Regulation of an idiotype+ B cell lymphoma. Effects of antigen and anti-idiotopic antibodies on proliferation and Ig secretion

The B cell surface Ig molecule plays an important regulatory role in delivering inductive/tolerogenic signals to the cell. In this paper, the effect of Ag and anti-idiotopic antibodies on the in vitro proliferation and Ig secretion of a B cell tumor was studied. The tumor (BCL1), which had been transfected with the TEPC-15 VH and VL Ig genes, expresses surface Ig and secretes antibody that binds the hapten phosphorylcholine. We found that Ag (C polysaccharide and phosphorylcholine carrier Ag) and two different anti-idiotopic antibodies, in the absence of T cells, all inhibited the proliferation of the T15+ transfectant cell line. The anti-idiotopic antibodies, but not Ag, also inhibited the secretion of T15 Ig by this cell line, suggesting different functional roles for Ag vs anti-Id in the regulation of B cell inactivation. The inhibition of secretion and proliferation appears to be cell cycle phase related. In addition, mouse rIL-4 could override the inhibition of proliferation induced in these studies. These phenomena, demonstrating that binding of surface Ig can result in the transduction of negative growth signals to a B cell tumor, can be viewed as a manifestation of immunologic tolerance. These findings collectively demonstrate that Ag and anti-Id mediate different signals to B cells via interaction with the surface Ig. Because of the monoclonal nature of the T15 transfectant and the anti-idiotypic antibodies, this system can be used to investigate the underlying molecular reactions involved in the B cell response and induction of tolerance.

A conserved heptamer upstream of the IgH promoter region octamer can be the site of a coordinate protein-DNA interaction

Immunoglobulin genes contain a conserved eight base sequence element 5' to the site of transcription initiation. This octamer can serve as a site for the binding of nuclear proteins which are presumably involved in the cell type specific expression of this family of genes. In studying the binding of nuclear proteins to this conserved sequence element, we have detected a protein interaction that involves, in addition to the octamer, nucleotides which are immediately upstream. We have characterized this additional contact as a sequence specific interaction with a heptameric sequence element (CTCATGA) that is conserved among Ig heavy chain promoters. Protein binding to the heptamer is unique in that it is dependent upon the proximity and orientation of, as well as protein interaction with, the conserved octamer.

Alternative expression of secreted and membrane forms of immunoglobulin mu-chain is regulated by transcriptional termination in stable plasmacytoma transfectants

During B cell differentiation the mu H chain of IgM is produced initially as a cell surface (m) and later as a secreted (s) form. Production of either the secreted (microseconds) or the membrane (micron) mRNA results from selection of either the promoter proximal (microseconds) or distal (micron) poly(A)/cleavage site. We have transfected lymphoid cell lines of disparate developmental stages with productively rearranged Ig mu-chain constructs to determine the mechanism by which alternative 3' end selection is achieved. Stable transfectants of lymphomas (early stage, microseconds less than or equal to micron) and a plasmacytoma (late stage, microsecond much greater than micron) produce the anticipated levels of microsecond and micron mRNA. Transcription termination between the microsecond and micron poly(A) sites occurs only in the plasmacytoma transfectants, and appears to be the rate-limiting step in the production of the final steady state microsecond/micron mRNA ratio. We propose that as the B cell becomes terminally committed to secretion, the mechanism responsible for the regulation of the steady state microsecond/micron mRNA ratio shifts from exclusively post-transcriptional to predominantly transcriptional termination.

Alternative expression of secreted and membrane forms of immunoglobulin mu-chain is regulated by transcriptional termination in stable plasmacytoma transfectants

During B cell differentiation the mu H chain of IgM is produced initially as a cell surface (m) and later as a secreted (s) form. Production of either the secreted (microseconds) or the membrane (micron) mRNA results from selection of either the promoter proximal (microseconds) or distal (micron) poly(A)/cleavage site. We have transfected lymphoid cell lines of disparate developmental stages with productively rearranged Ig mu-chain constructs to determine the mechanism by which alternative 3' end selection is achieved. Stable transfectants of lymphomas (early stage, microseconds less than or equal to micron) and a plasmacytoma (late stage, microsecond much greater than micron) produce the anticipated levels of microsecond and micron mRNA. Transcription termination between the microsecond and micron poly(A) sites occurs only in the plasmacytoma transfectants, and appears to be the rate-limiting step in the production of the final steady state microsecond/micron mRNA ratio. We propose that as the B cell becomes terminally committed to secretion, the mechanism responsible for the regulation of the steady state microsecond/micron mRNA ratio shifts from exclusively post-transcriptional to predominantly transcriptional termination.

Regional localization of the human c-rel locus using translocation chromosome analysis

The human cellular homolog of v-rel, the transforming gene of reticuloendotheliosis virus, strain T, was previously localized to 2 cent-2p13 by a combination of somatic cell hybrid and in situ hybridization analyses. In this study, we use translocation chromosome analysis to refine c-rel's genetic assignment to 2p12-2p13.

Identification of murine nuclear proteins that bind to the conserved octamer sequence of the immunoglobulin promoter region

Sequence-specific DNA-affinity chromatography was used to purify a nuclear protein from the B-cell leukemia cell line BCL1 that specifically binds to the octamer sequence ATTTGCAT, previously shown to be important in the regulation of immunoglobulin genes. This protein has a molecular mass of approximately 70 kDa and is responsible for the protein-DNA interaction specific to lymphoid cells. Other proteins of molecular mass 80-90 kDa and 50-55 kDa that specifically bind to the octamer sequence were also identified. These results demonstrate that the octamer is recognized by several biochemically distinct nuclear proteins, perhaps to differentially regulate the expression of immunoglobulin genes.

