Reversion of tumorigenicity and decreased agarose clonability after EBV conversion of an IgH/myc translocation-carrying BL line

Epstein-Barr virus-encoded latent membrane protein-1 upregulates 14-3-3σ and Reprimo to confer G(2)/M phase cell cycle arrest

Epstein-Barr virus (EBV) is a ubiquitous tumor-causing virus which infects more than 90% of the world population asymptomatically. Recent studies suggest that LMP-1, -2A and -2B cooperate in the tumorigenesis of EBV-associated epithelial cancers such as nasopharygeal carcinoma, oral and gastric cancer. In this study, LMPs were expressed in the HEK293T cell line to reveal their oncogenic mechanism via investigation on their involvement in the regulation of the cell cycle and genes that are involved. LMPs were expressed in HEK293T in single and co-expression manner. The transcription of cell cycle arrest genes were examined via real-time PCR. Cell cycle progression was examined via flow cytometry. 14-3-3σ and Reprimo were upregulated in all LMP-1 expressing cells. Moreover, cell cycle arrest at G(2)/M progression was detected in all LMP-1 expressing cells. Therefore, we conclude that LMP-1 may induce cell cycle arrest at G(2)/M progression via upregulation of 14-3-3σ and Reprimo.

The ubiquitin C-terminal hydrolase UCH-L1 regulates B-cell proliferation and integrin activation

The ubiquitin C-terminal hydrolase-L1 (UCH-L1) is a deubiquitinating enzyme that catalyses the hydrolysis of polyubiquitin precursors and small ubiquitin adducts. UCH-L1 has been detected in a variety of malignant and metastatic tumours but its biological function in these cells is unknown. We have previously shown that UCH-L1 is highly expressed in Burkitt's lymphoma (BL) and is up-regulated upon infection of B lymphocytes with Epstein-Barr virus (EBV). Here we show that knockdown of UCH-L1 by RNAi inhibits the proliferation of BL cells in suspension and semisolid agar and activates strong LFA-1-dependent homotypic adhesion. Induction of cell adhesion correlated with cation-induced binding to ICAM-1, clustering of LFA-1 into lipid rafts and constitutive activation of the Rap1 and Rac1 GTPases. Expression of a catalytically active UCH-L1 promoted the proliferation of a UCH-L1-negative EBV transformed lymphoblastoid cell line (LCL) and inhibited cell adhesion, whereas a catalytic mutant had no effect, confirming the requirement of UCH-L1 enzymatic activity for the regulation of these phenotypes. Our results identify UCH-L1 as a new player in the signalling pathways that promote the proliferation and invasive capacity of malignant B cells.

Epstein-Barr virus microRNAs are evolutionarily conserved and differentially expressed

The pathogenic lymphocryptovirus Epstein–Barr virus (EBV) is shown to express at least 17 distinct microRNAs (miRNAs) in latently infected cells. These are arranged in two clusters: 14 miRNAs are located in the introns of the viral BART gene while three are located adjacent to BHRF1. The BART miRNAs are expressed at high levels in latently infected epithelial cells and at lower, albeit detectable, levels in B cells. In contrast to the tissue-specific expression pattern of the BART miRNAs, the BHRF1 miRNAs are found at high levels in B cells undergoing stage III latency but are essentially undetectable in B cells or epithelial cells undergoing stage I or II latency. Induction of lytic EBV replication was found to enhance the expression of many, but not all, of these viral miRNAs. Rhesus lymphocryptovirus, which is separated from EBV by ≥13 million years of evolution, expresses at least 16 distinct miRNAs, seven of which are closely related to EBV miRNAs. Thus, lymphocryptovirus miRNAs are under positive selection and are likely to play important roles in the viral life cycle. Moreover, the differential regulation of EBV miRNA expression implies distinct roles during infection of different human tissues.

