Influences on the lifespan of B cell subpopulations defined by different phenotypes

Fate Mapping Quantifies the Dynamics of B Cell Development and Activation throughout Life

Follicular mature (FM) and germinal center (GC) B cells underpin humoral immunity, but the dynamics of their generation and maintenance are not clearly defined. Here, we exploited a fate-mapping system in mice that tracks B cells as they develop into peripheral subsets, together with a cell division fate reporter mouse and mathematical models. We find that FM cells are kinetically homogeneous, recirculate freely, are continually replenished from transitional populations, and self-renew rarely. In contrast, GC B cell lineages persist for weeks with rapid turnover and site-specific dynamics. Those in the spleen derive from transitional cells and are kinetically homogeneous, while those in lymph nodes derive from FM B cells and comprise both transient and persistent clones. These differences likely derive from the nature of antigen exposure at the different sites. Our integrative approach also reveals how the host environment drives cell-extrinsic, age- related changes in B cell homeostasis.

Verheijen and Rane et al. combine fate mapping and mathematical models to quantify the development and dynamics of follicular mature B cells and germinal center B cells in spleen and lymph nodes, and show how these processes vary across the mouse lifespan.

Essential Role for Survivin in the Proliferative Expansion of Progenitor and Mature B Cells

Survivin is a member of the inhibitor of apoptosis family of proteins and a biomarker of poor prognosis in aggressive B cell non-Hodgkin's lymphoma. In addition to its role in inhibition of apoptosis, survivin also regulates mitosis. In this article, we show that deletion of survivin during early B cell development results in a complete block at the cycling pre-B stage. In the periphery, B cell homeostasis is not affected, but survivin-deficient B cells are unable to mount humoral responses. Correspondingly, we show that survivin is required for cell division in response to mitogenic stimulation. Thus, survivin is essential for proliferation of B cell progenitors and activated mature B cells, but is dispensable for B cell survival. Moreover, a small-molecule inhibitor of survivin strongly impaired the growth of representative B lymphoma lines in vitro, supporting the validity of survivin as an attractive therapeutic target for high-grade B cell non-Hodgkin's lymphoma.

Murine B cell development and antibody responses to model antigens are not impaired in the absence of the TNF receptor GITR

The Glucocorticoid-Induced Tumor necrosis factor Receptor GITR, a member of the tumor necrosis factor receptor superfamily, has been shown to be important in modulating immune responses in the context of T cell immunity. B lymphocytes also express GITR, but a role of GITR in humoral immunity has not been fully explored. To address this question, we performed studies to determine the kinetics of GITR expression on naïve and stimulated B cells and the capacity of B cells to develop and mount antibody responses in GITR^−/− mice. Results of our studies indicate that all mature B cells express GITR on the cell surface, albeit at different levels. Expression of GITR on naïve mature B cells is upregulated by BCR signaling, but is counteracted by helper T cell-related factors and other inflammatory signals in vitro. In line with these findings, expression of GITR on germinal center and memory B cells is lower than that on naïve B cells. However, the expression of GITR is strongly upregulated in plasma cells. Despite these differences in GITR expression, the absence of GITR has no effect on T cell-dependent and T cell-independent antibody responses to model antigens in GITR^−/− mice, or on B cell activation and proliferation in vitro. GITR deficiency manifests only with a slight reduction of mature B cell numbers and increased turnover of naïve B cells, suggesting that GITR slightly contributes to mature B cell homeostasis. Overall, our data indicate that GITR does not play a significant role in B cell development and antibody responses to T-dependent and independent model antigens within the context of a GITR-deficient genetic background.

In vivo control of B-cell survival and antigen-specific B-cell responses

Targeted modification of the mouse genome provides the capability to manipulate complex physiological processes in a precise and controlled manner. Investigation of B-lymphocyte biology has benefited not only from the targeted modification of genes controlling B-cell survival and responsiveness, but also from the manipulation of antigen specificity made possible by targeting endogenous immunoglobulin loci. In this review, we discuss recent results obtained from our laboratory using gene-targeted mouse models to investigate the in vivo regulation of B-cell survival and responsiveness. The control of BAFF-dependent survival signals by the TRAF2- and TRAF3-signaling proteins is discussed as is the potential involvement of these molecules in B-lineage malignancies. We also outline the development and use of the SW(HEL) model for analyzing antigen-specific B-cell responses in vivo. This includes insights into the control of early decision-making during T-dependent B-cell differentiation, the affinity maturation and plasma cell differentiation of germinal center B cells, and the identification of EBI2 as a key regulator of B-cell migration and differentiation.

