Non-specific responses of Turbot (Scophthalmus maximus L.) adherent cells to microsporidian spores

We investigated non-specific responses of turbot spleen- and pronephros-resident adherent cells to spores of fish microsporidians, and the effects of the glucocorticoid dexamethasone (DX) on these responses. On average, 65% of adherent cells from the spleen and pronephros showed esterase activity (as characteristic of macrophages); 32% showed peroxidase activity (as characteristic of neutrophils), and 19% of peroxidase-positive cells were capable of phagocytosing microsporidian spores. A significantly higher proportion of adherent cells showed phagocytic activity when viable spores were the target than when non-viable spores were the target. Microsporidian spores stimulated adherent cells to produce reactive oxygen and nitrogen intermediates (ROIs and RNIs), though less effectively than the other stimulants tested. Adherent cells exposed to viable spores produced significantly less intracellular superoxide than adherent cells exposed to non-viable spores. Daily injection of fish with DX over 6 days significantly inhibited both phagocytosis of microsporidian spores and spore-induced ROI production, and similar effects were observed when adherent cells were exposed to DX in vitro.