B cells are programmed to activate kappa and lambda for rearrangement at consecutive developmental stages

Unfolding the symbiosis of AID, chromatin remodelers, and epigenetics-The ACE phenomenon of antibody diversity

Activation-induced cytidine deaminase (AID) is responsible for the initiation of somatic hypermutation (SHM) and class-switch recombination (CSR), which result in antibody affinity maturation and isotype switching, thus producing pathogen-specific antibodies. Chromatin dynamics and accessibility play a significant role in determining AID expression and its targeting. Chromatin remodelers contribute to the accessibility of the chromatin structure, thereby influencing the targeting of AID to Ig genes. Epigenetic modifications, including DNA methylation, histone modifications, and miRNA expression, profoundly impact the regulation of AID and chromatin remodelers targeting Ig genes. Additionally, epigenetic modifications lead to chromatin rearrangement and thereby can change AID expression levels and its preferential targeting to Ig genes. This interplay is symbolized as the ACE phenomenon encapsulates three interconnected aspects: AID, Chromatin remodelers, and Epigenetic modifications. This review emphasizes the importance of understanding the intricate relationship between these aspects to unlock the therapeutic potential of these molecular processes and molecules.

Kidins220 regulates the development of B cells bearing the λ light chain

The ratio between κ and λ light chain (LC)-expressing B cells varies considerably between species. We recently identified Kinase D-interacting substrate of 220 kDa (Kidins220) as an interaction partner of the BCR. In vivo ablation of Kidins220 in B cells resulted in a marked reduction of λLC-expressing B cells. Kidins220 knockout B cells fail to open and recombine the genes of the Igl locus, even in genetic scenarios where the Igk genes cannot be rearranged or where the κLC confers autoreactivity. Igk gene recombination and expression in Kidins220-deficient B cells is normal. Kidins220 regulates the development of λLC B cells by enhancing the survival of developing B cells and thereby extending the time-window in which the Igl locus opens and the genes are rearranged and transcribed. Further, our data suggest that Kidins220 guarantees optimal pre-BCR and BCR signaling to induce Igl locus opening and gene recombination during B cell development and receptor editing.

Contribution of Immunoglobulin Enhancers to B Cell Nuclear Organization

B cells undergo genetic rearrangements at immunoglobulin gene (Ig) loci during B cell maturation. First V(D)J recombination occurs during early B cell stages followed by class switch recombination (CSR) and somatic hypermutation (SHM) which occur during mature B cell stages. Given that RAG1/2 induces DNA double strand breaks (DSBs) during V(D)J recombination and AID (Activation-Induced Deaminase) leads to DNA modifications (mutations during SHM or DNA DSBs during CSR), it is mandatory that IgH rearrangements be tightly regulated to avoid any mutations or translocations within oncogenes. Ig loci contain various cis-regulatory elements that are involved in germline transcription, chromatin modifications or RAG/AID recruitment. Ig cis-regulatory elements are increasingly recognized as being involved in nuclear positioning, heterochromatin addressing and chromosome loop regulation. In this review, we examined multiple data showing the critical interest of studying Ig gene regulation at the whole nucleus scale. In this context, we highlighted the essential function of Ig gene regulatory elements that now have to be considered as nuclear organizers in B lymphocytes.

Induced B Cell Development in Adult Mice

We employed the B-Indu-Rag1 model in which the coding exon of recombination-activating gene 1 (Rag1) is inactivated by inversion. It is flanked by inverted loxP sites. Accordingly, B cell development is stopped at the pro/pre B-I cell precursor stage. A B cell-specific Cre recombinase fused to a mutated estrogen receptor allows the induction of RAG1 function and B cell development by application of Tamoxifen. Since Rag1 function is recovered in a non-self-renewing precursor cell, only single waves of development can be induced. Using this system, we could determine that B cells minimally require 5 days to undergo development from pro/preB-I cells to the large and 6 days to the small preB-II cell stage. First immature transitional (T) 1 and T2 B cells could be detected in the bone marrow at day 6 and day 7, respectively, while their appearance in the spleen took one additional day. We also tested a contribution of adult bone marrow to the pool of B-1 cells. Sublethally irradiated syngeneic WT mice were adoptively transferred with bone marrow of B-Indu-Rag1 mice and B cell development was induced after 6 weeks. A significant portion of donor derived B-1 cells could be detected in such adult mice. Finally, early VH gene usage was tested after induction of B cell development. During the earliest time points the VH genes proximal to D/J were found to be predominantly rearranged. At later time points, the large family of the most distal VH prevailed.

Foxo3 Promotes Apoptosis of B Cell Receptor-Stimulated Immature B Cells, Thus Limiting the Window for Receptor Editing

Central tolerance checkpoints are critical for the elimination of autoreactive B cells and the prevention of autoimmunity. When autoreactive B cells encounter their Ag at the immature B cell stage, BCR cross-linking induces receptor editing, followed by apoptosis if edited cells remain autoreactive. Although the transcription factor Foxo1 is known to promote receptor editing, the role of the related factor Foxo3 in central B cell tolerance is poorly understood. We find that BCR-stimulated immature B cells from Foxo3-deficient mice demonstrate reduced apoptosis compared with wild type cells. Despite this, Foxo3^-/- mice do not develop increased autoantibodies. This suggests that the increased survival of Foxo3^-/- immature B cells allows additional rounds of receptor editing, resulting in more cells "redeeming" themselves by becoming nonautoreactive. Indeed, increased Igλ usage and increased recombining sequence recombination among Igλ-expressing cells were observed in Foxo3^-/- mice, indicative of increased receptor editing. We also observed that deletion of high-affinity autoreactive cells was intact in the absence of Foxo3 in the anti-hen egg lysozyme (HEL)/membrane-bound HEL model. However, Foxo3 levels in B cells from systemic lupus erythematosus (SLE) patients were inversely correlated with disease activity and reduced in patients with elevated anti-dsDNA Abs. Although this is likely due in part to increased B cell activation in these SLE patients, it is also possible that low-affinity B cells that remain autoreactive after editing may survive inappropriately in the absence of Foxo3 and become activated to secrete autoantibodies in the context of other SLE-associated defects.

