Characterization of Piscirickettsia salmonis infection and technology development for optimal vaccines against intracellular fish pathogens

Piscirickettsiosis caused by the intracellular bacterium P.salmonis has been shown to cause mortalities up to 90 % in salmonide fish farms. Since its initial description in Chile in 1989, the disease has also been recorded in salmonids in Norway, Canada, Ireland and Scotland. In addition, Piscirickettsia- and Rickettsia-like organisms has also been shown to cause disease in a wide variety of fish species worldwide. P.salmonis is closely related to Francisella spp., another intracellular bacterium which ha s been shown to cause disease in Atlantic cod in Norway, both being members of the class Gammaproteobacteria.. Vaccine development against intracellular bacteria has proven to be very demanding, due to their complex pathogenesis. Antibody responses rarel y provide protection and may promote some aspects of the pathology as well as intracellular survival of certain bacteria. Therefore, we believe that it will be more relevant to identify virulence factors using proteomics than identifying antigens recogniz ed by antibodies from immunized fish or rabbits. We believe that increased knowledge about the progression of infection in cell culture and in fish and how P.salmonis evades the host immune system is essential in order to develop optimal vaccines with lon g term protection. We aim at further characterizing the intracellular bacterial infection and replication inside primary cell cultures. We also aim at developing methods for antigen release into the cytosol in order to increase presentation on MHC class I and activation of CD8+ T cells. Furthermore it is important to characterize the immune response in infected fish. Immunological responses following immunisation and challenge will be measured as expression of immune relevant genes by quantitative (real-t ime) reverse transcriptase PCR (qRT-PCR). Differences between high- and low virulent isolates will be examined using proteomic and genomic methods.