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From the Department of Pathology,*
University Medical
School of Pécs, Hungary, and the Department of
Pathology,
Weill Medical College of Cornell
University, New York, New York
| Abstract |
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| Introduction |
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Although the immunophenotypic and genotypic characteristics of PELs confirm their B cell lineage, the normal counterpart and stage of differentiation of the tumor cells remain controversial. The combination of the pleomorphic morphology, null cell immunophenotype, and CD30 antigen expression has led some authors to link PEL to anaplastic large cell lymphoma.10 In contrast, other investigators have suggested that PEL cells represent a mature stage of B cell development closely associated with mature plasma cells based on the findings that the tumor cells coexpress CD30, CD38, CD138, and EMA.6,9 Finally, some of the PELs display Ig heavy chain but not light chain gene rearrangement, causing some investigators to suggest that PELs may arise early in B cell development, following heavy chain gene but prior to light chain gene rearrangement.4
Analysis of nucleotide sequences of Ig heavy chain variable region (VH) genes can provide insight into the stage of B cell development at which clonal expansion occurs in B cell tumors. It has been shown that pregerminal center B cells contain IgVH region genes with germ line sequence and postgerminal center B cells contain mutated Ig VH regions. Furthermore, antigen selected postgerminal center B cells display somatic mutations clustered in the complementary determining regions (CDR) sequences. Germinal center B cells are characterized by ongoing mutation, which is absent in postgerminal center B cells.14,15
In order to determine the stage of B cell development at which clonal expansion of the lymphoma cells occurs in PEL, we analyzed nucleotide sequences of IgVH regions expressed by seven PELs. Our results suggest that the development of PELs may not be restricted to one stage of B cell differentiation.
| Materials and Methods |
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A group of seven non-Hodgkin's lymphomas containing KSHV sequences meeting the clinical, morphological, immunophenotypic, and molecular genetic criteria for PEL were included in this study.5,6,10 Case 1 represents the KSHV-positive, EBV-negative BC-3 cell line established from a PEL occurring in an HIV-negative patient.16 Case 2 represents the KSHV- and EBV-positive BC-1 cell line established from a PEL occurring in a patient with AIDS.17 Cases 37 represent KSHV- and EBV-positive PELs collected from adult HIV-positive patients. Heparin-treated samples of the lymphomatous effusions were collected under sterile conditions during standard diagnostic procedures. Mononuclear cells were isolated and cryopreserved as described.5
Immunophenotypic Analyses
The immunophenotype of the PEL tumor cells and cell lines was
determined by direct and indirect immunofluorescent flow cytometry
using the FACScan fluorescent activated cell sorter (Becton-Dickinson,
Mountain View, CA) as previously described.18
Monoclonal
antibodies used include leukocyte common antigen (LCA, CD45), BerH2,
Ki-1 (CD30), epithelial membrane antigen (EMA, Dako Corp, Santa
Barbara, CA), B4 (CD19), B1 (CD20), B2 (CD21, Coulter Immunology,
Hialeah, FL), Leu1 (CD5), Leu14 (CD22), Leu20 (CD23, Becton-Dickinson
Immunocytometry Systems, Mountain View, CA), and T3 (CD3, United
Biomedical, Hauppauge, NY). Antisera to total Ig, kappa (
) and
lambda (
) Ig light chains were obtained from Dako.
Isolation of RNA and First-Strand cDNA Synthesis
Total RNA was extracted from cryopreserved mononuclear cell suspensions using guanidine isothiocyanate techniques.19 Five mg of RNA was reverse transcribed into cDNA using M-MLV reverse transcriptase (Superscript RNase H Reverse Transcriptase, GIBCO-BRL, Grand Island, NY), in conjunction with a poly(dT)1213 primer according to the manufacturer's instructions.
PCR Amplification, Cloning, and Sequencing of the Expressed Ig VH-D-JH Genes
cDNAs were amplified by PCR using sense Ig VH gene family-specific (VH1, VH2, VH3, VH4, VH5, and VH6) leader primers in conjunction with an antisense consensus JH primer in independent reactions, as previously described.20 After amplification, PCR products were electrophoresed in 2% agarose and visualized by ethidium bromide staining. Appropriately sized PCR products were isolated. PCR products were cloned in the pCR II vector using the TA cloning system (Invitrogen, San Diego, CA), following the manufacturer's instructions. Plasmid DNA was isolated from overnight cultures of randomly selected white colonies using Wizard Mini-preps (Promega, Madison, WI). DNA sequencing was performed using the Sequenase version 2.0 (United States Biochemical, Cleveland, OH) system according to the manufacturer's instructions. DNA sequences were analyzed using the MacVector version 4.5 (Eastman Kodak, New Haven, CT) software and the EMBL/GenBank data base.
