Control of the B cell-intrinsic tolerance programs by ubiquitin ligases Cbl and Cbl-b

B cell receptor (BCR) signaling plays a critical role in B cell tolerance and activation. Here, we show that mice with B cell-specific ablation of both Cbl and Cbl-b (Cbl-/-Cblb-/-) manifested systemic lupus erythematosus (SLE)-like autoimmune disease. The Cbl double deficiency resulted in a substantial increase in marginal zone (MZ) and B1 B cells. The mutant B cells were not hyperresponsive in terms of proliferation and antibody production upon BCR stimulation; however, B cell anergy to protein antigen appeared to be impaired. Concomitantly, BCR-proximal signaling, including tyrosine phosphorylation of Syk tyrosine kinase, Phospholipase C-gamma2 (PLC-gamma2), and Rho-family GTP-GDP exchange factor Vav, and Ca2+ mobilization were enhanced, whereas tyrosine phosphorylation of adaptor protein BLNK was substantially attenuated in the mutant B cells. These results suggested that the loss of coordination between these pathways was responsible for the impaired B cell tolerance induction. Thus, Cbl proteins control B cell-intrinsic checkpoint of immune tolerance, possibly through coordinating multiple BCR-proximal signaling pathways during anergy induction.

B-1 B cell development

CD5+ B cells have attracted considerable interest because of their association with self-reactivity, autoimmunity, and leukemia. In mice, CD5+ B cells are readily generated from fetal/neonatal precursors, but inefficiently from precursors in adult. One model proposed to explain this difference is that their production occurs through a distinctive developmental process, termed B-1, that enriches pre-B cells with novel germline VDJs and that requires positive selection of newly formed B cells by self-Ag. In contrast, follicular B cells are generated throughout adult life in a developmental process termed B-2, selecting VDJs that pair well with surrogate L chain, and whose maturation appears relatively independent of antigenic selection. In the present study, I focus on processes that shape the repertoire of mouse CD5+ B cells, describing the differences between B-1 and B-2 development, and propose a model encompassing both in the generation of functional B cell subpopulations.

B-1 B cells: development, selection, natural autoantibody and leukemia

B-1 (CD5+) B cells constitute a phenotypic and functionally distinct population of B cells in mouse that show enriched expression of autoreactive B-cell antigen receptors and that produce several types of natural autoantibodies. Recently, there has been much progress in this field of research. Evidence has appeared for the existence of distinctive B-cell precursors that preferentially generate B-1 B cells, and the crucial requirement for strong B-cell antigen receptor signaling in the maturation of B-1 B cells has been established. Other work focuses on a phenotypically similar population that lacks CD5, termed 'B-1b', which shows similarities and differences from most CD5+ B cells in both development and function. The relationship of normal B-1 cells with B-cell lymphomas and leukemias continues to be a subject of interest and debate.

Lineage specification and plasticity in CD19- early B cell precursors

We describe here three CD19- B cell precursor populations in mouse bone marrow identified using 12-color flow cytometry. Cell transfer experiments indicate lineage potentials consistent with multilineage progenitor (MLP), common lymphoid progenitor (CLP), and B lineage-restricted pre-pro-B (Fr. A), respectively. However, single cell in vitro assays reveal lineage plasticity: lymphoid/myeloid lineage potential for CLP and B/T lineage potential for Fr. A. Despite myeloid potential, recombination activating gene 2 reporter activation is first detected at low levels in most MLP cells, with 95% of CLPs showing 10-fold increased levels. Furthermore, single cell analysis shows that half of CLP and 90% of Fr. A cells contain heavy chain DJ rearrangements. These data, together with expression profiles of lineage-specific genes, demonstrate progressive acquisition of B lineage potential and support an asynchronous view of early B cell development, in which B lineage specification initiates in the MLP/CLP stage, whereas myeloid potential is not lost until the pre-pro-B (Fr. A) stage, and B/T lymphoid plasticity persists until the CD19+ pro-B stage. Thus, MLP, CLP, and Fr. A represent progressively B lineage-specified stages in development, before the CD19+ B lineage-committed pro-B stage.

