Lysine residue 117 of the FasG adhesin of enterotoxigenic Escherichia coli is essential for binding of 987P fimbriae to sulfatide

Type II NKT Cell Agonist, Sulfatide, Is an Effective Adjuvant for Oral Heat-Killed Cholera Vaccines

Oral vaccination has the potential to offer a safer and more efficacious approach for protection against enteric pathogens than injection-based approaches, especially in developing countries. One key advantage is the potential to induce intestinal immune responses in addition to systemic immunity. In general, antigen delivery via the oral route triggers weak immune responses or immunological tolerance. The effectiveness of oral vaccination can be improved by co-administering adjuvants. However, a major challenge is the absence of potent and safe oral adjuvants for clinical application. Here, the Type II NKT cell activator sulfatide is shown for the first time to be an effective oral adjuvant for Vibrio cholerae vaccine antigens in a mouse model. Specifically, administration of sulfatide with the oral cholera vaccine Dukoral^® resulted in enhancement of intestinal antigen-specific IgA in addition to Th1 and Th17 immune responses. In summary, sulfatide is a promising adjuvant for inclusion in an oral cholera vaccine and our data further support the potential of adjuvants targeting NKT cells in new vaccine strategies.

Passive antibodies derived from intramuscularly immunized toxoid fusion 3xSTaN12S-dmLT protect against STa+ enterotoxigenic Escherichia coli (ETEC) diarrhea in a pig model

Enterotoxigenic Escherichia coli (ETEC) strains are among the most common causes of children's diarrhea and travelers' diarrhea. Developing effective vaccines against ETEC associated diarrhea becomes a top priority. ETEC heat-labile toxin (LT) and heat-stable toxin (STa) toxoid fusion 3xSTa_N12S-dmLT was demonstrated recently to induce neutralizing antitoxin antibodies in intraperitoneally or subcutaneously immunized mice. However, whether antibodies derived from this toxoid fusion are protective against ETEC diarrhea has not been examined. In this study, we intramuscularly immunized pregnant gilts with toxoid fusion 3xSTa_N12S-dmLT, challenged suckling piglets with a STa-positive ETEC strain, and assessed protective efficacy of passive acquire antitoxin antibodies against ETEC diarrhea. Data showed all three immunized gilts developed anti-STa IgG and IgA antibodies, and piglets born to the immunized dams acquired anti-STa and anti-LT antibodies. When challenged with a STa+ ETEC strain, none of the piglets born to the immunized dams developed watery diarrhea, with 20 piglets remained normal and the other 8 piglets developed mild diarrhea indicated with stained butt. In contrast, the control dams and born piglets had no anti-STa or anti-LT antibodies detected, and 26 out 32 piglets developed watery diarrhea after challenge of the STa+ ETEC strain. These results indicated that passive acquired anti-STa antibodies are protective against ETEC diarrhea, and suggested potential application of toxoid fusion 3xSTa_N12S-dmLT in ETEC vaccine development.

Animal Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of E. coli diarrhea in farm animals. ETEC are characterized by the ability to produce two types of virulence factors: adhesins that promote binding to specific enterocyte receptors for intestinal colonization and enterotoxins responsible for fluid secretion. The best-characterized adhesins are expressed in the context of fimbriae, such as the F4 (also designated K88), F5 (K99), F6 (987P), F17, and F18 fimbriae. Once established in the animal small intestine, ETEC produce enterotoxin(s) that lead to diarrhea. The enterotoxins belong to two major classes: heat-labile toxins that consist of one active and five binding subunits (LT), and heat-stable toxins that are small polypeptides (STa, STb, and EAST1). This review describes the disease and pathogenesis of animal ETEC, the corresponding virulence genes and protein products of these bacteria, their regulation and targets in animal hosts, as well as mechanisms of action. Furthermore, vaccines, inhibitors, probiotics, and the identification of potential new targets by genomics are presented in the context of animal ETEC.

