Incorrect us of "immortalization" for B-lymphoblastoid cell lines transformed by Epstein-Barr virus

Long-term Culture of EBV-induced Human Lymphoblastoid Cell Lines Reveals Chromosomal Instability

To preserve material for future genetic studies, human B-lymphocytes from whole blood samples are routinely transformed into lymphoblastoid cell lines (LCLs) by in vitro infection with Epstein-Barr virus. To determine the rate and frequency of chromosomal changes during long-term culture, we established 10 LCLs (from eight individuals). Before transformation, these cases showed a normal karyotype (three cases), a small supernumerary marker chromosome (three cases), or an aberrant karyotype (four cases). Chromosome analyses were performed at 8-week intervals over a period of at least 1 year, up to 3 years. Surprisingly, we demonstrate that chromosomal instability is the rule, rather than the exception, during long-term culture of LCLs. The most commonly observed acquired clonal aberration was trisomy 12, which emerged in all cell lines within 21 to 49 weeks after infection. Telomeric fusions indicating telomere shortening were found after ~21 weeks. After 1 year of cultivation, the proportion of cells with the original karyotype decreased to ≤10% in 7 of the 10 cell lines. To preserve cells with aberrant genomes, we conclude the cultivation time of LCLs must be restricted to the absolute minimum time required.

A novel antibody discovery platform identifies anti-influenza A broadly neutralizing antibodies from human memory B cells

Monoclonal antibody isolation directly from circulating human B cells is a powerful tool to delineate humoral responses to pathological conditions and discover antibody therapeutics. We have developed a platform aimed at improving the efficiencies of B cell selection and V gene recovery. Here, memory B cells are activated and amplified using Epstein-Barr virus infection, co-cultured with CHO-muCD40L cells, and then assessed by functional screenings. An in vitro transcription and translation (IVTT) approach was used to analyze variable (V) genes recovered from each B cell sample and identify the relevant heavy/light chain pair(s). We achieved efficient amplification and activation of memory B cells, and eliminated the need to: 1) seed B cells at clonal level (≤1 cell/well) or perform limited dilution cloning; 2) immortalize B cells; or 3) assemble V genes into an IgG expression vector to confirm the relevant heavy/light chain pairing. Cross-reactive antibodies targeting a conserved epitope on influenza A hemagglutinin were successfully isolated from a healthy donor. In-depth analysis of the isolated antibodies suggested their potential uses as anti-influenza A antibody therapeutics and uncovered a distinct affinity maturation pathway. Importantly, our results showed that cognate heavy/light chain pairings contributed to both the expression level and binding abilities of our newly isolated VH1-69 family, influenza A neutralizing antibodies, contrasting with previous observations that light chains do not significantly contribute to the function of this group of antibodies. Our results further suggest the potential use of the IVTT as a powerful antibody developability assessment tool.

A cascade leading to premature aging phenotypes including abnormal tumor profiles in Werner syndrome (review)

This perspective review focused on the Werner syndrome (WS) by addressing the issue of how a single mutation in a WRN gene encoding WRN DNA helicase induces a wide range of premature aging phenotypes accompanied by an abnormal pattern of tumors. The key event caused by WRN gene mutation is the dysfunction of telomeres. Studies on normal aging have identified a molecular circuit in which the dysfunction of telomeres caused by cellular aging activates the TP53 gene. The resultant p53 suppresses cell growth and induces a shorter cellular lifespan, and also compromises mitochondrial biogenesis leading to the overproduction of reactive oxygen species (ROS) causing multiple aging phenotypes. As an analogy of the mechanism in natural aging, we described a hypothetical mechanism of premature aging in WS: telomere dysfunction induced by WRN mutation causes multiple premature aging phenotypes of WS, including shortened cellular lifespan and inflammation induced by ROS, such as diabetes mellitus. This model also explains the relatively late onset of the disorder, at approximately age 20. Telomere dysfunction in WS is closely correlated with abnormality in tumorigenesis. Thus, the majority of wide and complex pathological phenotypes of WS may be explained in a unified manner by the cascade beginning with telomere dysfunction initiated by WRN gene mutation.

