Surface mu heavy chain signals down-regulation of the V(D)J-recombinase machinery in the absence of surrogate light chain components

Early B cell development is characterized by stepwise, ordered rearrangement of the immunoglobulin (Ig) heavy (HC) and light (LC) chain genes. Only one of the two alleles of these genes is used to produce a receptor, a phenomenon referred to as allelic exclusion. It has been suggested that pre–B cell receptor (pre-BCR) signals are responsible for down-regulation of the VDJ_H-recombinase machinery (Rag1, Rag2, and terminal deoxynucleotidyl transferase [TdT]), thereby preventing further rearrangement on the second HC allele. Using a mouse model, we show that expression of an inducible μHC transgene in Rag2^−/− pro–B cells induces down-regulation of the following: (a) TdT protein, (b) a transgenic green fluorescent protein reporter reflecting endogenous Rag2 expression, and (c) Rag1 primary transcripts. Similar effects were also observed in the absence of surrogate LC (SLC) components, but not in the absence of the signaling subunit Ig-α. Furthermore, in wild-type mice and in mice lacking either λ5, VpreB1/2, or the entire SLC, the TdT protein is down-regulated in μHC⁺LC⁻ pre–B cells. Surprisingly, μHC without LC is expressed on the surface of pro–/pre–B cells from λ5^−/−, VpreB1^−/−VpreB2^−/−, and SLC^−/− mice. Thus, SLC or LC is not required for μHC cell surface expression and signaling in these cells. Therefore, these findings offer an explanation for the occurrence of HC allelic exclusion in mice lacking SLC components.

Selection of Ig mu heavy chains by complementarity-determining region 3 length and amino acid composition

Although it is generally accepted that Ig heavy chains (HC) are selected at the pre-B cell receptor (pre-BCR) checkpoint, the characteristics of a functional HC and the role of pre-BCR assembly in their selection have remained elusive. We determined the characteristics of HCs that successfully passed the pre-BCR checkpoint by examining transcripts harboring V(H)81X and J(H)4 gene segments from J(H)(+/-) and lambda5(-/-)mice. V(H)81X-J(H)4-HC transcripts isolated from cells before or in the absence of pre-BCR assembly had no distinguishing complementarity-determining region 3 traits. In contrast, transcripts isolated subsequent to passage through the pre-BCR checkpoint had distinctive complementarity-determining regions 3 of nine amino acids in length (49%) and a histidine at position 1 (73%). Hence, our data define specific structural requirements for a functional HC, which is instrumental in shaping the diverse B cell repertoire.

Cutting edge: signaling and cell surface expression of a mu H chain in the absence of lambda 5: a paradigm revisited

Pre-B cell receptor (pre-BCR) signals are essential for pro-B cells to mature efficiently into pre-B cells. The pre-BCR is an Ig-like transmembrane complex that is assembled from two mu H chains (mu HC) and two surrogate L chains consisting of the non-covalently associated polypeptides VpreB and lambda5. In lambda5(-/-) mice, pro-B cell maturation is impaired, but not completely blocked, implying that a mu HC induces differentiation signals in the absence of lambda5. Using a mouse model, in which transgenic mu HC expression can be controlled by tetracycline, we show that in the absence of lambda5, the transgenic mu HC promotes in vivo differentiation of pro-B cells, induces IL-7-dependent cell growth, and is expressed on the surface of pre-B cells. Our findings not only show that an incomplete pre-BCR can initiate signals, but also challenge the paradigm that an IgHC must associate with an IgLC or a SLC to gain transport and signaling competency.

Immunoglobulin mu heavy chains do not mediate tyrosine phosphorylation of Ig alpha from the ER-cis-Golgi

Signals delivered by Ig receptors guide the development of functional B lymphocytes. For example, clonal expansion of early mu heavy chain ( mu HC)-positive pre-B cells requires the assembly of a signal-competent pre-B cell receptor complex (pre-BCR) consisting of a mu HC, a surrogate L chain, and the signal dimer Ig alpha beta. However, only a small fraction of the pre-BCR is transported to the cell surface, suggesting that pre-BCR signaling initiates already from an intracellular compartment, e.g., the endoplasmic reticulum (ER). The finding that differentiation of pre-B cells and allelic exclusion at the IgH locus take place in surrogate L chain-deficient mice further supports the presence of a mu HC-mediated intracellular signal pathway. To determine whether a signal-competent Ig complex can already be assembled in the ER, we analyzed the consequence of pervanadate on tyrosine phosphorylation of Ig alpha in J558L plasmacytoma and 38B9 pre-B cells transfected with either a transport-competent IgL chain-pairing or an ER-retained nonpairing micro HC. Flow cytometry, combined Western blot-immunoprecipitation-kinase assays, and confocal microscopy revealed that both the nonpairing and pairing mu HC assembled with the Ig alpha beta dimer; however, in contrast to a pairing mu HC, the nonpairing mu HC was retained in the ER-cis-Golgi compartment, and neither colocalized with the src kinase lyn nor induced tyrosine phosphorylation of Ig alpha after pervanadate treatment of cells. On the basis of these findings, we propose that a signal-competent Ig complex consisting of mu HC, Ig alpha beta, and associated kinases is assembled in a post-ER compartment, thereby supporting the idea that a pre-BCR must be transported to the cell surface to initiate pre-BCR signaling.

