Immunoregulation in fish. I: Intramolecular-induced suppression of antibody responses to haptenated protein antigens studied in Atlantic salmon (Salmo salar L).

We report here evidence for intramolecular-induced suppression of the in vivo antibody response in fish, using a panel of T-dependent hapten-carrier antigens. Atlantic salmon were immunized intraperitoneally with protein antigens (Limulus polyphemus hemocyanin, chicken gamma globulin, and Aeromonas salmonicida A-layer protein) given in their native form or haptenated with either 4-hydroxy-3-iodo-5-nitrophenyl-acetic acid (NIP), 2,4,6-trinitrophenyl-acetic acid (TNP), or fluorescein-5-iso-thiocyanate (FITC). The salmon immune system responds to these hapten-carrier antigens by eliciting high anti-hapten titers whereas the antibody titers against protein determinants were suppressed 87-99%, determined by ELISA. NIP also induced suppression of the anti-FITC response when NIP and FITC were intramolecularly conjugated to Limulus polyphemus hemocyanin (LPH). The suppression was found to be independent of haptenation ratios and time after immunization. The possibility that haptenation interferes with or blocks the protein determinants is not likely because antisera raised against native LPH recognize LPH-specific epitopes even on heavily NIP-substituted LPH. Although the mechanism behind intramolecular-induced suppression is poorly understood, even in mammals, this study demonstrates that intramolecular-induced suppression may be one means by which antibody responses in fish are regulated. The possible impact of antigen-induced suppression on immune responses against vaccine antigens in fish is discussed.