Crucial Role of Increased Arid3a at the Pre-B and Immature B Cell Stages for B1a Cell Generation

The Lin28b⁺Let7⁻ axis in fetal/neonatal development plays a role in promoting CD5⁺ B1a cell generation as a B-1 B cell developmental outcome. Here we identify the Let7 target, Arid3a, as a crucial molecular effector of the B-1 cell developmental program. Arid3a expression is increased at pro-B cell stage and markedly increased at pre-B and immature B cell stages in the fetal/neonatal liver B-1 development relative to that in the Lin28b⁻Let7⁺ adult bone marrow (BM) B-2 cell development. Analysis of B-lineage restricted Lin28b transgenic (Tg) mice, Arid3a knockout and Arid3a Tg mice, confirmed that increased Arid3a allows B cell generation without requiring surrogate light chain (SLC) associated pre-BCR stage, and prevents MHC class II cell expression at the pre-B and newly generated immature B cell stages, distinct from pre-BCR dependent B development with MHC class II in adult BM. Moreover, Arid3a plays a crucial role in supporting B1a cell generation. The increased Arid3a leads higher Myc and Bhlhe41, and lower Siglec-G and CD72 at the pre-B and immature B cell stages than normal adult BM, to allow BCR signaling induced B1a cell generation. Arid3a-deficiency selectively blocks the development of B1a cells, while having no detectable effect on CD5⁻ B1b, MZ B, and FO B cell generation resembling B-2 development outcome. Conversely, enforced expression of Arid3a by transgene is sufficient to promote the development of B1a cells from adult BM. Under the environment change between birth to adult, altered BCR repertoire in increased B1a cells occurred generated from adult BM. However, crossed with B1a-restricted V_H/D/J IgH knock-in mice allowed to confirm that SLC-unassociated B1a cell increase and CLL/lymphoma generation can occur in aged from Arid3a increased adult BM. These results confirmed that in fetal/neonatal normal mice, increased Arid3a at the pre-B cell and immature B cell stages is crucial for generating B1a cells together with the environment for self-ligand reactive BCR selection, B1a cell maintenance, and potential for development of CLL/Lymphoma in aged mice.

Early Generated B-1-Derived B Cells Have the Capacity To Progress To Become Mantle Cell Lymphoma-like Neoplasia in Aged Mice

In mice, fetal/neonatal B-1 cell development generates murine CD5⁺ B cells (B1a) with autoreactivity. We analyzed B1a cells at the neonatal stage in a V_H11/D/J_H knock-in mouse line (V_H11t) that generates an autoreactive antiphosphatidylcholine BCR. Our study revealed that antiphosphatidylcholine B1a cells develop in liver, mature in spleen, and distribute in intestine/colon, mesenteric lymph node (mLN), and body cavity as the outcome of B-1 cell development before B-2 cell development. Throughout life, self-renewing B-1 B1a cells circulate through intestine, mesenteric vessel, and blood. The body cavity-deposited B1a cells also remigrate. In old age, some B1a cells proceed to monoclonal B cell lymphocytosis. When neonatal B-1 B1a cells express an antithymocyte/Thy-1 autoreactivity (ATA) BCR transgene in the C.B17 mouse background, ATA B cells increase in PBL and strongly develop lymphomas in aging mice that feature splenomegaly and mLN hyperplasia with heightened expression of CD11b, IL-10, and activated Stat3. At the adult stage, ATA B cells were normally present in the mantle zone area, including in intestine. Furthermore, frequent association with mLN hyperplasia suggests the influence by intestinal microenvironment on lymphoma development. When cyclin D1 was overexpressed by the Eμ-cyclin D1 transgene, ATA B cells progressed to further diffused lymphoma in aged mice, including in various lymph nodes with accumulation of IgM^hiIgD^loCD5⁺CD23^-CD43⁺ cells, resembling aggressive human mantle cell lymphoma. Thus, our findings reveal that early generated B cells, as an outcome of B-1 cell development, can progress to become lymphocytosis, lymphoma, and mantle cell lymphoma-like neoplasia in aged mice.

