Abstract:
This invention relates to a mutant  E. coli  heat-labile enterotoxin (LT) subunit A that can be used as an adjuvant. This subunit A mutant contains an amino acid substitution at a position corresponding to position 61 of a wild-type LT. An LT containing this mutated subunit A exhibits reduced toxicity compared to its wild type counterpart.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Taiwanese Patent Application No. 95139707 filed Oct. 27, 2006, the content of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Enterotoxigenic  Escherichia coli  strains cause diarrhea in humans and domestic animals by producing two types of enterotoxins, i.e., heat-labile toxin (LT) and heat-stable toxin (ST) (Hofstra et al., 1984,  J. Bio. Chem.  259:15182-15187). LT is functionally, structurally, and immunologically related to cholera toxin (CT) (Clements et al., 1978,  Infect. Immun.  21:1036-1039). LT and CT are synthesized as holotoxin molecules composed of five identical subunits B and an enzymatically active subunit A (AB 5 ) (Spangler, 1992,  Microbio. Rev.  56(4):622-647). The B pentamer binds ganglioside GM1 in the membrane of intestinal epithelial cells or any other cell that contains GM1 (van Heyningen, 1974,  Science  183:656-657). Following binding of subunits B to GM1 on the cell surface, subunit A is inserted into cytosal and proteolytically cleaved and reduced at its single disulfide bond to produce an enzymatically active A1 peptide and a smaller A2 peptide (Fishman PH, 1982,  J. Membr. Biol.  69:85-97; Mekalanos et al., 1979,  J. Biol. Chem.  254:5855-5861; Moss et al., 1981,  J. Biol. Chem.  256:12861-12865). The A1 peptide is capable of binding NAD and catalyzing the ADP-ribosylation of Gsα, a GTP-binding regulatory protein associated with adenylate cyclase (Spangler, 1992,  Microbio. Rev.  56(4):622-647). The resulting increase in cAMP eventually leads to the release of electrolytes and fluids from affected cells (Cheng et al., 2000,  Vaccine  18:38-49). 
     LT has been shown to function as mucosal adjuvant and induce immune responses against mucosally co-administered antigens (Clements et al., 1988,  Vaccine  6:269-277; Elson C O. 1989,  Immunol. Today  146:29-33; Spangler, 1992,  Microbio. Rev.  56(4):622-647). However, the high toxicity of wild-type LT has limited its clinical use. Thus, it is desirable to generate mutated LTs having reduced toxicity while retaining immunogenicity. 
     The term “ E. coli  heat-labile enterotoxin” or “LT” used herein refers to a heat-labile enterotoxin produced by any enterotoxigenic  E. coli  strain. The term “ E. coli  heat-labile enterotoxin subunit A” or “LT A ” refers to subunit A of LT. It includes both precursor LT A , which contains a signal peptide, and mature LT A  which has the signal peptide removed. 
     SUMMARY 
     This invention is based on the unexpected discovery that an LT containing a mutated LT A  exhibits reduced toxicity compared to its wild type counterpart while retaining immunogenicity. This mutated LT A  has an amino acid substitution at the position corresponding to position 61 of a wild-type LT A , whose amino acid sequence, SEQ ID NO:5, is shown below. 
     Accordingly, this invention features an isolated polypeptide including a mutated LT A  that contains an amino acid residue other than S, T, and F, at the position corresponding to position 61 of SEQ ID NO:5. The substituting amino acid residue can be D, E, H, I, K, L, N, P, Q, R, Y or W. It can be a naturally occurring amino acid or a non-naturally occurring amino acid, e.g., a D-amino acid or a β-amino acid. In one example, the LT A  has the amino acid sequence of SEQ ID NO:2, 4, 8 or 10. An LT containing this mutated LT A  exhibits reduced toxicity, i.e., &lt;10 −5 -fold that of a wild-type LT containing SEQ ID NO:5. 
     In another aspect, this invention features a vaccine containing an antigen and the mutated LT A  described above. The vaccine can further include LT subunit B, which forms with the mutated LT A  whole LT protein. The antigen can be derived from either a bacterium or a virus. 
     In yet another aspect, this invention features an isolated nucleic acid including a nucleotide sequence that encodes the above-described LT A  mutant. In one example, the nucleotide sequence is SEQ ID NOs: 1, 3, 7, or 9. Also within the scope of this invention is a vector including the just-described nucleic acid and a transformed cell containing the vector. 
     The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims. 
    
