Opsonizing activity of anti-Aeromonas salmonicida antibodies after inactivation of complement in rainbow trout.

Young rainbow trout (Oncorhynchus mykiss) were immunized with killed Aeromonas salmonicida, bled 5 wk later, and tested for the presence of serum agglutinins. The positive antisera were pooled and fractionated in a Ultrogel AcA 34 column. Seven fractions were collected and concentrated. The higher agglutinin titres were found in the 2nd fraction, which also contained IgM revealed by immunoelectrophoresis. The 7 fractions were heated for 30 min at 45 degrees C to inactivate the complement system, and used to opsonize non-virulent cells of A salmonicida, which were subsequently put in contact with trout adherent cell cultures. Controls were carried out with non-heated immune and heated or non-heated normal sera. Non-opsonized bacteria and bacteria opsonized with heated normal trout serum or with fractions 4-7 were not phagocytized. Non-heated sera allowed observation of approximately 33% of phagocytic cells. A noticeable opsonizing activity (approximately 28% of phagocytic cells) was observed only in fraction 2, which contained the highest titres of specific antibodies. It appears that in the absence of complement opsonizing properties clearly persist in the fraction which contains specific antibodies.