Distinct and Overlapping Functions of TEC Kinase and BTK in B Cell Receptor Signaling

The Tec tyrosine kinase is expressed in many cell types, including hematopoietic cells, and is a member of the Tec kinase family that also includes Btk. Although the role of Btk in B cells has been extensively studied, the role of Tec kinase in B cells remains largely unclear. It was previously shown that Tec kinase has the ability to partly compensate for loss of Btk activity in B cell differentiation, although the underlying mechanism is unknown. In this study, we confirm that Tec kinase is not essential for normal B cell development when Btk is present, but we also found that Tec-deficient mature B cells showed increased activation, proliferation, and survival upon BCR stimulation, even in the presence of Btk. Whereas Tec deficiency did not affect phosphorylation of phospholipase Cγ or Ca²⁺ influx, it was associated with significantly increased activation of the intracellular Akt/S6 kinase signaling pathway upon BCR and CD40 stimulation. The increased S6 kinase phosphorylation in Tec-deficient B cells was dependent on Btk kinase activity, as ibrutinib treatment restored pS6 to wild-type levels, although Btk protein and phosphorylation levels were comparable to controls. In Tec-deficient mice in vivo, B cell responses to model Ags and humoral immunity upon influenza infection were enhanced. Moreover, aged mice lacking Tec kinase developed a mild autoimmune phenotype. Taken together, these data indicate that in mature B cells, Tec and Btk may compete for activation of the Akt signaling pathway, whereby the activating capacity of Btk is limited by the presence of Tec kinase.

Kidins220/ARMS binds to the B cell antigen receptor and regulates B cell development and activation

Fiala et al. report that Kidins220/ARMS is a novel interactor of the B cell antigen receptor (BCR) and its deletion impairs B cell development and B cell functioning.

B cell antigen receptor (BCR) signaling is critical for B cell development and activation. Using mass spectrometry, we identified a protein kinase D–interacting substrate of 220 kD (Kidins220)/ankyrin repeat–rich membrane-spanning protein (ARMS) as a novel interaction partner of resting and stimulated BCR. Upon BCR stimulation, the interaction increases in a Src kinase–independent manner. By knocking down Kidins220 in a B cell line and generating a conditional B cell–specific Kidins220 knockout (B-KO) mouse strain, we show that Kidins220 couples the BCR to PLCγ2, Ca²⁺, and extracellular signal-regulated kinase (Erk) signaling. Consequently, BCR-mediated B cell activation was reduced in vitro and in vivo upon Kidins220 deletion. Furthermore, B cell development was impaired at stages where pre-BCR or BCR signaling is required. Most strikingly, λ light chain–positive B cells were reduced sixfold in the B-KO mice, genetically placing Kidins220 in the PLCγ2 pathway. Thus, our data indicate that Kidins220 positively regulates pre-BCR and BCR functioning.

Long-Range Regulation of V(D)J Recombination

Given their essential role in adaptive immunity, antigen receptor loci have been the focus of analysis for many years and are among a handful of the most well-studied genes in the genome. Their investigation led initially to a detailed knowledge of linear structure and characterization of regulatory elements that confer control of their rearrangement and expression. However, advances in DNA FISH and imaging combined with new molecular approaches that interrogate chromosome conformation have led to a growing appreciation that linear structure is only one aspect of gene regulation and in more recent years, the focus has switched to analyzing the impact of locus conformation and nuclear organization on control of recombination. Despite decades of work and intense effort from numerous labs, we are still left with an incomplete picture of how the assembly of antigen receptor loci is regulated. This chapter summarizes our advances to date and points to areas that need further investigation.

Surrogate light chain is required for central and peripheral B-cell tolerance and inhibits anti-DNA antibody production by marginal zone B cells

Selection of the primary antibody repertoire takes place in pro-/pre-B cells, and subsequently in immature and transitional B cells. At the first checkpoint, μ heavy (μH) chains assemble with surrogate light (SL) chain into a precursor B-cell receptor. In mice lacking SL chain, μH chain selection is impaired, and serum autoantibody levels are elevated. However, whether the development of autoantibody-producing cells is due to an inability of the resultant B-cell receptors to induce central and/or peripheral B-cell tolerance or other factors is unknown. Here, we show that receptor editing is defective, and that a higher proportion of BM immature B cells are prone to undergoing apoptosis. Furthermore, transitional B cells are also more prone to undergoing apoptosis, with a stronger selection pressure to enter the follicular B-cell pool. Those that enter the marginal zone (MZ) B-cell pool escape selection and survive, possibly due to the B-lymphopenia and elevated levels of B-cell activating factor. Moreover, the MZ B cells are responsible for the elevated IgM anti-dsDNA antibody levels detected in these mice. Thus, the SL chain is required for central and peripheral B-cell tolerance and inhibits anti-DNA antibody production by MZ B cells.

Pre-B cell receptor signaling induces immunoglobulin κ locus accessibility by functional redistribution of enhancer-mediated chromatin interactions

Chromatin conformation analyses provide novel insights into how variable segments in the immunoglobulin light chain gene become accessible for recombination in precursor B lymphocytes.

During B cell development, the precursor B cell receptor (pre-BCR) checkpoint is thought to increase immunoglobulin κ light chain (Igκ) locus accessibility to the V(D)J recombinase. Accordingly, pre-B cells lacking the pre-BCR signaling molecules Btk or Slp65 showed reduced germline V_κ transcription. To investigate whether pre-BCR signaling modulates V_κ accessibility through enhancer-mediated Igκ locus topology, we performed chromosome conformation capture and sequencing analyses. These revealed that already in pro-B cells the κ enhancers robustly interact with the ∼3.2 Mb V_κ region and its flanking sequences. Analyses in wild-type, Btk, and Slp65 single- and double-deficient pre-B cells demonstrated that pre-BCR signaling reduces interactions of both enhancers with Igκ locus flanking sequences and increases interactions of the 3′κ enhancer with V_κ genes. Remarkably, pre-BCR signaling does not significantly affect interactions between the intronic enhancer and V_κ genes, which are already robust in pro-B cells. Both enhancers interact most frequently with highly used V_κ genes, which are often marked by transcription factor E2a. We conclude that the κ enhancers interact with the V_κ region already in pro-B cells and that pre-BCR signaling induces accessibility through a functional redistribution of long-range chromatin interactions within the V_κ region, whereby the two enhancers play distinct roles.

