Distribution of K88 Escherichia coli-adhesive and nonadhesive phenotypes among pigs of four breeds

Evaluation of receptor binding specificity of Escherichia coli K88 (F4) fimbrial adhesin variants using porcine serum transferrin and glycosphingolipids as model receptors

Diarrheal disease caused by enterotoxigenic Escherichia coli expressing the K88 (F4) fimbrial adhesin (K88 ETEC) is a significant source of mortality and morbidity among newborn and weaned piglets. K88 fimbrial adhesins are filamentous surface appendages whose lectin (carbohydrate-binding) activity allows K88 ETEC to attach to specific glycoconjugates (receptors) on porcine intestinal epithelial cells. There are three variants of K88 adhesin (K88ab, K88ac, and K88ad), which possess different, yet related, carbohydrate-binding specificities. We used porcine serum transferrin (pSTf) and purified glycosphingolipids (GSL) to begin to define the minimal recognition sequence for K88 adhesin variants. We found that K88ab adhesin binds with high affinity to pSTf (dissociation constant, 75 microM), while neither K88ac nor K88ad adhesin recognizes pSTf. Degradation of the N-glycan on pSTf by extensive metaperiodate treatment abolished its interaction with the K88ab adhesin, indicating that the K88ab adhesin binds to the single N-glycan found on pSTf. Using exoglycosidase digestion of the pSTf glycan, we demonstrated that K88ab adhesin recognizes N-acetylglucosamine (GlcNAc) residues in the core of the N-glycan on pSTf. All three K88 variants were found to bind preferentially to GSL containing a beta-linked N-acetylhexosamine (HexNAc), either GlcNAc or N-acetylgalactosamine, in the terminal position or, alternatively, in the penultimate position with galactose in the terminal position. Considering the results from pSTf and GSL binding studies together, we propose that the minimal recognition sequence for the K88 adhesin variants contains a beta-linked HexNAc. In addition, the presence of a terminal galactose beta-linked to this HexNAc residue enhances K88 adhesin binding.

Inhibition of adhesion of Escherichia coli k88ac fimbria to its receptor, intestinal mucin-type glycoproteins, by a monoclonal antibody directed against a variable domain of the fimbria

Strains of enterotoxigenic Escherichia coli that express K88 fimbriae are among the most common causes of diarrhea in young pigs. Adhesion of bacteria to receptors on intestinal epithelial cells, mediated by K88 fimbriae, is the initial step in the establishment of infection. Three antigenic variants of K88 fimbriae exist in nature: K88ab, K88ac, and K88ad. K88ac is the most prevalent and may be the only variant of significance in swine disease. Each K88 fimbrial variant is composed of multiple antigenic determinants. Some of these determinants are shared among the three variants and may be referred to as conserved epitopes, whereas others are unique to a specific variant and may be referred to as variable epitopes. In this study, monoclonal antibodies (MAbs) specific to either variable or conserved epitopes of K88ac fimbriae were produced. The specificity of each MAb was tested by enzyme-linked immunosorbent and immunoblot assays. Fab fragments were prepared from these MAbs and were tested for their ability to block the binding of K88-positive bacteria and purified fimbriae to porcine enterocyte brush border vesicles and purified K88 receptors, respectively. The purified receptors were intestinal mucin-type sialoglycoproteins (IMTGP) isolated from porcine enterocytes (A. K. Erickson, D. R. Baker, B. T. Bosworth, T. A. Casey, D. A. Benfield, and D. H. Francis, Infect. Immun. 62:5404-5410, 1994). Fab fragments prepared from MAbs specific for variable epitopes blocked the binding of bacteria to brush borders and of fimbriae to IMTGP. However, those from MAbs specific for a conserved epitope did not. These observations indicate that the receptor-binding domain of a K88ac fimbria is contained, at least in part, within the antigenically variable epitopes of that fimbria. Epitope mapping for one of the MAbs, which recognizes a linear epitope on K88ac fimbriae, indicated that this MAb binds to the region from amino acid no. 64 to no. 107 on the major subunit of K88ac fimbriae.

