The immunoglobulin heavy-chain locus in zebrafish: identification and expression of a previously unknown isotype, immunoglobulin Z

The only immunoglobulin heavy-chain classes known so far in teleosts have been mu and delta. We identify here a previously unknown class, immunoglobulin zeta, expressed in zebrafish and other teleosts. In the zebrafish heavy-chain locus, variable (V) gene segments lie upstream of two tandem diversity, joining and constant (DJC) clusters, resembling the mouse T cell receptor alpha (Tcra) and delta (Tcrd) locus. V genes rearrange to (DJC)(zeta) or to (DJC)(mu) without evidence of switch rearrangement. The zebrafish immunoglobulin zeta gene (ighz) and mouse Tcrd, which are proximal to the V gene array, are expressed earlier in development. In adults, ighz was expressed only in kidney and thymus, which are primary lymphoid organs in teleosts. This additional class adds complexity to the immunoglobulin repertoire and raises questions concerning the evolution of immunoglobulins and the regulation of the differential expression of ighz and ighm.

A novel VH to VHDJH joining mechanism in heavy-chain-negative (null) pre-B cells results in heavy-chain production

During B-cell development, the VH genes of immunoglobulin heavy (H) chains are assembled from three different germline components: the variable (VH) segment, the diversity (D) segment and the joining (JH) segment. The joining between two segments involves the recognition of conserved nonamer-heptamer sequences bordering each segment, double-stranded cuts at the heptamer-segment border, and the re-ligation of the two segment ends which have frequently been modified by the deletion and addition of nucleotides. The flexibility of the joint increases VHDJH variability. However, it also results in many pre-B cells which do not produce immunoglobulin H chains and have non-functional VHDJH complexes carrying the VH and JH coding sequences in different reading frames. We show here that such 'null cells' are not dead-end products of the B-cell developmental pathway but can perform a novel VH to VHDJH joining using a 5' VH segment to replace the VH sequence of the VHDJ-H complex. This process can result in the generation of a VHDJ+H complex and the subsequent expression of an immunoglobulin heavy chain.

The role of DNA rearrangement and alternative RNA processing in the expression of immunoglobulin delta genes

We have established the exon-intron structure of the gene coding for the constant (C) region of the mouse immunoglobulin delta heavy chain, using DNA clones isolated from BALB/c embryos and the delta mRNA extracted from two delta-producing hybridomas, B1-8. delta 1 and GCL2.8. At least three types of C delta gene structures are identified. A 2.7 kb delta mRNA reveals six exons. This delta mRNA may code for a membrane-bound delta chain. A second delta mRNA of 1.8 kb shares the first (5' side relative to direction of transcription) three exons with the 2.7 kb delta mRNA and in addition contains a fourth exon unique to this mRNA species. This delta mRNA most likely codes for a secreted delta chain. A third delta mRNA, also of 1.8 kb, shares the first four exons and a part of the fifth exon with the 2.7 kb mRNA. Its function, if any, remains unclear. We investigated the question of how a lymphocyte can produce the mu and delta heavy chains simultaneously, using the hybridoma GCL 2.8, which makes both IgM and IgD. Results of Southern gel blot analysis and gene cloning experiments indicate that this cell utilizes the same rearranged VH gene for the synthesis of the mu and delta chains, and yet maintains the embryonic configuration for the C mu and C delta genes and for the intervening region. Based on these results, we conclude that the VH sequence is spliced alternatively to the C mu or C delta sequence during processing of the primary RNA transcript. An alternative mechanism for the expression of the delta gene is found in hybridoma B1-8. delta 1, which actively secretes delta chains and synthesizes no mu chain. This mechanism involves deletion of the C mu gene, which brings the complete VH gene closer to the C delta gene.

IgA production without mu or delta chain expression in developing B cells

Surface, membrane-bound, immunoglobulin M (IgM) or IgD expression early in B cell ontogeny is considered essential for the differentiation of antibody-producing cells in mammals; only in IgM+ B cells is the heavy chain locus rearranged to express antibodies of other classes. We show here that IgA is selectively expressed in muMT mice, which lack IgM or IgD expression and have a pro-B cell developmental block. muMT IgA binds proteins of commensal intestinal bacteria and is weakly induced by Salmonella infection, although not through conventional immunization. This muMT IgA pathway requires extrasplenic peripheral lymphoid tissues and may be an evolutionarily primitive system in which immature B cells switch to IgA production at peripheral sites.

