Antibody responses of turbot Psetta maxima against various antigen formulations of scuticociliates Ciliophora

The kinetics of the antibody production and the protection at challenge were studied in turbot inoculated with various scuticociliate antigen preparations: live ciliates putatively attenuated through long-term in vitro culture (Trial 1) and formalin-killed ciliates without or with GERBU adjuvant in Trials 2, 3, and 4. Antigen used in killed preparations was a mixture of 3 different ciliate isolates (V3) in the case of Trials 2 and 3, whereas in Trial 4, monovalent (V1), trivalent (V3) or pentavalent (V5) antigens were used. A booster injection was administered 28 to 29 d post-priming in all trials. Fish were challenged with virulent live ciliates after the immunization protocol, testing 2 challenge times in Trial 2 (t1 and t2). No protection was obtained in Trial 1 with live ciliates, which in turn were not completely attenuated. Using killed-ciliate formulations, protection was high only in Trial 3 when a low dose (50 000 ciliates fish(-1)) was used for challenge. In Trial 1, heat-inactivated sera of antigen-inoculated fish agglutinated the homologous ciliate, although no specific antibodies were detectable by ELISA. In contrast, high specific antibody levels were detected in antigen-inoculated fish in Trials 2 and 4, and their amount increased progressively, usually peaking after challenge. No advantage was obtained from the use of V5 antigens compared to V1 or V3. No good correlation was observed in most cases between serum antibody levels and protection. Although the use of GERBU adjuvant generally increased the specific immune response, some undesired side effects indicate a need to adjust dosage and/or improve the formulation.