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Journal of Virology, February 2000, p. 1900-1907, Vol. 74, No. 4
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Modulation of Major Histocompatibility Class II
Protein Expression by Varicella-Zoster Virus
Allison
Abendroth,1
Barry
Slobedman,1
Eunice
Lee,1
Elizabeth
Mellins,1,1
Mark
Wallace,2 and
Ann M.
Arvin1,*
Departments of Pediatrics and Microbiology & Immunology, Stanford University School of Medicine,
Stanford,1 and San Diego Naval Hospital,
San Diego,2 California
Received 3 August 1999/Accepted 8 November 1999
 |
ABSTRACT |
We sought to investigate the effects of varicella-zoster virus
(VZV) infection on gamma interferon (IFN-
)-stimulated expression of
cell surface major histocompatibility complex (MHC) class II molecules
on human fibroblasts. IFN-
treatment induced cell surface MHC class
II expression on 60 to 86% of uninfected cells, compared to 20 to 30%
of cells which had been infected with VZV prior to the addition of
IFN-
. In contrast, cells that were treated with IFN-
before VZV
infection had profiles of MHC class II expression similar to those of
uninfected cell populations. Neither IFN-
treatment nor VZV
infection affected the expression of transferrin receptor (CD71). In
situ and Northern blot hybridization of MHC II (MHC class II DR-
)
RNA expression in response to IFN-
stimulation revealed that MHC
class II DR-
mRNA accumulated in uninfected cells but not in cells
infected with VZV. When skin biopsies of varicella lesions were
analyzed by in situ hybridization, MHC class II transcripts were
detected in areas around lesions but not in cells that were infected
with VZV. VZV infection inhibited the expression of Stat 1
and Jak2
proteins but had little effect on Jak1. Analysis of regulatory events
in the IFN-
signaling pathway showed that VZV infection inhibited
transcription of interferon regulatory factor 1 and the MHC class II
transactivator. This is the first report that VZV encodes an
immunomodulatory function which directly interferes with the IFN-
signal transduction via the Jak/Stat pathway and enables the virus to
inhibit IFN-
induction of cell surface MHC class II expression. This
inhibition of MHC class II expression on VZV-infected cells in vivo may
transiently protect cells from CD4+ T-cell immune
surveillance, facilitating local virus replication and transmission
during the first few days of cutaneous lesion formation.
 |
INTRODUCTION |
Major histocompatibility complex
(MHC) class II molecules are highly polymorphic heterodimers consisting
of an
and
chain which present exogenous peptides to
CD4+ T lymphocytes. The
and
chains form a
heterodimer in the endoplasmic reticulum, and this complex, when
associated with the invariant chain (Ii), is transported through the
Golgi and trans-Golgi reticulum to cytosolic endosomes. At
this site, limited Ii chain proteolysis occurs and results in a complex
of 
with Ii-derived peptides, termed CLIPs (class II
invariant-chain peptides). At an endosomal site, CLIP is exchanged for
antigenic peptides derived by proteolysis of endocytosed proteins, a
process which is improved by HLA-DM molecules. The peptide-loaded MHC
class II molecule is then presented on the cell surface (10, 39,
42). Constitutive expression of cell surface MHC class II is
restricted to specialized cell types including B cells, monocytes,
dendritic cells, and those of thymic epithelium, yet gamma interferon
(IFN-
) has been shown to be a potent inducer of MHC class II
expression on many cell types, including fibroblasts (14,
44).
Varicella-zoster virus (VZV) is a human herpesvirus that causes
varicella (chickenpox) as the primary infection in susceptible individuals, establishes latency in sensory ganglia, and may reactivate as herpes zoster (shingles) (4, 13). Multiple components of
the innate and antigen-specific immune responses are activated during
the course of primary VZV infection. The early host responses to VZV
are nonspecific and involve natural killer cells and interferons which
function to restrict virus replication and spread (2, 3,
51). VZV-specific T-cell recognition is critical for host recovery from varicella, and both MHC class I-restricted
CD8+ and MHC class II-restricted CD4+ T cells
are sensitized during primary VZV infection. VZV-specific CD4+ T cells that are elicited during primary infection are
predominantly of the Th1 type (7, 54) and function to
produce high levels of IFN-
, which potentiates the clonal expansion
