Germinal center founder cells display propensity for apoptosis before onset of somatic mutation

Biology of Hodgkin's lymphoma

Significant progress has been made in recent years in our understanding of the cellular origin of Hodgkin and Reed-Sternberg (HRS) cells in Hodgkin's lymphoma (HL). It is now clear that in most instances HRS cells represent clonal populations of transformed germinal centre (GC) B cells. While the tumour cells in the lymphocyte predominant type of the disease resemble mutating and antigen-selected GC B cells, there is evidence that HRS cells in classical HL originate from pre-apoptotic GC B cells. HRS cells of the recently defined novel subtype lymphocyte-rich classical HL moleculary resemble HRS cells of the other types of classical HL, but there appear to be phenotypic differences. In rare cases, HRS cells derive from T cells. In contrast to previous speculations, cell fusion apparently does not play a role in the generation of the tumour clone. By gene expression profiling of HL cell lines, it became evident that HRS cells have lost most of the B cell-typical gene expression program, which may explain why these cells can persist without B cell receptor expression and which suggests that at least one of the transforming events involved in HL pathogenesis affects a master regulator of cell lineage identity.

Molecular biology of Hodgkin's lymphoma

Hodgkin's lymphoma (HL) is characterized by typical mononucleated Hodgkin and multinucleated Reed-Sternberg cells, which occur at low frequency in a mixed cellular infiltrate in the tumor tissue. Because of the rarity of these cells and their unusual immunophenotype, which is strikingly different from those of all normal hematopoietic cell types, the origin of these cells and their clonality have long been unclear. Single-cell studies of rearranged immunoglobulin genes showed that Hodgkin and Reed-Sternberg (HRS) cells represent clonal tumor-cell populations derived from germinal center B cells. In classical HL, the detection of obviously crippling immunoglobulin gene mutations in a fraction of the cases suggests that HRS cells may derive from germinal center B cells that have lost the capacity to be positively selected by antigen and that normally would have undergone apoptosis. In rare cases, HRS cells represent transformed T lymphocytes. The transforming events involved in malignant transformation of HRS cells are still largely unknown. Constitutive activation of the transcription factor NFkappaB, which can, for example, be induced through Epstein-Barr virus transformation of HRS cells or destructive somatic mutations of the inhibitor of NFkappaB, is likely to be a key event in HL pathogenesis. Significant progress has been made in our understanding of the cellular interactions in HL tissues, which are mainly mediated by a large variety of cytokines and chemokines.

Human genetic defects in class-switch recombination (hyper-IgM syndromes)

Several genetic defects in class-switch recombination, leading to hyper-IgM syndromes, have been recently described in humans. Besides the well-known role of interaction between CD40-ligand and CD40, these pathological conditions definitively demonstrate the requirement of CD40-mediated NF-kappaB activation and the essential role of a newly described molecule, activation-induced cytidine deaminase (AID), in B cell terminal differentiation.

Increased frequency of pre-germinal center B cells and plasma cell precursors in the blood of children with systemic lupus erythematosus

We have analyzed the blood B cell subpopulations of children with systemic lupus erythematosus (SLE) and healthy controls. We found that the normal recirculating mature B cell pool is composed of four subsets: conventional naive and memory B cells, a novel B cell subset with pregerminal center phenotype (IgD(+)CD38(+)centerin(+)), and a plasma cell precursor subset (CD20(-)CD19(+/low)CD27(+/++) CD38(++)). In SLE patients, naive and memory B cells (CD20(+)CD38(-)) are approximately 90% reduced, whereas oligoclonal plasma cell precursors are 3-fold expanded, independently of disease activity and modality of therapy. Pregerminal center cells in SLE are decreased to a lesser extent than conventional B cells, and therefore represent the predominant blood B cell subset in a number of patients. Thus, SLE is associated with major blood B cell subset alterations.

