Is the epidermal club cell part of the innate immune system in fathead minnows?

Abstract

Fishes in the superorder Ostariophysi, including fathead minnows (Pimephales promelas), possess specialized epidermal club cells that contain an alarm substance. Damage to these cells, as would occur during a predator attack, causes the release of the alarm substance and can indicate the presence of actively foraging predators to nearby conspecifics. For nearly 70 years, research involving epidermal club cells has focused on the alarm substance and the role it plays in predator/prey interactions. However, recent studies have indicated that there may be a connection between epidermal club cells and the fish immune system. Fish increase investment in epidermal club cells upon exposure to skin penetrating pathogens and parasites. In this study I tested for differences in epidermal club cell investment by fathead minnows exposed to the immunosuppressive effects of the glucocorticoid hormone cortisol. In experiment 1, fathead minnows were exposed to either a single intraperitoneal injection of corn oil or no injection at all. The purpose of this experiment was to determine whether corn oil, the vehicle for cortisol injections in later experiments, had an effect on epidermal club cell density. The treatments had no effect on epidermal club cell size, cell area, or epidermal thickness. In experiment 2, skin extract was prepared from the skin of corn oil injected and non injected fathead minnows as in experiment 1 to determine whether corn oil had an effect on the epidermal club cell alarm substance concentration. The treatments showed no significant differences in observed anti-predator behaviour, including change in shelter use, dashing and freezing. In experiment 3, fathead minnows were exposed to either a single intraperitoneal injection of cortisol or corn oil. The purpose of this experiment was to determine whether cortisol, a known immunosuppressant, had an effect on epidermal club cell investment. Fathead minnows exposed to a single cortisol injection had significantly reduced respiratory burst activity of kidney phagocytes indicating that there was suppression of the innate immune system. Furthermore, cortisol treated fathead minnows showed significantly lower numbers of epidermal club cells. The treatments had no effect on individual epidermal club cell area, epidermal thickness and serum cortisol levels after 12 days. The results from this experiment suggest that pharmacological cortisol injections in fathead minnows have a suppressive effect on the fish innate immune system. Furthermore, the findings that cortisol induced immunosuppression also influences epidermal club cell investment provides support for the hypothesis that epidermal club cells may function as part of the fish immune system.