Seven neutralizing monoclonal antibodies were used to characterize 30 escape mutants of a type O foot-and-mouth disease (FMD) virus (O Kaufbeuren) selected with the five most active antibodies. Three non-overlapping antigenic sites were found by ELISA and cross-neutralization studies. Within two of the sites the epitopes of two or more monoclonal antibodies overlapped. Two of the sites were conformation-dependent and could not be detected on virus subunits or isolated denatured polypeptides. The third site was less conformation-dependent since the appropriate monoclonal antibodies were able to bind to 12S subunits, isolated VP1 protein and a synthetic peptide containing residues 141 to 160 of VP1 in ELISA. Electrofocusing of mutants of that site showed a high frequency of electrophoretic alterations in VP1. The sequence of most or all of the VP1 coding region of 10 escape mutants of that site plus three parental isolates was determined by primer extension sequencing. At least five amino acids were found to be involved but in only one case (residue 148 of VP1) did a change at that residue produce complete resistance to neutralization. Partial resistance was produced by changes at residues 144, 154 or 208 of VP1 or another residue(s), as yet undefined, that is probably in one of the other capsid polypeptides. Thus the site defined by these mutants was made up of at least three regions, the region involving residues 144 to 154 of VP1, the region encompassing residue 208 from the COOH terminus of VP1, plus a region, probably of VP2 or VP3, encompassing the undefined residue(s).


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