The expression of Vpre-B/lambda 5 surrogate light chain in early bone marrow precursor B cells of normal and B cell-deficient mutant mice

A human non-XLA immunodeficiency disease characterized by blockage of B cell development at an early proB cell stage

We report a detailed analysis of a B cell defect affecting a patient girl born from first cousin parents, characterized by a severe non-X-linked agammaglobulinemia with a total absence of CD19- cells in the periphery. In the bone marrow, CD19 expression was also highly impaired, resulting in the absence of both B and preB compartments. By contrast, CD34+CD10+, CD34psiL+, and some CD19+CD10+ mostly CD34+ early proB cells were present, although diminished. Semiquantitative RT-PCR analysis performed on mononuclear bone marrow cells indicated that lambda-like, VpreB, Rag-1, Rag-2, and TdT transcripts expressed during proB cell stages were found at normal levels whereas E2A, CD10, Syk, Pax-5, CD19, Igalpha, Igbeta, VH-Cmu, and Vkappa-Ckappa transcripts characteristic of later stages were severely depressed. This phenotype resembles that of Pax-5 knock-out mice, but since the coding sequence of the patient Pax-5 cDNA was shown to be normal, the defect might rather result from an altered regulation of this gene. All these data indicate that the patient suffers from a new genetic defect that results in an arrest of differentiation within the proB cell compartment, i.e., earlier than X-linked agammaglobulinemia, before the onset of Ig gene rearrangements.

Cell surface expression of surrogate light chain (psi L) in the absence of mu on human pro-B cell lines and normal pro-B cells

Surrogate light chains (psi L) encoded by lambda-like (lambda 5) and VpreB genes play a critical role in controlling the early steps of B cell differentiation. We prepared new anti-VpreB monoclonal antibodies (mAb) (3C7/6F6) which preferentially recognize the VpreB epitope at the cell surface of human cell lines that do not express the mu chain. These mAb provide the first characterization of human pro-B cell lines expressing surface psi L. We demonstrate that surface psi L expression is considerably enhanced upon interleukin-7 stimulation and that the psi L complex is formed independently of the Ig alpha/Ig beta heterodimer. Finally, using these antibodies, we confirm the existence of a normal pro-B cell population in human adult bone marrow. These cells are CD34+ CD38+ psi L+, do or do not express CD19, CD10, or both epitopes, and may represent the earliest cell population committed to B cell differentiation.

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.

Interleukin 7 receptor-deficient mice lack gammadelta T cells

The interleukin 7 receptor (IL-7R) plays a crucial role in early B- and T-cell development. It consists of a unique a chain and a common gamma chain [IL-2 receptor gamma chain (IL-2Rgamma)]. Gene inactivation of IL-7, IL-7R, and IL-2Rgamma resulted in severe impairment of B and T lymphopoiesis in mice. In addition, IL-2Rgamma-deficient mice lack gammadelta T cells in the skin and have the impaired development of natural killer (NK) cells and intraepithelial lymphocytes. To explore the role of IL-7/IL-7R system in gammadelta T- and NK-cell development, we have generated and analyzed IL-7R-deficient mice. gammadelta T cells were absent from skin, gut, liver, and spleen in the deficient mice. In contrast, alphabeta T and B cells were detected in reduced, but certain, numbers, and NK cells developed normally. The gammadelta T-cell development in fetal and adult thymus was also completely blocked. These results clearly demonstrate that the signal from IL-7R is indispensable for gammadelta T-cell development in both thymic and extrathymic pathways. On the contrary, it is suggested that NK-cell development requires cytokine(s) other than IL-7.

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.

Regulation of an early developmental checkpoint in the B cell pathway by Ig beta

Many of the cell fate decisions in precursor B cells and more mature B cells are controlled by membrane immunoglobulin (Ig)M heavy chain (mu) and the Ig alpha-Ig beta signal transducers. The role of Ig beta in regulating early B cell development was examined in mice that lack Ig beta (Ig beta-/-). These mice had a complete block in B cell development at the immature CD43+B220+ stage. Immunoglobulin heavy chain diversity (DH) and joining (JH) segments rearranged, but variable (VH) to DJH recombination and immunoglobulin messenger RNA expression were compromised. These experiments define an unexpected, early requirement for Ig(beta) to produce B cells that can complete VDJH recombination.

