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Model of Epstein-Barr Virus Infection of Human Thymocytes: Expression of
Viral Genome and Impact on Cellular Receptor Expression in the
TLymphoblastic Cell Line, HPB-ALL
By Robin L. Kaufman Paterson, Colm Kelleher, Thomas D. Amankonah, Joanne E. Streib, Jing Wu Xu,
James F. Jones, and Erwin W. Gelfand
Infection of B lymphocytes and epithelial tissue by EpsteinBarr virus (EBV) is associated with malignancy and autoimmunity. The cellular receptor for EBV has been identified as
CD21 (CR2). A molecule, which is biochemically and immunologically similar t o B-cell CD21, has been identified on a
subpopulation of immature thymocytes, suggesting a role
for thismolecule in the regulationof T-cell development and
further suggesting that immatureT cells might besusceptible t o EBV infection. A growing bodyof literature nowdocuments the presence of EBV in tumors of T-cell origin. We
have evaluated the susceptibility of the human immatureT
cell line, HPB-ALL, t o infection byEBV. Electron microscopy
studies showed a rapid internalization of virusby HPB cells.
Southern blotting showed theintracellular presence of lin-
ear EBV genomes, and components of the virusreplicative
cycle were identified. Expression of the BumHl Z region of
the genome, encoding the nuclear protein, ZEBRA, which is
strictly associated with productive infection in B cells, was
detected in HPB-ALL cells. A spliced variant of 2, RAZ, was
also identified. Cell surface expression of EBV late antigens
was observed t o occur transiently. Infection of HPB cells
was also accompanied by altered expression of T-cell surface
molecules involved in antigen recognition, a process critical
t o normal development of the T-cell repertoire. Delineation
of the outcome of T-cell infection by EBV may lead t o a
virus in autoimmune
better understanding of the role of this
processes and malignancy.
0 1995 by The American Society of Hematology.
C
number of recent reports now confirm the presence of EBV
in tumors and benign proliferations of T-cell
and
in thymoma of patients with myasthenia g r a ~ i s . ” , ~ ~
In the present study, we have examined the potential for
EBV infection of the T-cell line, HPB-ALL, which expresses
an immature phenotype characteristic of T cells developing
within the thymus. We report that HPB cells are infectable
by EBV, which interacts with cell surface CD21 and leads
to altered expression of cellular proteins involved in antigen
recognition. Components of the virus replicative pathway
were expressed in infected HPB cells. These studies suggest
that infection of thymocytes in vivo may interrupt the normal
pathways of differentiation, thereby contributing to the development of autoimmunity and to T-cell malignancy.
D21 (CR2) IS WIDELY expressed on B lymphocytes
where it functions as a receptor for complement component C3d,’ CD23’and the pathogen, Epstein-Barr virus
(EBV).’.3EBV binds CD21 via the outer envelope glycoprotein, g~350/220.~
The virus enters the cell following endocytosis of the CD2Uvirus complex4 and persists in a latent
form in which its genome is circularized.’ Alterations of Bcell function and growth following infection by EBV include
malignancy and autoantibody p r o d u c t i ~ n . ~ . ~ , ~
Several recent studies have shown that a molecule identical to or very similar to CD21 is expressed on a population
of human T cells and thymocytes.’”’ Additionally, CD21 is
highly expressed on approximately 50% of childhood T-cell
acute lymphoblastic leukemias (T-ALL),’*-I4many of which
express an immature phenotype. The functional role of CD21
on T lymphocytes hasnotbeen studied. Recently, itwas
shown that T-cell CD21 could bind C3d.I3 Other studies
have shown that soluble CD23 influences the growth of thymocytes,’’ although it was not determined whether this resulted from interaction with CD21, as described for B cells2
Interestingly, a subset of normalhuman thymocytes have
been shown to be infectable by EBVI6 (R. K. Paterson, C.
Kelleher, J. Streib, T. Amankonah, J. Xu, J. Jones, and E.
Gelfand, manuscript submitted). The consequences of EBV
infection in these immature T cells is unknown, although a
From the Divisions of Basic Sciences and Allergy Immunology,
Department of Pediatrics, National Jewish Center for Immunology
and Respiratory Medicine, Denver, CO.
