Studies on the role of VP4 of G serotype 10 rotavirus (B223) in the induction of the heterologous immune response in calves.

In a previous study, convalescent antisera from gnotobiotic calves (GC) infected with the G10 serotype bovine rotavirus (B223) were found to neutralize a number of rotaviruses representing G1-G6 and G8-G10 serotypes, except for a G8 serotype 69M (Z. Xu et al., Vet. Microbiol., 1993). In order to determine the immunodominant antigen, a panel of reassortants between B223 and 69M was generated and tested with the B223 GC antisera. It was found that the antisera neutralized the infectivities of the reassortants containing VP7 of B223 at titers similar to those with the parental virus B223, whereas the neutralization titers with the reassortants containing VP4 but not VP7 from B223 were significantly lower. These titers were close to the heterologous titers obtained with B641 (a G6 serotype rotavirus) against which B223 induced protective immunity. In contrast, the reassortants with both VP4 and VP7 from 69M were not neutralized by the B223 GC antisera. This indicates that the immunodominant neutralizing antigen during the primary immune response in calves is VP7 and the heterologous response is probably directed at VP4. Evidence was obtained through a cross-reactive B223 VP4 monoclonal antibody (MAB), B223-N6, that a shared epitope exists among B223, B641, and various G serotype rotaviruses. These data support the conclusion that VP4 is an important antigen for G10 as well as G6 rotavirus immunity. The B223/69M reassortants were also assayed with two cross-reactive VP4 MABs B223-N6 and 2G4. B223 VP7 was found to enhance the neutralization titers of the MABs with rotaviruses containing either homologous (B223) VP4 or the heterologous (69M) VP4.