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Journal of Virology, October 1999, p. 8179-8184, Vol. 73, No. 10
Institut für Medizinische Immunologie,
Received 26 April 1999/Accepted 28 June 1999
Cell-mediated immunity plays an essential role in the control of
infection with the human cytomegalovirus (HCMV). However, only a few
CD8+-T-cell epitopes are known, with the majority being
contained in the pp65 phosphoprotein, which is believed to dominate the CD8+-T-cell response to HCMV. Here, we have readdressed the
issue of CD8+ T cells specific for the 72-kDa major
immediate-early protein (IE-1), which is nonstructural but is found
very early and throughout the replicative cycle. Using a novel
flow-cytometric assay, we were able to identify CD8+-T-cell
epitopes (by IE-1 peptide-specific induction of cytokine synthesis) and
simultaneously measure the frequency of cells directed against them.
For this purpose, 81 pentadecamer peptides covering the complete
491-amino-acid sequence of IE-1 were tested on peripheral blood
mononuclear cells of anti-HCMV immunoglobulin G-seropositive donors. At
least 10 new epitopes were identified, and the fine specificity and
presenting HLA molecule of the first of them was determined. The
frequencies of CD8+ T cells directed against IE-1 were
similar to those directed against pp65 in donors tested with known
pp65-derived peptides. Importantly, additional testing of a
corresponding set of peptides covering the complete sequence of pp65 on
10 of these donors identified individuals whose CD8+ T
cells recognized IE-1 but not pp65 and vice versa, clearly illustrating
that either protein may be a major target. In summary, our results
suggest that IE-1 is far more important as a CD8+-T-cell
target than current opinion suggests.
Primary infection with or
reactivation of the human cytomegalovirus (HCMV) is a major
complication after bone marrow transplantation and solid-organ
transplantation, and in persons infected with human immunodeficiency
virus (11, 19-21, 24). Cell-mediated immunity plays an
essential role in the control of persistent infection, induction of
latency, and recovery from acute disease (11, 14, 15, 19-21,
24). Adoptive transfer of HCMV-specific CD8+-T-cell
clones in bone marrow transplant recipients has successfully prevented
viremia and disease (11, 20, 21, 24), illustrating the
importance of a sufficient CD8+-T-cell response in
particular. Boosting the CD8+-T-cell response with
HCMV-derived peptides could potentially be a useful alternative to
adoptive T-cell transfer in these patients. In this regard, a
previously defined HLA-2.1-presented cytotoxic T-lymphocyte (CTL)
epitope from the HCMV pp65 protein is being evaluated as a candidate
for vaccination therapy (5).
Unfortunately, to date, our knowledge of HCMV CD8+-T-cell
epitopes is limited to only a few, most of which originate from the pp65 lower matrix phosphoprotein (UL83) (25) and some of
which originate from the 72-kDa major immediate-early protein (IE-1) (1, 9). The presenting HLA molecules, as far as they are known, are HLA-A2, HLA-B7, HLA-B8, HLA-B18, and HLA-B35 (1, 9,
25), so that only approximately 50 to 60% of a Caucasian population could theoretically benefit from vaccinations or immune therapy based on these known epitopes. Despite the existence of several
CTL epitopes in IE-1 (1, 9) and the observation of
IE-1-directed CTL activity in early studies on the anti-HCMV T-cell
response (2), the pp65 phosphoprotein is presently believed to largely dominate the anti-HCMV CTL response (13, 25).