Histological assessment of cellular half-life in tissues in vivo

The assessment of cellular half-life is of fundamental importance for cell biology and biomedicine. Here, we show that cellular half-life in tissues can be histologically measured under steady state conditions in vivo by analyzing the loss of 5-bromo-2'-deoxyuridine (BrdU)-labeled cells over time after withdrawal of long-term BrdU labeling. To achieve efficient continuous cell labeling, we implanted BrdU-containing subcutaneous slow-release pellets into 12-month-old male Fischer 344 rats, delivering BrdU at a dose of 75 mg/kg per day over 1 (n=20) or 3 weeks (n=20). Four to five rats each were killed directly after the labeling or 1, 3, and 7 weeks post-labeling. Cellular half-life after withdrawal of BrdU was analyzed by nonlinear regression analysis of the labeling index, using a model of one-phase exponential decay. We initially validated our technique in the duodenum, where we determined a half-life of 2.4 days for crypt cells. Next, we applied this method to other tissues, and found a half-life of 2.2 weeks for cardiac endothelial cells, and of 5-6 days for pancreatic duct cells. In conclusion, we believe that this novel approach is an important step forward in the histological assessment of cellular half-life.

The recirculating B cell pool contains two functionally distinct, long-lived, posttransitional, follicular B cell populations

Disparate models for the development of peripheral B cells may reflect significant heterogeneity in recirculating long-lived B cells that have not been previously accounted for. We show in this study that the murine recirculating B cell pool contains two distinct, long-lived, posttransitional, follicular B cell populations. Follicular Type I IgM(low) B cells require Ag-derived and Btk-dependent signals for their development and make up the majority of cells in the recirculating follicular B cell pool. Follicular type II B cells do not require Btk- or Notch-2-derived signals, make up about a third of the long-lived recirculating B cell pool, and can develop in the absence of Ag. These two follicular populations exhibit differences in basal tyrosine phosphorylation and in BCR-induced proliferation, suggesting that they may represent functionally distinct populations of long-lived recirculating B cells.

Regulation of B cell self-tolerance by BAFF

To avoid the generation of pathogenic autoantibodies, self-reactive lymphocytes are deleted at several distinct checkpoints during B cell maturation. BAFF is required for mature B cell development and survival but causes B cell hyperplasia and autoimmunity when it is overexpressed. Self-reactive B cells have reduced responsiveness to BAFF and therefore die due to the limiting levels of BAFF available in vivo. Elevated BAFF expression subverts B cell self-tolerance by rescuing self-reactive B cells normally deleted relatively late during maturation. Strongly self-reactive B cells are deleted prior to expression of BAFF-R and are therefore resistant to rescue by BAFF.

Modelling cell lifespan and proliferation: is likelihood to die or to divide independent of age?

In cell lifespan studies the exponential nature of cell survival curves is often interpreted as showing the rate of death is independent of the age of the cells within the population. Here we present an alternative model where cells that die are replaced and the age and lifespan of the population pool is monitored until a steady state is reached. In our model newly generated individual cells are given a determined lifespan drawn from a number of known distributions including the lognormal, which is frequently found in nature. For lognormal lifespans the analytic steady-state survival curve obtained can be well-fit by a single or double exponential, depending on the mean and standard deviation. Thus, experimental evidence for exponential lifespans of one and/or two populations cannot be taken as definitive evidence for time and age independence of cell survival. A related model for a dividing population in steady state is also developed. We propose that the common adoption of age-independent, constant rates of change in biological modelling may be responsible for significant errors, both of interpretation and of mathematical deduction. We suggest that additional mathematical and experimental methods must be used to resolve the relationship between time and behavioural changes by cells that are predominantly unsynchronized.

Loss of the proapoptotic protein, Bim, breaks B cell anergy

Although B cells that respond with high avidity to self-antigen are eliminated early in their development, many autoreactive B cells escape elimination and are tolerized later in their lives via anergy. Anergic B cells are unresponsive to antigen and die prematurely. It has been suggested that the proapoptotic protein, Bim, controls the fate of anergic B cells. To test this idea, mice lacking Bim were crossed with mice that express soluble hen egg lysozyme and whose B cells bear receptors specific for the protein. In Bim^+/+ mice these B cells are anergic and die rapidly. If the mice lack Bim, however, the B cells live longer, are more mature, respond to antigen, and secrete anti–hen egg lysozyme antibodies. This break of tolerance is not due to expression of endogenous B cell receptors, nor is it dependent on T cells. Rather, it appears to be due to a reduced requirement for the cytokine BAFF. Normal B cells require BAFF both for differentiation and survival. Bim^−/− B cells, on the other hand, require BAFF only for differentiation. Therefore, autoreactive B cells are allowed to survive if they lack Bim and thus accumulate sufficient signals from differentiating factors to drive their maturation and production of autoantibodies.

