Hypoallergen-Encoding DNA Plasmids As Immunoprophylactic Vaccines of Shrimp Tropomyosin Hypersensitivity

Background: Shellfish is the second most common food allergen that causes sensitization in children under 6 years old. Despite its prevalence, clinical management of shellfish allergy is limited to conventional approaches and avoidance. Although tropomyosin has been identified as the major shellfish allergen, no allergen specific immunotherapy (SIT) currently exists to treat or prevent shellfish allergies. To investigate the possibility of DNA vaccines, we constructed two hypoallergens of the shrimp tropomyosin Met e 1: MEM49 and MED171 (Wai et al. 2014 PLoS One 9: e111649) and expressed them in plasmid pCI-Neo. Here we used an established mouse model of shrimp hypersensitivity to examine the immunoprophylactic potential of these two hypoallergen-based DNA vaccines.

Methods: 3-4 week old Balb/c mice were randomly divided into five groups (n = 8 per group). Two groups were intradermally injected with 100 μg pCI-Neo clones of MEM49 or MED171 thrice at weekly interval. The remaining groups were injected with naked pCI-Neo or PBS and served as vector, PBS or negative controls. One week after the last injection, all groups (except the negative control mice which were injected with PBS throughout the experiment) were sensitized subcutaneously with Met e 1 adsorbed to Freund’s complete and incomplete adjuvant and then orally challenged using a high dose of Met e 1. Blood, spleen and small intestine were collected for antibody, cytokine, gene expression and histological analysis.

Results: The PBS and vector control groups displayed typical Th2 responses upon Met e 1 challenge. These mice exhibited high levels of specific IgE and Th2-linked cytokines (IL-4, IL-5 and IL-13), as well as inflammatory responses (mast cell and eosinophil infiltration, goblet cell hyperplasia) in the gut. In contrast, vaccinated groups did not show any systemic allergic symptoms or inflammatory responses in the gut upon challenge. Met e 1-specific IgE and Th2-linked cytokines in these mice remained at basal levels, with the MEM49-encoding DNA vaccine providing more robust protection against Th2 responses. The protection offered by both vaccines included higher levels of specific IgG2a antibodies that possess both in vitro and in vivo blocking abilities, as well as higher splenic levels of Th1-linked cytokines (IL-12 and IFN-γ). MEM49 vaccination increased expression of TGF-β in the small intestine, while MED171 vaccination increased Foxp3 expression. 

Conclusions: Hypoallergen-based DNA vaccines could effectively protect against tropomyosin sensitization in mice via the establishment of Th1-oriented responses, recruitment of regulatory T cells, and induction of blocking IgG antibodies.

[This work was fully supported by the Health and Medical Research Fund (02130206), HKSAR Government.]