Cytoplasmic micro heavy chain confers sensitivity to dexamethasone-induced apoptosis in early B-lineage acute lymphoblastic leukemia

Most childhood acute lymphoblastic leukemia (ALL) arises from early B-lineage cells,and response to steroid treatment is critical to successful ALL therapy. To investigate the effect of the pre-B cell receptor (pre-BCR) complex on the response of leukemic cells to steroids, cytoplasmic micro protein (cyto mu) was transfected into cyto mu-, steroid-resistant early B cell lines. The presence of cyto mu and the assembled pre-BCR complex conferred sensitivity to dexamethasone-induced apoptosis. Both intrinsic and extrinsic apoptosis pathways are involved in this cell death. However, if the transfected cyto micro protein is unable to assemble the pre-BCR complex, the cells remain resistant to dexamethasone. These findings suggest a role for the pre-BCR complex in the response of ALL cells to treatment and provide insight into the mechanism of steroid response in the treatment of pre-B ALL.

An Ig mu-heavy chain transgene inhibits systemic lupus erythematosus immunopathology in autoimmune (NZB x NZW)F1 mice

Intrinsic defects in the B lymphoid lineage are involved in predisposition for systemic lupus erythematosus in (NZB x NZW)F(1) (NZB/W) mice. In addition, a contribution of CD4(+) T cells has been shown to be crucial for the development of fatal glomerulonephritis. To further dissect the role of B and T cells in lupus immunopathology we used Ig mu-heavy chain (muHC) transgenic (Tg) NZB/W mice that we recently established to study mechanisms of B cell tolerance. The Tg NZB/W mice have a very restricted B cell repertoire and only a very minor population of B cells having endogenously rearranged muHC Ig loci are able to undergo isotype switch. Here we analyzed the influence of the restricted B cell repertoire on the development of IgG anti-DNA antibodies and glomerulonephritis as well as the hyperactivation of T(h) cells. IgG anti-DNA antibodies developed delayed but consistently in the Tg NZB/W mice, suggesting that a strong selective mechanism for the development of these autoantibodies is operative. Despite significant autoantibody titers in Tg NZB/W mice, very little immune deposits in the glomeruli and no evidence for renal inflammation were found. The Tg mice have a significantly prolonged survival time and most of the Tg mice lived much longer than 1 year. Interestingly, the generalized T cell activation that normally correlates and coincides with the progression of the disease in NZB/W mice is strongly reduced in older Tg animals. The absence of IgG3 anti-DNA antibodies and the strong reduction of IgG2a anti-DNA antibodies in the Tg mice suggests that particularly the activation of T(h)1 cells is inhibited. This result shows that a significant restriction in the B cell repertoire prevents hyperactivation of T(h) cells and supports the model that T cell hyperactivation in NZB/W mice is secondary to specific interactions with a subpopulation of presumably autoreactive B lymphocytes.

The level of expression of mu heavy chain modifies the composition of peripheral B cell subpopulations

The B cell receptor (BCR) has a decisive role in transducing signals required for the development of B cells and their survival in the periphery. However, the processes that initiate these signals remain unclear and concepts of constitutive and ligand-dependent signaling have been proposed. Using a mu-transgenic mouse model, we have analyzed the impact of high surface IgM expression on the composition of the splenic B cell population. kappa-deficient mice homozygous for the H3-mu transgene have B cells with a higher BCR surface density than H3 heterozygous mice. This higher BCR expression is associated with an increase in the percentage and the total number of splenic B cells. In addition, an important proportion of CD23(-)CD21(+) marginal zone (MZ) B cells can be observed in H3 homozygous mice. However, these modifications operate in the absence of impairment of the positive selection process of the H3-mu/lambda1 combination over the H3-mu/lambda2 + 3 ones. These results suggest that (i) a constitutive BCR signaling directly correlated with BCR surface density is responsible for the efficient B cell colonization of the periphery with an accumulation of B cells in the MZ and (ii) a ligand-dependent BCR signal is responsible for the clonotype composition of the mature B cell repertoire.

Interleukin-4 transgenic mice develop glomerulosclerosis independent of immunoglobulin deposition

Mice with constitutive transgenic (tg) expression of IL-4 develop autoimmune-type disorders resembling human lupus nephritis. The kidneys show progressive glomerulosclerosis with immunoglobulin (Ig) and complement deposition. This study investigated the roles of renal IL-4 expression and glomerular Ig deposition in the pathogenesis of glomerulosclerosis in IL-4 tg mice. Treatment of these mice with IL-4 neutralizing antibody prevented renal disease. IL-4 tg mice treated with methylprednisolone (MP) showed increased mesangial collagen deposition with only trace amounts of glomerular Ig. To analyze the relevance of Ig deposition in the development of the renal lesions, IL-4 tg mice were cross-bred with mu chain-deficient mice (muMT-/-), which are unable to produce Ig. IL-4 tg/muMT-/- mice developed progressive glomerulosclerosis with mesangial accumulation of collagen types I, IV and V despite complete absence of glomerular Ig deposits. Renal IL-4 expression was observed in both anti-IL-4- and MP-treated IL-4 tg mice as well as in IL-4 tg/muMT-/- mice. No statistical difference in the number of glomerular T cells and macrophages between any of the groups was evident. Our data demonstrate that in this model glomerulosclerosis can develop independently of and prior to Ig deposition, and suggest that the initial accumulation of glomerular extracellular matrix is due to renal IL-4 expression. Our results point to a novel mechanism for the development of glomerulosclerosis which may have implications for human disease.