Transcriptome study of differential expression in schizophrenia

Schizophrenia genome-wide association studies (GWAS) have identified common SNPs, rare copy number variants (CNVs) and a large polygenic contribution to illness risk, but biological mechanisms remain unclear. Bioinformatic analyses of significantly associated genetic variants point to a large role for regulatory variants. To identify gene expression abnormalities in schizophrenia, we generated whole-genome gene expression profiles using microarrays on lymphoblastoid cell lines (LCLs) from 413 cases and 446 controls. Regression analysis identified 95 transcripts differentially expressed by affection status at a genome-wide false discovery rate (FDR) of 0.05, while simultaneously controlling for confounding effects. These transcripts represented 89 genes with functions such as neurotransmission, gene regulation, cell cycle progression, differentiation, apoptosis, microRNA (miRNA) processing and immunity. This functional diversity is consistent with schizophrenia's likely significant pathophysiological heterogeneity. The overall enrichment of immune-related genes among those differentially expressed by affection status is consistent with hypothesized immune contributions to schizophrenia risk. The observed differential expression of extended major histocompatibility complex (xMHC) region histones (HIST1H2BD, HIST1H2BC, HIST1H2BH, HIST1H2BG and HIST1H4K) converges with the genetic evidence from GWAS, which find the xMHC to be the most significant susceptibility locus. Among the differentially expressed immune-related genes, B3GNT2 is implicated in autoimmune disorders previously tied to schizophrenia risk (rheumatoid arthritis and Graves' disease), and DICER1 is pivotal in miRNA processing potentially linking to miRNA alterations in schizophrenia (e.g. MIR137, the second strongest GWAS finding). Our analysis provides novel candidate genes for further study to assess their potential contribution to schizophrenia.

Genotype instability during long-term subculture of lymphoblastoid cell lines

Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines (LCLs) promise to address the challenge posed by the limited availability of primary cells needed as a source of genomic DNA for genetic studies. However, the genetic stability of LCLs following prolonged culture has never been rigorously investigated. To evaluate genotypic errors caused by EBV integration into human chromosomes, we isolated genomic DNA from human peripheral blood mononuclear cells and LCLs collected from 20 individuals and genotyped the DNA samples using the Affymetrix 500K SNP array set. Genotype concordance measurements between two sources of DNA from the same individual indicated that genotypic discordance is negligible in early-passage LCLs (<20 passages) but substantial in late-passage LCLs (>50 passages). Analysis of concordance on a chromosome-by-chromosome basis identified genomic regions with a high frequency of genotypic errors resulting from the loss of heterozygosity observed in late-passage LCLs. Our findings suggest that, although LCLs harvested during early stages of propagation are a reliable source of genomic DNA for genetic studies, investigations that involve genotyping of the entire genome should not use DNA from late-passage LCLs.

Effect of one year of cryopreservation on the activity of lysosomal hydrolases from EBV-transformed lymphocytes

Background. The Epstein-Barr virus (EBV) was used as an agent of B lymphocyte proliferation for subsequent diagnosis of lysosomal storage disease. Due to the constant handling of long-preserved samples in our cell bank, we decided to observe the behavior and then compare cultured and frozen samples for at least one year's cryopreservation. Methods. Twenty-five samples from healthy individuals were used to assess the possible changes in activity of enzymes β-galactosidase, β-glucosidase, α-iduronidase, α-galactosidase, and α-glucosidase. Transmission electron microscopy was used to confirm cell transformation of B lymphocytes into EBV-infected cells, generating lymphoblastoid cell lines. Results. Transmission electron microscopy findings confirmed previous reports in the literature that is, significant and evident morphological changes in the nucleus occur after day 12 and the consequent cell transformation into EBV-infected cells. After thawing and subsequent treatment with the five enzymes utilized, we observed no significant changes in samples cryopreserved for more than one year, as compared to samples cultured for 12 days.

Sustained viral activity of epstein-Barr virus contributes to cellular immortalization of lymphoblastoid cell lines

EBV-transformed lymphoblastoid cell lines (LCLs) are used as a resource for human genetic, immunological, and pharmacogenomic studies. We investigated the biological activity of 20 LCL strains during continuous long-term subculture up to a passage number of 160. Out of 20 LCL strains, 17 proliferated up to a passage number of 160, at which point LCLs are generally considered as "immortalized". The other three LCL strains lost the ability to proliferate at an average passage number of 41, during which these LCLs may have undergone cellular crisis. These non-immortal LCL strains exhibited no telomerase activity, decreased EBV gene expression, and a lower copy number of the EBV genome and mitochondrial DNA when compared with immortal LCLs. Thus, this study suggests that sustained EBV viral activity as well as telomerase activity may be required for complete LCL immortalization.

