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.

B-cell commitment, development and selection

Here we review three areas in B-cell development in the mouse, with a focus on relevance to B-1/CD5+ B cells. Multiparameter flow cytometry has allowed the dissection of intermediate stages of developing B cells, both in fetal liver and bone marrow. In the first area, we present recent work that has delineated a fraction of pre-pro-B cells, committed to the B lineage, but lacking any immunoglobulin rearrangements. Next, the role of the pre-B-cell receptor in B-cell repertoire selection has become clear in the past few years, but we present work suggesting that the action of this process during fetal life is different, resulting in selection of a very distinct repertoire compared with adult. Finally, we describe a new VH3609 antithymocyte Ig transgenic mouse model system that has provided the first definitive evidence for the role of self-antigen in development and maintenance of natural autoreactive B cells.

A VH11V kappa 9 B cell antigen receptor drives generation of CD5+ B cells both in vivo and in vitro

B lymphocytes can be divided into different subpopulations, some with distinctive activation requirements and probably mediating specialized functions, based on surface phenotype and/or anatomical location, but the origins of most of these populations remain poorly understood. B cells constrained by transgenesis to produce an Ag receptor derived from a conventional (B-2) type cell develop a B-2 phenotype, whereas cells from mice carrying a B-1-derived receptor acquire the B-1 phenotype. In this study transgenic enforced expression of a B cell receptor (mu/kappa) originally isolated from a CD5+ (B-1a) B cell generates B-1 phenotype cells in bone marrow cultures that show a distinctive B-1 function, survival in culture. Despite their autoreactivity, we find no evidence for receptor editing or that the paucity of B-2 cells is the result of tolerance-induced selection. Finally, Ca2+ mobilization studies reveal a difference between transgenic B-1 cells in spleen and peritoneal cavity, with cells in spleen much more responsive to anti-B cell receptor cross-linking. We discuss these results in terms of specificity vs lineage models for generation of distinctive B cell subpopulations.

Functional consequences of the developmental arrest and follicular exclusion of anti-double-stranded DNA B cells

Anti-dsDNA B cells are actively tolerized in nonautoimmune BALB/c mice, as manifested by their developmental arrest, follicular exclusion, and rapid turnover rate. Previously, we have documented changes in the maturation status and follicular localization of anti-dsDNA B cells in autoimmune-prone MRL (+/+ and lpr/lpr) mice. To determine whether these differences in developmental status and follicular localization affect the functional capacity of anti-dsDNA B cells, we have now compared their in vivo life spans and their responses to in vitro stimuli. Our study shows that although anti-dsDNA B cells from both BALB/c and MRL-+/+ mice are localized to the T/B interface, only those in BALB/c mice have a rapid turnover rate. Therefore, the immature status and not the exclusion from the B cell follicle correlates with a shortened life span. Interestingly, apoptotic anti-dsDNA B cells were not detected at the T/B interface in BALB/c mice, suggesting that they are not dying there. This study also demonstrates that anti-dsDNA B cells, regardless of maturation status or follicular localization, are able to proliferate and up-regulate the costimulatory molecule B7-2 in response to CD40 ligand and IL-4. Therefore, one of the critical in vivo differences between anti-dsDNA B cells in BALB/c and MRL-+/+ mice compared with MRL-lpr/lpr mice may be the availability of T cell help.

Peripheral CD4+ T cell maturation recognized by increased expression of Thy-1/CD90 bearing the 6C10 carbohydrate epitope

The SM6C10 IgM autoantibody recognizes a surface determinant, 6C10, that is highly expressed on all immature thymocytes. In contrast, its expression on peripheral T cells appears developmentally regulated, i.e., absent from most naive T cells in spleen of neonatal mice, but expressed on 40-80% of naive CD4+ T cells in adult. In this paper, we demonstrate that SM6C10 recognizes a carbohydrate epitope on the Thy-1 glycoprotein using immunoprecipitation analysis, by binding to affinity-purified Thy-1 in an ELISA, and by sensitivity to N-glycosidase-F treatment. Retroviral Thy-1 gene transduction experiments into Thy-1- variant T cell lines and a pro-B cell line provide evidence that 6C10 glycosylated Thy-1 expression is not restricted to T cells but depends on the recipient cell. Therefore, differences in 6C10 levels among Thy-1+ T cells in mice likely reflect developmental regulation of posttranslational modification of the Thy-1 glycoprotein. The ability of naive CD4+ T cells to respond to anti-Thy-1 stimulation increases from neonate to adult, and 6C10- naive cells from adult mice respond poorly compared with 6C10+ cells, similar to the cells in neonatal mice. These results suggest that there is functional maturation by peripheral CD4+ T cells that coincides with 6C10 glycosylated Thy-1 up-regulation, and natural autoantibody recognizes this 6C10 carbohydrate epitope.