Molecular single-cell analysis of Hodgkin- and Reed-Sternberg cells harboring unmutated immunoglobulin variable region genes

Hodgkin- and Reed-Sternberg (H/RS) cells in classical Hodgkin's disease of the B lineage are the clonal progeny of antigen-experienced B cells harboring highly mutated immunoglobulin variable (V) region genes. Based on the detection of obviously destructive somatic mutations in a fraction of cases, we speculated that H/RS cells may be derived from a pre-apoptotic germinal center B cell. Seemingly contradicting this speculation, we present here the first case of classical Hodgkin's disease with H/RS cells harboring unmutated, potentially functional V region genes, which may indicate the derivation of the H/RS clone from a naive B cell. However, germinal center founder cells, which have not yet acquired somatic mutations, already have the intrinsic propensity to die by apoptosis. Thus, the rare occurrence of H/RS cells with unmutated V genes is expected if the H/RS cells are derived from the pool of pre-apoptotic germinal center B cells.

B-cell development in progressively transformed germinal centers: similarities and differences compared with classical germinal centers and lymphocyte-predominant Hodgkin disease

Progressively transformed germinal centers (PTGCs) are histologic structures mainly composed of small resting B cells and intermingled proliferating centroblast-like cells. The B-cell differentiation processes within PTGCs and their relation to classical germinal centers (GC) and to lymphocyte-predominant Hodgkin disease (LPHD), with which PTGCs are often associated, are largely unknown. To address these issues, single small resting (Ki67-) and proliferating (Ki67+) centroblast-like cells were isolated from 7 PTGCs of 5 lymph nodes, and rearranged immunoglobulin genes were amplified and sequenced. Most small resting B cells were clonally unrelated, and most carried unmutated immunoglobulin gene rearrangements resembling mantle zone B cells. Small resting B cells with mutated immunoglobulin gene rearrangements may represent centrocytes, memory B cells, or both. Among the centroblast-like Ki67+ cells, expanded B-cell clones were observed in 6 of 7 PTGCs analyzed. Clonally related V region genes showed extensive intraclonal diversity, and the mutation pattern indicated stringent selection of the cells for the expression of functional antigen receptors. Thus, somatic hypermutation, clonal expansion, and selection occur also in the disorganized PTGC microenvironment, as in classical GCs. In lymph nodes affected by PTGCs, no clonal expansion across the borders of individual PTGCs was observed, distinguishing PTGCs from LPHD.

Increase in tonsillar germinal centre B-1 cell numbers in IgA nephropathy (IgAN) patients and reduced susceptibility to Fas-mediated apoptosis

IgAN is a common form of primary glomerulonephritis and also a disease of tonsillar focal infection. The comprehensive mechanism underlying this disease remains to be defined. To better understand its pathogenesis, we investigated tonsillar CD5+ B cells (B-1 cells) with respect to IgA synthesis. Germinal centre (GC) B cells were isolated from the tonsils of IgAN patients and the number of B-1 cells in the GC determined by flow cytometry. GC B-1 and B-2 (CD5- B) cells were purified by cell sorter, the cells were incubated with agonist anti-CD40 MoAb and the ability for antibody production by B-1 and B-2 cells determined by ELISPOT assay. GC B-1 cells and B-2 cells were incubated with agonist anti-Fas MoAb, and apoptosis in GC B-1 cells and B-2 cells was analysed by flow cytometry. Although B-1 cells do not usually take part in the GC reaction, an increase in B-1 cell numbers was observed in the GC of tonsils from IgAN patients. These B-1 cells were likely IgA1 antibody-producing cells, since the prominent IgA subclass in IgAN is generally considered to be IgA1. Although Fas-dependent apoptosis is essential for the elimination of activated B cells, these B-1 cells showed a reduced susceptibility to Fas-mediated apoptosis. It is conceivable that activated B-1 cells may survive in the GC due to impaired apoptosis and thus produce abnormal antibodies. These findings suggest that the immune responses of B-1 cells in the tonsillar GC could thus have an impact on the pathogenesis of IgAN.

M cell pockets of human Peyer's patches are specialized extensions of germinal centers