NLR Nod1 signaling promotes survival of BCR-engaged mature B cells through up-regulated Nod1 as a positive outcome

The microenvironment, including microbial products, plays a role in mature B cell survival. Hayakawa et al. show that B cell antigen receptor ligand–mediated Nod1 up-regulation in vivo in B cell development leads to preferential mature B cell survival as a competitive survival, increasing the Nod1⁺ B cell pool with age.

Although B cell development requires expression of the B cell antigen receptor (BCR), it remains unclear whether engagement of self-antigen provides a positive impact for most B cells. Here, we show that BCR engagement by self-ligand during development in vivo results in up-regulation of the Nod-like receptor member Nod1, which recognizes the products of intestinal commensal bacteria. In anti-thymocyte/Thy-1 autoreactive BCR knock-in mice lacking self–Thy-1 ligand, immunoglobulin light chain editing occurred, generating B cells with up-regulated Nod1, including follicular and marginal zone B cells with natural autoreactivity. This BCR editing with increased Nod1 resulted in preferential survival. In normal adult mice, most mature B cells are enriched for Nod1 up-regulated cells, and signaling through Nod1 promotes competitive survival of mature B cells. These findings demonstrate a role for microbial products in promoting survival of mature B cells through up-regulated Nod1, providing a positive effect of BCR engagement on development of most B cells.

Zebrafish B Cell Development without a Pre-B Cell Stage, Revealed by CD79 Fluorescence Reporter Transgenes

CD79a and CD79b proteins associate with Ig receptors as integral signaling components of the B cell Ag receptor complex. To study B cell development in zebrafish, we isolated orthologs of these genes and performed in situ hybridization, finding that their expression colocalized with IgH-μ in the kidney, which is the site of B cell development. CD79 transgenic lines were made by linking the promoter and upstream regulatory segments of CD79a and CD79b to enhanced GFP to identify B cells, as demonstrated by PCR analysis of IgH-μ expression in sorted cells. We crossed these CD79-GFP lines to a recombination activating gene (Rag)2:mCherry transgenic line to identify B cell development stages in kidney marrow. Initiation of CD79:GFP expression in Rag2:mCherry⁺ cells and the timing of Ig H and L chain expression revealed simultaneous expression of both IgH-μ- and IgL-κ-chains, without progressing through the stage of IgH-μ-chain alone. Rag2:mCherry⁺ cells without CD79:GFP showed the highest Rag1 and Rag2 mRNAs compared with CD79a and CD79b:GFP⁺ B cells, which showed strongly reduced Rag mRNAs. Thus, B cell development in zebrafish does not go through a Rag^hi CD79⁺IgH-μ⁺ pre-B cell stage, different from mammals. After the generation of CD79:GFP⁺ B cells, decreased CD79 expression occurred upon differentiation to Ig secretion, as detected by alteration from membrane to secreted IgH-μ exon usage, similar to in mammals. This confirmed a conserved role for CD79 in B cell development and differentiation, without the requirement of a pre-B cell stage in zebrafish.

Early generated B1 B cells with restricted BCRs become chronic lymphocytic leukemia with continued c-Myc and low Bmf expression

Hayakawa et al. show that distinctive B-lineage progression from B-1 development allows for generation of B1a cells with restricted BCRs and self-renewal capacity, both contributing to potential for CLL progression.

In mice, generation of autoreactive CD5⁺ B cells occurs as a consequence of BCR signaling induced by (self)-ligand exposure from fetal/neonatal B-1 B cell development. A fraction of these cells self-renew and persist as a minor B1 B cell subset throughout life. Here, we show that transfer of early generated B1 B cells from Eμ-TCL1 transgenic mice resulted in chronic lymphocytic leukemia (CLL) with a biased repertoire, including stereotyped BCRs. Thus, B1 B cells bearing restricted BCRs can become CLL during aging. Increased anti-thymocyte/Thy-1 autoreactive (ATA) BCR cells in the B1 B cell subset by transgenic expression yielded spontaneous ATA B-CLL/lymphoma incidence, enhanced by TCL1 transgenesis. In contrast, ATA B-CLL did not develop from other B cell subsets, even when the identical ATA BCR was expressed on a Thy-1 low/null background. Thus, both a specific BCR and B1 B cell context were important for CLL progression. Neonatal B1 B cells and their CLL progeny in aged mice continued to express moderately up-regulated c-Myc and down-regulated proapoptotic Bmf, unlike most mature B cells in the adult. Thus, there is a genetic predisposition inherent in B-1 development generating restricted BCRs and self-renewal capacity, with both features contributing to potential for progression to CLL.

