DIAGNOSTIC AND PROTECTIVE ANTIGEN GENE SEQUENCES OF ICHTHYOPHTHIRIUS

Abstract: An i-antigen protein from Ichthyophthirius multifiliis is effective to induce a protective immune response in fish. The invention includes antigenic and membrane-targeting sequences of i-antigen proteins, nucleic acid molecules that encode antigenic or membrane-targeting portions of i-antigen proteins, DNA and protein subunit vaccines, methods of inducing an immune response in fish and methods for detection and characterization of I. multifiliis.

Description: DIAGNOSTIC AND PROTECTIVE ANTIGEN GENE SEQUENCES OF ICHTHYOPHTHIRIUS This application claims the benefit of U.S. Provisional Application Serial No. 60/131,121, filed April 27, 1999; U.S. Provisional Application 60/118,634, filed February 4, 1999; U.S. Provisional Application 60/122,372, filed March 2, 1999; and U.S. Provisional Application 60/124,905, filed March 17, 1999, each of which is incorporated herein by reference in its entirety. Statement of Government Rights This invention was made with government support under grants from the United States Department of Agriculture (USDA) CSRS NRICGP, Grant No. 95-37204-2139. The U.S. government has certain rights in this invention. Background of the Invention Ichthyophthirius multifiliis is a holotrichous ciliated protozoan which is an obligate parasite of freshwater fish. The life cycle of the parasite includes a free-living infectious stage (the theront or tomite) and an obligate fish-associated feeding stage (the trophont or trophozoite). The infective theront invades the skin and the gill epithelia, resulting in disturbances in respiratory and excretory functions. Once in epithelial tissue, the theront differentiates into the fish- associated feeding form known as a trophont. When the trophont is mature, it is released from the surface of the fish and secretes material forming a gelatinous cyst. Within the cyst, the mature trophont undergoes multiple cell divisions to produce hundreds of theronts, which are then released from the matrix to begin a new cycle of infection. Protection from Disease Caused by I. multifiliis Ichmyophthiriasis, the disease caused by this parasite, is commonly referred to as "Ich" or "white spot disease." Under conditions of intensive aquaculture, Ich frequently has a high morbidity and mortality, resulting in significant financial losses to fish producers. Treatments are available for Ich-infected fish. However, the chemical treatments are effective only against the free-living theronts; there is no known agent for eliminating trophonts associated with the host. Furthermore, some of the chemotherapeutic agents used to treat Ich are suspected to leave residues in treated fish and to be carcinogenic. As a result, certain of the available treatments, e.g., malachite green, are not permitted for fish raised for human consumption. In addition, chemical treatments result in physiological stress to the infected fish beyond that resulting directly from the infection. Those fish which survive infection by I. multifiliis are generally immune to further infection by the live parasite. Early reports suggested that fish (mirror carp) were successfully immunized by exposure to sublethal doses of the parasite (Hines et al., J. Fish. Biol. 6:373-378 (1974)) and by exposure to the live parasite in conjunction with chemical treatment. There have also been reports of at least partial protective immunity in fish vaccinated using the killed parasite. For example, a substantial decrease in the number of infective parasites on the body surface of goldfish which had been previously injected with killed theronts was observed when the fish were challenged with a measured dose of live theronts (Parker, Studies on the Natural History of Ichthyophthirius multifiliis Fouquet 1876. an Ectomarasitic Ciliate of Fish. Ph.D. Dissertation, The University of Maryland, College Park, Maryland (1965)). Areerat ("The Immune Response of Channel Catfish, Ictalurus punctatus (Rafinesque), to Ichthyophthirius multifiliis", unpublished Master's thesis, Auburn University) reported that channel catfish injected with formalin-fixed trophonts were protected when challenged with a lethal dose of infective theronts. Goven et al. (J. Fish Biol. 17:311-316 (1980)) reported initial protection against lethal infection when fish were injected intraperitoneally with theront cilia. The experiment was discontinued when all control fish had died and the fate of the vaccinated fish was not followed fiirther. More recently, however, Burkhardt et al. f J. Fish Pis. 13:401-410 (1990)) reported that neither immersion exposure nor intraperitoneal injection with killed I. multifiliis theronts conferred protective immunity to challenge doses of live theronts, although there was a delay in mortality of the vaccinated fish was observed. Likewise, intraperitoneal injection with theront cilia preparations did not prevent mortality, only delayed it. Only intraperitoneal injection with live theronts was effective in preventing mortality after challenge with infective parasites. Those fish which had been injected with live theronts remained protected against infection for an extended time, as evidenced by their resistance to challenge infections at 3 and 13 months after the original injection. Attempted immunization with formalin-fixed trophonts led to some delay in mortality but had an unclear effect on ultimate mortality. The mucus coating of an immune fish participates in protection from Ich infection. Hines et al. (J. Fish. Biol. 6:373-378 (1974)) showed that both sera and mucus from immune fish was capable of immobilizing the infective form of I. multifiliis. These authors also noted that fish recovering from Ich had a different distribution of the parasite than did newly infected fish. Newly infected fish exhibited parasites all over the body while a recovering fish exhibits parasites primarily at edges of the fish. These are the parts of the fish which are least well supplied with blood, and therefore, would be less well supplied with antibodies. Clark et al. fDevel. Comp. Immun. 1-2:581-594 (1988)) studied the sera of channel catfish that had been rendered immune to further Ich infection by exposure to sublethal surface infection and treated with chemotherapy. The sera of these immune catfish contain antibodies which specifically bind to I. multifiliis cilia; little cross-reactivity was observed for cilia prepared from the free-living ciliate Tetrahymena thermophila. Whole I. multifiliis cilia and a ciliary membrane fraction gave similar reactions with the immune sera, but axoneme fractions showed little differential reaction in comparisons between immune and preimmune sera. However, attempts to identify the ciliary proteins with which the antibodies reacted using blots from SDS gel electropherograms were not successful. Sera from immune fish also immobilize the parasite in vitro, with an apparent positive correlation between specific antibodies and immobilization of theronts in vitro. There were early reports that fish vaccinated with Tetrahymena pyriformis and with T. thermophila or with cilia prepared from Tetrahymena were protected from Ich infection (e.g., U.S. Pat. No. 4,309,416, Dawe et al.). It had been proposed that the ciliary membrane antigens from Tetrahymena showed cross-reactivity with those of I. multifiliis. However, more recent reports showed that attempted vaccination of channel catfish with T. thermophila Lwoff cilia did not protect the fish from subsequent challenge with I. multifiliis (Burkhardt et al., J. Fish Pis. 13:401-410 (1990)). It has been postulated that the previous cross-reactivity was due to the conserved axoneme proteins, rather than due to serologically related ciliary membrane proteins. I. multifiliis i-antigens A novel mechanism of humoral immunity against I. multifiliis was recently described. Rather than being killed on the host, a majority of parasites are forced to exit fish prematurely in response to antibody binding (M. Cross, J. Fish Pis.. 15:497-505 (1992))(T. Clark et al., Parasitol. Today. 13:477-480 (1997)). While the precise mechanism underlying this phenomenon is unknown, the target antigens responsible for premature exit have been identified as a class of abundant surface membrane proteins known as immobilization antigens, or i- antigens (T. Clark et al., Annu. Rev. Fish Pis.. 5:113-131 (1995)). Antibodies against these proteins rapidly immobilize cells in vitro. I-antigens are common to a variety of hymenostomatid ciliates and have been intensively studied in Paramecium and Tetrahymena where their expression undergoes marked variation in response to environmental stimuli (F. Caron et al., Annu. Rev. MicrobioL. 43:23-42 (1989); Smith et al., J. Protozool.. 39:420-428 (1992)). Antigenic switching in these cells results from the differential expression of multiple i-antigen genes under defined sets of conditions and represents one of the most striking examples of antigenic shift in nature. To date, there is little evidence that this type of variation occurs in Ichthyophthirius; however, steady-state levels of i-antigen transcripts vary as much as 50 fold during transition from the host-associated trophont to the infective theront stage, and it is clear that the genes for these proteins are developmentally regulated through the parasite life cycle (T. Clark et al., Proc. Nat. Acad. Sci. USA. 89:6363-6367 (1992)). Furthermore, serotypic variants of the i-antigens have been described among geographic isolates of the parasite (H. Pickerson et al., J. Euk. MicrobioL. 40:816-820 (1993)). Although i-antigens have gained considerable attention with regard to their mode of expression, their biological function remains obscure. In Paramecium and Tetrahymena, i-antigens are linked to the plasma membrane through a glycosylphosphatidylinositol (GPI) anchor, and in some cases, form a thick layer that coats the plasma and ciliary membranes (F. Caron et al., Annu. Rev. MicrobioL. 43:23-42 (1989)). This has led to speculation that their primary function is to shield the cell membrane from environmental insult; indeed, this fits a general model for the role of GPI-anchored proteins in lower eukaryotes. The fact that cross-linking of i-antigens at the surface of Ichthyophthirius elicits a physiological response in the parasite also suggests that these proteins may play a role in transmembrane signaling (T. Clark et al., Parasitol. Today. 13:477- 480 (1997)). Consistent with this idea, i-antigen antibodies trigger mucocyst discharge in both I. multifiliis (T. Clark et al., J. Fish Biol.. 31(A):203-208 (1987)) and Tetrahymena thermophila (J. Alexander, Trans. Amer. Microsc. Soc. 86:421-427 (1967)), as well as trichocyst discharge in Paramecium ssp. The cPNA sequence associated with a 48 kP i-antigen from an isolate of I. multifiliis (Gl isolate, serotype A) was reported by Clark et al. (Proc. Nat. Acad. Sci. USA. 89:6363-6367 (1992)); serotyping was reported by Pickerson et al. (Annu. Rev. Fish Pis. 6:107-120 (1996)). A recombinant subunit vaccine derived from this cPNA sequence was reported by He et al. (Aquaculture 158: 1- 10 (1997)). This subunit vaccine was engineered as a recombinant glutathione sulfotransferase (GST) fusion with a 105 amino acid fragment of the protein that the researchers identified as a potential antigenic epitope, corresponding to one of several tandemly repetitive amino acid sequence domains identified by Clark et al. fProc. Nat. Acad. Sci. USA. 89:6363-6367 (1992)). The nucleotide sequence encoding the fusion construct was chemically synthesized and used for expression of the recombinant peptide in bacteria. Two amino acid substitutions relative to the native sequence were required in order to provide restriction sites in the corresponding PNA; moreover, protozoan glutamine codons TAA and TAG (which function as stop codons in E. coli and other conventional protein expression systems) were replaced by the universal glutamine codons C AG or CAA in order to allow expression of the fusion construct in E. coli. The fusion vaccine gave weak protection against an undefined isolate of I. multifiliis; 50% of the vaccinated fish were heavily infected with I. multifiliis upon challenge with the live parasite, compared to 75% of control fish. The 48 kP i-antigen protein has been isolated from cultures of I. multifiliis (Clark et al., Annu. Rev. Fish Pis. 5:113-131 (1995); Lin et al., L Protozoology 39:457-463 (1992)). In addition, a 55 kP i-antigen protein has been isolated from cultures of I. multifiliis and affinity purified and used in studies on passive immunity (T. L. Lin et al., Inf. Immun. 64:4085-4090 (1996)). Mouse monoclonal antibodies raised against this protein were effective to immobilize G5 isolates of I. multifiliis. However, a native i-antigen protein would be very difficult to obtain from I. multifiliis in large quantity because this obligate parasite cannot be easily cultured. It is clear that an inexpensive and effective vaccine against Ich would be of great benefit to the aquaculture industry. Summary of the Invention The present invention is directed to novel i-antigen polypeptides and nucleic acid molecules that encode them. Examples of novel i-antigen polypeptides include polypeptides having SEQ IP NO: 6 and SEQ IP NO: 7 derived from I. multifiliis. The nucleic acid molecule provided by the invention contains a polynucleotide fragment having a nucleotide sequence that encodes at least a portion of an i-antigen polypeptide, exemplified by nucleotide sequences SEQ IP NOs: 1, 3 and 5. In one embodiment, the polynucleotide fragment encodes at least a C-terminal portion of an i-antigen polypeptide having amino acid sequence SEQ IP NO:6; in another it encodes at least one terminal portion of an i-antigen polypeptide having amino acid sequence SEQ LP NO:7; in yet another it encodes at least an antigenic portion an i-antigen polypeptide having amino acid sequence SEQ IP NO:7. The nucleic acid molecule of the invention can take the form of a vector that is capable of expression in an organism or in a cell, preferably a fish and/or in a conventional protein expression system, including bacteria, such as E. coli, yeast, mammalian cell culture or insect cells. The invention includes a host cell transformed with the vector of the invention, and further includes an organism, preferably a fish, comprising a nucleic acid molecule of the invention. Also included is an antibody capable of binding at least one of the i-antigen polypeptides of the invention. The invention further includes a composition for inducing an immune response in a fish comprising a nucleic acid molecule that has a nucleotide sequence that encodes an antigenic portion of an i-antigen polypeptide of the invention. Likewise, the invention includes a composition for inducing an immune response in a fish comprising an antigenic i-antigen polypeptide of the invention. The invention further includes a method for causing an immune response in the fish by administering an immunogenic composition as described, for example as a prophylactic or therapeutic vaccine. The composition preferably prevents or controls I. multifiliis infection in fish. Also included in the invention is a method for detecting Ichthyophthirius in an aquaculture. A sample containing nucleic acid is obtained from an aquaculture fish or from water present in the aquaculture, then at least one primer oligonucleotide having a sequence complementary to at least a portion of SEQ IP NO:6 or SEQ IP NO:7 is added to the nucleic acid sample. A polymerase chain reaction amplification is conducted, and the amplified product is analyzed for the presence of a product amplified by the at least one oligonucleotide primer. Advantageously, the amplified product can then be used to formulate or customize a vaccine effective to treat or prevent Ichthyophthirius infection. The method thus optionally includes making a polynucleotide vaccine that contains at least a portion of the amplified product, or a protein subunit vaccine that includes an antigenic polypeptide encoded by the portion of the amplified product; and administering the vaccine to treat or prevent Ichthyophthirius infection in a fish or fish population. Further, the invention provides a method for identifying an I. multifiliis serotype. A sample containing I. multifiliis nucleic acid is combined with at least one primer oligonucleotide that has a sequence complementary to a region of an J multifiliis nucleotide sequence encoding an i-antigen that is unique to and selective for that serotype, then a polymerase chain reaction amplification is conducted. The reaction mixture is analyzed for the presence of a product amplified by the serotype-selective oligonucleotide primer. Brief Description of the Figures Figure 1 shows the nucleotide sequence (SEQ IP NO:2) and the deduced amino acid sequence (SEQ LD NO:6) of the 48 kD i-antigen gene (IAG48[Glf); a guanine (G) nucleotide that marks the start of the 1.2 kb cDNA is indicated by the arrow; the adenosine (A) nucleotide in the gene's putative ATG start codon for the 48 kD i-antigen is assigned the number +1 ; a stretch of 14 mostly hydrophobic amino acids at the C-terminus of the deduced protein is boxed; the 3 small amino acids (Cys-Ala-Ser, denoted with asterisks) may represent the site at which cleavage and GPI-anchor addition occurs. Figure 2 shows nucleotide sequences for (a) the native G5 55 kP i- antigen coding region including stop codons (SEQ IP NO:44); and (b) a synthetic 55 kP i-antigen coding region, including stop codons (SEQ IP NO: 102), useful as a PNA vaccine and in conventional protein expression systems. Figure 3 shows (a) an alignment of the deduced amino acid sequences of the genomic 48 kP (upper line) (SEQ IP NO:6) and 55 kP (lower line) (SEQ IP NO:7) i-antigens of I. multifiliis; where asterisks indicate identities between the two deduced protein sequences, double dots indicate highly homologous amino acids, and single dots indicate moderately homologous amino acids; boxes indicate conserved regions; and (b) an alignment of the nucleotide sequences of the coding regions of the IAG48 [GIJ gene (upper line) (SEQ LD NO: 1) and the IAG55 [G5] gene (lower line) (SEQ LD NO:3) of/ multifiliis, where asterisks indicate