Notch1 expression in early lymphopoiesis influences B versus T lineage determination

Notch receptors regulate fate decisions in many cells. One outcome of Notch signaling is differentiation of bipotential precursors into one cell type versus another. To investigate consequences of Notch1 expression in hematolymphoid progenitors, mice were reconstituted with bone marrow (BM) transduced with retroviruses encoding a constitutively active form of Notch1. Although neither granulocyte or monocyte differentiation were appreciably affected, lymphopoiesis was dramatically altered. As early as 3 weeks following transplantation, mice receiving activated Notch1-transduced BM contained immature CD4+ CD8+ T cells in the BM and exhibited a simultaneous block in early B cell lymphopoiesis. These results suggest that Notch1 provides a key regulatory signal in determining T lymphoid versus B lymphoid lineage decisions, possibly by influencing lineage commitment from a common lymphoid progenitor cell.

Coordinate regulation of RAG1 and RAG2 by cell type-specific DNA elements 5' of RAG2

RAG1 and RAG2 are essential for V(D)J recombination and lymphocyte development. These genes are thought to encode a transposase derived from a mobile genetic element that was inserted into the vertebrate genome 450 million years ago. The regulation of RAG1 and RAG2 was investigated in vivo with bacterial artificial chromosome (BAC) transgenes containing a fluorescent indicator. Coordinate expression of RAG1 and RAG2 in B and T cells was found to be regulated by distinct genetic elements found on the 5' side of the RAG2 gene. This observation suggests a mechanism by which asymmetrically disposed cis DNA elements could influence the expression of the primordial transposon and thereby capture RAGs for vertebrate evolution.

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.

Commitment to the B lymphoid lineage occurs before DH-JH recombination

Lineage commitment in B lymphopoiesis remains poorly understood due to the inability to clearly define newly committed B lineage progenitors and their multipotential descendants. We examined the potential of three recently described progenitor populations in adult mouse bone marrow to differentiate into each hematopoietic lineage. The earliest of these, termed fraction (Fr.) A0, exhibited myeloid, erythroid, and B and T lymphoid progenitor activity and included individual cells with myeloid/B lymphoid potential. In sharp contrast, two later populations, termed Frs. A1 and A2 and characterized by surface B220 expression and transcription of the germline immunoglobulin heavy chain (IgH) locus, lacked progenitor activity for all hematopoietic lineages except B lymphocytes. These observations, together with single cell polymerase chain reaction analysis showing a lack of DHJH rearrangements in each population and experiments showing identical precursor potentials when these populations were derived from recombination activating gene (Rag)-1(-/-) and JH-/- mice, demonstrate that commitment to the B lymphoid lineage occurs before and independently of VHDHJH recombination.

c-Rel is essential for B lymphocyte survival and cell cycle progression

c-Rel is a lymphoid-specific member of the NF-kappaB/Rel family of transcriptional factors. To investigate the role of c-Rel in B lymphocyte function, we generated a c-Rel(-/-) mouse via a gene targeting approach. Although early lymphocyte development is normal in c-Rel(-/-) mice, there are significantly fewer B cells displaying a memory (IgM/IgD-) phenotype. Upon immunization, c-Rel(-/-) mice generate fewer B cells with a germinal center (PNAhi) phenotype. In vitro, c-Rel(-/-) B cells proliferate poorly upon ligation of their surface IgM or CD40 receptors or when stimulated with either lipopolysaccharide (LPS) or T cell help. Early molecular events that precede proliferation, such as increases in RNA synthesis as well as IL-2 receptor alpha chain expression, are greatly diminished in c-Rel(-/-) B cells. Furthermore, c-Rel(-/-) B cells are impaired in the ability to receive survival signals generated by anti-IgM or LPS. In contrast, CD40-mediated cell survival is normal in c-Rel(-/-) B cells, suggesting the involvement of a survival-signaling pathway that is independent of c-Rel. When c-Rel (-/-) B cells are co-stimulated with either anti-IgM and CD40 or LPS and CD40, they are rendered capable of progressing through the cell cycle. Finally, co-culture experiments suggest that the defects observed in c-Rel(-/-) B cells are intrinsic to the cell and can not be rescued through either cell-cell contact or addition of soluble factors. Thus, c-Rel is requisite for differentiation to the germinal center and memory B cells in vivo and is required for the transduction of survival and cell cycle progression signals mediated by anti-IgM and LPS in vitro. Furthermore, while c-Rel is involved in CD40-induced proliferation, it is apparently dispensable for the survival signals transduced by CD40.