of VZV-specific T cells (3, 26, 51). Although the classical
cytotoxic T-lymphocyte (CTL) response is mediated by CD8+ T
cells that recognize viral peptides in association with MHC class I
molecules, VZV-specific CTLs can also exhibit MHC class II
(CD4+)-restricted killing of infected target cells
(15, 17, 19, 20, 23, 25, 49). Based on these observations,
immunomodulatory mechanisms that limit the initial presentation of VZV
peptides by MHC class I or class II pathways are likely to have an
important effect on viral pathogenesis.
It has been postulated that interference with MHC class II-restricted
T-cell recognition may promote viral infection in the host by enabling
virus-infected cells to resist a crucial arm of the immune response
(38, 45). In this respect, several viruses including
adenovirus, murine and human cytomegalovirus (MCMV and HCMV), mouse
hepatitis virus, human immunodeficiency virus, Kirsten murine sarcoma
virus, and measles virus have been shown to inhibit IFN-
upregulation of MHC class II expression (9, 21, 22, 27, 29-31,
33, 34, 37, 41, 46-48). The mechanism of interference with
IFN-
-induced MHC class II expression has been studied for HCMV,
MCMV, and adenovirus. In these cases, the effect is primarily at the
level of mRNA transcription (21, 29, 37), although
posttranscriptional modulation has also been described (18,
28).
In this study, we found that VZV inhibits IFN-
-mediated induction of
cell surface MHC class II expression on infected human fibroblasts. We
applied a combination of reverse transcriptase (RT)-mediated PCR
(RT-PCR), Northern and Western blot analyses, and in situ hybridization
to demonstrate that VZV interferes with the IFN-
signal transduction
pathway to block MHC class II transcription. Examination of skin
biopsies of early varicella and herpes zoster lesions from healthy
adults by in situ hybridization showed that MHC class II transcripts
were not detected in infected cells.
 |
MATERIALS AND METHODS |
Cells and viral culture.
Human foreskin fibroblasts (HFF)
were grown in tissue culture medium (TCM; Dulbecco's modified Eagle's
medium; Gibco, Gaithersburg, Md.) supplemented with heat-inactivated
fetal calf serum, 2 mM L-glutamine (Gibco), 50 IU of
penicillin, 50 µg of streptomycin (ICN Biomedicals, Inc., Calif.),
and 0.5 µg of amphotericin B (Fungizone; Flow Laboratories, McLean,
Va.). 8.1.6 cells are clonally derived B-lymphoid cells grown in RPMI
1640 (Gibco)-containing TCM (32). A fresh clinical isolate,
designated strain Schenke and passed in HFF, was stored in TCM
supplemented with the above reagents plus 10% dimethyl sulfoxide.
Antibodies.
Monoclonal antibodies specific for human MHC
class II DR-
(TU 36) and human transferrin receptor (CD71; T56/14),
as well as goat anti-human antibody-fluorescein isothiocyanate
(FITC)-conjugated F(ab')2 fragments, were obtained from
CalTag Laboratories (South San Francisco, Calif.). Western blots were
performed with the following primary antibodies: rabbit anti-CD71
(Zymed), rabbit anti-Jak1 (Pharmingen), mouse anti-Stat 1
(Santa
Cruz Biotechnology), and rabbit anti-Jak2 (Santa Cruz Biotechnology).
VZV-immune (immunoglobulin G-purified) polyclonal human serum was used
for the detection of VZV-infected cells.
Plasmids.
pBS-62C was constructed by cloning a 2,835-bp
KpnI fragment (containing the VZV immediate-early 62 [IE62] gene) from pMS62 (40) into the KpnI site
of pBluescript-SK. Transcripts from the T 7 promoter are complementary
(antisense) to VZV IE62 transcripts. pBS-DR-
was constructed by
inserting the 800-bp EcoRI fragment from pCVneo-DR-
(kindly provided from E. Mellins, Stanford University, Stanford,
Calif.) containing the human MHC class II DR-
cDNA into the
EcoRI site of pBluescript-SK. Transcripts from the T7 promoter are complementary (antisense) to MHC class II DR-
transcripts.
Primers.
The primers used in this study were as follows:
Jak1 sense (5' GAAACTTTGACAAAACATTACGGTGC 3') and antisense
(5' TCCTTCTTGAGGATCCGATCG 3' [37]), Jak2
sense (5' ATGGGGATGGCTTGCCTTACGATGACAGAA 3') and antisense
(5' TCATCCAGCCATGTTATCCCTTACTTGATC 3'
[16]), Stat 1
sense (5'
AATGTGGACCAGCTGAACARG 3') and antisense (5'
GCTCTATACTGTGTTCATCA 3' [52]), transferrin
receptor fragment (CD71) sense (5' AAGTGGTTCGTGGACAGGCCGGA 3')
and antisense (5' TTCAAGTGTATGGTGGTCCCTGCA 3'),
interferon regulatory factor 1 (IRF-1) sense (5'
CTTCCCTCTTCCACTCGGAGTC 3') and antisense (5'
CTGGTCTTTCACCTCCTCCTCGATATCT 3' [37]), Ii sense
(5' TCCCAAGCCTGTGAGCAAGATG 3') and antisense (5'