    
     DETAILED DESCRIPTION 
     A detoxified LT, e.g., containing a mutated LT A , is a desired vaccine adjuvant due to its high immunogenicity. The mutated LT A  of this invention, designed to achieve this purpose, is made by introducing an amino acid substitution ir an LT A  at a position corresponding to the position 61 of SEQ ID NO:5. LT A  produced by different enterotoxigenic  E. coli  strains are highly homologous. Thus, by comparing the amino acid sequence of an LT A  with SEQ ID NO:5, a skilled artisan can find out which position in this LT A  corresponds to position 61 of SEQ ID NO:5. Just like the LT A  of SEQ ID NO:5, almost all wild-type LT A s include a serine at the position corresponding to position 61 of SEQ ID NO:5. An amino acid that is different from serine in, e.g., size or polarity, can be used as a substitutient substituent. Examples of the substituent amino acids include hydrophobic amino acid residues (e.g., I and L), charged amino acid residues (e.g., D, E, K, R, and H), or amino acid residues with bulky side chains (e.g., N, Q, Y, and W). Proline also can be used as a substituent as it generally alters the local structure of a polypeptide. These substituent amino acids include non-naturally occurring amino acids, e.g., D-amino acids or β-amino acids. 
     The LT A  mutant of this invention can be prepared by methods well known in the art. For example, the mutant is produced by a recombinant method as follows. A cDNA that encodes a wild-type LT A  is isolated from an enterotoxigenic  E. coli  strain and subjected to site-directed mutagenesis to produce a cDNA encoding the desired LT A  mutant. See Ho et al., Gene, 77:51-59, 1989. The cDNA bearing the mutation is then inserted into an expression vector for transforming cells. Finally, the LT A  mutant produced in the transformed cells is purified and assembled with LT subunit B to form whole LT protein. 
     The toxicity of an LT containing an LT A  mutant described above can be assessed using assays such as the Y-1 adrenal cell assay. See Cheng et al., Vaccine, 18:38-49, 2000. 
     The LT A  mutant of this invention can be used as an adjuvant in a vaccine. The vaccine (i.e., a human vaccine or a veterinary vaccine) can contain an antigen and the LT A  mutant itself or an LT containing the LT A  mutant. The antigen can be derived from a bacterium, e.g.,  Streptococcus pyogenes, Streptococcus pneumoniae, Neiseria gonorrhoea, Neisseria meningitides, Corynebacterium diphtheriae, Clostridium botulinum, Clostridium perfringens, Clostridium tetani, Haemophilus influenzae, Klebsiella pneumoniae, Klebsiella ozaenae, Staphylococcus aureus, Vibrio cholerae, Escherichia coli, Pseudomonas aeruginosa, Campylobacter jejuni, Aeromonas hydrophila, Bacillus cereus, Yersinia enterocolitica, Yersinia pestis, Salmonella typhimurium, Treponema pallidum, Borrelia vincentii, Borrelia burgdorferi, Mycobacterium tuberculosis, Pneumocystis carinii, Mycoplasma  spp.,  Rickettsia prowazeki, Chlamydia  spp.,  Helicobacter pylori . It also can be derived from a virus, e.g., influenza, herpes simplex virus, human immunodeficiency virus, cytomegalovirus, hepatitis c virus, delta hepatitis virus, poliovirus, hepatitis A virus, hepatitis B virus, Epstein-barr virus, varicella zoster virus, respiratory syncytial virus, enterovirus or Japanese encephalitis virus. 
     The vaccine can further contain a pharmaceutically acceptable carrier such as phosphate buffered saline or a bicarbonate solution. The carrier is selected on the basis of the mode and route of administration, and standard pharmaceutical practice. Suitable pharmaceutical carriers and diluents, as well as pharmaceutical necessities for their use, are described in Remington&#39;s Pharmaceutical Sciences. 
     Methods for preparing vaccines are generally well known in the art, as exemplified by U.S. Pat. Nos. 4,601,903; 4,599,231; 4,599,230; and 4,596,792. Vaccines may be prepared as injectables, as liquid solutions or emulsions. The antigen and the LT A  mutant or the LT containing it may be mixed with physiologically acceptable excipients, which may include, water, saline, dextrose, glycerol, ethanol, and combinations thereof. The vaccine may further contain minor amounts of auxiliary substances such as wetting or emulsifying agents, or pH buffering agents to enhance the effectiveness of the vaccines. Vaccines may be administered parenterally, by injection subcutaneously or intramuscularly. Alternatively, other modes of administration including suppositories, oral, or topical formulations may be desirable. For suppositories, binders and carriers may include, for example, polyalkalene glycols or triglycerides. Oral formulations may include normally employed incipients such as, for example, pharmaceutical grades of saccharine, cellulose, magnesium carbonate and the like. These compositions take the form of solutions, suspensions, tablets, pills, capsules, sustained release formulations or powders. 
     The vaccine is administered in a manner compatible with the dosage formulation, and in an amount that is therapeutically effective, protective and immunogenic. The quantity to be administered depends on the subject to be treated, including, for example, the capacity of the individual&#39;s immune system to synthesize antibodies, and if needed, to produce a cell-mediated immune response. Precise amounts of active ingredient required to be administered depend on the judgment of the practitioner. However, suitable dosage ranges are readily determinable by one skilled in the art and may be of the order of micrograms of the polypeptide of this invention. Suitable regimes for initial administration and booster doses are also variable, but may include an initial administration followed by subsequent administrations. The dosage of the vaccine may also depend on the route of administration and varies according to the size of the host. 
     The specific examples below are to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. Without further elaboration, it is believed that one skilled in the art can, based on the description herein, utilize the present invention to its fullest extent. All publications cited herein are hereby incorporated by reference in their entirety. 
     Example 1 
     Constructing Genes Encoding Wild Type LT A  and LT Mutated LT A    
     A 1.8-kb DNA fragment of an LT gene, including both subunit A and subunit B, was isolated from human entero-toxigenic  E. coli  H10407 and cloned into pBluescript II KS(−) vector (pBluescript-LThWT). The nucleotide sequence (SEQ ID NO:6) of the LT gene (encoding both Subunits A and B) and the amino acid sequence (SEQ ID NO:5) of subunit A of this LT are shown below: 
     