Perspectives on fetal derived CD5+ B1 B cells

CD5(+) B-cell origins and their predisposition to lymphoma are long-standing issues. Transfer of fetal and adult liver BM Pro-B cells generates B cells with distinct phenotypes: fetal cells generate IgM(high) IgD(low) CD5(+) , whereas adult cells IgM(low) IgD(high) CD5(-) . This suggests a developmental switch in B lymphopoiesis, similar to the switch in erythropoiesis. Comparison of mRNA and miRNA expression in fetal and adult Pro-B cells revealed differential expression of Lin28b mRNA and Let-7 miRNA, providing evidence that this regulatory axis functions in the switch. Recent work has shown that Arid3a is a key transcription factor mediating fetal-type B-cell development. Lin28b-promoted fetal development generates CD5(+) B cells as a consequence of positively selected self-reactivity. CD5(+) B cells play important roles in clearance of apoptotic cells and in protective immune responses, but also pose a risk of progression to leukemia/lymphoma. Differential Lin28b expression in fetal and adult human B-cell precursors showed that human B-cell development may resemble mouse, with self-reactive "innate-like" B cells generated early in life. It remains to be determined whether such human B cells have a higher propensity to leukemic progression. This review describes our recent research with CD5(+) B cells and presents our perspective on their role in disease.

Mutagenesis Screen Identifies agtpbp1 and eps15L1 as Essential for T lymphocyte Development in Zebrafish

Genetic screens are a powerful tool to discover genes that are important in immune cell development and function. The evolutionarily conserved development of lymphoid cells paired with the genetic tractability of zebrafish make this a powerful model system for this purpose. We used a Tol2-based gene-breaking transposon to induce mutations in the zebrafish (Danio rerio, AB strain) genome, which served the dual purpose of fluorescently tagging cells and tissues that express the disrupted gene and provided a means of identifying the disrupted gene. We identified 12 lines in which hematopoietic tissues expressed green fluorescent protein (GFP) during embryonic development, as detected by microscopy. Subsequent analysis of young adult fish, using a novel approach in which single cell suspensions of whole fish were analyzed by flow cytometry, revealed that 8 of these lines also exhibited GFP expression in young adult cells. An additional 15 lines that did not have embryonic GFP+ hematopoietic tissue by microscopy, nevertheless exhibited GFP+ cells in young adults. RT-PCR analysis of purified GFP+ populations for expression of T and B cell-specific markers identified 18 lines in which T and/or B cells were fluorescently tagged at 6 weeks of age. As transposon insertion is expected to cause gene disruption, these lines can be used to assess the requirement for the disrupted genes in immune cell development. Focusing on the lines with embryonic GFP+ hematopoietic tissue, we identified three lines in which homozygous mutants exhibited impaired T cell development at 6 days of age. In two of the lines we identified the disrupted genes, agtpbp1 and eps15L1. Morpholino-mediated knockdown of these genes mimicked the T cell defects in the corresponding mutant embryos, demonstrating the previously unrecognized, essential roles of agtpbp1 and eps15L1 in T cell development.

Selection of natural autoreactive B cells

Natural antibodies produced by CD5+ B1 B cells include anti-thymocyte autoantibody (ATA). Transgenic mice bearing the Ig-μ heavy chain of a prototypic ATA, V(H)3609Vκ21c, demonstrated a critical requirement for self-antigen in the accumulation of ATA B cells and production of high levels of serum ATA. Further work with ATA-μκ transgenic mice revealed that, while development of most B cells were blocked at an immature stage in spleen, some mature ATA B cells were present. ATA-μκ transgenic mice with varying levels of Thy-1 autoantigen showed a clear relationship between BCR crosslinking and B cell fate, with low levels generating marginal zone ATA B cells and complete antigen absence allowing maturation to follicular ATA B cells. Finally, different fates of developing ATA B cells encountering high levels self-antigen may be accounted for by variations in the response of newly formed B cells arising from foetal and adult development.