CCAGTTCCAGTGACTCTTTCG 3' [11]); and MHC class II
transactivator (CIITA) sense (5' CAAGTCCCTGAAGGATGTGGA 3')
and antisense (5' ACGTCCATCACCCGGAGGGAC 3'
[12]).
Infection and IFN-
treatment of cells.
Cells were
infected with VZV strain Schenke by adding VZV-infected cells to an
uninfected cell monolayer at a ratio of 1 infected cell to 5 uninfected
cells for 2 h at 37°C. The infected monolayers were washed three
times with phosphate-buffered saline (PBS), incubated with TCM for
12 h, and then incubated with TCM containing human IFN-
(100 U/ml) for a further 36 h.
Flow cytometry.
Cells were harvested, and aliquots of
approximately 106 cells obtained from mock- or VZV-infected
cells were washed and resuspended in 100 µl of FACS
(fluorescence-activated cell sorting) staining buffer (PBS with 1% FCS
and 0.2% sodium azide). Primary antibodies, VZV-immune or nonimmune
polyclonal immunoglobulin G, were diluted 1:40; secondary antibodies,
anti-MHC class II DR-
or anti-CD71, were diluted 1:20; goat
anti-human FITC was diluted 1:100. As a negative control, cells were
incubated with appropriate isotype control antibodies. All antisera
were diluted in FACS staining buffer, and all reactions were done in
the dark on ice for 30 min. The cells were washed between each antibody
step with 2 ml of FACS staining buffer. After the final wash, cells
were resuspended either in orthofixative (PBS with 1% EM-grade
formaldehyde) or in PBS for FACS sorting. Cell suspensions were either
analyzed with a Becton Dickinson FACScan apparatus or sorted by FACS
(Becton Dickinson, San Jose, Calif.).
In situ hybridization for VZV IE62 and MHC class II
transcripts.
Cell populations that were separated by FACS were
washed and resuspended in PBS and then cytospun onto
poly-L-lysine-coated glass microscope slides. Cells were
fixed in 4% paraformaldehyde for 30 min at room temperature, washed
twice in PBS for 5 min, and treated with 1% Triton X-100 in PBS for 2 min. Tissue sections (10 µm) were dewaxed with xylene (40 min),
rehydrated in graded ethanol solution (100, 70, and 50%), and washed
in PBS. To improve access of probe to target sequences, tissue sections
were incubated with proteinase K (10 µg/ml) for 10 min at 37°C in
20 mM Tris (pH 7.5)-2 mM CaCl2. Cell cytospins and tissue
sections were then washed twice in PBS, treated with 0.25% acetic
anhydride-0.1 M triethanolamine for 10 min, washed in PBS, and
dehydrated through graded (50, 70, 90, and 100%) ethanol; 20 µl of
hybridization buffer (50% deionized formamide, 1× SSC [0.15 M NaCl
plus 0.015 M sodium citrate], 100 mM Tris-HCl [pH 7.6], 10 mM
NaH2PO4, 10 mM Na2HPO4,
0.02% Ficoll, 0.02% polyvinylpyrrolidone, 500 µg of sheared
denatured salmon sperm DNA per ml, 500 µg of yeast tRNA per ml, 20 mM
dithiothreitol, 1 U of RNase inhibitor per ml, 30 pg of a
strand-specific digoxigenin [DIG]-labeled riboprobe generated from
pBS-62C or pBS-DR
C) was added to each section, covered with a
siliconized coverslip, sealed with rubber cement, and hybridized for
16 h at 55°C. Sections were then washed for 30 min each in 2×
SSC-10 mM Tris-HCl (pH 7.5) and 0.1× SSC-10 mM Tris-HCl (pH 7.5) for
30 min at room temperature followed by a stringent wash for 30 min at
55°C in 0.1× SSC-10 mM Tris-HCl (pH 7.5)-30% deionized formamide.
Finally, sections were washed for 15 min at room temperature in 0.1×
SSC-10 mM Tris-HCl (pH 7.5), and bound probe was detected using
anti-DIG antibody coupled to alkaline phosphatase and developed with
nitroblue tetrazolium chloride plus 5-bromo-4-chloro-3-indolyl phosphate according protocol of the manufacturer (Boehringer, Mannheim, Germany).
Northern blot hybridization.
Total RNA was isolated using a
commercial kit (Ambion, Austin, Tex.), and 2.5-µg samples were
separated on a 2% agarose-2% formaldehyde gel before being
transferred to nitrocellulose membranes by Northern blotting. Membranes
were incubated for 4 h at 42°C in prehybridization solution (5×
Denhardt's solution, 5× SSC, 0.1% sodium dodecyl sulfate [SDS],
50% deionized formamide, 200 µg of sheared and denatured salmon
sperm DNA per ml, 50 mM Na2HPO4, 50 mM
NaH2PO4) before being hybridized for 16 h
at 42°C in a hybridization solution (1× Denhardt's solution 5×
SSC, 0.1% SDS, 50% deionized formamide, 100 µg of sheared and
denatured salmon sperm DNA per ml, 10% dextran sulfate) and
random-primed 32P-labeled probe. Unbound probe was removed
by sequential washes at 60°C in 2× SSC-0.1% SDS (twice for 45 min
each time) and 0.1× SSC-0.1% SDS (twice for 45 min each time), and
