Progeny of sheep persistently infected with border disease virus

An immunohistochemical study of the distribution of border disease virus in persistently infected sheep

Three seronegative sheep persistently infected with Border disease virus and six seropositive, non-viraemic sheep were examined for the cellular distribution of the agent. These animals originated from a closed flock which had been kept in an isolation facility for 5 years. They were killed and immediately necropsied. There were no gross abnormalities other than reduced body weight of the persistently infected sheep. Two samples of each major organ were collected. The first sample was fixed by immersion in formalin and processed for histological examination, which showed no lesions unequivocally attributable to the viral infection. The second sample was snap-frozen for immunohistochemical examination. This revealed viral antigen in all organs of the persistently infected, but in none of the seropositive animals. The infected cells included smooth muscle cells of hollow organs and blood vessels, epithelial cells of the alimentary tract and urogenital organs, lymphocytes in lymphoid organs, endocrine cells, neurons and glial cell.

Distribution of Border disease virus antigen in lymphocyte subpopulations in the peripheral blood of experimentally infected lambs

Ten lambs were experimentally infected with Border disease virus and the distribution of viral antigen in lymphocyte subpopulations studied by flow cytometry. The virus was isolated in culture from the peripheral blood mononuclear cells (MNC) of all experimentally infected lambs for a mean period of 9.8 +/- 1.03 days. The peak virus titre of 3.26 log10 TCID50 per 10(6) MNC was attained Day 9 post-inoculation (pi). Viral antigen was present in peripheral blood lymphocytes of experimentally infected lambs as early as 24 h pi and continued to be detected up to Day 10 pi. The number of lymphocytes expressing viral antigen rose from 12.38 +/- 1.22% in samples taken Day 3 pi to 23.21 +/- 2.82% on those collected Day 7 pi, dropping gradually thereafter. During the peak period of infection, 12.46 +/- 2.09% of B cells, 37.71 +/- 10.96% of T cells and 52.33 +/- 8.27% of lymphocytes which were neither B nor T lymphocytes expressed viral antigen. Most of the infected lymphocytes expressed the OvCD5 (T cell) molecule. The virus affected all T cell subsets but the suppressor/cytotoxic (OvCD8+) cells appeared to be the main targets. During the peak period of infection, 54.20 +/- 6.16% of the infected T cells expressed the OvCD8 molecule, 31.58 +/- 7.12% were OvCD4+, and 12.67 +/- 6.50% were OvWC1+ (T-19+, gamma/delta).

Border disease virus antigens in lymphocyte subpopulations in the peripheral blood of persistently infected sheep

Biotinylated virus-specific antibodies were used to detect Border disease virus antigen by flow cytometry in the mononuclear cells of the peripheral blood of 15 sheep persistently infected with Border disease virus. The viral antigen was present in 12.86 +/- 4.65% (mean +/- SD) of mononuclear cells (MNC). The percentage of MNC that contained viral antigen was higher in lambs than in adult sheep, with mean rates of 22.68 +/- 5.02% and 11.65 +/- 4.39%, respectively. Depletion methods were used to estimate the distribution of viral antigen in peripheral blood lymphocyte subsets. The viral antigen was present in T-cells (OvCD5+), B-cells (LCAp220+) and non-T- and non-B-cells. Depletion studies revealed that 5.39 +/- 2.47% of the cells expressing the LCAp220 epitope (B-cells), 23.38 +/- 11.38% of those expressing the OvCD5 epitope (T-cells) and 55.07 +/- 10.93% of those which were neither B- nor T-cells were positive for viral antigen. Most (57.18 +/- 5.41%) of the T-cells containing viral antigen were cytotoxic/suppressor (OvCD8), 25.63 +/- 2.97% were helper (OvCD4) cells and 12.24 +/- 3.21% expressed the gamma/delta (OvWC1) epitope.