B cells generated by B-1 development can progress to chronic lymphocytic leukemia

B cells generated early during fetal/neonatal B-1 development in mice include autoreactive cells with detectable CD5 upregulation induced by B cell receptor (BCR) signaling (B1a cells). A fraction of B1a cells are maintained by self-renewal for life, with the potential risk of dysregulated growth and progression to chronic lymphocytic leukemia (CLL)/lymphoma during aging. In studies using the Eμ-hTCL1 transgenic mouse system, it became clear that this B1a subset has a higher potential than other B cell subsets for progression to CLL. We have generated several autoreactive germline BCR gene models to compare B cells generated under conditions of natural exposure to autoantigen. Analysis of the mice has been key in understanding the importance of the BCR and BCR signaling for generating different B cell subsets and for investigating the cellular origin of B-CLL.

Lin28b promotes fetal B lymphopoiesis through the transcription factor Arid3a

Zhou et al. demonstrate a requirement for the Let-7–Lin28b axis regulating a shift in development between fetal liver and bone marrow B lymphocyte progenitors in the generation of B1 versus B2 B cells. Specifically, the transcription factor Arid3a, induced by Lin28b and a target of Let-7 miRNA, is sufficient to recapitulate fetal B cell development from bone marrow progenitors.

Mouse B cell precursors from fetal liver and adult bone marrow (BM) generate distinctive B cell progeny when transplanted into immunodeficient recipients, supporting a two-pathway model for B lymphopoiesis, fetal “B-1” and adult “B-2.” Recently, Lin28b was shown to be important for the switch between fetal and adult pathways; however, neither the mechanism of Lin28b action nor the importance of B cell antigen receptor (BCR) signaling in this process was addressed. Here, we report key advances in our understanding of the regulation of B-1/B-2 development. First, modulation of Let-7 in fetal pro-B cells is sufficient to alter fetal B-1 development to produce B cells resembling the progeny of adult B-2 development. Second, intact BCR signaling is required for the generation of B1a B cells from Lin28b-transduced BM progenitors, supporting a requirement for ligand-dependent selection, as is the case for normal B1a B cells. Third, the V_H repertoire of Lin28b-induced BM B1a B cells differs from that of normal B1a, suggesting persisting differences from fetal progenitors. Finally, we identify the Arid3a transcription factor as a key target of Let-7, whose ectopic expression is sufficient to induce B-1 development in adult pro-B cells and whose silencing by knockdown blocks B-1 development in fetal pro-B cells.

Natural anti-intestinal goblet cell autoantibody production from marginal zone B cells

Expression of a germline VH3609/D/JH2 IgH in mice results in the generation of B1 B cells with anti-thymocyte/Thy-1 glycoprotein autoreactivity by coexpression of Vk21-5/Jk2 L chain leading to production of serum IgM natural autoantibody. In these same mice, the marginal zone (MZ) B cell subset in spleen shows biased usage of a set of Ig L chains different from B1 B cells, with 30% having an identical Vk19-17/Jk1 L chain rearrangement. This VH3609/Vk19-17 IgM is reactive with intestinal goblet cell granules, binding to the intact large polymatrix form of mucin 2 glycoprotein secreted by goblet cells. Analysis of a μκ B cell AgR (BCR) transgenic (Tg) mouse with this anti-goblet cell/mucin2 autoreactive (AGcA) specificity demonstrates that immature B cells expressing the Tg BCR become MZ B cells in spleen by T cell-independent BCR signaling. These Tg B cells produce AGcA as the predominant serum IgM, but without enteropathy. Without the transgene, AGcA autoreactivity is low but detectable in the serum of BALB/c and C.B17 mice, and this autoantibody is specifically produced by the MZ B cell subset. Thus, our findings reveal that AGcA is a natural autoantibody associated with MZ B cells.