Composition and function of T-cell receptor and B-cell receptor complexes on precursor lymphocytes

The precursor T-cell receptors (TCRs) and B-cell receptors (BCRs) direct lymphocyte development to the mature T-cell and B-cell stage, respectively. Recent genetic and biochemical experiments reveal the striking parallel in structure and function of these receptors. They consist of TCR beta and BCR mu chains paired with surrogate TCR alpha and BCR light chains. Both receptors employ a two-component signal transduction unit: CD3 gamma epsilon for the pre-TCR, and CD79ab for the pre-BCR. Plasma membrane levels of pre-TCR/BCR complexes are kept extremely low, most probably by a mechanism involving specific retention in the endoplasmic reticulum. This mechanism may control the signalling activity of pre-TCR/BCR and therewith the lymphocyte differentiation process.

Function of the pre-T-cell receptor alpha chain in T-cell development and allelic exclusion at the T-cell receptor beta locus

The pre-T-cell receptor, composed of the T-cell receptor (TCR) beta chain (TCRbeta), pre-Talpha (pTalpha) chain, and CD3 molecules, has been postulated to be a transducer of signals during the early stages of T-cell development. To examine the function of the transmembrane pTalpha chain during tbymocyte development, we generated pTalpha-/- embryonic stem cells and assayed their ability to differentiate into lymphoid cells in vivo after injection into recombination-activating gene (RAG)-2-deficient blastocysts. Thymocytes representing all stages of T-cell differentiation were detected in the thymus of pTalpha-/- chimeric mice, indicating that thymocyte development can occur without pTalpha. However, greatly reduced thymocyte numbers and substantially increased percentages of both CD4-CD8- thymocytes and TCRgammadelta+ thymocytes suggest that pTalpha plays a critical role in thymocyte expansion. To investigate the role of the pTalpha chain in allelic exclusion at the TCRbeta locus, a functionally rearranged TCRbeta minigene was introduced into pTalpha-/- and pTalpha+/- embryonic stem cells, which were subsequently assayed by RAG-2-deficient blastocyst complementation. In the absence of pTalpha, expression of the transgenic TCRbeta inhibited rearrangement of the endogenous TCRbeta locus to an extent similar to that seen in normal TCRbeta transgenic mice, suggesting that pTalpha may not be required for signaling allelic exclusion at the TCRbeta locus.

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.

bcl-x exhibits regulated expression during B cell development and activation and modulates lymphocyte survival in transgenic mice

We have assessed during B cell development, the regulation and function of bcl-x, a member of the bcl-2 family of apoptosis regulatory genes. Here we show that Bcl-xL, a product of bcl-x, is expressed in pre-B cells but downregulated at the immature and mature stages of B cell development. Bcl-xL but not Bcl-2 is rapidly induced in peripheral B cells upon surface immunoglobulin M (IgM) cross-linking, CD40 signaling, or LPS stimulation. Transgenic mice that overexpressed Bcl-xL within the B cell lineage exhibited marked accumulation of peripheral B cells in lymphoid organs and enhanced survival of developing and mature B cells. B cell survival was further increased by simultaneous expression of bcl-xL and bcl-2 transgenes. These studies demonstrate that Bcl-2 and Bcl-xL are regulated differentially during B cell development and activation of mature B cells. Induction of Bcl-xL after signaling through surface IgM and CD40 appears to provide mature B cells with an additional protective mechanism against apoptotic signals associated with antigen-induced activation and proliferation.

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.

Down-regulation of RAG1 and RAG2 gene expression in preB cells after functional immunoglobulin heavy chain rearrangement

Two waves of immunoglobulin gene rearrangements, first of the heavy, then of the light chain chain gene loci form functional immunoglobulin genes during B cell development. In mouse bone marrow the differential surface expression of B220 (CD45R), c-kit, CD25, and surrogate light chain as well as the cell cycle status allows FACS separation of the cells in which these two waves of rearrangements occur. The gene products of two recombination activating genes, RAG1 and RAG2 are crucial for this rearrangement process. Here, we show that the expression of the RAG genes is twice up- and down-regulated, at the transcriptional level for RAG1 and RAG2, and at the postranscriptional level for RAG2 protein. Expression levels are high in D-->JH and VH-->DJH rearranging proB and preB-I cells, low in preB cells expressing the preB cell receptor on the cell surface, and high again in VL-->JL rearranging small preB-II cells. In immature B cells expressing on the cell surface RAG1 and RAG2 mRNA is down-regulated, whereas RAG2 protein levels are maintained. Down-regulation of RAG1 and RAG2 gene expression after productive rearrangement at one heavy chain allele might be part of the mechanisms that prevent further rearrangements at the other allele.