Submitted February 23, 1994; accepted September 13, 1994.
Supported by Grants No. AI-29704 and Pol-AI 29903 from the
National Institutes of Health and Otsuka Pharmaceutical Corp.
Address reprint requeststo Erwin W. Gelfand,MD, National
JewishCenter for Immunology and RespiratoryMedicine,1400
Jackson St, Denver, CO 80206.
The publication costsof this article were defrayed in part by page
chargepayment. This article must therefore be hereby marked
“advertisement” in accordance with I8 U.S.C. section 1734 solely to
indicate this fact.
0 1995 by The American Society of Hematology.
0006-4971/95/8502-0016$3.00/0
456
MATERIALS AND METHODS
Antibodies and reagents. Phycoerythrin (PE)-conjugated antiCD4, anti-CD3, and anti-CDla monoclonal antibodies (MoAb) and
fluoroisothiocyanate (FITC)-conjugated anti-CD8 MoAb were obtained from Coulter (Hialeah, F’L).PE-conjugated anti-CD2 and antiCD5 MoAbs were obtained from Olympus (Lake Success, NY). The
anti-CD21 MoAbs, OKB7 (Ortho Diagnostics, Raritan, NJ) and HB5
(ATCC, Rockville, MD), were unconjugated and, for flow cytometry, were detected using, as a secondary reagent, FITC-conjugated
goat-antimouse Ig (KPL, Gaithersburg, MD). The anti-CD21 MoAb,
B2, was obtained as a PE conjugate (Coulter). FITC-conjugated antiCD23 MoAb was obtained from The Binding Site CO (San Diego,
CA). IgGZbcontrol isotype antibody was obtained from Coulter. The
gp 350/220 secreting hybridoma was obtained from ATCC. The 125
kD VCA MoAb was obtained from Cambridge Biotech (Rockville,
MD) and was detected flow cytometrically after secondary staining
with FITC-conjugated goat-antimouse Ig. B95-8 EBV was obtained
from Tampa Bay Research (Tampa Bay, FL). The P3HR-1 virus
strain, which cannot transform cells due to a deletion in the genome,
was prepared as described3’ and titered for its ability to upregulate
CD23 on Raji cells. EBV preparations were depleted of virus by
treatment with anti-gp350/220 MoAb for 30 minutes at room temperature, followed by incubation with S. aureus, Cowan I strain (SAC)
and centrifugation to remove immune complexes. Cellular morphology (HPB cells grew in tight clumps when exposed to active virus)
was assessed to monitor removal of virus activity.
Flow cytometry. Cells were stained with various MoAb for 30
Blood, Vol 85, No 2 (January 15), 1995: pp 456-464
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457
EBVINFECTIONOFT-LYMPHOBLASTOIDCELLS
minutes at 4°C in phosphate-buffered saline (PBS) (0.2% bovine
serum albumin [BSA]). Unconjugated MoAb were labeled in a second step with FITC-conjugated goat-antimouse Ig. Fluorescence was
measured on a Coulter Epics Profile.
Electron microscopy. HPB cells were resuspended in cold
RPMI-1640 and incubated with B95-8 virus at 4°C for 15 min to
allow binding to the cell surface. Virus-bound cells were then cultured at 37°C for varying time periods before fixation. Cells were
fixed in 1.5% glutaraldehyde in 0.1 m o m cacodylate buffer pH 7.3
and post-fixed in 1% osmium tetroxide in 0.1 m o m cacodylate
buffer. Dehydration was proceeded through a graded series of acetone and infiltration into Luft’s 3:7 embedding resin. Uranyl acetate
and Reynold’s lead-stained sections were evaluated and photographed using a Philips 400T electron microscope at an accelerating
voltage of 60 kv. B95-8 cells, which are productively infected with
virus, were fixed directly.
Cell culture. HPB-ALL cells were originally derived from a
child with T-ALL.” The Burkitt’s lymphoma B-cell lines, Ramos
(EBV-negative) and Raji (latent EBV’), were obtained from ATCC.