identities between the two nucleotide sequences. Figure 4 shows the deduced amino acid sequence of the 55 kD i-antigen encoded by the LAG 55 [G5J coding region in Fig. 2(a). Figure 5 shows an amino acid sequence alignment of five homologous tandemly repeated amino acid sequence domains of (a) the deduced 48 kD i- antigen genomic coding region and (b) the deduced 55 kD i-antigen coding region; amino acids that are shared by three or more repeats are boxed; asterisks denote cysteine residues conserved in all repeats. Figure 6 is a restriction map of the cloned genomic DNA fragment encoding the 48 kD i-antigen; Swa I, (S); EcoR I, (Ε); Nsi, (N); and Hrød HI, (H); the filled portion indicates the coding region of the i-antigen gene. Figure 7 shows a comparison between the IAG48 [GIJ gene sequence and 1.2 kb cDNA sequence. Panel (A) is a schematic illustrating the basic differences between the gene and cDNA sequences (boxed); the filled regions of the gene sequence are not present in the 1.2 kb cDNA sequence; ATG, start codon; TGA, stop codon; nt, nucleotides; inverted triangle indicates the site of the observed C/T transversion. Panel (B) diagrams equivalent regions of the 1.2 kb cDNA (nucleotides 1-1172) and IAG48 [GIJ gene (nucleotides 56-1227); the C/T transversion is indicated at nucleotide 897 (cDNA) and 952 (genomic DNA). Panel (C) shows the nucleotide sequences specified by the 3' ends of the 1.2 kb cDNA (SΕQ LD NO:64), the IAG48 [GIJ gene (SΕQ ID NO:66), the ICH5/ΕPB 3' RACE product (SEQ LD NO:67), and two additional cDNAs designated 1-3 (SEQ ID NO: 68) and 1-1 (SEQ YD NO:69); also shown are the corresponding deduced amino acid sequences of the 1.2 kb cDNA (SEQ LD NO:63) and the IAG48 [GIJ gene (SEQ ID NO:65); a point mutation in the RACE product is boxed and is most likely attributable to the use of a low fidelity thermostable DNA polymerase during synthesis; the bracket covering nucleotides +1409 through +1413 in the IAG48[G1] transcript indicates a putative polyadenylation site. Figure 8 shows an amino acid sequence alignment of/ multifiliis and Giardia surface proteins; the IAG48[G1J (amino acids 20-428) (SEQ LD NO:61) and Giardia lamblia vspA6-Sl (amino acids 61-459) (SEQ D NO:62) gene products were compared using CLUSTALW multiple sequence alignment software; common cysteine residues are boxed; the segment of the Giardia VSP shown here (accession no. Q24970) comprises 67% of the predicted protein. Figure 9 is a schematic illustrating the method used to isolate the 1 kb cDNA encoding a portion of the G5 55 kD i-antigen sequence. Figure 10 is a schematic illustrating a method utilizing inverse PCR that was used to obtain the nucleotide sequences flanking the 1 kb cDNA sequence in the gene encoding the G5 55 kD i-antigen. Figure 11 shows an SDS-PAGE gel and a Western blot of solubilized and GPI-phospholipase C treated membrane proteins from / multifiliis (strain G5). Figure 12 lists oligonucleotide primers used for DNA shuffling-based synthesis of a G5 synthetic i-antigen gene. Figure 13 shows the nucleotide sequence encoding the synthetic G5 proline mutant i-antigen protein (L6P) (SEQ LD NO:53); the arrow indicates the mutation position. Figure 14 shows the amino acid sequence of a synthetic G5 proline mutant i-antigen protein (L6P) (SEQ LD NO:54); the arrow indicates the position of the mutation. Figure 15 is a Western blot showing expression of a recombinant synthetic 55 kD i-antigen protein in E. coli. Figure 16 shows survival and days to death of channel catfish vaccinated with purified 55 kD i-antigen subunit protein. Figure 17 is a Western blot showing expression and secretion from a transformed Tetrahymena of a Gl i-antigen with C-terminal truncation. Figure 18 shows Western blotting analysis of sera from fish vaccinated with the 55 kD plasmid vaccines. Figure 19 shows ELIS A results for fish vaccinated with Tetrahymena expressing (a) membrane associated Gl i-antigen and (b) secretary form of Gl i- antigen. Figure 20 is a 1 OX magnification of serotype A / multifiliis an immobilization test conducted using sera (1:20 dilution) from (a) fish vaccinated with live Tetrahymena expressing neo ("anti-live Tneo," the negative control) and (b) fish vaccinated with live Tetrahymena expressing the full-length 48 kP i- antigen protein from a Gl / multifiliis isolate ("anti-live TGI). Figure 21 is a gel showing the use of universal primers P2 and P4 to amplify nucleotide sequences in several different / multifiliis serotypes. Detailed Description of the Invention The present invention provides a nucleic acid molecule comprising a polynucleotide fragment having a nucleotide sequence that encodes at least a portion of an i-antigen protein. In one aspect of the invention, the nucleotide sequence of the polynucleotide fragment encoding the i-antigen protein is derived from / multifiliis (Gl isolate), and encodes a putative i-antigen having a molecular weight of about 48 kP. In a preferred embodiment of this aspect of the invention, the polynucleotide fragment is at least about 20 nucleotides in length and has at least a 3' terminal portion of a nucleotide sequence as shown in Fig. 1, nucleotides 1 through 1326 (SEQ IP NO:l), wherein nucleotides 1 through 1326 represent the coding region of the IAG48[G1J gene of / multifiliis. In a particularly preferred embodiment, the polynucleotide fragment is SEQ IP NO:l. The term "3' terminal portion" of SEQ IP NO: 1 includes at least one nucleotide contiguous to and located 3' of nucleotide 1226, exemplified by nucleotides 1227 through 1326 (SEQ LP NO:4) in SEQ IP NO:l. A "coding region" is a linear string of nucleotides that encodes a polypeptide, usually via mRNA, when placed under the control of appropriate regulatory sequences. The boundaries of a coding region are generally determined by a translation start codon at its 5' end and a translation stop codon at its 3' end. Optionally, the polynucleotide fragment of the invention includes one or more flanking nucleotides on one or both of the 5' and 3' ends of the coding region, representing untranslated regions (UTRs) as shown in Fig. 1. A polynucleotide fragment of the invention can, for example, have a nucleotide sequence represented by nucleotides -432 through 2054, as shown in Fig. 1 (SEQ IP NO:2). In another aspect of the invention, the nucleotide sequence of the polynucleotide fragment encoding an i-antigen protein is derived from / multifiliis (G5 isolate), and encodes an i-antigen having a molecular weight of about 55 kP. In a preferred embodiment of this aspect of the invention, the polynucleotide fragment is at least about 20 nucleotides in length and has at least one terminal portion of a nucleotide sequence as shown in Fig. 2(a), nucleotides 1 through 1404 (SEQ IP NO:3), wherein nucleotides 1 through 1404 represent the coding region of the IAG55[G5] gene of / multifiliis. A terminal portion of SEQ LP NO:3 can be a 5' terminal portion or a 3' terminal portion of SEQ LP NO:3. A 5' terminal portion of SEQ IP NO:3 includes at least one nucleotide contiguous to and located 5' of nucleotide 61, exemplified by nucleotides 1 through 60 (SEQ LP NO: 19) in Fig. 2(a). Likewise, a 3' terminal portion of SEQ ID NO: 3 includes at least one nucleotide contiguous to and located 3' of nucleotide 1344, exemplified by nucleotides 1345 through 1404 (SEQ ID NO:20) in Fig. 2(a). In a particularly preferred embodiment, the polynucleotide fragment is SEQ ID NO:3. Optionally, the polynucleotide fragment of the invention includes one or more flanking nucleotides on one or both of the 5' and 3' ends of the coding region. The invention further includes a nucleic acid molecule comprising a polynucleotide fragment having a nucleotide sequence selected from the class of nucleotide sequences that encodes a polypeptide having at least a C-terminal portion of SEQ IP NO:6 (Fig. 1). Preferably, the polynucleotide is at least about 20 nucleotides in length. The term "polypeptide" as used herein refers to a polymer of amino acids and does not refer to a specific length of a polymer of amino acids. Thus, for example, the terms peptide, oligopeptide, and protein are included within the definition of polypeptide. A "C-terminal portion" of SEQ LP NO: 6 includes at least one amino acid contiguous to and located to the C- terminal side of amino acid 409 (Ala409) in SEQ LP NO:6, and is exemplified by amino acids 410 (Lys410) through 442 (Leu442) (SEQ LP NO:18) (Figs. 1 and 3(a)). Preferably, a C-terminal portion of SEQ IP NO: 6 includes at least about 6 contiguous amino acids from SEQ LP NO: 18, more preferably at least about 14, even more preferably at least about 20. In a particularly preferred embodiment of this aspect of the invention, a C-terminal portion of SEQ LP NO:6 includes the last 14 amino acids of SEQ LP NO:6 (-Phe-Leu-Ser-Ile-Ser- Leu-Leu-Phe-Ile-Ser-Phe-Tyr-Leu-Leu, SEQ LP NO: 13), which represents a novel hydrophobic signaling sequence for targeting to a ciliate membrane; even more preferably the C-terminal portion includes the last 23 amino acids of SEQ LP NO:6 (-Gln-Cys-Ala-Ser-Thr-Thr-Phe-Ala-Lys-Phe-Leu-Ser-Ile-Ser-Leu- Leu-Phe-Ile-Ser-Phe-Tyr-Leu-Leu, SEQ LP NO: 14). Most preferably, the polynucleotide fragment of this aspect of the invention has a nucleotide sequence selected from the class of nucleotide sequences that encodes a polypeptide having SEQ IP NO:6. It is envisioned that the C-terminal membrane targeting sequence of SEQ IP NO:6, exemplified by SEQ LP NOs: 13 and 14, can be fused to other proteins and use to direct membrane surface display of the fused proteins in other ciliates known to utilize GPI anchors, such as Tetrahymena and Paramecium. Conservative substitutions of hydrophobic residues, discussed in more detail below, such as substitutions of leucine, isoleucine, and phenylalanine with each other, are expected to be well tolerated within these signal sequences, and nucleotide sequence encoding SEQ LP NOs: 13 and 14 that have been modified with one or more of these conservative substitutions are also included in the invention. One example of the class of nucleotide sequences that encodes a polypeptide having amino acid SEQ IP NO: 6 is SEQ IP NO: 1. This class of nucleotide sequences is large but finite, and the nucleotide sequence of each member of the class can be readily determined by one skilled in the art by reference to the standard genetic code, as used in E. coli. Additional members of this class of nucleotide sequences, for use in ciliate expression systems, can be determined using the modified genetic code for ciliates, as described herein. Likewise, the invention includes a nucleic acid molecule comprising a polynucleotide fragment having a nucleotide sequence selected from the class of nucleotide sequences that encodes a polypeptide having at least a terminal portion of amino acid sequence SΕQ IP NO:7. Preferably, the polynucleotide is at least about 20 nucleotides in length. A terminal portion of SΕQ LP NO: 7 can be an N-terminal portion or a C-terminal portion. An "N-terminal portion" of SΕQ LP NO: 7 includes at least one amino acid that is contiguous to, and located to the N-terminal side of, amino acid 21 (Ala21) in SΕQ LP NO:7, and is exemplified by amino acids 1 (Metl) through 20 (Ser20) (SΕQ IP NO: 15). Preferably, an N-terminal portion of SΕQ IP NO: 7 includes at least about 6 contiguous amino acids from SΕQ LP NO: 15, more preferably at least about 10, even more preferably at least about 15. In a particularly preferred embodiment of this aspect of the invention, an N-terminal portion of SΕQ LP NO:7 includes the first 20 amino acids of SΕQ LP NO:7 (Met-Lys-Asn-Asn-Ile-Leu-Val-Ile- Leu-Ile-Ile-Ser-Leu-Phe-Ile-Asn-Gln-Ile-Lys-Ser, SΕQ LP NO: 15), which constitutes a novel membrane targeting sequence. A "C-terminal portion" of SΕQ IP NO: 7 includes at least one amino acid that is contiguous to, and located to the C-terminal side of amino acid 448 (Ile448) in SΕQ IP NO:7, and is exemplified by amino acids 449 (Gln449) through 468 (Leu468) (SΕQ LD NO: 17). Preferably, a C-terminal portion of SΕQ LD NO: 7 includes at least about 6 contiguous amino acids from SΕQ ID NO: 17, more preferably at least about 14, even more preferably at least about 20. In a particularly preferred embodiment of this aspect of the invention, a C-terminal portion of SΕQ LD NO:7 includes the last 14 amino acids of SΕQ ID NO:7 (Phe-Leu-Ser-Ile-Ser- Leu-Leu-Leu-Ile-Ser-Tyr-Tyr-Leu-Leu, SΕQ LD NO: 16), which represents a novel hydrophobic signaling sequence for targeting to a ciliate membrane; more preferably the C-terminal portion includes the last 20 amino acids of SΕQ LP NO:7 (Gln-Cys-Asp-Phe-Ala-Asn-Phe-Leu-Ser-Ile-Ser-Leu-Leu-Leu-Ile-Ser- Tyr-Tyr-Leu-Leu, SEQ LP NO: 17). It is envisioned the N-terminal and C-terminal membrane targeting sequences of SEQ LP NO:7, exemplified by SEQ IP NOs: 15- 17, can be fused to other proteins and used to direct secretion or membrane surface display of the proteins in other ciliates known to utilize GPI anchors, such as Tetrahymena and Paramecium. Additionally, the N-terminal membrane targeting sequence may be more generally applicable to other protein expression systems. Conservative substitutions of hydrophobic residues, such as substitutions of leucine, isoleucine, and phenylalanine with each other, are expected to be well tolerated within these signal sequences, and nucleotide sequence encoding SEQ LP Nos: 15- 17 that have been modified with one or more of these conservative substitutions are also included in the invention. Examples of the class of nucleotide sequences that encode a polypeptide having amino acid SEQ IP NO:7 are SEQ LP NOs:3 and 5. This class of nucleotide sequences is likewise large but finite, and the nucleotide sequence of each member of the class can also be readily determined by reference to the standard genetic code, as used in E. coli. Additional members of this class of nucleotide sequences, for use in ciliate expression systems, can be determined using the modified genetic code for ciliates, as described herein. The nucleic acid molecule of the invention can be PNA, RNA, or a combination thereof, and can include any combination of naturally occurring, chemically modified or enzymatically modified nucleotides. The nucleic acid molecule can be equivalent to the polynucleotide fragment encoding an i-antigen protein, or it can include said polynucleotide fragment in addition to one or more additional nucleotides or polynucleotides. For example, the nucleic acid molecule of the invention can be a vector, such as an expression or cloning vector. A vector useful in the present invention can be circular or linear, single- stranded or double stranded, and can include PNA, RNA, or any modification or combination thereof. The vector can be a plasmid, a cosmid, or a viral vector, such as baculovirus. Preferably, the nucleic acid molecule of the invention takes the form of an expression vector that is capable of expression in an organism or in a cell of the organism, in culture or in vivo. An organism or cell in which the coding sequence of the vector can be expressed can be eukaryotic or prokaryotic, and can be, without limitation, a bacterium, a yeast, an insect, a protozoan, preferably a ciliate such as Tetrahymena, or animal, such as a fish or a mammal. Preferably, the vector is expressible in a fish and/or in a conventional protein expression system, including bacteria, such as E. coli, yeast, such as Pischia pastoris, mammalian cell culture or insect cells. When the vector is intended for use in bacterial, yeast, mammalian or insect expression systems, the coding sequences of the vector are preferably engineered to utilized the conventional genetic code rather than the ciliate genetic code that is employed in the native / multifiliis coding sequences. Thus, in preferred embodiments of these aspects of the invention, the nucleotide sequence of the polynucleotide fragment that encodes an i-antigen protein is altered such that each ciliate glutamine codon TAA and TAG in the nucleotide sequence derived from the / multifiliis isolate is replaced with a universal glutamine codons, either CAG or CAA. Accordingly, a particularly preferred embodiment of the nucleic acid molecule of the invention includes a polynucleotide fragment having SΕQ LP NO:5. It should be understood that the nucleic acid molecule of the invention can be single-stranded or double-stranded, and further that a single-stranded nucleic acid molecule of the invention includes a polynucleotide fragment having a nucleotide sequence that is complementary to a nucleotide sequence that encodes an i-antigen protein or portion thereof according to the invention. As used herein, the term "complementary" refers to the ability of two single stranded polynucleotide fragments to base pair with each other, in which an adenine on one nucleic acid fragment will base pair to a thymine on the other, and a cytosine on one nucleic acid fragment will base pair to a guanine on the other. Two polynucleotide fragments are complementary to each other when a nucleotide sequence in one nucleic acid fragment can base pair with a nucleotide sequence in a second nucleic acid fragment. For instance, 5'-ATGC and 5'- GCAT are fully complementary, as are 5'-ATGC and 5'-GCAT. Further, the single-stranded nucleic acid molecule of the invention also includes a polynucleotide fragment having a nucleotide sequence that is substantially complementary to a nucleotide sequence that encodes an i-antigen protein or portion thereof according to the invention, or to the complement of the nucleotide sequence that encodes an i-antigen or portion thereof. Substantially complementary polynucleotide fragments can include at least one base pair mismatch, such that at least one nucleotide present on a first polynucleotide fragment will not base pair to at least one nucleotide present on a second polynucleotide fragment, however the two polynucleotide fragments will still have the capacity to hybridize. For instance the middle nucleotide of each of the two PNA fragments 5'-AGCAAATAT and 5'-ATATATGCT will not base pair, but these two nucleic acid fragments are nonetheless substantially complementary as defined herein. Two polynucleotide fragments are substantially complementary if they hybridize under