Multiple whole bacterial antigens in polyvalent vaccine may result in inhibition of specific responses in rainbow trout (Oncorhynchus mykiss).

The goal of fish vaccination today is to protect fish against multiple bacterial fish pathogens simultaneously using polyvalent vaccines. However, many immunological processes such as antigenic cross-reaction, antigenic competition, affinity maturation and antigen-induced suppression may affect the specificity, avidity and level of antibodies. Consequently, the biological function of antibodies may be markedly different from that predicted by conventional serologic tests. Here, we investigated the effects of vaccination and composition of vaccine on the plasma antibody levels, biological function of antibodies in opsonophagocytosis as well as the effects of vaccination on the blood leucocyte counts. Rainbow trout were vaccinated with saline or with two different polyvalent, mineral oil-adjuvanted vaccines. Vaccine 1 contained Aeromonas salmonicida, Listonella anguillarum and both Th and Fd serotypes of Flavobacterium psychrophilum antigens and vaccine 2 contained A. salmonicida, L. anguillarum and only Fd serotype of Fl. psychrophilum. The antibody-mediated opsonophagocytosis was determined as the respiratory burst (RB) activity of blood monocytes and granulocytes against the tested bacterial antigens. Three weeks after vaccination both vaccine groups and the control group showed increased RB activity against all bacterial strains. However, the increase in RB activities was non-specific and originated from the increased number of circulating granulocytes and monocytes. On the other hand, at 6 weeks post-vaccination both specific antibodies and antibody-dependent opsonophagocytosis appeared in both vaccine groups. However, the composition of the vaccine had a marked effect on the magnitude of specific responses. The Fd+Th vaccine enhanced the target specific opsonophagocytosis, to a lesser extent than the Fd vaccine. Both polyvalent vaccines appeared to mainly affect the numbers of circulating monocytes and our results suggest that the monocytes play a more significant role than the granulocytes in antibody-dependent opsonophagocytosis. Our results also suggest that the presented opsonophagocytic assay is an advantageous method to predict vaccine efficiency and that the number, and properties, of bacterial antigens in polyvalent vaccines should be carefully selected in order to avoid inhibitory effects of antigens on the specific response of fish.