Complement-mediated opsonisation and phagocytosis of Renibacterium salmoninarum.
R. salmoninarum depletes complement-mediated haemolytic activity of human and chinook salmon (Oncorhynchus tshawytscha) serum, and the extent of depletion depends on bacterial density. The bacterium also consumes human, chinook salmon, channel catfish (Ictalurus punctatus), and rainbow trout (O. mykiss) complement in the presence of Mg-EGTA indicating the involvement of the alternative complement pathway. Bacteria incubated in human serum with or without Mg-EGTA cause human erythrocytes to agglutinate. This immune adherence is blocked by incubation of the opsonised bacteria in the presence of a goat anti-human C3 antibody or incubation of the human erythrocytes with a monoclonal antibody against the human complement receptor CR1. Both results indicate that the bacteria have been opsonised with C3b. Furthermore, incubation of R. salmoninarum in haemolytically-active rainbow trout serum is required for the adherence of the bacterium to rainbow trout kidney macrophages before their phagocytosis.