The fetal origin of B-precursor leukemia in the E-mu-ret mouse

Before the clinical onset of B-precursor lymphoblastic leukemia, E-mu-ret mice have an expansion of late pro-B cells (CD45R+CD43(+)CD24(+)BP-1(+)) within the bone marrow. To characterize the early effects of the transgene product on lymphopoiesis, we initially sequenced the Ig heavy chain (IgH) rearrangements within the late pro-B cells in 24-day-old E-mu-ret and transgene negative mice. In both mouse populations, the IgH rearrangements were polyclonal, predominately nonproductive, and exhibited similar V, D, and J gene usage. However, the frequency of N regions, a marker of postnatal lymphopoiesis, was notably different. At the VD junction, N regions were found in 25 of 25 (100. 0%) rearrangements from transgene-negative mice compared with 12 of 36 (33.3%) rearrangements from Emicro-ret mice. At the DJ junction, N regions were found in 21 of 25 (84.0%) rearrangements from transgene negative mice compared with 4 of 36 (11.1%) rearrangements from E-mu-ret mice. Subsequently, we sequenced the clonal IgH rearrangements from 9 leukemias that developed in 10-to 38-week-old mice and found that 7 leukemias had a least 1 rearrangement that lacked N regions at the DJ junction. In addition, V replacement events were observed in the 1 leukemia studied in detail. Terminal deoxynucleotidyl transferase, the enzyme responsible for N region addition, was expressed at markedly lower levels in late pro-B cells from 7- to 10-day-old E-mu-ret mice compared with transgene-negative mice. Examination of fetal lymphopoiesis in E-mu-ret mice identified a relative increase in early (CD45R+CD43(+)CD24(+)BP-1(-)) and late pro-B cells and a decrease in more differentiated CD43(-) B-lineage cells. Fetal early pro-B cells from Emicro-ret mice proliferated threefold to fivefold greater but differentiated to a lesser extent than those from transgene negative mice when cultured in vitro with interleukin-7. These data suggest that the B precursor leukemias in adult E-mu-ret mice arise from the progeny of pro-B cells generated in utero.

Regulation of anti-DNA B cells in recombination-activating gene-deficient mice

Anti-DNA antibodies are regulated in normal individuals but are found in high concentration in the serum of systemic lupus erythematosus (SLE) patients and the MRL lpr/lpr mouse model of SLE. We previously studied the regulation of anti-double-stranded (ds)DNA and anti-single-stranded (ss)DNA B cells in a nonautoimmune background by generating mice carrying immunoglobulin transgenes coding for anti-DNAs derived from MRL lpr/lpr. Anti-dsDNA B cells undergo receptor editing, but anti-ssDNA B cells seem to be functionally silenced. Here we have investigated how anti-DNA B cells are regulated in recombination- activating gene (RAG)-2-/- mice. In this setting, anti-dsDNA B cells are eliminated by apoptosis in the bone marrow and anti-ssDNA B cells are partially activated.

HD mice: a novel mouse mutant with a specific defect in the generation of CD4(+) T cells

We have identified a spontaneous mutation in mice, which we term HD for "helper T cell deficient." This mouse is distinguished by the virtual absence of peripheral T cells of the CD4(+)8(-) major histocompatibility complex (MHC) class II-restricted T helper subset due to a specific block in thymic development. The developmental defect is selective for CD4(+)8(-) cells; the maturation of CD4(-)8(+) and gamma delta T cells is normal. The autosomal recessive mutation underlying the HD phenotype is unrelated to MHC class II, since it segregates independently of the MHC class II locus. Moreover, the HD phenotype is not caused by a defect of the CD4 gene. Bone marrow transfer experiments demonstrate that the defect is intrinsic to cells of the hematopoietic lineage, i.e., most likely to developing thymocytes themselves. The frequency of CD4(+)8(low) intermediate cells is markedly increased in HD mice, suggesting that class II-restricted thymocytes are arrested at this stage. This is the first genetic defect of its kind to be described in the mouse and may prove highly informative in understanding the molecular pathways underlying lineage commitment.

The evolution of B precursor leukemia in the Emu-ret mouse

Emu-ret mice carrying an RFP/RET fusion gene under the transcriptional control of the immunoglobulin heavy chain enhancer develop B lineage leukemias/lymphomas. We have characterized B-cell development in these mice before the onset of clinical disease to determine the steps involved in leukemogenesis. Flow cytometry reveals that the CD45R+CD43(+)CD24(+)BP-1(+) late pro-B-cell population is markedly expanded in the bone marrow of 3- to 5-week-old Emu-ret mice. Compared with late pro-B cells from transgene-negative mice, Emu-ret late pro-B cells have a limited capacity to differentiate in interleukin (IL)-7 and a higher incidence of VDJ rearrangements, but a similar cell cycle profile. In contrast, CD45R+CD43(+)CD24(+)BP-1(-) early pro-B cells from 3- to 5-week-old Emu-ret mice, which also express the RFP/RET transgene, differentiate in IL-7 similarly to their normal counterparts. Furthermore, early pro-B cells from Emu-ret and transgene-negative mice have an identical pattern of growth inhibition when exposed to interferons (IFNs)-alpha/beta and -gamma, whereas, pro-B-cell leukemia lines derived from Emu-ret mice are markedly less sensitive to growth inhibition by these IFNs. In 13-week-old well-appearing Emu-ret mice, late pro-B cells upregulate CYCLIN D1 expression and downregulate CASPASE-1 expression in a pattern that correlates with the emergence of B precursor cells in the peripheral blood and the loss of other B lineage subsets in the bone marrow. Taken together, these results suggest that the expression of the RFP/RET transgene initially prevents the normal elimination of late pro-B cells with nonproductive rearrangements. Secondary events that simultaneously disturb the normal transcriptional regulation of genes involved in the control of the cell cycle and apoptosis may allow for subsequent malignant transformation within the expanded late pro-B-cell population.

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.

