Crosslinking of the cell surface immunoglobulin (mu-surrogate light chains complex) on pre-B cells induces activation of V gene rearrangements at the immunoglobulin kappa locus

Discovery and Function of B-Cell IgD Low (BDL) B Cells in Immune Tolerance

It is now appreciated that in addition to their role in humoral immunity, B cells also exert regulatory mechanisms that lead to attenuation of inflammatory responses. The concept of B-cell regulation became well recognized when mice deficient in B cells due to genetic disruption were shown to be refractory to recovery from the signs of experimental autoimmune encephalomyelitis (EAE), the mouse model of multiple sclerosis. This seminal study spurred the search for B-cell regulatory phenotypes and mechanisms of action. Our approach was to utilize differential B-cell depletion with anti-CD20 to retain B cells whose presence were required to achieve EAE recovery. Utilizing flow cytometry, adoptive cell therapy and genetic approaches, we discovered a new B-cell subset that, upon adoptive transfer into B cell-deficient mice, was sufficient to promote EAE recovery. This B-cell subset is IgM⁺, but due to low/negative IgD cell surface expression, it was named B-cell IgD low (BD_L). Mechanistically, we found that in the absence of BD_L, the absolute cell number of CD4⁺Foxp3⁺ T regulatory cells (Treg), essential for immune tolerance, was significantly reduced. Furthermore, we found that BD_L expression of glucocorticoid-induced tumor necrosis factor ligand (GITRL) was essential for induction of Treg proliferation and maintenance of their homeostasis. Thus, we have identified a new B-cell subset that is critical for immunological tolerance through interactions with Treg.

The study of B cells and antibodies in Japan: a historical perspective

Japanese scientists were involved in pioneering work on therapeutic antisera and have made huge contributions to the characterization of the antibody molecules that are responsible for this and many other biological activities, as well as working back to understand the B cells that produce these Igs. This review emphasizes the role of Japanese immunologists in this field, starting with their work in developing antisera and studying the structure of Igs. It describes the molecular mechanisms that generate the enormous antibody repertoire and regulate B-cell development and signaling. It also details the importance of the germinal center in generating B-cell memory and the terminal differentiation of B cells as antibody-secreting plasma cells.

A unique role for the lambda5 nonimmunoglobulin tail in early B lymphocyte development

Precursor BCR (pre-BCR) signaling governs proliferation and differentiation of pre-B cells during B lymphocyte development. However, it is controversial as to which parts of the pre-BCR, which is composed of Igmu H chain, surrogate L chain (SLC), and Igalpha-Igbeta, are important for signal initiation. Here, we show in transgenic mice that the N-terminal non-Ig-like (unique) tail of the surrogate L chain component lambda5 is critical for enhancing pre-BCR-induced proliferation signals. Pre-BCRs with a mutated lambda5 unique tail are still transported to the cell surface, but they deliver only basal signals that trigger survival and differentiation of pre-B cells. Further, we demonstrate that the positively charged residues of the lambda5 unique tail, which are required for pre-BCR self-oligomerization, can also mediate binding to stroma cell-associated self-Ags, such as heparan sulfate. These findings establish the lambda5 unique tail as a pre-BCR-specific autoreactive signaling motif that could increase the size of the primary Ab repertoire by selectively expanding pre-B cells with functional Igmu H chains.

The Ig-alpha ITAM is required for efficient differentiation but not proliferation of pre-B cells

Signals from the pre-B cell receptor (pre-BCR) mediated by the cytoplasmic tails of Ig-alpha/Ig-beta are essential for developing B cells. To analyze the role of Ig-alpha ITAM and non-ITAM tyrosines in pre-BCR signaling, we reconstituted individual tyrosine mutants of Ig-alpha in src homology 2 domain-containing leukocyte protein of 65 kDa (SLP-65)/Ig-alpha double-deficient pre-B cells. We show that the Ig-alpha mutants led to comparable pre-BCR expression on the cell surface, while the pre-BCR-induced tyrosine phosphorylation was different. We further show that the reconstitution of Ig-alpha and the resulting pre-BCR expression led to enrichment of the pre-BCR-expressing cells in vitro irrespective of the introduced Ig-alpha mutation. We show that, even though the enrichment rate increased by lowering the IL-7 concentration, residual amounts of IL-7 were required for optimal enrichment. Our results indicate that surface IL-7 receptor expression is modulated by the pre-BCR, thereby increasing the IL-7 sensitivity of the respective cells. In contrast to the comparable pre-B cell proliferation, however, the Ig-alpha mutants differed in their capacity to induce calcium flux and activate efficient pre-B cell differentiation. Together, our data suggest that ITAM tyrosines and Y204 are required for efficient pre-B cell differentiation but not proliferation.

Mechanism and control of V(D)J recombination at the immunoglobulin heavy chain locus

V(D)J recombination assembles antigen receptor variable region genes from component germline variable (V), diversity (D), and joining (J) gene segments. For B cells, such rearrangements lead to the production of immunoglobulin (Ig) proteins composed of heavy and light chains. V(D)J is tightly controlled at the Ig heavy chain locus (IgH) at several different levels, including cell-type specificity, intra- and interlocus ordering, and allelic exclusion. Such controls are mediated at the level of gene segment accessibility to V(D)J recombinase activity. Although much has been learned, many long-standing questions regarding the regulation of IgH locus rearrangements remain to be elucidated. In this review, we summarize advances that have been made in understanding how V(D)J recombination at the IgH locus is controlled and discuss important areas for future investigation.

The κ-like pre-B receptor: Surplus biology or a missing link?

After the demonstration that surrogate JCkappa polypeptides could covalently bind mu heavy chain and upon the characterization of the Vkappa-like component of the kappa-like pre-B cell receptor, it became evident that germline transcription is not sterile. The present review discusses the concept of the alternative usage of kappa-like pre-B cell receptors and classical pre-B cell receptors utilizing the lambda-like surrogate light chain composed of lambda5 and VpreB. We propose that both kappa-like and lambda-like pre-B cell receptors work in concert in a fail-safe mechanism to promote light chain rearrangement, heavy chain allelic exclusion and B-lymphocyte maturation.

