The adaptive immune system arose about 500 million years ago and is common in all vertebrates from sharks to humans. Vaccines protect against disease by stimulating the immune system to recognize and combat specific pathogens, such as viruses or bacteria. Vaccines contain either weakened or killed pathogens, or parts of them (such as proteins or polysaccharides), and work by activating the adaptive immune system so that specific antibodies and immunological memory are established. For the body's B cells to produce specific antibodies, they usually receive help from T-helper cells.
Until a few years ago, it was believed that evolution had preserved the adaptive immune system intact in all animals. DNA sequencing of the entire genome of the Atlantic cod (Gadus morhua) revealed that this species has lost important genes that are present in most other vertebrates: MHC class II, which is usually found on B cells, and CD4-positive T cells. These genes are necessary for cooperation between B and T cells, which means that cod have a different way of handling infections and developing antibodies. This finding explains why earlier vaccination attempts in cod had not shown the expected development of specific antibodies.
In our research, we have uncovered several fascinating and unique features of the cod's immune system, which are highly relevant for both aquaculture and our understanding of immunology. We have identified different types of lymphocytes, which are a type of white blood cell that plays a crucial role in the immune response. We have found five subgroups of T cells and eight variants of B cells. Some of these T cells are particularly interesting because they do not express the CD8 molecule, which is usually present in other fish and animals on T cells that do not have CD4. This diversity of cells could help explain how cod adapts to its unique environment and fights diseases.
We have also closely examined how cod respond immunologically to vaccines. Despite this fish species often showing an inconsistent reaction to vaccination, we have found that it can still produce high levels of IgM antibodies through a T cell-independent mechanism. This is significant for the aquaculture industry, which is looking for ways to enhance the fish's immune defenses through vaccination.
One of the most interesting parts of our work has been looking at how vaccination can protect cod from diseases. When we vaccinated cod against the bacterium Vibrio anguillarum, we observed that they began to develop specific IgM antibodies that provided them with protection against the disease. We isolated these antibodies from vaccinated cod and injected them into cod that had not been vaccinated before exposing them to the pathogenic bacteria. What we discovered was that cod that received IgM antibodies from vaccinated fish were protected from the disease, while those that received IgM antibodies from unvaccinated fish were not protected. This indicates that specific IgM antibodies are responsible for protection and that it is possible to develop vaccines that can help protect cod from common diseases that arise in aquaculture.
At the same time, we have explored the possibilities of genetically manipulating cod using modern technologies like CRISPR. To create a simple test system, we designed changes that would give fish larvae an albino-like appearance and developed an effective method for altering the cod's genes. This opens up new opportunities in both research and aquaculture, where we can focus on higher survival rates and better yields.
Overall, our extensive research on Atlantic cod provides new perspectives on the fish's immune system and demonstrates how genetic tools can be applied in aquaculture. By understanding how cod reacts to diseases and how it can be genetically improved, we hope to contribute to a more sustainable and efficient aquaculture in the future.
MHC class I and MHC class II genes for antigen presentation and antigen receptor genes and their mechanism of somatic diversification evolved 550-600 mya. Codfishes specifically lost MHC class II and function of the associated molecule CD4 100 mya, i.e. about 450-500 million years later.
Immunologic research of non-model organisms have been difficult due to lack of specific antibodies. We will inject partially purified cod leukocytes into mice and generate hybridomas for monoclonal antibody production and make polyclonal antibodies to synthetic peptide sequences derived from cod proteins. With these novel tools and with established technologies, VACSACOD will characterize the unique immune system of cod. We will isolate and purify leukocytes from cod and characterize T-cell responses to establish signatures of protective immunity to monitor vaccination regimes. We will determine the role of T cells in graft rejection and antibody responses. Pathogen-specific antibodies derived from B cells are essential for immune protection in other teleosts and mammals, but their role in codfishes is questioned. We will investigate how these antibodies contribute to protection in the established vaccine against vibriosis. We will determine whether cod B cells can present foreign antigen to T cells in spite of the lack of MHC class II in this species. We will determine the location in tissues of other antigen-presenting cells and investigate their function in vivo.
We will develop Atlantic cod immunologic research and vaccinology. Because of the unique immune system of cod, vaccinology based on research in model fish or other cold-water species is likely to come short in developing efficient vaccines for cod. Conversely, the immune system of cod could reveal novel mechanisms that also operate in other species but have escaped notice due to a minor role in MHC class II-positive organisms. Such mechanism may nevertheless be exploited for therapeutic benefit.