Vertebrate cells express a large family of diverse small RNAs, called microRNAs, that can inhibit the expression of specific target genes. Recently, it has become apparent that several pathogenic human viruses, and in particular herpes viruses, also encode microRNAs that these viruses likely use to prevent infected cells and individuals from mounting effective antiviral responses. Here, we demonstrate that Epstein–Barr virus (EBV), which causes infectious mononucleosis and also some cancers in humans, makes 17 different microRNAs in infected human cells. These microRNAs are found in two clusters in the viral genome, one of three microRNAs, the second of 14 microRNAs, that are differentially expressed in different kinds of EBV-induced human tumors. Analysis of the closely related rhesus lymphocryptovirus shows that seven of these EBV microRNAs have been conserved in this simian virus across >13 million years of divergent evolution. This argues that these microRNAs likely play an important role in EBV replication and represents the first demonstration of the evolutionary conservation of viral microRNAs.

Identification of virus-encoded microRNAs

RNA silencing processes are guided by small RNAs that are derived from double-stranded RNA. To probe for function of RNA silencing during infection of human cells by a DNA virus, we recorded the small RNA profile of cells infected by Epstein-Barr virus (EBV). We show that EBV expresses several microRNA (miRNA) genes. Given that miRNAs function in RNA silencing pathways either by targeting messenger RNAs for degradation or by repressing translation, we identified viral regulators of host and/or viral gene expression.

Gene array identification of Epstein Barr virus-regulated cellular genes in EBV-converted Burkitt lymphoma cell lines

Epstein Barr virus (EBV) is associated with various B-cell neoplasms such as post-transplant lymphoproliferative disease or Burkitt lymphoma. B-lymphocyte reprogramming by EBV involves the control of numerous cellular genes. To identify such EBV-deregulated genes, we have compared the gene expression profile of EBV-negative Burkitt lymphoma cell lines (BL) (BL2, BL30, BL70) with their EBV-converted counterpart (BL2-B95, BL30-B95, BL70-B95) by cDNA array. Statistical analysis of the results was made using Ward's cluster analysis method. Results showed that the expression of up to 26% of the 1176 cellular genes analyzed may be modified in EBV-converted BL cells. Within this set of genes, a subset of genes markedly regulated in EBV-converted BL cells was defined as those for which expression in EBV+ cells was increased or decreased more than 2-fold. Expression of various genes was modulated in agreement with their previously reported regulation by EBV or by transcription factors activated by EBV. Numerous genes were newly identified as modulated in EBV-converted BL cells. Some of these results were verified by both semiquantitative RT-PCR and Western blotting, and were consistent with functional studies. Functional classification of EBV-regulated genes gave a comprehensive picture of cellular reprogramming by EBV in BL, by pointing out cellular modules such as cell cycle, apoptosis, and signal transduction pathways, including BCR and TNF receptor family and interferon pathways. Furthermore, and perhaps most importantly, cDNA array results point to three families of transcription factors, Rel/NF-kappaB, STAT1, and Ets-related proteins Spi-B, Elf-1, and Ets-1 as putative cellular targets of EBV.

Epstein-barr virus-induced changes in B-lymphocyte gene expression

To elucidate the mechanisms by which Epstein-Barr virus (EBV) latency III gene expression transforms primary B lymphocytes to lymphoblastoid cell lines (LCLs), the associated alterations in cell gene expression were assessed by using 4,146 cellular cDNAs arrayed on nitrocellulose filters and real-time reverse transcription-PCR (RT-PCR). A total of 1,405 of the 4,146 cDNAs were detected using cDNA probes from poly(A)(+) RNA of IB4 LCLs, a non-EBV-infected Burkitt's lymphoma (BL) cell line, BL41, or EBV latency III-converted BL41 cells (BL41EBV). Thirty-eight RNAs were consistently twofold more abundant in the IB4 LCL and BL41EBV than in BL41 by microarray analysis. Ten of these are known to be EBV induced. A total of 23 of 28 newly identified EBV-induced genes were confirmed by real-time RT-PCR. In addition, nine newly identified genes and CD10 were EBV repressed. These EBV-regulated genes encode proteins involved in signal transduction, transcription, protein biosynthesis and degradation, and cell motility, shape, or adhesion. Seven of seven newly identified EBV-induced RNAs were more abundant in newly established LCLs than in resting B lymphocytes. Surveys of eight promoters of newly identified genes implicate NF-kappaB or PU.1 as potentially important mediators of EBV-induced effects through LMP1 or EBNA2, respectively. Thus, examination of the transcriptional effects of EBV infection can elucidate the molecular mechanisms by which EBV latency III alters B lymphocytes.