       
         
               
             
               
               
             
           
               
                 Nucleotide sequence (SEQ ID NO: 6)of LT (Subunit 
               
               
                 A: 1-777; Subunit B: 774-1148) 
               
             
          
           
               
                 atgaaaaata taactttcat tttttttatt ttattagcat 
                 60 
               
               
                 cgccattata tgcaaatggc 
                   
               
               
                   
               
               
                 gacaaattat accgtgctga ctctagaccc ccagatgaaa 
                 120 
               
               
                 taaaacgttc cggaggtctt 
                   
               
               
                   
               
               
                 atgcccagag ggcataatga gtacttcgat agaggaactc 
                 180 
               
               
                 aaatgaatat taatctttat 
                   
               
               
                   
               
               
                 gatcacgcga gaggaacaca aaccggcttt gtcagatatg 
                 240 
               
               
                 atgacggata tgtttccact 
                   
               
               
                   
               
               
                 tctcttagtt tgagaagtgc tcacttagca ggacagtcta 
                 300 
               
               
                 tattatcagg atattccact 
                   
               
               
                   
               
               
                 tactatatat atgttatagc gacagcacca aatatgttta 
                 360 
               
               
                 atgttaatga tgtattaggc 
                   
               
               
                   
               
               
                 gtatacagcc ctcacccata tgaacaggag gtttctgcgt 
                 420 
               
               
                 taggtggaat accatattct 
                   
               
               
                   
               
               
                 cagatatatg gatggtatcg tgttaatttt ggtgtgattg 
                 480 
               
               
                 atgaacgatt acatcgtaac 
                   
               
               
                   
               
               
                 agggaatata gagaccggta ttacagaaat ctgaatatag 
                 540 
               
               
                 ctccggcaga ggatggttac 
                   
               
               
                   
               
               
                 agattagcag gtttcccacc ggatcaccaa gcttggagag 
                 600 
               
               
                 aagaaccctg gattcatcat 
                   
               
               
                   
               
               
                 gcaccacaag gttgtggaaa ttcatcaaga acaattacag 
                 660 
               
               
                 gtgatacttg taatgaggag 
                   
               
               
                   
               
               
                 acccagaatc tgagcacaat atatctcagg aaatatcaat 
                 720 
               
               
                 caaaagttaa gaggcagata 
                   
               
               
                   
               
               
                 ttttcagact atcagtcaga ggttgacata tataacagaa 
                 780 
               
               
                 ttcggaatga attatgaata 
                   
               
               
                   
               
               
                 aagtaaaatg ttatgtttta tttacggcgt tactatcctc 
                 840 
               
               
                 tctatgtgca tacggagctc 
                   
               
               
                   
               
               
                 cccagtctat tacagaacta tgttcggaat atcgcaacac 
                 900 
               
               
                 acaaatatat acgataaatg 
                   
               
               
                   
               
               
                 acaagatact atcatatacg gaatcgatgg caggcaaaag 
                 960 
               
               
                 agaaatggtt atcattacat 
                   
               
               
                   
               
               
                 ttaagagcgg cgcaacattt caggtcgaag tcccgggcag 
                 1020 
               
               
                 tcaacatata gactcccaaa 
                   
               
               
                   
               
               
                 aaaaagccat tgaaaggatg aaggacacat taagaatcac 
                 1080 
               
               
                 atatctgacc gagaccaaaa 
                   
               
               
                   
               
               
                 ttgataaatt atgtgtatgg aataataaaa cccccaattc 
                 1140 
               
               
                 aattgcggca atcagtatg 
                   
               
               
                   
               
               
                 aaaactag 
                   
               
             
          
         
       
     
     
       
         
               
               
             
           
               
                 Amino acid sequence (SEQ ID NO: 5) of mature LT A   
                   
               
               
                 NGDKLYRADS RPPDEIKRSG GLMPRGHNEY FDRGTQMNIN LYDHARGTQT GFVRYDDGYV  60 
               
               
                   
               
               
                   S TSLSLRSAH LAGQSILSGY STYYIYVIAT APNMFNVNDV LGVYSPHPYE QEVSALGGIP 120 
               
               
                   
               
               
                 YSQIYGWYRV NFGVIDERLH RNREYRDRYY RNLNIAPAED GYRLAGFPPD HQAWREEPWI 180 
               
               
                   
               
               
                 HHAPQGCGNS SRTITGDTCN EETQNLSTIY LRKYQSKVKR QIFSDYQSEV DIYNRIRNEL 240 
               
             
          
         
       
