Rainbow trout (Oncorhynchus mykiss) were immunized with plasmid
DNA vaccine constructs encoding selected
antigens from the parasite Ichthyophthirius multifiliis. Two
immobilization antigens (I-ags) and one
cysteine protease were tested as genetic
vaccine antigen candidates. Antigenicity was evaluated by immunostaining of transfected fish cells using I-ag specific mono- and polyclonal
antibodies. I. multifiliis specific antibody production, regulation of immune-relevant genes and/or protection in terms of parasite burden or mortality was measured to evaluate the induced immune response in vaccinated fish. Apart from
intramuscular injection, needle free injection and gene gun delivery were tested as alternative administration techniques. For the I-ags the
complement protein fragment C3d and the termini of the
viral haemorrhagic septicaemia virus glyco(
G)protein (VHSV G) were tested as opsonisation and cellular localisation mediators, respectively, while the full length viral
G protein was tested as molecular adjuvant. Expression of I-ags in transfected fish cells was demonstrated for several constructs and by immunohistochemistry it was possible to detect expression of a secreted form of the Iag52B in the muscle cells of injected fish. Up-regulations of
mRNA coding for
IgM, MHC I, MHC II and TCR β, respectively, were observed in muscle tissue at the injection site in selected trials. In the spleen up-regulations were found for IFN-γ and
IL-10. The highest up-regulations were seen following co-administration of I-ag and
cysteine protease plasmid constructs. This correlated with a slight elevation of an I. multifiliis specific antibody response. However, in spite of detectable
antigen expression and immune reactions, none of the tested vaccination strategies provided significant protection. This might suggest an insufficiency of
DNA vaccination alone to trigger protective mechanisms against I. multifiliis or that other or additional parasite
antigens are required for such a
vaccine to be successful.