Cells were maintained at 106/mL in “1-1640
(GIBCO, Grand
Island, NY) supplemented with 10% fetal calf serum (FCS), Lglutamine (2 mmol/L), penicillin (100 U/mL) and streptomycin (100
pg/mL), in an atmosphere of 5% C02 at 37°C. EBV was added at
a final concentration of lo6 transforming units/mL for varying time
periods. In some experiments, cells were pretreated with the antiCD21 MoAb, OKB7 (5 or 10 pg/mL), which blocks the EBV binding
site on the receptor, for 20 minutes at 37°C before EBV addition.
As a control for the effects of OKB7, an irrelevant IgG2,, MoAb was
examined in parallel and did not influence CDla or viral capsid
antigen (VCA) expression.
Detection of RA2 and BZLF-I mRNA transcripts by polymerase
chain reaction. Cells were cultured in the presence or absence of
EBV as described above. Cellular RNA was prepared by phenol
extraction as de~cribed.~’
First strand cDNA was made using avian
myeloblastosis virus reverse transcriptase using, for RAZ, the downstream primer: 5’ AG GCC TAA AAA GGA TGG CTT 3’, positions
105180 to 105161. Polymerase chain reaction (PCR) was then carried out in a Perkin Elmer Thermocycler (Cetus, Emeryville, CA)”
for 35 cycles (denaturing ‘P, 94°C for 1 minute, annealing at 54°C
for 1 minute, ramping to 72°C over 2 minutes and extension at 72°C
for 2 minutes). The upstream primer in the PCR reaction was 5’
ACC AAT GTC TGC TAG CTG T
l
‘ 3’, from positions 102721 to
102740. The primers span the junction between BZLF-1 and BRLF1 open reading frames. They also span a spliced region of BRLF-l
encoding the recently described, RAZ.34.35The PCR products were
then subjected to electrophoresis on a 3:l NuSieve: Seakem agarose
gel (FMC, Rockland ME) and transferred to nylon membrane (Zeta
Probe) by Southern blotting. Amplified RAZ products were detected
by hybridization of end-labeled oligonucleotide Z-probe (5’GGT
GCT GCA TAA GCT TGA TA 3’: positions 102881 to 102900).
The primers for RA2 amplify a 451-bp product,
For detection of BZLF-l (BamHI Z EBV replication activator),
the downstream primer used to initiate first strand cDNA synthesis
was: 5’ AGG TGC CTT l T G TAC AAG CT 3’: positions 103100
to 103081. The upstream BZLF-I primer was 5’ ATA ATG GAG
TCA ACA TCC AG 3‘: positions 102256 to 102275. Amplified
BZLF- 1 products were detected by hybridization of end-labeled oligonucleotide BZLF-l-probe (5‘ ATA CAA GAA TCG GGT GGC
‘lT 3’: positions 102495 to 102479). The primers span a spliced
region of BZLF-l. The primers for BZLF-l amplify a 634-bp product.
Detection of EBV genome. Cells were cultured in complete medium for various time periods in the presence or absence of EBV
and then washed extensively to remove residual virus. For Southern
blotting, DNA was extracted as d e ~ c r i b e d digested
,~~
with BamHI
Table 1. Influence of EBV on Expression
of HPB Cell Sudace Receptors
96 Control MFI
695-8
Experiment
1
2
3
4112
96 5
6
Mean t- SEM
51
CD4
ND
29
ND
38
41
106
58
9865
9277
f7
PIHR-1
CDla
158
245
164
162
218
ND
189 t- 18
CD4
CDla
ND
ND
ND
ND
99 IT 4
ND
104
HPB cells were cultured in the presence of EBV for 1 or 2 days. Cells
were stained for CDla and CD4 and fluorescence was measured by
flow cytometry. Values represent the % control relative MFI. Control
cells were 100% positive for both CDla and CD4. CDla expression
was significantly increased in the presence of EBV ( P < .01) and CD4
expression was reduced ( P .01). HPB cells responded as a homogeneous population tothe receptor-altering influences of EBV. In experiments 1 through 3, cultures were maintained for 1 day before assessment of cell surface receptors. In experiments 4 through 6, cultures
were maintained for 2 days before assessment of cell surface receptors. For CD4, absolute MFI values were 58 2 6 and for CDla were
30 f 4.
