Induction of apoptosis in EHV5 antigen-expressing T lymphocytes (A) and B lymphocytes (B) following inoculation at a MOI of 10 (72 hpi). Left panels; the percentage of mock- (black bars) or EHV5- (white bars) inoculated cells showing signs of apoptosis (left) or necrosis (right). Data are represented as means + SD and asterisks indicate significant differences (***P < 0.001) between mock- and EHV5-inoculated cells. Experiments were performed on cells from 3 individual horses. Cell death was analysed on living cells by the “Dead Cell Apoptosis Kit” from ThermoFisher Scientific. Apoptosis was characterized by the binding of annexin V to cellular phosphatidyl serine and necrosis was identified by binding of propidium iodide to nucleic acids in the cell. Right panels; after incubation with annexin V-FITC® (green), cells were fixed and stained for EHV5 antigens (Sultan; red). Cell nuclei are counterstained in blue. The scale bars represent 5 µm.