B lymphocyte development involves the generation of a functional antigen receptor, comprising two heavy chains and two light chains arranged in a characteristic “Y” shape. To do this, the receptor genes must first be assembled by ordered genomic recombination events, starting with the immunoglobulin heavy chain (IgH) gene segments. On successful rearrangement, the resulting IgH μ protein is presented on the cell surface as part of a preliminary version of the B cell receptor—the “pre-BCR.” Pre-BCR signaling then redirects recombination activity to the immunoglobulin κ light chain gene. The activity of two regulatory κ enhancer elements is known to be crucial for opening up the gene, but it remains largely unknown how the hundred or so Variable (V) segments in the κ locus gain access to the recombination system. Here, we studied a panel of pre-B cells from mice lacking specific signaling molecules, reflecting absent, partial, or complete pre-BCR signaling. We identify gene regulatory changes that are dependent on pre-BCR signaling and occur via long-range chromatin interactions between the κ enhancers and the V segments. Surprisingly the light chain gene initially contracts, but the interactions then become more functionally redistributed when pre-BCR signaling occurs. Interestingly, we find that the two enhancers play distinct roles in the process of coordinating chromatin interactions towards the V segments. Our study combines chromatin conformation techniques with data on transcription factor binding to gain unique insights into the functional role of chromatin dynamics.

Transcription-coupled eviction of histones H2A/H2B governs V(D)J recombination

Initiation of V(D)J recombination critically relies on the formation of an accessible chromatin structure at recombination signal sequences (RSSs) but how this accessibility is generated is poorly understood. Immunoglobulin light-chain loci normally undergo recombination in pre-B cells. We show here that equipping (earlier) pro-B cells with the increased pre-B-cell levels of just one transcription factor, IRF4, triggers the entire cascade of events leading to premature light-chain recombination. We then used this finding to dissect the critical events that generate RSS accessibility and show that the chromatin modifications previously associated with recombination are insufficient. Instead, we establish that non-coding transcription triggers IgL RSS accessibility and find that the accessibility is transient. Transcription transiently evicts H2A/H2B dimers, releasing 35-40 bp of nucleosomal DNA, and we demonstrate that H2A/H2B loss can explain the RSS accessibility observed in vivo. We therefore propose that the transcription-mediated eviction of H2A/H2B dimers is an important mechanism that makes RSSs accessible for the initiation of recombination.

Checkpoints of B cell differentiation: visualizing Ig-centric processes

The generation of antibody responses and B cell memory can only take place following multiple steps of differentiation. Key molecular processes during precursor B cell differentiation in bone marrow generate unique antibodies. These antibodies are further optimized via molecular modifications during immune responses in peripheral lymphoid organs. Multiple checkpoints ensure proper differentiation of precursor and mature B lymphocytes. Many of these checkpoints have been found disrupted in patients with a primary immunodeficiency. Based on studies in these patients and in mouse models, new insights have been generated in B cell differentiation and antibody responses. Still, in many patients with impaired antibody formation, it remains unclear how B cells are affected. In this perspective, we present 11 critical processes in B cell differentiation. We discuss how defects in these processes can result in impaired checkpoint selection and how they can be visualized in healthy subjects and patients with immunodeficiency or other immunological disease.

The DNA-binding protein CTCF limits proximal Vκ recombination and restricts κ enhancer interactions to the immunoglobulin κ light chain locus

Regulation of immunoglobulin (Ig) V(D)J gene rearrangement is dependent on higher-order chromatin organization. Here, we studied the in vivo function of the DNA-binding zinc-finger protein CTCF, which regulates interactions between enhancers and promoters. By conditional deletion of the Ctcf gene in the B cell lineage, we demonstrate that loss of CTCF allowed Ig heavy chain recombination, but pre-B cell proliferation and differentiation was severely impaired. In the absence of CTCF, the Igκ light chain locus showed increased proximal and reduced distal Vκ usage. This was associated with enhanced proximal Vκ and reduced Jκ germline transcription. Chromosome conformation capture experiments demonstrated that CTCF limits interactions of the Igκ enhancers with the proximal V(κ) gene region and prevents inappropriate interactions between these strong enhancers and elements outside the Igκ locus. Thus, although Ig gene recombination can occur in the absence of CTCF, it is a critical factor determining Vκ segment choice for recombination.

Direct reduction of antigen receptor expression in polyclonal B cell populations developing in vivo results in light chain receptor editing

Secondary Ab V region gene segment rearrangement, termed receptor editing, is a major mechanism contributing to B lymphocyte self-tolerance. However, the parameters that determine whether a B cell undergoes editing are a current subject of debate. We tested the role that the level of BCR expression plays in the regulation of receptor editing in a polyclonal population of B cells differentiating in vivo. Expression of a short hairpin RNA for κ L chain RNA in B cells resulted in reduction in levels of this RNA and surface BCRs. Strikingly, fully mature and functional B cells that developed in vivo and efficiently expressed the short hairpin RNA predominantly expressed BCRs containing λ light chains. This shift in L chain repertoire was accompanied by inhibition of development, increased Rag gene expression, and increased λ V gene segment-cleavage events at the immature B cell stage. These data demonstrated that reducing the translation of BCRs that are members of the natural repertoire at the immature B cell stage is sufficient to promote editing.

Epigenetic and 3-dimensional regulation of V(D)J rearrangement of immunoglobulin genes

V(D)J recombination is a crucial component of the adaptive immune response, allowing for the production of a diverse antigen receptor repertoire (Ig and TCR). This review will focus on how epigenetic regulation and 3-dimensional (3D) interactions may control V(D)J recombination at Ig loci. The interplay between transcription factors and post-translational modifications at the Igh, Igκ, and Igλ loci will be highlighted. Furthermore, we propose that the spatial organization and epigenetic boundaries of each Ig loci before and during V(D)J recombination may be influenced in part by the CTCF/cohesin complex. Taken together, the many epigenetic and 3D layers of control ensure that Ig loci are only rearranged at appropriate stages of B cell development.

