Piscirickettsia salmonis induces apoptosis in macrophages and monocyte-like cells from rainbow trout

Piscirickettsia salmonis is the etiologic agent of the salmonid rickettsial septicemia (SRS) which causes significant losses in salmon production in Chile and other and in other regions in the southern hemisphere. As the killing of phagocytes is an important pathogenic mechanism for other bacteria to establish infections in vertebrates, we investigated whether P. salmonis kills trout macrophages by apoptosis. Apoptosis in infected macrophages was demonstrated by techniques based on morphological changes and host cell DNA fragmentation. Transmission electron microcopy showed classic apoptotic characteristics and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling showed fragmented DNA. Programmed cell death type I was further confirmed by increased binding of annexin V to externalized phosphatidylserine in infected macrophages. Moreover, significant increases of caspase 3 activation were detected in infected cells and treatment with caspase inhibitor caused a decrease in levels of apoptosis. This is the first evidence that P. salmonis induces cell death in trout macrophages. This could lead to bacterial survival and evasion of the host immune response and play an important role in the establishment of infection in the host.