Abstract
Lepeophtheirus salmonis infections in Atlantic salmon, S almo salar , have been characterized by little to no hyperplastic response and a biphasic immune response that results in chronic inflammation with tissue repair as the infection progresses. We hypothesized that C p G administration with prior lice exposure would enhance epithelial inflammatory mechanisms and boost the A tlantic salmon immune response to L . salmonis , leading to greater protection against infection. We administered multiple exposures of L . salmonis to two groups of A tlantic salmon and compared responses against first‐time exposed A tlantic salmon. Following re‐exposure, C p G fed fish exhibited increased skin expression of interleukin ( IL )‐1β and IL ‐12 β compared to control previously exposed ( CPE ) and control first‐time exposed ( CFE ) animals, respectively. This inflammatory enhancement occurred with significantly lower expression of matrix metalloproteinase‐9 ( MMP 9), both systemically (spleen) and locally (skin). Reduced MMP 9 expression was a hallmark of the re‐infected fish (occurred in both tissues at both times). When significant differences were present in the skin or spleen, the two re‐exposed groups showed greater similarity than with the first exposure group. Lice numbers on C p G fed fish were significantly lower than CFE fish at 7 days post‐re‐infection (dpri), and although they were not significantly different at 17 dpri, the trend of lower lice levels remained. CpG fed fish also showed nearly twofold greater protection than CPE when compared to the CFE group (48.5% vs. 27.0% reductions at 7 dpri and 27.2% vs. 13.1% reductions at 17 dpri, respectively). The enhanced protection of C p G oligodeoxynucleotide administration to previous exposure was consistent across all body surfaces and suggests that C p G can not only enhance innate responses to L . salmonis in A tlantic salmon, but also further stimulate adaptive responses.