B-1 derived anti-Thy-1 B cells in old aged mice develop lymphoma/leukemia with high expression of CD11b and Hamp2 that different from TCL1 transgenic mice

Human old aged unmutated chronic lymphocytic leukemia U-CLL are the TCL1+ZAP70+CD5+ B cells. Since CD5 makes the BCR signaling tolerance, ZAP70 increased in U-CLL not only TCL1+ alone. In mice, TCL1 (TCL1A) is the negative from neonate to old aged, as TC-. VH8-12/Vk21-5 is the anti-thymocyte/Thy-1 autoreactive ATA B cell. When ATA μκTg generation in mice, ATA B cells are the neonate generated CD5+ B cells in B-1, and in the middle age, CD5+ can be down or continuously CD5+, then, old aged CLL/lymphoma generation with increased CD11b in TC-ZAP70-CD5- or TC-ZAP70+CD5+. In this old aged TC-ATA B microarray analysis showed most similar to human CLL and U-CLL, and TC-ZAP70+CD5+ showed certain higher present as U-CLL. Original neonate ATA B cells showed with several genes down or further increase in old aged tumor, and old aged T-bet+CD11c+, CTNNB1hi, HMGBhi, CXCR4hi, DPP4hi and decreased miR181b. These old aged increased genes and down miR181b are similar to human CLL. Also, in old age ATA B cell tumor, high CD38++CD44++, increased Ki67+ AID+, and decreased CD180- miR15Olow are similar to U-CLL. In this old aged ATA B, increased TLR7,9 and Wnt10b. TC+Tg generated with ATAμκTg mice occurred middle age tumor as TC+ZAP70-CD5+ or TC+ZAP70+CD5+, with high NF-kB1, TLR4,6 and Wnt5b,6 without increased CD11b. Since neonatal state to age with TC+Tg continuously, middle age CLL/lymphoma generation is not similar to old aged generated, however, some increased in TC+ZAP70+ are similar to the old age TC- ATA B tumor. Then, TC- ATA B old age tumor showed some difference to human CLL. ATA B cells showed CD11b+CD22++, CD24 down, and hepcidin Hamp2++ with iron down. This mouse V8-12 similar to human V2-5, and V2-5 showed several cancers with macrophages/neutrophils generated hepcidin+ ironlow or some showed hepcidin- iron+ with tumor, and mouse V8-12 with different Vk19-17 generate MZ B cells strongly increased macrophage++ in old aged and generated intestine/colon tumor. Conclusion, neonate generated TC-ATA B1 cells in old aged tumor generation are CD11b+ in the leukemia CLL together with lymphoma cancer with hepcidin-related Hamp2++ in B-1 cell generation to control iron.

Pla2g2a promotes innate Th2-type immunity lymphocytes to increase B1a cells

Newborns require early generation of effective innate immunity as a primary physiological mechanism for survival. The neonatal Lin28⁺Let7^– developmental pathway allows increased generation of Th2-type cells and B1a (B-1 B) cells compared to adult cells and long-term maintenance of these initially generated innate cells. For initial B1a cell growth from the neonatal to adult stage, Th2-type IL-5 production from ILC2s and NKT2 cells is important to increase B1a cells. The Th17 increase is dependent on extracellular bacteria, and increased bacteria leads to lower Th2-type generation. Secreted group IIA-phospholipase A2 (sPLA2-IIA) from the Pla2g2a gene can bind to gram-positive bacteria and degrade bacterial membranes, controlling microbiota in the intestine. BALB/c mice are Pla2g2a⁺, and express high numbers of Th2-type cells and B1a cells. C57BL/6 mice are Pla2g2a-deficient and distinct from the SLAM family, and exhibit fewer NKT2 cells and fewer B1a cells from the neonatal to adult stage. We found that loss of Pla2g2a in the BALB/c background decreased IL-5 from Th2-type ILC2s and NKT2s but increased bacterial-reactive NKT17 cells and MAIT cells, and decreased the number of early-generated B1a cells and MZ B cells and the CD4/CD8 T cell ratio. Low IL-5 by decreased Th2-type cells in Pla2g2a loss led to low early-generated B1a cell growth from the neonatal to adult stage. In anti-thymocyte/Thy-1 autoreactive μκ transgenic (ATAμκ Tg) Pla2g2a⁺ BALB/c background C.B17 mice generated NKT2 cells that continuously control CD1d⁺ B1 B cells through old aging and lost CD1d in B1 B cells generating strong B1 ATA B cell leukemia/lymphoma. Pla2g2a-deficient ATAμκTg C57BL/6 mice suppressed the initial B1a cell increase, with low/negative spontaneous leukemia/lymphoma generation. These data confirmed that the presence of Pla2g2a to control bacteria is important to allow the neonatal to adult stage. Pla2g2a promotes innate Th2-type immunity lymphocytes to increase early generated B1a cells.