Inhibition of intracellular transport of B cell antigen receptor complexes by Kaposi's sarcoma-associated herpesvirus K1

The B cell antigen receptor (BCR) is a large complex that consists of a disulfide-linked tetramer of two transmembrane heavy (mu) chains and two light (lambda or kappa) chains in association with a heterodimer of Igalpha and Igbeta. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a transforming protein called K1, which has structural and functional similarity to Igalpha and Igbeta. We demonstrate that K1 downregulates the expression of BCR complexes on the surface. The NH(2)-terminal region of K1 specifically interacts with the mu chains of BCR complexes, and this interaction retains BCR complexes in the endoplasmic reticulum, preventing their intracellular transport to the cell surface. Thus, KSHV K1 resembles Igalpha and Igbeta in its ability to induce signaling and to interact with mu chains of the BCR. However, unlike Igalpha and Igbeta, which interact with mu chains to direct BCR complexes to the cell surface, K1 interacts with mu chains to block the intracellular transport of BCR complexes to the cell surface. These results demonstrate a unique feature of the K1 transforming protein, which may confer virus-infected cells with a long-term survival advantage.

Investigation of anti-WI-1 adhesin antibody-mediated protection in experimental pulmonary blastomycosis

Infection with Blastomyces dermatitidis elicits strong antibody responses to the surface adhesin WI-1. The antibodies are directed chiefly against the adhesive domain, a 25-amino-acid repeat. Tandem-repeat-specific monoclonal antibodies (mAbs) were studied for their opsonic activity in vitro and their capacity to adoptively transfer protection in murine experimental blastomycosis. mAbs to WI-1 enhanced binding and entry of B. dermatitidis yeasts into J774. 16 cells but did not enhance killing or growth inhibition of the yeast. Passive transfer of 8 mAbs to WI-1 into 3 different inbred strains of mice also did not improve the course of experimental infection and sometimes worsened it. mu-deficient mice were more resistant to experimental blastomycosis than were intact littermates, and passive transfer of the mAbs into these mice did not protect them against experimental infection. Thus, antibody to WI-1 does not appear to improve the outcome of murine blastomycosis and may enhance the infection.

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 Ig heavy chain intronic enhancer core region is necessary and sufficient to promote efficient class switch recombination

The intronic IgH enhancer E(mu), which consists of the core enhancer (cE(mu) flanked by 5' and 3' matrix attachment regions (MAR), has been implicated in the control of IgH locus recombination and transcription. Both cE(mu) and the MAR are required to enhance transcription of an IgH transgene. To elucidate the regulatory functions of cE(mu) versus its associated MAR in IgH class switch recombination (CSR), we have assayed ES cell lines which have targeted deletions of these elements, both individually and in combination, by the Rag-2-deficient blastocyst complementation method. Mutant B cells from chimeric mice were activated in culture and the influence of the mutations on CSR was assessed by analysis of B cell hybridomas. We find that the cE(mu) is necessary and sufficient for providing the functions of E(mu) required for efficient CSR at the IgH locus. However, the 5' and 3' MAR sequences, as well as the known I(mu) transcription start sites and the bulk of I(mu) coding sequences, were dispensable for the process.

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.

Biogenesis and function of IgM: the role of the conserved mu-chain tailpiece glycans

The tailpiece of secretory Ig-mu-chains (mu(s)tp) is highly conserved throughout evolution: in particular, a carboxy-terminal cysteine residue (Cys575) and a glycan linked to Asn563 are found in all species sequenced so far. Here we show that the mu(s)tp oligosaccharide moieties are important for the binding of J-chains and for the process of IgM polymerization. In the absence of the mu(s)tp glycans, pentamers cannot be assembled and polymers containing six or more subunits are secreted. Despite their increased valency, these molecules have a lower association rate with antigen than wild-type polymers. Unexpectedly, the C-terminal oligosaccharides also affect kinetic parameters on unpolymerized subunits. Thus, monomers lacking the C-terminal sugars because of either site-directed mutagenesis or selective enzymatic deglycosylation with endoglycosidase H, have a lower k(on) for the antigen. Taken together, our results indicate that the C-terminal mu-chain glycans can shape the structure of mu(s2)L2 subunits and their further assembly into polymers.

Role of mu heavy chain in B cell development. I. Blocked B cell maturation but complete allelic exclusion in the absence of Ig alpha/beta

There is good evidence for a signaling role played by Ig heavy chain in the developmental transition through the pre-B cell stage. We have previously described signal-capable or signal-incapable mutants of mu heavy chain in which a signaling defect is caused by failure to associate with the Ig alpha/beta heterodimer. To further characterize the role of Ig heavy chain-mediated signaling in vivo, as well as in B cell development and allelic exclusion, we have created transgenic mice in which the B cells express these signal-capable and signal-incapable mutant mu chains. Failure of mu to signal via Ig alpha/beta results in a block in B cell development in mice expressing the signal-incapable mu. A small number of B cells in these animals do escape the developmental block and are expressed in the spleen and the periphery as B220+ transgenic IgM+ cells. These cells respond to LPS by proliferating but show no response to T-independent-specific Ag. In contrast, B cells expressing the signal-capable B cell receptor show a strong signaling response to Ag-specific stimulus. There is no Ig alpha seen in association with signal-deficient IgM. Thus, the B cell receptor complex is not assembled, and no signal can be delivered. Despite the block in developmental signaling, allelic exclusion is complete. There is no detectable coexpression of transgenic IgM and endogenous murine IgM, nor is there rearrangement of the endogenous heavy chain genes. This suggests that differing signaling mechanisms are responsible for the developmental transition and allelic exclusion and thus allows for separate examination of these signaling mechanisms.

