Interaction of Epstein-Barr virus with leukaemic B cells in vitro. II. Cell line establishment from prolymphocytic leukaemia and from Waldenström's macroglobulinaemia

Preclinical models of Waldenström's macroglobulinemia and drug resistance

Newer therapeutic strategies are emerging in Waldenström's Macroglobulinemia (WM), which has traditionally been an orphan disease diagnosis. Ibrutinib, a Bruton's tyrosine kinase (BTK) inhibitor was FDA-approved in 2015 as the first ever drug for the treatment of WM. This being a targeted therapy, has given rise to increased research into novel agents and pathways that can be exploited for clinical benefit in WM. In order to understand the underlying mechanisms of disease behavior as well as to test the benefit of various drugs, appropriate preclinical models are required. Historically there had been a lack of representative preclinical models in WM, but in recent years this has dramatically changed. This review highlights the currently available preclinical models and data regarding drug resistance pathways in WM. Knowledge from these will certainly help in paving the future course of treatment in this rare disorder which is indolent and yet, so far incurable.

Selective inhibition of protein arginine methyltransferase 5 blocks initiation and maintenance of B-cell transformation

Epigenetic events that are essential drivers of lymphocyte transformation remain incompletely characterized. We used models of Epstein-Barr virus (EBV)-induced B-cell transformation to document the relevance of protein arginine methyltransferase 5 (PRMT5) to regulation of epigenetic-repressive marks during lymphomagenesis. EBV(+) lymphomas and transformed cell lines exhibited abundant expression of PRMT5, a type II PRMT enzyme that promotes transcriptional silencing of target genes by methylating arginine residues on histone tails. PRMT5 expression was limited to EBV-transformed cells, not resting or activated B lymphocytes, validating it as an ideal therapeutic target. We developed a first-in-class, small-molecule PRMT5 inhibitor that blocked EBV-driven B-lymphocyte transformation and survival while leaving normal B cells unaffected. Inhibition of PRMT5 led to lost recruitment of a PRMT5/p65/HDAC3-repressive complex on the miR96 promoter, restored miR96 expression, and PRMT5 downregulation. RNA-sequencing and chromatin immunoprecipitation experiments identified several tumor suppressor genes, including the protein tyrosine phosphatase gene PTPROt, which became silenced during EBV-driven B-cell transformation. Enhanced PTPROt expression following PRMT5 inhibition led to dephosphorylation of kinases that regulate B-cell receptor signaling. We conclude that PRMT5 is critical to EBV-driven B-cell transformation and maintenance of the malignant phenotype, and that PRMT5 inhibition shows promise as a novel therapeutic approach for B-cell lymphomas.

Trisomy 12 in Epstein-Barr virus-transformed lymphoblastoid cell lines of normal individuals and patients with nonhematologic malignancies

Karyotypes of 36 lymphoblastoid cell lines established by Epstein-Barr virus (EBV) transformation of peripheral blood lymphocytes (PBL) of eight normal individuals and 28 patients with various nonhematologic malignancies were analyzed. In seven lines (19.4%), cells with trisomy 12 were noted, with clonality in two of these lines. In two of 11 metaphases with such trisomy, chromosome 12 was involved in structural rearrangements [t(8;12)(q12;p12) and t(12;12)(q11;q24)]. No cells with trisomy 12 were observed in phytohemagglutinin (PHA)-stimulated PBL cultures of these individuals. In 250 individuals (normal and with nonhematologic malignancies) examined in our laboratory in the last 5 years, extra copies of chromosome 12 in PHA-stimulated PBL cultures were observed in only five of 23,216 cells (0.02%). There were no cases of clonality in these samples. The frequency of an extra chromosome 12 was comparable to that of the other chromosomes except 21 and X, whose frequency of occurrence was 0.08% and 0.09%, respectively. These findings should be considered random events in PHA-stimulated PBL. On the contrary, in lymphoblastoid cell lines established by EBV transformation, trisomy of chromosome 12 was the most frequent numerical abnormality. It was observed in 64.7% of all cases with chromosome gains and therefore could not be considered a random occurrence. The specificity of this phenomenon for EBV transformation is supported by the results of cytogenetic analysis of eight lymphoblastoid cell lines established by an alternative procedure in our laboratory [1]. In 400 cells analyzed not a single cell with trisomy 12 was observed. We suggest that EBV transformation might either randomly induce formation of such cells in immortalized B-cell populations or show potentially blastomogenic cells or proneness to their formation in certain individuals who could be predisposed to develop lymphoproliferative diseases, especially chronic lymphocytic leukemia (CLL) in which trisomy of chromosome 12 is the most common alteration.