Evidence of marginal-zone B cell-positive selection in spleen

Antigen receptor-mediated signaling is critical for the development and survival of B cells. However, it has not been established whether B cell development requires a signal from self-ligand engagement at the immature stage, a process known as "positive selection." Here, using a monoclonal B cell receptor (BCR) mouse line, specific for the self-Thy-1/CD90 glycoprotein, we demonstrate that BCR crosslinking by low-dose self-antigen promotes survival of immature B cells in culture. In spleen, an increase in BCR signaling strength, induced by low-dose self-antigen, directed naive immature B cells to mature, not into the default follicular B cell fate, but instead into the marginal-zone B cell subset. These data indicate that positive selection can occur in developing B cells and that BCR signal strength is a key factor in deciding between two functionally distinct mature B cell compartments in the microenvironment of the spleen.

Association of B-1 B cells with follicular dendritic cells in spleen

Although CD5(+) B-1 B cells have been recognized as an infrequent B cell subset in mice for many years, attempts to identify their histologic location in normal mouse spleen have proven difficult due to both their paucity and low level expression of CD5. In this study we have studied V(H)11/D(H)/J(H) gene-targeted mice, V(H)11t, that develop elevated numbers of CD5(+) V(H)11/V(k)9 B cells with an anti-phosphatidylcholine (anti-PtC) autoreactive specificity, allowing B-1 B cell detection by anti-PtC Id-specific Abs in spleen section staining. Using this approach we found that anti-PtC B-1 cells first appear within the white pulp in neonates, expand in association with follicular dendritic cells (FDC), and localize more centrally than other (non-B-1) IgD(high) follicular B cells in adults. Among neonatal B cells, CD5(+) B-1 cells in both normal and V(H)11t mouse spleen and peritoneal cavity express the highest levels of CXCR5, which is important for FDC development. Injection of purified spleen or peritoneal B-1 cells into RAG knockout mice resulted in B-1 cell follicle formation in spleen, inducing FDC development and plasma cell generation. These results indicate that B-1 B cells are the first B cells to express fully mature levels of CXCR5, thereby promoting the development of FDC.

Basal immunoglobulin signaling actively maintains developmental stage in immature B cells

In developing B lymphocytes, a successful V(D)J heavy chain (HC) immunoglobulin (Ig) rearrangement establishes HC allelic exclusion and signals pro-B cells to advance in development to the pre-B stage. A subsequent functional light chain (LC) rearrangement then results in the surface expression of IgM at the immature B cell stage. Here we show that interruption of basal IgM signaling in immature B cells, either by the inducible deletion of surface Ig via Cre-mediated excision or by incubating cells with the tyrosine kinase inhibitor herbimycin A or the phosphatidylinositol 3-kinase inhibitor wortmannin, led to a striking “back-differentiation” of cells to an earlier stage in B cell development, characterized by the expression of pro-B cell genes. Cells undergoing this reversal in development also showed evidence of new LC gene rearrangements, suggesting an important role for basal Ig signaling in the maintenance of LC allelic exclusion. These studies identify a previously unappreciated level of plasticity in the B cell developmental program, and have important implications for our understanding of central tolerance mechanisms.

Gene rearrangement is a hallmark of B cell maturation. By interrupting basal cell signaling through the rearranged IgM receptor, immature B cells "back-differentiate" to an earlier stage in their development

Development of B cells producing natural autoantibodies to thymocytes and senescent erythrocytes