Adhesins of Enterotoxigenic Escherichia coli Strains That Infect Animals

The first described adhesive antigen of Escherichia coli strains isolated from animals was the K88 antigen, expressed by strains from diarrheic pigs. The K88 antigen was visible by electron microscopy as a surface-exposed filament that was thin and flexible and had hemagglutinating properties. Many different fimbriae have been identified in animal enterotoxigenic E. coli (ETEC) and have been discussed in this article. The role of these fimbriae in the pathogenesis of ETEC has been best studied with K88, K99, 987P, and F41. Each fimbrial type carries at least one adhesive moiety that is specific for a certain host receptor, determining host species, age, and tissue specificities. ETEC are the most frequently diagnosed pathogens among neonatal and post-weaning piglets that die of diarrhea. Immune electron microscopy of animal ETEC fimbriae usually shows that the minor subunits are located at the fimbrial tips and at discrete sites along the fimbrial threads. Since fimbriae most frequently act like lectins by binding to the carbohydrate moieties of glycoproteins or glycolipids, fimbrial receptors have frequently been studied with red blood cells of various animal species. Identification and characterization of the binding moieties of ETEC fimbrial adhesins should be useful for the design of new prophylactic or therapeutic strategies. Some studies describing potential receptor or adhesin analogues that interfere with fimbria-mediated colonization have been described in the article.

Porcine intestinal glycosphingolipids recognized by F6-fimbriated enterotoxigenic Escherichia coli

One important virulence factor of enterotoxigenic Escherichia coli is their ability to adhere via fimbrial adhesins to specific receptors located on the intestinal mucosa. Here, the potential glycosphingolipid receptors of enterotoxigenic F6-fimbriated E. coli were examined by binding of purified F6 fimbriae, and F6-expressing bacteria, to glycosphingolipids on thin-layer chromatograms. When intestinal mucosal non-acid glycosphingolipids from single pigs were assayed for F6 binding capacity, a selective interaction with two glycosphingolipids was observed. The binding-active glycosphingolipids were isolated and characterized as lactotriaosylceramide (GlcNAcβ3Galβ4Glcβ1Cer) and lactotetraosylceramide (Galβ3GlcNAcβ3Galβ4Glcβ1Cer). Further binding assays using a panel of reference glycosphingolipids showed a specific interaction between the F6 fimbriae and a number of neolacto core chain (Galβ4GlcNAc) glycosphingolipids. In addition, an occasional binding of the F6 fimbriae to sulfatide, galactosylceramide, lactosylceramide with phytosphingosine and/or hydroxy fatty acids, isoglobotriaosylceramide, gangliotriaosylceramide, and gangliotetraosylceramide was obtained. From the results we conclude that lactotriaosylceramide and lactotetraosylceramide are major porcine intestinal receptors for F6-fimbriated E. coli.

Oral immunization of a live attenuated Escherichia coli strain expressing a holotoxin-structured adhesin-toxoid fusion (1FaeG-FedF-LTA₂:5LTB) protected young pigs against enterotoxigenic E. coli (ETEC) infection

ETEC strains expressing K88 (F4) or F18 fimbriae and enterotoxins are the predominant cause of porcine post-weaning diarrhea (PWD). PWD continues causing significant economic losses to swine producers worldwide. Vaccines effectively protecting against PWD are needed. Our recent study revealed that a tripartite adhesin-toxin monomer (FaeG-FedF-LT(A2-B)) elicited protective antibodies. In this study, we constructed a new adhesin-toxoid fusion, expressed it as a 1A:5B holotoxin-structured antigen (1FaeG-FedF-LT(192A2):5LT(B)) in an avirulent Escherichia coli strain, and evaluated its vaccine potential in pig challenge studies. Piglets orally inoculated with this live strain showed no adverse effects but developed systemic and mucosal antibodies that neutralized cholera toxin and inhibited adherence of K88 and F18 fimbriae in vitro. Moreover, the immunized piglets, when were challenged with ETEC strain 3030-2 (K88ac/LT/STb), had significant fewer bacteria colonized at small intestines and did not develop diarrhea; whereas the control piglets developed severe diarrhea and died. These results indicated the 1FaeG-FedF-LT(192A2):5LT(B) fusion antigen induced protective antiadhesin and antitoxin immunity in pigs, and suggested a live attenuated vaccine can be potentially developed against porcine ETEC diarrhea. Additionally, presenting antigens in a holotoxin structure to target host local mucosal immunity can be used in vaccine development against other enteric diseases.