Establishment of a new Epstein-Barr virus nuclear antigen-positive B-cell line, BALL-2, with t(8;14) (q24;q32) chromosome abnormality from B-cell acute lymphoblastic leukemia, L2

A new Epstein-Barr virus nuclear antigen (EBNA)-positive B-cell line, designated BALL-2, was spontaneously established from the peripheral blood of a 14-year-old boy with an EBNA-negative B-cell acute lymphoblastic leukemia (B-ALL), L2 in the French-American-British classification. The BALL-2 cell line grew in suspension with or without forming clumps of cells. The cultured cells exhibited lymphoid morphology with indented or lobulated nuclei, prominent nucleoli, and relatively abundant cytoplasm. Immunologic and cytogenetic studies showed that the BALL-2 cell line expressed the B-cell phenotype, CpIg+, SmIg+, CD19+, CD20+, CD38-, Ia+, and had chromosome translocation, t(8;14) (q24;q32). The same phenotypic and chromosome markers were present in original leukemia cells. These results indicated that the cell line was derived from the patient's leukemia cells. Unexpectedly, however, BALL-2 cells were positive for EBNA and EB virus DNA. Gene analysis of the BALL-2 cell line showed biallelic rearrangements in the JH locus. One of the JH rearrangement comigrated with a rearranged c-myc gene, indicating the translocation had occurred between JH and c-myc loci. The t(8;14) abnormality is a known chromosome marker of Burkitt lymphoma and L3 type ALL. Our studies revealed that this translocation and myc gene rearrangement can also be found in L2 type B-ALL.

Characterization of a newly established human Burkitt's lymphoma cell line, OMA-BL-1

Using culture techniques, we have been able to grow occult tumor cells from the bone marrow from cancer patients and have developed a new malignant lymphoid cell line, OMA-BL-1, from the bone marrow of a 17-year-old patient with recurrent Burkitt's lymphoma. The tumor cells grew rapidly in vitro in suspension culture, and very aggressively in vivo in athymic nude mice with metastases to the liver and abdominal cavity. The morphological, chromosomal, immunophenotypic and molecular biologic characteristics of fresh uncultured tumor cells from the patient and tumor cells grown in culture and in athymic nude mice were very similar. The cells were positive for Epstein-Barr virus-associated nuclear antigens (EBNA) and chromosome analysis of the cells revealed an atypical chromosomal abnormality of 45,X,-X,i(8q), HSR(18)(q21),t(8;14)(q24;q32). Southern analysis demonstrated that c-myc was rearranged and amplified in these cells. Immunophenotypic analysis of the cells using flow cytometry showed monoclonal B cells expressing a surface IgG-kappa isotype. The tumor cells grown in nude mice had a significant decrease in CD24 expression when compared to cultured tumor cells. Electron microscopy of the fresh and cultured cells revealed Herpes virus, most likely Epstein-Barr virus, particles. This cell line has been maintained in culture for over 18 months. The aggressive growth and metastatic properties of this cell line in athymic nude mice make it a potentially useful experimental model to study the biology of human lymphoma.

Establishment of an EBV-positive lymphoblastoid cell line that grows as a lymphoma in nude mice and expresses membrane CD2 molecules

We describe a human lymphoblastoid cell line (LCL), called ZS, that originated spontaneously from the cultures of gamma-irradiated (50 Gy) peripheral-blood mononuclear cells of a normal donor. When injected subcutaneously in sublethally irradiated, splenectomized and anti-asialo-GM1-treated nude mice, ZS cells invaded the lymph nodes, that appeared 10 to 50-fold enlarged in all of the mice tested. Furthermore, ZS cells expressed a typical T-cell surface structure, the CD2 molecule, detectable by a variety of different anti-CD2 monoclonal antibodies (MAbs). However, other T-cell markers were not found, with the possible exception of a truncated messenger of the beta chain of the T-cell receptor and ZS cells could be identified as B cells since they (i) expressed a battery of markers of the resting and activated B cells, (ii) displayed a monoclonal rearrangement of the IgH chain locus and (iii) synthesized IgM K molecules. The Epstein-Barr virus (EBV) genome was detected in ZS cells in approximately ten copies per cell by DNA hybridization techniques. Furthermore, the cells were positive for EBV nuclear antigens (EBNA). Western blotting analysis of EBV encoded antigens demonstrated clear differences with those present in the B 95.8 virus-producer cell line, indicating that ZS cells were not infected by EBV in vitro and that they already harbored the virus in vivo. ZS cells formed colonies in vitro with a high cloning efficiency and displayed chromosomal abnormalities in all of the mitoses (karyotype 47, xy, +13, -14, 8p+, 21p+, +m). In spite of these malignant features, ZS cells expressed the full range of EBV latent proteins as usually do "normal" LCSs and did not have any of the chromosomal abnormalities that juxtapose the c-myc oncogene to one of the genes coding for immunoglobulin molecules.

