Immunological characterization of onion (Allium cepa) allergy

Abstract

Introduction:
Onion (Allium cepa) handling can induce contact dermatitis, rhinoconjunctivitis and asthma. However, only sporadic reports exist on allergic reactions to onion consumption.

Aim:
We describe herein a case of a 35-year-old man who had an episode of anaphylaxis following cooked onion ingestion. We evaluated onion-specific IgE, the possible cross-reactivity between onion and peach and lymphocyte proliferation in response to onion.

Material and methods:
Specific IgE was evaluated using two techniques: skin test and ImmunoCAP technology. Cross-reactivity between onion and peach was evaluated by IgE-ELISA inhibition test. As for lymphocyte proliferation, blood mononuclear cells were stained with CFSE dye and cultured with an in-house onion extract. Proliferation and phenotype was assessed by flow-cytometry.

Results:
The skin test and ImmunoCAP confirmed the IgE-dependent response towards onion. The incubation of the patient serum with increasing concentrations of the peach extract reduced only scarcely (~30%) onion-specific IgE. Interestingly, B cells but not T cells showed proliferation in response to onion extract.

Conclusions:
In conclusion, our report shows that cooked onion can induce severe allergic reactions, suggesting the presence of thermostable components. Moreover, we applied for the first time a B-cell-based approach to the diagnosis of food allergy. This latter approach might also be applied to other allergic conditions.

Full text

Advances in Dermatology and Allergology 1, February / 201998
Original paper
Address for correspondence: Dr. Marcello Albanesi, Department of Emergency and Organ Transplantation, Section of Allergology,
School and Chair of Allergology and Clinical Immunology, University of Bari-Aldo Moro, Piazza Giulio Cesare, Policlinico, 70124 Bari, Italy,
phone: +39 080 5478817, fax: +39 080 5593576, e-mail: marcello.albanesi@uniba.it
Received: 19.10.2017, accepted: 28.12.2017.
Immunological characterization of onion (Allium cepa)
allergy
Marcello Albanesi1, Carlo Pasculli1, Lucia Giliberti1, Maria Pia Rossi1, Danilo Di Bona1, Maria Filomena Caiaa2,
Luigi Macchia1
1
Department of Emergency and Organ Transplantation, School and Chair of Allergology and Clinical Immunology,
University of Bari-Aldo Moro, Bari, Italy
2
Department of Medical and Surgical Sciences, School and Chair of Allergology and Clinical Immunology, University of Foggia, Foggia, Italy
Adv Dermatol Allergol 2019; XXXVI (1): 98–103
DOI: https://doi.org/10.5114/ada.2019.82829
Abstract
Introduction: Onion (Allium cepa) handling can induce contact dermatitis, rhinoconjunctivitis and asthma. However,
only sporadic reports exist on allergic reactions to onion consumption.
Aim: We describe herein a case of a 35-year-old man who had an episode of anaphylaxis following cooked onion
ingestion. We evaluated onion-specic IgE, the possible cross-reactivity between onion and peach and lymphocyte
proliferation in response to onion.
Material and methods: Specic IgE was evaluated using two techniques: skin test and ImmunoCAP technology.
Cross-reactivity between onion and peach was evaluated by IgE-ELISA inhibition test. As for lymphocyte prolifera-
tion, blood mononuclear cells were stained with CFSE dye and cultured with an in-house onion extract. Proliferation
and phenotype was assessed by ow-cytometry.
Results: The skin test and ImmunoCAP conrmed the IgE-dependent response towards onion. The incubation of
the patient serum with increasing concentrations of the peach extract reduced only scarcely (~30%) onion-specic
IgE. Interestingly, B cells but not T cells showed proliferation in response to onion extract.
Conclusions: In conclusion, our report shows that cooked onion can induce severe allergic reactions, suggesting
the presence of thermostable components. Moreover, we applied for the rst time a B-cell-based approach to the
diagnosis of food allergy. This latter approach might also be applied to other allergic conditions.
Key words: food allergy, onion allergy, lymphocyte proliferation, IgE-ELISA inhibition, lipid transfer proteins.
Introduction
Onion belongs to the Allioideae, a sub-family in the
family of Amaryllidaceae that includes onion (Allium cepa),
garlic (A. sativum), chive (A. schoenoprasum), leek (A. am-
peloprasum) and rakkyo (Allium chinense) [1]. These plants
are largely used for culinary purposes worldwide and, par-
ticularly, in the Mediterranean diet. Onion can induce both
immediate and delayed hypersensitivity reactions. In fact,
in sensitized subjects, onion handling can induce not only
rhinoconjunctivitis and asthma but also contact dermatitis
[2]. However, despite its wide dietary use, only sporadic
reports exist on allergic reactions to onion ingestion [3–5].
Moreover, there have been no reports yet on anaphylactic
reactions following consumption of cooked onion.
In clinical practice, both skin reactivity towards food
allergens and food-specic immunoglobulins E (IgE) lev-
els in serum are used for the diagnosis and management
of food allergy [6]. Taken together, these two parameters
dene the global levels of allergen-specic IgE as skin re-
activity is quantitatively related to mast cell bound IgE.
Indeed, IgE interact via the Fc fragment with FceRI, ex-
pressed on different cell types, in particular mast cells.
FceRI is a high anity receptor that has a dissociation con-
stant (Kd) of 10–9 [6]. As a consequence, the vast majority
of IgE (i.e. the bound pool) is linked to tissue resident mast
cells. This larger bound pool of IgE can be assessed only by
the skin testing technique. The remaining IgE molecules
(i.e. the much lesser unbound pool) circulate in plasma. In
allergic subjects, the amount of unbound allergen-specic
IgE may be evaluated by ImmunoCAP technology [7, 8].
Three main allergens have been characterized for on-
ion: i) All c 3, a 12 kDa lipid transfer protein (LTP); ii) All c 4,
a prolin; and iii) All c, an alliin lyase. Allergenic cross-re-