K88 adhesins of enterotoxigenic Escherichia coli and their porcine enterocyte receptors

The three antigenic variants of the K88 fimbrial adhesin (K88ab, K88ac, and K88ad) of enterotoxigenic Escherichia coli (ETEC) each exhibit unique specificity with regard to their hemagglutination characteristics. The variants are also unique in the specificity of their binding to the brush borders of enterocytes isolated from pigs with different genetic backgrounds. Diversity in enterocyte binding specificity suggests the existence of several K88 receptors, expressed individually or in various combinations on porcine enterocytes. Three candidate receptors have been identified that may explain the adhesion of K88 fimbrial variants to various porcine enterocytes. These receptors are an intestinal mucin-type sialoglycoprotein (IMTGP), an intestinal transferrin (GP74), and an intestinal neutral glycosphingolipid (IGLad). The IMTGP binds K88ab and K88ac, but not K88ad. The GP74 binds K88ab, but not K88ac or K88ad, and the IGLad binds K88ad, but not K88ab or K88ac. Each of the candidate receptors has been found in brush borders that are adhesive for the fimbriae that bind the respective receptor. They have not been found in brush borders that are not adhesive for those same fimbriae. The presence of IMTGP was highly correlated with susceptibility of neonatal gnotobiotic pigs to ETEC expressing K88ab or K88ac.

The F4 fimbrial antigen of Escherichia coli and its receptors

F4 or K88 fimbriae are long filamentous polymeric surface proteins of enterotoxigenic Escherichia coli (ETEC), consisting of so-called major (FaeG) and minor (FaeF, FaeH, FaeC, and probably FaeI) subunits. Several serotypes of F4 have been described, namely F4ab, F4ac, and F4ad. The F4 fimbriae allow the microorganisms to adhere to F4-specific receptors present on brush borders of villous enterocytes and consequently to colonize the small intestine. Such ETEC infections are responsible for diarrhea and mortality in neonatal and recently weaned pigs. In this review emphasis is put on the morphology, genetic configuration, and biosynthesis of F4 fimbriae. Furthermore, the localization of the different a, b, c, and d epitopes, and the localization of the receptor binding site on the FaeG major subunit of F4 get ample attention. Subsequently, the F4-specific receptors are discussed. When the three variants of F4 (F4ab, F4ac, and F4ad) are considered, six porcine phenotypes can be distinguished with regard to the brush border adhesiveness: phenotype A binds all three variants, phenotype B binds F4ab and F4ac, phenotype C binds F4ab and F4ad, phenotype D binds F4ad, phenotype E binds none of the variants, and phenotype F binds F4ab. The following receptor model is described: receptor bcd is found in phenotype A pigs, receptor bc is found in phenotype A and B pigs, receptor d is found in phenotype C and D pigs, and receptor b is found in phenotype F pigs. Furthermore, the characterization of the different receptors is described in which the bcd receptor is proposed as collection of glycoproteins with molecular masses ranging from 45 to 70 kDa, the bc receptor as two glycoproteins with molecular masses of 210 an 240 kDa, respectively, the b receptor as a glycoprotein of 74 kDa, and the d receptor as a glycosphingolipid with unknown molecular mass. Finally, the importance of F4 fimbriae and their receptors in the study of mucosal immunity in pigs is discussed.

Escherichia coli K88 receptor expression in intestine of disease-susceptible weaned pigs

A challenge trial was carried out in which Escherichia coli O157 K88ac was administered to a litter of weaned pigs and the development of the disease monitored over a five-day experimental period. The eight animals in the trial were assigned to two groups depending on whether they exhibited disease symptoms. Six pigs developed diarrhoea and two appeared unaffected; these were designated as the test (or K88-susceptible) group and the control (or K88-resistant) group, respectively. The animals were euthanised and the intestine was removed and sections processed for brush border membrane vesicle preparation. Microscopic and biochemical assays were undertaken on tissue samples from each animal and a strong correlation was observed between the expression of a glycoprotein receptor complex associated with the brush border membrane and the development of disease symptoms. Further investigation revealed the presence of an analogous glycoprotein complex in the K88-resistant group which did not bind the K88-fimbriae antigen. These results suggest that genetic differences in the glycosyl moieties of the receptor complex provide the basis for disease susceptibility to K88-positive E. coli.

