The immune response to bluetongue virus infection.

The outcome following the single, serial and simultaneous inoculation of different bluetongue virus (BTV) types into sheep and cattle was examined in terms of temperature, viraemia and neutralising antibody responses. Animals inoculated with one BTV type and challenged with the same type were shown to be protected from this challenge and to have produced only homotypic neutralising antibodies. Following the inoculation of one type and challenge with a different BTV type no protection could be demonstrated. When rechallenged these animals were found to be resistant to a third BTV type. Animals serially inoculated with two BTV types were shown to produce a transient heterotypic neutralising antibody response to a number of BTV types and although the level of this heterotypic response diminished with time the inoculation of a third BTV type gave rise to a further heterotypic response. The simultaneous inoculation of three BTV types however, resulted in replication of only two of the three inoculated viruses and a neutralising antibody response to only those two types. The ability of BTV to induce a cell-mediated immune response, in terms of the production of cytotoxic T lymphocytes (CTL's) was examined in both mice and sheep. After inoculation with live BTV, mice produced CTL's which showed virus and H[2] restriction. On secondary in vitro stimulation, specifically stimulated sensitised memory cells also produced high numbers of CTL's. Inactivation of virus preparations, either by BPL or glutaraldehyde induced only a low level response and the use of Freunds adjuvants and double immunisation procedures failed to improve that response. However, mice immunised with a single BTV type were shown to produce CTL's which cross-reacted with a number of BTV types. These cross-reactive CTL's could be induced by both primary in vivo and secondary in vitro stimulation and a varying degree of cross-reactivity occurred with the six BTV types examined. Following these observations in mice, the ability of BTV to induce CTL's in ovines was investigated. Presumptive ovine CTL's were shown to occur, their activity to be maximal around day 14 post inoculation (pi) of virus and their activity to be genetically restricted. BTV immune sera appeared incapable of participating in antibody dependent cell-mediated cytotoxicity (ADCC) reactions and although BTV induced high levels of interferon it was itself not susceptible to the effects of this interferon. Following the demonstration that the serial inoculation of two BTV types protected against challenge with a third type and that both the humoral and cellular components of the immune response were capable of heterotypic activity, work was carried out to evaluate the relative importance of antibody and CTL's in recovery and protection from reinfection with BTV. Using antibody transfer techniques it was possible to show that immune sera has a role to play in protection from challenge with homologous virus type although that protection did not appear to correlate with neutralising antibody levels and no protection could be demonstrated against heterologous virus challenge. Cellular transfer experiments in monozygotic sheep showed that BTV-induced thoracic duct lymphocytes were able to afford some degree of protection from homologous virus challenge and that this activity was probably T cell-mediated. It was also shown that this T cell-mediated response gave solid protection against challenge with a BTV type different from that used to induce these cells. The work in this thesis is discussed with relevance to other virus diseases, BTV immunology and the present BTV vaccine policies