Advances in Dermatology and Allergology 1, February / 2019
Immunological characterization of onion (Allium cepa) allergy
99
activity has been demonstrated between members of the
Allioideae family [9, 10]. Moreover, cross-reactivity with
other food allergens such as peach was postulated [3].
Aim
We describe herein a case of a 35-year-old man who had
an episode of severe systemic reaction to cooked onion inges-
tion, which required treatment at the emergency department.
We evaluated both mast cell bound and unbound onion-spe-
cic IgE levels (along with other food and respiratory aller-
gens) and assessed the possible cross-reactivity with peach.
Moreover, in allergic subjects, it has been recently dem-
onstrated that a subset of circulating allergen-specic B cells,
with a memory phenotype, exists. Importantly, these cells are
able to proliferate in response to the cognate allergen [10].
Based on this observation, to further characterize the on-
ion sensitization, we developed a cytouorimetric approach
seeking demonstration of proliferation of circulating B cell in
response to onion extract.
Material and methods
Patient
We studied a 35-year-old man who reported an epi-
sode of anaphylaxis/severe systemic reaction (intense
itching, urticaria, dyspnoea, dysphonia and malaise)
a few minutes after cooked onion consumption.
Quantitative skin testing
Skin testing was carried out in a strictly quantitative
fashion, in a single session, on the volar side of the fore-
arm. A collection of 34 commercial food allergen extracts
(Lofarma, Milan, Italy), 15 respiratory allergen extracts (ALK-
Abellò, Milan, Italy; on the back of the patient) and an in-
house semi-puried extract of onion were used (see below
for extract preparation). Both commercial and in-house
semi-puried onion extracts were tested in duplicate.
As for skin reactivity quantitative assessment, the area
of the wheals generated was calculated as described. In
order to achieve normalization, results (the duplicate for
onion) were expressed in terms of the ratio between the
wheal area and the histamine area, referred to as Skin Index
[7–9].
The quantitative skin prick testing procedure was also
carried out with both raw and microwaved cooked onion
and with garlic and Muscari comosum, a locally consumed
edible bulb [11].
As for the latter, we subjected 50 g of onion to heating
treatment in a Philips M305 microwave oven at 2540 MHz
for 7’.
Preparation of onion and peach “in-house” extracts
A suitable amount (50 g) of the edible part of fresh
A. cepa was washed properly, minced and homogenated
for 15’ by a Heidolph DIAX 900 homogenizer with a Hei-
dolph 10 F probe, at 25,000 rpm, on ice, in the presence
of 5 ml PBS 10X w/o Ca++, 200 µl ethylenediaminetet-
raacetic acid (EDTA) 0.5 M and 350 µl of plant cell-spe-
cic protease inhibitor cocktail (Sigma, Milan, Italy). The
inhibitor cocktail contained 4-(2-aminoethyl) benzene-
sulfonyl fluoride hydrochloride (AEBSF), 1,10-phenan-
troline, pepstatin A, bestatin and trans-epoxysuccinyl-
L-leucylamido-(4-guanidino)butane (E64), at unknown
concentrations. The homogenate was then centrifuged
at 12,000 × g, for 20’ at 4°C and the supernatant was re-
covered. This step was repeated two more times. Succes-
sively, the sample was ultracentrifuged at 100,000 × g,
for 2 h, at 4°C. Thus, 4 ml of 100,000 × g supernatant
were obtained from 50 g of fresh A. cepa. The extract was
kept at –80°C, until used.
The protein content was determined according to the
colorimetric Bradford method [12].
The same procedure was used for the generation of
the in-house peach extract.
Serum IgE measurement
Circulating IgE levels were measured by ImmunoCAP
technology (ImmunoCAP Thermo Fischer, Milan, Italy) in
serum collected from the patient.
Amino acid sequence identity assessment
The amino acid sequences of All c 3 (Q41258) and Pru
p3 (AAV40850) are available in GenBank. The percent-
age of sequence identity was calculated using “Needle”
program 6.0 version.
IgE-ELISA inhibition test
IgE-ELISA inhibition experiments were performed
to evaluate the possible cross-reactivity between onion
and peach extracts. To perform the IgE-ELISA inhibition
experiments, 80 µl of serum from the patient were pre-
incubated for 24 h at 4°C, while gently shaking, with
a series of 3 samples of 20 µl, containing either onion or
peach proteins in PBS w/o Ca++ and Mg++, 2.5 mM EDTA.
For both allergens, samples with an increasing protein
content were used. Thus, 0.4, 2 and 8 µg, respectively for
onion (auto-inhibition) and 0.23, 1.14 and 4.56 µg, respec-
tively, for peach (cross-inhibition) were loaded.
Thereafter, specic-IgE levels were assessed using an
ELISA-capture assay marketed by RADIM, Pomezia, Italy. To
this aim, an anti-IgE coated 96-well plate was used. Fifty µl
of the patient serum were incubated together with 100 µl
of either biotinylated onion allergen or biotinylated peach
allergen (both obtained from RADIM, Pomezia, Italy). After
incubation (1 h, at 37°C) and extensive washing, 150 µl of
Conjugate E2 was added to all wells and incubated for 30
min, at 37°C, in the dark. Upon washing, the addition of
100 µl of Substrate Solution revealed the presence of spe-
cic IgE. Optical density (O.D.) measurement was carried