The human PD-1 gene: complete cDNA, genomic organization, and developmentally regulated expression in B cell progenitors

We report the complete cDNA sequence and the genomic structure of the human PD-1 homologue. An analysis of the expression pattern of the human PD-1 gene (hPD-1) and the murine PD-1 gene (mPD-1) in developing bone marrow B-lineage cells was also undertaken. The full length hPD-1 cDNA is 2106 nucleotides long and encodes a predicted protein of 288 amino acid residues. The hPD-1 and mPD-1 genes share 70% homology at the nucleotide level and 60% homology at the amino acid level. Four potential sites for N-linked glycosylation are conserved, as are a stretch of amino acids between two cysteine residues resembling a V-set immunoglobulin domain, and another region containing a motif similar to an immunoreceptor tyrosine-based inhibitory motif. Isolation of the genomic locus of the hPD-1 gene reveals that the gene is composed of five exons located on human chromosome 2 at band q37. The 5' flanking region lacks TATA and CAAT cis-acting elements, but includes a number of potential transcription factor binding sites and a dominant transcription start site. The mPD-1 gene was preferentially expressed in pro-B cells from murine adult bone marrow. Although hPD-1 was not preferentially expressed in pro-B cells from human fetal bone marrow, treatment of isolated pro-B cells with interleukin-7 resulted in a dramatic increase in expression. These data suggest that PD-1 may play a role in B-cell differentiation during the pro-B cell stage.

The human heavy chain Ig V region gene repertoire is biased at all stages of B cell ontogeny, including early pre-B cells

The expressed human Ig repertoire is not an equal representation of all V(H) segments present in genomic DNA. Studies have shown that a restricted set of V(H) gene segments are over-represented in Ab repertoires of fetal/neonatal and adult B cells. Additionally, this restricted set of V(H) genes is frequently expressed by autoimmune and tumor B cells. To investigate at which developmental stage a bias in the repertoire begins, we compared the V(H)3 and V(H)4 family repertoires of pre-B and immature B cells from bone marrow and mature B cells from peripheral blood of two adults. We found that the V4-34 and V4-59 gene segments of the V(H)4 family and the V3-23 gene segment of the V(H)3 family dominate the repertoires of the surface Ig-negative early pre-B as well as immature and mature B cells. Furthermore, the pattern of utilization of other V(H)3 family members suggests that certain genes that are frequently rearranged during early stages of B cell development are subsequently disfavored during later stages of B cell maturation. We conclude that the over-representation of certain V genes could arise from sequential mechanisms operating at both early and later stages of B cell development. These V(H)-mediated mechanisms might include preferential rearrangement and/or efficiency of pairing with the surrogate light chain at the surface Ig-negative, early pre-B cell stage and ligand selection at more mature, surface Ig-positive, B cell stages.

The human heavy chain Ig V region gene repertoire is biased at all stages of B cell ontogeny, including early pre-B cells

The expressed human Ig repertoire is not an equal representation of all V(H) segments present in genomic DNA. Studies have shown that a restricted set of V(H) gene segments are over-represented in Ab repertoires of fetal/neonatal and adult B cells. Additionally, this restricted set of V(H) genes is frequently expressed by autoimmune and tumor B cells. To investigate at which developmental stage a bias in the repertoire begins, we compared the V(H)3 and V(H)4 family repertoires of pre-B and immature B cells from bone marrow and mature B cells from peripheral blood of two adults. We found that the V4-34 and V4-59 gene segments of the V(H)4 family and the V3-23 gene segment of the V(H)3 family dominate the repertoires of the surface Ig-negative early pre-B as well as immature and mature B cells. Furthermore, the pattern of utilization of other V(H)3 family members suggests that certain genes that are frequently rearranged during early stages of B cell development are subsequently disfavored during later stages of B cell maturation. We conclude that the over-representation of certain V genes could arise from sequential mechanisms operating at both early and later stages of B cell development. These V(H)-mediated mechanisms might include preferential rearrangement and/or efficiency of pairing with the surrogate light chain at the surface Ig-negative, early pre-B cell stage and ligand selection at more mature, surface Ig-positive, B cell stages.

Down-regulation of terminal deoxynucleotidyl transferase by Ig heavy chain in B lineage cells

The enzyme terminal deoxynucleotidyl transferase (TdT) adds nontemplate-derived nucleotides (N regions) to the junctions between recombining variable, diversity, and joining segments of Ig genes. The relative paucity of N regions in Ig light chains, together with the down-regulation of TdT transcription in pre-B cells (prior to light chain production), suggested that production of IgM heavy chain (mu) protein might negatively regulate TdT expression. In this study, we examined the effect of mu production on TdT gene expression in B lineage subsets from normal mice, from recombination-deficient mice (SCID and Rag-1-) carrying mu transgenes, and in transformed pro-B cell lines transfected with mu constructs. In normal mice, TdT is sharply down-regulated at the early pre-B stage in which cells have just completed productive mu rearrangement. Furthermore, the expression of mu transgenes in pro-B stage cells from recombination-deficient mice results in a similar decrease. Finally, transfection of genomic constructs encoding mu into pro-B cell lines results in a marked reduction of TdT expression. Taken together, these findings indicate that mu protein production results in the down-regulation of TdT. The ability of mu transgenes to alter TdT expression in cell lines also suggests that signaling through the pre-B receptor does not necessarily require interaction with an external stromal cell-derived ligand.

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.