Genetic fusions of heat-labile (LT) and heat-stable (ST) toxoids of porcine enterotoxigenic Escherichia coli elicit neutralizing anti-LT and anti-STa antibodies

Enterotoxigenic Escherichia coli (ETEC) strains are a major cause of diarrheal disease in humans and farm animals. E. coli fimbriae, or colonization factor antigens (CFAs), and enterotoxins, including heat-labile enterotoxins (LT) and heat-stable enterotoxins (ST), are the key virulence factors in ETEC diarrhea. Unlike fimbriae or LT, STa has not often been included as an antigen in development of vaccines against ETEC diarrhea because of its poor immunogenicity. STa becomes immunogenic only after being coupled with a strongly immunogenic carrier protein. However, native or shorter STa antigens either had to retain toxic activity in order to become antigenic or elicited anti-STa antibodies that were not sufficiently protective. In this study, we genetically mutated the porcine LT (pLT) gene for a pLT(192(R-->G)) toxoid and the porcine STa (pSTa) gene for three full-length pSTa toxoids [STa(11(N-->K)), STa(12(P-->F)), and STa(13(A-->Q))] and used the full-length pLT(192) as an adjuvant to carry the pSTa toxoid for pLT(192):pSTa-toxoid fusion antigens. Rabbits immunized with pLT(192):pSTa(12) or pLT(192):pSTa(13) fusion protein developed high titers of anti-LT and anti-STa antibodies. Furthermore, rabbit antiserum and antifecal antibodies were able to neutralize purified cholera toxin (CT) and STa toxin. In addition, preliminary data suggested that suckling piglets born by a sow immunized with the pLT(192):pSTa(13) fusion antigen were protected when challenged with an STa-positive ETEC strain. This study demonstrated that pSTa toxoids are antigenic when fused with a pLT toxoid and that the elicited anti-LT and anti-STa antibodies were protective. This fusion strategy could provide instructive information to develop effective toxoid vaccines against ETEC-associated diarrhea in animals and humans.

Evolution of the chaperone/usher assembly pathway: fimbrial classification goes Greek

Many Proteobacteria use the chaperone/usher pathway to assemble proteinaceous filaments on the bacterial surface. These filaments can curl into fimbrial or nonfimbrial surface structures (e.g., a capsule or spore coat). This article reviews the phylogeny of operons belonging to the chaperone/usher assembly class to explore the utility of establishing a scheme for subdividing them into clades of phylogenetically related gene clusters. Based on usher amino acid sequence comparisons, our analysis shows that the chaperone/usher assembly class is subdivided into six major phylogenetic clades, which we have termed alpha-, beta-, gamma-, kappa-, pi-, and sigma-fimbriae. Members of each clade share related operon structures and encode fimbrial subunits with similar protein domains. The proposed classification system offers a simple and convenient method for assigning newly discovered chaperone/usher systems to one of the six major phylogenetic groups.

Enterotoxigenic Escherichia coli in veterinary medicine

Enterotoxigenic Escherichia coli (ETEC) infection is the most common type of colibacillosis of young animals (primarily pigs and calves), and it is a significant cause of diarrhoea among travellers and children in the developing world. The main virulence attributes of ETEC are adhesins and enterotoxins, which are mostly regulated on large plasmids. Almost all ETEC bacteria are known to adhere to receptors on the small intestinal epithelium by their proteinaceous surface appendages (fimbriae, pili) or by afimbrial proteins without inducing significant morphological changes. Furthermore, they secrete protein toxins (enterotoxins) to reduce absorption and to increase fluid and electrolyte secretion of small intestinal epithelial cells. Regarding details of epidemiology, pathogenesis, diagnosis and prevention of ETEC infections and diarrhoea in animals, readers are referred to an earlier more extensive review [Nagy and Fekete, 1999. Enterotoxigenic Escherichia coli (ETEC) in farm animals. Vet. Res. 30, 259-284]. This paper intends to summarise our basic knowledge and to highlight the new developments and most actual research topics in the area of ETEC infections in veterinary medicine. Attention is paid to recently described new virulence factors and to new genetic vectors in ETEC bacteria. Applications of our knowledge in the diagnosis and prevention of ETEC diarrhoea in animals will also be discussed.

