Four of five RAG-expressing JCkappa-/- small pre-BII cells have no L chain gene rearrangements: detection by high-efficiency single cell PCR

Two Distinct Pathways in Mice Generate Antinuclear Antigen-Reactive B Cell Repertoires

The escape of anti-self B cells from tolerance mechanisms like clonal deletion, receptor editing, and anergy results in the production of autoantibodies, which is a hallmark of many autoimmune disorders. In this study, we demonstrate that both germline sequences and somatic mutations contribute to autospecificity of B cell clones. For this issue, we investigated the development of antinuclear autoantibodies (ANAs) and their repertoire in two different mouse models. First, in aging mice that were shown to gain several autoimmune features over time including ANAs. Second, in mice undergoing a chronic graft-versus-host disease (GVHD), thereby developing systemic lupus erythematosus-like symptoms. Detailed repertoire analysis revealed that somatic hypermutations (SHM) were present in all Vh and practically all Vl regions of ANAs generated in these two models. The ANA B cell repertoire in aging mice was restricted, dominated by clonally related Vh1-26/Vk4-74 antibodies. In the collection of GVHD-derived ANAs, the repertoire was less restricted, but the usage of the Vh1-26/Vk4-74 combination was still apparent. Germline conversion showed that the SHM in the 4-74 light chain are deterministic for autoreactivity. Detailed analysis revealed that antinuclear reactivity of these antibodies could be induced by a single amino acid substitution in the CDR1 of the Vk4-74. In both aging B6 and young GVHD mice, conversion of the somatic mutations in the Vh and Vl regions of non Vh1-26/Vk4-74 using antibodies showed that B cells with a germline-encoded V gene could also contribute to the ANA-reactive B cell repertoire. These findings indicate that two distinct pathways generate ANA-producing B cells in both model systems. In one pathway, they are generated by Vh1-26/Vk4-74 expressing B cells in the course of immune responses to an antigen that is neither a nuclear antigen nor any other self-antigen. In the other pathway, ANA-producing B cells are derived from progenitors in the bone marrow that express B cell receptors (BCRs), which bind to nuclear antigens and that escape tolerance induction, possibly as a result of crosslinking of their BCRs by multivalent determinants of nuclear antigens.

Phenotyping of autoreactive B cells with labeled nucleosomes in 56R transgenic mice

The phenotypic characterization of self-reactive B cells producing autoantibodies is one of the challenges to get further insight in the physiopathology of autoimmune diseases. We took advantage of our previously developed flow cytometry method, using labeled nucleosomes, prominent autoantigens in systemic lupus erythematosus, to analyze the phenotype of self-reactive B cells in the anti-DNA B6.56R mouse model. We showed that splenic anti-nucleosome B cells express mostly kappa light chains and harbor a marginal zone phenotype. Moreover, these autoreactive B cells fail to acquire a germinal center phenotype and are less abundant in the transitional T3 compartment. In conclusion, the direct detection of autoreactive B cells helped determine their phenotypic characteristics and provided a more direct insight into the B cell tolerance process in B6.56R mice. This method constitutes an interesting new tool to study the mechanisms of B cell tolerance breakdown in B6.56R mice crossed with autoimmune prone models.

B-cell progenitors and precursors change their microenvironment in fetal liver during early development

The microenvironments, in which B lymphocytes develop in fetal liver, are largely still unknown. Among the nonhematopoietic cells, we have identified and FACS-separated two subpopulations, CD45(-) TER119(-) VCAM-1(+) cells that are either CD105(high) LYVE-1(high) or CD105(low) ALCAM(high) . Immunohistochemical analyses find three of four c-Kit(+) IL-7Rα(+) B220(low) CD19(-) SLC(-) B progenitors in contact with vascular endothelial-type LYVE-1(high) cells on embryonic day 13.5. One day later c-Kit(+) IL-7Rα(+) cells develop to CD19(- and +) , SLC-expressing, DHJH-rearranged pre/pro and pro/preB-I cells. Less than 10% are still in contact with LYVE-1(high) cells, but half of them are now in contact with mesenchymally derived ALCAM(high) liver cells. All of these ALCAM(high) cells, but not the LYVE-1(high) cells produce IL-7 and CXCL12, while both produce CXCL10. Progenitors and pro/preB-I cells are chemoattracted in vitro toward CXCL10 and 12, suggesting that lymphoid progenitors with Ig gene loci in germline configuration enter the developing fetal liver at E13.5 from vascular endothelium, attracted by CXCL10, and then migrate within a day to an ALCAM(high) liver cell microenvironment, differentiating to DHJH-rearranging, surrogate light chain-expressing pre/proB and pro/preB-I cells, attracted by CXCL10 and 12. Between E15.5 and E16.5 preB-I cells expand 10-fold in continued contact with ALCAM(high) cells and begin VH- to DHJH-rearrangements in further differentiated c-Kit(-) IL-7Rα(-) preBII cells. STEM Cells 2013;31:2800-2812.