Poly(A) site choice rather than splice site choice governs the regulated production of IgM heavy-chain RNAs

Alternative processing of the immunoglobulin mu primary transcript results in regulated production of mRNAs encoding the secreted (microseconds) and membrane-bound (micro m) form of IgM heavy chain during B-cell development. To elucidate the basis for this control, we analyzed the expression of altered forms of the mu transcription unit. Deletion of intron sequence between the microseconds and micro m exons, which reduces the distance between the two poly(A) sites as well as the distance between micro m splice sites, enhances production of micro m RNA. Correct expression is restored by insertion of heterologous sequences, demonstrating that spacing is indeed the critical aspect. The altered spacing appears to affect poly(A) site usage rather than splice site usage, since it was the distance between the poly(A) sites rather than the distance between splice sites that was found to be decisive. Finally, removal of either the C mu 4 splice donor or the m1 splice acceptor, thus eliminating normal micro m splicing, does not increase usage of the microseconds poly(A) site. We therefore conclude that the major factor in determining the ratio of microseconds to micro m is a poly(A) site choice rather than a splicing choice.

A new human immunoglobulin VH family preferentially rearranged in immature B-cell tumours

The prevalent forms of adult and childhood B-cell neoplasia are chronic lymphocytic (CLL) and acute lymphocytic (ALL) leukaemia, and are typified by a nearly monoclonal accumulation of cells expressing a single heavy (H) and light (L) chain variable (V) region. V gene selection could be random, or quite biased if the disease or the developmental status of the transformed cell somehow influenced DNA rearrangement. We have cloned and sequenced three germ-line VH gene segments that constitute a new human VH family (subgroup V) linked within 160 kilobase pairs of the DH-JH complex. One VH(V) member is rearranged in about 30% of patients with CLL and ALL, but not in IgM-expressing B-cell lines from peripheral blood. In some tumours, we detect a truncated (VH(V) RNA devoid of constant regions that originates from unrearranged VH(V) genes. In other tumours and in resting splenocytes, we detect large amounts of normally sized VH(V)-associated mRNA, although stimulation by mitogen of splenic B cells results in loss of VH(V)-hybridizing RNA. These features suggest that biased rearrangement of subgroup V may be under developmental selection.

Diversity of the mouse T cell receptor C gamma 1 gene: structural analysis in C57BL/Ka

We have isolated an unusual T cell receptor gamma chain cDNA clone (gamma 7.1) from a library made from RNA derived from adult thymus of C57BL/Ka mice. This cDNA clone corresponds to the appropriately processed C gamma 1 constant region exons preceded by 1.5 kb of J-C gamma 1 intron. The gamma 7.1 coding region is extremely homologous to the C gamma 1 gene of BALB/c mice, differing at the protein level by a single deletion (alanine 139) and a single substitution. This latter change eliminates the sole N-linked sugar attachment site, providing a basis for strain-specific glycosylation patterns. The J-C gamma 1 intronic region contains two DNA segments (termed psi J gamma 1 and psi J gamma 2) that are highly reminiscent of joining (J) segments; both have potentially functional recombination and donor splice sequences flanking an open reading frame. Northern analysis suggests that gamma 7.1 may be derived from a large, variable region-containing precursor.

Delta is the Cx-gene product in the gamma/delta antigen receptor of dendritic epidermal cells

Most T cells bear an antigen receptor that is a protein of a disulphide-linked heterodimer composed of an alpha chain and a beta chain associated with the non-polymorphic CD3 (T3) complex. A small subpopulation of thymic and peripheral T cells, as well as Thy-1+dendritic epidermal cells (dEC), express an alternative CD3-associated dimeric receptor composed of the product of the T-cell antigen receptor (TCR) gamma gene and a fourth chain, designated delta. Recently a new murine TCR constant-region gene, designated Cx, has been cloned and proposed as a candidate for the C delta gene. We have previously demonstrated that murine Thy-1+ dEC cell lines express a CD3-associated disulphide-linked heterodimer composed of a relative molecular mass Mr 41,000 (41K) gamma chain and a 50K delta chain. We have further analysed the receptor of one of these cloned dEC lines, 7-17.1, by endoglycosidase treatment of the isolated gamma and delta chains. The gamma chain was found to contain two N-linked oligosaccharide residues, consistent with the expression of a chain encoded by the V gamma 3 and C gamma 1 gene segments. The delta chain contains at least three N-linked oligosaccharides and has a core size of 38K. Northern blot analysis indicated the presence of abundant Cx messenger RNA in 7-17.1 cells. Immunoprecipitation with two antisera to peptides comprising distinct regions of the Cx sequence indicates that the delta chain is encoded by the Cx gene.