hybridization conditions exemplified by 2X SSC (SSC: 150 mM NaCl, 15 mM trisodium citrate, pH 7.6) at 55 °C. Substantially complementary polynucleotide fragments for purposes of the present invention preferably share at least one region of at least about 50 nucleotides in length, which shared region has at least about 85% nucleotide identity, preferably at least about 90% nucleotide identity. More preferably, substantially complementary polynucleotide fragments share a plurality of regions of between about 50 nucleotides and about 150 nucleotides in length, which shared regions have at least about 85% nucleotide identity, preferably at least about 90% nucleotide identity. In a particularly preferred embodiment, the substantially complementary nucleotide sequence encodes at least one of SEQ IP NOs:90-100 as shown on Fig. 3(a). Locations and levels of nucleotide sequence identity between two nucleotide sequences can be readily determined using CLUSTALW multiple sequence alignment software. The invention further includes a nucleic acid molecule comprising a polynucleotide fragment that hybridizes to at least a portion of the complement of either or both of SEQ LP NO:l or SEQ IP NO:3, under standard hybridization conditions, provided that the polynucleotide fragment encodes a polypeptide comprising at least a membrane targeting portion or an antigenic portion of an i- antigen protein. A membrane targeting portion of an i-antigen protein is one that targets the polypeptide to either the endoplasmic recticulum (e.g., an N-terminal signal sequence) or to the plasma membrane (e.g., a GPI anchor sequence). Standard hybridization conditions are exemplified by 2X SSC (SSC: 150 mM NaCl, 15 mM trisodium citrate, pH 7.6) at 55 °C. The invention further includes a nucleotide molecule comprising a polynucleotide fragment encoding an antigenic analog or modification of a polypeptide represented by SEQ IP NO: 6, or an antigenic fragment thereof that includes at least a C-terrriinal portion of SEQ IP NO:6; further, the invention includes a nucleotide molecule comprising a polynucleotide fragment encoding an antigenic analog, fragment, or modification of a polypeptide represented by SEQ LP NO:7, as described in more detail below. Also provided by the invention is a novel i-antigen protein. In a further aspect of the invention, therefore, the i-antigen protein is encoded by a nucleotide sequence derived from I. multifiliis (Gl isolate), and has a molecular weight of about 48 kP. In a particularly preferred embodiment of this aspect of the invention, the i-antigen protein is encoded by the nucleotide sequence as shown in Fig. 3(b), nucleotides 1 through 1326 (SEQ IP NO:l), representing the coding region of the IAG48[G1] gene of / multifiliis, and has the amino acid sequence SEQ IP NO:6 (Fig. 1). In another aspect of the invention, the i- antigen protein is encoded by a nucleotide sequence derived from / multifiliis (G5 isolate), and has a molecular weight of about 55 kP. In a particularly preferred embodiment of this aspect of the invention, the i-antigen is encoded by the nucleotide sequence as shown in Fig. 2(a), nucleotides 1 through 1404 (SEQ IP NO:3), representing the coding region of the IAG55[G5] gene, and has the amino acid sequence SEQ IP NO:7 (Fig. 4). The i-antigen polypeptide of the invention includes an i-antigen polypeptide having SEQ ID NO:6; an i-antigen polypeptide having SEQ JD NO:7; an analog or modification of an i-antigen polypeptide having SEQ ID NO:6; a fragment of an i-antigen polypeptide having SEQ ID NO:6 having at least a C-terminal portion of SEQ YD NO:6; an antigenic analog, fragment, or modification of an i-antigen polypeptide having SEQ ID NO:7; and an analog, fragment, or modification of an i-antigen polypeptide having SEQ ID NO: 7 wherein said analog, fragment or modification has at least one terminal portion of SEQ LD NO:7. An antigenic analog, fragment, or modification of a polypeptide having SEQ ID NOs: 6 or 7 is one that generates an immune response in fish against / multifiliis. Antigenicity of an polypeptide can be evaluated in vitro by performing a Western blot on the purified polypeptide (for example, an affinity purified polypeptide) using polyclonal antisera from a rabbit that was vaccinated with at least an antigenic portion of a native / multifiliis i-antigen protein, preferably with a complete / multifiliis i-antigen protein (e.g., SEQ ID NO:6 or SEQ ID NO:7). Antigenic analogs of polypeptide having SEQ IP NO:6, SEQ IP NO:7 include i-antigen polypeptides having amino acid substitutions that do not eliminate polypeptide antigenicity in fish. Substitutes for an amino acid may be selected from other members of the class to which the amino acid belongs. For example, nonpolar (hydrophobic) amino acids include alanine, leucine, isoleucine, valine, proline, phenylalanine, tryptophan, and tyrosine. Polar neutral amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine and glutamine. The positively charged (basic) amino acids include arginine, lysine and histidine. The negatively charged (acidic) amino acids include aspartic acid and glutamic acid. Examples of preferred conservative substitutions include Lys for Arg and vice versa to maintain a positive charge; Glu for Asp and vice versa to maintain a negative charge; Ser for Thr so that a free -OH is maintained; and Gin for Asn to maintain a free NH 2 . Fragments of an i-antigen polypeptide of the invention include i-antigen polypeptides containing deletions or additions of one or more contiguous or noncontiguous amino acids that do not eliminate the antigenicity of the i-antigen in fish are also contemplated. Fragments of an i- antigen polypeptide contain at least about six amino acids, preferably at least about 10 amino acids, more preferably at least about 60 amino acids. Modified i-antigens include i-antigens that are chemically and enzymatically derivatized at one or more constituent amino acid, including side chain modifications, backbone modifications, and N- and C- terminal modifications including acetylation, hydroxylation, methylation, amidation, and the attachment of carbohydrate or lipid moieties, cofactors, and the like. The invention further includes an antigenic polypeptide that shares a significant level of primary structure with either or both of SEQ LP NO: 6 or SEQ IP NO:7. Preferably, the antigenic polypeptide of this aspect of the invention is a synthetic polypeptide. A synthetic polypeptide is one that does not have the amino acid sequence of a polypeptide that is isolated from an organism; i.e., it is not a naturally occurring polypeptide. An antigenic polypeptide shares a significant level of primary structure with either or both of SEQ LP NO: 6 or SEQ ID NO: 7 if it has a plurality of amino acid sequence domains, each domain having about 60 to about 100 amino acids, of which six are cysteines that fall into register when the sequence domains are aligned with the amino acid sequence domains of SEQ LP NOs:6 or 7, as exemplified in Fig. 5. The predominant primary structure motif in an antigenic polypeptide of the invention is -Cys-Xaa 23 -Cys- (SEQ LP NOs:31, 32), where Xaa is any amino acid, and where Xaa^ means Xaaj or Xaa 3 , that is, where pairs of cysteines are separated by two or three amino acids. Preferably, the antigenic polypeptide of the invention has larger scale repeating motifs characterized by -Cys-Xaa j -Cys- Xaa m -Cys-Xaa 3 -Cys-Xaa n -Cys-Xaa 2 -Cys- where m = 15 - 25, preferably 20-22, and n = 15-25, preferably 19-20 (e.g., SEQ LP NO:33 where m = 20, n = 20). More preferably, the larger scale repeating motifs are characterized by -Cys- Xaa 2 -Cys-Xaa m -Cys-Xaa 3 -Cys-Pro-Xaa p -Cys-Xaa 2 -Cys- where m = 15-25, preferably 20-22 and p = 14-24, preferably 18-19 (e.g., SEQ LD NO:88 where m = 20, p = 18). In a particularly preferred embodiment, the larger scale repeating motifs are characterized by -Cys-Xaa 2 -Cys-Xaa q -Gln-Cys-Xaa 3 -Cys-Pro-Xaa- Gly-Thr-Xaa r -Cys-Xaa 2 -Cys-, where q = 14-24, preferably 19-21, and r = 11-21, preferably 15-16 (e.g., SEQ LD NO:89 where q = 20, r = 20). In a particularly preferred embodiment of the invention, each amino acid sequence domain of an antigenic polypeptide of the invention has at least 90% amino acid identity with at least one of SEQ ID NOs:8-12 (Fig. 5(a)) and 55-60 (Fig. 5(b)); most preferably, each amino acid sequence domain of an antigenic polypeptide of the invention has at least 95% amino acid identity with at least one of SEQ LD NOs:8-12 and 55-60 (Fig. 5). The location and level of amino acid sequence identity between two amino acid sequences can be readily determined using CLUSTALW multiple sequence alignment software. Alternatively, me two amino acid sequences (i.e., the amino acid sequence of the candidate domain sequence of the antigenic polypeptide and the reference amino acid sequence selected from SEQ JD NOs:8-12 and 55-60) are aligned such that the cysteines are in register, then further aligned to maximize the number of amino acids that they have in common along the lengths of their sequences; gaps in either or both sequences are permitted in making the alignment in order to place the cysteines in register and to maximize the number of shared amino acids, although the amino acids in each sequence must nonetheless remain in their proper order. The percentage amino acid identity is the higher of the following two numbers: (a) the number of amino acids that the two sequences have in common within the alignment, divided by the number of amino acids in the reference sequence, multiplied by 100; or (b) the number of amino acids that the two sequences have in common within the alignment, divided by the number of amino acids in the candidate polypeptide, multiplied by 100. The invention further includes a polypeptide having a membrane targeting sequence selected from a C-terminal portion of SEQ ID NO: 6, an N- terminal portion of SEQ ID NO: 7, and a C-terminal portion of SEQ ID NO: 7, as described above. The present invention further provides a vaccine for use in preventing or controlling disease in fish caused by / multifiliis. The polynucleotide vaccine comprises a polynucleotide fragment, preferably a PNA fragment, having a nucleotide sequence encoding an antigenic polypeptide comprising at least an antigenic portion of an i-antigen protein derived from / multifiliis. The polynucleotide vaccine optionally further comprises a promoter, preferably the CMV promoter, operably linked to the coding sequence for the i-antigen polypeptide (e.g., U.S. Pat. No. 5,708,448, Pavis). There are numerous plasmids known to those of ordinary skill in the art useful for the production of polynucleotide vaccines. A specific embodiment employs constructs using the plasmid pcPNA3.1 as the vector (InNitrogen Corporation, Carlsbad, CA). In addition, the vector construct can contain immunostimulatory sequences (ISS), such as unmethylated dCpG motifs, that stimulate the animal's immune system. Other possible additions to the polynucleotide vaccine constructs include nucleotide sequences encoding cytokines, such as granulocyte macrophage colony stimulating factor (GM-CSF), interleukin-12 (IL-12) and co-stimulatory molecules such B7-1, B7-2, CO40. The cytokines can be used in various combinations to fine-tune the response of the animal ' s immune system, including both antibody and cytotoxic T lymphocyte responses, to bring out the specific level of response needed to affect the animal's reproductive system. The polynucleotide vaccine can also encode a fusion product containing the antigenic polypeptide and a molecule, such as CTLA-4, that directs the fusion product to antigen-presenting cells inside the host. Plasmid PNA can also be delivered using attenuated bacteria as delivery system, a method that is suitable for PNA vaccines that are administered orally. Bacteria are transformed with an independently replicating plasmid, which becomes released into the host cell cytoplasm following the death of the attenuated bacterium in the host cell. An alternative approach to delivering the polynucleotide to an animal involves the use of a viral or bacterial vector. Examples of suitable viral vectors include adenovirus, polio virus, pox viruses such as vaccinia, canary pox, and fowl pox, herpes viruses, including catfish herpes virus, adenovirus-associated vector, and retroviruses. Exemplary bacterial vectors include attenuated forms of Salmonella, Shigella, Edwardsiella ictaluri, and Yersinia ruckerii. Preferably, the polynucleotide is a vector, such as a plasmid, that is capable of autologous expression of the nucleotide sequence encoding an i-antigen. In a particularly preferred embodiment, the vaccine is a PNA vaccine comprising a PNA fragment having a nucleotide sequence that encodes a polypeptide having amino acid sequence SEQ IP NO:6 or SEQ LP NO:7, an antigenic analog, fragment, or modification of a polypeptide having SEQ LP NO:7, an antigenic analog or modification of a polypeptide having SEQ LP NO:6, or an antigenic fragment of a polypeptide having SEQ LP NO:6 provided that the fragment includes at least a C-terminal portion of SEQ LP NO:6. An antigenic analog, fragment, or modification of an i-antigen polypeptide is one that generates an immune response in fish against / multifiliis. For example, a preferred PNA vaccine comprises a PNA fragment having a nucleotide sequence encoding an amino acid sequence selected from the group consisting of SEQ IP NOs:8-12 and 55-60; these eleven amino acid sequences represent tandemly repeated amino acid sequence domains found in the deduced amino acid sequence SEQ IP NO:6 and are expected to be antigenic. More preferably, the PNA vaccine comprises all or a portion of nucleotide sequence SEQ IP NO: 1, SEQ LP NO:3 or SEQ IP NO:5, provided that the portion of SEQ YD NO:l, SEQ ID NO:3 or SEQ ID NO:5 encodes an antigenic polypeptide and further provided that, in SEQ LD NO: 1 and SEQ ID NO:3 codons TAA and TAG that code for glutamine in / multifiliis are changed to CAG or CAA. Most preferably, the DNA vaccine includes a synthetic DNA fragment having a nucleotide sequence that encodes a polypeptide SEQ LD NOs:6 or 7 using codons that are biased in favor of the host's codon usage. Polynucleotide-based immunization induces an immune response to an antigen expressed in vivo from a heterologous polynucleotide fragment introduced into the fish. This method can be advantageous over other methods because heterologous nucleic acid expression may continue for a length of time sufficient to induce a relatively strong and sustained immune response without the need for subsequent "booster" vaccinations, as is common when portions of the protein antigen itself have been injected into the animal. A polynucleotide vaccine comprising a polynucleotide fragment having a nucleotide sequence encoding an i-antigen protein can be administered to a fish using biolistic bombardment, bath immersion, ingestion or direct injection, as described in U.S. Pat. No. 5,780,448 (Davis), preferably intraperitoneal or intramuscular injection. A preferred method of administration is biolistic bombardment, as with a "gene gun." A polynucleotide vaccine formulated for oral administration preferably contains DNA encapsulated in a biodegradable polymer. Examples of a suitable biodegradable polymer include chitosan and homo- or co-polymers of polylactic acid and polyglycolic acid. The invention thus further provides a method for immunizing freshwater fish against / multifiliis by administering to the fish a polynucleotide vaccine of the invention, preferably a DNA vaccine. The amount of polynucleotide vaccine to be administered to an animal depends on the type and size of animal, the condition being treated, and the nature of the polynucleotide, and can be readily determined by one of skill in the art. In fish, for example, if the polynucleotide vaccine is to be injected, the amount per injection is preferably at least about 10 ng; at most it is preferably about 50 μg, more preferably it is less than about 1 μg. If the polynucleotide vaccine is to be administered using a gene gun, the amount per dose is preferably at least about 1 ng; at most it is preferably about 10 μg, more preferably it is less than about 1 μg. For administration by immersion, the concentration of the polynucleotide in the aquatic medium is preferably at least about 10 ng/mL; at most it is preferably about 50 μg/mL, preferably it is less than about 1 μg/mL. For oral administration the amount per dose is preferably at least about 10 ng; at most it is preferably about 10 μg, preferably less than about 1 μg. In some applications, one or more booster administrations of the vaccine at time periods subsequent to the initial administration are useful to create a higher level of immune response in the animal. In another aspect, the vaccine of the invention comprises an i-antigen polypeptide having amino acid sequence SEQ ID NO: 6 or SEQ ID NO: 7, an antigenic analog, fragment, or modification of a polypeptide having SEQ ID NO: 7, an antigenic analog or modification of a polypeptide having SEQ ID NO:6, or an antigenic fragment of a polypeptide having SEQ LD NO:6 provided that the fragment includes at least a C-terminal portion of SEQ LD NO:6. This type of vaccine is referred to herein as a "protein subunit vaccine" even if it contains the entire i-antigen sequence. The i-antigen or antigenic analog, fragment, or modification thereof for use in the protein subunit vaccine of the invention can be naturally occurring (i.e., isolated from / multifiliis) or recombinant. A protein subunit vaccine of the invention are conveniently administered to fish using bath immersion, ingestion, topical administration, or direct injection, preferably intraperitoneal or intramuscular injection. A protein subunit vaccine formulated for oral aclministration preferably contains polypeptide encapsulated in a biodegradable polymer as described above in connection with the polynucleotide vaccine of the invention. In addition, the protein subunit vaccine can be administered to an animal via a live vector, such as recombinant Tetrahymena. Tetrahymena can be transformed such that it expresses the i-antigen or antigenic analog, fragment, or modification thereof, either in the cytosol, as a