NK cell activation in the IEL population and spleen of broiler chickens upon SE infection. A Gating strategy for NK cells expressing surface CD107 and intracellular IFNγ in the IEL population (first and second panels) and spleen (third and fourth panels). B Percentages of intraepithelial NK cells expressing CD107 and C IFNγ in uninfected (uninf) and SE-infected (SE-inf) chickens in the course of time. D Percentages of splenic NK cells expressing CD107 and E IFNγ in uninfected and SE-infected chickens. Mean + SEM per treatment and time point is shown (n = 5), for uninfected chickens at 7 dpi in spleen n = 4. Statistical significance is indicated as *** p < 0.001 and **** p < 0.0001.