The role of cell-mediated immunity in the pathogenesis of bluetongue virus serotype 11 in the experimental infection of vaccine/sensitized calves

The cellular immune response of cattle to virulent and avirulent (inactivated) bluetongue virus (BTV) was studied. Each of three calves received three vaccinations (sensitizations) with binary ethyleneimine (BEI)-inactivated BTV, 3 weeks apart. The sensitized animals were challenged with BTV-11 strain UC8 3 weeks after the last vaccination. BTV-seropositive and BTV-seronegative calves were used as controls. The animals were bled weekly for virus isolation and for evidence of cell-mediated immunity (CMI) as determined by the lymphocyte stimulation test (LST). Peripheral blood mononuclear leukocytes (PBML) cultures were induced with purified BTV antigen; the phytomitogens phytohemagglutinin (PHA), Concanavalin A (ConA) and pokeweed (PKW) mitogen, and combinations of phytomitogens and BTV antigen. LST data were analysed by ANOVA and reported as counts per minute (CPM) and stimulation index (SI). Following BTV challenge exposure, significant SI to mitogens were found in PBML cultures for all animals. BTV antigen induced a weak CMI response. There was evidence of perturbations in lymphocyte response as characterised by a sharp decrease in lymphocyte response to mitogens following combined BTV-antigen and mitogen PBML induction. The SI diminished in PBML cultures after a 4 day incubation period, except for ConA. These results provide evidence that the cell-mediated immune response could be affected by BTV and that inhibitory mediators might play an important role in the pathogenesis of BTV in cattle.