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CUTTING EDGE |

,
*
The G. W. Hooper Foundation, Department of Microbiology and Immunology, and Departments of
Biopharmaceutical Sciences and
Pharmaceutical Chemistry, University of California, San Francisco, CA 94143; and
§
Laboratories of Viral Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, MD, 20892
1 This work was supported by National Institutes of Health Grants GM38093 and GM57657 (to F.M.B.), Training Grant GM07175-23 (to E.M.B.), and by a scholarship from Achievement Rewards for College Scientists (ARCS) Foundation (to E.M.B.).
| Abstract |
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E19, an E19 mutant
lacking the ER-retention signal, delays maturation of class I
molecules, indicating that E19s inhibition of class I/TAP interaction
is sufficient to delay class I expression. These data identify tapasin
inhibition as a novel mechanism of viral immune evasion and suggest
that, through this secondary mechanism, adenovirus can affect Ag
presentation by MHC alleles that it can only weakly affect by direct
retention. | Introduction |
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E19, which lacks the retention motif, in murine mastocytoma cells
(7). In the presence of
E19, a portion of H2-Kd
molecules travel to the cell surface, where they elicit a CTL response.
No such response is detected when wild-type E19 is expressed. While
this result highlights the ability of E19 to act as an ER-retention
molecule, the data also suggest that E19 may be affecting class I
expression by an additional mechanism. In the presence of
E19, the
kinetics of Kd maturation are delayed almost 10-fold, and a
significant portion of Kd molecules fail to reach the cell
surface. Additionally, the maturation of those human and murine class I
alleles that are bound only weakly by E19 is still greatly delayed in
the presence of the viral protein (8, 9). These observations prompted
our further investigation into the effect of E19 on the class I
assembly pathway.
Class I MHC molecules are assembled in the ER as a membrane-bound heavy
chain and a soluble light chain, ß2-microglobulin. Class
I heterodimers acquire peptides that are generated in the cytosol by
proteasome-mediated degradation and transported into the ER lumen by
TAP. Tapasin, an ER-resident membrane protein, facilitates the
interaction of class I molecules with TAP by joining the two proteins
and cementing the formation of a class I assembly complex, which also
includes calreticulin and ERp57. (For a recent review see Lehner and
Trowsdale, 10). We hypothesized that E19 could be inhibiting an
additional step in the class I assembly pathway because of the
persistent effect of
E19 on the export of murine class I molecules.
To address this question, E19 was expressed in a panel of human cell
lines, and the interactions of E19, class I molecules, and TAP were
studied. We report here that E19 binds both class I molecules and TAP.
Unlike tapasin, however, E19 binds class I molecules and TAP
independently rather than simultaneously and, thereby, causes a
decrease in class I/TAP association, which can explain the delay in
class I maturation we observe in the presence of
E19.
| Materials and Methods |
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EBV-transformed human B lymphoblastoid cell lines (LCL)
were cultured in RPMI 1640 supplemented with 2 mM
L-glutamine and 10% FBS at 37°C in 8% CO2.
The TAP1-/TAP2-negative LCL 721.174 (11), the HLA-A-, -B-, -C-negative
LCL 721.221 (12), and the tapasin-negative LCL 721.220 (13, 14)
transfected with HLA-B8 were from Dr. Robert DeMars (University of
Wisconsin, Madison, WI). HeLa 229 cells (American Type Culture
Collection, Manassas, VA) were cultured in DMEM supplemented with 2 mM
L-glutamine and 10% FBS at 37°C in 6% CO2.
The specificities of mAb W6/32 (anti-human class I MHC (15)) and
Tw1.3 (against adenovirus E19 and
E19 (7)) and of polyclonal Abs
R.RING4C (16) and R.gp48N (14) (against TAP1 and tapasin, respectively,
both from Dr. Peter Cresswell, Howard Hughes Medical Institute, Yale
University School of Medicine, New Haven, CT), P129-143 (anti-E19,
from Dr. William Wold (St. Louis University School of Medicine, St.
Louis, MO) (17)), anti-TAP2 (18), and UCSF#2
(anti-class I) (18) have been described. Vaccinia virus constructs
encoding the E3 19-kDa protein of adenovirus type 5 (E19-Vac), a
deletion mutant of E19 lacking the C-terminal 6 amino acids
(
E19-Vac), and the nucleoprotein of influenza virus (NP-Vac) are
described elsewhere (7, 8, 19).