Organization of the murine immunoglobulin VH complex in the inbred strains

Deletion mapping analyses have been employed to order the heavy chain variable region (VH) gene families in three inbred murine strains. These nine VH gene families have been positioned with respect to the J558 and 3660 VH families in A/J (Ighe) as follows: 3609-J558-(J606,VGAM3-8,S107)-3660-(X24,Q52,7183 )-DH. Maps generated with respect to J558 in the BALB/c (Igha) and C57BL/6 (Ighb) strains are consistent with these results. The organization of the VH complex produced by deletion mapping is quite different from the accepted map generated by other methods, particularly in that J558 is more DH distal and 3660 is more DH proximal than previously thought. The order presented here is compatible with VH rearrangement frequencies suggesting preferential utilization of DH-proximal VH gene segments. Our data also indicate that interspersion of some VH family members may be a common feature of the murine VH complex since the 3609 VH family is interdigitated in the three strains and a Q52 VH gene segment is interspersed in C57BL/6.

Regulation of T-cell receptor gamma-chain RNA expression in murine Thy-1+ dendritic epidermal cells

The epidermis of normal mice contains two distinct populations of dendritic cells derived from the bone marrow, Ia+ Langerhans cells and Ia- cells that express the Thy-1 alloantigen. The Thy-1-bearing dendritic epidermal cells (Thy-1+ dEC) have a surface phenotype similar to that of very early T-lineage cells, produce IL-2-like growth factors and exhibit cytotoxicity which is not restricted by the major histocompatibility complex (MHC). The relationship of Thy-1+ dEC to the T-cell lineage is unclear. Most T lymphocytes bear a receptor for antigen composed of an alpha chain and a beta chain associated with a nonpolymorphic complex termed CD3 (T3). A minor population carries a receptor in which CD3 is associated with a gamma/delta complex. We have analysed clones of Thy-1+ dEC for rearrangement and expression of the genes for the alpha-, beta- and gamma-chains of the T-cell receptor (TCR). They do not express alpha or beta but do carry a gamma/delta complex. Activation of the cells with Con A is associated with a rapid decrease in the steady-state level of gamma-chain RNA. Because Thy-1+ dEC resemble early stage T lymphocytes, down-regulation of TCR expression may reflect a necessary event during T cell differentiation.

Relative position and strengths of poly(A) sites as well as transcription termination are critical to membrane versus secreted mu-chain expression during B-cell development

During B-cell differentiation, there is a dramatic switch in the RNA products of the immunoglobulin mu heavy chain transcription unit. In the mature B cell there is roughly equal production of the microseconds and the micron RNA, whereas in the antibody-secreting plasma cell there is nearly exclusive production of the microseconds RNA. A plasmid containing the entire mu transcription unit was properly regulated when assayed by transient transfection in a B lymphoma and a plasmacytoma. In contrast, no such regulation was observed with separate plasmids that could produce only one or the other RNA. Instead, the micron poly(A) site was utilized more efficiently than the microseconds poly(A) site, irrespective of the cell type. We also found that transcription termination prior to the micron poly(A) site in plasmacytomas contributes to preferential production of microseconds RNA in these cells. Finally, reducing the distance between the two poly(A) sites improved the use of the micron site at the expense of the use of the microseconds in B lymphoma cells, suggesting a competition for a limiting factor. Such competition was not apparent in plasmacytomas. We conclude that relative poly(A) site strength and the position of the poly(A) sites within the transcription unit, coupled with a changing concentration of a limiting factor, as well as transcription termination prior to the micron poly(A) site, all play a role in determining the expression of the mu locus during B-cell development.

Protein-nucleotide contacts in the immunoglobulin heavy-chain promoter region

Immunoglobulin heavy-chain variable-region genes contain the octanucleotide ATGCAAAT upstream from the site of transcription initiation. The complement of this sequence, in the reverse orientation, is found at an identical location in light-chain variable-region genes. This sequence element is thought to be involved in the lymphoid-specific expression of immunoglobulin genes. Analysis of nuclear extracts from both lymphoid and nonlymphoid cells in a gel migration inhibition assay, using an immunoglobulin promoter region fragment containing the octamer, reveals multiple migration-retarded species that represent specific DNA-protein complexes. The number and relative level of these complexes vary with cell type; some complexes are detected with all extracts, whereas one complex is lymphoid-specific and may represent an interaction involved in the lymphoid-restricted expression of immunoglobulin genes. Mitogenic stimulation of a B-lymphoid line can increase the level of the protein responsible for this lymphoid-specific complex. Analysis of the complexes detected in the gel migration inhibition assay by DNase I protection ("footprinting") has revealed that all of these DNA-protein complexes involve contact of the protein with the nucleotides of the octamer. One complex, present in both lymphoid and nonlymphoid cells, displays an additional DNA-protein contact adjacent to the octamer. Our results also indicate that the interaction of proteins with the octameric sequence can cause a local alteration in the structure of the DNA helix.

The effects of cytokines and adherent cells on the interleukin 4-mediated induction of Ia antigens on resting B cells

In this report we have extended our previous studies on interleukin 4 (IL-4) [previously termed B-cell stimulatory factor-1 (BSF-1)]. Our results demonstrate that 8 hr of exposure to IL-4 is sufficient to induce maximal expression of Ia antigens. This increase in expression of Ia antigens on resting B cells is due to the direct action of IL-4 on the B cells since adding or removing adherent cells or utilizing low density cultures of B cells at 50-100/culture had no effect on the IL-4-mediated increase in Ia. Monoclonal anti-IL-4 antibody completely abrogated the Ia-inducing activity of IL-4. A variety of other purified lymphokines including interleukin 2 (IL-2), interleukin 1 (IL-1), and a source of either B-cell differentiation factor for IgM (BCDF mu), or B-cell growth factor II (BCGF II), did not alter the expression of Ia antigens on resting B cells. However, interferon-gamma can partially inhibit the IL-4-mediated induction of Ia.

