Introduction:
Cat allergies are predominantly caused by the major cat allergen Fel d 1. Vaccination against Fel d 1 is an emerging approach for cat allergy immunotherapy, with the goal of generating strong Fel d 1-specific IgG responses, which protect from allergy. In this study, we assessed the immunogenicity of a novel vaccine based on Eggplant Mosaic Virus (EMV) virus-like particles (VLPs) engineered to display Fel d 1. Our aim was to compare the impact of administration routes and prime-boost strategies on immune responses in mice.

Methods:
Fel d 1 was genetically fused to the EMV capsid protein and expressed in E. coli. Mice were immunized with EMV-Fel d 1 VLPs, control EMV VLPs alone, or recombinant Fel d 1 protein using homologous or heterologous prime-boost regimens via subcutaneous or intranasal routes. Serum samples were collected to evaluate the Fel d 1-specific IgG responses using ELISA.

Results:
We found that both homologous and heterologous vaccination strategies led to strong Fel d 1-specific IgG responses, with levels increasing after the booster dose. The average log(OD₅₀) value rose from ~2.2 on day 14 (after prime) to ~3.6 on day 28 (after boost). By day 28, the IgG1, IgG2b, and IgG3 subclasses were prominently elevated, whereas IgG2a levels remained comparatively low. The avidity of Fel d 1-specific IgG improved following the booster doses. Interestingly, priming with the EMV-Fel d 1 VLPs resulted in robust IgG responses, regardless of the boosting agent—even when boosted with recombinant Fel d 1 protein.

Conclusion:
EMV-Fel d 1 VLP vaccines triggered robust and specific IgG responses, regardless of the prime-boost strategy used. These results support the continued development of EMV-based VLP platforms as a promising approach for cat allergy immunotherapy.