Synthesis of immunoglobulin mu chain gene products precedes synthesis of light chains during B-lymphocyte development

Sixty Years of Discovery

Each of us is a story. Mine is a story of doing science for 60 years, and I am honored to be asked to tell it. Even though this autobiography was written for the Annual Review of Immunology, I have chosen to describe my whole career in science because the segment that was immunology is so intertwined with all else I was doing. This article is an elongation and modification of a talk I gave at my 80th birthday celebration at Caltech on March 23, 2018.

Discovering NF-kappaB

David Baltimore recalls the experiments that led to the discovery of the NF-kappaB transcription factor more than 20 years ago.

From gene amplification to V(D)J recombination and back: a personal account of my early years in B cell biology

I have been invited to write a short historical feature in the context of being a co-recipient with Klaus Rajewsky and Fritz Melchers of the 2007 Novartis Prize in Basic Immunology that was given in the general area of the molecular biology of B cells. In this feature, I cover the main points of the short talk that I presented at the Award Ceremony at the International Immunology Congress in Rio de Janeiro, Brazil. This talk focused primarily on the work and people involved early on in generating the models and ideas that have formed the basis for my ongoing efforts in the areas of V(D)J recombination and B cell development.

Deletion of the RAG2 C terminus leads to impaired lymphoid development in mice

The recombination-activating gene (RAG)1 and RAG2 proteins comprise the lymphocyte-specific components of the V(D)J recombinase and are required for the assembly of antigen-receptor variable-region genes. A mutant truncated RAG2 protein ("core" RAG2) lacking the C-terminal 144 amino acids, together with core RAG1, is able to mediate the basic biochemical steps required for V(D)J recombination in vitro and in transfected cell lines. Here we examine the effect of replacing the endogenous RAG2 locus in mice with core RAG2. These mice generate substantial numbers of B and T cells, demonstrating that the core RAG2 protein retains significant in vivo function. However, core RAG2 mice display a reduction in the total number of B and T cells, reflecting impaired lymphocyte development at the progenitor stage associated with reduced chromosomal V(D)J recombination. We discuss potential roles of the RAG2 C terminus in mediating rearrangement of endogenous antigen-receptor loci.

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.

Deletion of the Ig kappa light chain intronic enhancer/matrix attachment region impairs but does not abolish V kappa J kappa rearrangement

Roles of the kappa intronic enhancer (iE kappa) and its associated matrix attachment region (MAR) during B cell development were examined using mutant embryonic stem (ES) cell lines in which the entire region on both chromosomes was replaced with either a recombined LoxP site (E kappa ND) or the PGK-neomycin resistance (PGK-neo(r)) gene (E kappa NI). B cells derived from E kappa ND ES cells had greatly impaired V kappa J kappa rearrangement, normal levels of kappa expression, and kappa:lambda ratios of 1:1 instead of the usual 10:1. Furthermore, lambda-producing hybridomas derived from E kappa ND cells displayed little kappa rearrangement. Thus, the MAR and iE kappa are quantitatively significant for kappa rearrangement but not necessary. In addition, little V kappa J kappa rearrangement could be detected in B cells derived from E kappa NI ES cells, demonstrating that an inserted PGK-neo(r) gene dominantly suppresses V kappa J kappa rearrangement.

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.

Involvement of wild-type p53 in pre-B-cell differentiation in vitro

Wild-type p53 protein is a growth modulator whose inactivation has been found to be a key event in malignant transformation. Reconstitution of wild-type p53 in the p53-nonproducer, Abelson murine leukemia virus-transformed pre-B-cell line L12 gave rise to stably growing clones. Wild-type p53-producer derived cell lines exhibit an altered cell cycle, however. More cells with an extended G0/G1 phase were found than in the p53-nonproducer parental cell line. Furthermore, when injected into syngeneic mice, these cells induced a lower incidence of tumors and these tumors were less aggressive. Analysis of immunoglobulin expression revealed that wild-type p53 induced the expression of cytoplasmic immunoglobulin mu heavy chain. In addition, these derived cells lines exhibited increased levels of a B-cell-specific surface marker, B220. These results suggest that wild-type p53 may function as a cell differentiation factor that can induce development of pre-B cells into a more advanced stage in the pathway of B-cell maturation. In these pre-B cells, wild-type p53 may induce cell differentiation without terminal growth arrest of the cell population.