Infection and metabolic labeling
HeLa cells (1.5 x 107) were infected with vaccinia virus (25 PFU/cell) in 2 ml 0.2% BSA/PBS for 45 min at 37°C. Virus was removed, and cells were maintained in DMEM/10% FBS until 2 h postinfection. LCL (5 x 106) were infected with 30 PFU/cell in 1 ml RPMI 1640/1% FBS for 2 h at 37°C. RPMI 1640/10% FBS was added (to 106 cells/ml), and infection proceeded for 12 h. For 35S metabolic labeling, cells were starved in cysteine/methionine-free medium supplemented with 5% dialyzed FBS for 1 h at 37°C. Cells were labeled by the addition of 1 mCi per 107 cells Tran35S-Label (ICN Pharmaceuticals, Irvine, CA) for 10 min at 37°C, washed three times in ice-cold PBS, and chased in medium containing five times excess methionine at 37°C.
Immunoprecipitation and Endoglycosidase H treatment
Cells were lysed in 1% digitonin (Calbiochem, La Jolla, CA), 10 mM Tris (pH 7.4), and 150 mM NaCl on ice for 20 min. After pelleting the nuclei, lysates were precleared with protein G-Sepharose for 1 h at 4°C, then incubated with 2 µg of Ab and fresh protein G-Sepharose for 2 h at 4°C. Immunoprecipitates were washed and analyzed under reducing conditions by SDS-PAGE. For Endoglycosidase H (EndoH) treatment, cells were lysed in 1% Nonidet P-40 (ICN Biomedicals, Aurora, OH) and immunoprecipitated as above. Immunoprecipitates were washed and resuspended in 200 µl 100 mM sodium citrate (pH 5.6), 0.2% SDS, and 150 mM 2-ME. Samples were divided in half and incubated at 37°C for 16 h in the presence or absence of 5 mU EndoH (Boehringer Mannheim, Indianapolis, IN) followed by SDS-PAGE analysis. For immunoblotting, proteins were transferred to nitrocellulose, incubated with Ab followed by HRP-conjugated secondary Ab (Zymed Laboratories, South San Francisco, CA), and visualized by enhanced chemiluminescence (ECL) and exposure to film. Bands were scanned and quantitated using the NIH Image 1.61 program.
| Results |
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E19 delays the maturation of human class I MHC
molecules
Adenovirus E19 has been shown to bind both human and murine class
I MHC molecules (20) and to prevent their cell surface expression (3, 4, 21). Though it lacks an ER-retention motif,
E19 caused a
significant delay in the maturation of murine class I molecules (7). We
wanted to determine whether the same delay is observed in human class I
expression in the presence of
E19 and where the delay occurs in the
secretory pathway. When
E19 was expressed in HeLa cells using a
vaccinia virus vector (
E19-Vac), class I molecules remained
sensitive to treatment with EndoH throughout a 2-h chase period (Fig. 1
A). EndoH cleaves
N-linked carbohydrates that have not been fully trimmed and
terminally glycosylated, modifications which take place during passage
through the Golgi complex. Thus, EndoH sensitivity can be used as a
measure of a proteins progression from the ER through the secretory
pathway. Class I molecules typically begin to acquire EndoH resistance
within 30 min (Fig. 1
B). Our data show that in the presence
of
E19, human class I molecules, like murine class I molecules, are
significantly delayed in export from the ER, remaining EndoH sensitive
for longer than 2 h. When cells were infected with a control
vaccinia construct encoding the nucleoprotein of influenza virus
(NP-Vac), class I molecules acquired EndoH resistance without
delay, indicating that vaccinia infection itself does not affect class
I maturation (Fig. 1
C).