Differential regulation of Epstein-Barr virus (EBV) latent gene expression in Burkitt lymphoma cells infected with a recombinant EBV strain

Epstein-Barr virus (EBV)-negative Burkitt lymphomas (BLs) can be infected in vitro with prototype EBV strains to study how the virus may affect the phenotype of tumor cells. Studies thus far have concentrated on the use of transforming B95-8 and nontransforming P3HR1 strains. Immunological and phenotypic differences between the sublines infected with these two strains were reported. The majority of these differences, if not all, can be attributed to the lack of EBNA-2 coding sequences in the P3HR1 strain. The recent development of a selectable Akata strain has opened up new possibilities for infecting epithelial and T cells as well. We infected five EBV-negative BL lines with the recombinant Akata virus. Our results indicate that the infected cell lines BL28, Ramos, and DG75 express EBNA-1, EBNA-2, and LMP1, the viral proteins associated with type III latency, and use both YUK and QUK splices. In contrast, two EBV-negative variants of Akata and Mutu when reinfected displayed restricted type I latency and expressed only EBNA-1. All clones of infected Mutu cells used the QUK splice exclusively. The usage of Qp was observed in a majority of Akata clones. Some Akata clones, however, were found to have double promoter usage (Qp and C/Wp) but at 4 months after infection did not express EBNA-2. The results demonstrate differential regulation of EBV latency in BLs with the same recombinant viral strain and suggest that the choice of latency type may be cell dependent. The restricted latency observed for infected Akata and Mutu cells indicates that a BL may opt for type I latency in the absence of immune pressure as well.

Differential effect of TPA on cell growth and Epstein-Barr virus reactivation in epithelial cell lines derived from gastric tissues and B cell line Raji

We characterized the cell growth and Epstein-Barr virus (EBV) reactivation for EBV infected epithelial cell lines, GT38, GT39, and GTC-4 using 12-O-tetradecanoylphorbol-13-acetate (TPA). These cell lines grew similarly in liquid medium, and formed colonies in soft agar. The cell growth was inhibited with TPA, dose-dependently in liquid medium. The colony formation was enhanced with low concentrations of TPA, but was inhibited with high concentrations. The latent EBV was reactivated with high concentrations of TPA as shown by the expression of EBV BZLF1 gene product ZEBRA. The effects of TPA on GTC-4 were compared with a Burkitt's lymphoma cell line Raji. The mode of actions of TPA in GTC-4 was different from Raji in terms of cell growth and EBV reactivation. The effective concentrations of TPA for cell growth inhibition and EBV reactivation were higher in Raji than GTC-4. Cell cycle analysis showed that TPA (20 ng/ml) induced cell cycle arrest to Raji but not to GTC-4; however, the rate of trypan blue stained cells increased in the TPA treated GTC-4 but not Raji. These results demonstrated that TPA affects differentially for the stimulation and inhibition of cell growth, and also EBV reactivation depends on TPA concentrations and cell types.

HLA-A11-mediated protection from NK cell-mediated lysis: role of HLA-A11-presented peptides

