In vitro interactions between rainbow trout (Oncorhynchus mykiss) macrophages and Vibrio anguillarum serogroup O2a

The sensitivity of Vibrio anguillarum serogroup O2a to killing by rainbow trout macrophages in the presence or absence of specific antibodies and complement components was evaluated using an in vitro assay. Fluorescence microscopy revealed that V. anguillarum serogroup O2a was phagocytosed by rainbow trout macrophages. In the absence of specific antibodies and complement components the bacteria were killed to a limited extent by the macrophages and there was no increased killing if the bacteria were opsonised with either antibodies or antibodies and complement. Furthermore, activated macrophages did not show enhanced ability to kill the bacteria. Vibrio anguillarum serogroup O2a were susceptible to both cell-free superoxide anion (O2-) and hydrogen peroxide (H2O2), which might be generated during the macrophage respiratory burst and the bacteria did not quench cell-free O2-. However, the production of O2- by macrophages was undetectable during the first 30 min following infection and no respiratory burst was inducible by phorbol myristate acetate (PMA) 4 h after infection with V. anguillarum. This suggests that the bacteria were able to inhibit the production of O2- by the infected macrophages. Naive fish were protected when passively immunised with anti-V. anguillarum serogroup O2a antiserum. However, previous results suggest that antibodies are unlikely to provide the fish with protective immunity directly through activation of the complement system and lysis of the bacterial cells. The present in vitro findings suggest that the protective mechanisms of antibody against V. anguillarum serogroup O2a may not involve the opsonising effect of antibodies for enhanced killing by macrophages. However, the possibility exists that such antibodies may prevent the attachment of the pathogen to the host's tissues.