Immunoglobulin JH, C mu, and C gamma gene rearrangements in human B lymphocytes clonally transformed by Epstein-Barr virus

Tumor cells of hairy cell leukemia express multiple clonally related immunoglobulin isotypes via RNA splicing

Hairy cell leukemia (HCL) derives from a mature B cell and expresses markers associated with activation. Analysis of immunoglobulin variable region genes has revealed somatic mutation in most cases, consistent with an origin from a cell that has encountered the germinal center. One unusual feature of hairy cells (HCs) is the frequent expression of multiple immunoglobulin heavy chain isotypes, with dominance of immunoglobulin (Ig)--G3, but only a single light chain type. The origin and clonal relationship of these isotype variants have been unclear. In order to probe the isotype switching status of HCL, RNA transcripts of V(H)DJ(H)--constant region sequences from 5 cases of typical HCL, all expressing multiple surface immunoglobulin isotypes, were analyzed. Tumor V(H)DJ(H)--C(mu) sequences were identified and found to be somatically mutated (range, 1.4% to 6.5%), with a low level of intraclonal heterogeneity. Additional immunoglobulin isotypes of identical V(H)DJ(H) sequence were also identified, including IgD (5 of 5), IgG3 (5 of 5), IgG1 (3 of 5), IgG2 (2 of 5), IgA1 (4 of 5), and IgA2 (1 of 5). Derivation of multiple isotypes from individual cells was demonstrated by analyzing transcripts in single sorted cells from one patient, with evidence for coexistence of isotype variants in 10 of 10 cells. These findings indicate that clonally related multiple isotypes coexist in single HCs, with individual isotypes presumably generated via RNA splicing. Production of IgG3 appears common, but IgG1, IgG2, IgA1, and IgA2 also arise, indicating a continuing influence of a directed process on the tumor clone. These HCs appear to be arrested at the point of isotype switch, where RNA processing may precede deletional recombination. (Blood. 2001;98:1174-1181)

DNA Fiber Fluorescence In Situ Hybridization Analysis of Immunoglobulin Class Switching in B-Cell Neoplasia: Aberrant CH Gene Rearrangements in Follicle Center-Cell Lymphoma

Immunoglobulin class switching usually involves deletion of part of the immunoglobulin CH region. By DNA fiber fluorescence in situ hybridization (FISH) with a barcode of probes covering the DH, JH, and CH genes, the configuration of the entire CH region can be visualized on single DNA molecules. Using this technique, we have studied class switching in three types of B-cell neoplasia, mantle-cell lymphoma (MCL), follicular lymphoma (FL) and hairy cell leukemia (HCL), representing B cells in, respectively, pregerminal center, germinal center, and postgerminal center stages of development. In MCL and FL, simultaneous detection of the t(11;14) and t(14;18) breakpoint with probes for the BCL-1 and BCL-2 loci, respectively, allowed differentiation between productive and nonproductive alleles. In none of 10 MCL cases was class switching detected. In 21 HCL, all nonimmunoglobulin M (IgM) cases had class-switch deletion consistent with the expressed isotype on at least one allele. In FL, however, a peculiar pattern of CH rearrangement was observed. In IgM expressing FL, the translocated alleles had switched in 11 of 13 cases, and the nontranslocated allele showed complex rearrangements downstream from the Cμ-Cδ genes in 9 of 13 cases. These downstream rearrangements may reflect tumor-specific deregulation of the class-switch machinery. All seven immunoglobulin G (IgG) expressing FL showed class switching on both alleles. Fiber FISH analysis also showed several polymorphisms. The most frequent one, present on 38% of all analyzed alleles, consisted of an extra Cγ gene or pseudogene in the 3′ cluster.

© 1998 by The American Society of Hematology.

DNA Fiber Fluorescence In Situ Hybridization Analysis of Immunoglobulin Class Switching in B-Cell Neoplasia: Aberrant CH Gene Rearrangements in Follicle Center-Cell Lymphoma

