The Role of T Cells in the Immune Response to Dictyocaulus viviparus in Calves

The aim of this study was to further characterise the immune responses to infection with normal L3 of D. viviparus in calves and, in particular, to investigate the role of T cells in these responses. Infection with D. viviparus induces a strong acquired immunity so this parasite provided an ideal model in which to study immune-regulation in the bovine host. Various immunological parameters were measured in calves after single and multiple infections and attempts were made to correlate these parameters with the clearance of infection and the development of resistance. Both primary infection, and challenge infection, of immune calves resulted in an increase in mRNA for IL-4, but not EL-2, in peripheral blood mononuclear cells and cells collected by bronchoalveolar lavage (BAL). There were significant increases in serum parasite-specific IgG1, IgE and IgA antibodies in serum, both after primary infection and in BAL fluid after challenge. Together, these findings suggest that there was a Th2-type response following primary and challenge infections with D. viviparus. Some of the calves had high IgG2a(Al) levels in serum and BAL fluid and these calves secreted more first stage larvae in their faeces than calves with low IgG2a(Al). This perhaps indicates that these calves were unable to clear the primary infection as efficiently as those with low IgG2a(Al). Compared with the other animals, unstimulated PBMC from one of the calves with high IgG2a(Al) produced very low levels of spontaneous IL-4 but produced higher levels of IL-2 and IFNgamma, suggesting a bias in the Th1 direction. Primary infection with D. viviparus stimulated the proliferation of gammadelta+ T cells to parasite antigen, both peripherally and locally. Associated with this was a down-regulation of proliferative responses of CD4+ and CD8+ cells to parasite antigen. This reciprocal relationship implies that activated gammadelta+ T cells may have induced factors which down-regulated CD4+ and CD8+ cells. However, after tertiary infection there was proliferation of CD4+ and CD8+ cells but not gammadelta+ T cells. These findings suggest that the proposed down-regulatory activity of gammadelta+ T cells may have been induced by the presence of parasites. In conclusion, the results presented demonstrate that a Th2-type response was important in the immune response to D. viviparus and that gammadelta+ T cells appeared to play a prominent role