The capacity of MHC class I to protect target cells from NK is well established, but the mechanism by which these molecules influence NK recognition and the physical properties associated with this function remain poorly defined. We have examined this issue using as a model the HLA-A11 allele. HLA-A11 expression correlated with reduced susceptibility to NK and interferon-activated cytotoxicity in transfected sublines of the A11-defective Burkitt's lymphoma WW2-BL and the HLA class I A,B-null C1R cell line. Protection was also achieved by transfection of HLA-A11 in the peptide processing mutant T2 cells line (T2/A11), despite a very low expression of the transfected product at the cell surface. Induction of surface HLA-A11 by culture of T2/A11 cells at 26 degrees C or in the presence of beta 2m did not affect lysis, whereas NK sensitivity was restored by culture in the presence of HLA-All-binding synthetic peptides derived from viral or cellular proteins. Acid treatment rendered T2/A11 and C1R/A11 cells sensitive to lysis, but protection was restored after preincubation with peptide preparations derived from surface stripping of T2/A11 cells. Similar peptide preparations from T2 cells had no effect. The results suggest that NK protection is mediated by HLA-A11 molecules carrying a particular set of peptides that are translocated to the site of MHC class I assembly in the ER in a TAP-independent fashion.

Induction of the lytic viral cycle in Epstein Barr virus carrying Burkitt lymphoma lines is accompanied by increased expression of major histocompatibility complex molecules

Six Epstein Barr virus (EBV) genome-carrying Burkitt lymphoma (BL) cultures (P3HR-1, Raji, Akata, Daudi, Rael and Jijoye) were induced to enter the lytic cycle. Phorbol esther (TPA), n-butyrate, 5-azacytidine (5AzaC) or anti-IgG were used according to their known inducing capacity on these cell lines. Concomitantly with the appearance of the viral early antigens (EA) in a proportion of cells, the expression of major histocompatibility complex (MHC) class II antigens increased in the cultures. On P3HR-1 and Raji cells class I expression also increased. The enhancement of MHC expression correlated with the efficiency of induction and required only an early event of the viral lytic cycle. Treatment of 3 EBV-negative lymphoma lines (BJAB, Ramos and BL41) with TPA plus n-butyrate or 5AzaC did not influence MHC expression. Moreover, BL lines which carry the EBV genome after having been infected in vitro and which cannot be induced for the viral lytic cycle did not change MHC expression after treatment with the inducing agents. In mixed cultures the allo-stimulatory capacity of induced cells with elevated MHC expression was stronger compared to the untreated ones.

A trypsin-like serine protease activity on activated human B cells and various B cell lines

We have studied the trypsin-like serine protease activity of human tonsillar B lymphocytes. The lysate of the low-density, in vivo activated B cells as well as the lysate of cells stimulated with anti-human IgM F(ab')2 show elevated trypsin-like serine protease activity compared to the resting subset as monitored by the cleavage of Tos-Gly-Pro-Arg-pNA. The cleavage is sensitive to N-tosyl-L-lysyl-chloromethyl ketone and benzamidine but not to iodoacetamide. Experiments with intact cells give similar results. The finding that the intact cells hydrolyze the substrate, while their supernatant does not, suggests that the protease activity is cell membrane associated. It is possible that C3 is a substrate of the enzyme since the activated B cells cleave C3, whereas the resting B cells do not, and also C3 inhibits the enzyme-substrate reaction. In addition to the ex vivo B cells, we studied the serine protease activity of certain well-characterized B cell lines. The results show a correlation between the phenotype and the enzyme expression of the cell lines. BL41, an Epstein-Barr virus (EBV)-negative Burkitt lymphoma line, with a resting phenotype, has low activity, while its EBV genome-carrying convertants E95-A-BL41, E95-C-BL41, EHR-A-BL41 and BL41/95 that have the phenotype of activated B cells, have high proteolytic activity. The lymphoblastoid cell line WW-1-LCL which has the phenotype of an immunoblast, has the highest serine protease activity. On the basis of the above data, we suggest that a rather tight correlation exists between the degree of activation and the appearance of serine protease(s) on the surface of human B cells.