Basal Igalpha/Igbeta signals trigger the coordinated initiation of pre-B cell antigen receptor-dependent processes

The pro-B to pre-B transition during B cell development is dependent upon surface expression of a signaling competent pre-B cell Ag receptor (pre-BCR). Although the mature form of the BCR requires ligand-induced aggregation to trigger responses, the requirement for ligand-induced pre-BCR aggregation in promoting B cell development remains a matter of significant debate. In this study, we used transmission electron microscopy on murine primary pro-B cells and pre-B cells to analyze the aggregation state of the pre-BCR. Although aggregation can be induced and visualized following cross-linking by Abs to the pre-BCR complex, our analyses indicate that the pre-BCR is expressed on the surface of resting cells primarily in a nonaggregated state. To evaluate the degree to which basal signals mediated through nonaggregated pre-BCR complexes can promote pre-BCR-dependent processes, we used a surrogate pre-BCR consisting of the cytoplasmic regions of Igalpha/Igbeta that is targeted to the inner leaflet of the plasma membrane of primary pro-B cells. We observed enhanced proliferation in the presence of low IL-7, suppression of V(H)(D)J(H) recombination, and induced kappa light (L) chain recombination and cytoplasmic kappa L chain protein expression. Interestingly, Igalpha/Igbeta-mediated allelic exclusion was restricted to the B cell lineage as we observed normal TCRalphabeta expression on CD8-expressing splenocytes. This study directly demonstrates that basal signaling initiated through Igalpha/Igbeta-containing complexes facilitates the coordinated control of differentiation events that are associated with the pre-BCR-dependent transition through the pro-B to pre-B checkpoint. Furthermore, these results argue that pre-BCR aggregation is not a requirement for pre-BCR function.

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.

Positive selection focuses the VH12 B-cell repertoire towards a single B1 specificity with survival function

B cells of varying antigen specificities are consistently present in the unmanipulated repertoire. These B cells appear to belong to the marginal zone (MZ) and B1 B-cell subsets and provide protection to the blood and lymph, respectively. Some are specific for self-antigens, suggesting that they are selected based on specificity for self but have a protective role against foreign pathogens. One of these specificities is for phosphatidylcholine (PtC). Anti-PtC B cells comprise 5-8% of the B1 repertoire and are protective against bacterial pathogens. In general, they are restricted to the expression of two VH/Vkappa combinations, VH11/Vkappa9 and VH12/Vkappa4/5H. This review focuses on the differentiation of VH12 anti-PtC B cells. They undergo a series of positive selection events beginning at the pre-B-cell stage that enriches for those with a VHCDR3 and L chain required for PtC binding and eliminating the majority of VH12 B cells that lack the ability to bind PtC. Thus, positive selection focuses the VH12 repertoire toward PtC, ensuring that anti-PtC VH12 B cells are a significant component of the B1-cell repertoire in all individuals.

LAT links the pre-BCR to calcium signaling

SLP-65(-/-) mice display a partial block at the pre-B cell stage of development. Here, we show that LAT is required for the differentiation of SLP-65(-/-) pre-B cells. We show that LAT and SLP-76 are recruited to the pre-BCR and associated with Ig-alpha upon pre-BCR engagement, whereas LAT interaction with SLP-76 is already detected in untreated pre-B cells. Reconstitution of LAT or SLP-65 expression in SLP-65/LAT(-/-) pre-B cells restored their calcium (Ca2+) mobilization capacity, led to downregulation of surface pre-BCR, and induced differentiation to BCR+ cells. Together, our results suggest that the adaptor proteins LAT and SLP-76 are involved in pre-BCR signaling, thereby rescuing arrested murine SLP-65(-/-) pre-B cells.

Molecular mechanism of serial VH gene replacement

The molecular mechanism of serial VH replacement was analyzed using a human B cell line, EU12, that undergoes continuous spontaneous differentiation from pro-B to pre-B and then to B cell stage. In earlier studies, we found that this cell line undergoes intraclonal V(D)J diversification. Analysis of the IgH gene sequences in EU12 cells predicted the occurrence of serial VH replacement involving the cryptic recombination signal sequences (cRSS) embedded within framework 3 regions and concurrent extension of the CDR3 region. Detection of double-stranded DNA breaks at the cRSS site and different VH replacement excision circles confirm the ongoing nature of this diversification process in the EU12 cells. In vitro binding and cleavage assays using recombinant RAG-1 and RAG-2 proteins further validated the cRSS participation in this RAG-mediated recombination process. Serial VH replacements may represent an additional mechanism for diversification of the primary B cell repertoire.

The variable, C(H)1, C(H)2 and C(H)3 domains of Ig heavy chain are dispensable for pre-BCR function in transgenic mice

The pre-BCR consists of Ig micro protein, the product of a heavy chain gene assembled by V(D)J recombination in pro-B cells, the surrogate light chains V(pre-B) and lambda 5, and the signaling chains Ig alpha and Ig beta. Signaling by the pre-BCR is a checkpoint required for further maturation of pro-B cells in the adult bone marrow. However, it is currently not known whether an extracellular ligand is required to initiate pre-BCR signaling. We reasoned that if the ectodomain of the pre-BCR is required to interact with a ligand, then a truncated heavy chain protein would not support B cell development. To test this notion, we produced transgenic mice expressing a heavy chain protein whose extracellular domains except for C(H)4 were replaced by an irrelevant Ig superfamily ectodomain from the human CD8 alpha protein. This transgene resulted in pre-BCR-like signaling since it rescued development of pre-B cells in recombinase-activating gene (RAG)1-deficient mice and resulted in allelic exclusion of the endogenous Ig heavy chain gene in RAG-proficient mice. These findings lead us to suggest that the majority of the extracellular region of the pre-BCR is not required for pre-BCR function and, thus, ligand binding is unlikely to be required for pre-BCR function.