Advances in Dermatology and Allergology 1, February / 2019100
Marcello Albanesi, Carlo Pasculli, Lucia Giliber ti, Maria Pia Rossi, Danilo Di Bona, Maria Filomena Caiaa, Luigi Macchia
out at l 450 nm. To this aim, a microplate reader (Biorad,
model 450, Milan, Italy) was used.
Peripheral blood mononuclear cells (PBMCs)
preparation, carboxyuorescein diacetate
succinimidyl ester labelling (CFSE) and ow cytometry
The PMCs were obtained from the patient and 2 non-
allergic healthy donors buy coats by dextran sedimen-
tation followed by centrifugation on Lymphoprep (Ny-
comed Pharma, Oslo, Norway) and hypotonic lysis of
contaminating erythrocytes.
Peripheral blood mononuclear cells (PBMCs) were la-
belled with 1 µM of the cytoplasmic dye carboxyuores-
cein diacetate succinimidyl ester (CFSE) (Sigma). CFSE-
stained PBMCs (2 × 106 cells/ml) were cultured in RPMI
media, with streptomycin (100 U/ml), and supplemented
with 5% autologous plasma. Antigen titration studies of
cultures incubated for 3 and 5 days, respectively, were
performed to optimize culture conditions. Thus, 2 × 105
cell samples were distributed to triplicate wells of a 96-
well at-bottom plate and cultured (200 µl nal volume)
in the presence of an in-house onion extract (10 µg/ml).
After 3-day culture, lymphocyte subsets were anal-
ysed by ow cytometry using: anti-CD3-Allophycocyanin
(eBioscience) or anti-CD19-Pacic Blue (eBioscience). Ac-
tively proliferating cells were distinguished by excluding
cells with high CFSE.
Ethics approval and consent to participate
This study was approved by the ethical committee of
“Ospedali Riuniti-Foggia”, Via Luigi Pinto 1, 70100, Foggia.
Oral informed consent was obtained and recorded in
the clinical le of the patient.
Results
Onion-specic IgE response
As shown in Figure 1 A, the skin prick testing tech-
nique revealed multiple allergenic sensitizations. In par-
Figure 1. Evaluation of onion-specic IgE. A – Quantitative skin prick tests for a commercially available array of food
allergens. B – Quantitative skin prick tests for a commercially available onion extract and semi-puried in-house onion
extract; prick tests with raw and cooked onion, raw garlic and M. comosum; C – circulating food allergen-specic IgE, as
measured by ImmunoCAP. The Skin Index represents the ratio between the area of the allergen wheal and the area of
the exogenous histamine reference wheal
10
8
6
4
2
0
Skin Index
Commercial
Raw onion
In-house
Cooked onion
Garlic
M. comosum
10
8
6
4
2
0
Skin Index
Wheat
Fennel
Bean
Hazelnut
Tomato
Orange
Garlic
Peanut
Almond
Pea
Celery
Peach
Onion
AB
9
8
7
6
5
4
3
2
1
0
IgE [kU/l]
Wheat
Fennel
Bean
Hazelnut
Tomato
Orange
Garlic
Peanut
Almond
Pea
Celery
Peach
Onion
C