Atypical disease after Bordetella pertussis respiratory infection of mice with targeted disruptions of interferon-gamma receptor or immunoglobulin mu chain genes

Using a murine respiratory challenge model we have previously demonstrated a role for Th1 cells in natural immunity against Bordetella pertussis, but could not rule out a role for antibody. Here we have demonstrated that B. pertussis respiratory infection of mice with targeted disruptions of the genes for the IFN-gamma receptor resulted in an atypical disseminated disease which was lethal in a proportion of animals, and was characterized by pyogranulomatous inflammation and postnecrotic scarring in the livers, mesenteric lymph nodes and kidneys. Viable virulent bacteria were detected in the blood and livers of diseased animals. An examination of the course of infection in the lung of IFN-gamma receptor-deficient, IL-4-deficient and wild-type mice demonstrated that lack of functional IFN-gamma or IL-4, cytokines that are considered to play major roles in regulating the development of Th1 and Th2 cells, respectively, did not affect the kinetics of bacterial elimination from the lung. In contrast, B cell-deficient mice developed a persistent infection and failed to clear the bacteria after aerosol inoculation. These findings demonstrate an absolute requirement for B cells or their products in the resolution of a primary infection with B. pertussis, but also define a critical role for IFN-gamma in containing bacteria to the mucosal site of infection.

Roles of calnexin and Ig-alpha beta interactions with membrane Igs in the surface expression of the B cell antigen receptor of the IgM and IgD classes

Membrane Igs (mIgs) of all five classes are associated with Ig-alpha beta dimers on the B cell surface. While mIgM requires the presence of these two associated molecules for its surface expression, mIgD does not. To study the structural basis for this differential Ig-alpha beta dependence, we created mutant mIgM and mIgD molecules (chimeras and those with reciprocal point mutations in their transmembrane sequences) and identified two amino acid residues in the transmembrane region of the mIg heavy chains responsible for this transport difference. Without Ig-alpha beta, mIgM and mutant mIgD molecules remained endoglycosidase H sensitive, consistent with endoplasmic reticulum (ER) localization. The molecular chaperone calnexin has previously been implicated in retaining unassembled mIgs in the ER. However, we found that inhibition of the association between calnexin and newly synthesized mIgs by castanospermine treatment did not lead to surface expression of normally retained mIgs. Therefore, calnexin cannot be the only retention molecule. Furthermore, for both wildtype and mutant mIgDs, association with calnexin was rather transient, thereby ruling out calnexin being a major ER retention molecule for mIgDs. Our study with castanospermine also showed that calnexin is required for wildtype mIgD surface expression only if Ig-alpha beta is absent, while the latter alone can function to promote mIg folding, assembly, and transport. Further study using our system will help to identify novel molecules and characterize their involvement in the control of mIg transport.

Gamma 2b provides only some of the signals normally given via mu in B cell development

B cell development is a complex process involving interactions between B cell precursors, stroma, and known and unknown ligands and cytokines. In order to more fully understand the requirements for Ig in that development we have created transgenic mice that carry a gamma 2b transgene and express it early in B cell development. Previously it was believed that these B cells arrested in their development prior to the pro- to pre-B cell transition. We show here that in conventional gamma 2b mice, B cell development actually arrests later, at the pre-B cell stage. This shows for the first time that a constant region different from mu can allow signaling through the pre-B cell receptor, but cannot promote complete development. The pro- and pre-B cells in the conventional gamma 2b transgenics are not fully functional since they cannot grow in IL-7 without stromal cells. This is a novel phenotype, separating development from stroma independence. The few, mature B cells that do develop in these mice express both mu and gamma 2b simultaneously, and are CD5+. Expression of a Bcl-2 transgene allows survival of gamma 2b transgenic immature B cells, but does not promote full maturation, indicating that normally mu provides both an anti-apoptotic signal and a differentiation signal. One line of gamma 2b mice, the C line, does not have this phenotype. B cell development is accelerated in this unconventional line, and the developing B cells have a very different phenotype from both normal mice and conventional gamma 2b mouse lines, but are very similar to mu transgenics. Mature B cells are largely CD5-, gamma 2b-only expressing. This unique phenotype is apparently due to the activation in B cell precursors of a gene at the insertion site of the transgene, circumventing the need for mu. Comparison of conventional gamma 2b transgenics with the C line and mu transgenics reveals the multiple signals required throughout 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.

Control of the sizes and contents of precursor B cell repertoires in bone marrow

Ordered rearrangements of immunoglobulin (Ig) gene loci, first as DH to JH, then as VH to DHJH, and finally as VL to JL segment-specific recombinations occur 'in-frame' and 'out-of-frame'. 'In-frame' rearrangements lead to the expression of truncated DHJH-microC proteins and to microH chains. These H chain proteins have two major effects on precursor B cells. They suppress (as DJC mu proteins) or enhance (as full microH chain) the proliferation of precursor cells at the point where these precursors express these proteins. At the same time, they signal allelic exclusion of the microH chain alleles, so that VH to DHJH rearrangement at the second allele is suppressed. Regulation of precursor B cell proliferation and H chain allelic exclusion is mediated by a pre-B cell receptor that is composed of the microH chains and a surrogate L chain. This surrogate L chain is made up of two proteins encoded by the Vpre-B and lambda 5 genes that are expressed only at the early precursor cell stages just before and when H chain genes are first expressed. They are not found in later B cell development, when L chains are expressed, nor in any other cell of the body tested so far. The physiological roles of surrogate L chain and of the pre-B receptor have been clarified by generating mutant mice in which the lambda 5 gene has been inactivated by targeted disruption. Molecular mechanisms and cellular developments, by which the pre-B receptor controls proliferation and allelic exclusion, are discussed.