The pre-B cell receptor signaling for apoptosis is negatively regulated by Fc gamma RIIB

Many studies have shown that FcgammaRIIB is a negative regulator of B cell receptor signaling, and even though FcgammaRIIB is expressed through all developmental stages of the B cell lineage, its involvement in pre-B cell receptor (pre-BCR) signaling has not been examined. To investigate FcgammaRIIB function at the pre-B cell stage, we have established pre-BCR positive pre-B cell lines from normal mice and FcgammaRIIB-deficient mice, named PreBR and Fcgamma(-/-)PreBR, respectively. These cell lines are able to differentiate into immature B cells in vitro by removal of IL-7. In PreBR, apoptosis was moderately induced by F(ab')(2) anti-mu Ab, but not by intact anti-mu Ab. Phosphorylation of SH2-containing inositol 5-phosphatase (SHIP) and Dok, which are involved in FcgammaRIIB signaling, was induced by anti-mu cross-linking in PreBR. In contrast, apoptosis was strongly induced by both the F(ab')(2) and intact anti-mu Abs in Fcgamma(-/-)PreBR, and the level of phosphorylation of SHIP or Dok was much lower in Fcgamma(-/-)PreBR than those observed in PreBR. Restoration of FcgammaRIIB to Fcgamma(-/-)PreBR followed by anti-mu cross-linking blocked severe apoptosis, and up-regulated SHIP and Dok phosphorylation. The results demonstrate that FcgammaRIIB negatively regulates pre-BCR-mediated signaling for apoptosis.

Truncated immunoglobulin Dmu causes incomplete developmental progression of RAG-deficient pro-B cells

Early stages of B cell development are dependent on the expression of a pre-B cell receptor (BCR), composed of a mu heavy chain (HC) in association with surrogate light chain (SLC) proteins and the signaling molecules, Igalpha and Igbeta. During the formation of the variable region of the mu chain by somatic gene rearrangement, a truncated form of the mu protein (called Dmu) is sometimes produced by the rearrangement of a D(H) segment to a J(H) segment using one of three reading frames (designated rf2). When a Dmu protein is formed, subsequent B cell development is blocked by down-regulation of further HC rearrangements, so that a full-length muHC cannot be formed. In this study, we demonstrate that in recombinase activating gene (RAG)-2-deficient B220(+) CD43(+) pro-B cells in which B lymphopoiesis has been arrested at fraction C, transgenic expression of Dmu promoted partial developmental progression to fraction C', but was unable to mediate the pro-B to pre-B cell transition to fraction D effected by full-length muHC protein. These data suggest that the intracellular signaling pathways engaged by the Dmu pre-BCR are insufficient to facilitate the expansion and/or survival of pre-B cells, and are distinct from those engaged by the pre-BCR-containing full-length muHC.

Truncation of the mu heavy chain alters BCR signalling and allows recruitment of CD5+ B cells

Ig are multifunctional molecules with distinct properties assigned to individual domains. To assess the importance of IgM domain assembly in B cell development we generated two transgenic mouse lines with truncated muH chains by homologous integration of the neomycin resistance gene (neo(r)) into exons C(mu)1 and C(mu)2. Upon DNA rearrangement shortened muH chain transcripts, V(H)-D-J(H)-C(mu)3-C(mu)4, are produced independent of the transcriptional orientation and termination signals provided by neo(r). The truncated muH chain of approximately 52 kDa associates non-covalently with the L chain to form a monovalent HL heterodimer. Surface IgM is assembled into a defective BCR complex which has lost important signalling capacity. In immunizations with T-dependent and T-independent antigens, specific IgM antibodies cannot be detected, whilst IgG responses remain normal. B cell development in the bone marrow is characterized by an increase in early B cells, but a decrease of B220(+) cells from the stage when muH chain rearrangement is completed. The peritoneal lymphocyte population has elevated levels of CD5(+) B cells and their expansion may be the result of a negative feedback mechanism. The results show that antigenic stimulation is compromised by truncated monovalent IgM and that this deficit in stimulation leads to reduced levels of conventional B-2 lymphocytes, but dramatically increased levels of B-1 cells.

Surrogate light chain-mediated interaction of a soluble pre-B cell receptor with adherent cell lines

Signals initiated by the precursor B cell receptor (pre-BCR) are critical for B cell progenitors to mature into precursor B cells. The pre-BCR consists of a homodimer of microH chains, the covalently associated surrogate L (SL) chain composed of VpreB and lambda5, and the transmembrane signal molecules Ig(alpha) and Igbeta. One way to explain how maturation signals are initiated in late progenitor B cells is that the pre-BCR is transported to the cell surface and interacts from there with a ligand on stroma cells. To address this hypothesis, we first produced soluble Fab-like pre-BCR and BCR fragments, as well as SL chain, in baculovirus-infected insect cells. Flow cytometry revealed that, in contrast to Fab-like BCR fragments, the soluble pre-BCR binds to the surface of stroma and several other adherent cell lines, but not to B and T lymphoid suspension cells. The specific binding of the soluble pre-BCR to stroma cells is saturable, sensitive to trypsin digestion, and not dependent on bivalent cations. The binding of pre-BCR seems to be independent of the H chain of IgM (microH chain), because SL chain alone was able to interact with stroma cells. Finally, soluble pre-BCR specifically precipitated a 135-kDa protein from ST2 cells. These findings not only demonstrate for the first time the capacity of a pre-BCR to specifically bind to a structure on the surface of adherent cells, but also suggest that the pre-BCR interacts via its SL chain with a putative ligand on stroma cells.

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.

Dissociation of Pax-5 from KI and KII sites during kappa-chain gene rearrangement correlates with its association with the underphosphorylated form of retinoblastoma

The KI and KII sites play a crucial role in kappa-chain gene rearrangement, which was investigated in mice deficient for these sites. Previously, we found that Pax-5 can bind to the KI and KII sites; however, the function of Pax-5 in kappa-chain gene rearrangement has not been investigated. Here, we have used an in vitro culture system in which differentiation from pre-B cells to immature B cells is induced by removing IL-7. We showed that, after the induction of differentiation, Pax-5 dissociated from the KI and KII revealed by EMSA analyses, and this dissociation occurred specifically at the KI and KII sites, but not at the Pax-5 binding site, in the CD19 promoter because of a lower binding affinity of Pax-5 for the KI and KII sites. During differentiation induced by removing IL-7, the underphosphorylated form of retinoblastoma preferentially associated with Pax-5, which caused dissociation of Pax-5 from KI and KII sites. These results suggest that the dissociation of Pax-5 from the KI and KII sites is important in the induction of kappa-chain gene rearrangement.