B-cell kinetics in humans: rapid turnover of peripheral blood memory cells

Information about the kinetic behavior and lifespan of lymphocytes is crucial to understanding the mechanisms that regulate processes such as immunologic memory. We have used in vivo labeling of dividing cells with 6,6-(2)H(2)-glucose, combined with cell sorting and gas-chromatography-mass spectrometry for deuterium enrichment, in order to analyze the kinetics of human total, naive, or memory B lymphocytes, separated from peripheral blood using monoclonal antibodies. We show that total blood B cells of young adults divide at an average rate of 1.9% (+/-1.0%) per day and at a similar though slightly slower rate, 1.5% (+/-1.3%) per day, in the elderly. Separation of naive and memory B cells according to expression of CD27 indicates that naive peripheral blood B cells divide slowly (0.46% per day), while memory cells proliferate more rapidly (2.66% per day). These data are compatible with the view that B-cell memory may be maintained by clones of proliferating B cells.

The role of Bcl-2 family members in tumorigenesis

The Bcl-2 family consists of about 20 homologues of important pro- and anti-apoptotic regulators of programmed cell death. The established mode of function of the individual members is to either preserve or disturb mitochondrial integrity, thereby inducing or preventing release of apoptogenic factors like Cytochrome c (Cyt c) from mitochondria. Recent findings also indicate further Bcl-2-controlled mitochondria-independent apoptosis pathways. Bcl-2 represents the founding member of the new and growing class of cell death inhibiting oncoproteins. In this review, we try to briefly summarize current models of Bcl-2 family function and to outline the work demonstrating the influence of deregulated Bcl-2 family member expression on tumorigenesis and cancer therapy. Since several Bcl-2 homologues, in addition to influencing apoptotic behaviour, also impinge on cell cycle progression, we discuss possible implications of this additional role for the expression of Bcl-2 family members in tumor cells.

B Lymphocyte Development in Rabbit: Progenitor B Cells and Waning of B Lymphopoiesis

In mammals that use gut-associated lymphoid tissues for expansion and somatic diversification of the B cell repertoire, B lymphopoiesis occurs early in ontogeny and does not appear to continue throughout life. In these species, including sheep, rabbit, and cattle, little is known about the pathway of B cell development and the time at which B lymphopoiesis wanes. We examined rabbit bone marrow by immunofluorescence with anti-CD79a and anti-mu and identified both proB and preB cells. The proB cells represent the vast majority of B-lineage cells in the bone marrow at birth and by incorporation of 5-bromo-2'-deoxyuridine, they appear to be a dynamic population. PreB cells reach maximum levels in the bone marrow at 3 wk of age, and B cells begin to accumulate at 7 wk of age. We cloned two VpreB and one lambda5 gene and demonstrated that they are expressed within B-lineage cells in bone marrow. VpreB and lambda5 coimmunoprecipitated with the mu-chain in lysates of 293T cells transfected with VpreB, lambda5, and mu, indicating that VpreB, lambda5, and mu-chains associate in a preB cell receptor-like complex. By 16 wk of age, essentially no proB or preB cells are found in bone marrow and by PCR amplification, B cell recombination excision circles were reduced 200-fold. By 18 mo of age, B cell recombination excision circles were reduced 500- to 1000-fold. We suggest that B cell development in the rabbit occurs primarily through the classical, or ordered, pathway and show that B lymphopoiesis is reduced over 99% by 16 wk of age.

