Cloning And Expression Of Surface I-Antigen Genes Of Ichthyophthirius multifiliis     Yuankai Lin, Chia-Cheng Wang, Qiang Chen, and Theodore Clark   Department of Microbiology and Immunology, Cornell University, Ithaca, NY     The i-antigens of Ichthyophthirius multifiliis comprise a family of structurally related GPI-anchored membrane proteins that coat the parasite surface.  Antibodies against these proteins protect against infection, and the antigens themselves represent important vaccine candidates for the prevention of ‘white-spot’ disease in freshwater fish.  I-antigens vary among parasite isolates, and at least 5 serotypes have been distinguished based on immobilization of swimming cells with type-specific mono- and polyclonal antibodies.  Immunity elicited by the purified antigens is serotype-specific, and since multivalent vaccines may be required for protection against field isolates, we are attempting to isolate and characterize i-antigen genes from a wide range of parasite strains.  These efforts have led to the identification of two new genes from the most common i-antigen serotype, namely, D.  These genes encode ~ 52/55 kDa antigens that share roughly 40% sequence identify at the deduced amino acid sequence level.  Comparisons of the products of these genes with the deduced sequence of a previously characterized 48 kDa antigen from serotype A, have revealed both conserved and unique elements among the i-antigens of different stains.  We are using this information to 1) map protective epitopes associated with these proteins, and 2) design PCR primers for the amplification of i-antigen genes from diverse serotypes.  Finally, we have developed methods that circumvent the non-standard genetic code used by Ichthyophthirius (where UAA and UAG “stop” codons specify glutamine).  Such methods will permit large-scale expression of recombinant proteins for use in structural studies and vaccine trials.