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Center for Molecular and Vascular Biology, University of Leuven, Leuven, Belgium
| Abstract |
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| Introduction |
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However, most defined T cell-stimulating regions have limited activity across divergent MHC class II haplotypes, i.e., different animal strains preserve and present different Ag regions for stimulation of T cells. This genetically restricted T cell stimulatory activity of peptides has been a serious obstacle to the development of synthetic vaccines, which practically should be effective in genetically diverse populations. Recently, however, T cell-stimulating peptides were described that are active across multiple mouse haplotypes and/or in association with most human class II MHC DR molecules (4, 5, 6, 7), thereby providing a significant step forward in the design of universally active T cell activators. A universal T cell epitope refers to a Th peptide that is recognized by MHC class II molecules of almost all animal strains. Such T cell epitopes could be used either to induce T cell anergy directly or to provide help to B cells for the production of specific Abs to weak immunogens and, in particular, to peptides (6, 8, 9). Unfortunately, not all universal T cell epitopes are suitable for therapy. For therapeutic efficacy, such epitopes must also dominate the response to a protein Ag in a given animal species, a property referred to as immunodominance (10). Putative promiscuous T cell epitopes have already been defined in a few allergens, but, with the possible exception of bee venom phospholipase (11), these are not immunodominant, because only a fraction of sensitized individuals recognize such epitopes (12).
The therapeutic potential of peptide-containing T cell epitopes, which would be both immunodominant and universal, prompted us to investigate the immune properties of Dermatophagoides pteronyssinus group 2 allergen (Der p 2),2 a major allergen from the house dust mite D. pteronyssinus. T cell epitopes of Der p 2 have been described in animals (13) and humans (14, 15), but data vary widely because recognition is restricted by different MHC class II haplotypes or different HLA molecules (13, 16). In this report, we describe the properties of a major T cell epitope, which is presented in a seemingly MHC-permissive manner.
| Materials and Methods |
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Peptides, synthesized by Eurogentec (Seraing, Belgium) and shown
in Table I
, include the following: 1)
15-mer peptides encompassing the entire Der p 2 sequence with 5-aa
overlaps, 2) 12-mer peptides covering residues 1039 of Der p 2 with
an 11-aa overlap, 3) 15-mer peptides covering residues 2135 of Der p
2, with each residue substituted by Ala, and 4) a 15-mer peptide
containing residues 830844 of tetanus toxoid.
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The coding sequence for full-length Der p 2 was produced by
RT-PCR and inserted into the pPICZ
C expression vector (Invitrogen,
Leek, The Netherlands). Positive clones were confirmed by PCR using Der
p 2 specific primers (17) and were further verified by DNA
sequencing. Plasmid cDNA was purified and transformed into Pichia
pastoris using the Easy Select Pichia Expression kit
(Invitrogen). Cells were first cultured in buffered minimal glycerol
medium and transfered into minimal methanol medium containing
methanol for induction of protein expression (18).
Recombinant Der p 2 was purified on a Superdex 75 column (Pharmacia
Biotech, Uppsala, Sweden) and was assessed by SDS-PAGE. Protein
concentration was determined by light absorption at 280 nm. Correct
folding of rDer p 2 was evaluated using conformation-dependent specific
mAbs produced in our laboratory.
Animals and immunization
BALB/c (H-2d) and C57BL/6 (H-2b) mice were obtained from the University of Leuven animal facilities. C3H (H-2k) and SJL (H-2s) were purchased from Harlan (Zeist, The Netherlands). Mice were immunized when 68 wk old by s.c. footpad injection of 100 µl containing 50 µg/ml peptide or 10 µg/ml rDer p 2 emulsified in CFA, followed by a second injection at day 14 with the same dose of Ags in IFA. All animal experiments were conducted in accordance with the regulations of the University of Leuven Review Board.
APCs
Splenocytes from syngeneic naive animals were used for Ag presentation after irradiation (2500 rad). APCs were added to 96-well culture plates at a concentration of 4 x 105/well and were incubated for 45 h at 37°C with 30 µg/ml test Ag before addition of T cells. This concentration was selected because preliminary experiments with 0.3, 3, and 30 µg/ml peptide or rDer p 2 showed that 30 µg/ml yielded the most reproducible results. A total of 5 x 106 nonirradiated cells per milliliter were fixed with 0.05% glutaraldehyde in PBS for 30 s at room temperature, after which an equal volume of 0.2 M lysine in PBS, pH 7.4, was added. The cells were then centrifuged and washed twice before use. Ag processing by APCs was blocked by treatment with the lysosomotropic agent chloroquine. Thus, 10 mM chloroquine (Sigma-Aldrich, St. Louis, MO) in RPMI 1640 medium supplemented with 2 mM L-glutamine, 100 U/ml penicillin, 100 µg/ml streptomycin, 5 x 10-5 M 2-ME, and 5% heat-inactivated FCS was filtered and stored at -20°C for subsequent use. Irradiated naive spleen cells were incubated in culture medium containing 40 µM chloroquine for 15 min at 37°C before addition of Ag. Four hours later, APCs were washed twice with RPMI 1640, and their capacity to activate T cells was tested as described below.