Distinct roles for E12 and E47 in B cell specification and the sequential rearrangement of immunoglobulin light chain loci

The E2A gene products, E12 and E47, are critical regulators of B cell development. However, it remains elusive whether E12 and E47 have overlapping and/or distinct functions during B lymphopoiesis. We have generated mice deficient for either E12 or E47 and examined their roles in B cell maturation. We show that E47 is essential for developmental progression at the prepro–B cell stage, whereas E12 is dispensable for early B cell development, commitment, and maintenance. In contrast, both E12 and E47 play critical roles in pre–B and immature B cells to promote immunoglobulin λ (Igλ) germline transcription as well as Igλ VJ gene rearrangement. Furthermore, we show that E12 as well as E47 is required to promote receptor editing upon exposure to self-antigen. We demonstrate that increasing levels of E12 and E47 act to induce Igλ germline transcription, promote trimethylated lysine 4 on histone 3 (H3) as well as H3 acetylation across the Jλ region, and activate Igλ VJ gene rearrangement. We propose that in the pre–B and immature B cell compartments, gradients of E12 and E47 activities are established to mechanistically regulate the sequential rearrangement of the Ig light chain genes.

Development of immunoglobulin lambda-chain-positive B cells, but not editing of immunoglobulin kappa-chain, depends on NF-kappaB signals

By genetically ablating IκB kinase (IKK)-mediated NF-κB activation in the B cell lineage, and by analyzing a mouse mutant in which Igλ⁺ B cells are generated in the absence of rearrangements in Igk, we define two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-κB signaling. During the first phase, in which NF-κB signaling is dispensable, predominantly Igκ⁺ B cells are generated and undergo efficient receptor editing. In the second phase, predominantly Igλ⁺ B cells are generated, whose development is ontogenetically timed to occur after Igk rearrangements. This second phase of development is dependent on NF-κB signals, which can be substituted by transgenic expression of the pro-survival factor Bcl2.

The epigenetic profile of Ig genes is dynamically regulated during B cell differentiation and is modulated by pre-B cell receptor signaling

Ag receptor loci poised for V(D)J rearrangement undergo germline transcription (GT) of unrearranged genes, and the accessible gene segments are associated with posttranslational modifications (PTM) on histones. In this study, we performed a comprehensive analysis of the dynamic changes of four PTM throughout B and T cell differentiation in freshly isolated ex vivo cells. Methylation of lysines 4 and 79 of histone H3, and acetylation of H3, demonstrated stage and lineage specificity, and were most pronounced at the J segments of loci poised for, or undergoing, rearrangement, except for dimethylation of H3K4, which was more equally distributed on V, D, and J genes. Focusing on the IgL loci, we demonstrated there are no active PTM in the absence of pre-BCR signaling. The kappa locus GT and PTM on Jkappa genes are rapidly induced following pre-BCR signaling in large pre-B cells. In contrast, the lambda locus shows greatly delayed onset of GT and PTM, which do not reach high levels until the immature B cell compartment, the stage at which receptor editing is initiated. Analysis of MiEkappa(-/-) mice shows that this enhancer plays a key role in inducing not only GT, but PTM. Using an inducible pre-B cell line, we demonstrate that active PTM on Jkappa genes occur after GT is initiated, indicating that histone PTM do not make the Jkappa region accessible, but conversely, GT may play a role in adding PTM. Our data indicate that the epigenetic profile of IgL genes is dramatically modulated by pre-BCR signaling and B cell differentiation status.

Chromatin opening is tightly linked to enhancer activation at the kappa light chain locus

Enhancers play an important role in chromatin opening but the temporal relationship between enhancer activation and the generation of an accessible chromatin structure is poorly defined. Recombination enhancers are essential for chromatin opening and subsequent V(D)J recombination at immunoglobulin loci. In mice, the kappa light chain locus displays an open chromatin structure before the lambda locus yet the same proteins, PU.1/PIP, trigger full enhancer activation of both loci. Using primary B cells isolated from distinct developmental stages and an improved method to quantitatively determine hypersensitive site formation, we find the kappa and lambda recombination enhancers become fully hypersensitive soon after transition to large and small pre-B-II cells, respectively. This correlates strictly with the stages at which these loci are activated. Since these cells are short-lived, these data imply that there is a close temporal relationship between full enhancer hypersensitive site formation and locus chromatin opening.

Mutations of the Igbeta gene cause agammaglobulinemia in man

Agammaglobulinemia is a rare primary immunodeficiency characterized by an early block of B cell development in the bone marrow, resulting in the absence of peripheral B cells and low/absent immunoglobulin serum levels. So far, mutations in Btk, mu heavy chain, surrogate light chain, Igalpha, and B cell linker have been found in 85-90% of patients with agammaglobulinemia. We report on the first patient with agammaglobulinemia caused by a homozygous nonsense mutation in Igbeta, which is a transmembrane protein that associates with Igalpha as part of the preBCR complex. Transfection experiments using Drosophila melanogaster S2 Schneider cells showed that the mutant Igbeta is no longer able to associate with Igalpha, and that assembly of the BCR complex on the cell surface is abrogated. The essential role of Igbeta for human B cell development was further demonstrated by immunofluorescence analysis of the patient's bone marrow, which showed a complete block of B cell development at the pro-B to preB transition. These results indicate that mutations in Igbeta can cause agammaglobulinemia in man.

Phospholipase Cgamma2 contributes to light-chain gene activation and receptor editing

Phospholipase Cgamma2 (PLCgamma2) is critical for pre-B-cell receptor (pre-BCR) and BCR signaling. Current studies discovered that PLCgamma2-deficient mice had reduced immunoglobulin lambda (Iglambda) light-chain usage throughout B-cell maturation stages, including transitional type 1 (T1), transitional type 2 (T2), and mature follicular B cells. The reduction of Iglambda rearrangement by PLCgamma2 deficiency was not due to specifically increased apoptosis or decreased proliferation of mutant Iglambda+ B cells, as lack of PLCgamma2 exerted a similar effect on apoptosis and proliferation of both Iglambda+ and Igkappa+ B cells. Moreover, PLCgamma2-deficient IgHEL transgenic B cells exhibited an impairment of antigen-induced receptor editing among both the endogenous lambda and kappa loci in vitro and in vivo. Importantly, PLCgamma2 deficiency impaired BCR-induced expression of IRF-4 and IRF-8, the two transcription factors critical for lambda and kappa light-chain rearrangements. Taken together, these data demonstrate that the PLCgamma2 signaling pathway plays a role in activation of light-chain loci and contributes to receptor editing.

