Interactions between salmon macrophages and pathogenic bacteria in the presence of secretions isolated from Lepeophtheirus salmonis

In response to stimuli (i.e., salmon mucus) the sea louse, Lepeophtheirus salmonis, produces pharmacologically active substances (prostaglandin E₂, trypsin-like proteases and cathepsin). Lice-derived secretions impair the genetic expression of pro-inflammatory mediators in the commercial salmon-head kidney (SHK-1) cell line and head kidney macrophages isolated from Atlantic salmon (Salmo salar); however, effects on the functionality of these cells has not been explored. Related to the development of an inflammatory response, salmon species (Oncorhynchus spp. and Salmo spp.) exhibit differences in infection rates and threshold tolerances to L. salmonis. The objective of this study was to determine if the presence of L. salmonis secretory and excretory products (SEPs) alters the innate immune response of salmon. More specifically, the present study examined if the presence of SEPs altered phagocytic activity and respiratory burst response of salmon macrophages. Phagocytosis assays were performed using SHK-1 cells and the bacterial pathogen, Aeromonas salmonicida, in the presence/absence of SEPs. To address species-specific differences, phagocytosis and respiratory burst assays were completed using macrophages isolated from pink (Oncorhynchus gorbuscha), chum (O. keta), and Atlantic (S. salar) salmon in the presence/absence of SEPs. SHK-1 cells incubated with SEPs plus A. salmonicida had a significantly higher phagocytic index (223.2 %) than cells incubated with A. salmonicida alone (136.5 %). Macrophages isolated from pink salmon had a pronounced production of superoxide (O₂) in the presence of SEPs that was not observed in chum or Atlantic salmon macrophages. Interestingly, pink salmon macrophages had a lower phagocytic index (15.8 %) than the more L. salmonis-susceptible species, chum (55.1 %) and Atlantic (26.4 %) salmon. Furthermore, the presence of PGE₂, proteins and other undetermined molecules in SEPs appear to have a biologically relevant concentration at which they no longer exert an effect on phagocytosis in SHK-1 cells. This study provides the first evidence of altered macrophage function in response to L. salmonis secretions and provides insight into the complex interactions that occur at the parasite-host interface (i.e., the skin).Land and Food Systems, Faculty ofGraduat