Rational design of vaccination strategies for Atlantic cod (VACSACOD)

Immunological memory describes the phenomenon that the immune system remembers an invading bacterium or virus and responds with a strong and effective defense the second time the same microbe attacks. B and T lymphocytes that are activated by a disease-causing bacterium will form memory cells, which recognize it if it reappears, thus effectively repelling a new attack. This adaptive immune system originated about 500 million years ago and is common in all vertebrates from sharks to humans. Until a few years ago, it was thought that evolution had kept the adaptive immune system intact in all animals, but DNA sequencing of the entire genome of Atlantic cod made it known that this species has lost genes that lead to severe immune failure if lost in mice or humans. In this project, we investigate how cod are able to obtain immunological memory without these genes. We have established a vaccine protocol that demonstrates that cod has a protective immunological memory. We are working to map the expression of all genes in individual cells in the immune system of cod, in the normal state, during vaccination and during a serious infection. With advanced bioinformatics tools, we will find out which cell types respond to the vaccine and the nature of the response. In humans and other animals, vaccines usually elicit strong antibody responses that characterize immunological protection, but cod do not have good antibody responses to vaccines. Therefore, we will look at other immunological responses and determine what kind of response correlates with an effective vaccine. Such knowledge is necessary to develop better vaccines for Atlantic cod, which is necessary for effective commercialization of cod farming.

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.