M cells in follicle-associated epithelium of Peyer's patches (PP) mediate antigen entrance into the underlying lymphoid tissue. To investigate the functional potential of B cells in this unique microcompartment, the expression of co-stimulatory molecules necessary for B-T cell interaction was examined in histologically normal human PP by three-color immunohistochemistry. In the M cell areas, CD80 / CD86 expression was much more frequent on memory (sIgD(-)CD20(+)) B cells than on naive (sIgD(+)CD20(+)) B cells. M cell areas identified by such co-expression of CD20 and CD80 / CD86 were always spatially related to germinal centers (GC). Contrary to the GC B cell phenotype (sIgD(-)CD20(+)CD80 / 86(hi)CD10(+)Bcl-2(-)), however, M cell-associated B cells with a high level of CD80 / CD86 were CD20(lo)CD10(-)Bcl-2(+), and adjacent memory T cells (CD3(+)CD45R0(+)) often expressed CD40L (CD154). Autologous peripheral blood B-T cell cocultures with purified protein derivative as antigen showed that the sIgD(-)CD80 / CD86(hi)CD20(lo) phenotype could indeed be generated during cognate B-T interactions, concurrent with CD40L up-regulation on memory T cells. Thus, this M cell-associated phenotype might result from B-T cell interactions in the course of antigen presentation by memory B cells, with subsequent CD40 engagement by CD40L-expressing cognate memory T cells. We propose that this M cell-associated event contributes to memory B cell survival and diversification of intestinal immunity, representing a specialized limb of GC function.

Novel diversity in IL-4-mediated responses in resting human naive B cells versus germinal center/memory B cells

Recent studies have defined several phenotypic and molecular changes associated with the maturation of naive human B cells within the milieu of germinal centers. Although naive B cells serve as natural precursors to germinal center (GC)/memory (M) subpopulations, little is known about the physiological requirements for the survival of the naive B cell pool in the absence of cell-cell contact or Ag-mediated activation. Because IL-4 induces expression of several membrane receptors such as CD23 which are uniquely present on resting human naive B lymphocytes, we hypothesized that these cells might be intrinsically programmed to respond to IL-4 in the absence of cell division. Using buoyant density-dependent isolation and further enrichment by negative/positive selection of human naive and GC/M subpopulations, we characterized cytokine receptor moieties on these cells and analyzed their survival and growth in the presence of IL-4 or IL-10. Resting naive B cells expressed significantly higher IL-4 receptor alpha-chain on their cell surface than the combined GC/M subpopulation. The IL-10 receptor and the IL-2 receptor gammac chain were almost equally expressed on both subpopulations. When cultured in vitro, the addition of IL-4, but not IL-10, protected naive B cells from apoptosis in the absence of activation and growth. However, IL-4 exerted no such effect on resting GC/M B cells. These data support the hypothesis that IL-4 plays a pivotal role in the survival and maintenance of resting human naive B cells.

Relative roles of somatic and Darwinian evolution in shaping the antibody response

The need for a highly specific system of recognition in immunity has resulted in the evolution of several somatic mechanisms such as V(D)J recombination, to diversify the repertoire of B cells. Therefore, repertoire diversification is the driving force for the cells that constitute the bulk of the response to unpredictable pathogens, the B2 naive B cells. Predictability of antigen, on the other hand, has played a major role in shaping the neonatal repertoire, in which evolution to recognize commonly encountered pathogens has driven the germline sequence of several VH segments that are used frequently in the neonatal repertoire. A third population, the memory B cell population, is generated to respond to a known pathogen, but predictability of the pathogen is not acquired until after a first exposure. Therefore, it is somatic evolution in germinal centers that drives the generation of high-affinity memory B cells.

EBV-infected B cells in infectious mononucleosis: viral strategies for spreading in the B cell compartment and establishing latency

Infection of humans with Epstein-Barr virus (EBV) may cause infectious mononucleosis (IM). Analysis of single EBV-infected cells from tonsils of IM patients for rearranged immunoglobulin genes revealed two strategies of EBV for rapid and massive spread in the B cell compartment: the direct infection of many naive as well as memory and/or germinal center B cells and the expansion of the latter cells to large clones. In IM, the generation of virus-harboring memory B cells from naive B cells passing through a germinal center reaction likely plays no role. Members of clones can show distinct morphologies and likely also EBV gene expression patterns, and this ability implies a mechanism by which EBV-harboring cells can evade immune surveillance and establish a pool of persisting EBV-infected B cells.

Activation-induced cytidine deaminase (AID) deficiency causes the autosomal recessive form of the Hyper-IgM syndrome (HIGM2)

The activation-induced cytidine deaminase (AID) gene, specifically expressed in germinal center B cells in mice, is a member of the cytidine deaminase family. We herein report mutations in the human counterpart of AID in patients with the autosomal recessive form of hyper-IgM syndrome (HIGM2). Three major abnormalities characterize AID deficiency: (1) the absence of immunoglobulin class switch recombination, (2) the lack of immunoglobulin somatic hypermutations, and (3) lymph node hyperplasia caused by the presence of giant germinal centers. The phenotype observed in HIGM2 patients (and in AID-/- mice) demonstrates the absolute requirement for AID in several crucial steps of B cell terminal differentiation necessary for efficient antibody responses.