transmembrane protein, as a GPI-anchored protein or as a secreted protein. Recombinant Tetrahymena can be injected into the animal or, in the case of an aquatic animal such as a fish, can be administered via immersion. Oral adrninistration of Tetrahymena if also envisioned. The invention thus further provides a method for immunizing freshwater fish against / multifiliis by adπiinistering to the fish a protein subunit vaccine of the invention. The amount of protein subunit vaccine to be administered to an animal depends on the type and size of animal, the condition being treated, and the nature of the protein, and can be readily determined by one of skill in the art. In fish, for example, if the protein subunit vaccine is to be injected, the amount per injection is preferably between about 0.1 μg and about 1000 μg per lOg fish; more preferably it is between about 1 μg and about 100 μg per 10 g offish. For administration by immersion, the concentration of the protein in the aquatic medium is preferably at least about 10 ng/mL; at most it is preferably about 50 μg/mL, preferably it is less than about 1 μg/mL. For oral aα^ninistration the amount per dose is preferably between about 0.1 μg and about 100 μg per lOg fish; more preferably it is between about 1 μg and about 10 μg per 10 g offish. Preferably, the protein subunit vaccine also includes an adjuvant. Further, one or more boosters are preferably administered at time periods subsequent to the initial administration to create a higher level of immune response in the animal. In yet another aspect, the vaccine of the invention comprises a fusion protein comprising a carrier polypeptide and an i-antigen polypeptide of the invention or an analog, fragment, or modification thereof. An i-antigen analog, fragment, or modified i-antigen for use in this aspect of the invention can itself be antigenic or nonantigenic; in embodiments wherein the i-antigen analog, fragment or modified i-antigen is nonantigenic, the carrier polypeptide provides the necessary antigenicity by stimulating the fish's immune system to react to the fusion protein thereby generating an immune response in fish against / multifiliis. A nonantigenic analog, fragment, or modification of the i-antigen thus function as a hapten. An example of an antigenic carrier polypeptide is KLH. Conventional fusion constructs between carriers such as glutathione sulfotransferase (GST) and i-antigens of the invention or antigenic analog, fragment, or modifications thereof are also included as protein subunit vaccines according to the invention, as are fusions of the i-antigen and an affinity tag such as a polyhistidine sequence. A fusion construct may be preferred for use as a protein subunit vaccine when the antigenic i-antigen analog, fragment, or modification thereof is small. The invention further provides a method for immunizing freshwater fish against / multifiliis by administering to the fish a fusion protein vaccine of the invention. Monoclonal antibodies that recognize immobilizing epitopes on i- antigens are protective in passive immunization experiments, but their activity is serotype-specific. On the other hand, fish that are actively immune following exposure to one serotype are cross-protected against heterologous strains. Thus, in one embodiment, the vaccine of the invention (whether in the form of a protein vaccine or a polynucleotide vaccine) is monovalent in that it is derived from a particular i-antigen from a particular serotype of/ multifiliis and effective to treat or prevent infection of the vaccinated animal by that serotype. Preferably, the monovalent vaccine contains at least one antigenic determinant that is shared by i-antigens of different serotypes, such that it also prevents infection by other I. multifiliis of other serotypes, thus offering broad protection. In another embodiment, the vaccine of the invention (whether in the form of a protein vaccine or a polynucleotide vaccine) is a combined vaccine or a multivalent vaccine that prevents infection by other / multifiliis of more than one serotype. The combined or multivalent vaccine can contain or encode, for example, a plurality of serotype-specific i-antigen polypeptides or antigenic portions thereof, derived from multiple serotypes of/ multifiliis, or can contain or encode a synthetic or fusion i-antigen polypeptide containing multiple antigenic determinants that together generate an immune response against multiple serotypes of/ multifiliis. In a preferred embodiment of the vaccine of the invention, i-antigen or an antigenic portion thereof is linked at its carboxy-terminus to at least two molecules of the C3d component of complement, using molecular cloning techniques. Preferably, the i-antigen or antigenic portion thereof is linked to about three molecules of the C3d component of complement. The C3d molecule can be either homologous or heterologous with respect to the species to be vaccinated. Complement genes have been cloned and characterized in salmonids (J. Lambris et al., J. Immunol. 151:6123 (1993); J. Sunyer et al., Proc. Natl. Acad. Sci USA 93:8546 (1996)). For vaccinations offish, the i-antigen or antigenic portion thereof is preferably linked to a salmonid C3d, such as trout C3d or catfish C3d. In the case of a protein subunit vaccine, the recombinant protein is conveniently expressed in bacteria, then administered to fish. This technique has been shown to generate in increase in the immune response in mice (P. Dempsey et al., Science 271:48 (1996)). The receptor for C3d, namely CD21, is expressed primarily on B cells and the follicular dendritic cells of lymphoid tissues. In the case of a polynucleotide vaccine, a plasmid encoding a fusion protein that incorporates an i-antigen or antigenic portion thereof, linked at its carboxy-terminus to at least two molecules of the C3d component, is administered to the fish. The active immune-stimulating ingredients are optionally mixed with excipients or diluents that are pharmaceutically acceptable as carriers and compatible with the active ingredient. The term "pharmaceutically acceptable carrier" refers to a carrier(s) that is "acceptable" in the sense of being compatible with the other ingredients of a composition and not deleterious to the recipient thereof. Suitable excipients are, for example, water, saline, dextrose, glycerol, ethanol, or the like and combinations thereof. In addition, if desired, the immune-stimulating composition (including vaccine) may contain minor amounts of auxiliary substances such as wetting or emulsifying agents, pH buffering agents, and/or adjuvants which enhance the effectiveness of the immune-stimulating composition. Fish that can be vaccinated against Ich infection include freshwater teleosts, preferably fish that are widely farmed in aquaculture including channel catfish, trout, salmon, tilapia and eels. The goal of vaccination against Ich infection is to elicit a population of lymphocytes, which upon subsequent exposure to the parasite proliferate and produce antibodies and/or effector cells specific to the parasite, resulting in protection against lethal infections. A vaccine effective for the prevention of Ich infection in freshwater fish is thus one which elicits the production of protective antibodies in a fish exposed to said vaccine. Those protective antibodies will prevent lethal infection of the vaccinated fish upon challenge with / multifiliis. In the present invention, protective antibodies generated in the fish are specific for / multifiliis i-antigens. Fish that can be immunized include ornamental and food fish. The present invention further includes monoclonal or polyclonal antibodies, whether derived from fish, rodents, mammals, avians, or other organisms, that bind to the i-antigen proteins described herein, including antigenic analogs, fragments and modifications thereof. Production and isolation of monoclonal and polyclonal antibodies to a selected polypeptide sequence is routine in the art. Sera from immune fish is known to confer passive immunity against both viral and bacterial pathogens when injected into non-immune fish (Hedrick et al. Trans. Amer. Fish Soc. 116:277 (1987); Viele et al. J. Fish Biol. 17:379 (1980)). We have carried out similar experiments in which mouse monoclonal antibodies specific for Ich i-antigen(s) of the A and D serotypes were transferred into non- immune 10-15 g catfish fingerlings. Monoclonal antibodies which have the ability to immobilize these serotypes in vitro conferred passive protection against the parasite in vivo. Fish weighing 10-15 g were injected intraperitoneally with 20 -200 μg of purified mouse monoclonal antibody (MAb) and challenged with infective theronts after 24 hours. A serotype (Gl -specific) MAbs included 10H3, 3H12, 8E11, 6A11 and 5A8; D serotype (G5 and G3-specific) MAbs included G3-27 and G3-61. Animals injected with immobilizing MAbs survived lethal infection while controls were completely overwhelmed. The passive protection achieved in the channel catfish model supports the use of i-antigen protein(s) as vaccines for eliciting active immunity (Lin et al., Inf. Immun. 4085- 4090 (1996); see also He et al. (Aquaculture 158:1-10 (1997)). Comparison of newly discovered amino acid sequences SEQ ID NO: 6 (the Gl / multifiliis isolate) and SEQ ID NO:7 (the G5 / multifiliis isolate) shows regions of amino acid identity that were heretofore unknown and unpredictable, because the sequence of only one / multifiliis i-antigen (i.e., the sequence encoded by the cDNA for the 48 kP protein, Clark et al. (Proc. Nat. Acad. Sci. USA. 89:6363-6367 (1992) was known. Fig. 3(a) shows regions of identity or high homology, which are boxed. It is expected that an oligonucleotide probe or primer having a sequence that encodes a conserved region of the i-antigens as identified herein will be useful for identifying additional i-antigens in / multifiliis having other serotypes; molecular methods for constructing these probes and primers and using them to screen genomic libraries are routine in the art. For example, amino acid sequences at the N and C- termini are highly conserved (see Fig. 3(a)). The corresponding regions of the genes would likely serve as sites for the design of diagnostic PCR primers that could be used to amplify i-antigen sequences from any parasite isolate. Moreover, using mouse monoclonal antibodies against the i-antigens, we have been able to define differences between parasite isolates, and have shown that protection following passive antibody transfer is strain (i.e., isolate) specific. Thus, the identification of additional i-antigens using nucleotide probes and primers derived from conserved regions as described herein will likely provide amino acid and nucleotide sequence information that allows the production of additional serotype-specific i-antigen PNA and protein subunit vaccines. Accordingly, the invention further provides for an oligonucleotide probe or primer (including its complement) having a nucleotide sequence encoding at least a portion of an i-antigen conserved region as shown in Fig. 3(a) (boxed regions). Preferably, the oligonucleotide probes or primers are represented by nucleotide sequences that encode conserved / multifiliis i-antigen regions of at least about eight amino acids in length, which conserved regions are selected from portions of SEQ IP NOs: 90- 100 (Fig. 3(a)) that contain at least about eight amino acids. More preferably, the oligonucleotide probes or primers are represented by the sequences MKYNLLLT (SEQ IP NO:36), FLSISLLF (SEQ IP NO:38), GTALOPGV (SEQ LP NO:46), AGTOTCT (SEQ IP NO:48), CTKKLTSGA (SEQ IP NO:50) and FAKFLSISL (SEQ IP NO:52). Further, the invention provides a method for identifying i-antigens in / multifiliis by using the oligonucleotide probe or primer of the invention to identify and isolate novel nucleotide sequences encoding other i-antigens, for example by probing a genomic PNA or cPNA library of an isolate of/ multifiliis or by conducting polymerase chain reaction. Vaccines that utilize the nucleotide and amino acid sequences of the i-antigens so discovered, which as a result are effective against other serotypic variants of/ multifiliis, are also provided. Knowledge of the i-antigen nucleotide and amino acid sequences set forth herein also opens up new possibilities for detecting, diagnosing and characterizing Ichthyophthirius in fish populations. For example, an oligonucleotide probe or primer based on a conserved region of the i-antigen protein can be used to detect the presence of Ichthyophthirius in a fish or in water, and an oligonucleotide probe or primer based on a less conserved region can be used to identify a specific Ichthyophthirius serotype. The invention therefore includes methods for detecting and characterizing Ichthyophthirius, for example in aquaculture facilities. Recombinant Expression of I-antigens As already noted, the principal difficulty associated with making a vaccine against / multifiliis is the fact that this obligate parasite cannot be easily cultured. Thus, production of i-antigen in an recombinant system is highly desirable. Ciliated protozoans, however, including Ichthyophthirius, utilize TAA and TAG as codons for the amino acid glutamine, while most other organisms recognize those as termination codons. Therefore, according to the invention, either the native nucleotide sequence is expressed in a ciliated protozoan, or the native nucleotide sequence is altered to use a universal glutamine codon (either CAG or CAA) in place of any TAA and TAG triplets used in the native sequence. Bacterial expression systems. Selection of a vector or plasmid backbone depends upon a variety of desired characteristics in the resulting construct, such as a selection marker, plasmid reproduction rate, and the like. Suitable plasmids for expression in E. coli, for example, include pUC(X), pKK223-3, pKK233-2, pTrc99A, and pΕT-(X) wherein (X) denotes a vector family in which numerous constructs are available. pUC(X) vectors can be obtained from Pharmacia Biotech (Piscataway, NH) or Sigma Chemical Co. (St. Louis, MO). pKK223-3, pKK233-2 and pTrc99A can be obtained from Pharmacia Biotech. pET-(X) vectors can be obtained from Promega (Madison, WI) Stratagene (La Jolla, CA) and Novagen (Madison, WI). To facilitate replication inside a host cell, the vector preferably includes an origin of replication (known as an "ori") or replicon. For example, ColEl and PI 5 A replicons are commonly used in plasmids that are to be propagated in E. coli. The expression vector optionally includes a promoter sequence operably linked to the nucleotide sequence encoding i-antigen protein. A promoter is a PNA fragment which causes transcription of genetic material. Transcription is the formation of an RNA chain in accordance with the genetic information contained in the PNA. The invention is not limited by the use of any particular promoter, and a wide variety are known. Promoters act as regulatory signals that bind RNA polymerase in a cell to initiate transcription of a downstream (3' direction) coding sequence. A promoter is "operably linked" to a nucleic acid sequence if it is does, or can be used to, control or regulate transcription of that nucleic acid sequence. The promoter used in the invention can be a constitutive or an inducible promoter. It can be, but need not be, heterologous with respect to the host cell. Preferred promoters for bacterial transformation include lac, lac\JV5, tac, trc, T7, SP6 and ara. The nucleotide sequence encoding the i-antigen protein can advantageously be fused, at either the 5 ' or 3' end, to a nucleotide sequence encoding an affinity tag, such as a polyhistidine amino acid sequence. The resulting fusion construct can be conveniently purified using the affinity tag for subsequent use. Affinity tags and methods for protein purification using affinity tags are well-known in the art. Optionally, a cleavage site, such as a Factor X cleavage site, can be introduced between the affinity tag and the amino acid sequence of the i-antigen to facilitate large-scale preparation of the i-antigen protein free of the carrier polypeptide. The expression vector optionally includes a Shine Oalgarno site (i.e., a ribosome binding site), and a start site (e.g., the codon ATG) to initiate translation of the transcribed message to produce the enzyme. It can also include a termination sequence to end translation. A termination sequence is typically a codon for which there exists no corresponding aminoacetyl-tRNA, thus ending polypeptide synthesis. The nucleic acid fragment used to transform the host cell can optionally further include a transcription termination sequence. The rrnB terminators, which is a stretch of PNA that contains two terminators, Tl and T2, is the most commonly used terminator that is incorporated into bacterial expression systems (J. Brosius et al., J. Mol. Biol. 148:107-127 (1981)). The TAA and TAG codons in the native coding sequence of the i-antigen can be substituted with conventional glutamine codons either by site-directed mutagenesis (see e.g., Sambrook et al. (1989) Molecular Cloning. Second Edition, Cold Spring Harbor Laboratory Press, Plainview, NY) or by creating a synthetic coding sequence using chemical synthesis of the desired coding sequence. Manual PNA synthetic techniques are well known (see, e.g., Caruthers (1983) in Methodology of PNA and RNA Sequencing. Weissman (ed.), Praeger Publishers, New York, Chapter 1), as is automated PNA synthesis using any of several commercially available systems. An alternative strategy is to express the i-antigen coding sequence with the UAA and UAG glutamine codons in a suppressor strain of E. coli. Cohen et al. (J. Molec. Biol. 216:189-194 (1990)) has reported a plasmid vector (pAP205) which provides an inducible suppressor tRNA which recognizes UAA and UAG codons and which is charged with glutamic acid. The plasmid pAP205 contains the gene encoding the artificial suppressor tRNA 81 " su oc205, previously described by Raftery et al. (EMBO J. 6:1499-1506 (1987)) which gene is expressed in pAO205 under the regulatory control of an isopropyl-β-P- thiogalactopyranoside (IPTG)-inducible tac promoter. IPTG induces the expression of the tRNAs and thus allows the readthrough of UAG and UAA