B cell receptor-independent stimuli trigger immunoglobulin (Ig) class switch recombination and production of IgG autoantibodies by anergic self-reactive B cells

In both humans and animals, immunoglobulin (Ig)G autoantibodies are less frequent but more pathogenic than IgM autoantibodies, suggesting that controls over Ig isotype switching are required to reinforce B cell self-tolerance. We have used gene targeting to produce mice in which hen egg lysozyme (HEL)-specific B cells can switch to all Ig isotypes (SWHEL mice). When crossed with soluble HEL transgenic (Tg) mice, self-reactive SWHEL B cells became anergic. However, in contrast to anergic B cells from the original nonswitching anti-HEL x soluble HEL double Tg model, self-reactive SWHEL B cells also displayed an immature phenotype, reduced lifespan, and exclusion from the splenic follicle. These differences were not related to their ability to Ig class switch, but instead to competition with non-HEL-binding B cells generated by VH gene replacement in SWHEL mice. When activated in vitro with B cell receptor (BCR)-independent stimuli such as anti-CD40 monoclonal antibody plus interleukin 4 or lipopolysaccharide (LPS), anergic SWHEL double Tg B cells proliferated and produced IgG anti-HEL antibodies as efficiently as naive HEL-binding B cells from SWHEL Ig Tg mice. These results demonstrate that no intrinsic constraints to isotype switching exist in anergic self-reactive B cells. Instead, production of IgG autoantibodies is prevented by separate controls that reduce the likelihood of anergic B cells encountering BCR-independent stimuli. That bacteria-derived LPS could circumvent these controls may explain the well-known association between autoantibody-mediated diseases and episodes of systemic infection.

The Bcl-2 protein family: sensors and checkpoints for life-or-death decisions

Members of the Bcl-2 family are crucial integrators of survival and death signals in higher eukaryotes. Although recent studies have provided novel and quite unexpected insights into the mechanisms by which these proteins might issue life permits or death sentences in cells, we are still on the way to fully understand their modes of action. This review provides a snapshot on where we are on this journey and how we may exploit our knowledge on this family of proteins to unveil the mysteries of immune regulation.

IkappaB kinase signaling is essential for maintenance of mature B cells

Nuclear factor (NF)-kappaB proteins play crucial roles in immune responses and cellular survival. Activation of NF-kappaB is mediated by the IkappaB kinase (IKK) complex, which is composed of two kinases, IKK1 and IKK2, and a regulatory subunit termed NF-kappaB essential modulator (NEMO). IKK2- and NEMO-deficient mice die at early embryonic stages. We therefore used conditional gene targeting to evaluate the role of these proteins in B cells in adult mice. B lineage-specific disruption of either IKK signaling by deletion of NEMO, or of IKK2-specific signals by ablation of IKK2 activity leads to the disappearance of mature B lymphocytes. We conclude that maintenance of mature B cells depends on IKK-mediated activation of NF-kappaB.

Estrogen alters thresholds for B cell apoptosis and activation

Estrogen is thought to contribute to the increased frequency of autoimmune disorders occurring in females, but a molecular basis for its effects on autoimmunity remains to be elucidated. We have shown previously that estrogen leads to the survival and activation of autoreactive cells in the naive repertoire. To identify the molecular pathways involved in B cell tolerance, we sought to identify genes that are differentially regulated by estrogen in mouse B cells. Several genes involved in B cell activation and survival, including cd22, shp-1, bcl-2, and vcam-1, were upregulated by estrogen in B cells. We found that overexpression of CD22 and SHP-1 in B cells decreased B cell receptor signaling. Estrogen receptors alpha and beta are expressed on B cells and are functional, since they can directly upregulate expression of CD22, SHP-1, and Bcl-2. Estrogen treatment protected isolated primary B cells from B cell receptor-mediated apoptosis. These results suggest that estrogen induces a genetic program that alters survival and activation of B cells in a B cell-autonomous fashion and thus skews the naive immune system toward autoreactivity.

xid mice reveal the interplay of homeostasis and Bruton's tyrosine kinase-mediated selection at multiple stages of B cell development

Human X-linked agammaglobulinemia (XLA) and murine X-linked immune defect (XID) are both immunodeficiencies mediated by mutations in Bruton's tyrosine kinase (Btk), yet the developmental stage(s) affected remain controversial. To further refine the placement of the XID defect(s), we used bromodeoxyuridine labeling to determine turnover, production and transition rates of developing B cell subsets in normal, xid and xid mice expressing a human Bcl-2 transgene (xid/bcl-2). We find the xid mutation manifest at two stages of B cell development. The first is early, reducing pre-B cell production by restricting pro-B to pre-B cell transit. Surprisingly, this impairment is offset by increased survival of cells progressing from the pre- to immature B cell pool, suggesting that Btk-independent homeostatic mechanisms act to maintain this compartment. The second point of action is late, substantially reducing mature B cell production. Together, these findings reconcile apparent discrepancies in the developmental stage affected by the murine versus human lesions and suggest previously unappreciated homeostatic processes that act at the pre-B to immature B cell transition. Finally, Btk likely functions differently at these two checkpoints, since ectopic Bcl-2 expression fails to directly complement the early xid lesion, yet reverses the defect impeding final B cell maturation.