Immature stage B cells enter but do not progress beyond the early G1 phase of the cell cycle in response to antigen receptor signaling

In contrast to mature B cells, immature stage B cells do not proliferate following Ag receptor cross-linking with anti-Ig Abs. To determine where in the cell cycle immature B cells arrest, we have examined the expression of specific G, cell cycle regulators. Following surface IgM (sIgM) cross-linking on mature B cells, we observed increased expression of the early G1 kinase, cyclin-dependent kinase 4 (cdk4), and one of its regulatory subunits, cyclin D2. Mature B cells also showed increased expression of components required for G1/S transition, including cyclin E and cdk2. Whereas immature stage B cells increased expression of cyclin D2 and cdk4 after anti-IgM stimulation, unlike mature stage B cells they failed to express cyclin E and cdk2. Expression of cyclin D2 and cdk4 indicates that these cells can exit G0 and enter the initial G1 phase following sIgM ligation. Interestingly, IL-4, which by itself does not stimulate proliferation of immature B cells, induced expression of cyclin E and cdk2. These latter results suggest that IL-4 complements sIgM, signaling for proliferation by increasing the basal levels of late G1 cell cycle regulators. Consistent with this idea, IL-4 synergizes with anti-Ig Abs to promote cell cycle progression and proliferation of immature B cells. Finally, c-myc, a transcriptional regulator of some members of the cell cycle machinery, is not induced following sIgM cross-linking of immature cells. This lack of inducible expression contrasts with that seen in mature stage B cells, and in immature stage cells stimulated to proliferate with LPS. These results suggest that c-myc may be a component of the signaling pathway that induces cyclin E and cdk2 expression.

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.

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.

The surrogate light chain in B-cell development

The proteins encoded by the VpreB and lambda 5 genes associate with each other to form a light (L) chain-like structure, the surrogate L chain. It can form Ig-like complexes with three partners-the classical heavy (H) chain, the DHJHC mu-protein, or the newly discovered p55 chain; these are expressed on the surface of pre-B cells at different stages of development. Here, Fritz Melchers and colleagues review the structures of the VpreB and lambda 5 genes in mouse and their relatives in humans, describe their pattern of expression, and speculate on their possible evolution and functions.

Involvement of the avian mu heavy chain in recolonization of the bursa of Fabricius

In the chicken, the B cells develop in a specialized organ, the bursa of Fabricius. Earlier it was shown that neonatal bursal cells treated with polyclonal anti-chicken immunoglobulin antibodies are not able to recolonize the bursa when transferred into cyclophosphamide-treated chicks. In this study, 4-day-old bursal cells were treated with different polyclonal and monoclonal anti-immunoglobulin antibodies and transferred into 4-day-old cyclophosphamide-treated chickens. Two monoclonal anti-chicken IgM antibodies, CVI-59.7 and 21-2B2, recognizing distinct epitopes of the mu heavy chain, were inhibitory. Incubation of cells with 21-2B2 antibody caused about 90% inhibition of bursal recolonization. After incubation with CVI-59.7 antibody the inhibition was 50%. The high inhibition by 21-2B2 antibody was also seen when F(ab')2 fragments of the antibody were used. These results suggest that the entry of the cells needed for bursal recolonization is inhibited almost totally by 21-2B2 antibody, or that this antibody blocks further proliferation of the cells in bursal follicles. In conclusion, we have shown that a mu heavy chain epitope is intimately involved in the recolonization of bursal follicles, and distinct epitopes of the mu heavy chain are not equally important in this process.

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

We constructed an expression vector encoding a truncated Ig mu chain that lacks both VH and CH1 domains (mu delta m chain) and introduced the mu delta m vector into the Ig negative Abelson pre-B cell line P17-27. The transfectants expressed a large amount of the mu delta m chain on their surface, which was not complexed with the lambda 5 and VpreB surrogate light chain molecules. While P17-27 transfected with a vector for the intact micron chain (P17-27 micron) shows V kappa rearrangements in culture, V kappa rearrangements were not detected in P17-27 mu delta m cells. When the mu delta m chains on the cell surface were crosslinked by anti-mu antibodies, V kappa gene rearrangements were induced in P17-27 mu delta m. These results strongly suggest that crosslinking of the micron-lambda 5-VpreB complex on the pre-B cell surface generates a signal that activates V kappa gene rearrangement, and that the lambda 5 and VpreB molecules are necessary for the spontaneous crosslinking of surface Ig on pre-B cells.

Surface mu heavy chain expressed on pre-B lymphomas transduces Ca2+ signals but fails to cause growth arrest of pre-B lymphomas

Little is known about the role of signals transduced by cell surface IgM (sIgM) expressed during early B cell development. A subclone (1.6) of the late pre-B cell lymphoma 70Z/3.12 was used to study signal transduction by surface mu heavy (H) chain before and after transition to the early immature B cell stage, and the functional consequences thereof. Although kappa L chain expression can be induced on 1.6 cells by LPS or cytokines, immunoprecipitations indicated that the non-induced 1.6 cells expressed mu H chain with an alternative protein(s) which may be a surrogate light chain(s). Consistent with this, anti-mu but not anti-kappa or anti-lambda antibodies caused transient Ca2+ mobilization in noninduced 1.6 cells. The Ca2+ signal was derived from both intracellular stores and Ca2+ influx in either noninduced cells or in cells that had been preinduced to express kappa L chain. Thus, the ability of mu H chain to mobilize Ca2+ as a second messenger does not depend upon the expression of mature L chains. The immature B lymphomas, WEHI-231 and CH1, express mature forms of IgM and undergo growth arrest when stimulated by anti-mu antibody. In contrast, signals generated by mu H chain on either noninduced or preinduced 1.6 cells or in the sIgM+ pre-B cell transfectant 300-19 mu lambda 36/8 did not cause growth arrest. These results suggest that mu H chain expressed on pre-B cells is capable of mobilizing Ca2+, but that this signal alone is insufficient to induce growth arrest in the pre-B cell.