     
     Various LT A  mutants were constructed using a site-directed mutagenesis method (Ho et al., 1989,  Gene  77, 51-59). More specifically, LT A  mutants, including LT A (S61K), LT A (S61R), LT A (S61H), LT A (S61Y) and LT A (S61F) were constructed by replacing the serine at position 61 of SEQ ID NO:5 with K, R, H, Y, and F, respectively. The following oligonucleotide primers were used to construct these mutants: LT A (S61K) [5′ TCT CAA ACT AAG AGA AGT TTT AAC ATA TCC GTC ATC ATA 3′](SEQ ID NO:11), LT A (S61R) [5′ ACT TCT CAA ACT AAG AGA AGT TCT AAC ATA TCC GTC ATC 3′](SEQ ID NO:12), LT A (S61F) [5′ ACT TCT CAA ACT AAG AGA AGT GAA AAC ATA TCC GTC ATC 3′](SEQ ID NO:13), LT A  (S61Y) [5′ ACT TCT CAA ACT AAG AGA AGT ATA AAC ATA TCC GTC ATC 3′](SEQ ID NO:14), LT A (S61H) [5′ ACT TCT CAA ACT AAG AGA AGT ATG AAC ATA TCC GTC ATC 3′](SEQ ID NO:15). The nucleotide sequences and amino acid sequences of these LT A  are shown below: 
     
       
         
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
           
               
                 Nucleotide sequence (SEQ ID NO: 1) of LT A (S61K) 
               
             
          
           
               
                 atgaaaaata taactttcat tttttttatt ttattagcat 
                 60 
               
               
                 cgccattata tgcaaatggc 
                   
               
               
                   
               
               
                 gacaaattat accgtgctga ctctagaccc ccagatgaaa 
                 120 
               
               
                 taaaacgttc cggaggtctt 
                   
               
               
                   
               
               
                 atgcccagag ggcataatga gtacttcgat agaggaactc 
                 180 
               
               
                 aaatgaatat taatctttat 
                   
               
               
                   
               
               
                 gatcacgcga gaggaacaca aaccggcttt gtcagatatg 
                 240 
               
               
                 atgacggata tgttaaaact 
                   
               
               
                   
               
               
                 tctcttagtt tgagaagtgc tcacttagca ggacagtcta 
                 300 
               
               
                 tattatcagg atattccact 
                   
               
               
                   
               
               
                 tactatatat atgttatagc gacagcacca aatatgttta 
                 360 
               
               
                 atgttaatga tgtattaggc 
                   
               
               
                   
               
               
                 gtatacagcc ctcacccata tgaacaggag gtttctgcgt 
                 420 
               
               
                 taggtggaat accatattct 
                   
               
               
                   
               
               
                 cagatatatg gatggtatcg tgttaatttt ggtgtgattg 
                 480 
               
               
                 atgaacgatt acatcgtaac 
                   
               
               
                   
               
               
                 agggaatata gagaccggta ttacagaaat ctgaatatag 
                 540 
               
               
                 ctccggcaga ggatggttac 
                   
               
               
                   
               
               
                 agattagcag gtttcccacc ggatcaccaa gcttggagag 
                 600 
               
               
                 aagaaccctg gattcatcat 
                   
               
               
                   
               
               
                 gcaccacaag gttgtggaaa ttcatcaaga acaattacag 
                 660 
               
               
                 gtgatacttg taatgaggag 
                   
               
               
                   
               
               
                 acccagaatc tgagcacaat atatctcagg aaatatcaat 
                 720 
               
               
                 caaaagttaa gaggcagata 
                   
               
               
                   
               
               
                 ttttcagact atcagtcaga ggttgacata tataacagaa 
                 777 
               
               
                 ttcggaatga attatga 
                   
               
               
                   
               
             
          
           
               
                 Amino acid sequence (SEQ ID NO: 2) of LT A (S61K) 
               
             
          
           
               
                 NGDKLYRADS RPPDEIKRSG GLMPRGHNEY FDRGTQMNIN 
                 60 
               
               
                 LYDHARGTQT GFVRYDDGYV 
                   
               
               
                   
               
               
                   K TSLSLRSAH LAGQSILSGY STYYIYVIAT APNMFNVNDV 
                 120 
               
               
                 LGVYSPHPYE QEVSALGGIP 
                   
               
               
                   
               
               
                 YSQIYGWYRV NFGVIDERLH RNREYRDRYY RNLNIAPAED 
                 180 
               
               
                 GYRLAGFPPD HQAWREEPWI 
                   
               
               
                   
               
               
                 HHAPQGCGNS SRTITGDTCN EETQNLSTIY LRKYQSKVKR 
                 240 
               
               
                 QIFSDYQSEV DIYNRIRNEL 
                   
               
               
                   
               
             
          
           
               
                 Nucleotide sequence (SEQ ID NO: 3) of LT A (S61R) 
               
             
          
           
               
                 atgaaaaata taactttcat tttttttatt ttattagcat 
                 60 
               
               
                 cgccattata tgcaaatggc 
                   
               
               
                   
               
               
                 gacaaattat accgtgctga ctctagaccc ccagatgaaa 
                 120 
               
               
                 taaaacgttc cggaggtctt 
                   
               
               
                   
               
               
                 atgcccagag ggcataatga gtacttcgat agaggaactc 
                 180 
               
               
                 aaatgaatat taatctttat 
                   
               
               
                   
               