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.

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.

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.

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.

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.

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

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.

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.

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.

Positive selection of natural autoreactive B cells

Lymphocyte development is critically influenced by self-antigens. T cells are subject to both positive and negative selection, depending on their degree of self-reactivity. Although B cells are subject to negative selection, it has been difficult to test whether self-antigen plays any positive role in B cell development. A murine model system of naturally generated autoreactive B cells with a germ line gene-encoded specificity for the Thy-1 (CD90) glycoprotein was developed, in which the presence of self-antigen promotes B cell accumulation and serum autoantibody secretion. Thus, B cells can be subject to positive selection, generated, and maintained on the basis of their autoreactivity.

A novel mechanism for B cell repertoire maturation based on response by B cell precursors to pre-B receptor assembly

The expression of different sets of immunoglobulin specificities by fetal and adult B lymphocytes is a long-standing puzzle in immunology. Recently it has become clear that production of immunoglobulin mu heavy chain and subsequent assembly with a surrogate light chain to form the pre-B cell receptor complex is critical for development of B cells. Here we show that instead of promoting pre-B cell progression as in adult bone marrow, this complex inhibits pre-B cell growth in fetal liver. Curiously, we identify a fetal-associated VH11 mu heavy chain that allows continued pre-B proliferation in fetal liver. Interestingly, this heavy chain does not associate efficiently with a surrogate light chain, providing a previously unrecognized mechanism for skewing the expression of distinctive VH genes toward fetal through early neonatal life.

Both E12 and E47 allow commitment to the B cell lineage

The E2A gene products, E12 and E47, are required for proper B cell development. Mice lacking the E2A gene products generate only a very small number of B220+ cells, which lack immunoglobulin DJ(H) rearrangements. We have now generated mice expressing either E12 or E47. B cell development in mice expressing E12 but lacking E47 is perturbed at the pro-B cell stage, and these mice lack IgM+B220+ B cells in both bone marrow and spleen. IgM+B220+ B cells can be detected, albeit at significantly reduced levels, in the bone marrow and spleen of mice lacking E12. Ectopic expression of both E12 and E47 in a null mutant background shows that E12 and E47 act in concert to promote B lineage development. Taken together, the data indicate that both E12 and E47 allow commitment to the B cell lineage and act synergistically to promote B lymphocyte maturation.

Functional immunoglobulin transgenes guide ordered B-cell differentiation in Rag-1-deficient mice

We have examined the regulatory role of the individual components of the immunoglobulin antigen receptor in B-cell development by transgenic complementation of Rag-1 deficient (Rag-1-) mice. Complementation with a membrane mu heavy chain (mu HC) gene allows progression of developmentally arrested Rag-1- pro-B-cells to the small pre-B cell stage, whereas the introduction of independently integrated mu HC and kappa light chain (kappa LC) transgenes promotes the appearance of peripheral lymphocytes which, however, remain unresponsive to external stimuli. Complete reconstitution of the B-cell lineage and the emergence of functionally nature Rag-1- peripheral B cells is achieved by the introduction of cointegrated heavy and light chain transgenes encoding an anti-H-2k antibody. This experimental system demonstrates the competence of the mu HC and kappa LC to direct and regulate the sequential stages of B-cell differentiation, defines the time at which negative selection of self-reactive B cells occurs, and shows that elimination of these cells occurs equally well in the absence of Rag-1 as in its presence. These data also support the hypothesis that Rag-1 directly participates in the V(D)J recombination process.

Resolution and characterization of pro-B and pre-pro-B cell stages in normal mouse bone marrow

We have resolved B220+ IgM- B-lineage cells in mouse bone marrow into four fractions based on differential cell surface expression of determinants recognized by S7 (leukosialin, CD43), BP-1, and 30F1 (heat stable antigen). Functional differences among these fractions can be correlated with Ig gene rearrangement status. The largest fraction, lacking S7, consists of pre-B cells whereas the others, expressing S7, include B lineage cells before pre-B. These S7+ fractions, provisionally termed Fr. A, Fr. B, and Fr. C, can differentiate in a stromal layer culture system. Phenotypic alteration during such culture suggests an ordering of these stages from Fr. A to Fr. B to Fr. C and thence to S7- pre-B cells. Using polymerase chain reaction amplification with pairs of oligonucleotide primers for regions 5' of JH1, DFL16.1, and Jk1, we find that the Ig genes of Fr. A are in germline configuration, whereas Fr. B and C are pro-B cell stages with increasing D-J rearrangement, but no V-D-J. Finally, functional analysis demonstrates that the proliferative response to IL-7, an early B lineage growth factor, is restricted to S7+ stages and, furthermore, that an additional, cell contact-mediated signal is essential for survival of Fr. A.