T cell receptor genes do not rearrange or express functional transcripts in natural killer cells of scid mice

The lineage of natural killer (NK) cells is poorly understood. To examine the relationship between NK cells and cells of the T lineage we have examined purified NK cells from a mutant mouse strain (scid) with severe combined immunodeficiency. Approximately 40% of the lymphoid cells in the spleens of scid mice expressed the NK-specific marker NK-2.1. All NK activity of the scid spleen cells could be accounted for by the NK-2.1+ population, similar to results obtained by using normal C57BL/6 X DBA/2 (B6D2F1) mice. Sorted NK-2.1+ cells proliferated in response to human recombinant interleukin 2 (r-IL 2) but not to concanavalin A (Con A), and were maintained in culture for 2 to 3 wk. Cultured NK-2.1+ cells displayed a cell surface phenotype (Asialo-GM1+, Thy-1.2+, L3T4-, Lyt-2-) and lytic activity similar to that described for freshly isolated NK cells of normal mice. Furthermore, T cell receptor (TCR) genes of the TCR-gamma and TCR-beta loci were in germline configuration, and no functional transcripts of TCR-gamma, TCR-beta, or TCR-alpha were detected. We propose that the expression of the TCR is not necessary for functional NK activity, and NK cells are distinct from both mature cytotoxic T lymphocytes and the earliest identifiable T cells.

T cell receptor genes do not rearrange or express functional transcripts in natural killer cells of scid mice

The lineage of natural killer (NK) cells is poorly understood. To examine the relationship between NK cells and cells of the T lineage we have examined purified NK cells from a mutant mouse strain (scid) with severe combined immunodeficiency. Approximately 40% of the lymphoid cells in the spleens of scid mice expressed the NK-specific marker NK-2.1. All NK activity of the scid spleen cells could be accounted for by the NK-2.1+ population, similar to results obtained by using normal C57BL/6 X DBA/2 (B6D2F1) mice. Sorted NK-2.1+ cells proliferated in response to human recombinant interleukin 2 (r-IL 2) but not to concanavalin A (Con A), and were maintained in culture for 2 to 3 wk. Cultured NK-2.1+ cells displayed a cell surface phenotype (Asialo-GM1+, Thy-1.2+, L3T4-, Lyt-2-) and lytic activity similar to that described for freshly isolated NK cells of normal mice. Furthermore, T cell receptor (TCR) genes of the TCR-gamma and TCR-beta loci were in germline configuration, and no functional transcripts of TCR-gamma, TCR-beta, or TCR-alpha were detected. We propose that the expression of the TCR is not necessary for functional NK activity, and NK cells are distinct from both mature cytotoxic T lymphocytes and the earliest identifiable T cells.

Murine natural killer cells do not express functional transcripts of the alpha-, beta-, or gamma-chain genes of the T cell receptor

Murine natural killer (NK) cells were purified by sorting with the NK cell-specific monoclonal antibody anti-NK-1.1. Sorted NK-1.1+ cells contained all splenic NK activity and could be propagated in human recombinant IL 2 for 3 to 4 wk. Short-term cultured NK-1.1+ cells maintained the cell-surface phenotype, morphologic appearance, and lytic activity characteristic of freshly isolated splenic NK cells. RNA extracted from this purified and propagated population of NK cells was probed for T cell receptor gene transcripts by Northern blot analysis. No functional transcripts of the alpha-, beta-, or gamma-chain genes of the T cell receptor were detected. These data dissociate NK cells from mature cytotoxic T cells, as well as the earliest identifiable cells committed to the T lineage.

Murine natural killer cells do not express functional transcripts of the alpha-, beta-, or gamma-chain genes of the T cell receptor

Murine natural killer (NK) cells were purified by sorting with the NK cell-specific monoclonal antibody anti-NK-1.1. Sorted NK-1.1+ cells contained all splenic NK activity and could be propagated in human recombinant IL 2 for 3 to 4 wk. Short-term cultured NK-1.1+ cells maintained the cell-surface phenotype, morphologic appearance, and lytic activity characteristic of freshly isolated splenic NK cells. RNA extracted from this purified and propagated population of NK cells was probed for T cell receptor gene transcripts by Northern blot analysis. No functional transcripts of the alpha-, beta-, or gamma-chain genes of the T cell receptor were detected. These data dissociate NK cells from mature cytotoxic T cells, as well as the earliest identifiable cells committed to the T lineage.

Interaction of cell-type-specific nuclear proteins with immunoglobulin VH promoter region sequences

All human and murine immunoglobulin heavy chain variable region (VH) genes contain the sequence ATGCAAAT approximately 70 nucleotides 5' from the site of transcription initiation. This octanucleotide, in reverse orientation, is also found in all light chain variable region (VL) genes, and in the immunoglobulin heavy chain transcriptional enhancer. Transfection studies have established that this octamer is involved in the lymphoid-specific transcription of immunoglobulin genes. Octamer-containing fragments have been reported to bind a factor present in nuclear extracts of human cell lines; however, identical binding activity was detected in both B lymphoid and non-lymphoid cells. Here we establish that nuclear extracts from distinct cell types differ in their ability to interact with octamer-containing fragments. We have also detected a DNA-protein interaction that may be involved in the cell-type specificity of immunoglobulin expression, and we have determined that a sequence upstream of the octamer participates in an interaction with a nuclear protein(s).

