The non-specific activation of rainbow trout, Salmo gairdneri Richardson, complement by Aeromonas salmonicida extracellular products and the correlation of complement activity with the inactivation of lethal toxicity products

Abstract The possible mechanism of inactivation of the toxicity of Aeromonas salmonicida extracellular products (ECP) by normal rainbow trout serum was investigated using juvenile rainbow trout. ECP was prepared from culture supernatant by an acetone precipitation method. The ECP was incubated with normal rainbow trout serum at 20°C for 2 h, and the interrelationship between ECP proteolytic activity and immune complex-initiating, haemolytic complement activity (CH₅₀) of normal serum against antibody-sensitized goldfish red blood cells was evaluated. When normal serum was incubated with increasing concentrations of ECP, the CH₅₀ activity of serum decreased. The CH₅₀ activity was completely abolished in serum treated with undiluted ECP. ECP treated with serum was administered to trout intraperitoneally to determine mortality. All the fish receiving untreated ECP (0.05 ml = 0.5 mg protein) alone died within 24 h. When ECP was treated with serum at 1:1 to 4:1 (serum: ECP) in volume a similar high mortality was produced. These inocula possessed high protease activity and no or low CH₅₀ activity. However, mortality decreased and finally no mortality was recorded as ECP was treated with large volumes of serum (9:1 to 19:1). These inocula had lower protease activity and considerably higher CH₅₀ activity. Fish receiving ECP treated with heat-inactivated serum at 19:1 showed 100% mortality. A serum: ECP inoculum derived from fish which had been administered lipopolysaccharide from Salmonella enteritidis and which possessed a low CH₅₀ activity also gave a high mortality when used at 19:1. These results suggest that rainbow trout complement is implicated in the inactivation of toxicity of A. salmonicida ECP.