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Departments of
*
Medicine and Microbiology and Immunology and
Pathology, College of Medicine, State University of New York, Syracuse, NY 13210;
Wistar Institute and the University of Pennsylvania, Philadelphia, PA 19104; and
§
Department of Biomedical Engineering, Boston University, Boston, MA 02215
| Abstract |
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helices
(residues 7598 and 302329). Four immunodominant peptides harboring
charged amino acid residues with topographically exposed side chains
were identified by sera from 13 multiple sclerosis patients with
predetermined autoreactivity to transaldolase. Autoantibodies binding
to the most prominent human transaldolase epitope, between residues 271
and 285, showed cross-reactivity with Epstein-Barr and herpes simplex
virus type 1 capsid-derived peptides. Molecular mimicry between
immunodominant autoepitopes and viral Ags may be a decisive factor in
directing autoimmunity to transaldolase in multiple sclerosis
patients. | Introduction |
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Previous studies from this laboratory, pursuing the goal to isolate
autoantigens containing epitopes cross-reactive with viral proteins,
demonstrated that an autoantigen, partially encoded by a
retrotransposon and selectively expressed in oligodendrocytes at high
levels (8), corresponds to transaldolase (9),
a rate-limiting enzyme of the pentose phosphate pathway (PPP). A full
length cDNA clone (4/2-4/1) was isolated and the 38-kDa protein was
identified as the human transaldolase (TAL-H) enzyme (9, 10). Involvement of PPP in myelination provided a physiological
explanation for the high level of transaldolase expression in
oligodendrocytes (8, 11). PPP also plays an essential role
in neutralization of reactive oxygen intermediates (ROIs) (12, 13). Although ROIs have long been considered as toxic
by-products of aerobic existence, evidence is now accumulating that
controlled levels of ROIs modulate various aspects of cellular
function and are necessary for signal transduction pathways, including
those mediating apoptosis (14). A normal reducing
atmosphere, required for cellular integrity, is maintained by
glutathione, which protects the cell from damage by excess ROIs. In
turn, synthesis of glutathione from its oxidized form depends on NADPH
produced by the PPP. TAL activity has a profound impact on the balance
between the two branches of PPP and the ultimate output of NADPH and
glutathione (14). These findings are in agreement with a
dominant role of TAL within the metabolic network that controls
intracellular NADPH levels and neutralization of ROIs
(15). Oligodendrocyte-specific expression of TAL-H is
possibly linked to production of large amounts of lipids, as a major
component of myelin and vulnerability of the vast network of myelin
sheaths to oxygen radicals. Although exons 2 and 3 in the 5'-region of
the cDNA are highly repetitive (9), the functional TAL-H
gene locus (TALDO1) is a single-copy element that has been
mapped to the short arm of human chromosome 11 at p15.4
p15.5
(16). Based on differential segregation of the
microsatellite marker, D11S922, 11p15 was identified as 1 of 8 loci
linked to MS in a large scale genome-wide study of MS families with the
use of 443 markers on all chromosomes with an average spacing of 9.6 cM
(17). TAL-H is presently the genetic marker mapped nearest
to D11S922 at 11p15. Therefore, TALDO1 may correspond to or
be located near a susceptibility gene influencing the development of
MS. Patients with MS have Abs to transaldolase in their blood and
cerebrospinal fluid. TAL-H autoantibodies recognize immunoblotted and
three-dimensional epitopes and inhibit enzymatic activity of TAL-H
(10). By contrast, TAL-H Abs are absent in normal
individuals and patients with other autoimmune and neurological
diseases (8, 11). In the present study, four
immunodominant B cell epitopes were identified in 13 MS patients with
concurrent B and T cell responses to TAL-H. Each of these peptides
contain topographically exposed epitopes homologous to viral proteins.
This raises the possibility that molecular mimicry may be involved in
inducing autoimmunity to TAL-H in MS.
| Materials and Methods |
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Serum samples were studied from 13 patients with MS previously documented to have concurrent B and T cell responses to TAL-H (11). All patients satisfied the criteria for a definitive diagnosis (18). Sera from 8 TAL-H-seronegative MS patients and 8 normal blood donors were used as negative control. Polyclonal rabbit Ab 12484 (10) was used as positive control for testing recognition of TAL-H epitopes. MS sera were tested for Abs to EBV capsid Ag with a commercial ELISA assay (SmithKline Beecham, Philadelphia, PA).