Effects of a phorbol ester on activation and immortalization of leukaemic B cells by Epstein-Barr virus

Immortalization of chronic leukaemic B cells by Epstein-Barr virus (EBV) was investigated. Immortalization resistant and susceptible cell populations were defined by chronic lymphocytic leukaemia (CLL) and prolymphocytic leukaemia (PLL) cells respectively. These cell types could be distinguished by the effects of a phorbol ester on [3H]-thymidine incorporation by EBV-infected cells. A synergistic effect was observed in the non-immortalizing CLL samples, whereas this response was inhibited in the immortalizing PLL. Under the conditions used in the study, TPA did not cooperate with EBV to immortalize CLL cells.

Activation and immortalization of leukaemic B cells by Epstein-Barr virus

We have studied the responses of chronic leukaemic B cells to infection by Epstein-Barr virus (EBV). Our results define one population of B lymphocytes, represented by prolymphocytic leukaemic (PLL) cells, which are highly susceptible to immortalization by EBV. Another B-cell type, represented by chronic lymphocytic leukaemic (CLL) cells, can be readily infected by the virus but is resistant to immortalization. Comparative studies of viral and cellular related events early after infection in these 2 cell types reveal that both express the EB viral nuclear antigen (EBNA) complex, but the immortalization-resistant CLL cells fail to express the latent membrane protein (LMP), which can be detected in PLL cells 48 hr after infection. Circularization of the linear viral genome could be detected at 7 days post infection in the PLL cells, but only in 2 out of 4 CLL cells tested. Both CLL and PLL cells show increased surface expression of CD23 and HLA-DR molecules after infection but, whereas PLL cells show an increase in size, together with RNA and DNA synthesis indicative of cell cycle progression, CLL cells appear to be arrested in the G1/S phase of the cycle. The results suggest that the outcome of infection by EBV is determined by the nature of the target cell rather than by random virus-related events. The correlation between the block in immortalization of CLL cells and their failure to express LMP suggests that expression of this protein is essential for in vitro immortalization of B cells. The failure to detect circularization in some EBV-infected CLL cells suggests that this, as well as LMP expression, may be dependent on prior activation of the B cell by EBV, an event which may vary between the different CLL samples tested.

The establishment of Epstein-Barr virus nuclear antigen-positive (SP-50B) and Epstein-Barr virus nuclear antigen-negative (SP-53) cell lines with t(11;14)(q13;q32) chromosome abnormality from an intermediate lymphocytic lymphoma

Two lymphoma cell lines, SP-50B and SP-53, were established from peripheral blood of a 58-year-old woman with leukemic conversion of intermediate lymphocytic lymphoma. These cell lines grew in suspension with or without forming clumps of cells. SP-50B was morphologically similar to the common Epstein-Barr (EB) virus-transformed lymphoblastoid cell lines and was positive for EB virus nuclear antigen (EBNA), whereas SP-53 closely resembled the patient's lymphoma cells and was negative for EBNA. Both cell lines expressed the same phenotypic markers as original lymphoma cells (CpIg+, SmIg+, OKIa1+, Leu12+) and possessed t(11;14)(q13;q32) chromosome translocation. These results indicate that although morphologically different, SP-50B and SP-53 were both derived from patient's lymphoma cells. The long-term cultivation of EBNA-positive and EBNA-negative B-cell lymphoma lines from a single donor has not been previously reported. These cell lines would provide useful tools for studying the oncogenic role of EB virus and bcl-1 oncogene that is located on chromosome 11q13.

Phenotypic, cytogenetic and molecular characterization of a new B-chronic lymphocytic leukaemia (B-CLL) cell line

A lymphoid cell line was established from a patient with B-cell chronic lymphocytic leukaemia (B-CLL) by infecting blood lymphocytes with Epstein-Barr virus (EBV). Immunoglobulin gene rearrangement studies and the presence of a chromosome marker (isochromosome 17q) provided the formal proof that the line has originated from the neoplastic B cells. The morphology and phenotype indicate that the EBV-induced cell line has reached a plasma cell-like stage of differentiation.

