Serum amyloid A (SAA) is an integral part of the innate immune response in general and in particular the acute phase response. SAA belongs to a highly conserved group of apolipoproteins reported from different groups of organisms such as mammals, birds, fish and even invertebrates.
The present study was undertaken to elucidate the role of SAA protein in the innate immune response of rainbow trout. For this purpose a monoclonal antibody was raised against a recombinant peptide of rainbow trout SAA. The antibody was characterized using Western blot, immunohistochemistry and ELISA techniques. SAA was found to be associated with high density lipoprotein (HDL) which complicated band identification in Western blot, but delipidization of the SAA-HDL isolate, using a solvent extraction method, highly increased the quality of reaction in the Western blot. Inhibition ELISA indicated the presence of SAA in serum and tissues (head kidney, liver and spleen) of rainbow trout. Rainbow trout fry (87 days post hatch) infected with Yersinia ruckeri showed a significant up-regulation of the SAA gene at 72 h post infection with further increase at 96 h post infection. Non-significant up-regulations were seen at earlier time points i.e. 4 and 24 h. A weak staining with the monoclonal antibody was seen at 4 h post infection which increased in intensity during the course of infection i.e. 24, 72 and 96 h post infection. The expression pattern of SAA in the infected fry, analysed by qPCR, significantly correlated with the results obtained by immunohistochemical methods. From the present study it can be concluded that the SAA may act as an acute phase protein in rainbow trout and its expression increases significantly during the course of infection.