Preparation of T cells and proliferation assays
Ten days after the last immunization, popliteal lymph nodes
(LNs) were removed and T cell-enriched fractions were purified by
negative selection using Abs against CD45R, CD11b, and CD11c coupled to
magnetic beads (MACS System; Miltenyi Biotec, Bergisch Gladbach,
Germany). The purity of T cell suspensions by FACS analysis was
97%.
Purified T cell suspensions were adjusted to a concentration of 4
x 106 cells/ml in RPMI 1640, and 100 µl were
cultured in 100 µl of RPMI 1640 containing the test Ag and irradiated
syngeneic naive spleen cells. After 4 days, the cells were pulsed with
[3H]thymidine for an additional 18 h. All
tests were performed in triplicate. Proliferative responses are
expressed as
cpm x 10-3 after
subtracting background levels in the absence of Ag, which varied
between 800 and 1000 cpm.
Cytokine assays
Cytokine concentrations were measured in supernatants of cell
cultures obtained after 48 or 72 h for IL-4 and IFN-
,
respectively. The sensitivity of the ELISAs (Biosource, Nivelles,
Belgium) was 1 pg/ml for IFN-
and 2.5 pg/ml for IL-4.
| Results and Discussion |
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We have previously shown that BALB/c mice
(H-2d) produce high titers of specific Abs upon
immunization with Der p 2 and that a T cell epitope is located between
residues 21 and 35 (19). To determine whether the T cell
epitope is dominant, T cells were prepared from LN cells of six BALB/c
mice primed by s.c. footpad injection of rDer p 2 and were pooled and
restimulated in vitro with a series of 15-mer overlapping peptides
spanning the entire Der p 2 sequence. T cells responded almost
exclusively to two epitopes contained within residues 2135 and
7185, respectively (Fig. 1
A). Only minor,
nonsignificant T cell proliferative responses were observed with the
remaining peptides. The proliferative response of T cells was
completely blocked by addition of anti-I-Ek and I-Ab mAb, which
cross-react with cells of the H-2d haplotype,
thereby confirming that proliferation of T cells is dependent on
binding of the peptide to MHC class II molecules (data not shown).
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To confirm these findings, six mice of each MHC haplotype were
immunized with p2135. The T cells were collected and purified as
described above. Significant dose-dependent T cell-proliferative
responses to both p2135 and rDer p 2 (data not shown) were now
observed in all cases (Fig. 2
). These
results demonstrate that p2135 exhibits the structural
characteristics of a universal T cell epitope, which can be recognized
by T cells in the context of at least four different class II
MHC-encoded alleles. Interestingly, a search for sequence homology
identified FIS, a peptide from sperm whale myoglobin encompassing
residues 106118 (FISEAIIHVLHSR), as having the sequence SE-X
(X)-IIH in common with p2135 (CHGSEPCIIHRGKPF). FIS contains a Th
cell epitope for restriction elements H-2d,
H-2k (22),
H-2s (23), and
H-2b (data not shown), and as such is considered
to be universal (8).
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p2135 presentation requires processing by APCs
A majority of the short peptides described so far do not require
processing (25, 26), unless they contain one or more
cysteines (27). In our study, p2135 contains two such
cysteines. The strong T cell-activating properties of p2135 observed
in the context of several MHC class II haplotypes prompted us to verify
whether p2135 required processing by APCs. Thus, spleen cells from
naive mice were treated with glutaraldehyde or were irradiated before
incubation with either p2135 or PHA. The cells were then incubated
with purified T cells from LNs of p2135 immunized BALB/c mice. The
results suggested that p2135 required cell processing for specific T
cell activation, because its stimulatory effect persisted in the
presence of irradiated APCs, but not in the presence of
glutaraldehyde-fixed APCs (data not shown). However, because MHC
molecules could have been altered by glutaraldehyde treatment, assays
were also conducted using chloroquine-treated APCs. Data shown in Fig. 3
confirm that p2135 required
processing for effective presentation by MHC class II
determinants.