stop codons. Basal levels of the suppressor tRNAs are kept low by the presence of a leaky transcription termination signal between the promoter and the tRNA gene to minimize potentially lethal effects on the cells. J. Cohen et al. (J. Mol. Biol. 216:189 (1990)) reported the expression of a Paramecium tubulin gene in E. coli using this vector. Eukaryotic expression systems. The insect baculovirus vector, Autographa californica, is capable of high level expression of foreign gene inserts and posttranslational modifications (see, e.g., International Patent Publication WO 90/14428). In terms of qualitative considerations, posttranslational modifications, including glycosylation, may be crucial in stimulating appropriate antigenic responses in vaccinated animals. In this regard yeast vectors can be useful since both glycosylation and secretion of foreign proteins are possible (Innis et al., Science 228:21 (1985)). A commercially available system (Immunex Corp., Seattle, WA) which is advantageous from both qualitative and quantitative standpoints has recently been described (Hopp et al., Biotechnology 6:1204 (1988)). This system allows both the detection and simplified purification of fusion proteins from yeast supernatants or E. coli extracts. Protozoan expression systems. Tetrahymena is a ciliated protozoan which is taxonomically related to / multifiliis. T. thermophila recognizes UAA and UAG codons as glutamine codons as does / multifiliis. In addition, posttranslational modifications, particularly glycosylation, are expected to occur more normally in organisms related to Ichthyophthirius (for example, Tetrahymena) than in procaryotes such as E. coli, or more distantly related eucaryotes. Because such modifications can play a critical role in immune recognition, Tetrahymena can have an advantage on this level as well. Vectors for and gene expression in Tetrahymena thermophila have been reported. For example, T. thermophila has been successfully transformed using self- replicating palindromic ribosomal PNA (rPNA) purified from macronuclei (Brunk et al., Εxp. Cell Res. 162:390 (1988)); Lovlie et al., Proc. Natl. Acad. Sci. USA 85:5156 (1988); Tondravi et al., Proc. Natl. Acad. Sci. USA 83:4369 (1986)). A selectable paromomycin resistance marker has been isolated and characterized; the resistant phenotype is due to a point mutation in the 17S rRNA gene. Resistance to hydromycin is conferred by this mutation as well (Sprangler et al., J. Biol. Chem. 260:6334 (1985)). Subsequently shuttle vectors capable of autonomously replicating as plasmids in Tetrahymena as well as in E. coli have been developed (Yu et al, Proc. Natl. Acad. Sci USA 86:8487 (1989)); Yu et al., Proc. Natl. Acad. Sci. USA 85:5151 (1988)). Such plasmids have been stably maintained at high copy number for more than 65 generations, and there has been at least one homologous gene expressed from a Tetrahymena shuttle vector. Successful expression of an Ich gene in T. thermophila allows the production of relatively large amounts of antigen in a purified form, at relatively low cost. In addition to its use in the production of purified antigens, transformed T. thermophila can also be used as a live vector in the animal host, as described above. This second approach provides the most efficient means of exposing fish to / multifiliis surface antigens (short of infecting them with the live parasite itself). Up to 500,000 T. thermophila cells can be injected into the peritoneal cavity of 15 g channel catfish, where they survive for several days without causing adverse effects. EXAMPLES The present invention is illustrated by the following examples. It is to be understood that the particular examples, materials, amounts, and procedures are to be interpreted broadly in accordance with the scope and spirit of the invention as set forth herein. Example 1 Isolation and Characterization of the /. multifiliis (Gl Isolate) Gene Encoding the 48 kD I-antigen Parasite culture. Ichthyophthirius multifiliis (isolate Gl) was maintained on juvenile channel catfish (Ictalurus punctatus) as previously described (H. Pickerson et al., J. Euk. MicrobioL. 40:816-820(1993)). Briefly, the infection is passsaged on fingerling channel catfish by incubation of one infected fish with five uninfected fish in a 10 gallon aquarium. Fish were gently rubbed and host- associated trophonts that were released from the skin were allowed to adhere to the sides of collecting dishes. Cells were then washed once in carbon-filtered tap water and permitted to divide to form infective theronts (18-24 hours at 23 °C). After filtration through 0.0015 inch wire mesh (Fisher Scientific, Pittsburgh, PA) cells were used for PNA isolation. Genomic PNA isolation. Total PNA was obtained using a modification of procedures described by Kavenoff et al. (Chromosoma. 41:1-27 (1973)). Theronts were harvested by centrifugation at 1 ,000 X g for 2 minutes and the resulting cell pellet lysed in 2 volumes HET buffer (0.5 M EPTA, 0.01 M TrisΗCl, pH 9.5) containing 1% SPS at 65 °C. After 30 minutes, samples were cooled to 50 °C and additional HET containing pronase (2 mg/ml) was added to bring the final volume to 4 times that of the original cell pellet. Lysates were incubated 4-6 hours, diluted with 0.5 vol 0.1 X SSC (0.1X SSC: 15 mM NaCl 1.5 mM sodium citrate, pH 7.0) and then extracted with an equal volume of phenol:chloroform:isoamyl alcohol (25:24: 1). After separation from the aqueous layer, the organic phase was back-extracted with 0.5 volume of 10 mM Tris-HCl, 1 mM EPTA, pH 8.0 (TE) and the aqueous layers combined. Samples were re-extracted with phenol:chloroform:isoamyl alcohol (25:24:1) and chloroformύsoamyl alcohol (24:1), and nucleic acids were precipitated by addition of 2 volumes ethanol. The precipitate was collected by centrifugation at 10,000 x g for 15 minutes (4°C), washed in cold 70% ethanol and air dried. The pellet was dissolved in 2 X SSC containing 1 mM EPTA, and contaminating RNA was removed by digestion with 100 μg/ml RNase A (Promega) at 37 °C for 1 hour. PNA was precipitated by addition of 2 volumes ethanol and collected by centrifugation as above. The pellet was washed for several hours in cold 70% ethanol and the PNA dissolved in TE to a final concentration of 50-250 μg/ml. Genomic library construction. Total PNA (20 μg) was digested overnight at 37 °C with the restriction endonuclease Swa I (4.2 units enzyme/μg PNA) (Boehringer-Mannheim Biochemicals, Indianapolis, LN). This enzyme has an 8 base pair recognition sequence containing all As and Ts. Because the non-coding regions of ciliate PNA are extremely A-T rich, Swa I cuts preferentially and relatively often in intergenic regions yielding roughly gene- sized pieces of PNA. After phenol extraction, restriction fragments were ethanol precipitated and dissolved in TE (10 mM Tris/1 mM EPTA pH 8.0. Potential staggered ends were repaired by incubation with T4 PNA polymerase (0.2 unit/μl for 30 minutes at 37 °C) (Promega, Madison, WI), and small fragments (<271 base pairs (bp)) removed by chromatography on Sephacryl 400 (Amersham Pharmacia Biochemical, Piscataway NJ). Genomic PNA (average size - 1.5-3 kb) was combined with a 20-fold molar excess of 10 mer EcoR I adaptors (e.g., Cat. No. C-1291, Promega) and the mixture incubated for 18 hours at 15 °C in the presence of T4 PNA ligase (0.25 unit/μl) (Promega). T4 polynucleotide kinase (to 0.25 unit/μl; Promega) and ATP (to 10 nM) were added and the reactions incubated an additional 30 minutes at 37° C to phosphorylate adaptor ends. Fragments were phenol extracted and ethanol precipitated as above, and again fractionated on Sephacryl 400 to remove excess adaptors. Resulting PNA was then ligated with λZAP II arms (Stratagene, La Jolla, CA) which had been EcoR I digested and dephosphorylated. Ratios of insert to vector PNA were varied according to the recommendations of the manufacturer. Individual ligation reactions were packaged using Gigapack II packaging extracts (Stratagene) and libraries titered on E. coli XL 1 -Blue MRF' in the presence of X-gal and IPTG. A library containing 4 X 10 5 phage plaques (> 90% white) was amplified and used for gene isolation. Library Screening. The amplified library was plated at 5 X 10 4 plaques/ 150 mm plate and the resulting phage particles lifted onto nylon filters (Micron Separations Inc., Westborough, MA) as described by Sambrook et al., 1989. A total of 8 filters were hybridized under conditions of high stringency (5X SSC at 68C overnight in 2% blocking reagent (Boehringer Mannheim Biochemicals, Indianapolis, IN), followed by two 5 minute washes in 2X SSC, 0.1 % SPS, followed by two 15 minute washes in 0.5X SSC, 0.1 % SPS at 68C) with a 1.2 kb cPNA probe against the 48 kP i-antigen (Clark et al. Proc. Nat. Acad. Sci. USA. 89:6363-6367 (1992)) labeled with digoxigenin (Genius System; Boehringer-Mannheim Biochemicals). This probe was previously shown to encode most of the 48 kP i-antigen of parasite isolate Gl . Filters were reacted with an alkaline phosphatase-tagged anti-digoxigenin antibody (Boehringer-Mannheim Biochemicals) and developed with CSPP chemiluminescent substrate (Tropix). Twelve positive clones were isolated and subjected to two rounds of plaque purification, wherein a positive clone is selected, diluted, replated and rescreened. Inserts were then subcloned into pBluescript SK(-) by in vivo excision in the presence R408 helper phage (Stratagene). Plasmid PNA was purified from bacterial cultures by alkaline lysis and anion-exchange chromatography using modified silicagel resins (Qiagen, Valencia, CA). Cloned inserts were mapped by digestion with appropriate restriction enzymes using techniques known to one of skill in the art (see, for instance, Sambrook et al., 1989). All positive recombinants contained a 3.3 kb Swa I fragment. A restriction map of the cloned insert from one recombinant (GL3-8) is shown in Fig. 6. Both strands of the region extending from the left end through the distal EcoR I site of the insert were subjected to nucleotide sequencing. DNA sequencing was performed with a Perkin Elmer/Applied Biosystems Division 377 automated DNA sequencer using dye terminator chemistry and AmpliTaq-FS DNA polymerase (Perkin Elmer, Norwalk, CT). Sequence analysis showed that the insert encoded the entire 48 kD i-antigen. Transcript Mapping. The 5' end of i-antigen transcripts were mapped by primer-extension analysis. Briefly, 10 pM of antisense primer H4 (AGCAGCACCTACATCAGTCAATCC, SEQ LD NO:21) complimentary to a sequence near the putative ATG start codon of the 48 kD i-antigen gene (nucleotides +94-117), was end-labeled with y 32 ? (Amersham, Arlington Heights, TL) in the presence of T4 polynucleotide kinase (10 units; Promega) (Sambrook et al., 1989). The labeled primer (2 pM) was hybridized with 10 μg total RNA from / multifiliis theronts (Gl isolate) and extended at 42° for 1.5 hours in the presence of AMV reverse transcriptase (26 Units; Boehringer- Mannheim). Mussel glycogen was added as a carrier (200 μg/ml; Boehringer- Mannheim) and the extension product was precipitated by addition of 0.5 volume 7.5 M ammonium acetate and 2.5 volumes ethanol. Following centrifugation (14,000 X g for 10 minutes), the pellet was air dried and dissolved in H 2 0. The size of primer extension products was determined relative to a 32 P- labeled sequencing ladder run on parallel lanes of a 6% polyacrylamide/urea sequencing gel. The ladder was prepared in a dideoxy sequencing reaction (Sequenase kit; United Sates Biochemicals) using single-stranded M13mpl8 PNA as a template, and Ml 3 universal primer GTAAAACGACGGCCAGT (SEQ IP NO:22) labeled with 32 P. After electrophoresis, gels were fixed, dried and autoradiographed for visualization of labeled PNA fragments (Sambrook et al., 1989). The 3' ends of i-antigen transcripts were mapped using the RACE (rapid amplification of cPNA ends) protocol (M. Frohman, In RACE: Rapid amplification of cPNA ends. In: PCR Protocols: A Guide to Methods and Applications. Innis, M. A., et al., (eds.) Academic Press, San Piego, pp. 28-38 (1990)). Total RNA was prepared from / multifiliis (Gl) theronts by lysis in guanidine thiocyanate, and poly(A) + mRNA was purified by two rounds of chromatography on oligo(dT)-cellulose (Clark et al. Proc. Nat. Acad. Sci. USA. 89:6363-6367 (1992)). First-strand cPNA was then prepared from 1 μg poly(A) + RNA, using 25 pmol of primer EPBdT 18 (GCGAATTCTGCAGGATCCAAACT 18 , SEQ LP NO:23); kindly provided by Or. Royal McGraw, University of Georgia), and AMV reverse transcriptase (0.25 unit/μl; Boehringer-Mannheim) (Sambrook et al., 1989). Following incubation at 42 °C for 2 hours, a fraction of the first-strand product was used as template in a second-round PCR reaction containing either of two forward primers, namely, ICH5 (GTGTCGACAGCAGGTACTGATACATG, SEQ IP NO:24) or H5 (CGAAAACAGTGGTGGTAGTACCTT, SEQ LP NO:25) in combination with the reverse primer, EPB (GCGAATTCTGCAGGATCCAAAC, SEQ IP NO:26). The ICH5 primer corresponded to a region of the gene that lay proximal to the breakpoint between the gene and 1.2 kb cPNA sequence, while H5 corresponded to a region of the 1.2 kb cPNA that lay distal to that site. PCR was carried out under standard conditions using 20 pmol primer/100 μl reaction and 5 U Taq PNA polymerase (Promega) (1 minute at 94°C, 1 minute at 52°C, 1 minute at 72°C; 30 cycles). The ICH5/EPB product was electrophoresed on a 1.4% agarose gel and ran as a broad band of -295-375 bp. PNA was eluted from the gel, digested with Sal I and EcøR I, and then directionally cloned into pBluescript SK(-) for subsequent sequence analysis. cPNA Clones. A λZAP cPNA library prepared from / multifiliis trophont RNA was screened with a 32 P-labeled 24 mer oligonucleotide (5'- AGCAGCACCA ACATCAGTCA AACC, SEQ IP NO:27) encoding eight amino acids near the N-terminus of the 48 kP i-antigen, as previously described (Clark et al. Proc. Nat. Acad. Sci. USA. 89:6363-6367 (1992)). In addition to a clone that provided the 1.2 kb i-antigen cPNA (designated 2-3), the library screen yielded several additional positive recombinants. Two such clones (designated 1-1 and 1-3) were chosen for sequence analysis. cPNA inserts were subcloned into pBluescript SK(-), and plasmid PNA sequenced on the positive strand using the ICH5 primer. Southern blotting analysis. 5' and 3 '-specific probes were generated using the polymerase chain reaction. The 5 '-specific probe was a 475 bp fragment spanning nucleotides +188-662 of the gene and was amplified using the 1.2 kb cPNA as a template in conjunction with the forward and reverse primers, ATGGTAATTAACCTTTCGCAGCAAATAA (SEQ LP NO:28) and GGTCTGCATTTAACACATAA (SEQ LP NO:29), respectively. The 3'- specific probe was a 495 bp fragment amplified from genomic PNA using H5 as the forward primer, and the reverse primer AGATACATCAGTATACGAAA (SEQ IP NO:30). The later sequence was derived from the H5/EPB RACE product. Probes were purified by agarose gel electrophoresis, and labeled using random oligonucleotide synthesis (High Prime PNA labelling kit™; Boelrringer-Mannheim) in the presence of α 32 P-dCTP (Amersham). Genomic PNA (5 μg) from / multifiliis theronts (Gl isolate) was digested with either EcoR I, Hind LLI or Swa I, fractionated on a 0.7% agarose gel and transferred to nylon. The filter was then hybridized with probes specific for either the 5' or 3' ends of the 1.2 kb cPNA in separate reactions. The probes were radiolabeled to 10 9 cpm/μg andused at 10 6 cpm/mL. Hybridization was carried out under conditions of high stringency (washed overnight in 6X SSC, 10X Penhardt's reagent, 0.1% SPS, 10 μg/mL denatured herring sperm PNA at 65C; followed by one 15 minute wash at room temperature in 2X SSC, 0.1% SPS, followed by one 15 minute wash at 65C in 2X SSC, 0.1% SPS, followed by one 15 minute wash at 65C in 0.5X SSC, 0.1 % SPS, followed by one 15 minute wash at 65C in 0.2X SSC, 0.1% SPS) as previously described (Clark et al. Proc. Nat. Acad. Sci. USA. 89:6363-6367 (1992)). Comparisons between gene and cPNA sequences. Results of nucleotide sequencing reactions were complied and analyzed using DNASIS software (version 2.0; Hitachi Software Engineering, Yokahama, Japan). When the nucleotide sequence of the 1.2 kb cPNA used as a probe in this study was compared with the coding region of the 48 kP i-antigen gene, a number of differences were observed. These included two single-base substitutions (nucleotides +781 and 952), as well as complete divergence in the region distal to nucleotide +1227 (nucleotide 1172 of the cPNA). To determine whether these differences were real, the 1.2 kb i-antigen cPNA was resequenced and found an error at position +781 (position 726 of the cPNA should read A rather than C as originally reported) (GenBank accession no. M92907 updated). The transversion at nucleotide 952, and the divergent sequence at the 3' end of the cPNA were, nevertheless, corLfirmed. As indicated in Fig. 7, these differences would result in an amino acid substitution at position 318 (phenylalanine for leucine), and an entirely different sequence at the carboxy-terminus of the gene product. Specifically, the cPNA product would lack the C-terminal hydrophobic amino acid residues specified by the gene. Hydrophobic domains at the C-termini of GPI-anchored proteins are necessary for covalent attachment of glycosylphosphatidylinositol moieties (P. Englund, Annu. Rev. Biochem.. 62:121-138 (1993)), and the deletion of relevant coding sequences from transgenes that encode such proteins usually results in secretion rather than membrane binding. Furthermore, there are a number of instances in which alternative splicing of endogenous mRNAs gives rise to transcripts that either specify, or fail to encode hydrophobic C-terminal peptides (I. Caras et al., Nature. 325:545-548 (1987); H. Gower et al., Cell, 55:955- 964(1988)). In the first case, such products are retained at the plasma membrane (as GPI-anchored proteins), while in the second, they are exported from the cell. Both secreted and membrane bound forms of a 48 kPa i-antigen have been described in / multifiliis (C. Xu et al., J. Euk. MicrobioL. 