Abbreviation: MFI, mean fluorescence intensity.
and run on a 0.6% agarose gel. Transfer to nitrocellulose was performed by capillary diffusion in 20x SSC overnight. Southern blots
were hybridized in 50% formamide, 0.25 m o m sodium phosphate,
0.25 m o m sodium chloride, 7% SDS and 1 mmom EDTA overnight. Blots were probed with the 9.4 kb NJhet probe:’ which was
32P-labeledby random priming. The NJhet probe encompasses both
ends of a fused EBV genome; it contains, in addition to three copies
of terminal repeats, unique sequences from both ends. Visualization
was carried out by autoradiography.
Statistics. Statistical analyses were performed by ANOVA.
RESULTS
EBV-induced alterations of cell surface markers on HPB
cells. CD21 is expressed on a minor subpopulation of immature T cells’ and is believed to mediate infection by EBV.
The phenotype of the HPB-ALL T-cell line is similar to that
(unpublished observaof an immature, human thym~cyte’~
tions). HPB cells are 100% double positive for CD4 and
CD8 and express the cortical thymocyte antigen, CDla. They
are 100% surface positive for TCR, CD3, and CD2 and have
homogeneous, high-density staining for CD21 using three
MoAbs, B2, OKB7, and HBS.
The expression of CD21 on the HPB-ALL cell line suggested its use as a model for infection of immature human
T cells by EBV. In B lymphocytes, EBV infection leads
to a variety of cellular changes that reflect virally-induced
activation and differentiation. Therefore, we examined the
effect of EBV on the surface phenotype of HPB cells. Expression of the molecules, CD4, CD8, and CDla, which
are involved in antigen recognition and presentation, was
examined. EBV dramatically upregulated the surface density
of CDla on HPB cells by 1 day in culture (Table 1). The
EBV-mediated upregulation of CDla was inhibited by prein-
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PATERSONETAL
-
T"
Fig 1. Internalization of EBV by HPB cells. HPB cells were incubated with EBV for 15 minutes at4°C t o allow bindingof virus to the
cell membrane. Virus-bound cells were then cultured in medium at
37°C for 5 minutes or15 minutes. Virus waspresent in close association with thecell membrane andwithin invaginations at both time
points, A ( x 200.000). B I x 64,0001, and C I x 225,000).
_I
C
.
.
n
_,.
6..
.W,
.-
.-
a
.
-
Y
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EBV INFECTION OF T-LYMPHOBLASTOID CELLS
459
Fig 1. (Cont'd) Virus was found within the cytoplasm of HPB cells inside cellular vesicles at 5 and 15 minutes, D ( x 220,0001 and
210,000). In (Dl, the virus-containing vesicle appearsto be incompletely sealed off from the extracellular milieu.
cubation of the cells with the anti-CD21 MoAb, OKB7
(189% t 18% v 125% t 8% control P < .02). In contrast,
EBV downregulated the levels of cell surface CD4 (Table
l ) . There was little or no effect of EBV on CD8 expression.
Depletion of EBV from the virus preparation removed the
ability to alter expression of cellular proteins (data not
shown). Similarly, mutated virus fromthe P3HR- 1 cell line3'
did not influence the expression of CDla or CD4 on HPB
cells (Table l ) . The regulatory effect of €395-8 on CD4 expression was diminished by ultraviolet (UV)-inactivation of
virus (88% ? 2% control), further suggesting thatintact
genome, rather than cell surface signaling, was required to
alter cellular surface markers.
Internalization of EBV by HPB-ALL cells. We examined
the association betweenHPB cells andEBVby
electron
microscopy (Fig l ) . HPB cells were first bound with EBV
at4°Candthen
cultured for various times. As early as S
minutes following culture, virus was observed in close association with the HPBcell membrane, oftenwithin invaginations (Fig 1A, B, and C). Figure IA shows a virion associated with the outer cell membrane of HPB. The capsid with
its electron dense core is surrounded by a bilayer membrane.
Fine projections are visible between the virus and the cell
surface. In Fig IB and C, the virion is partially surrounded
by cell membrane, apparently engaged in internalization. At
E
(X
5 and 15 minutes, intact virions were also observed within
the cytoplasm. enclosed in large, membrane-bound cellular
vesicles (Fig IC and D). Intact virions associated with HPB
cell membranes and vesicles were similar in appearance to
virions shed in culture by the productively infected, B95-8
cellline (data not shown). The average size of the EBV
virion was -200 nm.