bound probe was detected by autoradiography. Random-primed probes were
generated either from pBS-DR-
or from products derived from RT-PCR
and were radiolabeled with 32P, using a random prime DNA
labeling kit (Boehringer) according to manufacturer's directions.
Western blot analysis.
Cells were washed once in PBS,
sonicated for 1 min in cell extract buffer containing protease
inhibitors (50 mM Tris [pH 7.4], 240 mM NaCl, 0.5% NP-40, 10%
glycerol, 0.1 mM EDTA, 1 mM dithiothreitol, 0.5 mM phenylmethylsulfonyl
fluoride), and centrifuged (14,000 × g), and
supernatants were collected. The cell supernatants (5 × 104 cells/lane) were separated by SDS-polyacrylamide gel
electrophoresis (PAGE) in 7% gels, followed by electrotransfer to
Immobilon-P polyvinylidene difluoride membranes (Millipore, Bedford,
Mass.). Membranes were stained with amido black (1% amido black
[naphtho; blue black], 45% methanol, 10% acetic acid) to reveal
total protein before Western blot analysis.
Membranes were incubated in blocking solution (5% nonfat milk in PBS)
for 1 h. Jak1, Jak2, Stat 1
, and CD71 proteins were detected
with appropriate antibodies diluted at 1:100, 1:100, 1:250, and 1:500,
respectively, in blocking solution. Secondary goat anti-rabbit or goat
anti-mouse immunoglobulin G-horseradish peroxidase conjugates
(Amersham, Buckinghamshire, England) were diluted 1:2,000 and used for
enhanced chemiluminescence (ECL) detection of bound antibodies
according to the protocol of the manufacturer (Amersham). Molecular
weights of proteins were determined using protein reference standards
(Bio-Rad, Hercules, Calif.).
Human skin biopsy sections.
Skin biopsies of VZV lesions
from healthy adult donors were collected within 4 to 96 h after
the onset of varicella or herpes zoster rash, fixed in 10% formalin,
and embedded in paraffin, and 10-µm sections were collected onto
poly-L-lysine-coated glass microscope slides. Specimens
were collected with the informed consent of the donors.
RT-PCR.
Total RNA samples (200 ng) were reverse transcribed
using Superscript RT (Gibco) and oligo(dT) (Gibco) according to
manufacturer's directions. Duplicate samples had no RT as a control
for genomic DNA contamination in subsequent PCRs. Reaction products (3 µl) were subjected to PCR amplification for 30 cycles (94°C for 1 min, 55°C for 1 min, and 72°C for 3 min; final extension at 72°C for 10 min) with primers for CIITA and CD71. After amplification, 20%
of each reaction product was analyzed by 2% agarose gel
electrophoresis and ethidium bromide staining.
 |
RESULTS |
VZV inhibits IFN-
-induced MHC class II expression.
We used
HFF and FACS analysis to investigate the effect of VZV infection on
IFN-
-stimulated MHC class II expression. Fibroblasts do not
constitutively express MHC class II but rather can be induced to
express MHC class II by treatment with IFN-
. HFF were infected with
VZV strain Schenke by mixing VZV-infected and uninfected cells at a
ratio of 1:5. Twelve hours postinfection, cells were treated with human
IFN-
(100 U/ml) for 36 h to stimulate MHC class II expression,
stained for VZV and MHC class II expression using polyclonal VZV-immune
serum and mouse monoclonal antibody to MHC class II (MHC class II
DR-
), respectively, and analyzed by FACS (Fig.
1A). Negative controls included
mock-infected cells and incubation of both mock- and VZV-infected cells
with isotype control antibodies. At 48 h postinfection, 32% of
the cells tested positive for VZV (i.e., were VZV+), and
68% of the cells remained VZV
as determined by FACS. Of
the VZV
cell population, 86% expressed MHC II (i.e.,
were MHC II+). In contrast, only 26% of VZV+
cells were MHC II+ (Fig. 1C). To ensure that the inhibition
of MHC class II expression on VZV-infected cells following IFN-
treatment was selective and not due to a general effect on host cell
surface molecules, these cells were also assessed by FACS for
transferrin receptor (CD71) expression, using an anti-CD71 monoclonal
antibody (Fig. 1B). At 48 h postinfection, more than 98% of both
VZV+ and VZV
cell populations expressed
transferrin receptor. Taken together, these data show that VZV
selectively inhibits IFN-
induction of MHC class II cell surface
expression. In a total of five experiments, this specific inhibition of
MHC class II was consistent with the mean ± standard error for
MHC II+ cells in the VZV+ cell population being
26.3% ± 2.5%, whereas of the VZV
cell population,
74.8% ± 5.1% were MHC class II positive.