Down-regulation of terminal deoxynucleotidyl transferase by Ig heavy chain in B lineage cells

The enzyme terminal deoxynucleotidyl transferase (TdT) adds nontemplate-derived nucleotides (N regions) to the junctions between recombining variable, diversity, and joining segments of Ig genes. The relative paucity of N regions in Ig light chains, together with the down-regulation of TdT transcription in pre-B cells (prior to light chain production), suggested that production of IgM heavy chain (mu) protein might negatively regulate TdT expression. In this study, we examined the effect of mu production on TdT gene expression in B lineage subsets from normal mice, from recombination-deficient mice (SCID and Rag-1-) carrying mu transgenes, and in transformed pro-B cell lines transfected with mu constructs. In normal mice, TdT is sharply down-regulated at the early pre-B stage in which cells have just completed productive mu rearrangement. Furthermore, the expression of mu transgenes in pro-B stage cells from recombination-deficient mice results in a similar decrease. Finally, transfection of genomic constructs encoding mu into pro-B cell lines results in a marked reduction of TdT expression. Taken together, these findings indicate that mu protein production results in the down-regulation of TdT. The ability of mu transgenes to alter TdT expression in cell lines also suggests that signaling through the pre-B receptor does not necessarily require interaction with an external stromal cell-derived ligand.

Identification of the earliest B lineage stage in mouse bone marrow

We have identified a very early stage of B lineage cells in the CD45R (B220)+CD24 (HSA) pre-pro-B fraction of mouse bone marrow delineated by expression of AA4.1, a molecule found on stem cells and early B lineage cells. These cells are B lineage precursors based on their capacity to generate B lineage cells rapidly in stromal-dependent culture and their expression of high levels of germline IgH transcripts in the absence of Rag-1/2. Half of these AA4.1+ precursors express low levels of CD4, characteristic of lymphoid progenitors, but few if any have up-regulated CD19, a molecule expressed very early in the B lineage. Furthermore, expression of genes encoding pre-B and B cell receptor components (mb-1, B29, and lambda 5) and transcription factors necessary for B lineage differentiation (BSAP, E12, E47, and Id) provide further support for designating these cells as the earliest B cell precursors.

Frequent aberrant immunoglobulin gene rearrangements in pro-B cells revealed by a bcl-xL transgene

During B lymphocyte development, pro-B cells that fail to rearrange an immunoglobulin heavy (IgH) chain allele productively are thought to undergo developmental arrest and death, but because these cells are short-lived in vivo they are not well characterized. Transgenic mice expressing the apoptosis regulatory gene bcl-xL in the B lineage developed large expansions of pro-B cells in bone marrow. V(D)J rearrangements in the expanded populations were nearly all nonproductive, and DJH rearrangements were enriched for joints in DH reading frame 2 and for aberrant joints with extensive DH or JH deletions. Thus, the death of pro-B cells with failed immunoglobulin rearrangements occurs by apoptosis, and bcl-xL can deliver a strong survival signal at the pro-B stage. This analysis also demonstrated that immunoglobulin gene rearrangement is less precise than previously appreciated.

Transcription factor B cell lineage-specific activator protein regulates the gene for human X-box binding protein 1

The transcription factor human X-box binding protein 1 (hXBP-1) is a basic region-leucine zipper protein implicated in the regulation of major histocompatibility complex class II gene expression as well as in exocrine gland and skeletal development. Multiple regulatory elements in the hXBP-1 promoter lie 3' to the transcription start site, including the hX2 site, whose core sequence is an AP-1-like element identical to the hXBP-1 target sequence in the HLA-DRA promoter. One complex identified by electrophoretic mobility shift assay (EMSA), complex 3, was previously shown to protect the hX2 site and more 3' bases. Sequence analysis now shows that this region contains a consensus binding site for transcription factor BSAP (B cell lineage-specific activator protein). Complex 3 and BSAP have identical cell-type specificities, as they are found only in pre-B and mature B cell lines. In EMSAs, BSAP antibody specifically recognized complex 3, and in vitro translated BSAP could bind to an hXBP promoter fragment. Cotransfections using an hXBP-1 reporter construct indicated that BSAP downregulates the hXBP-1 promoter. The highest levels of hXBP-1 mRNA were found when BSAP was not expressed, in pre-Pro-B cells and in plasma cell lines. In addition, hXBP-1 and BSAP levels were inversely correlated along the early stages of B cell development. In the regulation of the hXBP-1 promoter, a strong positive transcriptional influence at the hX2 site is opposed by the downregulatory actions of BSAP.

Distinctive developmental origins and specificities of the CD5+ B-cell subset

In this review we present our recent work defining the pathway of B-cell development in the adult bone marrow of the mouse. We next present similarities and contrasts between fetal and adult B lymphopoiesis. Finally, considering the striking biases in specificity and V-gene usage between the progeny of fetal B lymphopoiesis, CD5+ B cells, and the bulk of adult-derived CD5- B cells, we suggest a model for developmentally distinct generation of this subset.