Natural antibodies produced by CD5+ B-1 B cells include those with specificity for senescent erythrocytes (anti-BrMRBC, anti-PtC) and for thymocytes (anti-thymocyte autoantibody, ATA). Here we describe work from our laboratories studying two prototypic examples, V(H)11Vkappa9-encoded anti-BrMRBC and V(H)3609Vkappa21c-encoded ATA. Using V(H)11-mu transgenic mice, we discovered that certain natural autoantibodies utilize V(H) genes that are selected against in bone marrow B cell development, but not fetal liver, effectively restricting their generation to fetal/neonatal life. Studies with ATA-mu transgenic mice demonstrated a critical requirement for self antigen in the accumulation of B cells with this specificity and for the production of high levels of serum ATA. Finally, analysis of B cell development in ATA-mu kappa transgenic mice revealed two distinct responses by B cells to expression of this B cell receptor (BCR): most developing B cells in spleen of adult mice were blocked at an immature stage and only escaped apoptosis by editing their BCR to eliminate the ATA specificity; nevertheless, high levels of serum ATA were observed, indicating that some B cells differentiated to antibody-forming cells without altering their specificity. Thus, our studies reveal mechanisms for restricting the generation of B cells producing natural autoantibodies, demonstrate a key positive selection step in their development, and show that most developing B cells in adult mice bearing such specificities fail to reach a mature stage.

Selection during development of VH11+ B cells: a model for natural autoantibody-producing CD5+ B cells

Natural autoantibodies constitute a large portion of serum immunoglobulin M (IgM) and bridge the adaptive and innate immune systems, serving as a rapid response to common pathogens. Many arise from a distinctive subset of B cells, termed B-1, that express CD5. Here, we describe our studies with a representative CD5+ B-cell-derived natural autoantibody, the VH11Vkappa9 B-cell receptor (BCR) that binds a determinant on senescent erythrocytes. This specificity represents 5-10% of the CD5+ B-cell subset, with a large portion accounted for by two novel BCRs, VH11Vkappa9 and VH12Vkappa4. We have found that the development of B-lineage cells with a VH11 rearrangement is surprisingly restricted at several crucial bottlenecks: (i). one of the most common VH11 rearrangements generates a heavy-chain protein that only inefficiently assembles a pre-BCR, key for recombinase-activating gene downregulation/allelic exclusion and pre-B-clonal expansion; (ii). cells containing VH11- micro chains lacking N-addition are favored for progression to the B-cell stage, eliminating most bone marrow VH11 rearrangements; and (iii). only a subset of Vkappa-light chains combine with VH11 heavy chain to foster progression to the mature B-cell stage. Together, these constrain VH11 generation to fetal development and may favor production of B cells with the prototype VH11Vkappa9 BCR.

Early B cell factor cooperates with Runx1 and mediates epigenetic changes associated with mb-1 transcription

Cd79a (called mb-1 here) encodes the Ig-alpha signaling component of the B cell receptor. The early B cell-specific mb-1 promoter was hypermethylated at CpG dinucleotides in hematopoietic stem cells but became progressively unmethylated as B cell development proceeded. The transcription factor Pax5 activated endogenous mb-1 transcription in a plasmacytoma cell line, but could not when the promoter was methylated. In this context, early B cell factor (EBF), a transcription factor required for B lymphopoiesis, potentiated activation of mb-1 by Pax5. EBF and the basic helix-loop-helix transcription factor E47 each contributed to epigenetic modifications of the mb-1 promoter, including CpG demethylation and nucleosomal remodeling. EBF function was enhanced by interaction with the transcription factor Runx1. These data suggest a molecular basis for the hierarchical dependence of Pax5 function on EBF and E2A in B lymphocyte development.

Characterization of B lymphopoiesis in mouse bone marrow and spleen

This chapter provides information on the application of flow cytometry for analysis of B-cell development, describing in detail the particular surface proteins that can serve as markers for recognizing distinct stages in this process. These cell fractions range from just prior to initial heavy chain rearrangement, the germline pro-B stage, through D-J rearranged pro-B and heavy chain expressing pre-B stages, to the maturing surface BCR positive B-cell stages. It also outlines assays for the characterization of these cells, including procedures for testing functional lineage restriction, determination of rearrangement status, analyses of gene expression at the ribonucleic acid (RNA) and protein level, and assessment of cell cycle state.