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FIG. 1.
FACS analysis of MHC class II, transferrin receptor
(CD71), and VZV proteins on VZV-infected cells treated with IFN- .
HFF were, at 12 h after VZV infection, treated with IFN- for
36 h (A and B) or were treated with IFN- for 36 h and then
infected with VZV (D and E); cell preparations were stained with
antibodies and fluorescent conjugates to MHC class II and VZV proteins
(A and D) or to transferrin receptor and VZV proteins (B and E). The
percentage of VZV+ and VZV cell populations
expressing cell surface MHC class II is shown with VZV infection and
subsequent IFN- treatment (C) and IFN- treatment followed by VZV
infection (F).
|
|
To determine whether VZV downregulates MHC class II expression on
fibroblasts that were pretreated with IFN-

, HFF were treated
with
IFN-

(100 U/ml) for 36 h before being infected with VZV
strain
Schenke. After 48 h, the cells were analyzed by immunofluorescent
staining and FACS as described above (Fig.
1D). At 48 h
postinfection,
83% of both VZV

and VZV
+ cell
populations expressed MHC class II (Fig.
1F). In four separate
experiments, the mean ± standard error of MHC class II positive
cells was 85.0% ± 6.5% of the VZV
+ population, compared
with 82.0% + 8.6% of the VZV

population. These data
indicate that VZV inhibits IFN-

induction
of MHC class II cell
surface expression but does not downregulate
MHC class II surface
expression on cells treated with IFN-

before
infection.
VZV inhibits IFN-
induced MHC class II RNA expression.
IFN-
induction of MHC class II expression occurs at the level of
gene transcription (8). We therefore evaluated the effect of
VZV infection on IFN-
-induced upregulation of MHC class II RNA
expression, using both Northern blot and in situ hybridization. Twelve
hours after infection with VZV strain Schenke, HFF were treated with
IFN-
(100 U/ml) and incubated for a further 36 h. Cells were
stained with antibodies to MHC class II (MHC class II DR) and VZV
proteins and sorted by FACS into VZV
and
VZV+/MHC class II DR-
cell populations. Of
the VZV+ cells, typically 70 to 85% were identified as MHC
class II DR
by FACS. Cells were subjected to total RNA
extraction and analyzed by Northern blot hybridization or were cytospun
onto microscope slides and analyzed by in situ hybridization.
Samples of total RNA (2.5 µg) were separated by agarose gel
electrophoresis and subjected to Northern blot hybridization using
an
MHC class II DR-

-specific random-primed
32P-labeled
probe derived from pBS-DR-

. Positive controls were
8.1.6 cells and
mock-infected HFF treated with IFN-

(100 U/ml)
for 48 h.
Negative controls were mock-infected and VZV-infected
HFF which had not
been treated with IFN-

. MHC class II DR-

transcripts
(1.6 kb)
were detected in the VZV

cells treated with IFN-

as
well as 8.1.6 cells and mock-infected
cells treated with IFN-

. In
contrast, VZV
+/MHC class II DR

cells and
untreated mock or VZV-infected cells did not express
detectable MHC
class II DR-

transcripts (Fig.
2A). To
determine
whether the inhibition of MHC class II DR-

mRNA expression
in
VZV-infected cells treated with IFN-

was due to a general effect
on host mRNA, we evaluated expression of CD71 mRNA after stripping
the
nitrocellulose filter of bound probe and reprobing with a
random-primed
32P-labeled probe derived from a partial-length CD71 cDNA.
The probe
hybridized similarly to all RNA samples (Fig.
2B), indicating
that VZV specifically inhibited IFN-

-stimulated expression of
MHC
class II DR-

transcripts.

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FIG. 2.
Detection of MHC class II DR- and CD71 mRNA by
Northern blot hybridization in VZV-infected cells treated with IFN- .
Northern blot hybridization for MHC class II DR- (A) and CD71 (B)
was performed on total cell RNA extracted from 8.1.6 cells (lane 1),
uninfected HFF (lane 2), uninfected HFF treated with IFN- (lane 3),
VZV-infected HFF (lane 4), and HFF exposed to VZV, treated with
IFN- , and sorted by FACS into VZV (lane 5) and
VZV+/MHC class II DR (lane 6) cell
populations.
|
|
Sorted populations of cells infected with VZV and treated with IFN-

were analyzed by in situ hybridization, with either strand-specific
DIG-labeled riboprobes designed to detect MHC class II (MHC class
II
DR-

) transcripts or VZV IE62 transcripts. MHC class II DR-
transcripts were readily detected in the majority of VZV

FACS-sorted cells (2,620/3,080; 85%) (Fig.
3A) but were not detected
in
VZV
+/MHC class II DR

cells (0/1,696; 0%)
(Fig.
3B). VZV probe hybridized to the majority
of VZV
+/MHC
class II DR

sorted cells (1,624/1,936; 84%) (Fig.
3C)
but only sporadically
to those defined by FACS as VZV

(92/2,230; 4%) (Fig.
3D). No staining was detected in either
cell
population after hybridization to a control riboprobe generated
in the
orientation opposite that used to detect MHC class II (MHC
class II
DR-

) transcripts (data not shown). MHC class II transcripts
were
undetectable in VZV
+ cells which did not express cell
surface MHC class II.

