4∣Discussion
This is the first study to demonstrate an epicutaneous anti-allergic DNA
vaccine that is effective in treating an established allergic condition
and preventing the development of an allergic condition using biting
midge allergy as a model. After epicutaneous DNA vaccination, not only
the For t 2 allergen-induced itchy response decreased significantly, the
biomarkers for allergic inflammation, including allergen-specific IgE,
allergen-specific IgG2a, allergen-challenge induced eosinophil
infiltration in the skin, Th2 cytokines from the splenocytes, and
regulatory T cell-related transcription factors also suggested that
immune tolerance was induced after three patches of the epicutaneous DNA
vaccine.
It is known that only lipophilic molecules with a molecular weight below
500 Dalton are able to pass though the stratum corneum of the
skin.23 Therefore, vaccination into the dermal
component usually requires penetration techniques, such as microneedles,
jet injection, DNA tattooing, thermal ablation, or addition of chemical
enhancers to modify the stratum corneum.6,24-26 Our
data showed that though the molecular weight of the For t 2 DNA vaccine
is as high as 4000 base pairs, it is able to penetrate the dermal
barrier and translates the corresponding protein in the targeted skin as
well as the spleen of the vaccinated mice. It is possible that the DNA
vaccine passes the epidermis via the hair follicles as the skin is
tape-stripped before epicutaneous vaccination.27Further experiments may be required to explore the delivery mechanism of
this epicutaneous DNA vaccine.
This epicutaneous anti-allergic DNA vaccine induced a similar immune
response to that of the intra-muscular anti-allergic DNA vaccine
investigated in our previous studies.20,21 As we did
not compare the effect of intra-muscular and epicutaneous DNA vaccine
directly in this study, it is not known whether the epicutaneous DNA
vaccine is superior to the intra-muscular modality with regard to its
effectiveness. However, the convenience of epicutaneous delivery is
certainly a major advantage.
This study did not address the duration of effectiveness of the
epicutaneous DNA vaccine and whether it is comparable to the
intra-muscular modality or the conventional protein-based vaccine.
Further studies on the duration of the protective effect of epicutaneous
DNA vaccine are required.
Recent studies surveying the conditions of allergen-specific
immunotherapy among allergic clinics during the COVID-19 pandemic showed
that a significant number of patients discontinued their
ASIT28 and up to 60% of allergists stopped receiving
new ASIT patients.4 This situation highlights the need
for an easily administered anti-allergic vaccine with a short course,
such as the epicutaneous DNA vaccine presented in the current study.
In conclusion, we demonstrate the first needle-free epicutaneous DNA
vaccine patch that is effective in preventing as well as treating biting
midge allergy in a murine model. The mice showed improvements in their
allergic condition both clinically as well as in terms of allergic
inflammation. The mode of this anti-allergic DNA vaccine may have the
potential for application in specific immunotherapies for other
allergens.