Self-renewal of B-1 lymphocytes is dependent on CD19

The B-1 subset of B lymphocytes is maintained by self-renewal of mature cells, and this process may involve signaling through membrane immunoglobulin (mIg). We determined whether CD19, a membrane protein that co-stimulates B cells by mIg, has a role in this process. Pre-natal treatment of mice with 1D3, a rat anti-mouse CD19 monoclonal antibody, down-regulated CD19 expression and reduced by sixfold the number of B-1a cells at birth; B-2 cells were relatively unaffected. Prolonged treatment of adult mice with 1D3 caused the loss of approximately 2% per day of peritoneal B-1a cells, without diminishing the recovery of splenic B-2 cells. The loss of B-1a cells was associated with inhibition of their replication rather than with accelerated turnover. Therefore, CD19 is involved in the development and self-renewal of B-1a cells, perhaps through its ability to amplify signaling through mIgM.

The 3' enhancer region determines the B/T specificity and pro-B/pre-B specificity of immunoglobulin V kappa-J kappa joining

Using transgenic substrates, we found that the immunoglobulin kappa gene 3' enhancer (E3') acts as a negative regulator in V kappa-J kappa joining. Although the E3' was originally identified as a transcriptional enhancer, it acts in a suppressive manner for recombinational regulation. Base substitution analysis has shown that the PU.1-binding site within the E3' regulates the B/T specificity of V kappa-J kappa joining. In a substrate with a mutated PU.1-binding site (GAGGAA to TCTTCG), V kappa-J kappa joining occurred not only in B cells, but also in T cells. The E3' region is also responsible for determining the pro-B/pre-B specificity of V kappa-J kappa joining. When the E3' region was deleted, kappa gene rearrangement actively occurred at the early pro-B stage of B cell development: nongermline (N) nucleotides were common at recombination junctions.

Differential expression of the blk and ret tyrosine kinases during B lineage development is dependent on Ig rearrangement

Expression of Ig transgenes in recombination-deficient mutant scid and Rag-1-mice results in the generation of pre-B and B cells, which are normally absent from these animals. In screening for protein tyrosine kinases (PTKs) that may play a role in this progression beyond the pro-B stage, we have identified five differentially regulated PTKs and compared their gene expression in defined stages of early B-lineage cells from normal, mutant, and Ig-transgenic mutant mice. Three PTKs (fgr, flk2/flt3, and tsk) show a comparable decrease at an early stage in all mice. In contrast, the decreasing expression of ret and the increasing expression of blk seen in differentiating B cells from normal mice are not observed in the mutant mice, unless they carry Ig transgenes. Therefore, our results show that the expression of certain PTKs is dependent on productive Ig rearrangement and suggest important roles for both Ret and Blk at distinct stages in the Ig-dependent progression of B cell differentiation.

Differential expression of the blk and ret tyrosine kinases during B lineage development is dependent on Ig rearrangement

Expression of Ig transgenes in recombination-deficient mutant scid and Rag-1-mice results in the generation of pre-B and B cells, which are normally absent from these animals. In screening for protein tyrosine kinases (PTKs) that may play a role in this progression beyond the pro-B stage, we have identified five differentially regulated PTKs and compared their gene expression in defined stages of early B-lineage cells from normal, mutant, and Ig-transgenic mutant mice. Three PTKs (fgr, flk2/flt3, and tsk) show a comparable decrease at an early stage in all mice. In contrast, the decreasing expression of ret and the increasing expression of blk seen in differentiating B cells from normal mice are not observed in the mutant mice, unless they carry Ig transgenes. Therefore, our results show that the expression of certain PTKs is dependent on productive Ig rearrangement and suggest important roles for both Ret and Blk at distinct stages in the Ig-dependent progression of B cell differentiation.

The site and stage of anti-DNA B-cell deletion

Antibodies to DNA and nucleoproteins are found in sera of individuals with systemic autoimmune disease. In the population (and in the autoimmune mouse strain MRL/lpr) there is a great variety of such antinuclear antibodies, but individuals with systemic lupus erythematosus or single MRL mice express a subset only of the antinuclear specificities found in the population. These observations have been interpreted to mean that these antibodies arise by immunization. The oligoclonal nature of the autoantibody response and the evidence of selection acting on somatically mutated autoantibodies favour this interpretation. Specific activation of autoantibodies in disease implies either that autoantibodies are regulated in non-diseased individuals or that autoantigen availability is variable. The former has been demonstrated in anti-DNA transgenic mice. In normal mice, transgene-encoded antibodies against double-stranded (ds) DNA are not expressed in serum or on B cells. Here we describe modified anti-dsDNA transgenic mice which allow us to study the site and developmental stage at which such B-cell regulation occurs. This model shows that in normal mice B cells expressing anti-DNA specificity are deleted in the bone marrow at a pre-B to immature B transitional stage.

