Detection of cellular cytokine mRNA expression during orf virus infection in sheep: differential interferon-γ mRNA expression by cells in primary versus reinfection skin lesions

In sheep infected with the parapoxvirus orf virus, primary infection orf skin lesions developed and resolved within 8 weeks. Reinfection lesions were smaller and resolved within 3 weeks. The host response in the skin was characterized by an accumulation of neutrophils, dendritic cells, CD4+ T cells, CD8+ T cells, B cells and T19+ γδ T cells. The magnitude of this accumulation paralleled orf virus replication in the skin. In situ hybridization was used to detect cells expressing interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α) and interleukin-4 (IL-4) mRNAs in orf skin. Cells expressing IL-4 mRNA were not detected at any time after infection. Cells expressing IFN-γ mRNA were detected after reinfection but not after primary infection. Cells expressing TNF-α mRNA included epidermal cells, vascular endothelium and uncharacterized cells that increased more rapidly in the skin after reinfection compared to primary infection. The results are consistent with a prominent role for IFN-γ in the host immune response controlling the severity of the disease.