Msh6 protects mature B cells from lymphoma by preserving genomic stability

Most human B-cell non-Hodgkin's lymphomas arise from germinal centers. Within these sites, the mismatch repair factor MSH6 participates in antibody diversification. Reminiscent of the neoplasms arising in patients with Lynch syndrome III, mice deficient in MSH6 die prematurely of lymphoma. In this study, we characterized the B-cell tumors in MSH6-deficient mice and describe their histological, immunohistochemical, and molecular features, which include moderate microsatellite instability. Based on histological markers and gene expression, the tumor cells seem to be at or beyond the germinal center stage. The simultaneous loss of MSH6 and of activation-induced cytidine deaminase did not appreciably affect the survival of these animals, suggesting that these germinal center-like tumors arose by an activation-induced cytidine deaminase-independent pathway. We conclude that MSH6 protects B cells from neoplastic transformation by preserving genomic stability.

Allelic exclusion of immunoglobulin genes: models and mechanisms

The allelic exclusion of immunoglobulin (Ig) genes is one of the most evolutionarily conserved features of the adaptive immune system and underlies the monospecificity of B cells. While much has been learned about how Ig allelic exclusion is established during B-cell development, the relevance of monospecificity to B-cell function remains enigmatic. Here, we review the theoretical models that have been proposed to explain the establishment of Ig allelic exclusion and focus on the molecular mechanisms utilized by developing B cells to ensure the monoallelic expression of Ig kappa and Ig lambda light chain genes. We also discuss the physiological consequences of Ig allelic exclusion and speculate on the importance of monospecificity of B cells for immune recognition.

Prolactin alters the mechanisms of B cell tolerance induction

OBJECTIVE

Autoimmune diseases predominantly affect women, suggesting that female sex hormones may play a role in the pathogenesis of such diseases. We have previously shown that persistent mild-to-moderate elevations in serum prolactin levels induce a break in self tolerance in mice with a BALB/c genetic background. The aim of this study was to evaluate the effects of hyperprolactinemia on the mechanisms of B cell tolerance induction.

METHODS

Effects of prolactin on splenic B cell subsets were studied in female BALB/c mice. B cell receptor (BCR)-mediated apoptosis and proliferation of transitional B cells were analyzed by flow cytometry. Expression of apoptotic genes was examined by microarrays and real-time polymerase chain reaction analysis. B cells coexpressing kappa/lambda light chains were assessed by flow cytometry and immunohistochemistry. Activation status of transitional type 3 (T3) B cells was evaluated by BCR-induced calcium influx studies.

RESULTS

BCR-mediated apoptosis of the T1 B cell subset, a major checkpoint for negative selection of autoreactive specificities, was decreased in prolactin-treated mice. Microarray studies indicated that this event may be mediated by the prolactin-induced up-regulation of the antiapoptotic gene interferon-gamma receptor type II and down-regulation of the proapoptotic gene Trp63. Prolactin treatment also altered the amount of receptor editing, as indicated by the increased number of transitional B cells coexpressing kappa/lambda light chains. Additionally, hyperprolactinemia modified the level of B cell anergy by increasing the degree of BCR-induced calcium influx in the T3 B cells.

CONCLUSION

Persistently elevated serum prolactin levels interfere with B cell tolerance induction by impairing BCR-mediated clonal deletion, deregulating receptor editing, and decreasing the threshold for activation of anergic B cells, thereby promoting autoreactivity.

Selective regulation of autoreactive B cells by FcgammaRIIB

FcgammaRIIB is an inhibitory receptor which plays a role in limiting B cell and DC activation. Since FcgammaRIIB is known to dampen the signaling strength of the BCR, we wished to determine the impact of FcgammaRIIB on the regulation of BCRs which differ in their affinity for DNA. For these studies, FcgammaRIIB deficient BALB/c mice were bred with mice expressing the transgene-encoded H chain of the R4A anti-DNA antibody which gives rise to BCRs which express high, low or no affinity for DNA. The deletion of FcgammaRIIB in R4A BALB/c mice led to an alteration in the B cell repertoire, allowing for the expansion and activation of high affinity DNA-reactive B cells. By 6-8 months of age, R4A x FcgammaRIIB-/- BALB/c mice spontaneously developed anti-DNA antibody titers. These mice also displayed an induction of IFN-inducible genes and an elevation in levels of the B cell survival factor, BAFF. These data demonstrate that FcgammaRIIB preferentially limits activation of high affinity autoreactive B cells and can influence the activation of DC through an immune complex-mediated mechanism.