Identification of murine germinal center B cell subsets defined by the expression of surface isotypes and differentiation antigens

Germinal centers (GCs) are inducible lymphoid microenvironments that support the generation of memory B cells, affinity maturation, and isotype switching. Previously, phenotypic transitions following in vivo B cell activation have been exploited to discriminate GC from non-GC B cells in the mouse and to delineate as many as seven distinct human peripheral B cell subsets. To better understand the differentiative processes occurring within murine GCs, we sought to identify subpopulations of GC B cells corresponding to discrete stages of GC B cell ontogeny. We performed multiparameter flow-cytometric analyses of GC B cells at consecutive time points following immunization of BALB/c mice with SRBC. We resolved the murine GC compartment into subsets based on the differential expression of activation markers, surface Ig isotypes, and differentiation Ags. Class-switched and nonswitched GC B cells emerged contemporaneously, and their relative frequencies remained nearly constant throughout the GC reaction, perhaps reflecting the establishment of a steady state. A significant percentage of the nonswitched B cells with a GC phenotype exhibited surface markers associated with naive B cells, including CD23, surface IgD, and high levels of CD38 consistent with either prolonged recruitment into the GC reaction or protracted expression of these markers during differentiation within the GC. Expression of the activation marker BLA-1 was dynamic over time, with all GC B cells being positive early after immunization, followed by progressive loss as the GC reaction matured into the second and third week. Implications of these results concerning GC evolution are discussed.

Apoptosis or plasma cell differentiation of CD38-positive B-chronic lymphocytic leukemia cells induced by cross-linking of surface IgM or IgD

Previously, we demonstrated that B-chronic lymphocytic leukemia (B-CLL) cells could be divided into 2 groups depending on the expression of CD38 by the malignant cells. The 2 groups differed in their signal-transducing capacities initiated by cross-linking of surface IgM; only in CD38-positive cells was an efficient signal delivered, invariably resulting in cell apoptosis. In this study, we investigated the effect of surface IgD cross-linking in 10 patients with CD38-positive B-CLL. Exposure of the malignant cells to goat antihuman δ-chain antibodies (Gaδ-ab) caused [Ca++]i mobilization and tyrosine kinase phosphorylation in a manner not different from that observed after goat antihuman μ-chain antibody (Gaμ-ab) treatment in vitro. However, Gaδ-ab-treated cells failed to undergo apoptosis and instead displayed prolonged survival in culture and differentiated into plasma cells when rIL2 was concomitantly present. Cross-linking of surface IgD failed to induce proliferation of the malignant cells in vitro. Moreover, treatment with Gaδ-ab did not prevent apoptosis of B-CLL cells induced by Gaμ-ab. Collectively, these experiments demonstrated that IgM and IgD expressed by the same cell may deliver opposite signals under particular circumstances and provide some clues for the understanding of the pathophysiology of B-CLL.

A model for persistent infection with Epstein-Barr virus: the stealth virus of human B cells

Most adult humans are infected benignly and for life with the herpesvirus Epstein-Barr virus. EBV has been a focus of research because of its status as a candidate tumor virus for a number of lymphomas and carcinomas. In vitro EBV has the ability to establish a latent infection in proliferating B lymphoblasts. This is the only system available for studying human herpesvirus latency in culture and has been extremely useful for elucidating how EBV promotes cellular growth. However, to understand how EBV survives in the healthy host and what goes awry, leading to disease, it is essential to know how EBV establishes and maintains a persistent infection in vivo. Early studies on the mechanism of EBV persistence produced inconclusive and often contradictory results because the techniques available were crude and insensitive. Recent advances in PCR technology and the application of sophisticated cell fractionation techniques have now provided new insights into the behavior of the virus. Most dramatically it has been shown that EBV in vivo does not establish latency in a proliferating lymphoblast, but in a resting memory B cell. The contrasting behaviors of being able to establish a latent infection in proliferating B blasts and resting memory B cells can be resolved in terms of a model where EBV performs its complete life cycle in B lymphocytes. The virus achieves this not by disrupting normal B cell biology but by using it.