Regulation of the expression of multiple class II genes in murine B cells by B cell stimulatory factor-1 (BSF-1)

We have studied the mechanisms governing the induction of class II major histocompatibility complex (MHC) antigens on murine B lymphocytes by B cell stimulatory factor-1 (BSF-1). BSF-1 induced a dramatic (10- to 15-fold) and selective increase in class II MHC antigen expression, as judged by flow cytometry. Analysis of radiolabeled membrane proteins on BSF-1-induced B cells also showed that the expression of class II MHC antigens was increased 10- to 15-fold. Biosynthetic labeling studies established that a selective increase in the translation of class II MHC molecules could be detected at 5 to 6 hr post-addition of BSF-1 to culture. After 16 hr of culture, when cell surface expression was induced 12- to 15-fold, biosynthesis rates of class II MHC antigens was induced fivefold to sixfold. The biosynthesis of the I region-associated invariant chain (Ii) was also enhanced. Actinomycin D abrogated the increased biosynthetic labeling of class II products, suggesting transcriptional regulation of expression. At 6 hr after addition of BSF-1, there was a twofold increase in the steady-stage level of class II mRNA, which was slightly increased at 16 hr. The BSF-1-induced increase in surface Ia antigen is dependent on new transcription and translation. However, posttranslational events that lead to a decrease in membrane antigen turnover rate or to an increase in protein stability probably also play a significant role in the hyperexpression of class II antigens on BSF-1-treated B lymphocytes.

Regulation of the expression of multiple class II genes in murine B cells by B cell stimulatory factor-1 (BSF-1)

We have studied the mechanisms governing the induction of class II major histocompatibility complex (MHC) antigens on murine B lymphocytes by B cell stimulatory factor-1 (BSF-1). BSF-1 induced a dramatic (10- to 15-fold) and selective increase in class II MHC antigen expression, as judged by flow cytometry. Analysis of radiolabeled membrane proteins on BSF-1-induced B cells also showed that the expression of class II MHC antigens was increased 10- to 15-fold. Biosynthetic labeling studies established that a selective increase in the translation of class II MHC molecules could be detected at 5 to 6 hr post-addition of BSF-1 to culture. After 16 hr of culture, when cell surface expression was induced 12- to 15-fold, biosynthesis rates of class II MHC antigens was induced fivefold to sixfold. The biosynthesis of the I region-associated invariant chain (Ii) was also enhanced. Actinomycin D abrogated the increased biosynthetic labeling of class II products, suggesting transcriptional regulation of expression. At 6 hr after addition of BSF-1, there was a twofold increase in the steady-stage level of class II mRNA, which was slightly increased at 16 hr. The BSF-1-induced increase in surface Ia antigen is dependent on new transcription and translation. However, posttranslational events that lead to a decrease in membrane antigen turnover rate or to an increase in protein stability probably also play a significant role in the hyperexpression of class II antigens on BSF-1-treated B lymphocytes.

Double isotype production by a neoplastic B cell line. I. Cellular and biochemical characterization of a variant of BCL1 that expresses and secretes both IgM and IgG1

We have subcloned the in vitro-adapted murine B cell leukemia, BCL1.B1, to obtain a variant that expresses both IgM and IgG1. By fluorescence analysis, radioiodination, and immunoprecipitation of cell surface Ig, and by RIA of medium from limiting dilution cultures, we have shown that: (a) all the cells express and secrete both isotypes. The heavy chains of both IgG1 and IgM have the apparent molecular weights of membrane mu and gamma 1 chains; (b) both isotypes bear the same idiotype as determined by immunoprecipitation with antiidiotypic antibody, and both use the same VDJ rearrangement as shown by Southern blotting; and (c) the cells express the membrane and secreted forms of mRNA for both mu and gamma 1 but not gamma 2b or gamma 3. Taken together, the data suggest that all the cells are synthesizing, expressing on their surface, and secreting two isotypes that use the same VDJ rearrangement in the DNA and express the same serologically-defined idiotype. The molecular basis responsible for the production of the two isotypes in a single cell is the subject of the accompanying paper.

Double isotype production by a neoplastic B cell line. II. Allelically excluded production of mu and gamma 1 heavy chains without CH gene rearrangement

In our accompanying paper, we described a switch variant (BCL1.2.58) that expresses membrane and secreted forms of IgM and IgG1. Both IgM and IgG1 share the same idiotype and use the same VDJ rearrangement. Here, a detailed Southern blot analysis of the entire constant region of the Ig heavy chain (Ig CH) locus of parental (BCL1.B1) and variants (BCL1.B2) DNA showed no detectable rearrangement. Similar analysis of the JH-C mu region led to the conclusion that two heavy chain alleles present in the IgM/IgG1-producing variants carried the same VDJ rearrangement but differed in their 3' flanking regions. One chromosome 12 did not carry any Ig CH genes, whereas, the other chromosome 12 carried one copy of CH genes. In BCL1.B1, however, each of the chromosome 12 alleles carried a full copy of CH genes. Karyotypic analysis confirmed the presence of two translocated t(12;16) chromosomes in both BCL1.2.58 and BCL1.B1 cells, with a break 5' to the VH locus at the distal region (12F2) of chromosome 12, and at the proximal region below the centromere (16B3) of chromosome 16. We conclude that double production of IgM and IgG1 in BCL1.B2 is accomplished by transcription of the corresponding CH genes in germline configuration using a single VDJ on the same chromosome 12.