Analysis of the B-cell progenitor compartment at the level of single cells

BACKGROUND

During B-cell development in the mouse, the VH, DH and JH elements of the immunoglobulin heavy chain (IgH) locus are rearranged, firstly by DH-JH joining, and then by VH-DHJH joining. In-frame ('productive') VHDHJH joints and DHJH joints in reading frame 2 (one of the three possible DH reading frames) allow the expression of mu and truncated mu chains (D mu proteins), respectively. The expression of such molecules from one of the two IgH loci of a cell is thought to interfere with VH-DHJH recombination on the other IgH locus, and to guide the cells through further development.

RESULTS

We have developed a gene amplification assay that permits the examination of rearranged immunoglobulin genes in single cells. Using this assay, we monitored cells bearing DHJH and/or VHDHJH joints at early stages of development: in CD43+ B-cell progenitors, subdivided into fractions A, B, C and C' by flow cytometry, and in CD43- pre-B cells (fraction D). Fraction C was enriched for cells with two non-productive VHDHJH joints. Cells containing both a DHJH joint in DH reading frame 2 and a VHDHJH joint were not seen in any fraction. All fraction D cells harbored an in-frame VHDHJH joint. Cells with two productive VHDHJH joints appear to be selected against throughout development.

CONCLUSIONS

Cells expressing D mu proteins appear to be arrested in development as a result of inhibited VH-DHJH joining. Expression of the mu chain is required for maturation into CD43- pre-B cells; accordingly, cells carrying two non-productive VHDHJH joints accumulate in the CD43+ compartment. Such a developmental arrest may also affect cells that express self-reactive VHDHJH antibody domains. Our results indicate further that allelic exclusion at the IgH locus is already established at the pre-B cell stage.

Mapping of heavy chain genes for mouse immunoglobulins M and D

A single DNA fragment containing both mu and delta immunoglobulin heavy chain genes has been cloned from normal BALB/c mouse liver DNA with a new lambda phage vector Charon 28. The physical distance between the membrane terminal exon of mu and the first domain of delta is 2466 base pairs, with delta on the 3' side of mu. A single transcript could contain a variable region and both mu and delta constant regions. The dual expression of immunoglobulins M and D on spleen B cells may be due to alternate splicing of this transcript.

Mode of regulation of immunoglobulin mu- and delta-chain expression varies during B-lymphocyte maturation

The transcription, processing, and accumulation of mu and delta mRNA was studied in several cell lines representing different stages of B-lymphocyte maturation. Our results indicate that the relative content of mu and delta mRNA is the major determinant of the IgM versus IgM + IgD phenotype and that the production of delta mRNA is regulated at distinctly different levels of gene expression in early and later developmental stages. In B cell lymphomas typical of early stages, transcription extends over the entire 25 kb of the mu- delta locus and the relative content of mu and delta mRNA is determined at the level of mRNA processing. In contrast, in cells typical of mature IgM secretors, transcription is terminated abruptly between the mu and delta genes, precluding the production of delta mRNA. We propose a model that accounts for qualitative and quantitative changes in mu-delta expression in the developing B lymphocyte.

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.

Immunoglobulin heavy chain variable region family usage is independent of tumor cell phenotype in human B lineage leukemias

During B cell development, immunoglobulin heavy chain (IgH) variable region (VH) genes are rearranged and expressed in a programmed manner and accumulating evidence suggests recurrent utilization of developmentally restricted VH genes in malignant B lymphoid populations. We have used polymerase chain reaction gene amplification in conjunction with a panel of VH family-specific amplimers to directly compare the repertoire of VH region rearrangement in mature, CD5+ B cell chronic lymphocytic leukemia with that in immature, CD5 B lineage acute lymphoblastic leukemia. The results revealed a diverse pattern of VH family utilization common to both disease groups in which VH regions most proximal to the IgH joining locus were preferentially rearranged relative to their family sizes with recurrent utilization of several known developmentally restricted VH genes in close to germ-line configuration. These results indicate that biased VH family usage is independent of tumor cell phenotype in B lineage leukemias. This bias may reflect similar stages or compartments in normal B lymphopoiesis from which diverse types of B cell malignancy may arise. Moreover, since blast cells in acute lymphoblastic leukaemia do not express functional immunoglobulin, we infer that the tumor cell-associated VH family repertoire is determined through antigen-independent mechanisms.