The transcriptional landscape of αβ T cell differentiation

αβT cell differentiation from thymic precursors is a complex process, explored here with the breadth of ImmGen expression datasets, analyzing how differentiation of thymic precursors gives rise to transcriptomes. After surprisingly gradual changes though early T commitment, transit through the CD4⁺CD8⁺ stage involves a shutdown or rare breadth, and correlating tightly with MYC. MHC-driven selection promotes a large-scale transcriptional reactivation. We identify distinct signatures that mark cells destined for positive selection versus apoptotic deletion. Differential expression of surprisingly few genes accompany CD4 or CD8 commitment, a similarity that carries through to peripheral T cells and their activation, revealed by mass cytometry phosphoproteomics. The novel transcripts identified as candidate mediators of key transitions help define the “known unknown” of thymocyte differentiation.

Positive and negative selection of natural autoreactive B cells

Naturally occurring antibodies (NAbs) produced by CD5(+) B-1 B cells include those with specificity for thymocytes (anti-thymocyte autoantibody, ATA). Here we describe a prototypic example, encoded by an unmutated immunoglobulin μ/κ heavy chain/light chain. Studies with ATA-μ ("heavy chain only") 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 NAb. Furthermore, analysis of B-cell development in ATA-μκ ("heavy and light chain") transgenic mice revealed two distinct responses by B cells to expression of this B-cell receptor (BCR). (1) Most B cells developing from bone marrow of adult mice were blocked at an immature stage in spleen and only escaped apoptosis by editing their BCR to eliminate the ATA specificity. (2) Some B cells differentiated to antibody-forming cells without altering their specificity, produced high levels of serum ATA, and many ATA-secreting plasma cells were observed in spleen. Finally, examination of B-cell development and ATA NAb production in ATA-μκ transgenic mice with levels of Thy-1 autoantigen varying from very low to above physiologic reveals a clear relationship between BCR crosslinking by antigen and B-cell fate. Low levels of Thy-1 autoantigen resulted in diversion of ATA B cells into the marginal zone B-cell compartment, presumably because of reduced BCR signaling. Thus, our studies demonstrate a key positive selection step in the development of NAb-producing B cells and show that most of these cells in adult mice bearing such specificities fail to reach a mature stage. Importantly, because these specificities are isolated from B-1 B cells and, when expressed as transgenes, guide development into the B-1 or marginal zone B-cell pool, we identify these B cells as a major source of natural autoantibodies in mice.

Transcriptomes of the B and T lineages compared by multiplatform microarray profiling

T and B lymphocytes are developmentally and functionally related cells of the immune system, representing the two major branches of adaptive immunity. Although originating from a common precursor, they play very different roles: T cells contribute to and drive cell-mediated immunity, whereas B cells secrete Abs. Because of their functional importance and well-characterized differentiation pathways, T and B lymphocytes are ideal cell types with which to understand how functional differences are encoded at the transcriptional level. Although there has been a great deal of interest in defining regulatory factors that distinguish T and B cells, a truly genomewide view of the transcriptional differences between these two cells types has not yet been taken. To obtain a more global perspective of the transcriptional differences underlying T and B cells, we exploited the statistical power of combinatorial profiling on different microarray platforms, and the breadth of the Immunological Genome Project gene expression database, to generate robust differential signatures. We find that differential expression in T and B cells is pervasive, with the majority of transcripts showing statistically significant differences. These distinguishing characteristics are acquired gradually, through all stages of B and T differentiation. In contrast, very few T versus B signature genes are uniquely expressed in these lineages, but are shared throughout immune cells.

NEDD9 promotes oncogenic signaling in mammary tumor development

In the past 3 years, altered expression of the HEF1/CAS-L/NEDD9 scaffolding protein has emerged as contributing to cancer metastasis in multiple cancer types. However, whereas some studies have identified elevated NEDD9 expression as prometastatic, other work has suggested a negative role in tumor progression. We here show that the Nedd9-null genetic background significantly limits mammary tumor initiation in the MMTV-polyoma virus middle T genetic model. Action of NEDD9 is tumor cell intrinsic, with immune cell infiltration, stroma, and angiogenesis unaffected. The majority of the late-appearing mammary tumors of MMTV-polyoma virus middle T;Nedd9(-/-) mice are characterized by depressed activation of proteins including AKT, Src, FAK, and extracellular signal-regulated kinase, emphasizing an important role of NEDD9 as a scaffolding protein for these prooncogenic proteins. Analysis of cells derived from primary Nedd9(+/+) and Nedd9(-/-) tumors showed persistently reduced FAK activation, attachment, and migration, consistent with a role for NEDD9 activation of FAK in promoting tumor aggressiveness. This study provides the first in vivo evidence of a role for NEDD9 in breast cancer progression and suggests that NEDD9 expression may provide a biomarker for tumor aggressiveness.

