Immune response in Porcellio scaber (Isopoda: Oniscidea): copper revisited

Abstract

Despite of numerous descriptive and experimental approaches, during the last 40 years, the function of vast copper stores in the midgut glands (hepatopancreas) of terrestrial isopods is still unknown. We tested the hypothesis that copper is involved in immune defence through the action of phenol-oxidising enzymes. We used Porcellio scaber fed on artificial diets with or without copper and with or without tyrosine for 2 months to reduce the copper store in the midgut glands and the tyrosine concentration in the hemolymph. To activate the immune response we implanted nylon filaments as artificial pathogens; we used phenoloxidase activity in hemolymph in vitro, and both cellular encapsulation and melanisation of filaments in vivo as measures of immune response. Isopods were capable of efficient immune reactions. Already 1 h after implantation of a nylon filament as an artificial pathogen, we observed heavy cellular encapsulation. At the same time, the implanted nylon filaments became partially covered with melanin that was likely due to hemolymph phenol oxidase (PO) activity. Hemolymph obtained from P. scaber also showed phenoloxidase activity in vitro, but to induce such activity, the addition of a denaturing detergent was necessary. Based on this, we propose that the hemocyanine of P. scaber can exert PO activity upon activation in vitro; whether, however, this process is of any significance in vivo is unclear. Our results on the effects of pre-experimental diets on immune response indicate that a deficiency of dietary copper might lead to the inability to raise and to regulate an efficient immune response, and thus, suggest an involvement of copper in immune response.