EFFECTS OF THE SECRETORY/EXCRETORY PRODUCTS OF Lepeophtheirus salmonis ON ATLANTIC SALMON LEUKOCYTE MIGRATION

The salmon louse, Lepeophtheirus salmonis, evades elimination by its host, Atlantic salmon, by inhibiting inflammatory responses; however, the mechanisms of this activity are not well characterized.  Studies from other arthropod ectoparasites have identified chemokine binding proteins in the saliva and salivary gland extracts that inhibit neutrophil recruitment and activation in vitro and in vivo by binding to chemokines such as leukotriene B₄ (LTB4) preventing interaction with their receptors on the surfaces of neutrophils.  To determine if L. salmonis may be using a similar mechanism to evade the host immune response the effects of L. salmonis secretory/excretory products (SEPs) on LTB4 stimulated leukocyte migration in vitro were examined.  Results showed an increase in leukocyte migration in response to LTB4 immune stimulation when compared to the control L-15 media.  However, pre-incubation of LTB4 with SEPs for 1 hour significantly reduced cell migration compared to LTB4 immune stimulation alone (p=0.0003).  This data suggests that L. salmonis may inhibit inflammatory responses in Atlantic salmon by secreting proteins with chemokine-binding or chemokine-degrading activity.  Furthermore, we measured the effects of SEPs on host response to chitin, a component of the exoskeleton of arthropods known to induce inflammation, in vivo.  Histological analysis of epithelial tissues collected 6 hours post chitin exposure showed a significant increase in leukocyte recruitment to chitin + dopamine seawater treatment compared to the no injection control (p=0.0137).  SEPs + chitin and SEPs only treatments were similar, with fewer leukocytes present at injections sites, however, theses treatments were not significantly different from the positive control, chitin + dopamine seawater (p=0.1930 and p=0.4003, respectively).  To gain a better understanding of the chemotactic pathways disrupted by SEPs in vivo, we will also measure the expression of genes involved in neutrophil activation and cellular chemotaxis in epithelial tissues collected from the in vivo chitin challenge.  Gene expression data will be presented at the meeting.  The gene expression work will provide clues as to which steps in the chemokine signaling cascade are disrupted by the secreted/excreted products of the sea louse.  In addition, the results may further support our previous in vitro migration data.