Expression and characterization of the two outer capsid proteins of African horsesickness virus: the role of VP2 in virus neutralization.

African horsesickness virus (AHSV) is a gnat-transmitted member of the Orbivirus genus of the Reoviridae family. The virus has a genome of 10 double-stranded RNA species (L1-L3, M4-M6, S7-S10). The L2 and M6 genes of AHSV serotype 4 (AHSV-4) which encode the outer capsid proteins VP2 and VP5, respectively, were inserted into recombinant baculoviruses downstream of the baculovirus polyhedrin, or p10 promoters. Recombinant baculoviruses expressing VP2, VP5, or VP2 and VP5 proteins of AHSV-4 were isolated. The expressed AHSV proteins were similar in size and antigenic properties to those of viral AHSV-4. Expressed VP2 and VP5 proteins were purified to homogeneity and utilized to differentiate sera from vaccinated and infected horses. Antigens were also used to determine whether any other AHSV serotypes are related to AHSV-4. The results indicated that AHSV-4 is distantly related to some serotypes (e.g., AHSV-2, -6, and -9) but not to others (e.g., AHSV-5 and -7). Hyperimmune monospecific antisera raised in rabbits with purified VP2 neutralized the infectivity of a virulent strain of AHSV-4 isolated from an infected horse during a recent outbreak of the disease in Spain.