Two new cell lines from B-prolymphocytic leukaemia: characterization by morphology, immunological markers, karyotype and Ig gene rearrangement

Leukaemic cells from 2 patients with B-prolymphocytic leukaemia were immortalized in vitro by means of Epstein-Barr virus and phorbol-ester TPA. The resulting cell lines, named JVM-2 and JVM-3, have been growing continuously in liquid culture for more than one year. JVM-2 is characterized cytogenetically by t(11;14)(q13; q32), and JVM-3 by trisomy 12. The immunoglobulin (Ig) heavy- and light-chain genes showed the same pattern of rearrangement in both lines as in the original prolymphocytes from each case. The cells from these lines showed a spectrum of morphological and immunological features corresponding to different stages of B-cell maturation. The expression of Ig, IgM-lambda in JVM-2 and IgMD-K in JVM-3, changed from a predominantly membrane pattern in the original cells to a cytoplasmic one in the cell lines. By comparison with their original progenitors, the cells from both lines showed reduced reactivity with the monoclonal antibody (MAb) FMC7, and increased expression of the antigens recognized by the MAbs OKT10, alpha-Tac, FMC53 and Ki-I. The availability of cell lines from this rare type of lymphoid leukaemia offers a potential tool for the study of molecular events associated with the expression of Ig and other antigens by neoplastic cells.

Burkitt-like lymphoma in an English child: characterisation of tumour biopsy cells and of the derived tumour cell line

An eight year old English boy presented with an abdominal undifferentiated 'Burkitt-like' lymphoma. Lymphoma cells from ascitic fluid were cultured on a human embryo fibroblast feeder layer and, after a short lag period, a cell line (DH-BL) was established which, like the original tumour, was both negative for the Epstein-Barr nuclear antigen (EBNA) and expressed a monoclonal pattern of surface immunoglobulin (alpha lambda). DH-BL also possessed the Burkitt-related 8:14 chromosome translocation in all metaphases analysed; no other chromosomal abnormalities were present. The cell surface phenotype of the original biopsy cells and the cultured tumour cells in early passage were investigated using a panel of monoclonal antibodies to B lineage-associated antigens. These antibodies had recently been used to characterise African 'endemic' Burkitt's lymphoma (BL) biopsy cells and their derived cell lines. The cell surface phenotype of this English EBNA negative Burkitt-like lymphoma biopsy was indistinguishable from that previously shown by biopsies of EBNA positive endemic BLs. It therefore appears that both the endemic and sporadic forms of BL, as illustrated by this case, may be derived from the same subset of progenitor cells.

Epstein-Barr virus status and tumour cell phenotype in sporadic Burkitt's lymphoma

Burkitt's lymphoma (BL) biopsy cells and derived cell lines can be grouped according to their patterns of reactivity with 6 selected monoclonal antibodies (MAbs) against B cell-associated surface antigens. Group I cells react only with MAbs J5 and 38.13, recognising the common acute lymphoblastic leukaemia antigen and a BL-associated antigen respectively; group II cells react with J5 and 38.13 and with one or more of a set of MAbs (Ki-24, MHM6, AC2, Ki-1) against "lymphoblastoid" antigens; group III cells react only with these anti-"lymphoblastoid" MAbs. Tumour biopsy cells from 17 cases of sporadic BL, 9 positive for the Epstein-Barr (EB) virus genome and 8 negative, have been analysed during the process of cell line establishment in vitro. In early passage the EB virus-negative BL cells showed either a group I phenotype or gave an additional reactivity with MAb Ki-24 which placed them in group II; these phenotypes remained essentially stable with continued growth of the cell lines for up to 50 passages. By contrast the EB virus-positive BL cells were much more susceptible to phenotypic change in vitro. Although such cells displayed a group I or group II phenotype in early passage, many of the lines soon moved into group III whilst retaining the karyotypic markers indicative of their malignant origin. These observations suggest that a resident EB virus genome can drive the in vitro progression of BL cells towards a more "lymphoblastoid" phenotype. This was confirmed in subsequent experiments where virus-negative BL cell lines were converted to EB virus positivity by in vitro infection. Clearly, therefore, phenotypic analysis of long-established lines can lead to false distinctions being drawn between the EB virus-positive and -negative forms of sporadic BL; both may derive from the same sub-population of target B cells in vivo.