Two types of mu chain complexes are expressed during differentiation from pre-B to mature B cells

Immunoglobulin mu chains synthesized in murine pre-B cells are known to be associated with surrogate light chains designated as omega (omega), iota (iota) and B34. In addition to these molecules, we identified the complexes of polypeptides (50, 40, 27 and 15.5 kd) associated with surface or intracellular mu chains of pre-B cell lines. Most of these polypeptides were continuously synthesized and associated with mu chains in virgin B cells lines, although some of them scarcely bound to the mu kappa dimer or mu 2 kappa 2 tetramer concomitantly present in the same clone or population. However, in mature B cells they were no longer detectable except B34. Cross-linking of micron chains on the surface of pre-B cells resulted in an increase in intracellular free Ca2+, indicating that the micron chain complex on the surface of pre-B cell lines acted as a signal transduction molecule. However, the receptor cross-linkage of pre-B cell lines did not induce the increased inositol phospholipid metabolism usually observed in virgin and mature B cell lines. These results suggest that, during the differentiation from pre-B to mature B cells, the cells express two types of mu chain complexes which exhibit different structures as a whole and possess different signal transducing capacities.

Biased expression of JH-proximal VH genes occurs in the newly generated repertoire of neonatal and adult mice

We have previously demonstrated a dramatic preference for utilization of the most JH-proximal VH gene segments in the newborn liver versus adult spleen. We now examine in detail the relative expression of different VH gene families throughout ontogeny and in immunodeficient mice to gain insight into factors that cause the shift in VH usage. We find that the relative expression of VH gene families remains constant and biased throughout fetal and neonatal liver development. In addition, the primary VH repertoire expressed in neonatal spleen displays a similarly biased, position-dependent VH repertoire. The pattern of VH gene expression begins to change at 5-7 d postnatally and reaches the adult randomized pattern at approximately 2 wk of age. We also find biased expression of JH-proximal VH gene families in adult bone marrow and in spleens of adult leaky scid mice, suggesting that the spontaneously generated repertoire of adult mice is similar to that observed in neonates. Together, these data suggest that a position-dependent repertoire is generated in differentiating pre-B cells at all stages of ontogeny, at least in part, as a result of preferential rearrangement of proximal VH gene segments. Therefore, mechanisms subsequent to V gene rearrangement, such as regulatory interactions and antigen selection, must play a major role in normalizing the repertoire.

neu protooncogene fused to an immunoglobulin heavy chain gene requires immunoglobulin light chain for cell surface expression and oncogenic transformation

The protein encoded by the neu protooncogene (human gene symbol NGL for neuro/glioblastoma-derived) is a member of the surface receptor/tyrosine kinase family. Though its structure suggests that it can transduce a transmembrane signal, neither its extracellular ligand nor its critical intracellular substrates are known. To explore the functional properties of the protein encoded by neu, we created a fusion gene that joins the cytoplasmic domain of neu to the extracellular portion of an immunoglobulin heavy chain. The localization of the fusion polypeptide can then be controlled by coexpression with immunoglobulin light chain. In the absence of light chain, the heavy chain-neu polypeptide is expressed intracellularly and has no transforming activity. By contrast, in the presence of light chain the fusion polypeptide is expressed at the cell surface and produces tumorigenic foci. Thus, transformation apparently requires expression at the cell surface, where the neu intracellular domain can interact with components that are localized to the plasma membrane. The fusion protein is active in cellular transformation when the transmembrane domain is derived either from neu or from immunoglobulin, indicating that the neu transmembrane domain is not specifically required for transformation, although neu activation in tumors is known to result from a point mutation in this region. The extracellular immunoglobulin heavy and light chain domains of the fusion protein form a functional binding site that allows antigen to modulate its activity, reversing the transforming effect.

Developmentally regulated and strain-specific expression of murine VH gene families

We have devised a simple assay that provides an instantaneous representation of VH family usage in primary and peripheral lymphoid tissues. This assay lacks complex manipulations out of the animal and thus minimizes the risk of in vitro artifacts. We have used this assay to demonstrate a dramatic preference for utilization of the most JH-proximal VH segments in the newborn liver of BALB/c and C57BL/6 mice. Furthermore, we find that VH segments from across the entire VH locus are utilized early in development, but at frequencies directly related to their JH proximity. A major shift away from the position-dependent VH repertoire of the neonate is seen in unprimed or polyclonally-activated adult spleen cells, in which relative utilization of the various VH families is related to family size. We also report consistent strain-specific differences in the expression of certain VH families. Our data indicate that a position-dependent VH repertoire is generated in differentiating pre-B lymphocytes (probably reflecting constraints imposed by the immunoglobulin gene assembly process), and that mechanisms that operate subsequent to rearrangement then randomize this position-dependent repertoire in a strain-specific manner.