Surface Molecule Loss and Bleb Formation by Human Germinal Center B Cells Undergoing Apoptosis: Role of Apoptotic Blebs in Monocyte Chemotaxis

Human tonsil germinal center (GC) B cells rapidly undergo apoptosis in culture. Annexin-V binding shows an early event in this process. In the present study, this method has been used to label apoptotic GC B cells and to analyze additional surface molecules. The expression of all of the molecules studied was reduced in apoptotic (annexin-V+) GC B cells, and the reduction was more marked for CD11a, CD21, CD22, CD49d, and CD54, molecules that participate in survival interaction for GC B cells. The analysis of CD54, one of the molecules that was more drastically reduced, showed that GC, but not mantle zone, B cells actively secrete CD54 to the culture supernatant (SN). The secreted CD54 was partly released from the GC B cells in a particulate form as demonstrated by centrifugation. Further experiments using filtration, fluorescence microscopy, electron microscopy, and flow cytometry analysis showed that GC B cells released to the culture SN a population of spherical membranous vesicles of about 0.18 μm in size, similar to the blebs described in other apoptosis systems. Bleb formation depended on active metabolism, Ca2+, and, in part, on microfilament integrity. GC B-cell–derived blebs were clearly associated with apoptosis, as antiapoptotic stimuli prevented their formation. In addition, GC B-cell–derived blebs contained the adhesion molecules previously studied. Consequently, bleb formation might contribute to the surface molecule loss occurring in apoptotic GC B cells. Finally, a chemotaxis assay showed that GC B-cell blebs were chemotactic for human monocytes, suggesting that this mechanism might operate in vivo.

B cell memory and the long-lived plasma cell

The germinal center reaction is pivotal to the induction of B cell memory. The signals that regulate this complex microenvironment, with their cellular and molecular consequences, underpin long-term protective immunity. Recent studies have identified many key regulators of the germinal center cycle and have revealed an array of cellular outcomes that further define the memory B cell compartment.

OPG/FDCR-1, a TNF Receptor Family Member, Is Expressed in Lymphoid Cells and Is Up-Regulated by Ligating CD40

We have cloned a TNFR family member from a follicular dendritic cell (FDC)-like cell line, FDC-1. This molecule, FDC-derived receptor-1 (FDCR-1), is identical to osteoprotegerin (OPG), a soluble cytokine that regulates osteoclast differentiation. Recently, OPG/FDCR-1 has been characterized as a second receptor for receptor activator of NF-κB ligand (RANKL)/TNF-related activation-induced cytokine (TRANCE), a primarily T-cell restricted TNF family member that augments dendritic cell (DC) function. In this report, we demonstrate that OPG/FDCR-1 is membrane bound on the surface of transfected baby hamster kidney (BHK) and untransfected FDC-1 cells. We also found a restricted OPG/FDCR-1 expression pattern in lymphoid cells, specifically in B cells, DCs and FDC-enriched fractions, which in B cells and DCs is up-regulated by CD40 stimulation. Because OPG/FDCR-1 shares some properties with RANK, the first RANKL/TRANCE receptor, we discuss how the balance between RANK and OPG/FDCR-1 expression could influence immune responses and, ultimately, germinal center formation.

B Cell Responses to a Peptide Epitope. VII. Antigen-Dependent Modulation of the Germinal Center Reaction

Germinal center responses to two analogous peptides, PS1CT3 and G32CT3, that differ in sequence only at one position within the B cell epitopic region were examined. In comparison with peptide PS1CT3, peptide G32CT3 elicited a poor germinal center response. By demonstrating equal facility of immune complexes with IgM and IgG Ab isotypes to seed germinal centers, we excluded differences in isotype profiles of early primary anti-PS1CT3 and anti-G32CT3 Ig as the probable cause. Quantitative differences in germinal center responses to the two peptides were also not due to either qualitative/quantitative differences in T cell priming or variation in the frequency of the early Ag-activated B cells induced. Rather, they resulted from qualitative differences in the nature of B cells primed. Analysis of early primary anti-PS1CT3 and anti-G32CT3 IgMs revealed that the latter population was of a distinctly lower affinity, implying the existence of an Ag affinity threshold that restricts germinal center recruitment of G32CT3-specific B cells. The impediment in anti-G32CT3 germinal center initiation could be overcome by making available an excess of Ag-activated Th cells at the time of immunization. This resulted in the appearance of a higher affinity population of G32CT3-specific B cells that, presumably, are now capable of seeding germinal centers. These data suggest that the strength of a germinal center reaction generated is Ag dependent. At least one regulatory parameter represents the quality of B cells that are initially primed.