               
                 gatcacgcga gaggaacaca aaccggcttt gtcagatatg 
                 240 
               
               
                 atgacggata tgttagaact 
                   
               
               
                   
               
               
                 tctcttagtt tgagaagtgc tcacttagca ggacagtcta 
                 300 
               
               
                 tattatcagg atattccact 
                   
               
               
                   
               
               
                 tactatatat atgttatagc gacagcacca aatatgttta 
                 360 
               
               
                 atgttaatga tgtattaggc 
                   
               
               
                   
               
               
                 gtatacagcc ctcacccata tgaacaggag gtttctgcgt 
                 420 
               
               
                 taggtggaat accatattct 
                   
               
               
                   
               
               
                 cagatatatg gatggtatcg tgttaatttt ggtgtgattg 
                 480 
               
               
                 atgaacgatt acatcgtaac 
                   
               
               
                   
               
               
                 agggaatata gagaccggta ttacagaaat ctgaatatag 
                 540 
               
               
                 ctccggcaga ggatggttac 
                   
               
               
                   
               
               
                 agattagcag gtttcccacc ggatcaccaa gcttggagag 
                 600 
               
               
                 aagaaccctg gattcatcat 
                   
               
               
                   
               
               
                 gcaccacaag gttgtggaaa ttcatcaaga acaattacag 
                 660 
               
               
                 gtgatacttg taatgaggag 
                   
               
               
                   
               
               
                 acccagaatc tgagcacaat atatctcagg aaatatcaat 
                 720 
               
               
                 caaaagttaa gaggcagata 
                   
               
               
                   
               
               
                 ttttcagact atcagtcaga ggttgacata tataacagaa 
                 777 
               
               
                 ttcggaatga attatga 
                   
               
               
                   
               
             
          
           
               
                 Amino acid sequence (SEQ ID NO: 4) of LT A (S61R) 
               
             
          
           
               
                 NGDKLYRADS RPPDEIKRSG GLMPRGHNEY FDRGTQMNIN 
                 60 
               
               
                 LYDHARGTQT GFVRYDDGYV 
                   
               
               
                   
               
               
                   R TSLSLRSAH LAGQSILSGY STYYIYVIAT APNMFNVNDV 
                 120 
               
               
                 LGVYSPHPYE QEVSALGGIP 
                   
               
               
                   
               
               
                 YSQIYGWYRV NFGVIDERLH RNREYRDRYY RNLNIAPAED 
                 180 
               
               
                 GYRLAGFPPD HQAWREEPWI 
                   
               
               
                   
               
               
                 HHAPQGCGNS SRTITGDTCN EETQNLSTIY LRKYQSKVKR 
                 240 
               
               
                 QIFSDYQSEV DIYNRIRNEL 
                   
               
               
                   
               
             
          
           
               
                 Nucleotide sequence (SEQ ID NO: 7) of LT A (S61H) 
               
             
          
           
               
                 atgaaaaata taactttcat tttttttatt ttattagcat 
                 60 
               
               
                 cgccattata tgcaaatggc 
                   
               
               
                   
               
               
                 gacaaattat accgtgctga ctctagaccc ccagatgaaa 
                 120 
               
               
                 taaaacgttc cggaggtctt 
                   
               
               
                   
               
               
                 atgcccagag ggcataatga gtacttcgat agaggaactc 
                 180 
               
               
                 aaatgaatat taatctttat 
                   
               
               
                   
               
               
                 gatcacgcga gaggaacaca aaccggcttt gtcagatatg 
                 240 
               
               
                 atgacggata tgttcatact 
                   
               
               
                   
               
               
                 tctcttagtt tgagaagtgc tcacttagca ggacagtcta 
                 300 
               
               
                 tattatcagg atattccact 
                   
               
               
                   
               
               
                 tactatatat atgttatagc gacagcacca aatatgttta 
                 360 
               
               
                 atgttaatga tgtattaggc 
                   
               
               
                   
               
               
                 gtatacagcc ctcacccata tgaacaggag gtttctgcgt 
                 420 
               
               
                 taggtggaat accatattct 
                   
               
               
                   
               
               
                 cagatatatg gatggtatcg tgttaatttt ggtgtgattg 
                 480 
               
               
                 atgaacgatt acatcgtaac 
                   
               
               
                   
               
               
                 agggaatata gagaccggta ttacagaaat ctgaatatag 
                 540 
               
               
                 ctccggcaga ggatggttac 
                   
               
               
                   
               
               
                 agattagcag gtttcccacc ggatcaccaa gcttggagag 
                 600 
               
               
                 aagaaccctg gattcatcat 
                   
               
               
                   
               
               
                 gcaccacaag gttgtggaaa ttcatcaaga acaattacag 
                 660 
               
               
                 gtgatacttg taatgaggag 
                   
               
               
                   
               
               
                 acccagaatc tgagcacaat atatctcagg aaatatcaat 
                 720 
               
               
                 caaaagttaa gaggcagata 
                   
               
               
                   
               
               
                 ttttcagact atcagtcaga ggttgacata tataacagaa 
                 777 
               
               
                 ttcggaatga attatga 
                   
               
               
                   
               
             
          
           
               