A VH12 transgenic mouse exhibits defects in pre-B cell development and is unable to make IgM+ B cells

V(H)12 B cells undergo stringent selection at multiple checkpoints to favor development of B-1 cells that bind phosphatidylcholine. Selection begins with the V(H) third complementarity-determining region (CDR3) at the pre-B cell stage, in which most V(H)12 pre-B cells are selectively eliminated, enriching for those with V(H)CDR3s of 10 aa and a fourth position Gly (designated 10/G4). To understand this selection, we compared B cell differentiation in mice of two V(H)12 transgenic lines, one with the favored 10/G4 V(H)CDR3 and one with a non-10/G4 V(H)CDR3 of 8 aa and no Gly (8/G0). Both H chains drive B cell differentiation to the small pre-BII cell stage, and induce allelic exclusion and L chain gene rearrangement. However, unlike 10/G4 pre-B cells, 8/G0 pre-B cells are deficient in cell division and unable to differentiate to B cells. We suggest that this is due to poor 8/G0 pre-B cell receptor expression and to an inability to form an 8/G0 B cell receptor. Our findings also suggest that V(H)12 H chains have evolved such that association with surrogate and conventional L chains is most efficient with a 10/G4 CDR3. Thus, selection for phosphatidylcholine-binding B-1 cells is most likely the underlying evolutionary basis for the loss of non-10/G4 pre-B cells.

The effects of the Epstein-Barr virus latent membrane protein 2A on B cell function

Epstein-Barr Virus (EBV) infects B-lymphocytes circulating through the oral epithelium and establishes a lifelong latent infection in a subset of mature-memory B cells. In these latently infected B cells, EBV exhibits limited gene expression with the latent membrane protein 2A (LMP2A) being the most consistently detected transcript. This persistent expression, coupled with many studies ofthe function of LMP2A in vitro and invivo, indicates that LMP2A is functioning to control some aspect of viral latency. Establishment and maintenance of viral latency requires exquisite manipulation of normal B cell signaling and function. LMP2A is capable of blocking normal B cell signal transduction in vitro, suggesting that LMP2A may act to regulate lytic activation from latency in vivo. Furthermore, LMP2A is capable of providing B cells with survival signals in the absence of normal BCR signaling. These data show that LMP2A may help EBV-infected cells to persist in vivo. This review discusses the advances that have been made in our understanding of LMP2A and the effects it has on B cell development, activation, and viral latency.

Immunoglobulin beta signaling regulates locus accessibility for ordered immunoglobulin gene rearrangements

The antigen receptor gene rearrangement at a given locus is tightly regulated with respect to cell lineage and developmental stage by an ill-defined mechanism. To study the possible role of precursor B cell antigen receptor (pre-BCR) signaling in the regulation of the ordered immunoglobulin (Ig) gene rearrangement during B cell differentiation, a newly developed system using mu heavy (H) chain membrane exon (microm)-deficient mice was employed. In this system, the antibody-mediated cross-linking of Igbeta on developmentally arrested progenitor B (pro-B) cells mimicked pre-BCR signaling to induce early B cell differentiation in vivo. Analyses with ligation-mediated polymerase chain reaction revealed that the Igbeta cross-linking induced the redirection of Ig gene rearrangements, namely, the suppression of ongoing rearrangements at the H chain locus and the activation of rearrangements at the light (L) chain locus. Upon the cross-linking, the kappaL chain germline transcription was found to be upregulated whereas the V(H) germline transcription was promptly downregulated. Notably, this alteration of the accessibility at the H and L chain loci was detected even before the induction of cellular differentiation became detectable by the change of surface phenotype. Thus, the pre-BCR signaling through Igbeta appears to regulate the ordered Ig gene rearrangement by altering the Ig locus accessibility.

Inducible differentiation and apoptosis of the pre-B cell receptor-positive pre-B cell line

The function of the pre-B cell receptor (pre-BCR) during B cell differentiation is not precisely defined. To investigate the pre-BCR receptor activity, we have established pre-BCR-positive pre-B cell lines that are able to differentiate into immature B cells in vitro. Antibody cross-linking of the pre-BCR induced apoptosis and differentiation accompanied with tyrosine phosphorylation. A specific tyrosine-phosphorylated 43 kDa protein (p43) was found down-stream of the pre-BCR. The results demonstrated the receptor function of pre-BCR, which indicates that a ligand-like molecule or a cross-linking structure on the cell surface is possibly present.

Receptor selection in B and T lymphocytes

The process of clonal selection is a central feature of the immune system, but immune specificity is also regulated by receptor selection, in which the fate of a lymphocyte's antigen receptor is uncoupled from that of the cell itself. Whereas clonal selection controls cell death or survival in response to antigen receptor signaling, receptor selection regulates the process of V(D)J recombination, which can alter or fix antigen receptor specificity. Receptor selection is carried out in both T and B cells and can occur at different stages of lymphocyte differentiation, in which it plays a key role in allelic exclusion, positive selection, receptor editing, and the diversification of the antigen receptor repertoire. Thus, the immune system takes advantage of its control of V(D)J recombination to modify antigen receptors in such a way that self/non-self discrimination is enhanced. New information about receptor editing in T cells and B-1 B cells is also discussed.

B-cell-receptor-dependent positive and negative selection in immature B cells

This review touches on only a small part of the complex biology of B cells, but serves to illustrate the point that the antigen receptor is the most important of many cell-surface receptors affecting cell-fate decisions. Receptor expression is necessary, but not sufficient, for cell survival. It is also essential that a B cell's antigen-receptor specificity be appropriate for its environment. The need to balance reactivity with self tolerance has resulted in an intricate feedback control (affected by both the recombinase and cell survival) that regulates independent selection events at the level of the receptor and the cell.