Immunoglobulin class switching usually involves deletion of part of the immunoglobulin CH region. By DNA fiber fluorescence in situ hybridization (FISH) with a barcode of probes covering the DH, JH, and CH genes, the configuration of the entire CH region can be visualized on single DNA molecules. Using this technique, we have studied class switching in three types of B-cell neoplasia, mantle-cell lymphoma (MCL), follicular lymphoma (FL) and hairy cell leukemia (HCL), representing B cells in, respectively, pregerminal center, germinal center, and postgerminal center stages of development. In MCL and FL, simultaneous detection of the t(11;14) and t(14;18) breakpoint with probes for the BCL-1 and BCL-2 loci, respectively, allowed differentiation between productive and nonproductive alleles. In none of 10 MCL cases was class switching detected. In 21 HCL, all nonimmunoglobulin M (IgM) cases had class-switch deletion consistent with the expressed isotype on at least one allele. In FL, however, a peculiar pattern of CH rearrangement was observed. In IgM expressing FL, the translocated alleles had switched in 11 of 13 cases, and the nontranslocated allele showed complex rearrangements downstream from the Cμ-Cδ genes in 9 of 13 cases. These downstream rearrangements may reflect tumor-specific deregulation of the class-switch machinery. All seven immunoglobulin G (IgG) expressing FL showed class switching on both alleles. Fiber FISH analysis also showed several polymorphisms. The most frequent one, present on 38% of all analyzed alleles, consisted of an extra Cγ gene or pseudogene in the 3′ cluster.

© 1998 by The American Society of Hematology.

Immortalization of immunologically committed Epstein-Barr virus-transformed human B-lymphoblastoid cell lines accompanied by a strong telomerase activity

The immunological characteristics and immortalization processes of three EBV-transformed human B-lymphoblastoid cell lines, N0003, N0005 and N6803, with strong telomerase and infinitively proliferating activities are described. The three cell lines were apparently immortalized: they developed a strong telomerase activity at the population doubling levels (PDLs) between 11 and 135, and continued proliferation over 250 PDLs. All the cell lines expressed CD22, CD19 and CD20 antigens. They were uniformly stained with IgM (N0005), IgG (N6803) or IgA (N0003) at early PDLs between 17 and 20, and they secreted the corresponding class of Ig into the medium; the N6803 and N0003 cell lines continued to secrete each class of Ig at decreased levels while the N0005 cell line expressed or secreted virtually no Ig after immortalization. Karyotype analysis of the immortalized cell lines showed that they were derived from a single cell because they shared a set of abnormal chromosomes within each cell population, and two of the cell lines attained clonal characteristics before they developed a strong telomerase activity. These results indicate that the three immortalized cell lines with a strong telomerase activity correspond to the intermediate stages of B-cell differentiation naturally committed to a specific Ig class, and suggest that they were derived from a B-lymphoblastoid cell committed to a specific class of Ig with poor telomerase activity, rather than from a strongly telomerase-positive B-lymphoblastoid cell either committed or multipotential.

A deletion map of the human immunoglobulin heavy chain variable region

Analysis of VH gene segments deleted in the process of immunoglobulin heavy chain (IGH) variable region assembly in three series of monoclonal B cell lines has been used to determine the human VH region organization. A deletion map of the relative positions of 21 different VH gene segments has been determined. The characterization of B cell lines from three unrelated adults of two racial groups yielded the same relative VH gene segment order, suggesting that the overall order of VH genes in the normal population is constant. This VH gene segment order was consistent with what we had previously generated from physical mapping techniques. DH segments from the second DH cluster, distinct from the major DH locus 3' of the VH region, were not observed to be used in 32 different rearrangements. Approximately 77% of the VH-(D)JH rearrangements involved VH gene segments within 500 kb of the JH region, indicating that human B cell lines preferentially rearrange JH-proximal VH gene segments. The switch, observed in mice, from the fetal use of JH-proximal VH gene segments to an adult VH use dependent upon VH family size may therefore not occur in humans. This detailed map of the VH gene segments is a necessary prerequisite for understanding VH usage in development and disease.

Isotype commitment of human B cells that are transformed by Epstein-Barr virus

Epstein-Barr virus (EBV) can transform a subpopulation of preactivated B cells thus promoting their growth and differentiation into plasma cells. In EBV-transformed clones of IgM-producing cells, the heavy chain constant region (CH) genes on the productive allele are fixed in germ-line configuration, whereas in isotype-switched clones the CH genes proximal to the expressed CH gene are deleted. In order to define more precisely the EBV-susceptible B cells, we sorted subpopulations of B cells on the basis of their cell surface Ig (sIg) isotypes, infected them with EBV, and determined which isotypes they could produce following transformation. Most precursors of IgM-producing plasma cells expressed both IgM and IgD on their surface, while a minority expressed IgM alone. Some B cell precursors of IgG- and IgA-producing cells also expressed sIgM, but surprisingly none expressed IgD. Those precursors of IgG and IgA producers, which bore sIgM, expressed it in relatively low levels, whereas B cells expressing high levels of sIgM were incapable of generating IgG and IgA producers. All of the precursors of IgG and IgA plasma cells expressed these isotypes on their cell surface. Interestingly, precursor B cells capable of producing the IgG3 and IgA2 subclasses could be respectively enriched on the basis of the presence or absence of cell sIgM. These results demonstrate the isotype precommitment of EBV-transformable B cells. They further suggest that residual IgM is transiently expressed on the surface of the IgG- and IgA-committed B cell precursors, whereas sIgD expression is extinguished earlier in the process of isotype switching via CH gene deletion.

