Discussion
We found that barley-OFC results were associated with wheat intake at
the time of OFC. Based on these results, OIT for wheat was conducted for
patients with a wheat allergy who had positive barley-OFC results, and
it was found that barley allergy was improved after wheat OIT. Although
the cross-reactive protein between wheat and barley was not identified
from the results of immunoblotting inhibition, we have shown for the
first time, both clinically and immunologically, that barley allergy
associated with wheat allergy is caused by cross-reactivity from wheat.
The positive rate of barley-OFC performed at our hospital (36.3%) was
lower than previous studies (47.6%, 48.1%)4,18,
which reported that the sIgE titers to wheat and
omega-5
gliadin18 and sIgE titers to barley4were useful predictors of the result of a barley-OFC. In addition to
these titers, we found that the results of barley-OFC were related to
wheat intake at the time of barley-OFC. Therefore, the relatively lower
positive rate of barley-OFC is due to the high proportion of cases with
high wheat intake at the time of OFC. Moreover, when the results of
barley-OFC were examined in patients with low wheat intake (wheat
protein, ≤260 mg) at the time of barley-OFC, the sIgE titers to wheat
and omega-5 gliadin were higher in patients with a positive barley OFC
result (p =0.03 and 0.01, respectively, data not shown).
Based on the barley-OFC results, the change of barley-OFC results in the
earlier stage of wheat OIT and after OIT were evaluated. The second
barley-OFC, performed after reaching a desensitized state of at least
2,000 mg of wheat protein by wheat OIT, was negative in all patients.
These results suggested that barley allergy may be clinically caused by
cross-reactivity with a wheat allergy. In addition, it was proven by
ELISA inhibition that barley allergy complicated by patients with a
wheat allergy was caused by cross-antigenicity of both antigens, and
wheat was the source of common antigenic sensitization.
Several components of barley allergy have been reported in the past. The
water-soluble fractions of barley, α-amylase/trypsin inhibitor,
α-amylase, and β-amylase, are known to be allergenic components of
baker’s asthma.19-21 Heat-stable LTP and protein Z4
are the components of beer that can cause allergic
symptoms.22-26 Adult patients with a barley allergy
whose allergic symptoms are triggered by beer consumption are rarely
complicated by wheat allergy, and the pattern of barley allergy
development in children and adults may be different. Using serum from
patients with WDEIA, it was reported that γ-3 hordein has
cross-antigenicity with omega-5 gliadin.7
This study detected various spots in immunoblotting with barley extract
using the serum of patients with a barley allergy complicated with the
wheat immediate-type allergy. In particular, the spots of 30–35,
40–45, 60–80 kDa were detected in all cases and may be the main
component. Furthermore, according to the WHO/IUIS database, the
molecular weight of γ-3 hordein is 34 kDa, which may correspond to the
30–35 kDa spot detected in this study. In the future, we plan to
identify the barley allergens for the spots detected in this study.
If the amino acid sequences of the epitope sites are highly homologous
even for different allergen components, the IgE antibody binds to both,
causing cross-antigenicity. Omega-5 gliadin has a repeating structure
containing many glutamine residues in the molecule and has many
continuous epitopes, QQX1PX2QQ
(X1=L, F, S, I; X2=Q, E,
G).15,27 On the other hand, γ-3 hordein is also a
protein-rich in glutamine residues and has an amino acid sequence that
is similar but not identical to the epitopes of omega-5
gliadin.28 Although the amino acid sequence similarity
between omega-5 gliadin and γ-3 hordein was not high (23.9%) according
to a Basic Local Alignment Search Tool search, the high amino acid
homology between the epitopes of both components suggests that
cross-antigenicity is likely to occur between wheat and barley
allergens. Furthermore, we showed by ELISA inhibition that the response
to barley extracts was more inhibited in wheat than in barley at low
inhibitor concentrations. This may be because the key amino acid
sequences of the epitopes for omega-5 gliadin and γ-3 hordein binding to
IgE antibodies are identical, but the epitope sequences are not
completely identical, resulting in a difference in affinity, or because
the number of similar epitopes is higher in omega-5 gliadin than in γ-3
hordein. However, in the immunoblotting inhibition, most of the spots of
barley disappeared due to inhibition by wheat extracts, and it is likely
that other barley proteins are also involved in the cross-antigenicity
with wheat proteins. In the future, it is necessary to identify barley
allergens and conduct epitope analysis to elucidate the mechanism of
cross-reactivity.
There are several limitations to this study. First, no patient underwent
a second barley-OFC without OIT for wheat. Hence it cannot be denied
that the barley allergy was not improved by the wheat OIT but was the
acquisition of tolerance during the natural course. Therefore, in the
future, patients with a positive barley OFC result without OIT for wheat
will also be evaluated for barley allergy status over time. Second, the
protocol for barley-OFC was not consistent in all cases. However,
patients with negative OFC results were instructed to consume sufficient
amounts of cooked barley at home to confirm the negative result once
again.
In conclusion, we showed that wheat OIT increased the symptom-inducing
threshold of barley allergy. In addition, barley allergy complicated by
patients with an immediate-type wheat allergy was caused by
cross-antigenicity of both antigens, and wheat was found to be the
source of sensitization. In the future, it is expected that epitopes to
which IgE antibodies react in cases of immediate-type wheat and barley
allergy will be identified, and the mechanism of cross-antigenicity will
be clarified.