Advances in Dermatology and Allergology 1, February / 2019
Immunological characterization of onion (Allium cepa) allergy
101
ticular, the Skin Index in response to the commercial
onion extract was 3.66 (Figures 1 A, B). Interestingly, the
Skin Index obtained in response to raw onion was 2-fold
higher compared to the commercial extract (i.e. 7.25).
Likewise, the Skin Index obtained after the skin test per-
formed with the in-house extract was 8.15 (Figure 1 B).
As mentioned before, the patient developed an ana-
phylactic reaction following the ingestion of cooked on-
ion. In order to test the allergenic activity of cooked onion
we also performed skin testing with cooked onion. Im-
portantly, although substantially reduced, cooked onion
was still able to induce a valid cutaneous response with
a Skin Index of 0.91 (Figure 1 B). Garlic (Figures 1 A, B)
and M. comosum (Figure 1 B) were also positive.
Finally, we evaluated the presence of soluble specic
IgE towards the same food allergens deemed positive
with skin testing, in the patient’s serum. Thus, circulating
IgE levels for all these allergens were detected (Figure 1 C).
The results obtained confirmed the poly-sensitization
status revealed with skin testing and conrmed the pres-
ence of onion-specic IgE (3.99 kU/l). Notably, the peach-
specic IgE level was 7.71 kU/l, as opposed to more mod-
est skin reactivity.
Skin tests for an array of 15 respiratory allergens
were also performed, revealing sensitizations for: Der-
matophagoides pteronyssinus (Skin Index: 2.5); D. fari-
nae (3.5); Olea europaea (2.2); Parietaria judaica (0.3);
Artemisia vulgaris (0.35); Cupressus arizonica (1.3). The
relevant RAST values were (in kU/L) 20.2; 11.5; 13.8; 0.42;
0.67; 0.30, respectively. Total IgE was 153 kU/l.
Onion and peach cross-reactivity assessment
Three allergens have been characterized for onion. In
particular, All c 3, one of the main onion allergens is a LTP
that might show cross-reactivity with other LTPs. Thus, in
the serum of the patient we evaluated the presence of
specic IgE for LTPs of the allergens that were deemed
positive in both skin tests and ImmunoCAP. In particu-
lar, as shown in Figure 2 A, we found IgE for Ara h 9
(11.2 kU/l; LTP from peanuts), Cor a 8 (1.07 kU/l; LTP from
hazelnut), Pru p 3 (12.3 kU/l; LTP from peach), Tri a 14
(3.4 kU/l; LTP from wheat), Art v 3 (2.18 kU/l; LTP from A.
absinthium) and Ole e 7 (14.1 kU/l; LTP from O. europaea).
In order to investigate the possible cross-reactivity
between onion and peach we performed IgE-ELISA inhibi-
tion tests with in-house semi-puried onion and peach ex-
tracts. Pre-incubation of the patient serum with increasing
concentrations of onion proteins abolished the anti-onion
IgE titre already after pre-incubation with 0.4 mg of pro-
teins (Figure 2 B). In contrast, the anti-onion IgE titre was
only scarcely reduced upon pre-incubation with an excess
of peach proteins (4.56 µg; Figure 2 C). Accordingly, inhibi-
tion of IgE-ELISA for peach with onion extract led to no
decrease in the peach-specic IgE titres (data not shown).
In order to corroborate these ndings, we assessed
the amino acid sequence homology of All c 3 and
Figure 2. Evaluation of onion cross-reactivity. A – Circulating LTP-specic IgE as measured by ImmunoCAP. B – IgE-ELISA
inhibition by onion proteins. Serum of the patient was incubated for 24 h at 4°C with the indicated amount of onion
protein from an in-house extract. C – IgE-ELISA inhibition by peach proteins. Serum of the patient was incubated for 24 h
at 4°C with the indicated amount of peach protein from an in-house extract. D – All c 3 and Pru p 3 amino acid sequence
identity. Vertical lines between the two sequences indicate the identical amino acid residues
mvrvvsllaastfillimiisspyansqnicprvnrivtpcvayg lgrapiapccralndlrfvntrnl rraacrclvgvvnrnpglrr nprfqniprdcrntfvrpfwwrpriqcgrinltdkliyldaee
maysamtklalvva lcmvvsvpiaqait-cgqassslapcipyv rgggavppac cngirnvn nlarttpdrqaacnclkqlsasvpgvnp n-naaalpgkcgvsi pykisastncatvk
15
10
5
0
8
6
4
2
0
8
6
4
2
0
IgE [kU/l]
Onion-specic IgE [kU/l]
Onion-specic IgE [kU/l]
Ara h 9
Cor a 8
Tri a 14
Ole e 7
Pru p 3
Art v 3
0 0.4 4 8 0 0.23 1.14 4.56
Onion extract [µg] Peach extract [µg]
AB C
D
All c 3
Pru p 3

