Immunogenicity analysis of LrrG-Sip fusion protein of Streptococcus agalactiae in tilapia

To evaluate the immunogenicity of LrrG-Sip recombinant fusion protein, we immunized Nile tilapia (Oreochromis niloticus) by intraperitoneal (IP) injection with 200 μL LrrG-Sip recombinant fusion protein at dose of 0.5 μg·g^-1 (R1 group), 1.0 μg·g^-1 (R2 group) and 1.5 μg·g^-1 (R3 group), respectively. Meanwhile, control groups of Nile tilapia were IP injected with 200 μL Sip protein (1.0 μg·g^-1, S group), 200 μL LrrG protein (1.0 μg·g^-1, L group) and 200 μL sterile phosphate buffered saline (PBS, P group), respectively. On 15^th day, all of the immunized fish (groups of R, S, L and P) were challenged with Streptococcus agalactiae by IP injection at LD₅₀ dose of 100 μL (containing 4.0×10⁸ CFU·mL^-1). Results show that the relative percent survival (RPS) of fish from group R1 (0.5 μg·g^-1) was the highest (89.14 %). The OD_{450 nm} values of serum antibody of fish from group R1 were 0.63 and 0.64 on 14^th day and 28^th day post-immunization, respectively, significantly higher than those of the other groups (S and L)(P < 0.05).The activity of peroxidase and alkaline phosphatase of fish serum from this group was also significantly higher than that of the other groups at the above two time points (P < 0.05). There was no significant difference in the activity of lysozyme and total superoxide dismutase of fish serum among different protein immunization groups(P>0.05). The results suggest that LrrG-Sip fusion protein has better immunogenicity than single protein LrrG or Sip, which can also reduce the injection dose effectively.