PDCD1 and infection: Moreover, during the acute infection, Qa1 was 4-fold higher in esophagus of BALB/c strain compared to C57BL/6 animals, and PD-1 expression was 2-fold higher in heart of C57BL/6 mice compared to BALB/c strain (P < 0.05; Figures 3(a) and 3(b)).