Internalization of B cell and pre-B cell receptors is regulated by tyrosine kinase and phosphatase activities

Prior to the expression of the B cell antigen receptor, the mu heavy chain associates with two non-polymorphic polypeptides, lambda like and VpreB, which form a pseudo-light chain complex in pre-B cells and pre-B cell lines. Surface expression of the so-called pre-B cell receptor (pre-BCR) occurs only in the presence of Ig alpha and Ig beta, known to be involved both in B cell antigen receptor (BCR) signaling and trafficking. Although the pre-BCR organization is consistent with an efficient transport to the cell surface, most of the newly synthesized receptor remains within the cells, and so far, no data are available concerning the rate of exit from the endoplasmic reticulum. Using the human pre-B cell line Nalm-6, we found that only a small fraction (2%) of newly synthesized pre-BCR is transported to the cell surface within 4-6 h after synthesis, where it is constitutively re-internalized. Membrane Ig-heavy chain cross-linking induced internalization of surface pre-BCR within a few minutes, and the mechanisms underlying endocytosis were analyzed by immunofluorescence and confocal microscopy. Preincubation of the cells with either genistein or orthovanadate, which inhibit, respectively, tyrosine kinases and tyrosine phosphatases, blocked pre-BCR internalization in a dose-dependent manner, indicating that both activities are required for endocytosis. BCR internalization was also inhibited in a reversible manner by the drugs. In contrast, neither drug affected the size of the steady-state pool of internalized transferrin receptors. Thus, our data show that tyrosine phosphorylation and dephosphorylation are both required for cross-linking-induced pre-BCR and BCR internalization.

Pre-B-cell development in the absence of lambda 5 in transgenic mice expressing a heavy-chain disease protein

BACKGROUND

Heavy-chain diseases (HCDs) are human lymphoproliferative neoplasias that are characterized by the secretion of truncated immunoglobulin heavy chains devoid of light chains. We have previously proposed--by analogy to the process by which mutated growth factor receptors can be oncogenic--that because the genetic defects in HCDs result in the production of abnormal membrane-associated heavy chains lacking an antigen-binding domain, these abnormal B-cell antigen receptors might engage in ligand-independent signalling. Normal pre-B-cell development requires the presence of the pre-B-cell receptor, formed by the association of mu heavy chains with two polypeptides--so-called surrogate light chains, Vpre-B and lambda 5--that are homologous to the variable and constant portions of immunoglobulin light chains, respectively. To assess whether amino-terminal truncation of membrane-associated heavy chains results in their constitutive activation, we have examined the ability of a HCD-associated mu protein to promote pre-B-cell development in transgenic mice.

RESULTS

When the mu HCD transgene is introduced into SCID mice, CD43- pre-B cells develop normally. To determine whether this pre-B-cell development requires surrogate light chains, we backcrossed mice expressing full-length or truncated mu transgenes with lambda 5-deficient mice. Our results show that the truncated heavy chain, but not the normal chain, is able to promote pre-B-cell development in the absence of lambda 5. We also show that truncated mu chains spontaneously aggregate at the surface of bone marrow cells.

CONCLUSIONS

Expression of the truncated mu heavy chain overrides a tightly controlled step of pre-B-cell development, which strongly suggests that a constitutive signal is delivered by the truncated mu chain disease protein. The self-aggregation of mu chain disease proteins might account for this constitutive activation. We conclude that amino-terminal truncation of heavy chains could play a role in the genesis of HCD neoplasia if it occurs at an appropriate stage of B-cell differentiation, namely in a mature B cell.

Resident bone marrow macrophages produce type 1 interferons that can selectively inhibit interleukin-7-driven growth of B lineage cells

Type 1 interferons alpha and beta are found to be potent inhibitors of IL-7-induced growth of early B lineage cells, while having no effect on cell growth induced by IL-2, IL-3, IL-4, or autogenous factors. The combination of IL-7 and interferons alpha/beta induces bcl-2 down-regulation and cell death by apoptosis. These conclusions were derived initially from experiments employing exogenous cytokines, but functional type 1 interferons are also shown to be produced by resident bone marrow macrophages. As physiological modulators of IL-7-driven proliferation and cell survival, interferons alpha/beta may cooperate with other homeostatic factors to maintain the balanced production of normal B lineage cells.