Border disease virus transmitted to sheep and cattle by a persistently infected ewe: epidemiology and control

In a Swedish sheep flock comprising 202 ewes and 13 rams, a pair of twin lambs born in the spring of 1990 demonstrated signs of border disease (BD) and were persistently infected (PI) with border disease virus (BDV). Investigation showed that BDV had been introduced in the preceding tupping period 5-6 months earlier by a bought-in ewe which, on the basis of immunoperoxidase- and polymerase chain reaction techniques, was shown to be PI with BDV. Only 7 of the ewes, all of which had been in close contact with the PI ewe, seroconverted during the subsequent gestation. Apart from the PI twin lambs the losses caused by BDV were restricted to 2 barren ewes. The twin lambs, the PI ewe and lambs from the other 4 ewes that seroconverted were removed from the flock. The flock was thereafter free from an ongoing infection with BDV as shown by the absence of seroconversion. In addition, 5 heifers in late pregnancy most probably seroconverted to bovine virus diarrhoea virus (BVDV) when kept in close contact with the same PI ewe during the winter of 1989-90. When these heifers were reintroduced to the BVDV-free dairy herd from which they originated, their serum antibody titres ranged between 1:250 and 1:1250. Neither these heifers--not their calves--caused any spread of the infection in the herd, as indicated by the absence of seroconversion in 70 cows. The present investigation shows that in the control of both BDV in sheep and BVDV in cattle, it is important to ensure that the risk of transmission of pestivirus between the 2 species is minimized.

Cytotoxic T cell responses in lambs experimentally infected with Border disease virus

Mononuclear cells cytotoxic to a noncytopathic strain of Border disease virus (BDV) but not to uninfected cells were detected in the peripheral blood of experimentally infected lambs, 10 days after experimental infection, reaching peak levels of cytotoxicity 15 days post-inoculation. The specificity of the cytotoxic activity to BDV-infected cells was tested by including autologous targets infected with another virus (bovine respiratory syncytial virus, BRSV) and normal uninfected autologous targets. The cytotoxic T cells were virus-specific, as only autologous BDV-infected targets were lysed, whereas autologous targets infected with BRSV and uninfected targets were not lysed. The present results also suggest that the cytotoxic activity was largely MHC-restricted as the specific release from BDV-infected autologous targets was significantly higher than that of BDV-infected heterologous targets (P < 0.001) and the cytotoxicity against BDV-infected homologous targets was significantly reduced by the selective depletion of OvCD8+ T cells (P < 0.001).

Lymphocyte responses to viral antigens and phytohaemagglutinin in persistently viraemic sheep and lambs experimentally infected with Border disease virus

Lymphocytes obtained from lambs 5 to 21 days after experimental infection with Border disease virus (BDV) showed significant blastogenic responses to live or inactivated BDV. Live virus stimulated significantly higher lymphocyte transformation (LT) responses than inactivated virus. Lymphocytes obtained from uninfected control and persistently infected lambs had no significant response to live or inactivated antigen. Lymphocyte responses to phytohaemagglutinin were significantly higher in control lambs than in experimentally infected lambs in samples obtained 5 to 10 days post-inoculation.

Replication of border disease virus in ovine lymphocytes and monocytes in vitro

Adherent and non-adherent mononuclear cells obtained from the peripheral blood of normal sheep supported the in vitro replication of a non-cytopathic and a cytopathic strain of Border disease virus (BDV) with no apparent cytopathic effects. There was a significant rise in virus titres in adherent mononuclear cells (monocyte) and non-adherent (lymphocyte) cultures infected with both non-cytopathic and cytopathic strains of BDV 24 hours after inoculation. Peak virus titres of 5.36 log10 TCID50 ml-1 were recorded in adherent samples incubated for 48 hours while peak titres of 6.17 log10 TCID50 ml-1 were recorded in lymphocyte culture after 72 hours of incubation. Both the non-cytopathic and the cytopathic strains of BDV produced significantly higher titres in non-adherent (lymphocyte) cultures than in adherent (monocyte) cultures (P < 0.001) but the replication in adherent cells was faster than in nonadherent cell cultures. The addition of virus on both types of mononuclear cell cultures had no effect on cell viability but it had a significant inhibitory effect on the blastogenic responses of lymphocytes to phytohaemagglutinin.