Mutations in Igalpha (CD79a) result in a complete block in B-cell development

Mutations in Btk, mu heavy chain, or the surrogate light chain account for 85-90% of patients with early onset hypogammaglobulinemia and absent B cells. The nature of the defect in the remaining patients is unknown. We screened 25 such patients for mutations in genes encoding components of the pre-B-cell receptor (pre-BCR) complex. A 2-year-old girl was found to have a homozygous splice defect in Igalpha, a transmembrane protein that forms part of the Igalpha/Igbeta signal-transduction module of the pre-BCR. Studies in mice suggest that the Igbeta component of the pre-BCR influences V-DJ rearrangement before cell-surface expression of mu heavy chain. To determine whether Igalpha plays a similar role, we compared B-cell development in an Igalpha-deficient patient with that seen in a mu heavy chain-deficient patient. By immunofluorescence, both patients had a complete block in B-cell development at the pro-B to pre-B transition; both patients also had an equivalent number and diversity of rearranged V-DJ sequences. These results indicate that mutations in Igalpha can be a cause of agammaglobulinemia. Furthermore, they suggest that Igalpha does not play a critical role in B-cell development until it is expressed, along with mu heavy chain, as part of the pre-BCR.

The C(H)1 and transmembrane domains of mu in the context of a gamma2b transgene do not suffice to promote B cell maturation

Mice carrying a gamma2b transgene have been shown previously to be deficient in B cell development. In particular, a developmental block exists at the pre-B cell stage. The few B cells that develop all express endogenous micro heavy chains. The phenotype suggests that gamma2b exerts a strong feedback inhibition on endogenous Ig gene rearrangement, but, unlike micro, cannot support further B cell development. In this study we have created hybrid transgenes between gamma2b and micro. Transgenic mice with a C(H)1 domain of micro, or both a C(H)1 and transmembrane/cytoplasmic domain of micro replacing the respective domains of a gamma2b transgene, have the same B cell defect as gamma2b transgenic mice. Interestingly, the severity of the defect is correlated with the level of expression of the transgene, suggesting that the degree of feedback inhibition of Ig gene rearrangement depends on the level and timing of Ig production. Crossing the gamma2b/micro transgenes into a Bcl-x(L) transgenic line allows immature gamma2b B cells to survive, but not to develop to maturity. Therefore, the missing function of micro is not simply an anti-apoptotic effect.

Lambda5 is required for rearrangement of the Ig kappa light chain gene in pro-B cell lines

Lambda5 associates with V(pre-B) to form the surrogate light (L) chain. The phenotype of lambda5 knockout mice showed severe impairment of B cell development from pro-B to immature B cell stages. To investigate the function of the surrogate L chain at this stage, we restored expression of lambda5 to lambda5-deficient pro-B cell lines which were established from bone marrow cells of lambda5 knockout mice in the presence of IL-7 and a stromal cell line. Some of these lines are severely impaired in B cell development from pro-B to immature B cell stages as is seen in vivo in lambda5 knockout mice. Restoration of lambda5 protein by retroviral-mediated gene transfer into established lambda5-deficient pro-B cell lines induced rearrangement of the Ig kappa L chain genes after removal of IL-7 from the culture. Immunoprecipitation revealed that the restored lambda5 in the cell line is coupled with V(pre-B) to form the surrogate L chain. The results demonstrate that formation of a complete surrogate L chain, consisting of both lambda5 and V(pre-B), stimulates efficient rearrangement of the kappa L chain genes.

Rearrangement and expression of immunoglobulin light chain genes can precede heavy chain expression during normal B cell development in mice

In mouse mutants incapable of expressing mu chains, VkappaJkappa joints are detected in the CD43(+) B cell progenitors. In agreement with these earlier results, we show by a molecular single cell analysis that 4-7% of CD43(+) B cell progenitors in wild-type mice rearrange immunoglobulin (Ig)kappa genes before the assembly of a productive VHDHJH joint. Thus, mu chain expression is not a prerequisite to Igkappa light chain gene rearrangements in normal development. Overall, approximately 15% of the total CD43(+) B cell progenitor population carry Igkappa gene rearrangements in wild-type mice. Together with the results obtained in the mouse mutants, these data fit a model in which CD43(+) progenitors rearrange IgH and Igkappa loci independently, with a seven times higher frequency in the former. In addition, we show that in B cell progenitors VkappaJkappa joining rapidly initiates kappa chain expression, irrespective of the presence of a mu chain.

μ-Surrogate Light Chain Physicochemical Interactions of the Human PreB Cell Receptor: Implications for VH Repertoire Selection and Cell Signaling at the PreB Cell Stage

The surrogate light chain (SL) composed of the λ-like and VpreB polypeptides is organized as two Ig domains and an extra-loop structure. It associates to the μ-chain in preB cells. We have produced human VpreB, SL, two Fdμ (VH-CH1), and the two corresponding Fab-like (Fdμ-SL) recombinant proteins in baculovirus. The correctness of the general conformation of the proteins was assessed by epitope mapping and affinity measurements using a new batch of anti-VpreB mAbs. Plasmon resonance analysis showed that both VpreB and the entire SL associated with the Fdμ fragments, with Kd values of 3 × 10−8 M for VpreB-Fdμ and of 10−9 to 10−10 M, depending upon the VH, for SL-Fdμ. These results indicate that the λ-like chain, in addition to be covalently bound to the Cμ1 domain, also interacts with the VH domain. Therefore, a dual role of the SL emerges: 1) interaction of the C-domain of λ-like would release the μ-chain from its interaction with binding protein in the endoplasmic reticulum, and 2) interaction of a part of λ-like and most of VpreB would bind to VH, ensuring a “quality control” of the native heavy chain that represents the first step of selection of the B cell repertoire. We also demonstrated that two Fab-like fragments did not interact with each other, suggesting that activation of the cell surface preB receptor does not involve aggregation neither in cis nor in trans of the Fab-like structures.