Immunoglobulin light chain repertoire in hairy cell leukemia

Of 166 hairy cell leukemia (HCL) patients, 81 had kappa and 80 had lambda expression. IGKV-J and IGLV-J rearrangement structure was analyzed in 21 HCL patients (11 kappa, 10 lambda). For kappa, IGKV1-5 was most frequent, and the KJ2 gene was over-utilized. For lambda HCL, LJ3 was over-utilized compared to normal. This study significantly adds to previous studies of light chain usage in HCL and is the first to report light chain gene usage. In HCL, we confirm the lack of kappa predominance observed in normal lymphocytes and in chronic lymphocytic leukemia, and note over-representation of several light chain genes.

Scarcity of lambda 1 B cells in mice with a single point mutation in C lambda 1 is due to a low BCR signal caused by misfolded lambda 1 light chain

The presence of valine-154 instead of glycine in the constant region of lambda1 causes a severe lambda1 B cell defect in SJL and lambda1-valine knock-in mice with a compensatory increase in lambda2,3 B cells. The defect is due to low signaling by the lambda1-valine BCR. lambda1-Valine B cells deficient in the SHP-1 phosphatase survive better than lambda2,3 B cells in these mice, or lambda1 B cells in lambda1 wildtype mice. Low signaling is apparently due to misfolding of the lambda1-valine light chain as demonstrated by the absence of a regular beta-sheet structure determined by circular dichroism, the sedimentation of the light chain in solution, and the association of valine-valine constant regions in a yeast two-hybrid assay. lambda1-Valine B cells that survive apparently have a higher BCR signal, presumably because of their specific lambda1-heavy chain combination or having encountered a high-affiniy antigen. lambda1-Valine mice have increased B1 cells which were shown by others to have a higher signaling potential. Valine mice crossed with non-conventional gamma2b transgenic mice, in which B cell development is accelerated and in which B1 cells and high signaling cells are greatly reduced, have essentially no, lambda2,3 B cells, but increased numbers of lambda1-valine B cells. This supports the conclusion that the major defect in lambda1-valine mice is the inability of valine-preB cells to produce a threshold signal for B cell development. The reduction of lambda2,3 B cells in valine mice with a gamma2b transgene shows that the majority of their compensatory increase is almost entirely of the B1 cell type.

Bruton's tyrosine kinase and SLP-65 regulate pre-B cell differentiation and the induction of Ig light chain gene rearrangement

Bruton's tyrosine kinase (Btk) and the adapter protein SLP-65 (Src homology 2 domain-containing leukocyte-specific phosphoprotein of 65 kDa) transmit precursor BCR (pre-BCR) signals that are essential for efficient developmental progression of large cycling into small resting pre-B cells. We show that Btk- and SLP-65-deficient pre-B cells have a specific defect in Ig lambda L chain germline transcription. In Btk/SLP-65 double-deficient pre-B cells, both kappa and lambda germline transcripts are severely reduced. Although these observations point to an important role for Btk and SLP-65 in the initiation of L chain gene rearrangement, the possibility remained that these signaling molecules are only required for termination of pre-B cell proliferation or for pre-B cell survival, whereby differentiation and L chain rearrangement is subsequently initiated in a Btk/SLP-65-independent fashion. Because transgenic expression of the antiapoptotic protein Bcl-2 did not rescue the developmental arrest of Btk/SLP-65 double-deficient pre-B cells, we conclude that defective L chain opening in Btk/SLP-65-deficient small resting pre-B cells is not due to their reduced survival. Next, we analyzed transgenic mice expressing the constitutively active Btk mutant E41K. The expression of E41K-Btk in Ig H chain-negative pro-B cells induced 1) surface marker changes that signify cellular differentiation, including down-regulation of surrogate L chain and up-regulation of CD2, CD25, and MHC class II; and 2) premature rearrangement and expression of kappa and lambda light chains. These findings demonstrate that Btk and SLP-65 transmit signals that induce cellular maturation and Ig L chain rearrangement independently of their role in termination of pre-B cell expansion.

IL-7: a key regulator of B lymphopoiesis

Interleukin-7 plays important roles in the B cell developmental pathway including events leading to commitment, survival, proliferation, and maturation. Because of its central role in adult murine B lymphopoiesis, IL-7 is frequently used to generate B cell progenitors in vitro. We have shown that differentiation of IL-7-responsive cells in these cultures is influenced by CD45, pre-B cell receptor, and other downstream signaling molecules. A common, but often overlooked aspect of IL-7 containing cultures is the routine maturation of cells to the sIgM(+) stage. The production of B cells in IL-7 containing cultures is balanced by cell death, since such cells fail to survive for long without additional stimulation.

Ig Gene Rearrangement Steps Are Initiated in Early Human Precursor B Cell Subsets and Correlate with Specific Transcription Factor Expression

The role of specific transcription factors in the initiation and regulation of Ig gene rearrangements has been studied extensively in mouse models, but data on normal human precursor B cell differentiation are limited. We purified five human precursor B cell subsets, and assessed and quantified their IGH, IGK, and IGL gene rearrangement patterns and gene expression profiles. Pro-B cells already massively initiate D(H)-J(H) rearrangements, which are completed with V(H)-DJ(H) rearrangements in pre-B-I cells. Large cycling pre-B-II cells are selected for in-frame IGH gene rearrangements. The first IGK/IGL gene rearrangements were initiated in pre-B-I cells, but their frequency increased enormously in small pre-B-II cells, and in-frame selection was found in immature B cells. Transcripts of the RAG1 and RAG2 genes and earlier defined transcription factors, such as E2A, early B cell factor, E2-2, PAX5, and IRF4, were specifically up-regulated at stages undergoing Ig gene rearrangements. Based on the combined Ig gene rearrangement status and gene expression profiles of consecutive precursor B cell subsets, we identified 16 candidate genes involved in initiation and/or regulation of Ig gene rearrangements. These analyses provide new insights into early human precursor B cell differentiation steps and represent an excellent template for studies on oncogenic transformation in precursor B acute lymphoblastic leukemia and B cell differentiation blocks in primary Ab deficiencies.

