Cortisol and the immune system of juvenile Pacific salmon during stress and development

Graduation date: 1989Plasma concentrations of cortisol, a glucocorticoid hormone that\ud is immunosuppressive in mammals, increase during stress and\ud smoltification of salmonids. There was a reduction in the ability of\ud lymphocytes to generate antibody producing cells (APC) in vivo\ud concurrent with increased plasma cortisol titers during smoltification\ud of coho salmon (Oncorhvnchus kisutch). Cortisol implants increased\ud circulating cortisol concentrations, reduced immune response and\ud increased fish mortality to vibriosis. Acute stress of chinook salmon\ud (0. tshawytscha) in laboratory and field trials caused transient\ud increases in plasma cortisol that were concurrent with decreased\ud ability to generate APC in vitro 4 h after stress. However, 24 h\ud after stress cortisol had returned to resting levels and immune\ud response was enhanced. Disease challenges validated the APC assay as\ud an accurate measure of immunocompetence as there was decreased\ud survival at 4 h, but enhanced survival at 24 h after stress.\ud Fish that were acutely stressed or treated with cortisol had\ud transient reductions in number of cells in spleen and peripheral\ud blood and increases in thymus and anterior kidney. Chronically\ud stressed fish responded in a similar manner, except that the number of\ud cells in the anterior kidney was reduced throughout the experiment.\ud High affinity, low capacity glucocorticoid receptors (GR) were\ud identified in gill and whole leukocytes from spleen and anterior\ud kidney. Leukocytes from the anterior kidney had 2- to 3-fold more GR\ud than did those from the spleen. The dissociation constants (Kd) and\ud numbers of GR in leukocytes increased as a result of stress; the Kd of\ud GR in gill of stressed fish also increased, but the number decreased.\ud During smoltification, number and Kd of GR in anterior kidney\ud leukocytes increased and Kd of GR in splenic leukocytes decreased with\ud no change in number. Anterior kidney leukocytes were initially\ud insensitive to cortisol added to cell cultures, but developed\ud sensitivity concurrent with changes in GR.\ud Similar to a model proposed for mammals, I speculate that the\ud functional significance of the immunosuppressive effects of cortisol\ud in salmon may be to downregulate the immune system to avoid\ud autoimmunity during recovery from stress, or as a result of tissue\ud changes associated with smoltification