42:558-564 (1995)). It should also be noted that the parasite isolate designated Gl contains a second i-antigen gene believed to encode an antigenically related 60 kPa protein (T. Clark et al., Annu. Rev. Fish Pis.. 5:113-131 (1995)); this gene is recognized by the 5 '-specific probe used in this study and appears as a 5 kb fragment in the Swa I digest as discussed below. The 3 '-specific probe fails to recognize this fragment indicating that the 1.2 kb cPNA is not a product of this gene. Interestingly, when the 3 '-end of i-antigen transcripts were mapped by RACE using a sense primer upstream of nucleotide +1227 (primer ICH5), the resulting PCR product had essentially the same sequence as the gene (Fig. 6). Consistent with this observation, two independent i-antigen cPNAs were isolated from the same library used to prepare the 1.2 kb probe and found that these contained the same sequence as the gene at their 3' ends (Fig. 6). By contrast, when a sense primer corresponding to the 1.2 kb cPNA downstream of nucleotide 1172 (primer H5; see herein) was used, the RACE product that was generated was much larger than expected (~600 bp) and had a sequence entirely different from that of the gene. Furthermore, it was not possible to generate PCR products with genomic PNA as a template in standard reactions using sense and antisense primer pairs that flanked nucleotide 1172 of the 1.2 kb cPNA. Since this suggested that the sequences on either side of nucleotide 1172 were discontinuous within the genome, Southern hybridization analysis was carried out using probes described above corresponding to 5' and 3' regions of the 1.2 cPNA. The restriction fragments recognized by the two probes were different. A 3.3 kb band recognized by the 5 '-specific probe in the Swa I digest corresponded to the genomic PNA fragment described herein. A larger (5 kb) band was also observed, which may represent the gene for a related 60 kP i- antigen expressed by the Gl isolate (Clark et al., Proc. Nat. Acad. Sci. USA. 89:6363-6367 . 1992 ^ 1: T.Clark et al.. Annu. Rev. Fish Pis.. 5:113-131 Q995Ϊ). Example 2 Identification of the Open Reading Frame and Analysis of the Deduced 48 kD I-antigen Sequence from /. multifiliis (Gl Isolate) Computer-based sequence analysis. Homology searches were performed at the ISREC World Wide Web server (Swiss Institute for Experimental Cancer Research) using the BLAST network service (BLAST, basic local alignment search tool; WU-BLAST server version 2.0al3) and the SwissProt+Trembl+TrUpdates peptide sequence databases. Predictions of potential signal peptides and their cleavage sites were made using the Signalp World Wide Web server (http://www.cbs.dtu.dk/services/SignalP/) version 1.0. Peptide mass was determined using the PEPTLPE MASS tool accessed through the ExPASy molecular biology World Wide Web server of the Swiss Institute of Bioinformatics (^ttp://expasy.hcuge.ch/sprot/peptide-mass.html). Amino acid sequence alignment was carried out using the CLUSTALW (1.74) multiple sequence alignment program accessed through the ExPASy web server. Identification of the open reading frame. The cPNA used to screen the genomic library in Example 1 begins with a 5 '-terminal G that lies several nucleotides upstream of a valine codon (GTT) marking the N-terrninus of the mature 48 kP i-antigen (Clark et al., Proc. Nat. Acad. Sci. USA. 89:6363-6367 (1992)). This G residue corresponds to nucleotide +56 of the genomic PNA sequence shown in Fig. 1. Translating downstream from this nucleotide (and in- frame with the original cPNA), the gene was found to contain a single, uninterrupted reading frame extending through nucleotide +1326, followed by two adjacent TGA stop codons. / multifiliis, along with other hymenostomatid ciliates, utilize TGA as the only stop codon; the standard TAA and TAG triplets specify glutamine instead. Translating in the opposite direction, the region immediately upstream of the 5 '-terminus of the cPNA was found to encode a methionine (predicted by the ATG triplet at nucleotides +1-3), followed by a stretch of 19 mostly hydrophobic amino acids. The hydrophobic nature of these amino acids (along with the fact that the i-antigens are membrane polypeptides) would suggest that this region specifies a signal peptide that targets the protein to the plasma membrane. To investigate this further, neural network algorithms trained on signal peptides and their cleavage sites (H. Nielsen et al., Prot. Engin.. 10:1-6 (1997)) were used to examine the first 50 amino acids of the deduced protein sequence beginning with the methionine residue cited above. Such algorithms identified the first 20 amino acids as a signal peptide (S mean = 0.839), and predicted a cleavage site between the alanine and valine residues (amino acids 20 and 21, respectively) of the deduced amino acid sequence. The N-terminal amino acid of the 48 kP antigen protein corresponds to the valine residue predicted above (Clark et al., Proc. Nat. Acad. Sci. USA. 89:6363-6367 (1992)). Taken together, these observations argue strongly that amino acids 1-20 of the deduced protein constitute a signal peptide. Assignment of the methionine residue (amino acid position 1) as the start site of the 48 kP protein is also supported by the results of primer extension analysis on poly A + RNA from the infective theronts. I-antigen transcripts appear to initiate at two sites located -33 and -34 nucleotides upstream of the A nucleotide of the methionine codon. Because no other ATG triplets are predicted within this region, the methionine residue at position 1 almost certainly represents the translational start site of the protein itself. Based on these considerations, the coding region of the gene extends 1326 nucleotides and specifies a protein precursor of 442 amino acids having a theoretical MW r (molecular mass) of 45,025 daltons. The gene is designated IAG48[G1J. A total of 23 UAA and UAG triplets (encoding glutamine in / multifiliis but functioning as stop codons in most other eukaryotes and prokaryotes) are present within IAG48[G1J. Curiously, the third position of the codons shared a strong preference for either A or T (84.9%). While the significance of this is not understood, such bias appears to be a common feature of i-antigen genes (D. Martindale, Protozool.. 36:29-34 (1989); J. Deak et al., Gene. 164:163-166 (1995)) and may reflect important constraints on RNA structure. The polyA tract at the 3' end of the RACE product, shown in Fig. 7, most likely represents the site at which polyadenylation of IAG48[G1] mRNA occurs. Based on this (and assiiming an average length of 100-200 nt for the polyA tail), the predicted size of RNA transcripts from this gene would be in the range of 1 ,543-1643 nt. Amino acid sequence motifs. Analysis of the deduced protein sequence of the gene revealed a minor discrepancy between the predicted mass of the preprocessed protein (45 kDa), and the size of the mature i-antigen based on SDS-PAGE (48 kDa) (T. Clark et al., Annu. Rev. Fish Pis.. 5:113-131 (1995)). Potential secondary modifications could appreciably alter the electrophoretic mobility of the processed protein. Furthermore, structural anomalies associated with repetitive sequence motifs could lead to incorrect estimates of size based on SPS-PAGE. This aside, the most interesting structural features of the deduced protein are hydrophobic regions at the N- and C- terminus, a consensus P-loop domain, and tandemly repetitive cysteine-rich motifs. A stretch of 14 mostly hydrophobic amino acids separated from a short spacer from three small amino acids (-Cys-Ala-Ser-) was predicted at the extreme carboxy-terminus of the protein (Fig. 1). This type of sequence is highly characteristic of an addition site for a glycosylphosphatidylinositol (GPL) anchor (P. Englund, Annu. Rev. Biochem.. 62:121-138 (1993)). Other evidence shows that the 48 kP i-antigen is, in fact, GPI-anchored. The hydrophobic region at the ammo-terminus of the protein is consistent with i-antigens being membrane associated proteins; presumably, the N-terminus targets the protein to the endoplasmic reticulum. The P-loop domain at position 316-323 [Gly-[Xaa 4 ]-Gly-Lys-Ser] (SEQ LP NO:34) may or may not be significant. While this type of structure is generally associated with proteins that bind ATP or GTP, the presence of such motifs does not insure a role in nucleotide binding (M. Saraste et al., Trends Biochem. Sci.. 15:430-434 (1990)). Indeed, proteins with closest similarity to the 48 kPa i- antigen in terms of primary structure (i.e., the Lembadion L-factor, Tetrahymena SerH, and Giardia NSP surface antigens) all lack a consensus phosphate-binding loop. Instead, these proteins, like the deduced 48 kP i-antigen, have numerous Cys-Xaa 23 -Cys (SEQ ID NO:31, 32) motifs embedded within higher order tandemly repeated amino acid sequence domains. Repetitive sequence domains. Beginning with a cysteine residue at position 23, five homologous segments with an average of 80 amino acids each were identified within the deduced 48 kD i-antigen protein sequence (Fig. 5(a)). Adjacent repeats had the greatest degree of homology, with the second and third being ~ 90% identical. As shown in Fig. 5(a) and (b), the repeats are characterized by 6 invariant cysteines that fall into register when the segments are aligned. The predominant spacing between cysteines has the order Cys- Xaa j 3 -Cys (SEQ LD NO:31, 32). There are 14 such motifs in the 48 kD i- antigen protein sequence, as well as 4 larger elements having the order, Cys- Xaa 2 -Cys-Xaa 20 -Cys-Xaa 3 -Cys-Xaa 20 -Cys-Xaa 2 -Cys (SEQ ID NO:34). The spacing between the last cysteine of each repeat, and the first cysteine of the next repeat is Cys-Xaa 3 -Cys (SEQ LD NO:32). Cysteine motifs with the order Cys-Xaa j 3 -Cys (SEQ ID NO:31, 32) are common to a large and diverse family of proteins that bind zinc and other metal ions (J. Berg et al., Science. 271:1081-1085 (1996)). A search of the SWISS- PROT/TrEMBL database showed similarities (smallest sum probability level 2.8e-06) between the 48 kD i-antigen and several other entries, five of which were protozoan membrane proteins. These included the SerH immobilization antigen of the free-living ciliate Tetrahymena thermophila (accession no. Q27197), a putative membrane protein (L-factor) from the predatory ciliate Lembadion bullinum (accession no. Q94589), and three variant-specific surface proteins (VSPs) of the mammalian gut parasite, Giardia lamblia (accession nos. Q24977; Q24970; P21849). Homologies with two additional entries (an antifreeze protein from Arctic cod [accession no. Q13028], and the product of an unidentified gene from Caenorhabditis elegans [accession no. Q 17084]) were considered less significant because of unusual bias in the amino acid composition of these proteins. With regard to the protozoan membrane proteins, all contained numerous Cys-Xaaj 3 -Cys (SEQ LD NO:31, 32) motifs. The product of the Giardia vspA6-Sl gene (accession no. Q24970) showed the greatest similarity to the 48 kD i-antigen in terms of the overall spacing of these motifs, with 29 of a possible 30 cysteine residues in the / multifiliis protein overlapping with the identical amino acid in the vspA6-Sl gene product (Fig. 8). This region has eight Cys-Xaa 2>3 -Cys (SEQ LP NO:31 , 32) motifs that do not align with similar domains in the / multifiliis polypeptide. Example 3 Identification of an L multifiliis (G5 Isolate) Gene Encoding a 55 kD I- antigen Construction of G5 cPNA library. A G5 cPNA library was made as follows. First-strand cPNA synthesis was primed with a 20-fold molar excess of oligodT 18 in a reaction containing 5 μg of poly A + RNA from / multifiliis theronts (strain G5) and AMV reverse transcriptase as described by Krug et al. (Meth. EnzymoL 152:316-325 (1987)). Second-strand synthesis was carried out in the presence of PNA polymerase I and Rnase H, followed by hairpin loop cleavage with mung bean nuclease as described by Gubler (Meth. Enzymo 152:330-335 (1987)). The second-strand product was size-fractionated and adaptors were added as in construction of the Gl genomic PNA library (Example 1). After removal of excess adaptors, material > 500 bp was cloned into the lambda phage vector λZAPLI as for Gl genomic PNA. Screening of the G5 cPNA library. The G5 cPNA library was first screened under conditions of reduced stringency with a digoxigenin-labelled probe corresponding to the coding region of the 48 kP i-antigen gene of parasite isolate Gl. No positive clones were identified. Previous attempts to screen a custom-made genomic PNA library from another parasite isolate (Gl.l) with the same probe had also resulted on no positive clones. Pegenerate PCR primers were then produced against the 5' and 3' ends of the 48 kP i-antigen gene. These primers, designated #35 and #37, corresponded to the 5' and 3' regions of the coding sequence of IAG48 [Gl], and the primer sequences were as follows: #35: ATGAAATA(C/T)AA(C/T)ATTTTATTAATT (SEQ LD NO:35), which is a 4-fold degenerate sense primer corresponding to all or part of the amino acid sequence MKYNLLLT (SEQ LD NO:36) at the N- terminus of the IAG48[G1J gene product; and #37: AAATAATAA(G / A)GAAAT(A / QGATAAAAA (SEQ LD NO:37), which is a 4-fold degenerate antisense primer corresponding to all or part of the amino acid sequence FLSISLLF (SEQ ID NO:38) at the C-terminus of the IAG48[G1] gene product. PCR conditions were 30 cycles of 94 C for 1 minute, 52 C for 1 minute, 72 C, for 1 minute. A single band corresponding to an amplified i- antigen coding region (55 kD) was expected. However, this reaction yielded several bands on agarose gels visible by ethidium staining, none of which had the expected size of a product from a gene encoding a 55 kD protein (Fig. 9). To determine which band, if any, corresponded to an amplified i-antigen coding region, the amplification products were Southern blotted and probed under conditions of reduced stringency with a radiolabelled cDNA probe corresponding to the 48 kD i-antigen of isolate, i.e., radiolabelled IAG48[G1J. Hybridization conditions were 6X SSC (Denhardt's, SDS, herring sperm DNA) at 55° C overnight. Blots were washed in 2X SSC at RT (2 X 15 minutes), followed by 2X SSC at 55° C (2 X 15 minutes). The IAG48[Gl] probe revealed a band of about 1.1 kb on the blot. The amplification products were again resolved by agarose gel electrophoresis, the region of the gel corresponding to DNA fragments of approximately 1.1 kb was excised, and the fragment was labeled with 32 P-dCTP. The radiolabelled fragment was used to probe the G5 cDNA library under high stringency conditions as for the Gl genomic DNA library (Example 1). A single positive clone containing a 1 kb cDNA insert was identified. On sequencing, this cDNA predicted a protein with all the hallmarks of an i-antigen, viz. tandemly repetitive amino acid sequence domains (approximately 80 amino acids each) containing 6 periodic cysteine residues (C-X 23 -C motifs), in addition to a stretch of hydrophobic amino acids at the C-terminus virtually identical to those predicted by the gene for the 48 kD antigen. The 1 kb cDNA was nevertheless truncated and lacked the region coding for the N-terminal part of the protein. RACE (Rapid Amplification of cDNA Ends) was carried out in an attempt to determine the missing 5' sequence. An antisense primer designated G5-11 (TGCTCGAGAATCTGTTGCTCCACCTG, SEQ LD NO:39) was used for first-strand cDNA synthesis, then a polyG tail was added and the resulting cDNA was amplified with the forward and reverse primers Q 0 (CCAGTGAGCAGAGTGACGAGGACTCGAGCTCAAGCCCCCCCCCCCC CCCCCC, SEQ LD NO:40) and, G5-11 (TGCTCGAGAATCTGTTGCTCCACCTG, (SEQ LP NO:39) respectively (PCR conditions: 1 cycle of 94° C for 3 minutes, 30° C for 2 minutes, 72° C for 40 minutes, then 30 cycles of 94 C for 45 seconds, 53° C for 45 seconds, 72° C for 90 seconds (e.g., Frohman, Meth. EnzymoL 218:340-356 (1993)). This was followed by a second round of (nested) PCR using the forward and reverse primers Q, (GAGGACTCGAGCTCAAGC, SEQ LP NO:41) and G5-12 (AACTCGAGTACCAGCAGGGCATTTAAC, SEQ IP NO:42), respectively (PCR conditions: 30 cycles of 94° C for 45 seconds, 53° C for 45 seconds, 72° C for 2 minutes). This produced a RACE product that approached the extreme 5' end of the coding region of the G5 gene (IAG55(G5)), but still failed to reach it; no ATG start codon was observed. We then resorted to a procedure referred to as inverse PCR (see Fig. 10). Based on the known sequence of the 1 kb cPNA (the shaded bar in Fig. 10), two oppositely oriented primers, G5-11 (SEQ IP NO:39) and G5-4 (CACACCTTGTCCGGCAATTAAAC, SEQ LD NO:43) were designed and used to amplify the regions flanking the 5'- and 3'-ends of the 1 kb cDNA. Genomic DNA (1 μg) from the G5 parasite isolate was incubated with a variety of different restriction enzymes (including Afl JJ and Swa I) in separate 20 μl reactions for 6 hours. DNA fragments were brought to a volume of 200 μl in IX DNA ligation buffer (Gibco-BRL Life Technologies, Gaithersburg, MD), then chloroform extracted and allowed to self-anneal at 22 C for 12 hours in the presence of T4 DNA ligase (40U) (Gibco-BRL). DNA was ethanol precipitated and amplified using the forward and reverse primer pair G5-11 and G5-4 under the following conditions: 10 cycles of 94 C for 10 seconds, 60 C for 30 seconds, 68 C for 4 minutes, followed by 20 cycles of 94° C for 10 seconds, 60° C for 30 seconds, 68 C for 4 minutes (+ 20 seconds for each cycle). DNA fragments of about 1.6 and about 1.9 kb were produced in the samples generated by the restriction enzymes Afl LI and Swa I, respectively. The amplified products were cloned into the pCR-Script™ plasmid vector (Stratagene) and sequenced. Both clones contained the 5' end of the coding sequence of the 55 kDa i-antigen gene, along with additional sequences flanking the 5' and 3' ends of the coding regions. New forward and reverse primers were constructed based upon nucleotide sequences of the 5' and 3' regions flanking the 55 kD coding region, and these primers were used to amplify the entire coding region of the gene using G5 genomic DNA as a template. A comparison between the deduced amino acid sequence encoded by the 48 kD i-antigen gene and the deduced amino acid sequence encoded by the 55 kP i-antigen gene, and of the coding regions of the two genes, is shown in Fig. 3. Example 4 Glycolipid Anchoring of I. multifiliis (G5 Isolate) 55 kD I-antigen Protein