Concatameric replication of Epstein-Barr virus: structure of the termini in virus-producer and newly transformed cell lines

The linear form of Epstein-Barr virus (EBV) DNA has homologous direct tandem repeats of approximately 500 bp at each terminus (TR). After infection, EBV DNA circularizes via the TR to form the intracellular episomal DNA. To analyze the mechanism of the synthesis of linear DNA through possible replicative intermediates, the terminal fragments were identified in the total intracellular DNA and the covalently closed circular DNA from a productively infected cell line after induction of replication or after treatment with an inhibitor of viral DNA synthesis. These studies indicate that some of the fused terminal fragments detected in the total intracellular DNA are replication-dependent forms which are selectively excluded from the covalently closed circular fraction and are eliminated after treatment with acyclovir. The EBV terminal restriction enzyme fragments were identified in three producer cell lines, each with a characteristic number of TR in the intracellular episomal DNA. Identification of the termini in cell lines established with the three virus strains revealed that the newly transformed cell lines had a greater number of TR than did the template DNA in the producer cell line. The increase in the number of TR in progeny episomes indicates that linear DNA is produced from concatameric replicative intermediates rather than from amplified catenated circular intermediates.

Clonal and molecular characteristics of the human IgE-committed B cell subset

We have followed the pathway of the IgE-committed B lymphocyte from fresh, unstimulated peripheral blood, through EBV activation, transformation, and eventual cloning. Using cell sorting in conjunction with limiting dilution culture systems, we found that: (a) cells that are selected in the cell sorter and secrete IgE in culture are sIgM+/sIgD+. They secrete all three isotypes after EBV activation and continue to do so stably in culture; (b) individual IgE+ cells in culture coproduce IgM, IgD, and IgE and cytoplasmic Ig of each isotype can be detected in single cells; (c) no rearrangement was observed of VDJ to epsilon in any of six lines tested. DNA between the rearranged VDJ-mu and -epsilon appears to be overall intact, including a region 10.5 kb upstream and 18 kb downstream of the 2-kb epsilon coding region and; (d) mRNA of mu and epsilon species is of normal and comparable size. In contrast to IgG- and IgA-producing clones, multiple isotype expression appears to be both frequent and stable in cells committed to IgE production. We propose that IgE-committed cells represent a unique B cell sublineage whose differentiation pathway may be more strictly regulated than that of other isotypes with regard to the signals required for classical, deletional switch recombination that has been observed in rare IgE-producing myeloma cell lines.

Comparative study of DNase I sensitivity at the X-linked human HPRT locus

To examine the association between chromatin structure and gene expression at the human hypoxanthine phosphoribosyltransferase (HPRT) locus, DNase I sensitivity of active and inactive genes was analyzed. In a set of human-hamster hybrid lines containing either an active or an inactive human X chromosome, or a derivative of the latter in which the HPRT gene was reactivated by 5-azacytidine treatment, only the promoter region of the gene was found to contain a hypersensitive domain, and its presence was strictly correlated with gene activity. An S1 nuclease-sensitive site was mapped upstream from the DNase I hypersensitive domain using supercoiled plasmids. The overall level of DNase I sensitivity in the interior of the HPRT gene was also assessed by comparing the degradation of polymorphic restriction fragments on active and inactive alleles in both polyclonal and monoclonal lines of female human cells. In these internally controlled experiments, the active X chromosome was found to be approximately twofold more susceptible to DNase I digestion than the inactive X chromosome.

B-cell lymphoproliferation and lymphomagenesis are associated with clonotypic intracellular terminal regions of the Epstein-Barr virus

We analyzed 17 B-cell lineages cloned from two patients with infectious mononucleosis and found that different B-cell lineages exhibited notable variation in the length of the fused Epstein-Barr virus (EBV) terminal region on intracellular EBV episomes. EBV termini in different B-cell clones from the same person differed by as many as 15 to 20 reiterations of the ca. 500-base-pair terminal repeat sequence. In contrast, analysis of seven B-cell lineages cloned from a patient with a fatal, oligoclonal lymphoma revealed that three of the cell clones had the same-sized EBV terminal region. These three clones had previously been shown, by immunoglobulin gene analysis, to be metastatic daughter cells descended from a common progenitor. Similarity of the EBV terminal regions in the three daughter clones suggested that EBV infected the progenitor cell before proliferation and metastasis. Individual, EBV-infected cells from a single individual showed sufficient heterogeneity in their EBV termini to allow use of terminal fragment size as a clonal marker in studies addressing the contribution of EBV to the clonal pathogenesis of tumors with which this virus has been associated.

