Supra-physiological levels of cortisol suppress lysozyme but not the antibody response in Atlantic salmon, Salmo salar L., following vaccine injection

Using key immunological parameters including lysozyme activity and specific antibody titres, we examined the influence of chronic, supra-physiological levels of cortisol on the vaccine-induced immune responses of farmed Atlantic salmon, Salmo salar L. Individual fish were injected with a polyvalent, oil-adjuvanted bacterial vaccine 400 degree days prior to sea water entry, and subsequently injected with a cortisol implant at either 53 or 212 degree days post-vaccine injection. All fish were sampled 74 degree days post-cortisol injection (at 127 and 286 degree days post-vaccine injection). Separate groups of Atlantic salmon were injected with a DNA vaccine against infectious haematopoietic necrosis virus (IHNV) either by itself or concurrently with the polyvalent, oil-adjuvanted bacterial vaccine and treated as described above. Because we were unable to detect a significant increase in lysozyme activity as a result of the DNA vaccine, we cannot determine whether there was a cortisol-induced suppression of this innate immune response. Our results indicate, however, that if Atlantic salmon are exposed to significant elevations in plasma cortisol following the injection of a polyvalent, oil-adjuvanted bacterial vaccine alone or concurrently with the IHNV DNA vaccine, lysozyme activity is suppressed while induction of pathogen-specific antibody titres is unchanged. We conclude, therefore, that chronically elevated, supra-physiological levels of plasma cortisol suppress the lysozyme-mediated innate immune response in Atlantic salmon injected with a polyvalent, oil-adjuvanted bacterial vaccine, but do not affect antibody production associated with the adaptive immune response.