Differential gene expression in Atlantic salmon infected with Lepeophtheirus salmonis

Lepeophtheirus salmonis is an ectoparasitic copepod that can cause serious disease outbreaks in salmonids. Previous work has shown that Atlantic salmon Salmo salar show very little, if any, tissue response to infection, whereas less-susceptible host species exhibit strong inflammatory responses. The objective of this study was to examine whether a low-level L. salmonis infection in the absence of a stress response would have an effect on expression of Atlantic salmon immune-related genes over time. The effects of low-level infection (approximately 8-11 lice/fish) on kidney tissue and head kidney leukocyte immune-related gene expression were studied at two postinfection time points. At this level of infection, there was no evidence of a Cortisol stress response. By use of real-time polymerase chain reaction, constitutive expression of the major histocompatibility (MH) class I gene was shown to be 2-10-fold lower in infected Atlantic salmon head kidneys by 21 d postinfection (dpi) than in head kidneys of uninfected fish held under the same conditions. Conversely, by 14 and 21 dpi, constitutive MH class II expression was significantly increased (> 10-fold) in infected fish. Constitutive expression of interleukin (IL) 1β also increased threefold in head kidneys of infected fish by 21 dpi; however, no differences were observed in cyclooxygenase (COX)-2 expression over the course of the infection. Adherent head kidney leukocytes from control Atlantic salmon showed an increase in MH class I and COX-2 expression after 3 h of lipopolysaccharide (LPS) stimulation for all three time points collected. The use of LPS stimulation on similar leukocytes from infected fish did not further increase levels of MH class I and COX-2 expression. This is the first report on host gene expression during an ectoparasitic copepod infection. The implications of these expression changes will be discussed with respect to the host-parasite relationship.