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.