Identification of an intestinal neutral glycosphingolipid as a phenotype-specific receptor for the K88ad fimbrial adhesin of Escherichia coli

In this study, we identified a receptor for the K88ad fimbrial adhesin of Escherichia coli in neutral glycosphingolipid preparations from intestinal epithelial cells of K88ad-adhesive pigs, which was absent in preparations from K88ad-nonadhesive pigs. Neither K88ab nor K88ac adhesin variants bound to this neutral glycosphingolipid. Because this receptor is an intestinal glycosphingolipid that binds K88ad adhesin, it has been designated IGLad. Carbohydrate compositional analysis of a partially purified preparation of IGLad identified galactose, glucose, and N-acetylglucosamine in a ratio of 1.5:1.0:0.5 as the major monosaccharides. Preliminary characterization experiments using lectins showed that IGLad contains the terminal glycanic structure Galbeta1-4GlcNAc. Removal of terminal beta-linked galactose residues from IGLad decreased the recognition of IGLad by the K88ad adhesin, indicating that terminal beta-linked galactose is an essential component of the K88ad adhesin recognition site on IGLad. Studies with purified glycosphingolipid standards demonstrated that K88ad adhesin binds to neolactotetraosylceramide (nLc4Cer) (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glcbeta1-1Cer) , lactotriosylceramide (GlcNAcbeta1-3Galbeta1-4Glcbeta1-1Cer) and lactotetraosylceramide (Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glcbeta1-1Cer) . Based on these studies, IGLad appears to be nLc4Cer.

Pathogenicity of an enterotoxigenic Escherichia coli hemolysin (hlyA) mutant in gnotobiotic piglets

Pigs infected with hemolytic F4(+) strains of enterotoxigenic Escherichia coli often develop septicemia secondary to intestinal infection. We tested the hypothesis that inactivation of hemolysin would reduce the ability of F4(+) enterotoxigenic E. coli to cause septicemia in swine following oral inoculation. Inactivation of the hemolysin structural gene (hlyA) did not decrease the incidence of septicemia in the gnotobiotic piglet model.

Expression of mucin-type glycoprotein K88 receptors strongly correlates with piglet susceptibility to K88(+) enterotoxigenic Escherichia coli, but adhesion of this bacterium to brush borders does not

Three antigenic variants of the K88 fimbrial adhesin exist in nature, K88ab, K88ac, and K88ad. Enterotoxigenic Escherichia coli (ETEC) strains that produce these fimbriae cause life-threatening diarrhea in some but not all young pigs. The susceptibility of pigs to these organisms has been correlated with the adherence of bacteria to isolated enterocyte brush borders. Whether that correlation holds for multiple K88 variants and over a broad genetic base of pigs is unknown and was the impetus for this study. We also desired to examine the correlation of the expression of a porcine intestinal brush border mucin-type glycoprotein (IMTGP) which binds K88ab and K88ac with the susceptibility of piglets to K88(+) ETEC. Of 31 neonatal gnotobiotic pigs inoculated with K88ab+ or K88ac+ ETEC, 13 developed severe diarrhea, became dehydrated, and died or became moribund. Another pig became severely lethargic but not dehydrated. In vitro brush border adherence analysis was not possible for 10 of the severely ill pigs due to colonization by challenge strains. However, of the 17 pigs that did not become severely ill, 8 (47%) had brush borders that supported the adherence of K88ab+ and K88ac+ bacteria in vitro, suggesting a poor correlation between in vitro brush border adherence and piglet susceptibility to K88(+) ETEC. By contrast, the expression of IMTGP was highly correlated with susceptibility to K88(+) ETEC. Of the 12 pigs that produced IMTGP, 11 developed severe diarrhea. The other pig that produced IMTGP became lethargic but not severely diarrheic. Only 2 of 18 pigs that did not produce IMTGP became severely diarrheic. Colonizing bacteria were observed in histologic sections of intestines from all pigs that expressed IMTGP except for the one that did not develop severe diarrhea. However, colonizing bacteria were observed in histologic sections from only one pig that did not produce IMTGP. The bacterial concentration in the jejuna and ilea of pigs expressing IMTGP was significantly greater (P < 0.005) than that in pigs not expressing IMTGP. These observations suggest the IMTGP is a biologically relevant receptor for K88ab+ and K88ac+ E. coli or a correlate for expression for such a receptor.