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FIG. 3.
Detection of MHC class II DR- RNA and VZV IE62 RNA by
in situ hybridization in VZV-infected cells stimulated with IFN- .
VZV-infected cells treated with IFN- were antibody stained and FACS
sorted into two populations: VZV (A and D) and
VZV+/MHC class II DR (B and C); cells were
then hybridized with strand-specific DIG-labeled riboprobes to MHC
class II DR- transcripts (A and B) and VZV IE62 transcripts (C and
D).
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|
MHC class II transcription in varicella skin lesion biopsies.
To confirm the relevance of observations about VZV effects on MHC class
II expression in HFF, we assessed the distribution of MHC class
II-positive cells in skin biopsies taken from individuals who had acute
varicella or herpes zoster. To determine whether VZV-infected cells
express MHC class II in vivo, we performed nonisotopic in situ
hybridization for MHC class II and VZV IE62 transcripts. Skin biopsies
of VZV lesions from two healthy donors were taken 4 h after onset
of varicella and 96 h after onset of herpes zoster. Consecutive
10-µm paraffin-embedded sections were collected onto slides and
hybridized with strand-specific DIG-labeled riboprobes designed to
detect MHC class II DR-
and VZV IE62 transcripts. VZV IE62
transcripts were readily detectable within cells where tissue damage
was most obvious, as well as in the glandular cells and fibroblasts of
the dermis (Fig. 4A). In contrast, on the
consecutive tissue sections, MHC class II DR-
transcripts were
detected only in infiltrating inflammatory cells. MHC class II DR-
transcripts were detected in cells in proximity to VZV+
cells but were never detected in VZV+ cells (Fig. 4B and
C). In addition, a riboprobe generated in the orientation opposite MHC
class II DR-
transcripts did not hybridize to consecutive sections,
confirming the RNA specificity of the MHC class II DR-
staining.
These data indicate that MHC class II transcripts are not expressed in
VZV-infected cells in vivo, but are expressed in cells proximal to
those which are infected, during the initial phase of cutaneous lesion
formation.

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FIG. 4.
MHC class II DR- and VZV IE62 RNA expression in
cutaneous varicella skin lesions. Biopsies of skin lesions from
subjects with varicella were sectioned and hybridized with
strand-specific DIG labeled riboprobes to VZV IE62 transcripts (A) and
MHC class II DR- transcripts (B and C). Positive hybridization for
VZV IE62 was detected in the lesion and deeper in the dermis (black
arrows), whereas MHC class II DR- was detected in areas of
infiltrating cells (boxed area) adjacent to VZV+ cells.
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VZV inhibits IFN-
-stimulated CIITA expression.
CIITA is
induced by IFN-
and is essential for MHC class II gene expression
(11, 50). Since we did not detect MHC class II DR RNA in
VZV-infected IFN-
-treated cells, we hypothesized that CIITA
expression may be deficient in these cells. To address this hypothesis,
we assessed CIITA RNA expression by RT-PCR in FACS-sorted populations
of cells infected with VZV and treated with IFN-
.
Cells were infected with VZV, treated with IFN-

, antibody stained as
previously described, and FACS sorted into two populations:
cells that
remained VZV

, and those that were VZV
+/MHC
II

. Total RNA (200 ng) was reverse transcribed with
oligo(dT), and
cDNAs were subjected to 30 rounds of PCR amplification
with CIITA-specific
primers. Controls for reverse transcription and PCR
included no
RT and no DNA, respectively. Following amplification, 20%
of each
reaction product was separated on 2% agarose gels and stained
with ethidium bromide (Fig.
5A). A 700-bp
product was readily
visualized in samples from VZV

cells;
in contrast, no CIITA sequences were detected in VZV
+/MHC
II

cell samples. To ensure that the inability to detect
CIITA expression
in VZV-infected cells treated with IFN-

was not due
to a general
effect on host mRNA, expression of CD71 RNA was evaluated
by using
the same cDNA samples and subjecting them to 30 rounds of PCR
amplification with CD71-specific primers. In all samples, CD71-specific
samples were amplified (Fig.
5A). These data indicate that VZV
specifically inhibited IFN-

-induced CIITA expression.

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FIG. 5.
Analysis of CIITA and IRF-1 RNA expression in
VZV-infected IFN- -treated cells. (A) RT-PCR for CIITA and CD71 was
performed on total cell RNA extracted from VZV-infected HFF treated
with IFN- , antibody stained, and sorted by FACS into
VZV (lanes 2 and 3) and VZV+/MHC class II
DR (lanes 4 and 5). No DNA was included as a PCR
control (lane 1); no-RT control for each sample is shown in lanes 3 and
5. (B) Northern blot hybridization for IRF-1 was performed on total
cell RNA extracted from 8.1.6 cells (lane 1), uninfected HFF (lane 2),
uninfected HFF treated with IFN- (lane 3), VZV-infected HFF (lane
4), and VZV-infected HFF treated with IFN- , antibody stained, and
sorted by FACS into VZV (lane 5) and VZV+/MHC
class II DR- (lane 6) cell populations.
|
|
VZV inhibits IFN-
-stimulated IRF-1 expression.
IRF-1 is
also required for transactivation of MHC class II in response to
IFN-
(24). To determine the effect of VZV infection on
IRF-1 expression, we assessed RNA levels by Northern blot
hybridization. Cells were infected, antibody stained, and sorted by
FACS into VZV
and VZV+/MHC class II
DR
populations as described earlier. The positive control
consisted of mock-infected HFF treated with IFN-
(100 U/ml) for
36 h; negative controls were mock- or VZV-infected HFF which had
not been treated with IFN-
and 8.1.6 cells. Total RNA (2.5 µg) was
resolved by gel electrophoresis, transferred to nitrocellulose, and
hybridized to 32P-labeled probe specific for IRF-1. Figure
5B shows the result of stripping and reprobing the nitrocellulose
membrane depicted in Fig. 2. IRF-1 RNA (2 kb) was detected in
IFN-
-treated mock-infected and VZV
sorted cells but
not in VZV+/MHC class II DR-
sorted cells,
8.1.6 cells, or mock- or VZV-infected HFF which were not treated with
IFN-
. These data demonstrate that IRF-1 transcription is inhibited
in VZV-infected cells which do not express cell surface MHC class II.
VZV restricts IFN-
induction of Jak2 and Stat 1
.
CIITA
and IRF-1 expression is induced after IFN-
treatment by Stat 1
(35, 43). Stat 1
is a component of the Jak/Stat signal
transduction pathway, which includes Jak1 and Jak2. The failure to
detect CIITA and IRF-1 RNA in VZV-infected IFN-
-treated cells
suggested that these cells may have a disruption of the Jak/Stat
pathway. To address this hypothesis directly, we used Western blotting
to assess the expression of Jak1, Jak2, and Stat 1
protein in
FACS-sorted populations of cells infected with VZV and treated with
IFN-
.
Cells were infected, antibody stained, and sorted by FACS into
VZV

