humanized antibody that binds to and neutralizes BAFF, inhibiting the BAFF-mediated stimulation (16). In autoimmune diseases, long-lived plasma cells secreting autoantibodies are resistant to conventional treatments such as immunosuppressants and anti-inflammatory drugs. In this regard, Hoyer et al. (8) propose more specific treatments for elimination of autoreactive, long-lived plasma cells to avoid the temporary immunoincompetence induced by immunoablation. Recent progress in the study of B cells and antibodies has been remarkable, and in this volume I have selected several timely and wide ranging reviews, from the detailed molecular dynamics inside B cells to the B-cell-targeted therapy of autoimmune diseases. Reading through the reviews, a common viewpoint is how the balance is maintained in each developmental stage of B cells (B-cell intrinsic) and how this balance can be influenced by surrounding microenvironment (B-cell extrinsic). The reviews here clearly and unanimously indicate that there is still much work left to do to precisely clarify supracellular and intracellular communication pathways in B cells and other immune cells.