Structure of a class I gene from Syrian hamster

Syrian hamsters possess a multigene class I family yet fail to perform several associative immunologic functions. In an attempt to determine whether representative hamster genes are structurally functional, we have cloned two closely linked class I-like genes and determined the complete sequence of the 5' member. Its exon organization is similar to that seen in mouse and man, although only two intracytoplasmic domains are encoded instead of the usual three. Comparison of the predicted amino acid sequence and the 3' untranslated region to mouse and human genes suggest along with the linkage data that the hamster gene may be related to either or both K and Qa region genes but probably not to D and L region genes.

Germ-line sequence of the DH segment employed in Ars-A antibodies: implications for the generation of junctional diversity

The majority of antibodies produced by A/J mice in response to p-azophenylarsonate belong to the Ars-A family. These antibodies have the conserved sequence cys-ala-arg-ser-x-tyr-tyr (in which x is variable) spanning the V-D junction of the heavy chain. The cys-ala-arg residues are accounted for in the sequence of the A/J VH gene; the tyr-tyr are believed to be specified by the A/J DH segment, although this assumption is based on the DFL16.1 sequence derived from BALB/c mice. This implies that the ser-x is generated by joining imprecision and/or N segment addition. More recent data have revealed that the codon specifying the junctional serine residue is highly conserved (TCN, where N is usually G), suggesting a germline origin. Because there is no obvious way to generate this codon from the A/J Ars-A VH gene, we examined the involvement of the A/J DH segment in the generation of this junctional residue by cloning and sequencing the A/J equivalent to DFL16.1. We have established that this DH segment is polymorphic among BALB/c and A/J at the nucleic acid sequence level, and that it does not encode the junctional serine. This implies that a mechanism other than joining imprecision or random N segment addition operates at V-D junctions of Ars-A heavy chains.

Germ-line sequence of the DH segment employed in Ars-A antibodies: implications for the generation of junctional diversity

The majority of antibodies produced by A/J mice in response to p-azophenylarsonate belong to the Ars-A family. These antibodies have the conserved sequence cys-ala-arg-ser-x-tyr-tyr (in which x is variable) spanning the V-D junction of the heavy chain. The cys-ala-arg residues are accounted for in the sequence of the A/J VH gene; the tyr-tyr are believed to be specified by the A/J DH segment, although this assumption is based on the DFL16.1 sequence derived from BALB/c mice. This implies that the ser-x is generated by joining imprecision and/or N segment addition. More recent data have revealed that the codon specifying the junctional serine residue is highly conserved (TCN, where N is usually G), suggesting a germline origin. Because there is no obvious way to generate this codon from the A/J Ars-A VH gene, we examined the involvement of the A/J DH segment in the generation of this junctional residue by cloning and sequencing the A/J equivalent to DFL16.1. We have established that this DH segment is polymorphic among BALB/c and A/J at the nucleic acid sequence level, and that it does not encode the junctional serine. This implies that a mechanism other than joining imprecision or random N segment addition operates at V-D junctions of Ars-A heavy chains.

Structure of a class I gene from Syrian hamster

Syrian hamsters possess a multigene class I family yet fail to perform several associative immunologic functions. In an attempt to determine whether representative hamster genes are structurally functional, we have cloned two closely linked class I-like genes and determined the complete sequence of the 5' member. Its exon organization is similar to that seen in mouse and man, although only two intracytoplasmic domains are encoded instead of the usual three. Comparison of the predicted amino acid sequence and the 3' untranslated region to mouse and human genes suggest along with the linkage data that the hamster gene may be related to either or both K and Qa region genes but probably not to D and L region genes.

Molecular analysis of the t(2;14) translocation of childhood chronic lymphocytic leukemia

Two rare cases of chronic lymphocytic leukemia (CLL) in children have been studied; both are associated with a previously undescribed chromosomal translocation [t(2;14) (p13;q32)]. In one patient the translocation was reciprocal and the breakpoint on chromosome 14 occurred just 5' of the C gamma 2 region on the productive immunoglobulin heavy-chain allele. The breakpoint on chromosome 2 does not involve the K locus but lies within an uncharacterized region that coincides with the position of a constitutive fragile site that occurs within normal lymphocytes. Data on the second patient are consistent with these findings and suggest that these cases represent a rare but distinct subgroup of CLL's with a specific cytogenetic change.

Syrian hamster DNA shows limited polymorphism at class I-like loci

The class I gene products of the Syrian hamster major histocompatibility complex are unique in that they lack functionally detectable polymorphism. Mouse cDNA and hamster genomic probes were used to analyze the hamster class I gene family using genomic Southern hybridization. These studies revealed that the hamster possesses a complex class I multigene family and that it shares extensive sequence homology with the corresponding mouse sequences. Unlike the mouse, however, the Syrian hamster demonstrates only limited restriction endonuclease polymorphism in these genes. These results suggest that the lack of detectable polymorphism in this species is directly related to limited DNA polymorphism. The data presented here support the hypothesis that this species has undergone an evolutionary bottleneck, i.e., that all surviving members of the species arose from a limited number of progenitors.