Activation and anergy in bone marrow B cells of a novel immunoglobulin transgenic mouse that is both hapten specific and autoreactive

Available evidence indicates that B cell tolerance is attained by receptor editing, anergy, or clonal deletion. Here, we describe a p-azophenylarsonate (Ars)-specific immunoglobulin transgenic mouse in which B cells become anergic as a consequence of cross-reaction with autoantigen in the bone marrow. Developing bone marrow B cells show no evidence of receptor editing but transiently upregulate activation markers and appear to undergo accelerated development. Mature B cells are present in normal numbers but are refractory to BCR-mediated induction of calcium mobilization, tyrosine phosphorylation, and antibody responses. Activation marker expression and acquisition of the anergic phenotype is prevented in bone marrow cultures by monovalent hapten. In this model, it appears that induction of anergy in B cells can be prevented by monovalent hapten competing with autoantigen for the binding site.

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.

The normal counterpart of IgD myeloma cells in germinal center displays extensively mutated IgVH gene, Cmu-Cdelta switch, and lambda light chain expression

Human myeloma are incurable hematologic cancers of immunoglobulin-secreting plasma cells in bone marrow. Although malignant plasma cells can be almost eradicated from the patient's bone marrow by chemotherapy, drug-resistant myeloma precursor cells persist in an apparently cryptic compartment. Controversy exists as to whether myeloma precursor cells are hematopoietic stem cells, pre-B cells, germinal center (GC) B cells, circulating memory cells, or plasma blasts. This situation reflects what has been a general problem in cancer research for years: how to compare a tumor with its normal counterpart. Although several studies have demonstrated somatically mutated immunoglobulin variable region genes in multiple myeloma, it is unclear if myeloma cells are derived from GCs or post-GC memory B cells. Immunoglobulin (Ig)D-secreting myeloma have two unique immunoglobulin features, including a biased lambda light chain expression and a Cmu-Cdelta isotype switch. Using surface markers, we have previously isolated a population of surface IgM-IgD+CD38+ GC B cells that carry the most impressive somatic mutation in their IgV genes. Here we show that this population of GC B cells displays the two molecular features of IgD-secreting myeloma cells: a biased lambda light chain expression and a C&mu-Cdelta isotype switch. The demonstration of these peculiar GC B cells to differentiate into IgD-secreting plasma cells but not memory B cells both in vivo and in vitro suggests that IgD-secreting plasma and myeloma cells are derived from GCs.

Aging-dependent exclusion of antigen-inexperienced cells from the peripheral B cell repertoire

Aging is accompanied by greatly reduced B cell production in the bone marrow, yet peripheral B cell numbers do not decline. We hypothesize that this may reflect filling of the peripheral pool with B cells that are long-lived as a consequence of specificity for, and chronic stimulation by, environmental Ags. To begin to explore this possibility, we analyzed the effects of aging on B cell population dynamics in the anti-H2(k/b) 3-83 mu-delta Ig-transgenic mouse. We predicted that, because they presumably do not bind environmental Ags, B cells bearing the transgenic receptor may be lost in aged animals. As seen in nontransgenic animals, total splenic B cell numbers remained constant with age in the Ig-transgenic animals despite reduced B cell production. Importantly, although the few newly produced B cells in the bone marrow of aged mice are 3-83 positive, the peripheral compartment of these mice is dominated by B cells that express endogenous Ig genes rather than the transgenes. This population includes large numbers of marginal zone-like and CD21(low/-)CD23(low/-)IgM(low) B cells, as well as elevated numbers of CD5+ B cells. Many of these cells express only non-B220 CD45 isoforms, suggesting that they may be memory cells. A significant proportion of aged transgenic animals produce autoantibodies that are reactive with ssDNA, dsDNA, or histones. Results support the hypothesis that, in the face of severely reduced production with age, B cells are selected based on reactivity to environmental Ags, accumulate, and display activated phenotypes. Cells bearing 3-83-transgenic receptors are excluded from this population due to their specificity. Beyond their importance in aging, these findings define a novel form of receptor revision in which B cells are selected rather than deleted based on Ag reactivity.