Regulation of acetylcholine receptor transcript expression during development in Xenopus laevis

The level of transcripts encoding the skeletal muscle acetylcholine receptor (AChR) was determined during embryonic development in Xenopus laevis. cDNAs encoding the alpha, gamma, and delta subunits of the Xenopus AChR were isolated from Xenopus embryo cDNA libraries using Torpedo AChR cDNAs as probes. The Xenopus AChR cDNAs have greater than 60% amino acid sequence homology to their Torpedo homologues and hybridize to transcripts that are restricted to the somites of developing embryos. Northern blot analysis demonstrates that a 2.3-kb transcript hybridizes to the alpha subunit cDNA, a 2.4-kb transcript hybridizes to the gamma subunit cDNA, and that two transcripts, of 1.9 and 2.5 kb, hybridize to the delta subunit cDNA. RNase protection assays demonstrate that transcripts encoding alpha, gamma, and delta subunits are coordinately expressed at late gastrula and that the amount of each transcript increases in parallel with muscle-specific actin mRNA during the ensuing 12 h. After the onset of muscle activity the level of actin mRNA per somite remains relatively constant, whereas the level of alpha subunit and delta subunit transcripts decrease fourfold per somite and the level of gamma subunit transcript decreases greater than 50-fold per somite. The decrease in amount of AChR transcripts per somite, however, occurs when embryos are paralyzed with local anaesthetic during their development. These results demonstrate that AChR transcripts in Xenopus are initially expressed coordinately, but that gamma subunit transcript levels are regulated differently than alpha and delta at later stages. Moreover, these results demonstrate that AChR transcript levels in Xenopus myotomal muscle cells are not responsive to electrical activity and suggest that AChR transcript levels are influenced by other regulatory controls.

Immunoglobulin gene expression is a normal differentiation event in embryonic thymocytes

By in situ hybridization to frozen sections of mouse embryos, we have localized cells transcribing the Ig C mu gene during ontogeny. Transcripts were detected from before day 14 of gestation in individual pre-B cells in the liver and, surprisingly, in a large proportion of thymocytes between days 15 and 18. The level of mu RNA sequences in the thymus at day 17 was much higher than has been observed for adult thymocytes; from grain counts, the amount of mu RNA was similar to that observed for Ti gamma RNA. These findings suggest that Ig and Ti genes are under similar transcriptional controls during Ti gene recombination and that elevated mu RNA production is a normal event early in the intrathymic differentiation of T cells.

Modification of the cell surface expression of histocompatibility antigens induced by the neurotoxin 2,5 hexanedione

Class I histocompatibility antigens (HLA) are expressed on the surface of almost all nucleated mammalian cells; the expression of this surface antigenic molecule may be changed or abrogated by several factors. In this paper, a modification in HLA expression in a human carcinoma cell line following exposure to the neurotoxicant 2,5 hexanedione is reported. This compound is known to produce a wide spectrum of subcellular pathological events; in this study, we describe an effect on the surface and cytoplasmic distribution of both light and heavy subunits of HLA antigens, demonstrated by immunocytochemical and immunoelectron microscopy techniques. Human carcinoma cells, which under normal growing conditions express the HLA, abrogate the surface expression of this glycoprotein after exposure to 2,5 hexanedione and an intracytoplasmic accumulation seems to occur. Several possibilities are discussed, such as an effect of the toxicant on the transport of the nascent glycoprotein.

Phorbol ester induces class II gene expression in pre-B cell lines

The effect of phorbol myristate acetate on the induction of major histocompatibility complex class II gene expression was studied in four Abelson murine leukemia virus-transformed pre-B cell lines. In three cell lines, low concentrations of PMA (0.1-10 ng/ml) induced the expression of high levels of surface Ia molecules, and this effect was mediated at the transcriptional level. PMA induced a program of coordinated transcription of all four genes involved in the biosynthesis of Ia molecules. A spontaneous Ia-positive variant pre-B cell line was derived from an Ia-negative parental cell line. This variant was highly sensitive to the toxic effects of PMA, and highly responsive to the Ia-inductive effects of phorbol ester. Our findings suggest that in pre-B cells the appearance of Ia molecules and the regulation of their expression are controlled, at least partially, by variations in the activity of protein kinase C, which is the cellular receptor for phorbol esters.

Active lambda and kappa antibody gene rearrangement in Abelson murine leukemia virus-transformed pre-B cell lines