Mice heterozygous for germ-line mutations in methylthioadenosine phosphorylase (MTAP) die prematurely of T-cell lymphoma

Large homozygous deletions of 9p21 that inactivate CDKN2A, ARF, and MTAP are common in a wide variety of human cancers. The role for CDKN2A and ARF in tumorigenesis is well established, but whether MTAP loss directly affects tumorigenesis is unclear. MTAP encodes the enzyme methylthioadenosine phosphorylase, a key enzyme in the methionine salvage pathway. To determine if loss of MTAP plays a functional role in tumorigenesis, we have created an MTAP-knockout mouse. Mice homozygous for a MTAP null allele (Mtap(lacZ)) have an embryonic lethal phenotype dying around day 8 postconception. Mtap/Mtap(lacZ) heterozygotes are born at Mendelian frequencies and appear indistinguishable from wild-type mice during the first year of life, but they tend to die prematurely with a median survival of 585 days. Autopsies on these animals reveal that they have greatly enlarged spleens, altered thymic histology, and lymphocytic infiltration of their livers, consistent with lymphoma. Immunohistochemical staining and fluorescence-activated cell sorting analysis indicate that these lymphomas are primarily T-cell in origin. Lymphoma-infiltrated tissues tend to have reduced levels of Mtap mRNA and MTAP protein in addition to unaltered levels of methyldeoxycytidine. These studies show that Mtap is a tumor suppressor gene independent of CDKN2A and ARF.

A role for cFLIP in B cell proliferation and stress MAPK regulation

Fas/Apo-1 signals through the FADD (Fas-associated death domain) adaptor protein, which recruits and activates the apical caspase 8 and leads to apoptosis. Cellular FLIP (cFLIP) is a homolog of caspase 8 and is also capable of binding to FADD. Previous studies suggest that cFLIP could either enhance or inhibit apoptosis and lead to NF-kappaB and Erk1/2 activation. Like FADD or caspase 8 deficiency, a lack of cFLIP disrupts embryogenesis and T cell proliferation. It has been demonstrated that B cells lacking either FADD or caspase 8 were defective in both Fas-induced apoptosis and TLR-induced proliferation, which indicates that these death-inducing proteins have an additional role in regulating innate immunity. To analyze the function of cFLIP in B cells, conditional deletion of cFLIP was induced by using CD19(Cre). The resulting B cell-specific cFLIP-deficient mice were found to have reduced numbers of peripheral B cells that were hypersensitive to Fas-induced apoptosis and impaired in proliferation induced by TLRs and the BCR. Furthermore, there was aberrant expression of costimulatory proteins and activation markers in cFLIP-deficient B cells. Whereas LPS-induced activation of NF-kappaB and Erk1/2 appears to be unaffected, p38 and Jnk were spontaneously activated and hyperinduced in cFLIP-deficient B cells. Therefore, these data revealed novel functions of cFLIP in B cells.

B cell receptor basal signaling regulates antigen-induced Ig light chain rearrangements

BCR editing in the bone marrow contributes to B cell tolerance by orchestrating secondary Ig rearrangements in self-reactive B cells. We have recently shown that loss of the BCR or a pharmacologic blockade of BCR proximal signaling pathways results in a global "back-differentiation" response in which immature B cells down-regulate genes important for the mature B cell program and up-regulate genes characteristic of earlier stages of B cell development. These observations led us to test the hypothesis that self-Ag-induced down-regulation of the BCR, and not self-Ag-induced positive signals, lead to Rag induction and hence receptor editing. Supporting this hypothesis, we found that immature B cells from xid (x-linked immunodeficiency) mice induce re-expression of a Rag2-GFP bacterial artificial chromosome reporter as well as wild-type immature B cells following Ag incubation. Incubation of immature B cells with self-Ag leads to a striking reversal in differentiation to the pro-/pre-B stage of development, consistent with the idea that back-differentiation results in the reinduction of genes required for L chain rearrangement and receptor editing. Importantly, Rag induction, the back-differentiation response to Ag, and editing in immature and pre-B cells are inhibited by a combination of phorbol ester and calcium ionophore, agents that bypass proximal signaling pathways and mimic BCR signaling. Thus, mimicking positive BCR signals actually inhibits receptor editing. These findings support a model whereby Ag-induced receptor editing is inhibited by BCR basal signaling on developing B cells; BCR down-regulation removes this basal signal, thereby initiating receptor editing.

