Gene expression and functional characterization of serum amyloid P component 2 in rock bream, Oplegnathus fasciatus

Mammalian serum amyloid P component (SAP) recognizes a wide range of exogenous pathogenic substances and activates a complementary pathway leading to pathogen clearance. To determine the potential roles of SAP in the fish immune system, SAP (RbSAP2) gene was cloned from ESTs analysis of rock bream (Oplegnathus fasciatus), which consisted of a signal peptide and pentraxin domain. Phylogenetic analysis revealed that the RbSAP2 gene was classified with other known fish SAPs. RbSAP2 was highly expressed in the liver of healthy rock bream. Overall, pathogen exposure led to an induction of RbSAP2 in the liver and spleen, although this effect was not observed in the spleen following infection with Edwardsiella tarda. A high concentration of recombinant RbSAP2 (rRbSAP2) showed lower growth Streptococcus iniae than control in the absence of Ca(2+), whereas E. tarda growth was decreased by high concentration of rRbSAP in the presence of the Ca(2+). These results suggest that RbSAP plays an important role in the immune response against invading pathogens.