An immunoglobulin C kappa-reactive single chain antibody fusion protein induces tolerance through receptor editing in a normal polyclonal immune system

Understanding immune tolerance mechanisms is a major goal of immunology research, but mechanistic studies have generally required the use of mouse models carrying untargeted or targeted antigen receptor transgenes, which distort lymphocyte development and therefore preclude analysis of a truly normal immune system. Here we demonstrate an advance in in vivo analysis of immune tolerance that overcomes these shortcomings. We show that custom superantigens generated by single chain antibody technology permit the study of tolerance in a normal, polyclonal immune system. In the present study we generated a membrane-tethered anti-Igkappa-reactive single chain antibody chimeric gene and expressed it as a transgene in mice. B cell tolerance was directly characterized in the transgenic mice and in radiation bone marrow chimeras in which ligand-bearing mice served as recipients of nontransgenic cells. We find that the ubiquitously expressed, Igkappa-reactive ligand induces efficient B cell tolerance primarily or exclusively by receptor editing. We also demonstrate the unique advantages of our model in the genetic and cellular analysis of immune tolerance.

Cellular maturation defects in Bruton's tyrosine kinase-deficient immature B cells are amplified by premature B cell receptor expression and reduced by receptor editing

In the mouse, Bruton's tyrosine kinase (Btk) is essential for efficient developmental progression of CD43(+)CD2(-) large cycling into CD43(-)CD2(+) small resting pre-B cells in the bone marrow and of IgM(high) transitional type 2 B cells into IgM(low) mature B cells in the spleen. In this study, we show that the impaired induction of cell surface changes in Btk-deficient pre-B cells was still noticeable in kappa(+) immature B cells, but was largely corrected in lambda(+) immature B cells. As lambda gene rearrangements are programmed to follow kappa rearrangements and lambda expression is associated with receptor editing, we hypothesized that the transit time through the pre-B cell compartment or receptor editing may affect the extent of the cellular maturation defects in Btk-deficient B cells. To address this issue, we used 3-83 mu delta transgenic mice, which prematurely express a complete B cell receptor and therefore manifest accelerated B cell development. In Btk-deficient 3-83 mu delta mice, the IgM(+) B cells in the bone marrow exhibited a very immature phenotype (pre-BCR(+)CD43(+)CD2(-)) and were arrested at the transitional type 1 B cell stage upon arrival in the spleen. However, these cellular maturation defects were largely restored when Btk-deficient 3-83 mu delta B cells were on a centrally deleting background and therefore targeted for receptor editing. Providing an extended time window for developing B cells by enforced expression of the antiapoptotic gene Bcl-2 did not alter the Btk dependence of their cellular maturation. We conclude that premature B cell receptor expression amplifies the cellular maturation defects in Btk-deficient B cells, while extensive receptor editing reduces these defects.

Receptor editing and marginal zone B cell development are regulated by the helix-loop-helix protein, E2A

Previous studies have indicated that the E2A gene products are required to initiate B lineage development. Here, we demonstrate that E2A^+/− B cells that express an autoreactive B cell receptor fail to mature due in part to an inability to activate secondary immunoglobulin (Ig) light chain gene rearrangement. Both RAG1/2 gene expression and RS deletion are severely defective in E2A^+/− mice. Additionally, we demonstrate that E2A^+/− mice show an increase in the proportion of marginal zone B cells with a concomitant decrease in the proportion of follicular B cells. In contrast, Id3-deficient splenocytes show a decline in the proportion of marginal zone B cells. Based on these observations, we propose that E-protein activity regulates secondary Ig gene rearrangement at the immature B cell stage and contributes to cell fate determination of marginal zone B cells. Additionally, we propose a model in which E-proteins enforce the developmental checkpoint at the immature B cell stage.

Pax-5 Is Essential for κ Sterile Transcription during Igκ Chain Gene Rearrangement

Pax-5 is the key regulator in B cell development. Pax-5-deficient mice show defects in B cell commitment and recombination of IgH chain gene rearrangement from DJ to VDJ. Previously, we found that Pax-5 bound to KI and KII sites, which play a crucial role in kappa-chain gene rearrangement. However, the function of Pax-5 in Ig kappa chain gene rearrangement has not been investigated. To address this issue, we newly established pre-BI cell lines expressing the pre-B cell receptor from Pax-5-deficient mice and used them in an in vitro culture system, in which kappa-chain gene rearrangement is induced by removing IL-7. By examining the Pax-5-deficient pre-BI (knockout (KO)) cells, we show in this study that, despite recombination-activating gene 1 and 2 expression, these KO cells did not rearrange the kappa-chain gene following the absence of kappa sterile transcription. Consistent with these data, fluorescent in situ hybridization analyses revealed that the J(kappa) locus in KO cells was located at the nuclear periphery as a repressive compartment. Transfection of KO cells with Pax-5 constructs indicated that the transactivation domain of Pax-5 was required for kappa sterile transcription and kappa-chain gene rearrangement. Moreover, the hormone-inducible system in KO cells demonstrated that Pax-5 directly functioned in kappa sterile transcription. These results indicate that Pax-5 is necessary for kappa sterile transcription during Ig kappa chain gene rearrangement.

Analysis of Gene Expression and Ig Transcription in PU.1/Spi-B-Deficient Progenitor B Cell Lines

A number of presumptive target genes for the Ets-family transcription factor PU.1 have been identified in the B cell lineage. However, the precise function of PU.1 in B cells has not been studied because targeted null mutation of the PU.1 gene results in a block to lymphomyeloid development at an early developmental stage. In this study, we take advantage of recently developed PU.1(-/-)Spi-B(-/-) IL-7 and stromal cell-dependent progenitor B (pro-B) cell lines to analyze the function of PU.1 and Spi-B in B cell development. We show that contrary to previously published expectations, PU.1 and/or Spi-B are not required for Ig H chain (IgH) gene transcription in pro-B cells. In fact, PU.1(-/-)Spi-B(-/-) pro-B cells have increased levels of IgH transcription compared with wild-type pro-B cells. In addition, high levels of Igkappa transcription are induced after IL-7 withdrawal of wild-type or PU.1(-/-)Spi-B(-/-) pro-B cells. In contrast, we found that Iglambda transcription is reduced in PU.1(-/-)Spi-B(-/-) pro-B cells relative to wild-type pro-B cells after IL-7 withdrawal. These results suggest that Iglambda, but not IgH or Igkappa, transcription, is dependent on PU.1 and/or Spi-B. The PU.1(-/-)Spi-B(-/-) pro-B cells have other phenotypic changes relative to wild-type pro-B cells including increased proliferation, increased CD25 expression, decreased c-Kit expression, and decreased RAG-1 expression. Taken together, our observations suggest that reduction of PU.1 and/or Spi-B activity in pro-B cells promotes their differentiation to a stage intermediate between late pro-B cells and large pre-B cells.

