Development and Efficacy of a Chitosan Nanoparticle-Based Immersion Vaccine Targeting Segment 4 of Tilapia Lake Virus

Tilapia lake virus disease (TiLVD), which is caused by tilapia lake virus (TiLV), has resulted in significant damage to global tilapia farming. TiLV is a negative-sense single-strand RNA virus consisting of 10 genome segments. To date, no commercial vaccine against TiLVD has been developed, and effective strategies to control and prevent TiLVD are lacking. In this study, we developed and tested a chitosan nanoparticle-based immersion recombinant protein targeting segment 4 (S4) of TiLV under both laboratory and field conditions. The open reading frame of S4 of TiLV was cloned into pET28a (+) and expressed by Escherichia coli BL21(DE3). The size of the nanoTiLV-S4 (CNS4) vaccine was 284 ± 9.2 nm, which is smaller than the pre-nanoencapsulation vaccine size of 2268 ± 41.8 nm. Transmission electron microscopy revealed that the nanoS4 particles had a round shape, uniform appearance and positive zeta potential of 17.7 ± 0.7 mV. Further analysis showed that the nanoS4 antigen was deposited on the fish gills and intestines and taken up into the epithelial cells within 30 min of immersion. Under laboratory infection using a cohabitation challenge model, the CNS4 vaccine demonstrated a relative percent survival (RPS) of 25%. In field conditions, the vaccine showed an RPS of 31.88% compared to the unvaccinated group. Overall, our study demonstrates that the new nanoTiLV-S4 vaccine can be absorbed by the fish epithelium and reduces mortality caused by TiLV. However, further optimisation and field trials are necessary to improve the efficacy of the CNS4 vaccine and to test it under various farm conditions.