Pre-B Cell Receptor-Mediated Selection of Pre-B Cells Synthesizing Functional μ Heavy Chains

Ig gene rearrangements could generate VH-D-JH joining sequences that interfere with the correct folding of a μ-chain, and thus, its capability to pair with IgL chains. Surrogate light (SL) chain might be the ideal molecule to test the capacity of a μ-chain to pair with a L chain early in development, in that only pre-B cells that assemble a membrane μ-SL complex would be permitted to expand and further differentiate. We have previously identified two SL chain nonpairing VH81X-μ-chains with distinct VH-D-JH joining regions. Here, we show that one of these VH81X-μ-chains does not rescue B cell development in JH knock-out mice, because flow cytometric analysis of bone marrow cells from VH81X-μ transgenic JH knock-out mice revealed normal numbers of pro-B cells, but essentially no pre-B and surface IgM+ B cells. Immunoprecipitation analysis of transfected pre-B and hybridoma lines revealed that the same μ-chain fails to pair not only with SL chain but also with four distinct κ L chains. These findings demonstrate that early pre-B cells are selected for maturation on the basis of the structure of a μ-chain, in particular its VH-D-JH joining or CDR3 sequence, and that one mechanism for this selection is the capacity of a μ-chain to assemble with SL chain. Therefore, we propose a new function of SL chain in early B cell development: SL chain is part of a quality control mechanism that tests a μ-chain for its ability to pair with conventional L chains.

Regulation of B lymphocyte development by the truncated immunoglobulin heavy chain protein Dmu

The development of B lymphocytes from progenitor cells is dependent on the expression of a pre-B cell-specific receptor made up by a mu heavy chain associated with the surrogate light chains, immunoglobulin (Ig)alpha, and Igbeta. A variant pre-B cell receptor can be formed in which the mu heavy chain is exchanged for a truncated mu chain denoted Dmu. To investigate the role of this receptor in the development of B cells, we have generated transgenic mice that express the Dmu protein in cells of the B lineage. Analysis of these mice reveal that Dmu expression leads to a partial block in B cell development at the early pre-B cell stage, probably by inhibiting VH to DHJH rearrangement. Furthermore, we provide evidence that Dmu induces VL to JL rearrangements.

Precursor B cells showing H chain allelic inclusion display allelic exclusion at the level of pre-B cell receptor surface expression

Within the pools of muH chain-producing precursor and mature B cells from normal and lambda5-defective mice, the frequency of cells in which both H chain alleles were productively VHDJH rearranged was determined. An equally high percentage (2%-4%) of cells carrying two productively VHDJH-rearranged H chain loci was found in precursor and mature B cell pools of both mouse strains. In all of these cells, one allele encodes a muH chain incapable of forming a surface-expressed pre-B cell receptor. Hence, allelic exclusion is maintained at the level of pre-B cell receptor expression. The surprising conservation of H chain allelic exclusion in lambda5-defective B cells suggests that an alternative form of pre-B cell receptor might function to ensure this allelic exclusion.

The Ig alpha/Igbeta heterodimer on mu-negative proB cells is competent for transducing signals to induce early B cell differentiation

The immunoglobulin alpha (Ig alpha)/Ig beta heterodimer was detected on the surface of mu-negative proB cell lines in association with calnexin. The cross-linking of Ig beta on proB cells freshly isolated from bone marrow of recombination activating gene (RAG)-2-deficient mice induced a rapid and transient tyrosine-phosphorylation of Ig alpha as well as an array of intracellular proteins including Syk, PI3-kinase, Vav, and SLP-76. It also elicited the phosphorylation and activation of a MAP kinase ERK but not JNK/SAPK or p38. When RAG-2-deficient mice were treated with anti-Ig beta monoclonal antibody, developmentally arrested proB cells were induced to differentiate to the small preB cell stage as observed when the mu transgene was expressed in RAG-2-deficient mice. Thus, the cross-linking of Ig beta on proB cells appears to elicit differentiation signals analogous to those delivered by the preB cell receptor in normal B cell development.

Pax-5 is identical to EBB-1/KLP and binds to the VpreB and lambda5 promoters as well as the KI and KII sites upstream of the Jkappa genes

During B cell differentiation, the pre-B cell stage plays a significant role in immunoglobulin gene rearrangement in the context of the allelic exclusion and kappa chain gene rearrangement. We previously reported that the early B cell-specific binding protein (EBB)-1 transcription factor binds to the promoters of two pre-B cell-specific genes, VpreB and lambda5, and regulates their pre-B cell-specific expression. Here, we demonstrate that EBB-1 binds to the KI and KII sites in the upstream of Jkappa region, which are crucial for kappa chain gene rearrangement. Gene transfer and gel-shift assays demonstrate that EBB-1 is identical to Pax-5 and binds to promoters of VpreB and lambda5 as well as the KI and KII sites. Our results suggest that Pax-5 plays an important role in the coordinate regulation of several immunoglobulin gene family members that are crucial in B cell development.

Constitutive Bcl-2 expression during immunoglobulin heavy chain-promoted B cell differentiation expands novel precursor B cells

To test for effects on B cell differentiation, we introduced immunoglobulin mu heavy chain (HC) and Bcl-2 transgenes, separately or together, into recombination-activating gene 2 (RAG-2)-deficient mice. Transgenic Bcl-2 expression led to increased numbers of RAG-deficient pro-B cells, but did not promote their further differentiation. Expression of the mu HC transgene promoted the differentiation of RAG-deficient pro-B cells into pre-B cells that also expressed certain differentiation markers characteristic of even more mature B cell stages. However, the extent of the mu HC-dependent differentiation effects was greatly enhanced by coexpression of the transgenic Bcl-2 gene, and a subset of pre-B cells from both HC and HC, Bcl-2-transgenic RAG-2-deficient animals expressed surface mu HCs that were functional as judged by cross-linking experiments. These experiments demonstrate that the pro-B to pre-B transition in vivo cannot be effected by the expression of Bcl-2 alone, and that nontransformed immature B-lineage cells are competent to receive signals through a surface mu complex.

Counterselection against D mu is mediated through immunoglobulin (Ig)alpha-Igbeta

The pre-B cell receptor is a key checkpoint regulator in developing B cells. Early events that are controlled by the pre-B cell receptor include positive selection for cells express membrane immunoglobulin heavy chains and negative selection against cells expressing truncated immunoglobulins that lack a complete variable region (D mu). Positive selection is known to be mediated by membrane immunoglobulin heavy chains through Ig alpha-Ig beta, whereas the mechanism for counterselection against D mu has not been determined. We have examined the role of the Ig alpha-Ig beta signal transducers in counterselection against D mu using mice that lack Ig beta. We found that D mu expression is not selected against in developing B cells in Ig beta mutant mice. Thus, the molecular mechanism for counterselection against D mu in pre-B cells resembles positive selection in that it requires interaction between mD mu and Ig alpha-Ig beta.