Homeostasis of peripheral B cells in the absence of B cell influx from the bone marrow

To study homeostasis of peripheral B lymphocytes in the absence of B cell influx from the bone marrow, we generated a mouse mutant in which the recombination-activating gene (RAG)-2 can be inducibly deleted. When RAG-2 was deleted at the age of 8-10 wk, splenic naive follicular B cells were gradually lost over a year of observation, with a half-life of approximately 4.5 mo. By contrast, the pool of marginal zone B cells in the spleen and of B-1 cells in the peritoneal cavity were kept at normal level. In lymph nodes, approximately 90% of the B cells were lost within 4 mo, and B cell numbers remained constant thereafter. Mice in which RAG-2 was deleted at birth maintained a small population of activated B cells with an increased proportion of marginal zone B cells. Additionally, an increase of the pool of IgM secreting cells and B-1a cells was observed.

BAFF binds to the tumor necrosis factor receptor-like molecule B cell maturation antigen and is important for maintaining the peripheral B cell population

The tumor necrosis factor (TNF) family member B cell activating factor (BAFF) binds B cells and enhances B cell receptor-triggered proliferation. We find that B cell maturation antigen (BCMA), a predicted member of the TNF receptor family expressed primarily in mature B cells, is a receptor for BAFF. Although BCMA was previously localized to the Golgi apparatus, BCMA was found to be expressed on the surface of transfected cells and tonsillar B cells. A soluble form of BCMA, which inhibited the binding of BAFF to a B cell line, induced a dramatic decrease in the number of peripheral B cells when administered in vivo. Moreover, culturing splenic cells in the presence of BAFF increased survival of a percentage of the B cells. These results are consistent with a role for BAFF in maintaining homeostasis of the B cell population.

Mixed chimerism induced without lethal conditioning prevents T cell- and anti-Gal alpha 1,3Gal-mediated graft rejection

Gal alpha 1,3Gal-reactive (Gal-reactive) antibodies are a major impediment to pig-to-human xenotransplantation. We investigated the potential to induce tolerance of anti-Gal-producing cells and prevent rejection of vascularized grafts in the combination of alpha 1,3-galactosyltransferase wild-type (GalT(+/+)) and deficient (GalT(-/-)) mice. Allogeneic (H-2 mismatched) GalT(+/+) bone marrow transplantation (BMT) to GalT(-/-) mice conditioned with a nonmyeloablative regimen, consisting of depleting CD4 and CD8 mAb's and 3 Gy whole-body irradiation and 7 Gy thymic irradiation, led to lasting multilineage H-2(bxd) GalT(+/+) + H-2(d) GalT(-/-) mixed chimerism. Induction of mixed chimerism was associated with a rapid reduction of serum anti-Gal naturally occurring antibody levels. Anti-Gal-producing cells were undetectable by 2 weeks after BMT, suggesting that anti-Gal-producing cells preexisting at the time of BMT are rapidly tolerized. Even after immunization with Gal-bearing xenogeneic cells, mixed chimeras were devoid of anti-Gal-producing cells and permanently accepted donor-type GalT(+/+) heart grafts (>150 days), whereas non-BMT control animals rejected these hearts within 1-7 days. B cells bearing receptors for Gal were completely absent from the spleens of mixed chimeras, suggesting that clonal deletion and/or receptor editing may maintain B-cell tolerance to Gal. These findings demonstrate the principle that induction of mixed hematopoietic chimerism with a potentially relevant nonmyeloablative regimen can simultaneously lead to tolerance among both T cells and Gal-reactive B cells, thus preventing vascularized xenograft rejection.

The central effectors of cell death in the immune system

The immune system relies on cell death to maintain lymphoid homeostasis and avoid disease. Recent evidence has indicated that the caspase family of cysteine proteases is a central effector in apoptotic cell death and is absolutely responsible for many of the morphological features of apoptosis. Cell death, however, can occur through caspase-independent and caspase-dependent pathways. In the case of cells that are irreversibly neglected or damaged, death occurs even in the absence of caspase activity. In contrast, healthy cells require caspase activation to undergo cell death induced by surface receptors. This review summarizes the current understanding of these two pathways of cell death in the immune system.