VH Gene Analysis of Primary Cutaneous B-Cell Lymphomas: Evidence for Ongoing Somatic Hypermutation and Isotype Switching

Primary cutaneous B-cell lymphomas are B-cell non-Hodgkin’s lymphomas that arise in the skin. The major subtypes discerned are follicle center cell lymphomas, immunocytomas (marginal zone B-cell lymphomas), and large B-cell lymphomas of the leg. In this study, we analyzed the variable heavy chain (VH) genes of 7 of these lymphomas, ie, 4 follicle center cell lymphomas (diffuse large-cell lymphomas) and 3 immunocytomas. We show that all these lymphomas carry heavily mutated VH genes, with no obvious bias in VH gene usage. The low ratios of replacement versus silent mutations observed in the framework regions of 5 of the 7 lymphomas suggest that the structure of the B-cell antigen receptor was preserved, as in normal B cells that are selected for antibody expression. Moreover, evidence for ongoing mutation was obtained in 3 immunocytomas and in one lymphoma of large-cell type. In addition, in 1 immunocytoma, both IgG- and IgA-expressing clones were found, indicative of isotype switching. Our data provide insight into the biology of primary cutaneous B-cell lymphomas and may be of significance for their classification.

VH Gene Analysis of Primary Cutaneous B-Cell Lymphomas: Evidence for Ongoing Somatic Hypermutation and Isotype Switching

Primary cutaneous B-cell lymphomas are B-cell non-Hodgkin’s lymphomas that arise in the skin. The major subtypes discerned are follicle center cell lymphomas, immunocytomas (marginal zone B-cell lymphomas), and large B-cell lymphomas of the leg. In this study, we analyzed the variable heavy chain (VH) genes of 7 of these lymphomas, ie, 4 follicle center cell lymphomas (diffuse large-cell lymphomas) and 3 immunocytomas. We show that all these lymphomas carry heavily mutated VH genes, with no obvious bias in VH gene usage. The low ratios of replacement versus silent mutations observed in the framework regions of 5 of the 7 lymphomas suggest that the structure of the B-cell antigen receptor was preserved, as in normal B cells that are selected for antibody expression. Moreover, evidence for ongoing mutation was obtained in 3 immunocytomas and in one lymphoma of large-cell type. In addition, in 1 immunocytoma, both IgG- and IgA-expressing clones were found, indicative of isotype switching. Our data provide insight into the biology of primary cutaneous B-cell lymphomas and may be of significance for their classification.

Human immunoglobulin (Ig)M+IgD+ peripheral blood B cells expressing the CD27 cell surface antigen carry somatically mutated variable region genes: CD27 as a general marker for somatically mutated (memory) B cells

Immunoglobulin (Ig)M+IgD+ B cells are generally assumed to represent antigen-inexperienced, naive B cells expressing variable (V) region genes without somatic mutations. We report here that human IgM+IgD+ peripheral blood (PB) B cells expressing the CD27 cell surface antigen carry mutated V genes, in contrast to CD27-negative IgM+IgD+ B cells. IgM+IgD+CD27(+) B cells resemble class-switched and IgM-only memory cells in terms of cell phenotype, and comprise approximately 15% of PB B lymphocytes in healthy adults. Moreover, a very small population (<1% of PB B cells) of highly mutated IgD-only B cells was detected, which likely represent the PB counterpart of IgD-only tonsillar germinal center and plasma cells. Overall, the B cell pool in the PB of adults consists of approximately 40% mutated memory B cells and 60% unmutated, naive IgD+CD27(-) B cells (including CD5(+) B cells). In the somatically mutated B cells, VH region genes carry a two- to threefold higher load of somatic mutation than rearranged Vkappa genes. This might be due to an intrinsically lower mutation rate in kappa light chain genes compared with heavy chain genes and/or result from kappa light chain gene rearrangements in GC B cells. A common feature of the somatically mutated B cell subsets is the expression of the CD27 cell surface antigen which therefore may represent a general marker for memory B cells in humans.