Altered major histocompatibility complex restriction in the NK1.1+Ly-6Chi autoreactive CD4+ T cell subset from class II-deficient mice

We previously demonstrated selective enrichment of major histocompatibility complex (MHC) class II-specific autoreactive T cells in a subset of mouse CD4+ thymocytes. Here we show that a significant fraction of these autoreactive cells in the normal adult thymus expresses NK1.1 and high levels of Ly-6C and also exhibits flexibility in MHC restriction. In normal mice, this NK1.1+Ly-6Chi subfraction accounts for 10-50% of the CD4+ autoreactive subset and is enriched for MHC class II-restricted autoreactive cells as determined by mixed leukocyte reaction frequency analysis, similar to NK1.1-Ly-6C-CD4+ autoreactive cells. In contrast, in the thymus of class II-deficient littermate mice, NK1.1+Ly-6Chi cells account for most of the mature heat stable antigen (HSA)-CD4+ fraction and exhibit MHC-restricted non-class II autoreactivity. Thus, NK1.1+Ly-6ChiCD4+ T cells show flexibility in MHC class restriction, but their autoreactivity remains MHC dependent.

T cell receptor V beta 8.2 gene germ-line transcription: an early event of lymphocyte differentiation

Rearrangement of the T cell antigen receptor (TCR) beta chain genes is highly regulated in both a developmental and a tissue-specific manner. T cell precursors originate from the yolk sac or fetal liver during gestation and from the bone marrow during adulthood. They initiate the recombination of TCR genes primarily during differentiation in the thymus. It has previously been suggested that transcription of immunoglobulin genes in germ-line configuration is linked to recombination events within these loci. Here, we examine whether germ-line transcription of TCR variable genes coincides with their rearrangement or whether it marks even earlier stages of T lymphocyte development. During gestation, we found V beta 8.2 germ-line transcripts in the fetal liver and the fetal thymus, but not in the yolk sac. This transcription precedes V beta 8.2 to D beta J beta rearrangement. In adult animals, we found these transcripts in the thymus, the spleen, the liver and the bone marrow. However, in the liver, this transcription is dependent on the presence of mature lymphocytes. This transcription does not happen in non-lymphoid cells. In the B lymphocyte lineage, V beta 8.2 germ-line transcripts are detected only in the earliest stages of differentiation (pre-pro- and pro-B cells), but not in pre-B cells and mature B lymphocytes. Altogether, our results show that transcription of the unrearranged V beta 8.2 gene is an early event of lymphocyte development, taking place in lymphocyte precursors, long before V beta 8.2 rearrangement.

Absence of MHC class II expression distinguishes fetal from adult B lymphopoiesis in mice

Early B-lineage progenitor cells (Pro-B) isolated from murine fetal liver and adult bone marrow can differentiate to the immature B cell stage in a stromal cell-dependent culture system and to mature B cells upon transfer into immunodeficient SCID mice. By using immunofluorescence analysis, we found that progenitor cells from day 16 fetus differentiating in culture lacked MHC class II expression during the Pre-B and immature B cell stages, whereas such expression was readily apparent on the surface of corresponding adult-derived populations. RT-PCR analysis of RNA message levels for the four class II genes (A alpha, A beta, E alpha, E beta) yielded completely concordant results. B cells of fetal progeny did eventually express class II upon further maturation in vivo. Thus, the onset of class II expression is uniquely delayed during fetal B cell differentiation. This result explains an apparent paradox, i.e., that class II expression is absent from B cells in neonatal spleen but present as early as the Pre-B cell stage in adult bone marrow. Furthermore, we suggest that such distinct programs of class II expression during fetal and adult lymphopoiesis could result in differences in susceptibility to tolerance.

Absence of MHC class II expression distinguishes fetal from adult B lymphopoiesis in mice

Early B-lineage progenitor cells (Pro-B) isolated from murine fetal liver and adult bone marrow can differentiate to the immature B cell stage in a stromal cell-dependent culture system and to mature B cells upon transfer into immunodeficient SCID mice. By using immunofluorescence analysis, we found that progenitor cells from day 16 fetus differentiating in culture lacked MHC class II expression during the Pre-B and immature B cell stages, whereas such expression was readily apparent on the surface of corresponding adult-derived populations. RT-PCR analysis of RNA message levels for the four class II genes (A alpha, A beta, E alpha, E beta) yielded completely concordant results. B cells of fetal progeny did eventually express class II upon further maturation in vivo. Thus, the onset of class II expression is uniquely delayed during fetal B cell differentiation. This result explains an apparent paradox, i.e., that class II expression is absent from B cells in neonatal spleen but present as early as the Pre-B cell stage in adult bone marrow. Furthermore, we suggest that such distinct programs of class II expression during fetal and adult lymphopoiesis could result in differences in susceptibility to tolerance.

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.

Two distinct non-T helper type 2 interleukin-4+ cell subsets in mice as revealed by single-cell cytokine analysis