Accelerated Emigration of B Lymphocytes in the Xid Mouse

The B cell receptor is required for the emigration of newly generated B lymphocytes and for their maintenance in the periphery. A specific maintenance defect was noted in fraction I (IgDhighIgMlow) B cells in Xid mice (which harbor a mutation in Btk). Although Bcl-2 levels in fractions I and II (IgDhighIgMhigh) are equivalent in normal and Xid B cells, a novel peak of Bcl-2low fraction III (IgDlowIgMhigh) B cells was noted in the Xid mouse. Since this B cell population resembled bone marrow immature B cells, we examined the emigration of newly formed B cells in normal and Xid mice. These studies revealed the accelerated emigration of newly formed Xid B cells. We conclude that distinct Btk-independent and Btk-dependent signals mediate emigration and maintenance events during peripheral B cell maturation.

B Cell Maintenance in Aged Mice Reflects Both Increased B Cell Longevity and Decreased B Cell Generation

In aged mice the population of mature peripheral B cells is maintained despite a severalfold decrease in the population of bone marrow B cell progenitors. The analysis of the rate of accumulation of 5′-bromo-2-deoxyuridine (BrdU)-labeled splenic B cells in mice fed BrdU for 8 days to 8 wk demonstrated a severalfold increase in the half-life of mature B cells in aged mice. Consistent with a role for decreased B cell turnover in maintaining the mature B cell population of aged mice, several findings indicate that fewer newly generated B cells enter the spleen from the bone marrow in aged vs young adult mice. These include 1) a fourfold decrease in the population of relatively immature splenic B cells, defined as cells that express high levels of heat-stable Ag and accumulate BrdU within 8 wk of labeling; and 2) an equivalent decrease in the population of bone marrow cells representative of later stages of B cell maturation (sIgD−sIgMint-high). Surprisingly, despite a four- to sixfold decrease in pre-B cells, the population of least mature bone marrow B cells (IgD−sIgMvery low) remains intact. Because this population accumulates BrdU-labeled cells more slowly in aged mice than in younger mice, and bone marrow B cells at more mature developmental stages are diminished, it appears that in aged mice B cell development beyond the sIgMvery low stage may be retarded and that cells, therefore, accumulate within this population.

Severe B cell deficiency and disrupted splenic architecture in transgenic mice expressing the E41K mutated form of Bruton's tyrosine kinase

To identify B-cell signaling pathways activated by Bruton's tyrosine kinase (Btk) in vivo, we generated transgenic mice in which Btk expression is driven by the MHC class II Ea gene locus control region. Btk overexpression did not have significant adverse effects on B cell function, and essentially corrected the X-linked immunodeficiency (xid) phenotype in Btk- mice. In contrast, expression of a constitutively activated form of Btk carrying the E41K gain-of-function mutation resulted in a B cell defect that was more severe than xid. The mice showed a marked reduction of the B cell compartment in spleen, lymph nodes, peripheral blood and peritoneal cavity. The levels in the serum of most immunoglobulin subclasses decreased with age, and B cell responses to both T cell-independent type II and T cell-dependent antigens were essentially absent. Expression of the E41K Btk mutant enhanced blast formation of splenic B cells in vitro in response to anti-IgM stimulation. Furthermore, the mice manifested a disorganization of B cell areas and marginal zones in the spleen. Our findings demonstrate that expression of constitutively activated Btk blocks the development of follicular recirculating B cells.

Independent and opposing roles for Btk and lyn in B and myeloid signaling pathways

Transphosphorylation by Src family kinases is required for the activation of Bruton's tyrosine kinase (Btk). Differences in the phenotypes of Btk-/- and lyn-/- mice suggest that these kinases may also have independent or opposing functions. B cell development and function were examined in Btk-/-lyn-/- mice to better understand the functional interaction of Btk and Lyn in vivo. The antigen-independent phase of B lymphopoiesis was normal in Btk-/-lyn-/- mice. However, Btk-/-lyn-/- animals had a more severe immunodeficiency than Btk-/- mice. B cell numbers and response to T cell-dependent antigens were reduced. Btk and Lyn therefore play independent or partially redundant roles in the maintenance and function of peripheral B cells. Autoimmunity, hypersensitivity to B cell receptor (BCR) cross-linking, and splenomegaly caused by myeloerythroid hyperplasia were alleviated by Btk deficiency in lyn-/- mice. A transgene expressing Btk at approximately 25% of endogenous levels (Btklo) was crossed onto Btk-/- and Btk-/-lyn-/- backgrounds to demonstrate that Btk is limiting for BCR signaling in the presence but not in the absence of Lyn. These observations indicate that the net outcome of Lyn function in vivo is to inhibit Btk-dependent pathways in B and myeloid cells, and that Btklo mice are a useful sensitized system to identify regulatory components of Btk signaling pathways.

