Host responses against the fish parasitizing ciliate Ichthyophthirius multifiliis

Recent studies have shown that fish are able to mount protective immune res ponses against various parasites. One of the best characterized parasite-ho st system in this context is the ciliate Ichthyophthirius multifiliis (Ich) parasitizing a range of freshwater fishes. Both specific and non-specific host defence mechanisms are responsible for the protection of fish against challenge infections with this ciliate. The specific humoral components com prise at least specific antibodies. The non-specific humoral elements inclu ded are the alternative complement pathway and probably lectins. Cellular f actors involved in the specific response are B-cells and putative T-cells. The non-specific effector cells recognized are various leukocytes. In addit ion, goblet-cells and mast cells (EGC-cells) may have a function. The NCC-c ell (suggested analogue to NK-cells in mammals) seems to play a role in the non-specific response. This well documented protective response in freshwa ter fishes against Ich has urged the development of anti-parasitic vaccines . Indeed, such products based on formalin killed parasites have been develo ped and found to offer the vaccinated host a satisfactory protection. Howev er, the collection of parasites for vaccine production is extremely laborio us. It involves keeping infected fish due to the fact that in vitro propaga tion of the parasite is still insufficiently developed. Gaining knowledge o f amino acid sequences and its encoding DNA-sequences for the protective an tigens (i-antigens) in the parasite was a major breakthrough. That achievem ent made it possible to produce a recombinant protein in E. coli and prelim inary results indicated a certain protection of fish vaccinated with this p roduct. Recent work has shown that the free-living and easily cultivated ci liate Tetrahymena can be transformed and express the i-antigen. This path s eems to be promising for future development of vaccines against Ich. A nove l approach in fish is the development of DNA-vaccines. Successful DNA-vacci nation trials have been conducted in fish against viral infections and the technology also makes it possible to develop a DNA-vaccine against Ich. Oth er approaches to immuno-protection against Ich have been the use of heterol ogous vaccines. Thus, both bath and injection vaccination using live or kil led (un-transformed) Tetrahymena has been reported to offer treated fish a certain level of protection. Such protection could be explained by non-spec ific reactions and the efficacy and duration of this vaccination type shoul d be further evaluated. (C) 2001 Elsevier Science B.V. All rights reserved.