The two Abelson murine leukemia virus (A-MuLV)-transformed cell lines, BM18-4 and ABC-1, undergo immunoglobulin L-chain gene recombination during passage in tissue culture. BM18-4 cells are capable of kappa gene recombination, whereas ABC-1 cells are capable of both kappa and lambda gene recombination. The expression of H chains is apparently not necessary for continuing L chain gene recombination in either of these cells, although H-chain expression may have been involved in the initiation of L-chain gene recombination. All ABC-1 cells that have lambda gene rearrangements also display recombined kappa alleles, supporting the hypothesis that kappa and lambda gene recombination are initiated in an ordered, developmentally regulated manner in maturing B cells. However, analyses of the ABC-1 line indicate that pre-B cells that have initiated lambda gene recombination do not terminate kappa gene rearrangement. The lambda gene recombinations that occur in the ABC-1 cell line indicate that the germline order of lambda gene segments is: 5' ... V lambda 2 ... J lambda 2C lambda 2-J lambda 4C lambda 4 ... V lambda 1 ... J lambda 3C lambda 3-J lambda 1C lambda 1 ... 3'. In addition, the frequencies of lambda 1, lambda 2, and lambda 3 gene recombinations among ABC-1 cells are quite different than the frequencies of B cells producing lambda 1, lambda 2, and lambda 3 L-chains in the mouse. RS DNA recombinations also occur in the BM18-4 and ABC-1 cell lines, supporting the notion that Ig gene recombinases are involved in RS rearrangement. Recombined RS segments are infrequent among BM 18-4 cells but common among ABC-1 cells, suggesting that RS recombinational events often occur in maturing pre-B cells just before initiation of lambda gene rearrangements. This developmental timing is consistent with the hypothesis that RS recombination may be involved in the initiation of lambda gene assembly.

Progenitor and pre-B lymphocytes transformed by Epstein-Barr virus

By rosetting with SRBC coupled to rabbit-anti-human IgM, the surface IgM-negative cells of human fetal bone marrow were enriched, and subsequently infected and transformed by Epstein-Barr virus (EBV). Single clones of the transformed cells were obtained. Ninety percent of the resulting cell clones were surface-immunoglobulin-negative, and of 8 clones which were further studied, 5 lacked intracellular, cytoplasmic Ig as measured by immunofluorescence. Control cell clones derived from the same material without pre-selection expressed surface Ig and also secreted Ig. Utilization of a panel of B-cell-specific monoclonal antibodies (MAbs) showed no difference between the cell clones expressing surface Ig and those that did not. The progenitor B-cell lines did not show a phenotype resembling that of cell lines derived from B-cell malignancies, such as high agarose clonability. In spite of their immature Ig-phenotype, these clones showed rearrangement of at least one heavy chain Ig-allele. Efforts to induce differentiation in these clones were unsuccessful. These clones may represent progenitor B cells, or B cells with faulty heavy-chain rearrangement. EBV can apparently be used as a tool to derive cell lines representing different levels of B-cell differentiation, and can also transform immature B cells, which may be useful in the analysis of B-cell differentiation.

Abelson virus potentiates long-term growth of mature B lymphocytes

Abelson murine leukemia virus (A-MuLV) infection of mouse bone marrow cells usually leads to transformation of pre-B cells. However, when the environment is modified by the continuous presence of lipopolysaccharide (LPS), two novel types of membrane immunoglobulin (mIg)-positive B cell lines are generated. Because the cells which give rise to these cell lines copurify with mIg-positive bone marrow cells, the cell lines arise as a result of A-MuLV interaction with a new type of in vitro target cell. The cell lines generated fall into two groups which differ in several phenotypic characteristics. Group 1 cells are more differentiated than the typical pre-B cell transformant in that they synthesize mIgM and appear to resemble virgin B cells. The group 1 cells do not secrete immunoglobulin and are independent of LPS for growth. In addition, these cell lines synthesize the Abelson P160 protein, contain integrated abl proviral DNA, and are highly tumorigenic in syngeneic animals. The group 2 cell lines differ markedly from both the group 1 cells and from typical, pre-B cell A-MuLV transformants. These cells are mIgG positive and secrete large amounts of immunoglobulin into the culture medium. The cell lines are comprised of both adherent and nonadherent cells and do not synthesize P160 or contain integrated v-abl sequences. The group 2 cells are nontumorigenic in syngeneic animals and require LPS for growth and viability. Both types of cells have remained in culture for over 2 years with no changes in their phenotypic characteristics. This A-MuLV infection system and the novel mIg-positive cell lines may serve as useful models for studying biochemical and molecular properties of mature B cells.