Rearrangement and selection of VH11 in the Ly-1 B cell lineage

One of the predominant VH genes utilized to encode the anti-BrMRBC specificity is a member of the small VH11 family rearranged to JH1. Using the polymerase chain reaction (PCR) we have determined that the frequency of B cells with a VH11 rearrangement is 10-20 times higher in Ly-1 B than in Ly-1- "conventional" B cells regardless of location (spleen or peritoneal cavity). Conventional B cells rearrange this gene at comparable levels in pre-B cells and in mature B cells utilizing all JH gene segments. In contrast, the increased levels of VH11 rearrangement in Ly-1 B are restricted to JH1 (and some JH2) and therefore appear to be the result of selection. Furthermore, most peritoneal Ly-1 B cells with VH11 rearrangements fall in a fraction stained by anti-BrMRBC antibody, likely bearing multivalent natural (likely self) antigen constitutively bound to their surface Ig receptors. Thus, we suggest that autoantigens are largely responsible for the accumulation of autoantibody specificities in the Ly-1 B cell lineage with time, whereas they do not exert this effect in the conventional B cells.

A single VH gene is utilized predominantly in anti-BrMRBC hybridomas derived from purified Ly-1 B cells. Definition of the VH11 family

By establishing hybridomas from two distinct surface IgM+ splenic B cell populations, Ly-1 B cells and "conventional" (Ly-1-) B cells, we found that the Ly-1 B population includes a 30 to 70 times higher frequency (1 to 2%) of cells with specificity for bromelain treated autologous red blood cells (anti-BrMRBC) when compared with conventional B cells (0.03%). We cloned and sequenced the V genes encoding anti-BrMRBC antibody from two hybridomas made with Ly-1 B cells sorted from the spleen of SM/J mice. The VH sequence (for both) is identical with the previously reported sequence associated with this specificity and belongs to a new VH gene family. This gene family, defined here as VH11, has only two members and is the predominant VH rearranged in a collection of Ly-1 B derived anti-BrMRBC hybridomas, always in association with a single VL gene (a member of the V kappa 9 family). Furthermore, analysis of hybridomas made with Ly-1 B cells sorted from the peritoneum reveals a yet higher increased frequency of VH11-encoded anti-BrMRBC specificity (30%). This variation in frequency of anti-BrMRBC in the Ly-1 population depending on location, together with the repeated association of VH11 with a particular V kappa gene suggest that antigen driven selection is (at least in part) responsible for the biased V gene expression seen in this population. Furthermore, a mechanism that might contribute to biased expression, preferential rearrangement due to close proximity to J (as seen in pre-B lines), is excluded by localization of VH11 5' to several of the more J-proximal families (Q52, 7183).

A single VH gene is utilized predominantly in anti-BrMRBC hybridomas derived from purified Ly-1 B cells. Definition of the VH11 family

By establishing hybridomas from two distinct surface IgM+ splenic B cell populations, Ly-1 B cells and "conventional" (Ly-1-) B cells, we found that the Ly-1 B population includes a 30 to 70 times higher frequency (1 to 2%) of cells with specificity for bromelain treated autologous red blood cells (anti-BrMRBC) when compared with conventional B cells (0.03%). We cloned and sequenced the V genes encoding anti-BrMRBC antibody from two hybridomas made with Ly-1 B cells sorted from the spleen of SM/J mice. The VH sequence (for both) is identical with the previously reported sequence associated with this specificity and belongs to a new VH gene family. This gene family, defined here as VH11, has only two members and is the predominant VH rearranged in a collection of Ly-1 B derived anti-BrMRBC hybridomas, always in association with a single VL gene (a member of the V kappa 9 family). Furthermore, analysis of hybridomas made with Ly-1 B cells sorted from the peritoneum reveals a yet higher increased frequency of VH11-encoded anti-BrMRBC specificity (30%). This variation in frequency of anti-BrMRBC in the Ly-1 population depending on location, together with the repeated association of VH11 with a particular V kappa gene suggest that antigen driven selection is (at least in part) responsible for the biased V gene expression seen in this population. Furthermore, a mechanism that might contribute to biased expression, preferential rearrangement due to close proximity to J (as seen in pre-B lines), is excluded by localization of VH11 5' to several of the more J-proximal families (Q52, 7183).