Expression of the Epstein-Barr virus (EBV)-encoded membrane protein LMP1 impairs the in vitro growth, clonability and tumorigenicity of an EBV-negative Burkitt lymphoma line

In a previous study on several independently established Epstein-Barr virus (EBV)-converted sublines of the EBV-negative Burkitt lymphoma (BL) line BL41, we found that expression of the virally encoded membrane protein LMP1 was accompanied by reduced agarose clonability and tumorigenicity. In order to investigate whether LMP1 can induce these phenotypic changes by itself, we have now studied the growth in suspension culture, the clonability in agarose and the tumorigenicity in immunosuppressed and SCID mice of 4 LMP1-transfected sublines of BL41 that carry the gene under the control of the ZnSO4-inducible metallothionein promoter. Expression of LMP1 at levels comparable to those detected in EBV-transformed lymphoblastoid cell lines (LCL) correlated with impairment of growth in suspension and reduction of clonability and tumorigenicity. Only minor changes were observed in transfectants expressing low LMP1 levels. Up-regulation of LMP1 by ZnSO4 treatment of the low LMP1 clone MTLM5 was accompanied by a slowing down of proliferation, increased cell clumping and decreased clonability. The results suggest that expression of LMP1 at levels which are compatible with immortalization of normal B-cells antagonizes the ability of BL cells to grow in vitro and in vivo, and illustrate a possible mechanism by which down-regulation of this viral antigen may favor tumorigenicity in EBV-carrying BLs.

Search for the critical characteristics of phenotypically different B cell lines, Burkitt lymphoma cells and lymphoblastoid cell lines, which determine differences in their functional interaction with allogeneic lymphocytes

Burkitt lymphoma (BL) lines can be grouped according to phenotypic characteristics. Group I cells exhibit the phenotype of resting B cells and grow as single cells. Such lines can be Epstein-Barr-virus(EBV)-negative or -positive. Group II and group III cells are always EBV-positive, they express B cell activation markers, grow in aggregates and resemble in varying degrees lymphoblastoid cell lines (LCL). We studied three groups of BL lines for their capacity to interact with allogeneic lymphocytes. The results showed that as long as the lines have the group I phenotype, they do not stimulate allogeneic T lymphocytes irrespective whether they carry the EBV genome. The group II and III cells are stimulatory. Generally there was no correlation between sensitivity ot lymphocyte-mediated lysis and the phenotype of the lines. In one set of lines, the group I cells had higher sensitivity to both natural killer and lymphokine-activated killer effectors compared to the group II or III lines. However, such correlation could not be seen with the other two sets of lines. Among the phenotypic features investigated, expression of the adhesion molecules LFA-1 and LFA-3 correlated with the tendency for cell aggregation.

Effect of transforming growth factor-beta 1 and -beta 2 on the proliferation of Burkitt lymphoma and lymphoblastoid cell lines

We tested the effect of transforming growth factor (TGF)-beta 1 and TGF-beta 2 on the proliferation of human B cell lines. The panel was selected to give information whether (1) their origin, (2) their phenotype, (3) their Epstein-Barr virus (EBV) carrier state, influence their responsiveness. The growth of lymphoblastoid cell lines (LCL) was not inhibited by TGF-beta 1. The EBV-carrying Burkitt lymphoma (BL) lines, Daudi, Jijoye, Rael but not Raji were inhibited. Three EBV-negative BL lines and the majority of their converted sublines were sensitive. The cell lines tested expressed TGF-beta receptors and TGF-beta 1 transcripts. The proliferation of EBV-infected B cells was inhibited by TGF-beta, their sensitivity decreased, however, after 3 days. The results suggest that the activation state of the B cells is decisive for TGF-beta sensitivity and EBV influences it indirectly by changing the cell phenotype.

Expression of Epstein-Barr virus-encoded proteins and B-cell markers in fatal infectious mononucleosis

We assessed 33 lymphoid tissues from 15 patients, including 7 with X-linked lymphoproliferative disease (XLP) and 8 patients with sporadic fatal infectious mononucleosis (IM), to determine whether the cellular infiltrate had the immunophenotype and expressed Epstein-Barr virus (EBV)-encoded proteins characteristic of either EBV-immortalized lymphoblastoid cell lines (LCL) or EBV-carrying Burkitt lymphoma (BL) cells. The results of these studies revealed that in 13 cases the proliferating B cells were polyclonal, LCL-like, and in 2 cases they were monoclonal, malignant lymphoma-like.