Germline transcripts of immunoglobulin light chain variable regions are structurally diverse and differentially expressed

The murine pre-B cell line R2-bfl, which can be induced to differentiate in vitro, was used to study germline transcription of variable regions of the light chain loci. RNA from these cells was subjected to a 3'-RACE and germline transcripts from 17 individual Vkappa gene segments belonging to 12 Vkappa families were characterized. Germline transcripts of all three Vlambda regions were similarly analyzed. The synchronous differentiation of R2-bfl cells was then used to investigate the order of appearance of germline transcripts of the V and JC clusters of both light chain loci. This was taken as indicator for accessibility of a particular locus to rearrangement. Germline transcripts of the JCkappa cluster and the Vkappa family most proximal to JCkappa was detectable already at day 0, while transcripts of the most distal Vkappa family became apparent after initiation of differentiation at day 1. Transcripts of the JClambda cluster could be found at day 2, whereas transcripts of the Vlambda region were already present at day 1. Thus, the lambda locus becomes accessible to rearrangement later during development than kappa, confirming and extending our previous findings. The V and JC clusters open at the same stage of development although slight asynchronicities were found for the Vlambda and the distal Vkappa gene segments.

Monoclonal IgE antibodies against birch pollen allergens: novel tools for biological characterization and standardization of allergens

BACKGROUND

IgE antibodies are key players in immediate hypersensitivity reactions. Allergen characterization and standardization is usually based on the sera of allergic patients, whereas monoclonal IgE antibodies specific for clinically relevant allergens are very rare.

OBJECTIVE

The aim of this study was to establish IgE mAbs specific for birch pollen allergens, because these are important inhalant allergens.

METHODS

IgE-producing hybridomas were identified by using the highly sensitive rat basophilic leukemia cell mediator release assay with enhanced allergen stimulation by additional cross-linking with birch pollen-specific IgG antibodies. The obtained IgE mAbs were characterized by immunologic methods and by cDNA sequencing.

RESULTS

Seven IgE mAbs specific for the birch pollen allergens Bet v 1 or Bet v 6 were obtained and were all biologically active in mast cell-based assays. Mediator release experiments with mAb combinations indicated that 2 different epitope regions were recognized on Bet v 1, whereas the 2 Bet v 6-specific mAbs bound to the same epitope region. After sensitization of rat basophilic leukemia cells with IgE mAbs, different amounts of Bet v 1 or Bet v 6 were detected in commercial diagnostic allergen reagents, whereas sensitization with polyclonal IgE resulted in similar allergenic potency of all products.

CONCLUSIONS

IgE mAbs represent promising novel tools for allergen characterization and component-resolved standardization of allergen extracts.

Expression of a targeted lambda 1 light chain gene is developmentally regulated and independent of Ig kappa rearrangements

Immunoglobulin light chain (IgL) rearrangements occur more frequently at Ig kappa than at Ig lambda. Previous results suggested that the unrearranged Ig kappa locus negatively regulates Ig lambda transcription and/or rearrangement. Here, we demonstrate that expression of a VJ lambda 1-joint inserted into its physiological position in the Ig lambda locus is independent of Ig kappa rearrangements. Expression of the inserted VJ lambda 1 gene segment is developmentally controlled like that of a VJ kappa-joint inserted into the Ig kappa locus and furthermore coincides developmentally with the occurrence of Ig kappa rearrangements in wild-type mice. We conclude that developmentally controlled transcription of a gene rearrangement in the Ig lambda locus occurs in the presence of an unrearranged Ig kappa locus and is therefore not negatively regulated by the latter. Our data also indicate light chain editing in approximately 30% of lambda 1 expressing B cell progenitors.

Block in development at the pre-B-II to immature B cell stage in mice without Ig kappa and Ig lambda light chain

Silencing individual C (constant region) lambda genes in a kappa(-/-) background reduces mature B cell levels, and L chain-deficient (lambda(-/-)kappa(-/-)) mice attain a complete block in B cell development at the stage when L chain rearrangement, resulting in surface IgM expression, should be completed. L chain deficiency prevents B cell receptor association, and L chain function cannot be substituted (e.g., by surrogate L chain). Nevertheless, precursor cell levels, controlled by developmental progression and checkpoint apoptosis, are maintained, and B cell development in the bone marrow is fully retained up to the immature stage. L chain deficiency allows H chain retention in the cytoplasm, but prevents H chain release from the cell, and as a result secondary lymphoid organs are B cell depleted while T cell levels remain normal.

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.

Haplotype exclusion and receptor editing: irreconcilable differences?

Features of antibody genes and their regulation hinder two properties thought to be critical for clonal selection: haplotype exclusion and receptor diversity. These properties include: (1) the retention of multiple independent L-chain isotypes, which compounds the problem of allelic exclusion with one of isotype exclusion; (2) the process of receptor editing, in which recombination continues in cells already expressing antigen receptors; and (3) non-random associations and quasi-ordered rearrangements of the elements that generate light chain genes, which promote editing at the expense of allelic exclusion and receptor diversification. In contrast, heavy chain gene structure seems to promote haplotype exclusion and receptor diversity. It appears that requirements of receptor selection, such as the need for receptor editing as an immune tolerance mechanism and positive selection as a quality control checkpoint for receptor functionality, impose independent selections that shape the organization and regulation of the antibody genes. Despite these features, B cell development still achieves a significant level of phenotypic haplotype exclusion, suggesting that there is indeed significant selection for antibody monospecificity that is accommodated along with receptor editing. Thus, the immune system achieves both receptor selection and clonal selection, despite their partly antagonistic mechanisms.

Differential surrogate light chain expression governs B-cell differentiation

Surrogate light chain expression during B lineage differentiation was examined by using indicator fluorochrome-filled liposomes in an enhanced immunofluorescence assay. Pro-B cells bearing surrogate light chain components were found in mice, but not in humans. A limited subpopulation of relatively large pre-B cells in both species expressed pre-B cell receptors. These cells had reduced expression of the recombinase activating genes, RAG-1 and RAG-2. Their receptor-negative pre-B cell progeny were relatively small, expressed RAG-1 and RAG-2, and exhibited selective down-regulation of VpreB and λ5expression. Comparative analysis of the 2 pre-B cell subpopulations indicated that loss of the pre-B cell receptors from surrogate light chain gene silencing was linked with exit from the cell cycle and light chain gene rearrangement to achieve B-cell differentiation.