Phylogenetic conservation of immunoglobulin heavy chains: direct comparison of hamster and mouse Cmu genes

We have analyzed the JH-Cmu locus of the Syrian hamster by DNA cloning and sequencing. The single Cmu gene is highly homologous to that of the mouse. The hamster equivalents of the JH and switch (S) recombination regions are arranged as in the mouse, but surprisingly are not highly conserved. Also unlike its close murine relative, the Smu regions among inbred hamster strains are not polymorphic. The complete nucleotide sequence of hamster and mouse Cmu genes have been compared to partial Cmu sequences of other species. Conservation within a portion of the 3' untranslated region may signify functional requirements for 3' end processing. Mutational frequencies within exons and introns of hamster and mouse do not support the theory that the rate of DNA transitions to transversions decreases with evolutionary distance.

Regulation of expression of immunoglobulins M and D in murine B cells

Immunoglobulins M (IgM) and D (IgD) are expressed in a coordinantly regulated and differentiation-dependent fashion on the surface of B lymphocytes. We have studied the role of DNA configuration of their linked constant region genes C mu and C delta, as well as their transcription and posttranscriptional processing, in the regulation of these changes. After rearrangement of variable region segments, IgD can be singularly expressed in plasmacytoma cells by a DNA deletion of the C mu gene that is mediated by illegitimate recombination. However, IgM and IgD are usually expressed jointly without further DNA rearrangement downstream of VDJ. In pre-B cells, C delta apparently is not transcribed before light-chain expression. However, in early neonates (2 days old), C delta is transcribed at approximately one-third the level of C mu even though IgD is not detectable on the cell surface. This same ratio of transcription is preserved in older neonates (12 days old), which express only modest quantities of IgD, and in mature resting B cells, which express far higher densities of cell surface IgD than IgM. On activation by mitogens, transcription of C mu is preferentially enhanced, but it is surprising that C delta transcription remains at the baseline level even though cytoplasmic delta mRNA is virtually undetectable. The apparent discrepancy in transcription and ultimate expression can be explained by further modifications of both the RNA and polypeptide chains. Collectively, our data show that the differential expression of IgM and IgD is regulated by complex mechanisms at several levels.

Human immunoglobulin D: genomic sequence of the delta heavy chain

The DNA coding for the human immunoglobulin D(IgD) heavy chain (delta, delta) has been sequenced including the membrane and secreted termini. Human delta, like that of the mouse, has a separate exon for the carboxyl terminus of the secreted form. This feature of human and mouse IgD distinguishes it from all other immunoglobulins regardless of species or class. The human gene is different from that of the mouse; it has three, rather than two, constant region domains; and its lengthy hinge is encoded by two exons rather than one. Except for the third constant region, the human and mouse genes are only distantly related.

Structural analysis of the murine IgG3 constant region gene

We have determined the complete sequence of the gamma 3 heavy chain constant (C gamma 3) region gene of the BALB/c mouse including the 5'-flanking region up to the switch site and the 3'-flanking region past the end of the membrane exons. The C gamma 3 coding region, typical of other IgGs, is divided into six exons corresponding to the protein domains (C gamma (3)1, hinge, C gamma (3)2, and C gamma (3)3) and to the membrane carboxyl terminus (M1 and M2). The predicted amino acid sequence of the gamma 3 chain has three potential N-linked carbohydrate addition sites (including one in the membrane spacer segment), as compared with a single occurrence in the other mouse IgGs. Between the switch recombination region and the body of the C gamma 3 gene, there is a remarkable homology with a sequence between C mu and C delta which provides a rationale for an alternative, T cell-independent class-switch mechanism. We have used a computer to analyze the secondary structure of the gamma 3 mRNA precursor for the membrane form. We predict that this RNA precursor (approximately 12 000 bp) folds into four leaf-like domains which correspond to the variable region, the large IVS, the body of the constant region, and the membrane exons. This organization may have a role to play in the function of the mRNA precursor.

Transcriptional regulation of the mu-delta heavy chain locus in normal murine B lymphocytes

The heavy chain genes for IgM (C mu) and IgD (C delta) are expressed differentially during B cell maturation and activation. We have determined the role that transcription plays in the regulation of these changes by using the method of in vitro nascent RNA chain elongation. In neonatal cells that express much lower densities of IgD than IgM on their surface, transcription of C delta is observed at half the level of C mu. This 3:1 transcriptional ratio of mu to delta is preserved in mature resting cells, which express higher densities of IgD on the surface than IgM. When activated by the mitogen, lipopolysaccharide (LPS), transcription of C mu is preferentially enhanced. However, C delta transcription is not shut off even though the expression of IgD in the stimulated cells is greatly decreased. In all three differentiative stages, polymerase unloading occurs in the vicinity of a large inverted repeat sequence, 5' to C delta and 3' to the mu membrane exons. This suggests that the developmental selection of secreted vs. membrane-bound carboxyl-terminal exons is controlled by RNA cleavage. The data presented here, together with our previous analysis of mRNA and protein synthesis, show that the differential expression of IgM and IgD in normal B lymphocytes is regulated at the transcriptional, translational, and posttranslation levels.

Illegitimate recombination generates a class switch from C mu to C delta in an IgD-secreting plasmacytoma

We present here the sequence characterization of a C mu----C delta immunoglobulin (Ig) heavy chain class switch. In the murine IgD-secreting plasmacytoma TEPC 1017, deletion of most of the mu switch recombination region (S mu) and the entire C mu gene occurred in the absence of switch region sequences 5' to C delta. This unique rearrangement resulted from an illegitimate recombination of sequences with only patchy homology to each other. The infrequent and variable nature of illegitimate recombination may explain the low frequency of IgD-secreting plasma cells in normal mouse tissues.

