The mRNA expressions of PD-1 and PD-L1 in tumor samples of chickens with MD in the field. A total of 4 tumor samples were analyzed, and then, PBMCs from uninfected chickens (n = 3) were also analyzed as controls. The expressions of PD-1 (A), PD-L1 (B), IFN-γ (C), and meq (D) mRNA were determined by real-time RT-PCR. The concentration of each mRNA was normalized to that of β-actin mRNA. The extent of gene expression (−fold) was calculated by dividing the value of each sample by that of the value of each control (A-D). Error bars represent standard deviations.