We have previously defined four murine CD4+ peripheral T cell subsets, fractions (Fr.) I-IV, based on expression of the 6C10 and 3G11 determinants (Hayakawa, K. and Hardy, R. R., J. Exp. Med. 1988. 168: 1825). These subsets also show distinctive levels of other cell surface markers: the two minor subsets, Fr. III and Fr. IV, are both CD45RBlow/-, L-selectin (Mel-14)- and CD44hi, characteristic of secondary T cells. The patterns and levels of cytokine production by individual cells in each subset were determined by bioassay for interleukin (IL)-2/IL-4 or IL-4/interferon (IFN)-gamma production after anti-CD3 stimulation. Our data revealed that these four phenotypically defined subsets largely coincide with clusters of cells showing uniform distinctive cytokine profiles, i.e. IL-2+/IFN-gamma-/IL-4- (Fr. I and Fr. II, L-selectin+), IL2+/IFN-gamma +/IL-4+ (Fr. III, L-selectin-), and IL-2-/IFN-gamma low/-/IL-4+ (Fr. IV, L-selectin-). Besides these subsets, an L-selectin-negative cell subfraction within Fr. II appears to represent a transitional population between the IL-2+/IFN-gamma-/IL-4- stage and the IL-2+/IFN-gamma +/IL-4+ stage. Taken together, these results demonstrate the presence of two IL-4+ secondary T cell subsets with distinct cytokine production patterns, and show that the majority of IL-4+ cells found in healthy adult laboratory mice co-produce IFN-gamma, and thus are not typical T helper type 2 cells.

Monoclonal anti-lambda 5 antibody FS1 identifies a 130 kDa protein associated with lambda 5 and Vpre-B on the surface of early pre-B cell lines

mAbs specific for mouse lambda 5 protein were prepared by fusion of spleen cells from a hamster immunized with recombinant lambda 5 protein synthesized in bacteria and the mouse myeloma cell line SP2/0-Ag14. Here we report the characteristics of the antibodies produced by the FS1 hybridoma. FS1 antibody stains a variety of mouse pre-B cell lines but not B cell lines or T cell lines. The staining of the pre-B cell lines A-1 and C-7 by phycoerythrin (PE)-conjugated FS1 (FS1-PE) can be blocked by preincubation of these cells with unconjugated FS1 antibody or with affinity purified polyclonal lambda 5 specific Ig but not with normal hamster or mouse IgG or with affinity purified polyclonal anti-Mb-1 Ig. From these experiments we concluded that FS1 specifically recognizes lambda 5 protein. We used FS1-PE to probe for surface (s) lambda 5+ cells in normal BALB/c mouse bone marrow. Such cells were undetectable when total bone marrow or FACS sorted subpopulations were analyzed. However, when B220+, CD43+, s lambda 5-bone marrow cells were cultured for 4 days on the stromal cell line FLST2 in the presence of IL-7, s lambda 5 expression became apparent. Further expansion of these cells in IL7 alone augmented the s lambda 5 expression to readily detectable levels. This modulation may indicate that s lambda 5 expression on normal bone marrow cells in vivo is transient and that at any given moment only a small fraction of bone marrow cells expresses low levels of lambda 5 protein on the surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Deletion and editing of B cells that express antibodies to DNA

We previously demonstrated that in mice transgenic for genes coding for an anti-ssDNA autoantibody B cells were functionally inactivated but not physically deleted. We have now extended this model by introducing an arginine into the CDR2 of the heavy chain transgene. This change alters the specificity of the Ab from anti-ssDNA to anti-dsDNA and increases the affinity for ssDNA. Mice carrying this transgene displayed a significant reduction of peripheral B cells and anti-dsDNA B cells were not recovered from the spleens. The remaining B cells escape deletion by revising their Ag receptors in several ways: 1) elimination of the transgenic heavy chain gene via intrachromosomal recombination, followed by rearrangement and expression of endogenous VH genes; 2) ongoing rearrangement of endogenous kappa light chain genes to generate a non-dsDNA-binding Ab; and 3) expression of a rare V lambda gene, V lambda x, to generate a non-DNA-binding Ab.

Deletion and editing of B cells that express antibodies to DNA

We previously demonstrated that in mice transgenic for genes coding for an anti-ssDNA autoantibody B cells were functionally inactivated but not physically deleted. We have now extended this model by introducing an arginine into the CDR2 of the heavy chain transgene. This change alters the specificity of the Ab from anti-ssDNA to anti-dsDNA and increases the affinity for ssDNA. Mice carrying this transgene displayed a significant reduction of peripheral B cells and anti-dsDNA B cells were not recovered from the spleens. The remaining B cells escape deletion by revising their Ag receptors in several ways: 1) elimination of the transgenic heavy chain gene via intrachromosomal recombination, followed by rearrangement and expression of endogenous VH genes; 2) ongoing rearrangement of endogenous kappa light chain genes to generate a non-dsDNA-binding Ab; and 3) expression of a rare V lambda gene, V lambda x, to generate a non-DNA-binding Ab.

Distinctive developmental origins and specificities of murine CD5+ B cells

CD5+ B cells constitute a small fraction of cells in the spleen of adult mice that exhibit numerous features serving to distinguish them from the bulk of IgD++CD5- "conventional" B cells. In this review we focus on two major questions relating to this population: 1) the relationship of CD5+ B cells to other B cells; and 2) the distinctive enrichment of particular autoreactive specificities in this subset. The nature of their origins is clarified by a thorough analysis of intermediate stages of early B-cell development in both fetal and adult tissues. The reactivity to bromelain-treated mouse red blood cells serves as a prototype system for the investigation of biased specificities in CD5+ B cells. These lines of investigation lead us to propose that CD5+ B cells in the adult are the remnant of a distinct fetal B-cell differentiation pathway wherein selection of cells from this fetal/neonatal population into the adult long-lived pool results in the over-expression of certain germline-encoded autoreactivities.