Fetal versus adult PreB or B cells: the human VH repertoire

At the preB stage, when only the IGH locus has rearranged, mu chains become expressed in association with the psi L chains, lambda-like and VpreB, thus forming the preB receptor. By the use of a monoclonal anti VpreB antibody, preB cells were isolated from two adult bone marrow samples, and the VH repertoire was analyzed and compared to fetal, XLA (X-linked agammaglobulinemia), and adult B repertoires. Most VH genes identified were also expressed in fetal liver, XLA bone marrow, and adult PBLs, with similar predominant usage of certain germline genes. Multiple D/D fusions, limited N diversity, and preferential use of JH4 with a low level of DQ52 usage were also identified. Few mutations could be observed, not specifically localized in CDR regions, that could be interpreted as not positively selected. Conversely, a shorter length of CDR3 appeared to be the hallmark of the preB step. Thus, the association of psi L chains with mu does not bring about a bias in the VH gene usage, but a first selection on the CDR3 region could be the result of recognition by given autoantigens or ligands different for preB cells and B cells.

Impaired expansion of mouse B cell progenitors lacking Btk

Mutations in the gene encoding the protein tyrosine kinase Btk are associated with the human B cell immunodeficiency X-linked agammaglobulinemia (XLA). In the mouse, a point mutation in the Btk pleckstrin homology domain segregates with a milder X-linked immunodeficiency (xid). To assess the importance of Btk function in murine lymphopoiesis, we generated multiple embryonic stem cell clones bearing a targeted disruption of the btk gene and examined their potential to produce lymphocytes in both C57BL/6 and RAG2-/- host chimeric animals. These mice provide a complementary set of in vivo competition assays that formally establish the genetic basis for the xid phenotype. Although the null mutation yields a phenotype quite similar to that of xid, it also compromises expansion of B cell precursors. Our results suggest that the murine and human consequences of Btk deficiency differ only quantitatively, and represent the same disease process.

V(D)J recombination and the cell cycle

V(D)J recombination is a major source of antigen receptor diversity and represents the only known form of site-specific DNA rearrangement in vertebrates. V(D)J recombination is initiated by specific DNA cleavage at recombinational signal sequences and requires components of the general machinery used for double-strand (DS)-break repair. The involvement of DS cleavage and repair mechanisms suggests that V(D)J recombination might be coupled to the cell cycle, as introduction or persistence of DS breaks during DNA replication or mitosis could interfere with faithful transmission of genetic information to daughter cells. Here, Weei-Chin Lin and Stephen Desiderio review recent evidence indicating that this is indeed the case and consider some biological implications of this linkage.

Making the in-vitro model closer to actual B lymphopoiesis in the bone marrow

After more than a decade since Whitlock and Witte established an in-vitro long-term culture of bone marrow B lineage cells, c-kit+IL-7-receptor(R)+B precursor cells in fresh bone marrow are now able to be grown under a fully defined culture condition containing only BSA, transferrin, IL-7 and the ligand for c-kit(Kit-ligand;KL) as protein components. On the other hand, previous studies indicated that the actual intramarrow B-cell-genesis is a complex process involving multiple stromal cell-derived molecules. Thus, the next step for the culture of B-cell genesis is to develop this simple culture into a new defined culture of B-cell-genesis that is closer to the actual process. In this article, we will describe how this defined culture condition has developed from the original Whitlock-Witte type culture, how the B precursors under this culture-condition are different from that in the bone marrow, and finally our biased view on the future direction to which this defined culture should develop.

In-vitro models of B-lineage commitment

The development of mature B lymphocytes from multipotent progenitors follows a pathway of differentiation marked by a progressive restriction in lineage options. The requirements for progression through the B lineage developmental pathway have been investigated intensively and a number of critical components of the differentiation process have been identified. However, the genetic basis for lineage determination remains unresolved. Recently, a number of in-vitro assays have been established which support the development of committed B cell progenitors from multipotent cells. These assays have provided a novel system in which the process of B lineage commitment can be followed and manipulated. In this review we present a model of B-lineage progression from multipotent progenitors to committed B-cell progenitors and discuss potential mediators of the commitment process.