Lymphokine-activated killer (LAK) cells discriminate between Epstein-Barr virus (EBV)-positive Burkitt's lymphoma cells

We have generated in vitro lymphokine-activated killer (LAK) cells from healthy donors by stimulating their mononuclear leukocytes with recombinant interleukin-2 (rIL-2) (100 U/ml). After 6 days in culture, the lytic properties of the LAK cells were analyzed in the 51Cr-release assay by utilizing a target panel of 6 paired lines consisting of an Epstein-Barr virus (EBV)-positive Burkitt's lymphoma (BL) cell line and an EBV-transformed lymphoblastoid cell line (LCL) from the same donor, the Raji BL line and the natural killer (NK) cell-sensitive K562 line. The patterns of lysis showed that the LAK cells discriminated between two categories of BL cell lines. Group I/II BL tumor cells which expressed the common acute lymphoblastic leukemia antigen (CALLA), the BL-associated glycolipid antigen (BLA) and phenotypically resembled biopsy cells were strongly lysed whereas group III BL cells which had assumed an LCL-like phenotype during culture and lacked the CALLA and BLA surface markers were only poorly lysed. The LCL targets were generally resistant to lysis but the K562 cell line was particularly sensitive. The outcome of cell depletion and monoclonal antibody (MAb) studies indicated that the LAK cell populations were phenotypically and functionally heterogeneous and consisted of at least 2 subpopulations of effector cells; a tumor-specific component and an NK-cell-mediated component.

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.

Host cell-dependent regulation of growth transformation-associated Epstein-Barr virus antigens in somatic cell hybrids

We have analyzed the expression of the three major known growth transformation-associated Epstein-Barr virus (EBV) proteins, EBNA-1, EBNA-2, and latent membrane protein (LMP), in a series of somatic cell hybrids derived from the fusion of EBV-carrying Burkitt lymphoma (BL) lines with EBV-positive or EBV-negative B-cell lines. Independently of the cell phenotype, EBNA-1 was invariably coexpressed in all EBV-carrying hybrids. In hybrids between EBV-carrying, LMP-positive and LMP-negative Burkitt lymphoma lines, LMP was expressed, indicating positive control. Two EBV-negative lymphoma lines, Ramos and BJAB, differed in their ability to express LMP after B95-8 virus-induced conversion and after hybridization with Raji cells. BJAB was permissive while Ramos was nonpermissive for LMP, although both expressed EBNA-2. The EBNA-2-deleted P3HR-1 virus gave the same pattern of LMP expression in these two cells. Our findings indicate that the expression of EBNA-1, EBNA-2, and LMP is regulated by independent mechanisms.

Immunophenotypic characterization of follicle-center-cell-derived non-Hodgkin's lymphomas

The distribution of the BLA, CALLA (CD 10), AC-2 (CD 39), MHM-6 (CD 23), LB-I, and 351C5 (CD 45R) antigens in 40 non-Hodgkin's lymphomas was demonstrated by immunohistochemical staining of frozen tissue sections. Nine out of 10 centroblastic and centrocytic follicular and diffuse type of lymphomas (CB/CC F/D) and all 10 cases of CB/CC follicular lymphomas were BLA+ and CALLA+. A few cases also showed weak expression of activation antigens (AC-2, MHM-6 and LB-I) and 351C5. In contrast, 3 CC and 3 lymphoblastic (non-Burkitt) lymphomas showed a heterogeneous pattern of distribution with dominating activation antigen expression. A single case of lymphoblastic lymphoma of Burkitt-like type expressed BLA and CALLA but not activation antigens. In reactive follicular center and FCC lymphomas different cell populations appeared to express BLA and activation antigens, respectively. Assessment of staining intensity and proportion of the stained cells indicated that almost all BLA+ cells are CALLA+. CALLA+ BLA- cells were regularly present, in addition. The co-expression of BLA and CALLA in the same cell was confirmed by double immuno-enzymatic staining. By the same technique, BLA+ and CALLA+ cells were shown to be activation-antigen negative.