Studies on immunoglobulin gene rearrangement in formalin-fixed, paraffin-embedded pathology specimens

Studies on immunoglobulin gene rearrangement in lymphoid lesions are an increasingly important application of molecular biology in diagnostic medicine. The authors have therefore examined the possibility of detecting such rearrangements in formalin-fixed, paraffin-embedded pathology specimens. Southern blots of DNA obtained from optimally fixed tissues were very similar to blots of unfixed material, except that the electrophoretic mobility of the fixed DNA fragments was sometimes slightly reduced. High-molecular-weight DNA was not recovered from suboptimally fixed partially autolysed samples. Increasing the time of exposure to formalin resulted in loss of hybridizable DNA. Monoclonal rearrangements of heavy and light chain immunoglobulin genes could be detected in formalin-fixed specimens provided that these fixation artifacts were taken into consideration. This technique expands the pool of material available for studies on gene rearrangement and should facilitate the use of such studies in clinical medicine.

B-cell lymphoma in severe combined immunodeficiency not associated with the Epstein-Barr virus

Patients with congenital and acquired immunodeficiencies are at increased risk for the development of B-cell lymphoproliferative disorders. When the appropriate tests have been performed, the Epstein-Barr virus (EBV) nuclear antigen (EBNA) or EBV DNA has been found in tissues from these tumors. These data have provided support for the idea that these tumors are associated with EBV. In this article we report about a child with severe combined immunodeficiency (SCID) who developed a malignant B-cell lymphoma that was not associated with EBV. The B-cell lymphoma in the patient proved to be, by hybridization analysis of immunoglobulin (Ig) heavy chain gene rearrangements of tumor-cell DNA, of clonal origin. However, neither EBNA nor EBV DNA could be detected in tumor tissue by anticomplement immunofluorescence or in situ cytohybridization with an EBV DNA probe. Furthermore, EBV DNA could not be detected by Southern blot hybridization using two EBV DNA hybridization probes on the same DNA blots that clearly contained the clonal Ig gene rearrangement. This case represents a clonal B-cell lymphoma occurring in a severely immunodeficient patient that was not associated with EBV. Antiviral chemoprophylaxis has been recommended for the prevention of EBV-related B-cell lymphoproliferations in transplant patients. Such prophylaxis may be ineffective in patients with B-cell lymphoproliferative disorders not associated with EBV.

Clonal origins of lymphoproliferative disease induced by Epstein-Barr virus

Analysis of immunoglobulin gene rearrangements in 16 B-cell lineages clonally propagated from two mononucleosis patients supported the notion that mononucleosis is a polyclonal B-lymphoproliferative disorder. Three of seven cell clones from a patient with a fatal B lymphoma revealed the same pattern of immunoglobulin gene rearrangement, indicating that this patient's disease was oligoclonal. The three similar clones were propagated from two sites (blood and spleen), indicating that they represent a metastatic cell lineage which arose during the patient's fatal B lymphoproliferation.

Immunoglobulin gene rearrangements and deletions in human Epstein-Barr virus-transformed cell lines producing different IgG and IgA subclasses

During differentiation B lymphocytes may switch from the expression of surface IgM to the synthesis of IgG, IgA, or IgE isotypes by using a different heavy chain constant region (CH) gene. The molecular mechanisms by which switching occurs remain controversial. Rearrangements and deletions of CH genes 5' to the expressed gene have often been observed in the mouse and, more recently, in human cells that have switched isotypes. We have used human JH, C micro, C gamma, and C alpha probes to examine the extent of the deletions and rearrangements in clones of Epstein-Barr virus-transformed human cells that produce IgG1, IgG3, IgG4, or IgA1. Though deletions of CH genes 5' to the expressed CH gene were consistently observed, the rearrangement process appeared to be highly variable for the nonproductive CH gene locus: deletion or persistence of 5' CH genes, combinations of deletion and duplication of 5' genes, and deletions extending to 3' CH genes. Our results reveal an unexpected lack of specificity in the DNA deletions in cells that have undergone isotype switching.