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.

The role of Ig beta in precursor B cell transition and allelic exclusion

Lymphocytes express multicomponent receptor complexes that mediate diverse antigen-dependent and antigen-independent responses. Despite the central role of antigen-independent events in B cell development, little is known about the mechanisms by which they are initiated. The association between the membrane immunoglobulin (Ig) M heavy chair (micron) and the Ig alpha-Ig beta heterodimer is now shown to be essential in inducing both the transition from progenitor to precursor B cells and subsequent allelic exclusion in transgenic mice. The cytoplasmic domain of Ig beta is sufficient to induce these early antigen-independent events by a mechanism that requires conserved tyrosine residues in this protein.

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.

Precursor B cells of mouse bone marrow express two different complexes with the surrogate light chain on the surface

Two monoclonal antibodies raised against the complex of mu heavy (H) chain and Vpre-B/lambda 5 surrogate light (L) chains recognize surrogate L chain in different conformations on normal pre-B cells. One, LM34 recognizes free lambda 5 protein and free lambda 5/Vpre-B surrogate L chains and binds to surrogate L chains on the surface of early, pro-B and pre-B-I cells where the surrogate L chain is associated with a gp130/gp35-65 complex of proteins. It also recognizes the surrogate L chain associated with the mu H chain on pre-B-II cells. The other monoclonal antibody, SL156, does not recognize free surrogate L chain or its components, nor its complex with gp130/gp35-65 on pro-B and pre-B-I cells. However, it does bind to a conformational epitope on the surrogate light chain/mu H chain complex on a subpopulation of pre-B-II cells and on mu H chain-positive pre-B cell lines. On mouse precursor B cells prepared ex vivo on ice, expression of the surrogate L chain is very low and almost undetectable. Incubation of the precursor cells for 1 h at 37 degrees C up-regulates the surface expression of surrogate L chain associated with gp130/gp35-65 (early complex) as well as the mu H chain/surrogate L chain complex. These results reconcile some of the apparently discrepant results on surface expression of the surrogate L chain obtained with human and mouse bone marrow pre-B cells, and show that a surrogate L chain/mu H chain-containing pre-B cell receptor can be expressed also on the surface of mouse pre-B-II cells.

Murine B cell development: commitment and progression from multipotential progenitors to mature B lymphocytes

B lymphocytes, the cellular source of antibody, are critical components of the immune response. They develop from multipotential stem cells, progressively acquiring the traits that allow them to function as mature B lymphocytes. This developmental program is dependent on appropriate interactions with the surrounding environment. These interactions, mediated by cell-cell and cell-matrix interactions, provide the growth and differentiation signals that promote progression along the developmental pathway. This chapter addresses the properties of developing B lineage cells and the nature of the environmental signals that support B lineage progression.

Light chain loss and reexpression leads to idiotype switch. Surrogate light chains are probably responsible for this process

The murine B-lymphocyte cell line 38C-13 is characterized by several cell surface markers typical for an early stage of B-cell differentiation. Cells of this cell line possess cell surface membrane IgM molecules composed of mu and kappa polypeptide chains. They also produce "surrogate" or "pseudo" light chains (psi L) coded by the lambda 5 and VpreB genes. Variants of the 38C-13 cell line which do not synthesize kappa chains can be isolated from the 38C-13 population by the use of anti-idiotype antibodies in vivo and in vitro. In some kappa chain-deficient variant cell lines, cells which have regained surface IgM expression but have lost the original idiotype specificity, can be isolated. This idiotype switch is probably due to a secondary rearrangement of the kappa gene. In the kappa chain-deficient variant cells, the mu chains assemble with the surrogate light chains but the assembled IgM-like molecules are not expressed on the cell surface. It is suggested that surrogate light chains play an important role in the induction of kappa gene rearrangement but that surface expression of mu-psi L complexes is not required for this process.