Analysis of B cell selection in the germinal center reaction during a T-dependent antibody response at a single cell level

The quasimonoclonal mouse is useful to examine B cell selection during T-dependent antibody (Ab) responses because of its limited B cell populations mainly expressing the knockin 17.2.25 V(H)-encoded H chain (V(H)T) paired with the lambda1 or lambda2 L chain. It has been reported that both two V(H)T/lambda1 and V(H)T/lambda2 B cell populations responded to a T-dependent antigen conjugated with a hapten p-nitrophenylacetyl (pNP), but only V(H)T/lambda2 B cells differentiated to secrete high affinity anti-pNP IgG Abs by acquiring a critical mutation (T313A) in the V(H)T. The V(H)T/lambda2 B cells may be more potent in migrating to the germinal centers (GCs) due to about 50-fold higher affinity for pNP than V(H)T/lambda1 B cells. Here, to uncover how V(H)T/lambda2 B cells were preferentially recruited for affinity maturation during the anti-pNP Ab response, we examined the L chain usage and mutation frequency of V(H)T(+) GC B cells at a single cell level. V(H)T/lambda2 B cells bearing the unmutated V(H)T gene were found in the GCs more frequently than V(H)T/lambda1 and mutated V(H)T/lambda2 counterparts in an early phase of the Ab response. In the course of the GC reaction, the number of V(H)T/lambda2 B cells that mutated their V(H)T genes preferentially expanded, and finally V(H)T/lambda2 B cells bearing the T313A mutation occupied V(H)T(+) GC B cell population. Thus, it is suggested that B cells with a higher affinity were selected not only for entry to the GCs but also in the affinity maturation process during a T-dependent Ab response.

Generation of peripheral B cells occurs via two spatially and temporally distinct pathways

We have identified a population of newly formed bone marrow (BM) B cells that shares multiple characteristics with late transitional B cells in the spleen. Both late splenic transitional B cells and cells within this uncharacterized BM population expressed the cell-surface phenotype AA4(+) CD23(+), yet the developmental kinetics and the renewal rate of AA4(+) CD23(+) BM B cells mirrored recently formed BM B cells. Further, unlike the least mature B cells in the BM and spleen, AA4(+) CD23(+) BM B cells expressed the homing receptor CD62L, were dependent on the antiapoptotic cytokine receptor BR3 and the tec family kinase Btk, and proliferated in response to IL-4 plus CD40 stimulation. Finally, frequencies of lambda light chain-positive B cells declined among AA4(+) CD23(+) B cells in both the BM and spleen, suggesting that V-gene selection events correlate with CD23 expression in both compartments. These observations indicate that the first step in B-cell maturation occurs in both the BM and the periphery and suggest that recently formed B cells exit the BM as a heterogeneous pool of immature and semimature B cells.

Hormonal regulation of B cell development: 17 beta-estradiol impairs negative selection of high-affinity DNA-reactive B cells at more than one developmental checkpoint

There are increasing data suggesting that sex hormones, such as estrogen, have immunomodulatory effects and play a role in disease progression and pathogenesis in patients with the autoimmune disorder systemic lupus erythematosus. We have shown previously that treatment with 17beta-estradiol (E2) induces a lupus phenotype in BALB/c mice that express a transgene-encoded H chain of an anti-DNA Ab. Because E2 treatment interferes with normal tolerance of naive DNA-reactive B cells, we elected to study the effects of hormonal modulation on the regulation of autoreactive B cells at early developmental checkpoints. Single-cell PCR was performed to study the repertoire of DNA-reactive B cell subsets. High-affinity DNA-reactive B cells were rescued at both the immature and transitional B cell stage in E2-treated mice. Interestingly, although low-affinity DNA-reactive B cells survive negative selection in control mice, the frequency of these cells was significantly reduced in the mature pool of E2-treated mice, suggesting that the high-affinity DNA-reactive cells that mature to immunocompetence out-compete the low-affinity population for survival as mature B cells. These data provide evidence that an elevation in serum levels of E2 facilitates the maturation of a pathogenic naive autoreactive B cell repertoire and hampers the maturation of a potentially protective autoreactive B cell repertoire. Furthermore, these data show that both positive and negative selection occur within the transitional B cell stage.