Role of Werner syndrome gene product helicase in carcinogenesis and in resistance to genotoxins by cancer cells

Werner syndrome (WS) is an autosomal recessive genetic disorder causing premature aging, and WRN has been identified as the causative gene of WS. The product of the WRN gene (WRN) acts as a DNA helicase with exonuclease activity, and data have accumulated showing that the WRN gene strongly participates in carcinogenesis: (1) the normal WRN gene likely participates in the immortalization of B-lymphoblastoid cell lines through telomeric crisis caused by telomere shortening, (2) a much higher incidence of rare cancers occurs in WS patients than in other kinds of patients, and (3) levels of WRN expressed in virus-transformed cells and cancer cells are usually markedly up-regulated and are inversely correlated with the sensitivity of these cells against various genotoxins, including camptothecin. In this paper, we review the events that show a close correlation of the WRN gene and WRN with carcinogenesis and their underlying molecular mechanisms.

Increased chemotherapeutic activity of camptothecin in cancer cells by siRNA-induced silencing of WRN helicase

Werner syndrome helicase (WRN) participates in a wide range of DNA activities, including replication, double-strand DNA break repair, telomere and retrovirus long terminal repeat maintenance. Mutations of the WRN gene cause Werner syndrome (WS), an autosomal recessive premature ageing disorder associated with various symptoms related to ageing. In this study, we investigated the siRNA that specifically down-regulates WRN expression. WRN silencing increased markedly the chemotherapeutic activity of camptothecin (CPT) on cancer cells in terms of the extent of efficacy and lowering effective drug dosage, accompanied by suppressing recovery from DNA damage caused by CPT. Here, we propose a potential combination therapy of WRN-siRNA and CPT, looking forward to minimizing the inevitable adverse effects associated with cancer chemotherapy.

[Mechanism of cell proliferation--cell cycle, oncogenes, and senescence]

Cell proliferation is regulated through a transition between the G0 phase and cell cycle. We isolated a mammalian temperature-sensitive mutant cell line defective in the function from the G0 phase to cell cycle. Senescent human somatic cells fail to enter into the cell cycle from the G0 phase with stimulation by any growth factor. Telomere shortening was found to be a cause of cellular senescence, and reexpression of telomerase immortalized human somatic cells. Immortalized human somatic cells showed normal phenotypes and were useful not only for basic research but also for clinical and applied fields. The importance of p53 and p21 activation/induction i now well accepted in the signal transduction process from telomere shortening to growth arrest, but the precise mechanism is largely unknown as yet. We found that the MAP kinase cascade and histone acetylase have an important role in the signaling process to express p21. Tumor tissues and cells were found to have strong telomerase activity, while most normal somatic human tissues showed very weak or no activity. Telomerase activity was shown to be a good marker for early tumor diagnosis because significant telomerase activity was detected in very early tumors or even in some precancerous tissues compared with adjacent normal tissues. Telomere/telomerase is a candidate target for cancer chemotherapeutics, and an agent that abrogated telomere functions was found to kill tumor cells effectively by inducing apoptosis whereas it showed no effect on the viability of normal cells.

The Siblings With Ischemic Stroke Study (SWISS): a progress report

There is increasing evidence that genetic factors are associated with ischemic stroke, including multiple recent reports of association with the gene PDE4D, encoding phosphodiesterase 4D, on chromosome 5q12. Genetic studies of stroke are important but can be logistically difficult to perform. This article reviews the design of the Siblings With Ischemic Stroke Study (SWISS) and discusses problems in performing a sibling-based pedigree study where proband-initiated consent is used to enroll pedigree members. Proband-initiated enrollment optimizes privacy protections for family members, but it is associated with a substantial pedigree non-completion rate such that 3 to 4 probands must be identified to obtain one completed sibling pedigree. This report updates the progress of enrollment in the SWISS protocol, discusses barriers to pedigree completion and describes innovative approaches used by the SWISS investigators to enhance enrollment.

Genetically Characterized Positive Control Cell Lines Derived from Residual Clinical Blood Samples

Background: Positive control materials for clinical diagnostic molecular genetic testing are in critically short supply. High-quality DNA that closely resembles DNA isolated from patient specimens can be obtained from Epstein–Barr virus (EBV)–transformed peripheral blood lymphocyte cell lines. Here we report the development of a process to (a) recover residual blood samples with clinically important mutations detected during routine medical care, (b) select samples likely to provide viable lymphocytes for EBV transformation, (c) establish stable cell lines and confirm the reported mutation(s), and (d) validate the cell lines for use as positive controls in clinical molecular genetic testing applications.

