Lepeophtheirus salmonis secretory/excretory products and their effects on Atlantic salmon immune gene regulation

We have previously shown that Lepeophtheirus salmonis produces trypsin and prostaglandin E₂ (PGE₂) that are most likely responsible for the limited inflammatory response of Atlantic salmon to infection. After removal of the dopamine and PGE₂, the immunomodulatory activity of unfractionated and pools of the fractionated secretions was determined by examining the effects of the secretions on Atlantic salmon immune gene expression. Incubation of macrophage-enriched isolates of Atlantic salmon head kidney cells with the unfractionated secretion + PGE₂ revealed a significant inhibition of interleukin-1β (IL-1β) and major histocompatibility class I gene expression. Inhibition of lipopolysaccharide- induced IL-1β expression in the Atlantic salmon head kidney cell line (SHK-1) was observed when three pools of the secretory/excretory products were tested. Further purification of products within these pools revealed that fraction 1-2 could account fully for the inhibition of IL-1β expression in SHK-1 cells observed in pooled fraction 1. This study demonstrates that there are other immunomodulatory compounds produced by L. salmonis, in addition to PGE₂ and trypsin, that can inhibit the expression of Atlantic salmon immune-related genes in vitro.