Histone H1 proteins act as receptors for the 987P fimbriae of enterotoxigenic Escherichia coli

The tip adhesin FasG of the 987P fimbriae of enterotoxigenic Escherichia coli mediates two distinct adhesive interactions with brush border molecules of the intestinal epithelial cells of neonatal piglets. First, FasG attaches strongly to sulfatide with hydroxylated fatty acyl chains. This interaction involves lysine 117 and other lysine residues of FasG. Second, FasG recognizes specific intestinal brush border proteins that migrate on a sodium-dodecyl sulfate-polyacrylamide gel like a distinct set of 32-35-kDa proteins, as shown by ligand blotting assays. The protein sequence of high performance liquid chromatography-purified tryptic fragments of the major protein band matched sequences of human and murine histone H1 proteins. Porcine histone H1 proteins isolated from piglet intestinal epithelial cells demonstrated the same SDS-PAGE migration pattern and 987P binding properties as the 987P-specific protein receptors from porcine intestinal brush borders. Binding was dose-dependent and shown to be specific in adhesion inhibition and gel migration shift assays. Moreover, mapping of the histone H1 binding domain suggested that it is located in their lysine-rich C-terminal domains. Histone H1 molecules were visualized on the microvilli of intestinal epithelial cells by immunohistochemistry and electron microscopy. Taken together these results indicated that the intestinal protein receptors for 987P are histone H1 proteins. It is suggested that histones are released into the intestinal lumen by the high turnover of the intestinal epithelium. Their strong cationic properties can explain their association with the negatively charged brush border surfaces. There, the histone H1 molecules stabilize the sulfatide-fimbriae interaction by simultaneously binding to the membrane and to 987P.

Differential carbohydrate epitope recognition of globotriaosyl ceramide by verotoxins and a monoclonal antibody. Role in human renal glomerular binding

The role of renal expression of the glycosphingolipid verotoxin receptor, globotriaosylceramide, in susceptibility to verotoxin-induced hemolytic uremic syndrome is unclear. We show that a single glycosphingolipid can discriminate multiple specific ligands. Antibody detection of globotriaosylceramide in renal sections does not necessarily predict verotoxin binding. The deoxyglobotriaosylceramide binding profile for verotoxin 1, verotoxin 2 and monoclonal anti-globotriaosylceramide are distinct. Anti-globotriaosylceramide had greater dependency on the intact alpha-galactose and reducing glucose of globotriaosylceramide than verotoxin 1, while verotoxin 2 was intermediate. These ligands differentially stained human kidney sections. Glomerulopathy is the primary verotoxin-associated pathology in hemolytic uremic syndrome. For most samples, verotoxin 1 immunostaining within adult glomeruli was observed (type A). Some samples, however, lacked glomerular binding (type B). Anti-globotriaosylceramide (and less effectively, verotoxin 2) stained all glomeruli. Verotoxin 1/anti-globotriaosylceramide tubular staining was comparable. Type B glomerular/tubular globotriaosylceramide showed minor, but significant, fatty acid compositional differences. Verotoxin 1 type B glomerular binding became evident following pretreatment with cold acetone, or methyl-beta-cyclodextrin, used to deplete cholesterol. Direct visualization, using fluorescein isothiocyanate-verotoxin 1B, showed paediatric, but no adult glomerular staining; this was confirmed by anti-fluorescein isothiocyanate immunostaining. Acetone induced fluorescein isothiocyanate-verotoxin 1B glomerular staining in type A, but poorly in type B samples. Comparison of fluorescein isothiocyanate-verotoxin 1B and native verotoxin 1B deoxyglobotriaosylceramide analogue binding showed an alteration in subspecificity. These studies indicate a marked heterogeneity of globotriaosylceramide expression within renal glomeruli and differential binding of verotoxin 1/verotoxin 2/anti-globotriaosylceramide to the same glycosphingolipid. Verotoxin 1 derivatization can induce subtle changes in globotriaosylceramide binding to significantly affect tissue binding. Heterogeneity in glomerular globotriaosylceramide expression may play a significant (cholesterol-dependent?) role in determining renal pathology following verotoxemia.