Immunoglobulin D (IgD) of Atlantic cod has a unique structure

A new immunoglobulin heavy-chain gene with some homology to mammalian IgD was recently cloned from the channel catfish and Atlantic salmon, two species of teleost fish. We have cloned and sequenced a new H-chain gene from Atlantic cod (Gadus morhua L.) which has clear similarities to these genes, but which also differs in several ways. The similarities of catfish, salmon, and cod delta to the mammalian delta genes are sequence homology, location immediately downstream of IgM (mu), and expression by alternative splicing rather than class switching. A unique feature of catfish, salmon, and cod delta is the chimeric nature of the gene product, as the mu1 exon is spliced to the delta1 exon. Several unique features of cod IgD were found: (1) a deletion of the delta3, delta4, delta5, and delta6 domains described in catfish and salmon IgD, (2) a tandem duplication of a part of the delta locus including the delta1 and delta2 domains, (3) the presence of a truncated delta7 domain downstream of the deltaTM exons, and (4) the separation of the duplicated domains by a short exon (deltay) which has homology to a conserved part of the transmembrane exon 1 (TM1) of some H-chain isotypes. This unique organization of the delta locus of cod probably developed after the evolutionary split from the catfish and salmon branches.

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.

Transcriptional and posttranscriptional control of immunoglobulin mRNA production during B lymphocyte development

A variety of cell lines representing different maturation stages of the B lymphocyte were used to analyse developmental changes in the transcriptional pattern through the mu-delta locus and the relationship between mu mRNA accumulation and transcriptional activity. As anticipated from earlier studies, we observed that RNA polymerase loading in the region between the mu m cleavage/poly A addition site and the delta 1 exon is markedly decreased in IgM secreting cells compared to cells bearing surface IgM or surface IgM and IgD. In several IgM secreting hybridomas, transcriptional termination mainly occurred downstream of the first mu m exon. Thus, the predominance of mu s-terminated transcripts in these cells would appear to be principally determined by RNA processing events, most likely by more efficient cleavage at the mu s poly A site and/or less efficient splicing of the C mu and mu m exons. In two plasmacytoma lines, polymerase unloading between the mu s and mu m sites also contributed significantly to the high mu s mRNA phenotype. Our results further indicate that posttranscriptional regulation is largely responsible for the greatly increased accumulation of mu mRNA in the IgM secretors. Interestingly, the sterile-mu RNA components do not seem to be subject to this posttranscriptional regulation.

The IgH locus of the channel catfish, Ictalurus punctatus, contains multiple constant region gene sequences: different genes encode heavy chains of membrane and secreted IgD

The delta-chain of catfish IgD was initially characterized as a unique chimeric molecule containing a rearranged VDJ spliced to C micro 1, seven C domain-encoding exons (delta1-delta7), and a transmembrane tail. The presence of cDNA forms showing splicing of delta7 to an exon encoding a secretory tail was interpreted to indicate that membrane (deltam) and secreted (deltas) forms were likely expressed from a single gene by alternative RNA processing. Subsequent cloning and sequence analyses have unexpectedly revealed the presence of three delta C region genes, each linked to a micro gene or pseudogene. The first (IGHD1) is located 1.6 kb 3' of the functional C micro (IGHM1). The second (IGHD3) is positioned immediately downstream of a pseudo C micro (IGHM3P), approximately 725 kb 5' of IGHM1. These two delta genes are highly similar in sequence and each contains a tandem duplication of delta2-delta3-delta4. However, IGHD1 has a terminal exon encoding the transmembrane region, whereas IGHD3 has a single terminal exon encoding a secreted tail. The occurrence of IGHD3 immediately downstream of a micro pseudogene indicates that the putative deltas product may not be expressed as a chimeric micro delta molecule. Western blots and protein sequencing data indicate that an IGHD3-encoded protein is expressed in catfish serum. Thus, catfish deltam transcripts appear to originate from IGHD1, whereas deltas transcripts originate from IGHD3 rather than, as previously inferred, from a single expressed delta gene. The third delta (IGHD2) is associated with a pseudo C micro (IGHM2P); its presence is inferred by Southern blot analyses.

Analysis of the targeting of the hypermutational machinery and the impact of subsequent selection on the distribution of nucleotide changes in human VHDJH rearrangements

B cells are unique in that they generate and tolerate a high rate of mutations in their antigen receptor genes and employ these mutations as a basis of avidity maturation. The precise role of the mutational machinery versus subsequent selection in determining the frequency and distribution of mutations has not been fully analyzed. To address these issues, the influence of the intrinsic mutational machinery and subsequent selection on the frequency and distribution of mutations in the expressed human immunoglobulin repertoire was analyzed. Analysis of non-productively rearranged VH genes from individual human B cells provided an opportunity to examine the immediate impact of somatic hypermutation without superimposed selective influences. Comparison with the frequency and distribution of mutations in the productively rearranged human VH genes permitted an estimate of the influences of subsequent selection.

