Immunoregulation in fish II: intermolecular-induced suppression of antibody responses studied by haptenated antigens in atlantic salmon (Salmo salar L).

Here we report evidence for T cell dependent intermolecular-induced suppression of antibody responses in fish, using a panel of T cell dependent (TD) and T cell independent (TI) hapten-carrier antigens. Atlantic salmon were immunized intraperitoneally either with protein antigens: Limulus polyphemus hemocyanin (LPH), chicken gammaglobulin (CGG), A. salmonicida surface A-layer protein (ALPAsal) or lipopolysaccharide (LPS) antigens isolated from A. salmonicida and Escherichia coli. The various antigens were given as a mixture of the native and haptenated (4-hydroxy-3-iodo-5-nitrophenyl-acetic acid, NIP; 2,4,6-trinitrophenyl-acetic acid, TNP; fluorescein-5-iso-thiocyanate, FITC) forms. The salmon immune system responded to the antigen mixtures by eliciting high anti-hapten titers whereas the antibody titers against protein determinants were low (suppress 65-95%) as determined by ELISA. The suppression was induced between haptens (NIP and FITC) and between heterologous antigens (NIP-CGG and LPH) indicating that the mechanisms involved were non-specific. Moreover, suppression was induced by TD antigens only, indicating that the mechanism was T cell dependent. Injection of antigen mixtures containing variable amounts of the competing antigens showed that the kinetics of suppression was dose-ratio and dose dependent. In a time-course study it was found that the suppressed anti-LPH response was unchanged until native LPH was injected almost 2 years after the primary immunization, showing that permanent tolerance had not been induced. Sequential immunization showed that the antibody response was most sensitive to suppression during the initial 10 days after immunization. Moreover, the carrier antigen was also able to induce suppression of hapten epitopes, but only if the anti-carrier response was allowed to develop for 14 days before the hapten-carrier antigen was injected. This shows that AIS in fish is elicited as a result of the immune response to the dominant antigen, and can be induced against either antigens if the temporal order of administration is reversed. A possible model for AIS as a normal immunoregulatory process in fish is proposed and discussed.