Engagement of MHC-class II molecules by staphylococcal exotoxins delivers a comitogenic signal to human B cells

The staphylococcal exotoxin toxic shock syndrome toxin-1 (TSST-1) has been demonstrated to bind to monomorphic determinants on MHC-class II molecules. In this study, we have used TSST-1 to probe the role of MHC-class II molecules in the activation and differentiation of resting human B cells. Highly purified B cells were stimulated with TSST-1, alone or in combination with PMA or with anti-human IgM antibodies (anti-mu) and the resulting B cell proliferation and Ig production were monitored. On its own, TSST-1 failed to induce B cell proliferation or Ig production. However, TSST-1 synergized with PMA and with anti-mu in inducing B cell proliferation in the absence of any added T cells or T cell factors. TSST-1 did not induce or potentiate early activation events associated with anti-mu treatment such as phosphoinositide hydrolysis and Ca2+ mobilization. Also, TSST-1 did not potentiate the capacity of anti-mu to induce the transcription of early activation genes such as c-myc. Finally, in contrast to its capacity to promote mitogen-trigered B cell proliferation, TSST-1 failed to induce the differentiation of B lymphocytes into Ig-secreting cells in the absence of added T cells. These results indicate that TSST-1 delivers a comitogenic signal to B cells via MHC-class II molecules that is distinct from signals delivered via surface mu and further strengthens the role of MHC-class II molecules as signal transducing structures.

Evidence for differential responsiveness of human CD5+ and CD5- B cell subsets to T cell-independent mitogens

Tonsillar resting B cells were separated into CD5+ and CD5- cell subsets and stimulated with the thymus-independent mitogens, Staphylococcus aureus Cowan strain I (SAC) or insolubilized anti-mu monoclonal antibodies (a mu Ab). CD5+ cells incorporated [3H]thymidine more efficiently than unfractionated cells when stimulated with SAC and their response was augmented by the addition of interleukin (IL) 2 to the cultures. CD5+ cells also proliferated in response to a mu Ab provided that IL 2 was present, SAC-, but not a mu Ab-stimulated CD5+ cells produced IgM and IgG molecules when IL 2 was added to the cultures and also secreted autoantibodies with rheumatoid factor activity and sometimes also with anti-single-stranded, but not double-stranded, DNA activity. The efficient response of CD5+ cells was not explained by the fact that they contained cells already activated in vivo. Thus, they did not express the CD23, CD69, CD71 and CD39 activation markers, failed to incorporated [3H]thymidine and to secrete Ig spontaneously or in response to IL 2 and were found to be in a quiescent state by cell cycle flow cytometric analysis. In contrast to CD5+ cells, CD5- cells displayed very little or no [3H]thymidine incorporation in response to SAC or to a mu Ab and their poor responsiveness was not altered by changing either the doses of the stimulants, the timing of the cultures, by co-culturing the cells together with CD5+ cells, or by adding IL 2 or IL 4. Immunofluorescence studies showed that freshly prepared CD5- cells did not have surface activation markers but that they expressed them following SAC stimulation. Thus, unlike that observed for CD5+ cells, SAC seems to be capable of activating CD5- cells but does not appear to be a sufficient stimulus for driving the cells into the subsequent phases of the cell cycle. The above findings, that demonstrate marked differences in the response to CD5+ and CD5- cells to thymus-independent stimuli, may bear relevance for the understanding of the normal clonal expansion of CD5+ cells as well as for the pathogenesis of autoimmune diseases.

Analysis of cell proliferation and mu-RNA processing during activation of mouse B-cells by anti-mu and T lymphokines

Anti-immunoglobulin (anti-Ig, anti-mu is commonly used) activates resting mouse B-cells to proliferate but not to differentiate and secrete Ig. Differentiation requires additional help from T-cells including soluble factors such as lymphokines. The capability of lymphokines, alone and in combination, to promote the differentiation of anti-mu activated B-cells has been investigated. Some lymphokines, like interleukin (IL) 2 and 3, as well as human-interferon beta-2 (IL-6), have no significant effect on differentiation. IL-4 and 5 maintain cell growth but do not lead to differentiation, which requires multiple factors present in ConA supernatant or partially purified TRF. Anti-mu and interferon-gamma (IFN-gamma) exert both positive and negative effects on B-cell maturation. Anti-mu induces cell proliferation. IFN-gamma enhances Ig transcription, but it has no apparent proliferation or differentiation activity. Anti-mu and IFN-gamma inhibit Ig secretion by causing the accumulation of nuclear mu-RNA precursors. Although phorbol ester plus ionomycin induce cell proliferation, the negative effect of anti-mu in RNA processing could not be mimicked by these reagents. I show that anti-mu and IFN-gamma interfere with the splicing of nuclear hnRNA. This phenomenon is independent of known 2'-5'(A)n synthetase activity. The data suggest that post-transcriptional regulation of mu-RNA processing might be a critical event in controlling the generation of the plasma cells (which secrete IgM), memory precursor cells or abortive cells (both of which do not secrete IgM).

Ligation of membrane immunoglobulin leads to inactivation of the signal-transducing ability of membrane immunoglobulin, CD19, CD21, and B-cell gp95

We have examined the ability of membrane immunoglobulin-binding ligands to desensitize several human B-cell surface molecules that normally transduce signals leading to Ca2+ mobilization. Ligation of membrane IgM or IgD leads to heterologous desensitization of the reciprocal receptor in Epstein-Barr virus-transformed B-cell lines and peripheral blood B cells, as evidenced by a failure of cells to mobilize in response to receptor ligation. Under these conditions CD19, CD21, and B-cell gp95 ligation also did not lead to normal Ca2+ mobilization, indicating that these transducers are also desensitized. The desensitization does not reflect receptor modulation from the cell surface or reduced accessibility to ligand and is long lived, lasting greater than 16 hr. Finally, data that indicate that desensitized cells remain responsive to the G protein activating agent AIF4-, as measured by Ca2+ mobilization, suggest that desensitization reflects uncoupling of these receptors from G proteins that are intermediaries in their transduction of signals. We hypothesize that the molecular target of desensitization may be a recently described membrane immunoglobulin-associated and inducibly tyrosine-phosphorylated protein complex that may function as a master transducer in B cells, analogous to CD3 in T cells.

