In vivo T-cell subset depletion suggests that CD4+ T-cells and a humoral immune response are important for the elimination of orf virus from the skin of sheep.

In vivo lymphocyte subset depletion offers a unique opportunity to study the roles of different cellular components of the immune system of sheep during infection with orf virus. Lambs were depleted of specific lymphocyte subsets by the intravenous administration of monoclonal antibodies against ovine lymphocyte surface markers and then challenged with orf virus. The skin lesions that developed were scored visually as to their severity. Blood samples were collected to monitor the lymphocyte depletions and to measure orf-virus-specific antibody levels. Skin biopsies were collected from the lesion site and studied to determine the course of the infection and the presence of various cell types and orf virus. All the sheep developed orf virus lesions after infection. All three of the CD4-depleted lambs were unable to clear virus from their skin and did not have an antibody response to the virus. Virus was also detected in the skin of one each of the three CD8-depleted, WC1-depleted and control sheep on the final day of the trial. CD8(+) lymphocytes did not appear to be essential for viral clearance later in the infection. Depletion of the majority of gammadelta(+) T-cells did not affect the outcome of orf virus infection. In sheep with high orf-virus-specific antibody titres at the time of infection, orf lesions healed faster than lesions in sheep with low antibody levels, and this occurred regardless of the lymphocyte depletion status of the animals. This study suggests that the presence of CD4(+) T-cells and orf-virus-specific antibodies are important for the control of viral replication in the skin of infected sheep.