LinearviralDNAiscontainedwithinHPR
cells. By
Southern blotting, EBV DNA could be detected within HPB
cells (Fig 2). When cells were kept at 4°C after virus exposure to prevent internalization, only a faint band of smaller
size was observed. indicating that surface bound virus contributed minimally to the total amount of genome we detected. By comparison with the B cell line, Namalwa, which
contains a single copy of EBV DNA, it was estimated that
HPB contained approximately 25 copies of virusgenome
per cell atday 4(data not shown). In situ hybridization
studies confirmed the presence of EBV DNA withinapproximately 75% of HPB cells exposed to the virus (unpublished
observations). The NJhet probe is specific for the terminal
repeat portions of the genome, and thus it was also possible
to determine the configuration of virus DNA detected in
our blots. Whenvirus DNA circularizes. as it does in B
lymphocytes,' closure brings together theterminalrepeat
regions at either end of the molecule. Thus, episomal DNA,
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PATERSON ET AL
460
1
2
3
4
5
3
1 2
23,130-
941 665574361-
2322-
Fig 2. EBV DNA is expressed in a linear configuration in HPB cells.
HPB cells were culturedin thepresence or absence of EBV for 4 days
and DNA extracted and analyzed by Southern blotting. EBV DNA
was detected by hybridization with the NJhet probe, which recognizes the terminalrepeat portions of the
genome and can thus distinguish between circular and linear DNA configurations. Lane 1, uninfected HPB cells, lane 2, HPB cells exposed t o EBV at 4°C for 30
minutes, lane 3, EBV-infected HPB cells at 4 days. The B-lymphoblastoid cell lines, Raji (lane 5) and 895-8 (lane4). were studiedas controls
expressing only episomal or both episomal and linear genome, respectively. Following digestion, linear EBV DNA appeared as a ladder
of small (lessthan 6 kb) bands, while circular DNA produced larger
bands (>6 kb). The variability in banding patterns is accounted for
by the multiplicity of terminal repeat regions in the EBV genome.
The experiment isrepresentative of seven separate experiments.
after digestion and Southern blotting, exhibits larger (>6
kb) bands than does linear EBV DNA molecules. The EBV
infected, B95-8 cell line is partially replicative and exhibited
both episomal and linear genome configurations (Fig 2). The
B cell line, Raji, is latently infected and expressed only
episomal virus DNA. Linear, but not episomal, EBV DNA
was detectable within infected HPB cells (Fig 2). even when
tested up to 30 days postinfection.
Expression of virally encoded mRNA by EBV infected
HPB cells. Having demonstrated the internalization of
EBV by HPB cells, we examined whether transcription from
the viral genome took place. Using message-specific primers,
Fig 3. Expression of BZLF-1 mRNAby EBV-infected HPB cells. HPB
cells were infected with EBV and cultured for3 days. BZLF-1 mRNA
was detected by PCR, using message-specific primers which amplified a 634-bp product. The productively infected, B95-8 B-cell line
was studied as a positive control for expression of BZLF-1 mRNA.
Lane 1, uninfected HPB cells,lane 2, EBV-infected HPB cells at 3 days,
and lane 3, 895-8 cells. BZLF-1 mRNA was not expressed by the
EBV-negative B-lymphoblastoid cellline, BJAB (data not shown). The
experiment isrepresentative of three separate experiments.
we assessed expression of the BamHIZ region of the genome
by PCR. BZLF-I mRNA, which encodes the 33 k D , transactivating transcription factor ZEBRA, was expressed within
HPB cells that had been cultured with virus (Fig 3). The
expression of BZLF-l mRNA indicated that, indeed, HPB
cells were infected by EBV andthat the intracellular environmentwas permissive for expression of the viral genome.