The role of class II molecules in human B cell activation. Association with phosphatidyl inositol turnover, protein tyrosine phosphorylation, and proliferation

Cross-linking class II molecules on resting human B cells can initiate phosphatidyl inositol turnover and an increase in intracellular calcium concentration levels comparable with that seen with the cross-linking of surface Ig receptors. The calcium response is most evident on dense B cell fractions: buoyant cells are less responsive, even though the levels of class II expression are similar on dense and buoyant tonsillar B cells. Human B cell lines exhibit the same absence of correlation between intensity of the calcium signal and levels of surface class II expression, indicating that responsiveness is related to the state of differentiation of the cell rather than the amount of class II expressed. Cross-linking class II on normal B cells or B cell lines caused accumulation of inositol phosphates, suggesting class II induces calcium release from intracellular stores, rather than through direct regulation of calcium channels. The calcium response mediated through class II was completely abolished by bringing the protein tyrosine phosphatase, CD45, into close proximity with surface class II. This result indicated that protein tyrosine phosphorylation might regulate the signal transduced through this molecule. In support of this notion we found that tyrosine phosphorylation is induced when small dense tonsillar B cells are stimulated with either anti-Ig or with antibodies to class II. Finally, in B cell proliferation assays we show that cross-linking class II molecules on dense tonsillar B cells synergize strongly with suboptimal concentrations of PMA or IL-4. The significance of these results is discussed with regard to the cognate signal between B and T lymphocytes.

The effects of bacterial lipopolysaccharide, anti-receptor antibodies and recombinant interferon on mouse B cell cycle progression using 5-bromo-2'-deoxyuridine/Hoechst 33258 dye flow cytometry

Polyclonal stimulation of resting B cells with anti-antigen receptor antibodies [anti-IgM mu chain antibody (anti-mu)] or with bacterial lipopolysaccharide (LPS) stimulates a proportion of B cells to proliferate. Exposure of resting B cells to both LPS and anti-mu activates a larger population of resting B cells than either alone, suggesting a synergistic effect of these two stimuli. Although recombinant interferon (rIFN) either alone or in combination with anti-mu has no apparent proliferative activity (as measured by [3H]thymidine incorporation), application of 5-bromo-2'-deoxyuridine/Hoechst 33258 dye flow cytometry reveals a distinct effect of rIFN on B cell growth. In the presence of anti-mu plus LPS, rIFN causes the majority of B cells to enter the cell cycle (CC), but a subset of B cells remains in the resting stage. Another subset of B cells has extremely rapid CC transit times, with a CC duration of less than 10 h. These studies show that both anti-mu and LPS are competence factors (which move cells from the G0 phase to the G1 phase). LPS acts also as a CC progression factor, while rIFN is a CC potentiating factor.

Intermolecular disulfide bonding in IgM: effects of replacing cysteine residues in the mu heavy chain

The conventional model of polymeric IgM depicts a unique structure in which the mu heavy chains and J chain are joined by well defined disulfide bonds involving cysteine residues at positions 337, 414 and 575 of the mu chain. To test this model, we have used site directed mutagenesis to produce IgM in which these cysteines have been replaced by serine. In each case the single mutants were able to assemble polymeric IgM, which was analyzed for its size, morphology, J chain content and activity in complement dependent cytolysis. Whereas normal polymeric IgM is composed predominantly of pentameric and hexameric molecules, the mutant IgM-Ser414 is covalently assembled as pentamers and smaller forms; IgM-Ser575 is assembled as covalent hexamers. IgM-Ser337 appears to include the same pentameric and hexameric forms as normal IgM except that, unlike normal polymeric IgM, most pentameric/hexameric IgM-Ser337 is not covalently assembled. J chain is present in polymeric IgM-Ser337 but absent in polymeric IgM-Ser414 and IgM-Ser575. IgM-Ser414 is defective in activating the classical pathway of complement dependent cytolysis. Our observations are consistent with models in which the covalent linkages between mu chains are mediated by disulfide bonded Cys337-Cys337, Cys414-Cys414 and Cys575-Cys575 but indicate that the arrangement of these Cys-Cys pairs in series and in parallel varies among and within IgM molecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Large granular lymphocytes from B-chronic lymphocytic leukemia patients inhibit normal B cell proliferation

Large granular lymphocytes (LGL) may exert regulatory influences on B cell immunoglobulin synthesis. We, therefore, investigated the influence of LGL from controls and B cell chronic lymphocytic leukemia patients (B-CLL) on control B cell proliferation to costimulation with the F(ab')2 fragment of goat antihuman mu and B cell growth factor (BCGF). Purified LGL (greater than 90% by morphology) from control and B-CLL peripheral blood were added in various concentrations to purified control B cells and incubated with anti-mu and BCGF for 3 days. [3H]-thymidine uptake of B cells was then measured. There was no proliferation of control or CLL LGL alone to the costimulatory signals of the F(ab')2 fragments of goat antihuman mu chain and BCGF. Addition of control LGL to equal numbers of control B cells did not blunt control B cell responsiveness to BCGF (with control LGL 8,649 +/- 298 cpm vs. control B cells alone 8,336 +/- 556 cpm, mean +/- SEM). When control LGL were increased to 10:1 LGL:B cell ratio, the maximal inhibition by control LGL of control B cell proliferative response to BCGF was 23%. In contrast, addition of CLL LGL at a 1:1 LGL:B cell ratio resulted in marked impairment of the control B cell proliferative response to BCGF (with CLL LGL 3,586 +/- 954 cpm vs. control B cells alone 8,649 +/- 298 cpm). Inhibition by CLL LGL occurred in a cell-concentration-dependent manner. No difference in CLL LGL's inhibitory effect on either resting or activated control B cell responsiveness to BCGF was noted. Inhibition of de novo protein synthesis (by cycloheximide inhibition) of CLL LGL did impair CLL LGL's inhibitory capacity for BCGF-induced B cell proliferation. A possible explanation for these findings includes the possibility that a subgroup of LGL with B cell suppressive activity may have expanded as a host response to the B cell leukemia or as part of the disordered cell regulation in B-CLL.

