Discussion
In this study, we aimed to evaluate the potential of EPIT to treat
cashew allergy. To that end, we first developed a clinically relevant
mouse model of anaphylaxis, for which IgE is the main mediator and in
which challenge is performed by oral administration of allergens,
similar to what occurs in humans. Using this model, we demonstrated that
epicutaneous patches are able to deliver cashew allergens to skin DCs,
especially Langerhans cells and cDC2 that have been demonstrated as the
main promotors of tolerance to topical allergens.14,15More importantly, EPIT with cashew patches affords a substantial level
of protection against IgE-mediated anaphylaxis to cashew. In this study,
we used a 50-µg dose per patch. This dose was selected based upon
preliminary experiments, showing that 100, 250 or 500 µg doses did not
afford additional benefits for protection (data not shown). This is, to
our knowledge, the first study demonstrating the efficacy of a prototype
immunotherapy against cashew allergy. Recently, Pereira et al. ,
evaluated the immunogenicity of orally-administered poly(anhydride)
nanoparticles loaded with cashew allergens in BALB/c
mice.16 These authors showed that nanoparticles were
able to promote a strong specific Th1 response to cashew in association
with an induction of FoxP3+ and LAP+ T regulatory cells (T-Reg).
Unfortunately, the clinical efficacy of this treatment has not been
reported. In our preclinical model, we showed that EPIT induced a
transient peak of cashew-specific IgE at the beginning of immunotherapy.
Interestingly, the same phenomenon has been observed in patients during
clinical trials investigating EPIT against peanut
allergy.17 Moreover, and in a similar way to what has
been observed in humans, this increase is followed by a gradual
progressive decrease of IgE titers with continued treatment.
Additionally, our results revealed that EPIT induces an increase of
cashew-specific IgG (IgG1, mouse equivalent of human IgG4 and IgG2a,
mouse equivalent of human IgG1). Again, these data should be viewed in
the context of clinical data investigating EPIT for peanut allergy, in
which a progressive increase of peanut specific IgG4 levels has been
observed during the first 12 months of treatment.17This increase in IgG4 response in humans is usually interpreted as a
beneficial impact of the treatment since IgG4 antagonizes effector
functions mediated by IgE through competition and neutralization of
allergens.18 The permanent increase of IgG2a observed
in our study should also be interpreted as a positive outcome since it
is the main marker of Th1 immune orientation in mouse, which is known to
be associated with non-allergic responses.19,20Mast-cells express a high amount of high affinity IgE receptors (FcεRI)
and are unequivocally considered as key players in IgE-dependent
anaphylaxis.21 Additionally, it has been previously
shown that IgE-dependent activation of mast-cells plays an important
role in disease induction in mouse models of
allergy.22,23 Here, we demonstrated that EPIT strongly
reduced mast-cell stimulation and degranulation following oral
challenge. Interestingly, this inhibition occurs despite a relatively
high level of IgE, suggesting that EPIT modulates mast-cell reactivity
to IgE signaling. Beyond the competition between IgE and IgG for
allergen binding we described above, this modulation could be linked to
an inhibition of the signal triggered by FcεRI as described in previous
studies.24,25 However, it is unlikely that this
desensitization results from constant stimulation of gut mast-cell with
small, sub-activating amounts of allergen, since EPIT does not lead to
systemic diffusion of allergen. Therefore, we are currently performingin vivo experiments to elucidate the mechanisms underlying this
modulation and the induction of tolerance to cashew, with a special
focus on regulatory T-cells which have been demonstrated as key players
in murine models of peanut allergy.26,27 Despite
almost complete suppression of mMCP-1 and mMCP-7 levels in blood
following 8 and 12 weeks of treatment, EPIT mice still presented
substantial anaphylactic symptoms following oral challenge. Therefore,
we cannot exclude a role for specific IgG in anaphylactic reactions, as
suggested by Figure 2. Consequently, our mouse model may underestimate
the potential efficacy of EPIT in humans, for which the role of IgG in
anaphylaxis is still a matter of considerable debate.
EPIT may offer a safe approach to treat food
allergy,28 primarily due to the absence of systemic
dissemination of allergens, thus avoiding direct contact with
anaphylaxis triggering effector cells, e.g. mast-cells. Therefore, we
suggest that EPIT may be a relevant treatment strategy for cashew
allergy. Indeed, the unique severity of reactions to cashew reported in
allergic patients could be a source of concerns regarding other
immunotherapeutic strategies that require subcutaneous injections or
oral administration of allergens. A recent study demonstrated that many
patients suffering from cashew allergy are IgE-sensitized to other nuts
of the Anacardiaceae family.29 Thus, it would
be interesting in future experiments to evaluate the capacity of
cashew-specific EPIT to afford cross-protection against anaphylaxis to
pistachio or mango, and therefore determine its spectrum of coverage
among Anacardiaceae family members.
Overall, our data demonstrate that EPIT could be a relevant strategy to
treat cashew allergy, bringing hope to patients suffering from this
potentially life-threatening immune dysregulation.