Development and characterization of an innovative Flavobacterium oreochromis antigen-encapsulated hydrogel bead for enhancing oral vaccine delivery in hybrid red tilapia (Oreochromis spp.)

Traditional oral vaccines often face several challenges, such as antigen degradation and poor immune activation due to harsh gastrointestinal conditions. To address this, an effective oral vaccine was developed using hydrogel beads made of sodium alginate (SA), calcium bentonite (BN), and chitosan (CS) to generate microspheres that can encapsulate F. oreochromis (Fo) cells as an oral vaccine (SA/BN/CS hydrogel Fo-OV) for hybrid red tilapia. This study demonstrates that oral administration of a Fo-encapsulated hydrogel bead vaccine (Fo-OV hydrogel) can effectively protect the antigen from gastric degradation and release it to fish intestine, as indicated by in vitro studies showing color and structural changes under different pH conditions, allowing the controlled antigen release. This led to significant enhancements in both systemically and predominantly mucosal immune responses in hybrid red tilapia (Oreochromis spp.) after 14 and 28 days post consecutive 7-day vaccination. The vaccine increased the specific IgM antibody to F. oreochromis in serum and mucosal tissues, and increased the expression of immunoglobulin repertoire genes, IgM, IgT, and IgD in mucosal tissues, indicating activation of the fish immune system. Histological examinations revealed the beneficial changes in the intestinal mucosa by increasing the thickness of lamina propria and causing an increasing number of Goblet cells without any pathological abnormalities in the liver or intestines. Moreover, the Fo-OV hydrogel group showed significantly higher survival rates after the challenge with F. oreochromis than control groups and traditional formalin-killed Fo vaccine. These findings suggest that the hydrogel formulation can protect the antigen from degradation and enhance their uptake by mucosal-associated lymphoid tissues (MALTs), leading to stronger and more specific immune responses. The outcome of this research could be an invaluable tool as the potential application of hydrogel-based vaccines in aquaculture to improve fish health and disease resistance.