Isolation of Ly-1+/CD5+ B cells by cell sorting

Ly-1/CD5 is a 68-kDa glycoprotein that was originally thought to mark the helper subset of T cells. Later it was shown to be present on all T cells and, more recently, on a subset of B cells. Although its function remains the subject of speculation, CD5 expression serves as a useful marker for a functionally distinct population of B cells that has attracted a considerable amount of interest from investigators of both murine and human immune systems. Of critical importance in much of this work is the isolation of pure populations of CD5+ B cells along with appropriate control populations. A flow cytometry technique is presented in this unit, which results in the selection and isolation of two populations of cells from a complex mixture based on physical properties (e.g., size and internal granularity) and correlated expression of several surface molecules.

B-cell commitment: deciding on the players

Considerable progress has been made over the past few years in determining the key molecular players that regulate the commitment and development of early B-lineage cells in mouse bone marrow (transcription factors, cytokines and their receptors). However, confusion remains as to the precise stages, surface phenotypes and lineage restrictions that are observed early in B-cell development. Increasingly powerful flow cytometry analysis of genetically altered animals (knockouts and transgenics) combined with sensitive functional assays may provide an answer, a "scorecard" for the cell stages.

Positive selection of anti-thy-1 autoreactive B-1 cells and natural serum autoantibody production independent from bone marrow B cell development

A natural serum autoantibody specific for the Thy-1 glycoprotein (anti-Thy-1 autoantibody [ATA]) is produced by B-1 cells that are positively selected by self-antigen. Here, using ATA micro kappa transgenic mice we show that cells with this B cell receptor are negatively selected during bone marrow (BM) development. In a Thy-1 null environment, BM ATA B cells progress to a normal follicular stage in spleen. However, in a self-antigen-positive environment, development is arrested at an immature stage in the spleen, concomitant with induction of CD5. Such cells are tolerant and short-lived, different from B-1. Nonetheless, ATA-positive selection was evident by self-antigen-dependent high serum ATA production, comprising approximately 90% of serum immunoglobulin M in ATA micro kappa mice. Splenectomy did not eliminate ATA production and transfer of tolerant splenic B cells did not induce it. These findings demonstrate that B-1 positive selection, resulting in the production of natural serum ATA, arises independently from the major pathway of BM B cell development and selection.

Human chronic lymphocytic leukemia modeled in mouse by targeted TCL1 expression

The TCL1 gene at 14q32.1 is involved in chromosomal translocations and inversions in mature T cell leukemias. These leukemias are classified either as T prolymphocytic leukemias, which occur very late in life, or as T chronic lymphocytic leukemias, which often arise in patients with ataxia telangiectasia (AT) at a young age. In transgenic animals, the deregulated expression of TCL1 leads to mature T cell leukemia, demonstrating the role of TCL1 in the initiation of malignant transformation in T cell neoplasia. Expression of high levels of Tcl1 have also been found in a variety of human tumor-derived B cell lines ranging from pre-B cell to mature B cell. Here we describe the phenotype of transgenic mice, E mu-TCL1, established with TCL1 under the control of a V(H) promoter-Ig(H)-E mu enhancer to target TCL1 expression to immature and mature B cells. Flow cytometric analysis reveals a markedly expanded CD5(+) population in the peritoneal cavity of E mu-TCL1 mice starting at 2 mo of age that becomes evident in the spleen by 3-5 mo and in the bone marrow by 5-8 mo. Analysis of Ig gene rearrangements indicates monoclonality or oligoclonality in these populations, suggesting a preneoplastic expansion of CD5(+) B cell clones, with the elder mice eventually developing a chronic lymphocytic leukemia (CLL)-like disorder resembling human B-CLL. Our findings provide an animal model for CLL, the most common human leukemia, and demonstrate that deregulation of the Tcl1 pathway plays a crucial role in CLL pathogenesis.