Characterization of the carbohydrate moiety of intestinal mucin-type sialoglycoprotein receptors for the K88ac fimbrial adhesin of Escherichia coli

We have previously identified two mucin-type sialoglycoproteins from porcine intestinal epithelial cells with approximate molecular masses of 210 (intestinal mucin-type glycoprotein IMTGP-1) and 240 kDa (IMTGP-2) as receptors for the K88ab and K88ac fimbrial adhesins of Escherichia coli. These receptors are detected in intestinal brush border membrane preparations from pigs with adhesive phenotypes but not from pigs with nonadhesive phenotypes and are postulated to be important determinants of the susceptibility of pigs to K88ab+ and K88ac+ enterotoxigenic E. coli infections. Using exoglycosidase digestion studies, we have now determined that beta-linked galactose is an important component in the recognition of IMTGP-1 and IMTGP-2 by the K88ac adhesin. In addition, we observed a differential distribution of the K88ac adhesin binding activity of IMTGP-1 and IMTGP-2 along the crypt-villus axis, suggesting that receptor activity is dependent on the maturation state of the intestinal epithelial cells. Brush borders from immature intestinal epithelial cells possessed the highest concentrations of IMTGP-1 and IMTGP-2 receptor activity, with a progressive decrease in receptor activity as the cells mature. To characterize the differences in the carbohydrate moieties of IMTGP-1 and IMTGP-2, we developed a procedure for purifying the receptors, using phenol extraction followed by serial lectin affinity chromatography. Carbohydrate compositional analysis of the purified receptors indicated that the carbohydrate moieties of IMTGP-1 and IMTGP-2 consist of both N- and O-glycans containing galactose, glucose, sialic acid, mannose, N-acetylgalactosamine, N-acetylglucosamine, and fucose. The major difference between the two receptors is that IMTGP-2 contains a higher percentage of monosaccharides (mannose and glucose) commonly found in N-glycans.

Multiple receptors on porcine intestinal epithelial cells for the three variants of Escherichia coli K88 fimbrial adhesin

We evaluated intestinal epithelial membrane preparations from five phenotypes of pigs, distinguished by the variant of K88 fimbrial adhesin (K88ab, K88ac, K88ad) which bind to their intestinal epithelial cells (A-all three variants, B-K88ab and K88ac, C-K88ab and K88ad, D-K88ad, and E-none of the variants), for the presence of K88 adhesin receptors. Intestinal brush border membranes were prepared from 20 animals (four from each phenotype). Brush border proteins, that had been separated using SDS-PAGE and transferred to nitrocellulose membranes, were overlaid with biotinylated K88 adhesin, 35S-labelled K88+ Escherichia coli, or biotinylated K88+ E. coli. Biotinylated K88ab and K88ac fimbrial adhesins and labelled E. coli expressing K88ab or K88ac adhesin bound to 210- and 240-kDa receptors in phenotype A and B, but not phenotype C, D, or E animals. In contrast, no phenotype-specific receptors were identified for the K88ad adhesin. Previously, purified K88ab and K88ac fimbriae were shown to block K88ad binding, but purified K88ad fimbriae were unable to block K88ab or K88ac binding in phenotype A animals. These results point to the existence of three K88 adhesin receptors to account for the observed phenotypes: (1) Receptor bcd binds all three variants and is found in phenotype A pigs, (2) Receptor bc (210- and 240-kDa receptors) binds K88ab and K88ac and is found in phenotype A and B pigs, and (3) Receptor d binds K88ad and is found in phenotype C and D pigs.