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Elicitation
Of Protective
Immunity
Against
Ichthyophthirius multifiliis
In Channel Catfish
(Ictalurus
punctatus)
By Immunization
With Immobilization
Antigens
Harry W. Dickerson1,
Xuting Wang1, Yan Gao1, Jane Noe1, Jacek
Gaertig1, and Theodore G. Clark2
1University
of Georgia, Athens, GA and 2Cornell University, Ithaca, NY
The
ciliate Ichthyophthirius multifiliis
infects all species of freshwater fish and causes the common disease known as
“white spot”. Experiments have revealed
a novel form of protective immunity in which parasites exit the host in
response to binding of specific antibody.
The target antigens are a class of abundant surface membrane proteins
referred to as immobilization antigens (i-ags), which are a family of
antigenically related polypeptides in the 40 to 60 kDa size range bound to the
cell membrane by a glycolipid anchor.
Affinity-purified parasite i-ags elicit serotype-specific protective
immunity when injected intraperitoneally into naïve channel catfish. I-ag genes have been expressed in the
free-living ciliate Tetrahymena
thermophila for the production of recombinant vaccines. Gene constructs encoding the 48- and
52/55-kDa i-ags of I. multifiliis
serotypes A and D, respectively, were targeted by homologous recombination into
the b-tubulin-1
gene locus of T. thermophila, and
expressed using a cadmium inducible promoter from a T. thermophila metallothionein gene. Up to 10 ng of recombinant i-ag protein/10 mg
total cell protein was produced in this system. Furthermore, immunogenic recombinant i-ags were targeted and
displayed on the surface of the transformed Tetrahymena
cells as determined by indirect immunofluorescence and immobilization of live
cells by specific antibodies. To test
for the ability of transformed T.
thermophila to induce immunity, channel catfish were injected twice
intraperitoneally at a 2-week interval with 1 x 106 live cells per
fish. Immunized fish produced specific
serum antibodies and had an 80-100% survival rate when exposed to lethal
parasite challenge.
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