Acquired and innate resistance to the haemoflagellate cryptobia salmositica in sockeye salmon (Oncorhynchus nerka)

Juvenile sockeye salmon (Oncorhynchus nerka, Fulton River stock) were protected from otherwise lethal challenges with the haemoflagellate Cryptobia salmositica by acclimation to elevated water temperatures (20 degrees C). Fish treated in this manner displayed increased immunity to C. salmositica and yielded plasma showing enhanced lytic activity against the parasite. The acquired lytic activity was antibody- and complement-mediated. In contrast, a stock of naive O. nerka from Weaver Creek, previously identified as having a high innate resistance to the lethal effects of C. salmositica, also had plasma factors that destroyed the parasite in vitro. This anti-Cryptobia activity also involved complement because 1) it resulted in lysis of the parasite, 2) it was heat-labile (40 degrees C for 20 min), and 3) it was largely removed from the plasma by substances capable of activating (binding) complement by the classical pathway (an antigen:antibody complex of Renibacterium salmoninarum and its specific antibody) and the alternate pathway (Escherichia coli lipopolyssacharide). The complement-mediated lysis associated with innate resistance was apparently the result of activation by the alternate pathway because it occurred in fish lacking antibodies against the parasite. The reaction was unusual in that a long incubation period (about 2 days) was required for maximum lysis of the parasite. At least one component of the innate lytic system depended on disulphide bonds because lytic activity was destroyed by 2-mercaptoethanol.