A prematurely expressed Ig(kappa) transgene, but not V(kappa)J(kappa) gene segment targeted into the Ig(kappa) locus, can rescue B cell development in lambda5-deficient mice

We generated surrogate light chain (SLC)-deficient mice carrying either a V(kappa)J(kappa)-C(kappa) transgene under the control of the kappa promoter and intron enhancer or a V(kappa)J(kappa) gene segment targeted into its physiological position. Efficient rescue of B cell development was seen in the former and partial rescue in the latter. This difference corresponded to a developmentally earlier onset of kappa chain expression from the conventional than from the targeted transgene. Thus, a kappa chain can substitute for SLC in development. However, mechanisms controlling gene expression in addition to gene rearrangements appear to restrict kappa chain expression largely to a cellular compartment into which mu chain-expressing B cell progenitors are selected with the help of the SLC.

Signal transduction through mu kappa B-cell receptors expressed on pre-B cells is different from that through B-cell receptors on mature B cells

We introduced kappa light chain genes into pre-B cells to increase the surface mu HC expression, and established transfectants expressing mature type of B-cell receptors (BCR) on pre-B-cell surfaces. Since the cytoplasmic conformations of the reconstituted BCR and intrinsic pre-B-cell receptor (pre-BCR) are identical, they would be connected with the identical signal transduction pathways in pre-B cells. By using the transfectants, we revealed that the reconstituted BCR on pre-B cells was functionally equivalent to BCR on mature B cells in terms of the induction of intracellular Ca++ mobilization. However, we found that the signal-transduction pathways through BCR on pre-B cells were quantitatively different from those mature B cells in two ways. First, cross-linkage of the reconstituted BCR on pre-B cells induced preferential tyrosine phosphorylation of p120 and p100, which was not observed when BCR on mature B cells was cross-linked. Second, BCR in pre-B cells was physically associated with a larger amount of phosphatidylinositol-3 kinase (PI-3K) than BCR in mature B cells in spite of the fact that both pre-B and B cells expressed a similar amount of PI-3K in cytoplasm. Signals through pre-BCR and BCR are known to cause distinct biological effects in B-cell development. The biochemical features in the downstream of reconstituted BCR on pre-B cells, which we revealed in this study, will be of help in understanding the mechanism of functional differences between pre-BCR and BCR.

Clonal selection and learning in the antibody system

Each antibody-producing B cell makes antibodies of unique specificity, reflecting a series of ordered gene rearrangements which must be successfully performed if the cell is to survive. A second selection process occurs during immune responses in which a new antibody repertoire is generated through somatic hypermutation. Here only mutants binding antigen with high affinity survive to become memory cells. Cells expressing autoreactive receptors are counter-selected at both stages. This stringent positive and negative selection allows the generation and diversification of cells while rigorously controlling their specificity.

A novel anti-Vpre-B antibody identifies immunoglobulin-surrogate receptors on the surface of human pro-B cells

Vpre-B and lambda 5 genes, respectively, encode V-like and C-like domains of a surrogate immunoglobulin light chain (psi L). Such psi L complex is expressed in early progenitor B (pro-B) cells, before conventional immunoglobulin heavy (microH) and light (L) chains are produced. We raised a wide panel of monoclonal antibodies (mAbs) against soluble recombinant Vpre-B proteins to study early events in human B cell development. One of these antibodies, B-MAD688, labeled surrogate Ig-complexes on the surface of microH- pro-B cell lines and normal bone marrow cells in immunofluorescence assays. Immunoprecipitations using surface-labeled pro-B cells and B-MAD688 mAb indicated that human psi L is associated with high molecular weight components homologous to the surrogate heavy (psi H) chains described in mouse. Using B-MAD688 and SLC2 mAbs, we were able to distinguish between psi H psi L and microH psi L complexes on the surface of human pro-B and later precursor, pre-B, cells. The finding of psi H psi L complexes in mouse and man lead us to hypothesize a role for psi H-containing receptors in B cell development.

Fate of surrogate light chains in B lineage cells

Biosynthesis of the immunoglobulin (Ig) receptor components and their assembly were examined in cell lines representative of early stages in human B lineage development. In pro-B cells, the nascent surrogate light chain proteins form a complex that transiently associates in the endoplasmic reticulum with a spectrum of unidentified proteins (40, 60, and 98 kD) and Bip, a heat shock protein family member. Lacking companion heavy chains, the surrogate light chains in pro-B cells do not associate with either the Ig(alpha) or Ig(beta) signal transduction units, undergo rapid degradation, and fail to reach the pro-B cell surface. In pre-B cells, by contrast, a significant portion of the surrogate light chain proteins associate with mu heavy chains, Ig(alpha), and Ig(beta) to form a stable receptor complex with a relatively long half-life. Early in this assembly process, Bip/GRP78, calnexin, GRP94, and a protein of approximately 17 kD differentially bind to the nascent mu heavy chains. The 17-kD intermediate is gradually replaced by the surrogate light chain protein complex, and the Ig(alpha) and Ig(beta) chains bind progressively to the mu heavy chains during the complex and relatively inefficient process of pre-B receptor assembly. The results suggest that, in humans, heavy chain association is essential for surrogate light chain survival and transport to the cell surface as an integral receptor component.

Assembly of the truncated immunoglobulin heavy chain D mu into antigen receptor-like complexes in pre-B cells but not in B cells

Rearrangements of the IgH locus with JH joined to reading frame 2 of DH are greatly underrepresented in B cells. These rearrangements encode the truncated heavy chain D mu. In pre-B cells, we found D mu protein expressed on the cell surface and assembled into a complex with surrogate light chains, Ig alpha, and Ig beta. Cross-linking of either mu m- or D mu m- containing pre-B cell receptors triggered signal transduction reactions. In contrast, when expressed in mature B cell lines, D mu was not detected on the cell surface and did not efficiently bind kappa immunoglobulin light chains, but did associate with Ig alpha and Ig beta. These results characterize the interactions of D mu chain with other components of the B cell antigen receptor complex and suggest possible mechanisms by which D mu expression may interfere with B cell development.

