The immune response of blue tilapia, Oreochromis aureus, to the parasitic dinoflagellate, Amyloodinium ocellatu

A simplified technique for the isolation and purification of serum immunoglobulin from fish is first presented. Immunoglobulin of an IgM-like class from the serum of the tilapine fish, Oreochromis aureus, was determined to have a molecular weight between 780,000-788,000 daltons. When reduced, the molecular weight of the heavy chain was estimated by SDS-PAGE analysis to be about 77,000 daltons, while the two distinct light chains had molecular weights of 21,500 and 20,500 daltons. Rabbit antiserum to the affinity-purified tilapia immunoglobulin did not react with sera from distantly related species of fish, but did cross-react with sera from more closely related African cichlids, including species in the genera Oreochromis, Haplochromis and Pseudotropheus.;An enzyme-linked immunosorbent assay (ELISA) using antibody to the affinity-purified O. aureus immunoglobulin and sonicated antigens from the parasitic dinoflagellate Amyloodinium ocellatum was developed to evaluate the immune response of the fish to the parasite. Fish immunized with antigens of either sonicated or live dinospores produced a specific immune response that was detectable by this ELISA. Serial dilutions of A. ocellatum antigen and fish anti-A. ocellatum serum were examined to determine which combinations provided optimal differentiation of seropositive from seronegative fish. Fresh and heat-inactivated serum had similar optical density values indicating that complement did not significantly affect the assay.;Finally, intraperitoneal immunization of O. aureus with A. ocellatum dinospores stimulated the production of an immune response such that serum from immune fish inhibited the infectivity and growth of the parasite in cell culture. Fresh serum had a greater inhibitory effect than heat-inactivated serum. Also, immunization with live parasites induced a much stronger immune response than immunization with dead, sonicated parasites. Parasite infectivity was inversely related to serum antibody ELISA titers. Serum from fish immunized with live parasites immobilized the infective dinospores at serum concentrations of 5% and greater by 5 minutes, as well as in lower serum concentrations at longer time intervals. Agglutination of the parasite occurred in immune serum concentrations from 2.5% to 0.156%.