Apoptosis and macrophage-mediated cell deletion in the regulation of B lymphopoiesis in mouse bone marrow

Studies of cell population dynamics and microenvironmental organization of B lymphopoiesis in the bone marrow of normal mice and in various genetically modified states have shown that cell loss, involving processes of apoptosis and macrophage-mediated cell deletion, is a prominent feature of the primary genesis of B lymphocytes. Balanced against the influence of proliferative stimulants, the programmed death of precursor B cells provides a quantitative control, determining the magnitude of the final output of functional B lymphocytes to the peripheral immune system. The cell loss mechanisms can be readily set in motion by external or systemic influences, making the B-cell output particularly vulnerable to suppression by ionizing irradiation, stress or other systemic mediators. In addition, however, cell loss exerts an important quality control in the formation of the primary B-cell repertoire. The combination of apoptosis and macrophage-mediated deletion, acting at successive stages of B-cell differentiation, efficiently eliminates many precursors having non-productive Ig gene rearrangements, cell cycle dysregulations, and certain autoreactive Ig specificities. Outstanding areas of further work abound. Important questions concern the nature of mechanisms which underlie the processes of B-cell apoptosis and macrophage deletion in bone marrow, the microenvironmental signals involved in B-cell life or death decisions and genetic factors which may override these B-cell culling mechanisms. The answers will be relevant to problems of autoimmune disease, humoral immunodeficiency and B-cell neoplasia.

The Ikaros gene is required for the development of all lymphoid lineages

The Ikaros gene encodes a family of early hematopoietic- and lymphocyte-restricted transcription factors. Mice homozygous for a germline mutation in the Ikaros DNA-binding domain lack not only T and B lymphocytes and natural killer cells but also their earliest defined progenitors. In contrast, the erythroid and myeloid lineages were intact in these mutant mice. We propose that Ikaros promotes differentiation of pluripotential hematopoietic stem cell(s) into the lymphocyte pathways. In the absence of a functional Ikaros gene, these stem cells are exclusively diverted into the erythroid and myeloid lineages.

The B-cell antigen receptor complex: structure and signal transduction

The specificity of the immune response is determined by the interaction between the B-cell receptor (BCR) and its cognate structure, antigen. Recent studies have provided considerable insight into the compartmentalization of function within this extremely versatile hetero-oligomeric receptor complex. In this article, Christopher Pleiman, Daniele D'Ambrosio and John Cambier consolidate new findings regarding BCR structure and signal transduction.

Influence of immunoglobulin heavy- and light-chain expression on B-cell differentiation

To study the influence of immunoglobulin heavy-chain (HC) and light-chain (LC) expression in promoting B-cell differentiation, we have introduced functional immunoglobulin HC and/or LC transgenes into the recombinase activating gene-2-deficient background (RAG-2-/-). RAG-2-/- mice do not undergo endogenous V(D)J rearrangement events and, therefore, are blocked in B- and T-cell development at the early pro-B- and pro-T-cell stages. Introduction of immunoglobulin HC transgenes into the RAG-2-/- background promotes the development of a B-lineage cell population that phenotypically has the characteristics of pre-B cells. We have shown further that this population has altered growth characteristics as measured by interleukin-7 responsiveness in culture. Bone marrow cells from immunoglobulin HC transgenic RAG-2-/- mice have up-regulated expression of germ-line kappa LC gene transcripts and down-regulated expression of lambda 5 surrogate LCs (SLCs). Although mu HC/SLC complexes are detectable intracellularly in HC/RAG-2-/- pre-B-cell populations, HC expression is not readily detectable on the surface of these cells. lambda LC RAG-2-/- mice had a bone marrow B-lineage cell phenotype indistinguishable from that of RAG-2-/- littermates, indicating that LC expression by itself has no influence on pro-B cell differentiation. Strikingly, simultaneous introduction of mu HC and lambda LC transgenes into RAG-2-/- mice led to the generation of a substantial population of "monoclonal" peripheral B-cells that were functional with regard to immunoglobulin secretion, indicating that T cells or diverse immunoglobulin repertoires are not necessary for peripheral B-cell development.

Probing immune functions in RAG-deficient mice

Inactivation of recombination activating gene (RAG)-1 or RAG-2 in mice results in the inability of developing lymphocytes to initiate V(D)J recombination, leading to the arrest of lymphocyte differentiation at a very early stage. Introduction of functionally assembled antigen-receptor genes or other potentially relevant genes into the RAG-deficient background can bypass the V(D)J recombination block and promote differentiation of the lymphocytes of RAG-deficient mice to various stages. This approach offers new means for analyzing the control of lymphocyte differentiation. In addition, generation of somatic chimeric mice by injecting mutant embryonic stem cells into the RAG-2-deficient blastocysts has also provided a powerful new method for assaying the potential roles of genes or regulatory elements in lymphocyte development or function.