Regulation of IgM and IgD synthesis in B lymphocytes. I. Changes in biosynthesis of mRNA for mu- and delta-chains

Although IgD is expressed on the surface of resting B cells at a higher density than IgM, we determined that both the steady-state level and the biosynthetic rate of mu m-mRNA is higher than that of delta m-mRNA, suggesting that translational or post-translational processing of the Ig heavy chains may modulate the expression of cell surface Ig. After B cell activation by LPS, delta m-mRNA decreases drastically, accounting for the observed decrease in expression of IgD. Concomitantly, a dramatic increase in the level of mu s- and gamma s-mRNA corresponds to the observed increase in secretion of these Ig isotypes in LPS-stimulated cells.

Regulation of IgM and IgD synthesis in B lymphocytes. I. Changes in biosynthesis of mRNA for mu- and delta-chains

Although IgD is expressed on the surface of resting B cells at a higher density than IgM, we determined that both the steady-state level and the biosynthetic rate of mu m-mRNA is higher than that of delta m-mRNA, suggesting that translational or post-translational processing of the Ig heavy chains may modulate the expression of cell surface Ig. After B cell activation by LPS, delta m-mRNA decreases drastically, accounting for the observed decrease in expression of IgD. Concomitantly, a dramatic increase in the level of mu s- and gamma s-mRNA corresponds to the observed increase in secretion of these Ig isotypes in LPS-stimulated cells.

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.

Unusual sequences in the murine immunoglobulin mu-delta heavy-chain region

The delta heavy (H) chain of mouse immunoglobulin D (IgD) is unusual both in its structure and in its differential expression relative to immunoglobulin M (IgM; reviewed in ref. 1). The region of DNA between IgM and IgD H-chain constant-region genes is probably implicated in this control. So far only fragments of the area have been sequenced. Now, however, we present the complete sequence as well as the sequence of the introns of the C delta gene. We have found several interesting features (Fig. 1), including an open reading frame (ORF) between Cmu and C delta which encodes 146 amino acids that might represent a previously unsuspected domain-like protein; three blocks of simple repetitive sequences; a 162-base pair (bp) unique-sequence inverted repeat; and a domain-like pseudogene in the large intron of C delta. We have not found, however, any sequence 5' of C delta resembling the switch (S) recombination sequences associated with class switching in other heavy chains. Moreover, we have determined the 3' deletion end point of an IgD-producing myeloma and find no sequences reminiscent of switch sites nearby.

The murine immunoglobulin alpha gene expresses multiple transcripts from a unique membrane exon

Immunoglobulin heavy chains of each class exist in two forms, membrane (m) and secreted (s), which have different carboxy-terminal sequences encoded by different gene segments. We have sequenced the DNA spanning the coding region for the carboxy-terminal end of the alpha m chain in a BALB/c genomic clone and have examined alpha mRNA expression in transformed and normal B cells. Unlike the other murine heavy chain genes, which have two membrane exons (M1 and M2), the alpha gene has only one membrane exon (alpha M1). This unusual exon encodes a highly conserved transmembranal peptide plus flanking amino acids singular to IgA. The complexity of the alpha gene transcription is also unusual. Downstream of alpha M1, three AATAAA hexanucleotides are used to terminate as many as six distinct alpha membrane mRNAs. These appear to differ only in length and splicing pattern of their 3'-untranslated regions, and thus, encode the same alpha m protein. The alpha s protein is approximately 4000 daltons smaller than alpha m as judged by in vitro translation and is coded by a single 2.0-kb alpha s mRNA. We propose that the expression of alpha s and the multiple alpha m mRNAs is differentially controlled at the level of transcriptional termination.

Effect of lipopolysaccharide stimulation on the transcription and translation of messenger RNA for cell surface immunoglobulin M

Analysis of mu-specific mRNA in the B cell tumor line, BCL1, shows that the cells contain predominantly mRNA for mu chain of membrane-bound immunoglobulin M (IgM) (2.7 kb, mu m mRNA). Stimulation of the cells to Ig secretion by lipopolysaccharide (LPS) results in a 6-12 fold increase in amount of mRNA for the mu chain of secreted IgM (2.4 kb mu s mRNA). The increase in mu s mRNA is accompanied by a 3-4-fold increase in mu m mRNA. The rate of mu chain synthesis of membrane IgM in LPS-stimulated cells is, however, reduced by at least twofold, suggesting that both transcriptional and translational regulatory events are involved in the induction of B lymphocytes to secretion.

Simultaneous expression of immunoglobulin mu and delta heavy chains by a cloned B-cell lymphoma: a single copy of the VH gene is shared by two adjacent CH genes

The cloned murine B-cell lymphoma line (BCL1) that expresses surface IgM and IgD is considered to be a model for the immunoglobulin gene expression of the mature virgin B cell. Of particular interest is the mechanism by which a single VH gene is shared by two CH genes. We examined the organization of the immunoglobulin heavy chain genes in BCL1 DNA. A single arrangement of CH genes was found with the expressed VHDJH gene complex just 5' to the Cmu gene. The complete DNA sequence of the VH gene was determined. No rearrangement occurred in the intervening DNA between the JH and C mu genes or between the C mu and C delta genes. We conclude that dual expression of mu and delta heavy chains using a single VH gene is accomplished by alternate processing of a primary transcript that encompasses the the VHDJH complex and both CH genes.