Peptide Ags
A total of 33 peptides, 32 that was 15 aa long, and 1 that was
17 aa long, overlapping TAL-H by 5 aa (Table I
), have been synthesized
on cellulose membranes with the use of a spot synthesizer (Abimed,
Langenfeld, Germany) (19, 20). To optimally preserve
natural conformation, peptides were C-terminally attached to cellulose
via a (ß-Ala)2 spacer (21). Before
testing, the membranes were wetted in methanol for 10 min and,
subsequently, in 100 mM Tris (pH 7.5), 0.9% NaCl for 10 min. The
peptide-containing strips were incubated with sera of control donors
and of MS patients with Ab reactivities to human rTAL-H at a 1000-fold
dilution or positive control rabbit Abs 12484 and 170 at a 3000-fold
dilution in 100 mM Tris (pH 7.5), 0.9% NaCl, 0.1% Tween 20, and 5%
skim milk at room temperature overnight. Ab 12484 was raised against
affinity-purified and enzymatically active full length recombinant
TAL-H (10), whereas Ab 170 was raised against a
gel-purified N-terminal 140-amino acid polypeptide fragment of TAL-H in
rabbits (9). For detection of rabbit Abs, after washing,
the strips were incubated with HRP-conjugated goat anti-rabbit IgG
(Boehringer Mannheim, Indianapolis, IN). For detection of human Abs,
after washing, the strips were incubated with biotinylated goat
(Fab')2 fragments directed against human IgA,
IgG, and IgM and, subsequently, with HRP-conjugated avidin (Jackson
ImmunoResearch, West Grove, PA). In between the incubations, the strips
were vigorously washed in 0.1% Tween 20, 100 mM Tris (pH 7.5), and
0.9% NaCl. The blots were incubated with chemiluminescent substrate
(ECL, Amersham, Little Chalfont, U.K.), developed by exposure to x-ray
film, and analyzed with a computerized automated densitometer (Bio-Rad,
Hercules, CA). Cutoff levels were set at 3 SDs over reactivity of
negative control sera. Preimmune rabbit sera and TAL-H-seronegative
human sera were used as negative control for the rabbit and MS sera,
respectively. Reactivities at or below cutoff levels were set at 0.
Maximum reactivity with a single peptide was set a 2.0 for each Ab.
Binding activities were considered dominant at half-maximal reactivity
(1.0) or higher. Repeat experiments gave variations <10% for peptide
binding values.
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Based on the crystal structure of transaldolase from E.
coli (TAL-E) (22), a three-dimensional model of TAL-H
at 1.5 Å resolution was constructed according to the Modeller program
(23). A highly significant 60.3% amino acid identity was
noted between residues 11 and 327 of TAL-H and TAL-E (Fig. 2
). X-ray
studies of TAL-E did not provide experimental data for the structure of
the first 10 and last 11 amino acids of TAL-H. The solvent accessible
surface areas of amino acids in the TAL-H model have been calculated by
the algorithm of Lee and Richards (24), with a water
radius of 1.4 Å. An amino acid residue X has been considered at least
partially exposed if the solvent accessible surface area of its side
chain exceeded 50% of the solvent accessible surface area of the side
chain in the Gly-X-Gly tripeptide (24).
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Correlations of cross-reactivities between TAL-H and viral
peptides were analyzed with Pearsons multivariate
2 test (25). Differences were
considered significant at p < 0.05.
| Results and Discussion |
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Abs induced as a consequence of immunization with a whole protein
Ag are often directed toward three-dimensional epitopes, most often
composed of sets of residues that are discontinuous and brought
together by folding (26, 27). Alternatively, a number of
recent studies have demonstrated that discontinuous linear segments,
which represent significant antigenic sites and brought together as a
composite site by the folds, can be positively identified by testing
antigenicity of overlapping synthetic peptides (26, 27). B
cell epitopes are usually comprised of 5 aa or less in contiguity
(28, 29). Peptides
15 aa long can assume ordered
conformations mimicking the native protein (30).
A total of 33 peptides overlapping TAL-H by 5 amino acids have been
synthesized on cellulose membranes (Table I
) (19, 20). To map epitopes
exposed on native TAL-H, we used polyclonal rabbit Ab 12484, raised
against the full length and enzymatically active TAL-H
(10). Ab 12484 binds native TAL-H and inhibits its
catalytic activity (10). Recognition by Ab 12484 of
individual TAL-H peptides was assessed at a 3000-fold dilution by
chemiluminescent detection (Fig. 1
).