This is partly because structural proteins, such as pp65, which is presented by infected cells prior to virus protein synthesis, are
thought to be more effective targets per se than are nonstructural proteins (such as IE-1) (13). That major histocompatibility complex class I-presented peptides are typically 9 amino acids long
(16-18) was first revealed in 1991 (8) and later
reflected in the use of at least nine overlaps in the design of
peptides used for epitope mapping. The only major study attempting to
map CTL epitopes in IE-1 was in fact published in 1991 and used only six overlaps between adjacent peptides (1). Because pp65 had only recently been studied with regard to CTL epitopes by using a
modern peptide design (25), we chose IE-1 as the first HCMV protein to readdress the issue of CD8+-T-cell epitopes by
using a novel and very efficient flow cytometry-based method
(12). This new method examines rapid peptide-specific induction of effector cytokine synthesis at the single-cell level (12). For this purpose, 81 15-amino-acid peptides (nine
overlaps between adjacent peptides) derived from the 491-amino-acid
protein sequence of IE-1 (SwissProt accession no. P13202) were tested on peripheral blood mononuclear cells (PBMC) from 20 HCMV-seropositive healthy blood donors. At least 10 15-amino-acid peptides that induced
gamma interferon (IFN- Citrated blood was obtained from HLA-typed HCMV-seropositive
(immunoglobulin G IgG) blood donors. Following standard Ficoll-Paque (Pharmacia, Uppsala, Sweden) density centrifugation, cells were washed
with sterile phosphate-buffered saline (PBS; Gibco-BRL), resuspended in
RPMI 1640 (Biochrom, Berlin, Germany) containing 0.1% (wt/vol) bovine
serum albumin (Biochrom) and 50 mM glutamine (Biochrom), and adjusted
to 107 cells/ml. Then 200 µl each of cell suspensions and
peptide solutions (10 µg/ml in RPMI 1640 containing 0.1% bovine
serum albumin) were placed in Cellstar polystyrene tubes (Greiner,
Frickenhausen, Germany) and placed in an incubator (5° slant) at
37°C under a humidified 5% CO2 atmosphere. After 1 h, 1,600 µl of RPMI 1640 containing 12.5% (vol/vol) fetal calf serum
(Biochrom), 50 mM glutamine, and 12.5 µg of brefeldin A (Sigma,
Munich, Germany) per ml was added. After an additional 5 h, the
cells were washed with cold PBS, resuspended in PBS-1 mM EDTA (Merck,
Darmstadt, Germany), incubated for 10 min at 37°C, and washed again
with cold PBS. After surface staining with monoclonal antibodies (for 30 min at 4°C in the dark), the cells were fixed for 5 min at 37°C
in PBS containing 4% (wt/vol) paraformaldehyde (Merck) and washed in
PBS prior to permeabilization (permeabilizing solution; Becton
Dickinson, Heidelberg, Germany) as specified by the manufacturer. Following intracellular staining, the cells were washed in PBS and
analyzed on a FACScalibur flow cytometer (Becton Dickinson) by using
the CellQuestTM software package. Unstimulated samples were analyzed to
verify the effect of stimulation. Data files were analyzed with
CellQuest or Paint-a-Gate software (Becton Dickinson).
On day 1, candidate peptides were selected according to the results
obtained with pooled peptides. These candidate peptides were tested
individually on day 2. For that purpose, PBMC from day 1 were kept in a
standard incubator. Stimulation was performed with individual peptides
by the same method as on day 1, i.e., with the same concentration of
individual peptides as within the peptide pool. Parallel stimulation
with (noncandidate) control peptides and unstimulated samples was
always performed to rule out unspecific stimulation.
Antibodies and peptides.
Fluorescein isothiocyanate
(FITC)-conjugated anti-IFN- Measurement of peptide-specific induction of IFN- Among the IE-1-derived peptides, IE-1307-321
(EFCRVLCCYVLEETS), IE-1193-207 (ARAKKDELRRKMMYM), and
IE-1199-213 (ELRRKMMYMCYRNIE) were the most frequently
identified. All donors reactive to IE-1307-321 were
HLA-B7, HLA-Bw6, and HLA-Cw7 positive (D1, D2, D3, and D4); however,
donors who were HLA-Cw7 or HLA-Bw6 but not HLA-B7 positive did not
react to this peptide (for example D5, D6, D9, D10, and D11),
identifying HLA-B7 as the presenting allomorph.
