Palm oil-adjuvanted feed-based vaccination enhances humoral and macrophage responses of marine tilapia (Oreochromis sp.) against Vibrio harveyi and Vibrio alginolyticus.

This study evaluated the effect of a formalin-inactivated feed-based Vibrio harveyi vaccine on macrophage activities following challenge with live Vibrio spp. Three experimental groups, each comprising 105 marine tilapias, were established. Fish in Groups 1 and 2 were vaccinated with adjuvanted or non-adjuvanted formulations at weeks 0, 2, and 6, respectively, whereas Group 3 received PBS only. Serum samples were collected at 2-week intervals, whereas peritoneal macrophages were collected on weeks 0 (pre-vaccination) and 10 (post-vaccination). Serum IgM against V. harveyi and V. alginolyticus were measured by indirect ELISA. Cultured macrophages were exposed to V. harveyi and V. alginolyticus for 0, 30, 60, and 120 min. Following exposure, phagocytic activity, intracellular bacterial killing, and macrophage cell death were assessed. Serum IgM concentrations increased following the primary and first booster vaccinations and remained elevated up to week 4. Administration of the second booster sustained these elevated IgM levels through week 12 in vaccinated Groups 1 and 2. However, Group 1 developed significantly (p < 0.05) higher IgM levels against both V. harveyi and V. alginolyticus than Groups 2 and 3. Similarly, Group 1 demonstrated significantly (p < 0.05) higher rate of macrophage phagocytic activity than unvaccinated Group 3 at 30 min and 60 min after exposure to V. harveyi and V. alginolyticus, respectively. Group 1 also showed significantly (p < 0.05) higher intracellular killing of V. harveyi and V. alginolyticus than Groups 2 and 3 at 30, 60, and 120 min, but non-significant (p > 0.05) lower macrophage death rates at 30 min after V. harveyi challenge and at 60 min after V. alginolyticus challenge compared to Groups 2 and 3. In conclusion, the oil-adjuvanted, formalin-killed V. harveyi feed-based vaccine elicited effective systemic immunity, leading to enhanced macrophage-mediated phagocytosis and intracellular killing of V. harveyi and V. alginolyticus in marine tilapia, suggesting potential heterologous (cross-protective) immune responses against multiple Vibrio species.