and VZV
+/MHC class II DR

populations as previously described. Cell lysates were prepared,
and
total protein from 5 × 10
4 cells was analyzed by
SDS-PAGE and Western blotting in three
separate experiments. Membranes
were reacted with antibodies to
Jak1, and bound antibody was visualized
with an ECL detection
system (Fig.
6). In
both VZV

and VZV
+/MHC class II
DR

cell populations, levels of Jak1 protein expression
showed little
or no change. Membranes were stripped of bound antibody
and reacted
with antibody to Jak2, stripped again of bound antibody,
and then
reacted with antibody to Stat 1

. In contrast to the
expression
levels of Jak1, both Jak2 and Stat 1

protein expression
was significantly
reduced in VZV
+/MHC class II
DR-


cells compared to VZV

cells.
Stripped membranes were also reacted with antibody to
CD71, which
confirmed equivalent protein loading. The reduction
of Jak2 and Stat
1

protein levels in VZV
+/MHC class II
DR-


cells was observed in a further two experiments.
These data demonstrate
that VZV infection interferes with the Jak/Stat
signal transduction
pathway by reducing steady-state levels of Jak2 and
Stat 1

but
not Jak1 protein expression in IFN-

-treated cells.

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[in a new window]
|
FIG. 6.
Detection of Jak1, Jak2, Stat 1 , and CD71 protein
expression by Western blot analysis in VZV-infected IFN- -treated
cells. Protein lysates were obtained from VZV-infected HFF treated with
IFN- , antibody stained, and sorted by FACS into VZV
(lane 2) and VZV+/MHC class II DR- (lane 3) cell
populations. The positive control consisted of mock-infected HFF
treated with IFN- for 48 h (lane 1). Membranes were incubated
with antibodies to Jak1, Jak2, Stat 1 , and CD71, and bound
antibodies were visualized by ECL.
|
|
 |
DISCUSSION |
These experiments demonstrate that VZV encodes an immunomodulatory
function which enables the virus to inhibit the induction of MHC class
II expression by IFN-
. The persistence of VZV as a human pathogen
depends on its transmission from the cutaneous lesions that are
associated with varicella, caused by primary VZV infection, and herpes
zoster, which results from reactivation of the virus from neuronal
sites of latency (4). The ability of VZV to inhibit MHC
class II expression in most infected human fibroblasts, despite
exposure to high concentrations of IFN-
, provides a mechanism by
which the virus can limit the consequences of immune surveillance by
CD4+ T cells. Impaired recognition of VZV-infected cells
antigen by CD4+ T cells, which requires interaction of the
T-cell receptor and viral peptides complexed with MHC class II
molecules, can be predicted to allow transient viral replication in
dermal and epidermal cells that is necessary for VZV transmission to
susceptible individuals.
With regard to the mechanism of inhibition of IFN-
-induced MHC class
II gene expression, we found that VZV infection inhibits IFN-
-dependent transcription of the MHC class II DR-
gene. HCMV and MCMV also inhibit MHC class II expression at the level of transcription (26, 37). In our studies, MHC class II DR
, CIITA, and IRF-1 transcripts did not accumulate in VZV-infected cells
after treatment with IFN-
. Stat 1
and Jak2 protein synthesis was
reduced compared with Jak1 and CD71 synthesis, which remained unchanged. These observations indicate that the pathway by which VZV
infection alters induction of MHC class II by IFN-
differs from the
effects of HCMV and MCMV. HCMV inhibits MHC class II expression in
human fibroblasts by blocking Jak/Stat signal transduction through a
specific decrease in Jak1 expression. Interestingly, the adenovirus E1A
protein inhibits MHC class II expression in HeLa cells stimulated by
IFN-
at the level of Jak/Stat signal transduction by specifically
decreasing Stat 1
expression (30). In contrast, MCMV
inhibits IFN-
-stimulated MHC class II expression in murine
macrophages by a mechanism that does not involve Jak/Stat signal
transduction. Thus, among the herpesviruses, VZV, HCMV, and MCMV employ
different strategies to reduce MHC class II antigen presentation pathways.
The CD4+ T-cell response to VZV is predominantly of the Th1
type, with IFN-
being the major cytokine produced (26).
Despite the prolonged 10- to 21-day incubation period, VZV-specific T cells are usually not detected until 24 to 72 h after the
appearance of cutaneous varicella lesions. The kinetics of the
appearance of VZV-specific T cells suggests that their sensitization
requires the replication of VZV in skin cells (5).
Individuals who develop T cells that proliferate and release IFN-
within 72 h are likely to experience mild primary VZV infection,
whereas delayed acquisition of these responses is associated with more
extensive cutaneous disease and the risk of progressive varicella
(3, 6). A viral immunomodulatory effect that slows the
initial clonal amplification of antigen-specific CD4+