Transcription from the RAZ region of the EBV genome was
also detected by PCR (Fig 4).The RAZ mRNA we detected
in HPB cells encodes a28 kD protein possessing the C
terminal DNA binding domain of ZEBRA and the N terminal
portion of the transactivator,
Transient expression of VCA. Taken together, the finding that BZLF-l and RAZ are expressed in the presence of
linear genome suggests that the productive phase of the virus
life cycle may be expressed within the infected T cells. Indeed, we have found cell surface expression of the virally
encoded late replicative antigen, VCA. The percentage of
cells expressing VCA ranged from 35% to 65% (mean of
53% 2 14%). Figure Sa illustrates the time dependency of
the expression ofVCAbyEBV
infectedHPB cells. The
expression was maximal by day 2 and thereafter declined,
although low levels were detectable for up to 1 monthin
culture. Pretreatment ofHPB
cells with the anti-CD21
MoAb, OKB7, before virus exposure almost completely
1
2
3
4
Fig 4. Expression of R A 2 mRNA by EBV-infected HPB cells. HPB
cells were cultured in the absence (lane 3) or presence (lane 41 of
EBV for 2 days. R A 2 mRNA was detected by PCR, using messagespecific primers, which amplified a 451-bp product. The productively
infected, 895-8 B-cell line was studied
as a positive control for
expression ofRA2 mRNA (lane 2). The latently infectedRaji cell line did not
express RA2 mRNA (lane 1). The experiment is representative of
three separate experiments.
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46 1
EBV INFECTION OF T-LYMPHOBLASTOID CELLS
abolished the expression of VCA (from 33.1% C 5.6% to
1.9% ? 1.8% positive, n = 3, P < .01). When the B lymphoblastoid cell line, Ramos, was infected in parallel with HPB
cells, VCA staining was marginal (Fig 5b).
401
DISCUSSION
30
20
l0
0
0
10
S
15
Time (days)
iru
Fig 5. Expression oflate viral antigens by EBV infected HPB cells.
(a) HPB cellswere infected with EBV at 37°C for varyingtime periods,
as indicated. The 125 kD cell surface VCA wes detected by binding
of MoAb and analyzed by flow cytometry. The date are expressed as
percentage of VCA poaitive cdls. Uninfeded HPB cells did not express detecteble levels of VCA. The date are representative of five
similar experiments. (b) HPB cells (A through Cl and Ramos cells (D
through F) were infected with EBV and cultured for 1 day as described in theMaterials and Methods before staining for125 kD VCA.
Fluorescence is expressed along the x-axis on a logarithmic scala; (A
and D) FlTCconjugated goat-antimouseIg control (B and E) anti-VCA
MoAb + FITC-conjugated goat-antimouse Ig staining of uninfected
cells. (C and F) Anti-VCA MoAb FITC-conjugated goat-antimouse
Ig staining of EBV infected cells. The percentage of VCA' HPB cells
in (C) was 63%. The surface density of CD21 (HE51 was threefold
higher on Remos cells compared with HPB cells (not shown). Results
are representative of five similar experiments.
+
There has been increasing recognition that malignancies
of T-cell origin may be causally associated with EBV infect i ~ n . ' ~CD21
- ' ~ is expressed on human T lymphocytess-".26
and its expression on thymic T cells is inversely correlated
with age39(R. Paterson, C. Kelleher, J. Streib, T. Amankonah, J.W. Xu, J. Jones, and E. Gelfand, manuscript submitted). Because CD21 is expressed only at low levels on a
small subpopulation of developing T cells, it is difficult to
study its function. However, a large proportion of T-ALL,
many of which have an immature phenotype, express high
surface levels of CD21.8.'2 Additionally, a number of Tcell lines have been characterized as expressing a surface
molecule, which is immunologically, biochemically, and
functionally similar to CD21. These include HPB-ALL,I3.l4
CEM,*l4' MOLT-3,8*'4,41
J ~ r k a t ~ .and
' ~ ,MOLT-4.41-43
~
The
CD21-like molecule expressed by the MOLT-4 cell line differs immunologically from that of B cells,44which may contribute to the inability of EBV to infect these cells.42The
CD21 receptor on the immature, T-cell line, HPB, has been
characterized and shown to be similar to that of B cells. The
receptor has a molecular weight of 145 kD and binds the
complement component, C3d.I3 Thus, the HPB cell line provides a good model in which to examine the functional role
of CD21 on an immature, human T cell and to evaluate the
outcome of EBV infection.
