Sustained T cell-independent type 2 antibody response in a naturally MHC II-deficient teleost fish

Atlantic cod (Gadus morhua) is well-known to show poor specific antibody responses after immunization, despite having high natural IgM levels. Unlike mammals and nearly all examined other vertebrates, cod lack the MHC II-CD4 axis required for T cell-dependent (TD) antibody responses. We evaluated the cod's antibody response to hapten carriers inducing T cell-independent (TI) and TD responses in mammals. Remarkably, cod generated strong hapten-specific IgM levels against hapten linked to the TI-2 antigens ficoll or dextran. Specific IgM levels plateaued 6–9 weeks after a single immunization and remained stable for six months. The serum half-life of IgM was 4 days, indicating continuous IgM production during this period. These findings reveal unique immune mechanisms in Atlantic cod, enhancing our understanding of vertebrate immunology and aiding the development of novel vaccination strategies in aquaculture.