Peptide reactivity with Ab 12484 was quantified by automated
densitometry and expressed as relative binding intensity on a scale of
02.0 (Table I
). Immunodominant epitopes with high binding affinity to
Ab 12484 were defined as having a binding intensity
1.0, i.e.,
10-fold or more over background (Table I
).
|
Recognition of synthetic peptides was correlated with the
three-dimensional structure of TAL-H. The functional TAL-H protein is
comprised of 337 amino acids. Transaldolase from E. coli
(TAL-E) codes for 317 amino acids. A highly significant amino acid
identity of 60.3% was noted between residues 11 and 327 of TAL-H and
TAL-E (Fig. 2
). Based on the crystal
structure of TAL-E (22), a three-dimensional model of
TAL-H was constructed using the Modeller program (23).
Similar to the bacterial enzyme, TAL-H forms an
ß barrel with 8
helices around 8 ß sheets, 6 additional helices, and a large loop
from residues 265 to 290 (Fig. 3
). The
catalytic site is on strand ß4 around the Schiff base-forming
Lys142 (10). Replacement of
Lys142 with glutamine resulted in a complete loss
of enzyme activity, suggesting that Lys142 is
essential for catalytic activity of TAL-H (10). Studies of
TAL-E by x-ray did not provide experimental data for the structure of
the first 10 and last 11 amino acids of TAL-H. The solvent-accessible
surface areas of amino acids in the TAL-H model have been calculated by
the algorithm of Lee and Richards, using a water radius of 1.4 Å
(24). An amino acid residue X has been considered at least
partially exposed if the solvent-accessible surface area of its side
chain exceeded 50% of the solvent-accessible surface area of the side
chain in the Gly-X-Gly tripeptide (24). According to this
analysis, peptides recognized by Ab 12484 correspond to loops with
linear epitopes and
helices in which highly charged residues are at
least partially exposed (Fig. 3
, Table I
).
|
Abs to TAL-H were previously detected in patients with MS
(8, 11). In the present study, we examined recognition of
TAL-H peptides by sera of 13 TAL-H seropositive MS patients at a
1:1000-fold dilution. All patients showed immunoreactivity to the full
length TAL-H protein by Western blot and ELISA. As controls, sera from
eight control donors, eight TAL-H Western blot-negative MS patients,
and preimmune rabbit serum for Ab 12484 showed no significant binding
to the peptides. Sera of all but one patient (AAB) recognized 2 or more
(maximum, 14) TAL-H peptides. Four immunodominant B cell epitopes were
identified by autoantibodies of MS patients. Peptide 24 was recognized
by 9 of 13 TAL-reactive sera, 10 of 13 sera recognized peptide 28, and
peptides 11 and 32 were recognized by 5 of 13 sera (Table II
). Each of these peptides contained
topographically exposed epitopes with highly charged residues in side
chains (Fig. 3
, Table I
). Peptides 11 and 32 were recognized by both Ab
12484 and MS sera. In contrast, peptides 24 and 28 were not recognized
by Ab 12484, suggesting that these two peptides may be cryptic or
nonimmunogenic in the rabbit. TAL-H, which is released from sites of
demyelination in the brain of MS patients (11), may be
subject to proteolytic degradation, thus exposing cryptic epitopes.
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2 test: p = 0.00157). Abs to
HSV-1 have been found in sera and CSF of MS patients (31),
and >99% of MS patients are infected with EBV (32).
Based on an ELISA, with one exception (LIT), sera of all MS patients of
this study contained Abs to EBV capsid protein (not shown). HSV-1 have
been previously linked to inflammatory CNS disease
(33).
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Autoantibodies are deposited around disintegrating myelin components of
the MS brain (39). Patients with MS have Abs to
transaldolase in their blood and CSF (8, 11). Because
Ag-specific B cells can bind Ag and present it to T cells at free Ag
concentrations
1000 times lower than those required by conventional
APC, Abs may be a decisive factor in directing autoimmunity to TAL-H in
comparison with other oligodendroglial Ags in patients with MS.
Alternatively, autoantibodies may bind to TAL-H released from sites of
tissue injury (11) and form immune complexes which in turn
may facilitate Ag presentation by other types of APC.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Andras Perl, State University of New York Health Science Center, 750 East Adams Street, Syracuse, NY 13210. E-mail address: ![]()
3 Abbreviations used in this paper: MS, multiple sclerosis; PPP, pentose phosphate pathway; TAL-H, human transaldolase; TAL-E, Escherichia coli transaldolase; ROIs, reactive oxygen intermediates. ![]()
Received for publication April 28, 1999. Accepted for publication July 7, 1999.
| References |
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p15.5. Genomics 45:233.
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