Resolution of three nonproliferative immature splenic B cell subsets reveals multiple selection points during peripheral B cell maturation

Although immature/transitional peripheral B cells may remain susceptible to selection pressures before full maturation, the nature and timing of these selection events remain unclear. We show that correlated expression of surface (s) IgM (sIgM), CD23, and AA4 defines three nonproliferative subpopulations of immature/transitional peripheral B cells. We designate these populations transitional (T) 1 (AA4(+)CD23(-)sIgM(high)), T2 (AA4(+)CD23(+)sIgM(high)), and T3 (AA4(+)CD23(+)sIgM(low)). Cells within all three subsets are functionally immature as judged by their failure to proliferate following sIgM cross-linking in vitro, and their rapid rate of turnover in vivo as assessed by 5-bromo-2'-deoxyuridine labeling. These labeling studies also reveal measurable cell loss at both the T1-T2 and T2-T3 transitions, suggesting the existence of multiple selection points within the peripheral immature B cell pool. Furthermore, we find that Btk-deficient (xid) mice exhibit an incomplete developmental block at the T2-T3 transition within the immature B cell pool. This contrasts markedly with lyn(-/-) mice, which exhibit depressed numbers but normal ratios of each immature peripheral B cell subset and severely reduced numbers of mature B cells. Together, these data provide evidence for multiple selection points among immature peripheral B cells, suggesting that the B cell repertoire is shaped by multiple unique selection events that occur within the immature/transitional peripheral B cell pool.

TCR beta chain influences but does not solely control autoreactivity of V alpha 14J281T cells

CD1d-dependent accumulation of alphabeta T cells bearing a canonical Valpha14Jalpha281 alpha-chain (Valpha14+ T cells) is thought to model positive selection of lipid-specific T cells, based on their ability to recognize CD1d-presented self glycolipid(s). However, it has been difficult to demonstrate self ligand specificity in this system, as most Valpha14+ T cells do not exhibit significant autoreactivity despite high reactivity to alpha-galactosylceramide presented by CD1d (alpha-GalCer/CD1d). To assess the role of TCRbeta chain in determining the alpha-GalCer/CD1d vs autoreactive specificity of Valpha14+ T cells, we conducted TCRalpha or TCRbeta chain transduction experiments. In this study we demonstrate, by combining different TCRbeta chains with the Valpha14 alpha-chain in retrovirally transduced T cell lines, that the Valpha14 alpha-chain plays a primary role, necessary but not sufficient for imparting alpha-GalCer/CD1d recognition. beta-Chain usage alone is not the sole factor that controls the extent of autoreactivity in Valpha14+ T cells, since transduction of TCRalphabeta chains from a high CD1d autoreactive Valpha14+ T cell line conferred the alpha-GalCer/CD1d specificity without induction of autoreactivity. Thus, heterogeneity of Valpha14+ T cell reactivity is due to both beta-chain diversity and control mechanism(s) beyond primary TCR structure.

Probability binning comparison: a metric for quantitating multivariate distribution differences

BACKGROUND

While several algorithms for the comparison of univariate distributions arising from flow cytometric analyses have been developed and studied for many years, algorithms for comparing multivariate distributions remain elusive. Such algorithms could be useful for comparing differences between samples based on several independent measurements, rather than differences based on any single measurement. It is conceivable that distributions could be completely distinct in multivariate space, but unresolvable in any combination of univariate histograms. Multivariate comparisons could also be useful for providing feedback about instrument stability, when only subtle changes in measurements are occurring.

METHODS

We apply a variant of Probability Binning, described in the accompanying article, to multidimensional data. In this approach, hyper-rectangles of n dimensions (where n is the number of measurements being compared) comprise the bins used for the chi-squared statistic. These hyper-dimensional bins are constructed such that the control sample has the same number of events in each bin; the bins are then applied to the test samples for chi-squared calculations.

RESULTS

Using a Monte-Carlo simulation, we determined the distribution of chi-squared values obtained by comparing sets of events from the same distribution; this distribution of chi-squared values was identical as for the univariate algorithm. Hence, the same formulae can be used to construct a metric, analogous to a t-score, that estimates the probability with which distributions are distinct. As for univariate comparisons, this metric scales with the difference between two distributions, and can be used to rank samples according to similarity to a control. We apply the algorithm to multivariate immunophenotyping data, and demonstrate that it can be used to discriminate distinct samples and to rank samples according to a biologically-meaningful difference.