                 Amino acid sequence (SEQ ID NO: 8) of LT A (S61H) 
               
             
          
           
               
                 NGDKLYRADS RPPDEIKRSG GLMPRGHNEY FDRGTQMNIN 
                 60 
               
               
                 LYDHARGTQT GFVRYDDGYV 
                   
               
               
                   
               
               
                   H TSLSLRSAH LAGQSILSGY STYYIYVIAT APNMFNVNDV 
                 120 
               
               
                 LGVYSPHPYE QEVSALGGIP 
                   
               
               
                   
               
               
                 YSQIYGWYRV NFGVIDERLH RNREYRDRYY RNLNIAPAED 
                 180 
               
               
                 GYRLAGFPPD HQAWREEPWI 
                   
               
               
                   
               
               
                 HHAPQGCGNS SRTITGDTCN EETQNLSTIY LRKYQSKVKR 
                 240 
               
               
                 QIFSDYQSEV DIYNRIRNEL 
                   
               
               
                   
               
             
          
           
               
                 Nucleotide sequence (SEQ ID NO: 9) of LT A (S61Y) 
               
             
          
           
               
                 atgaaaaata taactttcat tttttttatt ttattagcat 
                 60 
               
               
                 cgccattata tgcaaatggc 
                   
               
               
                   
               
               
                 gacaaattat accgtgctga ctctagaccc ccagatgaaa 
                 120 
               
               
                 taaaacgttc cggaggtctt 
                   
               
               
                   
               
               
                 atgcccagag ggcataatga gtacttcgat agaggaactc 
                 180 
               
               
                 aaatgaatat taatctttat 
                   
               
               
                   
               
               
                 gatcacgcga gaggaacaca aaccggcttt gtcagatatg 
                 240 
               
               
                 atgacggata tgtttatact 
                   
               
               
                   
               
               
                 tctcttagtt tgagaagtgc tcacttagca ggacagtcta 
                 300 
               
               
                 tattatcagg atattccact 
                   
               
               
                   
               
               
                 tactatatat atgttatagc gacagcacca aatatgttta 
                 360 
               
               
                 atgttaatga tgtattaggc 
                   
               
               
                   
               
               
                 gtatacagcc ctcacccata tgaacaggag gtttctgcgt 
                 420 
               
               
                 taggtggaat accatattct 
                   
               
               
                   
               
               
                 cagatatatg gatggtatcg tgttaatttt ggtgtgattg 
                 480 
               
               
                 atgaacgatt acatcgtaac 
                   
               
               
                   
               
               
                 agggaatata gagaccggta ttacagaaat ctgaatatag 
                 540 
               
               
                 ctccggcaga ggatggttac 
                   
               
               
                   
               
               
                 agattagcag gtttcccacc ggatcaccaa gcttggagag 
                 600 
               
               
                 aagaaccctg gattcatcat 
                   
               
               
                   
               
               
                 gcaccacaag gttgtggaaa ttcatcaaga acaattacag 
                 660 
               
               
                 gtgatacttg taatgaggag 
                   
               
               
                   
               
               
                 acccagaatc tgagcacaat atatctcagg aaatatcaat 
                 720 
               
               
                 caaaagttaa gaggcagata 
                   
               
               
                   
               
               
                 ttttcagact atcagtcaga ggttgacata tataacagaa 
                 777 
               
               
                 ttcggaatga attatga 
                   
               
               
                   
               
             
          
           
               
                 Amino acid sequence (SEQ ID NO: 10) of LT A (S61Y) 
               
             
          
           
               
                 NGDKLYRADS RPPDEIKRSG GLMPRGHNEY FDRGTQMNIN 
                 60 
               
               
                 LYDHARGTQT GFVRYDDGYV 
                   
               
               
                   
               
               
                   Y TSLSLRSAH LAGQSILSGY STYYIYVIAT APNMFNVNDV 
                 120 
               
               
                 LGVYSPHPYE QEVSALGGIP 
                   
               
               
                   
               
               
                 YSQIYGWYRV NFGVIDERLH RNREYRDRYY RNLNIAPAED 
                 180 
               
               
                 GYRLAGFPPD HQAWREEPWI 
                   
               
               
                   
               
               
                 HHAPQGCGNS SRTITGDTCN EETQNLSTIY LRKYQSKVKR 
                 240 
               
               
                 QIFSDYQSEV DIYNRIRNEL 
               
               
                   
               
             
          
         
       
     