Methods: A network of 32 genetic testing laboratories was established to obtain anonymous, residual clinical samples for transformation and to validate resulting cell lines for use as positive controls. Three panel meetings with experts in molecular genetic testing were held to evaluate results and formulate a process that could function in the context of current common practices in molecular diagnostic testing.

Results: Thirteen laboratories submitted a total of 113 residual clinical blood samples with mutations for 14 genetic disorders. Forty-one EBV-transformed cell lines were established. Thirty-five individual point and deletion mutations were shown to be stable after 20 population doublings in culture. Thirty-three cell lines were characterized for specific mutations and validated for use as positive controls in clinical diagnostic applications.

Conclusions: A process for producing and validating positive control cell lines from residual clinical blood samples has been developed. Sustainable implementation of the process could help alleviate the current shortage of positive control materials.

Differential cytotoxicity of anticancer agents in pre- and post-immortal lymphoblastoid cell lines

We studied the cytotoxic effect of various anticancer agents on lymphoblastoid cell lines transformed by Epstein-Barr virus. Post-immortal N0005 (post-N0005) is an immortalized cell line derived from pre-immortal N0005 (pre-N0005) accompanied by increased telomerase activity, short-telomere, abnormal karyotypes, mutation of p53 gene, down regulation of p16/Rb and the ability to grow in soft agar medium. Compared with pre-N0005 cells, post-N0005 cells were significantly (p<0.001 by the Student t test) more resistant to the killing activity of seven DNA-modifying agents: camptothecin, etoposide, bleomycin, fluorouracil, thioguanine, melphalan and actinomycin D. However, both pre-N0005 and post-N0005 cells showed similar levels of cytotoxicity against four DNA-non-modifying agents: colchicine, paclitaxel, vincristine and methotrexate. DNA-modifying and DNA-non-modifying agents are distinguished by their different sensitivities with pre-N0005 and post-N0005. Based on these results, we propose that pre-N0005 and post-N0005 cell lines be used as a new method to assess and screen anticancer agents.

Characterization of the hsp70 response in lymphoblasts from aged and centenarian subjects and differential effects of in vitro zinc supplementation

Human centenarians attract increasing interest as they hold some still undefined molecular mechanisms resulting in the achievement of exceptional old age. Recent data suggest the ability of centenarians to efficiently counter the increased cellular stress normally associated with ageing. The ubiquitous heat shock (HS) protein HSP70, expressed under the control of the heat shock transcription factor 1 (HSF-1), is recognized as one of the main chaperones associated with cell protection against stresses. In fact, HSP70 protein induction by heat, a classic well characterized cellular stress, was recently reported to be reduced in cells of most aged humans but not in centenarians. In order to investigate the molecular basis of this feature, we analyzed in vitro the time course expression of the hsp70 gene and the activation of HSF-1 in heat treated Epstein Barr virus transformed B-lymphocytes of centenarians. Our study demonstrates that lymphoblasts from centenarians maintain the transcriptional response of hsp70 gene to heat stress similar to young subjects. Such normal induction of hsp70 is associated to higher binding activity of HSF-1 that compensates an age-dependent delay in HSF-1 phosphorylation. Moreover, in vitro zinc supplementation had an age-dependent effect on hsp70 expression, indicating a role for this nutritionally important molecule and suggesting its involvement in cellular stress responses.

Steps involved in immortalization and tumorigenesis in human B-lymphoblastoid cell lines transformed by Epstein-Barr virus

Epstein-Barr virus (EBV) is closely associated with the generation of various tumors, including Burkitt's lymphoma. Human resting B cells from peripheral blood are easily transformed by EBV to actively proliferating B-lymphoblastoid cell lines (LCLs). These LCLs with normal diploid karyotypes have been believed to be "immortal", without becoming tumorigenic. A series of recent studies, however, indicate that this initial, simple concept needs extensive reconsideration. Most LCLs from normal individuals are mortal because their telomeres shorten. Some LCLs are truly immortalized by developing strong telomerase activity and aneuploidy, accompanied by various other changes: down-regulation of p16/Rb; mutation of the p53 gene; modulation of apoptosis; and sensitivity to various chemical agents. Some post-immortal LCLs additionally develop the ability to form colonies in agarose and even become tumorigenic by developing the ability to grow in nude mice. The genetic background of LCLs markedly affects the frequency of immortalization. In summary, changes of B cells after infection by EBV are roughly divided into two steps: (a) transformation of B cells into LCLs caused by EBV proteins; and (b) immortalization and tumorigenesis of LCLs mainly regulated by the factors of host cells in cooperation with EBV proteins. The new concept as reviewed here is essential for the future study of tumorigenesis by EBV.

