BHV-1-specific CD4⁺, CD8⁺, and γδ T cells in calves vaccinated with one dose of a modified live BHV-1 vaccine

Expression of the high-affinity interleukin 2 receptor α chain (CD25) was used to monitor antigen-specific activation of T lymphocyte subsets (CD⁴⁺, CD⁸⁺, and γδ T cells) from cattle immunized with modified live bovine herpesvirus-1 (BHV-1). Calves seronegative for BHV-1 were either vaccinated with one dose of modified live vaccine containing BHV-1 or not vaccinated to serve as negative controls. Two animals vaccinated 7 and 5 weeks before the start of the experiment with two doses of modified live vaccine containing BHV-1 served as positive controls. Blood samples were taken from the nonvaccinate group, the positive control group, and the vaccinate group at 0, 21, 35, 60, and 90 days postinoculation (PI). Isolated peripheral blood mononuclear cells from immunized and control animals were incubated for 5 days with and without live BHV-1 ISU99. Compared to the nonvaccinates, a significant (p < 0.05) increase in expression of CD25 by CD⁴⁺, CD⁸⁺, and γδ T lymphocytes from the vaccinate group was detected following in vitro exposure to live BHV-1 after vaccination. This is apparently the first report using live BHV-1 to stimulate lymphocytes in vitro and showing CD⁸⁺ T cell activation. Peripheral blood from the positive control animals was depleted of CD⁴⁺, CD⁸⁺, or γδ T lymphocytes prior to incubation with BHV-1 to assess bystander activation in the CD25 expression assay. When incubated with live BHV-1, depletion of CD⁴⁺ T cells depressed the expression of CD25 by CD⁸⁺ T cells, but not γδ T cells. Depleting CD⁸⁺ or γδ T cells prior to in vitro culture with BHV-1 did not affect the expression of CD25 by the remaining T lymphocyte subsets. Vaccinates were protected from challenge with virulent BHV-1 at 110 days postvaccination compared to nonvaccinates. Expression of CD25 appears to be a useful marker for evaluating induction of antigen-specific T lymphocyte subset responses following vaccination.