In-vitro infection of chronic lymphocytic leukemia cells by Epstein-Barr virus (EBV)

We sought to determine the potential of infecting lymphoid cells from patients with chronic leukemia (CLL) with Epstein-Barr virus (EBV) by testing for EBV receptors (EBVR) by flow cytometry, assessing for infectability of these cells by culturing with B95-8-derived virus, and staining for EB nuclear-associated antigens (EBNA) at various times post-infection. EBVR were present on 54-91% of lymphoid cells in seven cases of CLL and on 46% of prolymphocytic leukemia cells. Dynamic changes regarding EBNA positivity, morphology, and viability occurred post-infection with the virus. On day 2 only a few EBNA-positive lymphoblasts were observed. On days 11-21 positivity increased from 2 to 34% of cells. Simultaneously, the viable cell number declined to approximately 1/10th of original number. A significant proportion of the EBNA-positive cells corresponded to the original CLL cells. In 3 of 7 cases of CLL a Pan T-cell phenotype was demonstrated by Leu-1 monoclonal antibody testing. The infected cells did not react with two monoclonal antibodies, EBV-CS 1 and 4, which react with B-cell lymphoblastoid cell lines (B-LCL). Moreover, the B-LCL derived at 1-2 months post-infection of CLL cells did not express the Leu-1 antigen, but expressed EBV-CS 1 or 4 defined antigens. In the prolymphocytic leukemia, 64% of the cells showed EBNA positivity on day 7 and giant cells with huge round or multiple nuclei appeared which were EBNA-positive. CLL and prolymphocytic leukemia cells can be infected as demonstrated by EBNA-positivity. This infection does not lead to immediate transformation, but evokes lymphoblast and multinucleated giant cell production prior to the death of cells.

Distinctions between endemic and sporadic forms of Epstein-Barr virus-positive Burkitt's lymphoma

Tumour cell lines were established in vitro from 16 cases of Epstein-Barr (EB) virus genome-positive Burkitt's lymphoma (BL), 7 of "endemic" origin (i.e. from holoendemic malarial areas of Africa and of New Guinea) and 9 of "sporadic" origin (i.e. from outside such high-incidence areas). All the BL cell lines thus established were monoclonal by immunoglobulin isotype expression and displayed a characteristic chromosomal translocation, t(8:14) or t(8:22), confirming their malignant origin. Clear differences observed between the individual BL cell lines appeared to be related to their endemic or sporadic status. All 7 endemic cell lines began growth as a carpet of single cells, often with small, loose clumps appearing in later passage. Whilst 3 lines of sporadic origin displayed a similar pattern to the above, the majority of sporadic lines grew as large, tight clumps of cells from the first passage onwards. These differences in growth pattern were reflected by differences in cell surface phenotype, as defined in indirect immunofluorescence tests using a panel of monoclonal antibodies (MAbs) specific for B-lineage-associated antigens. BL cell lines could be classified into 3 separate groups on the basis of their reactivity with 6 particular antibodies (MHM6, AC2, Ki-1, Ki-24, J5 and 38.13). All 7 endemic BL cell lines and 2 of the 3 sporadic BL cell lines which began growth as single cells showed a group-I cell-surface phenotype (MHM6, AC2, Ki-1, Ki-24 negative; J5, 38.13 positive) in early passage. In contrast, all 6 sporadic BL cell lines which began growth in large clumps displayed a distinct group-II phenotype (MHM6, AC2, Ki-1 positive/negative; Ki-24, J5, 38.13 positive); in later passage most of these sporadic lines progressed to a group-III phenotype (MHM6, AC2, Ki-1, Ki-24 positive; J5, 38.13 negative) without loss of those immunoglobulin and chromosomal markers identifying the cells' malignant origin. These clear differences between endemic BL cell lines on the one hand and the majority of sporadic BL cell lines on the other suggest that endemic BL arises from a more restricted range of progenitor B cells than does the sporadic form of the disease.