Cutting Edge: Hormonal milieu, not antigenic specificity, determines the mature phenotype of autoreactive B cells

Although both marginal zone and follicular B cells produce anti-DNA Abs in murine models of systemic lupus erythematosus, it has been unclear whether these distinct B cell subsets make identical or different Abs. Single-cell analysis demonstrates that the same DNA-reactive B cells can mature to either subset, depending on the hormonal environment. Anti-DNA B cells in estradiol-treated mice become marginal zone cells while identical cells from prolactin-treated mice become follicular cells. The B cell receptor signaling pathway is influenced by hormonal milieu. Thus, hormonal milieu and perhaps B cell receptor signaling, but not antigenic specificity, correlates with the differentiation pathway. These observations have implications for the pathogenesis and treatment of autoimmune disease.

Models for antigen receptor gene rearrangement. III. Heavy and light chain allelic exclusion

The extent of allelic exclusion in Ig genes is very high, although not absolute. Thus far, it has not been clearly established whether rapid selection of the developing B cell as soon as it has achieved the first productively rearranged, functional heavy chain is the only mechanism responsible for allelic exclusion. Our computational models of Ag receptor gene rearrangement in B lymphocytes are hereby extended to calculate the expected fractions of heavy chain allelically included newly generated B cells as a function of the probability of heavy chain pairing with the surrogate light chain, and the probability that the cell would test this pairing immediately after the first rearrangement. The expected fractions for most values of these probabilities significantly exceed the levels of allelic inclusion in peripheral B cells, implying that in most cases productive rearrangement and subsequent cell surface expression of one allele of the heavy chain gene probably leads to prevention of rearrangement completion on the other allele, and that additional mechanisms, such as peripheral selection disfavoring cells with two productively rearranged heavy chain genes, may also play a role. Furthermore, we revisit light chain allelic exclusion by utilizing the first (to our knowledge) computational model which addresses and enumerates B cells maturing with two productively rearranged kappa light chain genes. We show that, assuming that there are no selection mechanisms responsible for abolishing cells expressing two light chains, the repertoire of newly generated B lymphocytes exiting the bone marrow must contain a significant fraction of such kappa double-productive B cells.

Autoreactive B cells in the marginal zone that express dual receptors

Allotype and isotype exclusion is a property of most lymphocytes. The reason for this property is not known but it guarantees a high concentration of a single receptor, and threshold numbers of receptors may be required for efficient positive and negative selection. Receptor editing compromises exclusion by sustaining recombination even after a functional receptor is formed. Consequently, B cells expressing multiple receptors arise. We have studied such B cells in which one of the two receptors is anti-self, and find that these partially autoreactive B cells accumulate in the marginal zone. The restriction of these cells in this location may help to prevent them from undergoing diversification and developing into fully autoreactive B cells.

GL7 defines the cycling stage of pre-B cells in murine bone marrow

We have identified a novel subset of early B lineage cells in the mouse bone marrow (BM) by GL7 expression on cell surface. GL7(+)B220(low) BM cells have a large cell size and are CD43(-to low), CD95(-), Sca-1(-), I-A(low), IgM(-) and IgD(-), suggesting that they are large pre-B cells. These BM cells express lambda5 and VpreB but not terminal deoxytransferase (TdT) and Bcl-2, and approximately 50 % of them are in cell cycle. This fraction was not detected in BM cells of Rag-1-deficient and Scid mice, supporting that GL7(+)B220(low) BM cells belong to fraction C' and D according to Hardy's criteria or to an early large pre-B-II fraction according to Melchers-Rolink's criteria. Furthermore, GL7(+)B220(low) BM cells can differentiate into IgM(+) immature B cells in co-culture with stromal cells. These results suggest that B lymphocytes pass through the GL7(+) pre-B cell stage during differentiation in the BM. Thus, GL7 is the critical marker to define the proliferation stage of large pre-B cells.

Editors and editing of anti-DNA receptors

Receptor editing is a means by which immature bone marrow B cells can become self-tolerant. Rearrangements of heavy (H) and/or light (L) chain genes are induced by encounter with autoantigens to change the specificity from self to nonself. We have developed site-directed transgenic mice (sd-tg) whose transgenes code for the H chain of antibodies that bind DNA. B cells that express the transgenic H chain associate mainly with four of the 93 functional Vkappa genes of the mouse. Numerous aspartate residues that might inhibit DNA binding by the V(H) domain distinguish these L chain Vkappa sequences, but engaging these Vkappa editors often requires multiple rearrangements. Among the edited B cells is a subset of multispecific cells that express multiple receptors. One consequence of multispecificity is partial autoreactivity; these multispecific B cells may contribute to autoimmunity.