Membrane proteins from / multifiliis (strain G5) were extracted with the non-ionic detergent Triton X-l 14. Following Triton extraction, detergent micelles (containing amphipathic proteins) were treated with a recombinant GPI-phospholipase C (GPI-PLC) from African trypanosomes which cleaves the lipid anchor from GPI-linked membrane proteins. Fig 11, left panel, shows a Coomassie blue stained gel of theront membrane proteins fractionated by SPS- PAGE. Lane (A) contains total detergent soluble membrane proteins. The prominent band at 55 kP represents the i-antigens of the G5 strain. Lane (B) shows the detergent soluble membrane protein fraction after cleavage with GPI- PLC. Lane (C) contains proteins liberated into the aqueous phase afer enzyme treatment. Note the transfer of the 55 kP band from the detergent to the aqueous phase. Fig. 11, right panel, shows a Western Blot of fractions (A) and (C) reacted with an antibody conjugate that recognizes a cross-reacting determinant on the cleaved portion of the GPI-anchor that becomes exposed following enzyme treatment. Positive staining of a 55 kP band in lane (C) (but not A) argues strongly for GPI-linkage of Ichthyophthirius i-antigens with the plasma membrane. Example 5 Recombinant Production of a 55 kD I-antigen Protein in E. coli Synthesis of a synthetic gene. A synthetic G5 i-antigen gene sequence (SEQ LP NO: 102, Fig. 2(b)) was constructed using a method known as PNA shuffling essentially as described by Stemmer, et al. (Gene 164:49-53 (1995)). Eighteen overlapping primers (primers 3201-3218, Fig. 12) spanning the entire length of the G5 i-antigen coding sequence were combined in a single tube at a final concentration of 5 μM and allowed to assemble in a cycling reaction carried out 30 times at 94 C for 15 seconds, 52 C for 30 seconds and 70 C for 30 seconds (plus 10 additional seconds in each cycle). Following the assembly reaction, 2.5 μl of the product was amplified with the original flanking primers (3201 and 3218) to produce a full-length synthetic gene. The resulting product was cloned and sequenced. Mutations arising from error-prone synthesis PCR were corrected by site-directed mutagenesis (Transformer Site-Pirected Mutagenesis kit, Clontech Laboratories, Inc., Palo Alto, CA). Synthesis of modified versions of the synthetic gene. Based on its deduced sequence, the 55 kP protein contains hydrophobic sequences at its N- and C-terminus that are presumed to target the protein to the plasma membrane (the N-terminal sequence acting as a signal peptide for ER localization, and the C-terminal sequence acting as a GPI-anchor cleavage and addition site). To determine whether different gene products might be processed differentially by the immune system following their expression in fish, we made two modified versions of the gene, one lacking the GPI-anchor addition sequence and the other lacking the N-terminal signal peptide. Specifically, we wanted to determine whether the product of the C-terminal deletion might give rise to a stronger humoral immune response since it was expected be secreted from cells rather than be bound to the plasma membrane through a glycolipid anchor. Therefore, having made the synthetic gene for the 55 kP protein, we used PCR to construct the modified versions of the full-length gene. The first lacked the coding region for the signal peptide at the N-terminus (residues 1-20), and the second lacked the hydrophobic stretch at the extreme C-terminus of the protein (amino acids 453-468). I-antigen protein production in E. coli. A recombinant version of the 55 kP / multifiliis i-antigen protein of parasite isolate G5 (SΕQ LP NO: 7) was produced in E. coli strain XL-1 Blue using the plasmid expression vector pProΕX™-l (Gibco-BRL LifeSciences) into which a version (SEQ LP NO:53) of the synthetic gene (SEQ LP NO:5) had been inserted. This version (SEQ LP NO:53) was identical to SEQ LP NO:5 except that it contained a point mutation at nucleotide position 17 in the coding sequence (Fig.13), resulting in the substitution of a proline for a leucine at amino acid position 6 in the i-antigen protein sequence (L6P) (SEQ LP NO:54) (Fig. 14). Logarithmic cultures were incubated in LPTG to induce expression of the 55 kP antigen fused to a 6x- histidine tag and rTEV protease cleavage signal. Fig. 15 shows a Western blot of a 10% SPS-polyacrylamide gel on which equivalent amounts of bacterial protein from cells taken before (1) and above (2) induction with LPTG were run. The blot was reacted with monospecific rabbit antibodies against the 55 kP i- antigen of the G5 strain followed by goat anti-rabbit IgG couples to alkaline phosphatase. Color substrates were NBT an BCIP. Note the strong band at about 55 kP in the induced sample. The modified versions of the synthetic gene that lacked membrane targeting sequences at their 5'- or 3'- ends were cloned into several procaryotic expression vectors containing LPTG-inducible promoters (pProEX-1 (Gibco), pET22b(+) (Novagen), pQE-16 (Qiagen) and pGEX-4P- 1 (Pharmacia)). Recombinant proteins of the expected size that reacted strongly on Western blots with monospecific polyclonal antibodies against the 55 kPa antigen were produced in all bacterial transformants following the addition of LPTG. Example 6 Immune Response of Catfish to Vaccination with /. multifiliis (G5 Isolate) 55 kD I-antigen Protein It is known that naive fish are completely protected against infection by / multifiliis following passive transfer of immobilizing murine monoclonal antibodies (mAbs). To test whether / multifiliis i-antigens themselves can elicit protective immunity, vaccination trials were carried out with channel catfish (Ictalurus punctatus) immunized with 55 kP i-antigen purified by mAb affinity chromatography from / multifiliis serotype P. Fifty channel catfish (each weighing 10-15 g) were immunized by two intraperitoneal (i.p.) injections (two weeks apart) consisting of 10 μg of purified 55 kP i-antigen of the / multifiliis G5 isolate (i-Ag) in Freund's complete or incomplete adjuvant. The same number of negative control animals were immunized similarly with either an irrelevant 14 kPa I. multifiliis G5 protein (14kP) or bovine serum albumin (BSA). A fourth group of positive control fish was vaccinated by two i.p. injections of 8000 and 10,000 live G5 parasites (live Ich) without adjuvant (this treatment has previously been shown to elicit protective immunity). All groups were challenged with infective G5 theronts (15,000/fish) 8 weeks after the last injection. Seventy-two percent (72%) offish immunized with the i-antigen and 59.2% offish immunized with live parasites survived challenge (Fig. 16). All of the negative control animals died. In addition, there were significant differences (P>0.001, Kruskal-Wallis one way ANOVA) between the median days to death of controls and the fish that died following immunization with i-antigen or live theronts. In a separate trial, single i.p. injections of 10 μg of i-antigen mixed with either a CpG oligodeoxynucleotide (19-mer) adjuvant or Freunds complete adjuvant provided 33% and 40% protection, respectively. Control fish immunized with i-antigen given with a non-CpG oligonucleotide or BSA with a CpG oligodeoxynucleotide all died following challenge. Serum and mucus antibody production in vaccinated fishes corresponded with protection. These results demonstrate the efficacy of purified i-antigen with adjuvant in eliciting protective immunity following vaccination. Example 7 Immune Response of Channel Catfish to Vaccination with /. multifiliis (G5 Isolate) 55kD I-antigen Plasmid Vaccine Truncated versions of the 55kP i-antigen from / multifiliis (G5 isolate) containing N-terminal and C-terminal deletions (Example 5), along with the full-length sequence, were cloned into the eucaryotic expression vector pcPNA3.1 for use fish vaccine trials. Although we have not yet determined how the respective gene products localize in cultured fish cells, an equivalent C- terminal deletion in the gene for the 48 kP i-antigen from serotype A resulted in secretion of the corresponding protein following transformation of T. thermophila (Example 9, Fig. 17). Groups of 10 fish were vaccinated by intramuscular injection with either 10, 1 or 0. 1 μg of plasmid PNA (pcPNA3. 1) containing either the full-length, N-terminal or C-terminal deletion of the synthetic 55 kPa i-antigen gene. A positive control group vaccinated with the purified antigen in Freund's complete adjuvant (see Example 6), and a negative control group injected with the pcPNA3.1 containing the lacZ gene were included in the study. Sera were withdrawn from fish at two week intervals beginning 14 days after injection and antibody levels were determined by Western blotting and ELISA. SPS-PAGE was carried out under both reducing and non-reducing conditions, and Western blots were probed with pooled antisera from the different groups offish. The 55 kP i-antigen from parasite isolate G5 was affinity purified and fractionated by SPS-PAGE under non-reducing conditions (50 ng/lane). Protein was then transferred to PVDF membrane and reacted sequentially with pooled sera from fish vaccinated with pcDNA3.1 PNA vaccine constructs followed by a monoclonal antibody cornjugate (alkaline phosphatase) against the heavy chain of channel catfish lg. The Western blot is shown in Fig. 18. Lanes were probed with sera from fish immiinized as follows: Lanes 1-3, 10, 1, and 0.1 μg full length gene construct, respectively; lanes, 4-6: 10, 1, and 0.1 μg C-terminal deletion construct, respectively; lanes 7-9: 10, 1, and 0.1 μg N-terminal deletion construct, respectively; lane 10, 10 μg LacZ construct (negative control); lane 11, naive fish serum; lane 12, fish vaccinated with 20 μg affinity purified 55 kDa i-antigen (positive control). Sera from fish vaccinated with constructs harboring the C-terminal deletion (lanes 4-6) reacted positively with the 55 kD protein run under non-reducing conditions, as did sera from fish injected with the purified antigen itself (lane 12). In contrast, only weak signals were seen in equivalent blots of protein separated under reducing conditions, the one exception being with sera from fish injected with 10 μg of the full-length construct. These results suggest that antibodies against the product of the C-terminal deletion (and against the i-antigen itself) are directed primarily towards conformational epitopes, while those against the full-length protein are directed towards linear epitopes. Of further interest was the fact that the antibody response to the vector encoded antigens decreased over time and was not detectable by ELISA 6 weeks after vaccination. The animals were challenged with lethal numbers of parasites (15,000 theronts per fish) 9 weeks after immunization. The percent survival in all groups injected with the i-antigen constructs was higher than that in the negative control (LacZ). Still, not all fish died in the LacZ control group, and because the number offish injected was small, the relative differences between groups were statistically significant in only one case (Table 1). This experiment did not include a group offish injected with PBS alone. Table 1. Immunogen LacZ FIO FI FOA CIO Cl CO.l N10 Nl N0.1 GSiAg No- offish 9 9 10 10 9 10 10 9 10 10 10 challenged No. of fish 4 8 8 9 6 7 8 7 10 8 10 survived Survival % 44.4 88.9 80 90 66.7 70 80 77.8 100 80 100 RSP N.A. 50 44.5 50.1 33.4 57.7 44.5 42.9 55.6 44.5 55.6 *MDD 14.0 17.0 14.5 16.0 20.3 12.3 21.0 10.5 21.0 ± ± ± ± ± ± ± ± SD 4.5 13A 1.5 5.5 57 6.4 7.1 Groups offish were injected with 10, 1 or 0.1 μg pcDNA3.1 PNA containing the sequence for either the full-length 55 kP i-antigen (F10, FI, FO.l), the C-terminal deletion (CIO, Cll, CO.l), or the N-terminal deletion (N10, Nl, N0.1). Two additional groups offish were injected with plasmid PNA containing a LacZ insert (LacZ), and 20 μg the affinity purified 55kP protein (i- Ag). Fish were challenged 9 weeks after vaccination with 15,000 theronts per fish. *Mean days to death + /-Standard Peviation: no significant difference between groups by One Way Anova, **A significant difference in percent survival was seen between the negative control group (LacZ) and the test groups N I and G5 i-Ag (PO.5). Because animals were exposed to what should have been a lethal challenge, the fact that some fish survived in the group injected with the lacZ constructs might be interesting in light of recent evidence that methylated CpG motifs in bacterial PNA can stimulate both cytokine production and B-cell proliferation, and in some cases, can protect animals non-specifically against microbial pathogens. Example 8 Construction of a Multivalent /. multifiliis Vaccine A multivalent vaccine vector is constructed based on the i-antigens of/ multifiliis serotype A or serotype P. The gene for the 48 kP i-antigen of serotype A has been isolated and the parasite itself is currently available for vaccine trials. If the 48 kP i-antigen is used, a synthetic copy of the gene equivalent to the one made for the 55 kP antigen (Example 5) is constructed. The construction is straightforward and involves the design and synthesis of overlapping oligonucleotides (-100 bp each) that span the coding region of the gene. The oligonucleotides are mixed at equimolar concentrations in a single reaction and allowed to assemble into the full-length sequence using a thermostable PNA polymerase (High Fidelity Expand enzyme, Roche). A fraction of the assembled product is then used as a template for amplification of the gene in a standard PCR reaction using flanking primers to drive the reaction. The resulting full-length product is gel-purified, cloned into pcPN A3.1 and sequenced. Errors in the sequence are corrected by in vitro mutagenesis according to the method of Peng et al. (Anal. Biochem. 200:81 (1992)) and the final version is tested as a PNA vaccine. A combined vaccine containing plasmid vectors for both the 48 and 55 kD i-antigens is also tested on fish challenged with serotype A and D. Example 9 Immune Response of Channel Catfish to Live Vaccine: Transformed Tetrahymena Expressing Full-length or Truncated I. multifiliis (Serotype A) 48 kD I-antigen Protein and Heterologous Challenge with /. multifiliis (Serotype D) The IAG48[G1] gene of Ichthyophthirius multifiliis Gl encodes the GPI anchored 48- kDa i-antigen. The extreme 3' region of the gene encodes a stretch of 14 mostly hydrophobic amino acids separated by a short spacer from three small amino acids (CAS). This sequence encodes the protein's GPI anchor addition site. Tetrahymena thermophila cells transformed with the entire IAG48[G1] gene produce an intact i-antigen anchored to the cells' surface. Tetrahymena cells fransformed with a modified IAG48[G1] gene construct lacking the 3' sequence which includes the GPI addition site would be expected to produce a truncated protein lacking the GPI anchor. Tetrahymena cells were transformed with either the gene encoding the full-length Ichthyophthirius Gl 48- kPa i-antigen protein, or a truncated version of the gene that encodes the i-antigen protein lacking 19 amino acids at the carboxy terminus. Transformants encoding the intact or C-terminal truncated i- antigen were grown in standard Tetrahymena growth medium. Cell pellets and supernatant fluids were collected at the time points indicated. I-antigen was detected in cell cytosol, cell membrane or cell culture supernatants by Western blots using rabbit antisera against affinity purified Ichthyophthirius G5 i-antigen (see Fig. 17). It is clearly seen that the truncated protein is secreted into the culture medium. Groups of channel catfish (6 fish per group) were immunized by bath exposure (10 6 or 10 5 cells/fish) or intraperitoneal injection with T. thermophila transformants (10 6 , 10 5 , or 10 4 cells/fish) producing intact or truncated i-antigen. A third group offish was immunized with membrane protein extracts (1 mg or 0.1 mg/fish) from /. thermophila producing the full length protein. Immunization by bath exposure to Tetrahymena transformants. Two groups offish (6 fish in each group) were immunized by bath exposure. The fish were exposed to either 10 6 or 10 5 cells /fish for a period of 24 hours. Two immunogens were used: 1) fransformed Tetrahymena cells expressing the entire Ichthyophthirius Gl 48-kPa protein, and 2) fransformed cells secreting a truncated form of the i-antigen lacking the GPI anchor. Fish in the control group were exposed to Tetrahymena transformants expressing the neol gene product. Fish were exposed twice at a 30 day interval and challenged 30-60 days after the last immunization with the G5 Ichthyophthirius isolate. There were no significant differences (z test) between test and confrol groups (see Table 2). Immunized fish were challenged with a heterologous strain (G5 isolate) of Ichthyophthirius expressing a different i-antigen than that produced by the recombinant Tetrahymena used for vaccination. It is expected that challenge with a strain of Ichthyophthirius producing an i- antigen homologous to the Gl 48 i-antigen would show increased levels of protection. Table 2. Vaccination by bath exposure 1 Tetrahymena expressing intact membrane form of Ichthyophthirius Gl i- antigen. 2 Tetrahymena secreting truncated form of Gl i-antigen. 3 Relative Survival Percent = 1- (number of dead fish in test group/number of dead fish in confrol group) x 100% 4 Mean days to death 5 Standard deviation Immunization by injection of Tetrahymena transformants. Fish in each group were injected intraperitoneally with 10 6 , 10 5 , or 10 4 live fransformed Tetrahymena cells/fish. The same immunogens and controls were tested as in the immersion vaccinations. Fish were injected two times at a 30 day interval, and challenged 21 days after the last immunization with G5 Ichthyophthirius. A greater degree of protection was elicited in immunized fish compared to confrols (Table 3). Table 3. Vaccination by injection 1 Tetrahymena expressing intact membrane form of Ichthyophthirius Gl i- antigen. 2 Tetrahymena secreting truncated form of Gl i-antigen. 