RESUMEN
Aeromonas caviae is a zoonotic pathogen that can cause disease in aquatic organisms and mammals, including humans, and it is widespread in nature, especially in freshwater environments. Previous research has reported that extracellular products (ECPs) secreted by pathogens during growth are effective protective antigens that can induce the host immune response and protect the host from pathogens. However, little is known about how ECPs enhance immunity. Here, we prepared extracellular products by the cellophane plate method, determined the total protein concentration, and analysed the protein composition of the extracellular products by SDS-PAGE. Subsequently, their enzyme activity and pathogenicity were evaluated separately. Crucian carp were randomly divided into four groups to receive formalin-inactivated A. caviae vaccine (FKC), ECPs mixed with the same amount of Freund's complete adjuvant, the same amount of ECPs mixed with an equal volume of A. caviae inactivated vaccine (FKC + ECPs), sterile PBS alone via intraperitoneal injection. On Days 7, 14, 21, and 28 after immunization, the expression levels of IgM, SOD, and CAT and the lysozyme (LYS) activity in the serum were detected by ELISA, and the relative expression levels of the TNF-α, IFN-γ, IL-1ß, and IL-10 genes in the liver, kidney, spleen, intestine, and gills were measured by qPCR. The extracellular products generated five clearly visible protein bands and exhibited lipase, protease, amylase, DNase and lysozyme but no urease or lecithinase activities. In addition, the median lethal doses of A. caviae and ECPs to crucian carp were 411.64 µg/fish and 1.6 × 105 CFU/mL, respectively. Compared with those of the control group, the IgM, SOD, and CAT contents and serum LYS activity were significantly increased in the experimental groups, and the qRT-PCR results showed that the relative expression levels of TNF-α, IFN-γ, IL-1ß, and IL-10 genes in the liver, kidney, spleen, and intestine were significantly increased after injection immunization. In addition, the relative immune protection rates of the three experimental groups were 60%, 65%, and 45%, all of which were significantly higher than those of the control group. Collectively, our findings show that the extracellular products of A. caviae can be used as a vaccine to significantly improve the immune level of crucian carp and have obvious anti-infection ability. This may represent a promising approach to prevent and control infection by A. caviae and provides strong theoretical support for the development of new inactivated vaccines.