Autoreactive B-1 B cells: constraints on natural autoantibody B cell antigen receptors

B-1 B-cells constitute a distinctive population of cells that are enriched for self-reactive B cell receptors (BCRs). These BCRs are encoded by a restricted set of heavy and light chains, including heavy chains that lack nontemplated nucleotide additions at the V-D and D-J joining regions. One prototype natural autoantibody produced by B-1 B cells binds to a cryptic determinant exposed on senescent red blood cells that includes the phosphatidylcholine (PtC) moiety. The V(H)11Vkappa9 BCR, which accounts for a large fraction of the anti-PtC specificity, is underrepresented in other B-cell populations, including newly formed B cells in bone marrow, and the transitional B cells, follicular B cells, and marginal zone B cells in spleen. Previous work has shown that V(H)11 heavy chains pair ineffectively with surrogate light chain (SLC) and so do not promote development in bone marrow, but instead allow fetal liver maturation because of a fetal preference for weaker pre-BCR signaling. Such inefficient SLC pairing constitutes one constraint on the maturation of B cells containing V(H)11 rearrangements that biases their generation to fetal development. Here, we examine another possible bottleneck to the B1 cell repertoire: light chain pairing with V(H)11 heavy chain, finding very significant preferences.

Ablation of ribosomal protein L22 selectively impairs alphabeta T cell development by activation of a p53-dependent checkpoint

The alphabeta and gammadelta T lineages are thought to arise from a common precursor; however, the regulation of separation and development of these lineages is not fully understood. We report here that development of alphabeta and gammadelta precursors was differentially affected by elimination of ribosomal protein L22 (Rpl22), which is ubiquitously expressed but not essential for translation. Rpl22 deficiency selectively arrested development of alphabeta-lineage T cells at the beta-selection checkpoint by inducing their death. The death was caused by induction of p53 expression, because p53 deficiency blocked death and restored development of Rpl22-deficient thymocytes. Importantly, Rpl22 deficiency led to selective upregulation of p53 in alphabeta-lineage thymocytes, at least in part by increasing p53 synthesis. Taken together, these data indicate that Rpl22 deficiency activated a p53-dependent checkpoint that produced a remarkably selective block in alphabeta T cell development but spared gammadelta-lineage cells, suggesting that some ribosomal proteins may perform cell-type-specific or stage-specific functions.

The Protean Nature of Cells in the B Lymphocyte Lineage

The subdivision of bone marrow (BM) with surface markers and reporter systems and the use of multiple culture and transplantation assays to assess differentiation potential have led to extraordinary progress in defining stages of B lymphopoiesis between the hematopoietic stem cell and B cell receptor (BCR)-expressing lymphocytes. Despite the lack of standard nomenclature and a series of technical issues that still need to be resolved, there seems to be a general consensus regarding the major route to becoming a B cell. Nevertheless, evidence that additional, minor pathways through which B lineage cells are generated exists, and a new appreciation that lymphoid progenitors are protean and able to alter their differentiation potential during embryogenesis and after birth in response to infections suggests that a full understanding of B cell development and how it is regulated has not yet been attained.

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.

Development and function of B-1 cells

Results from immunoglobulin-transgenic mice and BCR-mutant mice have been widely interpreted in recent years as supporting a simple 'activation' model for the origin of CD5+/B-1 B cells. However cell transfer experiments over 10 years ago and recent work investigating pre-BCR signaling suggest striking differences between B cell development in fetal liver and adult bone marrow, lending support for a 'lineage' model that we favor. Recent progress has been made relating to the development and function of the CD5+/B-1 B cell subpopulation in mice; the data can be viewed in the context of the generation of this subpopulation by a distinctive fetal B cell developmental process.