To define the epitope contained in IE-1307-321 more
precisely, all seven consecutive 9-amino-acid peptides derived from its
sequence were synthesized (i.e., IE-1307-315,
IE-1308-316, IE-1309-317, through
IE-1313-321) and tested in two donors. Several of these
nonamer peptides were able to stimulate IFN-
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Target Structures of the CD8+-T-Cell
Response to Human Cytomegalovirus: the 72-Kilodalton Major
Immediate-Early Protein Revisited
![]()
ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
![]()
INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
) synthesis in subsets of donors were
identified. For the first one of these, we have now determined the
minimal epitope and the presenting HLA molecule. Interestingly, the
measured CD8+-T-cell frequencies were of the same order of
magnitude as those elicited by known pp65-derived peptides in HLA-A2-
and HLA-B7-positive donors (12). More importantly even,
additional testing with a corresponding set of 15-amino-acid peptides
covering the complete sequence of pp65 in some of the donors identified
several subjects with a strong CD8+-T-cell response against
IE-1 but not pp65. Conversely, we identified several donors who had no
CD8+-T-cell response to IE-1 but did show a response to
pp65-derived peptides. These results may have important implications
with regard to the putative role of IE-1 as a factor in HCMV-associated
immune system pathology. In summary, our results suggest that in some individuals IE-1 may be of the same importance as pp65, and in a subset
who do not have pp65-specific CD8+ T cells, IE-1 may even
dominate the CD8+ immune response (possibly together with
yet unidentified epitopes from other proteins).
![]()
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
, phycoerythrin (PE)-conjugated
anti-CD69, Peridinin-chlorophyl-protein (PerCP)-conjugated anti-CD8,
allophyocyanin (APC)-conjugated anti-CD3, and the corresponding
isotype- and fluorescent conjugate-matched control reagents were
purchased from Becton Dickinson. Peptides (synthesized by the standard
Fmoc method) were purchased from NMI (Reutlingen, Germany) or produced
at our own facility. Peptides were stored freeze-dried or in dimethyl
sulfoxide (DMSO) at 4 mg/ml. Peptide pools were generated from peptides
dissolved in DMSO. DMSO concentrations in all assay mixtures were kept
below 0.1% (vol/vol).
![]()
RESULTS AND DISCUSSION
Top
Abstract
Introduction
Materials and Methods
Results and Discussion
References
in
CD8+ T cells is a rapid and efficient way to identify
CD8+-T-cell epitopes (12). By testing 81 overlapping peptides, the complete amino acid sequence of the IE-1
protein was covered. Peptide pools were set up in such a way that every
peptide was contained in exactly two pools. Thus, it was possible to
identify candidate peptides by testing not more than 18 peptide pools
on day 1 (Fig. 1). The following day,
selected candidate peptides were tested individually. For this purpose,
PBMC from the same donors had been kept in complete medium in a
standard incubator overnight. Interestingly, stimulation with
previously identified pp65-derived peptides (12) on days 1 and 2 revealed that this preincubation slightly increased the frequency
of responding CD8+ T cells without increasing nonspecific
stimulation as tested with an irrelevant peptide (Fig. 1, top right).
We believe that this effect was due to preactivation of
antigen-presenting cells and/or a reduction of inhibitory monocytic
activity. The shading in Fig. 1 illustrates how candidate peptides were
chosen. The combination of the two positive pools, 7 and 15, clearly
identifies the candidate peptide 52 (i.e., IE-1307-321,
the only one contained in both pools). Figure
2 corresponds to Fig. 1 and shows the
CD8+-T-cell IFN-
responses obtained with six different
pools, the two positive pools and the four "neighboring" negative
pools, 6, 8, 14, and 16. Table 1
summarizes the results from 15 experiments with samples from healthy
HCMV IgG-seropositive donors, which led to the identification of
several CD8+-T-cell-inducing IE-1-derived peptides. Table 1
also shows the results obtained by testing a corresponding set of
15-amino-acid peptides covering the complete sequence of the pp65
protein (SwissProt accession no. P06725) in 10 of these donors.

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FIG. 1.
Design of peptide pools. The numbers of the pools (left
column and top row) are shown in bold. Individual peptides
(n = 81) in these 18 pools correspond to the numbers in
the respective columns and rows, with peptide 1 = IE-11-15, peptide 2 = IE-17-21, etc.,
according to the complete sequence of the 72-kDa major IE protein.

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FIG. 2.
Identification of a CD8+-T-cell inducing
peptide from peptide pools. Stimulation of PBMC from an HCMV
IgG-seropositive donor with overlapping 15-amino-acid peptides
originating from the IE-1 protein. The figure shows results obtained
with six different peptide pools on day 1, two of which (pool 7 and
pool 15) gave positive results, and stimulation with the candidate
peptide IE-1307-321 (EFCRVLCCYVLEETS), the only peptide
contained in both positive pools, on day 2. IFN-
-positive events are
highlighted. Results for 50,000 CD3 T cells are displayed in each
diagram. PBMC were stained with anti-IFN-
-FITC, anti-CD69-PE,
anti-CD8-PerCP, and anti-CD3-APC. Axes show log fluorescence
intensity.