Rapid elimination of mature autoreactive B cells demonstrated by Cre-induced change in B cell antigen receptor specificity in vivo

Developing autoreactive B cells edit their B cell antigen receptor (BCR) in the bone marrow and are clonally deleted when they fail to reexpress an innocent BCR. Here, inducible Cre-loxP-mediated gene inversion is used to change the specificity of the BCR on mature IgM+ IgD+ B cells in vivo to address the fate of lymphocytes encountering self-antigens at this developmental stage. Expression of an autoreactive BCR on mature B cells leads to their rapid elimination from the periphery, a process that is inhibited by constitutive bcl-2 transgene expression in an antigen dose-dependent manner. Thus, selection of mature B cells into the long-lived peripheral pool does not prevent their deletion upon encounter of self-antigens.

Transfer of Primitive Stem/Progenitor Bone Marrow Cells from LTα−/− Donors to Wild-Type Hosts: Implications for the Generation of Architectural Events in Lymphoid B Cell Domains

To analyze whether the phenotypic abnormalities observed in lymphotoxin-α−/− (LTα−/−) mice are intrinsic to the hemolymphoid system itself or dependent on stromal elements, wild-type (WT) mice were reconstituted with bone marrow (BM) cells enriched for hemopoietic stem cells from LTα−/− animals. WT mice reconstituted with LTα−/−c-kit+Lin−Sca-1+ BM cells do not maintain follicular dendritic cell (FDC) networks and do not form primary follicles, while clear segregation of B and T cells could be observed. Furthermore, IgM+IgD− B cells, MOMA-1 (anti-metallophilic macrophages), ERTR-9 (anti-marginal zone macrophages), and MECA-367 (anti-MAdCAM-1) were all absent from the splenic marginal zone. Surprisingly, however, the expression of MOMA-1, ERTR-9, and MAdCAM-1 was normal in the lymph nodes of mice reconstituted with LTα−/− cells. In addition, peanut agglutinin-positive germinal centers were observed in both the spleen and mesenteric lymph nodes, although in the absence of detectable FDC. Furthermore, in animals reconstituted with a mixture of LTα−/− and WT c-kit+Lin−Sca-1+, GC contained either predominantly LTα−/− B cells or WT B cells. These results suggest that although the formation of primary follicles, FDC networks, and the splenic marginal zone are all dependent on hemopoietically derived LTα, germinal center formation and the expression of MAdCAM-1, MOMA-1, and ERTR-9 in lymph nodes are not. Our results also suggest that the disturbed B-T cell separation in LTα−/− mice is unrelated to defects in the marginal zone.

Differential effect of B lymphocyte-induced maturation protein (Blimp-1) expression on cell fate during B cell development

The B lymphocyte-induced maturation protein (Blimp-1) upregulates the expression of syndecan-1 and J chain and represses that of c-myc. We have transfected Blimp-1 into two sublines of the BCL1 B cell lymphoma that represent distinct stages of B cell development in secondary lymphoid tissues. After interleukin (IL)-2 and IL-5 stimulation, the BCL1 3B3 cells differentiate into centrocyte-like cells, whereas the BCL1 5B1b cells blast and appear to be blocked at the centroblast stage. This blasting effect and the increase in IgM secretion that follows it can be blocked by a dominant negative form of Blimp-1. At the same time, the ectopic expression of Blimp-1 in these partially activated cells induces an apoptotic response that also can be suppressed by the same dominant negative protein. A similar effect was noticed when Blimp-1 was expressed in the mature L10A and the immature WEHI-231 lines, indicating this may be a general effect at earlier stages of the B cell development, and distinct from the ability of Blimp-1 to induce maturation in late stages of differentiation. Truncation mutants indicate that the induction of the apoptotic response relies mainly on 69 amino acids within Blimp-1's proline-rich domain. We propose that Blimp-1 expression defines a checkpoint beyond which fully activated B cells proceed to the plasma cell stage, whereas immature and partially activated cells are eliminated at this point.

