IDENTIFICATION OF A CROSS-REACTIVE CONTINUOUS B-CELL EPITOPE IN ENTEROTOXIGENIC ESCHERICHIA-COLI COLONIZATION FACTOR ANTIGEN-I

Enterotoxigenic Escherichia coli (ETEC) colonizes the intestine by mea ns of several antigenically distinct colonization factors (CFs). Sever al of these CFs have very significant amino acid sequence similarity o r identity, particularly in the N-terminal end. We have previously sho wn that a monoclonal antibody (MAb) raised against the subunits of col onization factor antigen I (CFA/I) fimbriae, which reacts with a pepti de corresponding to the 25 N-terminal amino acids of such subunits, ca n inhibit attachment to intestinal cells of ETEC expressing heterologo us as well as homologous CFs, with related amino acid sequences. In th is study we have, by means of Pepscan analysis, determined the sequenc e of the MAb-specific linear epitope to be (15)IDLLQ(19). Parenteral i mmunization of rabbits with an N-terminal 25-mer synthetic peptide of CFA/I fimbrial subunit, either covalently coupled to bovine serum albu min or uncoupled, induced high titers of specific antibodies against t his peptide as well as against CFA/I fimbriae. Increased titers agains t several heterologous CF fimbriae with a related N-terminal sequence were also induced, whereas no increase was seen against fimbriae with an unrelated sequence. Neither antisera against the coupled peptide no r antisera against the uncoupled peptide inhibited binding of CF-expre ssing bacteria to the human intestinal cell line Caco-2 in spite of hi gh titers. The difference in the inhibitory capabilities of the antipe ptide sera and the MAb might be due to slightly different epitope spec ificities. Thus, whereas the antipeptide sera bound to several continu ous epitopes in the N-terminal end, none of them reacted specifically with the epitope (15)IDLLQ(19).