WRN gene and other genetic factors affecting immortalization of human B-lymphoblastoid cell lines transformed by Epstein-Barr virus

The immortalization of human B-lymphoblastoid cell lines (LCL) transformed by Epstein-Barr virus (EBV) is accompanied by two major events: increase in telomerase activity and change in karyotype from normal diploid to aneuploidy. We investigated the effect of genetic factors on the incidence of immortalization by putting old and new data together to collect enough samples for statistical analysis. Among 50 LCL from normal individuals, 5 LCL (10.0%) were immortalized and the remaining 45 LCL were mortal. None of the 44 LCL (0%; P < 0.031 against normal individuals by chi square test) from patients having Werner syndrome (WS), a recessive genetic disorder showing premature aging, were immortalized. Among 11 LCL from a family with a tendency to have hereditary type 2 diabetes mellitus, 5 LCL (45.5%; P < 0.0040 against normal individuals, P < 0.00001 against WS patients) were immortalized. Duplicated measurements of the lifespan of 33 LCL showed a good coincidence (r=0.785) between the first and second estimations, indicating that each mortal LCL has a predetermined lifespan. These results strongly suggest that the normal WRN gene, the causative gene of WS, is essential for LCL to immortalize, and genetic factor(s) of a family having diabetes mellitus increases immortalization, implicating that host genetic factors affect immortalization of EBV and probably carcinogenesis by EBV.

Establishment of stably EBV-transformed cell lines from residual clinical blood samples for use in performance evaluation and quality assurance in molecular genetic testing

Positive control materials for clinical molecular genetic testing applications are currently in critically short supply or non-existent for many genetically based diseases of public health importance. Here we demonstrate that anonymous, residual, clinical blood samples are potential sources of viable lymphocytes for establishing Epstein-Barr virus (EBV)-transformed blood lymphocyte cell lines. We attempted to transform 34 residual blood samples, and analyzed transformation success with respect to sample age, anticoagulant, storage temperature, volume, hemolysis, and patient age and sex. In univariate analysis, sample age was significantly associated with transformation success (P = 0.002). The success rate was 67% (6 of 9) for samples 1 to 7 days old, 38% (3 of 8) for samples 8 to 14 days old and 0% for samples 15 to 21 (0 of 11) days old. When we controlled for sample age in multivariate logistic regression, anticoagulant and storage temperature approached significance (P = 0.070 and 0.087, respectively; samples in acid citrate dextrose (ACD) and refrigerated samples were more likely to transform). Based on these findings, we suggest that samples collected in either ACD or ethylene diamine tetraacetic acid, and up to 14 days old (refrigerated) or 7 days old (stored ambient), are reasonable candidates for EBV transformation. The transformation rate for samples that met these criteria was 63% (10 of 16). Implementation of this process could help alleviate the shortage of positive control materials for clinical molecular genetic testing.

In VitroEstablishment of Tumorigenic Human B-Lymphoblastoid Cell Lines Transformed by Epstein-Barr Virus

We studied tumorigenic and phenotypic characteristics of pre- and postimmortal human B-lymphoblastoid cell lines (LCLs) transformed by Epstein-Barr virus (EBV): preimmortal LCLs showed low telomerase activity and a normal diploid karyotype while postimmortal LCLs showed much higher telomerase activity and maintained a clonal aneuploidic state. Among five postimmortal LCLs tested, LCLs N0005 and N6803 formed colonies in agar medium and showed marked aneuploidy, and N6803 was transplantable into nude mice indicating that it had a complete malignant phenotype, but all preimmortal LCLs and the remaining three postimmortal LCLs lacked these characteristics. The products of tumor suppresser genes, p16(INK4A) and pRb, were downregulated in these two LCLs, and the p53 gene was mutated in N0005 LCL. We believe these results showed for the first time that some postimmortal EBV-transformed LCLs can become tumorigenic, contrary to previous reports, and that these LCLs provide an in vitro model of tumorigenesis induced by EBV.

