Antigenic variation among members of the tick-borne encephalitis complex

The antigenic relationships between seven members of the tick-borne encephalitis complex of flaviviruses (group B arboviruses) were examined by raising a library of 16 monoclonal antibodies against one of them and examining their biological and antigenic properties. These clones reacted with only one of two intracellular, virus-specific polypeptides. One polypeptide [mol. wt. 58 X 10(3) (58K)] is related to the major envelope protein E, but the identity of the other is at present unknown, even though it is a major immunogen in experimental infections and vaccinations. Only those clones specific for the 58K polypeptide contain either neutralizing or haemagglutinin-inhibiting activity, but these epitopes are not identical. In general, epitopes on the 51K polypeptide were more heavily conserved than those on the 58K polypeptide, although both conserved and variant epitopes were found on both polypeptides. One epitope was present on the 51K polypeptide which was conserved on all seven isolates studied and another epitope on the same polypeptide was specific for all the western isolates including one isolate of louping-ill virus. Using the monoclonal antibodies raised in the study, it was shown that louping-ill virus was closely related antigenically to isolates of the Western subtype of tick-borne encephalitis virus.