Infection of human B lymphocytes by EBV, which is
associated with malignancy and autoimmunity, leads to dramatic changes in cellular function in vivo and in ~ i t r o . ~ - ~
Dysregulated expression of cellular proteins may actually
contribute to EBV-mediated pathogenesis; for example, cellular CD23 may promote cell growth and inhibit apoptosis
in centrocyte~:~believed to be the precursors of Burkitt's
lymphoma cells.* Within the thymus, disruption of normal
cellular function could impact the development of the T-cell
repertoire. We examined cellular responses to EBV in the
HPB cell line. EBV induced an increase in cell surface CDla
expression. This change was mediated by the virus from the
B95-8 cell line, butnot the virus obtained from P3HR-1
cells, suggesting that intracellular events of viral origin are
required to influence T-cell surface receptor expression. Because of its tissue distribution and association with &microgl~bulin,"~ CDla
is thought to possess an antigen presenting
function and has been shown to stimulate major histocompatibility complex (MHC) unrestricted, cytotoxic T-lymphocyte
(CTL) responses among CD4-CD8-T cells.48CDla also associates covalently with CD8,49 and it is physically associated with CDlb and CDlc, both of which mediate calcium
signaling in HPB cell^.^^^^^
The cell surface receptors, CD4 and CD8, direct T-cell
receptor (TCR) recognition of MHC molecules during thymic selection as well as during a mature immune response
in the periphe~y.~'
Cell surface CD4 was downregulated by
virions from B95-8 cells, but not by virions from P3HR-1
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462
PATERSON ET AL
cient to terminate laten~y.".'~ ZEBRA isa 33 kDnuclear
cells. We have observed that B95-8 also induced downregulation of CD4 in a subpopulation of normal human doubleprotein encoded by the BZLF-1 reading frame.34We demonstrated transcription from both the Z and RAZ regions of
positive thymocytes (unpublishedobservations). The interaction between the TCWcoreceptor (ie, CD4 or CD8) complex the EBV genome in infected HPB cells. RAZ has recently
been characterized as a regulator of ZEBRA activity and its
and MHCin developing T cells leads to deletionof autoreactive cells or to positive selection depending, in part, on the
expression is correlated with replicative EBV
affinity of ligation. The level of surface coreceptor expresOur findings of linear virus genome and transcription of Z
and RAZ in HPB cells have so Pdr been paralleled by our
sionmayinfluence
the affinity andoutcome of the TCR
recognition event.'? CD4 and CD8 may alsocompetefor
findings for EBV infection of normal human thymocytes (R.
available intracellular signaling molecules,54 suggesting that
Paterson, C. Kelleher, J. Streib, T. Amankonah, J.W. Xu, J.
a relative decrease in one coreceptor could enhance produc- Jones, and E. Gelfand, manuscript submitted).
Taken together, the present findings suggest that infected
tive association between the TCR andthe remaining corecepT cells may be permissive for viral productivity. A transient
tor, in this case CD8. Indeed, markedly increased numbers
m o n o n ~ c l e o s i s , ~ ~expression of EBV structural proteins was indeed observed.
of CD8' T cells are found during infectious
although their source has not been identified.
The rapid appearance of VCA was strikingly different from
We found that the transforming strain of EBV, B95-8,
its expression in infected B lymphocytes, where it appears
altered the expression of cell surface markers on virtually
relatively late following disruption of l a t e n ~ y Despite
.~
the
100% of the CD21' HPB cells while the deletion mutant
early expressionof replicative cycle genes, infectious potenstrain of virus, P3HR-1, did not induce any such changes.
tial of the culture supernatants was not demonstrable, suggesting that productivity may be low or that EBV infection
This finding strongly suggested that intact viral genome was
of T cells may be abortive in vitro. Recently, it was shown
required to influence the cells and that, indeed, expression
that the protein encoded by RAZ blocks the transactivating
of the viral genome occurredwithin HPB-ALL cells. On
average, the frequency of infection of HPB cells was approx- activity of ZEBRA in vitro." We are currently evaluating
imately 50%, as determined by flow cytometric assessment
the relative expression of Z and RAZ in EBV-infected HPB
of EBV-encoded antigens (see below). That HPB cells recells under various culture conditions to determine whether
sponded homogeneously to the receptor-altering influences
there is a correlation with viral productivity.