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We next evaluated the cytokine secretion profile of
p2135-activated T cells. In vitro stimulation of purified T cells
isolated from regional LNs of p2135 footpad-immunized BALB/c mice
resulted in high levels of IFN-
, but undetectable levels of IL-4 and
IL-10 (data not shown), suggesting a Th1 subset phenotype (Fig. 4
). The same Th1-like cytokine profile
was observed after p2135 immunization of H-2b,
H-2k, and H-2s mice. We
have previously shown that substitution of Ile28
by Ala or Asn abolishes p2135-induced T cell proliferation
(19), and we now demonstrate that this is accompanied by a
significant reduction in IFN-
production. The latter was not
affected by the Lys33Ala substitution, which
eliminates the major p2135 B cell epitope (19). The
production of cytokines by polyclonal T cells toward p2135, however,
could have been biased by the use of CFA (28).
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The observation that p2135 contains an immunodominant universal
T cell epitope led us to further analyze the structural characteristics
of the epitope. Two types of universal T cell epitopes have been
identified (29). In the first, a single epitope anchors to
different MHC class II alleles. In the second, several overlapping
epitopes are present that can accommodate different MHC class II
alleles. To determine which type of T cell epitope was recognized in
the context of the four different MHC allelles used here, purified T
cells obtained from p2135 immunized mice were cultured in the
presence of a panel of 12-mer synthetic peptides covering Der p 2
residues 1039. As shown in Fig. 5
, significant proliferative responses were obtained to residues 2431
for BALB/c, 2128 for C57BL/6, and 2430 for C3H and SJL mice,
respectively. Although the immunodominant epitopes recognized in the
context of the different haplotypes overlap by only a few a residues,
they are clearly distinct. Therefore, at least three different T cell
epitopes appear to be located within the region encompassing residues
2131. A T cell line produced from BALB/c mice immunized with p2135
similarly proliferated when exposed to residues 2431 (data not
shown).
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The binding of a peptide to MHC molecules depends on only a few
residues, referred to as positions 1 (P1), 4 (P4), 6 (P6), and 9 (P9).
Because the P1 residue appears to be degenerate, peptide specificity
and binding capacity depend on P4 and P6. The contribution of single
amino acid residues for either MHC class II anchoring or TCR
recognition was determined using Ala scanning, in which each p2135
amino acid residue was substituted in turn by Ala. Several critical
residues in each MHC haplotype were identified by proliferation assays
performed with T cells from p2135 immunized mice (Fig. 6
A). Importantly, substitution
of Ile28 significantly reduced T cell responses
obtained with p2135 in all four murine haplotypes (Fig. 6
B). In a previous study, Ile28 had
been identified using known algorithms as a putative P4 anchoring
residue in H-2d haplotype (19, 30).
Because the T cell epitopes defined for H-2k and
H-2s mice are almost identical (Fig. 5
),
Ile28 can be considered the P4 anchoring
residue of p2135 in these two MHC haplotypes. In
H-2b, the T cell epitope is located at the N
terminus of the peptide (Fig. 5
), in which Pro26
is the putative P4 anchoring residue, because Pro shares attributes
with those of Ile (31).
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An interesting observation made in the present study is that Ala substitution of Cys27 increases T cell proliferation, as compared with the results obtained with the immunizing peptide. This effect might be mediated by peptide cysteinylation, as has been reported (33). The second Cys is located at residue 21, well outside of the actual T cell epitope. This increased T cell proliferation to the Cys27Ala p2135 mutant could have an impact on the capacity to induce tolerance by administration of p2135.
Therefore, we have identified a T cell epitope expressed on a major allergen that is both immunodominant and universal. Such an epitope could be an ideal candidate for induction of anergy to Der p 2. The fact that it is located in a conserved region of the Der p 2 molecule offers the further advantage of possibly modulating the immune response toward alternative Der p 2 isoforms or other group 2 allergens. It remains to be established whether injection of p2135 induces linked suppression of the immune response toward Der p 2, as observed in studies with epitopes from other allergens such as Der p 1 (34), and whether both B and T cell epitopes are required for optimal effect of specific allergen vaccination. This would obviously exert a direct influence on the design of therapies using single or multiple epitopes of Der p 2. Experiments performed with p2135 indicate that linked suppression could require both B and T epitopes (B. Wu, G. Toussaint, W. Janssens, M. Jacquemin, and J. M. R. Saint-Remy, manuscript in preparation). Finally, it remains to be determined whether induction of anergy to one major allergen would be sufficient to alleviate the symptoms of patients sensitized to multiple components of house dust mite.
| Acknowledgments |
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| Footnotes |
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2 Abbreviations used in this paper: Der p 2, Dermatophagoides pteronyssinus group 2 allergen; LN, lymph node; P, position. ![]()
Received for publication November 9, 2001. Accepted for publication June 26, 2002.
| References |
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-helical region of sperm whale myoglobin show differing fine specificities for synthetic peptides: a multiview/single structure interpretation of immunodominance. J. Exp. Med. 164:1779.This article has been cited by other articles:
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