Analysis of Mutations
We calculated the number of expected replacement (R) mutations in complementary determining regions (CDR) and framework regions (FR) for the Ig VH genes using the formula RCDR or RFR = n x CDR Rf or FR Rf x CDRf or FRf, where n is the total number of observed mutations; Rf is the replacement frequency intrinsic to each Ig VH gene,21 and CDRf and FRf are the relative sizes of the CDRs and FRs. A binomial probability model was used to evaluate whether the observed R mutations in CDRs and the scarcity of silent (S) mutations in the FRs were due to chance alone: p = n!/k!(n - k)!qk(1 - q)n-k, where q = probability that an R mutation will localize to CDR or FR (q = CDRrel or FRrel x CDR Rf or FR Rf), and k = number of observed R mutations in the CDR or FR.21
| Results |
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The immunophenotypic features of the PELs included in this study
are summarized in Table 1
. All but one
PEL (case 5) expressed CD45. However, only one case (case 3) expressed
surface Ig. Also, all seven cases lacked B cell-associated antigens
CD19 and CD20 and only case (case 6) expressed CD22. Five cases (cases
14 and 6) expressed CD30, three cases (cases 2, 6, 7) expressed EMA
and two cases (cases 2 and 6) expressed HLA-DR. All seven cases lacked
CD21 and CD23 and T cell-associated antigens.
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To analyze the Ig VH-D-JH genes expressed
by the seven PELs, first-strand cDNAs were synthesized from total RNA
of lymphoma cells and then PCR-amplified using the appropriate primers.
The amplified DNAs were cloned and sequenced. Eight sequences from
independent bacterial isolates were analyzed in each sample. The
sequences obtained were compared with the corresponding germline
sequences using EMBL/GenBank database to determine the respective
homology of each gene with its closest germline counterpart. The
closest germline VH genes and their degrees of similarity
are shown in Table 1
. The deduced amino acid sequences of the
VH-D-JH segments of the seven PELs and
predicted amino acid changes when compared with the reported closest
germline VH gene sequences are depicted in Figure 1
. All the
VH-D-JH sequences were in-frame and no stop
codons were found, suggesting that these cases expressed functional Ig.
In cases 16, only identical and unique sequences were detected from
analysis of the multiple isolates. Cases 35 used germline genes from
the VH1 family and showed 88.4%, 93.5%, and 90.1%
homology to their closest germline genes, respectively. Cases 1, 2, and
6 utilized germline genes from VH4, VH5, and
VH3 families and displayed 100%, 94.9%, and 90.1%
homology to their corresponding germline genes, respectively. In case
7, VH-D-JH gene sequences derived from eight
independent isolates revealed seven unique but collinear sequences.
Lymphoma cells of this case rearranged a member of VH3
family and sequences displayed 95.2% to 99.0% homology with the
germline precursors (Table 2)
. D segment
sequences in cases 1 and 4 displayed the highest degree of identity to
those of germline genes HD5-5 and HD2-15, respectively. The CDR3
sequences of the other PELs did not display any identity to those of
known germline D genes. The HJ4 gene was used in five cases and the HJ6
gene in two cases.
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In six cases, the expressed VH genes showed numerous
nucleotide differences from the closest germline gene sequences
obtained from EMBL/GenBank database. Because germline VH
genes were not identified in these patients, some of these nucleotide
differences may represent individual polymorphism. However, sequence
polymorphism among V genes is generally low23
and in cases
where we24
and others25,26
have analyzed
germline genes of the patients, sequences corresponded exactly to those
obtained from EMBL/GenBank database in most cases. Therefore, it has
been concluded that polymorphism in VH genes is not
sufficient to require analysis27-29
and nucleotide
differences in these cases have been classified as somatic mutation.
Analysis of the distribution of somatic mutations in each sequence has
been carried out by the method of Chang and
Casali.21
In this method, the binomial distribution
model is used to calculate the probability that an excess in CDRs or
scarcity in FRs of R mutations resulted from chance alone. The
results of the analysis are summarized in Table 2
. The distribution of
R and S mutations indicates a high R:S ratio in the CDRs of cases 47;
however, this ratio did not exceed 2.9 for cases 2 and 3. Applying the
binomial distribution model revealed more R mutations in the CDRs than
expected in five sequences, with significant (P
< 0.05) clustering. All mutated sequences had an R:S ratio equal to or
less than 2 in their FR. Using the binomial distribution model, there
were fewer R mutations in the FRs than expected due to chance in three
sequences, with significant (P < 0.05)
conservation of sequences.
