CD4 and infection: The results showed that there was a significant increase in PD-1 expression on CD4+ (CP BVDV, p < 0.01, Figure 1A; NCP BVDV, p < 0.001, Figure 2A) and CD8+ T cells (CP BVDV, p < 0.001, Figure 1B; NCP BVDV, p < 0.05, Figure 2B) after both the CP BVDV and the NCP BVDV infection.