Self heat shock and gamma delta T-cell reactivity

We have investigated the effects of heat shock on T-cell induction and selection in vitro. We find that when cell preparations containing T lymphocytes are incubated for 30 min at 42 degrees C, a selective proliferation of gamma delta + T cells bearing the gamma delta T-cell antigen receptor follows. A greater enrichment of gamma delta + T cells is observed, upon preexposure to mycobacterial antigens in vivo. By comparing the effects of heat shock with that of mitogen or specific T-cell triggering by conventional antigens and by analyzing the gamma delta T-cell receptor genes expressed in cells that proliferate as a result of heat shock induction, we conclude that a subset of murine gamma delta T cells react to antigens on self cells in which a heat shock response was induced.

Human immunoglobulin selection associated with class switch and possible tolerogenic origins for C delta class-switched B cells

Current paradigms of peripheral B cell selection suggest that autoreactive B cells are controlled by clonal deletion, anergy, and developmental arrest. We report that changes to the human antibody repertoire likely resulting from these mechanisms both for a well-characterized autoreactivity from antibodies encoded by the V(H)4-34 gene and for other hallmarks of an autoreactive repertoire are apparent mainly for class-switched B cells and not for IgM germinal center, IgM memory, or IgM plasma cells. Other possible indicators of autoreactivity found selected with immunoglobulin class include J(H)6 gene segment usage, increased frequency of B cells with long third hypervariable regions, and distal J(kappa) gene segment bias. Of particular interest is the finding that B cells with these same characteristics are selected into the lineage of B cells that have undergone the unusual class switch from constant region C mu to C delta (C delta-CS). The C delta-CS population also displays an increased frequency of charged amino acids localized to the complementarity-determining regions, further suggesting autoreactivity, and evidence is presented that these B cells had undergone extensive receptor editing. Thus, the C delta-CS lineage may be a "sink" for B cells harboring autoreactive specificities in normal humans. A model for a new tolerizing mechanism that could account for the C delta-CS lineage is presented.

Artiodactyl IgD: the missing link

IgD has been suggested to be a recently developed Ig class, only present in rodents and primates. However, in this paper the cow, sheep, and pig Ig delta genes have been identified and shown to be transcriptionally active. The deduced amino acid sequences from their cDNAs show that artiodactyl IgD H chains are structurally similar to human IgD, where the cow, sheep, and pig IgD H chain constant regions all contain three domains and a hinge region, sharing homologies of 43.6, 44, and 46.8% with their human counterpart, respectively. According to a phylogenetic analysis, the Cdelta gene appears to have been duplicated from the Cmu gene >300 million yr ago. The ruminant mu CH1 exon and its upstream region was again duplicated before the speciation of the cow and sheep, approximately 20 million yr ago, inserted upstream of the delta gene hinge regions, and later modified by gene conversion. A short Sdelta (switch delta) sequence resulting from the second duplication, is located immediately upstream of the bovine Cdelta gene and directs regular mu-delta class switch recombination in the cow. The presence of Cdelta genes in artiodactyls, possibly in most mammals, suggests that IgD may have some as yet unknown biological properties, distinct from those of IgM, conferring a survival advantage.

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.

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.

Drastic change in idiotypic but not antigen-binding specificity of an antibody by a single amino-acid substitution

In proliferating B lymphocytes, somatic mutation of rearranged antibody variable (V)-region genes occurs at high frequency and may have a key role in the selection of these cells. It is of interest in this context to learn in which way single mutations can affect antigen binding and/or idiotypic specificity of an antibody. Previous investigations have analysed spontaneous mutants of myeloma and hybridoma cells in which the mutation affected the antigen-binding specificity of the antibody. Here we describe an antibody mutant that has fully retained antigen-binding specificity but has lost or drastically changed all V-region antigenic determinants (idiotopes) of the wild type as defined by monoclonal anti-idiotope antibodies. The mutant phenotype is generated by a glycine to arginine exchange in the middle of the diversity (D) element, at position 103 of the heavy chain.