T-cell populations which is enhanced by IFN-
is likely to facilitate VZV replication at cutaneous sites transiently. VZV-specific
CD4+ T cells mediate MHC class II-restricted lysis of
autologous targets that express VZV envelope and structural proteins
(49). Although CD8+ CTL are also induced, VZV
encodes a viral gene product that causes downregulation of MHC class I
expression (1). Herpes simplex virus (HSV) is the
herpesvirus most closely related to VZV. Like VZV infection, HSV
infection induces CD4+ T cells that mediate cytotoxicity
against HSV-infected targets. HSV-specific CD4+ CTL have
been considered a potential alternative mechanism for clearing
virus-infected cells since HSV inhibits MHC class I expression and
impairs the cytotoxic function of CD8+ T cells
(53). Our experiments suggest that the alphaherpesviruses have also evolved mechanisms to minimize the cytotoxic potential of
CD4+ T cells by limiting the induction of MHC class II
expression by IFN-
. When VZV reactivates, the capacity of viral gene
products to block the upregulation of MHC class II expression triggered by IFN-
should permit a sufficient period of viral replication to
cause the lesions of herpes zoster despite the presence of circulating
VZV-specific, memory CD4+ T cells in the immune host. The
transmission of VZV from older individuals with herpes zoster causes
varicella in the naive contact and is critical for preservation of the
virus in the human population.
The significance of the in vitro studies is substantiated by
examination of human skin biopsies for MHC class II and VZV RNA synthesis by nonisotopic in situ hybridization. Cutaneous VZV lesions
showed a distinct separation of VZV-infected cells and cells positive
for MHC class II transcripts. These experiments demonstrated that
dermal and epidermal cells infected with VZV were not expressing MHC
class II transcripts in vivo at early stages of lesion formation.
In contrast to the failure to induce MHC class II expression on
VZV-infected cells, the upregulation of MHC class II expression that
was triggered by IFN-
treatment of fibroblasts was not reversed by
subsequent infection of the cells with VZV. These observations suggest
that when VZV-specific CD4+ T cells that produce IFN-
have been induced, their trafficking to the site of VZV replication can
control the spread of the virus. Even though VZV spreads to adjacent
uninfected cells, local release of IFN-
should act to make these
secondarily infected cells vulnerable to immune surveillance. Similar
observations have been made when cells were treated with IFN-
before
infection with HSV (36). These experiments also suggest that
it is unlikely that VZV exerts its effect on MHC class II expression at
the cell surface by inhibiting the transport of MHC class II
components. From the perspective of achieving viral persistence in the
population over time, the immunomodulatory effects of VZV function
optimally if they are transient and limited, so that the host recovers
and latency, with the potential for later reactivation, is established.
In conclusion, we have demonstrated that VZV infection inhibits MHC
class II expression in human fibroblasts by interfering with the
IFN-
-stimulated signal transduction (Jak/Stat) pathway (Fig.
7). This study represents the first
report of a mechanism for VZV-mediated disruption of IFN-
-inducible
MHC class II expression and the third report of a direct
virus-associated alteration in Jak/Stat protein components.
Significantly, these experiments revealed the inhibition of MHC class
II in VZV-infected skin cells in vivo. These cells do not express MHC
class II, despite the presence of inflammatory cells, suggesting that
this effect may transiently protect infected cells from
CD4+ T-cell immune surveillance. These findings provide
another example of the diverse immunomodulatory functions that VZV and
other viruses utilize to avoid immune surveillance and establish
persistent infection in the host.

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|
FIG. 7.
Schematic diagram of the sites of VZV-mediated
disruption of IFN- -induced MHC class II expression. The various
proteins that are affected in VZV-infected cells are crossed out.
|
|
 |
ACKNOWLEDGMENTS |
A. Abendroth is supported by a Katherine McCormick fellowship and
Stanford University School of Medicine Dean's postdoctoral award. This
study was supported by grant AI20459 from the National Institute of
Allergy and Infectious Diseases.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Pediatrics, Rm. G312, Stanford University School of Medicine, Stanford, CA 94305-5208. Phone: (650) 723-5682. Fax: (650) 725-8040. E-mail: arvinam{at}stanford.edu.
 |
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Journal of Virology, February 2000, p. 1900-1907, Vol. 74, No. 4
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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