of the virus, may indicate that soluble factor(s) mediated the
The present findings, that EBV infected HPB cells express
changes in cellular receptor expression.
components of the replicative cycle, are comparable with
reports by Shapiro et al,ho who studied the T-ALL cell line,
By electron microscopy we showed that EBV is rapidly
MOLT-4, manipulated toexpress B-cell CD21.They obinternalized by HPB cells in culture. Internalization was deservedrapid and transientexpression of replicative cycle
tected as early as 5 minutes after binding of the virus. The
antigens (early antigen and VCA) following primary infecvirus was detected in association with invaginations of the
tion by EBV. EBV DNA and virus particles could be deplasmalemma and wasdetected cytoplasmically within large
tected within the infected MOLT-4 cells, further suggesting
(-500 nm) membrane-bound vesicles. A very similar prothat a productive infection had taken place. Artificial introcess of virus internalization by normal, human B lymphoduction of the EBV genome into murine
T cells was also
cytes was reported by Nemerow et al.4
found tolead to the expression
of replicative cycle genes." In
Southern blotting showed the presence of linear EBV geother studies, transfection with EBV DNA led toan abortive
nomeswithininfected
HPB cells.When studied up to 30
infection in which immortalizationof T cells occurred simuldays in culture, we did not detect
circularization of the internalized EBV genome within infected HPB cells, as has been taneously with expression of replicative cycle antigens."
The demonstration that immature human T cells are indescribed to occur in infected B cells within approximately
fectable by EBV in vitro suggests that thymocytes develIncells,
Bcircularization
20
to
12
hours
post
oping in vivo are potential targets for the virus. A number
of the genome isvery tightly correlated withestablishment of
of studies have shown that the thymus isnot as protected as
viral latency and subsequent immortalization.56 In its latent,
once thought" andEBV+ thymictumorshave
now been
episomal configuration, transcription from the viral genome
d o c ~ m e n t e d . ~T* ,cell
~ ~ tumors have been described, which
is restricted and productivity does not occur.' Although in
apparently harbor replicating EBV'9.26although, recently, evB cells, episomal virus DNA replicates faster than cellular
idence for viral latency within EBV' T cell tumors has also
DNA,'6 we cannot rule out the possibility that unamplified
been demonstrated.",6' There are also descriptions of T-cell
episomes might have been present in a fractionof HPB cells,
tumors expressing early, but not late, replicative cycle-assonot reaching the level of detection. However, we detected
ciated EBV genes, suggesting that
infection may be abortive
expressionof viral genesthatcharacterize thereplicative
in vivo.20.2'Notably, ZEBRA upregulates cellular fos and is
phase of the EBV life cycle, suggesting that the linear gein fact biochemically and functionallysimilar to c-fos.6h
nomes present within infected HPB cells were transcriptionThus, this molecule may possess oncogenicproperties when
ally active.
expressed in an abortively infected T cell, as suggested in
Viral replication in B cells has been described to proceed
regard to its role within the infected Reed-Sternberg cells
only following activedisruption of the latent state with attenof Hodgkin's di~ease.'~ The present studies highlightthe
dant relinearization of the g e n ~ m e In
. ~ B cells, expression
potential usefulness of immature T-cell lines as models in
of the virally encoded, DNA binding protein, ZEBRA, is
which to studyregulation of EBV genome expression, as
associated with relinearization of viral genome and is suffi-
From www.bloodjournal.org by guest on February 6, 2015. For personal use only.
EBV INFECTION OF T-LYMPHOBLASTOID CELLS
well as to delineate its pathogenic impact on
T-cell development.
ACKNOWLEDGMENT
We are grateful to Drs J. Lucas, Tem Finkel, and K. Takase for
their review of the manuscript and Debbie Remley and Jane Watkins
for preparation of the manuscript.
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1995 85: 456-464
Model of Epstein-Barr virus infection of human thymocytes:
expression of viral genome and impact on cellular receptor
expression in the T- lymphoblastic cell line, HPB-ALL
RL Paterson, C Kelleher, TD Amankonah, JE Streib, JW Xu, JF Jones and EW Gelfand
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