| Discussion |
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The intraclonal heterogeneity of the expressed VH-D-JH gene sequences was variable among the six EBV-positive PELs. In five of these six cases, the expressed VH-D-JH sequences were extensively mutated but these sequences did not show intraclonal heterogeneity. These findings suggest that the tumor cells have traversed the germinal center but are no longer influenced by the somatic hypermutation mechanism. It could be concluded from this that the neoplastic event has occurred at a postgerminal center stage of B-cell maturation. The similar pattern of somatic mutation and antigen expression, including the absence of surface Ig in multiple myeloma and most of these PEL cases, suggests a plasma cell origin for some PEL cases.31
In contrast to homogenous Ig gene sequences of PELs, considerable intraclonal heterogeneity, similar to the situation in follicle center cell lymphoma, was detected in one case.32,33 The ongoing somatic mutation indicates that the tumor cells are still influenced by the mutation mechanism after malignant transformation. This finding is highly suggestive of a germinal center origin of this particular PEL. However, certain memory B cells may recirculate through the germinal center and undergo further rounds of somatic mutation and antigen selection that further improve and expand B-cell memory.34
The germinal center/postgerminal center cellular origin of EBV-positive PELs analyzed is also suggested by the distribution of mutations in the VH genes of the tumor cells. Replacement mutations that affect CDRs can be positively or negatively selected by antigen, as these regions form the antigen binding sites. If a DNA segment displays more R mutations than would be expected by chance, it is likely that positive pressure was exerted on the gene product to select for these mutations.21 In all but one EBV-positive PEL, the significant clustering of R mutations in the CDRs indicates that tumor cells have been selected by antigen or, in other words, that the tumor cells have undergone affinity maturation.
EBV-infected tumor cells of different lymphoid neoplasms frequently express mutated Ig VH genes. For example, it has been shown that the tumor cells of endemic Burkitt's lymphomas,35 posttransplantation lymphoproliferative disorders,36 and Reed-Sternberg cells of Hodgkin's disease37 express mutated Ig VH genes. Our findings strengthen this observation by providing evidence that EBV-positive tumor cells of PEL also express mutated VH genes. However, the exact role of EBV infection in Ig gene somatic mutation of neoplastic cells is controversial. It has been demonstrated that EBV-infected normal B lymphocytes display VH genes with somatic hypermutation and evidence intraclonal heterogeneity.38 Therefore, it appears that EBV infection plays a role in preneoplastic events by driving B cell proliferation, activation, and mutation of the Ig gene.
Several lines of evidence suggest that KSHV infection plays a central role in the development of PELs. KSHV carries different genes that may behave as oncogenes, including a gene homologous to bcl-2 (ORF/16), a gene homologous to the cellular D-type cyclins (ORF72/cyclin D), and a G-protein-coupled receptor displaying constitutive activation (ORF74/GPCR).39,40 However, nonneoplastic lymphoid cells may be infected by KSHV. Ambroziak et al41 have shown that the CD19-positive B cell population of HIV-infected and uninfected individuals may carry KSHV without clinical evidence of PEL or Kaposi's sarcoma, and Luppi et al42 found KSHV sequences in benign lymph nodes with florid germinal center hyperplasia. This finding suggests that latent infection of KSHV precedes malignant transformation of the cells and additional adequate stimuli may be necessary to initiate malignant transformation of the KSHV-infected cells, not unlike the EBV scenario. A possible explanation for the heterogeneous mutation profile of VH genes found in PELs is that malignant transformation is not restricted to specific stages of B cell development. Based on this hypothesis, B cells may carry KSHV in all stages of B cell maturation and events leading to the neoplastic transformation may occur independently of the stage of B cell development.
B cell tumors generally are thought to represent normal B cells that froze at a point in B cell differentiation.43 Analysis of Ig genes expressed by B cell tumors shows that most of the non-Hodgkin's lymphoma types represent a certain stage of B cell development at which clonal expansion occurred. Our results demonstrate that tumor cells of PEL may express unmutated, highly mutated, and intraclonal divergent Ig VH gene sequences suggesting that development of PELs is not restricted to one stage of B cell maturation. These results also suggest that different stages of B cell differentiation can be the targets of malignant transformation associated with KSHV infection.
| Footnotes |
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Supported by grants FKFP 0931/97, OTKA T25782, and ETT 365/96 from the Hungarian Ministry of Culture and Education (to AM) and NIH grants CA 68939 and CA 73531 (to EC).
Accepted for publication August 22, 1998.
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