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To determine whether the inhibition of class I MHC expression
observed in the presence of
E19 is due to a block in class I
assembly, we investigated whether class I molecules bound by E19
associate with TAP. Only properly assembled class I dimers interact
with TAP (22). Thus, TAP association can be used to assess whether
early steps in the class I assembly pathway are intact. During the
course of this investigation, we found an unexpected direct association
between E19 and TAP. E19 was expressed in a panel of human B LCL and
immunoprecipitated from cell lysates with mAb Tw1.3. Immunoblotting
analysis showed that, in the wild-type cell line JY, Tw1.3
coprecipitated both the class I heavy chain and TAP2 with E19 (Fig. 2
A). In the class I-negative
cell line 721.221 the coprecipitation of TAP with E19 was maintained,
indicating that the association of TAP with E19 is not dependent on
class I molecules. In the reciprocal experiment, the absence of TAP
(721.174 cells) did not prevent the coprecipitation of class I
molecules with E19. Identical results were obtained from cells
expressing
E19 (Fig. 2
B), showing that E19s
ER-retention signal is not required for E19/TAP association. These data
suggest that, in addition to binding class I molecules, E19 binds TAP.
|
Adenovirus E19 inhibits the interaction of class I MHC molecules with TAP
The fact that both class I MHC molecules and TAP
coimmunoprecipitated with E19 in tapasin-deficient .220/B8 cells
suggested that E19 might be acting as a tapasin mimic. To test this
hypothesis, we infected JY and .220/B8 cells with a vaccinia virus
vector encoding adenovirus E19 (E19-Vac) and immunoprecipitated TAP
from cell lysates with anti-TAP1 rabbit serum. In uninfected JY
cells, both tapasin and class I molecules coprecipitated with TAP (Fig. 3
A, lane 1). In the
absence of tapasin, class I molecules failed to coprecipitate with TAP
(Fig. 3
A, lane 3). The expression of E19 in
.220/B8 cells did not restore the coprecipitation of class I molecules
with TAP, indicating that E19 cannot substitute for tapasin (Fig. 3
A, lane 4). This is consistent with lack of
sequence homology between the two proteins (24). Interestingly, when
E19 was expressed in JY cells, the coprecipitation of class I molecules
with TAP was greatly reduced (Fig. 3
A, lane 2).
Quantitation showed that, in the presence of E19, the ratio of
coprecipitated class I molecules to TAP was decreased by
75%, while
the amount of tapasin bound to TAP was approximately the same (Fig. 3
B). Taken together, these data indicate that E19 binds to
both class I molecules and TAP independently, and in so doing, prevents
tapasin from bridging the two molecules. The result is a reduced steady
state association between class I molecules and TAP.
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| Discussion |
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E19. While tapasin inhibition
allows
E19 to reduce the efficiency of class I assembly, it does not
block it completely, as a small portion of class I molecules do become
EndoH-resistant over time (Fig. 1
Inhibition of class I MHC/TAP interaction should lead to a delay in
peptide loading of class I molecules. We are unable to detect such a
delay, primarily because assays that rely on the thermal instability of
empty class I dimers are difficult to interpret. E19 has been shown to
bind the
1/
2 domains of the class I molecule (9, 25), the same
region where peptide binds. E19 and
E19 remain bound to class I
molecules following detergent lysis and heat treatment (data not
shown), and, therefore, may be stabilizing empty dimers,
rendering them undetectable. However, since we observe both inhibition
of class I/TAP association in the presence of E19 and delayed kinetics
of class I maturation in the presence of
E19, the most viable
explanation is that E19 delays peptide loading.
Viral immune evasion strategies highlight the evolutionary pressure on viruses to prevent clearance by the host and ensure their continued spread. Blocking Ag presentation is one mechanism by which viruses escape immune detection. In addition to adenovirus, human cytomegalovirus, HSV, and HIV have all been shown to prevent Ag presentation by class I MHC molecules (reviewed by Ploegh, 26). However, adenovirus E19 is unique in that it is the only viral gene product identified so far that uses more than one mechanism to inhibit class I expression, inhibition of class I/TAP interaction and ER retention. The ability of E19 to inhibit tapasin function appears to be novel, although additional studies will determine whether other viruses use this strategy as well.
| Acknowledgments |
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| Footnotes |
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2 Abbreviations used in this paper: ER, endoplasmic reticulum; LCL, lymphoblastoid cell line; EndoH, Endoglycosidase H; ECL, enhanced chemiluminescence; NP-Vac, vaccinia virus vector encoding the nucleoprotein of influenza virus; E19-Vac, vaccinia virus vector encoding adenovirus E19;
E19-Vac, vaccinia virus vector encoding adenovirus E19 with the ER retention signal deleted. ![]()
Received for publication January 21, 1999. Accepted for publication March 1, 1999.
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