Cross-linking of membrane IgM on B CLL cells: dissociation between intracellular free Ca2+ mobilization and cell proliferation

It has been shown previously that chronic lymphocytic leukemia (CLL) B cells are frozen at different stages of activation with unique requirements for proliferation. Although most B CLL cells express surface IgM, anti-mu antibodies are able to trigger only some of them to proliferate and/or respond to B cell growth factor (BCGF) or interleukin 2 (IL2), as normal B cells. In this report we extend these observations using three different monoclonal antibodies (mAb) to human mu chain (one mitogenic in soluble form for normal B cells, the two others mitogenic only when coupled on Sepharose 4B beads). Cells from only 3 out of 11 B CLL patients proliferated in the presence of either mitogenic anti-mu. When the early events following surface Ig cross-linking, such as calcium mobilization (by flow cytometry on indo-1-labeled cells), were studied all three mAb in soluble form were able to induce a similar increase in the intracellular free calcium concentration ([Ca2+]i); analogous to [Ca2+]i rise after the mitogenic F(ab')2 anti-mu stimulation. This response was seen only in 8 out of the 12 CLL B cells studied. All B CLL cells, however, proliferate in response to a combination of phorbol ester 12,13-dibutyrate (PBt2) and ionomycin. Therefore, three patterns of response to sIg cross-linking by anti-mu could be distinguished: cells from 4 out of 12 cases proliferate and mobilize Ca2+ upon anti-mu triggering (behaving like resting B lymphocytes); in 4 other cases anti-mu lead to Ca2+ mobilization without cell proliferation; in the last 4 cases neither Ca2+ mobilization, IP3 generation (in the one case studied) nor cell proliferation are observed although these cells do proliferate directly in response to growth factors. Moreover, anti-mu stimulation in this group leads to increased proliferation in response to BCGF and IL2 suggesting an anti-Ig signaling independent of inositol phosphate metabolism. These results are interpreted in terms of differential anti-mu signaling on B cells at different stages of activation.

A role for human heavy chain binding protein in the developmental regulation of immunoglobin transport

Human EBV transformed lymphoblastoid cell lines and lymphomas representing various stages of B cell development were examined for heavy chain binding protein (BiP) expression and its association with immunoglobin (Ig) heavy chains. Human BiP was shown to migrate with an apparent mol. wt of 79,000 and to have a pI of approximately 5.5 in all the human cell lines examined. Both the mum and the mus heavy chains synthesized in a pre-B cell line (mu+, LC-) remained associated with BiP and were all found to be endo H sensitive, suggesting that this association occurred in the endoplasmic reticulum (ER). Surface Ig+ B cell lines produce membrane type heavy chains which are expressed on the cell surface and secretory type heavy chains which remain intracellular. The membrane type mu heavy chains produced by a surface Ig+ B cell line were not associated with BiP after assembling with light chains and processing in the Golgi. However, the secretory type mu heavy chains synthesized by these same cells did not combine efficiently with LC and a significant quantity remained associated with BiP and were not secreted suggesting that BiP is involved in the divergent transport of membrane and secretory mu heavy chains in surface Ig+ B cell lines. In Ig secreting plasmacytoid lines the heavy chains were only associated with BiP prior to assembling with LC. When LC assembly was inhibited, the association of heavy chains with BiP was prolonged and Ig secretion was blocked. Therefore, BiP was found to participate in the post-translational processing of mu heavy chains synthesized by human lymphoid cell lines representing all stages of B cell development. Further, heavy chains that remained associated with BiP were not transported to the cell surface or secreted while heavy chains that were only transiently associated with BiP chains were expressed on the cell surface or secreted.

The immunoglobulin mu chains of membrane-bound and secreted IgM molecules differ in their C-terminal segments

The B lymphocytes synthesizes two forms of IgM molecules during its development from a stem cell to a mature antibody-secreting plasma cell. The monomeric receptor IgM molecule is affixed to the plasma membrane and triggers the later stages of B cell differentiation, whereas the pentameric secreted IgM molecule is an effector of humoral immunity. The structural differences between membrane-bound and secreted IgM molecules are reflected in the differences between their heavy or mu chains. We have previously determined the complete amino acid sequence of a murine secreted mu (microsecond) chain. In this study, we have compared the structures of the secreted and membrane-bound mu (micron) heavy chains by peptide mapping, micro-sequence and carboxypeptidase analyses. These studies demonstrate that the micron and microsecond chains are very similar throughout their VH, C mu 1, C mu 2, C mu 3 and C mu 4 domains. The micron and microsecond chains differ in the amino acid sequence of their C-terminal segments. These studies in conjunction with those carried out on the micron and microsecond mRNAs and the C mu gene suggest that the micron and microsecond chains from a given B cell are identical except for their 41 and 20 residue C-terminal segments, respectively. The amino acid sequence of the 41 residue C membrane terminal segment predicted from the corresponding micron mRNA is in agreement with all the protein studies reported in this paper.