Analysis of transformation with Epstein-Barr virus and phenotypic characteristics of lymphoblastoid cell lines established from patients with hairy cell leukemia

In order to assess the role of Epstein-Barr virus (EBV) in patients with hairy cell leukemia (HCL), we have sought to characterize 1) the ability of EBV to infect and transform hairy leukemic cells in vitro and 2) the phenotypes of cell lines putatively derived from those leukemic cells. Analysis of EBV-induced transformation and the kinetics of Epstein-Barr nuclear antigen (EBNA) induction in leukemic preparations indicated that most leukemic cells were not susceptible to EBV infection but that at least a small subpopulation of leukemic cells could be infected with EBV. Lymphoblastoid cells lines were established after exposure of peripheral blood or splenic cells from HCL patients to B95-8 or QIMR-WIL EBV. Splenic leukemic cell preparations were more sensitive targets for EBV transformation than were peripheral blood cell samples. The newly established cell lines, but not long-established B lines such as Raji, demonstrated high levels of synthesis of p35, (a protein complex expressed abundantly by cells of a subset of HCL patients) and high levels of tartrate-resistant acid phosphatase (an enzyme relatively diagnostic for HCL). Lymphoblastoid lines from one patient with HCL expressed lambda light chains and no kappa chains as did the patient's leukemic cells. Virus expression in these lines showed that HCL-derived lines had spontaneous early antigen (EA) and viral capsid antigen (VCA) expression. Transforming EBV could be rescued from HCL-derived cell lines but not from cord blood-derived lines.

Chromosome abnormalities in B-cell prolymphocytic leukemia: a study of nine cases

Chromosome abnormalities were demonstrated in 50-100% of Giemsa-banded metaphases from nine cases of B-cell prolymphocytic leukemia (B-PLL). Mitoses were obtained with pokeweed mitogen following pretreatment of peripheral blood (PB) prolymphocytes with neuraminidase-galactose oxidase. Chromosome 14 was abnormal in eight of the nine cases: a marker 14q+, with breakpoint at band q32 in seven and trisomy 14 in one. In four cases the abnormal No. 14 was one of several primary abnormalities and in four others it was seen in secondary clones. The origin of the translocated material was unknown in three cases, in two it resulted from t(11;14), later becoming t(11;14;21) in one of them, t(1;14) in another, progressing later to t(1;14;17); in yet another patient, the 14q+ was the result of a complex rearrangement t(6;14;17). Abnormalities of chromosome 6 were seen in six cases: 6q- as the primary abnormality in three; trisomy 6 was part of secondary changes in one case. Structural abnormalities of chromosome 1 were seen in six cases: 1q- in four (in one as the only abnormality), 1q+ in one case, and 1p- in another, both in the main clone. Trisomy 12 was demonstrated in three cases but not as the primary change. Spleen cells in two patients showed a higher frequency of abnormalities than in the PB, supporting the concept of the spleen being the organ primarily involved in B-PLL. Evidence of karyotypic evolution was demonstrated in six patients, in some clearly associated with clinical progression of the disease. The type and frequency of the abnormalities observed in B-PLL resemble those seen in non-Hodgkin's lymphomas and suggest major differences from B-CLL, although a relationship with the latter can not be completely ruled out at present.

Restrictions upon Epstein-Barr virus infection of the leukemic cell are demonstrated in patients with hairy cell leukemia

We tested the hypothesis that Epstein-Barr virus (EBV) might actually infect leukemic hairy cells in vivo by examining those cells for the EBV-receptor, EBV nuclear antigen (EBNA) and membrane antigen (MA), for spontaneous transformation and rescue of infectious virus and for presence of EBV genome. EBV-receptors were found on subpopulations of leukemic cells from each of 7 patients with hairy cell leukemia (HCL) tested. MA was present on low numbers (1-5 per cent) of fresh leukemic cells of 7 patients and in some instances occurred with a greater frequency after 3 to 5 days in culture, with or without 12-O-tetradecanoylphorbol-13-acetate. In 11 fresh leukemic cell preparations from 8 HCL patients, no EBNA was demonstrated. However, 2 samples after 4 days in culture expressed low frequencies of EBNA-positive cells. Spontaneous, EBV-positive cell lines were established with a high transformation efficiency from 3 HCL blood samples but not from 8 other specimens. Infectious EBV could be rescued from some hairy leukemic cell preparations by co-cultivation with cord blood lymphocytes. These results demonstrated that leukemic cell populations harbored infectious EBV, that the leukemic cells expressed virus receptors and suggested that a small subpopulation of leukemic cells might become infected in vivo at least transiently and possibly transformed in vitro by EBV. To test for the extent of occult in vivo infection of leukemic cells with EBV, Southern type hybridization studies were performed with a probe for EBV genome (Bam HI W). At a sensitivity level of 0.1 genome per cell, EBV genome was not detected in the leukemic cell populations of 7 patients. We conclude that host defence mechanisms protecting these individuals from EBV also prevent infections of the leukemic cell and/or most hairy leukemic cells are not suitable targets for both infection and transformation.