Sequences 3' of immunoglobulin heavy chain genes influence their expression

The function of sequences 3' of Ig genes in controlling their expression has been investigated by analyzing mutants of Ig-producing cells and by gene transfection experiments. A mutant of an IgA producing myeloma was isolated whose steady-state level of heavy chain mRNA and protein was decreased. Analysis of the mutant showed it had deleted at least 4 kb of DNA immediately 3' of the alpha gene and introduced at least 5 kb of non-Ig sequence in its place. Examination of nuclear RNA showed no accumulation of aberrant transcripts or altered processing patterns. Instead, the transcription rate of heavy chain in the mutant was approximately 1/7 of that in its parent. This mutant suggests that sequences 3' of Ig genes facilitate their transcription; alternatively, the non-Ig sequences may act to depress transcription. A complete gamma 2b heavy chain gene containing both the secreted and membrane exons was transfected into lymphoid cells. The ratio of membrane/secreted Ig mRNA produced by the transfectants was found to reflect the phenotype of the recipient cell; myelomas made mostly secreted mRNA while lymphomas made only a slight excess of secreted mRNA. When a heavy chain was used with a deletion of sequences within the IVS between the secreted and membrane exons which left the AATAA poly A addition signal intact, but removed the site of cleavage and polyadenylation, the processing ratio was altered so that predominantly membrane Ig was produced in transfected myeloma cells. The alteration in processing could be a result of the deletion of the normal site for poly A addition; alternatively it could be a result of the deletion of another sequence which is recognized by processing enzymes. A 13 bp sequence (GTCCTGGTTCTTT), was found to be highly conserved both in position and sequence in human and mouse gamma chain genes. When a gene with a deletion which left the poly A addition site and the conserved sequence intact was used for transfection, the processing pattern was found to be identical to that of a wild type heavy chain gene.

Abelson virus potentiates long-term growth of mature B lymphocytes

Abelson murine leukemia virus (A-MuLV) infection of mouse bone marrow cells usually leads to transformation of pre-B cells. However, when the environment is modified by the continuous presence of lipopolysaccharide (LPS), two novel types of membrane immunoglobulin (mIg)-positive B cell lines are generated. Because the cells which give rise to these cell lines copurify with mIg-positive bone marrow cells, the cell lines arise as a result of A-MuLV interaction with a new type of in vitro target cell. The cell lines generated fall into two groups which differ in several phenotypic characteristics. Group 1 cells are more differentiated than the typical pre-B cell transformant in that they synthesize mIgM and appear to resemble virgin B cells. The group 1 cells do not secrete immunoglobulin and are independent of LPS for growth. In addition, these cell lines synthesize the Abelson P160 protein, contain integrated abl proviral DNA, and are highly tumorigenic in syngeneic animals. The group 2 cell lines differ markedly from both the group 1 cells and from typical, pre-B cell A-MuLV transformants. These cells are mIgG positive and secrete large amounts of immunoglobulin into the culture medium. The cell lines are comprised of both adherent and nonadherent cells and do not synthesize P160 or contain integrated v-abl sequences. The group 2 cells are nontumorigenic in syngeneic animals and require LPS for growth and viability. Both types of cells have remained in culture for over 2 years with no changes in their phenotypic characteristics. This A-MuLV infection system and the novel mIg-positive cell lines may serve as useful models for studying biochemical and molecular properties of mature B cells.

Discordance between surface and cytoplasmic expression of the Leu-4 (T3) antigen in thymocytes and in blast cells from childhood T lymphoblastic malignancies

We have examined the expression of the Leu-4 (T3) antigen on the cell surface and in the cytoplasm of blast cells from 23 patients with T cell acute lymphoblastic leukemia and T cell lymphoblastic lymphoma. In the majority of cases (17), the Leu-4 antigen was absent from the cell surface; however, in 16 of these 17 cases, blast cells demonstrated cytoplasmic expression of Leu-4. This discordance between surface and cytoplasmic expression of Leu-4 was also found in thymocytes and appeared to be restricted to Leu-4, in that tests of other T cell antigens rarely revealed discordance between surface and cytoplasmic expression. To study further the cytoplasmic determinant identified by anti-Leu-4 in malignant T lymphoblasts, immunoprecipitation studies were performed that utilized biosynthetic labeling of established T cell lines derived from T lymphoblastic malignancies. By one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis, identical Leu-4 polypeptide families were immunoprecipitated from surface Leu-4+ and surface Leu-4-/cytoplasmic Leu-4+ cell lines. Because T lymphoblastic malignancies represent proliferations of immature T cells, and because the cases studied demonstrated surface phenotypes corresponding to all of the proposed stages of T cell ontogeny, it appears that cytoplasmic expression of Leu-4 occurs early in T cell development. The reason for the failure of these immature T cells to transport the Leu-4 molecule to their surface remains to be elucidated.

Mott cells are plasma cells defective in immunoglobulin secretion

Plasma cells containing intracellular inclusions of immunoglobulin (Russell bodies) are known as Mott cells, and are found in large numbers in lymphoid organs in autoimmune mice. Hybridoma technique was used to produce cell lines of this phenotype by fusing spleen cells from a NZB mouse with a nonproducing hybridoma cell line (Sp2/0-Ag14), allowing us to carry out studies of this cell type at the biochemical level. Ultrastructurally the inclusions were distended cisternae of rough endoplasmic reticulum, suggesting a block in the secretory pathway of the cells. Biosynthetic labeling studies confirmed that these cell lines have either a complete or partial block of secretion of immunoglobulin, possibly due to an abnormal light chain.