Phase variation of the 987P-like CS18 fimbriae of human enterotoxigenic Escherichia coli is regulated by site-specific recombinases

The gene cluster of the CS18 (PCFO20) fimbriae of human enterotoxigenic Escherichia coli (ETEC) was found to include seven genes (fotA to fotG) that are similar to each of the seven structural and export proteins of the 987P fimbriae. However, no analogous gene to the fasH regulatory gene, which is located at the 3' end of the 987P gene cluster and encodes an AraC-like activator of transcription, could be detected. Surprisingly, two novel genes (fotS and fotT) encoding proteins similar to the site-specific recombinases of the type 1 fimbriae (FimB and FimE) were identified at the 5' end of the fot gene cluster. These genes were shown to be required for the catalysis of a 312 bp-inversion just upstream of fotA. The inversion determines CS18 fimbrial phase variation. FotS participates in inverting the 312 bp-segment in both the ON and OFF orientation, whereas FotT has a bias for the OFF oriented recombination. Similar regulators of fimbriation by phase variation were described in uropathogenic and commensal Enterobacteriaceae. In contrast, only AraC-like transcriptional activators were previously described as regulators of the intestinal colonization factors of human ETEC isolates. Thus, the CS18 and 987P gene clusters encode similar components for fimbrial biogenesis but different types of regulators for fimbriation. The combination of blocks of genes encoding similar structural products but different regulatory proteins underlines how modular DNA rearrangements can evolve by serving pathogen diversification. Acquisition of a phase variation module to regulate fimbrial genes is proposed to be beneficial for the adaptation and transmission of pathogens.

Mapping the binding domain of the F18 fimbrial adhesin

F18 fimbrial Esherichia coli strains are associated with porcine postweaning diarrhea and pig edema disease. Recently, the FedF subunit was identified as the adhesin of the F18 fimbriae. In this study, adhesion domains of FedF were further studied by constructing deletions within the fedF gene and expressing FedF proteins with deletions either together with the other F18 fimbrial subunits or as fusion proteins tagged with maltose binding protein. The region essential for adhesion to porcine intestinal epithelial cells was mapped between amino acid residues 60 and 109 of FedF. To map the binding domain even more closely, all eight charged amino acid residues within this region were independently replaced by alanine. Three of these single point mutants expressing F18 fimbriae exhibited significantly diminished capabilities to adhere to porcine epithelial cells in vitro. In addition, a triple point mutation and a double point mutation completely abolished receptor adhesiveness. The result further confirmed that the region between amino acid residues 60 and 109 is essential for the binding of F18 fimbriae to their receptor. In addition, the adhesion capability of the binding domain was eliminated after treatment with iodoacetamide, suggesting the formation of a disulfide bridge between Cys-63 and Cys-83, whereas Cys-111 and Cys-116 could be deleted without affecting the binding ability of FedF.

Mapping the binding domain of the F18 fimbrial adhesin

F18 fimbrial Esherichia coli strains are associated with porcine postweaning diarrhea and pig edema disease. Recently, the FedF subunit was identified as the adhesin of the F18 fimbriae. In this study, adhesion domains of FedF were further studied by constructing deletions within the fedF gene and expressing FedF proteins with deletions either together with the other F18 fimbrial subunits or as fusion proteins tagged with maltose binding protein. The region essential for adhesion to porcine intestinal epithelial cells was mapped between amino acid residues 60 and 109 of FedF. To map the binding domain even more closely, all eight charged amino acid residues within this region were independently replaced by alanine. Three of these single point mutants expressing F18 fimbriae exhibited significantly diminished capabilities to adhere to porcine epithelial cells in vitro. In addition, a triple point mutation and a double point mutation completely abolished receptor adhesiveness. The result further confirmed that the region between amino acid residues 60 and 109 is essential for the binding of F18 fimbriae to their receptor. In addition, the adhesion capability of the binding domain was eliminated after treatment with iodoacetamide, suggesting the formation of a disulfide bridge between Cys-63 and Cys-83, whereas Cys-111 and Cys-116 could be deleted without affecting the binding ability of FedF.

