Monoclonal antibodies (MAbs) were used to delineate the antigenic relationship between the three morbillivirus types: measles virus (MV), canine distemper virus (CDV) and rinderpest virus (RPV). Panels of six to 31 MAbs against the haemagglutinin (H), fusion (F),
nucleocapsid protein (NP),
phosphoprotein (P) and
matrix (M)
proteins of MV and the H, F, NP and P
proteins of CDV were employed. Nine strains of MV, three strains of CDV and four strains of RPV were examined by radioimmunoprecipitation assay and immune fluorescence for reactivity with the heterologous MAbs. Overall, the NP and in particular the F
proteins of the morbilliviruses showed a high degree of
epitopic homology; the P and M
proteins showed a partial
epitopic homology, with the greatest variation between the M
proteins of CDV and MV; the H
proteins showed a low degree of
epitopic homology and then only between MV and RPV. These data indicate that the major cross-protecting
antigen in heterotypic vaccination amongst morbilliviruses is the F
antigen. The
epitopic relationships found between morbilliviruses as identified by the MAbs were classified as follows. (i) Group-specific
epitopes were present on all strains of the three morbillivirus types. (ii) Group-cross-reactive
epitopes were present on only some of the strains from each morbillivirus type (these
epitopes identified the presence of intratypic strain variation in all
proteins of all three virus types). (iii) Type-specific
epitopes, i.e. MV unique or CDV unique, were found only on the homologous morbillivirus type. (iv) CDV-RPV intertypic and MV-RPV intertypic
epitopes were, respectively,
epitopes shared by CDV and RPV but not with any MV strain, and
epitopes shared by MV and RPV but not with any CDV strain. These cross-reactivities and type-specific reactions were obtained with the internal
viral proteins (M, P and NP). The
epitopes of the F
proteins were mainly group-specific and no CDV-RPV or MV-RPV intertypic
epitopes were found. The
epitopes of the H
protein were either type-specific or MV-RPV intertypic. These data support the proposed evolutionary relationship between the morbilliviruses.