     For comparison, another LT mutant, LTp(S63K), derived from EWDB299 (Dallas et al., 1979,  J. Bacteriol.  139, 850-859) was also constructed by replacement of serine at amino acid position 63 of subunit A with lysine. 
     Example 2 
     Preparing Wild Type LT and LT Containing Mutated LT A    
     pBluescript II KS(−) vectors containing native or mutant LT genes, including genes for both subunit A and subunit B, were transformed into  E. coli  HB101. The native and mutant LT&#39;s were purified from cultures grown overnight in a 3-liter flask containing L-broth supplemented with 100 μg of ampicillin per ml. The cells were harvested by centrifugation, resuspended in TEAN buffer (0.2 M NaCl, 50 mM Tris, 1 mM EDTA and 3 mM NaN3, pH 7.4), and lysed with microfluidizer (Microfluidics Corporation, USA). After the lysates were clarified by centrifugation, the LT was fractionated by adding solid ammonium sulfate to 65% saturation. The preparation was then suspended in TEAN buffer, dialyzed thoroughly against the same buffer and used as the crude LT. The crude LT was subjected to chromatography on Immobilized D-galactose (Pierce, Rockford, Ill.) columns equilibrated with TEAN buffer at 4° C. (Uesaka et al., 1994,  Microbial Pathogenesis  16:71-76). Native and mutant LT&#39;s were eluted with 0.3 M galactose in TEAN. Each purified toxin was dialyzed against PBS buffer for biological and immunological assays. 
     The purified wild-type and mutant LT were separated by SDS-PAGE, wherein the molecular weight of LT subunit A was about 28 to 29 kDa, and the molecular weight of LT subunit B was about 12 to 13 kDa. The yields of the entire LT containing LT A (S61K), i.e., LTh(S61K), and LT A (S61R), i.e., LTh(S61R) were similar to the LT containing the LT A  of SEQ ID NO:5. 
     AB 5  heterohexamers and B 5  pentamers of the LT were separated by pH 3-10 isoelectric focusing gel (invitrogen). Intensity of AB5 and B5 protein bands was assayed by UVIBand software (UVItec Limited) to calculate the percentage of the AB 5  heterohexamers and the B 5  pentamers (The isoelectric point (pI) value of AB 5  was between 8.0 and 7.8, and the pI value of B 5  was between 8.3 and 8.1), the results are listed in Table 1. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Preparation of wild-type LT and LT containing mutated LT A   
               
             
          
           
               
                   E. coli  heat-labile enterotoxin (LT) 
                 The ratio of AB5/AB5 + B5 (%) 
               
               
                   
               
               
                 LTh(WT) 
                 91 (%) 
               
               
                 LTh(S61K) 
                 92 (%) 
               
               
                 LTh(S61R) 
                 85 (%) 
               
               
                 LTh(S61H) 
                 60 (%) 
               
               
                 LTh(S61F) 
                 70 (%) 
               
               
                 LTh(S61Y) 
                 57 (%) 
               
               
                   
               
               
                 The percentages of AB 5  of purified LTh(S61K) and LTh(S61R) were about 90-100%. 
               
             
          
         
       
     
     Example 3 
     Determining Effect of Wild-Type LT and LT Containing Mutated LT A  on Intracellular cAMP Levels 
     Caco-2 cells (ATCC HTB-37) were maintained in MEM-α medium supplemented with 20% FBS in 24-well plate at a concentration of 5×104 cells per well, grown to near confluency, and incubated in MEM-α containing 1% FBS and 1 mM 3-isobutyl-1-methylxanthine (IBMX) for 30 min in 5% CO2 prior to addition of toxins (Grant et al., 1994, Infection and Immunity 62: 4270-4278). Native or mutant LT was added to each well and incubated for 4 hours. The cells were then washed twice with cold PBS. Intracellular cAMP was extracted by adding 200 μl of 0.1 N HCl to each well and incubated at room temperature for 15 minutes. Supernatant of cell lysates were collected following addition of 0.1 N NaOH to each well. (Cheng et al., 2000,  Vaccine  18: 38-49; Park et al., 1999,  Experimental and Molecular Medicine  31: 101-107). cAMP was measured with a cAMP enzyme immunoassay kit (Assay designs; Correlate-EIA). Results obtained from this example show that LTh(S61K) did not increase the concentration of intracellular cAMP. 
     Example 4 
     Determining Toxicity of LT Containing Mutated LT A  using Y1 Adrenal Tumor Cell Assay 
     In this study, LT samples, including wild-type LT, LT active site mutant (h61K), and LT subunit B complex, B5, were evaluated for enterotoxic effect. Mouse Y-1 adrenal tumor cells (ATCC CCL-79) maintained in Ham&#39;s F12 media supplemented with 15% horse serum, 2.5% fetal bovine serum, 2 mM L-glutamine, and 1.5 g/L sodium bicarbonate were seeded in 96-well flat-bottom plates at a concentration of 2×104 cells per well (200 ul/well) at 37° C. in 5% CO2 for 48 hrs. Cells in 96-well flat-bottom plates were washed twice with 1×PBS (pH 7.4) and then treated with serially diluted LT samples (10 μg/200 μl˜10−10 μg/200 μl) at 37° C. in 5% CO2 overnight. Cells were examined by light microscopy for typical cell rounding 24 hrs after toxin treatment. Activity is defined as the minimum concentration of the toxin required to initiate cell rounding (ECi) or the toxin concentration required for 50% cell rounding (EC50). See David et al., 1975, Infection and Immunity, 11:334-336; Cheng et al., 1999, Vaccine 18:38-49. The toxicity of LTh(S61K), LTh(S61R), LTh(S61H) is significantly lower compared to wild-type LT, i.e., 10 −6  versus 1. See Table 2 below. The toxicity of LTh(S61F) is also lower (i.e., 10 −5 ), but the reduction is not as significant as the other mutants. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Toxicity of LT containing wild-type or mutated LT A   
               