TABLE 1.
Listing of identified CD8+ T cell epitopes in
IE-1 and pp65a
induction. In both
donors, IE-1309-317 induced the highest frequency of
CD8+ T cells (0.84 and 0.64%),
IE-1310-318 gave a somewhat smaller response (0.50 and
0.42%), and two additional consecutive peptides, IE-1309-317 and IE-1310-318, gave
increasingly weak responses (Fig. 3).
These different frequencies of responding CD8+ T cells
obtained in response to stimulation with largely overlapping peptides
suggested that T cells of overlapping or different specificities were
addressed or that, owing to the relatively high concentrations of
peptides we used (on the order of 109 mol/liter), the
incomplete epitope also stimulated T-cell cytokine induction in much
the same way as incomplete epitopes can induce target cell lysis in CTL
assays if their concentrations are high enough (7). This
will have to be examined in additional experiments.

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FIG. 3.
Fine mapping of a 15-amino-acid peptide. According to
the amino acid sequence of the HLA-B7-presented peptide
IE-1307-321 (EFCRVLCCYVLEETS), seven additional nonamer
peptides were synthesized and tested on PBMC of an HLA-B7-positive
donor whose CD8+ T cells were reactive to the original
peptide (top). Several of these nonamer peptides led to IFN-
induction in CD8+ T cells, with IE-1309-317
giving the strongest response (middle and bottom). IFN-
-positive
events are highlighted. The activation marker CD69 is used to increase
the specificity of the analysis. Results for 50,000 CD8+ T
cells are displayed. PBMC were stained with anti-IFN-
-FITC,
anti-CD69-PE, anti-CD8-PerCP, and anti-CD3-APC. Axes show log
fluorescence intensity.
Finally, to show that previous infection with CMV was necessary for cytokine induction by IE-1307-321, we tested this peptide on PBMC from three HCMV-seronegative, HLA-B7-positive donors under identical conditions but observed no response (Table 1, bottom). Phorbol myristate acetate/ionomycin stimulation performed in parallel showed that the CD8+ T cells from all individuals were able to produce cytokines upon stimulation. We assumed, therefore, that reactivity to this peptide in our HLA-B7-positive donors was indeed dependent on immunization with HCMV and not likely to result from cross-reactivity with peptides of different origin.
Unlike IE-1307-321, the other identified epitopes did not allow for easy determination of their presenting HLA molecule. Notably, the peptides IE-1193-207 and IE-1199-213 (Table 1) share 9 amino acids, suggesting that in donors who were reactive to both peptides (D5 and D11 [Table 1]) this overlap may define the epitope. Interestingly, computerized binding motif analysis carried out by SYFPEITHI (16), a computer program based on listings of MHC ligands and binding motifs (17, 18) predicted HLA-B8 as the most likely presenting allomorph for this candidate epitope. An even higher score, however, was obtained for HLA-A1 and the decapeptide, IE-1198-207 (DELRRKMMYM [contained only in IE-1193-207]). Nevertheless, at this stage the data is inconclusive, because although HLA-A1 and HLA-B8 are found in all donors reactive to one or both of these peptides (D4, D5, D10, and D11), other donors with just HLA-A1 (D8 and D12), just HLA-B8 (D15), or both HLA-A1 and HLA-B8 (D6) did not respond to either. Fine mapping of the epitope(s) with smaller peptides, however, is likely to give additional clues.
It is important to note that despite the identification of many new
epitopes and high frequencies of CD8+ T cells directed
against them in some donors, samples from other individuals tested with
the complete set of IE-1 peptides showed no response to any of them.