Binding to sulfatide and enterotoxicity of various Escherichia coli STb mutants

Binding of the 48 amino acid polypeptide of the mature heat-stable Escherichia coli enterotoxin b (STb) to the functional receptor sulfatide (SFT) constitutes the first step in inducing secretory diarrhoea in the intestinal lumen of animals. The NMR structure of this toxin dictated the choice of amino acids for site-directed mutagenesis to delineate the binding site of STb to SFT. Amino acids facing the solvent either in the loop or the hydrophobic alpha-helix were selected. Seventeen site-specific mutants of STb toxin were produced and purified by high-pressure liquid chromatography. Enterotoxicity of the 17 mutants was determined using a rat loop assay and binding was evaluated using a microtitre plate binding assay. Both hydrophobic and electrostatic interactions are important for STb attachment. When mutations (F37K, I41S and M42S) were introduced into the hydrophobic alpha-helix to lessen hydrophobicity, binding activity and enterotoxicity decreased by more than sixfold. The loop defined by C21 and C36 also made specific contributions. Mutants generated at basic residues (K22, K23 and R29) within this region exhibited both reduced binding activities and reduced toxic activities. For all STb mutants constructed and analysed, when binding to SFT was reduced, a reduction in toxicity equivalent or greater was noted, indicating that binding to SFT is a step that precedes the toxic effect observed for STb toxin. Significantly, when the negatively charged D30 was substituted for either alanine or valine, the binding to SFT was about twice that of native STb, whereas the enterotoxicity was reduced by half.

Characterization of FasG segments required for 987P fimbria-mediated binding to piglet glycoprotein receptors

The 987P fimbriae of enterotoxigenic strains of Escherichia coli bind to both glycoprotein and glycolipid receptors on the brush borders of piglet enterocytes. A mutation in lysine residue 117 of the adhesive subunit FasG [fasG(K117A)] previously shown to abrogate 987P binding to the lipid receptor sulfatide did not affect the interaction with the glycoprotein receptors. Both the fimbriae and the FasG subunits of the wild type and the fasG(K117A) mutant bound to the glycoprotein receptors, confirming that lysine 117 was not required for binding to the glycoprotein receptors. Truncated FasG molecules were used to identify domains required for glycoprotein receptor recognition. At least two segments which did not include lysine117, namely, residues 211 (glutamine) to 220 (serine) and 20 (aspartic acid) to 41 (serine), were shown to be involved in the FasG-glycoprotein receptor interactions by ligand-blotting assays. Changing isoleucine 217 or leucine 215 of FasG to alanine abolished the property of a truncated FasG fusion protein to inhibit 987P recognition of its glycoprotein receptors. Thus, the K117 residue of FasG is required only for binding to the glycolipid receptor, whereas the newly identified hydrophobic residues of the FasG subunit are required specifically for the recognition of the glycoprotein receptor. Taken together, our data indicate that different residues of the FasG adhesin are important in 987P fimbrial binding to sulfatide and glycoprotein receptors, suggesting different mechanisms of interaction.

Enhanced immune responses to viral epitopes by combining macrophage-inducible expression with multimeric display on a Salmonella vector

In this study, the immunogenicity of chimeric 987P fimbriae on a Salmonella vaccine strain was improved by optimizing fimbrial expression. The constitutive tetA promoter and the in vivo activated nirB and pagC promoters were evaluated for their use to express two epitopes of the transmissible gastroenteritis virus (TGEV) spike protein carried by fimbriae which were displayed on a Salmonella vaccine strain. Constructs with the pagC promoter were shown to drive increased expression of chimeric 987P fimbriae in macrophages as well as in Mg(2+)-poor media, mimicking a major environmental signal found in Salmonella-containing endocytic vacuoles of macrophages. Mice immunized orally with a Salmonella vaccine strain which expressed chimeric fimbriae from the pagC promoter elicited significantly higher mucosal and systemic immune responses to both the 987P fimbriae and the TGEV epitopes than mice immunized with the same strain hosting a tetA or nirB promoter-driven expression plasmid. Moreover, only the Salmonella vaccine strains harboring a plasmid with the pagC promoter, with or without an additional tetA promoter in tandem, elicited neutralizing antibodies to TGEV. This indicated that the pagC promoter can be used successfully to improve epitope-display by chimeric fimbriae on Salmonella vaccine strains for the induction of a desired immune response.