Immunoglobulin heavy chain expression shapes the B cell receptor repertoire in human B cell development

Developing B cells must pass a series of checkpoints that are regulated by membrane-bound Ig(mu) through the Igalpha-Igbeta signal transducers. To determine how Ig(mu) expression affects B cell development and Ab selection in humans we analyzed Ig gene rearrangements in pro-B cells from two patients who are unable to produce Ig(mu) proteins. We find that Ig(mu) expression does not affect V(H), D, or J(H) segment usage and is not required for human Igkappa and Iglambda recombination or expression. However, the heavy and light chains found in pro-B cells differed from those in peripheral B cells in that they showed unusually long CDR3s. In addition, the Igkappa repertoire in Ig(mu)-deficient pro-B cells was skewed to downstream Jkappas and upstream Vkappas, consistent with persistent secondary V(D)J rearrangements. Thus, Ig(mu) expression is not required for secondary V(D)J recombination in pro-B cells. However, B cell receptor expression shapes the Ab repertoire in humans and is essential for selection against Ab's with long CDR3s.

Transcription factor NF-kappa B regulates Ig lambda light chain gene rearrangement

The tissue- and stage-specific assembly of Ig and TCR genes is mediated by a common V(D)J recombinase complex in precursor lymphocytes. Directed alterations in the accessibility of V, D, and J gene segments target the recombinase to specific Ag receptor loci. Accessibility within a given locus is regulated by the functional interaction of transcription factors with cognate enhancer elements and correlates with the transcriptional activity of unrearranged gene segments. As demonstrated in our prior studies, rearrangement of the Igkappa locus is regulated by the inducible transcription factor NF-kappaB. In contrast to the Igkappa locus, known transcriptional control elements in the Iglambda locus lack functional NF-kappaB binding sites. Consistent with this observation, the expression of assembled Iglambda genes in mature B cells has been shown to be NF-kappaB independent. Nonetheless, we now show that specific repression of NF-kappaB inhibits germline transcription and recombination of Iglambda gene segments in precursor B cells. Molecular analyses indicate that the block in NF-kappaB impairs Iglambda rearrangement at the level of recombinase accessibility. In contrast, the activities of known Iglambda promoter and enhancer elements are unaffected in the same cellular background. These findings expand the range of NF-kappaB action in precursor B cells beyond Igkappa to include the control of recombinational accessibility at both L chain loci. Moreover, our results strongly suggest the existence of a novel Iglambda regulatory element that is either directly or indirectly activated by NF-kappaB during the early stages of B cell development.

Selection events operating at various stages in B cell development

B cells have to progress through various checkpoints during their process of development. The three transcription factors E2A, EBF (early B cell factor) and Pax5 play essential roles in B cell commitment checkpoints. The various forms of the BCR and their downstream signaling molecules, which are expressed at different stages of B cell development, act as critical checkpoint guards allowing (positive selection) or preventing (negative selection) developmental progression. The recent advances on the molecular mechanisms operating at these various checkpoints are here summarized and discussed.

Antibody regulation of B cell development

Antibodies on the surface of B lymphocytes trigger adaptive immune responses and control a series of antigen-independent checkpoints during B cell development. These physiologic processes are regulated by a complex of membrane immunoglobulin and two signal transducing proteins known as Ig alpha and Ig beta. Here we focus on the role of antibodies in governing the maturation of B cells from early antigen-independent through the final antigen-dependent stages.

Frequencies of multiple IgL chain gene rearrangements in single normal or kappaL chain-deficient B lineage cells

PCR analyses of the kappaL chain locus in single B-lineage cells of wild-type, Ckappa-, or JCkappa-deficient homozygous or heterozygous mice often detect multiple in- and out-of-frame rearrangements at the kappaL and lambdaL loci. They are most frequent in small pre-BII cells and equally so in wild-type and kappaL chain-deficient cells. Hence, kappaL chain production appears not to inhibit secondary rearrangements. Around 20% of all small preBII cells express IgL chains in their cytoplasm. Cells with a first productive rearrangement on one allele are favored to enter the immature B cell compartment. Thus, allelic exclusion might be secured by control of accessibility of IgL chain loci for rearrangement and by rapid selection of cells with a fitting over those with a nonfitting IgL chain.