VP2 is the major exposed protein on orbiviruses

VP2 gene phylogenetic characterization of field isolates of African horsesickness virus serotype 7 circulating in South Africa during the time of the 1999 African horsesickness outbreak in the Western Cape

We present the first VP2-gene phylogenetic analysis of African horsesickness (AHS) viruses within a serotype. Thirteen AHSV 7 isolates were obtained from cases that occurred in South Africa during 1998-1999, and three were historical AHSV 7 isolates. The goals were to start a database of isolates of known location and time of isolation and to determine if we could identify the origin of an AHS outbreak in the surveillance area in the Western Cape. We prepared full-length cDNA copies of the VP2-genes of the isolates. Nucleic acid sequence data of a 786 bp region was used to characterize the genetic relationships between the isolates. The nucleic acid identities between the isolates ranged from 95.5 to 100%. Isolates from common geographical regions grouped together. Characterization of field isolates revealed the presence of two AHSV 7 lineages in South Africa during this period. The grouping of the viruses into two clades accurately reflected the geographical groupings of the isolates. The average nucleic acid divergence between the clades was 4.3%. Within the clades the divergence was 0.5 and 0.1%, respectively. The data suggests that the AHS outbreak in the Western Cape could have been an incursion from the Kwazulu Natal Province.

African horsesickness: pathogenesis and immunity

African horsesickness (AHS) is a serious, non-contagious disease of horses and other solipeds caused by an arthropod-borne orbivirus of the family Reoviridae. In horses, AHS causes three distinct clinicopathologic syndromes, the pulmonary, cardiac and fever forms of the disease. Recent work has shown that the primary determinant of the form of disease expressed by naive horses is the virulence of the virus inoculum. Horses which recover from AHS exhibit solid humoral immunity against homologous challenge. Protective antibodies appear to be directed towards neutralizing epitopes on AHS virus VP2. The relationship of neutralization to protection and vaccination is discussed.

Rapid generation of monoclonal antibody-secreting hybridomas against African horse sickness virus by in vitro immunization and the fusion/cloning technique

Splenocytes from non-immune mice were stimulated in vitro using an equimolar mixture of factors from mixed lymphocyte reaction (MLR) and from phorbol-12-myristate 13-acetate (PMA) stimulated EL-4 cells, and concomitantly immunized with inactivated African horse sickness virus (AHSV) antigen serotype 4 or viral proteins 2 and 5 from AHSV serotype 9. Fusion with NSO myeloma cells was performed five days after primary or secondary stimulation/immunization. The record of hybridoma growth after a standard method of fusion, expansion of cells and subsequent cloning was compared with a fusion/cloning method in which cells were cloned within 2 to 3 days of the fusion event. Detection of antigen specific antibodies in the hybridoma culture supernatants was successful only with cells derived from primary stimulation/immunizations. Antibodies were detected using an indirect ELISA with the immunizing antigen coated on to the surface of the plates. Monoclonal hybridomas were isolated within 2 to 3 weeks using the fusion/cloning method, compared with the standard method, where it took 4 to 5 weeks. Although the total number of clones isolated from the fusion/cloning method was less than that obtained through the standard method, the yield of specific antibody-producing hybridomas as a percentage of the total picked was often more efficient with the fusion/cloning method. With respect to the immunoglobulin isotype produced, not all of the antibodies could be classified by the ELISA system used; 14% of anti-AHSV positive clones were identified as IgG-secreting cells, 25% as IgM-secreting, 18% were cross-reacting with IgG and IgM, and 43% could not be classified. Similar results in all aspects of the work were obtained whether a crude infected cell extract or purified outer capsid polypeptides VP2/5, from serotype 4 and serotype 9 respectively, were used.

Identification and characterization of the structural and nonstructural proteins of African horsesickness virus and determination of the genome coding assignments

Proteins present in purified African horsesickness virus (AHSV) and in infected cells were analyzed by SDS-polyacrylamide gel electrophoresis. Twelve viral proteins were identified, one minor and four major structural proteins, three major and two minor nonstructural proteins, as well as variable amounts of two additional structural proteins. Cell-free translation of total AHS virion RNA in a rabbit reticulocyte system resulted in the synthesis of proteins which were qualitatively and quantitatively similar to those found in infected cells. The in vivo and in vitro synthesized proteins were viral specific as demonstrated by immunoprecipitation. The coding assignments of all the purified genome segments were determined by in vitro translation and confirmed by immunoprecipitation.