Epstein-Barr virus transforms precursor B cells even before immunoglobulin gene rearrangements

The very early stages of the human B-cell differentiation pathway are poorly understood, primarily because of the lack of appropriate permanent cell lines. Epstein-Barr virus (EBV) is a putative human oncogenic virus which transforms human B cells in vitro into continuously proliferating cells. It has been believed that EBV transforms mature B cells, but recently, transformation of immature pre-B-cell lines has been reported, suggesting that EBV might also transform cells much earlier in the B-cell lineage. We report here the establishment of cell lines transformed by EBV at various stages of the B-cell differentiation pathway. Interestingly, two lines showed the complete absence of immunoglobulin synthesis and the lack of immunoglobulin gene rearrangement despite containing EBV genome and surface markers of B cells. Our results indicate that EBV can infect and transform cells of the B lymphocyte lineage even before immunoglobulin gene rearrangement.

The molecular biology of immunoglobulin D

Immunoglobulin D (IgD) is co-expressed with immunoglobulin M (IgM) on the membranes of most B cells, yet its biological function has remained a mystery. Recent detailed information on the structure and transcription of the unusual IgD heavy chain (delta) gene in mouse suggests a complex genetic control. A model is presented for the developmental regulation of IgM and IgD and roles suggested for the membrane and secreted forms of IgD in the immune network.

Sequential generation of antibody diversity during B-cell development

It has become increasingly apparent that generation of and variation in antigen-combining sites of antibodies occurs sequentially during B-cell development. Allelic and isotypic exclusion mechanisms ensure that a B cell produces antibody molecules having a single kind of combining site. A major reason for evolution of mechanisms which result in asynchronous formation of functional H and L chain genes may be the need for allelic and isotypic exclusion; but this may not be the only advantage of asynchronous formation of H and L chain genes. The evolution of mechanisms causing nonjunctional somatic mutation late in B cell development - only after antigen exposure apparently - may result from the biological advantages of: (1) 'fine tuning' of the combining site; (2) a response to an anti-idiotype regulatory network, or (3) expanded memory.

Surface antigen expression and immunoglobulin gene rearrangement during mouse pre-B cell development

The first part of this article discusses the isolation and characterization of several monoclonal antibodies to the B lineage-specific surface molecule, B220. B220 was shown to be expressed on precursors of B cells by the demonstration that removal of B220+ cells from B cell-depleted bone marrow removes the ability of bone marrow to regenerate B cells. Although these antibodies recognize a broad range of differentiation stages within the B lineage, they can be used to isolate highly enriched populations of pre-B cells from mouse bone marrow. We also describe the use of antibodies to the surface markers B220 and ThB to define two sequential stages of pre-B cell differentiation. A simplified diagram of our current view of the B lineage differentiation sequence is shown in Figure 4. No attempt has been made to include the various functionally defined B cell subsets on this diagram since we know almost nothing about the expression of these two surface markers on them. This model reflects an assumption that the early part of B cell differentiation is a linear rather than a branching pathway. At present, there is no evidence for a branching pathway, but little evidence against it either. B220 is the first B cell-specific molecule known to be expressed during differentiation and it continues to be expressed on most subsequent B lineage cells. In this regard, it resembles the Thy-1 molecule on thymus-derived lymphocytes and, like Thy-1, B220 should be quite useful for identifying and classifying B lineage cells. One example of this is the use of B220 expression to clearly assign germinal center cells to the B lineage. The example of Thy-1+, RA3-2C2+ cells from mice with the lpr/lpr genotype, however, suggests that some caution should be used when interpreting data, especially with pathological samples. The availability of substantially purified pre-B cell populations has made it possible to follow changes in immunoglobulin gene organization and expression during differentiation. Our current understanding of these events is also shown in Figure 4, correlated with cell surface phenotype. The large pre-B cell population has extensive heavy chain rearrangements and synthesizes significant quantities of mu heavy chain, but does not yet have detectable light chain gene rearrangement. The small pre-B population consists of two cell types, some with kappa gene rearrangement and some without. This suggests that kappa rearrangement occurs within this cell population, which is homogeneous with respect to morphology and surface phenotype. The asynchrony of heavy and light chain gene rearrangement results in an asynchrony at the level of expression of these genes as well, but the purpose of this remains one of many unanswered questions about pre-B cell differentiation. Now that it is possible to identify, isolate, and manipulate pre-B cells as readily as B or T cells, many of these questions may now be addressed.