The ATPase domain of hsp70 possesses a unique binding specificity for 3'-sulfogalactolipids

The region(s) of hsp70 critical for sulfogalactolipid (SGL) recognition has been defined through deletion analysis and site-directed mutagenesis. Truncated polymerase chain reaction products of hsp70 generated N-terminal fragments of 43, 35, 29, and 22 kDa. The C terminus substrate-binding domain (28 kDa) was also expressed. The N-terminal ATPase domain (rP43) shared the binding specificity of hsp70, because only sulfogalactosyl ceramide and sulfogalactosyl glycerolipid were recognized by both TLC overlay and RELISA. The C-terminal domain showed no binding. SGL binding of rP29 and rP22 was severely reduced. The loss of SGL binding for rP35 by RELISA but not TLC overlay was considered as a function of receptor presentation. The truncation of rP43 to rP35 demonstrates that residues 318-387 (the base of the ATP binding cleft) are critical for high affinity SGL binding. Mutagenesis showed that Arg(342) and Phe(198) are crucial for this process. SGL binding, mediated by these conserved residues within the ATPase domain of hsp70, implies that this binding specificity is evolutionarily conserved.

Mucosal and systemic immune responses to chimeric fimbriae expressed by Salmonella enterica serovar typhimurium vaccine strains

Recombinant live oral vaccines expressing pathogen-derived antigens offer a unique set of attractive properties. Among these are the simplicity of administration, the capacity to induce mucosal and systemic immunity, and the advantage of permitting genetic manipulation for optimal antigen presentation. In this study, the benefit of having a heterologous antigen expressed on the surface of a live vector rather than intracellularly was evaluated. Accordingly, the immune response of mice immunized with a Salmonella enterica serovar Typhimurium vaccine strain expressing the Escherichia coli 987P fimbrial antigen on its surface (Fas(+)) was compared with the expression in the periplasmic compartment (Fas(-)). Orally immunized BALB/c mice showed that 987P fimbriated Salmonella serovar Typhimurium CS3263 (aroA asd) with pCS151 (fas(+) asd(+)) elicited a significantly higher level of 987P-specific systemic immunoglobulin G (IgG) and mucosal IgA than serovar Typhimurium CS3263 with pCS152 (fasD mutant, asd(+)) expressing 987P periplasmic antigen. Further studies were aimed at determining whether the 987P fimbriae expressed by serovar Typhimurium chi4550 (cya crp asd) could be used as carriers of foreign epitopes. For this, the vaccine strain was genetically engineered to express chimeric fimbriae carrying the transmissible gastroenteritis virus (TGEV) C (379-388) and A (521-531) epitopes of the spike protein inserted into the 987P major fimbrial subunit FasA. BALB/c mice administered orally serovar Typhimurium chi4550 expressing the chimeric fimbriae from the tet promoter in pCS154 (fas(+) asd(+)) produced systemic antibodies against both fimbria and the TGEV C epitope but not against the TGEV A epitope. To improve the immunogenicity of the chimeric fimbriae, the in vivo inducible nirB promoter was inserted into pCS154, upstream of the fas genes, to create pCS155. In comparison with the previously used vaccine, BALB/c mice immunized orally with serovar Typhimurium chi4550/pCS155 demonstrated significantly higher levels of serum IgG and mucosal IgA against 987P fimbria. Moreover, mucosal IgA against the TGEV C epitope was only detected with serovar Typhimurium chi4550/pCS155. The induced antibodies also recognized the epitopes in the context of the full-length TGEV spike protein. Hence, immune responses to heterologous chimeric fimbriae on Salmonella vaccine vectors can be optimized by using promoters known to be activated in vivo.