The F4 fimbrial antigen of Escherichia coli and its receptors

F4 or K88 fimbriae are long filamentous polymeric surface proteins of enter otoxigenic Escherichia coli (ETEC), consisting of so-called major (FaeG) an d minor (FaeF FaeH, FaeC, and probably FaeI) subunits. Several serotypes of F4 have been described, namely F4ab, F4ac, and F4ad. The Fl fimbriae allow the microorganisms to adhere to W-specific receptors present on brush bord ers of villous enterocytes and consequently to colonize the small intestine . Such ETEC infections are responsible for diarrhea and mortality in neonat al and recently weaned pigs. In this review emphasis is pur on the morpholo gy, genetic configuration, and biosynthesis of F4 fimbriae. Furthermore, th e localization of the different a, b, c, and d epitopes, and the localizati on of the receptor binding site on the FaeG major subunit of F4 get ample a ttention. Subsequently, the F4-specific receptors are discussed. When the t hree variants of F4 (F4ab, F4ac, and F4ad) are considered, six porcine phen otypes can be distinguished with regard to the brush border adhesiveness: p henotype A binds all three variants, phenotype B binds Flab and F4ac, pheno type C binds F-lab 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 be is fou nd 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 characteriz ation 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 be 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. Fina lly, the importance of F4 fimbriae and their receptors in the study of muco sal immunity in pigs is discussed. (C) 2000 Elsevier Science B.V. All right s reserved.