             
          
           
               
                   E. coli  heat-labile 
                 The 
                 Toxic intensity (wild type 
               
               
                 enterotoxin (LT) 
                 amount of LT (pg/well) 
                 LT is 1) 
               
               
                   
               
             
          
           
               
                 LTh(WT) 
                 0.1 
                  1 
               
               
                 LTh(S61K) 
                 100,000 
                 10 −6   
               
               
                 LTh(S61R) 
                 100,000 
                 10 −6   
               
               
                 LTh(S61H) 
                 100,000 
                 10 −6   
               
               
                 LTh(S61F) 
                 10,000 
                 10 −5   
               
               
                 LTh(S61Y) 
                 100,000 
                 10 −6   
               
               
                 LTh(S63K) 
                 10,000 
                 10 −5   
               
               
                   
               
             
          
         
       
     
     Example 5 
     Determining Toxicity of LT Containing Mutated LT A  by Rabbit Ileal Loop Assay 
     The assay was performed as previously described (Giannelli et al., 1997,  Infection and Immunity  65: 331-334; Giuliani et al., 1998,  J. Exp. Med.  187:1123-1132). The New Zealand adult rabbits, −2.5 Kg each, were used for this assay. Loops, each 5 cm long, were made by starting at the end of the rabbit&#39;s small intestines and moving toward the stomach. 0.5 ml samples with various amounts of LT or LT mutants were injected intro each loop and then the abdomen was closed. After 18 hours, the liquid accumulated in each loop was collected and measured. The experiment was performed three times and the results expressed in milliliters per centimeter. Results from this experiment show that 500 μg of LTh(S61K) only accumulated 1 ml fluid in the rabbit ileal loop, and the volume of the accumulated fluid was similar to native control. When 0.1 μg of wild-type LT was used, the volume of accumulated fluid of wild-type LT was considerably larger than that of LTh(S61K). In addition, the fluid accumulation of other LTp(S63K) also considerably larger than that of LTh(S61K). 
     Example 6 
     Determining Adjuvant Effect of LT Containing Mutated LT A  in Intranasal Immunization 
     Deactivated Influenza virus, A/Puerto Rico/8/34 (H1N1) (PR8) (ATCC VR-95), was used in this example. The viral particles were prepared as previously described in (Aitken et al., 1980,  Eur. J. Biochem.  107:51-56; Gallagher et al., 1984,  J. Clin. Microbiol.  20:89-93; Johansson et al., 1989,  J. Virol.  63:1239-1246). Briefly, the virus was propagated in the allantoic cavity of 10-day-old embryonated hen&#39;s eggs at 35° C. for two days. The allantoic fluid from eggs infected with PR8 was first centrifuged at low-speed and then centrifuged at 96,000×g for 1 hour to precipitate viral particles, which were resuspended in phosphate-buffered saline (PBS). These particles were loaded onto a 30-60% sucrose density gradient and centrifuged for 5 hours at 96,000×g. The fraction containing the virus was collected and diluted with PBS. The virus was further pelleted at 96,000×g for 1 hour and resuspended in PBS. Purified virus was treated with 0.025% formalin at 4° C. for a week. Protein concentration was measured and standardized based on the optical density, Bio-Rad Protein Assay, and the haemagglutinin (HA) content was determined by SDS-polyacryamide gel electrophoresis (Oxford et al., 1981,  J. Biol. Stand.  9:483-491). 
     Female BALB/c mice, between 6 and 8 weeks old, were obtained from Taiwan National Laboratory Animal Center. Groups consisting of five of mice each were immunized intranasally with 25 μl PBS containing 20 μg of inactivated influenza virus (flu.Ag) alone or in combination with 8 μg of native or mutant LT under anesthesia. The mice were re-immunized 3 weeks later. Control mice were given PBS under the same condition. Two weeks after the final immunization, mice in each group were sacrificed to obtain serum and conduct haemagglutination inhibition (HI) assay. Significantly enhanced HI titers were detected in mice intranasally immunized with the inactivated virus vaccine in combination with LTh(S61K) or LTp(S63K) when compared with mice intranasally immunized with inactivated virus vaccine alone. The HI titer of LTh(S61K) in combination with the inactivated virus was substantially increased. The HI titer of LTh(S61K) in combination with the virus is 813, similar to that of LTh(S63K) in combination with the virus but larger than that of the virus alone. 
     Example 7 
     Determining Adjuvant Effect of LT Containing Mutated LT A  in Intramuscular Immunization 
     The procedure carried out in Example 6 was repeated except that the immunization utilizes intramuscular delivery. Intramuscular vaccines were prepared in 50 μl PBS containing 10 μg of inactivated virus vaccine alone or in combination with 4 μg of native or mutant LT and were injected into the posterior thigh muscle. The immunization and sampling programs were performed as described. The HI titer of LTh(S61K) in combination with the inactivated virus was substantially increased. The HI titer of LTh(S61K) in combination with the virus was 512, which is significantly higher than that of wild type LT in combination with the virus or that of the virus alone. 
     Other Embodiments 
     All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features. 
     From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the scope of the following claims.