CONCLUSION

Probability binning, as shown here, provides a useful metric for determining the probability with which two or more multivariate distributions represent distinct sets of data. The metric can be used to identify the similarity or dissimilarity of samples. Finally, as demonstrated in the accompanying paper, the algorithm can be used to gate on events in one sample that are different from a control sample, even if those events cannot be distinguished on the basis of any combination of univariate or bivariate displays. Published 2001 Wiley-Liss, Inc.

Frequency difference gating: a multivariate method for identifying subsets that differ between samples

BACKGROUND

In multivariate distributions (for example, in 3- or more color flow cytometric datasets), it can become difficult or impossible to identify populations that differ between samples based only on a combination of univariate or bivariate displays. Indeed, it is possible that such differences can only be identified in "n"-dimensional space, where "n" is the number of parameters measured. Therefore, computer assisted identification of such differences is necessary. Such a method could be used to identify responses (i.e., by comparing cell samples before and after stimulation) in exquisite detail by allowing complete analysis of the collected data on only those events which have responded.

METHODS

Multivariate Probability Binning can be used to compare different datasets to identify the distance and statistical significance of a difference between the distributions. An intermediate step in the algorithm provides access to the actual locations within the n-dimensional comparison which are most different between the distributions. Gates based on collections of hyper-rectangular bins can then be applied to datasets, thereby selecting those events (or clusters of events) that are different between samples. We term this process Frequency Difference Gating.

RESULTS

Frequency Difference Gating was used in several test scenarios to evaluate its utility. First, we compared PBMC subsets identified by solely by immunofluorescence staining: based on this training data set, the algorithm automatically generated an accurate forward and side-scatter gate to identify lymphocytes. Second, we applied the algorithm to identify subtle differences between CD4 memory subsets based on 8-color immunophenotyping data. The resulting 3-dimensional gate could resolve cells subsets much more frequent in one subset compared to the other; no combination of two-dimensional gates could accomplish this resolution. Finally, we used the algorithm to compare B cell populations derived from mice of different ages or strains, and found that the algorithm could find very subtle differences between the populations.

CONCLUSION

Frequency Difference Gating is a powerful tool that automates the process of identifying events comprising underlying differences between samples. It is not a clustering tool; it is not meant to identify subsets in multidimensional space. Importantly, this method may reveal subtle changes in small populations of cells, changes that only occur simultaneously in multiple dimensions in such a way that identification by univariate or bivariate analyses is impossible. Finally, the method may significantly aid in the analysis of high-order multivariate data (i.e., 6-12 color flow cytometric analyses), where identification of differences between datasets becomes so time-consuming as to be impractical. Published 2001 Wiley-Liss, Inc.

B cell development pathways

B cell development is a highly regulated process whereby functional peripheral subsets are produced from hematopoietic stem cells, in the fetal liver before birth and in the bone marrow afterward. Here we review progress in understanding some aspects of this process in the mouse bone marrow, focusing on delineation of the earliest stages of commitment, on pre-B cell receptor selection, and B cell tolerance during the immature-to-mature B cell transition. Then we note some of the distinctions in hematopoiesis and pre-B selection between fetal liver and adult bone marrow, drawing a connection from fetal development to B-1/CD5(+) B cells. Finally, focusing on CD5(+) cells, we consider the forces that influence the generation and maintenance of this distinctive peripheral B cell population, enriched for natural autoreactive specificities that are encoded by particular germline V(H)-V(L) combinations.

Response by B cell precursors to pre-B receptor assembly: differences between fetal liver and bone marrow

The expression of different sets of immunoglobulin specificities by fetal and adult B lymphocytes is a longstanding puzzle in immunology. In the past few years it has become clear that production of mu heavy chain and subsequent assembly with surrogate light chain to form the pre-B cell receptor complex is critical to promote development of adult B cell precursors in mouse bone marrow. Recently we found that instead of promoting pre-B cell expansion as in adult bone marrow, this complex inhibits pre-B cell growth in fetal liver, providing a previously unrecognized mechanism for alteration of the B cell repertoire with age. The consequence is very distinct primary repertoires for development of fetal B1 cells and adult bone marrow B2 cells.