Regulated expression of an immunoglobulin kappa gene introduced into a mouse lymphoid cell line

We have introduced a functionally rearranged murine kappa light chain immunoglobulin (Ig) gene into an Abelson murine leukemia virus-transformed lymphoid cell line. Plasmid pSV2gpt-kappa 41, containing the kappa light chain gene from the myeloma MOPC41 and the selectable marker gene gpt, was introduced into 81A-2 cells by the calcium phosphate coprecipitation technique. Cells expressing the gpt gene were selected by growth in medium containing mycophenolic acid. One transfected cell line, kappa-2, was shown to make kappa mRNA and polypeptide chains and to assemble the kappa chain product with gamma 2b heavy chains to form an apparently complete IgG2b. When bacterial lipopolysaccharide was added to the growth medium, levels of kappa mRNA and polypeptide increased, showing regulated expression of the introduced kappa gene.

Differential regulation of membrane and secretory mu chain synthesis in human beta cell lines. Regulation of membrane mu or secreted mu

Regulation of membrane and secretory mu synthesis was examined in human lymphoblastoid cell lines representing various stages of differentiation. Immunoglobulin phenotype was determined by surface and cytoplasmic staining with fluorochrome-conjugated antibodies and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of anti-mu precipitable cellular products. The thymidine analogue, 5-bromo-2'-deoxyuridine (BUdR), which inhibits differentiation-specific proteins in a variety of systems, was used to examine regulation of immunoglobulin synthesis. We found that BUdR had a differential effect on membrane (mum) and secretory (mus) type mu heavy chains. Ig production in pre-B and plasma cell-like lines, which make mus, was unaffected by BUdR. However, surface expression of IgM (mum) in B cell lines was drastically inhibited at similar doses of BUdR without diminishing total Ig or protein synthesis. Examination of labeled mu chains from control and BUdR-treated B cell lines by SDS-PAGE revealed the production of two sizes of mu (mum and mus) in control cells and only the smaller size (mus) in BUdR-treated cells. This size difference could not be attributed to alterations in glycosylation of the molecules. These data show that BUdR inhibits the production of membrane mu chains without diminishing secretory mu chain synthesis in the same cell. Our findings suggest that thymidine-rich regions of the genome are involved in the regulation of mum vs. mus during B cell differentiation.

Multiple immunoglobulin heavy-chain gene transcripts in Abelson murine leukemia virus-transformed lymphoid cell lines

Lymphoid cells transformed by Abelson murine leukemia virus (A-MuLV) contain three classes of RNA transcripts from immunoglobulin mu genes. P mu-mRNAs (productive) correspond to the normal 2.7-kilobase (kb) membrane (mu m) and 2.4-kb secreted (mu s) mu mRNA species both in size and coding capacity and occur at approximately equal abundance in most mu-positive (pre-B-like) A-MuLV transformants. A mu-mRNAs (aberrant) generally fall into one of two categories--aberrantly small 2.3-kb mu m and 2.0-kb mu s mRNAs which encode aberrantly small mu polypeptide chains, or normal-sized, V H-containing mu RNAs which do not encode immunologically identifiable mu polypeptide chains. In one case, the latter type of A mu-mRNA was demonstrated to result from an in-phase termination codon in the D segment of the mu mRNA. Also, most, if not all, A-MuLV transformants express members of a 3.0 to 1.9-kb set of C mu-containing, but V H-negative S mu-RNAs (for sterile), the expression of which may occur simultaneously with but independently of P mu-mRNAs or A mu-mRNAs. The S mu-RNA sequences do not encode immunologically identifiable mu chains and can be produced by cells with unrearranged heavy-chain alleles, such as T-lymphocytes, although the structure of the S mu-RNAs from T-lymphoid cells appears to be different from that of B-lymphoid cell S mu-RNAs. Certain A-MuLV transformants also express gamma-RNA sequences that are probably analogous to the three different forms of mu RNA. These data support the concept that heavy-chain allelic exclusion, like that of light chains, is not mediated by control at the DNA or RNA levels but is probably a consequence of feedback control from cytoplasmic mu chains.

Normal human B cells display ordered light chain gene rearrangements and deletions

Human kappa-producing B cell lines and leukemias retain their excluded lambda light chain genes in the germ line configuration, whereas transformed lambda-producing B cells uniformly rearrange or delete their kappa genes (12). Whether the unexpected lambda gene recombinations within malignant lambda-producing B cells reflect a normal developmental process or are secondary to transformation and specific chromosomal translocations was uncertain. To resolve this issue, we purified circulating lambda-bearing B cells from a normal individual to 97% purity by using a series of negative selection steps and a final positive selection on a cell sorter. Over 95% of the collective kappa genes in these lambda B cells were no longer in their germ line form, with the majority (60%) deleted and the remainder present but in a rearranged state. The chromosomal loss of the germ line kappa genes included the joining (J kappa) segments as well as the constant (C kappa) region, yet the particular variable (V kappa) gene family studied was spared. In addition, the incidence of kappa gene deletions was higher in long-term than in freshly transformed lambda B cell lines. This implies that the deletion of aberrantly rearranged kappa genes may occur as a second event. Such a mechanism would serve to eliminate aberrant transcripts and light chain fragments that might interfere with the synthesis and assembly of effective immunoglobulin molecules. Thus, despite the nearly equal usage of kappa and lambda light chain genes in man, there appears to be a sequential order to their expression during normal B cell ontogeny in which kappa gene rearrangements precede those of lambda.