Advances in Dermatology and Allergology 1, February / 2019102
Marcello Albanesi, Carlo Pasculli, Lucia Giliber ti, Maria Pia Rossi, Danilo Di Bona, Maria Filomena Caiaa, Luigi Macchia
Pru p 3. Interestingly, these two proteins share only 23%
of sequence identity (Figure 2 D).
B cell specically proliferate in response to onion
extract
The existence of circulating allergen-specic B cells
was previously demonstrated in the allergic subject.
These cells have a memory phenotype and are able to
proliferate upon the cognate allergen encounter [10].
In order to investigate this possibility, we sought for
onion-specic B cells in the patient blood. Patient lym-
phocytes were stained with CFSE and assessed for their
capacity to proliferate in response to the semi-puried
in-house onion extract (10 µg). B cells were identied on
the basis of the expression of the B cell marker CD19,
whereas T cells were identied by the CD3 marker (Figure
3 A). Importantly, CD19+ (Figure 3 B) but not CD3+ cells
(Figure 3 C) of the patient proliferated when incubated
with the semi-puried in-house onion extract (Figures
3 A, B). Proliferation was not observed in healthy controls
(either CD19+ or CD3+ cells).
Discusssion
Onion and other edible plants belonging to the Al-
lioideae family are widely used in the Mediterranean
diet, as well as in other diets, either raw or cooked. So
far, despite its large consumption, allergic reactions due
to onion ingestion remain anecdotal and, to our knowl-
edge, severe allergic reactions following cooked onion
ingestion have never been described. Moreover, it was
reported that patients that experienced an allergic reac-
tion to raw onion ingestion tolerated cooked onion [2–5].
In our work we describe a case of a severe allergic
reaction after cooked onion ingestion.
We studied the allergic response towards onion us-
ing dierent techniques. At rst we performed skin prick
tests in order to assess the mast-cell bound pool of on-
ion-specic IgE. However, although a limited number of
well-standardized food allergen extracts exist, most food
allergen extracts are little or non-standardized. Thus, the
skin reactivity and thereby the evaluation of the amount
of allergen specic IgE might be inuenced by the qual-
ity of the extract (e.g. protein concentration, protein de-
naturation etc.). Moreover, the composition of non-stan-
dardized, commercially available extracts may vary greatly
between manufacturers. With respect to onion, we tested
both a commercial onion extract and an in-house onion
extract and, in order to achieve normalization, we then
expressed the results obtained as a Skin Index, previously
dened as the ratio between the area of the wheal gener-
ated by the allergen and the area of the wheal generated
by exogenous histamine [7–9]. Importantly, we found that
CD 3
SS-C
Figure 3. Onion-specic B-cell proliferation. A – Gating strategy for CD 19 and CD 3 cells. Representative CD 19+ (B) and
CD 3+ (C) cell proliferation assessed by reduction of CFSE intensity for the patient compared to healthy controls. Experi-
ments were done in triplicates
ABPatient
Untreated Onion
Proliferating Resting
Proliferating Resting Proliferating Resting
CTRL
Onion
CD 19
Patient
Untreated Onion
Proliferating Resting
Proliferating Resting Proliferating Resting
CTRL
Onion
SS-C
C
CFSE intensity
0
0.5 0.4 3.9
7.9 0100
98.4 98.7 94.3
88.1 100

