Cadmium effects onIchthyophthirius: evidence for metal-sequestration in fish tissues following administration of recombinant vaccines

Abstract

We are developingTetrahymena thermophilaas a delivery system for recombinant vaccines against parasitic protozoa, including the common fish parasite,Ichthyophthirius multifiliis. T. thermophilacell lines expressingI. multifiliisgenes under the control of a cadmium-inducible metallothionein gene promoter conferred strong protection against a lethal parasite challenge when administered parenterally to naïve fish. Nevertheless, given that heavy metals can be toxic to parasites, a question arose as to whether protection resulted from Cd residues carried over with the vaccine, rather than acquired immunityper se. To address this issue, we examined the sensitivity ofI. multifiliisto Cdin vitroand determined Cd concentrations in different host tissues following i.p. injection of juvenile channel catfish with the recombinant vaccine. We found that CdCl2at concentrations [ges ]50 ppb were lethal toI. multifiliistherontsin vitro. Furthermore, Cd concentrations were clearly elevated in fish tissues and reached levels equivalent to 74 ng/g wet weight (74 ppb) in the skin within 14 days of injection with recombinantT. thermophila. Nevertheless, fish injected with non-transformedTetrahymenagrown in the presence or absence of CdCl2showed no significant difference in either relative survival or parasite load following direct challenge withI. multifiliis.