Development of a Tetrahymena thermophila-Based Vaccine Expressing Miamiensis avidus Ciliary Proteins to Combat Scuticociliatosis.

Miamiensis avidus is a parasitic pathogen responsible for scuticociliatosis, a lethal infection affecting marine fish worldwide, including Japanese flounder. Immunisation with formalin-killed M. avidus has shown promise in inducing protective immunity, positioning it as a potential vaccine candidate against scuticociliatosis. However, challenges such as the high cost of producing sufficient cells and inconsistent quality due to the lack of cryopreservation methods hinder its development. In this study, we expressed M. avidus ciliary proteins in Tetrahymena, a culturable ciliate, and used these cells to immunise Japanese flounder. The immunised fish produced antibodies against M. avidus. Additionally, immunisation with two transgenic Tetrahymena strains, each expressing different ciliary proteins, induced the production of antibodies against two serotypes of the parasite. In challenge experiments, fish immunised with the transgenic Tetrahymena showed prolonged survival compared to the control group, highlighting the potential of this approach as a vaccine candidate. These findings suggest that transgenic Tetrahymena cells could be a viable platform for developing vaccines against multiple serotypes of M. avidus.