Advances in Dermatology and Allergology 1, February / 2019
Immunological characterization of onion (Allium cepa) allergy
103
the Skin Index obtained with the in-house onion extract
was higher compared to the one obtained with the com-
mercial extract. Our results conrm that extract quality
matters in food allergy diagnosis.
In order to test the allergenic activity of cooked onion
we then performed skin tests with both raw and cooked
onion. It is worth noticing that the allergenic activity of
onion after heating treatment was reduced but not lost,
suggesting the existence of one or more thermostable
components (Figure 1 B). Thus, among the characterized
onion allergens (All c 3, All c 4, All c), All c 3 belongs to
the LTP superfamily and should thereby be considered
thermostable [13–16]. In the case reported, this protein
could perhaps be responsible for the clinical symptoms
experienced by the patient.
As for the postulated cross-reactivity between onion
and peach, although some reports seem to convincingly
suggest that [3], our ndings indicate that this cross reac-
tivity is marginal since an excess of peach proteins inhib-
ited the IgE-ELISA for onion by approximately 30% only
(Figure 2 C). Amino acid sequence identity data (~23%)
seem to support this interpretation.
Diagnosis in food allergy may sometimes be diffi-
cult as pointed out by the skin test results (see above),
thereby new investigational tools may be helpful. It is
well known that production of IgE involves B cell and,
in particular, antibody-producing plasma-cells. Aside
plasma-cells, in pollen-allergic subjects, it has been re-
cently demonstrated that the circulating allergen-specic
B cell population exists [10]. Importantly, these cells have
a memory phenotype (i.e. CD19+, CD20+ and CD27+). Thus,
upon encounter with the cognate allergen, they can pro-
liferate and eventually give rise to antibody-producing
plasma cells. Taken into account these observations, in
order to conrm the onion sensitization in our patient,
we developed a lymphocyte proliferation test using ow
cytometry technique. In fact, we found for the rst time
an onion-specic CD19+ cell population, able to prolifer-
ate in response to onion extract (Figure 3 B). This latter
cell population is not present in healthy controls. These
ndings are further validated by the fact that similar ob-
servations could not be done with CD 3+ cells.
Conclusions
Our report shows that cooked onion can induce se-
vere allergic reactions that are likely due to the LTP All
c 3, a thermostable allergen. Moreover, we applied for
the rst time a B-cell-based approach to the diagnosis of
food allergy. Importantly, this latter approach might also
be applied to other allergic conditions such as hymenop-
tera venom allergy or drug allergy.
Conict of interest
The authors declare no conict of interest.
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