Nature versus nurture: contributions of developmental programming and the microenvironment to B cell tolerance

The original Burnet Lederberg and Bretscher Cohn models of immunological tolerance are essentially incompatible, one considering tolerance to be the obligatory outcome of antigen recognition by an immature lymphocyte and the other considering it as one of two possible responses to antigen, the crucial determinant being interaction with a second antigen-reactive cell. The early experimental evidence was confusing, in that it appeared to support both theories. In response to this situation, a hybrid model retaining some of the features of the original models was proposed. In particular, immature B cells were regarded as 'hypersensitive to tolerance induction', but could also make a positive response to antigen under some circumstances. More recent data from B cell transgenic mice have challenged even these hybrid models, stimulating renewed interest in the question of how B cell tolerance is regulated in vivo. This article presents a new interpretation of the data, in which the increased resistance of mature B cells to tolerance induction is postulated to result from partial receptor desensitization in response to environmental antigen, rather than from a developmentally programmed change in B cell signalling. Thus, it is suggested that Burnet's 'window of tolerance induction' is determined by the environment rather than developmental pre-programming. If this postulate is accepted, induction of B cell self-tolerance in both the bone marrow and periphery follows the simple and elegant rules originally laid down by Bretscher and Cohn.

CD1.1 Expression by Mouse Antigen-Presenting Cells and Marginal Zone B Cells

Mouse CD1.1 is an MHC class I-like, non-MHC-encoded, surface glycoprotein that can be recognized by T cells, in particular NK1.1+ T cells, a subset of αβ T cells with semiinvariant TCRs that promptly releases potent cytokines such as IL-4 and IFN-γ upon stimulation. To gain insight into the function of CD1.1, a panel of nine mAbs was generated and used to biochemically characterize and monitor the surface expression of CD1.1 on different cell types. CD1.1 is a heavily glycosylated, β2-microglobulin-associated surface protein. Its recognition by a panel of 12 Vα14-positive and -negative CD1-specific αβ T cell hybridomas was blocked by two groups of mAbs that bound to adjacent clusters of epitopes, indicating that different αβ TCRs bind to the same region of CD1.1, presumably above the groove. Remarkably, CD1.1 was mainly expressed by dendritic cells, B cells, and macrophages, suggesting a function in Ag presentation to Th cells. Furthermore, the cell type that expressed the highest levels of CD1.1 was the splenic marginal zone B cell, a distinct subset of B cells that also expresses CD21 (the C3d receptor) and may be involved in natural responses to bacterial Ags. Altogether, the results support the idea that CD1.1 may function in recruiting a form of innate help from specialized cytokine producer αβ T cells to APCs, a role that might be important at the preadaptive phase of immune responses to some microbial pathogens.

Receptor editing and commitment in B lymphocytes

B cells that fail to pass a developmental checkpoint, either as immature or mature B cells, can be rescued by creating a new B cell antigen receptor through nested secondary immunoglobulin gene rearrangements, a process termed receptor editing. Tolerance-mediated receptor editing occurs in self-reactive immature bone marrow B cells, while peripheral receptor editing probably occurs in low-affinity B cells competing for antigen and for survival signals within the germinal center response.

B-cell receptor regulation of peripheral B cells

Recent studies indicate that immature B cells compete with recirculating B cells for survival signals. The signals, delivered through the B-cell receptor for antigen, induce immature cells to differentiate into recirculating cells and maintain the survival of recirculating cells. They do not induce proliferation or differentiation to antibody-producing cells.