Proteome analysis of Epstein-Barr virus-transformed B-lymphoblasts and the proteome database

The proteome is the entire protein complement of the genome expressed in a particular cell, tissue, or organism at a given time under a specific set of environmental conditions. Proteomics is a combinatorial methodology to comprehensively analyze the proteome. The general protocol of the expression proteomics consists of advanced methods of high-resolution protein separation, high-quality image analysis and high-throughput protein identification. Although Epstein-Barr virus-transformed B-lymphoblastoid cell lines (LCLs) have long been believed to be immortalized, recent studies have provided ample evidence that a large proportion of LCLs have limited life spans due to shortening of telomeres, and that part of them are truly immortalized by developing strong telomerase activity to maintain telomeres. Differential proteome analysis of pre- and post-immortal LCLs would provide a powerful tool to analyze proteins participating in the process of immortalization. We focus in this review on cumulative data of proteomic information on pre- and post-immortal LCLs.

Innate apoptosis of human B lymphoblasts transformed by Epstein-Barr virus: modulation by cellular immortalization and senescence

B-lymphoblastoid cell lines (LCLs) transformed by Epstein-Barr virus have a phenotype corresponding to activated B-lymphoblasts. Although they are widely used as models in various biological and medical studies, their innate morphological differentiation and apoptosis has been little studied. We report here that a large proportion of LCL cells spontaneously differentiate into smaller lymphoid cells which ultimately undergo apoptosis during conventional cell culture. Two distinct types of apoptosis with some intermediate types exist: type 1 apoptosis in small and medium-size cells with shrunken nuclei having heavily condensed chromatin in the whole nucleus region accompanied by relatively large internucleosomally fragmented DNA (above 2 kbp); type 2 apoptosis in large lymphoblasts with extremely lobulated nuclei having chromatin condensation beneath the nuclear membrane alone accompanied by smaller internucleosomally fragmented DNA (below 2 kbp). Type 1 apoptotic cells were far more numerous than type 2 apoptotic cells. The incidence of type 1 apoptosis was suppressed by cellular immortalization and was extremely stimulated at the end of the lifespan (crisis). These results provide essential information for us to use LCLs for various biological and medical studies including cellular immortalization, tumorigenesis and senescence.

Chromosomal instability in B-lymphoblasotoid cell lines from Werner and Bloom syndrome patients

Werner's syndrome (WS) and Bloom's syndrome (BS) are rare autosomal genetic diseases that predispose to cancer and are associated with genomic instability. To characterize the genomic instability of WS and BS, we analyzed and compared the cytogenetics of B-lymphoblastoid cell lines (LCLs) from WS and BS patients and healthy donors. Although, similar spontaneous frequencies of micronuclei (MN) and sister chromatid exchanges (SCE) were observed in LCLs from WS patients and healthy donors, they were much higher in BS-LCLs. We also examined the cells' cytotoxic and cytogenetic formation (MN) response to camptothecin (CAM), etoposide (ETO), 4-nitroquinoline 1-oxide (4NQO), and mitomycin C (MMC). Compared to healthy donor LCLs, BS-LCLs but not WS-LCLs tended to be resistant to cytotoxicity and sensitive to MN induction by 4NQO and MMC. Spectrum karyotyping analysis revealed that most WS- and BS-LCLs generated "variegated translocation mosaicism" at high frequencies during cell culture. These findings support the idea that the basis of genomic instability in WS is different from that in BS.

Clonal chromosomal aberrations accompanied by strong telomerase activity in immortalization of human B-lymphoblastoid cell lines transformed by Epstein-Barr virus

Human B-lymphoblastoid cell lines transformed by Epstein-Barr (EBV-LCLs) are considered to be immortalized, although most of them show a normal diploid karyotype. Recently, we and others have shown that only part of EBV-LCLs is immortalized by developing strong telomerase activity that stabilizes the telomeres. In this study, we investigated the change in karyotypes during immortalization. All the eight immortalized cell lines developed clonal chromosomal aberrations accompanied by the development of strong telomerase activity. Interestingly, abnormal chromosomes were not shared among the immortalized cell lines. These results strongly suggest that chromosomal rearrangements and induction of strong telomerase activity are two events that take place in parallel in the process of immortalization of EBV-LCLs, and indicate that EBV-LCLs are clearly divided into two distinct groups, pre-immortal cell lines mostly with a normal diploid karyotype and post-immortal cell lines with a clonally abnormal karyotype.