The normal counterpart of IgD myeloma cells in germinal center displays extensively mutated IgVH gene, Cmu-Cdelta switch, and lambda light chain expression

Human myeloma are incurable hematologic cancers of immunoglobulin-secreting plasma cells in bone marrow. Although malignant plasma cells can be almost eradicated from the patient's bone marrow by chemotherapy, drug-resistant myeloma precursor cells persist in an apparently cryptic compartment. Controversy exists as to whether myeloma precursor cells are hematopoietic stem cells, pre-B cells, germinal center (GC) B cells, circulating memory cells, or plasma blasts. This situation reflects what has been a general problem in cancer research for years: how to compare a tumor with its normal counterpart. Although several studies have demonstrated somatically mutated immunoglobulin variable region genes in multiple myeloma, it is unclear if myeloma cells are derived from GCs or post-GC memory B cells. Immunoglobulin (Ig)D-secreting myeloma have two unique immunoglobulin features, including a biased lambda light chain expression and a Cmu-Cdelta isotype switch. Using surface markers, we have previously isolated a population of surface IgM-IgD+CD38+ GC B cells that carry the most impressive somatic mutation in their IgV genes. Here we show that this population of GC B cells displays the two molecular features of IgD-secreting myeloma cells: a biased lambda light chain expression and a C&mu-Cdelta isotype switch. The demonstration of these peculiar GC B cells to differentiate into IgD-secreting plasma cells but not memory B cells both in vivo and in vitro suggests that IgD-secreting plasma and myeloma cells are derived from GCs.

Amino acid insertions and deletions contribute to diversify the human Ig repertoire

The sequence analysis of Ig variable region genes transcribed within different B-cell subpopulations from human tonsil led us to identify a rare DNA sequence modification event consisting of bp insertions and/or deletions (I/D). Although these events were previously reported, they had never been formally associated with the somatic hypermutation process. I/D events share with more conventional somatic hypermutation events their localization within hypervariable regions and, most particularly, within DNA motifs known to be mutational hot spots. Repetitive DNA tracts or DNA elements capable of forming DNA loop intermediates seem to be the preferred substrate for I/D to occur. These characteristics suggest a model for somatic hypermutation reminiscent of the "polymerase slippage" model involved in replication and repair mutations in prokaryotes, yeast, and mammals.

Normal human B cell sub-populations and their malignant counterparts

Seven tonsillar B cell sub-sets have been isolated according to cell surface molecular markers. The molecular characteristics of their phenotype, cell cycle, survival, somatic mutations and isotype switch status permit their inter-relationships to be followed up until the plasma cell stage. Different lymphoma types correlate with all except one of the stages and only two out of nine non-Hodgkin's B cell lymphomas cannot be presently associated to a normal peripheral B cell sub-set. The assignment of lymphomas to their normal human B cell counterparts will facilitate the identification of the causative event(s) responsible for the malignant transformation.

Germinal center development

Using a set of surface markers including IgD and CD38, human tonsillar B cells were classified into discrete subpopulations. Molecular and functional analysis allowed us to identify: i) two sets of naive B cells (Bm1 and Bm2); ii) germinal center founder cells (Bm2'); iii) an obscure population of germinal center B cells, displaying a high load of somatic mutations in IgV genes, C mu to C delta switch and preferential Ig lambda light chain usage: these cells may represent the precursors of normal and malignant IgD-secreting plasma cells; iv) the centroblasts (Bm3) in which somatic mutation machinery is activated; v) the centrocytes (Bm4) in which isotype switch occurs; vi) the memory B cells. The characterization of these subpopulations showed that: i) programmed cell death is set before somatic mutations, possibly providing an efficient way for affinity maturation; ii) only high affinity centrocytes are allowed to switch isotype; iii) CD40-ligation inhibits plasmacytic differentiation of mature B lymphocytes; iv) memory B cells preferentially differentiate into plasma cells; v) IgD isotype switch occurs in normal B cells; vi) receptor editing may be induced by somatic mutations in germinal centers. We also characterized two types of antigen-presenting cells in germinal centers: follicular dendritic cells that select high affinity B cells, and a new subset of germinal center dendritic cells that activate germinal center T cells.

Mechanisms of selection and differentiation in germinal centers

The choice between death and survival, a feature of early B cell development, is a choice also faced by mature B cells. If they survive this early developmental decision, mature B cells then face a second choice: either to undergo either terminal differentiation into plasma cells or to differentiate into memory B cells. Antigens, T cell signals (cytokines, CD40 ligand and Fas ligand) as well as the activation state of B cells determine their ultimate fate.