The B-cell biology of aging

Although both the number and responsiveness of peripheral B cells in aged mice remain relatively intact, there are dramatic changes in B-cell generation. Alterations in B-cell development include both a skewing of V-gene utilization, especially in cells responsive to phosphorylcholine, and a decrease in the generation of various developmental B-cell subsets. The altered representation of these subsets appears to be the consequence of two developmental blocks. The first developmental block occurs during the maturation of pro-B cells and is evidenced by a decrease in the number of pre-B cells. The second developmental block occurs at the earliest stage of sIg(+)-cell maturation (sIgMvery lo). Because of this block in B-cell maturation, in spite of a decrease in incoming pre-B cells, the number of sIgMvery lo cells appears to increase in aged mice. Additionally, the time of residence of cells within this maturational stage increases dramatically, while the proportion of cells in more mature (sIgMhi) stages of bone marrow development are decreased. In addition to the decreased number of maturing bone marrow B cells, the population of splenic B cells that represent recent bone marrow émigrés (HSAvery hi) is markedly decreased. In the face of this decrease in newly emerging cells from the bone marrow, the population of mature splenic B cells is maintained by their increased longevity.

A re-evaluation of the effects of X-linked immunodeficiency (xid) mutation on B cell differentiation and function in the mouse

CBA/N (xid) mice have a point mutation in Bruton's tyrosine kinase (btk), which results in their failure to respond to T-independent type 2 (TI-2) antigens, and to several B cell mitogens [most notably anti-immunoglobulin (Ig)] in vitro. They have reduced numbers of peripheral (B2) B cells, which are regarded as being phenotypically and functionally immature. We show here that adult CBA/N mice in fact have two distinct B cell populations: some 60% of the cells are CD23+ HSAlo sIgDhi and hence resemble recirculating, follicular (RF) B cells from normal mice, except that they are sIgMhi. The remaining 40% of xid B cells are CD23- HSAhi sIgD-/lo and resemble immature transitional (TR) B cells. TR B cells from xid mice do not synthesize DNA when cultured with lipopolysaccharide (LPS), whereas those from normal mice do so. Only the RF cells from either xid or normal mice proliferate in response to ligation of CD40. In neonatal normal mice the emergence of mitogen responsiveness followed the chronological sequence LPS-->anti-CD40-->anti-Ig approximately anti-CD38. The same developmental sequence was seen with B cells from xid mice (for LPS and anti-CD40), but it occurred at a significantly slower tempo and this correlated with the later appearance of RF-type cells. TR xid B cells express very low levels of bcl-2 and we conclude that these cells resemble very immature (bone marrow) B cells, rather than normal transitional cells. We, therefore, propose that the xid mutation imposes a multistage brake on B cell differentiation in the mouse. The available data suggest that btk is required for the positive selection of B cells throughout their differentiation in the periphery. This in turn implies that low level signaling via surface Ig is needed throughout this process in order for peripheral B cells to become functionally mature.

B cell life span: a review

Debate has surrounded the subject of B cell life span since it was first measured in mice in the early 1970s. In the 25 years which have passed since then, it has become increasingly apparent that the methods employed to measure rates of B cell turnover, such as [3H]-thymidine labelling, cell transfer or cell ablation, brought about significant disruptions to normal physiology which in themselves might have affected B cell turnover. More recently the use of bromodeoxyuridine has overcome many of these methodological difficulties and has allowed rates of B cell renewal to be measured within B cell subpopulations defined by multiparameter flow cytometry. Such studies have largely resolved the issue, concluding that about 85% of peripheral B cells are phenotypically mature and display first-order exponential kinetics defined by a half-life of 5-6 weeks, whilst the remainder are short-lived with a life span of several days. This review examines both traditional and recent methods and discusses the influence of age, self-tolerance and randomness in the overall shaping of a kinetically stable mature B cell population.

Distinct roles for lymphotoxin-alpha and tumor necrosis factor in organogenesis and spatial organization of lymphoid tissue

Specialized roles for the pro-inflammatory cytokines tumor necrosis factor (TNF) and lymphotoxin (LT) were characterized in TNF/LT alpha -/- and TNF -/- mice established by direct gene targeting of C57BL/6 ES cells. The requirement for LT early in lymphoid tissue organogenesis is shown to be distinct from the more subtle and varied role of TNF in promoting correct microarchitectural organization of leukocytes in LN and spleen. Development of normal Peyer's patch (PP) structure, in contrast, is substantially dependent on TNF. Only mice lacking LT exhibit retarded B cell maturation in vivo and serum immunoglobulin deficiencies. A temporal hierarchy in lymphoid tissue development can now be defined, with LT being an essential participant in general lymphoid tissue organogenesis, developmentally preceeding TNF that has a more varied and subtle role in promotion of correct spatial organization of leukocytes in LN and spleen PP development in TNF -/- mice is unusual, indicating that TNF is a more critical participant for this structure than it is for other lymphoid tissues.