A point mutation in the constant region of Ig lambda1 prevents normal B cell development due to defective BCR signaling

Surface expression of B cell antigen receptors (BCRs) containing Ig and Igalpha/Igbeta generates signals required to transit discrete developmental checkpoints. The mechanism by which BCR components collaborate to initiate signals is still unclear. The expression of Iglambda1 in SJL mice is 50-fold lower than in other strains. Here, we demonstrate by gene targeting that a point mutation, which changes a glycine to a valine in the lambda1 constant region, is responsible for this defect. In vitro experiments show that Ig receptors bearing this mutation, while expressed normally, are deficient in signaling. These findings reveal a direct involvement of the Ig light chain (IgL) in B cell signaling and development beyond the requirement of light chains for BCR assembly.

Interference with immunoglobulin (Ig)alpha immunoreceptor tyrosine-based activation motif (ITAM) phosphorylation modulates or blocks B cell development, depending on the availability of an Igbeta cytoplasmic tail

To determine the function of immunoglobulin (Ig)alpha immunoreceptor tyrosine-based activation motif (ITAM) phosphorylation, we generated mice in which Igalpha ITAM tyrosines were replaced by phenylalanines (Igalpha(FF/FF)). Igalpha(FF/FF) mice had a specific reduction of B1 and marginal zone B cells, whereas B2 cell development appeared to be normal, except that lambda1 light chain usage was increased. The mutants responded less efficiently to T cell-dependent antigens, whereas T cell-independent responses were unaffected. Upon B cell receptor ligation, the cells exhibited heightened calcium flux, weaker Lyn and Syk tyrosine phosphorylation, and phosphorylation of Igalpha non-ITAM tyrosines. Strikingly, when the Igalpha ITAM mutation was combined with a truncation of Igbeta, B cell development was completely blocked at the pro-B cell stage, indicating a crucial role of ITAM phosphorylation in B cell development.

Germ-line transcripts of the immunoglobulin lambda J-C clusters in the mouse: characterization of the initiation sites and regulatory elements

Transcription of unrearranged immunoglobulin gene segments strongly correlates with their accessibility to the V(D)J recombination machinery. The regulatory mechanisms governing this germ-line transcription are still poorly defined. In order to identify new regulatory elements, we first carried out a detailed characterization of the transcription initiation sites for the J-C germ-line transcripts, using rapid amplification of 5' cDNA ends, assisted by a template switching mechanism at the 5'-end of the RNA. Transcripts were observed that initiated heterogeneously, starting up to 293 (lambda1), 116 bp (lambda2) and 79 bp (lambda3) upstream from the respective Jlambda gene segment. Additional RT-PCR analysis revealed the existence of germ-line transcripts of lambda and also of kappa that initiate even more upstream of these transcription initiation sites, although their frequencies were low. Promoter activity was detected in vitro 5' of Jlambda2, with the minimal promoter activity mapping to the region between positions -35 and -120. In addition, computer analysis allowed the prediction of a nuclear scaffold/matrix attachment (S/MAR) region between the two J-C gene clusters at each hemi-locus. This region between the lambda1/lambda3 clusters binds to the nuclear matrix in vitro, and J-C lambda1 germ-line transcription initiates a short distance downstream from this S/MAR element.

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.

Bruton's tyrosine kinase regulates the activation of gene rearrangements at the lambda light chain locus in precursor B cells in the mouse

Bruton's tyrosine kinase (Btk) is a nonreceptor tyrosine kinase involved in precursor B (pre-B) cell receptor signaling. Here we demonstrate that Btk-deficient mice have an ∼50% reduction in the frequency of immunoglobulin (Ig) λ light chain expression, already at the immature B cell stage in the bone marrow. Conversely, transgenic mice expressing the activated mutant Btk^E41K showed increased λ usage. As the κ/λ ratio is dependent on (a) the level and kinetics of κ and λ locus activation, (b) the life span of pre-B cells, and (c) the extent of receptor editing, we analyzed the role of Btk in these processes. Enforced expression of the Bcl-2 apoptosis inhibitor did not alter the Btk dependence of λ usage. Crossing 3-83μδ autoantibody transgenic mice into Btk-deficient mice showed that Btk is not essential for receptor editing. Also, Btk-deficient surface Ig⁺ B cells that were generated in vitro in interleukin 7-driven bone marrow cultures manifested reduced λ usage. An intrinsic defect in λ locus recombination was further supported by the finding in Btk-deficient mice of reduced λ usage in the fraction of pre-B cells that express light chains in their cytoplasm. These results implicate Btk in the regulation of the activation of the λ locus for V(D)J recombination in pre-B cells.

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.

Insertion of phosphoglycerine kinase (PGK)-neo 5' of Jlambda1 dramatically enhances VJlambda1 rearrangement

Gene-targeted mice were generated with a loxP-neomycin resistance gene (neo(r)) cassette inserted upstream of the Jlambda1 region and replacement of the glycine 154 codon in the Clambda1 gene with a serine codon. This insertion dramatically increases Vlambda1-Jlambda1 recombination. Jlambda1 germline transcription levels in pre-B cells and thymus cells are also greatly increased, apparently due to the strong housekeeping phosphoglycerine kinase (PGK) promoter driving the neo gene. In contrast, deletion of the neo gene causes a significant decrease in VJlambda1 recombination to levels below those in normal mice. This reduction is due to the loxP site left on the chromosome which reduces the Jlambda1 germline transcription in cis. Thus, the correlation between germline transcription and variable (V), diversity (D), and joining (J) recombination is not just an all or none phenomenon. Rather, the transcription efficiency is directly associated with the recombination efficiency. Furthermore, Jlambda1 and Vlambda1 germline transcription itself is not sufficient to lead to VJ recombination in T cells or early pre-B cells. The findings may suggest that in vivo: (a) locus and cell type-specific transactivators direct the immunoglobulin or T cell receptor loci, respectively, to a "recombination factory" in the nucleus, and (b) transcription complexes deliver V(D)J recombinase to the recombination signal sequences.