Surrogate light chain expression is required to establish immunoglobulin heavy chain allelic exclusion during early B cell development

Allelic exclusion at the IgH locus was examined in B lineage cells of wild-type mice and mice unable to express the surrogate light chain molecule lambda 5 using a single-cell PCR approach. By analyzing B precursor cells containing two VHDHJH rearrangements, we found that in wild-type animals, cells are allelically excluded as soon as mu chains are expressed. Furthermore, we provide evidence that in cells expressing D mu proteins VH-->DHJH rearrangement is inhibited. In contrast, in the absence of lambda 5 protein, B precursor cells were allelically "included", indicating that allelic exclusion at the IgH locus requires expression of the pre-B cell receptor either containing a mu chain or a D mu chain. However, although mu chain double-producing B precursor cells are generated in lambda 5-deficient mice, such cells were not detected among surface immunoglobulin positive B cells.

Cellular and molecular analysis of lymphoid development using Rag-deficient mice

The establishment of a functional immune system with diverse antigen receptors is dependent on the V(D)J recombination activating gene products Rag-1 and Rag-2. These two proteins constitute the key lymphoid components required for the activation of antigen receptor rearrangement. Both Rag-1 and Rag-2 are required for the catalysis of the initial stages of V(D)J recombination. Thus, functional disruption of either the Rag-1 or Rag-2 genes by homologous recombination, leads to immunodeficiency due to lymphoid arrest at a stage prior to the recombination of the antigen receptor loci. In Rag-deficient mice, both B- and T-cell differentiation is eliminated due to the absence of antigen receptors. Lymphoid development can be restored by the introduction of rearranged antigen receptor transgenes that give rise to monoclonal populations of fully mature B- or T-cells. The absence of the major conventional populations of B- and T-cells from the Rag-deficient mice provided an excellent background for studying the molecular and cellular mechanisms of lymphoid differentiation. The Rag-deficient background has been used as a system for: the functional analysis of Rag-1 and Rag-2; studying the developmental functions of antigen receptors and other molecules of the immune system; the molecular analysis of the early stages of the B- and T-cell lineages; the co-development of lymphocytes with stroma cells; the identification of minor subpopulations of the developing immune system; the involvement of lymphoid populations in the onset of pathogenesis. In addition, the development of the "blastocyst complementation assay" methodology, based on the phenotype of the Rag-/- mice, allowed the functional analysis of numerous lymphoid specific components.

In-vitro analyses of mechanisms of B-cell development

B-cell lymphopoiesis in vivo is very complex due to the influences of cooperating cells, cytokines and other receptor-ligand interactions which appear to occur developmentally at different cellular stages. Therefore in-vitro models will help to unravel this complex situation. Here, we review our and others' work on in-vitro models of B-cell development. The role of stromal cells, cytokines, surrogate light chain and products of rearranged Ig-loci in the developmentally different cellular stages will be discussed.

Positive and negative selection events during B lymphopoiesis

Early in B-cell development, large numbers of cells have to be generated, each of which expresses only one type of B-cell receptor (i.e. Ig) on its surface. This is achieved by the surface expression of a pre-B cell receptor containing a mu heavy chain/surrogate light chain which differentially provides signals for two responses of precursor B cells at this stage of development. On the one hand, it signals inhibition of further rearrangements of variable heavy chain to diverse-joining heavy chain loci to achieve allelic exclusion at the heavy-chain locus. On the other hand, it signals proliferative expansion by factors between 20 and 100. Later in B-cell development, tolerance to autoantigens must be established and maintained. Tolerance is achieved by developmental arrest and induction of secondary light-chain gene rearrangements in those IgM+ immature B cells that are reactive to autoantigens presented in the primary B-cell generating organs. Even later in development, when mature surface (s)IgM+/sIgD+ B cells encounter autoantigens presented to them in the periphery, either deletion or anergy of the autoantigen-reactive cells occurs. Anergic cells have a sIg-dependent, sIg-proximal defect in signaling and are short-lived. Anergy can be broken in vitro by polyclonal activation via ligation of CD40 in the presence of IL-4. A small part of the remaining immature B cells not reactive to autoantigens are selected to become mature, antigen-reactive sIgM+/sIgD+ B cells. Molecules which might guide such positive selection of B cells still remain to be identified.

lambda 5, but not mu, is required for B cell maturation in a unique gamma 2b transgenic mouse line

gamma 2b transgenic mice have a severe B cell defect, apparently caused by strong feedback inhibition of endogenous H-gene rearrangement coupled with an inability of gamma 2b to provide the survival/maturation functions of mu. A unique gamma 2b transgenic line, named the C line, was found to permit B cell development. When the C line is crossed with a mu-membrane knockout line, gamma 2b+ B cells develop in the homozygous knockout. In contrast, a transgenic line representative of all the other gamma 2b lines is completely B cell deficient when mu-mem is deleted. Strikingly, the C phenotype is dominant in C x other gamma 2b transgenic line crosses. There is no evidence for higher gamma 2b transgene expression or other position effects on the transgene in the C mouse. The sequences of the three gamma 2b transgene copies in the C line are identical to that of the original transgene. These results have led to the conclusion that in the C line the transgene integration constitutively induces a gene whose expression can replace mu. To more clearly delineate the stage at which the altered phenotype of the C line is expressed, C mice were crossed onto a lambda 5 knockout background. In the absence of lambda 5, the C line produces no B cells. Since it was also found that gamma 2b can associate with the surrogate light chain (sL; lambda 5/Vpre-B), the crosses between C line gamma 2b mice and lambda 5 knockout mice suggest that gamma 2b/sL is required for B cell maturation in this mouse line. Thus, gamma 2b alone is unable to replace mu for pre-B cell survival/maturation; however, in combination with an unknown factor and the sL, gamma 2b can provide these nurturing functions.