Testing some of our donors with a corresponding set of overlapping
peptides covering the complete 561-amino-acid sequence of pp65
identified several individuals who, despite a strong response to one or
several IE-1-derived peptides, did not respond to any of the
pp65-derived peptides and vice versa. By testing the pp65-derived
peptides, several new epitopes were identified, and some of these are
also shown in Table 1 (the presenting HLA allomorphs are currently
being determined). In some donors, the responses to certain peptides were clearly positive (a distinct population of IFN-
-positive events
not observed in the control) yet extremely small. For example, in D15,
the frequency of CD8+ T cells responsive to
pp65489-503 and pp65493-507 (both containing
the known epitope pp65495-503) was only 0.02%, while
0.78% responded to the newly identified epitope
pp65205-219. This probably illustrates that when several
different epitopes can be presented, individual peptides may become
dominant epitopes. The same phenomenon may also be observed in other
donors, where single peptides give much stronger responses than others
(Table 1).
In donors reactive to both pp65 and IE-1, the frequencies of responsive CD8+ T cells may be compared; however, it should be noted that the responses obtained with overlapping peptides cannot simply be added, since complete or partial epitopes may be contained in the overlap. More important than the difference in the frequencies of CD8+ T cells responding to one or the other protein in donors responsive to both seems to be the fact that some donors responded to only one of the two proteins. So far we have tested 12 donors with both complete sets of peptides. In total, 7 donors (58%) had IE-1-specific CD8+ T cells, 9 donors (75%) had pp65-specific T cells, and 4 donors (33%) had CD8+ T cells responsive to both proteins. Three donors (25%) responded only to IE-1, and 5 (42%) responded only to pp65. The finding that IE-1-specific CD8+ T cells exist in some but not all individuals may be very important with regard to IE-1-associated immune system pathology (3, 4). IE-1 is known to induce adhesion molecule upregulation in infected cells, which may lead to nonspecific activation of other cells (3, 4). This process is not disrupted by therapy with ganciclovir or foscarnet (4). The role of IE-1-recognizing CD8+ T cells in such a situation will be an interesting subject to study.
Notably, all donors nonresponsive to the pp65 peptides were HLA-A2 or HLA-B7 negative. Conversely, most individuals not responding to IE-1 were HLA-A2 positive and HLA-B7 negative. The data, though preliminary, suggests that preferences for IE-1 over pp65 and vice versa are directly related to the HLA type, and additional studies are in progress to corroborate these results. Dominant peptides may be derived from pp65 or IE-1, and clearly there is no obvious hierarchy of presenting allomorphs at this stage.
Interestingly, Gilbert et al. reported in 1996 that IE-1-specific CTL
were able to lyse IE-1-transfected but not HCMV-infected autologous
fibroblasts (10), and they found that pp65 apparently abrogated IE-1 peptide presentation by restricting its access to the
antigen-processing machinery or diverting it to a different degradation
pathway. This in contrast to earlier data obtained by Borysiewicz et
al., who found that in two subjects, 18 to 58% of the CTL clones able
to lyse HCMV-infected autologous fibroblasts were IE-1 specific
(2). In their study, only a small number of IE-1-specific
CTL clones were unable to lyse HCMV-infected targets. The phenomenon
described by Gilbert et al. may thus apply to only some IE-1-specific
clones. This could be explained if some but not all IE-1 epitopes
escaped presentation under the influence of pp65. Nevertheless, the
high frequencies of IE-1-specific CD8+ T cells in our
donors clearly argue that sufficient presentation of IE-1-derived
peptides takes place for these cells to be generated. It is important
to note in this regard that in HCMV reactivation, IE proteins are
expressed prior to the early protein pp65 (whose expression is
regulated by IE-1- and IE-2-dependent promoters) (22).
Recent studies indicate that HCMV reactivation is a frequent event even
in healthy donors (23), seems to be stress related, and is
probably mediated by tumor necrosis factor alpha (TNF-
) (6,
23).
To compile a selection of epitopes which may be useful for immunotherapy in a majority of the population, additional work must be done. Since the protein-coding content of HCMV is enormous, it will hardly be possible to scan all proteins for epitopes. However, the need to identify T-cell targets in HCMV proteins will increase if vaccination trials using, for example, the A2-presented pp65495-503 are successful (5). Our results should encourage the search for T-cell epitopes in other nonstructural HCMV proteins to complete the repertoire of peptides which may be potential candidates for vaccine development.
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FOOTNOTES |
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* Corresponding author. Mailing address: Institut für Medizinische Immunologie der Charité, Campus Mitte, COZ, 10098 Berlin, Germany. Phone: 030-28022858. Fax: 030-28025461. E-mail: florian.kern{at}charite.de.
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