The Effect of Site and Mode of Expression of a Heterologous Antigenic Determinant on the Properties of Poliovirus Hybrids

The antigenic site normally situated in the EF loop of VP2 (2EF) of poliovirus type 2 (Lansing) [PV2 (L)] was expressed in 2EF or in the BC loop of VP1 (1BC) of PV1 (Mahoney) [PV1 (M)]. A hybrid virus expressing the site in 2EF of PV1 (M) is known to be neutralizable by PV2 (L)-specific antisera and to induce neutralizing antibodies against PV-2 (L). In contrast, a hybrid expressing a related sequence in 1BC of PV1 (M), which maintained the length of the native BC loop, was not neutralizable by PV2 (L)-specific antisera and did not induce PV2 (L)-neutralizing antibodies. However, when 1BC was extended, so that it was longer than the native 1BC, the resulting hybrids induced low titers of PV2 (L)-neutralizing antibody although they were still not neutralizable by PV2 (L)-specific antisera. Synthetic peptides copying the extended sequences raised neutralizing antibodies which were able to distinguish between the sequence expressed in the extended 1BC, in the native-length 1BC and in 2EF. The growth rates of the hybrids with modifications to 1BC depended upon the nature of the modifications. The hybrid with the native length 1BC grew poorly, but extending 1BC tended to improve the growth rate, and extending 1BC with PV1 (M)-specific sequences nearly restored wild-type growth rates. Thus, the size, precise sequence and location of a heterologous antigenic site expressed on poliovirus have significant effects on the properties of that determinant and of the hybrid expressing it. Antigenicity of hybrids can be modified and their growth rate can be enhanced by appropriate choices of these parameters.