3 Relative Survival Percent = 1- (number of dead fish in test group/number of dead fish in control group) x 100% 4 Mean days to death 5 Standard deviation Serum antibody production. Fish serum antibody responses against recombinant Gl Ichthyophthirius i-antigen were determined by ELISA at 2, 4, and 6 weeks after immunization. Serum antibodies from immunized fish were detected with a sandwich ELISA technique that used wells coated with a cross- reactive rabbit antibody against Ichthyophthirius G5 i-antigen to capture recombinant Gl i- antigen produced in fransformed Tetrahymena. Sera from test and control fish were added to wells and antibodies that bound to the captured i-antigen were detected using an alkaline phosphatase labeled mouse mAb against the immunoglobulin heavy chain of channel catfish. ELISA controls consisted of antibody-coated wells reacted with membrane protein from Tetrahymena cells transformed with the neol gene. Fish injected with Tetrahymena membrane protein produced high levels of serum antibody against the recombinant i-antigen. The antibody response elicited by fish immunized with live cells was almost an order of magnitude lower. The antibody response offish immunized by bath or i.p. injection with live cells secreting recombinant i-antigen was approximately two-fold greater than the antibody response offish immunized with Tetrahymena producing the membrane-bound, intact i-antigen. In Fig. 19, the differences in antibody production between fish immunized with the (a) membrane associated or (b) secreted form of the i-antigen are shown. These results suggest that live cells secreting antigen are more efficatious in eliciting the production of serum antibodies. The mucosal antibody response was not determined in these experiments. Example 10 Immune Response of Channel Catfish to Live Vaccine: Transformed Tetrahymena Expressing Full-length or Truncated I. multifiliis (Serotype A) 48 kD I-antigen Protein and Homologous Challenge with I. multifiliis (Serotype A) Tetrahymena cells were fransformed with either the entire Ichthyophthirius Gl 48- kDa i-antigen protein, or a truncated gene sequence which encodes the i-antigen protein lacking 19 amino acids at the carboxy terminus as in Example 9. Groups of channel catfish (70 fish per group) were vaccinated by infraperitoneal injection with 10 6 T. thermophila transformants producing intact or truncated i-antigen. A third group offish (control group) was vaccinated with T. thermophila transformants expressing neo. No adjuvant was used in any of the vaccinations. The fish were boosted 2 weeks following the initial injection and bled at 3 weeks following the initial injection. Sera from 3 fish per group were pooled. A 96 well ELISA plate was seeded with a homologous sfrain (i.e., serotype A) of I. multifiliis (strain. NY1, a Gl isolate), 200 cells per well. Fish sera were serially diluted and added to the wells, and the effect on the motility of / multifiliis was observed. Immobilization of I. multifiliis was immediately evident at serum dilutions of 1 :20, and at higher concentrations the organisms exhibited clumping (Fig. 20). Sera from the control group did not cause any change in motility of 7. multifiliis. As another control, additional wells were seeded with a heterologous strain of 7. multifiliis (a G5 isolate). The motility of these organisms was not affected by sera from any of the groups of vaccinated fish, confirming that the immobilization epitopes on I. multifiliis i-antigens are highly specific. For comparison, two other groups offish were vaccinated with purified subunit proteins produced from recombinant Tetrahymena (either the full-length 48 kD i-antigen protein or the C-terminal truncated version). The subunit proteins were adjuvanted with Freund's Complete Adjuvant. In a plate assay similar to the one described above using the homologous strain of 7. multifiliis, some immobilization was observed but not to the degree caused by the "live vaccine." This observation lends support to the expectation that the "live vaccine" will prove to be more efficacious than the analogous protein subunit vaccine. Example 11 Degenerate Primers for Amplification of I-antigen Genes from Other Serotypes Forward and reverse primers for amplifying i-antigen gene sequences from different parasite isolates were designed based on regions of homology discovered between the 48 kDa (Gl sfrain) and 55 kDa i-antigens (G5 sfrain) of I. multifiliis. Forward primer P2 was successfully used to amplify sequences from a variety of different parasite isolates in combination with the reverse primers P3, P4 and P5. Primer sequences are as follows: P2: CCGAATTCTCTGG(C/T)ACTGCACTTGATGATGGAG (SEQ LD NO:45), which is 2-fold degenerate and corresponds to all or part of the amino acid sequence GTALDDGV (SEQ LP NO:46); it contains an EcoR I restriction site for cloning purposes; P3: GTGGATCCAGTACATGTTACA(A/G)TACCTGC (SEQ LP NO:47), which is 2-fold degenerate and corresponds to all or part of the amino acid sequence AGTOTCT (SEQ IP NO:48); it contains a BamH I restriction site for cloning purposes; P4: GTGGATCC(A G)CCAGAAGTTAATTTTTTA(T/G)TAC (SEQ ID NO:49), which is 4-fold degenerate and corresponds to all or part of the amino acid sequence CTKKLTSGA (SEQ LD NO:50); it contains a BamH I site; and P5: GTGGATCCAAGGAAAT(C/T)GATAAAAA(T/A)TTAGCG (SEQ LD NO:51), which is 4-fold degenerate and corresponds to all or part of amino acid sequence FAKFLSISL (SEQ LD NO:52); it contains a BamH I restriction site. PCR amplification of i-antigen gene sequences was carried out under standard conditions (30 cycles at 94 C for 1 minute, 52 C for 1 minute ,72 C for 1 minute). Fig. 21 shows PCR amplification of genomic DNA from several serotypic variants using the P2/P4 primer pair. Amplification products were run on 1.5% agarose gel and stained with ethidium bromide. Multiple bands seen in some of the lanes may be due to amplification from more than a single i-antigen gene (serotype A, for example, has two i-antigen genes) or to ntispriming from closely related sequences within tandem repeats of the same gene. The major band in the lane containing G5 isolate DNA is precisely the expected size (~800 bp) based on the sequence of the 55 kD i-antigen gene. Among the ten isolates represented in Fig. 21, two belong to serotype A (Gl and G10) and four to serotype D (G3, US, G7, and JI). Isolates Gl.l, G2 and 04 correspond to serotypes B, C and E, respectively (the CU1 isolate has not yet been typed). Example 12 Construction of an I-antigen/C3d DNA Vaccine Several distinct genes for the third component of trout complement have been identified. Although unique from one another, they share the same coding region for C3d (J. Sunyer et al., Proc. Natl. Acad. Sci USA 93, 8546 (1996)). Preparation of the hybrid i-antigen/C3d construct is carried out as follows. Three separate PCR reactions are performed to synthesize the basic elements of the construct. The first reaction involves amplification of the 55 kDa i-antigen gene using a forward primer that encodes restriction endonuclease cloning sites and replaces the existing N-terminal signal peptide of the i-antigen with the signal peptide of trout lg. The reverse primer in this case contains restriction sites as well, and omits the coding sequence for the last 15 amino acids at the C- terminus of the protein (ordinarily, the C-terminus acts as a GPI-anchor addition site). The second PCR reaction involves synthesis of the C3d fragment containing appropriate restriction sites at its ends. The forward primer in this case mutates the first cysteine in the C3d fragment to a serine (thereby assuring correct disulfide bridging between cysteine residues within C3d.. The final PCR reaction involves synthesis of a second C3d fragment containing a short linker sequence (GS[G 4 S] 2 ) (SEQ LD NO:101) at its C-terminus along with appropriate restriction sites that allow cloning between the i-antigen sequence and the first C3d PCR product. The template for synthesis of the C3d fragments is trout genomic DNA. The entire construct is assembled by first cloning the PCR product for the i-antigen gene into pcDNA3.1. The C3d products are inserted at the 3' end of the i-antigen gene using restriction sites built into the fragments. Following transformation of bacteria with the resulting construct, the vector is directly sequenced to determine the number of C3d fragments in the hybrid, as well as the accuracy of the overall sequence. The same basic strategy is employed in the construction of a HEL (hen egg-white lysozyme)/C3d hybrid control vaccine. The rationale for including the model antigen (HEL) in these studies relates to the uncertainty associated with correct folding of the i- antigen/C3d hybrid (and consequently the likelihood that it will induce the expected result). After correcting nucleotide sequence errors, groups offish (in this case 20-50 g rainbow trout) are injected with varying amounts of either the i-antigen or HEL hybrid constructs and the antibody response is determined in plate ELISAs (as described in other Examples) using secondary antibody conjugates against trout lg. Trout immunized with the i-antigen/C3d hybrid vaccine are also challenged with parasites of the G5 isolate to determine levels of protection. Example 13 Oral Delivery of a DNA Vaccine Oral delivery represents an extremely attractive alternative for administration of DNA vaccines, particularly in aquaculture species. Based on recent experiments in mice (P. Jones et al., Pev. Biol. Stand. 92:149 (1998); K. Roy et al., Nature Med. 3:387 (1999)) it may now be possible to administer genetic vaccines orally using biodegradable polymers. Fish are expected to be capable of developing cutaneous mucosal antibody responses against Ichthyophthirius following oral vaccination with vector-encoded antigens. Juvenile channel catfish are vaccinated orally using i-antigen based PNA vaccines complexed with chitosan. Encapsulation is performed substantially in accordance with the method of K. Leong et al., (J. Controlled Release 53:183 (1998)). Briefly, chitosan (MW 390,000) is dissolved at a concentration of 0.02% in 25 mM sodium acetate, pH 5-7. After prewarming to 55°C, 100 μl aliquots are added to an equivalent volume of vector PNA (1 ug/ml) in sodium sulfate buffer at 55 °C, and the mixture is vortexed rapidly for 20 seconds. Nanoparticles formed by complex coacervation are examined by scanning electron microscopy and the resulting preparations stored at room temperature. Preparations are combined with fish food and animals are fed the equivalent of 10- 100 μg plasmid. PNA. Antigen-specific antibody responses are determined in cutaneous mucus and serum by ELISA, and positive fish are challenged with live theronts. The complete disclosures of all patents, patent applications, publications, and nucleic acid and protein database entries, including for example GenBank accession numbers and EMBL accession numbers, that are cited herein are hereby incorporated by reference as if individually incorporated. Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.

Claims: WHAT IS CLAIMED IS: 1. A nucleic acid molecule comprising a polynucleotide fragment having a nucleotide sequence that encodes at least a C-terminal portion of an i- antigen polypeptide having amino acid sequence SEQ LP NO:6. 2. The nucleic acid molecule of claim 2 comprising SEQ IP NO: 1. 3. A nucleic acid molecule comprising a polynucleotide fragment having a nucleotide sequence that encodes at least an antigenic portion of an i- antigen polypeptide having amino acid sequence SEQ LP NO:7. 4. A nucleic acid molecule comprising a polynucleotide fragment having a nucleotide sequence that encodes at least one terminal portion of an i- antigen polypeptide having amino acid sequence SEQ LD NO:7. 5. The nucleic acid molecule of claim 4 comprising SEQ ID NO: 3 or SEQ LD NO:5. 6. The nucleic acid molecule of claims 1 , 3 or 4 that is a vector capable of expressing the polypeptide encoded by the nucleic acid sequence in a cell or an organism, wherein the cell or organism is selected from the group consisting of a bacterium, a protozoan, a yeast, an insect and an animal cell or organism. 7. The nucleic acid molecule of claim 6 that is capable of expressing the polypeptide encoded by the nucleic acid sequence in E. coli, Pischia pastoris or Tetrahymena. 8. A nucleic acid molecule encoding an antigenic polypeptide that shares a significant level of primary structure with at least one of a 55 kD i- antigen protein from a G5 isolate of Ichthyophthirius and a 48 kP i- antigen protein from a Gl isolate of Ichthyophthirius. 9. The nucleic acid molecule of claim 8 encoding an antigenic polypeptide that shares a significant level of primary structure with at least one of SEQ LP NO:6 and SEQ LP NO:7. 10. A nucleic acid molecule that is substantially complementary to the nucleic acid molecule of claims 1, 3, 4, or 8. 11. A nucleic acid molecule comprising a polynucleotide fragment that hybridizes to at least a portion of the complement of at least one of SEQ LP NO:l or SEQ LP NO:3, under standard hybridization conditions, wherein the polynucleotide fragment encodes a polypeptide comprising at least a membrane targeting portion or an antigenic portion of an i- antigen protein. 12. A polypeptide selected from the group consisting of: an i-antigen polypeptide having SEQ ID NO:6; an i-antigen polypeptide having SEQ LD NO:7; an analog or modification of an i-antigen polypeptide having SEQ LP NO:6; a fragment of an i-antigen polypeptide having SEQ LD NO:6 wherein the fragment comprises at least a C-terminal portion of SEQ LD NO:6; an analog or modification of an i-antigen polypeptide having- SEQ LD NO:7; a fragment of an i-antigen polypeptide having SEQ LP NO:7 wherein the fragment comprises at least at least one terminal portion of SEQ LP NO:7; and an antigenic fragment of an i-antigen polypeptide having SEQ ID NO:7; and an i-antigen polypeptide that shares a significant level of primary structure with at least one of SEQ LD NO:6 and SEQ LD NO:7. 13. The polypeptide of claim 12 that is antigenic. 14. A composition for inducing an immune response in a fish comprising at least one component selected from the group consisting of (a) a polypeptide comprising an antigenic portion of an i-antigen polypeptide and (b) a nucleic acid molecule comprising a polynucleotide fragment having a nucleotide sequence encoding an antigenic portion of an i- antigen polypeptide. 15. The composition of claim 14 comprising a polypeptide of claim 13. 16. The composition of claim 14 comprising at least one of the nucleic acid molecules of claims 1, 3, 4, 8 or 10. 17. The composition of claim 14 wherein administration of the composition to fish prevents or confrols 7. multifiliis infection. 18. The composition of claim 14 comprising a nucleic acid molecule comprising a polynucleotide fragment having a nucleotide sequence encoding an antigenic portion of an i-antigen polypeptide linked at its carboxy-terminus to a plurality of molecules of the C3d component of complement. 19. The composition of claim 14 formulated for oral administration. 20. The composition of claim 19 wherein the polypeptide or nucleic acid molecule is encapsulated in a biodegradable polymer. 21. A host cell transformed with the nucleic acid molecule of claim 6. 22. A fish comprising the nucleic acid molecule of any of claims 1 , 3 or 4. 23. Transformed Tetrahymena comprising the nucleic acid molecule of any of claims 1, 3 or 4. 24. An antibody capable of binding a polypeptide of claim 12. 25. A method for detecting Ichthyophthirius in an aquaculture comprising: obtaining a sample containing nucleic acid from an aquaculture fish or an aquaculture water; adding at least one primer oligonucleotide having a sequence complementary to at least a portion of SEQ LD NO: 1 or SEQ ID NO: 3 to the nucleic acid sample; conducting a polymerase chain reaction amplification with the sample; and analyzing the reaction mixture for the presence of a product amplified by the at least one oligonucleotide primer. 26. The method of claim 25 wherein the primer is capable of amplifying nucleotide sequences encoding i-antigens derived from least two different I. multifiliis serotypes. 27. The method of claim 26 wherein the primer has a nucleic acid sequence that encodes an amino acid sequence selected from the group consisting of MKYNILLT (SEQ ID NO:36), FLSISLLF (SEQ LP NO:38), GTALOPGV (SEQ L NO:46), AGTDTCT (SEQ ID NO:48), CTKKLTSGA (SEQ LD NO:50) and FAKFLSISL (SEQ ID NO:52). 28. The method of claim 25 further comprising making an polynucleotide vaccine comprising at least a portion of the amplified product, wherein the portion of the amplified product encodes an antigenic polypeptide. 29. The method of claim 25 further comprising making a protein subunit vaccine comprising an antigenic polypeptide encoded by at least a portion of the amplified product. 30. The method of claims 28 or 29 further comprising administering the vaccine to fish to treat or prevent Ichthyophthirius infection. 31. A method for identifying an 7. multifiliis serotype comprising: providing a sample comprising an 7. multifiliis nucleic acid molecule having a nucleotide sequence encoding an i-antigen; adding to the sample at least one primer oligonucleotide having a sequence complementary to a unique region of an I. multifiliis nucleotide sequence encoding an i-antigen; subjecting the sample to amplification conditions; and analyzing the sample to determine the presence of a product amplified by the at least one oligonucleotide primer. 32. A method for inducing an immune response in a fish comprising adniinistering to the fish the immunogenic composition of claim 14. 33. The method of claim 32 performed prophylactically in advance of exposure to / multifiliis. 34. The method of claim 32 performed therapeutically while the fish is infected with 7. multifiliis. 35. The method of claim 32 wherein adminisfration is performed by injection, immersion, or oral ingestion.

Inventors: CLARK THEODORE G, DICKERSON HARRY W JR, LIN TIAN-LONG, CLARK, THEODORE, G., DICKERSON, HARRY, W., JR., LIN, TIAN-LONG
Applicants: CLARK THEODORE G, CORNELL RES FOUNDATION INC, DICKERSON HARRY W JR, LIN TIAN LONG, UNIV GEORGIA, CLARK, THEODORE, G., CORNELL RESEARCH FOUNDATION, INC., DICKERSON, HARRY, W., JR., LIN, TIAN-LONG, UNIVERSITY OF GEORGIA RESEARCH FOUNDATION, INC.
Application year: 2000
Main classification: C12N15
IPC codes: A61K39, A61P33, C07K14, C12N1, C12N15