Biosynthesis and structure of membrane and secretory immunoglobulins

Almost all of the body's extracellular immunoglobulin (Ig) is derived from Ig-secreting plasma cells of lymphoid tissues. The secreted material is a heterogeneous mixture of different classes and specificities. Lymphoid tissues also contain a large number of essentially non-secretory cells--B lymphocytes--which bear Ig firmly associated with their plasma membranes. Ig molecules thus exist in two functionally different forms, as membrane-bound antigen receptors on the surface of B lymphocytes on the one hand, and as humoral secreted Ig antibodies on the other. On B cells, membrane-bound heavy chains have an apparent mol. wt. slightly larger than that of secreted heavy chains from plasma cells. Membrane-bound but not secreted heavy chains bind detergents, thus suggesting the presence of a hydrophobic region in membrane-bound heavy chains, which is absent in secreted heavy chains. Most investigations have dealt with immunoglobulin M. The two types of IgM heavy chains differ at their carboxy termini. Recent investigations at the nucleic acid level demonstrate that membrane-associated mu chains contain a 41-residue hydrophobic tail adjacent to the last constant domain, whereas secretory mu chains contain a 20-residue hydrophilic tail. At the present time, evidence is accumulating that all membrane-bound Ig heavy chain classes may contain similar hydrophobic structures necessary for anchorage of the molecules into the lipid bilayer.

Multiple immunoglobulin heavy-chain gene transcripts in Abelson murine leukemia virus-transformed lymphoid cell lines

Lymphoid cells transformed by Abelson murine leukemia virus (A-MuLV) contain three classes of RNA transcripts from immunoglobulin mu genes. P mu-mRNAs (productive) correspond to the normal 2.7-kilobase (kb) membrane (mu m) and 2.4-kb secreted (mu s) mu mRNA species both in size and coding capacity and occur at approximately equal abundance in most mu-positive (pre-B-like) A-MuLV transformants. A mu-mRNAs (aberrant) generally fall into one of two categories--aberrantly small 2.3-kb mu m and 2.0-kb mu s mRNAs which encode aberrantly small mu polypeptide chains, or normal-sized, V H-containing mu RNAs which do not encode immunologically identifiable mu polypeptide chains. In one case, the latter type of A mu-mRNA was demonstrated to result from an in-phase termination codon in the D segment of the mu mRNA. Also, most, if not all, A-MuLV transformants express members of a 3.0 to 1.9-kb set of C mu-containing, but V H-negative S mu-RNAs (for sterile), the expression of which may occur simultaneously with but independently of P mu-mRNAs or A mu-mRNAs. The S mu-RNA sequences do not encode immunologically identifiable mu chains and can be produced by cells with unrearranged heavy-chain alleles, such as T-lymphocytes, although the structure of the S mu-RNAs from T-lymphoid cells appears to be different from that of B-lymphoid cell S mu-RNAs. Certain A-MuLV transformants also express gamma-RNA sequences that are probably analogous to the three different forms of mu RNA. These data support the concept that heavy-chain allelic exclusion, like that of light chains, is not mediated by control at the DNA or RNA levels but is probably a consequence of feedback control from cytoplasmic mu chains.

Developmental hierarchy of immunoglobulin gene rearrangements in human leukemic pre-B-cells

We have used a special class of human acute lymphocytic leukemias, the common "non-T/non-B" cell type, to define a hierarchy of genetic rearrangements that occur during the earliest stages of B-cell maturation. This has allowed us to identify intermediate cells predicted by a hierarchial model in which immunoglobulin heavy chain variable region gene formation precedes that of light chain and in which kappa light chain gene formation precedes that of lambda. The model emphasizes the flexible nature of immunoglobulin gene recombination that not infrequently produces aberrant or null genes that are phenotypically excluded from expression. Remaining alleles or isotypic genes can then be utilized as "spares" undergoing recombination until a valid gene is formed. Significantly, the excluded allele or isotype is frequently deleted from the genome. In addition to defining a pathway of genetic maturation, this analysis provides a powerful means to further classify cases of non-T/non-B-cell acute lymphocytic leukemia, most of which seem to reside at early stages along the B-cell pathway of differentiation.