Abstract
So far, three subtypes of the F4 (K88) fimbrial antigen of porcine enterotoxigenic Escherichia coli, F4ab, F4ac, and F4ad, have been distinguished by using polyclonal antisera in agglutination and precipitation tests. The a factor represents one or more common epitopes, whereas each of the b, c, and d factors represents one or more subtype-specific epitopes. We further characterized the F4 antigen complex by using a panel of 40 F4-specific monoclonal antibodies (MAbs). The specificity of all MAbs was proven by enzyme-linked immunosorbent assays, agglutination and radioimmunoprecipitation tests, and immunoelectron microscopy. The MAbs either reacted with all F4 subtypes, reacted with two subtypes, or were subtype specific. Epitope analysis by competition enzyme-linked immunosorbent assays revealed at least 11 epitope clusters on the F4 antigen complex, designated a1 to a7, b1, b2, c, and d. The following antigenic formulas were found for the F4 subtypes: F4ab, a1a2a3a4a5a6b1b2; F4ac, a1a2a3(a4)a5a6a7c; and F4ad, a1a2a3a4a7d. All MAbs were directed against conformational epitopes located on the 27,500-dalton major fimbrial subunits. Consequences for the replacement of polyclonal antisera by MAbs in diagnostic tests are discussed.